Why are the nuclei of some galaxies more active than others? If most galaxies harbor a central massive black hole, the main difference is probably in how well it is fueled by its surroundings. We investigate the hypothesis that such a difference can be seen in the detailed circumnuclear morphologies of galaxies using several quantitatively defined features, including bars, isophotal twists, boxy and disky isophotes, and strong nonaxisymmetric features in unsharp-masiced images. These diagnostics are applied to 250 high-resolution images of galaxy centers obtained in the near-infrared with NICMOS on the Hubble Space Telescope. To guard against the influence of possible biases and selection effects, we have carefully matched samples of Seyfert 1, Seyfert 2, LINER, starburst, and normal galaxies in their basic properties, taking particular care to ensure that each was observed with a similar average scale (10-15 pc pixel-1)- Several morphological differences among our five different spectroscopic classifications emerge from the analysis. The H II/starburst galaxies show the strongest deviations from smooth elliptical isophotes, while the normal galaxies and LINERs have the least disturbed morphology. The Seyfert 2s have significantly more twisted isophotes than any other category, and the early-type Seyfert 2s are significantly more disturbed than the early-type Seyfert 1s. The morphological differences between Seyfert Is and Seyfert 2s suggest that more is at work than simply the viewing angle of the central engine. They may correspond to different evolutionary stages.

We have compared the morphological characteristics of the 891 galaxies in the Extended 12 μm Galaxy Sample (E12GS) and assessed the effect of the 12 μm selection criterion on galaxy properties. The normal spirals in the E12GS have the same axial ratios, morphological types, and bar and ring fractions as other normal spirals. The H II/starburst galaxies have a higher incidence of bars and more than twice the normal rate of "peculiar" morphologies, both of which are attributable to relatively recent disturbances. The 12 μm Seyfert galaxies show a small (10%) deficiency of edge-on disks. This is caused by extinction but is a much less severe effect than in optically selected samples. There is a similar modest deficit of highly inclined H II/starburst galaxies in the 12 μm sample. The galaxies with active nuclei (Seyfert galaxies and LINERs) have the same incidence of bars as normal spirals but show rings significantly more often than ormal galaxies or starbursts. The LINERs have elevated rates of inner rings, while the Seyfert galaxies have outer ring fractions several times those in normal galaxies. The different formation times of bars and rings suggest an interpretation of these differences. Bars form relatively quickly and indicate that material is recently being transported (by redistribution of angular momentum) to the center of the galaxy, where it is likely to trigger a short (e.g., ≲ 108 yr) burst of star formation. Outer rings may result from similar disturbances but require much more time to form. They would then be associated with more intense nuclear activity if it takes 109 yr or more for the mass transfer to reach the center and raise the black hole accretion rate, by which time the bar has dissolved or has begun to do so. Inner rings form before outer ones, with a formation time more comparable to bars. Thus, it may be that after an interaction or instability triggers an infall of gas, the galaxy in the earliest stage is likely to show enhanced star formation in its center, while later it is more likely to show LINER activity, and still later it is likely to be a Seyfert galaxy. The trends we find with morphology and nuclear activity are not biased either by the distances of the galaxies or by the slightly elevated recent star formation rates shown by the 12 μm galaxies in general.

We have derived the near-infrared structural components of a sample of Seyfert and starburst (SBN) host galaxies by fitting near-infrared images with a new two-dimensional decomposition algorithm. An analysis of the fitted parameters shows that Seyfert 1 and SBN bulges resemble normal early-type bulges in structure and color, with (J - K)cb about 0.1 mag redder than disk (J - K)cd. Seyfert 2 bulges, on the other hand, are bluer than normal, with (J - K)cb ∼ (J - K)cd. Seyfert disks (especially type 1), but not those of SBNs, are abnormally bright (in surface brightness), significantly more so than even the brightest normal disks. Seyfert disks are also compact, but similar to those in normal early-type spirals. For a given mass, Seyfert and particularly SBN galaxies are abnormally rich in neutral hydrogen, and there is strong, albeit indirect, evidence for lower mass-to-light (M/L) ratios in Seyfert and SBN disks, but normal M/L ratios in their bulges. In Seyfert and SBN galaxies, H I mass fractions and M/L ratios are anticorrelated, and we attribute the high gas mass fractions and low M/L ratios in SBNs and several Seyfert galaxies to ongoing star formation. Such abundant gas in Seyfert galaxies would be expected to inhibit bar formation, which may explain why active galaxies are not always barred.

We present 263 optical and near-infrared (NIR) images for 42 1s and 48 Seyfert 2s, selected from the Extended 12 μm Galaxy Sample. Elliptically averaged profiles are derived from the images, and isophotal radii and magnitudes are calculated from these. We also report virtual aperture photometry that, judging from comparison with previous work, is accurate to roughly 0.05 mag in the optical, and 0.07 mag in the NIR. Our B-band isophotal magnitude and radii, obtained from ellipse fitting, are in good agreement with those of Third Reference Catalogue of Bright Galaxies. When compared with the B band, V, I, J, and K isophotal diameters show that the colors in the outer regions of Seyfert galaxies are consistent with the colors of normal spirals. Differences in the integrated isophotal colors and comparison with a simple model show that the active nucleus + bulge are stronger and redder in the NIR than in the optical. Finally, roughly estimated Seyfert disk surface brightnesses are significantly brighter in B and K than those in normal spirals of similar morphological type.

We present Herschel/PACS 100 and 160 μm integrated photometry for the 323 galaxies in the Herschel Reference Survey (HRS), a K-band, volume-limited sample of galaxies in the local Universe. Once combined with the Herschel/SPIRE observations already available, these data make the HRS the largest representative sample of nearby galaxies with homogeneous coverage across the 100-500 μm wavelength range. In this paper, we take advantage of this unique data set to investigate the properties and shape of the far-infrared/submillimetre spectral energy distribution in nearby galaxies. We show that, in the stellar mass range covered by the HRS (8 ≤ log (M*/M⊙) ≤ 12), the far-infrared/submillimetre colours are inconsistent with a single modified blackbody having the same dust emissivity index β for all galaxies. In particular, either β decreases or multiple temperature components are needed, when moving from metal-rich/gas-poor to metal-poor/gas-rich galaxies. We thus investigate how the dust temperature and mass obtained from a single modified blackbody depend on the assumptions made on β. We show that, while the correlations between dust temperature, galaxy structure and star formation rate are strongly model dependent, the dust mass scaling relations are much more reliable, and variations of β only change the strength of the observed trends. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.