Volume 608, December 2017
|Number of page(s)||41|
|Published online||15 December 2017|
Molecular gas in the Herschel-selected strongly lensed submillimeter galaxies at z ~ 2–4 as probed by multi-J CO lines⋆,⋆⋆,⋆⋆⋆
1 Purple Mountain Observatory/Key Lab of Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China
2 Institut d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Bât. 121, 91405 Orsay Cedex, France
3 Graduate University of the Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, 10049 Beijing, PR China
4 CNRS, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
5 UPMC Univ. Paris 06, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
6 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
7 Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
8 European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany
9 Max Planck Institute for Astronomy, Konigstuhl 17, 69117 Heidel- berg, Germany
10 Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
11 Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
12 Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
13 Joint ALMA Observatory, 3107 Alonso de Córdova, Vitacura, Santiago, Chile
14 Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d’ Hères, France
15 Astronomy Department, Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
16 Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA
17 Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
18 Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
19 Centre for Extragalactic Astronomy, Durham University, Department of Physics, South Road, Durham DH1 3LE, UK
Received: 18 June 2017
Accepted: 13 September 2017
We present the IRAM-30 m observations of multiple-J CO (Jup mostly from 3 up to 8) and [C I](3P2 → 3P1) ([C I](2–1) hereafter) line emission in a sample of redshift ~2–4 submillimeter galaxies (SMGs). These SMGs are selected among the brightest-lensed galaxies discovered in the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS). Forty-seven CO lines and 7 [C I](2–1) lines have been detected in 15 lensed SMGs. A non-negligible effect of differential lensing is found for the CO emission lines, which could have caused significant underestimations of the linewidths, and hence of the dynamical masses. The CO spectral line energy distributions (SLEDs), peaking around Jup ~ 5–7, are found to be similar to those of the local starburst-dominated ultra-luminous infrared galaxies and of the previously studied SMGs. After correcting for lensing amplification, we derived the global properties of the bulk of molecular gas in the SMGs using non-LTE radiative transfer modelling, such as the molecular gas density nH2 ~ 102.5–104.1 cm-3 and the kinetic temperature Tk ~ 20–750 K. The gas thermal pressure Pth ranging from~105 K cm-3 to 106 K cm-3 is found to be correlated with star formation efficiency. Further decomposing the CO SLEDs into two excitation components, we find a low-excitation component with nH2 ~ 102.8–104.6 cm-3 and Tk ~ 20–30 K, which is less correlated with star formation, and a high-excitation one (nH2 ~ 102.7–104.2 cm-3, Tk ~ 60–400 K) which is tightly related to the on-going star-forming activity. Additionally, tight linear correlations between the far-infrared and CO line luminosities have been confirmed for the Jup ≥ 5 CO lines of these SMGs, implying that these CO lines are good tracers of star formation. The [C I](2–1) lines follow the tight linear correlation between the luminosities of the [C I](2–1) and the CO(1–0) line found in local starbursts, indicating that [C I] lines could serve as good total molecular gas mass tracers for high-redshift SMGs as well. The total mass of the molecular gas reservoir, (1–30) × 1010M⊙, derived based on the CO(3–2) fluxes and αCO(1–0) = 0.8 M⊙ ( K km s-1 pc2)-1, suggests a typical molecular gas depletion time tdep ~ 20–100 Myr and a gas to dust mass ratio δGDR ~ 30–100 with ~20%–60% uncertainty for the SMGs. The ratio between CO line luminosity and the dust mass L′CO/Mdust appears to be slowly increasing with redshift for high-redshift SMGs, which need to be further confirmed by a more complete SMG sample at various redshifts. Finally, through comparing the linewidth of CO and H2O lines, we find that they agree well in almost all our SMGs, confirming that the emitting regions of the CO and H2O lines are co-spatially located.
Key words: galaxies: high-redshift / galaxies: ISM / infrared: galaxies / submillimeter: galaxies / radio lines: ISM / ISM: molecules
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Based on observations carried out under project number 076-16, 196-15 and 079-15 (PI: C. Yang); 252-11 and 124-11 (PI: P. van de Werf) with the IRAM-30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
The reduced spectra (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/608/A144
© ESO, 2017
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