18075( 3) - 23-Sep-2025 20:25:23 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18075 Version: 3 Check-in Time: 23-Sep-2025 20:25:23 STScI Edit Number: 0 Title Multiple intermediate-mass black holes in the local dwarf galaxy UGCA320 ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Ms. Xin Li European Southern Observatory - Chile Y CoI: Dr. Fuyan Bian European Southern Observatory - Chile N CoI: Dr. Yong Shi Westlake University N CoI: Mr. Jialai Kang University of Science and Technology of N China CoI: Ms. Yanina Lopez Pontificia Universidad Catolica de N Valparaiso ------------------------------------------------------------------------------------ Abstract Identifying and studying intermediate-mass black holes (IMBHs; BH mass < 10^5 solar mass) in local dwarf galaxies offers crucial insights into the formation and growth of supermassive black holes (SMBHs), as IMBHs are believed to represent an early evolutionary stage. Determining their abundance and accretion properties is key to understanding how SMBHs assemble, and the UV emission is crucial in identifying IMBH and constraining the accretion disk properties. However, only a few IMBHs have been identified so far, and no more than one IMBH has been confirmed in any single dwarf galaxy, leaving their demographics and properties uncertain. In the dwarf galaxy UGCA 320, we identified two off-disk IMBH candidates with multiple evidence, and an off-center candidate in the disk but with inconclusive evidence. Although they have previous UV detection by Galex and Swift, large uncertainties prevent constraining the intrinsic UV slopes of the two off-disk candidates, and the broad PSF prevents resolving the candidate located in the disk. We propose HST observations using SBC/F140LP, UVIS/F225W, and UVIS/F300X to obtain cleaner AGN UV emission and construct high spatial resolution SEDs at three parsecs. These data will allow us to study IMBH accretion disk properties and provide robust evidence for them as IMBHs. ------------------------------------------------------------------------------------ Observations Description ------------------------ 1. Visit01: ACS/SBC F140LP observation for SBC-1. The pointing can cover two targets of interest simultaneously, regardless of orientation. The four-step dithering pattern is used to improve photometric accuracy, considering flat-field variation and PSF sampling. Each exposure lasting 500s, balancing the orbits' utilization and visibility. 2. Visit02: ACS/SBC F140LP observation for SBC-2. The pointing covers the target of interest and its surrounding environment. The "DITHER-BOX" main pattern is to improve photometric accuracy, and the "DITHERC-LINE" subpattern offsets the pointing by 3" to slightly increase FOV. We also set a pointing named SBC2-BACKGROUND to obtain one dedicated background observation. Considering the orbit utilization and the visibity, the first orbit without background observation with 450s per exposure, and the second orbit with 360s 18075( 3) - 23-Sep-2025 20:25:23 - [ 2] per exposure. The background observation will carried out at end of source exposure, with 360s observing time. Thus, this observation needs two consecutive orbits. 3. Vist03 and Vist04: WFC3/UVIS F225W and F300X observations for UVIS-HOST. The pointing covers all three targets of interest at the same time, regardless of orientation. The "UVIS2-C1K2C-CTE" aperture and a "FLASH=15" are used to migrate the CTE effects for the three targets. The "DITHER-BOX" pattern is to improve the accuracy of photometry. Each exposure lasting 450s considering the required S/N, orbit utilization, and visibility. For UVIS observation, while the orientation is unconstrained to avoid decreasing visibility, it would be better if the aperture covers as much of the host galaxy as possible. ------------------------------------------------------------------------------------ 18075( 3) - 23-Sep-2025 20:25:23 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SBC-1 ISM, NUCLEUS RA=13H03M15.7052S, J2000 V = 18.7 DEC=-17D25'38.08" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 2 SBC-2 ISM, NUCLEUS RA=13H03M15.1717S, J2000 V = 19.93 DEC=-17D25'13.76" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 3 SBC-2-BACKGR CALIBRATION, SKY RA=13H03M7.4697S, J2000 V = 19.93 OUND BACKGROUND DEC=-17D25'48.55" Reference Frame: ICRS Extended: YES 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 ------------------------------------------------------------------------------------------------------------------------------------ 4 UVIS-HOST GALAXY, MULTIPLE NUCLEI RA=13H03M15.6108S, J2000 V = 13.05 DEC=-17D25'36.47" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 18075( 3) - 23-Sep-2025 20:25:23 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-SBC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.179 Line_Spacing 0.116 Coordinate_Frame POS-TARG Pattern_Orient 20.02 Angle_Between_Sides 63.65 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 4 Primary_Pattern Secondary_Pattern Pattern_Type ACS-SBC-DITHER-BOX ACS-SBC-DITHER-LINE Pattern_Purpose DITHER DITHER Number_Of_Points 4 2 Point_Spacing 0.179 3 Line_Spacing 0.116 Coordinate_Frame POS-TARG POS-TARG Pattern_Orient 20.02 44.4 Angle_Between_Sides 63.65 Center_Pattern NO NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 3 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 ------------------------------------------------------------------------------------------------------------------------------------ 18075( 3) - 23-Sep-2025 20:25:23 - [ 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 SBC-1 ACS/SBC ACCUM SBC F140LP 1 500 S PATTERN 1 1 (2000 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 | ------------------------------------------------------------------------------------------------------------------------------------ SBC-1 1 ACS/SBC ACCUM SBC F140LP 1 none none none 1 N/A SBC-1 1 ACS/SBC ACCUM SBC F140LP 2 none none none 1 N/A SBC-1 1 ACS/SBC ACCUM SBC F140LP 3 none none none 1 N/A SBC-1 1 ACS/SBC ACCUM SBC F140LP 4 none none none 1 N/A 18075( 3) - 23-Sep-2025 20:25:23 - [ 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 SBC-2 ACS/SBC ACCUM SBC F140LP 1 800 S PATTERN 4 1 see below (3240 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SBC-2-BACKG ACS/SBC ACCUM SBC F140LP 1 360 S (360 ROUND 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 | ------------------------------------------------------------------------------------------------------------------------------------ SBC-2 1 ACS/SBC ACCUM SBC F140LP 1 1 none none 1 450.0 S SBC-2 1 ACS/SBC ACCUM SBC F140LP 1 2 none none 1 450.0 S SBC-2 1 ACS/SBC ACCUM SBC F140LP 2 1 none none 1 450.0 S SBC-2 1 ACS/SBC ACCUM SBC F140LP 2 2 none none 1 450.0 S SBC-2 1 ACS/SBC ACCUM SBC F140LP 3 1 none none 2 360 S SBC-2 1 ACS/SBC ACCUM SBC F140LP 3 2 none none 2 360 S SBC-2 1 ACS/SBC ACCUM SBC F140LP 4 1 none none 2 360 S SBC-2 1 ACS/SBC ACCUM SBC F140LP 4 2 none none 2 360 S SBC-2-BACKGROUND 2 ACS/SBC ACCUM SBC F140LP none none none none 2 N/A 18075( 3) - 23-Sep-2025 20:25:23 - [ 7] Visit: 03 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 UVIS-HOST WFC3/UVI ACCUM UVIS2 F225W FLASH=20 1 450 S PATTERN 3 1 S (1800 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 | ------------------------------------------------------------------------------------------------------------------------------------ UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F225W 1 none none none 1 N/A S UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F225W 2 none none none 1 N/A S UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F225W 3 none none none 1 N/A S UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F225W 4 none none none 1 N/A S 18075( 3) - 23-Sep-2025 20:25:23 - [ 8] Visit: 04 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 UVIS-HOST WFC3/UVI ACCUM UVIS2 F300X FLASH=20 1 450 S PATTERN 3 1 S (1800 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 | ------------------------------------------------------------------------------------------------------------------------------------ UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 1 N/A S UVIS-HOST 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 1 N/A S 18075( 3) - 23-Sep-2025 20:25:23 - [ 9] Summary Form for Proposal 18075 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures SBC, UVIS2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/SBC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=20 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 3 1, PATTERN 4 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F140LP, F225W, F300X ------------------------------------------------------------------------------------------------------------------------------------ Target Names SBC-1, SBC-2, SBC-2-BACKGROUND, UVIS-HOST ------------------------------------------------------------------------------------------------------------------------------------