Cosmic Archaeology With the Leader of a Group

May 5, 2015

A team of astronomers, including two from KIPAC, have created a map of X-ray emission from around the central galaxy of a galaxy group. Along with data from other wavelengths, it dramatically shows the effects of outbursts from the central active galactic nucleus that occurred millions of years ago.

Image of X-ray emission from around NGC 5813, with the galaxy itself in the center. The cavities are visible extending to the upper left and lower right in the image, at approximately .1, .25 and.5 in. from center (Credit: NASA/CXC/SAO/S.Randall et al.)

Groups are collections of galaxies, but contain fewer galaxies and less matter than the larger collections known as clusters. In larger groups, as in clusters, most of the ordinary matter is in the form of gas between the galaxies, which is very hot and glows in X-rays. Because they are smaller than clusters, these groups can be useful laboratories for studying the connections between the intergalactic matter and the activity in central galaxy.
Groups often have a large 'active' galaxy (AGN) at the center, where the interaction between matter and the central supermassive black hole results in jets of particles and radiation. A team of astronomers, including KIPAC postdocs Norbert Werner and Aurora Simionescu, along with colleagues from a number of other institutions lead by S. Randall of the Harvard-Smithsonian Center for Astrophysics, has carried out a detailed study of such a central AGN, NGC 5813, which is at the center of the group known as NGC 5846. Their analysis show three pairs of dramatic 'cavities' of lower X-ray brightness extending from the center that have been carved out by outbursts of the AGN.
The team used X-ray data from NASA's Chandra X-ray space observatory, along with radio observations and optical light at a frequency that comes from atomic Hydrogen gas, to study the region around NGC 5813. The two innermost pairs of X-ray cavities are filled with a plasma that emits radio waves, indicating that there are fast electrons moving away from the central AGN in these cavities, while the optical observations show filaments of cooler hydrogen gas that has been pushed along with the flow. The edges of the cavities are ringed by X-ray emitting gas that glows brighter, indicating that the edges of the cavities have been heated by shocks when the fast outward moving material hits the ambient medium.
Taken in total, the observations paint a dramatic picture of three past outburst of the central AGN, which sent out enhanced jets of particles and radiation that cleared the cavities out of the surrounding gas. The cavities have been propagating outwards ever since, providing a fossil record of past outbursts and allowing the team to estimate the time since each. They conclude that the outbursts occurred starting 3, 20, and 90 million years ago, and each outburst lasts approximately 10 million years.
This work is based on a paper accepted to the Astrophysical Journal and available from astro-ph at arXiv:1006.4397.
Science Contact:
Norbert Werner