The dragonfly nearby galaxies survey. Iv. A giant stellar disk in ngc 2841

TL;DR

This study reveals an extensive low-surface brightness stellar disk in NGC 2841, challenging the idea that gas dominates the outskirts of galaxies and suggesting complex formation processes involving migration, accretion, and star formation.

Contribution

It presents the discovery of a giant stellar disk extending to 70 kpc in NGC 2841, with a constant stellar-to-gas mass ratio beyond 20 kpc, highlighting new insights into galaxy outskirts.

Findings

Stellar disk extends to ~70 kpc, much farther than typical expectations.

Stellar mass surface density remains above gas density at all radii.

Outer disk shows warping, indicating possible interaction with circumgalactic medium.

Abstract

Neutral gas is commonly believed to dominate over stars in the outskirts of galaxies, and investigations of the disk-halo interface are generally considered to be in the domain of radio astronomy. This may simply be a consequence of the fact that deep HI observations typically probe to a lower mass surface density than visible wavelength data. This paper presents low surface brightness optimized visible wavelength observations of the extreme outskirts of the nearby spiral galaxy NGC 2841. We report the discovery of an enormous low-surface brightness stellar disk in this object. When azimuthally averaged, the stellar disk can be traced out to a radius of $\sim$70 kpc (5 $R_{25}$ or 23 inner disk scale lengths). The structure in the stellar disk traces the morphology of HI emission and extended UV emission. Contrary to expectations, the stellar mass surface density does not fall below that of the gas mass surface density at any radius. In fact, at all radii greater than $\sim$20 kpc, the ratio of the stellar to gas mass surface density is a constant 3:1. Beyond $\sim$30 kpc, the low surface brightness stellar disk begins to warp, which may be an indication of a physical connection between the outskirts of the galaxy and infall from the circumgalactic medium. A combination of stellar migration, accretion and in-situ star formation might be responsible for building up the outer stellar disk, but whatever mechanisms formed the outer disk must also explain the constant ratio between stellar and gas mass in the outskirts of this galaxy.