Star formation in high-redshift quasars: excess [O II] emission in the
  radio-loud population
  We investigate the [O II] emission line properties of 18,508 quasars at z<1.6
drawn from the Sloan Digital Sky Survey (SDSS) quasar sample. The quasar sample
has been separated into 1,692 radio-loud and 16,816 radio-quiet quasars (RLQs
and RQQs hereafter) matched in both redshift and i'-band absolute magnitude.
  We use the [O II]\lambda3726+3729 line as an indicator of star formation.
Based on these measurements we find evidence that star-formation activity is
higher in the RLQ population. The mean equivalent widths (EW) for [O II] are
EW([O II])_RL=7.80\pm0.30 \AA, and EW([O II])_RQ=4.77\pm0.06 \AA, for the RLQ
and RQQ samples respectively. The mean [O II] luminosities are \log[L([O
II])_RL/W]=34.31\pm0.01 and \log[L([O II])_RQ/W]=34.192\pm0.004 for the samples
of RLQs and RQQs respectively. Finally, to overcome possible biases in the EW
measurements due to the continuum emission below the [O II] line being
contaminated by young stars in the host galaxy, we use the ratio of the [O II]
luminosity to rest-frame i'-band luminosity, in this case, we find for the RLQs
\log[L([O II])_RL/L_opt]=-3.89\pm0.01 and \log[L([O
II])_RQ/L_opt]=-4.011\pm0.004 for RQQs. However the results depend upon the
optical luminosity of the quasar. RLQs and RQQs with the same high optical
luminosity \log(L_opt/W)>38.6, tend to have the same level of [O II] emission.
On the other hand, at lower optical luminosities \log(L_opt/W)<38.6, there is a
clear [O II] emission excess for the RLQs. As an additional check of our
results we use the [O III] emission line as a tracer of the bolometric
accretion luminosity, instead of the i'-band absolute magnitude, and we obtain
similar results.
  Radio jets appear to be the main reason for the [O II] emission excess in the
case of RLQs. In contrast, we suggest AGN feedback ensures that the two
populations acquire the same [O II] emission at higher optical luminosities.
