Host galaxies of luminous quasars: population synthesis of optical off-axis spectra

TL;DR

This paper introduces a population synthesis method for analyzing off-axis spectra of luminous quasar host galaxies, revealing differences in host galaxy colors and stellar ages related to radio luminosity.

Contribution

The paper presents a novel spectral modeling technique that accounts for scattered nuclear light, enabling detailed study of quasar host galaxy properties.

Findings

Elliptical quasar hosts are bluer than inactive ellipticals.

Radio luminous quasars tend to be in redder host galaxies.

Radio faint quasar hosts are similar to star-forming galaxies.

Abstract

There is increasing evidence of a connection between AGN activity and galaxy evolution. To obtain further insight into this potentially important evolutionary phase, we analyse the properties of quasar host galaxies. In this paper, we present a population synthesis modeling technique for off-axis spectra, the results of which constrain host colour and the stellar ages of luminous quasars (M_V(nuc)<-23). Our technique is similar to well established quiescent-galaxy models, modified to accommodate scattered nuclear light (a combination of atmospheric, instrumental and host galaxy scattered light) observed off axis. In our model, subtraction of residual scattered quasar light is performed, while simultaneously modeling the constituent stellar populations of the host galaxy. The reliability of this technique is tested via a Monte-Carlo routine in which the correspondence between synthetic spectra with known parameters and the model output is determined. Application of this model to a preliminary sample of 10 objects is presented and compared to previous studies. Spectroscopic data was obtained via long-slit and integral-field unit observations on the Keck and WIYN telescopes. We confirm that elliptical quasar hosts are distinguishable (bluer) from inactive ellipticals in rest frame B-V colour. Additionally, we note a trend for radio luminous (L_5GHz > 10^40 erg s^-1) quasars to be located in redder host galaxies in comparison to their less luminous radio counterparts. While the host colour and age of our radio luminous sample is in close proximity to the green valley, our radio faint sample is consistent with quiescent star-forming galaxies. However, further observations are needed to confirm these results. Finally, we discuss future applications for our technique on a larger sample of objects being obtained via SALT and WIYN telescope observing campaigns.