Heating of the intergalactic medium as a result of structure formation

We estimate the heating of the intergalactic medium as a result of shocks arising from structure formation. The heating of the gas outside the collapsed regions, with small overdensities [(n(b)/(n) over bar (b)) much less than 200] is considered here, with the aid of a Zel'dovich approximation. We estimate the equation of state of this gas, relating the density to its temperature and its evolution in time, considering the shock heating caused by 1 sigma density peaks as being the most dominant. We also estimate the mass fraction of gas above a given temperature as a function of redshift. We find that the baryon fraction above 10(6) K at z = 0 is similar to 10 per cent. We estimate the integrated Sunyaev-Zel'dovich distortion from this gas at the present epoch to be of the order of 10(-6).