! Proposal 6658, submission 1 ! PI: Henry C. Ferguson ! Received Fri Feb 9 16:35:05 EST 1996 ! From: ferguson@stsci.edu ! +====================+ ! | PED OUTPUT PRODUCT | ! +====================+ ! ! /planxty/data2/cyc6prop/virgo/6658/6658.prop ! Generated by PREPROCESSOR, version 6.0e ! Date: Tue Jan 30 23:15:47 EST 1996 Proposal_Information Title: Intergalactic Stars in the Virgo Cluster Proposal_Category: GO Scientific_Category: GALAXIES & CLUSTERS Cycle: 6 Investigators PI_Name: Henry C. Ferguson PI_Institution: Space Telescope Science Institute CoI_Name: Ted von Hippel CoI_Institution: University of Wisconsin Contact: CoI_Name: Nial Tanvir CoI_Institution: University of Cambridge Contact: 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. Questions Observing_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: Fixed_Targets Target_Number: 2 Target_Name: VIRGO-CLUS1 Alternate_Names: Description: CLUSTER OF GALAXIES,POOR CLUSTER,BLANK SKY,HALO,EXT-STAR,K III-I,M III-I,CARBON STAR,AGB STAR Position: RA=12H 33M 52S +/- 10",DEC=12D 21' 40" +/- 10" Equinox: J2000 RV_or_Z: Z = 0.004 RA_PM: 0.0 Dec_PM: 0.0 Epoch: J2000 Annual_Parallax: 0.0 Flux: I = 28.0+/-0.5 Comments: BLANK FIELD IN THE VIRGO CLUSTER Solar_System_Targets Generic_Targets Scan_Data Visits Visit_Number: 01 Visit_Requirements: On_Hold_Comments: Visit_Comments: Exposure_Number: 110 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 40M Special_Requirements: LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 120 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 130 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 140 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: LOW-SKY MAX DUR 60M EXPAND Comments: Visit_Number: 02 Visit_Requirements: On_Hold_Comments: Visit_Comments: Exposure_Number: 210 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 40M Special_Requirements: POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 220 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 230 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 240 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: POS TARG 0.233,0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Visit_Number: 03 Visit_Requirements: On_Hold_Comments: Visit_Comments: Exposure_Number: 310 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 40M Special_Requirements: POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 320 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 330 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 340 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Exposure_Number: 350 Target_Name: VIRGO-CLUS1 Config: WFPC2 Opmode: IMAGE Aperture: WFALL-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 42M Special_Requirements: POS TARG -0.233,-0.233 LOW-SKY MAX DUR 60M EXPAND Comments: Data_Distribution ! 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