A study of the central stellar populations of galaxies in SDSS-IV MaNGA: identification of a sub-sample with unusually young and massive stars

Abstract

This paper describes a search for galaxy centers with clear indications of unusual stellar populations with an initial mass function flatter than Salpeter at high stellar masses. Out of a sample of 668 face-on galaxies with stellar masses in the range 10^10- 10^11 M_sol, I identify 15 galaxies with young to intermediate age central stellar populations with unusual stellar population gradients in the inner regions of the galaxy. In these galaxies, the 4000 Angstrom break is either flat or rising towards the center of the galaxy, indicating that the central regions host evolved stars, but the H$\alpha$ equivalent width also rises steeply in the central regions. The ionization parameter [OIII]/[OII] is typically low in these galactic centers, indicating that ionizing sources are stellar rather than AGN. Wolf Rayet features characteristic of hot young stars are often found in the spectra and these also get progressively stronger at smaller galactocentric radii. These outliers are compared to a control sample of galaxies of similar mass with young inner stellar populations, but where the gradients in Halpha equivalent width and 4000 Angstrom break follow each other more closely. The outliers exhibit central Wolf Rayet red bump excesses much more frequently, they have higher central stellar and ionized gas metallicities, and they are also more frequently detected at 20 cm radio wavelengths. I highlight one outlier where the ionized gas is clearly being strongly perturbed and blown out either by massive stars after they explode as supernovae, or by energy injection from matter falling onto a black hole.