Showing papers on "Spiral galaxy published in 2008"

The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales

Abstract: We present a comprehensive analysis of the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at sub-kpc resolution in a sample of 18 nearby galaxies. We use high-resolution H I data from The H I Nearby Galaxy Survey, CO data from HERACLES and the BIMA Survey of Nearby Galaxies, 24 μm data from the Spitzer Space Telescope, and UV data from the Galaxy Evolution Explorer. We target seven spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2 dominated centers of spiral galaxies, their H I dominated outskirts and the H I rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0 ± 0.2 relates ΣSFR and ΣH2 across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2 × 109 years. The range of ΣH2 over which we measure this relation is ~3-50 M ☉ pc–2, significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in giant molecular clouds with approximately uniform properties and that ΣH2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σgas) and ΣSFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between ΣHI and ΣSFR. As a result, the star formation efficiency (SFE), ΣSFR/Σgas, varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low density, low metallicity, H I dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H I surface densities at ΣHI ≈ 9 M ☉ pc–2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.

Metallicity Calibrations and the Mass-Metallicity Relation for Star-forming Galaxies

Abstract: We investigate the effect of metallicity calibrations, AGN classification, and aperture covering fraction on the local mass-metallicity relation using 27,730 star-forming galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 4. We analyse the SDSS mass-metallicity relation with 10 metallicity calibrations, including theoretical and empirical methods. We show that the choice of metallicity calibration has a significant effect on the shape and y-intercept(12+log(O/H)) of the mass-metallicity relation. The absolute metallicity scale (y-intercept) varies up to �[log(O/H)] = 0.7 dex, depending on the calibration used, and the change in shape is substantial. These results indicate that it is critical to use the same metallicity calibration when comparing different luminosity-metallicity or mass-metallicity relations. We present new metallicity conversions that allow metallicities that have been derived using different strong-line calibrations to be converted to the same base calibration. These conversions facilitate comparisons between different samples, particularly comparisons between galaxies at different redshifts for which different suites of emission-lines are available. Our new conversions successfully remove the large 0.7 dex discrepancies between the metallicity calibrations, and we reach agreement in the mass-metallicity relation to within 0.03 dex on average. We investigate the effect of AGN classification and aperture covering fraction on the mass-metallicity relation. We find that different AGN classification methods have negligible effect on the SDSS MZ-relation. We compare the SDSS mass-metallicity relation with nuclear and global relations from the Nearby Field Galaxy Survey (NFGS). The turn over of the mass-metallicity relation at M∗ ∼ 10 10 M⊙ depends on aperture covering fraction. We find that a lower redshift limit of z 10 10 M⊙) galaxies. Subject headings: galaxies: starburst—galaxies: abundances—galaxies: fundamental parameters— galaxies: spiral—techniques: spectroscopic

The Resolved Properties of Extragalactic Giant Molecular Clouds

Abstract: We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and mass-luminosity relations of GMCs in a variety of extragalactic systems. Although the data are heterogeneous, we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. The property that shows the largest variability is their resolved brightness temperature, although even that is similar to the average Galactic value in most sources. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H2 factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the molecular cloud. We also find that the virial CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H2 envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of objects that exhibit a remarkably uniform set of properties from galaxy to galaxy.

Cold gas accretion in galaxies

Abstract: Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: (1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. This suggests ongoing minor mergers and recent arrival of external gas. It may be regarded, therefore, as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M 31. (2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is undoubtedly produced by galactic fountains, it is likely that a part of it is of extragalactic origin. Also the Milky Way has extra-planar gas complexes: the Intermediate- and High-Velocity Clouds (IVCs and HVCs). (3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. (4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The new gas could be added to the halo or be deposited in the outer parts of galaxies and form reservoirs for replenishing the inner parts and feeding star formation. The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean “visible” accretion rate of cold gas in galaxies of at least $${0.2\, M_{\odot} year^{-1}}$$ . In order to reach the accretion rates needed to sustain the observed star formation ( $${\approx 1 M_{\odot} year^{-1}}$$ ), additional infall of large amounts of gas from the IGM seems to be required.

Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks

Abstract: Stars in disks of spiral galaxies are usually assumed to remain roughly at their birth radii. This assumption is built into decades of modeling of the evolution of stellar populations in our own Galaxy and in external systems. We present results from self-consistent high-resolution N-body + smooth particle hydrodynamics simulations of disk formation, in which stars migrate across significant galactocentric distances due to resonant scattering with transient spiral arms, while preserving their circular orbits. We investigate the implications of such migrations for observed stellar populations. Radial migration provides an explanation for the observed flatness and spread in the age-metallicity relation and the relative lack of metal-poor stars in the solar neighborhood. The presence of radial migration also prompts rethinking of interpretations of extragalactic stellar population data, especially for determinations of star formation histories.

Cold gas accretion in galaxies

Abstract: Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: 1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. It may be regarded as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is produced by galactic fountains, it is likely that a part of it is of extragalactic origin. 3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. 4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates needed to sustain the observed star formation (~1 Msol/yr), additional infall of large amounts of gas from the IGM seems to be required.

Measuring the inclination and mass-to-light ratio of axisymmetric galaxies via anisotropic Jeans models of stellar kinematics

Abstract: We present a simple and efficient anisotropic generalization of the semi-isotropic (two-integral) axisymmetric Jeans formalism, which is used to model the stellar kinematics of galaxies. The following is assumed: (i) a constant mass-to-light ratio (MIL) and (ii) a velocity ellipsoid that is aligned with cylindrical coordinates (R, z) and characterized by the classic anisotropy parameter β z = 1 - υ 2 z /υ 2 R . Our simple models are fit to SAURON integral-field observations of the stellar kinematics for a set of fast-rotator early-type galaxies. With only two free parameters (β z and the inclination), the models generally provide remarkably good descriptions of the shape of the first (V) and second (V rms ≡ √V 2 + σ 2 ) velocity moments, once a detailed description of the surface brightness is given. This is consistent with previous findings on the dynamical structure of these objects. With the observationally motivated assumption that β z ≥0, the method is able to recover the inclination. The technique can be used to determine the dynamical MIL and angular momenta of early-type fast-rotators and spiral galaxies, especially when the quality of the data does not justify more sophisticated modelling approaches. This formalism allows for the inclusion of dark matter, supermassive black holes, spatially varying anisotropy and multiple kinematic components.

Beyond inside-out growth : formation and evolution of disk outskirts

Abstract: We have performed a high mass and force resolution simulation of an idealized galaxy forming from dissipational collapse of gas embedded in a spherical dark matter halo. The simulation includes star formation and effects of stellar feedback. In our simulation a stellar disk forms with a surface density profile consisting of an inner exponential breaking to a steeper outer exponential. The break forms early on and persists throughout the evolution, moving outward as more gas is able to cool and add mass to the disk. The parameters of the break are in excellent agreement with observations. The break corresponds to a rapid drop in the star formation rate associated with a drop in the cooled gas surface density, but the outer exponential is populated by stars that were scattered outward on nearly circular orbits from the inner disk by spiral arms. The resulting profile and its associated break are therefore a consequence of the interplay between a radial star formation cutoff and redistribution of stellar mass by secular processes. A consequence of such evolution is a sharp change in the radial mean stellar age profile at the break radius.

Uncovering Extremely Metal-Poor Stars in the Milky Way’s Ultrafaint Dwarf Spheroidal Satellite Galaxies*

Abstract: We present new metallicity measurements for 298 individual red giant branch stars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the Milky Way (MW) system. Our technique is based on medium-resolution Keck DEIMOS spectroscopy coupled with spectral synthesis. We present the first spectroscopic metallicities at [ Fe/H ] < − 3.0 of stars in a dwarf galaxy, with individual stellar metallicities as low as [ Fe/H ] = − 3.3. Because our [Fe/H] measurements are not tied to empirical metallicity calibrators and are sensitive to arbitrarily low metallicities, we are able to probe this extremely metal-poor regime accurately. The metallicity distribution of stars in these dSphs is similar to the MW halo at the metal-poor end. We also demonstrate that the luminosity-metallicity relation previously seen in more luminous dSph galaxies (MV = − 13.4 to –8.8) extends smoothly down to an absolute magnitude of MV = − 3.7. The discovery of extremely metal-poor stars in dSphs lends support to the ΛCDM galaxy assembly paradigm wherein dwarf galaxies dissolve to form the stellar halo of the MW.

Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size–luminosity relations

Abstract: While galactic bulges may contain no significant dust of their own, the dust within galaxy discs can strongly attenuate the light from their embedded bulges. Furthermore, such dust inhibits the ability of observationally determined inclination corrections to recover intrinsic (i.e. dust-free) galaxy parameters. Using the sophisticated 3D radiative transfer model of Popescu et al. and Tuffs et al., together with the recent determination of the average face-on opacity by Driver et al. in nearby disc galaxies, we provide simple equations to correct (observed) disc central surface brightness and scalelengths for the effects of both inclination and dust in the B, V, I, J and K passbands. We then collate and homogenize various literature data sets and determine the typical intrinsic scalelengths, central surface brightness and magnitudes of galaxy discs as a function of morphological type. All galaxies have been carefully modelled in their respective papers with a Sersic R l/n bulge plus an exponential disc. Using the bulge magnitude corrections from Driver et al., we additionally derive the average, dust-corrected, bulge-to-disc flux ratio as a function of galaxy type. With values typically less than 1/3, this places somewhat uncomfortable constraints on some current semi-analytic simulations. Typical bulge sizes, profile shapes, surface brightness and deprojected densities are provided. Finally, given the two-component nature of disc galaxies, we present luminosity-size and (surface brightness)-size diagrams for discs and bulges. We also show that the distribution of elliptical galaxies in the luminosity-size diagram is not linear but strongly curved.

Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks

Abstract: Stars in disks of spiral galaxies are usually assumed to remain roughly at their birth radii. This assumption is built into decades of modelling of the evolution of stellar populations in our own Galaxy and in external systems. We present results from self-consistent high-resolution $N$-body + Smooth Particle Hydrodynamics simulations of disk formation, in which stars migrate across significant galactocentric distances due to resonant scattering with transient spiral arms, while preserving their circular orbits. We investigate the implications of such migrations for observed stellar populations. Radial migration provides an explanation for the observed flatness and spread in the age-metallicity relation and the relative lack of metal poor stars in the solar neighborhood. The presence of radial migration also prompts rethinking of interpretations of extra-galactic stellar population data, especially for determinations of star formation histories.

Nearby stars of the Galactic disc and halo – V

Abstract: The Milky Way Galaxy has an age of about 13 billion years. Solar-type stars evolve all the long way to the realm of degenerate objects on essentially this time-scale. This, as well as the particular advantage that the Sun offers through reliable differential spectroscopic analyses, render these stars the ideal tracers for the fossil record of our parent spiral. Astrophysics is a science that is known to be notoriously plagued by selection effects. The present work ‐ with a major focus in this fourth contribution on model atmosphere analyses of spectroscopic binaries and multiple star systems ‐ aims at a volume-complete sample of about 300 nearby F-, G-, and K-type stars that particularly avoids any kinematical or chemical pre-selection from the outset. It thereby provides an unbiased record of the local stellar populations ‐ the ancient thick disc and the much younger thin disc. On this base, the detailed individual scrutiny of the long-lived stars of both populations unveils the thick disc as a single-burst component with a local normalization of no less than 20 per cent. This enormous fraction, combined with its much larger scaleheight, implies a mass for the thick disc that is comparable to that of the thin disc. On account of its completely different mass-to-light ratio the thick disc thereby becomes the dark side of the Milky Way, an ideal major source for baryonic dark matter. This massive, ancient population consequently challenges any gradual build-up scenario for our parent spiral. Even more, on the supposition that the Galaxy is not unusual, the thick disc ‐ as it emerges from this unbiased spectroscopic work ‐ particularly challenges the hierarchical cold-dark-matter-dominated formation picture for spiral galaxies in general.

