Showing papers on "Spiral galaxy published in 2000"

Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant

Abstract: We present here the final results of the Hubble Space Telescope Key Project to measure the Hubble constant. We summarize our method, the results and the uncertainties, tabulate our revised distances, and give the implications of these results for cosmology. The analysis presented here benefits from a number of recent improvements and refinements, including (1) a larger LMC Cepheid sample to define the fiducial period-luminosity (PL) relations, (2) a more recent HST Wide Field and Planetary Camera 2 (WFPC2) photometric calibration, (3) a correction for Cepheid metallicity, and (4) a correction for incompleteness bias in the observed Cepheid PL samples. New, revised distances are given for the 18 spiral galaxies for which Cepheids have been discovered as part of the Key Project, as well as for 13 additional galaxies with published Cepheid data. The new calibration results in a Cepheid distance to NGC 4258 in better agreement with the maser distance to this galaxy. Based on these revised Cepheid distances, we find values (in km/sec/Mpc) of H0 = 71 +/- 2 (random) +/- 6 (systematic) (type Ia supernovae), 71 +/- 2 +/- 7 (Tully-Fisher relation), 70 +/- 5 +/- 6 (surface brightness fluctuations), 72 +/- 9 +/- 7 (type II supernovae), and 82 +/- 6 +/- 9 (fundamental plane). We combine these results for the different methods with 3 different weighting schemes, and find good agreement and consistency with H0 = 72 +/- 8. Finally, we compare these results with other, global methods for measuring the Hubble constant.

Rotation Curves of Spiral Galaxies

Abstract: Rotation curves of spiral galaxies are the major tool for determining the distribution of mass in spiral galaxies. They provide fundamental information for understanding the dynamics, evolution and formation of spiral galaxies. We describe various methods to derive rotation curves, and review the results obtained. We discuss the basic characteristics of observed rotation curves in relation to various galaxy properties, such as Hubble type, structure, activity, and environment.

Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars

Abstract: We present a detailed analysis of the space motions of 1203 solar-neighborhood stars with metal abundances [Fe/H] ≤ -0.6, on the basis of a catalog, of metal-poor stars selected without kinematic bias recently revised and supplemented by Beers et al. This sample, having available proper motions, radial velocities, and distance estimates for stars with a wide range of metal abundances, is by far the largest such catalog to be assembled to date. We show that the stars in our sample with [Fe/H] ≤-2.2, which likely represent a pure halo component, are characterized by a radially elongated velocity ellipsoid (σU, σV, σW) = (141 ± 11, 106 ± 9, 94 ± 8) km s-1 and small prograde rotation V = 30 to 50 km s-1, consistent with previous analysis of this sample by Beers and Sommer-Larsen based on radial velocity information alone. In contrast to the previous analysis, we find a decrease in V with increasing distance from the Galactic plane for stars that are likely to be members of the halo population (ΔV/Δ|Z| = -52 ± 6 km s-1 kpc-1), which may represent the signature of a dissipatively formed flattened inner halo. Unlike essentially all previous kinematically selected catalogs, the metal-poor stars in our sample exhibit a diverse distribution of orbital eccentricities, e, with no apparent correlation between [Fe/H] and e. This demonstrates, clearly and convincingly, that the evidence offered in 1962 by Eggen, Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, an apparent correlation between the orbital eccentricity of halo stars with metallicity, is basically the result of their proper-motion selection bias. However, even in our nonkinematically selected sample, we have identified a small concentration of high-e stars at [Fe/H] ~ -1.7, which may originate, in part, from infalling gas during the early formation of the Galaxy. We find no evidence for an additional thick disk component for stellar abundances [Fe/H] ≤ -2.2. The kinematics of the intermediate-abundance stars close to the Galactic plane are, in part, affected by the presence of a rapidly rotating thick disk component with V 200 km s-1 (with a vertical velocity gradient on the order of ΔV/Δ|Z| = -30 ± 3 km s-1 kpc-1) and velocity ellipsoid (σU, σV, σW) = (46 ± 4, 50 ± 4, 35 ± 3) km s-1. The fraction of low-metallicity stars in the solar neighborhood that are members of the thick disk population is estimated as ~10% for -2.2 < [Fe/H] ≤ -1.7 and ~30% for -1.7 < [Fe/H] ≤ -1. We obtain an estimate of the radial scale length of the metal-weak thick disk of 4.5 ± 0.6 kpc. We also analyze the global kinematics of the stars constituting the halo component of the Galaxy. The outer part of the halo, which we take to be represented by local stars on orbits reaching more than 5 kpc from the Galactic plane, exhibits no systematic rotation. In particular, we show that previous suggestions of the presence of a counter-rotating high halo are not supported by our analysis. The density distribution of the outer halo is nearly spherical and exhibits a power-law profile that is accurately described as ρ ∝ R-3.55±0.13. The inner part of the halo is characterized by a prograde rotation and a highly flattened density distribution. We find no distinct boundary between the inner and outer halo. We confirm the clumping in angular-momentum phase space of a small number of local metal-poor stars noted in 1999 by Helmi et al. We also identify an additional elongated feature in angular-momentum phase space extending from the clump to regions with high azimuthal rotation. The number of members in the detected clump is not significantly increased from that reported by Helmi et al., even though the total number of the sample stars we consider is almost triple that of the previous investigation. We conclude that the fraction of halo stars that may have arisen from the precursor object of this clump may be smaller than 10% of the present Galactic halo, as previously suggested. The implications of our results for the formation of the Galaxy are discussed, in particular in the context of the currently favored cold dark matter theory of hierarchical galaxy formation.

