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Register a new userMeasuring Star Formation Histories, Distances, and Metallicities with Pixel Color-Magnitude Diagrams II: Applications to Nearby Elliptical Galaxies
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We present spatially-resolved measurements of star formation histories (SFHs), metallicities, and distances in three nearby elliptical galaxies and the bulge of M31 derived using the pixel color-magnitude diagram (pCMD) technique. We compute pCMDs from archival $textit{HST}$ photometry of M87, M49, NGC 3377 and M31, and fit the data using the new code $texttt{PCMDPy}$. We measure distances to each system that are accurate to $sim 10%$. The recovered non-parametric SFHs place reasonable ($pm 1$ dex) constraints on the recent (< 2 Gyr) star formation in M31 and NGC 3377, both of which show evidence of inside-out growth. The SFHs in M87 and M49 are constrained only at the oldest ages. The pCMD technique is a promising new avenue for studying the evolutionary history of the nearby universe, and is highly complementary to existing stellar population modeling techniques.
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We present a comprehensive study of the applications of the pixel color-magnitude diagram (pCMD) technique for measuring star formation histories (SFHs) and other stellar population parameters of galaxies, and demonstrate that the technique can also constrain distances. SFHs have previously been measured through either the modeling of resolved-star CMDs or of integrated-light SEDs, yet neither approach can easily be applied to galaxies in the semi-resolved regime. The pCMD technique has previously been shown to have the potential to measure stellar populations and star formation histories in semi-resolved galaxies. Here we present Pixel Color-Magnitude Diagrams with Python (pcmdpy), a GPU-accelerated package that makes significant computational improvements to the original code and including more realistic physical models. These advances include the simultaneous fitting of distance, modeling a Gaussian metallicity-distribution function, and an observationally-motivated dust model. GPU-acceleration allows these more realistic models to be fit roughly 7x faster than the simpler models in the original code. We present results from a suite of mock tests, showing that with proper model assumptions, the code can simultaneously recover SFH, [Fe/H], distance, and dust extinction. Our results suggest the code, applied to observations with HST-like resolution, should constrain these properties with high precision within 10 Mpc and can be applied to systems out to as far as 100 Mpc. pCMDs open a new window to studying the stellar populations of many galaxies that cannot be readily studied through other means.
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.
We adapt the L-Galaxies semi-analytic model to follow the star-formation histories (SFH) of galaxies -- by which we mean a record of the formation time and metallicities of the stars that are present in each galaxy at a given time. We use these to construct stellar spectra in post-processing, which offers large efficiency savings and allows user-defined spectral bands and dust models to be applied to data stored in the Millennium data repository. We contrast model SFHs from the Millennium Simulation with observed ones from the VESPA algorithm as applied to the SDSS-7 catalogue. The overall agreement is good, with both simulated and SDSS galaxies showing a steeper SFH with increased stellar mass. The SFHs of blue and red galaxies, however, show poor agreement between data and simulations, which may indicate that the termination of star formation is too abrupt in the models. The mean star-formation rate (SFR) of model galaxies is well-defined and is accurately modelled by a double power law at all redshifts: SFR proportional to $1/(x^{-1.39}+x^{1.33})$, where $x=(t_a-t)/3.0,$Gyr, $t$ is the age of the stars and $t_a$ is the loopback time to the onset of galaxy formation; above a redshift of unity, this is well approximated by a gamma function: SFR proportional to $x^{1.5}e^{-x}$, where $x=(t_a-t)/2.0,$Gyr. Individual galaxies, however, show a wide dispersion about this mean. When split by mass, the SFR peaks earlier for high-mass galaxies than for lower-mass ones, and we interpret this downsizing as a mass-dependence in the evolution of the quenched fraction: the SFHs of star-forming galaxies show only a weak mass dependence.
The spatially resolved star formation histories are studied for 32 normal star-forming galaxies drawn from the the Spitzer Extended Disk Galaxy Exploration Science survey. At surface brightness sensitivities fainter than 28 mag arcsec$^{-2}$, the new optical photometry is deep enough to complement archival ultraviolet and infrared imaging and to explore the properties of the emission well beyond the traditional optical extents of these nearby galaxies. Fits to the spectral energy distributions using a delayed star formation history model indicate a subtle but interesting average radial trend for the spiral galaxies: the inner stellar systems decrease in age with increasing radius, consistent with inside-out disk formation, but the trend reverses in the outermost regions with the stellar age nearly as old as the innermost stars. These results suggest an old stellar outer disk population formed through radial migration and/or the cumulative history of minor mergers and accretions of satellite dwarf galaxies. The subset of S0 galaxies studied here show the opposite trend compared to what is inferred for spirals: characteristic stellar ages that are increasingly older with radius for the inner portions of the galaxies, and increasingly younger stellar ages for the outer portions. This result suggests that either S0 galaxies are not well modeled by a delayed-$tau$ model, and/or that S0 galaxies have a more complicated formation history than spiral galaxies.
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