Molecular gas in theHerschel-selected strongly lensed submillimeter galaxies atz ~ 2–4 as probed by multi-JCO lines

Abstract

We present the IRAM-30 m observations of multiple-J CO (J up mostly from 3 up to 8) and [C I]( 3 P 2 → 3 P 1 ) ([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 J up ∼ 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 n H 2 ∼ 10 2.5 -10 4.1 cm −3 and the kinetic temperature T k ∼ 20-750 K. The gas thermal pressure P th ranging from ∼ 10 5 K cm −3 to 10 6 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 n H 2 ∼ 10 2.8 -10 4.6 cm −3 and T k ∼ 20-30 K, which is less correlated with star formation, and a high-excitation one (n H 2 ∼ 10 2.7 -10 4.2 cm −3 , T k ∼ 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 J up ≥ 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) × 10 10 M , derived based on the CO(3-2) fluxes and α CO(1-0) = 0.8M ( K km s −1 pc 2 ) −1 , suggests a typical molecular gas depletion time t dep ∼ 20-100 Myr and a gas to dust mass ratio δ GDR ∼ 30-100 with ∼ 20%-60% uncertainty for the SMGs. The ratio between CO line luminosity and the dust mass L CO /M dust 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 H 2 O lines, we find that they agree well in almost all our SMGs, confirming that the emitting regions of the CO and H 2 O lines are co-spatially located.

NOEMA redshift measurements of bright Herschel galaxies

“…Figure 13 displays the relation between the apparent CO luminosities, L ′ CO(1−0) , and the width (∆V) of the CO emission lines for the sources presented in this work and a compilation of high-z lensed and unlensed SMGs, as well as local ULIRGs from the literature (see figure caption for details and references). This relationship has already been presented and discussed in previous studies (e.g., Harris et al 2012;Bothwell et al 2013;Carilli & Walter 2013;Aravena et al 2016;Yang et al 2017;Dannerbauer et al 2017;Isbell et al 2018). Figure 13 includes CO measurements of ∼160 galaxies in total; it should be noted that none of the gravitationally lensed sources in this plot was corrected for lensing magnification.…”

Full of Orions: a 200-pc mapping of the interstellar medium in the redshift-3 lensed dusty star-forming galaxy SDP.81

“…The global source-plane CO SLED in SDP.81 peaks between Jupp = 5 and 9 (in units of L ), with a sharp drop at Jupp = 10 ( Fig. 8, see also Yang et al 2017, for a recent sky-plane analysis). Compared to the Rosenberg et al (2015) study of CO excitation in present-day ULIRGS, the sharp drop in LCO for Jupp ≥ 9 and minor CO contribution to the global neutral gas cooling makes SDP.81 akin to Rosenberg et al (2015) Class I sources, which have similarly low CO cooling contributions (≤ 10 per cent).…”

“…By including only Jupp ≥ 3 CO lines (critical density ncrit ≥ 8 × 10 3 cm −3 , we effectively discount the extended cold, low-density (n ≤ 10 3 cm −3 ) gas phase traced by CO(1-0) and CO(2-1) lines. Indeed, studies of the CO ladders including the Jupp = 1 − 2 lines have found significant cold gas components both in z ∼ 0 ULIRGs (e.g., Kamenetzky et al 2014;Greve et al 2014) and high-redshift DSFGs (e.g., Bothwell et al 2013;Yang et al 2017). While the cold gas component can contribute to the CO(3-2) and CO(5-4) luminosity, the Yang et al (2017) source-averaged CO SLED model for SDP.81 -which includes both a cold and a warm component -shows the cold-gas contribution to the CO(3-2) and CO(5-4) luminosities to be ≤30 and <10 per cent, respectively.…”

Probing the Full CO Spectral Line Energy Distribution (SLED) in the Nuclear Region of a Quasar-starburst System at z = 6.003

“…We also compare the CO SLED of J2310+1855 with high redshift systems, including high redshift (lensed) SMGs and quasars. Figure 6 (a) shows the CO SLED of J2310+1855 and high redshift SMGs: a sample of z ∼ 1.2 − 4.1 SMGs (Bothwell et al 2013) and a sample of 15 z ∼ 2 − 4 lensed SMGs from Carilli & Walter (2013) and Yang et al (2017). The high redshift SMG CO SLEDs peak at J 6, while J2310+1855 peaks at higher J (J = 8) than the two SMG samples.…”

“…Recent studies found a nearly linear relation between the water luminosity and the infrared luminosity in local and high redshift systems over three orders of magnitude (González-Alfonso et al 2010;? ;Riechers et al 2013;Yang et al , 2017Omont et al 2013;Jarugula et al 2019). The brightest water lines are detected in the presence of shocks or X-rays (González-Alfonso et al 2010;Pellegrini et al 2013).…”