The First RELHIC? Cloud-9 is a Starless Gas Cloud

Abstract

Five-hundred-meter Aperture Spherical Telescope (FAST) observations have recently identified a compact HI cloud (hereafter Cloud-9) in the vicinity of the spiral galaxy M94. This identification has been confirmed independently by Very Large Array (VLA) and Green Bank Telescope (GBT) observations. Cloud-9 has the same recession velocity as M94, and is therefore at a similar distance ($\sim$4.4 Mpc). It is compact ($\sim$1$'$ radius, or $\sim$1.4 kpc), dynamically cold ($W_{50}=12$ km/s), non-rotating, and fairly massive, with an HI mass of $\sim 10^{6}$ $M_{\odot}$. Here we present deep Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) imaging designed to search for a luminous stellar counterpart. We visually rule out the presence of any dwarf galaxy with stellar mass exceeding 10$^{3.5}$$M_{\odot}$. A more robust color-magnitude diagram-based analysis conservatively rules out a 10$^{4}$$M_{\odot}$ stellar counterpart with $99.5^{+0.5}_{-8.2}$$\%$ confidence. The non-detection of a luminous component reinforces the interpretation that this system is a Reionization-Limited HI Cloud (RELHIC); i.e., a starless dark matter halo filled with hydrostatic gas in thermal equilibrium with the cosmic ultraviolet background. Our results make Cloud-9 the leading RELHIC candidate of any known compact HI cloud. This provides strong support for a cornerstone prediction of the $Λ$CDM model, namely the existence of gas-filled starless dark matter halos on sub-galactic mass scales, and constrains the present-day threshold halo mass for galaxy formation.

Figures (5)

• Figure 1: Digitized Sky Survey image covering a 10$'\times$10$'$ region around Cloud-9. The VLA H1 contours Benitez-Llambay2024 are shown in red, and the footprint of our HST/ACS observations are shown in blue.
• Figure 2: Top Left: The spatial distribution of sources fulfilling our quality criteria. The green dashed circle indicates the effective radius (8.4$"$ or 180 pc projected) of a Leo T analog at the distance of M94, and is centered at the location of Cloud-9. Top Right: CMD of the field. The three sources within the green circle in the top panel are shown in green. We also show an RGB isochrone for an old, metal-poor stellar population (10 Gyr, [Fe/H] = $-$2.0 dex) drawn from MIST, and placed at the distance of M94. The green crosses on the right-hand side indicate typical photometric uncertainties for various F814W magnitudes at the color of the RGB isochrone. Bottom Left: Photometric completeness distributions for both of our filters, as determined by the artificial star experiments. Bottom Right: Difference between input and output magnitudes from the artificial star experiments for the primary CMD filter (F814W).
• Figure 3: Top: A color composite of our ACS/WFC1 imaging ($\sim$202$"$$\times$100$"$). The green circle ($r=8.4$$"$) marks the VLA H1 maximum column density, with a radius corresponding to the effective radius of a Leo T analog at the distance of M94. Bottom: Simulated dwarf galaxies spanning a range of stellar masses, generated under the same observing conditions and shown at the same physical scale as the ACS data. A Leo T analog ($M_{\star}=10^{5} \ M_{\odot}$) would be readily detected, but no stellar counterpart is visible down to at least $M_{\star}\sim10^{3.5}M_{\odot}$.
• Figure 4: Histograms of 10,000 simulations performed for each of a range of input stellar masses (shown in the legend). The x-axis values show the remaining number of visible stars in the simulated CMDs after applying our observational systematics.
• Figure 5: H1 -stellar mass relation for the ALFALFA-SDSS catalog Durbala2020. Newly identified dwarf galaxies in the local volume detected with FAST are shown as green points Karachentsev2024. For comparison, Leo T, KK 153, Leo P, and Cloud-9 are also shown. The stellar mass upper limit for Cloud-9 (red star symbol) underscores its extreme gas richness relative to its possible stellar mass, if interpreted as an ultra-faint galaxy.