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Session 3 - Source Surveys, Galaxy Surveys, Distance Scale I.
Display session, Wednesday, January 07
The luminosity of the red-giant branch tip (TRGB) is a highly precise standard candle for Population II stars, since it marks the point of the core helium flash in these low-mass halo stars. For stellar populations more metal-poor than [Fe/H] \simeq -0.7, the TRGB luminosity is at M_I = -4.0 \pm 0.1, and is visible as a sharp upturn in the luminosity function which is easily detected from photometry that penetrates a magnitude or more fainter than the tip (e.g., Lee, Freedman, amp; Madore 1995, ApJ, 417, 553). With ground-based photometry, the TRGB method can be used for distance calibration out to d \sim 3 Mpc (Madore amp; Freedman 1995, AJ, 109, 1645). With the higher resolution and depth of the Hubble Space Telescope, the Virgo and Fornax galaxies should be within reach. We have used WFPC2 on HST to obtain deep I-band images of a Virgo dwarf elliptical, VCC1104, to test the feasibility of this technique at the \sim 15-20 Mpc distance range. Our target galaxy, a nucleated dE, is moderately metal-poor ([Fe/H] = -1.0 according to integrated colors and spectra), and its radial velocity and central location in Virgo maximize the probability that it belongs to the `core' of Virgo defined by the gE and dE,N galaxies. Our composite I-band image (with a total integration time of 32,200 sec over 12 orbits) succeeds in cleanly resolving more than 1 magnitude of the red giant branch stellar population, and clearly shows the expected step-function luminosity function upturn at the RGB tip. We will show the composite image of the galaxy, and present a distance measurement based on a maximum-likelihood fit to the luminosity function.
Program listing for Wednesday