Resolving the Stellar Outskirts of M81: Evidence for a Faint, Extended Structural Component

TL;DR

This study uses deep stellar observations of M81 to identify a faint, extended structural component beyond its bright disk, revealing a flatter surface brightness profile and a metal-poor stellar population at large radii.

Contribution

It provides the first detailed analysis of the faint outskirts of M81, combining resolved star counts and diffuse light to characterize its structure and stellar populations.

Findings

Evidence for a faint, extended stellar component beyond the optical disk.

The extended component has a flatter surface brightness profile and a power-law distribution with gamma ~ 2.

The RGB stars in this component have a peak metallicity of approximately -1.1 dex.

Abstract

We present a wide field census of resolved stellar populations in the northern half of M81, conducted with Suprime-Cam on the 8-m Subaru telescope and covering an area ~ 0.3 square degrees. The resulting color-magnitude diagram reaches over one magnitude below the red giant branch (RGB) tip, allowing a detailed comparison between the young and old stellar spatial distributions. The surface density of stars with ages <~ 100 Myr is correlated with that of neutral hydrogen in a manner similar to the disk-averaged Kennicutt-Schmidt relation. We trace this correlation down to gas densities of ~ 2 x 10^20 cm^{-2}, lower than typically probed with H-alpha flux. Both diffuse light and resolved RGB star counts show compelling evidence for a faint, extended structural component beyond the bright optical disk, with a much flatter surface brightness profile. The star counts allow us to probe this component to significantly fainter levels than is possible with the diffuse light alone. From the colors of its RGB stars, we estimate this component has a peak global metallicity [M/H] ~ -1.1 +/- 0.3 at deprojected radii 32 - 44 kpc assuming an age of 10 Gyr and distance of 3.6 Mpc. The spatial distribution of its RGB stars follows a power-law surface density profile, I(r) ~ r^{-gamma}, with gamma ~ 2. [Abridged]