psfgan: a generative adversarial network system for separating quasar point sources and host galaxy light

Abstract

The study of unobscured active galactic nuclei (AGN) and quasars depends on the reliable decomposition of the light from the AGN point source and the extended host galaxy light. The problem is typically approached using parametric fitting routines using separate models for the host galaxy and the point spread function (PSF). We present a new approach using a Generative Adversarial Network (gan) trained on galaxy images. We test the method using Sloan Digital Sky Survey r-band images with artificial AGN point sources added that are then removed using the gan and with parametric methods using galfit. When the AGN point source is more than twice as bright as the host galaxy, we find that our method, psfgan, can recover point source and host galaxy magnitudes with smaller systematic error and a lower average scatter (⁠|$49\hbox{ per cent}$|⁠). psfgan is more tolerant to poor knowledge of the PSF than parametric methods. Our tests show that psfgan is robust against a broadening in the PSF width of |$\pm 50\hbox{ per cent}$| if it is trained on multiple PSFs. We demonstrate that while a matched training set does improve performance, we can still subtract point sources using a psfgan trained on non-astronomical images. While initial training is computationally expensive, evaluating psfgan on data is more than 40 times faster than galfit fitting two components. Finally, psfgan is more robust and easy to use than parametric methods as it requires no input parameters.