Limits on Dust and Metallicity Evolution of Lya Forest Clouds from COBE

We consider possible observational consequences of dust and metals in \lya forest clouds. We relate the dust content, $\Omega_d^{Lya}$, to the metal evolution of the absorbers and assume that dust is heated by the ultraviolet background radiation and by the CMB. We find that the dust temperature deviates from $T_{CMB}$ by at most 10% at redshift $z=0$. The \lya cloud dust opacity to redshift $\sim 5$ sources around the observed wavelength $\lambda_0 \sim 1 \mu$m is $\sim 0.13$, and could affect observations of the distant universe in that band. The expected CMB spectral distortions due to high-$z$ dust in \lya clouds is $\sim 1.25-10$ smaller than the current COBE upper limit, depending on the metallicity evolution of the clouds. If \lya clouds are clustered, the corresponding CMB anisotropy due to dust is $\sim 10^{-1}$ on angular scales $\theta \simlt 10''$ at frequencies probed by various future/ongoing FIR missions, which makes these fluctuations potentially detectable in the near future. Emission from CII fine-structure transitions could considerably contribute to submm range of the FIR background radiation. Depending on the ionization of carbon and on the density of metal enriched regions, this contribution can be comparable with the observed residual flux at $\lambda\approx 0.15$ mm, after CMB subtraction. We argue that constraints on metal evolution versus redshift can be obtained from the observed flux in that range.