The effect of spiral arms on the S\'ersic photometry of galaxies

TL;DR

This study investigates how spiral arms affect the accuracy of galaxy photometry when fitting with a Sérsic profile, revealing biases in flux and size estimates especially for bulge-dominated galaxies with prominent spiral arms.

Contribution

It quantifies the biases introduced by spiral arms in Sérsic profile fitting and provides guidelines for more accurate photometric measurements.

Findings

Spiral arms can cause overestimation of total flux by about 15%.

Half-light radius can be overestimated by approximately 30%.

Biases are smaller in disk-dominated galaxies.

Abstract

Context. The S\'ersic profile is a widely-used model to describe the surface brightness distribution of galaxies. Spiral galaxies, however, are qualitatively different from a S\'ersic model. Aims. The goal of this study is to assess how accurately the total flux and half-light radius of a galaxy with spiral arms can be recovered when fitted with a S\'ersic profile. Methods. I selected a sample of bulge-dominated galaxies with spiral arms. Using photometric data from the Hyper Suprime-Cam survey, I estimated the contribution of the spiral arms to their total flux. Then I generated simulated images of galaxies with similar characteristics, fitted them with a S\'ersic model, and quantified the error on the determination of the total flux and half-light radius. Results. Spiral arms can introduce biases on the photometry of galaxies in a way that depends on the underlying smooth surface brightness profile, the location of the arms, and the depth of the photometric data. A set of spiral arms accounting for 10% of the flux of a bulge-dominated galaxy typically causes the total flux and the half-light radius to be overestimated by 15% and 30%, respectively. This bias, however, is much smaller if the galaxy is disk-dominated. Conclusions. Galaxies with a prominent bulge and a non-zero contribution from spiral arms are the most susceptible to biases in the total flux and half-light radius, when fitted with a S\'ersic profile. If photometric measurements with high accuracy are required, then measurements over finite apertures are to be preferred over global estimates of the flux.