Active Galactic Nuclei in Four Metal-poor Dwarf Emission-Line Galaxies

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 H alpha luminosities, ranging from [image] to [image] erg s super(-1). The oxygen abundance in these galaxies is very low, varying in the range [image]. Such extraordinarily high broad H alpha 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 H alpha 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 [image] -[image]. 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] [lambda]3426 and He ii [lambda]4686 can be understood if the nonthermal radiation contributes less than [image]10% of the total ionizing radiation.