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NGC 4762: A Galaxy on the Edge

Explanation: Why is there a bright line on the sky? What is pictured above is actually a disk galaxy being seen almost perfectly edge on. The image from the Hubble Space Telescope is a spectacular visual reminder of just how thin disk galaxies can be. NGC 4762, a galaxy in the nearby Virgo Cluster of Galaxies, is so thin that it is actually difficult to determine what type of disk galaxy it is. Its lack of a visible dust lane indicates that it is a low-dust lenticular galaxy, although it is still possible that a view from on top would reveal spiral structure. The unusual stellar line spans about 100,000 light years from end to end. Near NGC 4762's center is a slight bulge of stars, while many background galaxies are visible far in the distance. Galaxies that appear this thin are rare mostly because our Earth must reside (nearly) in the extrapolated planes of their thin galactic disks. Galaxies that actually are this thin are relatively common -- for example our own Milky Way Galaxy is thought to be about this thin.

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It is fascinating to think that NGC 4762 might be a lot like NGC 891 or NGC 4565, minus the dust. The bright line along the disk of NGC 4762 appears to be the star-rich thin disk of it, after its dust has evaporated. (Here, by the way, is an Hubble image of the central parts of NGC 891, showing off its dust lane beautifully.)

Where has the dust gone in NGC 4762? It has probably disappeared along with most of the galaxy's gas supply. Alternatively, the gas and dust is so turbulent and "hot" in NGC 4762 that it can't settle down to form a central dust lane. Chances are that this galaxy is affected by the outbursts of black holes, either from a supermassive black hole of its own, or, just as likely, from the ongoing outbursts of the black hole of another Virgo galaxy, M87.

Ann

Isn't it ironic when you can barely tell stars from noise?

That's a very white galaxy!

And the galaxies in the background appear distinctly red. Is that visible red shift from stars very, very far away, or a product of some chromo-fiddling to get that very Thin, White Galactic Duke?

John

JohnD wrote:That's a very white galaxy!

And the galaxies in the background appear distinctly red. Is that visible red shift from stars very, very far away, or a product of some chromo-fiddling to get that very Thin, White Galactic Duke?

They are red shifted. The red things in the picture are representing near-infrared light. So they are either close things which are emitting that sort of light more strongly than the blue light of the other filter or they are red shifted galaxies.

i wonder if there is a thin mint galaxy

That's a very pretty picture. I do like it. Interesting to see the galaxy at that angle.

My question is: are the stars tightly packed in that line and the rest of the "glow" are also stars? Something must be reflecting and/or emitting the glow!

Guest wrote:My question is: are the stars tightly packed in that line and the rest of the "glow" are also stars? Something must be reflecting and/or emitting the glow!

The glow is just stars. You're not seeing scattered or re-emitted light.

JohnD wrote:
That's a very white galaxy!

Yes, it is quite white. It is certainly whiter than the light of a typical incandescent light bulb. But it is yellower than the Sun. If you believe that the Sun is yellow, then you should remember that this galaxy is yellower than the Sun.

Ann

Ann wrote:Yes, it is quite white. It is certainly whiter than the light of a typical incandescent light bulb. But it is yellower than the Sun.

That isn't apparent to me. Its b-v is greater than that of the Sun. I'd say that makes it less yellow (although not by a lot).

Chris Peterson wrote:

Ann wrote:Yes, it is quite white. It is certainly whiter than the light of a typical incandescent light bulb. But it is yellower than the Sun.

That isn't apparent to me. Its b-v is greater than that of the Sun. I'd say that makes it less yellow (although not by a lot).

It's the other way round, Chris. The larger the B-V index, the yellower is the galaxy. Anyway, this is a typical old population galaxy, where most of the light comes from stars that are either the color of the Sun, or yellower than the Sun.

It should be noted that a galaxy like this one is nothing like a globular cluster, whose B-V indexes may be comparatively low, in spite of the great age of the constituent stars. That is because the globulars typically contain a moderately large population of metal-poor blue horizontal branch stars. But a large galaxy like NGC 4762 is mostly made up of metal-rich stars, so the fraction of blue horizontal stars is very low.

Ann

Ann wrote:It's the other way round, Chris. The larger the B-V index, the yellower is the galaxy.

Of course. I was reading large B as brighter. Duh. Shouldn't post before breakfast.

APOD Robot wrote:is so thin that it is actually difficult to determine what type of disk galaxy it is

There is actually a category of galaxy called "superthin"!

My first thought on seeing today's APOD that it was a pulsar.
I suppose, should the day come that we could see one from a relatively close distance, it really wouldn't look this way?? More like below - but closer?

http://apod.nasa.gov/apod/ap000609.html

Probably be more like looking at a light-house beacon. More apt to see it if it was pointing at us.
The sideways beams would need something off which to reflect? If so, that would be quite a sight too!!

Chris Peterson wrote:

Guest wrote:My question is: are the stars tightly packed in that line and the rest of the "glow" are also stars? Something must be reflecting and/or emitting the glow!

The glow is just stars. You're not seeing scattered or re-emitted light.

With all that out there, for all to see.... there must be life...

I have to return to the color of NGC 4762. It really is very white. In this image by Josef Pöpsel and Beate Behle it looks definitely whiter than its apparent neighbour, NGC 4754. NGC 4762 is so white, indeed, that if we bear in mind that it must be somewhat reddened by interstellar dust along our line of sight, it hardly makes sense for it to be a large Virgo Cluster galaxy that has undergone many episodes of star formation and built up a large population of metal-rich stars. But if it is a relatively small galaxy, one that has undergone one or two violent bursts of star formation whose subsequent fireworks of supernovas blew all its gas and dust away, then it makes sense for this galaxy to have a metal-poor population that is large enough to affect the overall color of it. More precisely, it makes sense for it not to have built up a completely dominant metal-rich population that totally swamps the light of the metal-poor stars in it.