New velocimetry and revised cartography of the spiral arms in the milky way—a consistent symbiosis

Abstract: Recent advances in the determinations of the positions (pitch angle, shape, numbers, interarm separation) and velocities (rotation curve) of the spiral arms are evaluated and compared to previous determinations. Based on these results, an average cartographic model is developed that fits the means of basic input data and provides predictions for the locations of the arms in the Milky Way, for each galactic quadrant. For each spiral arm segment in each galactic quadrant, the LSR radial velocities are calculated for the radial distance as well as for its galactic longitude. From our velocimetric model, arm intercepts (between line of sights and spiral arms) are indicated in velocity space and may be used to find the distance and velocity to any arm, in a given longitude range. Velocity comparisons between model predictions and published CO velocity distribution are done for each galactic quadrant, with good results. Our velocimetric model is not hydromagnetic in character, nor is it a particle-simulation scheme, yet it is simple to use for comparisons with the observations and it is in symbiosis and consistent with our cartographic model (itself simple to use for comparisons with observations). A blending in velocity of the Perseus and Cygnus arms is further demonstrated, as well as an apparent longitude-velocity blending of the starting points of the four spiral arms near 4 kpc (not a physical ring). An integrated (distance, velocity) model for the mass in the disk is employed, to yield the total mass of 3.0 × 1011 M Sun within a galactic radius of 28 kpc.

Accretion of gas on to nearby spiral galaxies

Abstract: We present evidence for cosmological gas accretion onto spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extra-planar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extra-planar gas is produced by supernova feedback (galactic fountain) and only 10 20% comes from accretion. The accreting material must have low specific angular momentum about the disc’s spin axis, although the magnitude of the specific angular-momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extra-planar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona.

The Outer Scale of Turbulence in the Magnetoionized Galactic Interstellar Medium

Abstract: We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed Analysis of the partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum In the Galaxy's spiral arms, no rotation measure fluctuations on scales above a few parsecs are measured In the interarm regions fluctuations on larger scales than in spiral arms are present, and show power-law behavior with a shallow spectrum These results suggest that in the spiral arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and superbubble explosions are the main sources of energy on scales on the order of 100 pc

Accretion of gas onto nearby spiral galaxies

Abstract: We present evidence for cosmological gas accretion onto spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extra-planar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extra-planar gas is produced by supernova feedback (galactic fountain) and only 10-20 % comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular-momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extra-planar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona.

HI holes and high-velocity clouds in the spiral galaxy NGC 6946

Abstract: We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC 6946. The galaxy has been observed for 192 h at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity HI (up to about 100 km s(-1)) and find 121 HI holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with HI holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. We conclude that the high-velocity gas in NGC 6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the HI holes. There are also indications, especially in the outer regions, -an extended HI plume, velocity anomalies, sharp edges, and large-scale asymmmetries-pointing to tidal encounters and recent gas accretion.

The outer scale of turbulence in the magneto-ionized Galactic interstellar medium

Abstract: We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's spiral arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in spiral arms are present, and show power law behavior with a shallow spectrum. These results suggest that in the spiral arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and super bubble explosions are the main sources of energy on scales on the order of 100 parsecs.

Trigonometric Parallaxes of Massive Star Forming Regions: II. Cep A & NGC 7538

Abstract: We report trigonometric parallaxes for the sources NGC 7538 and Cep A, corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04] kpc, respectively. The distance to NGC 7538 is considerably smaller than its kinematic distance and places it in the Perseus spiral arm. The distance to Cep A is also smaller than its kinematic distance and places it in the Local arm or spur. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find significant deviations from circular Galactic orbits for these sources: both sources show large peculiar motions (> 10 km/s) counter to Galactic rotation and NGC 7538 has a comparable peculiar motion toward the Galactic center.

Color Profiles of Spiral Galaxies: Clues on Outer-Disk Formation Scenarios

Abstract: We have explored radial color and stellar surface mass density profiles for a sample of 85 late-type spiral galaxies with deep (down to ~27 mag arcsec−2) SDSS g'- and r'-band surface brightness profiles. About 90% of the light profiles have been classified as broken exponentials, exhibiting either truncations (Type II galaxies) or antitruncations (Type III galaxies). The color profiles of Type II galaxies show a " shape" with a minimum of (g' − r') = 0.47 ± 0.02 mag at the break radius. Around the break radius, Type III galaxies have a plateau region with a color of (g' − r') = 0.57 ± 0.02. Using the color to calculate the stellar surface mass density profiles reveals a surprising result. The breaks, well established in the light profiles of the truncated galaxies, are almost gone, and the mass profiles now resemble those of the pure exponential (Type I) galaxies. This result suggests that the origin of the break in Type II galaxies is more likely due to a radial change in stellar population than being associated with an actual drop in the distribution of mass. Type III galaxies, however, seem to preserve their shape in the stellar mass density profiles. We find that the stellar surface mass density at the break for truncated galaxies is 13.6 ± 1.6 M☉ pc−2 and for the antitruncated ones is 9.9 ± 1.3 M☉ pc−2. We estimate that the fraction of stellar mass outside the break radius is ~15% for truncated galaxies and ~9% for antitruncated galaxies.

