The Proper Motion Field along the Magellanic Bridge: a New Probe of the LMC-SMC interaction

0 Assigned ID: 861 ----- Attempting Submission 1 (Fri Apr 11 21:25:21 GMT 2014) ----- HST Phase I Proposal 861 (MCBridge1.apt) successfully submitted. ----- Attempting Submission 2 (Fri Apr 11 23:46:14 GMT 2014) ----- This is a resubmission for setting ORIENT values of visits 05 and 06 to ORIENT=289.70. 2099035690 ----- Attempting Submission 1 (Wed Jul 23 17:00:04 GMT 2014) ----- ----- Attempting Submission 2 (Mon Dec 01 14:37:33 GMT 2014) ----- ----- Attempting Submission 3 (Wed Feb 18 21:13:49 GMT 2015) ----- Repeat of visit 07 The Proper Motion Field along the Magellanic Bridge: a New Probe of the LMC-SMC interaction Our HST proper motion (PM) measurements of the LMC and SMC have revolutionized our understanding of the Magellanic System, and have spurred new research on its use as a cosmological probe of galaxy formation. The PMs imply that the Magellanic Clouds are likely on their first infall towards the Milky Way (MW). The disturbed nature of the Magellanic System is therefore likely due to the LMC-SMC interaction, and not to the MW influence. This has emphasized the importance of dwarf galaxy interactions for galaxy evolution. The Clouds are connected by a complex of gas and stars called the Magellanic Bridge. We propose to map the stellar PM field of the Bridge, similar to our prior HST mapping of the LMC PM rotation field. Our state-of-the-art N-body simulations show that the PM field will tightly constrain the impact parameter of LMC-SMC orbit at its last pericenter 100-300 Myr ago, which is the main uncertainty in our understanding of the LMC/SMC interaction history. This will test whether the tidal debris between the galaxies is due to a recent direct-hit collision. It will also test models in which the tidal debris is responsible for the observed microlensing events. We will observe once 3 fields for which first-epoch archival data already exists, and observe twice 5 other fields over a 2-cycle time baseline. With the established data reduction techniques of our successful HSTPROMO collaboration, this will yield PM accuracies of 10-25 km/s per field, well below the 130 km/s velocity difference between the Clouds. This will yield the best constraints to date on the LMC/SMC interaction, and will further test the importance of dwarf-dwarf interactions for galaxy evolution. To measure the absolute proper motion of stars in the Magellanic Bridge, we will use two different types of objects as stationary reference sources. For the 3 fields that have first-epoch data in the archive, we will use hundreds of distant background galaxies as stationary reference sources. The Cycle 22 observations will be used as the second-epoch data for these "background galaxy fields". For the other 5 fields, we will use quasars as stationary reference sources. Our Cycle 22 observations for these "quasar fields" will only provide one of the two epochs required for proper motion measurements, and the second-epoch data will be obtained in Cycle 24. [Background Galaxy Fields] We will observe each background galaxy field for 4 orbits (separated into 2 visits) with ACS/WFC. We will use the F775W filter, the same filter used for the first-epoch data, for the proper motion measurements. Each orbit sill start or end with short F606W exposure for constructing CMDs that will allow identification of Magellanic Bridge stars. Individual exposures will be dithered using customized patterns designed by J. Anderson (STScI) to maximize pixel phase coverage. These are specificied using POS TARGs on the individual exposures; no pre-specified patterns are used. To maximize the overlapping area and to minimize systematics dependent on location on the detector, we request the same orientations and coordinates as used by the first-epoch observations. We also require that each target field be imaged in 30 day windows so that they can be treated as single epoch data for astrometric analyses. During these second epoch observations, the WFC3/UVIS camera will be pointed ~6 arcmin away from our target fields. These parallel fields will be observed with F814W and F606W filters for constructing CMDs to study the stellar population of the Magellanic Bridge. [Quasar Fields] These will be the first epoch in a two epoch program to measure the proper motion of the 5 background QSOs in the Magellanic Bridge. Epoch field in this epoch will consist of a one orbit observation, obtaining multiple dithered UVIS exposures, in a pattern to optimize PSF sampling. We aim for a S/N >> 200 for the quasars in F606W, and also need the F814W data to make CMDs to separate the Magellanic Bridge stars from field stars as well as to study the stellar populations. To maximize the time baseline with respect to the second epoch, and to achieve random errors that are small enough to do the science, we have put a timing requirement that says "before Feb 1, 2015" for all our targets. RESOLVED STELLAR POPULATIONS 12 Regular false false false false false SUPPORTED GALAXY QSO B0202-765 J2000 J2000 GALAXY QSO B0242-7229 J2000 J2000 GALAXY QSO B0251-675 J2000 J2000 GALAXY QSO ESO31-8 J2000 J2000 B = 15.5 GALAXY QSO ESO55-2 J2000 J2000 B = 15.7 GALAXY MAGELLANIC IRREGULAR J2000 J2000 GALAXY MAGELLANIC IRREGULAR J2000 J2000 GALAXY MAGELLANIC IRREGULAR J2000 J2000 ACS/WFC Imaging No No F775W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F775W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F775W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W 1406129219525 Timing link suitability includes constraints ([Group (01 02) Within 30.0 days]) The scheduling data was generated by CASM 22.2. CASM was run in Phase II mode. This is the first visit for imaging MCBRIDGE-BG1. Two orbits are required to complete this visit. Since our goal is to measure proper motions of stars in the target field, our orientation is set to exactly match the previous observations of PID 12286. We adopt a customized dither pattern designed to optimally cover the pixel phase using the POS-TARG special requirements. We required that Visits 01 and 02 be observed within a 30 day window so that they can be treated as single epoch data for astrometric analysis. 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CASM was run in Phase II mode. This is the second visit for imaging MCBRIDGE-BG1. Two orbits are required to complete this visit. Since our goal is to measure proper motions of stars in the target field, our orientation is set to exactly match the previous observations of PID 12286. We adopt a customized dither pattern designed to optimally cover the pixel phase using the POS-TARG special requirements. We required that Visits 01 and 02 be observed within a 30 day window so that they can be treated as single epoch data for astrometric analysis. 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CASM was run in Phase II mode. This is the second visit for imaging MCBRIDGE-BG3. Two orbits are required to complete this visit. Since our goal is to measure proper motions of stars in the target field, our orientation is set to exactly match the previous observations of PID 9488. We adopt a customized dither pattern designed to optimally cover the pixel phase using the POS-TARG special requirements. We required that Visits 05 and 06 be observed within a 30 day window so that they can be treated as single epoch data for astrometric analysis. 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