Weak¹³CO in the Cloverleaf quasar: evidence for a young, early generation starburst

Abstract

Observations of 12 CO at high redshift indicate rapid metal enrichment in the nuclear regions of at least some galaxies in the early universe. However, the enrichment may be limited to nuclei that are synthesized by short-lived massive stars, excluding classical "secondary" nuclei like 13 C. Testing this idea, we used the IRAM Interferometer to tentatively detect the 13 CO J = 3 → 2 line at a level of 0.3 Jy km s −1 toward the Cloverleaf quasar at z = 2.5. This is the first observational evidence for 13 C at high redshift. The 12 CO/ 13 CO J = 3 → 2 luminosity ratio is with 40 +25 −8 much higher than ratios observed in molecular clouds of the Milky Way and in the ultraluminous galaxy Arp 220, but may be similar to that observed toward NGC 6240. Large Velocity Gradient models simulating seven 12 CO transitions and the 13 CO line yield 12 CO/ 13 CO abundance ratios in excess of 100. It is possible that the measured ratio is affected by a strong submillimeter radiation field, which reduces the contrast between the 13 CO line and the background. It is more likely, however, that the ratio is caused by a real deficiency of 13 CO. This is already apparent in local ultraluminous galaxies and may be even more severe in the Cloverleaf because of its young age ( < ∼ 2.5 Gyr). A potential conflict with optical data, indicating high abundances also for secondary nuclei in quasars of high redshift, may be settled if the bulk of the CO emission is originating sufficiently far from the active galactic nucleus of the Cloverleaf.

LUMINOUS INFRARED GALAXIES WITH THE SUBMILLIMETER ARRAY. IV.12COJ= 6-5 OBSERVATIONS OF VV 114

“…The observed 12 CO/ 13 CO line ratio was 40 and exceeds that seen in Arp 220 but is similar to that of NGC 6240 (Greve et al 2009). Henkel et al (2010) surmise that the high line ratio may be caused by a real deficiency in 13 …”

“…Wilson & Rood 1994). Casoli et al (1992) have suggested that the observed higher 12 CO/ 13 CO line ratios seen in U/LIRGs, than in our Galaxy are likely due to an under-abundance of 13 CO and/or an over-abundance of 12 CO. Casoli et al (1992) Henkel et al (2010) have observed 13 CO J = 3-2 in the Cloverleaf quasar and have compared it to the 12 CO J = 3-2. The observed 12 CO/ 13 CO line ratio was 40 and exceeds that seen in Arp 220 but is similar to that of NGC 6240 (Greve et al 2009).…”

How Dense Is Your Gas? On the Recoverability of LVG Model Parameters

“…Furthermore, isotopologue selective chemical effects such as selective photodissociation and isotope charge exchange reactions can lead to isotope fractionation. When coupled with real variations in the elemental isotope abundances, such as in accreted primordial gas or cosmic-ray-dominated star formation paradigms, can lead to large variations in the isotopologue abundance ratios from species to species and galaxy to galaxy (Henkel et al 2010(Henkel et al , 2014Papadopoulos 2010;Papadopoulos et al 2011;Ritchey et al 2011;Roueff et al 2015). We therefore explore the effects of assuming an incorrect abundance ratio on the recovered T k and n H 2 , using HCN.…”

Modeling the Molecular Gas in NGC 6240

“…Recently, it has been suggested by Papadopoulos et al (2011) that a high [ 12 C]/[ 13 C] ratio might be evidence for a cosmic ray dominated star formation paradigm, leading to a top heavy stellar initial mass function (IMF) and thereby an excess of massive, 12 C producing, stars. Typical values of [ 12 C]/[ 13 C] in the local universe are ∼25 in the central Milky Way, 68 in the Milky Way as a whole and up to ∼100 in local starbursts and 100  in high-redshift ULIRGs (Milam et al 2005;Henkel et al 2010Henkel et al , 2014. Typically, these ratios are measured using a combination of observations of CN, 13 CN, CO, 13 CO and C 18 O lines.…”