PC reported that visit 1 wouldn't schedule. This version has shortened
actual durations in that visit to remove visibility overruns. Also, in Visits 2 and 3 actual durations
were recalculated to remove visibility overruns.----- Attempting Submission 1 (Wed Jul 02 15:34:34 GMT 2008) -----
----- Attempting Submission 2 (Wed Oct 15 20:19:37 GMT 2008) -----
----- Attempting Submission 3 (Thu May 28 19:10:35 GMT 2009) -----
----- Attempting Submission 4 (Thu Sep 24 16:17:24 GMT 2009) -----Unused visibility of 0 sec is errorneously reported as an overrun.Assigned ID: 324
----- Attempting Submission 1 (Fri Mar 07 12:21:36 GMT 2008) -----The First Proper Motion Measurement for M31: Dynamics and Mass of the Local GroupWe will perform observations to determine the proper motion of the Andromeda galaxy M31, which has been sought for almost a century without success. While challenging, this measurement has now become possible due to the availability of existing deep ACS/WFC images of several M31 fields. The requested second epoch images will yield the average shift of the M31 stars with respect to compact galaxies in the background. Our observing strategy uses six different fields (three primary and three coordinated parallel) with two different instruments (ACS and WFC3) to provide a maximum handle on possible systematic effects. The expected result will be sufficiently accurate to: (a) discriminate between different histories for the dynamics of the Local Group; (b) constrain the mass distribution of the Local Group; (c) determine the details of the expected future merger between M31 and the Milky Way; (d) infer the past interaction history between M31 and M33; (e) constrain the internal proper motion kinematics of the M31 spheroid, outer disk, and tidal stream; and (f) obtain a pilot estimate of the M31 distance through the method of rotational parallax.To measure absolute PMs we will use compact background galaxies in the
field of view (FOV) of M31 as stationary reference sources. We will
measure the average motion of stars in M31 between epochs 1 and 2,
with respect to these reference sources. Our analysis techniques and
expected accuracies were described in the Phase I proposal. First
epoch data are available from previous observing programs by PI
T. Brown. The present project will obtain the second epoch data.
One could use all exposure time with a single field and instrument,
but then there would be no internal consistency checks on the
results. So instead, we will study three different M31 fields with two
different instruments (WFC3 and ACS). This will yield more or less the
same random error, but in addition will constrain possible systematic
errors.
The target fields are the spheroid, disk, and tidal stream fields from
Brown et al. (2003, 2006). These have similar stellar densities that
provide acceptable crowding. The first epoch data were taken by Brown
et al. in two filters, to make color magnitude diagrams that reach the
M31 main sequence turn-off. These data are very deep (30--60 orbits
per field per filter) so we already have excellent templates of all
the usable reference background galaxies in the field. Moreover, the
available broad-band colors and depth are sufficient for separating
M31 stars, foreground stars, and background galaxies. For the second
epoch it is therefore sufficient to take less deep data in only one of
the filters.
We will observe each field for 2 orbits with WFC3 and 1 orbit with
ACS, all in the F606W filter. Individual exposures will be sub-pixel
dithered and will last half an orbit. Tests have shown that the F606W
filter provides the best astrometric handle on the background
galaxies. The compact source detection efficiencies (taking into
account pixel size, read noise, and throughput characteristics) of
WFC3 and ACS are similar in this filter.
Brown et al. (2003, 2006) also observed fields with WFPC2 in parallel
with their primary ACS fields. We will use coordinated parallel
observations (with ACS when WFC3 is primary, and vice versa) to cover
exactly these same fields. We do not expect the same astrometric
accuracy as for the primary fields, because of the lower depth with
WFPC2, the larger pixel size, and the larger CTE. Nonetheless, this
will yield another handle on possible systematic errors, in addition
to the ones discussed below.
For our primary ACS observations we request the same telescope
orientation as for epoch 1. For our primary WFC3 observations we
request the orientation that will align the parallel ACS field with
the coordinated parallel WFPC2 fields of Brown et al. (2003, 2006).
Possible systematic errors are difficult to quantify a priori, but our
multi-faceted observing strategy should provide a good handle both on
such errors as well as on field-to-field variations in M31 internal
kinematics. Using WFC3 in epoch 2 has the advantage of providing a new
detector that is relatively free of Charge Transfer Efficiency (CTE)
effects. The disadvantage is that we will have to calculate PMs by
comparison of data from different instruments. While this has the
potential to introduce low-level systematic errors, we have already
performed studies that show that such cross-instrument comparisons can
be done to high accuracy. By contrast, using ACS in epoch 2 has the
disadvantage that it now has significant CTE degradation, which
affects astrometry at low but important levels. By comparing the
second epoch ACS and WFC3 results, and their dependence on detector
y-position, we will be able to perform a low-level correction for CTE
effects in both the second and the first epoch data. The large number
of M31 stars of different brightnesses will also constrain the
differential CTE between the first and second epoch.
The data need to be corrected for geometric distortion. For ACS this
is calibrated and stable with time to better than 0.005
pixels. However, any instabilities at this level would only affect our
results to much lower levels. This is because we will compare the
position of each galaxy only to its local neighboring
stars. Therefore, we are sensitive only to the highest spatial
frequencies of any geometric distortion uncertainty. Moreover,
averaging of many galaxies at different positions in the FOV will
further reduce these uncertainties. Being a new instrument, no
distortion calibration exists yet for WFC3. However, calibration
observations will be obtained soon after SM4 that should allow a
calibration of similar accuracy as what we have obtained in the past
for ACS.
We request that visit 4 be scheduled early in Cycle 17. This 1-orbit
visit contains ACS observations of the M31 spheroid field. This is the
only field for which we have already analyzed the epoch 1 data in some
detail. Early receipt and analysis of the visit 4 data will allow us
to do a quick sanity check on the results, and identify possible
problems, if any. If problems are identified then we will tweak the
other visits if necessary.N/AN/AN/AUNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE12false0.00.0falsefalsefalsefalsefalsefalseSUPPORTEDGALAXYDISKHALOSPIRALJ200000 46 8.1400+40 42 36.36J2000GALAXYDISKHALOSPIRALJ200000 49 8.5000+42 44 57.00J2000GALAXYDISKHALOSPIRALJ200000 44 18.0000+39 47 36.00J2000GALAXYDISKHALOSPIRALJ200000 46 8.1400+40 42 36.36J2000GALAXYDISKHALOSPIRALJ200000 49 8.5000+42 44 57.00J2000GALAXYDISKHALOSPIRALJ200000 44 18.0000+39 47 36.00J2000WFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF606WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF606WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF606WPOS-TARGDITHER0.173This is the default pattern WFC3-UVIS-DITHER-BOX.
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