Powerful Radio Emission From Low-mass Supermassive Black Holes Favors
  Disk-like Bulges
  The origin of spin of low-mass supermassive black hole (SMBH) is still a
puzzle at present. We here report a study on the host galaxies of a sample of
radio-selected nearby ($z<0.05$) Seyfert 2 galaxies with a BH mass of $10^{6-7}
M_\odot$. By modeling the SDSS $r$-band images of these galaxies through a
2-dimensional bulge+disk decomposition, we identify a new dependence of SMBH's
radio power on host bulge surface brightness profile, in which more powerful
radio emission comes from a SMBH associated with a more disk-like bulge. This
result means low-mass and high-mass SMBHs are spun up by two entirely different
modes that correspond to two different evolutionary paths. A low-mass SMBH is
spun up by a gas accretion with significant disk-like rotational dynamics of
the host galaxy in the secular evolution, while a high-mass one by a BH-BH
merger in the merger evolution.
