Robbie Christopher Dohm-Palmer

Deep Hubble Space Telescope Imaging of IC 1613. II. The Star Formation History

We have taken deep images of an outlying field in the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the standard broadband F555W (V, 8 orbits) and F814W (I,16 orbits) filters. The photometry reaches to V=27.7 (M-V=+3.4) and I=27.1 (M-I=+2.8) at the 50% completeness level, the deepest to date for an isolated dwarf irregular galaxy. We analyze the resulting color-magnitude diagram (CMD) and compare it with CMDs created from theoretical stellar models using three different methods to derive a star formation history (SFH) as well as constrain the chemical evolution for IC 1613. All three methods find an enhanced star formation rate (SFR), at roughly the same magnitude (factor of 3), over roughly the same period (from 3 to 6 Gyr ago). Additionally, all three methods were driven to similar age-metallicity relationships (AMR) that show an increase from [Fe/H]approximate to-1.3 at earliest times to [Fe/H]approximate to-0.7 at present. Good agreement is found between the AMR which is derived from the CMD analysis and that which can be inferred from the derived SFH at all but the earliest ages. The agreement between the three models and the self-consistency of the derived chemical enrichment history support the reality of the derived SFH of IC 1613 and, more generally, are supportive of the practice of constructing galaxy SFHs from CMDs. A comparison of the newly observed outer field with an earlier studied central field of IC 1613 shows that the SFR in the outer field has been significantly depressed during the last Gyr. This implies that the optical scale length of the galaxy has been decreasing with time and that comparison of galaxies at intermediate redshift with present-day galaxies should take this effect into account. Comparing the CMD of the outer field of IC 1613 with CMDs of Milky Way dSph companions, we find strong similarities between IC 1613 and the more distant dSph companions (Carina, Fornax, Leo I, and Leo II) in that all are dominated by star formation at intermediate ages. In particular, the SFH and AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr galaxies cannot be distinguished from dSphs by their intermediate-age stellar populations. This type of a SFH may also be evidence for slower or suppressed early star formation in dwarf galaxies due to photoionization after the reionization of the universe by background radiation. Assuming that IC 1613 is typical of a dIrr evolving in isolation, since most of the star formation occurs at intermediate ages, these dwarf systems cannot be responsible for the fast chemical enrichment of the intergalactic medium that is seen at high redshift. There is no evidence for any large-amplitude bursts of star formation in IC 1613, and we find it highly unlikely that analogs of IC 1613 have contributed to the excess of faint blue galaxies in existing galaxy redshift surveys.

The Recent Star Formation History of GR 8 from Hubble Space Telescope Photometry of the Resolved Stars

?????We have used the Hubble Space Telescope to observe the resolved stars in the dwarf irregular galaxy GR 8 (DDO 155, UGC 8091). The data consisted of dithered Wide Field Planetary Camera 2 images in three bands: F439W (1 hr), F555W (30 minutes), and F814W (30 minutes). The stellar photometry was extracted with a modified version of DoPHOT. Artificial star tests showed the data to be 50% complete to V = 26.3, B = 25.4, and I = 25.2. The color-magnitude diagrams contain well-defined populations, including a very young main sequence (MS) (<10 Myr), and a red giant branch as old as several Gyr. These features align well with stellar evolution models of the appropriate metallicity. The distance based on the tip of the red giant branch is in excellent agreement with the Cepheid determination of ? = 26.75 ? 0.35 (2.2 Mpc), which we adopted. An extended stellar halo was discovered well beyond the H I. Based on the MS and blue HeB luminosity function, we calculated the star formation rate (SFR) over the past 500 Myr. The SFR has been fairly constant, at 400 M? Myr-1 kpc-2 with up to 60% variations. The blue HeB stars were used as a tracer for the location of star formation over this time period. The star formation occurred in superassociation size regions (100?200 pc), which lasted ~100 Myr. These regions come and go with no obvious pattern, except that they seem to concentrate in the current locations of H I clumps. This suggested that the H I clumps are long-lived features that support several star-forming events over time. The most likely explanation is that the star-forming regions are gravitationally bound. We estimated the gas-to-star conversion efficiency to be 6%. We compared our results with those of three other dI galaxies: Sextans A, Pegasus DIG, and Leo A. There is a trend of higher SFR per area with larger MH I/LB. Also, the star formation pattern is similar in all four galaxies. Finally, none of the four dI galaxies contained a large starburst, comparable to those in BCD galaxies. Combining the histories of all four galaxies, this implies that less than 5% of dI galaxies are hosting a strong burst of star formation at any given time. Observations of more galaxies are needed to improve this statistic.

