9373( 2) - 02/04/02 10:56 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9373 Version: 2 Check-in Date: 04-Feb-2002 10:55:56 1.Proposal Title: Reaching the Horizontal Branch in NGC 5128: Deepest Probe of a Giant Elliptical ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Marina Rejkuba European Southern Observatory - Germany CoI: Dr. William Harris McMaster University N CoI: Dr. Gretchen Harris University of Waterloo N CoI: Mr. Eric Peng The Johns Hopkins University N CoI: Dr. Laura Greggio Osservatorio Astronomico di Bologna N ------------------------------------------------------------------------------------ 5. Abstract NGC 5128 is the nearest easily observable giant E galaxy, and is a unique testing ground for stellar population models. Previous WFPC2 photometry of its halo red- giant stars has shown that they are predominantly metal-rich (~-0.45), but little is yet known about their spread in ages. With the ACS/WFC camera, we propose to obtain deep (V, I) photometry down to the level of its horizontal-branch population, with the goal of refining the metallicity distribution function and gaining quantitative information on its age distribution. This will be unique data for any giant elliptical galaxy and will provide major new input to population synthesis techniques for such galaxies. ------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 2] Observations Description ------------------------ Target Location: We propose to use ACS in its Wide-Field mode to image a single NGC 5128 outer-halo field as deeply as possible. Since we also need the best possible photometric precision, the target field needs to be far enough from galaxy center that the projected stellar density will permit uncrowded stellar photometry even at the HB level. In the previously observed 31-kpc field (Harris & Harris 2000) there were ~650 stars per arcmin^2 brighter than M_I ~ - 2 (or I<26.1; the top 2 magnitudes of the RGB). Our new photometric limit will need to be at least 2 magnitudes fainter to reach past the red HB. Scaling from the M32 data of Grillmair et al.(1996), we estimate that this limit would give almost 10 times as many resolved stars in total. In our 31-kpc field this would have been equivalent to a density of 1.8 stars per arcsec^2. Although photometry at these densities with 0arcs1 resolution would be quite manageable with normal photometric codes, we conservatively prefer to reduce the crowding even further. Our target field location will be slightly further away, at 40 kpc projected distance, where (working from the r^1/4 halo profile) the expected stellar density will be ~1.0 stars per arcsec^2 brighter than I=28. This will yield 40, 000 (HB+RGB+AGB) stars over the whole ACS/WFC array, an easily manageable total. Our selected field is at a projected distance about 34' from the center of NGC 5128 near the southwestern major axis. This is just beyond the previous 31-kpc field and far away from any of the optical shells, plumes, etc. that are found at various locations farther in. Exposures: The red horizontal branch stars will be centered near (I=28, (V-I) = 1.0) with half- magnitude spread in both magnitude and color. Our goal will be to reach S/N > 5 at I=28.5, half a magnitude fainter than the nominal lower edge of the red HB. Since these are intermediate-color stars, the optimum filter combination is F606W (wide V) and F814W (I); exposures in the B filter would be prohibitively longer than either of these. These limiting magnitudes are feasible, but we will need the combination of full-orbit exposures in the darkest-sky conditions to reach them efficiently. Adopting 2400 sec exposures in Earth shadow, we estimate that a 12*2400 sec sequence in each filter (F606W, F814W) will give S/N=10 at I=28.0 and S/N=5 at I=28.5. These limits will definitively resolve the RHB population. If there is also a significant old, metal-poor blue HB population, it will show up as an extension at constant V~29. Stars near the blue side of the RR Lyrae gap should have S/N=8 in V and S/N=5 in I and thus will be visible if they are present. In summary, the proposed sequence of exposures will give us thorough coverage of the horizontal branch, as well as extremely precise photometry for the upper RGB and AGB. Parallel Imaging: HRC auto-parallel observations will be taken in a nearby part of the halo to obtain V and NUV data (F555W, F250W). These auto-parallel observations are not essential to the program, but would add useful extra parameters of the stellar populations in the halo of NGC 5128. This pair of filters will be sensitive to any very young population, and to the bright progeny of old stars in advanced, hot evolutionary phases (Post AGB, AGB Manque). Real Time Justification ----------------------- In order to be able to resolve the red horizontal branch stars in the nearest giant elliptical NGC 5128 and achieve our scientific goal of distinguishing the age and metallicity distribution of the stars in the halo, we need to reach S/N>5 at I=28.5. This limiting magnitude is feasible according to the ACS exposure time calculator, but the combination of full-orbit exposures in the darkest sky conditions is necessary to resolve the faint stars. Brighter sky makes the number of necessary orbits prohibitively larger. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5128-F0 GALAXY, ELLIPTICAL RA=13H 25M 15.10S +/- 0.1S, DEC= J2000 I = 24.1+/-0.1 -43D 34' 30.0" +/- 0.1" SURF(B) = 27.5+/-0.5 Coordinate Source: OTHER_SOURCE 9373( 2) - 02/04/02 10:56 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 5] Visit: 02 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 6] Visit: 03 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 7] Visit: 04 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 8] Visit: 05 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 9] Visit: 06 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 10] Visit: 07 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F606W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 11] Visit: 08 Visit Priority: Visit Requirements: ORIENT 97D TO 99D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2520 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5128-F0 ACS/WF ACCUM WFC F814W CR-SPLIT=NO,PAREXP=MULTIPLE 1 2600 S LOW-SKY C Comments: Auto-parallel observations with HRC requested. ------------------------------------------------------------------------------------------------------------------------------------ 9373( 2) - 02/04/02 10:56 - [ 12] Summary Form for Proposal 9373 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO PAREXP=MULTIPLE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 97D TO 99D LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5128-F0 ------------------------------------------------------------------------------------------------------------------------------------