17713( 3) - 16-Sep-2024 14:18:06 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17713 Version: 3 Check-in Time: 16-Sep-2024 14:18:06 STScI Edit Number: 1 Title Observing the Merger-induced Formation of a Nuclear Star Cluster in UGC 7346 ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Mr. Pablo M Sanchez-Alarcon Instituto de Astrofisica de Canarias Y CoI: Dr. Pamela M Marcum NASA Ames Research Center Y CoI: Mr. Javier Roman Instituto de Astrofisica de Canarias N CoI: Dr. Johan Knapen Instituto de Astrofisica de Canarias N CoI: Dr. Sebastien Comeron Universidad de La Laguna N CoI: Dr. Alejandro Serrano Borlaff NASA Ames Research Center N CoI: Reynier Frans Peletier Kapteyn Astronomical Institute N ------------------------------------------------------------------------------------ Abstract The formation of Nuclear Star Clusters are a key ingredient to understand fundamental aspects in galaxy evolution, such as super massive black hole growth, physics of dense stellar systems and galaxy morphology evolution. However, direct observational evidence of NSC formation is still lacking. Roman et al. (2023; R23) proposed that the dwarf galaxy UGC 7346 is an exceptional first case in which the formation of a Nuclear Star Cluster (NSC) is caught in the act of forming. This dwarf galaxy, located in the outskirts of the Virgo Cluster, appears to be a merging system of two dwarf galaxies. R23 postulate that the dynamical friction produced by a 1:5 merger would be the mechanism triggering the collapse of the globular clusters (GCs) towards the centre. Ground-based imaging hints that the GC surface density is much higher in the centre of the galaxy than other dwarfs galaxies in the Virgo cluster with a bright tail in the GC luminosity function which could be explain as already merged GC. Our proposed observations will have the resolution and depth needed to resolve the full GC population and study the complete GC luminosity function, critical and sufficient requirements to prove this scenario. With the resolution of HST, we will be able to resolve the GCs and estimate the morphological components of the point sources improving the detection and filetring of GCs while reducing the fraction of interlopers. The proposed observations would constitute the first observational evidence of ongoing NSC formation, which could serve as a unique prove for the feasibility of dwarf mergers as a mechanism to produce an NSC. ------------------------------------------------------------------------------------ Observations Description ------------------------ We aim to detect the whole GC luminosity function (GCLF) of UGC 7346 down to an absolute magnitude of -4 to ensure the detention of the fainter tail of GCs. At the assumed distance of ~16.5 Mpc this requirement translates into a source detection of ~27 magnitudes with an SNR>5 in two different bands (F814W and F606W). 17713( 3) - 16-Sep-2024 14:18:06 - [ 2] The APT indicates 53~minutes per orbit for the proposed observations. We will use a dithering pattern to fill the gap between the CCDs of the WFC/ACS and to correct for optical distortion. Withouth overhads we have 1920 seconds remain to observe. To optimise the overheads we use one filter per orbit. We expected a signal-to-noise ratio of 30 and 8 for point sources of 25.5 and 27 magnitudes in the i-band (AB system), respectively, which meet the requirements to study the full GCLF. We will observe UGC 7346 for two obrits, one for each filter, the F606W and F814W. In each orbit, 4 frames of 480 seconds will be taken using a dithering pattern. ------------------------------------------------------------------------------------ 17713( 3) - 16-Sep-2024 14:18:06 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 UGC-7346 GALAXY, DWARF RA=12H18M41.7920S +/- 0.1", J2000 V = 12.92 +/- 0.031 14.16 +- 0.02 ELLIPTICAL, LSB DEC=+17D43'8.73" +/- 0.1" (SDSS-g, AB), 13.36 +- 0.08 (SDSS-i, AB) Reference Frame: ICRS Extended: YES Comments: This object was generated by the target selector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 17713( 3) - 16-Sep-2024 14:18:06 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.2637 Line_Spacing 0.1856 Coordinate_Frame POS-TARG Pattern_Orient 20.7 Angle_Between_Sides 69.02 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 17713( 3) - 16-Sep-2024 14:18:06 - [ 5] Visit: 01 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 UGC-7346 ACS/WFC ACCUM WFC1-FIX F606W 1 487 S PATTERN 1 1 see below (1949 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UGC-7346 ACS/WFC ACCUM WFC1-FIX F814W 1 500 S PATTERN 1 2 see below (1956 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 | ------------------------------------------------------------------------------------------------------------------------------------ UGC-7346 1 ACS/WFC ACCUM WFC1-FIX F606W 1 none none none 1 487.0 S UGC-7346 1 ACS/WFC ACCUM WFC1-FIX F606W 2 none none none 1 487.0 S UGC-7346 1 ACS/WFC ACCUM WFC1-FIX F606W 3 none none none 1 487.0 S UGC-7346 1 ACS/WFC ACCUM WFC1-FIX F606W 4 none none none 1 488.0 S UGC-7346 2 ACS/WFC ACCUM WFC1-FIX F814W 1 none none none 2 489.0 S UGC-7346 2 ACS/WFC ACCUM WFC1-FIX F814W 2 none none none 2 489.0 S UGC-7346 2 ACS/WFC ACCUM WFC1-FIX F814W 3 none none none 2 489.0 S UGC-7346 2 ACS/WFC ACCUM WFC1-FIX F814W 4 none none none 2 489.0 S 17713( 3) - 16-Sep-2024 14:18:06 - [ 6] Summary Form for Proposal 17713 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names UGC-7346 ------------------------------------------------------------------------------------------------------------------------------------