6695( 3) - 02/07/96 19:11 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6695 Version: 3 Check-in Date: 07-Feb-1996 19:05:38 1.Proposal Title: The Stellar Populations in the LGS3 Dwarf Galaxy ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Bryan Miller Space Telescope Science Institute and Carnegie Institution of Washington, CoI: Myung Gyoon Lee Department of Astronomy, Seoul National N University CoI: Paul Hodge Astronomy Department, University of N Washington ------------------------------------------------------------------------------------ 6. Abstract We will study of the stellar populations in the nearby dwarf galaxy LGS3. This galaxy unique in that is has properties of both dwarf spheroidal and dwarf irregular systems, i.e. it is dominated by old or intermediate age stellar populations but it also contains a small amount of neutral hydrogen and a small young stellar population. The program is to obtain deep F555W and F814W photometry to V~27.5 to study the horizontal branch, intermediate-age main-sequence turnoff, and young main sequence. We will determine the star formation history of LGS3 has been episodic as in Carina and Leo I, or more continuous as in Local Group dwarf irregulars. The results will be important to our understanding of the morphology/density relation in nearby groups, the fate of the faint blue galaxies, and the evolution of dwarf galaxies in general. ------------------------------------------------------------------------------------ 6695( 3) - 02/07/96 19:11 - [ 2] Observations Description ------------------------ Deep WFPC2 images through filters F555W and F814W will be used to study the stellar populations and star formation histories of LGS3, a dwarf galaxy in the Local Group. The F555W and F814W bandpasses are chosen since they have good transformations to the Johnson/Cousins V and I system which is the standard for observing old stellar populations. The Cycle 4 TAC strongly recommended these filters for galaxy population studies. While it would be best to center the galaxy in the PC to take advantage of the superior resolution, the necessities of avoiding bright stars that can produce harmful scattered light and of having a large number of scheduling opportunities mean that we will need to center the galaxy in one of the wide field chips. The chip to be used as the center will depend on the orientation of the telescope at the time of the observation. For the given coordinates centering in PC1 is allowed only for orientations between 145D and 160D but according to RPS2 there are no scheduling opportunities if all 8 orbits are done in one visit. The most flexible scheduling is when the coordinates are centered in WF2 and this has been given as the default. In this case orientations between 0D and 80D are acceptable (though an offset may be needed for roll angles between 25D and 40D). Orientations between 265D and 350D are acceptable when centering in WF3, and centering in WF4 is needed for orientations between 125D and 265D. Because the majority of the stars will be in the WF chips we will incorporate a four-point dithering scheme. We use the same offset scheme as the DITHER-TYPE=BOX option but we use explicit POS TARGS so that only three spacecraft moves are needed during the entire visit. This procedure will significantly improve the undersampling of the WF PSF and therefore give more reliable magnitudes when using PSF-fitting routines. The goals of our program are to determine the morphology of the horizontal branch, to investigate the upper main sequence, and to detect the existence of a young or intermediate age turnoff. These goals can be attained with the allocated 8 orbits. The increased brightness of the stars in I is almost equally balanced by the lower system sensitivity using F814W, so 4 orbits will be given observation with each filter. Given a visibility of 53 minutes, we will have a approximately 9600 seconds of integration in each filter. Three exposures will be taken each orbit so that we will be able to unambiguously remove the cosmic rays at each dither position. The limiting magnitude in this exposure time at S/N=5 is V=27.8 according to the Exposure Time Calculator. However, dithering should allow us to push slightly fainter than this. This corresponds to M_V~3.5 for a distance modulus of 24.54 (Lee 1995, AJ, 110, 1129). The tip of the RGB is at V=21.5, so we will easily detect the horizontal branch at V=24.5. Young or intermediate age main-sequence turnoffs will be 2--3 magnitudes below the horizontal branch, and our observations will be able to detect their existence. With accurate photometry of the RGB, the horizonal branch, and the upper main sequence, we will be able to determine the star formation history of LGS3 over the last 3--5 Gyr. Real Time Justification ----------------------- None. None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6695( 3) - 02/07/96 19:11 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6695( 3) - 02/07/96 19:11 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LGS3 GALAXY,dwarf spheroidal RA=1H 03M 54.4S +/- 0.05S, J2000 V=-277 SURF(V) = 25 DEC=21D 52' 56.2" +/- 0.5", PLATE-ID=02BD 6695( 3) - 02/07/96 19:11 - [ 5] Visit: 01 Visit Requirements: PCS MODE Fine ORIENT 0D TO 80D On Hold Comments: Additional Comments: ORIENT 0D TO 80D using WF2-FIX ORIENT 125D TO 265D using WF4-FIX ORIENT 265D TO 350D using WF3-FIX ORIENT 145D TO 160D using PC1-FIX Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LGS3 WFPC2 IMAGE WF2-FIX F555W ATD-GAIN=7,CR- 2 800S SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 2 LGS3 WFPC2 IMAGE WF2-FIX F555W ATD-GAIN=7,CR- 1 700S SAME POS AS 1 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 3 LGS3 WFPC2 IMAGE WF2-FIX F814W ATD-GAIN=7,CR- 3 800S SAME POS AS 1 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 4 LGS3 WFPC2 IMAGE WF2-FIX F814W ATD-GAIN=7,CR- 3 800S POS TARG 0.5,0.25 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 5 LGS3 WFPC2 IMAGE WF2-FIX F555W ATD-GAIN=7,CR- 3 800S SAME POS AS 4 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 6 LGS3 WFPC2 IMAGE WF2-FIX F555W ATD-GAIN=7,CR- 3 800S POS TARG 0.75,0.75 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 7 LGS3 WFPC2 IMAGE WF2-FIX F814W ATD-GAIN=7,CR- 3 800S SAME POS AS 6 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 8 LGS3 WFPC2 IMAGE WF2-FIX F814W ATD-GAIN=7,CR- 3 800S POS TARG 0.25,0.5 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 9 LGS3 WFPC2 IMAGE WF2-FIX F555W ATD-GAIN=7,CR- 3 800S SAME POS AS 8 SPLIT=NO Comments: see visit comments for Aperture ------------------------------------------------------------------------------------------------------------------------------------ 6695( 3) - 02/07/96 19:11 - [ 6] Summary Form for Proposal 6695 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ATD-GAIN=7 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE Fine ORIENT 0D TO 80D SAME POS AS 1 POS TARG 0.5,0.25 SAME POS AS 4 POS TARG 0.75,0.75 SAME POS AS 6 POS TARG 0.25,0.5 SAME POS AS 8 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names LGS3 ------------------------------------------------------------------------------------------------------------------------------------