The influence of diffuse scattered light II. Observations of galaxy haloes and thick discs and hosts of BCGs

TL;DR

This study investigates how scattered light affects the observation of galaxy haloes and thick discs, revealing that it can mimic faint structures and thus complicate the interpretation of deep photometric data.

Contribution

It demonstrates the significant influence of scattered light on galaxy surface-brightness profiles and highlights the need for accurate PSF measurements in analyzing diffuse galactic structures.

Findings

Scattered light creates bright haloes and red excess at large radii.

Most previous studies underestimated the role of scattered light.

Accurate PSF measurements are essential for reliable analysis of galaxy haloes.

Abstract

Studies of deep photometry of galaxies have presented discoveries of excess light in surface-brightness and colour profiles at large radii in the form of diffuse faint haloes and thick discs. In a majority of the cases, it has seemed necessary to use exotic stellar populations or alternative physical solutions to explain the excess. Few studies have carefully scrutinized the role of scattered light in this context. I explore the influence of scattered light on ground-based observations of haloes and thick discs around edge-on galaxies, haloes around face-on disc galaxies, host galaxies around blue compact galaxies (BCGs), and haloes around elliptical galaxies. Surface-brightness structures of all considered types of galaxies are modelled and analysed to compare scattered-light haloes and thick discs with measurements. I simulate the influence of scattered light and accurate sky subtraction on simplified S\'ersic-type and face-on disc galaxy models. All galaxy models are convolved with both lower-limit and brighter point spread functions (PSFs); for a few galaxies it was possible to use dedicated PSFs. The results show bright scattered-light haloes and high amounts of red excess at large radii and faint surface brightnesses for nearly all types of galaxies; exceptions are the largest elliptical-type galaxies where the influence of scattered light is smaller. Studies have underestimated the role of scattered light to explain their surface-brightness profiles. My analysis shows surface-brightness profiles that include scattered light that are very similar to and overlap measurements at all radii. The derivation of physical properties of haloes, thick discs, and BCG hosts from diffuse data is misleading since accurate and radially extended PSFs are non-existent. Significantly improved analyses that include new measurements of PSFs are required to study diffuse haloes further.