Detection of A Pseudobulge Hidden inside the "Box-Shaped Bulge" of NGC 4565
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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 mu m wavelength images, both calibrated to Two Micron All Sky Survey 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.
CitationBarentine, John C., and John Kormendy. "Detection of a Distinct Pseudobulge Hidden Inside the" Box-Shaped Bulge" of NGC 4565." arXiv preprint arXiv:0902.0017 (Jun., 2009)
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