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Title: Examining the Nature of the Starless Dark Matter Halo Candidate Cloud-9 with Very Large Array Observations
Authors: Alejandro Benítez-Llambay et al.
First Author’s Institution: University of Milano-Bicocca
Status: Published in ApJ
Astronomy, in many ways, is the study of light — in every wavelength that exists — coming at us from light-years away. But most of the mass in our universe is made of dark matter, a material that neither emits nor reflects any light. So, how exactly are we supposed to find this dark matter? Well, the dominant theory of what our universe is made of, Lambda cold dark matter (ΛCDM), predicts that this dark matter can form small halos — regions of gravitationally bound matter — that host reservoirs of cold gas but no stars. While we cannot directly observe dark matter, this cold gas can reflect light at radio wavelengths. Finding these dark halos, referred to as reionization-limited neutral atomic hydrogen clouds (RELHICs), is therefore rather important to confirm ΛCDM. The authors of this article are attempting to do just that by taking a close look at Cloud-9, a potential RELHIC.
How to “See” in the Dark
Cloud-9 was first discovered in 2023 when a team of astronomers noticed an excess of neutral hydrogen (HI) gas with no bright stars around it, near the spiral galaxy Messier 94 (M94). This discovery was made with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), which doesn’t have the best resolution. For reference, this article found Cloud-9’s diameter to be roughly 3 arcminutes in the sky, while FAST’s resolution is 2.9 arcminutes. So, the authors of this article decided to look at Cloud-9 again, this time with the Very Large Array (VLA), an array of radio telescopes that can pick out more of the structure of this system.
With this upgraded resolution, the authors were able to get a clearer picture of the shape of Cloud-9 and compare it to the previous study (see Figure 1). They found that Cloud-9 was slightly smaller and much more lopsided than previously thought. This funky shape is likely due to gravitational interactions with the larger nearby galaxy M94, causing the gas in Cloud-9 to squish and stretch.
Figure 1: Left: The HI gas column density from the previous FAST observations. [Benítez-Llambay & Navarro 2023] Right: The HI gas column density from the VLA observation in this article. [Benítez-Llambay et al. 2024] Corner circles represent the resolution of each telescope and the x and y axes are the position on the sky. Note the scale of the axes in each plot; the right plot is a zoomed-in version of the left.
RELHIC or Just RELHIC-tively Dim?
To determine if Cloud-9 really is a RELHIC, the authors needed to compare its HI radial density profile to the profiles of simulated RELHICs. Density profiles are a good tool to use here since although we don’t fully understand what dark matter is, we do know how it will affect things around it gravitationally. If the only matter hosted by Cloud-9 is HI gas and dark matter, then that gas will have a specific distribution that we can observe. If other baryonic matter is hosted by Cloud-9 that we just aren’t picking up right now, it will affect that density profile.
The authors found that for a detailed, general simulation the profiles did not match well, specifically in the outer regions. But when they compared it to a sphere of gas with a constant, specific temperature, they were able to get the fits to line up (see Figure 2). However, the authors point out that these simulations are isolated, meaning there is no big galaxy next to them affecting their shape, unlike Cloud-9.
Figure 2: Left: Comparison of Cloud-9’s HI profile (grey dots) to a general simulated RELHIC’s profile (green line) and a simulated RELHIC at a specific temperature (red line). Right: Comparison of Cloud-9’s profile (black dots) to Leo T’s profile (orange line). Although the positions are shifted, the slopes of the profiles match. [Benítez-Llambay et al. 2024]
What Are the Next Steps?
The authors conclude that, as of right now, they can’t definitively say whether Cloud-9 is or is not a RELHIC, but there are a few things astronomers can do to clear things up. First, theorists can re-run the RELHIC simulations with halos that have been affected by gravity like Cloud-9. This will allow them to better compare their observations to the simulations.
Second, observers can image Cloud-9 with an optical telescope that is powerful enough to detect any stars that could be there. They suggest the Hubble Space Telescope since it would be able to detect the brightest handful of stars in the potential dim population. JWST would be an even better choice, with its even deeper imaging capabilities, but it is very difficult to get observing time on it. (JWST recently received the most proposals in one year for any telescope ever!)
We will likely have to wait a bit for a definitive answer on the nature of Cloud-9, but either way the authors are excited by this gas cloud. Either it is the first starless dark matter halo, a confirmation of one of the predictions of ΛCDM, or it is the most distant ultra-faint dwarf galaxy ever detected! Although Cloud-9 is not much to look at right now, you should still definitely keep your eye on it.
Original astrobite edited by Maggie Verrico and Cole Meldorf.
About the author, Veronika Dornan:
Veronika is a final-year PhD candidate at McMaster University. Her research is in observations of extragalactic globular star clusters and what they can tell us about galaxy evolution and dark matter distribution in the universe.