Intergalactic dust and its photoelectric heating

Abstract

We have examined dust photoelectric heating in the intergalactic medium (IGM). The heating rate in a typical radiation field of the IGM is represented by [Gamma]^sub pe^=1.2x10^sup -34^ erg s^sup -1^ cm^sup -3^ (D/10^sup -4^)(n ^sub H^/10^sup -5^ cm^sup -3^)43(J ^sub L^/10^sup -21^ erg s^sup -1^ cm^sup -2^ H^sub Z^ ^sup -1^ sr^sup -1^)2/3(T10^sup 4^ K)^sup -1/6^), where D is the dust-to-gas mass ratio, n ^sub H^ is the hydrogen number density, J ^sub L^ is the mean intensity at the hydrogen Lyman limit of the background radiation, and T is the gas temperature, if we assume the new X-ray photoelectric yield model by Weingartner et al. ( 2006 ) and the dust size distribution in the Milky Way by Mathis et al. ( 1977 ). This heating rate dominates the HI and HeII photoionization heating rates when the hydrogen number density is less than 10^sup -6^ cm^sup -3^ if D = 10-4 which is 1% of that in the Milky Way, although the heating rate is a factor of 2-4 smaller than that with the old yield model by Weingartner and Draine ( 2001 ). The grain size distribution is very important. If only large ([greater than or equal to]0.1 [mu]m) grains exist in the IGM, the heating rate is reduced by a factor of 5. Since dust heating is more efficient in a lower density medium relative to the photoionization heating, it may cause an inverted temperature-density relation in the low-density IGM, as suggested by Bolton et al. ( 2008 ). Finally, we have found that dust heating is not very important in the mean IGM before the cosmic reionization.