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Register a new userThe Star Formation Histories of Disk and E/S0 Galaxies from Resolved Stars
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The resolved stellar populations of local galaxies, from which it is possible to derive complete star formation and chemical enrichment histories, provide an important way to study galaxy formation and evolution that is complementary to lookback time studies. We propose to use photometry of resolved stars to measure the star formation histories in a statistical sample of galaxy disks and E/S0 galaxies near their effective radii. These measurements would yield strong evidence to support critical questions regarding the formation of galactic disks and spheroids. The main technological limitation is spatial resolution for photometry in heavily crowded fields, for which we need improvement by a factor of ~10 over what is possible today with filled aperture telescopes.
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The impact of HST photometry and European astronomy in studies concerning the star formation histories of resolved galaxies is described. Our current knowledge of the star formation history of systems within 10-20 Mpc, as derived from the colour-magnitude diagrams of their resolved stellar populations, is reviewed, as well as the impact of these results on our understanding of galaxy evolution.
We combine new data from the main sequence (M_* versus SFR) of star-forming galaxies and galaxy colors (from GALEX to Spitzer) with a flexible stellar population scheme to deduce the mass-to-light ratio (Upsilon_*) of star-forming galaxies from the SPARC and S^4G samples. We find that the main sequence for galaxies, particular the low-mass end, combined with the locus of galaxy colors, constrains the possible star formation histories of disk and dwarf galaxies to a similar shape found by Speagle et al. (2014). Combining the deduced star formation history with stellar population models in the literature produces reliable Upsilon_* values as a function of galaxy color with an uncertainty of only 0.05 dex. We provide prescriptions to deduce Upsilon_* for optical and near-IR bandpasses, with near-IR bandpasses having the least uncertainty (Upsilon_* from 0.40 to 0.55). We also provide the community with a webtool, with flexible stellar population parameters, to generate their own Upsilon_* values over the wavelength range for most galaxy surveys.
We reexamine the systematic properties of local galaxy populations, using published surveys of star formation, structure, and gas content. After recalibrating star formation measures, we are able to reliably measure specific star formation rates well below the main sequence of star formation vs mass. We find an unexpectedly large population of galaxies with star formation rates intermediate between vigorously star-forming main sequence galaxies and passive galaxies, and with gas content disproportionately high for their star formation rates. Several lines of evidence suggest that these quiescent galaxies form a distinct population rather than a low star formation tail of the main sequence. We demonstrate that a tight main sequence, evolving with epoch, is a natural outcome of most histories of star formation and has little astrophysical significance, but that the quiescent population requires additional astrophysics to explain its properties. Using a simple model for disk evolution based on the observed dependence of star formation on gas content in local galaxies, and assuming simple histories of cold gas inflow, we show that the evolution of galaxies away from the main sequence can be attributed to the depletion of gas due to star formation after a cutoff in gas inflow. The quiescent population is composed of galaxies in which the density of disk gas has fallen below a threshold for disk stability. The evolution of galaxies beyond the quiescent state to gas exhaustion requires another process, probably wind-driven mass loss. The SSFR distribution of the quiescent and passive implies that the timescale of this process must be greater than a few Gyrs but less than a few tens of Gyrs. The environmental dependence of the galaxy populations is consistent with recent theory suggesting that cold gas inflows into galaxies are truncated at earlier times in denser environments.
In this tutorial paper we summarize how the star formation (SF) history of a galactic region can be derived from the colour-magnitude diagram (CMD) of its resolved stars. The procedures to build synthetic CMDs and to exploit them to derive the SF histories (SFHs) are described, as well as the corresponding uncertainties. The SFHs of resolved dwarf galaxies of all morphological types, obtained from the application of the synthetic CMD method, are reviewed and discussed. In short: 1) Only early-type galaxies show evidence of long interruptions in the SF activity; late-type dwarfs present rather continuous, or gasping, SF regimes; 2) A few early-type dwarfs have experienced only one episode of SF activity concentrated at the earliest epochs, whilst many others show extended or recurrent SF activity; 3) No galaxy experiencing now its first SF episode has been found yet; 4) No frequent evidence of strong SF bursts is found; 5) There is no significant difference in the SFH of dwarf irregulars and blue compact dwarfs, except for the current SF rates. Implications of these results on the galaxy formation scenarios are briefly discussed.
We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using ULySS and STECKMAP, we derive radial profiles of the SSP-equivalent ages, metallicities and star-formation histories. The old stellar population of the dEs, which dominates their mass, is likely coeval with that of massive ellipticals or bulges, but the star formation efficiency is lower. Important intermediate age (1-5 Gyr) populations, and frequently tails of star formation until recent times are detected. These histories are reminiscent of their lower mass dSph counterparts of the Local Group. Most galaxies (10/16) show significant metallicity gradients, with metallicity declining by 0.5 dex over one half-light radius on average. These gradients are already present in the old population. The flattened (or discy), rotating objects (6/16) have flat metallicity profiles. This may be consistent with a distinct origin for these galaxies or it may be due to their geometry. The central SSP-equivalent age varies between 1 and 6 Gyr, with the age slowly increasing with radius in the vast majority of objects. The group and cluster galaxies have similar radial gradients and star-formation histories. The strong and old metallicity gradients place important constraints on the possible formation scenarios of dEs. Numerical simulations of the formation of spherical low-mass galaxies reproduce these gradients, but they require a longer time for them to build up. A gentle depletion of the gas, by ram-pressure stripping or starvation, could drive the gas-rich, star-forming progenitors to the present dEs.
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