Structural Analogs of the Milky Way Galaxy: Stellar Populations in the Boxy Bulges of NGC 4565 and NGC 5746

TL;DR

This study compares the stellar populations of the boxy bulges in NGC 4565 and NGC 5746 to the Milky Way, revealing that all three galaxies have old, alpha-enhanced stars indicating rapid early star formation.

Contribution

It demonstrates that NGC 4565 and NGC 5746 are structural and evolutionary analogs of the Milky Way through spectroscopic analysis of their bulges.

Findings

Bulges in NGC 4565 and NGC 5746 are composed of old, alpha-enhanced stars.

Star formation in these bulges likely finished within ~1 Gyr of start.

Structural similarities suggest similar evolutionary processes to the Milky Way.

Abstract

We present NGC 4565 and NGC 5746 as structural analogs of our Milky Way. All three are giant, SBb - SBbc galaxies with two pseudobulges, i. e., a compact, disky, star-forming pseudobulge embedded in a vertically thick, "red and dead", boxy pseudobulge that really is a bar seen almost end-on. The stars in the boxy bulge of our Milky Way are old and enhanced in alpha elements, indicating that star formation finished within ~ 1 Gyr of when it started. Here, we present Hobby-Eberly Telescope spectroscopy of the boxy pseudobulges of NGC 4565 and NGC 5746 and show that they also are made of old and alpha-element-enhanced stars. Evidently it is not rare that the formation of stars that now live in bars finished quickly and early, even in galaxies of intermediate Hubble types whose disks still form stars now. Comparison of structural component parameters leads us to suggest that NGC 4565 and NGC 5746 are suitable analogs of the Milky Way, because they show signatures of similar evolution processes.