5915( 4) - 11/20/95 13:48 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5915 Version: 4 Check-in Date: ********** 1.Proposal Title: Star Formation Histories of Dwarf Irregular Galaxies ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Stellar Populations GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Evan Skillman University of Minnesota CoI: Cesare Chiosi Padova N CoI: Reginald Dufour Rice University N CoI: Jay Gallagher University of Wisconsin N CoI: John Hoessel University of Wisconsin N CoI: Mario Mateo University of Michigan N CoI: Abhi Saha STScI N ------------------------------------------------------------------------------------ 6. Abstract We propose to obtain deep WFPC2 `BVI' color-magnitude diagrams (CMDs) for the dwarf irregular (dI) Local Group galaxies GR 8, Leo A, Pegasus, and Sextans A. In addition to resolved stars, we will use star clusters, and especially any globulars, to probe the history of intense star formation. These data will allow us to map the Pop I and Pop II stellar components, and thereby construct the first detailed star formation histories for non-interacting dI galaxies. Our results will bear on a variety of astrophysical problems, including the evolution of small galaxies, distances in the Local Group, age-metallicity distributions in small galaxies, ages of dIs, and the physics of star formation. The four target galaxies are typical dI systems in terms of luminosity, gas content, and H II region abundance, and represent a range in current star forming activity. They are sufficiently near to allow us to reach to stars at M_V = 0, have 0.1 of the luminosity of the SMC and 0.25 of its oxygen abundance. Unlike the SMC, these dIs are not near giant galaxies. This project will allow the extension of our knowledge of stellar populations in star forming galaxies from the spirals in the Local Group down to its smallest members. We plan to take maximum advantage of the unique data which this project will provide. Our investigator team brings extensive and varied experience in studies of dwarf galaxies, stellar populations, imaging photometry, and stellar evolution to this project. ------------------------------------------------------------------------------------ 5915( 4) - 11/20/95 13:48 - [ 2] Observations Description ------------------------ Our experience with the GR 8 HST data shows that deep BVI imaging will be adequate to address the scientific issues we wish to study. The B-I color will act as a very good discriminant, but it is important to have V so that very blue or very red stars are detected in at least two bands. Also, the color- color diagram is a useful diagnostic for reddening, foreground star rejection, and luminosity class. Multi-color data are very important for the false-star experiments required to determine completeness corrections (Mateo 1988, Saha & Hoessel 1990). We will take a minimum of six exposures per field to facilitate cosmic ray rejection (non trivial from our experience with the undersampled WFPC2 PSF). We will acquire WFPC2 imaging in B, V, and I of a central field in four Local Group Dwarf Irregular Galaxies. Each field will be imaged as follows, the first orbit is dedicated to acquisition and imaging in B (F439W), with subsequent orbits dedicated to imaging in B, V (F555W), and I (F814W). Assuming 52 usable minutes per orbit, 12 minutes for acquisition, and 5 minutes of overhead per C/R split, then the first orbit will be dedicated to two fifteen minute B integrations (all integrations are C/R split). The 2nd, 3rd, and 4th orbits will be dedicated to three twelve minute integrations in B, V, and I, respectively. This allows a total of 3960 seconds in B and 2160 seconds in V and I and a minimum of six exposures in each band. Following the WFPC2 handbook, we calculate S/N ratios of 7.2 for B = 27, 12.1 for V = 27, and 8.6 for I = 27. (Given the low ecliptic latitudes of Sextans A and Leo A, we conservatively assumed a V sky magnitude of 22.4.) This assumes all of the flux is gathered in a single pixel. Relaxing this final assumption gives larger integrations times as indicated in Table 6.4 of the WFPC2 manual. Given a realistic point spread function (which will be a function of the position on the chip) and the crowding of ``background" stars, we can reasonably expect to reach stars as faint as V=27 with these integration times. Thus we feel confident in our ability to reach below the HB and thus work on even the old Pop II components in these galaxies. Team members have a vast amount of experience with DAOPHOT and DoPHOT, especially with versions optimized for undersampled images. In addition to the actual photometry, a large effort will be required to carry out false star reductions. These are critical in producing the luminosity functions in each color. Analysis of the stellar photometry will consist of forming color-magnitude diagrams and color- color plots of the individual identifiable clusters, associations, and other regions in the galaxies. Comparison with theoretical evolutionary tracks (calculated at the appropriate abundances) will then allow us to assign ages (both differential and absolute) to these different features. Although the absolute ages one obtains in this way are model dependent (and therefore carry systematic uncertainties), the relative ages of features are quite reliable. The WFPC2 Team is planning to work with the Padova group with the goal of placing their models onto the main WFPC2 filter system in order to diminish CMD transformation problems. We will proceed in a similar manner to that reported in Bertelli et al.