Photoelectric heating for dust grains at high redshifts

Lyman-alpha absorption systems at z similar to 3 with N-HI greater than or similar to 3 x 10(14) cm(-2) have been found to be enriched with a mean met allicity of Z/Z. similar to 10(-2.5), and a large scatter in the metallicit y. It is reasonable to assume that the process of initial enrichment of the intergalactic medium (IGM) at z greater than or similar to 3 also produced dust grains, We explore the implications of the presence of dust grains in the IGM at high redshift, in particular, the contribution of photoelectric emission from grains by hard background photons to the net heating rate of the IGM. We show that (i) the charge on dust particles and the characteristics of ph otoemitted electrons differ substantially from those in the interstellar me dium (ISM) in several respects: (a) grains are exposed to and charged by ph otons beyond the Lyman limit, and (b) because of this, the photoelectrons h ave typical energy of tens of eV, We also show that: (ii) silicates are mor e efficient heating agents than graphites; (iii) small grains contribute mo stly to the net heating; (iv) at densities typical of the IGM at z similar to 3 and for Ly alpha absorbers, dust heating can be comparable to or excee d photoionization heating within an order of magnitude; and (v) this increa ses the temperature of overdense regions, compared to the case of no dust h eating, by a factor of similar to 2, We discuss the implications of this ex tra heating source in Ly alpha absorbing systems.