TWO PSEUDOBULGES IN THE “BOXY BULGE” GALAXY NGC?5746

摘要

Galaxy formation and growth under the ΛCDM 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 μm wavelength, Hubble Space Telescope H-band, Two Micron All Sky Survey Ks -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 Sérsic 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 ΛCDM universe.