X-Rays Examine Colliding Galaxies

Galaxies were once thought of as `island universes` evolving slowly in complete isolation. This is now known not to be the case. By using the world`s most powerful X-ray observatories, UK astronomers are discovering that most of these gigantic star systems interact with each other in a wide variety of ways.

During the UK National Astronomy Meeting in Bristol on Thursday 11 April, Dr. Andrew Read (University of Birmingham) will present new results from the Chandra and XMM-Newton spacecraft about the nature of these colliding galaxies. Famous examples of such cosmic collisions studied by Dr. Read include the `Antennae`, the `Mice` and Markarian 266.

“The X-ray properties of merging galaxies are the result of very hot and energetic phenomena -massive stars, supernovae and supernova remnants, collapsed objects (black holes, neutron stars) and hot diffuse gas,” said Read.

“With the recent launch of Chandra and XMM, we can now use the vastly superior power, sensitivity and resolution of these new X-ray telescopes to extend our research to study many kinds of merging galaxy systems.”

Several of the bright interacting systems reveal a very diffuse, extended X-ray emission that seems to arise from very hot (a few million degrees Celsius) material ejected from the galaxies.

This ejection process seems to begin very soon after the galaxies first encounter one another and rapidly creates distorted flows of outflowing gas. Meanwhile, vigorous star formation is seen in the galactic nuclei and within the merging disks.

Other phenomena, resulting from star formation and expulsion of hot gas in the early stages of interaction, are seen as the galactic disks collide. More massive gaseous X-ray ejections (perhaps involving up to 10 billion solar masses of hot gas) are seen at the `ultraluminous` peak of the interaction, as the galactic nuclei coalesce.

In the new Chandra X-ray images, red represents soft (cool) X-ray emission [0.2-0.9 keV], green intermediate [0.9-2.5 keV] and blue hard (hot) X-ray emission [2.5-10 keV].

Within Arp 270, a system in the early stages of a merger, the galactic disks are still very distinct. The X-ray image shows relatively cool (less than 1 million degrees Celsius) diffuse gaseous emission associated with the two disks and several point sources of varying X-ray energy (indicators of varying temperatures) are seen scattered within the galaxies. Of particular interest are the `hard` point sources visible where the disks collide. A new idea that is being considered is that they may be due to strong star-formation as the two gaseous disks rotate through each other.

The `Mice` show bright X-ray features from both galaxy nuclei, and further emission from within the northern tail. The southern nucleus appears harder in X-rays than the northern. The northern galaxy, however, shows what appears to be a small bipolar X-ray outflow – a great rarity in such a violently evolving, classical merger system.

Markarian 266 consists of two galaxies about to merge. The Chandra image resolves their nuclei, with the northern nucleus emitting higher energy X-rays and appearing significantly hotter. Interestingly, very soft, diffuse gaseous emission is seen to the north and to the east of
the system – thought to be due to a stronger, more evolved and more distorted outflow than seen in the `Mice`.

Observations such as these will enable astronomers to address important questions, such as when, where and how these ejection processes occur, and how much energy, gas and metal is injected by merging galaxies out into the intergalactic medium.

This has profound implications for galactic formation and evolution, for group/cluster evolution and for the total mass content of the universe.