An international team of astronomers presented the latest Hubble Space Telescope data on their census of dwarf galaxies in a series of poster presentations held in Atlanta Jan. 12 during the American Astronomical Society meeting. Dwarf galaxies are the most common type of galaxy in the universe, but their total numbers are very uncertain. The team uses what would otherwise be unused time on the Hubble Space Telescope to improve the census of nearby dwarf galaxies.
Patrick Seitzer, a U-M assistant research scientist and leader of the research team, explained that, Most of the suspected nearby galaxies in our survey were selected, based on close inspection of sky photographs, by team members Valentina E. Kara-chentseva (Ukraine) and Igor D. Karachentsev (Russia). These team members surveyed large sections of the sky looking for very faint, fuzzy objects, which might be nearby galaxies, but are so far away that the blurring effects of the Earths atmosphere make it impossible to resolve the objects image into individual stars.
Because the Hubble Space Telescope (HST) is above the atmosphere, its observations are unaffected by this blurring effect. Every HST image produces a very clear view of nearby galaxies, resolving the closest ones into stars. By measuring the brightnesses and colors of the brightest stars, we can determine an approximate distance to the galaxy and whether the brightest stars are very old or very young.
We use the Hubble Space Telescope in what is called snapshot mode. After the schedule for approved program observations has been determined, there are occasional gaps of one or two orbits where regular program observations are impossible to fit in. During these gaps, it is possible to take very short exposures of selected objects. This is the mode that our team uses. In a single orbit, we take one short 10-minute exposure of a candidate galaxy through a green filter and one 10-minute exposure through a red filter.
Although there is no guarantee of obtaining data for a snapshot proposal like ours, we have been very successful since the program began in July 1999. Since then, we have obtained data for 33 galaxies from our initial target list of 75 candidates. Of these, 23 are well-resolved into stars and within four Mpc.
Examples of survey data can be seen on the Web at www.astro.lsa.umich.edu/users/seitzer/snapshot.
Members of the Hubble Space Telescope Snapshot Galaxy Survey team, in addition to Seitzer, are:
Eva K. Grebel, University of Washington; Andrew E. Dolphin, Kitt Peak National Observatory, National Optical Astronomy Observatories; Doug Geisler, Universidad de Concepcion, Chile; Puragra Guhathakurta, Lick Observatory, University of California, Santa Cruz; Paul W. Hodge, University of Washington; Igor D. Karachentsev, Special Astrophysical Observatory, Russian Academy of Sciences; Valentina E. Karachentseva, Astronomical Observatory of Kiev University, Ukraine; and Ata Sarajedini, Wesleyan University.