Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
tatarjj
Pooh-Bah
Reged: 04/20/04
Posts: 1300
Loc: Austin, TX
|
|
Well, if you google "Largest galaxy in the universe" you'll get IC 1101 that is supposedly "5-6 million light years across". As I argued in this post:
Quote:
An example of a galaxy at one billion light years that I would THINK should be visible in an 8" would be IC 1101 in ACO 2029. According to wikipedia, it's 5-6 MILLION light years across. That sounds really, really big, so let's check it. Measuring IC 1101's angular diameter with the ruler tool in SIMBAD, it's a little over 1.3' across. That's ((1.3/60)/360)*2*pi = 0.00038 radians. Using the small angle approximation, that means its diameter is 0.00038*1e9 = 380,000 light years. So wikipedia fails again. Regardless, it's still a huge galaxy.
Multiple websites seem to quote that this galaxy is 5-6 million light years across, which would mean it would subtend an angle of (assuming 5.5 MLY diameter) [(5.5/1000)/(2*pi)] * 360*60 = 19', or almost TWO THIRDS the diameter of the moon. As can be seen by any image of this object, this object surely can't span any more than 2', and I like my number of 1.3'.
The math is overwhelming. Based on trignometry, you'd expect this object to be almost two thirds the diameter of the full moon, yet, on the POSS, it's clearly no more than 2'. So it looks like this object is NOT 5-6 MLY across.
Yet, some could argue "well, maybe it's got a really big outer halo..." To bad this arguement is incorrect as well. As shown in the SIMBAD Aladin Java Applet (http://simbad.u-strasbg.fr/simbad/) screen capture at the bottom of this page I just made, THE ENTIRE CLUSTER is on the order of 20' across. So IC 1101 cannot have an extended halo 5-6 MLY across simply because the other galaxies in the cluster would have to lie INSIDE this halo and would instantly capture, consume, or scatter it. If IC 1101 were TRUELY 5-6 MLY across, it would more than fill the entire image I just posted! Simply put, no matter how you shake it, the literature MUST be wrong and IC 1101 cannot be 5-6 MLY across.
It would be interesting tracking down how this absurdity got started or how to correct the literature? Even this website from Cornell University says it's 5-6 MLY across! I'd edit the Wikipedia on this object, but I don't think that my measurements would count as an official and citable data source. Did someone just accidentially add an extra zero? Or does 5-6 MLY refer to the size of the cluster and then someone thought they were talking just about the central galaxy IC 1101?
--------------------
John T.
Austin, TX
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
|
tatarjj
Pooh-Bah
Reged: 04/20/04
Posts: 1300
Loc: Austin, TX
|
|
Here's another reference, NYT:
http://www.nytimes.com/1990/10/26/us/sighting-of-largest-galaxy-hints-clues-on-the-clustering-of-matter.html
Seems like I need to look for some article from Science to see what's going on. I'm hoping that its an error by the NYT. Either that or these guys are using some kind of definition for galaxy that I'VE never heard of. Generally, (being sarcastic here) you wouldn't consider the entire local group a single galaxy. I just can't believe that it would be an error that was made in 1990 that has survived 19 years without anyone noticing... that's just too improbable.
Reading the NYT article carefully, it seems that PERHAPS the article is saying that the galaxy IC 1101 is really the ENTIRE cluster- that's REALLY stretching the definition of galaxy if true- I need to get ahold of that article to see exactly what they are claiming.
Edited by tatarjj (11/17/09 12:22 AM)
|
tatarjj
Pooh-Bah
Reged: 04/20/04
Posts: 1300
Loc: Austin, TX
|
|
Well, I think I've solved my riddle- and I DON'T like the answer. While I cannot access the article from Science, here at http://www.sciencemag.org/cgi/content/abstract/250/4980/539, here is what the abstract reads:
"A mosaic of images shows the extended structure of the cD galaxy that resides at the center of the rich cluster of galaxies Abell 2029. After correcting for the scattered light of nearby stars and galaxies, the faint halo of this giant can be traced out to a distance of more than 1 megaparsec, making it one of the largest and most luminous galaxies known. The smoothness of this halo suggests that it was formed early in the history of the cluster."
