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We present the discovery of a molecular cloud at zabs=2.5255 along the lineof sight to the quasar J0000+0048. We perform a detailed analysis of theabsorption lines from ionic, neutral atomic and molecular species in differentexcitation levels, as well as the broad-band dust extinction. We find that theabsorber classifies as a Damped Lyman-alpha system (DLA) withlogN(HI)(cm^-2)=20.8+/-0.1. The DLA has super-Solar metallicity with adepletion pattern typical of cold gas and an overall molecular fraction ~50%.This is the highest f-value observed to date in a high-z intervening system.Most of the molecular hydrogen arises from a clearly identified narrow (b~0.7km/s), cold component in which CO molecules are also found, with logN(CO)~15.We study the chemical and physical conditions in the cold gas. We find that theline of sight probes the gas deep after the HI-to-H2 transition in a ~4-5pc-size cloud with volumic density nH~80 cm^-3 and temperature of only 50 K.Our model suggests that the presence of small dust grains (down to about 0.001{\mu}m) and high cosmic ray ionisation rate (zeta_H a few times 10^-15 s^-1)are needed to explain the observed atomic and molecular abundances. Thepresence of small grains is also in agreement with the observed steepextinction curve that also features a 2175 A bump. The properties of this cloudare very similar to what is seen in diffuse molecular regions of the nearbyPerseus complex. The high excitation temperature of CO rotational levelstowards J0000+0048 betrays however the higher temperature of the cosmicmicrowave background. Using the derived physical conditions, we correct for asmall contribution (0.3 K) of collisional excitation and obtain TCMB(z =2.53)~9.6 K, in perfect agreement with the predicted adiabatic cooling of theUniverse. [abridged]

astronomy and astrophysics

Discovery of a Perseus-like cloud in the early Universe: HI-to-H2  transition, carbon monoxide and small dust grains at zabs=2.53 towards the  quasar J0000+0048