NGC 5135 is a barred spiral, similar in some ways to NGC 1365. Both galaxies are members of the IRAS "Bright Galaxy Sample," meaning that they are very luminous in the infrared as a result of vigorous star-forming activity. Both have very strong bars. Both harbor an active galactic nucleus at their core. (All large galaxies are believed to have a supermassive black hole at their core, but only a small fraction of those black holes are actually being fed; it is the feeding of the black hole that triggers the AGN.) And, both have been observed by Katie Chynoweth and I as part of our (sadly unfunded) project to build maps of the relative Doppler shifts and spectral line intensities of the ionized gas throughout the galaxy.
This image was taken in April 2007 at the CTIO 0.9m, on the same run as the image of ESO 264-G057 that I posted previously.
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Normally we think of active star forming regions as looking blue, because a lot of the luminosity is from O&B class stars. Presumably the really-really active star forming regions are so dense that most of the rediation is absorbed and rerediated by gas/dust?
Yeah, that's why these galaxies are IR luminous, for the most part.
This case is probably a bit different. You see the bar is reddish -- there's a lot of dense gas and dust along the bar, because the bar compresses it, and can stream gas down to the center. The center is quite red (although burnt out in this image) because there often tends to be a lot of dust around an AGN.
-Rob
Check out this paper for a neat image of the circumnuclear dust structure of NGC 5135. (Sadly, this object was not in the sample I used in a similar paper.)
"project to build maps of the relative Doppler shifts and spectral line intensities of the ionized gas throughout the galaxy."
Throughout which galaxy? Both of them?
Bright is relative. NGC1365 is listed as mag 9. It may be visible in a 10 inch dob at a dark sky site. From my 42 degrees North location, it will get as high as 18 degrees in a few months. That's alot of atmosphere to look through.
Bright is indeed relative. Given that most extragalactic astronomers nowadays tend to be working at z=1 and higher, anything with a NGC number is extremely bright.
More to the point, though, the "Bright Galaxy Sample" is defined via 12, 60, and 100 micron flux. (If memory serves; I may have gotten the IRAS bands wrong.) These are galaxies bright in the infrared-- and, "bright" being defined as "above a such and such flux cutoff." There may also be a luminosity (i.e. intrinsic energy output, not just how bright it is t ous here) cutoff; I'd have to go back and read the sample paper again to remember.
Many of the BGS galaxies won't be particularly bright in the optical, which is what defines "bright" for people looking for galaxies in their telescopes.
If you use "bright" as a qualifier, you really do have to specify what you mean, because it's relative to a lot of things. What are you comparing to, what wavelengths are you looking at, etc.
-Rob
@Rob : By 'extragalactic astronomers' do you mean astronomers outside this galaxy or outside any galaxy?
Sorry, couldn't resist, it just tickled me, kind of like theoretical physicists.