This is REALLY stretching the definition of what a galaxy is. Sounds like there is a thin smattering of stars unbounded to any particular galaxy throughout the whole cluster, throughout which many other galaxies orbit. These stars, just like the other galaxies of the cluster, would be orbiting the center of the cluster, which is IC 1101. So the astronomers are considering these stranded and isolated stars orbiting the center of ACO 2029 to be PART of IC 1101?! That's really a stretch. You might as well call the WHOLE cluster a single galaxy then. Sorry, but we've detected stranded and isolated stars flying around within the Virgo cluster, and we don't call the Virgo cluster a SINGLE GALAXY. Sure, one can talk about where the halo to a certain galaxy ends, but the problem is there are definately dozens of distinct OTHER GALAXIES orbiting throughout this star cloud in the center of ACO 2029. Those other galaxies will stir up this star cloud and locally be a of a greater gravitational influence than IC 1101. You can't tell me these isolated stars floating around between the galaxies of this cluster belong to a specific one of them.
--------------------
John T.
Austin, TX
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
Edited by tatarjj (11/17/09 12:41 AM)
|
astrotrf
scholastic sledgehammer
Reged: 09/30/07
Posts: 947
Loc: Rodeo, NM
|
|
The abstract you quoted *does* say "... after correcting for the scattered light of nearby stars and galaxies ...", so I think they are in fact saying that there is a faint remaining halo after said correction that stretches to a megaparsec from the galaxy's center. I don't think they believe this halo is "unbounded to any particular galaxy"; they mean the halo is part of IC 1101.
I've no idea what the size of the Milky Way's halo is, but if it were large enough to extend past the Magellanic Clouds, would that make it incorrect to say the halo was part of the Milky Way? It would, after all, be bound to the Milky Way, not the Clouds, except perhaps in the vicinity of the Clouds themselves.
You do make some interesting points, though; at a bare minimum, a lot more clarification needs to be provided.
--------------------
Terry (astrotrf)
|
tatarjj
Pooh-Bah
Reged: 04/20/04
Posts: 1300
Loc: Austin, TX
|
|
Quote:
I've no idea what the size of the Milky Way's halo is, but if it were large enough to extend past the Magellanic Clouds, would that make it incorrect to say the halo was part of the Milky Way? It would, after all, be bound to the Milky Way, not the Clouds, except perhaps in the vicinity of the Clouds themselves.
This is not equivalent. Why? Well, the LMC and SMC are really close to the Milky way. If there was a halo of stars surrounding and filling the entire local group, would you consider the entire local group a single galaxy?
Oh and by the way, the Milky Way does have a halo of stars, mostly grouped into things we call "dwarf galaxies". A thinner, outer halo exists where the stars are so mixed that they cannot really be grouped into individual dwarf galaxies. I don't remember offhand what the size of this halo is. However, how large is the Milky Way galaxy? The answer to that question will be, 99 times out of 100, the diameter of the MAIN STELLAR DISK. Thus, if you're going to say that IC 1101 is 5-6 million light years across, that is inconsistant with the VERY COMMON convention of giving galaxy diameters as their main stellar disk diameter (yes I know an elliptical doesn't have a stellar disk, but it still has edges that are fairly easy to define).
--------------------
John T.
Austin, TX
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
Edited by tatarjj (11/17/09 02:08 AM)
|
StarmanDan
professor emeritus
Reged: 08/27/07
Posts: 622
Loc: China Spring, Texas
|
|
Finding the "true" diameter of any galaxy is a subject of much debate. The question is where do you mark the line? Using the "main stellar disk" has its own problems as elliptical, irregular and physically interacting galaxies would not necessarily conform. Using the galaxy's halo is also problematic for reasons already stated. Even defining the diameter as a function of a galaxy's direct gravitational influence would be difficult for clusters of galaxies. Apparently, there is no "standard" for measuring this.
--------------------
"Starman" Dan Doyle
Texas Astronomical Society of Dallas
Central Texas Astronomical Society
8" LX200GPS w/ST80 guidescope, Canon 350D+DSI Pro
150mm f/8 Sky Watcher Refractor
10" f/4.5 Homemade Dob
RV-6 Criterion Dynascope
http://darcstar.wordpress.com
|
tatarjj
Pooh-Bah
Reged: 04/20/04
Posts: 1300
Loc: Austin, TX
|
|
I would think that, considering there are dozens of normal-sized galaxies orbiting within this halo of stars, that to call this shared halo of stars part of the largest galaxy in this cluster is not something most people would do.
--------------------
John T.
