Two Pseudobulges In The "Boxy Bulge" Galaxy NGC 5746

Abstract

Galaxy formation and growth under the Lambda CDM paradigm is expected to proceed in a hierarchical, bottom-up fashion by which small galaxies grow into large galaxies; this mechanism leaves behind large "classical bulges" kinematically distinct from "pseudobulges" grown by internal, secular processes. We use archival data (Spitzer Space Telescope 3.6 mu m wavelength, Hubble Space Telescope H-band, Two Micron All Sky Survey K-s-band, and Sloan Digital Sky Survey gri-band) to measure composite minor-and major-axis surface brightness profiles of the almost-edge-on spiral galaxy NGC 5746. These light profiles span a large range of radii and surface brightnesses to reveal an inner, high surface brightness stellar component that is distinct from the well-known boxy bulge. It is well fitted by Sersic functions with indices n = 0.99 +/- 0.08 and 1.17 +/- 0.24 along the minor and major axes, respectively. Since n < 2, we conclude that this innermost component is a secularly evolved pseudobulge that is distinct from the boxy pseudobulge. This inner pseudobulge makes up 0.136 +/- 0.019 of the total light of the galaxy. It is therefore considerably less luminous than the boxy structure, which is now understood to be a bar seen nearly end-on. The infrared imagery shows further evidence for secular evolution in the form of a bright inner ring of inner radius 9.1 kpc and width 1.6 kpc. NGC 5746 is therefore a giant, pure-disk SB(r) bc galaxy with no sign of a merger-built bulge. We do not understand how such galaxies form in a Lambda DM universe.