$\texttt{HostSub_GP}$: Precise Galaxy Background Subtraction in Transient Long-slit Spectroscopy with Gaussian Processes

TL;DR

This paper introduces a Gaussian process-based method for precise host galaxy subtraction in long-slit spectroscopy of transients, utilizing multi-band archival images to improve accuracy over classic linear interpolation techniques.

Contribution

The novel approach models the galaxy background with Gaussian processes informed by archival images, enabling more accurate transient spectra extraction in complex galaxy environments.

Findings

Outperforms classic methods on synthetic data with spatially resolved hosts.

Effectively recovers weak spectral features in real transient spectra.

Provides a scalable, open-source toolkit for the community.

Abstract

We present a novel host galaxy subtraction technique in long-slit spectroscopy for extragalactic transients. Unlike classic methods which generally estimate the background using a simple linear interpolation of local galaxy flux in the 2D spectrum, our approach leverages multi-band archival images of the host galaxies to model the background emission from the galaxy in the 2D spectrum. Such imaging encodes the wavelength-dependent galaxy profile along the slit, and is readily accessible through wide-field imaging surveys. We construct a smooth prior for the 2D galaxy profile with a Gaussian process (GP) based on these reference images, and use another GP to model the correlated deviations from the prior in the observed spectrum. This enables accurate inference of the galaxy flux blended with the transient. On synthetic long-slit data of a spiral galaxy extracted from a Multi Unit Spectroscopic Explorer hyper-spectral cube, the GP method remains robust as long as the host galaxy is spatially resolved and consistently outperforms classic methods. We apply the method to archival Keck spectra of two real transients, SN 2019eix and AT 2019qiz, to further demonstrate how the method uniquely recovers weak spectral features amid strong galaxy contamination, enabling refined constraints on the properties of both transients. We have released the software implementation, $\texttt{HostSub_GP}$, a scalable toolkit that leverages $\texttt{JAX}$, with an MIT license.