GAS kinematics in Lyα nebulae

Exploring the origin of Lyα nebulae ("blobs") at high redshift requires measurements of their gas kinematics that are impossible with only the resonant, optically thick Lyα line. To define gas motions relative to the systemic velocity of the blob, the Lyα line must be compared with an optically thin line like Hα λ6563, which is not much altered by radiative transfer effects and is more concentrated about the galaxies embedded in the nebula's core. We obtain optical and near-IR (NIR) spectra of the two brightest Lyα blobs (CDFS-LAB01 and CDFS-LAB02) from the Yang et al. sample using the Magellan/Magellan Echellette Spectrograph optical and Very Large Telescope/SINFONI NIR spectrographs. Both the Lyα and Hα lines confirm that these blobs lie at the survey redshift, z 2.3. Within each blob, we detect several Hα sources, which roughly correspond to galaxies seen in Hubble Space Telescope rest-frame UV images. The Hα detections show that these galaxies have large internal velocity dispersions (σ_v = 130-190kms^-1) and that, in the one system (LAB01), where we can reliably extract profiles for two Hα sources, their velocity difference is Δv 440kms^-1. The presence of multiple galaxies within the blobs, and those galaxies' large velocity dispersions and large relative motion, is consistent with our previous finding that Lyα blobs inhabit massive dark matter halos that will evolve into those typical of present-day rich clusters and that the embedded galaxies may eventually become brightest cluster galaxies. To determine whether the gas near the embedded galaxies is predominantly infalling or outflowing, we compare the Lyα and Hα line centers, finding that Lyα is not offset (Δv _Lyα = +0kms^-1) in LAB01 and redshifted by only +230kms^-1 in LAB02. These offsets are small compared to those of Lyman break galaxies, which average +450kms^-1 and extend to about +700kms^-1. In LAB02, we detect C II λ1334 and Si II λ1526 absorption lines, whose blueward shifts of 200kms^-1 are consistent with the small outflow implied by the redward shift of Lyα. We test and rule out the simplest infall models and those outflow models with super/hyperwinds, which require large outflow velocities. Because of the unknown geometry of the gas distribution and the possibility of multiple sources of Lyα emission embedded in the blobs, a larger sample and more sophisticated models are required to test more complex or a wider range of infall and outflow scenarios.