  In contrast to massive, bulge hosting galaxies, very few supermassive black
holes (SMBHs) are known in either low mass, or bulgeless galaxies. Such a
population could provide clues to the origins of SMBHs and to secular pathways
for their growth. Using the all-sky Wide-Field Infrared Survey Explorer (WISE)
survey, and bulge-to-disk decompositions from the Sloan Digital Sky Survey
(SDSS) Data Release 7, we report the discovery of a population of local (z<0.3)
bulgeless disk galaxies with extremely red mid-infrared colors highly
suggestive of a dominant active galactic nucleus (AGN), despite having no
optical AGN signatures in their SDSS spectra. Using various mid-infrared
selection criteria from the literature, there are between 30 to over 300
bulgeless galaxies with possible AGNs. Other known scenarios that can heat the
dust to high temperatures do not appear to explain the observed colors of this
sample. If these galaxies are confirmed to host AGNs, this study will provide a
breakthrough in characterizing the properties of SMBHs in the low bulge mass
regime and in understanding their relation with their host galaxies.
Mid-infrared selection identifies AGNs that dominate their host galaxy's
emission and therefore reveal a different AGN population than is uncovered by
optical studies. We find that the fraction of all galaxies identified as
candidate AGNs by WISE is highest at lower stellar masses and drops
dramatically in higher mass galaxies, in striking contrast to the findings from
optical studies.
