Title: A “Black Hole Star” Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

Authors: Rohan P. Naidu, Jorryt Matthee, Harley Katz, Anna de Graaff, Pascal Oesch, Aaron Smith, Jenny E. Greene, Gabriel Brammer, Andrea Weibel, Raphael Hviding, John Chisholm, Ivo Labbé, Robert A. Simcoe, Callum Witten, Hakim Atek, Josephine F. W. Baggen, Sirio Belli, Rachel Bezanson, Leindert A. Boogaard, Sownak Bose, Alba Covelo-Paz, Pratika Dayal, Yoshinobu Fudamoto, Lukas J. Furtak, Emma Giovinazzo, Andy Goulding, Max Gronke, Kasper E. Heintz, Michaela Hirschmann, Garth Illingworth, Akio K. Inoue, Benjamin D. Johnson, Joel Leja, Ecaterina Leonova, Ian McConachie, Michael V. Maseda, Priyamvada Natarajan, Erica Nelson, David J. Setton, Irene Shivaei, David Sobral, Mauro Stefanon, Sandro Tacchella, Sune Toft, Alberto Torralba, Pieter van Dokkum, Arjen van der Wel, Marta Volonteri, Fabian Walter, Bingjie Wang, Darach Watson

First Author’s Institution: MIT Kavli Institute for Astrophysics and Space Research, Cambridge, Massachusetts, USA

Status: Available on arXiv [open access]

A supermassive black hole lies at the center of nearly every massive galaxy. When matter falls onto these black holes, they switch on like cosmic lanterns, shining with such intensity that they can be seen far into the distant universe. Objects like this are called active galactic nuclei (AGN). In the nearby universe, they tend to have central black holes which are about 0.1% as massive as all their stars put together. Despite making up such a small fraction of the total mass of their host galaxies, they can have impacts on massive scales. For example, AGN may be responsible for shutting off the formation of new stars, and they can unleash jets which stretch thousands of light-years long.

For decades, the most distant AGN were discovered by large ground-based telescopes. That all changed with the launch of the James Webb Space Telescope (JWST). By observing the distant universe in infrared light, JWST has uncovered a plethora of astounding AGN at unfathomable distances, including a brand new class of objects called little red dots (LRDs). The unique spectra of LRDs make them very challenging to characterize. They feature typical signs of AGN, like broad emission lines and extremely compact sizes. However, they often fail to show other common signs of AGN activity like X-ray emission and variability. Moreover, many of them showcase a special kind of feature called a Balmer break, which is usually thought of as coming from galaxies whose light is dominated by an aged stellar population.

Black Holes in a Pressure Cooker

Exciting new theories have been crafted to explain these observations. One particular idea was motivated by the detection of absorption features in Balmer emission lines. These lines are produced when electrons drop to the first excited state (n=2) in a hydrogen atom. The main way Balmer photons can be absorbed is by interacting with another electron sitting at n=2. There’s just one problem: electrons really don’t like being there. Electrons in the n=2 state will transition to the n=1 ground state (emitting a Lyman alpha photon) after only 1-2 nanoseconds, on average.

However, there are some extreme physical situations where you actually can have enough electrons at n=2 to absorb Balmer photons. In a gas, collisions between atoms can sometimes jostle their electrons up to n=2. In an extremely dense environment, these collisions can happen frequently enough for the rate at which electrons jump up to equal the rate at which they drop down, creating a sustained population. Dense gas also has a ton of atoms packed into it, meaning that there are many chances for photons to be absorbed. Using computer simulations, astronomers have found that this kind of roiling chemical soup really can cook up just the right amount of absorption to explain what we’re seeing. Not only that – this concoction is seasoned just right to actually produce the Balmer break, with no stars required!

If you take these intriguing possibilities at face value, they paint a fascinating physical picture: black holes in the early universe may have been blanketed in cocoons of extremely dense gas. This theory seems to nicely explain many of the observations, including the fact that many early black holes appear to be gulping down matter at terrifying rates. The gas would disperse light from even a modestly sized black hole, producing the broad emission lines we see, and naturally explains the absorption features and Balmer breaks exhibited by several LRDs. It could even explain the lack of observed X-rays, since they would be largely absorbed in this dense shell.

Finding a Black Hole Shining like a Star

The authors of today’s paper investigated one special LRD, called MoM-BH*-1. They analyzed data from the PRISM and G395M grating on JWST’s NIRSpec instrument, finding some remarkable results. MoM-BH*-1 displays a remarkably strong Balmer break, which almost certainly cannot have been produced by stars alone. The authors argue that the theory discussed above can readily explain its observed spectrum. That would make MoM-BH*-1 a black hole covered in a cocoon of gas – which the paper dubs a “black hole star” due to the gaseous shell’s resemblance to a star’s photosphere.

The authors use a code called CLOUDY to generate a theoretical spectrum that could result from a gas enshrouded black hole. This model turns out to be a strikingly good fit to the observed data, especially in the rest-frame optical part of the spectrum, as can be seen in Figure 1. There are, however, some discrepancies. The model overpredicts the emission near the Balmer break (labeled H infinity in the figure), but this is an artifact of how CLOUDY models hydrogen atoms. What do appear to be real differences are the rest-frame UV continuum and the [OIII] emission line, which the model underpredicts. The authors argue that these parts of the spectrum come from the faint, yet still present, host galaxy.

Perhaps other LRDs, which have more mass in stars and display a spectrum that’s not quite as extreme as MoM-BH*-1, are instances where objects like this one have merged with a larger, more typical galaxy. Indeed, there is actually a massive galaxy near MoM-BH*-1, which it’s expected to merge with in about 100 million years. In the future, improved modeling of these extreme conditions and more data of breathtaking sources will help confirm or rule out the tantalizing possibility of cloaked black holes being the ultimate force behind LRDs. Until then, these enigmatic objects remain shrouded in mystery.

Astrobite edited by Neev Shah

Featured image credit: EHT Collaboration