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Paper: Detection of a Distinct Pseudobulge Hidden Inside the “Box-Shaped Bulge” of NGC 4565
Volume: 419, Galaxy Evolution: Emerging Insights and Future Challenges
Page: 149
Authors: Barentine, J. C.; Kormendy, J.
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 images and Spitzer Space Telescope 3.6 μm wavelength images to penetrate the dust in NGC 4565. We find a high surface brightness central stellar component that is clearly distinct from the boxy bar and from the galaxy’s disk. Its minor-axis profile has a Sérsic 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. This is in contrast to a scale height of ∼740 pc for the boxy bar plus thin disk. The disky pseudobulge is also much less luminous than the boxy bar, so the true pseudobulge-to-total luminosity ratio of the galaxy is much less than previously thought. We infer that the (pseudo)bulge-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 presents a challenge to 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 in turn supports our developing picture of the formation of pseudobulges—both edge-on bars and disky central components—by secular evolution in isolated galaxies.
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