Hubble Space Telescope WFPC2 imaging of three low surface brightness dwarfelliptical galaxies in the Virgo Cluster

Utilizing the F814W and F300W filters, Hubble Space Telescope Wide Field Pl anetary Camera 2 (WFPC2) images were taken of three low surface brightness dwarf elliptical galaxies in the Virgo Cluster. The intent of the observati ons was to determine the small-scale structure in these enigmatic galaxies and to attempt to learn something about the nature of their giant branches through the detection of luminosity fluctuations. In two of the three studi ed galaxies, V7L3 and V1L4, the luminosity fluctuations in the inner, const ant surface brightness regions were unambiguously detected. At the nominal distance of the Virgo Cluster, the measured luminosity fluctuations in the F814W band yields a density of 2-10 red giants per pixel. In the most extre me of these two cases, V7L3, we derive a surface density of giant stars of similar to 3 per 10 pc(2). Using the observed B - V and V - I colors as a c onstraint, we could find no model that would reproduce the observed fluctua tion signal and blue colors if there were a significant population of M gia nts in these systems. Overall, our results are consistent with a mean spect ral type of K0-K2, which implies a relatively metal-poor population. The th ird system, V2L8, did not have a detectable fluctuation signal, which possi bly implies that it is not in the Virgo Cluster. Interestingly, this system is highly nucleated. Our observations have resolved this nucleus, and if V 2L8 is in Virgo, then we have discovered what is likely the smallest bulge measured to date, having an effective radius of only 50 pc. This bulge is q uite red (as red as giant ellipticals), and it is entirely possible that th is nucleated dwarf elliptical galaxy, in fact, is a very large galaxy locat ed in the background. As such, it is highly reminiscent of the manner in wh ich Malin 1 was discovered. Optical spectroscopy of this nucleus is require d to confirm this. Finally, we find no evidence for small-scale clumping of stars in any of the studied systems at this much-improved spatial resoluti on. This implies that these systems are dynamically well relaxed and that t he physical cause of their observed low surface brightnesses is their low d ensity. When imaged at the high spatial resolution of the WFPC2 (similar to 6 pc pixel(-1)), the galaxies are easy to look right through without even knowing that they are present in the very middle of the WFPC2 frame. They a ppear only as elevated "sky noise."