9400( 5) - 02/13/02 11:09 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9400 Version: 5 Check-in Date: 13-Feb-2002 11:09:18 1.Proposal Title: ARE THERE YOUNG GALAXIES IN THE LOCAL UNIVERSE: THE AGE OF THE BLUE COMPACT DWARF GALAXY I ZW 18 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Trinh Thuan The University of Virginia CoI: YURI IZOTOV MAIN ASTRONOMICAL OBSERVATORY, UKRAINE N ------------------------------------------------------------------------------------ 5. Abstract The question of whether there exists young galaxies in the local universe is important for cosmology. Cold Dark matter models predict that low-mass galaxies could still be forming at the present epoch. In the hierarchical model of galaxy formation, large galaxies result from the merging of smaller structures. These primordial building-block galaxies are too faint and small to be studied at high redshifts, while we stand a much better chance of understanding them if we can find some local examples. One of the best candidates for being a young nearby galaxy forming stars for the first time at the present epoch, is the blue compact dwarf (BCD) galaxy I Zw 18 because of its extremely low heavy element content (2% that of the Sun). We propose to obtain deep $V$ and $I$ ACS images of I Zw 18. Our goal is to detect or put limits on the red giant branch (RGB) stellar population in this galaxy. If RGB stars are not detected, then we can set an upper limit for the age of I Zw 18 to be less than 1 Gyr. If they are detected, I Zw 18 is not young, and the RGB tip can be used to derive its distance and set limits on the metallicity of the pregalactic gas. ------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 2] Observations Description ------------------------ We wish to obtain V (F555W) and I (F814W) ACS images of the most metal- deficient star-forming galaxy known, I Zw 18. Assuming a distance of 12.5 Mpc, the expected V and I apparent magnitudes of stars on the red giant tip are respectively 27 mag and 26 mag (for a distance of 15.8 Mpc, the magnitudes are 0.5 mag fainter). To obtain a signal-to-noise ratio of 10 (corresponding to magnitude errors of 0.1 mag) for stars one magnitude fainter (V = 28 mag and I = 27 mag), five orbits in I and ten orbits in V are necessary, counting guide star and target acquisitions and other overhead, and using the continuous viewing zone (CVZ) observation mode. Our exposure estimates are obtained for a star of spectral type K0 with V = 28 mag and I = 27 mag, using the ACS imaging exposure time calculator. We thus ask for a total of 15 orbits. Real Time Justification ----------------------- It turns out that I Zw 18 only barely grazes the CVZ, a fact that was not immediately evident because of the coarse resolution of the Phase I CVZ tables. We have run RPS2 with the CVZ option with 2 x 2300 s per orbit, and with one orbit per visit, corresponding to a total of 15 visits for 15 orbits. We found that the total scheduling window for each visit with the CVZ mode is only a few days in Oct 2002, and a few days in Oct 2003. Because of the narrow total scheduling window, we were advised by Ray Lucas to drop the CVZ option, and add non-CVZ orbits so that the total integration time spent on source (1130 mn) in non-CVZ mode is the same as the time spent on source in CVZ mode. We found that 25 non-CVZ orbits are equivalent to 15 CVZ orbits. We have a total of 8 visits. We split each exposure into 2 subexposures to remove cosmic rays. To remove hot pixels, we perform 3-point dithering with an angle of 47.17 for visits 1 through 7, and 2-point dithering with an angle of 47.17 for visit 8. We have a total exposure time of 725 mn in V and 405 mn in I. We set the GROUP WITHIN parameter to 60 days. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 IZW18 GALAXY, Dwarf Compact RA=09H 34M 00.9S +/- 0.1S, J2000 Z=0.0025 V = 16.0 DEC=55D 14' 34.2" +/- 0.5", 0 B-V = 0.0 PLATE-ID=0009 Coordinate Source: GUIDE_STAR_CATALOG 9400( 5) - 02/13/02 11:09 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.5 Coordinate_Frame POS-TARG Pattern_Orient 47.17 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.5 Coordinate_Frame POS-TARG Pattern_Orient 47.17 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 5] Visit: 01 Visit Priority: Visit Requirements: GROUP 1-8 WITHIN 60D 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 IZW18 ACS/WF ACCUM WFC1 F555W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 6] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D 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 ------------------------------------------------------------------------------------------------------------------------------------ 2 IZW18 ACS/WF ACCUM WFC1 F814W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 7] Visit: 03 Visit Priority: Visit Requirements: SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D 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 ------------------------------------------------------------------------------------------------------------------------------------ 3 IZW18 ACS/WF ACCUM WFC1 F555W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 8] Visit: 04 Visit Priority: Visit Requirements: SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D 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 ------------------------------------------------------------------------------------------------------------------------------------ 4 IZW18 ACS/WF ACCUM WFC1 F814W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 9] Visit: 05 Visit Priority: Visit Requirements: SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D 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 IZW18 ACS/WF ACCUM WFC1 F555W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 10] Visit: 06 Visit Priority: Visit Requirements: SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D 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 ------------------------------------------------------------------------------------------------------------------------------------ 6 IZW18 ACS/WF ACCUM WFC1 F814W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 11] Visit: 07 Visit Priority: Visit Requirements: SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D 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 ------------------------------------------------------------------------------------------------------------------------------------ 7 IZW18 ACS/WF ACCUM WFC1 F555W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 1 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 12] Visit: 08 Visit Priority: Visit Requirements: SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D 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 ------------------------------------------------------------------------------------------------------------------------------------ 8 IZW18 ACS/WF ACCUM WFC1 F555W CR-SPLIT=NO ,GAIN=1 2 1350 S PATTERN 2 C ------------------------------------------------------------------------------------------------------------------------------------ 9 IZW18 ACS/WF ACCUM WFC1 F555W CR-SPLIT=NO ,GAIN=1 2 1430 S PATTERN 2 C ------------------------------------------------------------------------------------------------------------------------------------ 9400( 5) - 02/13/02 11:09 - [ 13] Summary Form for Proposal 9400 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO GAIN=1 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 1-8 WITHIN 60D PATTERN 1 SAME ORIENT AS 1 GROUP 1-8 WITHIN 60D PATTERN 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names IZW18 ------------------------------------------------------------------------------------------------------------------------------------