Starship Asterisk*

I was messing around with processing this galaxy when I decided there was something interesting enough to start a discussion.

First, there is an elliptical globular cluster next to this dwarf galaxy. In the attached picture, it's near the bottom of the frame, just below the dwarf galaxy. It even faintly looks as though it has an edge-on disk. Weird! I was incredulous enough at first to think that maybe it just had an edge-on galaxy behind it, but that does not seem to be the case.

Next, I was actually considering emailing CDS because the entry in SIMBAD calls NGC 5238 a pair of galaxies. Looking at the Hubble data, I dismissed it as being a pair because it looked like one dwarf galaxy to me. I have to be cautious because I could easily be wrong, so I thought some more before sending an email. Meanwhile, the mystery of the ellipsoid globular cluster is plaguing me. Could it be two recently merged dwarf galaxies and that cluster was part of one of them?

geckzilla wrote:I was messing around with processing this galaxy when I decided there was something interesting enough to start a discussion.

First, there is an elliptical globular cluster next to this dwarf galaxy. In the attached picture, it's near the bottom of the frame, just below the dwarf galaxy. It even faintly looks as though it has an edge-on disk. Weird! I was incredulous enough at first to think that maybe it just had an edge-on galaxy behind it, but that does not seem to be the case.

Assuming it is, in fact, a structure local to the galaxy (which seems plausible given that it apparently has resolved halo stars similar to the resolved stars in the galaxy), I'd hesitate to consider it a globular cluster. I did find a paper discussing elliptical globular clusters (posited to be created by a lack of strong tidal forces generating a spherical shape from the presumed initial oblate form), but I note that the ellipticities (1 - b / a) of objects examined are in the range of 0.1 - 0.2, whereas the object in your image is more like 0.4 - 0.5. Maybe there are such globulars, but I don't find quick reference to them. Your suggestion of the product of some sort of interaction seems more likely to me, especially as this entire area looks like something mixed it up pretty good. But it also looks like a low mass galaxy (or galaxies), and according to the paper, that's what we might expect to have elliptical globular clusters around it.

I tried to think of a more appropriate word for it but I don't know any better than globular cluster. Tiny galaxy would work but I've never seen one so compact. There are at least four obvious, normal globular clusters including a bright, youthful-looking one at the visual center of NGC 5238. They all provide a stark contrast for the thing in the southeast.

Assuming for a moment that it is a globular cluster, it is perhaps too difficult to get a good estimate of its oblateness, from its coarse pixel scale in this image. It might not be quite as oblate as 0.4 to 0.5.

Due to a strong resemblance to the galaxy type, perhaps a good term would be Lenticular Globular Cluster

I think you mean Football-Shaped Globular Cluster.

Bad geck! (Especially during the FIFA world cup.)

Given that GCs are either spherical or slightly oblate spheroidal (but not, I think, prolate spheroidal), "football shaped" and "ovoid" are both poor descriptors for GCs. "Lenticular" is better, as a lentil is roughly an oblate spheroid. Personally, I would prefer Oblate Globular Cluster.

Relax, it was a joke.

Obviously, we need to be able to define more accurately what it is before getting into nomenclature.

geckzilla wrote:Relax, it was a joke.

Obviously, we need to be able to define more accurately what it is before getting into nomenclature.

I was merely attempting to acknowledge your joke without resorting to emoticons.

There does seem to be a grey area in the classification of the full range of globular clusters and dwarf galaxies, and we can't even be sure if this thing is either.

World Cup isn't football, it's futbol.

bystander wrote:World Cup isn't football, it's futbol.

Ahem, that's the Fédération Internationale de Football Association World Cup, in all its evil splendour.

Very interesting, Geck.

Let's look at the thing. It is white in color. Therefore it is not a far-away background object, which are yellowish or orange.

NGC 5238 is not white in color, however. Its bright stars are mostly blue, with a few bright yellow-orange stars in the mix. The faint stars of NGC 5238 are mostly yellowish. The bright stars are undoubtedly young and quite bright stars, many of spectral class B and maybe even O, as can be deduced from the widespread presence of green (really red) emission nebulae. A small number of the bright yellow-orange stars just might be young M-type giants or (lesser) supergiants, or at least bright young K-type giants or supergiants. The faint yellowish stars are probably (that would be my guess anyway) modest old red giants.

