Authors: Scott C. Chapman, Ryley Hill, Manuel Aravena, Melanie Archipley, Arif Babul, James Burgoyne, Rebecca E. A. Canning, Roger P. Deane, Carlos De Breuck, Anthony H. Gonzalez, Christopher C. Hayward, Seon Woo Kim, Matt Malkan, Dan P. Marrone, Vincent McIntyre, Eric Murphy, Emily Pass, Ryan W. Perry, Kedar A. Phadke, Douglas Rennehan, Cassie Reuter, Kaja M. Rotermund, Douglas Scott, Nick Seymour, Manuel Solimano, Justin Spilker, Anthony A. Stark, Nikolaus Sulzenauer, Nick Tothill, Joaquin D. Vieira, David Vizgan, George Wang, and Axel Weiss
First Author’s Institution: Department of Physics and Atmospheric Science, Dalhousie University, Canada
Status: Accepted to ApJ [open access]
By Meghana Pannikkote
Meghana is a second year student in the Erasmus Mundus Joint Master’s program in Astrophysics and Space Science (MASS), interested in the studying high-redshift AGNs. Her research so far includes finding gamma-ray-emitting FR0 radio galaxies and investigating the radio variability of high-redshift quasars. Outside research, you can find her exploring museums or getting lost in a good who-dun-it mystery.
Protoclusters are like the early drafts of galaxy clusters – those massive, gravitationally-bound groups of galaxies that we see today. Observing these cosmic prototypes offer laboratories for testing cosmology, structure formation and the role of supermassive black holes (SMBHs) in the early universe. Active galactic nuclei (AGN) are a key player in this drama. Present at the central regions of most galaxy clusters, AGNs regulate gas accretion and star formation, essentially shaping the future of their host galaxy and its surroundings. Today’s paper uncovers a radio-loud AGN in one such high-redshift protocluster using data from the Australia Telescope Compact Array (ATCA) looking at radio frequencies.
Meet SPT2349−56
SPT2349−56, a protocluster at redshift z=4.3, hosts over 30 submillimeter galaxies (SMGs). These types of galaxies are known for their gas-rich environments and their role in star formation during the peak of cosmic evolution, often called “cosmic noon” (for more details check here). Previous observations of this protocluster using ALMA (350GHz) and LABOCA (870 micron) have also explored the gas and star formation in this region, revealing an environment that’s ripe for AGN activity. This paper detected a source at 2.2 GHz that could be from an AGN, that is closely aligned with the central trio of galaxies in the protocluster (B, C, and G in Figure 1). While the combination of data from both ATCA and MeerKAT (which collects data at 816 MHz) is strongest near galaxy C, the most massive member, we cannot be sure whether the emission is solely from C or a mix from some or all of the three galaxies. Other indicators of AGN activity, such as the equivalent width of the OH line at 163μm, the [CII]/FIR ratio, or optical spectra, were not detected (refer here for a detailed overview of AGN spectral diagnostics). However, this does not rule out the presence of an AGN but rather highlights the power of radio observations in detecting these objects when other methods fall short.
Who’s responsible for this cosmic toot?
The central galaxies that have been associated with the radio source (B, C, and G), are among the most massive in terms of gas and dynamical mass within the protocluster. High-resolution imaging from ALMA reveals that these galaxies are interacting and have multiple clumps of gas and stars, surrounded by a cloud of [CII] emission. This suggests that the abundance of gas and interactions between galaxies in dense environments like this one might fuel AGN activity. When the researchers stacked the radio emission from the next ten brightest SMGs, they found that it matched the typical radio emission expected from star formation, following a linear correlation between far-infrared (FIR) and radio luminosities, hinting there isn’t any AGN activity in these objects.
Another cool thing: AGN Feedback in Action
Radio jets are capable of influencing their host galaxy and surrounding environment. This process, known as AGN feedback, can regulate star formation by heating and redistributing gas within the protocluster. The radio jet power (calculated from the radio luminosity) from the AGN in SPT2349−56 was found to be strong enough to create a cavity in the surrounding intracluster medium (ICM), providing a powerful feedback mechanism. This process is one of the theorized solutions to the cooling flow problem where the gas in clusters is expected to cool too quickly, but the feedback from AGNs helps regulate this process. Figure 2 shows examples of cavities in the ICM formed by radio jets in nearby clusters. Further, the radio luminosity also suggests that the AGN in SPT2349−56 could be a strong X-ray source easily detectable by Chandra or XMM-Newton.
This discovery of a radio-loud AGN at the heart of a growing brightest cluster galaxy (BCG) with significant stellar mass makes this discovery an important new ingredient in understanding the formation and evolution of the clusters. Future observations with X-ray telescopes and JWST infrared spectroscopy are required to constrain the AGN in this system and compare to AGNs found in other protoclusters at similarly high redshifts.
Astrobite edited by Skylar Grayson
Featured image credit: NASA/CXC/Stanford/I. Zhuravleva et al