The Nature of Ultra-Luminous Compact X-Ray Sources in Nearby Spiral Galaxies

Abstract: Studies were made of ASCA spectra of seven ultra-luminous compact X-ray sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6 (Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al. 1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano & Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al. 1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed, or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all these seven sources have been described successfully with so called multi-color disk blackbody (MCD) emission arising from optically-thick standard accretion disks around black holes. Except the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ~100 solar masses. However, the observed innermost disk temperatures of these objects, Tin = 1.1--1.8 keV, are too high to be compatible with the required high black-hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole, and hence hotter.

Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis

Abstract: We describe observations and abundance analysis of a high-resolution, high signal-to-noise ratio survey of 168 stars, most of which are metal-poor dwarfs. We follow a self-consistent LTE analysis technique to determine the stellar parameters and abundances, and we estimate the effects of random and systematic uncertainties on the resulting abundances. Element-to-iron ratios are derived for key α-, odd-Z, Fe-peak, and r- and s-process elements. Effects of non-LTE on the analysis of Fe I lines are shown to be very small on average. Spectroscopically determined surface gravities are derived that are quite close to those obtained from Hipparcos parallaxes.

Deep galaxy counts, extragalactic background light and the stellar baryon budget

Abstract: We assess the constraints imposed by the observed extragalactic background light (EBL) on the cosmic history of star formation and the stellar-mass density today. The logarithmic slope of the galaxy number–magnitude relation from the Southern Hubble Deep Field imaging survey is flatter than 0.4 in all seven UBVIJHK optical bandpasses, i.e. the light from resolved galaxies has converged from the UV to the near-IR. We find a lower limit to the surface brightness of the optical extragalactic sky of about 15 nW m−2 sr−1, comparable to the intensity of the far-IR background from COBE data. Assuming a Salpeter initial mass function with a lower cut-off consistent with observations of M subdwarf disc stars, we set a lower limit of Ωg+sh2>0.0013 I50 to the visible (processed gas + stars) mass density required to generate an EBL at a level of 50 I50 nW m−2 sr−1; our ‘best-guess’ value is Ωg+sh2≈0.0031 I50. Motivated by the recent microlensing results of the MACHO collaboration, we consider the possibility that massive dark haloes around spiral galaxies are composed of faint white dwarfs, and show that only a small fraction (≲5 per cent) of the nucleosynthetic baryons can be locked in the remnants of intermediate-mass stars forming at zF≲5, as the bright early phases of such haloes would otherwise overproduce the observed EBL.

Identification of a-colored stars and structure in the halo of the Milky Way from Sloan digital sky survey commissioning data

Abstract: A sample of 4208 objects with magnitude 15 < g* < 22 and colors of main-sequence A stars have been selected from 370 deg2 of Sloan Digital Sky Survey (SDSS) commissioning observations. The data is from two long, narrow stripes, each with an opening angle of greater than 60°, at Galactic latitudes 36° < |b| < 63° on the celestial equator. Relative photometric calibrations good to 2% and consistent absolute photometry allows this uniform sample to be treated statistically over the large area. An examination of the sample's distribution shows that these stars trace considerable substructure in the halo. Large overdensities of A-colored stars in the north at (l, b, R) = (350°, 50°, 46 kpc) and in the south at (157, -58, 33 kpc) and extending over tens of degrees are present in the halo of the Milky Way. Ivezic et al. have detected the northern structure from a sample of RR Lyrae stars in the SDSS. Using photometry to separate the stars by surface gravity, both structures are shown to contain a sequence of low surface gravity stars consistent with identification as a blue horizontal branch (BHB). Both structures also contain a population of high surface gravity stars 2 mag fainter than the BHB stars, consistent with their identification as blue stragglers (BSs). The majority of the high surface gravity stars in the Galactic halo may be BS stars like these. A population of F stars associated with the A star excess in the southern structure is detected (the F stars in the northern structure at 46 kpc would be too faint for the SDSS to detect). From the numbers of detected BHB stars, lower limits to the implied mass of the structures are 6 × 106 M☉ and 2 × 106 M☉, although one does not yet know the full spatial extent of the structures. The fact that two such large clumps have been detected in a survey of only 1% of the sky indicates that such structures are not uncommon in the halo. Simple spheroidal parameters are fit to a complete sample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit to a halo distribution with flattening (c/a = 0.65 ± 0.2) and a density falloff exponent of α = -3.2 ± 0.3.

NICMOS Imaging of Infrared-Luminous Galaxies

Abstract: We present near-infrared images obtained with the Hubble Space Telescope NICMOS camera for a sample of nine luminous [LIGs: LIR(8?1000 ?m) ? 1011 L?] and 15 ultraluminous (ULIGS: LIR ? 1012 L?) infrared galaxies. The sample includes representative systems classified as warm (f25 ?m/f60 ?m > 0.2) and cold (f25 ?m/f60 ?m ? 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as H II (i.e., starburst), QSO, Seyfert, and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters. The colors and the luminosities of the observed clusters are consistent with them being young (107?108 yr), formed as a result of galactic interactions, and having masses much greater than those of Galactic globular clusters. In NGC 6090 and VV 114, they are preferentially situated along the area of overlap of the two galactic disks. With the exception of IR 17208-0018, all of the ULIGs have at least one compact (2.2??m FWHM ? 200 pc) nucleus. Analysis of the near-infrared colors (i.e., m1.1?1.6 vs. m1.6?2.2) derived from 11 diameter apertures suggests that the warm galaxies have near-infrared colors consistent with QSO+hot dust emission and the cold galaxies, as a group, have near-infrared colors consistent with reddened starlight. In addition, the cold ULIG UGC 5101 (and possibly three others) have near-infrared colors suggesting additional active galactic nucleus?like near-infrared components in their nuclei. In a 2 kpc diameter aperture measurement, the global colors of all of the cold galaxies except UGC 5101 are consistent with starlight with a few magnitudes of visual extinction. The general dichotomy of the near-infrared properties of the warm and the cold galaxies are further supported by the light distributions: seven of the eight warm galaxies have unresolved nuclear emission that contributes significantly (i.e., ?30%?40%) to the total near-infrared luminosity. The smooth, more extended light observed in all of the galaxies is most likely composed of giant and supergiant stars, but evidence at longer wavelengths suggests that these stars contribute little to the high 8?1000 ?m luminosity of these galaxies. Finally, light profiles of nine of the 24 systems were fitted well by an r1/4 law (and not so well by an exponential disk profile). Whether these star systems eventually become massive central bulges or giant elliptical galaxies will depend on how efficiently the present ISM is converted into stars.

