6 Global Properties, Morphology & Spiral Structure
The NIR global properties of the largest galaxies provide some insight into the optically-derived Hubble morphology. The central (effective) surface brightness is higher for E/S0/Sa types compared to the late types, Sb/Sc/Sd (Figure 16). As is visually apparent from the highly-resolved images (Figures 1-6), the core/bulge light of early-type vs. late-type spirals extends much further into the disk (Figures 9, 15 & 16), underscoring the radial mass distribution differences for galaxies along the Hubble sequence (see also De Jong 1996; Grauer & Rieke 1998). For color variations across Hubble type, it has been shown that there is only small differences in J-K color between early and late-type nearby galaxies (Jarrett 2000), although the most extreme spirals (e.g., Sd/Sdm) are significantly bluer than other spirals or ellipticals (see color results below).
The global properties of Milky Way globular, NGC104, bear a resemblance to E/S0 galaxies (e.g., Figure 9), although the size (too small) and surface brightness (too high) are deviant from the statistical trends for these galaxies (Figures 14-16). Put another way, globulars have a morphology like an elliptical galaxy, but more compact and without the extended stellar structure.
The effect of large-scale bars is much more elusive with global measurements since, in part at least, the nearby galaxy sample is heavily skewed toward non-barred or "transition barred" spirals (Figure 8). It may also be true that the bar-fraction is in fact much higher in the NIR than what is seen in the optical (cf. Eskridge et al, 2000), which would have the undesired effect of diluting the optically-defined results (see also Block et al 2001; Buta & Block 2001). A more meaningful examination of morphology, and of the range of gravitational potentials of bars, can be made by using the larger nearby galaxy sample and comparing with the RC3, presented below.
To enhance the statistical trends, we now include the more complete 2MASS Large Galaxy Atlas catalog, consisting of an additional ~400 objects, in the following plots and analysis. (Photometry for the entire Atlas catalog can be found at IRSA). Here we also compare with the RC3 measurements: total B mag (BT), isophotal 25 B mag arcsec-2 diameter (D25), effective or half-light diameter (Ae), and the effective surface brightness (µe). We have not corrected for internal extinction effects (galactic or otherwise) and we have not corrected for disk inclination, which can lead to erroneous results for highly inclined spirals due to the significant attenuation of NIR photons (cf. Terndrup et al 1994; Kuchinski et al 1998).