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Register a new userThe History of Star Formation in Galaxy Disks in the Local Volume as Measured by the ACS Nearby Galaxy Survey Treasury
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Benjamin F. Williams
Publication date
2011
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Physics
and research's language is
English
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We present a measurement of the age distribution of stars residing in spiral disks and dwarf galaxies. We derive a complete star formation history of the ~140 Mpc^3 covered by the volume-limited sample of galaxies in the Advanced Camera for Surveys (ACS) Nearby Galaxy Survey Treasury (ANGST). The total star formation rate density history is dominated by the large spirals in the volume, although the sample consists mainly of dwarf galaxies. Our measurement shows a factor of ~3 drop at z~2, in approximate agreement with results from other measurement techniques. While our results show that the overall star formation rate density has decreased since z~1, the measured rates during this epoch are higher than those obtained from other measurement techniques. This enhanced recent star formation rate appears to be largely due to an increase in the fraction of star formation contained in low-mass disks at recent times. Finally, our results indicate that despite the differences at recent times, the epoch of formation of ~50% of the stellar mass in dwarf galaxies was similar to that of ~50% of the stellar mass in large spiral galaxies (z>~2), despite the observed galaxy-to-galaxy diversity among the dwarfs.
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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 ACS on HST, supplemented with archival data and new 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_F475W=28.0 mag, m_F606W=27.3 mag, and m_F814W=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. 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 study the relationship between the field star formation and cluster formation properties in a large sample of nearby dwarf galaxies. We use optical data from the Hubble Space Telescope and from ground-based telescopes to derive the ages and masses of the young (t_age < 100Myr) cluster sample. Our data provides the first constraints on two proposed relationships between the star formation rate of galaxies and the properties of their cluster systems in the low star formation rate regime. The data show broad agreement with these relationships, but significant galaxy-to-galaxy scatter exists. In part, this scatter can be accounted for by simulating the small number of clusters detected from stochastically sampling the cluster mass function. However, this stochasticity does not fully account for the observed scatter in our data suggesting there may be true variations in the fraction of stars formed in clusters in dwarf galaxies. Comparison of the cluster formation and the brightest cluster in our sample galaxies also provide constraints on cluster destruction models.
[abridged] We study the resolved stellar populations and derive the SFH of the SDIG, a gas-rich dwarf galaxy member of the NGC7793 subgroup in the Sculptor group. We construct a CMD using archival HST observations and examine its stellar content. We derive its SFH using a maximum-likelihood fit to the CMD. The CMD shows that SDIG contains stars from 10Myr to several Gyr old, as revealed from the MS, BL, luminous AGB, and RGB stars. The young stars with ages less than ~250Myr show a spatial distribution confined to its central regions, and additionally the young MS stars exhibit an off-center density peak. The intermediate-age and older stars are more spatially extended. SDIG is dominated by intermediate-age stars with an average age of 6.4Gyr. The average metallicity inferred is [M/H]approx -1.5dex. Its SFH is consistent with a constant SFR, except for ages younger than ~200Myr. The lifetime average SFR is 1.3x10^{-3} Mo/yr. More recently than 100Myr, there has been a burst of SF at a rate ~2-3 times higher than the average SFR. The inferred recent SFR from CMD modelling is higher than inferred from the Ha flux of the galaxy; we interpret this to mean that the upper end of the IMF is not being fully sampled due to the low SFR. Additionally, an observed lack of bright blue stars in the CMD could indicate a downturn in SFR on 10^7-yr timescales. A previous SF enhancement appears to have occurred between 600-1100Myr ago, with amplitude similar to the most recent 100Myr. Older bursts of similar peak SFR and duration would not be resolvable with these data. The observed enhancements in SF suggest that SDIG is able to sustain a complex SFH without the effect of interactions with its nearest massive galaxy. Integrating the SFR over the entire history of SDIG yields a total stellar mass 1.77x10^{7}Mo, and a current V-band stellar mass-to-light ratio 3.2Mo/Lo.
Using the method of integral-field (3D) spectroscopy, we have investigated the kinematics and distribution of the gas and stars at the center of the early-type spiral galaxy with a medium scale bar NGC 7177 as well as the change in the mean age of the stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius R = 1.5 -- 2, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate; basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar age inside the star formation ring, in the galactic nucleus, is old, ~10 Gyr. Outside, at a distance R = 6 - 8 from the nucleus, the mean age of the stellar population is ~2 Gyr. If we agree that the bar in NGC 7177 is old, then, obviously, the star formation ring has migrated radially inward in the last 1-2 Gyr, in accordance with the predictions of some dynamical models.
We present the star formation history (SFH) of the isolated (D~970 kpc) Local Group dwarf galaxy WLM measured from color-magnitude diagrams constructed from deep Hubble Space Telescope imaging. Our observations include a central (0.5 $r_h$) and outer field (0.7 $ r_h$) that reach below the oldest main sequence turnoff. WLM has no early dominant episode of star formation: 20% of its stellar mass formed by ~12.5 Gyr ago (z~5). It also has an SFR that rises to the present with 50% of the stellar mass within the most recent 5 Gyr (z<0.7). There is evidence of a strong age gradient: the mean age of the outer field is 5 Gyr older than the inner field despite being only 0.4 kpc apart. Some models suggest such steep gradients are associated with strong stellar feedback and dark matter core creation. The SFHs of real isolated dwarf galaxies and those from the the Feedback In Realistic Environment suite are in good agreement for $M_{star}(z=0) sim 10^7-10^9 M_{odot}$, but in worse agreement at lower masses ($M_{star}(z=0) sim 10^5-10^7 M_{odot}$). These differences may be explainable by systematics in the models (e.g., reionization model) and/or observations (HST field placement). We suggest that a coordinated effort to get deep CMDs between HST/JWST (crowded central fields) and WFIRST (wide-area halo coverage) is the optimal path for measuring global SFHs of isolated dwarf galaxies.
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