A Multivariate Statistical Analysis of Spiral Galaxy Luminosities. II. Morphology-dependent Multiwavelength Emission Properties

This is the second of two papers based on a systematic multiwavelength (X, B, H, 12 μm, FIR, and 6 cm) statistical analysis of the Einstein Observatory Galaxy Catalog sample of 234 "normal" S0/a-Irr galaxies. This sample is representative of spiral galaxies, and its wide wavelength coverage provides a unique opportunity for a systematic exploration of the relations among different emission bands, which can give novel insight on the different emission processes and their relation to galaxian components, as a function of galaxy morphology. We find clear morphological differences in the overall relationships among different emission bands: while all wave bands tend to be well correlated in late-type Sc-Irr galaxies, in agreement with a general connection of all emission bands with the stellar component, in bulge-dominant early-type S0/a-Sab galaxies instead our results point to unrelated emission mechanisms for the two groups of wave bands: (12 μm, FIR, 6 cm) and (X, B, H). A dependence on galaxy morphology is also found in the 12 μm-FIR link, which was shown by the multivariate analysis of our first paper to be one the two strongest fundamental correlations in spiral galaxies, together with the well-known B-H correlation. We find that blue late-type dwarf galaxies appear to lack 12 μm emission, suggesting a lack of small-size dust grains in their interstellar medium (ISM), perhaps resulting from a particularly intense UV photon field. Also, the IR-B color-color plot of S0/a-Sab galaxies presents more scatter than those of later type galaxies, suggesting both significant red giant circumstellar dust emission and nuclear emission. Examining the well-known IR-radio-continuum link of spiral galaxies, we find surprising morphological differences: the "fundamental" radio-IR correlation flips from the mid-IR (12 μm) emission in S0/a-Sab galaxies to the FIR in Sc-Irr. These correlations also have different slopes: the 6 cm-12 μm correlation of S0/a-Sab galaxies is steep and follows the same trend as that of the bright active galactic nuclei (AGNs) in the sample, suggesting that low-luminosity AGNs embedded in these normal galaxies may be responsible for the bulk of their mid-IR and radio emission; the 6 cm-FIR correlation of Sc-Irr galaxies is linear, consistent with the well-known star formation link. The X-ray emission is not strongly correlated with any of the other variables, with the exception of the FIR in Sc-Irr, consistent with a general link with the young stellar population in these systems. Contrary to preliminary reports, we find that the X-ray-B correlation is similarly nonlinear (slope ~ 1.5) in early- and late-type spirals. We suggest that this apparent similarity stems from different processes in the two samples. In S0/a-Sab galaxies the steep slope may be due to the presence of some hot ISM in the more X-ray-luminous systems, as observed in E and S0 galaxies. In Sc-Irr galaxies instead it may be related to a luminosity dependence of intrinsically absorbed X-ray emission regions, connected with star formation activity. Obscured star-forming regions in higher luminosity galaxies can also explain other functional relations of the correlations found in the Sc-Irr sample.