Diffuse Light in the Virgo Cluster

Chris Mihos1, Paul Harding1, John Feldmeier2, and Heather Morrison1

1 Department of Astronomy, Case Western Reserve University, Cleveland, OH
2 National Optical Astronomy Observatories, Tucson, AZ

This research has been supported by grants from the National Science Foundation and
Research Corporation, and by Case Western Reserve University.

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As the nearest galaxy cluster to us, the Virgo Cluster gives us an up-close look at the evolution of galaxies in clusters. As galaxies collide and interact gravitationally, stars are stripped out of their parent galaxies and strewn throughout the cluster, giving rise to the very faint and diffuse "intracluster light" (ICL).

The goal of our project is to use Case's newly-upgraded Burrell Schmidt wide-field telescope at Kitt Peak National Observatory in Arizona to create a very deep image of the Virgo cluster and see if we can actually find stars being stripped from Virgo galaxies to form the ICL.

The Burrell Schmidt Telescope

An image of the core of the Virgo Cluster,
taken from the Digital Sky Survey.

Over the course of two months in March and April 2004, on clear moonless nights, we took a series of seventy two 15-minute exposures of the Virgo Cluster with the Burrell Schmidt (and twice as many calibration images).

The images were then combined to make a composite wide-field image, with the exquisite sensitivity needed to detect the very faint intracluster light -- light that is nearly 1000x fainter than the dark night sky itself.

The image to the right shows the Virgo Cluster in its true glory, revealing the complex, diffuse web of starlight that fills the space between the galaxies in the cluster. Many long streamers of stars can be seen, along with very faint extended halos surrounding the bright galaxies, and several groups of galaxies embedded in faint "common envelopes" of light.

This diffuse intracluster light, formed from the repeated collisions of galaxies within the cluster, represents an archaeological "history" of the formation and evolution of the Virgo Cluster.

(We also have available an mpeg movie showing light at progressively fainter and fainter levels in the Virgo Cluster.)

The deep, wide-field image of the Virgo Cluster, revealing its complex web of diffuse
intracluster light. The dark circles are areas where bright foreground stars in our own galaxy
have been masked out of the image.  (Click on the image for a larger version.)

We can also use computer simulations to study how intracluster light forms. When two galaxies collide (below left), the gravitational forces between the galaxies rip out long streamers of stars known as "tidal tails." When many galaxies collide inside a galaxy cluster (below right), these tidal tails get strewn throughout the cluster and form the diffuse intracluster light.

Computer simulation of two galaxies colliding
and merging, ejecting long "tidal tails" of stars
in the process. (Click on the image to see the movie.)

Simulation credit: Chris Mihos and Lars Hernquist;
Visualization credit: Chris Mihos and Sean Maxwell

Computer simulation of a cluster of galaxies,
and the creation of intracluster light.
(Click on the image to see the movie.)

Simulation and visualization credit:
Chris Mihos and Cameron McBride

More movies of colliding galaxies can be found here.
More movies of galaxy cluster simulations can be found here