The elusive stellar halo of the Triangulum Galaxy

TL;DR

This study revisits the existence of a stellar halo around the Triangulum Galaxy using improved data analysis techniques, ultimately finding no evidence for a large-scale smooth stellar halo and setting upper limits on its brightness and luminosity.

Contribution

The paper introduces advanced fitting algorithms and contamination models to better analyze the stellar environment of M33, clarifying the presence or absence of a stellar halo.

Findings

No detection of a large-scale smooth stellar halo around M33.

Established an upper limit on the halo's surface brightness at ${}_V$ = 35.5 mags/arcsec^2.

Placed a luminosity limit of less than $10^6L_{\u00f3dot}$ for the stellar halo.

Abstract

The stellar halos of large galaxies represent a vital probe of the processes of galaxy evolution. They are the remnants of the initial bouts of star formation during the collapse of the proto-galactic cloud, coupled with imprint of ancient and on-going accretion events. Previously, we have reported the tentative detection of a possible, faint, extended stellar halo in the Local Group spiral, the Triangulum Galaxy (M33). However, the presence of substructure surrounding M33 made interpretation of this feature difficult. Here, we employ the final data set from the Pan-Andromeda Archaeological Survey (PAndAS), combined with an improved calibration and a newly derived contamination model for the region to revisit this claim. With an array of new fitting algorithms, fully accounting for contamination and the substantial substructure beyond the prominent stellar disk in M33, we reanalyse the surrounds to separate the signal of the stellar halo and the outer halo substructure. Using more robust search algorithms, we do not detect a large scale smooth stellar halo and place a limit on the maximum surface brightness of such a feature of ${\mu}_V$ = 35.5 mags per square arcsec, or a total halo luminosity of $L < 10^6L_{\odot}$.