Free-bound emission from cosmological hydrogen recombination

Free access article

Issue		A&A Volume 458, Number 2, November I 2006


Page(s)		L29 - L32
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20066191

A&A 458, L29-L32 (2006)
DOI: 10.1051/0004-6361:20066191

Letter

Free-bound emission from cosmological hydrogen recombination

J. Chluba¹ and R. A. Sunyaev^{1, 2}

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 ( $n\geq 2$ ) during the epoch of cosmological hydrogen recombination. This contribution leads to a total of one photon per recombined hydrogen atom and therefore a ~ $30{-}88\%$ increase in the recombination spectrum within the frequency range $1~{\rm GHz}\leq \nu \leq 100~{\rm GHz}$ . In particular, the Balmer-continuum emission increases the distortion at $\nu\sim 690$ GHz by roughly $92\%$ . 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 $\nu\la 30$ GHz is completely featureless.

Key words: cosmic microwave background

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