Showing papers on "Spiral galaxy published in 2004"

The Geneva-Copenhagen survey of the Solar neighbourhood - Ages, metallicities, and kinematic properties of ~14 000 F and G dwarfs

Abstract: We present and discuss new determinations of metallicity, rotation, age, kinematics, and Galactic orbits for a com- plete, magnitude-limited, and kinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our ∼63 000 new, accurate radial-velocity observations for nearly 13 500 stars allow identification of most of the binary stars in the sample and, together with published uvbyβ photometry, Hipparcos parallaxes, Tycho-2 proper motions, and a few earlier radial velocities, complete the kinematic information for 14 139 stars. These high-quality velocity data are supplemented by effective temperatures and metallicities newly derived from recent and/or revised calibrations. The remaining stars either lack Hipparcos data or have fast rotation. Am ajor effort has been devoted to the determination of new isochrone ages for all stars for which this is possible. Particular attention has been given to a realistic treatment of statistical biases and error estimates, as standard techniques tend to under- estimate these effects and introduce spurious features in the age distributions. Our ages agree well with those by Edvardsson et al. (1993), despite several astrophysical and computational improvements since then. We demonstrate, however, how strong observational and theoretical biases cause the distribution of the observed ages to be very different from that of the true age distribution of the sample. Among the many basic relations of the Galactic disk that can be reinvestigated from the data presented here, we revisit the metallicity distribution of the G dwarfs and the age-metallicity, age-velocity, and metallicity-velocity relations of the Solar neighbourhood. Our first results confirm the lack of metal-poor G dwarfs relative to closed-box model predictions (the "G dwarf problem"), the existence of radial metallicity gradients in the disk, the small change in mean metallicity of the thin disk since its formation and the substantial scatter in metallicity at all ages, and the continuing kinematic heating of the thin disk with an efficiency consistent with that expected for a combination of spiral arms and giant molecular clouds. Distinct features in the distribution of the V component of the space motion are extended in age and metallicity, corresponding to the effects of stochas- tic spiral waves rather than classical moving groups, and may complicate the identification of thick-disk stars from kinematic criteria. More advanced analyses of this rich material will require careful simulations of the selection criteria for the sample and the distribution of observational errors.

The Star Formation Rate and Dense Molecular Gas in Galaxies

Abstract: HCN luminosity is a tracer of dense molecular gas, n(H(2)) greater than or similar to3 x 10(4) cm(-3), associated with star-forming giant molecular cloud (GMC) cores. We present the results and analysis of our survey of HCN emission from 65 infrared galaxies, including nine ultraluminous infrared galaxies (ULIGs, L(IR) greater than or similar to 10(12) L(circle dot)), 22 luminous infrared galaxies (LIGs, 10(11) L(circle dot) < L(IR) less than or similar to 10(12) L(circle dot)), and 34 normal spiral galaxies with lower IR luminosity (most are large spiral galaxies). We have measured the global HCN line luminosity, and the observations are reported in Paper I. This paper analyzes the relationships between the total far-IR luminosity (a tracer of the star formation rate), the global HCN line luminosity (a measure of the total dense molecular gas content), and the CO luminosity (a measure of the total molecular content). We find a tight linear correlation between the IR and HCN luminosities L(IR) and L(HCN) (in the log-log plot) with a correlation coefficient R = 0.94, and an almost constant average ratio L(IR)/L(HCN) = 900 L(circle dot) (K km s(-1) pc(2))(-1). The IR-HCN linear correlation is valid over 3 orders of magnitude including ULIGs, the most luminous objects in the local universe. The direct consequence of the linear IR-HCN correlation is that the star formation law in terms of dense molecular gas content has a power-law index of 1.0. The global star formation rate is linearly proportional to the mass of dense molecular gas in normal spiral galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation as the power source in ultraluminous galaxies since the star formation in these galaxies appears to be normal and expected given their high mass of dense star-forming molecular gas.

Stellar chemical signatures and hierarchical galaxy formation

Abstract: To compare the chemistries of stars in the Milky Way dwarf spheroidal (dSph) satellite galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. When kinematic information is available, we have assigned the stars to standard Galactic components through Bayesian classification based on Gaussian velocity ellipsoids. As found in previous studies, the [α/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm this for the Galactic halo, thin-disk, and thick-disk components. There is marginal overlap in the low [α/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits (V < -420 km s-1), but this is not supported by other element ratios. Other element ratios compared in this paper include r- and s-process abundances, where we find a significant offset in the [Y/Fe] ratios, which results in a large overabundance in [Ba/Y] in most dSph stars compared with Galactic stars. Thus, the chemical signatures of most of the dSph stars are distinct from the stars in each of the kinematic components of the Galaxy. This result rules out continuous merging of low-mass galaxies similar to these dSph satellites during the formation of the Galaxy. However, we do not rule out very early merging of low-mass dwarf galaxies, since up to one-half of the most metal-poor stars ([Fe/H] ≤ -1.8) have chemistries that are in fair agreement with Galactic halo stars. We also do not rule out merging with higher mass galaxies, although we note that the LMC and the remnants of the Sgr dwarf galaxy are also chemically distinct from the majority of the Galactic halo stars. Formation of the Galaxy's thick disk by heating of an old thin disk during a merger is also not ruled out; however, the Galaxy's thick disk itself cannot be comprised of the remnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarf galaxy like the LMC or Sgr, because of differences in chemistry. The new and independent environments offered by the dSph galaxies also allow us to examine fundamental assumptions related to the nucleosynthesis of the elements. The metal-poor stars ([Fe/H] ≤ -1.8) in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than [Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy. Predictions from the α-process (α-rich freeze-out) would be consistent with this result if there have been a lack of hypernovae in dSph galaxies. The α-process could also be responsible for the very low Y abundances in the metal-poor stars in dSph's; since [La/Eu] (and possibly [Ba/Eu]) are consistent with pure r-process results, the low [Y/Eu] suggests a separate r-process site for this light (first-peak) r-process element. We also discuss SNe II rates and yields as other alternatives, however. In stars with higher metallicities ([Fe/H] ≥ -1.8), contributions from the s-process are expected; [(Y, La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still much higher in the dSph stars than similar metallicity Galactic stars. This result is consistent with s-process contributions from lower metallicity AGB stars in dSph galaxies, and is in good agreement with the slower chemical evolution expected in the low-mass dSph galaxies relative to the Galaxy, such that the build-up of metals occurs over much longer timescales. Future investigations of nucleosynthetic constraints (as well as galaxy formation and evolution) will require an examination of many stars within individual dwarf galaxies. Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster is confirmed in Galactic halo stars, but we discuss this in terms of the general nucleosynthesis of neutron-rich elements. We do not confirm that the Na-Ni trend is related to the accretion of dSph galaxies in the Galactic halo.

