Discovery of a Perseus-like cloud in the early Universe: H I-to-H 2 transition, carbon monoxide and small dust grains at z abs ≈ 2.53 towards the quasar J0000+0048

P. Noterdaeme, J. K. Krogager, S. Balashev, J. Ge, N. Gupta, T. Krühler, C. Ledoux, M.T. Murphy, I. Pâris, P. Petitjean, H. Rahmani, R. Srianand, W. Ubachs

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We present the discovery of a molecular cloud at zabs ≈ 2.5255 along the line of sight to the quasar SDSS J 000015.17+004833.3. We use a high-resolution spectrum obtained with the Ultraviolet and Visual Echelle Spectrograph together with a deep multi-wavelength medium-resolution spectrum obtained with X-shooter (both on the Very Large Telescope) to perform a detailed analysis of the absorption lines from ionic, neutral atomic and molecular species in different excitation levels, as well as the broad-band dust extinction. We find that the absorber classifies as a Damped Lyman-α system (DLA) with log N(H i) (cm-2) = 20.8 ± 0.1. The DLA has super-solar metallicity (Z ~ 2.5 Z, albeit to within a factor of two to three) with a depletion pattern typical of cold gas and an overall molecular fraction f = 2N(H2)/(2N(H2) + N(H i)) ~ 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-1), cold component in which carbon monoxide molecules are also found, with log N(CO)≠15. With the help of the spectral synthesis code Cloudy, we study the chemical and physical conditions in the cold gas. We find that the line of sight probes the gas deep after the H i-to-H2 transition in a ~4-5 pc-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 μm) and high cosmic ray ionisation rate (ζH ~ a few times 10-15 s-1) are needed to explain the observed atomic and molecular abundances. The presence of small grains is also in agreement with the observed steep extinction curve that also features a 2175 Å bump. Interestingly, the chemical and physical properties of this cloud are very similar to what is seen in diffuse molecular regions of the nearby Perseus complex, despite the former being observed when the Universe was only 2.5 Gyr old. The high excitation temperature of CO rotational levels towards J0000+0048 betrays however the higher temperature of the cosmic microwave background. Using the derived physical conditions, we correct for a small 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 the Universe.

Original languageEnglish
Article numberA82
Pages (from-to)1-24
Number of pages24
JournalAstronomy and Astrophysics
Early online date6 Jan 2017
Publication statusPublished - Jan 2017


We thank Gargi Shaw for help with running Cloudy. P.N. and P.P.J. acknowledge support from the Agence Nationale de la Recherche under grant ANR-12-BS05-0015. P.N., N.G., P.P.J. and R.S. acknowledge support from the Indo-French Centre for the Promotion of Advanced Research under project No. 5504-2. J.K. acknowledges support from the European Union's Seventh Framework Programme for research and innovation under the Marie-Curie grant agreement No. 600207 with reference DFF-MOBILEX-5051-00115. S.B. acknowledges support from the Russian Science Foundation under grant 14-12-00955. T.K. acknowledges support through the Sofja Kovalevskaja Award to P. Schady from the Alexander von Humboldt Foundation of Germany. M.T.M. thanks the Australian Research Council for Discovery Project grant DP130100568 which supported this work. This work has made use of the SDSS-III/BOSS database. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the US Department of Energy Office of Science.

FundersFunder number
National Science Foundation
U.S. Department of Energy
Alfred P. Sloan Foundation
Alexander von Humboldt-Stiftung
Seventh Framework Programme600207
Australian Research CouncilDP130100568
Agence Nationale de la RechercheANR-12-BS05-0015
Indo-French Centre for the Promotion of Advanced Research5504-2
Seventh Framework ProgrammeDFF-MOBILEX-5051-00115
Russian Science Foundation14-12-00955


    • Cosmic background radiation
    • Cosmology: observations
    • Dust, extinction
    • ISM: clouds
    • ISM: molecules
    • Quasars: absorption lines


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