16186( 5) - 19-Oct-2020 15:29:12 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16186 Version: 5 Check-in Time: 19-Oct-2020 15:29:12 STScI Edit Number: 3 Title The extremely peculiar globular cluster system of UDG GAMA-526784 and its implications ------------------------------------------------------------------------------------ Type Cycle GO 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Remco van der Burg European Southern Observatory - Germany Y CoI: Dr. Michael Hilker European Southern Observatory - Germany N CoI: Dr. Richard McDermid Macquarie University N CoI: Dr. Giacomo Beccari European Southern Observatory - Germany N CoI: Dr. Daniel Prole Macquarie University N ------------------------------------------------------------------------------------ Abstract Recent observations of Ultra-Diffuse Galaxies (UDGs, which have the luminosities of dwarfs but sizes of giant galaxies) have picked up a lot of attention by the community. Two nagging problems that are being discussed in the context of UDGs, are 1) the apparent lack of dark matter of some UDGs, and 2) their anomalously abundant-and-bright globular cluster systems. The former is difficult to reconcile with a LCDM model of structure formation, while the latter is contrary to what we observe in essentially any other galaxy system. We have recently discovered a UDG candidate, that may provide valuable insight towards solving both problems. This object has very similar properties to typical UDGs, apart from a widely spread population of compact star-forming regions. Our ground-based photometry indicates that these regions have stellar masses similar to globular clusters (GCs), and at least three have radial profiles that are unresolved by our ground based imaging - in line with expectations for young massive clusters. The HST brings the high spatial resolution, and imaging capability in the near-UV, that are required to better constrain the physical sizes and formation ages of these compact regions. Interestingly and coincidentally, some properties of this galaxy are strikingly similar to those hypothesised by Silk 2019, who predicts the simultaneous formation of massive GCs, and links this to the formation of UDGs that lack dark matter and exhibit extreme GC systems. This UDG may just provide us a very rare local view on such processes in action. ------------------------------------------------------------------------------------ Observations Description ------------------------ This is an HST imaging program of a single ultra-diffuse galaxy in three filters: ACS/WFC F606W and F814W, and WFC3/UVIS F275W. Data will be combined to measure fluxes and sizes of candidate globular clusters (expected to be borderline resolved at HST spatial resolution) in this galaxy. One orbit is allocated to each of three filters. Each orbit is split in 4 exposures with offset positions of half-pixel plus several pixels (i.e. the standard DITHER BOX option for both instruments) so that the individual frames can be interlaced and combined in the reduction. This is to improve the spatial sampling of the PSF and to facilitate CR and hot pixel removal. The chip gap does not have to be filled, so those small dithers suffice. 16186( 5) - 19-Oct-2020 15:29:12 - [ 2] ORIENT constraints: The galaxy will fall on WFC3/UVIS chip 2 and ACS/WFC chip 1 (aperture UVIS2 and WFC1). The galaxy is strongly elliptical (PA ~35 degrees from North Eastwards) and we intend to align the major axis with the x-axes of both chips, to be able to probe all star clusters in this galaxy. We allow a tolerance of 20 degrees between the major axis and the x-axis of UVIS chip1, and the whole configuration can be flipped by 180 degrees. To ensure a maximum overlap between the WFC3 and ACS imaging, also for the second chips, we impose a 10 degrees tolerance between the x-axes of UVIS2 and WFC1. Given how the detectors are placed in the HST FoV, we split the orbits over two HST visits. We believe these ORIENT constraints strike a good balance between maximising science return and schedulability. NOTE Oct 2020: To increase schedulability, we had to relax the ORIENT constraints listed above. TARGET POSITION: The galaxy is off-set towards the readout amplifier on the UVIS chip 2 to reduce CTI