13735( 4) - 19-Jun-2015 20:25:06 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13735 Version: 4 Check-in Time: 19-Jun-2015 20:25:06 STScI Edit Number: 1 Title A New Dwarf Galaxy Associated with an Ultra-Compact High Velocity Cloud ------------------------------------------------------------------------------------ Type Cycle GO 22 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. David J. Sand Texas Tech University Y CoI: Prof. Anil C. Seth University of Utah N CoI: Dr. Denija Crnojevic Texas Tech University N CoI: Prof. Jay Strader Michigan State University N CoI: Dr. Beth Willman Haverford College N CoI: Dr. Nelson C. Caldwell Smithsonian Institution Astrophysical N Observatory CoI: Prof. Puragra Guhathakurta University of California - Santa Cruz N CoI: Dr. Joshua D. Simon Carnegie Institution of Washington N CoI: Dr. Elisa Toloba University of California - Santa Cruz N ------------------------------------------------------------------------------------ Abstract We request three filter HST imaging to study the properties of what may be the faintest gas-rich dwarf galaxy known (M_V ~ -5). This galaxy will provide crucial information on how the very lowest mass galaxies form in isolation, unaffected by local environment. It will provide a useful reference object for interpreting the ultrafaint dwarfs in the Local Group, and help to populate the poorly known luminosity function of faint galaxies in the Local Volume. The galaxy was found through deep ground based imaging of a ultracompact high velocity HI cloud, however with current data we can only constrain that it likely lies within the local volume at a distance of < 4.5 Mpc. We propose to combine optical and UV imaging to determine the galaxy's distance, extent, current metallicity, and star formation history. ------------------------------------------------------------------------------------ Observations Description ------------------------ Our primary science goal is to determine the basic physical properties -- distance, luminosity, metallicity and star formation history -- of the newly discovered dwarf associated with HI gas from Adams et al. (2013), HVC274pt68 (Fig. 1 & 2 from phase I). This goal requires excellent UV-optical colors for determining the extinction and stellar metallicity of the young stellar population (Fig 3; phase I). Optical magnitudes and colors (F606W, F814W) will let us measure the distance to HVC274pt68 via the tip of the red giant branch method, as well as a robust star formation history as we will image down to the red clump if the dwarf is at D<3.5 Mpc. Required Depth: We are confident that HVC274pt68 is between 2--4.5 Mpc away. Assuming the most distant estimated value, we need a S/N>20 at F814W=24.5 mag and F606W=26.0 mag (assuming M_I=-4.0 and M_V=-2.5 for the TRGB; Da Costa & Armandroff 1990) to measure a TRGB distance to ~10% (e.g. Karachentsev et al. 2007). If within D<3.5 Mpc, we can also obtain a robust star formation history 13735( 4) - 19-Jun-2015 20:25:06 - [ 2] of the dwarf in an economical amount of time; for instance, the red clump (M_I~-0.5 and M_V~0) at that distance is at F814W=27.2 mag and F606W=27.7 mag, and we will set our exposure limits to get a S/N=5 at that magnitude. The star formation histories will be measured with a color magnitude diagram fitting technique (e.g. MATCH; Dolphin 2002; Weisz et al. 2011). The UV/F275W observations are critical for determining the metallicity of the young stars in HVC274pt68. Once the distance to HVC274pt68 is determined via the TRGB distance, we can correct for any residual extinction via a UV/optical color-color plot, which is insensitive to the metallicity of the young stellar population but shifts strongly in the face of extinction (see Figure 3 phase I; right panel). Once corrected for extinction, the F275W-F606W colors will be a clean indicator of the metallicity of the young star population, where a ~0.5 dex metallicity difference will typically have a color difference of >0.15 mag (Figure 3 phase I; left panel). We illustrate and outline our thought process for measuring both the extinction and young population metallicity in Figure 3 in the Phase I document. To make these measurements, given the expected color differences, we will need a S/N=20 for our brightest main sequence star candidates, which will have a F275W~22.5 mag (assuming the largest distance