Ultraviolet and infrared diagnostics of star formation and dust in NGC 7331

We present images of NGC 7331 obtained with GALEX and Spitzer, tracing UV and IR signatures of star formation. NGC 7331's morphology at 8-850 μm is dominated by a central dust ring. This structure is a vigorous site of star formation (hosting one-third of the present activity) but remains inconspicuous in our GALEX UV imagery. Radial profile analysis and photometry for discrete UV- and UV+IR-selected substructures indicate a decline in UV extinction with increasing galactocentric distance, although highly attenuated star-forming regions can be found throughout the disk. UV-optical surface brightness profiles suggest a recent birthrate parameter (b₈) that is highest in the outer part of the disk, even though the local star formation intensity peaks in the ring. Bolometric luminosity and UV attenuation are correlated in substructures on 0.4 kpc scales, with a relationship similar to that established for starburst galaxies. The distribution of substructures in L(IR)/L(FUV), L_λ(FUV)/L_λ(NUV) space suggests that the majority of the disk is best characterized by Milky Way-type dust, with the exception of sources in the star-forming ring. As found by Calzetti et al. in M51, the observed 8 and 24 μm luminosity for substructures in NGC 7331 are correlated, showing a decline in L_v(8 μm)/L_v(24 μm) with increasing luminosity. We demonstrate the dependence of L_v(8 μm)/L_v(24 μm) on the local extinction-corrected Hα surface brightness (hence current Σ_SFR). A power law of slope 1.64 (1.87) accurately describes the Schmidt-law relation versus Σ_H2 (Σ_gas) for molecular-dominated environments. The same locations show no correlation between Σ_SFR and Σ_HI. For atomic-dominated regions above an apparent local star formation threshold, we found a trend for increasing Σ_SFR at higher Σ_HI, although the Schmidt-law correlation with molecular-only surface density persists in areas dominated by atomic gas.