8778( 3) - 03/03/00 10:53 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8778 Version: 3 Check-in Date: 03-Mar-2000 10:52:13 1.Proposal Title: Proper Motion of the Sagittarius Dwarf Galaxy-Cycle 9 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Rodrigo Ibata European Southern Observatory - Germany CoI: Gerard Gilmore Institute of Astronomy, Cambridge N CoI: Michael Irwin Royal Greenwich Observatory N CoI: Geraint Lewis State University of New York at Stony N Brook CoI: Nicholas Suntzeff Cerro Tololo InterAmerican Observatory N CoI: Rosie Wyse Johns Hopkins N ------------------------------------------------------------------------------------ 5. Abstract We propose to use the unique spatial resolution of the HST to determine the proper motion of the Sagittarius dwarf galaxy (Sgr). Three fields, previously imaged in May -- June 1996, will be imaged again in cycle 7 and late in cycle 9. This will yield a baseline of ~ 3-4 years from which the proper motion of Sgr will be measured to an accuracy of ~ 35 km/s. The proper motion information coupled with our extensive ground- based radial velocity and heliocentric distance data, will permit an accurate determination of the orbit of the dwarf galaxy. This crucial new information will allow us to study the interaction between Sgr and the Milky Way in detail. Among the scientific issues which depend on an accurate orbit for their analysis, we will test the possibility that Sgr generates the warp in the Milky Way disk; we will quantify the effect on the star formation and chemical evolution of both Sgr and the outer Milky Way disk of the successive passages of Sgr through the Galactic disk potential; and we will limit the flattening of the Galactic dark halo. ------------------------------------------------------------------------------------ 8778( 3) - 03/03/00 10:53 - [ 2] Observations Description ------------------------ We will image three fields in the Sagittarius dwarf galaxy that have been previously observed (May 5th, 1996; June 16th, 1996; June 25th, 1996) as part of proposal 6614 (please note that these fields were actually executed in cycle 5). The expected transverse velocity of Sgr is ~ 300 km/s (determined from fits to the radial velocity data, see above) and its heliocentric distance is 24 +/- 1 kpc, thus its expected proper motion is 2.7 mas/yr. This movement will be observed with respect to background galaxies. We have performed an experiment to determine the centering accuracy that we are likely to achieve. We have taken four pairs of ``dithered'' HST images in F555W of an uncrowded field in the globular cluster M4. The pairs were chosen to have less than 1 pixel offsets. The centroids of the stars in each frame were found using a slightly modified version of the finding algorithm in the DAOPHOT package. The mean x and y offsets between frames were calculated using unsaturated stars with photometric errors less than 0.05 mag (typically 400 stars); an iterative sigma -rejection scheme was used to remove very deviant points. The matching accuracy, determined by the rms scatter about the mean offset, was found to be better than 0.009 pixels. We repeated this experiment using the coordinates of objects which, judging by a ``roundness'' and ``peakiness'' parameter (Stetson 1987), had been found to be non-stellar. Typically 100 such objects were found (we performed a visual check of the image profiles to verify that the sample was indeed made up predominantly of galaxies), a number which should match that in our data. The mean offsets determined from the non-stellar samples agreed with those determined from the stellar samples to better than 0.017 pixels. Data in three epochs are required. The first epoch data (F555W and F814W) were taken in May -- June 1996, as part of proposal 6614; those data will allow easy discrimination between galactic and Sgr stars. We request imaging data in F814W at two extra epochs, the first of these should be taken during cycle 7, and the second during cycle 9, just before WFPC2 is decommisioned, yielding a ~ 3 year baseline. Since the PC chip has a scale of 45 mas/pixel, and we expect to be able to centroid the frames to ~ 0.02 pixels, the proposed observations will yield ~ 35 km/s resolution at the distance of Sgr. This accuracy is ideal for the problems we wish to solve. The pixels on a CCD are not perfectly aligned on a square grid, and also the WFPC2 camera introduces some optical distortions over its field of view. To maximize the accuracy of the experiment, it is therefore important that the second and third epoch exposures are taken with as close to an identical pointing to the first epoch exposure as is possible. We request a total of 6 orbits: one orbit, per epoch, in each of the three Sgr fields which were exposed in May -- June 1996. Data in three fields is needed since there will be relatively few Sgr stars on the PC chip (from which the best proper motion constraints will be obtained). The S/N of the second and third epoch exposures will then be marginally better than the first epoch observations (S/N~ 15 for I=25). We additionally request parallel data with NICMOS (all 3 cameras) and STIS (CCD, F25x50LP). We will use NICMOS to acquire JHK data to improve determination of the age and metallicity distribution near the turnoff in Sgr, and STIS to improve luminosity function statistics. We note that these will be relatively shallow observations, useful only near and above turnoff, and so do not compete with other projects studying the population structure in Sgr. