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.