\ (1992), and try to identify regions of different ages. We can illustrate the sensitivity of this approach by calculating the age resolution we can hope to attain from these data. Consider a population made up of two bursts: burst A contains stars of 80 Myr and burst B has stars of 800 Myr age. The relative numbers of stars in burst B divided by the number in burst A is delta. We will examine three cases, assuming an IMF slope of alpha. Defining the ratio R1 as the number of MS stars brighter than M(V)=1 divided by the number of MS stars brighter than M(V)=0, and the ratio R2, the ratio of the number of evolved (that is red) stars brighter than M(V)=0 and the MS stars brighter than the same limit. We then consider three cases: (1) delta equals 5 (that is the old burst dominates the population by mass but NOT by luminosity); (2) delta = 1 (equal bursts); and (3) delta = 0 (no old component). We derive: Case 1: alpha = 1.8; delta = 5: R1 = 7.8 and R2 = 2.7 Case 2: alpha = 1.8; delta = 1: R1 = 3.2 and R2 = 0.57 Case 3: alpha = 2.3; delta = 0: R1 = 3.3 and R2 = 0.06 The comparison between Cases 1 and 2 shows we are very sensitive to the older component even though it barely contributes to the MS stars we are able to directly measure (primarily observed in the differences in the red stars). Note that we'd be able to tell that an even older old component exists, although R1 would saturate at a value consistent with the IMF and we would be limited to determining the total number of stars older than about 1.5 Gyr. Of course, the deeper the photometry, the older the age we can directly detect in this manner. Note that the comparison of cases 2 and 3 shows that if you mimic R1 with a steeper IMF, you get an R1 similar to that as in Case 2, but R2 is completely different (too few red giants from the young population). Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 5915( 4) - 11/20/95 13:48 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5915( 4) - 11/20/95 13:48 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 DDO69 GALAXY, LSB RA=9H 56M 32.5S +/- 0.2S, DEC = 1950 +30D 58' 50.0" +/- 1.0" 2 DDO75 GALAXY, LSB RA=10H 8M 30.0S +/- 0.2S, DEC = 1950 -04D 27' 30.0" +/- 1.0" 3 DDO155 GALAXY, LSB RA=12H 56M 08.8S +/- 0.2S, DEC = 1950 +14D 28' 53.0" +/- 1.0" 4 DDO216 GALAXY, LSB RA=23H 28M 33.0S +/- 0.2S, DEC = 2000 +14D 44' 16.0" +/- 1.0" 5915( 4) - 11/20/95 13:48 - [ 5] Visit: 01 Visit Requirements: ORIENTation 90D TO 120D 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 DDO69 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 900S FIX ------------------------------------------------------------------------------------------------------------------------------------ 2 DDO69 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 900S POS TARG 0.2, 0.2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 3 DDO69 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 1100S POS TARG 0.15, 0.05 FIX ------------------------------------------------------------------------------------------------------------------------------------ 4 DDO69 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 1100S POS TARG 0.35, 0.25 FIX ------------------------------------------------------------------------------------------------------------------------------------ 5 DDO69 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S FIX ------------------------------------------------------------------------------------------------------------------------------------ 6 DDO69 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 FIX ------------------------------------------------------------------------------------------------------------------------------------ 7 DDO69 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 FIX ------------------------------------------------------------------------------------------------------------------------------------ 8 DDO69 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S FIX ------------------------------------------------------------------------------------------------------------------------------------ 9 DDO69 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 FIX ------------------------------------------------------------------------------------------------------------------------------------ 10 DDO69 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 FIX ------------------------------------------------------------------------------------------------------------------------------------ 5915( 4) - 11/20/95 13:48 - [ 6] Visit: 02 Visit Requirements: ORIENTation 270D TO 290D On Hold Comments: Additional Comments: Orientation angle was chosen to avoid the bright star in the field. When this is scheduled, check the orientation to be sure that the bright star is outside of the field of view. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DDO75 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 900S FIX ------------------------------------------------------------------------------------------------------------------------------------ 2 DDO75 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 900S POS TARG 0.2, 0.2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 3 DDO75 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 1100S POS TARG 0.15, 0.05 FIX ------------------------------------------------------------------------------------------------------------------------------------ 4 DDO75 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 1100S POS TARG 0.35, 0.25 FIX ------------------------------------------------------------------------------------------------------------------------------------ 5 DDO75 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S FIX ------------------------------------------------------------------------------------------------------------------------------------ 6 DDO75 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 FIX ------------------------------------------------------------------------------------------------------------------------------------ 7 DDO75 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 FIX ------------------------------------------------------------------------------------------------------------------------------------ 8 