estimate). Exposure Times and Final Orbit Count: With our desired S/N=5 at F814W, F606W=27.2, 27.7, we require 2300s in exposure time for each band, according to the ACS Exposure Time Calculator. A standard dither sequence will be incorporated. Each filter's observation can be accomplished in a single orbit. Similarly, we require a S/N=20 (with errors of 0.05 mag; see Phase I Figure 3 and the discussion above for justification) for our brightest main sequence stars in the WFC3/F275W band at F275W~22.5 mag, which requires 2000s of integration time. A two-exposure dithering strategy will be adopted and a single orbit is necessary to achieve our scientific goal. In all, we require 2 orbits of ACS imaging (one for each F606W and F814W) and 1 orbit of WFC3 UVIS imaging (F275W), for a total of three orbits for this proposal. Note added for phase II: We have put in a loose ORIENT constraint for the ACS imaging only. This is so that we can cover more of the HI radio observation's positional error circle. ------------------------------------------------------------------------------------ 13735( 4) - 19-Jun-2015 20:25:06 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HVC274PT68 GALAXY, AMORPHOUS RA=12H21M54.0550S +/- 5", J2000 V = -225 V = 22.0 +/- 0.5 IRREGULAR, DWARF DEC=+13D27'40.37" +/- 5" SPHEROIDAL Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 13735( 4) - 19-Jun-2015 20:25:06 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 3.011 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.28 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.145 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 3 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.265 Line_Spacing 0.187 Coordinate_Frame POS-TARG Pattern_Orient 20.67 Angle_Between_Sides 69.05 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 4 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 13735( 4) - 19-Jun-2015 20:25:06 - [ 5] Visit: 01 Visit Requirements: ORIENT 94.5D TO 270 D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HVC274PT68 ACS/WFC ACCUM WFC1 F606W 1 550 S PATTERN 3 1 see below (2196 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 HVC274PT68 ACS/WFC ACCUM WFC1 F814W 1 550 S PATTERN 3 2 see below (2336 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ HVC274PT68 1 ACS/WFC ACCUM WFC1 F606W 1 none none none 1 549.0 S HVC274PT68 1 ACS/WFC ACCUM WFC1 F606W 2 none none none 1 549.0 S HVC274PT68 1 ACS/WFC ACCUM WFC1 F606W 3 none none none 1 549.0 S HVC274PT68 1 ACS/WFC ACCUM WFC1 F606W 4 none none none 1 549.0 S HVC274PT68 2 ACS/WFC ACCUM WFC1 F814W 1 none none none 2 584.0 S HVC274PT68 2 ACS/WFC ACCUM WFC1 F814W 2 none none none 2 584.0 S HVC274PT68 2 ACS/WFC ACCUM WFC1 F814W 3 none none none 2 584.0 S HVC274PT68 2 ACS/WFC ACCUM WFC1 F814W 4 none none none 2 584.0 S 13735( 4) - 19-Jun-2015 20:25:06 - [ 6] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HVC274PT68 WFC3/UVI ACCUM UVIS1 F275W FLASH=12.0 1 550 S PATTERN 4 1 S see below (2480 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 1 none none none 1 620.0 S S HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 2 none none none 1 620.0 S S HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 3 none none none 1 620.0 S S HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 4 none none none 1 620.0 S S 13735( 4) - 19-Jun-2015 20:25:06 - [ 7] Visit: 52 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HVC274PT68 WFC3/UVI ACCUM UVIS1 F275W FLASH=12.0 1 550 S PATTERN 4 1 S see below (2480 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 1 none none none 1 620.0 S S HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 2 none none none 1 620.0 S S HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 3 none none none 1 620.0 S S HVC274PT68 1 WFC3/UVI ACCUM UVIS1 F275W 4 none none none 1 620.0 S S 13735( 4) - 19-Jun-2015 20:25:06 - [ 8] Summary Form for Proposal 13735 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1, WFC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=12.0 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 94.5D TO 270 D, PATTERN 3 1, PATTERN 3 2, PATTERN 4 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F275W, F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names HVC274PT68 ------------------------------------------------------------------------------------------------------------------------------------