The Evolution of the Galactic Morphological Types in Clusters

Abstract: The morphological types of galaxies in nine clusters in the redshift range 0.1 z 0.25 are derived from very good seeing images taken at the NOT and the La Silla-Danish telescopes, with all galaxies at MV < -20 and within the central ~1 Mpc2 area being classified. With the purpose of investigating the evolution of the fraction of different morphological types with redshift, we compare our results with the morphological content of nine distant clusters studied by the MORPHS group, five clusters observed with HST/WFPC2 at redshift z = 0.2-0.3, and Dressler's large sample of nearby clusters. After having checked the reliability of our morphological classification both in an absolute sense and relative to the MORPHS scheme, we analyze the relative occurrence of elliptical, S0, and spiral galaxies as a function of the cluster properties and redshift. We find a large intrinsic scatter in the S0/E ratio, mostly related to the cluster morphology. In particular, in our cluster sample, clusters with a high concentration of ellipticals display a low S0/E ratio and, vice versa, low concentration clusters have a high S0/E. At the same time, the trend of the morphological fractions (%E's, %S0's, %Sp's) and of the S0/E and S0/Sp ratios with redshift clearly points to a morphological evolution: as the redshift decreases, the S0 population tends to grow at the expense of the spiral population, whereas the frequency of E's remains almost constant. We also analyze the morphology-density (MD) relation in our clusters and find that—similarly to higher redshift clusters—a good MD relation exists in the high-concentration clusters, while it is absent in the less concentrated clusters. Finally, the comparison of the MD relation in our clusters with that of the MORPHS sample suggests that the transformation of spirals into S0 galaxies becomes more efficient with decreasing local density.

HST Photometry and Keck Spectroscopy of the Rich Cluster MS1054-03: Morphologies, Butcher-Oemler Effect and the Color-Magnitude Relation at z=0.83

Abstract: We present a study of 81 I selected, spectroscopically-confirmed members of the X-ray cluster MS1054-03 at z=0.83. Redshifts and spectral types were determined from Keck spectroscopy. Morphologies and accurate colors were determined from a large mosaic of HST WFPC2 images in F606W and F814W. Early-type galaxies constitute only 44% of this galaxy population. Thirty-nine percent are spiral galaxies, and 17% are mergers. The early-type galaxies follow a tight and well-defined color-magnitude relation, with the exception of a few outliers. The observed scatter is 0.029 +- 0.005 magnitudes in restframe U-B. Most of the mergers lie close to the CM relation defined by the early-type galaxies. They are bluer by only 0.07 +- 0.02 magnitudes, and the scatter in their colors is 0.07 +- 0.04 magnitudes. Spiral galaxies in MS1054-03 exhibit a large range in their colors. The bluest spiral galaxies are 0.7 magnitudes bluer than the early-type galaxies, but the majority is within +- 0.2 magnitudes of the early-type galaxy sequence. The red colors of the mergers and the majority of the spiral galaxies are reflected in the fairly low Butcher-Oemler blue fraction of MS1054-03: f_B=0.22 +- 0.05. The slope and scatter of the CM relation of early-type galaxies are roughly constant with redshift, confirming previous studies that were based on ground-based color measurements and very limited membership information. However, the scatter in the combined sample of early-type galaxies and mergers is twice as high as the scatter of the early-type galaxies alone. This is a direct demonstration of the ``progenitor bias'': high redshift early-type galaxies seem to form a homogeneous, old population because the progenitors of the youngest present-day early-type galaxies are not included in the sample.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XXVI. The Calibration of Population II Secondary Distance Indicators and the Value of the Hubble Constant

Abstract: A Cepheid-based calibration is derived for four distance indicators that utilize stars in old stellar populations: the tip of the red giant branch (TRGB), the planetary nebula luminosity function (PNLF), the globular cluster luminosity function (GCLF), and the surface brightness fluctuation method (SBF) The calibration is largely based on the Cepheid distances to 18 spiral galaxies within cz = 1500 km s-1 obtained as part of the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale, but relies also on Cepheid distances from separate HST and ground-based efforts The newly derived calibration of the SBF method is applied to obtain distances to four Abell clusters in the velocity range 3800-5000 km s-1 Combined with cluster velocities corrected for a cosmological flow model, these distances imply a value of the Hubble constant of H0 = 69 ? 4 (random) ? 6 (systematic) km s-1 Mpc-1 This result assumes that the Cepheid PL relation is independent of the metallicity of the variable stars; adopting a metallicity correction as in Kennicutt et al would produce a 5% ? 3% decrease in H0 Finally, the newly derived calibration allows us to investigate systematics in the Cepheid, PNLF, SBF, GCLF, and TRGB distance scales

