Detection of a Distinct 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 supporting secular evolution over hierarchical merging.

Contribution

It identifies a distinct, small-scale pseudobulge in NGC 4565 using infrared imaging, showing that many edge-on galaxies may lack classical bulges.

Findings

The pseudobulge has a Sersic index of 1.33.

The pseudobulge's scale height is about 90 pc.

The pseudobulge's luminosity is much less than the boxy bar.

Abstract

N-body simulations show that "box-shaped bulges" of edge-on galaxies are not bulges at all: they are bars seen side-on. The two components that we readily see in edge-on Sb galaxies like 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 K-band and Spitzer Space Telescope 3.6 um images to penetrate the dust in NGC 4565. We find a high surface brightness, central stellar component, distinct from the boxy bar and from the galaxy's disk. Its minor-axis profile has a Sersic index of 1.33+/-0.12, so it is a pseudobulge. The pseudobulge has the smallest scale height (~90 pc) of any component in the galaxy, in contrast to ~740 pc for the boxy bar plus thin disk. The disky pseudobulge is also much less luminous than the boxy bar, so the true (pseudo)bulge-to-total luminosity ratio of the galaxy is much less than previously thought. We infer that the pseudobulge-to-total luminosity ratios of edge-on galaxies with box-shaped bulges have generally been overestimated. Therefore more galaxies than we have recognized contain little or no evidence of a merger-built classical bulge. This challenges our picture of galaxy formation by hierarchical clustering, because it is difficult to grow big galaxies without also making a big classical bulge. Solving the puzzle of the "missing pseudobulge" in NGC 4565 further increases our confidence that we understand box-shaped bulges correctly as edge-on bars. This supports our developing picture of the formation of pseudobulges -- both edge-on bars and disky central components -- by secular evolution in isolated galaxies.