The cored distribution of dark matter in spiral galaxies

Abstract: We present the Hi data for 5 spiral galaxies that, along with their Hα rotation curves, are used to derive the distribution of dark matter within these objects. A new method for extracting rotation curves from Hi data cubes is presented; this takes into account the existence of a warp and minimises projection effects. The rotation curves obtained are tested by taking them as input to construct model data cubes that are compared to the observed ones: the agreement is excellent. On the contrary, the model data cubes built using rotation curves obtained with standard methods, such as the first-moment analysis, fail the test. The Hi rotation curves agree well with the Hα data, where they coexist. Moreover, the combined Hα + Hi rotation curves are smooth, symmetric and extended to large radii. The rotation curves are decomposed into stellar, gaseous and dark matter contributions and the inferred density distribution is compared to various mass distributions: dark haloes with a central density core, � Cold Dark Matter (�CDM) haloes (NFW, Moore profiles), Hi scaling and MOND. The observations point to haloes with con

Hcn survey of normal spiral, infrared-luminous, and ultraluminous galaxies

Abstract: We report systematic HCN J = 1-0 (and CO) observations of a sample of 53 infrared (IR) and/or CO-bright and/or luminous galaxies, including seven ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more than a dozen of the nearest normal spiral galaxies. This is the largest and most sensitive HCN survey of galaxies to date. All galaxies observed so far follow the tight correlation between the IR luminosity LIR and the HCN luminosity LHCN initially proposed by Solomon, Downes, & Radford, which is detailed in a companion paper. We also address here the issue of HCN excitation. There is no particularly strong correlation between LHCN and the 12 ?m luminosity; in fact, of all the four IRAS bands, the 12 ?m luminosity has the weakest correlation with the HCN luminosity. There is also no evidence of stronger HCN emission or a higher ratio of HCN and CO luminosities LHCN/LCO for galaxies with excess 12 ?m emission. This result implies that mid-IR radiative pumping, or populating, of the J = 1 level of HCN by a mid-IR vibrational transition is not important compared with the collisional excitation by dense molecular hydrogen. Furthermore, large velocity gradient calculations justify the use of HCN J = 1-0 emission as a tracer of high-density molecular gas (3 ? 104/? cm-3) and give an estimate of the mass of dense molecular gas from HCN observations. Therefore, LHCN may be used as a measure of the total mass of dense molecular gas, and the luminosity ratio LHCN/LCO may indicate the fraction of molecular gas that is dense.

A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X-Ray Emission

Abstract: We present arcsecond resolution Chandra X-ray and ground-based optical Hα imaging of a sample of 10 edge-on star-forming disk galaxies (seven starburst and three "normal" spiral galaxies), a sample that covers the full range of star formation intensity found in disk galaxies. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to more robustly than before remove X-ray emission from point sources and hence obtain the X-ray properties of the diffuse thermal emission alone. We have combined the X-ray observations with existing, comparable-resolution, ground-based Hα and R-band imaging and present a mini-atlas of images on a common spatial and surface brightness scale to aid cross-comparison. In general, the morphology of the extraplanar diffuse X-ray emission is very similar to the extraplanar Hα filaments and arcs, on both small and large scales (scales of tens of parsecs and kiloparsecs, respectively). The most spectacular cases of this are found in NGC 1482 (for which we provide the first published X-ray observation) and NGC 3079. We provide a variety of quantitative measures of how the spectral hardness and surface brightness of the diffuse X-ray emission varies with increasing height z above the plane of each galaxy. Of the eight galaxies in which diffuse X-ray emitting halos are found (the starbursts and the normal spiral NGC 891), significant spatial variation in the spectral properties of the extraplanar emission (|z| ≥ 2 kpc) is only found in two cases: NGC 3628 and NGC 4631. In general, the vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights of the sample galaxies lying in the range Heff ~ 2-4 kpc. The presence of extraplanar X-ray emission is always associated with the presence of extraplanar optical line emission of similar vertical extent. No X-ray emission was detected from the halos of the two low-mass normal spiral galaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present, appear to play no role in powering or shaping the outflows from the starburst galaxies in this sample. The Chandra ACIS X-ray spectra of extraplanar emission from all these galaxies can be fitted with a common two-temperature spectral model with an enhanced α-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. Our favored model is that SN feedback in the disks of star-forming galaxies create, via blow-out and venting of hot gas from the disk, tenuous exponential atmospheres of density scale height Hg ~ 4-8 kpc. The soft thermal X-ray emission observed in the halos of the starburst galaxies is either this preexisting halo medium, which has been swept up and shock-heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case, the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.

VLA HI Observations of Gas Stripping in the Virgo Cluster Spiral NGC 4522

Abstract: We present VLA HI observations at ~20"=1.5 kpc resolution of the highly inclined, HI-deficient Virgo cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing ICM-ISM stripping. HI is abundant and spatially coincident with the stellar disk in the center, but beyond R = 3 kpc the HI distribution in the disk is sharply truncated and the only HI is extraplanar, and all on the northwest side. The kinematics and the morphology of the HI appear more consistent with ongoing stripping, and less consistent with gas fall-back which may occur long after peak pressure. Much of the extraplanar gas exhibits a modest net blueshift with respect to the galaxy's disk rotational velocities, consistent with gas accelerated toward the mean cluster velocity. The SW side of the galaxy has less HI in the disk but more HI in the halo, suggesting more effective gas removal on the side of the galaxy which is rotating into the ICM wind. The galaxy is 3.3 degrees ~800 kpc from M87, somewhat outside the region of strongest cluster X-ray emission. The ram pressure may be significantly stronger than standard values, due to large bulk motions and local density enhancements of the ICM gas, which may occur in a dynamic, shock-filled ICM experiencing subcluster merging. The HI and H-alpha distributions are similar, implying that the star-forming molecular ISM has been effectively stripped from the outer disk of the galaxy along with the HI.

