5898( 7) - 04/30/96 14:49 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5898 Version: 7 Check-in Date: 30-Apr-1996 14:45:44 1.Proposal Title: Photometry of Candidate Dwarf Spheroidal Galaxies in the M81 Group ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Stellar Populations GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Dr.\ Nelson Caldwell F.L.\\ Whipple Observatory, Smithsonian Institution CoI: Dr.\ Taft Armandroff Kitt Peak National Observatory, NOAO N CoI: Dr.\ Gary Da Costa Anglo-Australian Observatory N CoI: Dr.\ Patrick Seitzer Dept.\\ of Astronomy, Univ.\\ of Michigan N ------------------------------------------------------------------------------------ 6. Abstract The M81 group is one of the nearest groups of galaxies to our own but its properties are quite different from that of the Local Group. It has therefore provided a different evolutionary environment for its member galaxies. This team has carried out a CCD survey of the M81 group to search for analogs to the Local Group dwarf spheroidal (dSph) galaxies. We have recovered all of the M81 dwarf candidates previously identified in photographic surveys and, in addition, have discovered six new systems whose surface brightnesses and apparent sizes place them within the range of values found for the Local Group dSphs. In this proposal we request HST time to image two of the candidate M81 group dSph systems. The proposed observations will allow construction of color- magnitude diagrams covering the upper two magnitudes of the red giant branch and will allow a determination of their distance (via the luminosity of the giant branch tip), an estimate of their mean abundance (via the mean giant branch color) and a first estimate of their star formation history (via the frequency of occurrence of upper-AGB stars). Comparison of these results with those for the Local Group dSphs will then help define the dependence of the formation and evolution of these low luminosity galaxies on the local conditions. ------------------------------------------------------------------------------------ 5898( 7) - 04/30/96 14:49 - [ 2] Observations Description ------------------------ The required color-magnitude diagrams will be derived from long exposures carried out with the WFPC2 using the F555W and F814W filters. These filters are perhaps the most used on WFPC2 and provide acceptable transformations to the ground- based V,I system while minimizing HST total exposure times. The center of each candidate galaxy will be placed on the nominal center of the PC1 field with the ``core'' of the galaxies filling the PC1 field. The WF fields then image the outskirts of the galaxies but since they cover more area, they will still contain significant numbers of target member stars. We require that the F555W frames and the F814W frames be taken with the same spacecraft orientation so that the same stars are imaged in both filters. For each filter, the exposures will be made in two sets with an offset of n+0.5 PC pixels (n ~ 20) between each set. This will minimize the effects of uncertainties in the flat fields and of undersampling on the final photometry while still allowing cosmic -ray discrimination. Assuming a distance modulus for the M81 group of 27.8 (Freedman et al.\\ 1994, ApJ, 427, 628), the tip of the red giant branch in an old stellar population should occur at an I magnitude of approximately 23.8, with V- I color ranging from ~1.4 to >= 2.0 depending on the abundance (see for example, Fig.\\ 6 of Da Costa & Armandroff 1990, AJ, 100, 162). Based on our experience with equivalent ground-based observations of Local Group dE and dSph systems (e.g.\\ Mould et al.\\ 1983, ApJ, 270, 471; Armandroff et al.\\ 1993, AJ, 106, 986), we require, in order to fulfill the science aims of the program, reasonably precise (Sigma ~ 0.10) magnitudes for stars at least two magnitudes fainter (in I) than the giant branch tip. Such stars have colors in the range 1.0 <= V-I <= 1.4. Consequently, we have calculated exposure times on the basis of achieving a S/N ~ 10 for stars with V=27.0 (SpT G0 - K0III). To maximize efficiency, the exposure times have also been chosen so that this S/N can be achieved within the 9 contiguous uninterrupted orbits applicable to a target in the CVZ. We find that 10x1800s exposures with the F555W filter and 14x1900s exposures with the F814W filter will just achieve this aim. In making the calculations, the correction factor applied to the signal-to-noise (calculated as required by assuming that all the light falls in one pixel) has been taken to be the average of the PC-Center and PC-Corner values (Table 6.4 WFPC2 Handbook) interpolated to the central wavelengths of the F555W and F814W filters. A set of short exposures (3x120s F555W, 3x180s F814W) will allow overlap between the HST photometry and the ground-based observations that will provide the photometric zeropoint. These short exposures are taken in groups of 3 to facilitate removal of cosmic rays and to extend the interval of overlap with the ground-based photometry to brighter magnitudes. Including the time for initial guide star acquisition and WFPC2 overheads (NO CR-SPLIT), the proposed set of observations fills 860 of the 864 minutes (9x96min) available for 9 orbits with the target in the CVZ. Thus we can achieve our science aims in a very efficient manner. Real Time Justification ----------------------- At the declination of these targets, the standard orbit visibility is given as 59 minutes. After subtracting 6mins for guide star re-acquisition and allowing 6 minutes for WFPC2 overhead (2x1400s exposures, NO CR-SPLIT), the effective time for integration is 47 minutes per orbit for the second and subsequent orbits. If, on the other hand, the observations are made when the targets are in the CVZ, the full 96min is available for the second and subsequent orbits (upto the ninth). If, for example, we take three exposures (1700s, again NO CR- SPLIT) in this time, then the