How a Dust Concentration Mimics Dynamical Signatures around the Nucleus of NGC 7331

Summary

Where the phenomenon of wavelength-dependent kinematics has been observed in galaxies, it has been well modelled as a dust extinction effect. Therefore “compact mass features” in rotation curves close to the nuclei of dusty inclined galaxies may not be caused by dynamical effects, but by differential extinction by circumnuclear dust. NGC 7331 exhibits this phenomenology.

Introduction

The kinematics of the zones around the nuclei of galaxies are of exceptional interest, especially in the context of tests for very compact central objects (black holes). A number of nearby galaxies show striking kinematic features around their nuclei and have been well modelled by adding a central point mass to an otherwise smoothly varying bulge distribution, yielding the steep velocity gradients, dispersions and “shoulders” in their rotation curves (Bower et al. 1993). Given the exceptional interest in the presence of supermassive compact objects, however, it may not be surprising that, in at least some cases, remarkable circumnuclear kinematical effects may have been overinterpreted. In previous spectroscopy of dusty, highly inclined galaxies we have found systematic steepening of velocity curves from the near-UV to the near-IR (McKeith et al. 1993), which is convincingly modelled via dust extinction (Prada et al. 1994).

Data and results

To see how the observable rotation curve of the dusty spiral inclined galaxy NGC 7331 varies with wavelength we took long-slit spectra with the ISIS spectrograph on the 4.2-m WHT (La Palma), with the slit along the major axis centred on the optical nucleus.