Hidden AGNs in Dwarf Galaxies Revealed by MaNGA: Light Echoes, Off-nuclear Wanderers, and a New Broad-line AGN

Abstract

Abstract Active galactic nuclei (AGNs) in dwarf galaxies could host the relics of those early universe seed black holes that did not grow into supermassive black holes. Using MaNGA integral field unit (IFU) spectroscopy we have found a sample of 37 dwarf galaxies that show AGN ionization signatures in spatially resolved emission line diagnostic diagrams. The AGN signatures are largely missed by integrated emission line diagnostics for 23 of them. The bolometric luminosity of these 23 new AGN candidates is ≲1040 erg s−1, fainter than that of single-fiber SDSS AGNs, X-ray AGNs, and radio AGNs in dwarf galaxies, which supports IFU spectroscopy as a powerful tool for identifying hidden and faint AGNs in dwarf galaxies. The AGN emission is in most cases offset from the optical center of the dwarf galaxy and shows a symmetric morphology, which indicates that either the AGNs are off-nuclear, that the central emission of the galaxy is dominated by star formation, or that the AGNs are turned off and we are observing a past ionization burst. One of the new AGNs shows a broad Hα emission line component, from which we derive a black hole mass in the realm of intermediate-mass black holes. This constitutes the first hidden type 1 AGN discovered in a dwarf galaxy based on IFU spectroscopy. The finding of this sample of hidden and faint AGNs has important implications for population studies of AGNs in dwarf galaxies and for seed black hole formation models.