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8778( 3) - 03/03/00 10:53 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SGR-DSPH1 GALAXY, DWARF RA=18H 55M 3.30S +/- 0.01S, DEC= J2000 V = 17 SPHEROIDAL -30D 16' 42.27" +/- 0.1" B-V = 0.6 Comments: DO NOT USE THE GLOBULAR CLUSTER M54 AS A GUIDE STAR. 2 SGR-DSPH2 GALAXY, DWARF RA=18H 55M 3.30S +/- 0.01S, DEC= J2000 V = 17 SPHEROIDAL -30D 14' 42.27" +/- 0.1" B-V = 0.6 Comments: DO NOT USE THE GLOBULAR CLUSTER M54 AS A GUIDE STAR. 3 SGR-DSPH4 GALAXY, DWARF RA=19H 6M 11.00S +/- 0.01S, DEC= J2000 V = 17 SPHEROIDAL -30D 23' 22.27" +/- 0.1" B-V = 0.6 8778( 3) - 03/03/00 10:53 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 269.40579D TO 269.40579D On Hold Comments: Additional Comments: The orientation of the images in this visit MUST match the first epoch images (SGR-DSPH1 images of program 6614) as closely as possible. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 11 SGR-DSPH1 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.000, +0.000 ------------------------------------------------------------------------------------------------------------------------------------ 12 SGR-DSPH1 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.224, -0.124 ------------------------------------------------------------------------------------------------------------------------------------ 13 SGR-DSPH1 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.350, +0.352 ------------------------------------------------------------------------------------------------------------------------------------ 15 SGR-DSPH1 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 500 S POS TARG +0.169, +0.231 ------------------------------------------------------------------------------------------------------------------------------------ 8778( 3) - 03/03/00 10:53 - [ 5] Visit: 02 Visit Priority: Visit Requirements: ORIENT 288.4091D TO 288.4091D On Hold Comments: Additional Comments: The orientation of the images in this visit MUST match the first epoch images (SGR-DSPH2 images of program 6614) as closely as possible. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 SGR-DSPH2 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.000, +0.000 ------------------------------------------------------------------------------------------------------------------------------------ 22 SGR-DSPH2 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.224, -0.124 ------------------------------------------------------------------------------------------------------------------------------------ 23 SGR-DSPH2 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.350, +0.352 ------------------------------------------------------------------------------------------------------------------------------------ 25 SGR-DSPH2 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 500 S POS TARG +0.169, +0.231 ------------------------------------------------------------------------------------------------------------------------------------ 8778( 3) - 03/03/00 10:53 - [ 6] Visit: 03 Visit Priority: Visit Requirements: ORIENT 89.9917D TO 89.9917D On Hold Comments: Additional Comments: The orientation of the images in this visit MUST match the first epoch images (SGR-DSPH4 images of program 6614) as closely as possible. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 31 SGR-DSPH4 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.000, +0.000 ------------------------------------------------------------------------------------------------------------------------------------ 32 SGR-DSPH4 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.224, -0.124 ------------------------------------------------------------------------------------------------------------------------------------ 33 SGR-DSPH4 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 400 S POS TARG +0.350, +0.352 ------------------------------------------------------------------------------------------------------------------------------------ 35 SGR-DSPH4 WFPC2 IMAGE PC1-FIX F814W CLOCKS=NO,ATD-GAIN=15,CR-SPLIT=NO 1 500 S POS TARG +0.169, +0.231 ------------------------------------------------------------------------------------------------------------------------------------ 8778( 3) - 03/03/00 10:53 - [ 7] Summary Form for Proposal 8778 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CLOCKS=NO ATD-GAIN=15 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 269.40579D TO 269.40579D POS TARG +0.000, +0.000 POS TARG +0.224, -0.124 POS TARG +0.350, +0.352 POS TARG +0.169, +0.231 ORIENT 288.4091D TO 288.4091D ORIENT 89.9917D TO 89.9917D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names SGR-DSPH1 SGR-DSPH2 SGR-DSPH4 ------------------------------------------------------------------------------------------------------------------------------------