Gas flow models in the Milky Way embedded bars

Abstract: Context. The gas distribution and dynamics in the inner Galaxy present many unknowns, such as the origin of the asymmetry of the lu-diagram of the Central Molecular Zone (CMZ). On the other hand, there is recent evidence in the stellar component of the presence of a nuclear bar that may be slightly lopsided. Aims. Our goal is to characterize the nuclear bar observed in 2MASS maps and to study the gas dynamics in the inner Milky Way taking into account this secondary bar. Methods. We have derived a realistic mass distribution by fitting the 2MASS star count map with a model including three components (disk, bulge and nuclear bar) and we have simulated the gas dynamics in the deduced gravitational potential using a sticky-particles code. Results. Our simulations of the gas dynamics successfully reproduce the main characteristics of the Milky Way for a bulge orientation of 20°-35° with respect to the Sun-Galactic Center (GC) line and a pattern speed of 30-40 km s -1 kpc -1 . In our models the Galactic Molecular Ring (GMR) is not an actual ring but the inner parts of the spiral arms, while the 3-kpc arm and its far side counterpart are lateral arms that extend around the bar. Our simulations reproduce, for the first time, the parallelogram shape of the lu-diagram of the CMZ as the gas response to the nuclear bar. This bar should be oriented by an angle of ∼60°-75° with respect to the Sun-GC line and its mass amounts to (2-5.5) 10 9 M ⊙ . We show that the observed asymmetry of the CMZ cannot be due to lopsidedness of the nuclear bar as suggested by the 2MASS maps. Conclusions. We do not find clear evidence of lopsidedness in the stellar potential. We propose that the observed asymmetry of the central gas layer can be due to the infalling of gas into the CMZ in the I = 1.3°-complex.

On the influence of ram-pressure stripping on the star formation of simulated spiral galaxies

Abstract: Aims. We investigate the influence of ram-pressure stripping on the star formation and the mass distribution in simulated spiral galaxies. Special emphasis is put on the question of where the newly formed stars are located. The stripping radius from the simulation is compared to analytical estimates. Methods. Disc galaxies are modelled in combined N-body/hydrodynamic simulations (GADGET-2) with prescriptions for cooling, star formation, stellar feedback, and galactic winds. These model galaxies move through a constant density and temperature gas, which has parameters comparable to the intra-cluster medium (ICM) in the outskirts of a galaxy cluster (T = 3k eV≈ 3.6 × 10 7 K and ρ = 10 −28 g/cm 3 ). With this numerical setup we analyse the influence of ram-pressure stripping on the star formation rate of the model galaxy. Results. We find that the star formation rate is significantly enhanced by the ram-pressure effect (up to a factor of 3). Stars form in the compressed central region of the galaxy, as well as in the stripped gas behind the galaxy. Newly formed stars can be found up to hundred kpc behind the disc, forming structures with sizes of roughly 1 kpc in diameter and with masses of up to 10 7 M� .A s they do not possess a dark matter halo due to their formation history, we name them “stripped baryonic dwarf” galaxies. We also find that the analytical estimate for the stripping radius from a Gunn & Gott (1972) criterion agrees well with the numerical value from the simulation. Like in former investigations, edge-on systems lose less gas than face-on systems, and the resulting spatial distribution of the gas and the newly formed stars is different.

