6352( 6) - 12/23/95 06:22 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6352 Version: 6 Check-in Date: 22-Dec-1995 19:00:02 1.Proposal Title: Dwarf Elliptical Galaxy Snapshot Survey ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS SNAP 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Henry Ferguson Space Telescope Science Institute CoI: Bryan Miller Space Telescope Science Institute N CoI: Brad Whitmore Space Telescope Science Institute N CoI: Massimo Stiavelli Space Telescope Science Institute N ------------------------------------------------------------------------------------ 6. Abstract Snapshot images in V and I will be obtained for 30 dwarf elliptical galaxies. This survey will provide important information on the globular cluster systems of such galaxies and the properties of their cores and nuclei. The data will provide a straightforward indication of whether dE's are true low-luminosity ellipticals, or simply dwarf irregulars that have lost their gas. The images will be searched for evidence that the nuclei formed recently, and for the existence of dust or low levels of star formation in the more extended dE envelopes. Comparison of this data set to HST images of luminous elliptical galaxies and their globular cluster systems should provide valuable clues to the formation of galaxies and their globular cluster systems. The data will be non-proprietary. ------------------------------------------------------------------------------------ 6352( 6) - 12/23/95 06:22 - [ 2] Observations Description ------------------------ We propose a snapshot survey of 30 galaxies from a dE sample of roughly 1000. As the database will be of immediate use in a variety of comparative studies to other HST data, we will waive our proprietary rights. Our optimum exposure sequence is two exposures of 230 sec each in F555W, and one exposure of 350 seconds in F814W. The total time per target will be 29 minutes including guide- star acquisition and instrument overheads. Fine lock is required for guiding. Shorter exposure times can be used for smaller observing windows; however windows shorter than 20 minutes are not useful due to our requirement for fine lock and good cosmic ray removal. As a primary goal is a first statistical assessment of the globular cluster dE population, we have chosen a large sample of potential targets and modest exposure times per galaxy. For a distance modulus of 31.7 (the largest of the dwarfs in our sample, for H_0 = 75 km s^-1 Mpc^-1), exposure times to reach S/N = 10 at the peak of the luminosity function (M_V = -7) are 300 sec in F814W. This program is thus ideal for a snapshot survey. The major sources of background in order of importance are (1) cosmic rays, (2) background galaxies, and (3) the dE galaxy itself. With three short exposures, even in different bands, we expect virtually no contamination from cosmic rays. Background galaxies are a much more important source of false detections. Typical effective radii of the dE galaxies in our sample are 15". At V=25, there are roughly 7 * 10^4 galaxies per square degree. Thus, within 15", we expect 4 background galaxies. At this magnitude faint galaxies are still in general well resolved by HST, with median half-light radii of 0.15 arcsec (Griffiths et al. 1994). In contrast, globular clusters at the Virgo Cluster distance are barely resolved, with r_e ~ 0.04" (Whitmore et al. 1995). Thus most background galaxies can be identified by morphology, as has proved to be the case with existing data. Colors will provide additional discrimination, but the median colors of galaxies at I ~ 25 is similar to that of globular clusters, so this must be used with care. If necessary, statistical corrections can be made to the measured values of S_N based on the number of globular- cluster like objects seen in archival blank-field observations. These corrections will dominate the uncertainties for galaxies fainter than M_B = -15 if is less than 1. However, we do not wish to bias our sample by selecting only high-luminosity dE's, as that presupposes that is independent of luminosity. We note that a globular cluster population the size of that in the Fornax dSph would be detectable at the 2-sigma level with only a statistical correction for background galaxies. The smooth background of the dE galaxy itself increases the exposure time to detect an unresolved source. For galaxies fainter than M_V = -14.5 even the central surface brightnesses are fainter than the sky background, so globular clusters can be detected all the way in to the center. For galaxies at the bright end of our sample (M_V = -17), central surface brightnesses are still considerably fainter than in the central regions of M87 surveyed by Whitmore et al. (1995). The expected globular cluster populations are high enough at M_B = -17 that the loss of the central few arcseconds will not significantly affect the determination of S_N. Real Time Justification ----------------------- None. None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6352( 6) - 12/23/95 06:22 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6352( 6) - 12/23/95 06:22 - [ 4] Summary Form for Proposal 6352 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category SNAP ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------