Observations of neutral carbon in 29 high-z lensed dusty star forming galaxies and the comparison of gas mass tracers

The nature and evolution of high-redshift dusty star-forming galaxies (high-z DSFGs) remain an open question. Their massive gas reservoirs play an important role in driving the intense star-formation rates hosted in these galaxies. We aim to estimate the molecular gas content of high-z DSFGs by using various gas mass tracers such as the [CI], CO, [CII] emission lines and the dust content. These tracers need to be well calibrated as they are all limited by uncertainties on factors such as aCO, XCI, aCII and GDR, thereby affecting the determination of the gas mass accurately. The main goal of our work is to check the consistency between the gas mass tracers and cross-calibrate the uncertain factors. We observe the two [CI] line transitions for 29 SPT-SMGs with the ALMA-ACA. Additionally, we also present new APEX observations of [CII] line for 9 of these galaxies. We find a nearly linear relation between the infrared luminosity and [CI] luminosity if we fit the starbursts and main-sequence galaxies separately. We measure a median [CI]-derived excitation temperature of 34.5+/-2.1 K. We probe the properties of the interstellar medium (ISM) such as density and radiation field intensity using [CI] to mid- or high-J CO lines and [CI] to infrared luminosity ratio, and find similar values to the SMG populations in literature. Finally, the gas masses estimated from [CI], CO, dust, and [CII] do not exhibit any significant trend with the infrared luminosity or the dust temperature. We provide the various cross-calibrations between these tracers. Our study confirms that [CI] is a suitable tracer of the molecular gas content, and shows an overall agreement between all the classical gas tracers used at high redshift. However, their absolute calibration and thus the gas depletion timescale measurements remain uncertain.Comment: Accepted for publication in A&A, 25 pages, 11 figures, 6 table

Milky Way-like Gas Excitation in an Ultrabright Submillimeter Galaxy at z = 1.6

Based on observations with the IRAM 30-m and Yebes 40-m telescopes, we report evidence of the detection of Milky Way-like, low-excitation molecular gas, up to the transition CO($J=5-4$), in a distant, dusty star-forming galaxy at $z_{CO}=1.60454$. WISE J122651.0+214958.8 (alias SDSSJ1226, the Cosmic Seahorse), is strongly lensed by a foreground galaxy cluster at $z=0.44$ with a source magnification of $\mu=9.5\pm0.7$. This galaxy was selected by cross-correlating near-to-mid infrared colours within the full-sky AllWISE survey, originally aiming to discover rare analogs of the archetypical strongly lensed submillimeter galaxy SMM J2135-0102, the Cosmic Eyelash. We derive an apparent (i.e. not corrected for lensing magnification) rest-frame 8-1000 $\mu$m infrared luminosity of $\mu L_\mathrm{IR}=1.66^{+0.04}_{-0.04}\times 10^{13}$ L$_\odot$ and apparent star-formation rate $\mu\mathrm{SFR}_\mathrm{IR}=2960\pm70$ M$_\odot$ yr$^{-1}$. SDSSJ1226 is ultra-bright at $S_{350\mu m}\simeq170$ mJy and shows similarly bright low-$J$ CO line intensities as SMM J2135-0102, however, with exceptionally small CO($J=5-4$) intensity. We consider different scenarios to reconcile our observations with typical findings of high-$z$ starbursts, and speculate about the presence of a previously unseen star-formation mechanism in cosmic noon submillimeter galaxies. In conclusion, the remarkable low line luminosity ratio $r_{5,2}=0.11\pm0.02$ is best explained by an extended, main-sequence star-formation mode -- representing a missing link between starbursts to low-luminosity systems during the epoch of peak star-formation history.Comment: 12 pages incl. one appendix, 5 figures, 2 tables. Accepted for publication in The Astrophysical Journal Letter