NGC 253 - an optical/infrared comparison

[madbadgalaxyman (Robert)]

Here are two images of NGC 253, not exactly in registration, but close enough to enable comparison between them.

The first is a visible-wavelengths image,
and the second is from the 4.1 metre VISTA telescope in the near-infrared
[[ The infrared image is composite of Z and Y bands (0.88 and 1.02 micrometers) and also the Ks and J bands (2.15 micrometers and 1.25 microns), possibly with a narrowband filter image also added to it. ]]

The highly-foreshortened bar structure and the two primary spiral arms are much more evident in the near-infrared, and it is consistent with the morphology of the largest-scale features that the bar structure sits within a ring structure.

Another interesting aspect of the NIR image from the VISTA telescope is that we can make a more informed hypothesis as to what parts of the heavy and rather-chaotic dust distribution (which is so very evident at visible wavelengths) are associated with major structural features in this galaxy (e.g. the bar, spiral arms, etc.) and what parts are not;
This sort of analysis may give some further insight into the structure of this galaxy, e.g. what is the nature of the unsettled-looking dust streamers on the right side of this galaxy that look like they could be rising above the principal plane of this galaxy?

The observed overall dust asymmetry & entropy would, considered as an isolated fact, probably not be interesting, because many other galaxies which have a high Current time-rate of formation of New Stars can also have this sort of heavy and semi-chaotic dust distribution, BUT........
I am also currently contemplating the nature of some other interesting peculiarities in NGC 253;
- its asymmetric outer halo.
- the unusual outermost distribution of the HI (neutral atomic hydrogen gas) in this galaxy, which is perhaps a sign of a gas inflow or outflow.
- the notably compact disk of HI, which is truncated (absent outside of the bright optical portions of this galaxy), something which is, under normal circumstances, the signature of a galaxy which has undergone multiple interactions in a dense Galaxy Cluster environment.

So, given that NGC 253 actually has several different peculiarities and anomalies, perhaps there is some correlation between all of these interesting facts??!???

[multiweb (Marc)]

The IR version is an eye opener. I never noticed it was a barred spiral not unlike the one face on in Fornax.

[madbadgalaxyman (Robert)]

Originally Posted by multiweb

The IR version is an eye opener. I never noticed it was a barred spiral not unlike the one face on in Fornax.

Marc

In optical imagery, and also in visual observation, there is the suggestion of a bar in NGC 253, but when I compared the so-called 'bar' seen at visual wavelengths to the actual bar that is plainly seen longward of 1 microns, I found that the bar seen in visual imagery and observations is not the real bar; the visual "bar" is a sort of "cutout" made of residual light that is surrounded by highly obscured areas.

It is extremely tough to figure out the actual structure of a galaxy like this, which is not far from being in an edge-on orientation, as all of the structure is foreshortened and compressed (the NGC 4945 case is even harder to figure out than NGC 253!). I am not up on the most recent estimates of the inclination of NGC 253, but some of the older literature gave values of 10-18 degrees from edge-on.

The "traditional" hubble class assignment of this galaxy, which is traditionally estimated from blue or visual imagery, is actually Sc [ or perhaps S(B)c ], but it is the very dusty nature of the images at these wavelengths that gives this galaxy the appearance of having large star-forming knots as are commonly seen in an Sc galaxy.
(The HII regions look quite small, so the Sc type is, in my view, highly questionable)

However, the near-infrared Hubble Type is probably earlier in the Hubble Sequence; SBbc or SBb

cheers,
bad galaxy man

Added in edit:
For fun and profit, here is another version of the NIR image, emphasizing the dust distribution:



And here is a negative version of a very high quality LRGB taken with a 24 inch RC at the Mt Lemmon SkyCenter by Adam Block. The heavy dust distribution all over the face of this galaxy is particularly obvious in this negative.