Spectroscopy of 7 radio-loud QSOs at 2 < z < 6: giant Lyman α emission nebulae accreting on to host galaxies

Abstract

We performed long-slit optical spectroscopy (GTC-OSIRIS) of 6 radio-loud QSOs at redshifts $2<z<3$, known to have giant ($\sim 50$-100 kpc) Lyman-$\alpha$ emitting nebulae, and detect extended Lyman-$\alpha$ emission for 4, with surface brightness $\sim10^{-16}$ ergs $\rm cm^{-2}s^{-1}arcsec^{-2}$ and line width FWHM 400-1100 (mean 863) km $\rm s^{-1}$. We also observed the $z\simeq 5.9$ radio-loud QSO, SDSS J2228+0110, and find evidence of a $\geq 10$ kpc extended Lyman-$\alpha$ emission nebula, a new discovery for this high-redshift object. Spatially-resolved kinematics of the 5 nebulae are examined by fitting the Lyman-$\alpha$ wavelength at a series of positions along the slit. We found the line-of-sight velocity $\Delta(v)$ profiles to be relatively flat. However, 3 of the nebulae appear systematically redshifted by 250-460 km $\rm s^{-1}$ relative to the Lyman-$\alpha$ line of the QSO (with no offset for the other two), which we argue is evidence for infall. One of these (Q0805+046) had a small ($\sim 100$ km $\rm s^{-1}$) velocity shift across its diameter and a steep gradient at the centre. Differences in line-of-sight kinematics between these 5 giant nebulae and similar nebulae associated with high-redshift radio galaxies (which can show steep velocity gradients) may be due to an orientation effect, which brings infall/outflow rather than rotation into greater prominence for the sources observed `on-axis' as QSOs.