APOGEE: The Apache Point Observatory Galactic Evolution Experiment

Abstract: APOGEE is a large-scale, NIR, high-resolution (R ∼ 20 000) spectroscopic survey of Galactic stars. It is one of the four experiments in SDSS-III. Because APOGEE will observe in the H band, where the extinction is six times smaller than in V, it will be the first survey to pierce through Galactic dust and provide a vast, uniform database of chemical abundances and radial velocities for stars across all Galactic populations (bulge, disk, and halo). The survey will be conducted with a dedicated, 300-fiber, cryogenic, spectrograph that is being built at the University of Virginia, coupled to the ARC 2.5-m telescope at Apache Point Observatory. APOGEE will use a significant fraction of the SDSS-III bright time during a three-year period to observe, at high signal-to-noise ratio (S /N > 100), about 100 000 giant stars selected directly from 2MASS down to a typical flux limit ofH < 13. The main scientific objectives of APOGEE are: (1) measuring unbiased metallicity distributions and abundance patterns for the different Galactic stellar populations, (2) studying the processes of star formation, feedback, and chemical mixing in theMilkyWay, (3) surveying the dynamics of the bulge and disk, placing constraints on the nature and influence of the Galactic bar and spiral arms, and (4) using extensive chemodynamical data, particularly in the inner Galaxy, to unravel its formation and evolution. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GHASP: an Hα kinematic survey of spiral and irregular galaxies - VI. New Hα data cubes for 108 galaxies

Abstract: We present the Fabry-Perot observations obtained for a new set of 108 galaxies in the frame of the Gassendi Ha survey of SPirals (GHASP). The GHASP survey consists of 3D Ha data cubes for 203 spiral and irregular galaxies, covering a large range in morphological types and absolute magnitudes, for kinematics analysis. The new set of data presented here completes the survey. The GHASP sample is by now the largest sample of Fabry-Perot data ever published. The analysis of the whole GHASP sample will be done in forthcoming papers. Using adaptive binning techniques based on Voronoi tessellations, we have derived Ha data cubes from which are computed Ha maps, radial velocity fields as well as residual velocity fields, position-velocity diagrams, rotation curves and the kinematical parameters for almost all galaxies. Original improvements in the determination of the kinematical parameters, rotation curves and their uncertainties have been implemented in the reduction procedure. This new method is based on the whole 2D velocity field and on the power spectrum of the residual velocity field rather than the classical method using successive crowns in the velocity field. Among the results, we point out that morphological position angles have systematically higher uncertainties than kinematical ones, especially for galaxies with low inclination. The morphological inclination of galaxies having no robust determination of their morphological position angle cannot be constrained correctly. Galaxies with high inclination show a better agreement between their kinematical inclination and their morphological inclination computed assuming a thin disc. The consistency of the velocity amplitude of our rotation curves has been checked using the Tully-Fisher relationship. Our data are in good agreement with previous determinations found in the literature. Nevertheless, galaxies with low inclination have statistically higher velocities than expected and fast rotators are less luminous than expected.

IMAGES - III. The evolution of the near-infrared Tully-Fisher relation over the last 6 Gyr

Abstract: Using the multi-integral field spectrograph GIRAFFE at VLT, we have derived the K-band Tully-Fisher relation (TFR) at z ∼ 0.6 for a representative sample of 65 galaxies with emission lines (W0(OII) ≥ 15 A). We confirm that the scatter in the z ∼ 0. 6T FR is caused by galaxies with anomalous kinematics, and find a positive and strong correlation between the complexity of the kinematics and the scatter that they contribute to the TFR. Considering only relaxed-rotating disks, the scatter, and possibly also the slope, of the TFR, do not appear to evolve with redshift. We detect an evolution of the K-band TFR zero point between z ∼ 0. 6a ndz = 0, which, if interpreted as an evolution of the K-band luminosity of rotating disks, would imply that a brightening of 0.66 ± 0.14 mag occurs between z ∼ 0. 6a ndz = 0. Any disagreement with the results of Flores et al. (2006, A&A, 455, 107) are attributed to both an improvement of the local TFR and the more detailed accurate measurement of the rotation velocities in the distant sample. Most of the uncertainty can be explained by the relatively coarse spatial-resolution of the kinematical data. Because most rotating disks at z ∼ 0.6 are unlikely to experience further merging events, one may assume that their rotational velocity, which is taken as a proxy of the total mass, does not evolve dramatically. If true, our result implies that rotating disks observed at z ∼ 0.6 are rapidly transforming their gas into stars, to be able to double their stellar masses and be observed on the TFR at z = 0. The rotating disks observed are indeed emission-line galaxies that are either starbursts or LIRGs, which implies that they are forming stars at a high rate. Thus, a significant fraction of the rotating disks are forming the bulk of their stars within 6 to 8 Gyr, in good agreement with former studies of the evolution of the mass-metallicity relationship.

