2023/09/06 by Benitez-Llambay, Alejandro, Navarro, Julio F. · 1 citation
#Astrophysics of Galaxies (astro-ph.GA) #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2309.03253
Observations with the Five-Hundred-Meter Aperture Spherical Telescope have revealed the presence of a marginally-resolved source of 21 cm emission from a location ∼50' from the M94 galaxy, without a stellar counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey (∼29.15 mag arcsec-2 in the g band). The system (hereafter Cloud-9) has round column density isocontours and a line width consistent with thermal broadening from gas at T∼2×104 K. These properties are unlike those of previously detected dark HI clouds and similar to the expected properties of REionization-Limited-HI Cloud (RELHICs), namely, starless dark matter (DM) halos filled with gas in hydrostatic equilibrium and in thermal equilibrium with the cosmic ultraviolet background. At the distance of M94, d∼4.7 Mpc, we find that Cloud-9 is consistent with being a RELHIC inhabiting a Navarro-Frenk-White (NFW) DM halo of mass, M200∼5×109 M\odot, and concentration, c\rm NFW∼13. Although the agreement between the model and observations is good, Cloud-9 appears to be slightly, but systematically, more extended than expected for ΛCDM RELHICs. This may imply either that Cloud-9 is much closer than implied by its recessional velocity, v\rm CL9∼300 km s-1, or that its halo density profile is flatter than NFW, with a DM mass deficit greater than a factor of 10 at radii r\lesssim1 kpc. Further observations may aid in constraining these scenarios better and help elucidate whether Cloud-9 is the first ever observed RELHIC, a cornerstone prediction of the ΛCDM model on the smallest scales.