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

“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 ofthe 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.


Collisions and mergers of galaxies are now thought to be one of the most
dominant mechanisms in the evolution of these giant star systems. Even our
own Milky Way galaxy is presently interacting with the nearby Small and Large
Magellanic Clouds, and these will be subsumed by our own galaxy within the
next few hundred million years.

If events during galaxy formation are counted, there are probably very few
galaxies that were not shaped by interactions or even outright mergers.

The position of a galaxy in Edwin Hubble’s famous morphological sequence of
galaxies may in fact depend mainly on the number and severity of merger
events in its past history. Pure disk spiral systems, formed from relatively
isolated protogalactic gas clouds, appear at one end of the Hubble sequence.
The giant ellipticals (spheroidal galaxies), possibly produced through
mergers of similar spirals, appear at the other end. In between, mergers
between galaxies of differing mass produce galaxies with a wide range of
shapes and sizes.

Arp 270 lies about 90 million light years away.

The ‘Mice’ is the popular name for two interacting galaxies – so-called
because of their distinctive tails. They are officially known as NGC 4676 A,
B or Arp 242, and lie about 290 million light years away.

Markarian 266 is a pair of interacting galaxies located some 365 million
light years from Earth.


During the National Astronomy Meeting, Dr. Read can be contacted via the NAM
press office (see above) on Wednesday 10 April and Thursday 11 April.

Normal contact details:

Dr. Andrew Read

Astrophysics & Space Research Group

School of Physics & Astronomy

University of Birmingham

Watson building: G16


B15 2TT

Tel: +44 (0)121-414-6466

Fax: +44 (0)121-414-3722