Star Formation and the Interstellar Medium in Four Dwarf Irregular Galaxies

We present new, high-sensitivity VLA observations of H i in four dwarf galaxies (UGCA 292, GR 8, DDO 210, and DDO 216), and we use these data to study interactions between star formation and the interstellar medium. H i velocity dispersions and line shapes in UGCA 292, GR 8, and DDO 210 show evidence that these three galaxies contain both warm and cool or cold H i phases. The presence of the cold neutral medium is indicated by a low-dispersion (3–6 km s � 1 )H i component or by the Gauss-Hermite shape parameter h4 > 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 H i 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 H i 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 H i phases. Subject headings: galaxies: dwarf — galaxies: individual (DDO 210, DDO 216, GR 8, UGCA 292) — galaxies: ISM — galaxies: kinematics and dynamics — stars: formation

A Wide Field Planetary Camera 2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

The stellar population of the Pegasus dwarf irregular galaxy is investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. With WFPC2 the Pegasus dwarf is highly resolved into individual stars to limiting magnitudes of about 25.5 in B and V and 25 in I. These and ground-based data are combined to produce color-magnitude diagrams that show the complex nature of the stellar population in this small galaxy. A young (<0.5 Gyr) main-sequence stellar component is present and clustered in two centrally located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of AV = 0.47 mag. The mean color of the well-populated red giant branch (RGB) is relatively blue, consistent with a moderate-metallicity young, or older metal-poor, stellar population. The RGB also has significant width in color, implying a range of stellar ages and/or metallicities. A small number of extended asymptotic giant branch stars are found beyond the RGB tip. Near the faint limits of our data is a populous red clump superposed on the RGB. Efforts to fit self-consistent stellar population models based on the Geneva stellar evolutionary tracks yield a revised distance of 760 kpc. Quantitative fits to the stellar population are explored as a means of constraining the star formation history. The numbers of main-sequence and core helium burning blue-loop stars require that the star formation rate was higher in the recent past, by a factor of 3–4 about 1 Gyr ago. Unique results cannot be obtained for the star formation history over longer time baselines without better information on stellar metallicities and deeper photometry. The youngest model consistent with the data contains stars with constant metallicity of Z = 0.001 that mainly formed 2–4 Gyr ago. If stellar metallicity declines with increasing stellar age, then older ages are allowed of up to ≈8 Gyr. However, even at its peak of star-forming activity, the intermediate-age–dominated model for the Pegasus dwarf most likely remained relatively dim, with MV ≈ -14.

YOUNG SUPERNOVA REMNANTS IN NONUNIFORM MEDIA

We investigate the dynamics of young supernova remnants (SNRs) in the presence of a nonuniform external density such as might be present at the edge of a molecular cloud. Using a two-dimensional, Total Variation Diminishing, hydrodynamic code, we perform high-resolution (1024 by 2048) simulations in cylindrical symmetry of explosions in the vicinity of a smooth density transition. We improve upon previous calculations by including the inertia and kinetic energy of the stellar ejecta. The evolution is followed until the outer blast wave has swept up roughly 20 times the initial ejected mass We (14.5 M☉). and that the presence of the density transition has important dynamical effects on the remnant during the pre-Sedov-Taylor evolution. When the remnant is smaller than the width of the density transition, the apparent SNR center shifts while maintaining a nearly circular shape. At this stage, the morphology of such a SNR would be difficult to distinguish from one expanding in a uniform medium. Once the remnant size exceeds the transition width, it expands as distinct lobes into the high- and low-density regions. The radii of these lobes, as measured from the explosion center, approach the self-similar Sedov-Taylor solution appropriate for the gas density that is local to each lobe. The nonuniform density also creates a pressure imbalance in the SNR interior that drives gas from the high-density side to the low-density side. This sets up asymmetric, nonradial flows in the remnant interior, which may be directly observable. At early stages, the nonspherical motions should serve as a better indicator of a nonuniform external density than the morphology.

Deep Hubble Space Telescope Imaging of Sextans A. III. The Star Formation History

We present a measurement of the star formation history of Sextans A, based on WFPC2 photometry that is 50% complete to V = 27.5 (MV ~ +1.9) and I = 27.0. The star formation history and chemical enrichment history have been measured through modeling of the color-magnitude diagram (CMD). We find evidence for increased reddening in the youngest stellar populations and an intrinsic metallicity spread at all ages. Sextans A has been actively forming stars at a high rate for ~2.5 Gyr ago, with an increased rate beginning ~0.1 Gyr ago. We find a nonzero number of stars older than 2.5 Gyr, but because of the limited depth of the photometry, a detailed star formation history at intermediate and older ages has considerable uncertainties. The mean metallicity was found to be [M/H] ≈ -1.4 over the measured history of the galaxy, with most of the enrichment happening at ages of at least 10 Gyr. We also find that an rms metallicity spread of 0.15 dex at all ages allows the best fits to the observed CMD. We revisit our determination of the recent star formation history (age ≤ 0.7 Gyr) using blue helium-burning (BHeB) stars and find good agreement for all but the last 25 Myr, a discrepancy resulting primarily from different distances used in the two analyses and the differential extinction in the youngest populations. This indicates that star formation histories determined solely from BHeB stars should be confined to CMD regions where no contamination from reddened main-sequence stars is present.