Bars and warps traced by the molecular gas in the seyfert 2 galaxy ngc 1068

Abstract: We present new interferometer observations of the 12CO (1¨0) and 12CO (2¨1) line emission of NGC 1068 with a resolution of The molecular gas in the inner 5A is resolved into a ring with two bright 0A knots east and west of the nuclear continuum emission. For the —rst time in NGC 1068, we can trace molecular gas at (13 pc) from the nucleus. The high velocities in this region imply an enclosed B0A.18 mass of D108 This value is consistent with a black hole mass of 1.7 ) 107 as estimated from M _ . M _ , nuclear maser emission, plus a contribution from a compact nuclear stellar cluster. Perpendicular H 2 O to the kinematic major axis, optical images of NGC 1068 show a bright, stellar, oval structure of eccen- tricity 0.8 and a deprojected length of 17 kpc. Analysis of the rotation curve shows the CO spiral arms are at the inner Lindblad resonance of this barlike structure. Inside the molecular spiral arms, 10A from the nucleus, the CO kinematic axis changes direction probably in response to the 2.5 kpc (deprojected) long stellar bar seen in the near-infrared (NIR). The low velocity dispersion indicates the molecular gas is in a disk with a thickness of 10 pc in the nuclear region and 100 pc in the spiral arms. We constructed kinematic models for the molecular gas using elliptical orbits caused by a D1A (72 pc) nuclear bar and using tilted rings resulting in a warp. We —nd that the gas motions are consistent with either the warp or the bar models. However, because there is no evidence for a D1A nuclear bar in NIR images, we favor the warp model. A warped CO disk can also explain the obscuration of the active galactic nucleus (AGN), the extinction of light from the nuclear stellar cluster, and the observed NIR and mid-IR polarization. The model predicts that the warped CO disk should become edge-on at a radius of 70 pc, thereby creating a cavity for the ionization cone. Subject headings: galaxies: individual (NGC 1068) ¨ galaxies: ISMgalaxies: nuclei ¨ radio lines: ISM

The Vienna-KPNO search for Doppler-imaging candidate stars - I. A catalog of stellar-activity indicators for 1058 late-type Hipparcos stars

Abstract: We present the results from a spectroscopic Ca II HK 23% with strong emission, 36% with moderate emission, and 41% with weak emission. These spectra are used to determine absolute H&K emission-line fluxes, radial velocities, and equivalent widths of the luminosity-sensitive Sr II line at 4077 ˚ A. Red-wavelength spectroscopic and Stromgren y photometric follow-up obser- vations of the 371 stars with H&K emission are used to ad- ditionally determine the absolute H-core flux, the lithium abundance from the Li I 6708 ˚ A equivalent width, the rotational velocity v sin i, the radial velocity, and the light variations and its periodicity. The latter is interpreted as the stellar rotation period due to an inhomogeneous surface brightness distribu- tion. 156 stars were found with photometric periods between 0.29 and 64 days, 11 additional systems showed quasi-periodic variations possibly in excess of 50 days. Further 54 stars had variations but no unique period was found, and four stars were essentially constant. Altogether, 170 new variable stars were discovered. Additionally, we found 17 new SB1 (plus 16 new candidates) and 19 new SB2 systems, as well as one definite and two possible new SB3 systems. Finally, we present a list of 21 stars that we think are most suitable candidates for a detailed study with the Doppler-imaging technique.

A Search for Environmental Effects on Type IA Supernovae

Abstract: We use integrated colors and B and V absolute magnitudes of Type Ia supernova (SN) host galaxies in order to search for environmental effects on the SN optical properties. With the new sample of 44 SNe we confirm the conclusion by Hamuy et al. that bright events occur preferentially in young stellar environments. We find also that the brightest SNe occur in the least luminous galaxies, a possible indication that metal-poor neighborhoods produce the more luminous events. The interpretation of these results is made difficult, however, because of the fact that galaxies with younger stellar populations are also lower in luminosity. In an attempt to remove this ambiguity, we use models for the line strengths in the absorption spectrum of five early-type galaxies, in order to estimate metallicities and ages of the SN host galaxies. With the addition of abundance estimates from nebular analysis of the emission spectra of three spiral galaxies, we find possible further evidence that luminous SNe are produced in metal-poor neighborhoods. Further spectroscopic observations of the SN host galaxies will be necessary to test these results and assist in disentangling the age and metallicity effects on Type Ia SNe.

The Halo Stars in NGC 5128. II. An Outer Halo Field and a New Metallicity Distribution

Abstract: We present new Hubble Space Telescope WFPC2 (V, I) photometry for field stars in NGC 5128 at a projected distance of 8 kpc from the galaxy center, which probes a mixture of its inner halo and outer bulge. The color-magnitude diagram shows an old red giant branch that is even broader in color than our two previously studied outer halo fields (at 21 and 31 kpc), with significant numbers of stars extending to solar metallicity and higher. The peak frequency of the metallicity distribution function (MDF) is at [m/H] -0.4, with even fewer metal-poor stars than in the outer halo fields. If we use the 21 and 31 kpc fields to define template "halo" MDFs and subtract these from the 8 kpc field, the residual "bulge" population has a mean [m/H] -0.2, similar to the bulges of other large spiral and elliptical galaxies. We find that the main features of the halo MDF can be reproduced by a simple chemical evolution model in which early star formation goes on simultaneously with an initial stage of rapid infall of very metal-poor gas, after which the infall dies away exponentially. Finally, by comparison with the MDFs for the NGC 5128 globular clusters, we find that in all the halo fields we have studied there is a clear decrease of specific frequency SN (number of clusters per unit halo light) with increasing metallicity. At the lowest-metallicity range ([Fe/H] -1 it has dropped to 1.5. This trend may indicate that globular cluster formation efficiency is a strong function of the metallicity of the protocluster gas. However, we suggest an alternate possibility, which is that globular clusters form preferentially sooner than field stars. If most of the cluster formation within a host giant molecular cloud takes place sooner than most of the distributed field-star formation and if the earliest most metal-poor star-forming clouds are prematurely disrupted by their own first bursts of star formation, then they would leave relatively few field stars with a high-SN population. The high specific frequency at low metallicity may therefore be related to the comparably large SN values found in the most metal-poor dwarf elliptical galaxies.