But if this is a smallish galaxy, then it can't belong to the Virgo Cluster proper. At the very least, it must be much closer to us than the typical Virgo Cluster galaxies. And indeed, according Principal Galaxy Catalog, the redshift and recession velocity of NGC 4762 suggests that it is only about 32 million light-years away, compared with about 56 million light-years for its apparent neighbour, and obvious Virgo Cluster member, NGC 4754. Admittedly the Virgo Cluster is so nearby that the proper motion of a galaxy in it may cancel out, or at least strongly counteract, the cosmological redshift of it.

But if NGC 4762 is a smallish foreground galaxy, then it makes sense for it to be whiter than NGC 4754, which certainly belongs to the Virgo Cluster proper.

I have to wonder if NGC 4762 can be compared with an older version of M82, the nearby starburst galaxy. M 82 is already losing copious amounts of gas and dust due to the strong winds of its brilliant OB stars and the violence of its many supernovas. Is it possible that M82 will one day end up looking like NGC 4762 does today?

Ann

You make a good case for NGC 4762 being both closer and smaller than advertised Ann, but how can uncertainty about such a galaxy's distance be resolved? I guess we would need a standard candle Supernova to pop off there before we could tell for sure.

Good question, Bruce. I'd say that any galaxy, old or young, must contain red giants, and in a galaxy where all the stars are old the brightest stars will be red giants. It seems to me that it should be possible to resolve a sufficient number of red giants in NGC 4762 in order to make a decent estimate about the distance to this galaxy, because red giants can be standard candles too. They typically reach a maximum brightness before they turn into white dwarfs - these are the brightest medium-mass M giants. But the red giants also reach another, lower, maximum brightness as they exhaust their supply of helium in their cores and "clump" together at the top of the red horizontal giant branch - these are the red clump stars of bright spectral class K.

If we could find a way to resolve a sufficient number of red giants, we should be able to say how far away NGC 4762 is. But I must say that the stellar population of this galaxy seems to be amazingly well mixed.

Ann

Gosh! The simian eye is as accurate as a telescope!

Please explain, Anne. When you say it is anomolously white, is that based on observations or on that image, which I suspect has been PhotoShopped to make it very white?

John
PS I imply no scientific skullduggery, just a wish for the APOD to impress.

Well, the fact that the galaxy looks extremely white in the Hubble photo doesn't necessarily mean much at all, because it may simply have been processed to look that way. But it is true that the color doesn't seem to vary at all, so the mix of stellar populations seems to be the same everywhere. There are no concentrations of particularly yellow stars anywhere, and no concentrations of blue stars anywhere.

But also the galaxy's so called B-V index (which is what astronomers use to measure the color of stars and galaxies) is relatively white, and definitely whiter than I would expect for a lenticular galaxy (that is, a galaxy with a disk but no gas or dust, like NGC 4762).

Finally, as you could perhaps see, NGC 4762 looks definitely whiter than its apparent neighbour NGC 4754 in Josef Pöppel and Beate Behle's image (the one I posted a link to).

So all in all, NGC 4762 strikes me as interestingly white. Well spotted, John!

Ann

Well, there is probably a good reason why lenticular galaxies are considered to be at some stage between elliptical and spiral. If ellipticals are older, it would figure that they are generally redder. Of course, when processing an image, a good goal to have is to assume something in the picture is white. It helps to balance the colors and is very generally correct but by no means perfect.

Geck wrote:
Well, there is probably a good reason why lenticular galaxies are considered to be at some stage between elliptical and spiral. If ellipticals are older, it would figure that they are generally redder.

I'm not sure that ellipticals are generally redder than lenticulars. I feel certain that very large galaxies with no star formation are generally redder than small galaxies with no star formation. Large galaxies are typically very metal-rich, and if the largest ellipticals are larger than the largest lenticulars, they will probably also be redder in color. Recently, it was suggested that giant elliptical galaxies contain so many small red dwarf stars that their abundance triples the the number of stars in the universe.

The reason why giant ellipticals contain so many red dwarfs is most likely that they have undergone so very many episodes of star formation, because every bout of star formation gives rise to a large number of tiny and incredibly long-lived red dwarfs. But every new generation of stars is typically, on average, more metal-rich and therefore somewhat redder than previous generations.

But as for the color of ellipticals versus lenticulars, please note that NGC 4754, the apparent neighbour of NGC 4762, is classified as a lenticular galaxy (SB0), and it is yellower than NGC 4762. Of course, I believe that NGC 4754 is also more distant than NGC 4762 and therefore more reddened by intergalactic dust. But if it is more distant, it is also larger than NGC 4762, and size, as I said, is a factor that often contributes to "redness".

Ann

geckzilla wrote:Well, there is probably a good reason why lenticular galaxies are considered to be at some stage between elliptical and spiral.

That's true morphologically, but there's not much consensus that it's true in terms of galactic evolution. Old galaxies (ellipticals and lenticulars) have almost all their stellar metrics in common. If spirals evolve to ellipticals via lenticulars, they must not spend much time in the lenticular phase.

Oh. Wikipedia makes it sound so certain...

Chris Peterson wrote:If spirals evolve to ellipticals via lenticulars, they must not spend much time in the lenticular phase.

I infer by that that lenticulars are rare then. What percentage of galaxies are classed as lenticular?