What conclusions can you draw from Hubble images?

[Ann]

Recently in the Found Images forum, bystander posted a Hubble picture of NGC 3921, brilliantly processed by Judy Schmidt. A previous Hubble picture of the same galaxy is said to show 1,000 bright young clusters at the heart of this pair of colliding galaxies. You can see the picture that claims to show 1,000 clusters here.

I'm baffled. I think I can see one cluster, at about 4 o'clock, at the edge of the bright central part.

Perhaps Hubble has measured the brightness of the center of this galaxy and concluded that only 1,000 young clusters could produce this much light? But if so, I'm surprised by this galaxy's colors. Its B-V index is 0.680 and its U-B index is 0.250. There is no suggestion here that the galaxy is full of young clusters, rich in blue and ultraviolet light.

Perhaps the galaxy is very dusty? That would make sense for a pair of merging galaxies that have formed a lot of hot new stars. But NGC 3921 really isn't very dusty. Its far infrared magnitude, which is a measure of its dust content, is about the same as its blue magnitude. Compare that with M82, the Cigar galaxy, which may contain about 150 bright clusters in its core, and which is 4.5 magnitudes brighter in the far infrared than in blue light.

It is possible that there are 1,000 young clusters in the core of NGC 3921, but they may not be young enough to have very blue and ultraviolet colors. But isn't it equally likely, then, that the central bulge of NGC 3921 is a mixture of old yellow stars and young blue stars that were born in clusters, but many of the clusters have now been disrupted due to the crowded conditions in the bulge?

And in any case, I'm still baffled as to how anyone can conclude that NGC 3921 contains 1,000 young clusters in its core, based on a Hubble picture showing a very bright central part and one definite cluster at the edge of it. Unless, of course, that is a foreground star.

Ann

[geckzilla]

That one was processed by ESA's guy, not me! You are not the first to be confused. I am kind of getting tired of correcting people.

Regarding the clusters, a look back in time reveals this paper released shortly before the caption mentioning 1000 clusters:
http://cdsads.u-strasbg.fr/cgi-bin/nph- ... etcookie=1

This paper states just over 100 candidates were discovered, so your trouble may in fact all be due to a typo.

[Ann]

Thanks, Geck!

I still have to wonder how anyone could spot even a hundred clusters in that blurry brightness at the center of NGC 3921, let alone say that the median half-light radius of candidate GCs is R_eff_ <~ 5 pc and that their median (V-I)_Cousins_ = 0.65.

I can only conclude that if they could conclude that much from the Hubble picture, they must have scrutinized some Hubble data that simply can't be translated into anything even reminiscent of a pretty picture.

Ann

[Chris Peterson]

Thanks, Geck!

I still have to wonder how anyone could spot even a hundred clusters in that blurry brightness at the center of NGC 3921, let alone say that the median half-light radius of candidate GCs is R_eff_ <~ 5 pc and that their median (V-I)_Cousins_ = 0.65.

I can only conclude that if they could conclude that much from the Hubble picture, they must have scrutinized some Hubble data that simply can't be translated into anything even reminiscent of a pretty picture.

The paper describes the method they used- a labor intensive visual examination of the image.

This is very old work. There are better tools available now which can examine all the objects in an image and identify those with non-stellar profiles, which are likely to be either clusters or background galaxies.