Is a recently discovered HI cloud near M94 a starless dark matter halo? (2309.03253v1)

Abstract: 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 $\sim50'$ from the M94 galaxy, without a stellar counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey ($\sim29.15$ 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 $T\sim2\times10^4$ $K$. 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, $d\sim4.7$ Mpc, we find that Cloud-9 is consistent with being a RELHIC inhabiting a Navarro-Frenk-White (NFW) DM halo of mass, $M_{200}\sim5\times10^{9}$ $M_{\odot}$, and concentration, $c_{\rm NFW}\sim13$. Although the agreement between the model and observations is good, Cloud-9 appears to be slightly, but systematically, more extended than expected for $\Lambda$CDM RELHICs. This may imply either that Cloud-9 is much closer than implied by its recessional velocity, $v_{\rm CL9}\sim300$ 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 $\Lambda$CDM model on the smallest scales.

• The paper establishes Cloud-9 as a candidate RELHIC by identifying its starless HI emission in line with ΛCDM predictions.
• The paper uses analytical models and FAST data to show that Cloud-9’s 21 cm line profile and column density match NFW halo parameters (M200≈5×10^9 M⊙, c≈13).
• The paper highlights discrepancies in size and profile that suggest either a distance underestimation or a shallow dark matter profile, prompting calls for further high-resolution follow-up observations.

Analyzing the Starless HI Cloud Near M94: Potential as a Reionization-Limited HI Cloud (RELHIC)

The observed HI emissions from certain locations offer fascinating opportunities for understanding dark matter (DM) and structure formation in the Universe. A paper led by Alejandro Benitez-Llambay probes the characteristics of a recently discovered starless HI cloud near the galaxy M94, designated Cloud-9 (CL-9), through observations captured by the Five-Hundred-Meter Aperture Spherical Telescope (FAST). This essay offers a precise review of the paper titled "Is a recently discovered HI cloud near M94 a starless dark matter halo?" that examines whether CL-9 could be classified as a Reionization-Limited HI Cloud (RELHIC), a significant prediction of the Lambda-Cold Dark Matter (ΛCDM) model.

Overview

Cloud-9 is identified as a potential RELHIC candidate due to several intriguing findings: it has no observable stellar counterpart, displays a distinct 21 cm line emission, and its isocontours suggest hydrostatic equilibrium with an NFW DM halo. These characteristics are consistent with simulations predicting that RELHICs, although bright in HI emissions, could remain starless if they fail to exceed a redshift-dependent critical mass for effective star formation post-reionization.

The paper correlates the properties of CL-9 with predictions for RELHICs using analytical models, asserting that the cloud's observed column density is compatible with being embedded within an NFW halo with mass $M_{200}\sim5\times10^9$ $M_{\odot}$ and concentration $c_{\rm NFW}\sim13$. However, it is noted that the actual scale of Cloud-9 appears somewhat more extended than what typical RELHIC models suggest.

Numerical Findings and Interpretations

The detected characteristics of CL-9 include a round column density profile and a line width indicative of thermal broadening around $T\sim2\times10^4$ K, resembling RELHIC predictions. Simulated RELHICs, constrained by the mass-concentration relation from ΛCDM statistics, suggest Cloud-9 as a plausible candidate for such a phenomenon. The model indicates a total HI mass consistent with observations ($M_{\rm HI}\sim7.2\times10^5$ $M_{\odot}$ assuming its placement at the same distance as M94).

While the fit between the model and observations holds adequately, the model highlights some systematic deviations — particularly in the extension beyond core radii, which might imply either an underestimation of its distance (placing CL-9 closer than assumed) or a shallow DM density profile that deviates from NFW expectations. The latter could point towards a DM mass deficit within smaller radii, presenting a potential challenge for standard ΛCDM interpretations.

Implications and Future Prospects

The confirmation of CL-9 as a RELHIC represents a significant step in validating ΛCDM predictions concerning low-mass DM halos without stellar formation. Furthermore, it opens a novel channel to probe both the presence and properties of DM on sub-galactic scales, potentially addressing known discrepancies in small-scale cosmology.

Future observations with enhanced resolution, particularly via instruments such as MeerKAT or the Very Large Array (VLA), are recommended to verify the structural nuances and distance anomalies suggested by the current paper. These should aim to increase precision in timing and spatial detail, which in turn could offer critical insights into DM distribution. Additionally, complementary studies targeting potential Hα emissions and further examination of CL-9's metallicity might provide crucial evidence for or against the presence of a hidden stellar structure.

In conclusion, while the research on CL-9's HI characteristics substantially supports its eligibility as the first observed RELHIC candidate, it also hints at potential complexities within the traditional framework of DM distribution, suggesting future empirical and theoretical exploration to elucidate remaining ambiguities. The documentation and examination of such extragalactic objects continue to offer profound implications for cosmic structure studies and the robustness of the ΛCDM framework, advancing our understanding of the Universe's composition.