Comments on: This is a galaxy?

By: Pete Jackson

Pete Jackson — Wed, 29 Jun 2011 15:13:49 +0000

No, Zach. The center of our Galaxy is about 30,000 ly distant, only one-tenth as far.

By: Zach

Zach — Wed, 29 Jun 2011 02:07:36 +0000

At 300,000 ly distant, we’re actually closer to this thing than we are to the center of our own galaxy!

By: TMB

TMB — Tue, 28 Jun 2011 17:02:03 +0000

@33 and 34: The faintest dwarf galaxies are indeed fainter than the brightest globular clusters, but the key difference is their dynamics: globular clusters are just bound together by the gravity of their stars, and so are very compact, while dwarf galaxies are bound together by the gravity of their parent dark matter halo, which contains much more mass so the galaxies are much more diffuse. But they can’t just be disrupted GCs because their velocities are random – they are not expanding, the stars are just whizzing around inside the dark matter halo (although some dwarf galaxies are tidally disrupting too – see the Sgr dSph, for the most dramatic example).

By: Captn Tommy

Captn Tommy — Tue, 28 Jun 2011 12:11:11 +0000

Is it possible that this is a dispersed/dispersing Globular Cluster? It probably (from the looks of it) has as many stars. whose to say a catastrophic disrupttion could not overcome the gravity holding a Cluster together. It would also explain the lack of gas.

Has the relative velocities and motions of the stars been studied to see if an origin point can be determined?

Interesting picture.

By: Thameron

Thameron — Tue, 28 Jun 2011 04:16:12 +0000

So how many stars does it take to stop being a globular cluster and start being a dwarf galaxy?

By: Messier Tidy Upper

Messier Tidy Upper — Tue, 28 Jun 2011 03:35:54 +0000

More info on the Carina dwarf on wikipedia here :

http://en.wikipedia.org/wiki/Carina_Dwarf

whilst this :

http://en.wikipedia.org/wiki/Local_Group

gives a list of the Local Group galaxies of which our inconspicuous Carina dwarf is but one member.

Mind you it’s not the faintest, hardest or most recently spotted as this :

http://kencroswell.com/UrsaMajorDwarf.html

article by Ken Croswell illustrates.

By: VinceRN

VinceRN — Tue, 28 Jun 2011 03:21:59 +0000

My favorite thing about this blog, besides the blogger himself, is how much good information the commentators come up with.

By: Messier Tidy Upper

Messier Tidy Upper — Tue, 28 Jun 2011 02:55:33 +0000

@ ^ Jon Hanford – & @ Pete Jackson too : Thanks.

By: Jon Hanford

Jon Hanford — Tue, 28 Jun 2011 01:34:01 +0000

@Simon #19, Grimbold #23,

Grimbold, yes, astronomers working on the ESO/SRC Southern Sky Survey, using the 1.2m UK Schmidt Telescope at Siding Spring Australia, found the galaxy by visual inspection of one of the plates taken for the survey (and partially seen on an adjoining plate). The discovery paper, along with the discovery image, can be found here: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1977MNRAS.180P..81C&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

By: Pete Jackson

Pete Jackson — Tue, 28 Jun 2011 01:27:27 +0000

I have taken the image as published by BA and adjusted the brightness and contrast to show the Carina Dwarf Galaxy much more clearly, at least to me. Many browser/monitor combinations render dark images with too low brightness. Let me know if this image shows it more clearly to you:

http://www.chinesemeridiansblueprintsoflife.com/Images/

As I have mentioned before, you will see much more detail in an image if the background is gray than if the background is black.

By: Ross Amans

Ross Amans — Tue, 28 Jun 2011 00:07:32 +0000

There should be a clear plot of galaxy diameter vs frequency oi galaxy, and similar galaxy mass/freq, galaxy number of stars/freq, galaxy brightness/ frequency. So an estimate of density of galaxies in starfields could be made, and also, density of visible galaxies in the starfield.
But I cannot find any such plots.
Anyone know where they are hiding?
It would be bad astronomy if they did not even exist.

By: Messier Tidy Upper

Messier Tidy Upper — Mon, 27 Jun 2011 23:36:02 +0000

Not much to it, is there?

Well, everything is relative. That’s still an awfully large region of space and number of stars and perhaps exoplanets. Even a tiny and comparitively unimpressive dwarf galaxy is an island universe of its own!

By: VinceRN

VinceRN — Mon, 27 Jun 2011 23:09:53 +0000

@21 Germs – excellent.

By: Checkmate1

Checkmate1 — Mon, 27 Jun 2011 23:05:26 +0000

Okay, I see it now, thanks to the overlay photo (#21). But it begs the question: What IS the small blue smudge around the central star? None of the other bright stars seem to have this appearance.

