Young stellar population gradients in central cluster galaxies from NUV
  and optical spectroscopy
  Central cluster galaxies are the largest and most massive galaxies in the
Universe. Although they host very old stellar populations, several studies
found the existence of blue cores in some BCGs indicating ongoing star
formation. We analyse VLT/X-Shooter stacked spectra of 6 nearby massive central
galaxies with high central velocity dispersions ($\sigma$>300 km/s) at
different galactocentric distances. We quantify the young stellar population
out to 4 kpc by fitting near-UV and optical absorption line indices with
predictions of composite stellar populations modelled by an old and a young
stellar component. We also use IMF-sensitive indices since these galaxies have
been found to host a bottom-heavy IMF in their central regions. We derive
negative young stellar populations gradients, with mass fractions of stars
younger than 1 Gyr decreasing with galactocentric distance, from 0.70% within
0.8 kpc to zero beyond 2 kpc. We also measure the mass fraction in young stars
for individual galaxies in the highest S/N central regions. All the galaxies
have young components of less than one percent. Our results clearly suggest
that the star formation in massive central cluster galaxies takes place in
their galaxy cores (<2 kpc), which, with deeper gravitational potential wells,
are capable of retaining more gas. Among the possible sources for the gas
required to form these young stars, our results are consistent with an in-situ
origin via stellar evolution, which is sufficient to produce the observed young
stellar populations.
