6658( 3) - 02/09/96 17:28 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6658 Version: 3 Check-in Date: 09-Feb-1996 17:25:34 1.Proposal Title: Intergalactic Stars in the Virgo Cluster ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle GALAXIES & CLUSTERS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Henry C. Ferguson Space Telescope Science Institute CoI: Ted von Hippel University of Wisconsin N CoI: Nial Tanvir University of Cambridge N ------------------------------------------------------------------------------------ 6. Abstract We propose deep imaging of a blank field in the Virgo cluster to search for intergalactic stars. If even 5\% of the integrated light in the Virgo cluster is in a diffuse population, it should be possible to detect individual red giant branch stars against the background of faint galaxies. The results will provide (1) an estimate of the mass of stars stripped off from galaxies in encounters during the formation of the Virgo cluster; (2) a critical test of distance estimates to the Virgo cluster based on Cepheids; (3) a sensitive search for intergalactic globular clusters; and (4) a sensitive search for galaxies far below the surface brightness limits of conventional surface photometry. ------------------------------------------------------------------------------------ 6658( 3) - 02/09/96 17:28 - [ 2] Observations Description ------------------------ We propose to observe intergalactic red giant stars (RGB) in the core of the Virgo cluster. These stars are expected to be found at I >= 27.16 if the Freedman et al. (1994) distance is correct, or at I >= 27.64 if the Sandage (1995) distance is correct. We propose to observe to I = 28.0 (S/N = 5.5) in order to count stars 0.8 magnitudes down the RGB (for the Freedman et al. distance) or 0.4 mag down the RGB (for the Sandage distance). We chose a S/N = 5.5 as after flat fielding uncertainties and combining the images (which will span multiple orbits), we seek S/N = 5 at our magnitude limits. We have chosen to use the F814W filter due to its maximal sensitivity to the very red stars at the tip of the RGB, our need for good calibration, good flat fields, maximum depth in a given integration time, and compatibility with the growing HST deep star count database. Note that our use of the I-band only makes the V-band apparant magnitude reported in the ``Observing Summary'' have little meaning for this project. Our S/N calculations are based on the WFPC2 Instrument Handbook and the WFPC2 Exposure Time Calculator available over the WWW. From this we calculate that for S/N = 5.5 at I = 28.0 through F814W, and given our object position on the sky, we require 32,300 seconds of integration time. This integration time comes from interpolating between an object falling at the pixel center, for which we calculate S/N = 6, and an object falling at a pixel corner, for which we calculate S/N = 5. The pixel subsampling also will affect completeness at the faint end, but would not cause the loss of too many stars across most of our magnitude sensitivity range (see von Hippel et al, 1996). For our object declination (+12 degrees) the HST orbit allows 52 minutes of visibility. We would acquire one frame per orbit with an associated instrument overhead of 3 minutes, and 6 minutes for guide star re-acquisition (7.5 minutes for initial guide star acquisition). Thus, 43 minutes would be available per orbit, and our exposure time therefore require 12.5 orbits. We request 13 orbits. Having detected the sources, our major source of background is faint galaxies. The surface density of galaxies at I = 28 is approximately 1 * 10^6 deg^-2 mag^ -1, corresponding to 493 per WF chip, on average. For a square field of angular size Theta on a side, the expected variance in the number counts is Delta N^2 = N + 2.24 N^2Atheta^1-Gamma where N is the mean number per field expected from Poisson statistics, and the angular correlation function follows the powerlaw w(Theta) = A theta^1-Gamma (Peebles 1975). Using A = 1.5 * 10^-4 and Gamma = 1.8 (Brainerd et al 1995), the expected standard deviation per WF chip is Delta N = 47. Thus 200 stars (corresponding to a surface brightness Mu_B = 31.5), would be a 5Sigma detection per chip, or 9Sigma for the 3 WF chips and 1 PC chip combined. Real Time Justification ----------------------- No special requirements. No supporting or coordinated observations. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6658( 3) - 02/09/96 17:28 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6658( 3) - 02/09/96 17:28 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 2 VIRGO-CLUS1 CLUSTER OF RA=12H 33M 52S +/- 10",DEC=12D J2000 Z = I = 28.0+/-0.5 GALAXIES,POOR 21' 40" +/- 10" 0.004 CLUSTER,BLANK SKY,HALO,EXT-STAR,K III Comments: BLANK FIELD IN THE VIRGO CLUSTER Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) J2000 0.0 0.0 0.0 6658( 3) - 02/09/96 17:28 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 110 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 40M LOW-SKY MAX DUR 60M EXPAND FIX ------------------------------------------------------------------------------------------------------------------------------------ 120 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M LOW-SKY MAX DUR 60M EXPAND FIX ------------------------------------------------------------------------------------------------------------------------------------ 130 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M LOW-SKY MAX DUR 60M EXPAND FIX ------------------------------------------------------------------------------------------------------------------------------------ 140 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M LOW-SKY MAX DUR 60M EXPAND FIX ------------------------------------------------------------------------------------------------------------------------------------ 6658( 3) - 02/09/96 17:28 - [ 6] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 210 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 40M POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 220 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 230 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 240 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 6658( 3) - 02/09/96 17:28 - [ 7] Visit: 03 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 310 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 40M POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 320 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 330 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 340 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 350 VIRGO-CLUS1 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 42M POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M FIX EXPAND ------------------------------------------------------------------------------------------------------------------------------------ 6658( 3) - 02/09/96 17:28 - [ 8] Summary Form for Proposal 6658 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements LOW-SKY MAX DUR 60M EXPAND POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M EXPAND POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M EXPAND ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names VIRGO-CLUS1 ------------------------------------------------------------------------------------------------------------------------------------