On the Evolutionary Status of Class I Stars and Herbig-Haro Energy Sources in Taurus-Auriga

Abstract: We present high-resolution (R ~ 34,000) optical (6330-8750 A) spectra obtained with the HIRES spectrograph on the W. M. Keck I telescope of stars in Taurus-Auriga whose circumstellar environment suggests that they are less evolved than optically revealed T Tauri stars. Many of the stars are seen only via scattered light. The sample includes 15 class I stars and all class II stars that power Herbig-Haro flows in this region. For 28 of the 36 stars observed, our measurements are the first high-dispersion optical spectra ever obtained. Photospheric features are observed in all stars with detected continuum, 11 of 15 class I stars (42% of known Taurus class I stars) and 21 of 21 class II stars; strong emission lines (e.g., Hα) are detected in the spectra of all stars. These spectra, in combination with previous measurements, are used to search for differences between stars that power Herbig-Haro flows and stars that do not and to reassess the evolutionary state of so-called protostars (class I stars) relative to optically revealed T Tauri stars (class II stars). The stellar mass distribution of class I stars is similar to that of class II stars and includes three spectroscopically confirmed class I brown dwarfs. Class I stars (and brown dwarfs) in Taurus are slowly rotating (v sin i < 35 km s^(-1)); the angular momentum of a young star appears to dissipate prior to the optically revealed T Tauri phase. The amount of optical veiling and the inferred mass accretion rates of class I stars are surprisingly indistinguishable from class II stars. Class I stars do not have accretion-dominated luminosities; the accretion luminosity accounts for ~25% of the bolometric luminosity. The median mass accretion rate of class I and class II stars of K7-M1 spectral type is 4 × 10^(-8) M_☉ yr^(-1), and the median mass outflow rate is 5% of the mass accretion rate. The large ranges in mass accretion rate (~2 orders of magnitude), mass outflow rate (~3 orders of magnitude), and the ratio of these quantities (~2 orders of magnitude) represent real dispersions in young accreting stars of similar mass. We confirm previous results that find larger forbidden-line emission associated with class I stars than class II stars. We suggest that this is caused by an orientation bias that allows a more direct view of the somewhat extended forbidden emission line regions than of the obscured stellar photospheres, rather than being caused by larger mass outflow rates. Overall, the similar masses, luminosities, rotation rates, mass accretion rates, mass outflow rates, and millimeter flux densities of class I stars and class II stars are best explained by a scenario in which most class I stars are no longer in the main accretion phase and are much older than traditionally assumed. Similarly, although stars that power Herbig-Haro flows appear to have larger mass outflow rates, their stellar and circumstellar properties are generally indistinguishable from those of similar mass stars that do not power these flows.

The ultraluminous x-ray source population from the chandra archive of galaxies

Abstract: One hundred fifty-four discrete non-nuclear Ultra-Luminous X-ray (ULX) sources, with spectroscopically-determined intrinsic X-ray luminosities greater than 1 e39 ergs/s, are identified in 82 galaxies observed with Chandra's Advanced CCD Imaging Spectrometer. Source positions, X-ray luminosities, and spectral and timing characteristics are tabulated. Statistical comparisons between these X-ray properties and those of the weaker discrete sources in the same fields (mainly neutron star and stellar-mass black hole binaries) are made. Sources above approximately le38 ergs per second display similar spatial, spectral, color, and variability distributions. In particular, there is no compelling evidence in the sample for a new and distinct class of X-ray object such as the intermediate-mass black holes. 83% of ULX candidates have spectra that can be described as absorbed power laws with index = 1.74 and column density = 2.24e21 l per square centimeter, or approximately 5 times the average Galactic column. About 20% of the ULX's have much steeper indices indicative of a soft, and likely thermal, spectrum. The locations of ULXs in their host galaxies are strongly peaked towards their galaxy centers. The deprojected radial distribution of the ULX candidates is somewhat steeper than an exponential disk, indistinguishable from that of the weaker sources. About 5--15% of ULX candidates are variable during the Chandra observations (which average 39.5 ks). Comparison of the cumulative X-ray luminosity functions of the ULXs to Chandra Deep Field results suggests approximately 25% of the sources may be background objects including 14% of the ULX candidates in the sample of spiral galaxies and 44% of those in elliptical galaxies implying the elliptical galaxy ULX population is severely compromised by background active galactic nuclei. Correlations with host galaxy properties confirm the number and total X-ray luminosity of the ULXs are associated with recent star formation and with galaxy merging and interactions. The preponderance of ULXs in star-forming galaxies as well as their similarities to less-luminous sources suggest they originate in a young but short-lived population such as the high-mass X-ray binaries with a smaller contribution (based on spectral slope) from recent supernovae. The number of ULXs in elliptical galaxies scales with host galaxy mass and can be explained most simply as the high-luminosity end of the low-mass X-ray binary population.

VLA H I Observations of Gas Stripping in the Virgo Cluster Spiral NGC 4522

Abstract: We present VLA H I observations at ~20'' 1.5 kpc resolution of the highly inclined, H I–deficient Virgo Cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing intracluster medium–interstellar medium (ICM-ISM) stripping. H I is abundant and spatially coincident with the stellar disk in the center, but beyond R = 3 kpc the H I distribution in the disk is sharply truncated, and the only H I is extraplanar and all on the northwest side. Forty percent of the total H I, corresponding to 1.5 × 108 M⊙, is extraplanar and has likely been removed from the galaxy disk by an ICM-ISM interaction. The kinematics and the morphology of the H I appear more consistent with ongoing stripping and less consistent with gas fall-back, which may occur long after peak pressure. Some of the extraplanar gas has line widths (FWZI) of 150 km s-1, including a blueshifted tail of weaker emission, and much of the extraplanar gas exhibits a modest net blueshift with respect to the galaxy's disk rotational velocities, consistent with gas accelerated toward the mean cluster velocity. The southwest side of the galaxy has less H I in the disk but more H I in the halo, suggesting more effective gas removal on the side of the galaxy that is rotating into the ICM wind. In recent simulations of ICM-ISM interactions large surface densities of extraplanar gas like that in NGC 4522 are seen at relatively early stages of active stripping and not during later gas fall-back stages. The galaxy is 33 800 kpc from M87, somewhat outside the region of strongest cluster X-ray emission. The ram pressure at this location, assuming a static smooth ICM and standard values for ICM density and galaxy velocity, appears inadequate to cause the observed stripping. We consider the possibility that the ram pressure is significantly stronger than standard values, because of large bulk motions and local density enhancements of the ICM gas, which may occur in a dynamic, shock-filled ICM experiencing subcluster merging. The H I and Hα distributions are similar, with both truncated in the disk at the same radius and H II regions located throughout much of the extraplanar H I. This implies that the star-forming molecular ISM has been effectively stripped from the outer disk of the galaxy along with the H I. The inferred peak stripping rate of ~10 M⊙ yr-1 is much larger than the galaxy's total star formation rate of ~0.1 M⊙ yr-1, implying that the rate of triggered star formation due to ICM pressure is presently minor compared with the rate of gas lost as a result of stripping.

