9363( 7) - 04/15/02 10:18 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9363 Version: 7 Check-in Date: 15-Apr-2002 10:17:45 1.Proposal Title: Ultra Low Surface Brightness Galaxies ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Nelson Caldwell Smithsonian Institution Astrophysical Observatory CoI: Dr Taft Armandroff National Optical Astronomy Observatories N ------------------------------------------------------------------------------------ 5. Abstract Extremely low surface brightness galaxies have been detected in the Virgo cluster, which are 2 magnitudes fainter than any previously known in that cluster or even in the Local Group. ACS images of three of these should resolve stars at the giant branch tip, and allow us to determine distances, mean metal abundance of the stars, and rough ages. Confirmation of the nature of these galaxies will provide evidence that dark matter halos are pervasive in the universe, extending to galaxies with stellar densities 6 times lower than currently known. These resolved stars would be the most distant yet observed accurately by HST. ------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 2] Observations Description ------------------------ Previous HST observers have attempted to resolve red giant stars in Virgo using WFPC2 and the F814W filter (I band) with some success. O'Neil et al. 1999 looked for surface brightness fluctuations in a few Virgo dwarfs. Harris et al. 1998 looked at a bright dwarf elliptical; Ferguson et al. 1998 searched for intergalactic stars. The latter two barely detected an emergent number of stars beginning at I~26.5. This is what is expected for the tip of the red giant branch if the Virgo distance modulus is 31.0. The V (F555W) magnitudes of such stars (as yet unmeasured) would be fainter than 28.0, because the tip stars have V-I ~ 1.5. Exposure times for those projects (in one filter) were around 33, 000s. With ACS, we can do better and indeed need to so that we can go down the giant branch 1 magnitude in I, and 0.5 magnitude in V. Sampling the giant branch in this way will give us a better handle on the luminosity function, giving us a better distance estimate. Simply detecting stars at the limit is insufficient for dwarf elliptical galaxies, because many such galaxies have a substantial population above the tip, due to intermediate age stars (``upper- AGB stars''). Detecting stars another magnitude deeper resolves any confusion between AGB and GB stars, particularly since the colors we can form will help separate the extremely red AGB stars from the regular GB stars. Resolving the stars in V of course allows colors to be formed, and thus the metallicity of the stars to be estimated. We don't expect to go as far down the giant branch in V, rather we ask for enough exposure time obtain 10 percent photometry for the tip of the giant branch (down to V=28.5), allowing reasonable statistics for stars another 0.5 mag fainter. Getting 10 percent photometry on those stars is essential for determining the astrophysically interesting quantities, based on experience with previous Cycle data. With the ACS ETC, using a K star template, an optimal S/N of 10 is reached for I = 27.5 (0.5mag down the giant branch) in 15, 000s, which we will break up into 6 separate 1 orbit exposures. For V of 28.5 (tip of the giant branch), 32, 000s is needed to get S/N=10, which we would break up into 13 exposures. Concerning crowding, we don't think there will be a problem, even at the distance of Virgo, because of the extremely low surface brightnesses of our candidates. The total light from giant branch stars in an old population is about 1.5 magnitudes less than the total light from all stars (Caldwell et al. 1992). So the central surface brightnesses in galaxies of stars we would hope to resolve in this program are about 27.3+1.5=28.8 mag arcsec^- 2, or 0.003L_RGB star pc^-2, where L_RGB is a star with luminosity 1.5 mags below the giant branch tip. The ACS scale is 0.05 arcsec pix^-1, or in area at Virgo 13 pc^2 pix^-1. So one pixel in one of our Virgo dwarfs has 0.03 GB star: the density is 1 giant branch star per 34 pixels. The average distance between stars at the center is thus sqrt(34) ~eq 6 pix. And the density is of course is lower outside the center. This won't present a problem for doing point-spread function photometry, and in fact one might say that these galaxies are ideal for a demonstration of the resolving power of the HST+ACS. Separate sets of exposures will be dithered by n+0.5 pixels (where n ~ 20) to aid in removing flat-field problems, bad pixels and undersampling. 0.2 truein Caldwell, N., Armandroff, T., Da Costa, G. & Seitzer, P. 1992, AJ, 103, 840., Ferguson, Henry C., Tanvir, Nial R., von Hippel, Ted 1998 Nature, 391, 461, Harris, W. E., Durrell, P. R., Pierce, M. J., & Secker, J. 1998 Nature, 395, 45, O'Neil, K., Bothun, G., & Impey, C. 1999, AJ, 118, 1618, Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- Orientation for all exposures on a specific target should be the same. ------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 GAL- GALAXY, DWARF RA=12H 26M 48.1S +/- 0.1S, J2000 SURF(R) = 25.7 122648+13211 ELLIPTICAL, DWARF DEC=13D 21' 17.5" +/- 1" SURF(B) = 27.0 8 SPHEROIDAL Coordinate Source: OTHER_SOURCE Comments: Orientation for all exposures on this target should be the same. 2 GAL- GALAXY, DWARF RA=12H 26M 20.1S +/- 0.1S, J2000 SURF(B) = 27.0+/-0.3 122620+12342 ELLIPTICAL, DWARF DEC=12D 34' 25.3" +/- 1" SURF(R) = 25.7+/-0.3 5 SPHEROIDAL Coordinate Source: OTHER_SOURCE Comments: Orientation for all exposures on this target should be the same. 9363( 7) - 04/15/02 10:18 - [ 4] Visit: 01 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 2 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT POS TARG 0.174, 132118 C 0.187 ------------------------------------------------------------------------------------------------------------------------------------ 