NGC 3628 : a boxy & X shaped Bulge, plus a peculiar dust lane

[madbadgalaxyman (Robert)]

Following on from the discussion between myself and Les in the Observation Reports post "AGCS 805 Observed"...... regarding the various types of bulges that can be found in spiral and S0 galaxies, I present a few images of NGC 3628 in the Leo Triplet, emphasizing various interesting features of this strange galaxy.

Firstly, a negative image showing the strange "x" shape projections above the plane of this galaxy:



Now I present a Near-infrared image from the 2MASS survey, which removes much of the complex and confusing extinction that is caused by the equatorial dust lane:



Now that the effect of much of the obscuring dust has been removed, you will note that the "X"-shaped bulge is actually symmetric, with all four arms of the X being present.
Another important feature revealed here is that the main body of this galaxy, seen here minus much of the dust, is not particularly distorted, despite the unusual bending or bifurcation of the dust lane that is seen in visual/optical imagery.

Thirdly, I present an image at high contrast, showing how very boxy ("rectangular") the bulge of this galaxy can look when seen at certain scalings:




The subtle structure of the bulge of this galaxy is relatively easy to discern in these images, because of its nearness.
But this bulge shape is common enough in some Galaxy Cluster and Galaxy Group environments!

However, the case for NGC 3628 being a peculiar galaxy is proven when it is imaged to faint isophotes and displayed at very high contrast:



As we all know, spiral and S0 galaxies, because they are thin and disk-like in three-dimensional space, show themselves to the eye and camera as ovals.
However, this galaxy is very noticeably rectilinear ("rectangular") at faint isophotes!!

cheers,
mad galaxy man

P.S.
At the website http://burro.cwru.edu/JavaLab/GalCrashWeb
, Chris Mihos and colleagues present the results of simulations showing that the apparent Warping & Tilting & Thickening of the disk component of NGC 3628 is plausibly caused by the accretion (cannibalization) of a companion galaxy which had about 10 percent of the mass of NGC 3628 .

[Lismore Bloke (Paul)]

Hello Robert,

Thanks so much for that very interesting information. It adds another aspect to our visual observations. Galaxy evolution and structure is really fascinating and I welcome posts that explain clearly the nature of the galaxies we are observing, especially if it saves us from having to wade through the SIMBAD interface!

The Wikipedia article below on NGC 5139, postulating the possibility of a black hole at its centre, and certain evidence that it may be the core of a cannabalised dwarf galaxy is just one example of a very familiar object with a truly remarkable history.

I'm sure I'm not the only visual observer thinking "more posts like that please!!!"

http://en.wikipedia.org/wiki/Omega_Centauri

[madbadgalaxyman (Robert)]

Thanks, Paul, for the encouraging comments.

My posts in iceinspace explaining the morphology & structure of galaxies are my first communications with my fellow amateurs in a long time. I used to write an e-newsletter explaining the results of extragalactic research to a select audience of Intermediate to Advanced level amateurs, but this fell by the wayside; because the required time commitment interfered too much with my personal astronomical projects.

I suspect that, in visual observation, the equatorial dust lane of NGC 3628 would look relatively normal. I think that, with sufficient aperture and with a really dark and transparent sky, the abnormal thickening of the dust lane could also be seen.
My bet is that the unusual bending or bifurcation of the dust lane (at its two ends) may be beyond the ability of visual observation to discern.....but I hope that I am wrong about this!

The typical very-stretched amateur images of galaxies, and the logarithmic scale images of galaxies that are popular with professional astronomers, are a poor guide to what can be seen by eye; partly because faint features are deliberately displayed at abnormally high brightness.
The pre-CCD era Photographic images in the Hubble Atlas of Galaxies and the Carnegie Atlas of Galaxies actually give a more realistic (true-to-life) view of what a galaxy really looks like, as there is no fancy processing done on these images.

[Paddy (Patrick)]

Great thread Robert. Those images are quite compelling. You've certainly added something to look for when observing.

[astroron (Ron)]

My posts in iceinspace explaining the morphology & structure of galaxies are my first communications with my fellow amateurs in a long time. I used to write an e-newsletter explaining the results of extragalactic research to a select audience of Intermediate to Advanced level amateurs, but this fell by the wayside; because the required time commitment interfered too much with my personal astronomical projects.





Hi Robert, I was one of the recipitents of your e-newsletter many moons back, great to see you back with your different way of reporting galaxy information
keep them coming
Cheers

[ngcles]

Hi Robert, Ron & All,

Originally Posted by astroron

I was one of the recipitents of your e-newsletter many moons back, great to see you back with your different way of reporting galaxy information
keep them coming
Cheers

Agree completely. I too was a subscriber and it was much enjoyed. Very glad to have you contributing here. For small nations (population wise), Australia and NZ pack a punch in both professional and amateur astronomy. There really are a substantial number of very talented and committed practitioners.

The dust lane in NGC 3628 does visually show some structure in moderate apertures at mod/high power. In my old 25cm at a pristine site at x276 you could see it was "crinkly". Also looks that way in 46cm in similar conditions at x247. Really is a quite underrated telescopic object.

Certainly looks to me like a merger-remnant eg; the snail-trail of molecular hydrogen (I've seen a 21cm map of it a few years ago in a paper I read), not to mention the huge tidal tail of stars (about a degree long in about PA 60) (in other words 500,000-odd ly long!!!) speaks loudly of fairly recent interaction.

Another one I've just remembered with a "boxy" bulge is another favourite (of mine) edge on spiral eg -- IC 2531 in Antlia. Love it! Dust-lane is visible in 50cm at high power in a dark sky. This is a very massive spiral 112 million ly away (infered from R/V of +2477km/sec) that is still 7 arc-mins long! (ie closing in on 200,000ly diameter). Seems to be no other eg in its immediate surroundings. May well be a field eg (ie "alone" in space).

Could *possibly* be related to the NGC 3100 group (almost 2 deg away south), though these eg have a slightly different R.V (what's +400km/sec between friends??) that argues likely non-membership of that group. (Though I haven't trawled through SIMBAD on this one yet (have I mentioned before how much I hate the SIMBAD interface??). May do it if I feel motivated enough tomorrow.

As Ron said, glad to have you!


Best,

Les D


Best,

Les D

[madbadgalaxyman (Robert)]

Thanks, Les and Ron, for the encouraging comments.

That detection of the dust lane in IC 2531 with the 50cm telescope was a good observation, Les.
According to Freeman and colleagues (see below), it is only about 10 arcseconds wide.

Here is an image of IC 2531 from the DSS version made by Odd Trondal (his supernova reference images):



Most images show the bulge of IC 2531 looking either Very Boxy...... or very peanut-shaped.
This bulge is one of the prototypes of a small boxy bulge that is actually a bar structure seen edge-on.

The bulge of IC 2531 looks to be High Surface Brightness, so its "Boxy and Peanut" shape might well be obvious in visual observations. However, the problem for most observers using smaller apertures than Les used on this object (50cm) will be actually detecting the light from this small bulge with our eyes, as the total Blue magnitude of this galaxy is only 13, and the bulge will be significantly fainter than this.

Here is another image of IC 2531, this one from the PhD thesis of M. Kregel (2003)(University of Groningen)



This image is at low contrast, so it should be closer to what we might see with the eye+telescope.

_________________________

A detailed imaging study of this galaxy, by Prof. Kenneth C. Freeman of ANU (and some colleagues), can be found in this paper:
(1989), Astrophysical Journal, Vol. 337, p.163

Here is an image of IC 2531 from this paper:



They note that the dust plane of IC 2531 is seen precisely in an edge-on orientation (it exactly bisects the bulge).