The Ultra-Luminous X-ray Source Population from the Chandra Archive of Galaxies

Abstract: One hundred fifty-four discrete non-nuclear Ultra-Luminous X-ray (ULX) sources, with intrinsic X-ray luminosities >1e39 ergs/s, are identified in 82 galaxies observed with Chandra's Advanced CCD Imaging Spectrometer. Source positions, X-ray luminosities, and spectral and timing characteristics are tabulated. Statistical comparisons between these X-ray properties and those of the weaker discrete sources in the same fields are made. Sources above ~1e38 erg/s display similar spatial, spectral, color, and variability distributions. In particular, there is no compelling evidence in the sample for a new and distinct class of X-ray object such as the intermediate-mass black holes. Comparison of the cumulative X-ray luminosity functions of the ULXs to Chandra Deep Field results suggests ~25% of the sources may be background objects including 14% of the ULX candidates in the sample of spiral galaxies and 44% of those in elliptical galaxies implying the elliptical galaxy ULX population is severely compromised by background active galactic nuclei. Correlations with host galaxy properties confirm the number and total X-ray luminosity of the ULXs are associated with recent star formation and with galaxy merging and interactions. The preponderance of ULXs in star-forming galaxies as well as their similarities to less-luminous sources suggest they originate in a young but short-lived population such as the high-mass X-ray binaries with a smaller contribution (based on spectral slope) from recent supernovae. The number of ULXs in elliptical galaxies scales with host galaxy mass and can be explained most simply as the high-luminosity end of the low-mass X-ray binary population.

The Anatomy of Star Formation in NGC 300

Abstract: The Spitzer Space Telescope was used to study the mid- to far-infrared properties of NGC 300 and to compare dust emission to Hα to elucidate the heating of the interstellar medium (ISM) and the star formation cycle at scales smaller than 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 μm dust emission with respect to 24 μm dust and to H II regions traced by Hα light. The 8 μm emission highlights the rims of H II regions, and the 24 μm emission is more strongly peaked in star-forming regions than 8 μm. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 μm, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star-forming regions. When averaging over regions larger than ~1 kpc, the ratio of Hα to aromatic feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid- to far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.

Gas physics, disk fragmentation, and bulge formation in young galaxies

Abstract: We investigate the evolution of star-forming gas-rich disks, using a 3D chemodynamical model including a dark halo, stars, and a two-phase interstellar medium with feedback processes from the stars. We show that galaxy evolution proceeds along very different routes depending on whether it is the gas disk or the stellar disk which first becomes unstable, as measured by the respective Q-parameters. This in turn depends on the uncertain efficiency of energy dissipation of the cold cloud component from which stars form. When the cold gas cools efficiently and drives the instability, the galactic disk fragments and forms a number of massive clumps of stars and gas. The clumps spiral to the center of the galaxy in a few dynamical times and merge there to form a central bulge component in a strong starburst. When the kinetic energy of the cold clouds is dissipated at a lower rate, stars form from the gas in a more quiescent mode, and an instability only sets in at later times, when the surface density of the stellar disk has grown sufficiently high. The system then forms a stellar bar, which channels gas into the center, evolves, and forms a bulge whose stars are the result of a more extended star formation history. We investigate the stability of the gas-stellar disks in both regimes, as well as the star formation rates and element enrichment. We study the morphology of the evolving disks, calculating spatially resolved colours from the distribution of stars in age and metallicity, including dust absorption. We then discuss morphological observations such as clumpy structures and chain galaxies at high redshift as possible signatures of fragmenting, gas-rich disks. Finally, we investigate abundance ratio distributions as a means to distinguish the different scenarios of bulge formation.

Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties

Abstract: We performed a compilation of more than 1000 published spectra of H  regions in spiral galaxies. The oxygen and nitrogen abundances in each H  region were recomputed in a homogeneous way, using the P-method. The radial dis- tributions of oxygen and nitrogen abundances were derived. The correlations between oxygen abundance and macroscopic properties are examined. We found that the oxygen abundance in spiral galaxies correlates with its luminosity, rotation velocity, and morphological type: the correlation with the rotation velocity may be slightly tighter. There is a significant difference be- tween the luminosity-metallicity relationship obtained here and that based on the oxygen abundances determined through the R23-calibrations. The oxygen abundance of NGC 5457 recently determined using direct measurements of Te (Kennicutt et al. 2003) agrees with the luminosity-metallicity relationship derived in this paper, but is in conflict with the luminosity-metallicity relationship derived with the R23-based oxygen abundances. The obtained luminosity-metallicity relation for spiral galaxies is compared to that for irregular galaxies. Our sample of galaxies shows evidence that the slope of the O/H − MB relationship for spirals (−0.079 ± 0.018) is slightly more shallow than that for irregular galaxies (−0.139 ± 0.011). The effective oxygen yields were estimated for spiral and irregular galaxies. The effective oxygen yield increases with increasing luminosity from MB ∼− 11 to MB ∼− 18 (or with increasing rotation velocity from Vrot ∼ 10 km s −1 to Vrot ∼ 100 km s −1 ) and then remains approximately constant. Irregular galaxies from our sample have effective oxygen yields lowered by a factor of 3 at maximum, i.e. irregular galaxies usually keep at least 1/3 of the oxygen they manufactured during their evolution.

