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wstevenbrown
2005-Feb-25, 05:19 AM
Hi, Gang! I am introducing this topic to provide some healthy contrast to the recent articles on galaxy formation, and to show that the recently discovered “black galaxy” is only a fairly extreme member of an already-known (albeit not very well-known) population

Here are some samples of papers on Low Surface Brightness Galaxies:

http://arxiv.org/abs/astro-ph/0405439

http://arxiv.org/abs/astro-ph/0304313

http://arxiv.org/abs/astro-ph/0303239

http://arxiv.org/abs/astro-ph/0502311

http://arxiv.org/abs/astro-ph/0501085

The concept of a Low Surface Brightness Galaxy is a relatively new one. By definition, an LSB’s surface is dimmer than visual magnitude 23—clearly, even the skyglow is brighter than that in many locations, so they are difficult to find and study. I hasten to add that most of the observational and theoretical work on galaxies and their evolution has been done on much brighter objects.

There is no consensus as to their formation mechanism—the only common element is that none of them seem to be very old, and most of them seem to be metal-poor thruout.

There is some speculation that the original gas clouds for these were less dense than older, brighter galaxies. There is some direct evidence that the average masses of their stars is lower. One would expect from this that their spectra would be somewhat redder than the average galaxy, but noooooo…
While there is a sub-population of LSB’s that are redder than normal, there is also a totally unexpected sub-population of LSB’s that are much bluer than normal.

Several of the authors suggest that new models of formation and evolution will be needed to explain these oddball objects. Lest you think that these late-bloomers are limited to scraps and wisps of gas, there are several that have double the mass of the Milky Way, but spread out over 20-30 times the surface area!

Does anybody know a specialist in this area? Or perhaps, want to speculate a bit? :rolleyes: Steve

isferno
2005-Feb-26, 10:52 PM
Here are some thoughts,

With LSBG's
- the eating habbits of blackholes are related to the star formation
- The merging of galaxies triggers star formation
(- Density wave star formation seems to be obsolete with the find of this True dark galaxy)
(- Fluffy or Leaded seems to me like normal volume explanations)

I believe with the Dark Galaxy that the question is: what made it be.
- Is it an eddy in intergalactic space?
- Is it the residue of a merging?
- Something to do with the vicinity of the Virgo Cluster?
- Just two local dense hydrogen clouds passing each other?

But I believe that astronomers might finaly give in that empty space is relative. At least I hope they induce that it's relative to speed or drag.

antoniseb
2005-Feb-27, 11:47 PM
Speculation only
I've been curious about LSBGs since the first time I read about them. Generally, I think they are formed from less dense collections of gas than are the more visible galaxies, and I agree that the dark galaxy in the recent news report is probably further down the spectrum of large un-dense objects.

Concerning the issue of some LSB's being bluer than others, I suspect that this is a sign that it has recently had a merger with another cloud of some sort.

I am incline to think that the main galaxies we see are formed along the lines forming the edges of the voids in space, and that the further you get from these lines the more likely you are to get a sparser [LSBG or Dark Galaxy] object. I say this having never actually looked at the locations of the known LSBGs to see where they lie with respect to the line from the Virgo cluster to us.

LSBailey
2005-Mar-01, 06:51 PM
Here's a link to an Oxford University page about LSBG's.

http://www-thphys.physics.ox.ac.uk/users/S...cience/lsb.html (http://www-thphys.physics.ox.ac.uk/users/SimonBerman/science/lsb.html)

wstevenbrown
2005-Mar-01, 09:26 PM
LS Bailey: Bravissimo!!! What a nice summary, thanx!

Antoniseb, this summary bears out your suspicion that their distribution favors the edges of clusters and the 'tween spaces. Shows you the effect of growing up in a poor neighborhood.


The suggestion that the night sky may be producing such a selection effect was first raised by Zwicky (1957). However, it wasn't until Disney (1976) that the effect was quantified. Disney suggested that this night sky selection effect could refute the findings of Freeman (1970) that all spiral galaxies have a roughly constant surface brightness of 21.65 +- 0.35 mag arcsec-2 in the blue. Since then, as McGaugh (1996) shows, Disney and Zwicky have been vindicated and `Freeman's Law' no longer holds. It turns out that the selection effect was so severe that there is still no representative catalogue of nearby galaxies.

I included this quote only to emphasize that it is at present unknown how much baryonic matter is bound up in LSBG's, since we cannot presently tell how many of them there are. Perversely, though, theorists have been unable to make any sense of their rotational dynamics without invoking more than the usual amount of dark matter.

So it goes-- we continue to discover new galaxies in the Local Group. Does anyone know of a more recent local galaxy discovery than Andromeda IX, our sister's satellite?

Best regards... Steve