Does the Milky Way Obey Spiral Galaxy Scaling Relations?

TL;DR

This study reassesses the Milky Way's placement on spiral galaxy scaling relations using new, robust measurements, finding it aligns with the Tully-Fisher relation but is unusually compact in the luminosity-velocity-radius relation.

Contribution

It provides updated, systematic comparisons of the Milky Way to galaxy scaling relations, revealing consistency with TFR but a significant deviation in the LVR relation.

Findings

Milky Way agrees with Tully-Fisher relation across multiple proxies.

Galactic disk is unusually compact compared to expectations from the LVR relation.

Observed disk scale length is nearly half of the predicted value, with moderate statistical tension.

Abstract

It is crucial to understand how the Milky Way, the galaxy we can study in the most intimate detail, fits in amongst other galaxies. Key examples include the Tully-Fisher relation (TFR) --- i.e., the tight correlation between luminosity ($L$) and rotational velocity ($V_\textrm{rot}$) --- and the 3-dimensional luminosity-velocity-radius ($LVR$) scaling relation. Several past studies have characterized the MW as a 1--1.5$\sigma$ outlier to the TFR. This study reexamines such comparisons using new estimates of MW properties that are robust to many of the systematic uncertainties that have been a problem in the past and are based on assumptions consistent with those used for other spiral galaxies. Comparing to scaling relations derived from modern extragalactic data, we find that our Galaxy's properties are in excellent agreement with TFRs defined using any SDSS-filter absolute magnitude, stellar mass, or baryonic mass as the $L$ proxy. We next utilize disk scale length ($R_\textrm{d}$) measurements to extend this investigation to the $LVR$ relation. Here we find that our Galaxy lies farther from the relation than $\sim$90\% of other spiral galaxies, yielding $\sim$9.5$\sigma$ evidence that it is unusually compact for its $L$ and $V_\textrm{rot}$ (based on MW errors alone), a result that holds for all of the $L$ proxies considered. The expected $R_\textrm{d}$ for the MW from the $LVR$ relation is $\sim$5 kpc, nearly twice as large as the observed value, with error estimates placing the two in tension at the $\sim$1.4$\sigma$ level. The compact scale length of the Galactic disk could be related to other ways in which the MW has been found to be anomalous.