Active galactic nuclei in four metal-poor dwarf emission-line galaxies

TL;DR

This study identifies four low-metallicity dwarf galaxies hosting active galactic nuclei with unusually high broad Halpha luminosities, likely powered by intermediate-mass black holes, representing a new class of low-metallicity AGN.

Contribution

First detection of very low-metallicity AGN in dwarf galaxies with persistent broad Halpha emission, suggesting intermediate-mass black holes as the central engine.

Findings

Broad Halpha luminosities remained constant over 3-7 years.

Black hole mass estimates range from ~5x10^5 to 3x10^6 solar masses.

Absence of high-ionization lines indicates low contribution of nonthermal radiation.

Abstract

We present 3.5m Apache Point Observatory second-epoch spectra of four low-metallicity emission-line dwarf galaxies discovered serendipitously in the Data Release 5 of the Sloan Digital Sky Survey (SDSS) to have extraordinary large broad Halpha luminosities, ranging from 3x10^41 to 2x10^42 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 Halpha luminosities cannot be accounted for by massive stars at different stages of their evolution. By comparing with the first-epoch SDSS spectra, we find that the broad Halpha luminosities have remained constant over a period of 3-7 years, 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 ~5x10^5 Msun-3x10^6 Msun. If so, these four objects form a new class of very low-metallicity AGN that have been elusive until now. The absence of the strong high-ionization lines [NeV] 3426 and He II 4686 can be understood if the nonthermal radiation contributes less than ~10% of the total ionizing radiation.