8725( 2) - 11/30/00 07:29 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8725 Version: 2 Check-in Date: 16-Mar-2000 15:36:59 1.Proposal Title: The Formation Epoch(s) of Globular Clusters Around Ellipticals from Ultraviolet Photometry ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Stephen Zepf Yale University CoI: Dr. Sukyoung Yi Caltech N CoI: Prof. Pierre Demarque Yale University N CoI: Dr. Keith Ashman University of Kansas N ------------------------------------------------------------------------------------ 5. Abstract We propose to study the formation history of elliptical galaxies by obtaining far-UV photometry of their globular clusters. The far-UV photometry will be used in conjunction with existing optical data to constrain the ages of these clusters, and thus the formation epochs of their host galaxies. Optical data show that globular cluster systems of ellipticals often have bimodal metallicity distributions, indicating an episodic formation history. However, the ages of these formation epochs are uncertain. The age distribution is critical because it directly tests models of the formation of elliptical galaxies and their globular clusters. Specifically, merger models predict that the metal-rich clusters are formed in the merger that made the elliptical and are therefore younger than the metal-poor clusters that come from the halos of the progenitor spirals. Alternatively, if the observed bimodality comes from accreting metal-poor clusters onto an already formed elliptical, then the metal-rich clusters will generally be older. Moreover, age- dating the major formation epochs of elliptical galaxies provides a critical link between the local fossil record and the formation history of ellipticals inferred from deep data. The far-UV to optical color provides this important age information because the stars that make up the horizontal branch are hotter in older stellar populations, with significant evolution for otherwise difficult ages of t > 8 Gyr. ------------------------------------------------------------------------------------ 8725( 2) - 11/30/00 07:29 - [ 2] Observations Description ------------------------ We propose to obtain photometry with the WFPC2 and the F170W filter for globular clusters around four nearby ellipticals (M87, NGC 4472, NGC 4406, and NGC 3610). These are chosen to be well-studied in the optical, particularly with available F555W and F814W observations so F170W-V, V-I colors for the globulars can be readily determined. The galaxies are also chosen so that the cluster systems of two have clearly bimodal color distributions (M87 and NGC 4472), while the other two are not obviously bimodal in their globular cluster colors (NGC 4406 and NGC 3610). For the bimodal cases, the prediction of the merger model is clear - the red, metal-rich clusters should be younger than the blue metal-poor clusters. For NGC 4406 and NGC 3610, which have color distributions that appear to be unimodal, we will test the idea that this is because the metal-rich clusters are even younger in these cases, and therefore not yet as red as in M87 or NGC 4472 (e.g.\ Whitmore et al.\ 1997). For these two, the merger model predicts that the F170W-V, V-I diagram should given an even wider spread of ages than M87 or NGC 4472. We request 3 orbits of integration in F170W for each of the Virgo ellipticals (M87, NGC 4406, and NGC 4472), and 6 orbits for NGC 3610 which is more distant (~eq 25 Mpc) and has a somewhat less populous cluster system. This will provide S/N = 7 in F170W for old clusters with V=22 in the first three galaxies, and reach V=22.5 for NGC 3610. We intentionally focus on modest S/N and bright clusters because the predicted differences are large and model uncertainties would dominate for highly accurate photometry. Moreover, there are a significant number of clusters within this magnitude limit. Specifically, within one WFPC2 frame, at these magnitudes about 120 globulars are accessible in M87, about half of that in NGC 4406 and NGC 4472, and about 20 in NGC 3610. These sample sizes will minimize any stochastic effects (already modest in Galactic clusters), as will the focus on more luminous (massive) clusters with a greater number of stars. The sample sizes are particularly aimed at a allowing a clear determination of the spread in ages in the cluster samples and a test of correlations of age with metallicity. Real Time Justification ----------------------- Calibration Justification ------------------------- + Additional Comments ------------------- ------------------------------------------------------------------------------------ 8725( 2) - 11/30/00 07:29 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M87 galaxy, elliptical RA=12H 30M 49.4S +/- 0.1S, 2000 V = 22 DEC=+12D 23' 28.1" +/- 1.0" Coordinate Source: OTHER_SOURCE Comments: Flux is for target globular clusters, 2 NGC4472 galaxy, elliptical RA=12H 29M 46.7S +/- 0.1S, 2000 V = 22 DEC=+08D 00' 01.8" +/- 1.0" Coordinate Source: OTHER_SOURCE Comments: Flux is for target globular clusters 3 NGC4406 galaxy, elliptical RA=12H 26M 11.8S +/- 0.1S, 2000 V = 22 DEC=+12D 56' 46.1" +/- 1.0" Coordinate Source: HST_IMAGE Comments: Flux is for target globular clusters 4 NGC3610 galaxy, elliptical RA = 11H 18M 25.2S +/- 0.1S, 2000 V = 22 DEC=+58D 47' 10.6" +/- 1.0" Coordinate Source: HST_IMAGE Comments: Flux is for target globular clusters 8725( 2) - 11/30/00 07:29 - [ 4] Visit: 01 Visit Priority: Visit Requirements: Orient 236.0D to 246.0D On Hold Comments: Additional Comments: Orient constraint comes from desire to match archival 555W and 814W data which has a V3 roll angle of 119 degrees Ideally, the observations would avoid times of strong dayglow emission in OI lines, although SHADOW is not required and would be too restrictive. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 M87 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2400 S ------------------------------------------------------------------------------------------------------------------------------------ 2 M87 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S POS TARG +0.25,+0.25 ------------------------------------------------------------------------------------------------------------------------------------ 3 M87 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S POS TARG +0.5,+0.5 ------------------------------------------------------------------------------------------------------------------------------------ 8725( 2) - 11/30/00 07:29 - [ 5] Visit: 02 Visit Priority: Visit Requirements: Orient 250.5D to 260.5D On Hold Comments: Additional Comments: Orient constraint comes from needing to match archival 555W and 814W data which has a V3 roll angle of 104.5 degrees Ideally, the observations would avoid times of strong dayglow emission in OI lines, although SHADOW is not required and would be too restrictive. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4472 WFPC2 IMAGE PC1 F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2400 S Comments: Aperture is intentionally PC1 to overlap more exactly with existing Cycle 4 F555W and F814W observations. ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4472 WFPC2 IMAGE PC1 F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S POS TARG +0.25,+0.25 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4472 WFPC2 IMAGE PC1 F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S POS TARG +0.5,+0.5 ------------------------------------------------------------------------------------------------------------------------------------ 8725( 2) - 11/30/00 07:29 - [ 6] Visit: 03 Visit Priority: Visit Requirements: Orient 228.6D to 238.6D On Hold Comments: Additional Comments: Orient constraint comes from needing to match archival 555W and 814W data which has a V3 roll angle of 126.4 degrees Ideally, the observations would avoid times of strong dayglow emission in OI lines, although SHADOW is not required and would be too restrictive. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4406 WFPC2 IMAGE PC1 F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2400 S Comments: Aperture is intentionally PC1 to overlap more exactly with existing Cycle 4 F555W and F814W observations ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4406 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S POS TARG +0.25,+0.25 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4406 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S POS TARG +0.5,+0.5 ------------------------------------------------------------------------------------------------------------------------------------ 8725( 2) - 11/30/00 07:29 - [ 7] Visit: 04 Visit Priority: Visit Requirements: Orient 225.8D to 235.8D On Hold Comments: Additional Comments: Orient constraint comes from needing to match archival 555W and 814W data which has a V3 roll angle of 129.2 degrees Ideally, the observations would avoid times of strong dayglow emission in OI lines, although SHADOW is not required and would be too restrictive. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3610 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC3610 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2800 S POS TARG +0.25,+0.25 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC3610 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2800 S POS TARG +0.5,+0.5 ------------------------------------------------------------------------------------------------------------------------------------ 8725( 2) - 11/30/00 07:29 - [ 8] Visit: 05 Visit Priority: Visit Requirements: Orient 225.8D to 235.8D On Hold Comments: Additional Comments: Orient constraint comes from needing to match archival 555W and 814W data which has a V3 roll angle of 129.2 degrees Ideally, the observations would avoid times of strong dayglow emission in OI lines, although SHADOW is not required and would be too restrictive. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3610 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2600 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC3610 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2800 S POS TARG +0.25,+0.25 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC3610 WFPC2 IMAGE PC1-FIX F170W CR-SPLIT=0.5,CR-TOLERANCE=0.0 1 2800 S POS TARG +0.5,+0.5 ------------------------------------------------------------------------------------------------------------------------------------ 8725( 2) - 11/30/00 07:29 - [ 9] Summary Form for Proposal 8725 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=0.5 CR-TOLERANCE=0.0 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements Orient 236.0D to 246.0D POS TARG +0.25,+0.25 POS TARG +0.5,+0.5 Orient 250.5D to 260.5D Orient 228.6D to 238.6D Orient 225.8D to 235.8D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F170W ------------------------------------------------------------------------------------------------------------------------------------ Target Names M87 NGC4472 NGC4406 NGC3610 ------------------------------------------------------------------------------------------------------------------------------------