Image of inter-galaxy light in The Virgo Cluster

[madbadgalaxyman (Robert)]

Here is an extremely deep image of the Virgo Cluster of Galaxies showing the extremely faint inter-galaxy (intracluster) light between the member galaxies.

It was taken with the Burrell Schmidt by Chris Mihos and Paul Harding of CWRU.



Here is another version of the same image, at higher contrast. It was also made by these two professional astronomers.



The depth of this image is to a V-band surface brightness of 28.5 magnitude per square arcsec.

Strange to think that an appreciable fraction of the total number of stars in the Virgo Cluster of Galaxies is located in between its galaxies.
However, as Obi Wan Kenobi said, "these are the truths that we must cling to".

The intracluster (inter-galaxy) light can contribute 10-20 Percent of the total light in certain clusters of galaxies!

See:
http://astroweb.case.edu/hos/Virgo
for more about how this image was made.
The associated scientific paper is:
Mihos et el., (2005), ApJ, 631, 41

In the scientific literature, you will find that the Intracluster Light is shortened to "ICL"
(Yet another obscure acronym!)

[SkyViking (Rolf)]

How awesome is that I love such deep images, and it's amazing what is really out there. Those vertical streaks look very intriguing.

[madbadgalaxyman (Robert)]

Hi Rolf,

Agreed, these images are awesome; but awesomeness doesn't always get the attention it deserves from professional astronomers.
For instance, I think that the intricate details in your ultra-deep N5128 image are also awesome......but it is very hard to find a "pro" who is willing to take on the task of modelling these features and their origin.
(mostly they obsess about the orbits of the stars, globulars, and planetaries in NGC 5128.....hard to get them to "look sideways" from this at the actual distribution of the stars in this galaxy......these are often narrowminded people)

I don't think anybody on this planet really understands the features in the intracluster light. It really came into prominence only since year 2000, and the number of astronomers who give it some thought is still small.

The study of this phenomenon is in its infancy;

at first, with their ever deeper CCD observations of elliptical galaxies located within clusters of galaxies, professional astronomers were looking for an outermost galactocentric radius at which there was no detectable light from a galaxy itself;
but they never found an outermost "no light" radius in many elliptical galaxies that are located within clusters;
instead, there was a changeover to a vanishingly-faint diffuse light with different properties (colours, stellar velocities & motions, etc) from the light of the galaxy itself.

M86 is a particularly interesting case of this; the integrated (total) magnitude of this galaxy keeps on increasing as you sum up its light to ever greater radii, and there is no sign of this trend stopping even at very very large radius!!

Cheers,
Robert

Image of the bizarre outermost light distribution of M86 (it is the object in the centre of this field):



(this is one of Malin's deep "co-added Schmidt films" images)

[SkyViking (Rolf)]

All very interesting Robert, thanks! A fair bit of this Intracluster Light also seems visible in this image: http://apod.nasa.gov/apod/image/1104...GCM_andreo.jpg

[madbadgalaxyman (Robert)]

I have found an early mention of the intracluster light (= ICL) by Fritz Zwicky in 1951. He was well aware that inter-galactic stars most probably do exist within clusters of galaxies, stars that do not belong to any of the galaxies in a cluster. (I believe that Zwicky plainly saw the inter-galactic light in an image of the Coma Cluster)

As with his discoveries of dark matter and many other things, Zwicky was far ahead of his time;
so far ahead, in fact, that some of his major discoveries were virtually ignored for decades...... simply because they seemed at the time to be 'too far out'.
(of course, Zwicky was also often wrong about things; but that is the occupational hazard if you are creative enough to come up with wild ideas that are not part of the current paradigm)

Intergalactic red giant stars & Planetary Nebulae & Globular Clusters have been detected within clusters of galaxies, objects that do not belong to any specific galaxy within a cluster of galaxies. The orbits and velocities of these intracluster (inter-galaxy) objects are very distinct from the orbital structures of objects belonging to the cluster galaxies.

I suspect that the actual proof that the intracluster light is composed of stars that move in such a way that they cannot be associated with any of the cluster galaxies, may have come very late in the 90 year history of extragalactic astronomy (with Time = 0 being the year of Hubble's first resolution of an external galaxy!)

For instance, in a 1996 paper , Arnaboldi and Freeman et al., ApJ, 472, 145,
were studying the planetary nebulae that belong to Messier 86 , which is a giant elliptical galaxy in the Virgo Cluster of Galaxies, and they found that three of their studied planetaries actually had redshifts similar to that of the Virgo Cluster Core rather than the redshift of M86.
[[ M86 actually has a velocity of approach (!!)(V equals
minus 240 km/s) due (it is believed) to the fact that it is falling into the Virgo Cluster Core. In contrast, the three planetaries with the anomalous velocities had an average recession velocity of 1573 km/s ! ]]

[madbadgalaxyman (Robert)]

To further facilitate comparison of the "ultra deep" Virgo image with a normal image of the same field, I include an ordinary exposure of the Virgo Cluster and also the "ultra deep" image, approximately in registration.





The ultra-deep image referred to in this thread was made by J. Christopher Mihos, Paul Harding, John Feldmeier, and Heather Morrison.
Very Deep optical imagery of Galaxy Clusters by this team, which is mainly associated with Case Western Reserve University, was one of the observations that caused interest in the ICL to renew, after a long period of time in which little study had been done.

The previously mentioned 1996 paper by Arnaboldi and Freeman and colleagues was the first to use the phrase "intracluster planetary nebulae";
therefore this paper marked the dawning realization that it was possible to study individual stars that belong to the gravitational potential of a galaxy cluster rather than stars belonging to its member galaxies.
In many ways, the culmination of this work that involves firstly tracing the stellar component of elliptical galaxies right out to their outermost parts (to maximum galactocentric radius), and then observing individual objects even beyond that radius (= the inter-galaxy stars), is a magnificent paper by a group of notable "elliptical galaxy" specialists which uses spectroscopy to trace the orbits of the outermost planetary nebulae surrounding 16 elliptical galaxies;

L.Coccato et al., 2009, MNRAS, 394, 1249
The title of this paper is "Kinematic Properties of early-type galaxy haloes using planetary nebulae" and you can find it at the ADS website. (It is even possible to download the final MNRAS paper rather than just the preprint of the paper)
http://adsabs.harvard.edu/abstract_service.html