0 Assigned ID: 1061 ----- Attempting Submission 1 (Fri Apr 07 22:59:05 GMT 2017) ----- added orient: U3 ORIENT 330D TO 145D U3 ORIENT 165D TO 318D -196944723 ----- Attempting Submission 1 (Thu Jul 20 12:42:54 GMT 2017) ----- ----- Attempting Submission 2 (Fri Feb 23 13:08:21 GMT 2018) ----- We request HST/ACS imaging to obtain a deep color-magnitude diagram of the newly discovered candidate Milky Way satellite Virgo I. With an estimated absolute magnitude of only M_V~-0.8 and a Galactocentric radius of ~90 kpc, Virgo I is one of the faintest and most distant dwarfs ever observed, and could be identified as a prototype ``hyper'' faint galaxy. The detailed characterization of the smallest inhabited dark matter subhalos is crucial to guide hierarchical galaxy formation models, and in particular to constrain reionization, the nature of the dark matter particle, etc. With the advent of deep wide-field, ground-based surveys, the potential of uncovering these lowest-mass galaxies is quickly turning into reality, as demonstrated by the discovery in the past two years of tens of new Local Group members in the ultra-faint regime (M_V>-8). Virgo I represents a new record in galaxy physical properties, and urges us to be prepared for the likely emergence of an entirely new class of such objects in the era of future wide-field surveys (e.g., LSST). Only high resolution HST observations can enable us to confirm the nature of Virgo I, providing significantly more accurate estimates for its distance and structural properties, when compared to the discovery Subaru/HyperSuprimeCam imaging. Our proposed dataset will constitute a fundamental step in the upcoming hunt for galaxies with similarly extreme properties. The observations are going to be performed with ACS/WFC (primary) and WFC3/UVIS (parallel). We request imaging in F606W and F814W filters (broad V and I) for both instruments, in order to derive an accurate MSTO distance for our target, resolve a significantly larger number of stars and derive improved structural parameters with respect to the ground-based discovery data, while optimizing the exposure times. The parallel pointing will allow us to observe a field region next to our target, which is extended and completely fills the ACS field of view: we will thus be able to estimate more precisely the number of background/foreground contaminants present in our ACS field. Our goal is to obtain a SNR of ~100 at ~0.5 mag below the MSTO, i.e. at F814W~23 and F606W~23.5, in order to derive a precise distance measurement and structural properties for our target. With the requested SNR, we will obtain a photometric uncertainty as low as ~0.02 mag at the MSTO, and we will be able to improve the discovery measurements by a factor of ~10. Our goal requires ~2400 sec in the F606W filter and ~2300 sec in the F814W filter, which nicely fit into one orbit per filter after taking into account overheads, and subdividing the exposures into three dithered observations to remove detector defects and improve the PSF sampling. We combine the two orbits into one visit in order to maintain the same orientation and thus target the same stars in both filters. Stellar Populations Cosmology 6 false false false false false false SUPPORTED GALAXY DWARF SPHEROIDAL UNDESIGNATED NO MSTO apparent magnitude. J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 3.034 LINE false false 3 85.29 ACS-WFC-DITHER-LINE 1 false 1500554396144 The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 15332 :visit-id "01" :proposal-version -1 :trans-version "20170613" :date "20-Jul-2017 14:39:56.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 3437840 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15332 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15332 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2869 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3202 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15332 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2980 :al-overhead-at-end 116 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3202 :pcs-gap 0 ) false 2 0