New CO detections of lensed submillimetre galaxies in A2218: probing molecular gas in the LIRG regime at high redshift

Context. Submillimetre galaxies (SMGs) are distant, dusty galaxies undergoing star formation at prodigious rates. Recently there has been major progress in understanding the nature of the bright SMGs (i.e. $S_{\rm 850~\mu m} > 5$ mJy). The samples for the fainter SMGs are small and are currently in a phase of being built up through identification studies. Aims. We study the molecular gas content in the two SMGs, SMMJ163555 and SMMJ163541, at redshifts $z=1.034$ and $z=3.187$ with unlensed submillimetre fluxes of 0.4 mJy and 6.0 mJy. Both SMGs are gravitationally lensed by the foreground cluster Abell 2218. Methods. We used the IRAM Plateau de Bure Interferometer to obtain observations at 3 mm of the lines CO(2–1) for SMMJ163555 and CO(3–2) for SMMJ163541. Additionally, we obtained CO(4–3) observations for the candidate $z=4.048$ SMMJ163556 with an unlensed submillimetre flux of 2.7 mJy. Results. The CO(2–1) line was detected for SMMJ163555 at redshift 1.0313 with an integrated line intensity of $1.2\pm0.2$ Jy km s -1 and a line width of $410\pm120$ km s -1 . From this a gas mass of $1.6\times10^{9}$ $M_\odot$ is derived and a star formation efficiency of 440 $L_\odot$/$M_\odot$ estimated. The CO(3–2) line was detected for SMMJ163541 at redshift 3.1824, possibly with a second component at redshift 3.1883, with an integrated line intensity of $1.0\pm0.1$ Jy km s -1 and a line width of $280\pm50$ km s -1 . From this a gas mass of $2.2\times10^{10}$ $M_\odot$ is derived and a star formation efficiency of 1000 $L_\odot$/$M_\odot$ is estimated. For SMMJ163556, the CO(4–3) is undetected within the redshift range $4.035{-}4.082$ down to a sensitivity of 0.15 Jy km s -1 . Conclusions. Our CO-line observations confirm the optical redshifts for SMMJ163555 and SMMJ163541. The CO-line luminosity $L'_{\rm CO}$ for both galaxies is consistent with the $L_{\rm FIR}-L'_{\rm CO}$ relation. SMMJ163555 has the lowest far-infrared luminosity of all SMGs with a known redshift and is one of the few high-redshift LIRGs whose properties can be estimated prior to ALMA.