The mightiest cluster of young blue stars is of course the structure to the lower left of the other blue stars in NGC 5238. There is a mighty green (really red) arc of nebulosity to the upper right of it. As for the cluster itself, it has an obvious core, and there are more widely scattered stars apparently flowing away in the opposite direction than the green arc of nebulosity. To me this suggests a young compact cluster whose outer parts are either scattering away, or else they are the products of earlier rounds of star formation.

My software calls this structure PGC 47857, which means it has been classified as an independent galaxy. I really don't think it looks like a galaxy - at best it is a very proto-proto-galaxy! - but please note that the green arc "above" this cluster looks a bit like a bow shock. It is just faintly possible that the cluster was formed farther away from the core of NGC 5238 than its present location, and then it just might have been acted on by the gravity of NGC 5238 so that it is now moving in towards the core, creating a bow shock in front of it. (Of course the green arc might just be the brightest part of the emission nebula surrounding this cluster.) Note that the core of NGC 5238 is obviously brighter than the cluster that may be moving towards it. The core is also slightly bluish in color. This may account for the fact that my software says that the effective B-V color of NGC 5238 is 0.130, which is absolutely extremely blue for a galaxy.

Let's return to the elongated globular cluster. It is an extremely high surface brightness object, and unlike any of the background galaxies, it appears to be surrounded by a halo of stars. This clearly suggests that the object is a globular cluster, although a very strange one.

Here is my guess. I would guess that the elongated globular cluster just possibly might be a comparatively high-metallicity cluster, so that it lacks blue horizontal stars, similar to 47 Tucanae. 47 Tuc is quite white in color, compared to the bluish color of apparently neighbouring dwarf galaxy, the Small Magellanic Cloud. The white color of the globular cluster next to NGC 5238, compared with the mostly bluish color of the dwarf galaxy itself, might be explained in a similar way.

(There is by the way another globular cluster in the picture of NGC 5238, right next to the black area at upper right. This makes the appearance of NGC 5238 eerily similar to the SMC and the two globular clusters that are seen in the direction of it, 47 Tuc and NGC 362.)

But what about the elongated shape of the cluster? Here is my guess. Quite a long time ago, this cluster might have merged with another globular cluster in a way that induced a disk formation in the cluster. And if it didn't merge with another globular cluster, then perhaps it merged with something else, like possibly a concentrated chunk of dark matter. Or something. All the messiness of tidal tails and other kinds of untidiness are gone by now, but the elongated disk shape of the globular cluster remains.

Just my two cents.

Ann

Ann wrote:My software calls this structure PGC 47857, which means it has been classified as an independent galaxy.

PGC 47857 is a synonym for NGC 5238. If you look up PGC 47857 at SIMBAD it takes you straight to the same entry for NGC 5238. There is no designation, much less a classification, for the thing in the corner. As usual, I would be cautious in making judgements based on the system's colors, especially when I have made no such attempt at measurable accuracy.

I'm feeling sheepish now because the appearance of elongation of the globular cluster is at least partially due to a diffraction spike which is visible along that axis but not the other one. I'm sleepy and annoyed that I could be so stupid, but at least I've caught it now before heading off into really unforgivably stupid territory (and no one else saw it yet, either, so there!). I have to look at it some more tomorrow and figure out what is artifact and what is not. It may still be slightly elliptical, but it's not certainly not as pronounced as I have foolishly presumed it to be. This unicorn is just a horse with a narwhal's tusk strapped to its forehead!

Geck wrote:
PGC 47857 is a synonym for NGC 5238.

My software says that PGC 47857 is located in the lower part of NGC 5238. According to my software, the magnitude of NGC 5238 is 13.7, while the magnitude of PGC 47857 is 16.2. So there must be some people who think that NGC 5238 and PGC 47857 are not the same.

Ann

This is an incredible image of the area around NGC 5238!

As points of clarification, a check of NED http://ned.ipac.caltech.edu shows that the "elongated globular" to the southeast is SDSS J133445.99+513547.6 while the other "globular" at the northwest edge of the image is SDSS J133441.02+513733.9. Neither of these have been studied in detail, yet. Spectroscopy would give a better indication of the true nature of these sources.

