Hidden AGN in dwarf galaxies revealed by MaNGA: light echoes,
  off-nuclear wanderers, and a new broad-line AGN
  Active galactic nuclei (AGN) in dwarf galaxies could possibly 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 ionisation 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 $\lesssim 10^{40}$ erg
s$^{-1}$, fainter than that of single-fiber SDSS AGN, X-ray AGN, and radio AGN
in dwarf galaxies, which stands IFU spectroscopy as a powerful tool for
identifying hidden and faint AGN 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 AGN are off-nuclear, that the
central emission of the galaxy is dominated by star formation, or that the AGN
are turned-off and we are observing a past ionisation burst. One of the new AGN
shows a broad H$\alpha$ 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 AGN has important implications
for population studies of AGN in dwarf galaxies and for seed black hole
formation models.
