The effects of an active galactic nucleus on host galaxy colour and morphology measurements

Abstract

We assess the effects of simulated active galactic nuclei (AGN) on the colour and morphology measurements of their host galaxies. To test the morphology measurements, we select a sample of galaxies not known to host AGN and add a series of point sources scaled to represent specified fractions of the observed V‐band light detected from the resulting systems; we then compare morphology measurements of the simulated systems to measurements of the original galaxies. AGN contributions >rsim 20 per cent bias most of the morphology measurements tested, though the extent of the apparent bias depends on the morphological characteristics of the original galaxies. We test colour measurements by adding to non‐AGN galaxy spectra a quasar spectrum scaled to contribute specified fractions of the rest‐frame B‐band light detected from the resulting systems. A quasar fraction of 5 per cent can move the NUV −r colour of an elliptical galaxy from the ultraviolet–optical red sequence to the green valley, and 20 per cent can move it into the blue cloud. Combining the colour and morphology results, we find that a galaxy/AGN system with an AGN contribution >rsim 20 per cent may appear bluer and more bulge dominated than the underlying galaxy. We conclude that (1) bulge‐dominated, E/S0/Sa, and early‐type morphology classifications are accurate for red AGN host galaxies and may be accurate for blue host galaxies, unless the AGN manifests itself as a well‐defined point source and (2) although highly unobscured AGN, such as the quasar used for our experiments, can significantly bias the measured colours of AGN host galaxies, it is possible to identify such systems by examining optical images of the hosts for the presence of a point source and/or measuring the level of nuclear obscuration.

CANDELS: THE PROGENITORS OF COMPACT QUIESCENT GALAXIES ATz∼ 2

“…Point-source contamination by a bright AGN component could also bias the structural parameters of otherwise larger galaxies toward more compact morphologies (Simmons & Urry 2008;Pierce et al 2010). We verified that these galaxies are genuinely compact by comparing to the (light profile independent) sizes estimated with SExtractor (Bertin & Arnouts 1996).…”

Cosmic X-ray surveys of distant active galaxies

“…While this makes it challenging to determine the properties of the AGN over that band pass, it allows for convenient measurements of the host-galaxy properties without significant contamination from the AGN (e.g., Simmons and Urry 2008;Silverman et al 2009b;Pierce et al 2010a;Xue et al 2010;Lusso et al 2011). See Fig.…”

HOST GALAXY PROPERTIES OF THESWIFTBAT ULTRA HARD X-RAY SELECTED ACTIVE GALACTIC NUCLEUS

“…A full listing of the morphological measurements can be found in Table 6. While we have limited our results to NL AGN because of the difficulty of subtracting the light distribution to make morphological measurements (Pierce et al 2010), we provide morphological measurements of broad-line AGN after AGN light subtraction with GALFIT in Table 6.…”

“…Other authors have used different fixed and floating Sérsic index models including a fixed bulge (n = 4), a fixed disk and bulge (n = 1 and n = 4), and a floating Sérsic index (Veilleux et al 2006(Veilleux et al , 2009aSchawinski et al 2009). While a detailed study of the most effective way to measure the AGN and galaxy light has been done for simulated HST images (Simmons & Urry 2008;Kim et al 2008;Pierce et al 2010), little has been done for ground-based images similar to the current study. Therefore, to determine the best modeling approach with GALFIT and the associated error, we simulated AGN galaxies for both our Kitt Peak and SDSS images.…”