The size of the Milky Way galaxy

TL;DR

This study measures the Milky Way's half-light radius using a new, detailed surface brightness profile, revealing it is larger than previous estimates and consistent with similar galaxies, indicating typical size and growth history.

Contribution

The paper introduces a new measurement of the Milky Way's size using an age-resolved surface brightness profile, challenging traditional scale length estimates and providing insights into its growth history.

Findings

Half-light radius of 5.75±0.38 kpc for the Milky Way

Surface brightness profile shows a nearly flat region between 3.5 and 7.5 kpc

Milky Way's size evolution aligns with high-redshift galaxies

Abstract

The size of a galaxy is one of the fundamental parameters that reflects its growth and assembly history. Traditionally, the size of the Milky Way has been characterized by the scale length of the disk, based on the assumption of an exponential density profile. Earlier scale length measurements suggest the Milky Way is an overly compact galaxy, compared to similar galaxies of its mass. These size measurements, however, ignore the presence of the bulge, and the assumption of a single-exponential disk profile faces growing challenges from the recent observations. The half-light radius is an alternative size measurement that is independent of the galaxy density profile and has been widely used to quantify the size of external galaxies. Here we report the half-light radius of the Milky Way, derived from a new measurement of the age-resolved Galactic surface brightness profile in an unprecedentedly wide radial range from ${\rm R=0}$ to 17~kpc. We find a broken surface brightness profile with a nearly flat distribution between 3.5 and 7.5 kpc, which results in a half-light radius of 5.75$\pm$0.38 kpc, significantly larger than the scale-length inferred from the canonical single-exponential disk profile but in good consistency with local disk galaxies of similar mass. Because our density profile can be decomposed by stellar age and extrapolated backwards in time, we can also confirm that the size history of the Milky Way is broadly consistent with high-redshift galaxies but with systematically smaller size at each look back time. Our results suggest that the Milky Way is a typical disk galaxy regarding its size and has likely experienced inefficient secular size growth.