Shining light on the invisible: the faint structures around galaxies in the local volume

Abstract

The low surface brightness component that envelops every galaxy - the stellar halo - is a crucial tool for galactic archaeology, as it holds the fossil record of past galactic mergers. Thus far, detailed studies of stellar halos have only been done on the nearest galaxies - Milky Way and M31. To broaden our understanding of galaxy assembly, it is necessary to extend this type of resolved star analysis to galaxies beyond the Local Group. However, this poses many challenges, as it requires deep wide-field observations with large telescopes to map individual stars and star clusters over large areas.

In this thesis, I present an analysis of the stellar halos of two high mass (~1011M☉) galaxies, M81 (D = 3.63 Mpc) and NGC 1052 (D = 19.2 Mpc), and a smaller ultra--diffuse galaxy F8D1 (D = 3.67 Mpc), using state-of-the-art wide-field data from Hyper Suprime-Cam on the 8.2m Subaru telescope and MegaCam on the Canada-France-Hawaii 3.6m telescope (CFHT). I study the stellar halos of these systems out to radii of >60 kpc using red giant branch (RGB) stars and globular clusters (GCs).

The Milky Way analogue, M81, sits at the centre of a small group of galaxies and has two close companions, M82 and NGC 3077 with which it is tidally interacting. In the first chapter, I examine the properties of the M81 stellar halo using RGB star count data from the Subaru telescope. I quantify the shape of the halo and extract star count profiles along several directions. Merging these with a diffuse light profile extracted from deep CFHT g-band observations, I construct a composite surface brightness profile that can be traced over 70 kpc. I use a multi-component model to derive the luminosity and mass of the stellar halo and quantify its radial behaviour. I find that the M81 stellar halo profile shows a shallow slope of -1.6 ± 0.1, similar to that of M31 but in contrast to smaller area studies which suggested a steeper fall-off. I also quantify the metal content [M/H] of the halo using the colours of the RGB stars and find evidence for some asymmetries along different axes, suggesting that the halo may not be well-mixed at the present epoch.

In the second chapter, I present the discovery of a significant ongoing accretion event in the halo of M81. Discovered more than 20 years ago, F8D1 is an ultra-diffuse galaxy that lies 115 kpc in projection to the Southwest of M81. My analysis of the distribution of RGB stars in the surrounding region uncovers a previously unknown giant tidal tail stretching for ≥60 kpc in the direction of NGC 2976 and M81. I quantify the structure of the tail across and along its length, and measure its photometric metallicity. I also use deep CFHT data to extract improved measurements for the main body of F8D1. The distance to NGC 2976 and the main body of F8D1 is estimated via the tip of the Red Giant Branch method to deduce the 3D distribution of the system. Although closer in projection, NGC 2976 was found not to be associated with the stream and was merely projected in the foreground. I found that the tail contains approximately 36% of F8D1’s luminosity, demonstrating that F8D1 is being severely disrupted, likely by M81.

In contrast to the M81 analyses which focus on RGB stars, the stellar halo of NGC 1052 has been studied via its population of GCs in the third chapter of this thesis. Using ugi-band data taken under excellent seeing conditions, I search for new candidate GCs using their photometric and morphological properties. The search criteria are devised by using the properties of a sample of spectroscopically-confirmed GCs in the halo of NGC 1052 and its neighbouring dwarf galaxies. I identify 643 GC candidates using their location in colour-colour space and characterise their spatial distribution, luminosity function and colour distribution. I show that GC candidates in the NGC 1052 stellar halo follow a smooth and shallow radial power law γ = -2.24 ± 0.21 out to ~120 kpc. In the inner stellar halo, the GCs show a striking correlation with faint tidal debris features associated with the ongoing merger between NGC 1052 and NGC 1047. No significant red/blue bimodality was found in the NGC 1052 stellar halo. The GC populations of dark matter deficient ultra-diffuse galaxies NGC 1052-DF2 and -DF4 have distinct properties compared to those of the NGC 1052 halo and hence are unlikely to be associated.