A ROSAT HRI survey of bright nearby galaxies

Abstract: We use the extensive public archive of ROSAT High Resolution Imager (HRI) observations to carry out a statistical investigation of the X-ray properties of nearby galaxies. Specifically we focus on the sample of 486 bright (B_T 10ks) exposure. The X-ray sources detected within the optical extent of each galaxy are categorised as either nuclear or non-nuclear depending on whether the source is positioned within or outside of a 25 arcsecond radius circle centred on the optical nucleus. A nuclear X-ray source is detected in over 70% of the galaxies harbouring either a Seyfert or LINER nucleus compared to a detection rate of only ~40% in less active systems. The correlation of the H alpha luminosity with nuclear X-ray luminosity previously observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the regime of ultra-low luminosity (L(x)~10^38 - 10^40 erg/s) active galactic nuclei (AGN). The inferred accretion rates for this sample of low-luminosity AGN are significantly sub-Eddington. In total 142 non-nuclear sources were detected. In combination with published data for M31 this leads to a luminosity distribution (normalised to an optical blue luminosity of L(B) = 10^10 L(solar)) for the discrete X-ray source population in spiral galaxies of the form dN/dL38 = 1.0 +/- 0.2 L38^-1.8, where L38 is the X-ray luminosity in units of 10^38 erg/s. The implied L(x)/L(B) ratio is ~1.1 x 10^39 erg/s/(10^10 L(solar)). The nature of the substantial number of ``super-luminous'' non-nuclear objects detected in the survey is discussed.

Optical Spectral Signatures of Dusty Starburst Galaxies

Abstract: We analyze the optical spectral properties of the complete sample of Very Luminous Infrared Galaxies presented by Wu et al., and we find a high fraction (~50%) of spectra showing both a strong Hδ line in absorption and relatively modest [O II] emission [e(a) spectra]. The e(a) signature has been proposed as an efficient method to identify dusty starburst galaxies, and we study the star formation activity and the nature of these galaxies, as well as the effects of dust on their observed properties. We examine their emission-line characteristics, in particular their [O II]/Hα ratio, and we find this to be greatly affected by reddening. A search for AGN spectral signatures reveals that the e(a) galaxies are typically H II/LINER galaxies. We compare the star formation rates derived from the FIR luminosities with the estimates based on the Hα line and find that the values obtained from the optical emission lines are a factor of 10-70 (Hα) and 20-140 ([O II]) lower than the FIR estimates (50-300 M☉ yr-1). We then study the morphological properties of the e(a) galaxies, looking for a near companion or signs of a merger/interaction. In order to explore the evolution of the e(a) population, we present an overview of the available observations of e(a) galaxies in different environments both at low and high redshift. Finally, we discuss the role of dust in determining the e(a) spectral properties and we propose a scenario of selective obscuration in which the extinction decreases with the stellar age.

A sample of 669 ultra steep spectrum radio sources to find high redshift radio galaxies

Abstract: Since radio sources with Ultra Steep Spectra (USS; ) are efficient tracers of high redshift radio galaxies (HzRGs), we have defined three samples of such USS sources using the recently completed WENSS, TEXAS, MRC, NVSS and PMN radio-surveys. Our combined sample contains 669 sources with mJy and covers virtually the entire sky outside the Galactic plane (). For our 2 largest samples, covering , we selected only sources with angular sizes . For 410 sources, we present radio-maps with 03 to ~5” resolution from VLA and ATCA observations or from the FIRST survey, which allows the optical identification of these radio sources. Comparison with spectrally unbiased samples at similar flux density levels, shows that our spectral index, flux density, and angular size selections do not affect the angular size distribution of the sample, but do avoid significant contributions by faint foreground spiral galaxies. We find that the spectral index distribution of 143 000 sources from the WENSS and NVSS consists of a steep spectrum galaxy and a flat spectrum quasar population, with the relative contribution of flat spectrum sources doubling from Jy to Jy. The identification fraction of our USS sources on the POSS () is as low as 15%, independent of spectral index . We further show that 85% of the USS sources that can be identified with an X-ray source are probably contained in galaxy clusters, and that sources are excellent Galactic pulsar candidates, because the percentage of these sources is four times higher in the Galactic plane. Our sample has been constructed to start an intensive campaign to obtain a large sample of high redshift objects () that is selected in a way that does not suffer from dust extinction or any other optical bias.

Chemo-spectrophotometric evolution of spiral galaxies – II. Main properties of present-day disc galaxies

Abstract: We study the chemical and spectrophotometric evolution of galactic discs with detailed models calibrated on the Milky Way and using simple scaling relations, based on currently popular semi-analytic models of galaxy formation. We compare our results with a large body of observational data on present-day galactic discs, including disc sizes and central surface brightness, Tully–Fisher relations in various wavelength bands, colour–colour and colour–magnitude relations, gas fractions versus magnitudes and colours and abundances versus local and integrated properties, as well as spectra for different galactic rotational velocities. Despite the extremely simple nature of our models, we find satisfactory agreement with all those observables, provided that the time-scale for star formation in low-mass discs is longer than for more massive ones. This assumption is apparently in contradiction with the standard picture of hierarchical cosmology. We find, however, that it is extremely successful in reproducing major features of present-day discs, like the change in the slope of the Tully–Fisher relation with wavelength, the fact that more massive galaxies are on average ‘redder’ than low-mass ones (a generic problem of standard hierarchical models) and the metallicity–luminosity relation for spirals. It is concluded that, on a purely empirical basis, this new picture is at least as successful as the standard one. Observations at high redshifts could help to distinguish between the two possibilities.

