Detection of a Pseudobulge Hidden Inside the "Box-Shaped Bulge" of NGC 4565

TL;DR

This study reveals a hidden pseudobulge in NGC 4565, challenging previous assumptions about galaxy bulge composition and suggesting many similar galaxies lack classical bulges, impacting galaxy formation theories.

Contribution

The paper identifies a pseudobulge in NGC 4565 using infrared imaging, showing it is much less luminous than the boxy bar and revising the galaxy's bulge-to-total luminosity ratio.

Findings

Detected a distinct pseudobulge with a Sersic index around 1.5.

Revised the bulge-to-total luminosity ratio to approximately 0.06.

Challenged the hierarchical galaxy formation model for large pure-disk galaxies.

Abstract

Numerical simulations show that box-shaped bulges of edge-on galaxies are not bulges: they are bars seen side-on. Therefore the two components that are seen in edge-on Sb galaxies such as NGC 4565 are a disk and a bar. But face-on SBb galaxies always show a disk, a bar, and a (pseudo)bulge. Where is the (pseudo)bulge in NGC 4565? We use archival Hubble Space Telescope H-band images and Spitzer Space Telescope 3.6 micron wavelength images, both calibrated to 2MASS K_s band, to penetrate the prominent dust lane in NGC 4565. We find a high surface brightness, central stellar component that is clearly distinct from the boxy bar and from the disk. Its brightness profile is a Sersic function with index n = 1.55 +- 0.07 along the major axis and 1.33 +- 0.12 along the minor axis. Therefore it is a pseudobulge. It is much less luminous than the boxy bar, so the true pseudobulge-to-total luminosity ratio of the galaxy is PB/T = 0.06 +- 0.01, much less than the previously believed value of B/T = 0.4 for the boxy bulge. We infer that published B/T luminosity ratios of edge-on galaxies with boxy bulges have been overestimated. Therefore, more galaxies than we thought contain little or no evidence of a merger-built classical bulge. From a formation point of view, NGC 4565 is a giant, pure-disk galaxy. This presents a challenge to our picture of galaxy formation by hierarchical clustering: it is difficult to grow galaxies as big as NGC 4565 without also making big classical bulges.