Are the Milky Way and Andromeda unusual? A comparison with Milky Way and Andromeda Analogs

TL;DR

This study compares the properties of Milky Way and Andromeda analogs to understand how well they match their real counterparts, highlighting the importance of including structural and star-formation criteria for accurate analog selection.

Contribution

It systematically evaluates different selection criteria for galaxy analogs, improving the methodology for identifying meaningful Milky Way and Andromeda analogs.

Findings

Milky Way is within 1σ of its analogs in star-formation rate and bulge-to-total ratio

Scale length differences suggest it should be included in analog selection

Analog selection based on morphology and star-formation rate affects similarity to Andromeda

Abstract

Our Milky Way provides a unique test case for galaxy evolution models, thanks to our privileged position within the Milky Way's disc. This position also complicates comparisons between the Milky Way and external galaxies, due to our inability to observe the Milky Way from an external point of view. Milky Way analog galaxies offer us a chance to bridge this divide by providing the external perspective that we otherwise lack. However, over-precise definitions of "analog" yield little-to-no galaxies, so it is vital to understand which selection criteria produce the most meaningful analog samples. To address this, we compare the properties of complementary samples of Milky Way analogs selected using different criteria. We find the Milky Way to be within 1$\sigma$ of its analogs in terms of star-formation rate and bulge-to-total ratio in most cases, but we find larger offsets between the Milky Way and its analogs in terms of disc scale length; this suggests that scale length must be included in analog selections in addition to other criteria if the most accurate analogs are to be selected. We also apply our methodology to the neighbouring Andromeda galaxy. We find analogs selected on the basis of strong morphological features to display much higher star-formation rates than Andromeda, and we also find analogs selected on Andromeda's star-formation rate to over-predict Andromeda's bulge extent. This suggests both structure and star-formation rate should be considered when selecting the most stringent Andromeda analogs.