0 Assigned ID: 589 ----- Attempting Submission 1 (Fri Apr 08 16:49:51 GMT 2016) ----- 0 ----- Attempting Submission 1 (Thu Jul 21 07:11:29 GMT 2016) ----- DDO 68: A flea with smaller fleas that on him prey With the Large Binocular Cameras on the LBT, we have recently discovered a stellar stream apparently connected to DDO 68, one of the most metal-poor and isolated star-forming dwarf galaxies, with a stellar mass of only 10^8 solar masses. Here we propose HST/WFC3 follow-up imaging of the stream to accurately measure, map and characterize its individual stars, in order to: a) constrain its precise distance through the red giant branch tip, and confirm its physical association with DDO 68; and b) infer the evolution of its stellar population. At DDO 68' s distance of about 12 Mpc, only HST can resolve the stream into individual stars. We have previously obtained ACS imaging of DDO 68. Those data, which only capture a small portion of the stream, provide tentative evidence that it is indeed at the distance of the galaxy. We also used the previous ACS data to derive DDO 68' s star formation history, and combined the results with new N-body simulations which reproduce both DDO 68' s distorted morphology and the position of the stream. This analysis suggests that DDO 68 has cannibalized a ten times smaller companion. Our proposed observations will image the full extent of the stream, and are critical for not only understanding the true nature of this structure, but may also enable us to trace it further, beyond the extent apparent in the LBT data. DDO 68 will be the least massive dwarf galaxy with direct evidence for accretion of a satellite thus far if confirmed with the proposed observations. This study will be extremely important to test the self-similarity of the hierarchical galaxy formation process at all scales. At the DDO 68's distance of ~12.6 Mpc, corresponding to a distance modulus of (m-M)_0~30.51 (Sacchi et al 2016), the tip of the Red Giant Branch (RGB) is detected at I ~ 26.4. The same distance is assumed for our targeted stellar stream apparently connected to DDO 68. In order to derive a robust determination of the stream's distance and stellar population properties, we require to detect an RGB star with a S/N~5 at ~1 mag below the RGB tip. This corresponds to a red giant star with I=27.4 and V=28.7 in Johnson-Cousins (conservatively assuming that the reddest RGB stars have V-I~1.3 at the metallicity of DDO 68). The WFC3 ETC provides an exposure time of ~12,000 sec in F606W and ~12,000 sec in F814W to reach a S/N~5 within a circular aperture of 4 pixel radius (0.016", ~2xFWHM). For each filter, the total exposure time will be split into two separate visits lasting respectively 2 and 3 orbits, for a total of 10 orbits for the entire program. Each visit will be organized into 8 sub-exposures executed with a primary dither pattern WFC3_UVIS_GAP_LINE to fill the gap between the chips, plus a secondary pattern WFC3_UVIS_DITHER_BOX to improve the PSF sampling (and get rid of cosmic rays, and hot/bad pixels). Within the longer 3 orbit visits, we added two short ~300 s exposures, executed with a WFC3_UVIS_DITHER_LINE pattern (with a point spacing ~ 4.8 ", or ~ 120 pixels) in order to get photometry of bright objects saturated in the long exposures. The targeted stellar stream has a projected size of ~20" x 80" on the LBT images. We require a specific orient for our observations in order to place the whole stream within one of the two WFC3 chips, avoiding the gap between the two detectors. The selected orient also maximizes the overlap with the portion of the LEGUS UV data lacking coverage in V and I. We require that the 4 visits will be all executed with the same position and same orient. We also require that all 10 orbits are taken within 2 weeks of each other. Carbon stars are an important component of color-magnitude diagrams, and many of these stars are long-period variables. Having all observations made within a two week window will reduce the effect of the long-term variability of the carbon stars on their inferred positions in the color-magnitude diagrams. Galaxies Stellar Populations 12 false false false false false false SUPPORTED GALAXY TIDAL TAIL This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 The scheduling data was generated by CASM 24.0. CASM was run in Phase I mode. WFC3/UVIS Imaging No No F606W The scheduling data was generated by CASM 24.0. CASM was run in Phase I mode. ACS/WFC Imaging No No F606W The scheduling data was generated by CASM 24.0. CASM was run in Phase I mode. WFC3/UVIS Imaging No No F814W The scheduling data was generated by CASM 24.0. CASM was run in Phase I mode. ACS/WFC Imaging No No F814W POS-TARG OTHER DITHER 4 2.414 WFC3-UVIS-DITHER-BOX false 81.785 true 0.112 0.173 2 85.759 POS-TARG WFC3-UVIS-GAP-LINE 23.884 1 true POS-TARG OTHER 4.828 WFC3-UVIS-DITHER-BOX false false 2 46.84 WFC3-UVIS-DITHER-LINE 2 false /Users/fannibali/science/Proposal/HST/cycle24/PhaseII/OP-cache/annibali-v01.xml Hst Same orient constraint links visits (01 02 03 04) The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 14716 :visit-id "01" :proposal-version -1 :trans-version "20160601" :date "18-Jul-2016 18:38:42.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2887440 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14716 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14716 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2892 :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 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14716 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2994 :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 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14716 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2994 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 3 0 /private/var/folders/5t/2kxw8f392h7fk23l3bkzhkt40000gp/T/OPVisit6823464557610610750.xml Hst Same orient constraint links visits (01 02 03 04) The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 14716 :visit-id "02" :proposal-version -1 :trans-version "20160601" :date "20-Jul-2016 17:36:12.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2887440 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14716 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14716 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2880 :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 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14716 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3006 :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 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14716 :visit-id "02" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3006 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 3 0 /Users/fannibali/science/Proposal/HST/cycle24/PhaseII/OP-cache/annibali-v03.xml Hst Same orient constraint links visits (01 02 03 04) The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 14716 :visit-id "03" :proposal-version -1 :trans-version "20160601" :date "18-Jul-2016 18:38:48.0" :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 2310000 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14716 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14716 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2896 :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 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14716 :visit-id "03" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3004 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 2 0 /Users/fannibali/science/Proposal/HST/cycle24/PhaseII/OP-cache/annibali-v04.xml Hst Same orient constraint links visits (01 02 03 04) The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 14716 :visit-id "04" :proposal-version -1 :trans-version "20160601" :date "18-Jul-2016 18:38:49.0" :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 2310000 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14716 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14716 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2896 :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 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14716 :visit-id "04" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3004 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 2 0