2008ApJ...687..133I

We present 3.5 m Apache Point Observatory second-epoch spectra of four low-metallicity emission-line dwarf galaxies discovered serendipitously in Data Release 5 of the Sloan Digital Sky Survey (SDSS) to have extraordinarily large broad Hα luminosities, ranging from 3x10⁴¹ to 2x10⁴² erg/s. The oxygen abundance in these galaxies is very low, varying in the range 12+logO/H=7.36-7.99. Such extraordinarily high broad Hα luminosities cannot be accounted for by massive stars at different stages of their evolution. By comparing these with the first-epoch SDSS spectra, we find that the broad Hα luminosities have remained constant over a period of 3-7 yr, which probably excludes Type IIn supernovae as a possible mechanism of broad emission. The emission most likely comes from accretion disks around intermediate-mass black holes, with lower mass limits in the range ∼5x10⁵-3x10⁶ M_☉. If this is the case, these four objects form a new class of very low metallicity active galactic nuclei (AGNs) that have been elusive until now. The absence of the strong high-ionization lines [Ne V] λ3426 and He II λ4686 can be understood if the nonthermal radiation contributes less than ∼10% of the total ionizing radiation.