View Full Version : What's The Best Theory On No Galaxies In Groups...

2010-Dec-19, 12:32 AM
having...this is hard for me to articulate...simular equatorial spins? Aligned rotations? Like Jupiter and most of the other gas giants share around the Sun for instance. Galaxies in associations are all haphazard with their spins and "faces" all of in different directions.

I'm sorry about the lack of vocabulary needed to really ask this question.

2010-Dec-19, 01:12 AM
Galaxy spin chirality correlation (http://adsabs.harvard.edu/abs/2009MNRAS.392.1225S). That's the technical term for it: the alignment of the spin axes of galaxies that are near each other.

We would expect galaxies less than some separation from each other to have correlated spin axes, particularly if they both formed at similar times from relatively close clouds of material. And that's what the Galaxy Zoo team found (http://adsabs.harvard.edu/abs/2010MNRAS.404..975J), using the spiral winding direction in galaxy images from the SDSS. Galaxies that formed early in the Universe's history have correlated spins, while those that formed more recently don't show such a correlation.

But it's not a very strong signal: you likely wouldn't notice it when looking at a bunch of images of galaxy groups or clusters with the Mk.1 Eyeball.

The citations of the first paper (http://adsabs.harvard.edu/cgi-bin/nph-ref_query?bibcode=2009MNRAS.392.1225S&refs=CITATIO NS&db_key=AST) above (Slosar 2009) give some other interesting recent studies on this topic, though I'm not at all familiar with any of the other studies. ngc3314 will probably be along to give some of the history of this topic: I suspect that it was only very recently that there was enough data to make a clear measurement.

2010-Dec-19, 07:40 PM
Wow! Thank you very much Mr. Parekoj. Looks like I have further reading on tidal torque theory.

I knew by your fourth or fifth post here at BAUT that you were going to be an asset to this board!

(Though if I recall correctly you first joined to refute somebodies ATM idea and I didn't know which side of the argument you were playing from.)

2010-Dec-19, 08:32 PM
What parejkoj said. The only other bit of history I want to add is that this question was apparently first looked at in the context of near-equal galaxy pairs, the simplest systems and a case where pre-Sloan data might tell something useful. The simplest version of tidal torque theory, in the case that these pairs are primordial and have always been two galaxies (rather than the merging-tree remnants of more), would be comfortable with spins mostly being opposite. Which they aren't - paired spirals with roughly aligned planes have almost equal fractions of parallel and antiparallel rotations (one example (http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1991ApJ...375L...5K&data_type=PDF_H IGH&whole_paper=YES&type=PRINTER&filetype=.pdf) - which reminds me of parejkoj's relief at finding that, while we were discussing a project, my oldest paper on the topic is not actually older than he is...). My favorite explanations to look into are that some of these galaxies are already merger products, or that environments got seriously scrambled during cosmic history so we may not always be seeing the actual perpetrators. Or as an observer, it doesn't bother me to speculate that there might be additional ways to get net angular momentum in protogalaxies, especially since there is good evidence that a lot of galaxies grew over a long period and big organized spirals only appeared after several Gyr.

Jeff Root
2010-Dec-19, 09:18 PM
This might be a good place and time to ask... Do galaxies
flatten out into disks for the same reason that (I believe)
protostellar disks form-- collisions between gas molecules
as the cloud cools off and shrinks in size, thus increasing
in density and raising the frequency of collisions? I can't
imagine how collisionless interactions between stars could
result in a disk. That should just result in virialization of
the stars' kinetic energies, so that the center of the galaxy
is dense and the outer parts not dense... right?

-- Jeff, in Minneapolis

2010-Dec-20, 02:17 AM
To a good approximation, a star's orbit is frozen when it forms (modulo encounters with clusters or molecular clouds, and very slow interactions between the orbit and shape of the galaxy's potential). That means that, more or less, a star now in the disk formed there. This fits with the classic Eggen/Lynden-Bell/Sandage picture, where the bulge and stellar halo formed early in the gravitational collapse of a spiral, and the gas that was still left over after that underwent energy-sapping collisions and flattened into a disk (where all subsequent star formation took place). This picture has needed modification with evidence that a lot of gas infall took place over a longer time and some came in as fully-formed dwarf galaxies, but the main idea that stars in the disk formed there still applies.