Authors: Josephine F.W. Baggen, Pieter van Dokkum, Gabriel Brammer, Anna de Graaff, Marijn Franx, Jenny Greene, Ivo Labbe´, Joel Leja, Michael V. Maseda, Erica J. Nelson, Hans-Walter Rix, Bingjie Wang, and Andrea Weibel
First Author’s Institution: Department of Astronomy, Yale University
Status: Preprint on Arxiv
Since its launch, JWST has been on a roll with observations that continue to shape our understanding of the universe. Recently, JWST saw a bunch of galaxies in the early universe that could each host a massive black hole. These galaxies are rightfully called “little red dots” (LRD) as they are compact (have a small radius) and appear red in the infrared as observed by JWST. However, we are not entirely sure about the identity of the LRDs. The spectra of these galaxies seem to contain features that suggest the presence of an accreting supermassive black hole (SMBH) or AGN. AGNs are known to produce X-ray emissions, and we have not detected X-rays in the LRDs. So, what exactly are the LRDs? Do they have an AGN or are they simply a collection of very massive and compact galaxies?
A recent finding showed that three LRDs have a relatively older, evolved stellar population. We know this because of the presence of a blamer break, a jump in the galaxy spectrum at the Balmer line, which is predominantly seen in older stars. Does this mean that the galaxy is dominated by older, evolved stars, and does that rule out the presence of an AGN? The authors in today’s paper argue that the three LRDs with a balmer break have no AGN and are just massive, compact galaxies. Have they truly unmasked the LRDs? Let’s find out.
These are some extremely dense galaxies
The authors find that the galaxies are extremely compact, and have small half-light radii around 100 parsecs, similar to ultra-compact dwarf galaxies in the local universe. While the galaxy spectra can give us an estimate of the stellar mass, the contribution from the AGN alone can be a significant fraction of a galaxy’s mass and must be accounted for. They fit the spectra to three models: 1) assuming most of the galaxy’s light comes from an old, evolved stellar population with no contribution from the AGN, 2) assuming maximum contribution from the AGN and minimal from the stars, and 3) a model that lies somewhere in between. The three models yield very different galaxy mass estimates, with the first model estimating 100 billion solar masses in stars, and the second model just estimating a billion solar masses.
The detection of the Balmer break points towards a source that has light mostly dominated by stars. Assuming the AGN contribution to the mass is minimal, the stellar mass density would be quite high. The authors show this in Figure 1, where the model with no AGN component is shown in purple and exhibits a very high stellar mass density near the galaxy’s center compared to other galaxies. This could be taken as evidence against the no-AGN model, but there is another observation we should consider.
Figure 1: The stellar mass density profile, stellar mass density versus the distance from the center for the three different models. The purple profile is for the no AGN model. The black dotted line shows the profiles of galaxies from the local universe. The cyan and purple dotted lines show the profiles at higher redshifts. The no AGN model’s stellar mass profile indicates a higher central density than others. Figure 5 in the paper
What about the broad emission line?
One of the clearest signatures of an AGN in the galaxy spectra comes from broad emission lines, which result from fast-moving gas near the SMBH moving both towards and away from us. This causes a Doppler broadening of the emission lines. This associated velocity dispersion was measured in the three LRD galaxies in today’s paper.
The authors argue that the calculated velocity dispersion can be explained by the dynamics of gas and stars in the galaxy itself, without the need for an AGN. This is because of the galaxy’s extreme density which can produce a width that is similar to that of an AGN’s broad emission line. In Figure 2, the authors show a plot of the observed velocity dispersion versus the predicted, assuming it is the gas and stars present in the galaxy that are contributing to the Doppler broadening. The predicted and observed velocity dispersions seem to match very well for the model with no AGN contribution. This also provides a natural explanation for the lack of X-ray signatures in these galaxies, which we would from AGNs.
Figure 2: (Left) A plot of the observed vs predicted stellar velocity dispersion assuming the kinematics of the gas and stars in the dense galaxy contributing to the broad line emission. In the no AGN model, the three LRDs (indicated by the purple square, rectangle, and pentagon) can explain the observed stellar velocity dispersion. (Right) A schematic figure showing that the observed H-beta line widths can be accounted for the velocity dispersion calculated from the kinematics. Figure 6 in the paper.
Verdict: we need more evidence
However, further investigation is needed to determine if this scenario can explain every aspect of the LRDs. Additionally, the origins of such extremely dense galaxies are uncertain, and we need to understand how they evolve into the “normal” galaxies with lower central densities and larger sizes that we see today. The authors stress that the explanation of the LRD spectrum being dominated by AGN rather than stars is still likely, and further observations are needed to uncover the truth.
Astrobite edited by William Smith
Featured image credit: Pranav Satheesh
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