The Recent Evolution of the Dwarf Starburst Galaxy NGC 625 from Hubble Space Telescope Imaging

New Hubble Space Telescope WFPC2 imaging of the dwarf starburst galaxy NGC 625 is presented. These data, which are 80% complete to V and I magnitudes of 26.0 and 25.3, respectively, allow us to study the recent star formation history of NGC 625. Using outlying red giant stars, we derive a tip of the red giant branch (TRGB) distance modulus of 27.95 ± 0.07. This corresponds to a distance of 3.89 ± 0.22 Mpc, placing this system on the far side of the Sculptor Group. NGC 625 has a well-defined radial stellar population gradient, evidenced by a central concentration of young main-sequence stars and a red giant branch (RGB)/asymptotic giant branch (AGB) ratio that increases with galactocentric distance. The prominent AGB is very red, similar to the population found in the Local Group dwarf irregular galaxy NGC 6822. The RGB stars can be detected out far from the central star-forming activity and show an elliptical distribution in agreement with the galaxys outer H I distribution. Using Hα and Hβ narrowband imaging and previous optical spectroscopy, we identify substantial and varying internal extinction associated with the central active star formation regions. This extinction, which varies from AV = 0.0 to 0.6 mag, hampers efforts to derive a detailed recent star formation history. To better understand the effects of internal extinction on the analysis of young stellar populations, synthetic models are presented that, for the first time, examine and account for this effect. Using the luminous blue helium-burning stars, we construct a simple model of the recent (<100 Myr) star formation in which an elevated but declining star formation rate has been present over this entire period. This is at odds with the presence of spectroscopic Wolf-Rayet (W-R) features in the major star formation region, which implies a short duration (≤5 Myr) for the recent starburst. This suggests that starbursts displaying W-R features are not necessarily all of a short duration. Finally, we speculate on the possible causes of the present burst of star formation in this apparently isolated galaxy and compare it with other nearby, well-studied dwarf starburst systems.

Wide field planetary camera 2 observations of Leo A: A predominantly young galaxy within the Local Group

The unprecedented detail of the WFPC2 colour-magnitude diagrams of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops and the tip of the red giant branch, we obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occured in this galaxy. Leo A provides a nearby laboratory for studying young metal poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.

Deep [ITAL]Hubble Space Telescope[/ITAL] Imaging of Sextans A. II. Cepheids and Distance

We have identified 82 short-period variable stars in Sextans A from deep WFPC2 observations. All of the periodic variables appear to be short-period Cepheids, with periods as small as 0.8 days for fundamental-mode Cepheids and 0.5 days for first-overtone Cepheids. These objects have been used, along with measurements of the RGB tip and red clump, to measure a true distance modulus to Sextans A of (m-M)_0 = 25.61 +/- 0.07, corresponding to a distance of d = 1.32 +/- 0.04 Mpc. Comparing distances calculated by these techniques, we find that short-period Cepheids (P < 2 days) are accurate distance indicators for objects at or below the metallicity of the SMC. As these objects are quite numerous in low-metallicity star-forming galaxies, they have the potential for providing extremely precise distances throughout the Local Group. We have also compared the relative distances produced by other distance indicators. We conclude that calibrations of RR Lyraes, the RGB tip, and the red clump are self-consistent, but that there appears to be a small dependence of long-period Cepheid distances on metallicity. Finally, we present relative distances of Sextans A, Leo A, IC 1613, and the Magellanic Clouds.

Deep Hubble Space Telescope Imaging of Sextans A. II. Cepheids and Distance

We have identified 82 short-period variable stars in Sextans A from deep WFPC2 observations. All of the periodic variables appear to be short-period Cepheids, with periods as small as 0.8 days for fundamental-mode Cepheids and 0.5 days for first-overtone Cepheids. These objects have been used, along with measurements of the RGB tip and red clump, to measure a true distance modulus to Sextans A of (m-M)_0 = 25.61 +/- 0.07, corresponding to a distance of d = 1.32 +/- 0.04 Mpc. Comparing distances calculated by these techniques, we find that short-period Cepheids (P < 2 days) are accurate distance indicators for objects at or below the metallicity of the SMC. As these objects are quite numerous in low-metallicity star-forming galaxies, they have the potential for providing extremely precise distances throughout the Local Group. We have also compared the relative distances produced by other distance indicators. We conclude that calibrations of RR Lyraes, the RGB tip, and the red clump are self-consistent, but that there appears to be a small dependence of long-period Cepheid distances on metallicity. Finally, we present relative distances of Sextans A, Leo A, IC 1613, and the Magellanic Clouds.