problems (-20'' in Y direction). Accordingly, we shift the target on the ACS/WFC chip 1 (-50'' in X direction and -20'' in Y direction) to maximise spatial overlap between the WFC3 and ACS data sets. We will apply a post-flash level of 19 electrons to the UVIS chips. Including the ~2 electrons of background per pixel we expect with our exposure times, this meets the updated recommended total background level of at least 20 electrons to reduce CTI problems. In the event of Reduced Gyro mode, our program would need a minor reduction of exposure times to accommodate a longer initial target acquisition. There are no bright sources in the field of view and there are no limitations for using WFC3/UVIS and ACS/WFC. ------------------------------------------------------------------------------------ 16186( 5) - 19-Oct-2020 15:29:12 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 GAMA-526784 GALAXY, DWARF RA=08H57M8.4000S, J2000 V = 23 ELLIPTICAL, LSB, STAR DEC=-00D58'33.96" FORMING REGION Reference Frame: ICRS Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2015.5 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 GAL-085709-0 GALAXY, DWARF RA=08H57M8.6350S, J2000 V = 23 05835 ELLIPTICAL, LSB, STAR DEC=-00D58'34.63" FORMING REGION Reference Frame: ICRS Comments: Elliptical UDG/LSB galaxy with major axis ~1 arcmin. Contains at least 10-20 bright clusters/star-forming regions. ------------------------------------------------------------------------------------------------------------------------------------ 16186( 5) - 19-Oct-2020 15:29:12 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 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 ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.262 Line_Spacing 0.192 Coordinate_Frame POS-TARG Pattern_Orient 18.39 Angle_Between_Sides 68.14 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 16186( 5) - 19-Oct-2020 15:29:12 - [ 5] Visit: 01 Visit Requirements: ORIENT 60D TO 115 D; ORIENT 260D TO 295 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 GAL-085709- WFC3/UVI ACCUM UVIS2 F275W FLASH=19 1 586 S POS TARG null, -20; 005835 S see below PATTERN 1 1 (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 | ------------------------------------------------------------------------------------------------------------------------------------ GAL-085709-005835 1 WFC3/UVI ACCUM UVIS2 F275W 1 none none none 1 584.0 S S GAL-085709-005835 1 WFC3/UVI ACCUM UVIS2 F275W 2 none none none 1 584.0 S S GAL-085709-005835 1 WFC3/UVI ACCUM UVIS2 F275W 3 none none none 1 584.0 S S GAL-085709-005835 1 WFC3/UVI ACCUM UVIS2 F275W 4 none none none 1 584.0 S S 16186( 5) - 19-Oct-2020 15:29:12 - [ 6] Visit: 02 Visit Requirements: ORIENT -7D TO 7D FROM 01 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 GAL-085709- ACS/WFC ACCUM WFC1 F814W 1 516 S POS TARG -50, -10; 005835 (2064 S) PATTERN 2 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 GAL-085709- ACS/WFC ACCUM WFC1 F606W 1 520 S POS TARG -50, -10; 005835 (2080 S) PATTERN 2 2 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ GAL-085709-005835 1 ACS/WFC ACCUM WFC1 F814W 1 none none none 1 N/A GAL-085709-005835 1 ACS/WFC ACCUM WFC1 F814W 2 none none none 1 N/A GAL-085709-005835 1 ACS/WFC ACCUM WFC1 F814W 3 none none none 1 N/A GAL-085709-005835 1 ACS/WFC ACCUM WFC1 F814W 4 none none none 1 N/A GAL-085709-005835 2 ACS/WFC ACCUM WFC1 F606W 1 none none none 2 N/A GAL-085709-005835 2 ACS/WFC ACCUM WFC1 F606W 2 none none none 2 N/A GAL-085709-005835 2 ACS/WFC ACCUM WFC1 F606W 3 none none none 2 N/A GAL-085709-005835 2 ACS/WFC ACCUM WFC1 F606W 4 none none none 2 N/A 16186( 5) - 19-Oct-2020 15:29:12 - [ 7] Summary Form for Proposal 16186 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2, WFC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=19 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT -7D TO 7D FROM 01, ORIENT 260D TO 295 D, ORIENT 60D TO 115 D, PATTERN 1 1, PATTERN 2 1, PATTERN 2 2, POS TARG -50, -10, POS TARG null, -20 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F275W, F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GAL-085709-005835 ------------------------------------------------------------------------------------------------------------------------------------