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Register a new userThe Evolutionary Status of Isolated Dwarf Irregular Galaxies II. Star Formation Histories and Gas Depletion
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L. van Zee
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The results of UBV and H alpha imaging of a large sample of isolated dwarf irregular galaxies are interpreted in the context of composite stellar population models. The observed optical colors are best fit by composite stellar populations which have had approximately constant star formation rates for at least 10 Gyr. The galaxies span a range of central surface brightness, from 20.5 to 25.0 mag arcsec^{-2}; there is no correlation between surface brightness and star formation history. Although the current star formation rates are low, it is possible to reproduce the observed luminosities without a major starburst episode. The derived gas depletion timescales are long, typically ~20 Gyr. These results indicate that dwarf irregular galaxies will be able to continue with their slow, but constant, star formation activity for at least another Hubble time. The sample of isolated dIs is compared to a sample of star bursting dwarf galaxies taken from the literature. The star bursting dwarf galaxies have many similar properties; the main difference between these two types of gas-rich dwarf galaxies is that the current star formation is concentrated in the center of the star bursting systems while it is much more distributed in the quiescent dIs. This results in pronounced color gradients for the starbursting dwarf galaxies, while the majority of the quiescent dwarf irregular galaxies have minor or non-existent color gradients. Thus, the combination of low current star formation rates, blue colors, and the lack of significant color gradients indicates that star formation percolates slowly across the disk of normal dwarf galaxies in a quasi-continuous manner.
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We present observations and analysis of nine dwarf irregular galaxies (dIs) in the M81 Group taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The nine galaxy sample (the Garland, M81 Dwarf A, DDO 53, Ho IX, Ho I, DDO 165, NGC 2366, Ho II, and IC 2574) spans 6 magnitudes in luminosity, a factor of 1000 in current star formation rate, and 0.5 dex in metallicity. Here we use color-magnitude diagrams of resolved stellar populations to study the star formation histories (SFHs) of these galaxies. We divide the sample into faint and bright galaxies, with a dividing line of M_${B}$ = -15, and then analyze the similarities and differences in the SFHs, birthrate parameters, fraction of stars formed per time interval, and spatial distribution of stellar components. Comparing these parameters as a function of luminosity, we find only minor differences in SF characteristics. We extend our comparison to select dIs in the Local Group (LG), and find only minor differences in SF parameters. The fraction of stars formed per time interval for an average M81 Group and LG dI is consistent with a constant SFH. However, individual galaxies can show significant departures from a constant SFH. Thus, we find this result underlines the importance of stochastic SF in dIs. We also compare possible formation scenarios of the less luminous and candidate tidal dwarfs in the M81 Group. The SFHs and the lack of an overdensity of associated red stars suggest that the Garland and Ho IX are not dIs and are potentially tidal dwarf galaxies. Interestingly, a noteworthy difference between the LG and the M81 Group is the lack of tidal dwarf candidates in the LG.
We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The resulting color-magnitude diagram (CMD) reaches more than a magnitude below the oldest main-sequence turnoff, allowing us to derive the star formation history (SFH) over the entire lifetime of the galaxy with a timing precision of ~10% of the lookback time. Using a maximum likelihood fit to the CMD we find that only ~10% of all star formation in Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift z ~2). The star formation rate increased dramatically ~6-8 Gyr ago (z ~ 0.7-1.1) and then declined until the present time. The only known galaxy with a more extreme confirmed delay in star formation is Leo A, a galaxy of similar M(HI)/M(stellar), dynamical mass, mean metallicity, and degree of isolation. The delayed stellar mass growth in these galaxies does not track the mean dark matter accretion rate from CDM simulations. The similarities between Leo A and Aquarius suggest that if gas is not removed from dwarf galaxies by interactions or feedback, it can linger for several gigayears without cooling in sufficient quantity to form stars efficiently. We discuss possible causes for the delay in star formation including suppression by reionization and late-time mergers. We find reasonable agreement between our measured SFHs and select cosmological simulations of isolated dwarfs. Because star formation and merger processes are both stochastic in nature, delayed star formation in various degees is predicted to be a characteristic (but not a universal) feature of isolated small galaxies.
We present a detailed study of the Magellanic irregular galaxy NGC 4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity ($D=3.82pm 0.27$ Mpc) we reach stars 3 magnitudes fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history spans the whole Hubble time, but due to the age-metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometry, i.e. $sim 3$ Gyr. The most recent peak of star formation is around 10 Myr ago. The average surface density star formation rate over the whole galaxy lifetime is $0.01$ M$_{odot}$ yr$^{-1}$ kpc$^{-2}$. From our study it emerges that NGC 4449 has experienced a fairly continuous star formation regime in the last 1 Gyr with peaks and dips whose star formation rates differ only by a factor of a few. The very complex and disturbed morphology of NGC 4449 makes it an interesting galaxy for studies of the relationship between interactions and starbursts, and our detailed and spatially resolved analysis of its star formation history does indeed provide some hints on the connection between these two phenomena in this peculiar dwarf galaxy.
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 present new, high sensitivity VLA observations of HI in four dwarf galaxies (UGCA 292, GR8, DDO 210, and DDO 216) and we use these data to study interactions between star formation and the interstellar medium. HI velocity dispersions and line shapes in UGCA 292, GR8, and DDO 210 show that these three galaxies contain both warm and cool or cold HI phases. The presence of the cold neutral medium is indicated by a low-dispersion (3--6 km/s) HI component or by the Gauss-Hermite shape parameter h_4 > 0. Contrary to expectations, we find no trend between the incidence of the low-dispersion (colder) phase and the star formation rate in five dwarf galaxies. The colder HI phase may be a necessary ingredient for star formation, but it is clearly not sufficient. However, there is a global trend between the star formation rate of a galaxy and the incidence of asymmetric HI profiles. This trend probably reflects kinetic energy input from young massive stars. Numerical simulations show that the effects of rotational broadening (finite angular resolution) are minimal for these galaxies. Simulations are also used to estimate the errors in the column densities of the high-dispersion and the low-dispersion HI phases.
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