APOD: NGC 4762: A Galaxy on the Edge (2014 Nov 05)

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Chris Peterson
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Re: APOD: NGC 4762: A Galaxy on the Edge (2014 Nov 05)

Post by Chris Peterson » Thu Nov 06, 2014 6:49 pm

BDanielMayfield wrote:
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?
No idea. My observation was based on the fact that both lenticulars and ellipticals are comprised of similar populations of old stars. If the lenticular phase were a long one ending in ellipticals, I'd expect different stellar ages.
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Ann
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Re: APOD: NGC 4762: A Galaxy on the Edge (2014 Nov 05)

Post by Ann » Fri Nov 07, 2014 1:03 am

BDanielMayfield wrote:
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?
I don't think they are very rare at all. When you look at a dense cluster of galaxies, where all the (centrally located) galaxies are obviously composed of old red stars only, several of these galaxies may well be lenticulars.

I made a quick check in James D Wray's The Color Atlas of Galaxies. Of course the selection of galaxies that you find in an atlas may not be very representative of the true population of galaxies out there. There are, however, many lenticulars in Wray's atlas. As I checked up two rather red ones, NGC 1533 and NGC 1543, I found that they were quite small. Ooops! :oops:

But in any case, I can easily imagine that spirals may evolve into lenticulars, but I don't think it necessarily follows that lenticulars evolve into ellipticals.

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Re: APOD: NGC 4762: A Galaxy on the Edge (2014 Nov 05)

Post by Ann » Fri Nov 07, 2014 2:02 am

As I leafed through James D Wray's The Color Atlas of Galaxies, I came across this beautiful specimen of a lenticular galaxy, NGC 4026. The discription in the link calls NGC 4026 an edge-on spiral galaxy, but that's not what it is. In James D Wray's closeup of the galaxy, taken with the McDonald 2.1 meter telescope, we can see that there is no dust in the galaxy and no structure at all, apart from the very prominent disk. James D. Wray wrote:
This is the type example of an S0 galaxy (disk, no evident dust and no recent star formation.)
As I checked up NGC 4026, I found that it was small, smaller indeed than NGC 4762. Yes, but it is noticeably redder than NGC 4762. It is farther away and therefore likely more affected by intergalactic dust, but on the other hand it is located in the direction of Ursa Major, where there is comparatively little dust between ourselves and other galaxies.

The only lenticular galaxy I could find that is comparative in color to NGC 4762 is NGC 205, the larger but fainter of the Andromeda galaxy's two obvious satellite galaxies. The B-V colors of NGC 205 and NGC 4762 are similar, but the U-B color of NGC 205 is noticeably bluer than the U-B color of NGC 4762. And indeed, Adam Block's image of NGC 205 shows the existence of not only two small dust clouds in Andromeda's satellite galaxy, but also the presence of a small bluish population near the center of this galaxy. So it could be that NGC 205 should not actually be classified as an S0 galaxy, since it does contain dust and even some moderately recent star formation.

I think I stand corrected when I said that small galaxies aren't very red. That can't be true, since I found some quite red and quite small S0 galaxies. But at least I was correct when I said that NGC 4762 is indeed a very white S0-type galaxy.

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Re: APOD: NGC 4762: A Galaxy on the Edge (2014 Nov 05)

Post by geckzilla » Fri Nov 07, 2014 3:23 am

From looking through Hubble's archive, I've found lenticulars to be interestingly varied in both structure and dust content. They invariably present with symmetry and sometimes some faint dust bits but the structures are also often barely discernible. I can make up for that with a couple of high pass filters in Photoshop. For instance, the core of NGC 4026 which you've just mentioned:
hst_09107_a1_wfpc2_f555w_pc_sci.jpg
Of course, it's really only possible to see these using Hubble or some other advanced telescope. Otherwise, lenticulars are indeed very plain looking.


Another example, NGC 5308:
Image

NGC 4111. Try not to let the yellow color get to you on this one. It probably doesn't mean as much as you might think. If you fancy yourself a keen observer, see if you can spot the slight dimming within the white part of the galactic disc. Think of it as something like Saturn's E ring viewed nearly edgewise. Of course, the conspicuous ring of dust encircling the nucleus at a perpendicular angle is especially noteworthy for this one.
Image
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Re: APOD: NGC 4762: A Galaxy on the Edge (2014 Nov 05)

Post by Ann » Fri Nov 07, 2014 12:15 pm

Interesting, Geck. It makes sense that many, if not most, lenticular galaxies must retain some structure from their dusty and starforming days.

Of the galaxies you picked, NGC 5308 is redder than NGC 4762, while NGC 4111 is very similar to NGC 4762 in color. The obvious dust structure in NGC 4111 might that there is a non-negligent "moderately young" to intermediate population in that galaxy, which would make the overall color bluer. But since the U-B color is relatively red, such a population can't be all that young and ultraviolet.

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