Determining star formation rates in active galactic nuclei hosts via stellar population synthesis

Abstract

The effect of active galactic nuclei (AGNs) 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 photoionization by young stars – cannot be used for active galaxies as the ionizing 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 H α emission line ($\rm \Sigma SFR_{Gas}$) for regions ionized by hot stars according to diagnostic diagrams with those obtained from stellar population synthesis ($\rm \Sigma SFR_\star$) over the last 1 to 100 Myr. We find that the $\rm \Sigma SFR_\star$ over the last 20 Myr closely reproduces the $\rm \Sigma SFR_{Gas}$, although a better match is obtained via the transformation: $\mbox{log($ \rm \Sigma SFR_\star $)} = (0.872\pm 0.004)\mbox{log($\rm \Sigma SFR_{Gas}$)} -(0.075\pm 0.006)$ (or $\mbox{log($\rm \Sigma SFR_{Gas}$)} = (1.147\pm 0.005)\mbox{log($ \rm \Sigma SFR_\star $)} +(0.086\pm 0.080)$), which is valid for both AGN hosts and non-active galaxies. We also compare the reddening obtained via the gas H α/H β 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 ≤AVg/AV⋆ ≤ 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 with a younger stellar population buried deeper in the dust.

Estimating Dust Attenuation From Galactic Spectra. II. Stellar and Gas Attenuation in Star-forming and Diffuse Ionized Gas Regions in MaNGA

“…In this subsection we compare our work with both studies. We note that, in a recent study based on MaNGA, Riffel et al (2021) also examined the correlation between E(B − V ) star and E(B − V ) gas , but focusing on AGN host galaxies and a control sample of star-forming galaxies. Here we will not make comparisons with this study, given the difference in sample selection.…”

“…A simple approach to estimate dust attenuation through a galactic spectrum is to match the attenuated spectrum with those of unattenuated galaxies that have similar stellar populations (e.g., Calzetti et al 2000;Wild et al 2011;Reddy et al 2015;Battisti et al 2017a,b). Alternatively, fitting the SED (or full spectrum) with a stellar population synthesis model is also widely adopted to obtain the stellar attenuation (e.g., Cid Fernandes et al 2005;Asari et al 2007;Riffel et al 2009;Conroy 2013;Wilkinson et al 2015;Ge et al 2018;Boquien et al 2019;Riffel et al 2021). In ionized gas regions, the dust attenuation on emission lines, E(B − V ) gas , is commonly estimated from the Balmer decrement Hα/Hβ.…”

“…Using a sample of 232 star-forming spiral galaxies from MaNGA and the full spectral fitting code STARLIGHT (Cid Fernandes et al 2005), Greener et al (2020) also found that the variation in dust attenuation properties is likely driven by local physical properties of galaxies, such as the SFR surface density. Riffel et al (2021) analyzed a sample of 170 active galactic nuclei (AGN) hosts with strong star formation and a control sample of 291 star-forming galaxies from MaNGA. They also found a strong correlation between E(B−V ) star (derived with STARLIGHT) and E(B − V ) gas , with f ∼ 0.37, which is close to, but slightly smaller than the typical value of f = 0.44 found for the general population of star-forming regions.…”

Stellar populations in local AGNs: evidence for enhanced star formation in the inner 100pc

“…Burtscher et al (2021) tested the capability of to distinguish between a FC and young SSPs, and the code proved capable of separating blue light originating from the FC from blue light from young SPs. Also, by comparing the SFR derived from with the SFR derived from the emission lines in their sample, Riffel et al (2021) reported that they closely reproduce each other.…”

The metal-poor dwarf irregular galaxy candidate next to Mrk 1172

“…To perform the stellar population synthesis we use the module, which was developed to work as a front-end for the code, operating in three main modules (Cid Fernandes et al 2005;Mallmann et al 2018;Riffel et al 2021). Since is designed to operate with ASCII-format files, the spectrum of each spaxel needs to be extracted from the original fits files, applying several pre-processing corrections, i.e., rest-frame spectrum shifting and galactic extinction correction.…”