Starship Asterisk*

The Densest Galaxy

Explanation: The bright core and outer reaches of giant elliptical galaxy M60 (NGC 4649) loom large at the upper left of this sharp close-up from the Hubble Space Telescope. Some 54 million light-years away and 120,000 light-years across, M60 is one of the largest galaxies in the nearby Virgo Cluster. In cosmic contrast, the small, round smudge at picture center is now recognized as an ultra-compact dwarf galaxy. Cataloged as M60-UCD1, it may well be the densest galaxy in the nearby universe. Concentrating half of its total mass of 200 million suns into a radius of only 80 light-years, stars in the inner regions of M60-UCD1 are on average 25 times closer together than in planet Earth's neighborhood of the Milky Way. Exploring the nature of M60-UCD1, astronomers are trying to determine if ultra-compact dwarf galaxies are the central remnants of larger galaxies that have been tidally stripped by gravitatonal encounters, or evolved as massive globular star clusters. Recently discovered, a bright X-ray source seen at its center could be due to a supermassive black hole. If so, that would favor a remnant galaxy origin for M60-UCD1.

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Gads Even though it's only a picture, ya almost gotta wear shades-->

Very interesting. The galaxy is so compact that it actually appears to show stellar diffraction spikes in the Hubble picture.

I commented on this galaxy at http://asterisk.apod.com/viewtopic.php?p=209551#p209551, and pointed out the similarity between this M60 satellite and M32, the small, compact satellite of giant Andromeda.

Ann

Indeed very interesting. I barely knew of M60, and now I'm being told about the existence of M60-UCD1. Where will it all end?

And thank goodness for the caption, as otherwise I might have assumed it was a lousy, overblown exposure of the Moon with a few washed-out stars and the odd aberration. (In other words, the sort of photo I would take.)

Nitpicker wrote:Indeed very interesting. I barely knew of M60, and now I'm being told about the existence of M60-UCD1. Where will it all end?

And thank goodness for the caption, as otherwise I might have assumed it was a lousy, overblown exposure of the Moon with a few washed-out stars and the odd aberration. (In other words, the sort of photo I would take.)

according to the APOD they are one and the same Nitpicker. However, on a plus note I guessed the APOD right the next clue is "Light Weekend."

Wonder what that could mean?

Ann wrote:Very interesting. The galaxy is so compact that it actually appears to show stellar diffraction spikes in the Hubble picture.

There are a lot of galaxies that do this in the archive. Usually they are catalogued as Markarian galaxies or otherwise galaxies with active nuclei.

So I have to wonder what it would be like to live on a planet that is orbiting a star in this galaxy. Nights would be much brighter than here. There would still be a lot of stars in the night sky, but they would all be brighter and bigger and closer than what we are used to. I wonder if we would see multiple stars during the daylight hours, too.

wonderboy wrote:according to the APOD they are one and the same Nitpicker.

M60 and M60-UCD1 are not one and the same. M60 (upper left) is a large elliptical galaxy, and M60-UCD1 (center) is an ultra-compact dwarf galaxy.

bystander wrote:

wonderboy wrote:according to the APOD they are one and the same Nitpicker.

M60 and M60-UCD1 are not one and the same. M60 (upper left) is a large elliptical galaxy, and M60-UCD1 (center) is an ultra-compact dwarf galaxy.

I read it wrong. sorry nitpicker.

geckzilla wrote:

Ann wrote:Very interesting. The galaxy is so compact that it actually appears to show stellar diffraction spikes in the Hubble picture.

There are a lot of galaxies that do this in the archive. Usually they are catalogued as Markarian galaxies or otherwise galaxies with active nuclei.

Quite so. Usually, though, it's just the cores of the Markarian galaxies that are extremely bright, and they often have relatively faint, extended envelopes.

What sets M60-UCD1 and M32 apart from most Markarian galaxies is that they lack this extended envelope. They are just extremely compact and extremely bright for their sizes, but really very small as galaxies go.

