Cooling radiation and the Lyα luminosity of forming galaxies

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Lyα line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z ≈ 2. However, in contrast to the classical picture of gas cooling from the ∼ 10⁶ K virial temperature of a typical dark matter halo, we find that most of the cooling radiation is emitted by gas with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation emerges in the Lyα line. While a galaxy's cooling luminosity is usually smaller than the ionizing continuum luminosity of its young stars, the two are comparable in the most massive systems, and the cooling radiation is produced at larger radii, where the Lyα photons are less likely to be extinguished by dust. We suggest, in particular, that cooling radiation could explain the two large (∼ 100 kpc), luminous (L_Lyα ∼ 10⁴⁴ ergs s^-1) "blobs" of Lyα emission found in the narrowband survey of a z = 3 protocluster by Steidel and collaborators. Our simulations predict objects of the observed luminosity at about the right space density, and radiative transfer effects can account for the observed sizes and line widths. We discuss observable tests of this hypothesis for the nature of the Lyα blobs, and we present predictions for the contribution of cooling radiation to the Lyα luminosity function of galaxies as a function of redshift.