Martin Bureau

Abstract:

We present high angular resolution (0.3 arcsec or 37 pc) Atacama Large Millimeter/submillimeter Array observations of the CO(2–1) line emission from a central disc in the early-type galaxy NGC 524. This disc is shown to be dynamically relaxed, exhibiting ordered rotation about a compact 1.3 mm continuum source, which we identify as emission from an active supermassive black hole (SMBH). There is a hole at the centre of the disc slightly larger than the SMBH sphere of influence. An azimuthal distortion of the observed velocity field is found to be due to either a position angle warp or radial gas flow over the inner 2. 5. By forward-modelling the observations, we obtain an estimate of the SMBH mass of 4.0+3.5 −2.0 × 108 M, where the uncertainties are at the 3σ level. The uncertainties are dominated by the poorly constrained inclination and the stellar mass-to-light ratio of this galaxy, and our measurement is consistent with the established correlation between SMBH mass and stellar velocity dispersion. Our result is roughly half that of the previous stellar dynamical measurement, but is consistent within the uncertainties of both. We also present and apply a new tool for modelling complex molecular gas distributions.

Abstract:

This is the first paper of a series exploring the multi-frequency properties of a sample of eleven nearby low excitation radio galaxies (LERGs) in the southern sky. We are conducting an extensive study of different galaxy components (stars, warm and cold gas, radio jets) with the aim of improving our understanding of the AGN fuelling/feedback cycle in LERGs. We present ALMA Band 6 12CO(2-1) and continuum observations of nine sources. Continuum emission from the radio cores was detected in all objects. Six sources also show mm emission from jets on kpc/sub-kpc scales. The jet structures are very similar at mm and cm wavelengths. We conclude that synchrotron emission associated with the radio jets dominates the continuum spectra up to 230 GHz. The 12CO(2-1) line was detected in emission in six out of nine objects, with molecular gas masses ranging from 2 × 107 to 2 × 1010 M⊙. The CO detections show disc-like structures on scales from ≈0.2 to ≈10 kpc. In one case (NGC 3100) the CO disc presents some asymmetries and is disrupted in the direction of the northern radio jet, indicating a possible jet/disc interaction. In IC 4296, CO is detected in absorption against the radio core as well as in emission. In four of the six galaxies with CO detections, the gas rotation axes are roughly parallel to the radio jets in projection; the remaining two cases show large misalignments. In those objects where optical imaging is available, dust and CO appear to be co-spatial.