0 Assigned ID: 6408 ----- Attempting Submission 1 (Thu Apr 10 10:10:31 GMT 2025) ----- HST Phase I Proposal 6408 successfully submitted. Receipt: # 6408-1 Added new orient ranges to avoid saturation spikes from nearby stars. The impact on the scheduling is limited, affecting only the observing windows in October 2025 and 2026 -909318138 ----- Attempting Submission 1 (Fri Aug 08 09:43:04 GMT 2025) ----- HST Phase II Proposal 18019 successfully submitted. Receipt: # 18019-1 ----- Attempting Submission 2 (Mon Aug 11 21:16:38 GMT 2025) ----- HST Phase II Proposal 18019 successfully submitted. Receipt: # 18019-2 ----- Attempting Submission 3 (Thu Aug 14 10:20:51 GMT 2025) ----- 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. 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$50 for the luminous satellite. As mentioned above, the AO data were not photometric and do not provide the luminosities or stellar masses of the lens and satellite. However, based on our previous comparisons of AO and HST data, we can estimate the HST exposure times required for our science. We propose the following observations: - one orbit in the F140W filter, which is close to Keck K$^\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. Our team has several years of experience in the field of strong gravitational lensing and its application with HST observations, and especially in the particular subfield of low-mass-halo lensing, which is the subject of this proposal. We have pioneered some of the most advanced lens modelling codes for the gravitational imaging of low-mass dark matter halos and developed the statistical formalism necessary to interpret our results in the context of different dark matter models. We also have access to and run high-resolution numerical cosmological simulations needed for the comparison between the observational results and theoretical predictions. Over the years, we have developed independent lens modelling codes that can be tested against each other to quantify the robustness of our results and the effect of systematics. Below, we provide more details on each member experience. Dr. Giulia Despali (PI) is an expert in numerical simulations, dark matter theories and strong gravitational lensing. She works both on theoretical predictions (via numerical simulations) and the modelling of observed gravitational lenses. She has led or co-authored many publications aimed at comparing the the amount of low-mass haloes and subhalos with predictions from different dark matter models (e.g. Despali & Vegetti 2017, Hsueh, Despali et al. 2017, Despali et al. 2018, Despali et al. 2019a,b). Her work on the contribution from line-of-sight haloes (rather than subhaloes within the lens galaxy viral halo) represents a major and the latest significant development of this field (Despali et al. (2018). Her recent paper (Despali et al. 2025a) analysed the density profiles of two subhalos detected via gravitational imaging, finding an important tension between observations and CDM. Moreover, she is the PI of a large numerical simulation project, the AIDA-TNG runs (Despali et al. 2025b) which includes cosmological hydrodynamical simulations with six different dark matter scenarios - these will be employed in this proposal to generate theoretical predictions. Dr Simona Vegetti is an expert in strong gravitational lensing. She developed one of the first strong gravitational lensing codes with pixellated sources and pioneered the gravitational imaging technique, which detects dark matter haloes via their gravitational effect on Einstein rings and strongly lensed arcs. By applying this technique to optical data, she has reported the first two gravitational detections of low-mass haloes at high redshift and derived the first constraints on dark matter from strong gravitational lensing. Other independent groups confirmed her results more than ten years later. In 2018, she received an ERC starting grant to study dark matter with strong lensing. Her group is developing key codes to interpret the observations and connect them to theory. Along the way, she is uncovering and quantifying challenges in the form of limitations and systematics that must be addressed. Her goal is to make strong gravitational lensing a tool for precision cosmology. Prof. Chris Fassnacht is a full professor at the University of California Davis. He is an expert in the observations and analysis of strong gravitational lenses. He has played a key role in the CLASS, SHARP, and STRIDES projects that have focused on using lenses as tools to study cosmology and astrophysics. He is the PI of the observational proposal to observe SHARP lenses with Keck, including the AO observations of the target of this proposal, and an expert in data reduction. Dr. Conor O'Riordan is an expert in strong gravitational lensing and machine learning, with a particular focus on space telescope observations. In O'Riordan et al. 2025, \&A 694, he modelled the first Einstein ring observed by the Euclid telescope at extremely high signal-to-noise ratio. He also modelled the first lens candidates in the Euclid survey to confirm their strong lensing nature (Acevedo Barroso et al 2024). He has made forecasts for dark matter constraints from strong gravitational lensing in Euclid (O'Riordan et al 2023, MNRAS 521) and HST (O'Riordan & Vegetti, MNRAS 2024) and worked extensively on machine learning developement for subhalo finding. He is an expert in data reduction for space-based observations, including HST. Dr. Cristiana Spingola is an expert in strong gravitational lensing and VLBI. She used microarcsec-level (global-)VLBI astrometry to construct precise lens models of gravitational arcs. This precise analysis demonstrated the presence of astrometric anomalies, which provide strong evidence for the presence of a population of subhaloes or wave-like dark matter (Spingola et al. 2018, MNRAS 478, 4816). These results are at the base of the subsequent sophisticated models published in Powell. et al. (2021, 2022, 2023). She has also performed the first search for strong lensing systems in a wide-field VLBI survey (Spingola et al. 2019a, MNRAS, 483, 2125). Her research has been proven to be fundamental, as it is consistently awarded observing time on a competitive basis for more than 1500 hours of observing time at the major facilities (which include JVLA, VLBA, LOFAR, ALMA, VLT-MUSE, MeerKAT). Consequently, she has experience in their data reduction and analysis. She exploits these data to also understand the nature of the background galaxies (Spingola et al. 2019). For pursuing these investigations she and her team were recently awarded NASA Chandra (25700001 and 25700002) and ADAP (ID: 23-ADAP23-0012) fundings. Mr. Prayaag Kyatta is a PhD student at the University of California Davis. He is currently leading, along with Dr. Despali, the analysis of the AO observations of J0040+2555, as well as a sample of other SHARP lenses. Galaxies 6 false false false false false false SUPPORTED GALAXY EINSTEIN RING GRAVITATIONAL LENS YES J2000 J2000 WFC3/IR Imaging No No F140W WFC3/UVIS Imaging No No F606W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false POS-TARG DITHER 1.716 LINE 1.095 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 2 false 1755166052295 /Users/gdespali/Library/CloudStorage/Dropbox/Work/5-Lenses/SHARP/Sharp_lenses/PHASE2/OP-cache/Hst_satellites-v01.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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