Halpha Morphologies and Environmental Effects in Virgo Cluster Spiral Galaxies

Abstract: We describe the various Halpha morphologies of Virgo Cluster and isolated spiral galaxies, and associate the Halpha morphologies with the types of environmental interactions which have altered the cluster galaxies. The spatial distributions of Halpha and R-band emission are used to divide the star formation morphologies of the 52 Virgo Cluster spirals into several categories: normal (37%), anemic (6%), enhanced (6%), and (spatially) truncated (52%). Truncated galaxies are further subdivided based on their inner star formation rates into truncated/normal (37%), truncated/compact (6%), truncated/anemic (8%), and truncated/enhanced (2%). The fraction of anemic galaxies is relatively small (6-13%) in both environments, suggesting that starvation is not a major factor in the reduced star formation rates of Virgo spirals. The majority of Virgo spiral galaxies have their Halpha disks truncated (52%), whereas truncated Halpha disks are rarer in isolated galaxies (12%). Most of the Halpha-truncated galaxies have relatively undisturbed stellar disks and normal-to-slightly enhanced inner disk star formation rates, suggesting that ICM-ISM stripping is the main mechanism causing the reduced star formation rates of Virgo spirals. In other galaxies, the Halpha morphologies are more consistent with a tidal origin or perhaps outer cluster HI accretion. These results indicate that most Virgo spiral galaxies experience ICM-ISM stripping, many experience significant tidal effects, and many experience both. (abridged).

Penetrating bars through masks of cosmic dust : the hubble tuning fork strikes a new note

Abstract: The Editors - Preface A Tribute to Cosmic Dust Pioneer J. Mayo Greenberg D.L. Block The Hubble Tuning Fork Strikes a New Note D.L. Block et al. Episodes in the Development of the Hubble Galaxy Classification A. Sandage Secular Evolution Versus Hierarchical Merging: Galaxy Evolution along the Hubble-sequence, in the Fields and Rich Environments F. Combes Dense Gas and Star Formation in Bars S. Huttemeister On the Origin of S0 Galaxies U.-Fritze v. Alvensleben Gravitational Bar Torques in the Spiral/S0 Divide R. Buta Direct Measurement of Pattern Speeds In Double-Barred SB0's E.M. Corsini et al. Gas Flows, Star Formation and Galaxy Evolution J.E. Beckman et al. Bar-Driven Evolution and 2D Spectroscopy of Bulges M. Bureau et al. Bar-Driven Fueling of Galactic Nuclei: a 2D view E. Emsellem Dust Penetrated Arm Classes: Insight form Rising and Falling Rotation Curves M.S. Seigar, D.L. Block, I. Puerari Bar Dissolution and Reformation Mechanisms F. Bournaud, F. Combes Dynamics of Doubly Barred Galaxies, also with the Inner Bar Retrograde W. Maciejewski A Coordinated Episode of AGB Star Production at Large Galactocentric Distances in the Andromeda Galaxy G. Worthey Fuelling Starbursts and AGN J.H. Knapen Penetrating Dust Tori in AGN G. Canalizo et al. Bars from the Inside Out: An HST Study of their Dusty Circumnuclear Regions P. Martini Morphology of Bar and Spiral Modes: do they relate? P.J. Grosbol Bar Formation by Galaxy-Galaxy Interactions M. Noguchi Triggering AGN's - Interactions or Bars? J.Lim et al. Triggered Star Formation: from Large to Small Scales J. Palous, J. Jachym, E. Ehlerova Investigation of Age and Metallicity Gradients in Spiral Galaxies B. Cunow Secular Evolution and the Growth of Pseudobulges in Disk Galaxies J. Kormedy, M.E. Cornell Bars and Lences in Spiral Galaxies: Clues for Secular Evolution L.K. Hunt, C. Giovanardi, M.A. Malkan Evolution and Impact of Bars over the last nine Gyr: Early Results from GEMS S.Jogee et al. First Phylogenetic Analysis of Galaxy Evolution D. Fraix-Burnet A Unified Picture of Disk Galaxies where Bars, Spirals and Warps Result from the Same Fundametal Causes D. Pfenninger, Y. Revez On the Generation of the Hubble Sequence trough an Internal Secular Dynamical Process X. Zhang The Angular Momentum Problem and the Formation of Bulgeless Galaxies E. D'Onghia, A, Burkert Disks Evolution in a Cosmological Framework A. Curir, P. Mazzel, G. Murante Galaxy Formation and the Cosmological Angular Momentum Problem A.M. Burkert, E. D'Onghia The Problems with Galxy Formation G. Lake The Interplay between Bars and Dark Matter Halos K. Holley-Bockelmann Resent Results from the Spitzer Space Telescope: A New View of Galaxy Morphology and Classification G.G. Fazio et al. Using Bars as Signposts of Galaxy Evolution at High and Low Redshifts K. Sheth et al. The VIMOS VLT Deep Survey: Redshift Distribution of a IAB greater than or equals 24 Sample, and the Effect of Environment on Galaxy Evolution O. Le Fevre et al. An HST AOS/WFC Ha Imaging Survey of Nearby Galaxies R.A. Jansen HST Mid-UV Imaging of Nearby Galaxies R.A. Windhorst, V.A. Taylor, R.A. Jansen Bulges, Disks and Kinematics of Galaxies at z ~ 1 D.C. Koo Fourier Decomposition of Galaxies S.C. Odewahn The Evolutionary Status of Clusters of Galaxies at z ~ 1 H. Ford et al. Distant z greater than 2 Protoclusters and their Galaxies G. Miley et al. The Galaxy Structure - Redshift Relationship C.J. Conselice The Cosmic Bacground: Evolution of Infrared Galaxies and Dust Properties. A lecture Dedicated to the Memory of Mayo Greenberg J.-L. Puget, G. Lagache, H. Dole The Physical Evolution of Mass and Dust in Distant Galaxies B. Rocca-Wolmarange The Warm, Cold and Very Cold Dusty Universe A. Li Turbulence and Galactic Structure B.G. Elmegreen Chaos in Spiral Galaxies F.H. Shu, S. Chakrabarti, G. Laughlin Obtaining Statistics of Turbulent Velocity from Astrophysical Spectral Line Data A. Lazarian Estimating Power Spectra of Galaxy Structure: Can Statistics Help? P.B. Stark From z greater than 6 to z~2: Unearthing Galaxies at the Edge of the Dark Ages G. Illingsworth, L. Bouwens Masks in the Milky Way K.C. Freeman The Hierarchical Formation of the Galactic Disk J.F. Navarro The SINGS view of Barred Galaxies M.W. Reagan et al. Quantifying Bar Strength: Morphology Meets Methodology P.B. Eskridge Estimation of Bar Strengths from nearby IR Images H. Salo, E. Laurikainen, R. Buta Bar Strengths Measured for the OSUBGS sample:active vs non-active galaxes E. Laurikanen, H. Salo, R. Buta Globular Clusters: Galactic and Internal Motions I. King Evolution of Self-Gravitating Gas Disks Driven by a Rotating Bar Potential C. Yuan, D.C.C. Yen Stellar Disk Trunkations:Where do we stand? M. Pohlen et al. On the unification of Dwarf and Giant Elliptical Galaxies A.W. Graham, R. Gizman Photodissociation and the morphology of HI in Galxies R.J Allen The True H2 Content of Spiral Galaxies F. Boulanger X-Ray Perspective of Early Type Galaxies D.-W. Kim Spiral Arm Star Formation in Barred Galaxies S. Baes-Fischlmair et al. Galaxy Types and Luminosity Functions in the Soan Digital Sky Survey using Artificial Neural Networks N.M.. Ball, J. Loveday Globular Cluster Systems and Supermassive Galactic Black Holes R. Capuzzo-Dolcetta Pseudobulges in Barred S0 Galaxies P. Erwin, J.E. Beckman, J.-C. Vega-Beltran Bar and Spiral Torques in the Triangulum Galaxy M33 R. Groess, D.L. Block, I. Puerari Bar Parameters from Ha Observations O. Hernandez et al. Molecular Gas in Classical Elliptical Radio Galaxies J. Lim et al. nBars and Dust Multi Wavelength Maps of Simulations of Galaxy Formation P. Mazzel, A. Curir How Barred is the NIR Nearby Universe? An Analysis using 2MASS K. Menendez-Delmestre et al. Truncation of Stellar Disks at high Redshift J. Perez Is there a Large Stellar Bar in the LSB Galaxy UGC7321? M. Pohlen, M. Balcells, L.D. Matthews Spiral structure of the Milky Way: The state of Affairs T. Steiman-Cameron, M. Wolfire, D. Hollenbach Uncovering Morphology from dust: a NIR View of the Interacting Galaxy Pair NGC5394/95 M. Valdez-Gutierrez, I. Puerari, I. Hernandez-Lopez SCUBA Local Universe Galaxy Survey V. Vlahakis, S. Eales, L. Dunne How JWST can measure First Light, Reionization and Galaxy Assembly R.A. Windhorst, H. Yan High Resolution Velocity Fields in the Strongly Barred Galaxy NGC 1530 A. Zurita et al. Panel Discussion -Closing Conference Thoughts by Vera Rubin The Noblest Scientific Problem of the Age: Perspectives on the Transit of Venus, 1882 and 2004 W. Sheehan Description of the Commemorative Medallion, Struck to Celebrate the Transition of Venus, Observed from South Africa on 8 June 2004 B.G. Elmegreen Caption for Colour plates The 16 colour plates List of Participants

