Tracking Down the Quasar Light Switch

Title: Powerful Radio-Loud Quasars are Triggered by Galaxy Mergers in the Cosmic Bright Ages

Authors: Peter Breiding, Marco Chiaberge, Erini Lambrides, Eileen T. Meyer, S. P. Willner, Bryan Hilbert, Martin Haas, George Miley, Eric S. Perlman, Peter Barthel, Christopher P. O’Dea, Alessandro Capetti, Belinda Wilkes, Stefi A. Baum, Duccio F. Macchetto, Grant Tremblay, Colin Norman

First Author’s Institution: The William H. Miller III Department of Physics & Astronomy, Johns Hopkins University, Baltimore MD

Status: Submitted to ApJ [open access]

Deep in the universe, some objects are shining so brightly in radio wavelengths that even from Earth, billions of light-years away, they look like stars. Before these objects were formally identified, that’s exactly how astronomers referred to them – they were known as QUAsi-StellAr Radio objects, or quasars. Now that we’ve studied them further, we know that these objects are Supermassive Black Holes (SMBHs) at the centers of galaxies, accreting material from their surroundings at near-light speeds. The energy with which this material is falling onto the object heats it up so much that enough thermal radiation is emitted to make quasars some of the brightest sources in the universe.

While we have a decent idea of what quasars are, there are a lot of details that we’re still missing. In particular, not all SMBHs are accreting enough material to make them bright, and not all bright, actively-accreting SMBHs are bright at radio wavelengths. Various subclasses were defined observationally over time, but, like many things in astronomy, these distinctions are foggy. Astrobites has a guide to the subclasses here; in general, ‘Active Galactic Nucleus’, or AGN, refers to all bright accreting SMBHs. ‘Quasar’ typically refers to the brightest types of AGN, which are often, but not always, radio-loud. Astronomers are fairly sure that all of these SMBH types are different phases of the same object’s lifecycle, and that some event must act as a ‘light switch’, turning a previously dim SMBH into a bright quasar, but exactly what that event is is still unclear.

In today’s paper, the authors explore a probable mechanism for creating radio-bright quasars from SMBHs – major mergers between galaxies. This study targets a large sample of known quasars specifically at redshifts between 1 and 2 (about 8 to 10 billion years ago), a period of the universe’s history known as ‘cosmic noon’ because most objects, including quasars, were particularly bright at these times.

The HST Observations

The quasars being explored in this study are from the Revised Third Cambridge Catalogue (3CR), a later version of the low-frequency (178 MHz) catalogue that identified the first quasars (all the way back in the 1960s!).  The authors also look at three control samples of radio-quiet AGN. All four samples were observed in near-infrared wavelengths with the Hubble Space Telescope (HST), in order to measure the distribution of starlight in galaxies containing the quasars (the ‘host galaxies’). The point of this study was to target specifically the morphology of the objects – to use the spatial distribution of light to identify the quasar, the galaxy containing the quasar, and any other galaxies nearby. This way, the authors can classify the host galaxies as merging or non-merging, and see what fraction of the quasar hosts are merging.

PSF Subtraction and Galaxy Fitting

The issue with these objects is that, like many other high-redshift targets, they’re really far away. This makes them appear so small on the sky that it’s pushing the limits of even HST’s angular resolution to spot merging galaxies. It’s actually even more difficult to work with small objects when they’re bright – hard edges in the telescope can catch and reflect parts of the incoming beam of light (this is why stars look spiky). In order to classify the objects as mergers or non-mergers, the authors therefore had to use some creative statistical techniques, exploiting two things that are very well-studied: the spatial distribution of starlight in galaxies (known as a Sérsic profile) and the unique way that HST distributes light from small, bright objects (known as the Point Spread Function, or PSF).

Figure 1: The HST data and the reduction process for one of the quasar sample. Panel (ii) shows the raw data, Panel (ii) shows the PSF model which is subtracted to produce panel (iii), and panel (iv) shows the residuals after fitting and removing the shape of a typical galaxy from the quasar host galaxy. This particular object is undergoing a merger. Adapted from Figure 3 of Breiding et al. (2023).
Figure 2: Examples of different merger morphologies found in the survey. The galaxies represent (a) a nearby but non-merging ‘companion galaxy’, (b) a galaxy showing two quasar-like objects, (c) a galaxy that is disturbed enough to suggest it has recently undergone a merger, and (d) a galaxy without sufficient spatial resolution to tell anything apart. The different panels are as in Figure 1. Adapted from Figures 3 and 4 of Breiding et al. (2023).

Using these well-studied shapes, the authors were able to isolate different galaxies in their observations of the quasars. The process is shown in Figure 1: the raw data is in panel (i), the glaring response of HST to the bright quasar (its PSF) is in panel (ii), and the data with the quasar’s glare removed is shown in panel (iii). Once this is done, the authors can fit the central galaxy to the normal galaxy shape described by the Sersic profile and remove it as well, so only unusual parts of its morphology remain (iv). In the case of the object shown in Figure 1, the remaining unusual emission suggests a whole other galaxy, which means a merger is happening! The additional galaxy is the object in the top right of the green circle.


Even after isolating the emission that can’t be explained by the quasar or a single, typical host galaxy, it’s not always easy to say for certain what’s happening with the hosts. Other galaxies could be nearby spatially but not merging (Figure 2a), a galaxy could look reasonably normal but have two quasar-like objects inside it (2b), or appear like a single galaxy but have enough atypical morphology to suggest that it probably underwent a merger recently (2c). There’s also always the possibility that the galaxy is simply too small for HST to say anything about its morphology (2d). These exact classifications pretty much have to be made qualitatively, by eye. To do this as scientifically as possible, six of the authors independently classified each galaxy in the sample. Once these individual classifications were made, the authors quantified the probability that each object was a merger statistically by looking at the distribution of individual classifications.


Figure 3: The fraction of galaxies that are undergoing a merger in the radio quasar sample (in black) and the three control samples (colours), shown as statistical distributions. The fraction of merging galaxies in the radio sample is significantly higher than the others. Figure 5 of Breiding et al. (2023).

With this classification process, the authors found that over 85% of the radio-bright quasars were inside galaxies that were merging, about to merge, or had recently merged. The other three radio-quiet samples had merger fractions around 40% (shown in Figure 3). This is extremely good evidence for the radio emission in quasars, at the very least, being triggered from galaxy mergers. The authors suggest that this is for two reasons: the variable gravitational fields in major galaxy mergers funnel gas towards the center of the resulting galaxy, providing fuel for an AGN. Both galaxies involved in the merger will also have their own SMBH, and these likely will merge together to form the product galaxy’s SMBH. This is an energetic merging process that could result in radio emission.

This seems like a very good explanation, but many details still remain. In particular, we have plenty of evidence for major galaxy mergers that don’t involve any AGN at all, so there is likely some additional criterion required to trigger quasars from galaxy mergers. Larger sample sizes of radio-loud and radio-quiet AGN, which could be gathered from other radio surveys such as LOFAR and the SKA, could make this criterion clearer. More information about the mergers in this quasar sample could also be determined with better spatial resolution, possible with the James Webb Space Telescope. However, even with these caveats, this is an excellent next step towards understanding how quasars work.

Astrobite edited by Isabella Trierweiler

Featured image credit: ESO/M. Kommesser

About Delaney Dunne

I'm a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency Line Intensity Mapping experiment based in California's Owens Valley.

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