6730( 8) - 12/22/98 16:17 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6730 Version: 8 Check-in Date: 22-Dec-1998 16:16:44 1.Proposal Title: Halos of Nearby Edge-on Spirals ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 6 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Heather Morrison Case Western Reserve University CoI: Paul Harding Steward Observatory, University of Arizona N ------------------------------------------------------------------------------------ 5. Abstract HST allows the detection of the tip of the halo field star giant branch in galaxies at distances of order 10 Mpc. Used in conjunction with recently developed techniques of deep CCD surface photometry, this will allow us to extend the (currently very small) sample of well-studied spiral galaxy halos and to begin to determine whether the Milky Way halo is typical. We are proposing to take deep WFPC2 exposures in F814W of a halo field in the nearby edge-on spiral NGC 5907. We have deep CCD surface photometry (to R and I ~ 27 mag/sq arcsec) of the galaxy, and propose to use deep WFPC2 images to check that the extended halos seen in the surface photometry originate from halo starlight and not from various phases of the ISM. In addition, these images will allow the identification of halo globular clusters and the measurement of an accurate distance via a globular cluster luminosity function and red giant tip luminosity. ------------------------------------------------------------------------------------ 6730( 8) - 12/22/98 16:17 - [ 2] Observations Description ------------------------ Exposure time calculations: The M31 halo luminosity distribution starts to rise above the foreground/background contaminants at M_V=-2.6 (PvdB). At a distance modulus of 30, this corresponds to V=27.4. We have calculated exposure times to obtain S/N of 10 at V = 28, 0.6 magnitudes below the giant branch tip. Filter F814W was chosen to combine the merits of efficient observing on these red stars (V-I ~ 1.8) and a well-established photometric calibration to simplify comparisons with other work, both ground-based and HST. We intend to dither the exposures by fractional pixel amounts to partially compensate for the WF CCD undersampling. Unless we have good S/N, the density gradient measurements will be completely dominated by completeness corrections. Therefore we have calculated exposure times for a S/N of 10 at V=28. Using the WFPC2 instrument manual, we find that since NGC 5907 is in the CVZ, a S/N of 6.6 is reached at V=28 in one orbit (5100s). Thus 3 orbits give S/N of 10. These exposure time estimates have been checked using 4 2200 sec WFPC2 exposures obtained for the H_0 Key Project on NGC 925, a galaxy at a similar distance to our objects. The tip of the giant branch is clearly visible in these data, despite the fact that the NGC 925 giants are seen against a higher background from the integrated light of the galaxy's disk. Foreground/Background Contamination: Foreground contamination due to faint stars of the Milky Way is not a problem at these magnitudes. A version of the Bahcall-Soniera Model (Bahcall and Soniera 1984) modified to include a thick disk was used to estimate that there should be of order 10 halo dwarfs and 4 thick disk dwarfs per WFPC2 field. Contamination due to faint unresolved background galaxies is potentially a more serious problem. Extrapolating the deep MDS galaxy counts of Driver et al. 1995, we estimate that there should be ~1400 galaxies with V<28 on a WFPC2 field. Most but not all of these galaxies will be resolved. We intend to obtain several deep MDS fields from the archive when these data become public in order to obtain better estimates of contamination due to faint unresolved galaxies and to measure its possible variation due to large-scale structure. Use of the MDS archive exposures will save many orbits of telescope time and allow a better measure of possible statistical fluctuations in the background due to large-scale structure. Data Reductions: Both investigators have considerable expertise in the use of DoPHOT for large sets of crowded field data, and Harding is experienced in the use of DoPHOT on HST (H_0 Key Program) data. At the faint magnitudes we plan to work, and working in fields where we expect stellar density gradients, good estimates of completeness will be critical. We plan to use ``fake star'' experiments to obtain estimates of photometric completeness as a function of distance along the minor axis and of stellar magnitude. We will use the better-sampled PC exposures to verify that we understand the effects of sample incompleteness and crowding due to marginally detected stars lower on the giant branch. We also plan to mask out extended sources in the images to ensure more uniform detection levels across the WFPC2 CCDs. Otherwise, stars superimposed on background galaxies will be harder to detect, and DoPHOT will detect some galaxies as collections of spurious stars. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6730( 8) - 12/22/98 16:17 - [ 3] Data Distribution Paper Products: Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: DITHER-LINE-STEPS=3 DITHER-LINE-SPACING=0.707 ------------------------------------------------------------------------------------ 6730( 8) - 12/22/98 16:17 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5907-HALO GALAXY, SPIRAL, EXT- RA=15H 16M 06.S +/- 0.04S, J2000 V=+667 V = 28 STAR, K III-I, HALO DEC=+56D 20' 32.00" +/- 1.0", B-V = 1.0 PLATE-ID = 01RT SURF-BKG(I) = 25.0+/-2.0 Comments: The brightest giants in NGC 5907's halo should be just visible on a single CR split exposure. 6730( 8) - 12/22/98 16:17 - [ 5] Visit: 01 Visit Priority: Visit Requirements: CVZ 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 NGC5907-HAL WFPC2 IMAGE WFALL-F F606W 1 5200 S O IX ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC5907-HAL WFPC2 IMAGE WFALL-F F606W 1 5200 S pos targ 0.5, 0.5 O IX ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC5907-HAL WFPC2 IMAGE WFALL-F F606W 1 5100 S pos targ 1.0, 1.0 O IX ------------------------------------------------------------------------------------------------------------------------------------ 6730( 8) - 12/22/98 16:17 - [ 6] Summary Form for Proposal 6730 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFALL-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ pos targ 0.5, 0.5 pos targ 1.0, 1.0 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5907-HALO ------------------------------------------------------------------------------------------------------------------------------------