Slow motion mergers in galaxy clusters provide conditions to transformspirals to smooth disks (Forwarded)

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EMBARGOED FOR 00:01 BST, WEDNESDAY, 5 APRIL 2006

Ref.: PN 06/21 (NAM14)

SLOW MOTION MERGERS IN GALAXY CLUSTERS PROVIDE CONDITIONS TO
TRANSFORM SPIRALS TO SMOOTH DISKS

Astronomers at Liverpool John Moores University may have solved
the mystery of how spiral galaxies in clusters are transformed
over time into smooth disks. Results from a study of galaxy
clusters confirms that the slow-motion conditions needed for the
transformation are occurring among populations of galaxies falling
towards the cluster centre.

Over the past several billion years the predominant shape of disc
galaxies in clusters has changed from a spiral to a smooth disk.
Theory suggests that this change occurs when two galaxies of
unequal mass merge and gravitational effects pull gas to the
galaxies nucleus, sweeping away the spiral structure and leaving
behind a smooth, barren, thickened disk known as a lenticular
galaxy. However, galaxies orbiting in clusters move at high speeds
and in random directions, which should mean that conditions needed
for these slow interactions rarely occur. Instead, multiple rapid
encounters between galaxies, known as 'galaxy harassment', are
dominant but these types of fast encounters cannot easily form
the smooth disks.

The group from Liverpool John Moores compared eight examples of
populations of galaxies falling towards the centres of galaxy
clusters with control samples of galaxies far from the clusters.
They found that the infalling galaxies in the cluster were
predominantly distorted in shape and had a higher than normal
rates of star formation. Between a half and three-quarters of these
galaxies were very close by to another galaxy or appeared to be
merging with a companion galaxy, which suggested that interactions
and mergers are more common in galaxies falling into the cluster
than in the control sample.

"Our findings are very exciting because these results suggest that
galaxies are more likely to merge when falling into a cluster and
this may explain why clusters today have so few spirals and so many
lenticular galaxies" said Dr Chris Moss, who will be presenting the
results at the Royal Astronomical Society's National Astronomy
Meeting on the 5th April.

The results suggest the conditions needed for slow galaxy
interactions and mergers are more likely to occur in galaxies
falling into a galaxy cluster compared to the general population
of galaxies outside clusters.

Since infalling of galaxies into clusters was greater in the past,
such interactions and mergers may have contributed significantly
to the transformation of the past population of cluster spirals
to lenticular galaxies in present-day clusters.

The observations were carried out over the past several years using
the JKT (Jacobus Kapteyn Telescope) and the Nordic Optical telescope,
La Palma based on earlier survey work using the Burrell Schmidt
Telescope, Kitt Peak National Observatory.

NOTES FOR EDITORS

Royal Astronomical Society's National Astronomy Meeting The 2006 RAS
National Astronomy Meeting is hosted by the University of Leicester.
It is sponsored by the Royal Astronomical Society, the UK Particle
Physics and Astronomy Research Council (PPARC), the University of
Leicester and the National Space Centre, Leicester.

IMAGES

For images, see:
http://www.astro.livjm.ac.uk/press/chrismoss.html

1. Cluster: Abell 347, galaxy CGCG 538-043

2. Cluster Abell 426, galaxy CGCG 540-112 = IC 316

3. Cluster Abell 1367, galaxy CGCG 127-012

CONTACTS

Chris Moss
Astrophysics Research Institute, Liverpool John Moores University
Twelve Quays House
Egerton Wharf Birkenhead, CH41 1LD
United Kingdom
Tel: 0151 231 2902
Fax: 0151 231 2921

From Wednesday 4th to Friday 7th April, Dr Moss can be contacted
through the NAM press office (see details at the top of the release)

Claire Thomas
Astrophysics Research Institute, Liverpool John Moores University
Twelve Quays House
Egerton Wharf Birkenhead, CH41 1LD
United Kingdom
Tel: 0151 231 2905
Fax: 0151 231 2921

Phil James
Astrophysics Research Institute, Liverpool John Moores University
Twelve Quays House
Egerton Wharf Birkenhead, CH41 1LD
United Kingdom
Tel: 0151 231 2916
Fax: 0151 231 2921