Determining star-formation rates in Active Galactic Nuclei hosts via stellar population synthesis

TL;DR

This study compares star-formation rates derived from stellar population synthesis and gas emission lines in 170 AGN host galaxies, finding a strong correlation and insights into dust reddening differences.

Contribution

It introduces a method to reliably estimate SFR in AGN hosts using stellar population synthesis, overcoming limitations of traditional emission-line indicators.

Findings

SFR from stellar populations closely matches gas-based SFR over last 20 Myr.

A specific logarithmic relation links SFRstars and SFRg for both AGN and non-active galaxies.

Gas reddening is approximately 2.7 times higher than stellar reddening, consistent with dust distribution models.

Abstract

The effect of active galactic nuclei (AGN) feedback on the host galaxy, and its role in quenching or enhancing star-formation, is still uncertain due to the fact that usual star-formation rate (SFR) indicators -- emission-line luminosities based on the assumption of photoionisation by young stars -- cannot be used for active galaxies as the ionising source is the AGN. We thus investigate the use of SFR derived from the stellar population and its relation with that derived from the gas for a sample of 170 AGN hosts and a matched control sample of 291 galaxies. We compare the values of SFR densities obtained via the Ha emission line (SFRg) for regions ionised by hot stars according to diagnostic diagrams with those obtained from stellar population synthesis (SFRstars) over the last 1 to 100~Myr. We find that the SFRstars over the last 20~Myrs closely reproduces the SFRg, although a better match is obtained via the transformation: log(SFRstars) = (0.870+/-0.004) log(SFRg) +(0.007+/-0.006), which is valid for both AGN hosts and non-active galaxies. We also compare the reddening obtained via the gas Ha/Hb ratio with that derived via the full spectral fitting in the stellar population synthesis. We find that the ratio between the gas and stellar extinction is in the range 2.64 < A_Vgas/A_Vstar} < 2.85, in approximate agreement with previous results from the literature, obtained for smaller samples. We interpret the difference as being due to the fact that the reddening of the stars is dominated by that affecting the less obscured underlying older population, while the reddening of the gas is larger as it is associated to a younger stellar population buried deeper in the dust.