effective integration time is 85min per orbit, or an efficiency that is 1.8 times higher. The increased efficiency resulting from CVZ observations is actually larger than this number, by the square root of the ratio of the exposure times (1.1), since the longer CVZ exposures allow the desired S/N to be reached in a shorter total integration time. That is, the total efficiency gain from observing these targets when they are in the CVZ is a factor of two and the program can be completed in half the number of orbits it would otherwise require. Put another way, we can observe both our targets in the time it would take for one, if they are both observed when they are in the CVZ. This represents a significant scientific gain. The program objects have been drawn from a survey carried out with a CCD camera mounted on the Burrell Schmidt telescope at Kitt Peak Observatory. These observations have yielded integrated colors, total magnitudes and surface brightnesses for the candidates. We have also been granted time on the Michigan-Dartmouth-MIT 2.4m telescope in an endeavor to resolve these galaxies into stars. These attempts have met with limited success so far but have indicated that one of the candidates in this proposal is indeed likely to be an M81 group member. Ground- based observations to confirm the photometric zeropoint of the HST data will be obtained through application for time on the facilities of Kitt Peak National Observatory and the Michigan-Dartmouth-MIT Observatory, as necessary. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 5898( 7) - 04/30/96 14:49 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5898( 7) - 04/30/96 14:49 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 GAL- Galaxy, Dwarf RA=09H 44M 47.055S +/- 0.1S, J2000 SURF(V) = 25.0+/-0.1 094447+67261 Elliptical DEC=67D 26' 18.69" +/- 1" B-V = 0.2+/-0.1 9 2 GAL- Galaxy, Dwarf RA=10H 04M 41.1S +/- 0.1S, J2000 SURF(V) = 25.0+/-0.1 100441+68152 Elliptical DEC=68D 15' 22" +/- 1" B-V = 0.2+/-0.1 2 5898( 7) - 04/30/96 14:49 - [ 5] Visit: 01 Visit Requirements: CVZ 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-094447+ WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 6 1900S 672619 ------------------------------------------------------------------------------------------------------------------------------------ 2 GAL-094447+ WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 5 1800S 672619 ------------------------------------------------------------------------------------------------------------------------------------ 5898( 7) - 04/30/96 14:49 - [ 6] Visit: 02 Visit Requirements: SAME ORIENTation AS 1 CVZ 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-094447+ WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 8 1900S POSition TARGet 0.94, 672619 0.94 ------------------------------------------------------------------------------------------------------------------------------------ 2 GAL-094447+ WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 5 1800S SAME POSition AS 1 672619 ------------------------------------------------------------------------------------------------------------------------------------ 3 GAL-094447+ WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 3 120S SAME POSition AS 1 672619 ------------------------------------------------------------------------------------------------------------------------------------ 4 GAL-094447+ WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 3 180S SAME POSition AS 1 672619 ------------------------------------------------------------------------------------------------------------------------------------ 5898( 7) - 04/30/96 14:49 - [ 7] Visit: 03 Visit Requirements: CVZ 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-100441+ WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 6 1900S 681522 ------------------------------------------------------------------------------------------------------------------------------------ 2 GAL-100441+ WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 3 1800S 681522 ------------------------------------------------------------------------------------------------------------------------------------ 5898( 7) - 04/30/96 14:49 - [ 8] Visit: 04 Visit Requirements: SAME ORIENTation AS 3 CVZ 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-100441+ WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 8 1900S POSition TARGet 0.94, 681522 0.94 ------------------------------------------------------------------------------------------------------------------------------------ 2 GAL-100441+ WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 5 1800S SAME POSition AS 1 681522 ------------------------------------------------------------------------------------------------------------------------------------ 3 GAL-100441+ WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 3 120S SAME POSition AS 1 681522 ------------------------------------------------------------------------------------------------------------------------------------ 4 GAL-100441+ WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 3 180S SAME POSition AS 1 681522 ------------------------------------------------------------------------------------------------------------------------------------ 5 GAL-100441+ WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 2 1800S 681522 ------------------------------------------------------------------------------------------------------------------------------------ 5898( 7) - 04/30/96 14:49 - [ 9] Summary Form for Proposal 5898 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Stellar Populations ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ SAME ORIENTation AS 1 CVZ POSition TARGet 0.94, 0.94 SAME POSition AS 1 SAME ORIENTation AS 3 CVZ ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GAL-094447+672619 GAL-100441+681522 ------------------------------------------------------------------------------------------------------------------------------------