3 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 4 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S POS TARG 0.174, 0.187 NO 132118 C SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 5] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 5 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 6 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT POS TARG 0.174, 132118 C 0.187 ------------------------------------------------------------------------------------------------------------------------------------ 7 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 8 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT POS TARG 0.174, 132118 C 0.187 ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 6] Visit: 03 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 9 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 10 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S POS TARG 0.174, 0.187 NO 132118 C SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 11 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 12 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S POS TARG 0.174, 0.187 NO 132118 C SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 13 GAL-122648+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 7] Visit: 04 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 14 GAL-122648+ ACS/WF ACCUM WFC F814W 1 2400 S 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 15 GAL-122648+ ACS/WF ACCUM WFC F814W 1 2400 S POS TARG 0.174, 0.187 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 16 GAL-122648+ ACS/WF ACCUM WFC F814W 1 2400 S 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 17 GAL-122648+ ACS/WF ACCUM WFC F814W 1 2400 S POS TARG 0.174, 0.187 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 8] Visit: 05 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 18 GAL-122648+ ACS/WF ACCUM WFC F814W 1 2400 S 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 19 GAL-122648+ ACS/WF ACCUM WFC F814W 1 2400 S POS TARG 0.174, 0.187 132118 C ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 9] Visit: 06 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 101 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 102 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT POS TARG 0.174, 123425 C 0.187 ------------------------------------------------------------------------------------------------------------------------------------ 103 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 104 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S POS TARG 0.174, 0.187 NO 123425 C SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 10] Visit: 07 Visit Priority: Visit Requirements: SAME ORIENT AS 06 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 ------------------------------------------------------------------------------------------------------------------------------------ 105 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 106 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT POS TARG 0.174, 123425 C 0.187 ------------------------------------------------------------------------------------------------------------------------------------ 107 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 108 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT POS TARG 0.174, 123425 C 0.187 ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 11] Visit: 08 Visit Priority: Visit Requirements: SAME ORIENT AS 06 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 ------------------------------------------------------------------------------------------------------------------------------------ 109 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 110 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S POS TARG 0.174, 0.187 NO 123425 C SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 111 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 112 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S POS TARG 0.174, 0.187 NO 123425 C SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 113 GAL-122620+ ACS/WF ACCUM WFC F555W CR-SPLIT=NO 1 2560 S NO SPLIT 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 12] Visit: 09 Visit Priority: Visit Requirements: SAME ORIENT AS 06 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 ------------------------------------------------------------------------------------------------------------------------------------ 114 GAL-122620+ ACS/WF ACCUM WFC F814W 1 2400 S 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 115 GAL-122620+ ACS/WF ACCUM WFC F814W 1 2400 S POS TARG 0.174, 0.187 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 116 GAL-122620+ ACS/WF ACCUM WFC F814W 1 2400 S 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 117 GAL-122620+ ACS/WF ACCUM WFC F814W 1 2400 S POS TARG 0.174, 0.187 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 13] Visit: 10 Visit Priority: Visit Requirements: SAME ORIENT AS 06 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 ------------------------------------------------------------------------------------------------------------------------------------ 118 GAL-122620+ ACS/WF ACCUM WFC F814W 1 2400 S 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 119 GAL-122620+ ACS/WF ACCUM WFC F814W 1 2400 S POS TARG 0.174, 0.187 123425 C ------------------------------------------------------------------------------------------------------------------------------------ 9363( 7) - 04/15/02 10:18 - [ 14] Summary Form for Proposal 9363 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements NO SPLIT NO SPLIT POS TARG 0.174, 0.187 POS TARG 0.174, 0.187 NO SPLIT SAME ORIENT AS 01 POS TARG 0.174, 0.187 SAME ORIENT AS 06 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GAL-122648+132118 GAL-122620+123425 ------------------------------------------------------------------------------------------------------------------------------------