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Galaxy Distribution by Type

Started by yahganlang, 08/23/2005 08:50PM
Posted 08/23/2005 08:50PM Opening Post
Hi folks. I've got a question regarding the distribution of galaxies by type in the sky. As I was getting myself a basic "up to speed" from my recently purchased set of Burnham's I started noticing that the different types of galaxies aren't spread uniformly in the various constellations- I'm NOT talking here about clumping of galaxies generally, which I'm aware of (such as the Virgo Cluster).

So I started crunching numbers- not only are there some pretty interesting skews in terms of how common different types of galaxies are across the sky, but also there is some real clumping of types as well (for instance ellipticals over here, spiral type 2 over there, and so on), as if something in the local region of formation statistically affected the entire group. Even some different types tend to be found together, at the expense of others.

I'm not aware of ever having read anything like this in any introductory books on cosmology I've read- anyone out there aware of similar findings?

Jess Tauber
Posted 08/24/2005 12:15AM #1
Jess,

Excellent question!

To rely on galaxy catalogues can be a dangerous business. Giant elliptics are more luminous than spirals, so in more distant galaxy clusters there must be zillions unicatalogued spirals. Reliable statistics can only be deduced from the nearest clusters. Is the elliptic share greater in Bootes than in Virgo?

Also, elliptics are the final phase of galaxy evolution, so there must be more of them in our vicinity, but this effect can be only seen in 5+ metre mirrors, I guess. Anyone with Obsession? smile

Dmitri
Posted 08/24/2005 06:21AM #2
Interesting issue, but quite complicated.

One suggestion is to take distance into account as well as the distribution over the celestial sphere. For this, Burnham's probably does not provide sufficient information. Sky Catalogue 2000 (Vol.2) would probably be a better source. At least the radial velocities of the listed galaxies can be found there.
Also notice that considerable variation can be observed when small samples are taken from a random distribution. I sometimes have the feeling that statistics contradict common sense.

Inge S
Posted 08/24/2005 12:26PM #3
I would also like to add that if you look at any piece of the sky not obscured by the galactic dust with a sufficiently big aperture, there will be lots and lots of unnamed and uncatalogued galaxies. Tens of them in any field of view in case of the Hale telescope. So any existing catalogue will be not enough.

Dmitri
Posted 08/25/2005 09:24AM #4
I posted the same issues to CloudyNight's deep sky discussion and apparently it is a concensus that ellipticals are OLD, and are themselves often found at the centers of galactic clusters. Spirals are found further out, along with irregulars and dwarf systems.

Sans having read the works suggested to me, it seemed that if the above structural trend is correct, it sort of parallels the structure of some galaxies themselves, just on a vaster, and more distributed, scale.

Dwarf galaxies resemble large, but loose globular clusters and are on the outskirts of the galactic clusters. Irregulars and spirals would perhaps find an analogue in gaseous nebuli and fresh star clusters within galactic arms.

The central bulges of spirals would then be analagous to elliptical galaxies.

The interesting thing here is that at both scales, very old stars are more common at the center and the very outskirts, and young ones in between, where there is more traffic?

It almost reminds me of planetary plate tectonic recycling and convection circuits. Since planets CAN get thrown clear of their parent stars if circumstances are right, I wonder whether stars (or groups of them, or their quasar-blasted remains) at the centers of galaxies ever manage to wend their way to the outside zone, and if dynamics are ever right to provide a conveyor belt to the center. Could the reworking of galactic innards be a bit more involved than the "snapshot" prejudice might expect?

Jess Tauber