Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 2370c Report Date: 09-May-96:17:04 Version: ********** Check-in Date: ********** 1.Proposal Title: GLOBULAR CLUSTER SYSTEMS IN THE COMA SUPERGIANT ELLIPTICALS ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID GALAXIES & CLUSTERS GO Sub Category DISTANCE SCALE ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Dr. William E. Harris MCMASTER UNIVERSITY CANADA 416-525-9140 PROFESSOR ------------------------------------------------------------------------------------ 7. Abstract We propose to use the Wide Field Camera in the V and I bands for imaging of the globular cluster systems around NGC 4874 and NGC 4889, the two central supergiant ellipticals in the Coma Cluster.Photometry of the globular clusters around these galaxies will accomplish three principal scientific goals: 1) We will obtain the globular cluster luminosity function (number of clusters per unit magnitude) to approximately Mv~-6.6, fully one magnitude fainter than LF peak frequency or "turnover." Fitting the LFs directly to those from the Local Group galaxies and the giant Virgo ellipticals in a single step will then yield a pure Population II distance calibration for Coma with an internal uncertainty of + 1-0.2 mag and a clean determination of HO, completely independent of the entire chain of Population I standard candles.2)We will obtain the total cluster populations (specific frequencies) in each galaxy, thus sampling the formation efficiency of globular clusters in a much richer and denser galaxy environment than any accessible from ground-based imaging.3) We will obtain the radial distribution and color (metallicity) gradient within each cluster system. By comparing these with the same quantities for the underlying halo light of the galaxies, we will determine how distinct these two components of the halo are in their structure and chemical enrichment history. ------------------------------------------------------------------------------------ 8. Scientific Key Words: GLOBULAR CLUSTERS, ELLIPTICAL GALAXIES, HALO, DISTANCE SCALE ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 11.60 par: 0 10. Num targs pri: 2 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: WF/PC ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 2370c PI: Dr. William E. Harris Proposal Title: GLOBULAR CLUSTER SYSTEMS IN THE COMA SUPERGIANT ELLIPTICALS ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ P.I. William E. Harris MCMASTER UNIVERSITY CANADA David A. Hanes QUEEN'S UNIVERSITY CANADA James E. Hesser DOMINION ASTROPHYSICAL CANADA OBSERVATORY Christopher J. Pritchet VICTORIA, UNIVERSITY OF CANADA Hugh C. Harris US NAVAL OBSERVATORY USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. For each of the primary targets NGC 4874 and 4889, we have identified one WFC field which places the galaxy center toward one edge of the frame and avoids any bright stars that would saturate in 2500 sec. Our primary filter for the deep GCLF study and the radial distribution will be 555W = V, which gives the best compromise for photometric S/N, reproducibility of a standard magnitude system, and contrast between the globular clusters and the underlying (redder) galaxy light. Our second filter for color index measurement is 791W = I to maximize the sensitivity to GC metallicity variations at a given S/N, and will allow strict comparability with GTO observations of other globular cluster systems. We choose the WFC for the following reasons: (i) A 2.6-arcmin field is large enough that the outer parts of it will provide its own background, thus allowing the radial structure and composition gradients in the GCSs to be studied on a single frame. For each of the two fields (NGC 4874 and 4889), the galaxy center will be positioned such that the farthest corners of the field will be 2.4 arcmin from galaxy center, or 70 kpc in linear scale. At this distance, the projected number density of globular clusters is 30 times lower than in the central regions, so that the radial distribution of the GCS can be derived for the most important (r < 30 kpc) region. (ii) Even with the WFC, image crowding will not be important for either galaxy even in the inner regions. Since Coma is 5.6 times more distant than Virgo, the projected number density of globular clusters around the Coma galaxies will be 30 times higher for the same limiting absolute magnitude. Scaling from the GCSs around the Virgo giants M87 and M49, we find that the maximum surface density of images around NGC 4874 will be sigma(r) = 1.5--2.0 images/arcsec**2 brighter than V=29 for r < 7" from galaxy center (in these giant ellipticals, sigma(r) does not increase all the way in to the center but rather is observed to level off inwards of r=3 kpc. These estimates apply if NGC 4874 has an anomalously populous GCS like M87; if the system is more normal, then the density will be 2--3 times lower. Even the extreme upper limit corresponds to 50 pixels/image, and since the FWHM for starlike images is 1 pixel, the frames will be relatively 'uncrowded' even fairly close to galaxy center. ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. (No relevant Phase II information in this item) Page 3 ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. The V data will be primarily for the deep GCLF and the radial distribution, while the I band will be used to obtain color indices (hence metallicities) for the brighter clusters. Our exposure times in V and I are calculated on the basis of the following: Given (S/N) = (r t) / [(r t) + n(eff) (s t + R**2)]**0.5 where r = count rate from star image (e-/sec) and s = count rate from background (e- /sec/pixel) and t = integration time (sec), R = read noise and n(eff) = no. of pixels per image (3.2 in V, 5.0 in I). Our goal is S/N of about 10 in V at the turnover level of the GCLF, which is expected to be at V=27.7 and (V-I) =1.0. The parameters listed above then yield S/N = 5.0 (V) in 2500sec, 3 (I) in 2400 sec for globular cluster images just at the turnover, or S/N = 11 (V) in 5 x 2500 sec and S/N = 4.3 (I) in 3 x 2400 sec. The I-band data will effectively reach about one magnitude shallower than V, but our intention is to use them primarily for the color-distribution program and therefore they need not be ultra-deep. For clusters brighter than V=27 (I=26, or <1 mag above the turnover), the measured V-I indices will have internal uncertainties of at most 0.10 mag, which is sufficient to trace out the intrinsic color distribution and metallicity gradient within the cluster system (the range between the metal-poorest and metal -richest globular clusters is delta(V-I)=0.5 mag). The effective useful limit of the photometry will be V=29, more than one magnitude past the turnover. Note that in practice, the actual S/N will vary slightly across the frame, since the background changes continuously with radius from the galaxy center and its contribution dominates over the readnoise in most of the field. The numbers here assume an `average' radial location s(V)=22 mag/arcsec**2, which corresponds to 3 -4 times dark sky. For NGC 4874 we request exposure sequences of (6 x 2500) sec (V) and (3 x 2400) sec (I). For NGC 4889 we request exposure sequences of (5 x 2500) sec (V) and (3 x 2400) sec (I), for a grand total of 11.6 hours of exposure, plus overhead spacecraft time (quite small in this case). Our maximum TAC-assigned spacecraft time is 13.3 hours. ------------------------------------------------------------------------------------ 6. Description of special calibration exposures. No special requirements for scheduling or calibration ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. Final absolute calibration of the pseudo-V,I data will be carried out by ground- based CCD imaging of the program fields, through copies of the HST broadband filters that we will have made for the purpose. The sequence of deep V exposures will allow us to filter out cosmic-ray events and to correctly identify all true starlike images on the frames (the catalog of images detected on the V frames can also be applied to the pair of I frames as a crosscheck). The subsequent photometry (including sample incompleteness and background noise as a function of location on the frame; characterizing stellar and nonstellar images) will require only the application of the codes that we already have in place and their modifications to apply specifically to the WFC CCDs. ------------------------------------------------------------------------------------ 8. Additional comments or special requests. No additional requests ------------------------------------------------------------------------------------ 9. Description of previous HST work. No previous HST time ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). Our home institutions will supply all necessary computing facilities and reduction software (IRAF, DAOPHOT and their updates, on Sun and VAX machines). ------------------------------------------------------------------------------------ 11. Address Information Page 4 Name: WILLIAM E. HARRIS Category: PI Institution: Address: PHYSICS DEPARTMENT MCMASTER UNIVERSITY City: HAMILTON L8S 4M1 State: ON Zip Code: Country: CANADA Telephone: Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 2370c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4874- GALAXY; ELLIPTICAL RA = 12H 59M 36.4S +/- 2.0", J2000 V=+7184 1 V = 27.7 +/- 0.1 NORTH GALAXY DEC = +27D 58' 20" +/- 2.0" 2 SURF-BKG(V)=22.0 +/- 2.0 Comments: V,I PHOTOMETRY OF GLOBULAR CLUSTERS IN HALO OF E GALAXY ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4889-WEST GALAXY; ELLIPTICAL RA = 13H 00M 4.1S +/- 2.0", J2000 V=+6512 1 V = 27.7 +/- 0.1 GALAXY DEC = +27D 58' 20" +/- 2.0" 2 SURF-BKG(V)=22.0 +/- 2.0 Comments: V,I PHOTOMETRY OF GLOBULAR CLUSTERS IN HALO OF E GALAXY ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 2370c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4874- WFC IMAGE ALL F555W CLOCKS=YES 6 2500S 5 1 1 DARK TIME NORTH 2 GUIDE TOL 0.020" POS TARG 0.0, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 ^ ^ ^ ^ F791W CLOCKS=YES 3 2400S 3 1 3 DARK TIME 2 GUIDE TOL 0.020" SAME POS FOR 2 AS 1 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4889- ^ ^ ^ F555W CLOCKS=YES 5 2500S 5 1 2 DARK TIME WEST 2 GUIDE TOL 0.020" POS TARG 0.0, 0.0 ORIENT 45D +/- 20D SEQ 3-4 NO GAP Comments: X-AXIS OFFSET OF 45D ASSUMED ANY ORIENTATION +/- 90D OR +/- 180D FROM SPECIFIED VALUE IS ACCEPTABLE ------------------------------------------------------------------------------------------------------------------------------------ 4 ^ ^ ^ ^ F791W PURGE=YES 3 2400S 3 1 3 DARK TIME CLOCKS=YES 2 GUIDE TOL 0.020" SAME POS FOR 4 AS 3 SAME ORIENT FOR 4 AS 3 ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 2370c [ 7] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC ^ ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ^ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CLOCKS=YES PURGE=YES ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 11.60 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category DISTANCE SCALE ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements DARK TIME; GUIDE TOL 0.020"; POS TARG 0.0, 0.0 SAME POS FOR 2 AS 1 POS TARG 0.0, 0.0; ORIENT 45D +/- 20D; SEQ 3-4 NO GAP SAME POS FOR 4 AS 3; SAME ORIENT FOR 4 AS 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F791W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC4874-NORTH NGC4889-WEST ------------------------------------------------------------------------------------------------------------------------------------