A near-infrared stellar census of the blue compact dwarf galaxy VII Zw 403.
SCHULTE-LADBECK R.E., HOPP U., GREGGIO L. and CRONE M.M.
Abstract (from CDS):
We present near-infrared single-star photometry for the low-metallicity blue compact dwarf galaxy VII Zw 403. We achieve limiting magnitudes of F110W~25.5 and F160W~24.5 using one of the NICMOS cameras with the Hubble Space Telescope equivalents of the ground-based J and H filters. The data have a high photometric precision (0.1 mag) and are more than 95% complete down to magnitudes of about 23, far deeper than previous ground-based studies in the near-IR. The color-magnitude diagram contains about 1000 point sources. We provide a preliminary transformation of the near-IR photometry into the ground system. We investigate the tip of the red giant branch method in the J and H bands to provide an empirical distance calibration. Combining our result with globular cluster data as well as stellar-evolution models, we recommend MH,TRGB=-5.5(±0.1) for -2.3<[Fe/H]←1.5. We proceed to discuss the stellar content of VII Zw 403 using evolutionary tracks as well as a classification scheme based on optical and near-IR colors, and we comment on the detection of asymptotic giant branch stars and the blue Hertzprung gap. We use MH as an indicator of Mbol for red stars after evaluating BCH at low metallicity. We calculate the fractional contribution of individual stars from our color-magnitude diagram to the integrated light of VII Zw 403 and determine which red stellar population dominates the integrated colors. We find that young red supergiants, and young and intermediate-age asymptotic giants, together provide about 50% of the light in I, J, and H bands, whereas the old red giant stars contribute less than 15%. Young, main-sequence stars and blue supergiants account for the remaining light and dominate in V. This explains the difficulties in discerning the nature of blue compact dwarf galaxies when only integrated photometry is available.