DDO 44 and UGC 4998: Distances, Metallicities, and Star Formation Histories

We have obtained deep HST imaging of two candidate dwarf systems in the nearby M81 Group, DDO 44 and UGC 4998. Both are isolated, low surface brightness systems, but with likely very different star formation histories based on their mean colors and color fluctuations. The color-magnitude diagrams of these galaxies have been used to estimate their distances using the tip of the red giant branch (RGB), and their mean metallicities from the colors of the RGB. For DDO 44 we find a distance of D=3.01+/-0.18 Mpc-confirming it to be a member of the M81 Group-and a mean metallicity of [Fe/H]=-1.54+/-0.14. We also find from the properties and numbers of a population of luminous asymptotic giant branch (AGB) stars that about 20% of the luminous population of DDO 44 consists of ``intermediate-age'' stars (with ages between about 2 and 8 Gyr) that give rise to the observed AGB. There is no difference in the spatial distribution of the RGB and AGB stars in this galaxy. For UGC 4998 we derive a metallicity of [Fe/H]=-1.58+/-0.21 and a distance of D=8.24+/-0.43 Mpc. The latter puts UGC 4998 well behind the M81 Group, confirming results from previous measurements. We find this galaxy to contain a population of young (<50 Myr old) stars, and there is evidence of older RGB stars. The young stars are considerably more centrally concentrated than the older stars. The current star formation rate in UGC 4998 is ~9×10^-4 M_solar yr^-1 over the entire galaxy, or ~5×10^-4 M_solar yr^-1 kpc^-2. Given its H I mass (1×10⁷ M_solar), this galaxy can continue to form stars at this rate for another ~11 Gyr.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal GO-8137.