The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds

Abstract: We combine new CCD UBV photometry and spectroscopy with those from the literature to investigate 19 Magellanic Cloud OB associations that contain Wolf-Rayet (W-R) and other types of evolved, massive stars. Our spectroscopy reveals a wealth of newly identified interesting objects, including early O-type supergiants, a high-mass, double-lined binary in the SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV; R85), a newly discovered W-R star (Sk -69 degrees 194), and a newly found luminous B[e] star (LH 85-10). We use these data to provide precise reddening determinations and construct physical H-R diagrams for the associations. We find that about half of the associations may be highly coeval, with the massive stars having formed over a short period (Delta tau < 1 Myr). The (initial) masses of the highest mass unevolved stars in the coeval clusters may be used to estimate the masses of the progenitors of W-R and other evolved stars found in these clusters. Similarly, the bolometric luminosities of the highest mass unevolved stars can be used to determine the bolometric corrections (BCs) for the evolved stars, providing a valuable observational basis for evaluating recent models of these complicated atmospheres. What we find is the following: (1) Although their numbers is small, it appears that the W-R stars in the SMC come from only the highest mass (greater than 70 M.) stars. This is in accord with our expectations that at low metallicities only the most massive and luminous stars will have sufficient mass loss to become W-R stars. (2) In the LMC, the early-type WN (WNE) stars occur in clusters whose turnoff masses range from 30 to 100 M. or more. This suggests that possibly all stars with mass greater than 30 M. pass through a WNE stage at LMC metallicities. (3) The one WC star in the SMC is found in a cluster with a turnoff mass of 70 M., the same as that for the SMC WN stars. In the LMC, the WC stars are found in clusters with turnoff masses of 45 M. or higher, similar to what is found for the LMC WN stars. Thus we conclude that WC stars come from essentially the same mass range as do WN stars and indeed are often found in the same clusters. This has important implications for interpreting the relationship between metallicity and the WC/WN ratio found in Local Group galaxies, which we discuss. (4) The LBVs in our sample come from very high mass stars (greater than 85 M.), similar to what is known for the Galactic LBV eta Car, suggesting that only the most massive stars go through an LBV phase. Recently, Ofpe/WN9 stars have been implicated as LBVs after one such star underwent an LBV-like outburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br 18, which we find in clusters with much lower turnoff masses (25-35 M.). We suggest that Ofpe/WN9 stars are unrelated to "true" LBVs: not all "LBV-like outbursts" may have the same cause. Similarly, the B[e] stars have sometimes been described as LBV-like. Yet, the two stars in our sample appear to come from a large mass range (30-60 M.). This is consistent with other studies, suggesting that B[e] stars cover a large range in bolometric luminosities. (5) The bolometric corrections of early WN and WC stars are found to be extreme, with an average BC(WNE) of -6.0 mag and an average BC(WC4) of -5.5 mag. These values are considerably more negative than those of even the hottest O-type stars. However, similar values have been found for WNE stars by applying Hillier's "standard model" for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars (BC = -2 to -4 mag), also consistent with recent analysis done with the standard model. Extension of these studies to the Galactic clusters will provide insight into how massive stars evolve at different metallicities.

The Origin of Extremely Metal-poor Carbon Stars and the Search for Population III.

Abstract: The number of known extremely metal-deficient stars has recently increased substantially, stimulating inquiry into the formation and initial chemical evolution of the Milky Way. In order to draw proper inferences from the observations, it is necessary to understand the evolution of these low-mass stars and the modifications in their surface elemental abundances that they have experienced during their long lifetimes. Among the observations to be explained is the fact that the incidence of carbon-enhanced stars increases with decreasing metallicity. We show that low-mass, extremely metal-poor stars evolve into carbon stars along paths that are quite different from those followed by more metal-rich stars of younger populations. This permits us, in principle, to distinguish the brightest survivors of the first generations of stars formed in the universe (Population III carbon stars) from stars belonging to younger populations.

Chemo-spectrophotometric evolution of spiral galaxies — III. Abundance and colour gradients in discs

Abstract: We study the relations between luminosity and chemical-abundance profiles of spiral galaxies, using detailed models for the chemical and spectrophotometric evolution of galactic discs. The models are ‘calibrated’ on the Milky Way disc and are successfully extended to other discs with the help of simple ‘scaling’ relations, obtained in the framework of semi-analytic models of galaxy formation. We find that our models exhibit oxygen abundance gradients that increase in absolute value with decreasing disc luminosity (when expressed in dex kpc−1) and are independent of disc luminosity (when expressed in dex scalelength−1), both in agreement with observations. We notice an important strong correlation between abundance gradient and disc scalelength. These results support the idea of ‘homologous evolution’ of galactic discs.

High-resolution rotation curves of low surface brightness galaxies

Abstract: High-resolution Halpha rotation curves are presented for five low surface brightness galaxies. These Halpha rotation curves have shapes different from those previously derived from H i observations, probably because of the higher spatial resolution of the Halpha observations. The Halpha rotation curves rise more steeply in the inner parts than the H i rotation curves, and they reach a flat part beyond about two disk scale lengths. With radii expressed in optical disk scale lengths, the rotation curves of the low surface brightness galaxies presented here and those of the high surface brightness galaxies have almost identical shapes. Mass modeling shows that the contribution of the stellar component to the rotation curves may be scaled to explain most of the inner parts of the rotation curves, albeit with high stellar mass-to-light ratios. On the other hand, well-fitting mass models can also be obtained with lower contributions of the stellar disk. These observations suggest that the luminous mass density and the total mass density are coupled in the inner parts of these galaxies.

