Observing H₂ emission in forming galaxies

We study the H₂ cooling emission of forming galaxies and discuss their observability using the future infrared facility Single Aperture Far-Infrared Observatory (SAFIR). Forming galaxies with mass ≲, 10 ¹¹ M⊙ emit most of their gravitational energy liberated by contraction in molecular hydrogen line radiation, although a large part of thermal energy at virialization is radiated away by the hydrogen Lyα emission. For more massive objects, the degree of heating due to dissipation of kinetic energy is so great that the temperature does not drop below 10 ⁴ K, and the gravitational energy is emitted mainly by the Lyα emission. Therefore, the total H₂ luminosity attains the peak value of L_H2 ∼ 10⁴² ergs s^-1 for forming galaxies whose total mass M_tot ∼ 10¹¹ M⊙. If these sources are situated at redshift z ∼ 8, they can be detected by rotational lines of 0-0 S(3) at 9.7 μm and 0-0 S(1) at 17 μm by SAFIR. An efficient way to find such H₂ emitters is to look at the Lyα emitters, since the brightest H₂ emitters are also luminous in Lyα emission.