Abundances of Metal-rich H II Regions in M51

Abstract: We have obtained multiobject spectroscopy of H II regions in the spiral galaxy M51 with the Keck I telescope and the Low Resolution Imaging Spectrometer. For 10 objects we have detected the auroral line [N II] λ5755, while [S III] λ6312 has been measured in seven of these. This has allowed us to measure the electron temperature of the gas and to derive oxygen, sulfur, and nitrogen abundances for the 10 H II regions. Contrary to expectations from previous photoionization models of a few H II regions in M51 and from strong-line abundance indicators, the O/H abundance is below the solar value for most objects, with the most metal-rich H II regions, P203 and CCM 72, having log(O/H) = -3.16 [~1.4(O/H)☉] and log(O/H) = -3.29 [~1.0 (O/H)☉], respectively. The reduction of O/H by factors of up to 2 or 3 with respect to previous indirect determinations has important consequences for the calibration of empirical abundance indicators, such as R23, in the abundance and excitation range found in the central regions of spiral galaxies. The abundance gradients in these galaxies can therefore be considerably flatter than those determined by using such empirical calibrations. The H II regions with a measured electron temperature span the range (0.19-1.04) R0 in galactocentric radius and indicate a shallow abundance gradient for M51: -0.02 ± 0.01 dex kpc-1. The S/O abundance ratio is found to be similar to previous determinations of its value in other spiral galaxies, log(S/O) ≈ -1.6. Therefore, we find no evidence for a variation in massive-star initial mass function or nucleosynthesis at high oxygen abundance. An overabundance of nitrogen is measured, with log(N/O) -0.6. On the basis of our new abundances, we revise the effective yield for M51, now found to be almost 4 times lower than previous estimates, and we discuss this result in the context of chemical evolution in galactic disks. Features from Wolf-Rayet stars (the blue bump at 4660 A and the C III line at 5696 A) are detected in a large number of H II regions in M51, with the C III λ5696 line found preferentially in the central, most metal-rich objects.

Masses of star clusters in the nuclei of bulge-less spiral galaxies

Abstract: In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10^6 - 10^8 Lsun) but compact (Re ~ 5 pc) nuclear star clusters in very late type spirals with UVES at the VLT. We find the velocity dispersions of the nine clusters in our sample to range from 13 to 34 km/s. Using photometric data from the HST/WFPC2 and spherically symmetric dynamical models we determine masses between 8*10^5 and 6*10^7 Msun. The mass to light ratios range from 0.2 to 1.5 in the I band. This indicates a young mean age for most clusters, in agreement with previous studies. Given their high masses and small sizes we find that nuclear clusters are among the objects with the highest mean surface density known (up to 10^5 Msun pc^-2). From their dynamical properties we infer that, rather than small bulges, the closest structural kin of nuclear clusters appear to be massive compact star clusters. This includes such different objects as globular clusters, "super star clusters", ultra compact dwarf galaxies and the nuclei of dwarf elliptical galaxies. It is a challenge to explain why, despite the wildly different current environments, all different types of massive star clusters share very similar and structural properties. A possible explanation links UCDs and massive globular clusters to nuclear star clusters through stripping of nucleated dwarf galaxies in a merger event. The extreme properties of this type of clusters would then be a consequence of their location in the centers of their respective host galaxies.

