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Register a new userConstraints on high-J CO emission lines in $zsim 6$ quasars
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We present Atacama Large Millimiter/submillimiter Array (ALMA) observations of eight highly excited CO (J$_{rm up}>8$) lines and continuum emission in two $zsim6$ quasars: SDSS J231038.88+185519.7 (hereafter J2310), for which CO(8-7), CO(9-8), and CO(17-16) lines have been observed, and ULAS J131911.29+095951.4 (J1319), observed in the CO(14-13), CO(17-16) and CO(19-18) lines. The continuum emission of both quasars arises from a compact region ($< 0.9$ kpc). By assuming a modified black-body law, we estimate dust masses of Log$(M_{rm dust}/M_{odot})=8.75pm0.07$ and Log$(M_{rm dust}/M_{odot})=8.8pm0.2$ and dust temperatures of $T_{rm dust}=76pm3~{rm K}$ and $T_{rm dust}=66^{+15}_{-10}~{rm K}$, respectively for J2310 and J1319. Only CO(8-7) and CO(9-8) in J2310 are detected, while $3sigma$ upper limits on luminosities are reported for the other lines of both quasars. The CO line luminosities and upper limits measured in J2310 and J1319 are consistent with those observed in local AGN and starburst galaxies, and other $zsim 6$ quasars, except for SDSS J1148+5251 (J1148), the only quasar at $z=6.4$ with a previous CO(17-16) line detection. By computing the CO SLEDs normalised to the CO(6-5) line and FIR luminosities for J2310, J1319, and J1149, we conclude that different gas heating mechanisms (X-ray radiation and/or shocks) may explain the different CO luminosities observed in these $zsim6$ quasar. Future J$_{rm up}>8$ CO observations will be crucial to understand the processes responsible for molecular gas excitation in luminous high-$z$ quasars.
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In bright photodissociation regions (PDRs) associated to massive star formation, the presence of dense clumps that are immersed in a less dense interclump medium is often proposed to explain the difficulty of models to account for the observed gas emission in high-excitation lines. We aim at presenting a comprehensive view of the modeling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and are analyzed using the Meudon PDR code. A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to Jup=23 in the Orion Bar (Jup=19 in NGC7023), can only originate from small structures of typical thickness of a few 1e-3 pc and at high thermal pressures (Pth~1e8 K cm-3). Compiling data from the literature, we found that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help rationalising the analysis of high-J CO emission in massive star formation and provides an observational constraint for models that study stellar feedback on molecular clouds.
The UV/optical variation, likely driven by accretion disc turbulence, is a defining characteristic of type 1 active galactic nuclei (AGNs) and quasars. In this work we investigate an interesting consequence of such turbulence using quasars in SDSS Stripe 82 for which the measurements of the UV/optical variability amplitude are available from $sim$ 10 years long light curves. We discover positive correlations between UV/optical variability amplitude $sigma_{rms}$ and equivalent widths of CIV, Mg II and [OIII]5007 emission lines. Such correlations remain statistically robust through partial correlation analyses, i.e., after controlling the effects of other variables including bolometric luminosity, central supermassive black hole mass, Eddington ratio and redshift. This, for the first time, indicates a causal link between disc turbulence and emission line production. We propose two potential underlying mechanisms both of which may be involved: 1) quasars with stronger disc turbulence have on average bluer/harder broadband SED, an expected effect of the disc thermal fluctuation model; 2) stronger disc turbulence could lead to launch of emission line regions with larger covering factors.
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