We have used a suite of simpliÐed spectrophotometric spiral galaxy evolution models to argue that there are substantial variations in stellar mass-to-light (M/L ) ratios within and among galaxies, amounting to factors of between 3 and 7 in the optical and factors of 2 in the near-infrared. Our models show a strong correlation between stellar M/L and the optical colors of the integrated stellar populations. Under the assumption of a universal spiral galaxy initial mass function (IMF), relative trends in model stellar M/L with color are robust to uncertainties in stellar population and galaxy evolution modeling, including the e †ects of modest bursts of star formation. Errors in the dust-reddening estimates do not strongly a †ect the Ðnal derived stellar masses of a stellar population. We examine the observed maximum disk stellar M/L ratios of a sample of spiral galaxies with accurate rotation curves and optical and near-infrared luminosity proÐles. From these observed maximum disk M/L ratios we conclude that a Salpeter IMF has too many low-mass stars per unit luminosity but that an IMF similar to the Salpeter IMF at the high-mass end with less low-mass stars (giving stellar M/L ratios 30% lower than the Salpeter value) is consistent with the maximum disk constraints. Trends in observed maximum disk stellar M/L ratios with color provide a good match to the predicted model relation, suggesting that the spiral galaxy stellar IMF is universal and that a fraction of (particularly high surface brightness) spiral galaxies may be close to maximum disk. We apply the model trends in stellar M/L ratio with color to the Tully-Fisher (T-F) relation. We Ðnd that the stellar mass T-F relation is relatively steep, has modest scatter, and is independent of the passband and color used to derive the stellar masses, again lending support for a universal IMF. The di †erence in slope between the optical (especially blue) and nearinfrared T-F relations is due to the combined e †ects of dust attenuation and stellar M/L variations with galaxy mass. Assuming the Hubble Space T elescope Key Project distance to the Ursa Major Cluster and neglecting the (uncertain) molecular gas fraction, we Ðnd that the baryonic T-F relation takes the form (with random and systematic 1 p slope errors of D0.2 each) when using a bisector Ðt and M baryon P V 3.5 rotation velocities derived from the Ñat part of the rotation curve. Since we have normalized the stellar M/L ratios to be as high as can possibly be allowed by maximum disk constraints, the slope of the baryonic T-F relation will be somewhat shallower than 3.5 if all disks are substantially submaximal.

The ACS Nearby Galaxy Survey Treasury (ANGST) is a systematic survey to establish a legacy of uniform multi-color photometry of resolved stars for a volume-limited sample of nearby galaxies (D < 4 Mpc). The survey volume encompasses 69 galaxies in diverse environments, including close pairs, small & large groups, filaments, and truly isolated regions. The galaxies include a nearly complete range of morphological types spanning a factor of ∼ 10 4 in luminosity and star formation rate. The survey data consists of images taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST), supplemented with archival data and new Wide Field Planetary Camera (WFPC2) imaging taken after the failure of ACS. Survey images include wide field tilings covering the full radial extent of each galaxy, and single deep pointings in uncrowded regions of the most massive galaxies in the volume. The new wide field imaging in ANGST reaches median 50% completenesses of m F 475W = 28.0 mag, m F 606W = 27.3 mag, and m F 814W = 27.3 mag, several magnitudes below the tip of the red giant branch (TRGB). The deep fields reach magnitudes sufficient to fully resolve the structure in the red clump (RC). The resulting photometric catalogs are publicly accessible and contain over 34 million photometric measurements of >14 million stars. In this paper we present the details of the sample selection, imaging, data reduction, and the resulting photometric catalogs, along with an analysis of the photometric uncertainties (systematic and random), for both the ACS and WFPC2 imaging. We also present uniformly derived relative distances measured from the apparent magnitude of the TRGB.

We combine deep optical and IR photometry for 326 spiral galaxies from two recent galaxy samples and report that the surface brightness profiles of late-type spirals are best fit by two exponentials. Moreover, the ratio of bulge and disk scale lengths takes on a restricted range of values and is uncorrelated with Hubble type. This suggests a scale-free Hubble sequence for late-type spirals. Careful numerical simulations ensure that our results are not affected by seeing or resolution effects. Many of these galaxies show spiral structure continuing into the central regions with a previously undetected small bar and slowly changing colors between the inner disk and the bulge. We invoke secular dynamical evolution and interpret the nature of disk central regions in the context of gas inflow via angular-momentum transfer and viscous transport. In this scenario, galaxy morphologies in late-type spirals are not imprinted at birth but are the result of evolution.Comment: Accepted for publication in the Astrophysical Journal Letters 11 pages LaTeX file, 2 PostScript figures, uses aaspp4.sty, also available at: ftp://sagittarius.tuc.noao.edu/pub/bulges95.ps.Z http://www.astro.su.se/~broeils/papers.html http://star-www.dur.ac.uk/~rdejong/papers.htm

