When Galaxies Collide

Supermassive Black Holes Belch

Artist’s conceptualization of the environment around the supermassive black hole at the center of Mrk 231. The broad outflow seen in the Gemini data is shown as the fan-shaped wedge at the top of the accretion disk around the black hole. This side-view is not what is seen from the Earth where we see it ‘looking down the throat’ of the outflow. A similar outflow is probably present under the disk as well and is hinted at in this illustration. The total amount of material entrained in the broad flow is at least 400 times the mass of the Sun per year. Note that a more localized, narrower jet is shown, this jet was known prior to the Gemini discovery of the broader outflow featured here. Credit:Gemini Observatory/AURA, artwork by Lynette Cook

Astronomers studying the effects of galaxy mergers — the events that occur when two (or more) galaxies collide and merge their materials together — have found that the black hole at the heart of the newly merged galaxy develops an incredible appetite for MORE matter.  However, the hungry black holes don’t get fed for long, due to something called a “galactic outflow.”  These outflows during the final stages of the galactic merger.

The scientists were studying the galaxy Markarian 231 (Mrk 231). It has a well-known and well-studied set of collimated jets (collimated means that the outflows are nearly in a straight line and do not spread out).  The astronomers studied the galaxy using the Gemini Observatory and found a much broader outflow from the central black hole — which is also known as an active galactic nucleus, or quasar.  It’s clearing gas out of the nuclear area in a cosmic belch that is speeding away from the black hole at about a thousand kilometers per second, more than  2.5 times the star formation rate. The speeds observed eliminate stars as the possible “engine” fueling the outflow. This leaves the black hole itself as the most likely culprit, and it can easily account for the tremendous energy required.

The energy involved is sufficient to sweep away matter  — that is, the gas and dust in the galaxy.  The galaxy is mostly stars at this stage in its life, and the outflow has no effect on them. However, the fireworks of new star formation that were ignited during the galaxy merger, will soon come to an end, as will the material that the black hole “feeds” on, most likely as a result of the outflow of matter.

Eventually, as it runs out of fuel, the AGN will become extinct, leaving behind an aging galaxy of old stars with few young stars to regenerate the stellar population. Ultimately, these old stars will make the galaxy appear redder giving these galaxies the moniker “red and dead.” The black hole will remain, but the action around it will have died down.  For more on this discovery, visit the Gemini web page for Markarian 231. Movie showing the gas in a galaxy merger with a quasar-driven “blowout”: https://www.cfa.harvard.edu/~phopkins/Site/Movies_files/collision_BH_run4.avi