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 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 Sérsic function with index n = 1.55 \pm 0.07 along the major axis and 1.33 \pm 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 \pm 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 .