A deep search for molecular gas in two massive Lyman break galaxies at z=3 and 4: vanishing CO-emission due to low metallicity

We present deep IRAM Plateau de Bure Interferometer (PdBI) observations, searching for CO-emission toward two massive, non-lensed Lyman break galaxies (LBGs) at z = 3.216 and 4.058. With one low significance CO detection (3.5σ) and one sensitive upper limit, we find that the CO lines are &3–4 times weaker than expected based on the relation between IR and CO luminosities followed by similarly, massive galaxies at z =0–2.5. This is consistent with a scenario in which these galaxies have low metallicity, causing an increased CO-to-H2 conversion factor, i.e., weaker CO-emission for a given molecular (H2) mass. The required metallicities at z > 3 are lower than predicted by the fundamental metallicity relation (FMR) at these redshifts, consistent with independent evidence. Unless our galaxies are atypical in this respect, detecting molecular gas in normal galaxies at z > 3 may thus remain challenging even with ALMA. Subject headings: galaxies: evolution — galaxies: high-redshift — galaxies: star formation Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Science, Nanjing 210008, China CEA Saclay, DSM/Irfu/Service d’Astrophysique, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France Graduate University of the Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, China Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany National Radio Astronomy Observatory, P. O. Box O, Socorro, NM 87801, USA Universität Wien, Institut für Astronomie Türkenschanzstrasse, 171160 Wien, Austria National Optical Astronomical Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA Institute for Astronomy, University of Hawaii, Honolulu, HI, 96822, USA Canada-France-Hawaii Telescope, Kamuela, HI, 96743, USA Department of Astronomy, Cornell University, 220