0 Assigned ID: 1557 ----- Attempting Submission 1 (Sat Sep 30 14:08:05 GMT 2017) ----- ----- Attempting Submission 2 (Sat Sep 30 14:17:46 GMT 2017) ----- We have chosen a 3-point line dither pattern, which differs from what we proposed in Phase I. For a single orbit, the 4-point pattern we initially proposed incurs a buffer dump penalty, so we changed the number of exposures to 3. 17669850 ----- Attempting Submission 1 (Fri Dec 08 08:19:22 GMT 2017) ----- Ultra-diffuse galaxies (UDGs) are low surface brightness galaxies with low stellar masses, but sizes more like L* galaxies. UDGs could be "failed galaxies" that have been extremely inefficient at forming stars, and thus extremely dark matter dominated. We have identified the galaxy VLSB-B in the nearby Virgo cluster as one of the lowest surface brightness UDGs, and have very recently obtained velocities of candidate satellite globular clusters (GCs) in an effort to measure its dynamical mass. A preliminary measurement of the velocity dispersion of VLSB-B's GCs indicates that it could have one of the highest dark matter fractions for a galaxy of its mass. VLSB-B's systemic velocity is close to zero, however, making confusion between its GCs and Milky Way foreground stars a problem. GCs at Virgo distance are mostly point sources from the ground, but are easily distinguished from stars by HST. We propose to use HST/WFC3 for one orbit to image VLSB-B and definitively separate GCs from foreground stars by spatially resolving the GCs. Coupled with our ground-based spectroscopy, we will then have a clean measurement of VLSB-B's dynamical mass. The imaging will also be used to study the GC system of this unique galaxy. The primary goal of our program is to obtain a clean, deep sample of globular clusters (GCs) in VLSB-B by spatially resolving them. We use WFC/UVIS with the F606W filter in a 3-point line dither pattern that fills the CCD gap. We also use ACS/WFC with F606W in parallel to observe a neighboring field in the Virgo cluster core that includes VCC 1077. Galaxies and the IGM 3 false false false false false false SUPPORTED GALAXY DWARF SPHEROIDAL LSB Targeting faint, marginally resolved globular clusters in a low surface brightness galaxy. J2000 J2000 WFC3/UVIS Imaging No No F606W ACS/WFC Imaging No No F606W POS-TARG DITHER 2.4 WFC3-UVIS-MOS-DITH-LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false /Users/ericpeng/Proposals/2017-09/hst-midcycle/OP-cache/VLSB-B-HST-MC-Phase2-v01.xml 1512718526571 The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 15417 :visit-id "01" :proposal-version -1 :trans-version "20171009" :date "08-Dec-2017 15:35:26.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15417 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15417 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2875 :al-overhead-at-end 85 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3209 :pcs-gap 0 ) true 1 0