Authors: Alejandro Benítez-Llambay, Rajeshwari Dutta, Michele Fumagalli, and Julio F. Navarro
First Author’s Institution: Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano Bicocca, Piazza della Scienza, Milano, Italy
Status: Published in The Astrophysical Journal [open access]
Astronomy, in many ways, is the study of light – in every wavelength that exists – coming at us across lightyears. 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, dark energy – cold dark matter (ΛCDM), predicts that this dark matter can form in 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 in the 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 paper 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 near the spiral galaxy M94, but with no bright stars around it. This discovery was made with the FAST (500m Aperture Spherical Telescope) radio telescope, which doesn’t have the best resolution. For reference, this paper 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 paper decided to look at Cloud-9 again, this time with the VLA (Very Large Array), a larger radio telescope which 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. They found that Cloud-9 was slightly smaller and much more lop-sided 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.
REHLIC or Just REHLIC-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.
They 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, which is the case with Cloud-9.
The authors also tried to test another explanation for Cloud-9’s apparent lack of stars: the stars are there but just too dim for us to see right now. There are many ultra faint dwarf galaxies near the Milky Way, which are small and have quite dim stellar populations. We’ve only just developed telescopes in the last few years that can take in enough light to see these galaxies, so if one of them were as far away from us as Cloud-9 is we might not be able to detect its stars. The authors compare the HI density profile of Cloud-9 to that of Leo-T, a nearby ultra faint dwarf galaxy. They find that these profiles match up better than for the general RELHIC simulations, particularly in the innermost regions of Cloud-9 and Leo-T.
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 but 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. The James Webb Space Telescope would be an even better choice, with its even deeper imaging capabilities, but it is very difficult to get observing time on it (it 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 it. Either it is the first starless dark matter halo, a confirmation of one of the predictions of ΛCDM, or it is the furthest away 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.
Astrobite edited by Maggie Verrico and Cole Meldorf
Featured image credit: Figure 2 from Benítez-Llambay et al. (2024)
