A study of 1000 galaxies with unusually young and massive stars in the
  SDSS: a search for hidden black holes
  We select 1076 galaxies with extinction-corrected Halpha equivalent widths
too large to be explained with a Kroupa (2001) IMF, and compare these with a
control sample of galaxies that is matched in stellar mass, redshift and 4000
AA break strength, but with normal Halpha equivalent widths. Our goal is to
study how processes such as black hole growth and energetic feedback processes
from massive stars differ between galaxies with extreme central Halpha emission
and galaxies with normal young central stellar populations. The stellar mass
distribution of Halpha excess galaxies is peaked at 3 \times 10^10 Msun and
almost all fall well within the star-forming locus in the [OIII]/Hbeta versus
[NII]/Halpha BPT disgram. Halpha excess galaxies are twice as likely to exhibit
Halpha line asymmetries and 1.55 times more likely to be detected at 1 GHz in
the VLA FIRST survey compared to control sample galaxies. The radio luminosity
per unit stellar mass decreases with the stellar age of the system. Using
stacked spectra, we demonstrate that [NeV] emission is not present in the very
youngest of the radio-quiet Halpha excess galaxies with detectable Wolf-Rayet
features, suggesting that black hole growth has not yet commenced in such
systems. [NeV] emission is detected in Halpha excess galaxies with radio
detections and the strength of the line correlates with the radio luminosity.
This is the clearest indication for a possible population of black holes that
may be forming in a subset of the Halpha excess population.