DDO75 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S FIX ------------------------------------------------------------------------------------------------------------------------------------ 9 DDO75 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 FIX ------------------------------------------------------------------------------------------------------------------------------------ 10 DDO75 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 FIX ------------------------------------------------------------------------------------------------------------------------------------ 5915( 4) - 11/20/95 13:48 - [ 7] Visit: 03 Visit Requirements: ORIENTation 55D TO 95D On Hold Comments: Additional Comments: This galaxy is aligned across two WF chips. If there are scheduling problems, other orientations are possible, but the target position and aperture choise would need to be changed. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DDO155 WFPC2 IMAGE WF3-FIX F439W CR-SPLIT=NO 1 900S ------------------------------------------------------------------------------------------------------------------------------------ 2 DDO155 WFPC2 IMAGE WF3-FIX F439W CR-SPLIT=NO 1 900S POS TARG 0.2, 0.2 ------------------------------------------------------------------------------------------------------------------------------------ 3 DDO155 WFPC2 IMAGE WF3-FIX F439W CR-SPLIT=NO 1 1100S POS TARG 0.15, 0.05 ------------------------------------------------------------------------------------------------------------------------------------ 4 DDO155 WFPC2 IMAGE WF3-FIX F439W CR-SPLIT=NO 1 1100S POS TARG 0.35, 0.25 ------------------------------------------------------------------------------------------------------------------------------------ 5 DDO155 WFPC2 IMAGE WF3-FIX F555W CR-SPLIT=NO 1 600S ------------------------------------------------------------------------------------------------------------------------------------ 6 DDO155 WFPC2 IMAGE WF3-FIX F555W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 ------------------------------------------------------------------------------------------------------------------------------------ 7 DDO155 WFPC2 IMAGE WF3-FIX F555W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 ------------------------------------------------------------------------------------------------------------------------------------ 8 DDO155 WFPC2 IMAGE WF3-FIX F814W CR-SPLIT=NO 1 600S ------------------------------------------------------------------------------------------------------------------------------------ 9 DDO155 WFPC2 IMAGE WF3-FIX F814W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 ------------------------------------------------------------------------------------------------------------------------------------ 10 DDO155 WFPC2 IMAGE WF3-FIX F814W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 ------------------------------------------------------------------------------------------------------------------------------------ 5915( 4) - 11/20/95 13:48 - [ 8] Visit: 04 Visit Requirements: ORIENTation 45D TO 70D 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 DDO216 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 900S FIX ------------------------------------------------------------------------------------------------------------------------------------ 2 DDO216 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 900S POS TARG 0.2, 0.2 FIX ------------------------------------------------------------------------------------------------------------------------------------ 3 DDO216 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 1100S POS TARG 0.15, 0.05 FIX ------------------------------------------------------------------------------------------------------------------------------------ 4 DDO216 WFPC2 IMAGE WFALL- F439W CR-SPLIT=NO 1 1100S POS TARG 0.35, 0.25 FIX ------------------------------------------------------------------------------------------------------------------------------------ 5 DDO216 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S FIX ------------------------------------------------------------------------------------------------------------------------------------ 6 DDO216 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 FIX ------------------------------------------------------------------------------------------------------------------------------------ 7 DDO216 WFPC2 IMAGE WFALL- F555W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 FIX ------------------------------------------------------------------------------------------------------------------------------------ 8 DDO216 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S FIX ------------------------------------------------------------------------------------------------------------------------------------ 9 DDO216 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S POS TARG 0.233, 0.067 FIX ------------------------------------------------------------------------------------------------------------------------------------ 10 DDO216 WFPC2 IMAGE WFALL- F814W CR-SPLIT=NO 1 600S POS TARG 0.067, 0.233 FIX ------------------------------------------------------------------------------------------------------------------------------------ 5915( 4) - 11/20/95 13:48 - [ 9] Summary Form for Proposal 5915 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Stellar Populations ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENTation 90D TO 120D POS TARG 0.2, 0.2 POS TARG 0.15, 0.05 POS TARG 0.35, 0.25 POS TARG 0.233, 0.067 POS TARG 0.067, 0.233 ORIENTation 270D TO 290D ORIENTation 55D TO 95D ORIENTation 45D TO 70D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F439W F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names DDO69 DDO75 DDO155 DDO216 ------------------------------------------------------------------------------------------------------------------------------------