I believe the confusion of whether NGC 5238 is a pair of galaxies or a single one can be traced to the two bright "hot spots". Zwicky in 1971 called it a "Pair of blue post-eruptive interconnected Sc's with compact nuclei." Karachentsev included these in his "Isolated Pairs of Galaxies Catalogue" in 1972. Obviously modern images are much better than the Palomar Sky Survey they worked from 40 years ago. The different PGC numbers assigned (PGC 47853 and 47857) refer to these two bright clumps. So, NGC 5238 is the combination of these two objects and nomenclature and classification can easily get confused.

FWIW!

Thanks, zb. I did look around in NED but I am much less familiar with it. I was looking through it at some point and ended up learning from this paper it is part of a group of 15 galaxies with the Whirlpool being the biggest. I guess that makes it one of the Whirlpool's satellites.

Anyway, having a night of rest and looking at the thing again, SDSS J133445.99+513547.6 is definitely a flattened blob, but that edge-on disk looking structure I think is just a diffraction spike. It's annoying, though, because the major axis is just about perfectly aligned with the diffraction spike which makes it very hard to tell what is spike and what is not.

geckzilla wrote:Anyway, having a night of rest and looking at the thing again, SDSS J133445.99+513547.6 is definitely a flattened blob, but that edge-on disk looking structure I think is just a diffraction spike. It's annoying, though, because the major axis is just about perfectly aligned with the diffraction spike which makes it very hard to tell what is spike and what is not.

I'm not convinced. It doesn't really look like a diffuse diffraction spike, and it doesn't look like it's quite lined up with other stellar diffraction spikes in the same image. Also, it seems to be present in all three channels, which I wouldn't expect for a diffraction artifact.

Do you have a link to the FITS files you used?

The blue channel in this case is slightly in use by F814W. Sorry I didn't explain that in the post. It usually doesn't matter. The blue is useful in this case because the telescope was oriented differently so the diffraction spikes don't align with it.

F606W http://hla.stsci.edu/cgi-bin/fitscut.cg ... ormat=fits
F814W http://hla.stsci.edu/cgi-bin/fitscut.cg ... ormat=fits

For F275W, F336W, F438W if you anonymously FTP to archive.stsci.edu the three FITS files will be in /stage/anonymous/anonymous75651 (They probably won't be there for more than a few days, or even much less...)
Edit: If they're not there anymore, I put them here. http://www.geckzilla.com/astro/NGC5238.zip (341 MB)

Thank you Geck for starting this thread and for zb for giving the info about the NED.IPAC database. I've just been exploring it with great satisfaction!
M

Blowing up parts of the image to 500% rules out, I think, the spike theory. The left part of this composite image is the spikey bit from very near the top of the image; the right side of this is the possible GC under discussion. I see the spikes of the former as distinctly linear, while lacking in any particular starriness. Whereas the GC is just the opposite, no really distinct linearity and definitely a sprinkling of bright stars along its 'waistline'. Have a look...
Rob

I think it's a little too coincidental that the linear, diffuse structure of the blob of stars lines up exactly with the diffraction spikes but maybe I did jump too quickly to that conclusion.

NED results for object SDSS J133445.99+513547.6
I may be being dense - but is this the object under consideration?

It says "object type - G". Which at the help page is said to mean Galaxy.

So where did 'globular cluster' come from?
As I said, it is perfectly possible that I'm being obtuse.

M

Yeah, that's the squishy football ellipsoid globular cluster wannabe. The more obscure an object is, the less likely it is to have been looked at thoroughly. Such entries in SED or SIMBAD could be wrong.

MargaritaMc wrote:NED results for object SDSS J133445.99+513547.6
I may be being dense - but is this the object under consideration?

It says "object type - G". Which at the help page is said to mean Galaxy.

So where did 'globular cluster' come from?
As I said, it is perfectly possible that I'm being obtuse.

It might be a galaxy. Or it might be an unusual form of cluster that simply hasn't been properly classified. And some of the classifications are automatically generated. I think most any automatic classifier would identify this object as a galaxy.

Interesting - it is extremely compact and indeed quite elongated at faint levels. It's almost certainly not a background galaxy as there is an enhancement of point sources at its location. (an example of a background galaxy is at 3677, 2802)
Perhaps a globular cluster that is being tidally disrupted?