Letter to the Editor

Free-bound emission from cosmological hydrogen recombination

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei München, Germany e-mail: jchluba@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 4 August 2006
Accepted: 7 September 2006

Abstract

In this letter we compute the emission coming from the direct recombination of free electrons to a given shell () during the epoch of cosmological hydrogen recombination. This contribution leads to a total of one photon per recombined hydrogen atom and therefore a ~ increase in the recombination spectrum within the frequency range . In particular, the Balmer-continuum emission increases the distortion at  GHz by roughly . With our 100 shell calculations for the hydrogen atom, we find that a total of ~5 photons per hydrogen atom are emitted when including all the bound-bound transitions, the 2s two-photon decay channel, and the optically thin free-bound transitions. Since the direct recombination continuum at high n is very broad, only a few n-series continuua are distinguishable and most of this additional emission below  GHz is completely featureless.