----- Submission 1 (Fri May 13 13:33:27 GMT 2005) ----- Assigned ID: 207 ----- Submission 1 (Fri Jan 21 15:37:08 GMT 2005) ----- Phase I Proposal 207 (phase14-GO-Disney1) successfully submitted. Validating a dark galaxy 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.
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. 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