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

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 α 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 α-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.

Based on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.