! Proposal 6778, submission 1 ! PI: Mark Dickinson ! Received Fri Feb 9 14:28:58 EST 1996 ! From: med@stsci.edu ! +====================+ ! | PED OUTPUT PRODUCT | ! +====================+ ! ! /tmp_mnt/babbar/u1/med/cycle6/3c220.1/6778.prop ! Generated by PREPROCESSOR, version 6.0e ! Date: Thu Feb 8 00:00:50 EST 1996 Proposal_Information Title: HST Observations of a `Clusterless' Giant Arc centered on 3C 220.1 Proposal_Category: GO Scientific_Category: COSMOLOGY Cycle: 6 Investigators PI_Name: Mark Dickinson PI_Institution: Space Telescope Science Institute CoI_Name: Tom Broadhurst CoI_Institution: University of California, Berkeley Contact: Abstract: In an imaging survey of the cluster environments of distant radio galaxies, we have discovered a giant arc centered on 3C 220.1. At z = 0.62, this is the most distant known lens responsible for a giant arc. We have spectroscopically measured the arc redshift to be z = 1.49. While the arc suggests a very large lensing mass, the radio galaxy is situated in, at most, a very poor group of galaxies. The lens mass and mass--to--light ratio within the critical radius of the arc are unprecedentedly large for a single elliptical galaxy. WFPC2 observations can resolve the structure of this arc and confirm the reality of a possible counter--image seen close to the radio galaxy, to produce a detailed model of the lensing mass. Moreover, the lens is strong enough that significant weak lensing distortions of the faint background galaxies are expected, thereby probing the mass distribution beyond the critical radius. Questions Observing_Description: We propose to obtain deep V and I--band images of the 3C 220.1 field using WFPC2 with the F555W and F814W filters. As many post--refurbishment observations have shown, in only a few orbits HST reaches remarkably deep magnitude limits, detecting hundreds of faint, compact galaxies within a single field of view. With 0\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\secspt1 angular resolution and well-- controlled imaging systematics, HST is the ideal tool for weak lensing studies. The F555W and F814W filters are broad and efficient, and span the redshifted wavelength of the 4000Angstrom\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ break at z = 0.62, so that the radio galaxy and other potential associated galaxies will be less prominent in the V--band image, while candidates for arclet lensing come mostly from the notoriously blue population of very faint field galaxies. The limiting depth is set by the requirement of detecting the weak field distortion at an interesting level. Above we showed that the limiting S/N for this measurement is independent of radius for constant fraction radial bins and is expected to be S/N=5 at a depth of I_AB=27.5 for bin of width 10\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\% of the radius. This we know empirically based on real field HST data to this same depth (Kaiser, Squires & Broadhurst 1995) which we have distorted as if lensed. This limiting magnitude in I can be reached in 8 orbits in F555W or F814W bands - sensitivities being comparable when taking into account the relative k-correction for distant objects (0.3 mags in the mean between V and I at z_s=1-2, as quantified for the HDF project (Williams et al) with a 10\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\% limiting flux error in 0.5'' diameter aperture for a flat F_Nu spectrum. Since the measurement of distortions does not depend strongly on passband, we propose to split the full 8 orbits equally between V and I so that we may look for any red colour sequence of E/SO galaxies potentially associated with the radio source at z=0.62, and also to aid the identification of the counter image to the giant arc. We will employ sub-pixel dithering to each set of data, of 0.5 pixel shifts in a box pattern to effectively double the resolution, at no loss of ability to reject defective pixels of cosmic rays. Only 1 pointing is necessary to reach our main goals. with the radio galaxy centered on PC chip we have 70\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\% circular coverage about the radio galaxy to 0.3Mpc/h. Real_Time_Justification: None. Ground--based multiwavelength imaging of the 3C 220.1 field is already in hand, including wide--field infrared (JHK) images obtained by Stanford, Eisenhardt and Dickinson as part of an infrared--optical study of the environments of distant radio galaxies. Preliminary multislit spectroscopy of the field around 3C 220.1 was carried out by Dickinson and McCarthy using the COSMIC spectrograph at the Palomar 200", and we plan much more extensive multislit work using the LRIS spectrograph at the Keck telescope. A 37 ksec ROSAT HRI x-- ray image is scheduled to be taken in mid--September 1995. This observation was proposed without prior knowledge of the lens arc, and was intended to study the x--ray properties of the radio galaxy itself. We are in close contact with the ROSAT PI (Charles Lawrence), and hope to share information in order to provide joint constraints from lensing, multislit spectroscopy, and x--ray data. We note here, however, that the radio galaxy itself has a strong, flat spectrum radio core. This is unfortunate from the point of view of the x-- ray observations, as it suggests that the active nucleus is likely to be a strong x--ray source in its own right, making reliable decomposition of the x--ray profile into point-- source and extended components difficult within the lens radius. Calibration_Justification: Additional_Comments: Fixed_Targets Target_Number: 1 Target_Name: 3C220.1 Alternate_Names: Description: GALAXY,RADIO GALAXY,BGM,CLUSTER OF GALAXIES,POOR CLUSTER,HIGH REDSHIFT CLUSTER (Z>0.5),GRAVITATIONAL LENS Position: RA=09H 26M 31.87S +/- 1.0",DEC=79D 19' 45.4" +/- 1.0" Equinox: B1950 RV_or_Z: Z = 0.620 RA_PM: Dec_PM: Epoch: Annual_Parallax: Flux: R = 20.5+/-0.5 Comments: Radio galaxy in poor cluster with a giant arc gravitational lens located ~9 arcsec to the north. Solar_System_Targets Generic_Targets Scan_Data Visits Visit_Number: 01 Visit_Requirements: ORIENT 115D TO 155D On_Hold_Comments: Visit_Comments: Image field for 8 consecutive orbits, to be executed within one visit. 4 orbits in F555W and four in F814W. For each filter, four exposures are taken in each of two dither positions, offset from one another by POS-TARGs which ensure a total offset of approximately 1 arcsecond in the x and y coordinates, including an 0.5 pixel fractional offset on the WFC. Exposure_Number: 1 Target_Name: 3C220.1 Config: WFPC2 Opmode: IMAGE Aperture: WF3 Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=no Number_of_Iterations: 4 Time_Per_Exposure: 1400S Special_Requirements: EXPAND Comments: Exposure_Number: 2 Target_Name: 3C220.1 Config: WFPC2 Opmode: IMAGE Aperture: WF3 Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=no Number_of_Iterations: 4 Time_Per_Exposure: 1400S Special_Requirements: POS TARG 1.046,1.046 EXPAND Comments: Exposure_Number: 3 Target_Name: 3C220.1 Config: WFPC2 Opmode: IMAGE Aperture: WF3 Sp_Element: F555W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 4 Time_Per_Exposure: 1400S Special_Requirements: EXPAND Comments: Exposure_Number: 4 Target_Name: 3C220.1 Config: WFPC2 Opmode: IMAGE Aperture: WF3 Sp_Element: F555W Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 4 Time_Per_Exposure: 1400S Special_Requirements: POS TARG 1.046,1.046 EXPAND Comments: Data_Distribution ! 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