We have used a large sample of low-inclination spiral galaxies with radially-resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and regarding the importance of dust all compromise the absolute ages and metallicities derived in this work, however our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass, however the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.Comment: 25 pages; 17 figures; re-submitted to MNRAS after replying to referee's repor

We made new estimates of the Galactic escape speed at various Galactocentric radii using the latest data release of the RAdial Velocity Experiment (RAVE DR4). Compared to previous studies we have a database that is larger by a factor of 10, as well as reliable distance estimates for almost all stars. Our analysis is based on statistical analysis of a rigorously selected sample of 90 highvelocity halo stars from RAVE and a previously published data set. We calibrated and extensively tested our method using a suite of cosmological simulations of the formation of Milky Way-sized galaxies. Our best estimate of the local Galactic escape speed, which we define as the minimum speed required to reach three virial radii R 340 , is 533 +54 −41 km s −1 (90% confidence), with an additional 4% systematic uncertainty, where R 340 is the Galactocentric radius encompassing a mean overdensity of 340 times the critical density for closure in the Universe. From the escape speed we further derived estimates of the mass of the Galaxy using a simple mass model with two options for the mass profile of the dark matter halo: an unaltered and an adiabatically contracted Navarro, Frenk & White (NFW) sphere. If we fix the local circular velocity, the latter profile yields a significantly higher mass than the uncontracted halo, but if we instead use the statistics for halo concentration parameters in large cosmological simulations as a constraint, we find very similar masses for both models. Our best estimate for M 340 , the mass interior to R 340 (dark matter and baryons), is 1.3 +0.4 −0.3 × 10 12 M (corresponds to M 200 = 1.6 +0.5 −0.4 × 10 12 M ). This estimate is in good agreement with recently published, independent mass estimates based on the kinematics of more distant halo stars and the satellite galaxy Leo I.

Combining the precise parallaxes and optical photometry delivered by Gaia's second data release (Gaia DR2) with the photometric catalogues of Pan-STARRS1, 2MASS, and AllWISE, we derived Bayesian stellar parameters, distances, and extinctions for 265 million of the 285 million objects brighter than G = 18. Because of the wide wavelength range used, our results substantially improve the accuracy and precision of previous extinction and effective temperature estimates. After cleaning our results for both unreliable input and output data, we retain 137 million stars, for which we achieve a median precision of 5% in distance, 0.20 mag in Vband extinction, and 245 K in effective temperature for G ≤ 14, degrading towards fainter magnitudes (12%, 0.20 mag, and 245 K at G = 16; 16%, 0.23 mag, and 260 K at G = 17, respectively). We find a very good agreement with the asteroseismic surface gravities and distances of 7000 stars in the Kepler, K2-C3, and K2-C6 fields, with stellar parameters from the APOGEE survey, and with distances to star clusters. Our results are available through the ADQL query interface of the Gaia mirror at the Leibniz-Institut für Astrophysik Potsdam (gaia.aip.de) and as binary tables at data.aip.de. As a first application, we provide distanceand extinction-corrected colour-magnitude diagrams, extinction maps as a function of distance, and extensive density maps. These demonstrate the potential of our value-added dataset for mapping the three-dimensional structure of our Galaxy. In particular, we see a clear manifestation of the Galactic bar in the stellar density distributions, an observation that can almost be considered direct imaging of the Galactic bar.

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

We present new high-resolution observations of the center of the late-type spiral M100 (NGC 4321) supplemented by 3D numerical modeling of stellar and gas dynamics, including star formation (SF). NIR imaging has revealed a stellar bar, previously inferred from optical and 21 cm observations, and an ovally-shaped ring-like structure in the plane of the disk. The K isophotes become progressively elongated and skewed to the position angle of the bar (outside and inside the `ring') forming an inner bar-like region. The galaxy exhibits a circumnuclear starburst in the inner part of the K `ring'. Two maxima of the K emission have been observed to lie symmetrically with respect to the nucleus and equidistant from it slightly leading the stellar bar. We interpret the twists in the K isophotes as being indicative of the presence of a double inner Lindblad resonance (ILR) and test this hypothesis by modeling the gas flow in a self-consistent gas + stars disk embedded in a halo, with an overall NGC4321-like mass distribution. We have reproduced the basic morphology of the region (the bar, the large scale trailing shocks, two symmetric K peaks corresponding to gas compression maxima which lie at the caustic formed by the interaction of a pair of trailing and leading shocks in the vicinity of the inner ILR, both peaks being sites of SF, and two additional zones of SF corresponding to the gas compression maxima, referred usually as `twin peaks').Comment: 31 pages, postscript, compressed, uuencoded. 21 figures available in postscript, compressed form by anonymous ftp from ftp://asta.pa.uky.edu/shlosman/main100 , mget *.ps.Z. To appear in Ap.