Local Swift-BAT active galactic nuclei prefer circumnuclear star formation

TL;DR

This study finds that local AGN host galaxies have more compact star-forming regions compared to non-AGN galaxies at similar far-infrared luminosities, supporting a link between gas concentration and black hole feeding.

Contribution

It provides the first detailed comparison of far-infrared emission sizes in AGN hosts versus non-AGN galaxies, highlighting the role of gas distribution in AGN-host coevolution.

Findings

AGN hosts have smaller far-infrared sizes at moderate luminosities

No size difference at higher luminosities

Star formation regions are more compact in AGN hosts

Abstract

We use Herschel data to analyze the size of the far-infrared 70micron emission for z<0.06 local samples of 277 hosts of Swift-BAT selected active galactic nuclei (AGN), and 515 comparison galaxies that are not detected by BAT. For modest far-infrared luminosities 8.5<log(LFIR)<10.5, we find large scatter of half light radii Re70 for both populations, but a typical Re70 <~ 1 kpc for the BAT hosts that is only half that of comparison galaxies of same far-infrared luminosity. The result mostly reflects a more compact distribution of star formation (and hence gas) in the AGN hosts, but compact AGN heated dust may contribute in some extremely AGN-dominated systems. Our findings are in support of an AGN-host coevolution where accretion onto the central black hole and star formation are fed from the same gas reservoir, with more efficient black hole feeding if that reservoir is more concentrated. The significant scatter in the far-infrared sizes emphasizes that we are mostly probing spatial scales much larger than those of actual accretion, and that rapid accretion variations can smear the distinction between the AGN and comparison categories. Large samples are hence needed to detect structural differences that favour feeding of the black hole. No size difference AGN host vs. comparison galaxies is observed at higher far-infrared luminosities log(LFIR)>10.5 (star formation rates >~ 6 Msun/yr), possibly because these are typically reached in more compact regions in the first place.