10562( 2) - 05/13/05 10:18 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 10562 Version: 2 Check-in Date: 13-May-2005 14:17:39 1.Proposal Title: Validating a dark galaxy ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle 4. Parallel Pointing Tolerance GO 14 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Dr. Mike J. Disney University of Wales, College of Cardiff (UWCC) CoI: Dr. Jonathan Davies University of Wales, College of Cardiff N (UWCC) CoI: Dr. Robert F. Minchin University of Wales, College of Cardiff N (UWCC) CoI: Dr. Sabina Sabatini INAF, Osservatorio Astronomico di Roma N CoI: Dr. Marco Grossi Universita di Roma La Sapienza N ------------------------------------------------------------------------------------ 6. Abstract VIRGOHI21 is an object detected in the Virgo Cluster HI survey of Davies et al (2004), with a velocity width typical of a disc galaxy (220 km/s) but which does not appear to have an optical counterpart down to a surface brightness level of 27.5 B mag/sq. arcsec. Altogether, it is the best ever candidate for a Dark Galaxy. We propose to image this object with the ACS through the F814W filter for 9 orbits to see if this object contains a population of individually very faint stars which would be missed by ground-based telescopes. ------------------------------------------------------------------------------------ 10562( 2) - 05/13/05 10:18 - [ 2] Observations Description ------------------------ We wish to obtain I (F814W) ACS images of the best candidate Dark Galaxy, VIRGOHI 21, ever discovered to establish whether or not there are Red Giant Branch stars at its position. Assuming it is in the Virgo Cluster with a distance modulus close to 31 there could be as many as 6 RGB stars per square arcsec without conflicting with ground based observations and the tip of the RGB would be close to 27.0 in I. We have now imaged the HI source at 21-cm with Westerbork and want to centre the image on 12 17 53, +14 46 50 (J2000) close to the greatest mass and column-density, and where there is what appears to be a faint background galaxy (B ~ 20) within 2.5 arcseconds of a 21st magnitude star. Since our aims are very close the aims of the IZw18 (proposal 9400) observations by Izotov and Thuan we have closely modelled our observations, and our Phase II proposal, on theirs. We have 9 orbits, to be split in 3 visits, and subsplit in 2 for CR removal, with each orbital exposure slightly offset to permit drizzling and hence avoid crowding and to go as deep as possible. All images to have the same orientation. Total observations to be spread over no more than 60 days. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 10562( 2) - 05/13/05 10:18 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 GAL- GALAXY, LSB, RA=12H17M53.0000S +/- 0.1", J2000 V = V = 26.0+/-0.5 121753+14465 PROTOGALAXY DEC=+14D46'50.00" +/- 0.1", 2000.0 0 PLATE-ID=0590 Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 10562( 2) - 05/13/05 10:18 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.16 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 47.17 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 10562( 2) - 05/13/05 10:18 - [ 5] Visit: 01 Visit Priority: Visit Requirements: GROUP 01, 02, 03 WITHIN 60.0D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 GAL-121753+ ACS/WFC ACCUM WFC F814W CR-SPLIT=NO,GAIN=1 2 2500 S POS TARG 0, 0; PATTERN 1 144650 see below 1 ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 1 none 1 none 1 1265.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 1 none 2 none 1 1265.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 2 none 1 none 2 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 2 none 2 none 2 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 3 none 1 none 3 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 3 none 2 none 3 1296.0 S 10562( 2) - 05/13/05 10:18 - [ 6] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 01; AFTER 01 BY 0 D TO 60 D; GROUP 02, 01, 03 WITHIN 60.0D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 GAL-121753+ ACS/WFC ACCUM WFC F814W CR-SPLIT=NO,GAIN=1 2 2500 S POS TARG 0, 0; PATTERN 1 144650 see below 1 ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 1 none 1 none 1 1265.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 1 none 2 none 1 1265.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 2 none 1 none 2 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 2 none 2 none 2 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 3 none 1 none 3 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 3 none 2 none 3 1296.0 S 10562( 2) - 05/13/05 10:18 - [ 7] Visit: 03 Visit Priority: Visit Requirements: SAME ORIENT AS 01; AFTER 01 BY 0 D TO 60 D; GROUP 03, 01, 02 WITHIN 60.0D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 GAL-121753+ ACS/WFC ACCUM WFC F814W CR-SPLIT=NO,GAIN=1 2 2500 S POS TARG 0, 0; PATTERN 1 144650 see below 1 ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 1 none 1 none 1 1265.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 1 none 2 none 1 1265.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 2 none 1 none 2 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 2 none 2 none 2 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 3 none 1 none 3 1296.0 S GAL-121753+144650 1 ACS/WFC ACCUM WFC F814W 3 none 2 none 3 1296.0 S 10562( 2) - 05/13/05 10:18 - [ 8] Summary Form for Proposal 10562 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO GAIN=1 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 01, 02, 03 WITHIN 60.0D POS TARG 0, 0; PATTERN 1 1 SAME ORIENT AS 01; AFTER 01 BY 0 D TO 60 D; GROUP 02, 01, 03 WITHIN 60.0D SAME ORIENT AS 01; AFTER 01 BY 0 D TO 60 D; GROUP 03, 01, 02 WITHIN 60.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GAL-121753+144650 ------------------------------------------------------------------------------------------------------------------------------------