Austin, TX
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
|
Kobayashi
professor emeritus
Reged: 07/10/08
Posts: 738
|
|
Does it really matter? Astronomical objects typically don't have a well-defined size; it depends on the definition. By some definitions you could claim that IC-1101 is over 5 million light years, by other definitions it's much smaller. Anyone who cares about the distinction would understand (or easily find out) the definitions.
|
GlennLeDrew
Carpal Tunnel
Reged: 06/18/08
Posts: 1651
Loc: Ottawa, Ontario, Canada
|
|
I see no problem with the extended halo of IC 1101. It's most likely the result of multiple mergers of smaller galaxies, which is expected to result in a volumetrically huge, diffuse swarm of stars. The other, compact galaxies in the vicinity can be thought of as analogues of a galaxy's family of globular clusters, a great many of which survive just fine over billions of years.
Just because a denser stellar system resides within the less-dense, outer portion of a larger system does not necessarily imply impending doom for either. It can take a very long time for dynamical friction to eventually cause a smaller body to spiral into the heart of its larger cannibalistic neighbor.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery (mostly DIY stuff)
Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover
|
Achernar
Postmaster
Reged: 02/25/06
Posts: 5209
Loc: Mobile, Alabama, USA
|
|
The halo of the Milky way extends over a volume more than 500,000 light years across, if you include the most distant globular clusters that are bound to it. Andromeda's halo is even larger. The Magellanic Clouds and a number of other dwarf galaxies are well within the Milky Way's halo, and the three that have been sheared apart left stars behind in it. They're destined to be eventually absorbed into the Milky Way. While looping though the halo in their orbits, I'm sure they gravitationally push the stars orbiting out there on their own around, even ejecting some to intergalactic space forever. However, I have a hard time imagining a galaxy big enough to allow the whole Local Group to fit inside of it.
Taras
--------------------
15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
One curious cat
|
GlennLeDrew
Carpal Tunnel
Reged: 06/18/08
Posts: 1651
Loc: Ottawa, Ontario, Canada
|
|
Taras,
Even if no galaxy has ever formed with such dimensions originally, what's to say that an active period of multiple mergers could not result in such a very *distended* halo? A big, easily-observed halo merely implies a highly 'heated', or disturbed system having a goodly fraction of its members given near-to-escape energy. (And it's certain that many have been flung out or the system at well beyond escape velocity, of course.)
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery (mostly DIY stuff)
Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover
|
Achernar
Postmaster
Reged: 02/25/06
Posts: 5209
Loc: Mobile, Alabama, USA
|
|
The halo is one thing, it's very thinly populated with stars but a galaxy whose central portion is several million light years is something else yet again. A galaxy like that would boggle the mind with the sheer number of stars such an object would have. Sure a massive galaxy can have a halo that large, after all it's gravitational field extends a long way outside of it.
Taras
--------------------
15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
One curious cat
|
OpalescentNebula
super member
Reged: 01/11/07
Posts: 188
Loc: Calgary, Alberta, Observing so...
|
|
Wikipedia says the stellar disk of the Milky Way is approximately 70,000 to 100,000 lights years in diameter, and is considered to be on average about 1,000 light years thick. Containing up to 400 billion stars. The Galactic Halo with the Two Magellanic Clouds is approximately 180,000 light years. The new galaxy’s rotational speed has the Milky Way with a total mass equivalent of around 3 trillion stars, about 50% more massive then previously thought.
Using the idea of a simple volume of a cylinder. V=pr2 h
180,000 being the upper limit and 70,000 being the lower limit of our diameter, our radius would be half of these values squared times pie times the thickness.
The upper limit would be V = 3.142 x 8,100,000,000 x 1800 = 45,810,360,000,000
The lower limit would be a Volume of 6,928,100,000,000
The Largest Galaxy would have a Volume of 981,875,000,000,000,000.
That would fit 21,433 to 141,723 Milky Ways
Giving a very upper limit of 1.4 quintillion suns or putting it another way of 1.4 million million million suns.
Some people can’t imagine when I say if you take all the sand grains on our planet and all the sand grains on all the planets in the solar system; that will give you an idea of how may stars are out there.
The above equation is simplistic but puts the largest galaxy into perspective. Yes it does boggle the mind.
--------------------
Yours truly,
Bill
"Good friends are like stars...you don't always see them, but you know that they are always there" - unknown
Binos : various binos, Zeiss 10x42 FL, Garrett Optical 20x110
telescope: WO Megrez 110 ED
Eyepieces: 13mm Ethos, 5mm Pentax XO, 28mm WO & 2x Powermate
|
|
1 registered and 0 anonymous users are browsing this forum.
Moderator: LivingNDixie, Shadowalker, llanitedave
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 551
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