By: Grimbold

Grimbold — Mon, 27 Jun 2011 21:47:42 +0000

@Simon #19
I was shown transparent negatives of photographic plates on a tour of Siding Spring observatory and one of these had a dwarf galaxy on it (may in fact have BEEN Carina, I can’t remember). On this negative it was very obvious where the galaxy was. With foreground stars some appear large, others small. With the dwarf galaxy ALL of its stars are very small due to its distance and somewhat close together. It stood out pretty obviously once you knew what to look for. Carina would likely have been found by an experienced astronomer with a magnifying glass and a light table.

It is not as obvious in the picture accompanying this article but I can still make it out.

By: HvP

HvP — Mon, 27 Jun 2011 21:43:28 +0000

VinceRN,

As I understand it, the discovery of this galaxy isn’t a discovery of the actual stars themselves – we had already recorded them a long time ago. It’s the discovery that some of the stars are gravitationally interacting as a galaxy that is new.

So no, this isn’t previously unidentified matter. The stars were seen and counted. It was simply assumed that they were part of our own galaxy instead of in their own galaxy.

By: Germs

Germs — Mon, 27 Jun 2011 20:52:33 +0000

Here’s an overlay (animated GIF) of the image from #13 VinceRN’s link and the ESO image.
http://tinypic.com/view.php?pic=2zhnx1c&s=7

By: VinceRN

VinceRN — Mon, 27 Jun 2011 19:53:58 +0000

@14 Gary – Sorry if I misunderstood you, but if there’s not enough “by about five times” to account for the excess momentum, wouldn’t that reduce the fraction of dark matter by about a fifth? I mean that would finding that much more matter reduce the amount of dark matter from 23% to 18.4%(supposing the “by about five times” that you stated)? Or would the amount of matter in these abundant dwarf galaxies be negligible compared to the amount of dark matter?

Perhaps you thought I was saying there would be no dark matter? Far from it, I’m just wondering about how all this previously unseen mass fits into the equation. Would it raise the amount of atoms in the universe by a percent or two, or would it be far less than that?

By: Simon Green

Simon Green — Mon, 27 Jun 2011 19:34:42 +0000

I’m interested in how it *was* finally discovered. I imagine someone (or some computer) with a lot of spare time made distance/velocity measurements on all the stars in the image? Perhaps as part of a larger survey? And then someone noticed a correlation?

Or a slightly different sequence of events whereby a small sample of the stars were measured and someone came up with a distance of 300K lightyears. Checking a few more yielded the same result, and *then* a more in-depth study revealed the whole lot?

By: MikeS

MikeS — Mon, 27 Jun 2011 19:20:17 +0000

Ah, Carina, my old friend. The commenters above illustrate why these are so hard to find. This doesn’t look like a galaxy in the traditional sense. It’s more of an overabundance of stars. If you look at them on a big Schmidt or POSS plate, covering a huge area, they become more obvious, looking like a thumbprint. But our surveys have shown that they are VERY extended — Carina actually extends well off of the image Phil has above but it as such low surface brightness (stellar density) you can only detect it statistically or with very precise methods. Sloan has detected a bunch of these guys that look like ‘nuthin.

By: chris j.

chris j. — Mon, 27 Jun 2011 18:47:26 +0000

i wonder what the milky way looks like, to someone observing from the carina dwarf. would a glorious dawn await?

By: Anchor

Anchor — Mon, 27 Jun 2011 18:46:42 +0000

Lotsa flotsam and jetsam out thar…

By: Gary Ansorge

Gary Ansorge — Mon, 27 Jun 2011 18:33:42 +0000

12. VinceRN

“If it were true that these were the most common type of galaxy in the universe, would all that previously unobserved mass appreciably reduce that fraction of dark matter that makes up the universe?”

No!

First off, if it shines, it isn’t “dark”. Second, it’s the excess momentum of stars around the galaxies that led to the “dark matter” hypothesis. Not enough visible/normal matter in the galaxies(by about five times) to account for the excess momentum.

Gary 7

By: Douglas Troy

Douglas Troy — Mon, 27 Jun 2011 18:29:42 +0000

Only 300,000 light years, well then, why don’t you just zip on over there and get some better pictures for us Phil. We’ll wait.

By: VinceRN

VinceRN — Mon, 27 Jun 2011 18:11:35 +0000

If it were true that these were the most common type of galaxy in the universe, would all that previously unobserved mass appreciably reduce that fraction of dark matter that makes up the universe?

@ #10 and #11 – thanks for clarifying that, I expected to to be smaller and partially obscured by the bright star, but couldn’t really make it out.
I found another picture at http://users.westconnect.com.au/~sjastro/carinadwf.html that shows that better, though it’s not nearly as pretty.