Maximum Disk Mass Models for Spiral Galaxies

Abstract: We present axisymmetric maximum disk mass models for a sample of 74 spiral galaxies taken from the southern sky Fabry-Perot Tully-Fisher survey by Schommer et al. The sample contains galaxies spanning a large range of morphologies and having rotation widths from 180 km s -1 to 680 km s -1. For each galaxy we have an I-band image and a two-dimensional Hα velocity field. We decompose the disk and bulge by fitting models directly to the I-band image. This method utilizes both the distinct surface brightness profiles and shapes of the projected disk and bulge in the galaxy images. The luminosity profiles and rotation curves are derived using consistent centers, position angles, and inclinations derived from the photometry and velocity maps. The distribution of mass is modeled as a sum of disk and bulge components with distinct, constant mass-to-light ratios. No dark matter halo is included in the fits. The models reproduce the overall structure of the rotation curves in the majority of galaxies, providing good fits to galaxies that exhibit pronounced structural differences in their surface brightness profiles. Of galaxies for which the rotation curve is measured to R23.5 or beyond 75% are well fitted by a mass-traces-light model for the entire region within R23.5. The models for about 20% of the galaxies do not fit well; the failure of most of these models is traced directly to nonaxisymmetric structures, primarily bars but also strong spiral arms. The median I-band M/L of the disk plus bulge is 2.4 ± 0.9 h75 in solar units, consistent with normal stellar populations. These results require either that the mass of dark matter within the optical disk of spiral galaxies is small or that its distribution is very precisely coupled to the distribution of luminous matter.

The evolution of early-type galaxies in distant clusters. III. M/L-v ratios in the z=0.33 cluster Cl 1358+62

Abstract: Internal kinematics, length scales, and surface brightnesses have been determined for a large sample of 53 galaxies in the cluster Cl 1358+62 at z = 0.33 from Keck spectroscopy and Hubble Space Telescope WFPC2 imaging over a 1.5 h-1 × 1.5 h-1 Mpc2 field of view. These data have been used to constrain the evolution of early-type galaxies in the cluster environment. We have constructed the fundamental plane using 30 E and S0 galaxies and draw the following conclusions. The fundamental plane at z = 0.33 has the form re ∝ σ1.31±0.13I, similar to that found locally. The 1 σ intrinsic scatter about this plane is ±14% in M/LV, comparable to that observed in Coma. We conclude that, even at intermediate redshifts, E and S0 galaxies are structurally mature and homogeneous, like those observed in nearby clusters. The M/LV ratios of these early-type galaxies are offset from the Coma fundamental plane by Δ log(M/LV) = -0.13 ± 0.03 (q0 = 0.1), indicative of mild luminosity evolution in the stellar populations in the Cl 1358+62 E and S0 galaxies. This level of evolution suggests a luminosity-weighted formation epoch for the stars of z > 1. The precise redshift depends on the initial mass function and on the cosmology. The scatter about the fundamental plane is consistent with that in the color-magnitude relation, indicating that the E/S0's have a scatter in luminosity-weighted ages of 15%. We have also analyzed the M/LV ratios of galaxies of type S0/a and later. These early-type spirals follow a different plane from the E and S0 galaxies: re ∝ σ0.66±0.29I, with a scatter that is twice as large as the scatter for the E/S0's. The difference in the tilt between the plane of the spirals and the plane of the E/S0's is shown to be due to a systematic correlation of velocity dispersion with residual from the plane of the early-type galaxies. These residuals also correlate with the residuals from the color-magnitude relation. Thus for the spirals in this cluster, as well as for the three E+A galaxies in the sample, we see a systematic variation in the luminosity-weighted mean properties of the stellar populations with central velocity dispersion. If this is a relative age trend, then luminosity-weighted age is positively correlated with σ; i.e., more massive spiral galaxies have older stars on average. The residuals from the color-magnitude relation were used to correct the surface brightnesses of the early-type spirals. After this correction for age effects, these spirals fall on the fundamental plane of E/S0's. We conclude that the early-type spirals may well evolve onto the scaling relations of the old cluster members. After correcting the spirals for the systematic trend with color, their scatter about the fundamental plane does not decrease and remains twice as large as that for the E/S0's. This large scatter should be seen in a subsample of present-day cluster galaxies, if these spirals evolve into contemporary S0's, unless some unknown process reduces their scatter. The colors and M/LV ratios imply that many cluster galaxies were forming stars at least up until z ~ 0.5, but that this activity was specific to the spiral population. The E+A's will likely evolve into low-mass present-day S0 and Sa galaxies but be slightly bluer than the present-day (B-V) color-magnitude relation by ~0.08 mag, while fading to ~1 mag below L*.

Exploring Halo Substructure with Giant Stars. I. Survey Description and Calibration of the Photometric Search Technique

Abstract: We have begun a survey of the structure of the Milky Way halo, as well as the halos of other Local Group galaxies, as traced by their constituent giant stars. These giant stars are identified via large-area, CCD photometric campaigns. Here we present the basis for our photometric search method, which relies on the gravity sensitivity of the Mg I triplet+MgH features near 5150 ? in F?K stars, and which is sensed by the flux in the intermediate-band DDO51 filter. Our technique is a simplified variant of the combined Washington/DDO51 four-filter technique described by Geisler, which we modify for the specific purpose of efficiently identifying distant giant stars for follow-up spectroscopic study: We show here that for most stars the Washington T1-T2 color is correlated monotonically with the Washington M-T2 color with relatively low scatter; for the purposes of our survey, this correlation obviates the need to image in the T1 filter, as originally proposed by Geisler. To calibrate our (M-T2, M-DDO51) diagram as a means to discriminate field giant stars from nearby dwarfs, we utilize new photometry of the main sequences of the open clusters NGC 3680 and NGC 2477 and the red giant branches of the clusters NGC 3680, Melotte 66, and ? Centauri, supplemented with data on field stars, globular clusters and open clusters by Doug Geisler and collaborators. By combining the data on stars from different clusters, and by taking advantage of the wide abundance spread within ? Centauri, we verify the primary dependence of the M-DDO51 color on luminosity and demonstrate the secondary sensitivity to metallicity among giant stars. Our empirical results are found to be generally consistent with those from analysis of synthetic spectra by Paltoglou & Bell. Finally, we provide conversion formulae from the (M, M-T2) system to the (V, V-I) system, corresponding reddening laws, as well as empirical red giant branch curves from ? Centauri stars for use in deriving photometric parallaxes for giant stars of various metallicities (but equivalent ages) to those of ? Centauri giants.

