Constraint on intergalactic dust from thermal history of intergalactic medium

ABSTRACT This Letter investigates the amount of dust in the intergalactic medium (IGM).The dust photoelectric heating can be the most efficient heating mechanism in theIGM where the density is very small and there are a lot of hard ultraviolet photons.Comparing the observational thermal history of IGM with a theoretical one takinginto account the dust photoelectric heating, we can put an upper limit on the dust-to-gas ratio, D, in the IGM. Since the rate of the dust photoelectric heating dependson the size of dust, we find the following results: If the grain size is & 100 ˚A, D atz ∼ 3 is . 1/100 Galactic value corresponding to Ω IGMdust . 10 −5 . On the other hand,if the grain size is as small as ∼ 10 ˚A, D is . 1/1000 Galactic value corresponding toΩ IGMdust . 10 −6 .Key words: cosmology: theory — dust, extinction — intergalactic medium —quasars: absorption lines 1 INTRODUCTIONThere are metals in the intergalactic medium (IGM) even inlow density regions such as Lymanα forest (e.g., Cowie et al.1995; Telfer et al. 2002). Since metal and dust are relatingeach other, it is sure that dust grains also exist in the IGM.However, the amount of the intergalactic (IG) dust is stillquite uncertain although it seems to be not so abundant. Wetry to put a new constraint on the amount of the IG dustby using the IGM thermal history suggested by the recentobservations of Lyman α forest (e.g., Schaye et al. 2000).After the early attempts to estimate the IG extinc-tion (Eigenson 1949; Humason, Mayall, & Sandage 1956),Crane & Hoffman (1973) have obtained an upper limit onthe amount of the IG dust in terms of the density parameteras Ω