Mapping of molecular gas inflow towards the Seyfert nucleus of NGC 4051 using Gemini NIFS

Abstract: We present 2D stellar and gaseous kinematics of the inner ∼130× 180 pc 2 of the Narrow-Line Seyfert 1 galaxy NGC 4051 at a sampling of 4.5 pc, from near-infrared K-band spectroscopic observations obtained with the Gemini's Near-infrared Integral Field Spectrograph (NIFS) operating with the ALTAIR adaptive optics module. We have used the CO absorption band heads around 2.3 μm to obtain the stellar kinematics which show the turnover of the rotation curve at only ≈55 pc from the nucleus, revealing a highly concentrated gravitational potential. The stellar velocity dispersion of the bulge is ≈60 km s -1 - implying on a nuclear black hole mass of ≈10 6 M ⊙ - within which patches of lower velocity dispersion suggest the presence of regions of more recent star formation. From measurements of the emission-line profiles we have constructed 2D maps for the flux distributions, line ratios, radial velocities and gas velocity dispersions for the H 2 , H II and [Ca VIII] emitting gas. Each emission-line samples a distinct kinematics. The Bry emission-line shows no rotation as well as no blueshifts or redshifts in excess of 30 km s -1 , and is thus not restricted to the galaxy plane. The [Ca VIII] coronal region is compact but resolved, extending over the inner 75 pc. It shows the highest blueshifts - of up to -250 km s -1 , and the highest velocity dispersions, interpreted as due to outflows from the active nucleus, supporting an origin close to the nucleus. Subtraction of the stellar velocity field from the gaseous velocity field has allowed us to isolate non-circular motions observed in the H 2 emitting gas. The most conspicuous kinematic structures are two nuclear spiral arms - one observed in blueshift in the far side of the galaxy (to the north-east), and the other observed in redshift in the near side of the galaxy (to the south-west). We interpret these structures as inflows towards the nucleus, a result similar to those of previous studies in which we have found streaming motions along nuclear spirals in ionized gas using optical Integral Filed Unit (IFU) observations. We have calculated the mass inflow rate along the nuclear spiral arms, obtaining M H2 ≈ 4 x 10 -5 M ⊙ yr -1 , a value ∼100 times smaller than the accretion rate necessary to power the active nucleus. This can be understood as due to the fact that we are only seeing the hot 'skin' (the H 2 emitting gas) of the total mass inflow rate, which is probably dominated by cold molecular gas. From the H 2 emission-line ratios we conclude that X-ray heating can account for the observed emission, but the H 2 λ2.1218 μm/Br y line ratio suggests some contribution from shocks in localized regions close to the compact radio jet.

GMC formation by agglomeration and self gravity

Abstract: We investigate the formation of giant molecular clouds (GMCs) in spiral galaxies through both agglomeration of clouds in the spiral arms, and self gravity. The simulations presented include two-fluid models, which contain both cold and warm gas, although there is no heating or cooling between them. We find agglomeration is predominant when both the warm and cold components of the interstellar medium are effectively stable to gravitational instabilities. In this case, the spacing (and consequently mass) of clouds and spurs along the spiral arms is determined by the orbits of the gas particles and correlates with their epicyclic radii (or equivalently spiral shock strength). Notably GMCs formed primarily by agglomeration tend to be unbound associations of many smaller clouds, which disperse upon leaving the spiral arms. These GMCs are likely to be more massive in galaxies with stronger spiral shocks or higher surface densities. GMCs formed by agglomeration are also found to exhibit both prograde and retrograde rotation, a consequence of the clumpiness of the gas. At higher surface densities, self gravity becomes more important in arranging both the warm and cold gas into clouds and spurs, and determining the properties of the most massive GMCs. These massive GMCs can be distinguished by their higher angular momentum, exhibit prograde rotation and are more bound. For a 20 M ⊙ pc -2 disc, the spacing between the GMCs fits both the agglomeration and self gravity scenarios, as the maximum unstable wavelength of gravitational perturbations in the warm gas is similar to the spacing found when GMCs form solely by agglomeration.

