Far-infrared and accretion luminosities of the present-day active galactic nuclei

TL;DR

This study examines the relationship between star formation and black hole accretion luminosities in local AGNs, finding a positive correlation and highlighting the rarity of AGNs in low star-forming galaxies, with implications for understanding galaxy evolution.

Contribution

It provides new insights into the correlation between FIR-based star formation luminosities and AGN luminosities, addressing biases in previous studies.

Findings

Positive linear correlation between FIR and AGN luminosities.

Pure-AGN candidates show significantly lower star formation activities.

Low-SF AGNs are rare, possibly due to observational limitations.

Abstract

We investigate the relation between star formation (SF) and black hole accretion luminosities, using a sample of 492 type-2 active galactic nuclei (AGNs) at z < 0.22, which are detected in the far-infrared (FIR) surveys with AKARI and Herschel. We adopt FIR luminosities at 90 and 100 um as SF luminosities, assuming the proposed linear proportionality of star formation rate with FIR luminosities. By estimating AGN luminosities from [OIII]5007 and [OI]6300 emission lines, we find a positive linear trend between FIR and AGN luminosities over a wide dynamical range. This result appears to be inconsistent with the recent reports that low-luminosity AGNs show essentially no correlation between FIR and X-ray luminosities, while the discrepancy is likely due to the Malmquist and sample selection biases. By analyzing the spectral energy distribution, we find that pure-AGN candidates, of which FIR radiation is thought to be AGN-dominated, show significantly low-SF activities. These AGNs hosted by low-SF galaxies are rare in our sample (~ 1%). However, the low fraction of low-SF AGN is possibly due to observational limitations since the recent FIR surveys are insufficient to examine the population of high-luminosity AGNs hosted by low-SF galaxies.