Title: “Beads on a String” Star Formation Tied to one of the most Powerful AGN Outbursts Observed in a Cool Core Galaxy Cluster
Authors: Osase Omoruyi, Grant R. Tremblay, Francoise Combes, Timothy A. Davis, Michael D. Gladders, Alexey Vikhlinin, Paul Nulsen, Preeti Kharb, Stefi A. Baum, Christopher P. O’Dea, Keren Sharon, Bryan A. Terrazas, Rebecca Nevin, Aimee L. Schechter, John A. Zuhone, Michael McDonald, Håkon Dahle, Matthew B. Bayliss, Thomas Connor, Michael Florian, Jane R. Rigby, Sravani Vaddi
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Published in The Astrophysical Journal [open access]
Billions of years ago, around 3.8 billion light years away, there was a black hole outburst so explosive that we can still see remnants of it today – and it may be the most powerful event of its kind we have ever observed. The authors identified this outburst when they noticed a remarkable structure that formed from the wreckage, a 28 kiloparsec arc of star formation that resembles beads on a string.
The “beads on a string” structure lies near the center of SDSS J1531+3414 (hereafter SDSS 1531), a cool core, strong-lensing galaxy cluster. In initial lower resolution images from the Subaru telescope, SDSS 1531 appeared to have one large central galaxy that was slightly bluer than expected, which was assumed to be an artifact of the strong gravitational lensing. The authors of this paper obtained new observations in multiple wavelengths, revealing that there are instead two large central galaxies in the process of merging and that the blue excess is actually a result of the string of star formation.
Multiwavelength studies are especially illuminating, because each wavelength range can provide unique information about the underlying physical processes in a system. The Chandra X-ray telescope was used to observe the hot Intracluster medium. Radio observations from the Low Frequency Array (LOFAR) and the Very Large Array (VLA) were used to search for activity associated with the black hole outburst. The Gemini Multi-Object Spectrograph (GMOS) observed the warm ionized gas and searched for ionization sources. Finally, the Atacama Large Millimeter and submillimeter Array (ALMA) observed the cold molecular gas from which stars form.
From these observations, the authors were able to piece together a picture of the outburst, the lasting effect it had on its environment, and specifically how it created the string of star formation. Some 200 million years before SDSS 1531 was as it appears today (in reality, billions of years ago when light travel time is accounted for), a supermassive black hole at the center of one of the large merging galaxies was actively accreting material and emitting two extremely powerful jets in opposite directions, making it an Active Galactic Nucleus (AGN). The jets blasted hot material away, forming massive cavities. One cavity was identified by Chandra and LOFAR observations, because it lacks hot X-ray emission and is filled with radio emission. The symmetric cavity created by the other jet was not observed, but the authors proposed it may have since faded away if the jet blew into a less dense region of gas, or the motion of the surrounding gas pushed the cavity away from its original location.
This AGN was not observed, so the authors know that it has since “turned off” and is no longer active. However, the major disruption caused by the AGN outburst and the complex dynamics of the ongoing galaxy merger are likely the cause of the “beads on a string” star formation. The GMOS and ALMA observations revealed warm ionized gas and cold molecular gas along the edge of the star formation arc. The authors believe that hot gas pushed away by the outburst’s jets eventually cooled and is now falling back on to the merging galaxies. This cold gas then began to collapse to form stars, initiated by a cooling wake, strong ram pressure forces, or tidal interactions from the galaxy merger.
To confirm their findings, the authors plan to obtain deeper observations and compare their work with simulations. This will allow them to verify the origin of the star formation, understand the complex interplay of gas in this system, and prove that billions of years ago an awesomely powerful black hole outburst boomed through SDSS 1531.
Astrobite edited by Abbe Whitford
Featured image credit: X-ray: NASA/CXC/SAO/O. Omoruyi et al.; Optical: NASA/ESA/STScI/G. Tremblay et al.; Radio: ASTRON/LOFAR; Image Processing: NASA/CXC/SAO/N. Wolk
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