0Assigned ID: 1640
----- Attempting Submission 1 (Fri Mar 09 02:28:36 GMT 2018) -----
HST Phase I Proposal 1640 (cvnI.apt) successfully submitted.
Receipt: # 1640-10----- Attempting Submission 1 (Tue May 29 22:50:40 GMT 2018) -----
----- Attempting Submission 2 (Thu Jun 07 12:20:12 GMT 2018) -----The Formation History of Milky Way Satellite Canes Venatici IWe propose deep ACS/WFC and WFC3/UVIS F606W and F814W imaging of Milky Way (MW) satellite Canes Venatici I (CVn I) to measure its star formation history (SFH) and establish first epoch imaging for its proper motion (PM) determination. CVn I (Mv = -7.9) is a distant ( ~220 kpc) MW satellite that straddles the boundary between ultra-faint and classical dwarf galaxy. It is among the most diffuse, faint satellites in the local Universe (r_h > 500pc, mu=28.1 mag/arcsec^2), but whether its low density is intrinsic or environmentally driven remains unknown.
The SFH and PM motion measurements enable a wide range of immediate and long term science including (1) determining if the timing and intensity of star formation can explain CVn I's low density through dark matter core creation; (2) examining how reionization impacts galaxy formation in this transitional mass regime; (3) checking if the sub-Solar stellar IMF follows the trends established in other ultra-faint galaxies; (4) determining if CVn I's orbital history includes a close pericentric passage (< 20kpc) to the MW's disk, which would have tidally disturbed CVn I; (5) providing one of the most precise measurements of the total mass of the MW's halo. Furthermore, this program will put CVn I on same observational footing as all other MW satellites.
Only HST can achieve any of these science goals. However, all archival HST observations of CVn I are inadequate. Maximizing the time baseline for PM measurement is scientifically imperative, but it will be 2 cycles until normal channels for small programs are restored. Waiting will substantially compromise the precision of the PM determination.We will image CVn I using the F606W and F814W filters of ACS/WFC and WFC3/UVIS. Our imaging will reach high S/N at the oldest main sequence turnoff in multiple fields, allowing us to measure the detailed star formation history (SFH) of CVn I across all cosmic time. Our imaging is well-dithered, and thus establishes a first epoch proper motion baseline, and is deep enough to explore the sub-Solar stellar initial mass function.
Our observing plan generally follows that specificed in the GO-14734, a Treasury survey of Milky Way satellites, which did not include CVn I. Our program consists of two adjacent ACS pointings located within the half-light radius. Each pointing is allocated 4 orbits, two in F606W and two in F814W, spread out over two visits. Each orbit is executed as a single filter and divided into long exposures (~1100-1300 sec per filter) in order to minimze overheads. For a single orbit in each filter, we add a short intergration (60 sec) to mitigate any potential saturation issues with bright stars. We use POS-TARG commands to implement the 4 point dither pattern specified by the ACS users handbook. This dither pattern satisfies PSF sampling requirements for consistent astrometic fidelity with observations of other MW satellites.
WFC3/UVIS observations are taken in parallel mode, and follow a similar observing strategy to ACS. For the 60 sec UVIS exposures, we use FLASH=10. To maximize schedulability of this program, we only constrain the roll angle to avoid bright foreground stars that may negatively affect the parallels. However, we do require that differnet visits for the same pointings have the same ORIENTATION. Given the minimal roll angle constraints and the diffuse nature of CVn I, there is a >50% chance that one of the UVIS fields will sample stars within the galaxy's half-light radius.Stellar Populations0falsefalsefalsefalsefalsefalsefalseSUPPORTEDGALAXYDWARF SPHEROIDALFaintest targets have V~28-29. A few foreground stars with V~13J2000J2000GALAXYDWARF SPHEROIDALFaintest targets have V~28-29. A few foreground stars with V~13J2000J2000ACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF814WACS/WFCImagingNoNoF606WACS/WFCImagingNoNoF814WWFC3/UVISImagingNoNoF606WWFC3/UVISImagingNoNoF814WThe scheduling data was generated by CASM 26.0.CASM was run in Phase II mode./private/var/folders/32/ytc3cnbn2dq5w8jqm7vynw8w0000gn/T/OPVisit7962988916910907741.xml(Tic-Visit
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