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 3 x 10(41) to 2 x 10(42) erg s(-1). The oxygen abundance in these galaxies is very low, varying in the range 12 + log O/H = 7.36-7.99. 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 similar to 5 x 10(5)-3 x 10(6) M-circle dot. 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 similar to 10% of the total ionizing radiation.