Unearthing foundations of a cosmic cathedral: searching the stars for M33's halo

Abstract

We use data from the Pan-Andromeda Archaeological Survey (PAndAS) to search for evidence of an extended halo component belonging to M33 (the Triangulum Galaxy). We identify a population of red giant branch (RGB) stars at large radii from M33's disk whose connection to the recently discovered extended "disk substructure" is ambiguous, and which may represent a "bona-fide" halo component. After first correcting for contamination from the Milky Way foreground population and misidentified background galaxies, we average the radial density of RGB candidate stars over circular annuli centered on the galaxy and away from the disk substructure. We find evidence of a low-luminosity, centrally concentrated component that is everywhere in our data fainter than µ V ∼ 33 mag arcsec −2 . The scale length of this feature is not well constrained by our data, but it appears to be of order r exp ∼ 20 kpc; there is weak evidence to suggest it is not azimuthally symmetric. Inspection of the overall CMD for this region that specifically clips out the disk substructure reveals that this residual RGB population is consistent with an old population with a photometric metallicity of around [Fe/H] ∼ -2 dex, but some residual contamination from the disk substructure appears to remain. We discuss the likelihood that our findings represent a bona-fide halo in M33, rather than extended emission from the disk substructure.We interpret our findings in terms of an upper limit to M33's halo that is a few percent of its total luminosity, although its actual luminosity is likely much less.

The Large-Scale Structure of the Halo of the Andromeda Galaxy. I. Global Stellar Density, Morphology and Metallicity Properties

“…A previous contribution (Cockcroft et al 2013), based also on PAndAS data, detected the halo of M33 (a satellite of M31) out to very large radii, revealing hints of asymmetric morphology. Recent spectroscopic studies have shown evidence of a metalpoor halo in M31 (Chapman et al 2006;Kalirai et al 2006;Koch et al 2008), which had been largely missed in earlier work due to the presence of substantial contamination from the so-called southern Giant Stellar Stream (Ibata et al 2001), which pollutes almost all the inner halo region.…”

The Large-scale Structure of the Halo of the Andromeda Galaxy. II. Hierarchical Structure in the Pan-Andromeda Archaeological Survey

“…We also note that satellites of M33 as faint as those recently discovered around the LMC are sufficiently faint that they would be extremely challenging to detect in PAndAS imaging. Chapman et al (2013) the absence of a spatially extended population of globular clusters around M33 (Cockcroft et al 2011), as well as an absence of any obvious M33 stellar halo component (Cockcroft et al 2013;McMonigal et al 2016). It also means that it is possible that some of the globular clusters and dwarf galaxies currently associated with M31 were in fact originally associated with M33.…”

Kinematics and mass modelling of M33: Hα observations

“…The particular structure (star streams) seen in M33 could be associated with this history of mergers and interactions. In fact, many of the structures presented in McConnachie et al (2009McConnachie et al ( , 2010; Cockcroft et al (2013) and Wolfe et al (2013) around M31 and M33 can only be explained by these interactions. Deep Hα observations of M33 reveal the presence of low density Hii regions outside the optical disk (Hoopes et al 2001).…”