Elucidating the Nature of a 'Starless' Dark Matter Halo Near M94

Fixed duplicated reference. 241435710 Assigned ID: 5353 ----- Attempting Submission 1 (Tue Mar 26 06:53:03 GMT 2024) ----- HST Phase I Proposal 5353 successfully submitted. Receipt: # 5353-1 ----- Attempting Submission 2 (Tue Mar 26 18:27:26 GMT 2024) ----- HST Phase I Proposal 5353 successfully submitted. Receipt: # 5353-2 ----- Attempting Submission 3 (Tue Mar 26 20:35:08 GMT 2024) ----- resubmission with adjusted dither pattern, orients, and new position centroid from VLA. -Gagandeep Anand 65245707 ----- Attempting Submission 1 (Fri Aug 16 18:31:07 GMT 2024) ----- ----- Attempting Submission 2 (Tue Oct 08 13:30:34 GMT 2024) ----- Elucidating the Nature of a 'Starless' Dark Matter Halo Near M94 Observations with the FAST radio telescope recently detected a marginally resolved 21 cm HI cloud (named Cloud-9) near the M94 galaxy without an optical counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey (~29.5 mag/arcsec^2 in the g band). Unlike previous candidate "dark" HI clouds, all known properties of Cloud-9 are consistent with RELHICs, namely starless dark matter halos filled with gas in hydrostatic equilibrium and thermal equilibrium with the cosmic ultraviolet background. However, the main doubts about its true nature concern the relatively poor angular resolution of the FAST data and the uncomfortably large upper limit in the stellar mass of any luminous stellar counterpart, M_* <= 10^5 Msun, a value comparable to the stellar mass of nearby HI-rich dwarfs. We thus propose to carry out deep photometric observations (8 orbits) with the HST/ACS instrument to bring down constraints to the stellar mass of this object to M_* < 10^4 Msun (and possibly 10^{3.5} Msun) and elucidate the true nature of Cloud-9. Either outcome will be important: (i) if we do not detect a stellar counterpart, we will reinforce the RELHIC interpretation for this system while confirming a cornerstone prediction of the LCDM model, namely the existence of starless collapsed dark matter structures on sub-galactic mass scales; (ii) if we do detect a stellar counterpart, we will have uncovered the faintest HI-rich dwarf galaxy known outside the Local Group. This program target a potential satellite galaxy of M94 for analysis of its resolved stellar populations. There is a chance the satellite is a LCDM RELHIC (a starless cloud containing just gas and dark matter), in which case there will be no stars seen in the images. This 8-orbit program is broken up into 4 2-orbit visits, with equal amounts of time to F606W and F814W with both ACS/WFC (prime, main target) and WFC3/UVIS (parallel, for analysis of a background field). Orient constraints (specified in Phase I) are present to obtain an optimal background field and avoid a bright foreground star, but can be adjusted for scheduling if needed. The dither pattern adopted is the 5x5 pixel ACS-WFC-DITHER-LINE pattern to optimize rejection of bad pixels/columns/artifacts in ACS/WFC. The dither patter is implemented via POS TARGs. Dr. Benitez-Llambay, Dr. Navarro, and Dr. D'Onghia have expertise in a wide range of astronomical simulation tools and theoretical expertise that span N-body modeling in an LCDM framework and detailed hydrodynamic modeling of individual systems. Their backgrounds were essential to identifying M94 Cloud-9 as a high-confidence REHLIC candidate based on its observed properties (in Radio and Optical observations) and the theoretical predictions of such objects -- for which these authors have been leaders. Their expertise will be vital to interpret the observations proposed here in the context of the properties of RELHICs. These individuals are also part of a VLA proposal studying the resolved shape and velocity distribution of Cloud-9, which will likely produce a detailed mass profile of Cloud-9. Dr. Beaton and Dr. Anand are experts in analyzing and characterizing optical and near-infrared images for the content of their stellar populations. Both have extensive experience with resolved stellar photometry of dwarf galaxies and galaxy halos for crowded and uncrowded regimes with ACS and WFC3/UVIS. Dr. Beaton was one of the central photometrists for the Carengie-Chicago Hubble Program. Dr. Anand has been a principal for the Extragalactic Distance Database for the last decade and, more recently, central for the SH0ES team. Dr. Anand is a member of the ACS team at STScI. Dr. Beaton is a member of the Roman team at STScI. Both have the skills and experience to perform the analysis required here. Dr. Fox is an expert on the study and interpretation of H1 Clouds in the Milky Way halo, which provides insight into classifying M94 Cloud-9 as a REHLIC candidate. His expertise will continue to be relevant in further interpreting any associated stellar component or in understanding the dynamical state of the H1 cloud if it lacks a stellar component relative to HVCs. Dr. Fox has extensive experience with a broad range of HST observations. Stellar Populations and the Interstellar Medium Galaxies 6 false true false false false false false false SUPPORTED GALAXY DWARF COMPACT J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W 1727992451881 3128 The scheduling data was generated by CASM 32.1.3. 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