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Using the Karl G. Jansky Very Large Array (VLA), we have detected absorptionlines due to carbon-monoxide, CO(J=0-1), and the cyano radical, CN(N=0-1),associated with radio galaxy B2 0902+34 at redshift z=3.4. The detection ofmillimeter-band absorption observed 1.5 Gyr after the Big Bang facilitatesstudying molecular clouds down to gas masses inaccessible to emission-lineobservations. The CO absorption in B2 0902+34 has a peak optical depth of$\tau$ $\ge$ 8.6% and consists of two components, one of which has the sameredshift as previously detected 21-cm absorption of neutral hydrogen (HI) gas.Each CO component traces an integrated H$_2$ column density of N(H2) $\ge$3x10$^{20}$ cm$^{-2}$. CN absorption is detected for both CO components, aswell as for a blueshifted component not detected in CO, with CO/CN line ratiosranging from $\le$0.4 to 2.4. We discuss the scenario that the absorptioncomponents originate from collections of small and dense molecular clouds thatare embedded in a region with more diffuse gas and high turbulence, possiblywithin the influence of the central Active Galactic Nucleus or starburstregion. The degree of reddening in B2 0902+34, with a rest-frame color B-K ~4.2, is lower than the very red colors (B-K > 6) found among other knownredshifted CO absorption systems at z<1. Nevertheless, when including also themany non-detections from the literature, a potential correlation between theabsorption-line strength and B-K color is evident, giving weight to theargument that the red colors of CO absorbers are due to a high dust content.

Absorption of Millimeter-band CO and CN in the Early Universe: Molecular Clouds in Radio Galaxy B2 0902+34 at Redshift 3.4