18019( 4) - 14-Aug-2025 16:07:56 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18019 Version: 4 Check-in Time: 14-Aug-2025 16:07:56 STScI Edit Number: 0 Title Gravitational lensing by a luminous satellite: a test of dark matter and galaxy formation models ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Giulia Despali Universita di Bologna Y CoI: Prayaag R Katta University of California - Davis N CoI: Prof. Christopher Fassnacht University of California - Davis N CoI: Dr. Cristiana Spingola INAF - Istituto di Radioastronomia N CoI: Dr. Conor O'Riordan Max Planck Institute for Astrophysics N CoI: Dr. Simona Vegetti Max Planck Institute for Astrophysics N ------------------------------------------------------------------------------------ Abstract Strong gravitational lensing enables precise measurements of galaxy mass distributions, including both dark matter and baryonic components. In some systems, small satellites perturb the lensed arcs, allowing us to measure their masses independently. Recent lensing detections of low-mass satellites show anomalously high concentrations, inconsistent with predictions from the Cold Dark Matter (CDM) model. These outliers cannot be explained by baryonic effects alone, as simulations that reproduce the observed densities require stellar masses exceeding luminosity constraints. This tension motivates the exploration of alternative dark matter models such as Self-Interacting Dark Matter (SIDM), which can alter central densities through particle interactions. To test these scenarios, we propose deep HST observations of a newly discovered lens system with a luminous satellite visibly perturbing the arc. This rare configuration allows us to disentangle stellar and dark components of the satellite, measuring its mass, light, and dark matter density profile. Preliminary Keck-AO modeling indicates a steep satellite mass profile (gamma~2), deviating from NFW expectations. Multi-band HST imaging will enable accurate stellar mass and luminosity estimates for both the lens and satellite, allowing us to precisely constrain the density profile. These data are critical to determine whether the high concentrations observed are a general feature of lensing satellites or indicate new dark matter physics. This system, part of a growing sample our team is compiling, offers a timely and high-impact opportunity to address fundamental questions about dark matter on galactic scales. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to observe the system J0040+2555 with HST to complement the existing Keck AO data and add this system to the sample built on AGEL data. The K$^\prime$ band Keck data (2.1 $\mu$m) were obtained with the imager of the OSIRIS instrument, as a set of 8 dithered images for a total exposure time of 1440 seconds. This led to a peak signal-to-noise ratio of $\sim$50 for the luminous satellite. As mentioned above, the AO data were not photometric and do not provide the luminosities or stellar masses of the lens and satellite. 18019( 4) - 14-Aug-2025 16:07:56 - [ 2] However, based on our previous comparisons of AO and HST data, we can estimate the HST exposure times required for our science. We propose the following observations: - one orbit in the F140W filter, which is close to Keck K$^\prime$ and thus should yield a similar morphology. Moreover, this is the same filter used for the available AGEL observations, allowing us to directly compare the systems. Given the low redshift of the lensing galaxy, we expect it to have a relatively smooth red continuum, bright in this filter. From this observation, we will calculate the luminosity and stellar mass of the lens and perturber, in addition to modelling the arc. - one orbit in the F606W filter. Given the redshift of the source galaxy (z=0.4), we expect to see bright emission from the arc also in this band and the emergence of additional detail in the surface brightness distribution. It could also reveal additional angular structure in the lens galaxy to explain the multipoles required to correctly model this system and reconstruct the source. - We will split each orbit into 4 exposures and use standard dithering patterns to improve PSF reconstruction and cosmic-ray rejection. ------------------------------------------------------------------------------------ 18019( 4) - 14-Aug-2025 16:07:56 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 J0040+2555 GALAXY, EINSTEIN RING, RA=00H40M18.2600S, J2000 Z = V = 19 GRAVITATIONAL LENS DEC=+25D55'35.88" 0.143 Reference Frame: ICRS Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 18019( 4) - 14-Aug-2025 16:07:56 - [ 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 WFC3-IR-DITHER-BOX-MIN Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 1.716 Line_Spacing 1.095 Coordinate_Frame POS-TARG Pattern_Orient 18.528 Angle_Between_Sides 74.653 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18019( 4) - 14-Aug-2025 16:07:56 - [ 5] Visit: 01 Visit Requirements: ORIENT 185D TO 255 D; ORIENT 5D TO 75 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 J0040+2555 WFC3/UVI ACCUM UVIS2-C1K F606W 1 550 S PATTERN 1 1 S 1C-SUB (2200 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 J0040+2555 WFC3/IR MULTIAC IR F140W NSAMP=12, SAMP-SEQ=SPARS50 1 DEF PATTERN 2 2 CUM (2211.749 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 | ------------------------------------------------------------------------------------------------------------------------------------ J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F606W 1 none none none 1 N/A S K1C-SUB J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F606W 2 none none none 1 N/A S K1C-SUB J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F606W 3 none none none 1 N/A S K1C-SUB J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F606W 4 none none none 1 N/A S K1C-SUB J0040+2555 2 WFC3/IR MULTIAC IR F140W 1 none none none 2 N/A CUM J0040+2555 2 WFC3/IR MULTIAC IR F140W 2 none none none 2 N/A CUM J0040+2555 2 WFC3/IR MULTIAC IR F140W 3 none none none 2 N/A CUM J0040+2555 2 WFC3/IR MULTIAC IR F140W 4 none none none 2 N/A CUM 18019( 4) - 14-Aug-2025 16:07:56 - [ 6] Summary Form for Proposal 18019 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures IR, UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/IR, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters NSAMP=12, SAMP-SEQ=SPARS50 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 185D TO 255 D, ORIENT 5D TO 75 D, PATTERN 1 1, PATTERN 2 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F140W, F606W ------------------------------------------------------------------------------------------------------------------------------------ Target Names J0040+2555 ------------------------------------------------------------------------------------------------------------------------------------