5.2 NGC 253
The nearby starburst galaxy, NGC 253, is one of the most spectacularly resolved disk galaxies owing to its size, star formation activity (e.g., bright IRAS galaxy) and proximity to the Sun (VH = 240 km/s; D < 5 Mpc). It is also a remarkable study showing the decoupling of Population I and II disksłthe contrast between the visual and near-infrared windows is extreme. Figure 6 shows NGC 253 as seen in the near-infrared and in the visual wavelengths. The upper panel shows the near-infrared mosaic, where the 2MASS J, H and Ks bands have been RGB combined to form a color image. The lower panel shows a 4-color photographic image of NGC 253 (courtesy of amateur astronomer Tony Hallas). For the NIR, the spirals arms are clearly present, including a large "bar" feeding a compact nucleus (this galaxy is classified as a late-type disk). 2MASS easily resolves the GMCs and sites of massive star formation; extinction and reddening from gas and dust lanes are plainly evident in this color image. Nevertheless, it is astonishing how radically different the galaxy looks in comparison to the visual "mask".
In the visual wavelengths, the stellar elements in and about the spiral arms are mostly hidden by HII nebulosity and dust extinction, while the "bar" is utterly invisible in NGC 253. Whereas the near-infrared wavelengths penetrate through this "mask of smoke" to reveal the underlying stellar structure (i.e., mass density backbone) of the galaxy. After M31, NGC253 is the largest galaxy in the NIR sky (as given by r20) and the brightest (as given by K20). It demonstrates the versatility of the NIR and 2MASS for detailed study of not only the older stellar populations (in the bulge of NGC253, and early-type galaxies, for example), but also the heavily extincted starburst galaxies. NGC253 is not the only example of clear decoupling between the young and old stellar populations; e.g., NGC309 (Block et al. 1994) and several other disk galaxies presented in this paper (see the detailed look at M51 at the end of this paper).
We can further delineate the structure of NGC 253 by constructing the radial surface brightness profile and J-Ks color distribution (Figure 7). It reveals a very bright and compact nucleus, "bar" plateau, exponential stellar distribution and greatly extended emission (well beyond what 2MASS can detect at the single pixel level). The surface brightness ranges over 14 magnitudes (~6 orders of magnitude) in 34 arcminutes of diameter. The nucleus is exceptionally red (J-K > 2 mag), indicative of the heavy extinction in the core (but also similar to the nuclear colors of Seyfert/AGNs; Jarrett 2000). The bar and ringed spiral arm that contain the bar are also quite red, J-K ~ 1.2 mag, in comparison to the disk in general. The outer disk is relatively blue (J-K < 0.9 mag), with the extended emission predominantly seen in the J-band. The plot shown here (Figure 7) is an example of the standard product that is constructed for each source in the Large Galaxy Atlas.