Quantitative Spectroscopy of 24 A Supergiants in the Sculptor Galaxy NGC 300:* Flux-weighted Gravity-Luminosity Relationship, Metallicity, and Metallicity Gradient

Abstract: A quantitative spectral analysis of 24 A supergiants in the Sculptor Group spiral galaxy NGC 300 at a distance of 1.9 Mpc is presented. A new method is introduced to analyze low-resolution (~5 A) spectra, which yields metallicities accurate to 0.2 dex including the uncertainties arising from the errors in Teff (5%) and log g (0.2 dex). For the first time the stellar metallicity gradient based on elements such as titanium and iron in a galaxy beyond the Local Group is investigated. Solar metallicity is measured in the center and 0.3 solar in the outskirts and a logarithmic gradient of –0.08 dex kpc−1. An average reddening of E(B − V) ~ 0.12 mag is obtained, however, with a large variation from 0.07 to 0.24 mag. We also determine stellar radii, luminosities, and masses and discuss the evolutionary status. Finally, the observed relationship between absolute bolometric magnitudes Mbol and flux-weighted gravities gF = g/T4eff is investigated. At high temperatures the strength of the Balmer lines depends solely on the flux-weighted gravity, which allows a precise direct determination of log gF with an accuracy of 0.05-0.1 dex. We find a tight relationship between Mbol and log gF in agreement with stellar evolution theory. Combining these new results with previous work on Local Group galaxies, we obtain a new flux-weighted gravity-luminosity relationship (FGLR), which is very well defined and appears to be an excellent alternative tool to determine distances to galaxies.

Ultraviolet dust attenuation in star‐forming galaxies – II. Calibrating the A(UV) versus LTIR/LUV relation

Abstract: We investigate the dependence of the total-infrared (TIR) to UV luminosity ratio method for calculating the UV dust attenuation A(UV) from the age of the underlying stellar populations by using a library of spectral energy distributions for galaxies with different star formation histories. Our analysis confirms that the TIR/UV vs. A(UV) relation varies significantly with the age of the underlying stellar population: i.e. for the same TIR/UV ratio, systems with low specific star formation rate (SSFR) suffer a lower UV attenuation than starbursts. Using a sample of nearby field and cluster spiral galaxies we show that the use of a standard (i.e. age independent) TIR/UV vs. A(UV) relation leads to a systematic overestimate up to 2 magnitudes of the amount of UV dust attenuation suffered by objects with low SSFR and in particular HI-deficient star forming cluster galaxies. This result points out that the age independent $TIR/UV$ vs. $A(UV)$ relation cannot be used to study the UV properties of large samples of galaxies including low star-forming systems and passive spirals. Therefore we give some simple empirical relations from which the UV attenuation can be estimated taking into account its dependence on the age of the stellar populations, providing a less biased view of UV properties of galaxies.

A New Spiral Arm of the Galaxy: The Far 3 kpc Arm

Abstract: We report the detection in CO of the far-side counterpart of the well-known expanding 3 kpc arm in the central region of the Galaxy. In a CO longitude-velocity map at b = 0° the far 3 kpc arm can be followed over at least 20° of Galactic longitude as a faint lane at positive velocities running parallel to the near arm. The far arm crosses l = 0° at +56 km s−1, quite symmetric with the –53 km s−1 expansion velocity of the near arm. In addition to their symmetry in longitude and velocity, we find that the two arms have line widths (~21 km s−1), linear scale heights (~103 pc FWHM), and H2 masses per unit length (~4.3 × 106 M☉ kpc−1) that agree to 26% or better. Guided by the CO, we have also identified the far arm in high-resolution 21 cm data and find, subject to the poorly known CO-to-H2 ratio in these objects, that both arms are predominately molecular by a factor of 3-4. The detection of these symmetric expanding arms provides strong support for the existence of a bar at the center of our Galaxy and should allow better determination of the bar's physical properties.