0 Assigned ID: 883 ----- Attempting Submission 1 (Fri Apr 11 21:40:31 GMT 2014) ----- HST Phase I Proposal 883 (findlgdgs.apt) successfully submitted. 0 Resolving the Tip of the Red Giant Branch of Two New Candidate Local Group Dwarf Galaxies We propose to use ACS/WFC to observe two faint dwarf galaxies recently discovered via their HI emission. Based on a blind HI search of 40 HI clumps from 7500 square degrees of the GALFA-HI survey, these two candidates are the only objects with optical counterparts. They show HI and Halpha emission consistent with nearby galaxies, and have blue stars that are barely resolved in ground-based optical imaging with good seeing. These resolved stars are consistent with the galaxies being at Local Group distances. If they are in the Local Group, these galaxies are both less luminous and more compact than the recently-discovered Leo P, also found first with HI observations. They may then also be the faintest known star-forming galaxies. The ground-based imaging leaves large distance uncertainty, however, because the tip of the red giant branch cannot be resolved. We propose one orbit per galaxy of ACS/WFC imaging in F606W and F814W to measure accurate TRGB distances and determine if they truly are Local Group galaxies. If so, these galaxies provide tests on both the efficacy of Lambda CDM in predicting the properties of dwarf galaxies in low density environments, and the lowest-luminosity data points on models of galaxy star formation. The observations for this program are aimed at detecting RGB stars in the two target galaxies, to apply the Tip of the Red Giant Branch method to determine their distance. There are two targets, HI22 and HI23, which are to be observed for one orbit each with ACS/WFC. Each orbit is to be split roughly evenly between F606W and F814W (slightly favoring more exposure time for F606W). Each target should be centered on the WHC1 chip because they both fit on a single chip (and this avoids some part of the galaxies being on the chip gap). They are also small enough that their long axes fit on the chip, so there is no need for a special orientation. Two exposures are to be done for each filter per target, with a dither between them. This serves to fill the chip gap, avoid hot pixels/CCD defects, and reject cosmic rays. This ends up providing about 1100 s per filter per exposure, which the ETC indicates should get down to around 27 mags at S/N~10. RESOLVED STELLAR POPULATIONS Default false false false false false SUPPORTED GALAXY AMORPHOUS IRREGULAR DWARF COMPACT EMISSION LINE NEBULA J2000 J2000 r~17.5 GALAXY AMORPHOUS IRREGULAR DWARF COMPACT EMISSION LINE NEBULA J2000 J2000 r~17.3 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W POS-TARG DITHER 3.011 LINE false false 2 85.28 ACS-WFC-DITHER-LINE 1 false The scheduling data was generated by CASM 22.2. CASM was run in Phase II mode. /solaris/user1/cmannfolk/work/OP-cache/572-v01.xml (Tic-Visit :proposal-id 572 :visit-id "01" :proposal-version -1 :trans-version "20140605" :date "16-Sep-2014 09:45:38.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 572 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 572 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2893 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3226 :pcs-gap 0 ) false 1 0 The scheduling data was generated by CASM 22.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 572 :visit-id "02" :proposal-version -1 :trans-version "20140605" :date "16-Sep-2014 12:04:31.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1136896 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 572 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 572 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2853 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3226 :pcs-gap 0 ) false 1 0