The local space density of SB-SDM galaxies as function of their scale size, surface brightness, and luminosity.

Abstract: We investigate the dependence of the local space density of spiral galaxies on luminosity, scale size, and surface brightness. We derive bivariate space density distributions in these quantities from a sample of about 1000 Sb-Sdm spiral galaxies, corrected for selection effects in luminosity and surface brightness. The structural parameters of the galaxies were corrected for internal extinction using a description depending on galaxy surface brightness. We find that the bivariate space density distribution of spiral galaxies in the (luminosity, scale size)-plane is well described by a Schechter luminosity function in the luminosity dimension and a log-normal scale-size distribution at a given luminosity. This parameterization of the scale-size distribution was motivated by a simple model for the formation of disks within dark matter halos, with halos acquiring their angular momenta through tidal torques from neighboring objects and the disk specific angular momentum being proportional to that of the parent halo. However, the fractional width of the scale-size distribution at a given luminosity is narrower than what one would expect from using the distribution of angular momenta of halos measured in N-body simulations of hierarchical clustering. We present several possible explanations for the narrowness of the observed distribution. Using our bivariate distribution, we find that determinations of the local luminosity function of spiral galaxies should not be strongly affected by the bias against low surface brightness galaxies, even when the galaxies are selected from photographic plates. This may not be true for studies at high redshift, where (1 + z)4 surface brightness dimming would cause a significant selection bias against lower surface brightness galaxies, if the galaxy population did not evolve with redshift.

The local space density of Sb-Sdm galaxies as function of their scalesize, surface brightness and luminosity

Abstract: We investigate the dependence of the local space density of spiral galaxies on luminosity, scalesize and surface brightness. We derive bivariate space density distributions in these quantities from a sample of about 1000 Sb-Sdm spiral galaxies, corrected for selection effects in luminosity and surface brightness. The structural parameters of the galaxies were corrected for internal extinction using a description depending on galaxy surface brightness. We find that the bivariate space density distribution of spiral galaxies in the (luminosity, scalesize)-plane is well described by a Schechter luminosity function in the luminosity dimension and a log-normal scale size distribution at a given luminosity. This parameterization of the scalesize distribution was motivated by a simple model for the formation of disks within dark matter halos, with halos acquiring their angular momenta through tidal torques from neighboring objects, and the disk specific angular momentum being proportional to that of the parent halo. However, the fractional width of the scalesize distribution at a given luminosity is narrower than what one would expect from using the distribution of angular momenta of halos measured in N-body simulations. We present several possible explanations for the narrowness of the observed distribution. Using our bivariate distribution, we find that determinations of the local luminosity function of spiral galaxies should not be strongly affected by the bias against low surface brightness galaxies, even when the galaxies are selected from photographic plates. This may not be true for studies at high redshift, where (1+z)^4 surface brightness dimming can cause a significant selection bias against lower surface brightness galaxies.

The Bar-Mode Instability in Differentially Rotating Neutron Stars: Simulations in Full General Relativity

Abstract: We study the dynamical stability against bar-mode deformation of rapidly spinning neutron stars with diUerential rotation. We perform fully relativistic three-dimensional simulations of compact stars with M/R " 0.1, where M is the total gravitational mass and R the equatorial circumferential radius. We adopt an adiabatic equation of state with adiabatic index ! \ 2. As in Newtonian theory, we —nd that stars above a critical value of b 4 T /W (where T is the rotational kinetic energy and W the gravita- tional binding energy) are dynamically unstable to bar formation. For our adopted choices of stellar compaction and rotation pro—le, the critical value of is D0.24¨0.25, only slightly smaller than b \ b dGR the well-known Newtonian value D0.27 for incompressible Maclaurin spheroids. The critical value depends only very weakly on the degree of diUerential rotation for the moderate range we surveyed. All unstable stars form bars on a dynamical timescale. Models with sufficiently large b subsequently form spiral arms and eject mass, driving the remnant to a dynamically stable state. Models with moderately large do not develop spiral arms or eject mass but adjust to form dynamically stable b Z b dGR ellipsoidal-like con—gurations. If the bar-mode instability is triggered in supernova collapse or binary neutron star mergers, it could be a strong and observable source of gravitational waves. We determine characteristic wave amplitudes and frequencies. Subject headings: dense matterrelativitystars: neutronstars: rotation

Surface photometry of nearby field galaxies: the data

Abstract: We have obtained integrated spectra and multifilter photometry for a representative sample of similar to 200 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M(B) = -14 to -22) and colors (B-R = 0.4-1.8). Here we describe the sample selection criteria and the U, B, R surface photometry for these galaxies. The spectrophotometric results will be presented in a companion paper. Our goals for the project include measuring the current star formation rates and metallicity of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high red-shift. We discuss the observing, data reduction, and calibration techniques and show that our photometry agrees well with previous work in those cases in which earlier data are available. We present an atlas of images, radial surface brightness profiles, and color profiles as well as tables of derived parameters. The atlas and tables of measurements will be made available electronically. We study the correlations of galaxy properties determined from the galaxy images. Our findings include the following: (1) colors determined within the effective radius correlate better with morphological type than with M(B) and (2) 50% of the low-luminosity galaxies are bluest in their centers.