ar X iv : a st ro - p h / 97 08 06 3 v 1 7 A ug 1 99 7 What causes the Ly α forest , clouds or large - scale velocity fields ?

If an additional large scale hydrodynamic velocity field is superposed on the general Hubble flow, the peculiar velocities have the effect that spatially separated volume elements along a given line of sight may contribute to the absorption at the same value of λ. This effect leads to a ‘line-like’ absorption structure rather than to a smooth GP-depression. From this it follows that at least part of the Lyα forest may be explained without invoking density fluctuations (clouds). Thus, there is no clear observational distinction between intergalactic clouds and the diffuse medium inbetween. We have investigated this effect by means of Monte Carlo simulations [1]. In Fig. 1 we present results based on a model in which we assumed the diffuse intergalactic medium to be (locally) homogeneous having at z = 3 the parameters n(HI) = 4 × 10 cm and Tkin = 10 K. For the large scale stochastic velocity field we assumed a rms velocity of σt = 300 km s , and a correlation length l = 6 Mpc. In calculating the expansion rate we assumed q0 = 0.5 and H0 = 70 km s −1 Mpc. The value of n(HI) was estimated from the continuum depression DA at z = 3 observed in low resolution spectra, Tkin corresponds to the width of the narrowest lines in the Lyα forest, σt is half the value found for the peculiar velocities in the local universe [2], and the value of l corresponds to the expected size of voids at z = 3. We performed similar calculations for He II Lyα (Fig. 2). Assuming a photoionizing rate of ΓHI = 10 −12 s [3] one derives from n(HI) a total baryon density of the order of 10 cm at z = 3 corresponding to Ω0,IGM ≃ 0.03, i.e. it is about 3 per cent of the mass needed to close the Universe. This result indicates that most of the baryonic matter is found in the IGM inbetween clouds.