Galaxy Cluster Illusion (APOD 5 Mar 2007)

[TimeTravel123456789]

I wrote an article last year that the arXiv would not post about mistakes in astronomical identification. One of the arguments was about the difficulty of making distinctions between globular clusters and galaxies or what the technical distinctions were between globular cluster and galaxies.

The object on the right side I question it as galaxy. On what basis is the identification made. Foreground cloud? Foreground gas?

There is little discussion in this resource of voids. The voids are notable in this picture. Why a certain amount of voids appear in an image is just as significant as ram pressure stripping.

Part of the concept was at what point does something go from being a globular cluster to a galaxy? How many more stars or how much more gas? Similar to the planet debate, I do not think the distinction has been clearly defined. I sent it to many resources IAU, AAS, Mercury, Physics Teacher, others, ApJ, ApJ Letters.

[orin stepanek]

http://antwrp.gsfc.nasa.gov/apod/ap070305.html
If it's not a galaxy than what is it? It has all the characteristics of a spiral galaxy.
Orin

[BMAONE23]

Every time I have noted a localized star or cluster in most images, the individual stars will appear to have defraction spikes but galaxies do not. If this is the cast then there is only one individual star in this image and all remaining light is from galaxies.

[astro_uk]

Speaking as someone who works on Globular Clusters the distinction between GCs and dwarf galaxies is usually an arbitrary choice of mass, people generally say a bound collection of stars much larger than about a million solar masses is a dwarf galaxy, anything below is a star cluster. Theorists often prefer the definition that Dwarf galaxies have their own Dark Matter Halos, whereas GCs do not, this tends to correspond to roughly a mass of around 1 million solar masses as well.

One of the arguments was about the difficulty of making distinctions between globular clusters and galaxies or what the technical distinctions were between globular cluster and galaxies.

If you mean it is difficult to determine what is a GC in a cluster and what is a distant background galaxy it is difficult if you only have one image. If you have spectra of the object then it becomes very easy, the background galaxies all have larger redshifts than the galaxies that make up the cluster, any GCs which may be in the cluster will also have redshifts consistent with the cluster.

This is of course the method used to determine that the lensed galaxy is in fact behind the cluster, it has a higher redshift than the galaxies doing the lensing, so it must be behind it.

I'm not sure what you mean by voids.

[NoelC]

Every time I have noted a localized star or cluster in most images, the individual stars will appear to have defraction spikes but galaxies do not. If this is the cast then there is only one individual star in this image and all remaining light is from galaxies.

You see the diffraction spikes because the exposures are made to such a depth as to be able to see tremendously dim objects, and the stars are being overexposed. The imagers and telescope are so good that you do not usually see the starlight very much spread out as a white blob as is the case with less sophisticated telescopes/imagers, such as amateur equipment. The only thing giving away the fact that the maxed-out pixels are so extremely overexposed is the diffraction effect of the telescope's secondary mirror support structure. Thus, the bright stars - in our own galaxy - are showing up with spikes.

However...

Do not infer that if there are no spikes the object must not be a local star. Indeed, it could well be a star in our galaxy that just isn't so bright/close as to overexpose to the level of exhbiting diffraction spikes.

Examining a Hubble image at full resolution, before processing into a presentation quality image, it is a bit easier - though notably not definitively so - to tell which point-like light sources are stars and which are galaxies or nebula components.

It should also be noted that energetic particles quite often leave marks in Hubble image data. Again, these are reasonably easy to differentiate when working on the images, but not absolutely so. If there is more than one exposure of the same object, then the knowledge that a light pixel or group of pixels shows up in one exposure and not the others can be used to help clean up these spots.

Looking closely at this image, I would say that there is definitely more than one star shown, but that the majority of light sources are extragalactic.

-Noel

[NoelC]

Part of the concept was at what point does something go from being a globular cluster to a galaxy?

Two things...

1. Don't confuse a cluster of galaxies with a cluster of stars. I don't think you have, but since the image is being presented as a cluster (of galaxies) the wording could confuse other readers.

2. I've always subscribed to the definition that a globular cluster is roughly millions or fewer stars, while a galaxy contains more like billions of stars. A 1000-fold difference is pretty significant.

The large object on the right notably isn't being resolved into individual stars, as with all the similar yellowish objects in the image (what speckling you DO see appears to be imager noise). If it were a globular cluster in our own galaxy or even nearby it would most definitely be resolved into stars by the Hubble. It can even resolve stars in globulars around the Andromeda Galaxy, which is a few million light-years away. Note: http://www.seds.org/messier/more/m031_g1hst.html

-Noel