Limits on the molecular gas content of z ~ 5 LBGs

L. J.M. Davies*, M. N. Bremer, E. R. Stanway, M. Birkinshaw, M. D. Lehnert

*Corresponding author for this work

Research output: Contribution to journalLetter (Academic Journal)peer-review

10 Citations (Scopus)


We present limits on the molecular gas content of Lyman break galaxies (LBGs) at z ~ 5 from observations targeting redshifted CO(1-0) and CO(2-1) line emission. We observed a single field containing eight spectroscopically confirmed z ~ 5 LBGs, seven of which are contained within a narrow (z = 4.95 ± 0.08) redshift range and the eighth is at z = 5.2. No source was individually detected. Assuming the CO to H2 conversion factor for vigorous starbursts, we place upper limits on the molecular gas content of individual z ~ 5 LBGs of M(H2) ≲ 1010 M. From a stacking analysis combining all of the non-detections, the typical z ~ 5 LBG has an H2 mass limit comparable to their stellar mass, <3.1 × 109 M. This limit implies that, given the star formation rates of these systems (measured from their UV emission), star formation could be sustained for at most ~100Myr, similar to the typical ages of their stellar populations. The lack of a substantially larger reservoir of cold gas argues against the LBGs being UV-luminous superstarbursts embedded in much larger UV-dark systems and as a result increases the likelihood that at least those LBGs with multiple components are starbursts triggered by mergers. The sources responsible for re-ionization are expected to be starbursts similar to these systems, but with lower luminosities, masses and consequently with star formation time-scales far shorter than the recombination time-scale. If so, the ionized bubbles expected in the IGM during the re-ionization era will only infrequently have UV-luminous sources at their centres.

Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
Publication statusPublished - 1 Oct 2010


  • Galaxies: high-redshift
  • Galaxies: star formation
  • Galaxies: starburst
  • Radio lines: galaxies


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