Old and Young X-Ray Point Source Populations in Nearby Galaxies

Abstract: We have analyzed Chandra ACIS observations of 32 nearby spiral and elliptical galaxies and present the results of 1441 X-ray point sources that were detected in these galaxies. The total point-source X-ray (0.3-8.0 keV) luminosity LXP is well correlated with the B-band, K-band, and FIR+UV luminosities of spiral host galaxies and is well correlated with the B-band and K-band luminosities of elliptical galaxies. This suggests an intimate connection between LXP and both the old and young stellar populations, for which K and FIR+UV luminosities are reasonable proxies for the galaxy mass M and star formation rate SFR. We derive proportionality constants ? = 1.3 ? 1029 ergs s-1 M and ? = 0.7 ? 1039 ergs s-1 (M? yr-1)-1, which can be used to estimate the old and young components from M and SFR, respectively. The cumulative X-ray luminosity functions for the point sources have significantly different slopes. For the spiral and starburst galaxies, ? ? 0.6-0.8, and for the elliptical galaxies, ? ? 1.4. This implies that the most luminous point sources?those with LX 1038 ergs s-1?dominate LXP for the spiral and starburst galaxies. Most of the point sources have X-ray colors that are consistent with soft-spectrum (photon index ? ~ 1-2) low-mass X-ray binaries, accretion-powered black hole high-mass X-ray binaries (BH HMXBs), or ultraluminous X-ray sources (ULXs, also known as IXOs). We rule out hard-spectrum neutron star HMXBs (e.g., accretion-powered X-ray pulsars) as contributing much to LXP. Thus, for spirals, LXP is dominated by ULXs and BH HMXBs. We find no discernible difference between the X-ray colors of ULXs (LX ? 1039 ergs s-1) in spiral galaxies and point sources with LX ? 1038-1039 ergs s-1. We estimate that 20% of all ULXs found in spirals originate from the older (Population II) stellar populations, indicating that many of the ULXs that have been found in spiral galaxies are in fact Population II ULXs, like those in elliptical galaxies. We find that LXP depends linearly (within uncertainties) on both M and SFR for our sample galaxies (M 1011 M? and SFR 10 M? yr-1).

The mass discrepancy-acceleration relation: disk mass and the dark matter distribution

Abstract: The mass discrepancy in disk galaxies is shown to be well correlated with acceleration, increasing systematically with decreasing acceleration below a critical scale a0 � 3700 km 2 s � 2 kpc � 1 ¼ 1:2 ; 10 � 10 ms � 2 .F or each galaxy, there is an optimal choice of stellar mass-to-light ratio that minimizes the scatter in this mass discrepancy–acceleration relation. The same mass-to-light ratios also minimize the scatter in the baryonic TullyFisher relation and are in excellent agreement with the expectations of stellar population synthesis. Once the disk mass is determined in this fashion, the dark matter distribution is specified. The circular velocity attributable to the dark matter can be expressed as a simple equation that depends only on the observed distribution of baryonic mass. It is a challenge to understand how this very fine-tuned coupling between mass and light comes about. Subject headings: dark matter — galaxies: kinematics and dynamics — galaxies: spiral On-line material: color figures

Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies

Abstract: We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the UV to the FIR/submm and can also account for the surface brightness distribution in both the optical/NIR and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912 AA and 2.2 micron, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Halpha/Hbeta line ratio.

The Hα Galaxy Survey ⋆ I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxies

Abstract: We discuss the selection and observations of a large sample of nearby galaxies, which we are using to quantify the star formation activity in the local Universe. The sample consists of 334 galaxies across all Hubble types from S0/a to Im and with recession velocities of between 0 and 3000 km s 1 . The basic data for each galaxy are narrow band H�+(Nii) and R-band imaging, from which we derive star formation rates, H�+(Nii) equivalent widths and surface brightnesses, and R-band total magnitudes. A strong correlation is found between total star formation rate and Hubble type, with the strongest star formation in isolated galaxies occurring in Sc and Sbc types. More surprisingly, no significant trend is found between H�+(Nii) equivalent width and galaxy R-band luminosity. More detailed analyses of the data set presented here will be described in subsequent papers.

The structure and environment of young stellar clusters in spiral galaxies

Abstract: A search for stellar clusters has been carried out in 18 nearby spiral galaxies, using archive images from the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. All of the galaxies have previously been imaged from the ground in UBVI. A catalogue of structural parameters, photometry and comments based on visual inspection of the clusters is compiled and used to investigate correlations between cluster structure, environment, age and mass. Least-squares fits to the data suggest correlations between both the full-width at half-maximum (FWHM) and half-light radius (Reff )o f the clusters and their masses (M) at about the 3σ level. Although both relations show a large scatter, the fits have substantially shallower slopes than for a constant-density relation (size ∝ M 1/3 ). However, many of the youngest clusters have extended halos which make the Reff determinations uncertain. There is no evidence for galaxy-to-galaxy variations in the mean cluster sizes. In particular, the mean sizes do not appear to depend on the host galaxy star formation rate surface density. Many of the youngest objects (age < 10 7 years) are located in strongly crowded regions, and about 1/3-1/2 of them are double or multiple sources. The HST images are also used to check the nature of cluster candidates identified in a previous ground-based survey. The contamination rate in the ground-based sample is generally less than about 20%, but some cluster identifications remain ambiguous because of crowding even with HST imaging, especially for the youngest objects.

Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies

Abstract: We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the ultraviolet (UV) to the Far-infrared (FIR)/submillimeter (submm) and can also account for the surface brightness distribution in both the optical/Near-infrared (NIR) and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912A and 2.2 μm, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Hα/Hβ line ratio.

Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties

Abstract: We performed a compilation of more than 1000 published spectra of HII regions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. The radial distributions of oxygen and nitrogen abundances were derived. The correlations between oxygen abundance and macroscopic properties are examined. There is a significant difference between the L-Z relationship obtained here and that based on the oxygen abundances determined through the R_23-calibrations. The oxygen abundance of NGC 5457 recently determined using direct measurements of Te (Kennicutt, Bresolin & Garnett 2003) agrees with the L-Z relationship derived here, but is in conflict with the L-Z relationship derived with the R_23-based oxygen abundances. The obtained L-Z relation for spirals is compared to that for irregulars. Our sample of galaxies shows evidence that the slope of the O/H-M_B relationship for spirals is slightly more shallow than that for irregulars. The effective oxygen yields were estimated for spiral and irregular galaxies. The effective oxygen yield increases with increasing luminosity from M_B=-11 to M_B=-18 (or with increasing rotation velocity from Vrot=10 km/s to Vrot=100 km/s) and then remains approximately constant. Irregular galaxies from our sample have effective oxygen yields lowered by a factor of 3 at maximum, i.e. irregular galaxies usually keep at least 1/3 of the oxygen they manufactured during their evolution.

