Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 2495c Report Date: 09-May-96:17:09 Version: ********** Check-in Date: ********** 1.Proposal Title: THE AGE(S) OF THE SCULPTOR DWARF SPHEROIDAL GALAXY ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID STELLAR POPULATIONS GO Sub Category OLD FIELD STARS ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Dr. Gary S. Da costa YALE UNIVERSITY USA 203-432-3022 ASSOC. PROFESSOR ------------------------------------------------------------------------------------ 7. Abstract PC imaging of two fields near the center of the Sculptor dwarf spheroidal galaxy will be used to construct c-m diagrams and luminosity functions that reach more than two magnitudes fainter than the turnoff expected for the oldest possible stars in this system. With these data we will investigate the mean age, and the age range of the stellar population of this dwarf galaxy. Knowledge of these quantities is a prerequisite for understanding the evolutionary history of this galaxy in particular, and of galaxies of this class in general. The HST is required for its faint limiting magnitude capability, which enables stars well below the turnoff to be detected, and for its high resolution capability that will allow such stars to be accurately measured. ------------------------------------------------------------------------------------ 8. Scientific Key Words: DWARF SPHEROIDAL GALAXY, LUMINOSITY FUNCTION, STELLAR POPULATION ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 4.92 par: 0 10. Num targs pri: 2 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: WF/PC ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 2495c PI: Dr. Gary S. Da costa Proposal Title: THE AGE(S) OF THE SCULPTOR DWARF SPHEROIDAL GALAXY ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ P.I. Gary S. Da Costa YALE UNIVERSITY USA Patrick Seitzer STSCI USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. The observations will be carried out with the PC since only it combines adequate sampling of the image profiles with large enough field size. The wide-band filters F555W and F791W will be employed since they provide satisfactory transformations to the ground-based V,I(Cousins) standard system, and, at their effective wavelengths, quantum efficiency hysteresis is not a concern. Deferred charge problems will be overcome by pre-flashing the CCDs by the standard amount. Inclusion of a small position offset (~=1-2") between exposures will help to minimize uncertainties from the flat field calibration. The instrumental photometry should then be internally quite precise. Both fields will be observed with ground-based telescopes to provide the zeropoint calibration for each individual CCD. Short exposures with HST will also be obtained of each field to allow measurement of stars saturated on the long exposures and to increase the magnitude overlap with the ground-based photometry. Scaling the number density of stars brighter than V=23 in the field studied by Da Costa (1984) with the major- axis density profile of Eskridge (1988, A.J.,95,1706) and the luminosity function of the metal-poor globular cluster NGC 6752 (Da Costa 1982,A.J.,87,990), indicates that a PC field centered within the core of this galaxy will contain about 2000 stars brighter than V~=25.7. Thus such fields will be only moderately crowded, yet there will be sufficient stars near the turnoff that any age range determined will not have any significant statistical uncertainty. As noted in the Scientific Justification, two fields will be imaged. One will be near the center of the galaxy and the second will be centered on the most prominent of the density enhancements discovered by Eskridge. Both fields lie within the galaxy's core. ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. With considerable effort and only under the very best of conditions, ground- based imaging can detect faint objects whose magnitudes approach those that will be routinely achieved with HST. However, detection is a much less stringent requirement than reasonably precise photometry of stars. Consequently, for those projects requiring such photometry, ground-based imaging is limited to V~=24 in uncrowded fields; this limit rapidly becomes brighter as image crowding becomes significant. The PI has published the results of faint ground-based imaging of a field in the outskirts of this galaxy (Da Costa 1984, Ap.J.,285,483). With the faintest stars measured approaching V~=24, these data represent the best that can be achieved from the ground for this galaxy. But to answer the questions raised in the Scientific Justification, this limit is simply too bright; we must, therefore, use HST. Indeed HST is essential because only it can provide the required relatively precise (magnitude errors~= 0.05 mag) photometry at the necessary faint (V~=25.7) magnitude levels. To calibrate the zeropoint of the HST observations, we will obtain ground- imaging of both PC fields. We also plan to observe from the ground a sample of well studied galactic globular clusters, such