And they both orbit a huge bully of a galaxy, too. What a coincidence.

Ann

wonderboy wrote:

bystander wrote:

wonderboy wrote:according to the APOD they are one and the same Nitpicker.

M60 and M60-UCD1 are not one and the same. M60 (upper left) is a large elliptical galaxy, and M60-UCD1 (center) is an ultra-compact dwarf galaxy.

I read it wrong. sorry nitpicker.

Bystander and Nitpicker are not one and the same. Bystander is one of our venerable retirees and Nitpicker is a fairly new member we don't know much about yet.

geckzilla wrote:Bystander and Nitpicker are not one and the same. Bystander is one of our venerable retirees and Nitpicker is a fairly new member we don't know much about yet.

In wonderboy's defense, he was picking nitpicker's nits, not mine. I just made an error correction.

I couldn't help following the pattern, though.

APOD Robot wrote:... Exploring the nature of M60-UCD1, astronomers are trying to determine if ultra-compact dwarf galaxies are the central remnants of larger galaxies that have been tidally stripped by gravitational encounters, or evolved as massive globular star clusters. Recently discovered, a bright X-ray source seen at its center could be due to a supermassive black hole. If so, that would favor a remnant galaxy origin for M60-UCD1.

Omega Centauri, orbiting our own Milky Way galaxy at a mere 16,000 light years from Earth, is similarly ambiguous. It has traditionally been classified as a globular cluster, but if it is truly a globular, it is by far the most massive of the Milky Way's globulars, equivalent to four million solar masses compared to a few hundred thousand for a typical globular cluster. It's also the brightest, easily visible to the naked eye from the tropics and the southern hemisphere (I had the pleasure of seeing Omega Centauri during a visit to Hawai'i for the June 2012 transit of Venus).

Omega Centauri has been known since antiquity -- Ptolemy described it as a star in the Almagest and Johann Bayer took Ptolemy's word for it, cataloguing the "star" as Omega Centauri in his 1603 Uranometria. In 1677 Edmond Halley (the comet guy) was the first person who saw that Omega Centauri looks too fuzzy to be a single star, and the Scottish astronomer James Dunlop classified it as a globular cluster in 1826. We now know that Omega Centauri contains stars of different ages and compositions, unlike other globular clusters, and in 2008 Omega Centauri was observed to have a central black hole, which strongly suggests it is a disrupted dwarf galaxy, many of whose stars have been stripped away by the tidal pull of the Milky Way.

So Omega Centauri is the only object that at different times in history has been a star, a globular cluster, and a galaxy!

Today's APOD makes me wonder if there is a clear demarcation or mass gap between the smallest dwarf elliptical galaxies and the largest globular clusters. Or are these two classes of objects the same, except for where they are located?

P. S. I wrote the above prior to seeing Anthony’s comment, which relates to my question too, since he points out another ambiguous case. Is there any difference between these objects other than size and location in or outside another galaxy?

BDanielMayfield wrote:Today's APOD makes me wonder if there is a clear demarcation or mass gap between the smallest dwarf elliptical galaxies and the largest globular clusters. Or are these two classes of objects the same, except for where they are located?

P. S. I wrote the above prior to seeing Anthony’s comment, which relates to my question too, since he points out another ambiguous case. Is there any difference between these objects other than size and location in or outside another galaxy?

My understanding is that the stars in a typical globular cluster are all the same very old age and have very low concentrations of elements heavier than hydrogen and helium. I.e. they're all "Population II" stars, which are older than "Population I" stars like our Sun (maintaining this arcane terminology is one way that astronomy professors torture their introductory astronomy students). When you see something that looks like a globular cluster but has stars of widely differing ages and compositions, you're probably looking at a dwarf galaxy. And if it has a central black hole, that suggests that it used to be bigger and has lost a lot of stars to tidal interaction with a larger galaxy.

