Expansion and Collapse in the Cosmic Web

Abstract

We study the kinematics of the gaseous cosmic web at high redshift using Ly forest absorption in multiple QSO sight lines. Observations of the projected velocity shifts between Ly absorbers common to the lines of sight to a gravitationally lensed QSO and three more widely separated QSO pairs are used to directly measure the expansion of the cosmic web in units of the Hubble velocity, as a function of redshift and spatial scale. The lines of sight used span a redshift range from about 2 to 4.5 and represent transverse scales from the subkiloparsec range to about 300 h À1 70 physical kpc. Using a simple analytic model and a cosmological hydrodynamic simulation, we constrain the underlying three-dimensional distribution of expansion velocities from the observed line-of-sight distribution of velocity shear across the plane of the sky. The shape of the shear distribution and its width (14.9 km s À1 rms for a physical transverse separation of 61 h À1 70 kpc at z ¼ 2, 30.0 km s À1 for 261 h À1 70 kpc at z ¼ 3:6) are found to be in good agreement with the IGM undergoing large-scale motions dominated by the Hubble flow, making this one of the most direct observations possible of the expansion of the universe. However, modeling the Ly clouds with a simple ''expanding pancake'' model, the average expansion velocity of the gaseous structures causing the Ly forest in the lower redshift (z $ 2) smaller separation (61 kpc) sample appears about 20% lower than the local Hubble expansion velocity.In order to understand the observed velocity distribution further we investigated the statistical distribution of expansion velocities in cosmological Ly forest simulations. The mean expansion velocity in the (z $ 2, separation $ 60 kpc) simulation is indeed somewhat smaller than the Hubble velocity, as found in the real data. We interpret this finding as tentative evidence for some Ly forest clouds breaking away from the Hubble flow and undergoing the early stages of gravitational collapse. However, the distribution of velocities is highly skewed, and the majority of Ly forest clouds at all redshifts from 2 to 3.8 expand with super-Hubble velocities, typically about 5%-20% faster than the Hubble flow. This behavior is explained if most Ly forest clouds in the column density range typically detectable are expanding filaments that stretch and drain into more massive nodes. The significant difference seen in the velocity distributions between the high-and low-redshift samples may conceivably reflect actual peculiar deceleration, the differences in spatial scale, or our selecting higher densities at lower redshift for a given detection threshold for Ly forest lines.We also investigate the alternative possibility that the velocity structure of the general Ly forest could have an entirely different, local origin, as expected if the Ly forest were produced or at least significantly modified by galactic feedback, e.g., winds from star-forming galaxies at high redshift. However, we find no evidence that the observed distribution of veloc...

Astrophysics in 2006

“…Its spectrum shows absorption features at the wavelengths expected for the Bridge gas (Smoker et al 2005). This also applies to direct evidence that things have moved apart from each other with time, deduced from statistics of the Ly α forest between z = 2 and 4.5 (Rauch et al 2005). …”

Triple-ionized carbon associated with the low-density neutral hydrogen gas at 1.7 <z< 3.3: the integratedN H I–N C ivrelation

“…This range was chosen on the basis of several observational findings. Studies of close QSO pairs have found a strong C iv clustering signal within ∼ 200 km s −1 , which might indicate the outflow velocity could be less than ∼ 200 km s −1 (Rauch et al 2005). A significant clustering signal of H i is also found at a transverse velocity separation at ∼ 500 km s et al 2006).…”

On the sizes of neutral hydrogen regions giving rise to damped Lyα absorption systems

“… e Rauch et al (2005), using VLT spectra, found ‘all of the systems’ are in common apart from differences in column densities. …”