6820( 6) - 02/05/97 14:32 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6820 Version: 6 Check-in Date: 05-Feb-1997 14:28:49 1.Proposal Title: A Very Deep Luminosity Function for the Galactic Bulge ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle STELLAR POPULATIONS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: ALVIO RENZINI ESO CoI: R. Michael Rich Columbia University N CoI: Sergio Ortolani Universita di Padova N CoI: Laura Greggio Universita di Bologna N CoI: Kenneth J. Mighell Columbia University N ------------------------------------------------------------------------------------ 6. Abstract We propose to obtain the deepest possible luminosity function in the Galactic bulge. This is the only old, metal rich stellar population that we can study down to M_I~eq +12, unaffected by the dynamical processes that are known to occur in globular clusters. The initial mass function (IMF) of a metal rich stellar population is of fundamental importance in modelling the formation, evolution, and integrated light of galaxies. The discovery of large numbers of microlensing events toward the bulge has greatly increased the need to define the bulge luminosity function to as faint a limit as possible, in order to constrain whether the high microlensing rate can be attributed to normal stars, instead of dark matter or brown dwarfs. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will image our bulge field for 5 orbits in both the R and I passband. Our goal is to solidly reach I=27. Care will be taken to do the observations in an optimal fashion (i.e. changing the pointing by a few pixels in between exposures). The WFPC2 exposure time calculator on WWW indicates that for a 13,000s exposure (about what one gets in 5 orbits) we reach I=27 at S/N = 5.1. The faint red end of the main sequence is 1.5 mag fainter in R; at R=28.5 we reach S/N=3.2. Expected color errors of 0.3 mag are large, but we should be able to clearly distinguish the main sequence at the faint end. For A_I=1 mag and (m-M)_0=14.5 (8kpc), then M_I=+12 at I=26.5; this gives us a good chance of reaching 0.1 M\\odot, depending on the adopted mass-luminosity relation (e.g. Burrows et al. 1993, ApJ, 406, 158). Extragalactic sources must be carefully considered; we will use color and shape criteria to remove suspicious objects, and also rely on faint galaxy counts (e.g. the Keck counts of Smail et al. 1995 ApJ 449, L105) as a check on possible galaxy contamination. With careful modelling, we should be able to get the last bin of the LF to within a factor of two. The encouraging point is that at I=26, S/N=13.9; errors will be less than the intrinsic width of the bulge main sequence. Given the width of features such as a slope or faint-end downturn, we expect this result to be quite unaffected by such factors as the 0.5 mag depth dispersion of the bulge, or the abundance spread (which is an intrinsic property of the population anyway). Contamination of the sample from the foreground disk main sequence will be addressed using 2 approaches. First, the ridgeline of the bulge main sequence should be well defined, and we will make every effort to purify the sample on the basis of color. But this may be difficult at the faint end. Our second approach is to take advantage of the rapidly growing knowledge of the faint end of the disk luminosity function from other HST observations. (e.g. Bahcall et al. 1994, ApJ 435, L51; Medium Deep Survey team project) and to statistically correct for the presence of the faint foreground main sequence. Note that our solid angle subtends a much larger physical volume (and samples a population of much higher density) in the bulge than it does in the disk, so we do not expect this correction to affect our measurements. Further, since the sight line rapidly drops out of the disk, we are confident that the Solar neighborhood disk lumninosity function will be useful for this correction. We will also reduce archival WFPC2 images of other bulge fields, which can give us additional insight on the bright end of the disk luminosity function. To summarize, we feel that this difficulty can be overcome in a very straightforward way and that its effect can be well quantified. Note that two teams (Westphal and Trauger) have obtained deep 2000s exposures in V and I of two Galactic bulge fields, Baade's Window and Sgr I. These fields are not only more crowded but also suffer more extinction, as they lie closer to the Galactic center than our proposed field. Since the fields are more reddened and they are imaged in V and I, the red colors of the faintest stars will truncate the detected faint end further. Parallel FOC Observations: while not absolutely required for the science, we believe that deep parallel observations should be obtained if it does not adversely affect the primary WFPC2 program. U,B,V exposures with FOC/96 will further constrain the reddening and will provide an alternate measurement of the brighter IMF with a different instrument. Real Time Justification ----------------------- We have obtained the deepest possible ground-based frames of bulge fields at b= -6 and -8 using the ESO NTT. These CCD images are in U, B, V, and I. We will use these to (1) improve the calibration and (2) as input for simulated fields. At b=-6 in 0.4 arcsec seeing, one can reach only 1 mag below the turnoff in the bulge fields. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: R. Michael Rich*538 W. 120th St.*NY NY 10027 Ship Via: UPS Email: arenzini@eso.org , ------------------------------------------------------------------------------------ 6820( 6) - 02/05/97 14:32 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6558-FLD1 STELLAR CLUSTER, RA=18H 11M 05.0S +/- 0.2S, DEC=- 2000 V = 28.0 GLOBULAR CLUSTER 31D 45' 49.0" +/- 0.5" B-V = 2.0 SURF(V) = 19.0 E(B-V) = 0.3+/-0.1 Comments: Galactic Bulge Field Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 6820( 6) - 02/05/97 14:32 - [ 2] Visit: 01 Visit Requirements: ORIENTation 83D TO 83D 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 NGC6558-FLD WFPC2 IMAGE PC1-FIX F606W CLOCKS=NO,ATD-GAIN=7 1 2000 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC6558-FLD FOC/96 IMAGE 512X512 F342W 1 2400 S PAR WITH 10 1 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=7 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC6558-FLD FOC/96 IMAGE 512X512 F430W 1 2500 S PAR WITH 20 1 ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6558-FLD WFPC2 IMAGE PC1-FIX F606W CLOCKS=NO,ATD-GAIN=7 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC6558-FLD FOC/96 IMAGE 512X512 F480LP 1 2500 S PAR WITH 30 1 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=7 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 42 NGC6558-FLD FOC/96 IMAGE 512X512 F342W 1 2500 S PAR WITH 40 1 ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC6558-FLD WFPC2 IMAGE PC1-FIX F606W CLOCKS=NO,ATD-GAIN=7 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 52 NGC6558-FLD FOC/96 IMAGE 512X512 F430W 1 2500 S PAR WITH 50 1 ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=7 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 62 NGC6558-FLD FOC/96 IMAGE 512X512 F480LP 1 2500 S PAR WITH 60 1 ------------------------------------------------------------------------------------------------------------------------------------ 6820( 6) - 02/05/97 14:32 - [ 3] Visit: 02 Visit Requirements: ORIENTation 83D TO 83D On Hold Comments: Additional Comments: All exposures with same orientation, both visits. Second visit due to SAA. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=7,CR-SPLIT=0.5 1 2000 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 72 NGC6558-FLD FOC/96 IMAGE 512X512 F342W 1 2400 S PAR WITH 70 1 ------------------------------------------------------------------------------------------------------------------------------------ 80 NGC6558-FLD WFPC2 IMAGE PC1-FIX F606W CLOCKS=NO,ATD-GAIN=7,CR-SPLIT=0.5 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 82 NGC6558-FLD FOC/96 IMAGE 512X512 F430W 1 2500 S PAR WITH 80 1 ------------------------------------------------------------------------------------------------------------------------------------ 90 NGC6558-FLD WFPC2 IMAGE PC1-FIX F606W CLOCKS=NO,ATD-GAIN=7,CR-SPLIT=0.5 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 92 NGC6558-FLD FOC/96 IMAGE 512X512 F480LP 1 2500 S PAR WITH 90 1 ------------------------------------------------------------------------------------------------------------------------------------ 100 NGC6558-FLD WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=7,CR-SPLIT=0.5 1 2200 S LOW-SKY 1 ------------------------------------------------------------------------------------------------------------------------------------ 102 NGC6558-FLD FOC/96 IMAGE 512X512 F480LP 1 2500 S PAR WITH 100 1 ------------------------------------------------------------------------------------------------------------------------------------ 6820( 6) - 02/05/97 14:32 - [ 4] Summary Form for Proposal 6820 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 FOC/96 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CLOCKS=NO ATD-GAIN=7 CR-SPLIT=0.5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENTation 83D TO 83D LOW-SKY PAR WITH 10 PAR WITH 20 PAR WITH 30 PAR WITH 40 PAR WITH 50 PAR WITH 60 PAR WITH 70 PAR WITH 80 PAR WITH 90 PAR WITH 100 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F342W F814W F430W F480LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6558-FLD1 ------------------------------------------------------------------------------------------------------------------------------------