If you have a clear dark sky and a good telescope, look at some globular clusters and some open clusters. The globulars have a more even mellow golden glow, because all the surviving stars are old yellow or red stars (nitpicker foil: you're not going to notice "blue stragglers" through an amateur telescope). Young open clusters look more blue and sparkly, because they still have some massive blue stars.

I just checked M32 with my software. It says that the surface brightness of M32 is 1 on a scale from 1 to 6, where 1 is brightest and 6 is faintest. Of course it's not so surprising that the surface brightness of M32 would be high. But my software also says that the surface brightness of "the first outer region" of M32 is 6, so its outer region is exceedingly faint! For comparison, the surface brightness of the "first outer region" of the big Andromeda galaxy is 2!

Ann

bystander wrote:

wonderboy wrote:according to the APOD they are one and the same Nitpicker.

M60 and M60-UCD1 are not one and the same. M60 (upper left) is a large elliptical galaxy, and M60-UCD1 (center) is an ultra-compact dwarf galaxy.

I'm confused. M60 appears absolutely dead center in the image. NGC 4647 is in the upper right, and M60-UCD1 is in the center of the lower right quadrant.

geckzilla wrote:Nitpicker is a fairly new member we don't know much about yet.

To borrow from the caption, "dense smudge" could equally be applied to me. Hope that helps.

Chris Peterson wrote: I'm confused. M60 appears absolutely dead center in the image. NGC 4647 is in the upper right, and M60-UCD1 is in the center of the lower right quadrant.

Are we looking at the same image? I see M60 as a big overexposed thing dominating the top-left quadrant. M60-UCD1 is the smallish, whitish "desnse smudge" just below image centre. A little up and to the right from M60-UCD1 is a reddish star, with a prominent cross-shaped diffraction spike (if that is the correct term).

Nitpicker wrote:

Chris Peterson wrote: I'm confused. M60 appears absolutely dead center in the image. NGC 4647 is in the upper right, and M60-UCD1 is in the center of the lower right quadrant.

Are we looking at the same image? I see M60 as a big overexposed thing dominating the top-left quadrant. M60-UCD1 is the smallish, whitish "desnse smudge" just below image centre. A little up and to the right from M60-UCD1 is a reddish star, with a prominent cross-shaped diffraction spike (if that is the correct term).

I think I get the confusion. You're looking at the cropped image on the main page? I'm looking at the actual APOD image that you get when you click on that image.

I almost never pay attention to the image on the main page, since it's usually an inferior representation of the original, for any number of reasons. Usually there's no confusion about the position of objects, but with a crop like this that isn't the case.

Chris Peterson wrote:

Nitpicker wrote:

Chris Peterson wrote: I'm confused. M60 appears absolutely dead center in the image. NGC 4647 is in the upper right, and M60-UCD1 is in the center of the lower right quadrant.

Are we looking at the same image? I see M60 as a big overexposed thing dominating the top-left quadrant. M60-UCD1 is the smallish, whitish "desnse smudge" just below image centre. A little up and to the right from M60-UCD1 is a reddish star, with a prominent cross-shaped diffraction spike (if that is the correct term).

I think I get the confusion. You're looking at the cropped image on the main page? I'm looking at the actual APOD image that you get when you click on that image.

I almost never pay attention to the image on the main page, since it's usually an inferior representation of the original, for any number of reasons. Usually there's no confusion about the position of objects, but with a crop like this that isn't the case.

It's reassuring to know that I'm not the only person who occasionally gets confused.

Chris Peterson wrote:I think I get the confusion. You're looking at the cropped image on the main page? I'm looking at the actual APOD image that you get when you click on that image.

Ah, I see. Well, the full, uncropped image certainly looks like nothing I've ever taken.

Anthony Barreiro wrote:It's reassuring to know that I'm not the only person who occasionally gets confused.

I'm just the opposite. I occasionally have bouts of un-confusion, but they pass quickly!