Gaia DR3 and nearby galaxies: where do foregrounds matter?

TL;DR

This study uses Gaia DR3 data to identify and quantify foreground contamination in observations of nearby galaxies, revealing that about half of the sources are foreground stars, which impacts stellar population analyses.

Contribution

It introduces a method to quantify foreground contamination in galaxy observations using Gaia DR3 astrometry, applicable to a large sample of nearby galaxies.

Findings

Approximately 50% of Gaia sources in galaxy directions are foreground stars.

Foreground sources are brighter, redder, and less concentrated than galaxy sources.

Foreground contamination is significant within 5 Mpc, affecting stellar population studies.

Abstract

Nearby galaxies provide populations of stellar and non-stellar sources at a common distance and in quantifiable environments. All are observed through the Milky Way foreground, with varying degrees of contamination that depend on observed Galactic latitude and the distance and size of the target galaxy. This work uses Gaia Data Release 3 (DR3) to identify foreground sources via astrometric measurements and thus quantify foreground contamination for a large sample of nearby galaxies. There are approximately half a million Gaia sources in the directions of 1401 galaxies listed in the Local Volume Galaxy catalogue (D<11 Mpc), excluding the largest Local Group galaxies. About two thirds of the Gaia sources have astrometric properties consistent with foreground sources; these sources are brighter, redder, and less centrally-concentrated than non-foreground sources. Averaged over galaxies, foreground sources make up 50 per cent of Gaia sources at projected radius r50=1.06*a26, where a26 is the angular diameter at the B=26.5 isophote. Foreground sources make up 50 per cent of Gaia sources at apparent magnitude m(G,50)=20.50. This limit corresponds to the tip of the red giant branch absolute magnitude at D = 450 kpc, and to the globular cluster luminosity function peak absolute magnitude at 5 Mpc. Gaia data provide a powerful tool for removing foreground contamination in stellar population studies of nearby galaxies, although Gaia foreground removal will be incomplete beyond distances of 5 Mpc.