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.