The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A

We present deep Hubble Space Telescope single-star photometry of Leo A in B, V, and I. Our new field of view is offset from the centrally located field observed by Tolstoy et al. (1998) in order to expose the halo population of this galaxy. We report the detection of metal-poor red horizontal branch stars, which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least as old as metal-poor Galactic Globular Clusters which exhibit red horizontal branches, and are considered to have a minimum age of about 9 Gyr. We discuss the distance to Leo A, and perform an extensive comparison of the data with stellar isochrones. For a distance modulus of 24.5, the data are better than 50% complete down to absolute magnitudes of 2 or more. We can easily identify stars with metallicities between 0.0001 and 0.0004, and ages between about 5 and 10 Gyr, in their post-main-sequence phases, but lack the detection of main-sequence turnoffs which would provide unambiguous proof of ancient (>10 Gyr) stellar generations. Blue horizontal branch stars are above the detection limits, but difficult to distinguish from young stars with similar colors and magnitudes. Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A. The models also suggest ~50% of the total astrated mass in our pointing to be attributed to an ancient (>10 Gyr) stellar population. We conclude that Leo A started to form stars at least about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, of only 3% of Solar, in its ionized interstellar medium. The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation.Comment: 44 pages, 18 figures, accepted for publication in the August 2002 issue of AJ. High resolution figures is available at http://www.astro.spbu.ru/staff/dio/preprints.htm

The Stellar Content of NGC 6789, A Blue Compact Dwarf Galaxy in the Local Void

We find that NGC6789 is the most nearby example of a Blue Compact Dwarf galaxy known to date. With the help of WFPC2 aboard the Hubble Space Telescope, we resolve NGC6789 into over 15,000 point sources in the V and I bands. The young stars of NGC6789 are found exclusively near the center of the galaxy. The red giant population identified at large galacticentric radii yields a distance of about 3.6 Mpc, a stellar metallicity [Fe/H] of about -2, and a minimum age of about 1 Gyr. Despite its isolated location in the Local Void,its low metallicity, and its active star formation, the properties of NGC6789 are clearly not those of a galaxy in formation.Comment: 8 pages, 4 figures, ApJL Accepte

The HII Regions of the Damped Lyman alpha Absorber SBS 1543+593

We report new imaging and spectroscopic observations of the damped Ly alpha (DLA) galaxy SBS 1543+593, a nearby dwarf galaxy whose stellar disk is intersected by the sightline to the bright background QSO HS 1543+5921. Hubble Space Telescope imaging observations with WFPC2 in the F450W and F702W bands are used to measure the DLA galaxy's properties and compile a catalog of its (candidate) HII regions. Ground-based long-slit spectroscopy of the brightest HII region in the galaxy yields estimates of the star formation rate (SFR) and of chemical abundances in the galaxy's interstellar medium. We find that SBS 1543+593 exhibits a SFR $\approx 0.006 h^{-2}_{70} M_{\odot}$ yr$^{-1}$, or a SFR per unit area of $\approx 1.4 \times 10^{-4} h^{-2}_{70} M_\odot$ yr$^{-1}$ kpc$^{-2}$. We derive gas-phase abundances in the ionized gas of $12 + \log {\rm (O/H)} = 8.2 \pm 0.2$, which is about 1/3 of the solar value, and $\log {\rm (N/O)} = -1.40^{+0.2}_{-0.3}$. These values are consistent with the morphologial appearance of SBS 1543+593, an Sm dwarf of $M_B -5 \log h_{70} = -16.8\pm0.2$ and of intermediate surface brightness. SBS 1543+593 is the first {\it bona fide} DLA for which abundances have been measured using emission-line diagnostics. When compared with future, high-resolution, ultraviolet spectroscopy, our results should prove key for interpreting abundance determinations in high redshift DLAs.Comment: ApJ, accepte

The Dwarf Irregular/Wolf-Rayet Galaxy NGC4214: I. A New Distance, Stellar Content, and Global Parameters

We present the results of a detailed optical and near-IR study of the nearby star-forming dwarf galaxy NGC4214. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old field stars, which are used as a distance indicator. On images obtained with the Hubble Space Telescope WFPC2 and NICMOS instruments in the equivalents of the V, R, I, J and H bands, the galaxy is well resolved into stars. We achieve limiting magnitudes of F814W ~27 in the WF chips and F110W ~25 in the NIC2. The optical and near-infrared color-magnitude diagrams confirm a core-halo galaxy morphology: an inner high surface-brightness young population within ~1.5' (~1 kpc) from the center of the galaxy, where the stars are concentrated in bright complexes along a bar-like structure; and a relatively low-surface-brightness, field-star population extending out to at least 8' (7 kpc). The color-magnitude diagrams of the core region show evidence of blue and red supergiants, main-sequence stars, asymptotic giant branch stars and blue loop stars. We identify some candidate carbon stars from their extreme near-IR color. The field-star population is dominated by the "red tangle", which contains the red giant branch. We use the I-band luminosity function to determine the distance based on the tip-of-the-red-giant-branch method: 2.7\pm0.3 Mpc. This is much closer than the values usually assumed in the literature, and we provide revised distance dependent parameters such as physical size, luminosity, HI mass and star-formation rate.Comment: 24 pages, 19 figures, accepted for publication in the July 2002 issue of AJ. Version with high resolution figures is available at http://www.astro.spbu.ru/staff/dio/preprints.htm

The Dwarf Irregular/Wolf-Rayet Galaxy NGC 4214: I. A New

We present the results of a detailed optical and near-IR study of the nearby star-forming dwarf galaxy NGC 4214. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old field stars, which are used as a distance indicator. On images obtained with the Hubble Space Telescope WFPC2 and NICMOS instruments in the equivalents of the V , R, I, J and H bands, the galaxy is well resolved into stars. We achieve limiting magnitudes of F814W # 27 in the WF chips and F110W # 25 in the NIC2. The optical and near-infrared color-magnitude diagrams confirm a core-halo galaxy morphology: an inner high surface-brightness young population within # 1