Cluster Gas Puts The Kibosh on AGN
A population study of active galaxies in X-rays shows that they get discouraged from living near the centers of galaxy clusters, and that there is an important interplay between gas inside and outside the galaxies.
Chandra X-ray counts image for the galaxy cluster MACS J1931.8-2634 showing the diffuse X-ray emisstion from the hit cluster gas, and the X-ray point sources highlighted in blue and magenta circles.
A major component of our understanding of galaxy evolution that has remained elusive is the influence of the local environment on the galaxy, especially when discussing galaxies in large clusters. Galaxy clusters, the largest gravitationally bound objects in the Universe, are the home of both significant concentrations of galaxies and hot diffuse intracluster gas that resides between the galaxies, as well as a lot of dark matter. The hot gas associated with clusters has been observed to span distances of a million light years or more from the cluster centers, and its influences on galaxies, in particular on active galaxies, is crucial for understanding how galaxies evolve. Active galaxies (AGN) are those where a supermassive black hole at the center with millions or billions of times the mass of the Sun is being pummeled by the infall and accretion of nearby gas, resulting in enormous relativistic jets of particles and radiation. Although active galaxies are observed in a number of clearly distinguished morphologies at optical and infrared wavelengths, a common feature observed in nearly all classes of AGN is point-like X-ray emission.
A team led by KIPAC PhD Student Steven Ehlert has now compiled one of the largest catalogs of X-ray point sources in the vicinity of galaxy clusters to date, with the hopes of elucidating the processes by which the cluster environment might influence the accretion onto the black holes in AGN. For instance, the motion of the cluster gas all around may disrupt and disperse any accreting gas near a black hole in a galaxy, similar to how your hat may blow away in the wind.
Ehlert worked with KIPAC members Professor Steve Allen, Anja von der Linden, and Glenn Morris, plus Adam Mantz (now at KIPAC's sister institute in Chicago), in collaboration with Niel Brandt and Bin Luo at Penn State University and Yongquan Xue at the University of Science and Technology in China. Their analysis, using X-ray data from NASA's Chandra X-ray Telescope, and optical observations with the Subaru telescope in Hawaii, also builds upon recent and state-of-the-art surveys of X-ray point sources that are not in galaxy clusters. It also takes into account the presence of coincidental background AGN in the vicinity of a galaxy cluster and the cluster gas which itself emits X-rays in a diffuse manner, both of which can significantly skew estimates for the number of point AGN X-ray sources associated with the cluster itself.
They estimate a total of approximately one X-ray AGN source per cluster within the central regions of each cluster field. By looking at the spatial distribution of these excess sources, they were also able to show that the fraction of galaxies hosting AGN must be roughly 8 times higher near the outskirt regions of the cluster than in the center, suggesting that the cluster environment is very efficient at stripping away the gas fuel from the galaxy centers as the galaxies fall into the cluster's deep gravitational well. Thus, these results show that the local environment around a galaxy is very important for determining whether it will be able to become an AGN, and that there is a powerful feedback between the cluster gas around the galaxy and the gas in the galaxy. An expansion of this work utilizing a roughly three times larger cluster sample is currently underway, and all of the clusters included in this first analysis also soon have deep optical imaging data, which will offer important new insights about the properties of the galaxies hosting these X-ray AGN.
This work is described in part in a paper submitted to the Monthly Notices of the Royal Astronomical Society (MNRAS) and available from astro-ph at arXiv:1209.2132. Research at KIPAC is supported by the Department of Energy, the Kavli Foundation, the National Aeronautics and Space Administration, the National Science Foundation and Stanford University, as well as private donors. We are grateful to each of these sponsors for their continued interest and support.
Tidbit Author: Steven Ehlert and Jack Singal