as M92 and M13 whose abundances bracket that of Sculptor. The resulting globular cluster c-m diagrams will cover the same absolute magnitude range as the HST observations. They are required to aid the interpretation of the HST observations and are presently not available. Page 3 ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. The horizontal branch stars in Sculptor have a mean V magnitude of 20.15 (Kunkel and Demers 1977,Ap.J.,214,71). Thus a population of stars in this galaxy that is as old as the galactic globular clusters has a turnoff magnitude some 3.5 to 3.6 magnitudes fainter (e.g. Sarajedini and King 1989,A.J.,in press) or V~=23.7. Total exposure times have then been calculated by requiring a S/N greater than 20 (i.e. minimum magnitude and color errors of 0.05 and 0.08 mag, respectively) at V=25.9, more than two magnitudes fainter than the "old" turnoff. This represents a minimum S/N for achieving the precision necessary to fulfill the scientific objectives. Assuming 2 subexposures for F555W and 3 for F791W, a preflash of 53 e - per exposure (Lauer 1989,PASP,101,445), a G0 flux distribution and the mean sensitivities of the 4 PC chips from the July '88 Newsletter, total integration times are 2400s (2x1200s) for F555W and 6300s (3x2100s) for F791W. Short exposure times have been calculated on the basis of requiring at least one mag of overlap between the short and long exposures at a S/N > 25. The amount of unocculted spacecraft time required is detailed on the attached RPSS Resource Summary Worksheet; the efficiency is 70 percent. ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. Point-Spread-Function (PSF) fitting codes will be used to produce instrumental (F555W vs F555W-F791W) c-m diagrams for each field. Both Da Costa and Seitzer have extensive experience in this type of work. The possible occurrence of radial variations in the PSF will not pose any difficulties because they presumably will be stable and well- calibrated. Ground-based observations of the brighter stars in the PC fields will be used to set the zeropoints of the instrumental magnitudes. Thus we will not be directly relying on any HST sensitivity calibration. The completeness of the photometry, and its errors, will be estimated as a function of magnitude by the process of finding and measuring artificial stars, modelled from the PSF, that have been randomly inserted into the data. Once these incompleteness functions are known, the data can then be used to construct a luminosity function for each field. Given the small size of the PC and the high galactic latitude of Sculptor galactic foreground stars will not be a serious contaminant of these data. Instead, the principle source of contamination will be faint background galaxies, but since in general they will be resolved, they can be easily removed in the reductions. Once the errors and completeness corrections are established, the analysis of the c-m diagrams and luminosity functions will proceed in three distinct steps. First, we will compare the results of the two fields. This will, of course, immediately reveal whether the stellar populations of the density enhancements observed by Eskridge (1988, A.J.,96,1336) are distinct or not. It will therefore provide definitive constraints on the probable origin of these anomalous high density regions. Second, we will compare the Sculptor data with the galactic globular clusters. The c-m diagrams will be overlaid by superposing the main sequences. Although this analysis is made somewhat complicated by the fact that Sculptor has a range in abundance (indeed our observations should be of sufficient accuracy to reveal the intrinsic width of the main sequence expected from the observed abundance spread), we will, by using clusters that cover this abundance range, be able to infer whether or not the bulk of the Sculptor stellar population is younger relative to the galactic globular clusters as suggested by Da Costa (1984). This comparison requires the not unreasonable assumption that the individual abundance ratios, such as \[O/Fe\], are the same in both Sculptor and the galactic globular clusters of comparable \[Fe/H\]. Third, we will compare the Sculptor data with the best available theoretical isochrones and luminosity functions. This technique may allow us, via Monte-Carlo simulations, to place limits on the age distribution of the stellar population in addition to providing estimates of the ages of the oldest and youngest stars present in these fields. Page 4 ------------------------------------------------------------------------------------ 8. Additional comments or special requests. Scheduling should take care to ensure that residual images from previous PC observations do not contaminate our frames. Our fields have been chosen as far as practical to avoid the generation of any residual images in our exposures. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). The computational, image display and software resources available at the Anglo- Australian Observatory (where the PI will be after Jan 1990) and at STScI will be used for the data reduction and analysis. Virtually all the necessary hardware and software is installed and tested through use in numerous ground-based CCD imaging projects. The necessary ground-based calibration and follow-up observations will also be obtained, at least in part, through the use of AAO facilities. ------------------------------------------------------------------------------------ 11. Address Information Name: PATRICK SEITZER Category: CON Institution: STSCI Address: 3700 SAN MARTIN DRIVE City: BALTIMORE State: MD Zip Code: 21218 Country: USA Telephone: Telex (or e-mail): ------------------------------------------------------------------------------------ Name: GARY S. DA COSTA Category: PI Institution: ANGLO-AUSTRALIAN OBSERVATORY Address: 167 VIMIERA ROAD EASTWOOD NSW 2122 City: State: Zip Code: Country: AUSTRALIA Telephone: Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 2495c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 1 SCULPTOR-F1 GALAXY; TYPE=DWARF PLATE-ID = 024K, 2000.0 1 V = 25.9 , TYPE = G0 SPHEROIDAL GALAXY RA = 0H 59M 46.384S +/- 2", 2 V = 21.8 , TYPE = G0 LOCAL GROUP; STELLAR DEC = -33D 38' 58.43" +/- 2" POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ 2 SCULPTOR-F2 GALAXY; TYPE=DWARF PLATE-ID = 024K, 2000.0 1 V = 25.9 , TYPE = G0 SPHEROIDAL GALAXY RA = 1H 00M 19.198S +/- 2", 2 V = 21.8 , TYPE = G0 LOCAL GROUP; STELLAR DEC = -33D 43' 32.65" +/- 2" POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 2495c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DEFINE # PC IMAGE ALL F555W PURGE=NO 1 50S 25.3 2 # VS CR-SPLIT=NO SUM=1X1 READ=YES PRE-FLASH=YES CLOCKS=NO ------------------------------------------------------------------------------------------------------------------------------------ 2 DEFINE # PC IMAGE ALL F555W PURGE=NO 1 1200S 14.8 1 # VL CR-SPLIT=NO SUM=1X1 READ=YES PRE-FLASH=YES CLOCKS=NO ------------------------------------------------------------------------------------------------------------------------------------ 3 DEFINE # PC IMAGE ALL F791W PURGE=NO 1 100S 24.2 2 # IS CR-SPLIT=NO SUM=1X1 READ=YES PRE-FLASH=YES CLOCKS=NO ------------------------------------------------------------------------------------------------------------------------------------ 4 DEFINE # PC IMAGE ALL F791W PURGE=NO 1 2100S 11.9 1 # IL CR-SPLIT=NO SUM=1X1 READ=YES PRE-FLASH=YES CLOCKS=NO ------------------------------------------------------------------------------------------------------------------------------------ 5 USE VS SCULPTOR-F1 1 SEQ 5-11 NO GAP POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 6 USE IS SCULPTOR-F1 1 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 7 USE VL SCULPTOR-F1 1 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 8 USE IL SCULPTOR-F1 1 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 9 USE IL SCULPTOR-F1 1 POS TARG -1.5,1.0 ------------------------------------------------------------------------------------------------------------------------------------ 10 USE IL SCULPTOR-F1 1 POS TARG 1.5,-1.0 ------------------------------------------------------------------------------------------------------------------------------------ 11 USE VL SCULPTOR-F1 1 POS TARG 1.5,-1.0 ------------------------------------------------------------------------------------------------------------------------------------ 12 USE VS SCULPTOR-F2 2 SEQ 12-18 NO GAP POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 13 USE IS SCULPTOR-F2 2 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 14 USE VL SCULPTOR-F2 2 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ 15 USE IL SCULPTOR-F2 2 POS TARG 0.0,0.0 ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 2495c [ 7] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 16 USE IL SCULPTOR-F2 2 POS TARG -1.5,1.0 ------------------------------------------------------------------------------------------------------------------------------------ 17 USE IL SCULPTOR-F2 2 POS TARG 1.5,-1.0 ------------------------------------------------------------------------------------------------------------------------------------ 18 USE VL SCULPTOR-F2 2 POS TARG 1.5,-1.0 ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 2495c [ 8] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations PC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters PURGE=NO CR-SPLIT=NO SUM=1X1 READ=YES PRE-FLASH=YES CLOCKS=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 4.92 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category OLD FIELD STARS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SEQ 5-11 NO GAP; POS TARG 0.0,0.0 POS TARG -1.5,1.0 POS TARG 1.5,-1.0 SEQ 12-18 NO GAP; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F791W ------------------------------------------------------------------------------------------------------------------------------------ Target Names SCULPTOR-F1 SCULPTOR-F2 ------------------------------------------------------------------------------------------------------------------------------------