Spiral Structure in the Circumstellar Disk around AB Aurigae

Abstract: We present a near-infrared image of the Herbig Ae star AB Aur obtained with the Coronagraphic Imager with Adaptive Optics mounted on the Subaru Telescope. The image shows a circumstellar emission extending out to a radius of r = 580 AU, with a double spiral structure detected at r = 200-450 AU. The surface brightness decreases as r-3.0±0.1, steeper than the radial profile of the optical emission possibly affected by the scattered light from the envelope surrounding AB Aur. This result, together with the size of the infrared emission similar to that of the 13CO (J = 1-0) disk, suggests that the spiral structure is indeed associated with the circumstellar disk but is not part of the extended envelope. We identified four major spiral arms, which are trailing if the brighter southeastern part of the disk is the near side. The weak gravitational instability, maintained for millions of years by continuous mass supply from the envelope, might explain the presence of the spiral structure at the relatively late phase of the pre-main-sequence period.

The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters

Abstract: We use archival {\it Hubble Space Telescope} observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of $\sim 7.3 \times 8.1$ kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). Using this dataset we find: {\it i}) that the cluster formation rate seems to have had a large increase $\sim$ 50-70 Myr ago, which is coincident with the suggested {\it second passage} of its companion, NGC 5195, {\it ii}) a large number of extremely young ($<$ 10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next $\sim$10 Myr, and {\it iii)} that the distribution of cluster sizescan be well approximated by a power-law with exponent, $ -\eta = -2.2 \pm 0.2$, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters.

On the Continuing Formation of the Andromeda Galaxy: Detection of H I Clouds in the M31 Halo

Abstract: Green Bank Telescope 21 cm observations have revealed a faint, yet extensive H I cloud population surrounding the Andromeda galaxy (M31). The newfound objects are likely analogs to the high-velocity H I clouds seen around the Milky Way. At least 20 discrete features are detected within 50 kpc of the M31 disk, with radial velocities that are comparable to those of outer disk rotation. In addition, a filamentary halo component of at least 30 kpc extent is concentrated at the M31 systemic velocity. Some of the discrete features are organized into elongated systems with velocity continuity, suggestive of tidal streams. The discrete population can be characterized by a steep power-law distribution of number versus H I mass in the range between 105 and 107 M☉. The velocity line width of discrete clouds is correlated with the cloud H I mass such that if the clouds are gravitationally bound this implies a dark matter to H I mass ratio of ~100 : 1. Possible origins for the discrete and halo M31 features include a Local Group cooling flow, tidal debris from recent mergers or interactions, and the gaseous counterparts of low-mass dark matter halos.

Implications for the formation of star clusters from extragalactic star formation rates

Abstract: Observations indicate that young massive star clusters in spiral and dwarf galaxies follow a relation between luminosity of the brightest young cluster and the star formation rate (SFR) of the host galaxy, in the sense that higher SFRs lead to the formation of brighter clusters. Assuming that the empirical relation between maximum cluster luminosity and SFR reflects an underlying similar relation between maximum cluster mass (M ecl,max ) and SFR, we compare the resulting SFR(M ecl,max ) relation with different theoretical models. The empirical correlation is found to suggest that individual star clusters form on a free-fall time-scale with their pre-cluster molecular-cloud-core radii typically being a few parsecs independent of mass. The cloud cores contract by factors of 5-10 while building up the embedded cluster. A theoretical SFR(M ecl,max ) relation in very good agreement with the empirical correlation is obtained if the CMF of a young population has a Salpeter exponent of β 2.35 and if this cluster population forms within a characteristic time-scale of a 1-10 Myr. This short time-scale can be understood if the interstellar medium is pressurized, thus precipitating rapid local fragmentation and collapse on a galactic scale. Such triggered star formation on a galactic scale is observed to occur in interacting galaxies. With a global SFR of 3-5 M ○. yr -1 , the Milky Way appears to lie on the empirical SFR(M ecl,max ) relation, given the recent detections of very young clusters with masses near 10 5 M ○. in the Galactic disc. The observed properties of the stellar population of very massive young clusters suggests that there may exist a fundamental maximum cluster mass, 10 6 < M ecl,max */M ○. < 10 7 .

STRONG EMISSION LINE H ii GALAXIES IN THE SLOAN DIGITAL SKY SURVEY.I. CATALOG OF DR1 OBJECTS WITH OXYGEN ABUNDANCES FROM T~e MEASUREMENTS

Abstract: We present the first edition of the SDSS H II galaxies with Oxygen abundances Catalog (SHOC), which is a listing of strong emission-line galaxies (ELGs) from the Sloan Digital Sky Survey (SDSS). Oxygen abundances have been obtained with the classic Te method. We describe the method exploiting the SDSS database to construct this sample. The selection procedures are described and discussed in detail, as well as some problems encountered in the process of deriving reliable emission line parameters. The method was applied to the SDSS Data Release 1 (DR1). We present 612 SDSS emission-line galaxies (624 separate SDSS targets in total), for which the oxygen abundances 12 + log(O/H) have rms uncertainties ≤0.20 dex. The subsample of 263 ELGs (272 separate SDSS targets) have an uncertainty ≤0.10 dex, while 459 ELGs (470 separate SDSS targets) have an uncertainty ≤0.15 dex. The catalog includes the main parameters of all selected ELGs, the intensities and equivalent widths of hydrogen and oxygen emission lines, as well as oxygen abundances with their uncertainties. The information on the presence of Wolf-Rayet blue and/or red bumps in 109 galaxies is also included. With the use of combined g, r, i SDSS images we performed visual morphological classification of all SHOC galaxies. Four hundred sixty-one galaxies (~75%) are classified as confident or probable blue compact galaxies (BCG/BCG?), 78 as irregular ones, 20 as low surface brightness galaxies (LSBG), 10 as obviously interacting, and 43 as spiral galaxies. In creating the catalog, 30 narrow-line active galactic nuclei and 69 LINERs were also identified; these are also presented apart of the main catalog. We outline briefly the content of the catalog, and the prospects of its use for statistical studies of the star formation and chemical evolution issues. Some of these studies will be presented in the forthcoming paper. Finally, we show that the method presented by Kniazev et al. for calculating O+/H+ using intensities of the [O II] λλ7320, 7330 lines for SDSS emission-line spectra in the absence of [O II] λ3727 line appears to yield reliable results over a wide range of studied oxygen abundances: 7.10 < 12 + log(O/H) < 8.5.