Kavli Affiliate: Ran Wang
| First 5 Authors: Fuxiang Xu, Ran Wang, Jianan Li, Roberto Neri, Antonio Pensabene
| Summary:
We present NOrthern Extended Millimeter Array observations of CO(8-7), (9-8),
and (10-9) lines, as well as the underlying continuum for two far-infrared
luminous quasars: SDSS J2054-0005 at $rm z=6.0389$ and SDSS J0129-0035 at $rm
z=5.7788$. Both quasars were previously detected in CO (2-1) and (6-5)
transitions, making them candidates for studying the CO Spectral Line Energy
Distribution (SLED) of quasars at $z sim 6$. Utilizing the radiative transfer
code CLOUDY, we fit the CO SLED with two heating mechanisms, including the
photo-dissociation region (PDR) and X-ray-dominated region (XDR) for both
objects. The CO SLEDs can be fitted by either a dense PDR component with an
extremely strong far-ultraviolet radiation field (gas density $ n_{rm H} sim
10^6 , rm cm^{-3}$ and field strength $G_0 gtrsim 10^6$) or a two-component
model including a PDR and an XDR. However, the line ratios, including tir and
previous cii and ci measurements, argue against a very high PDR radiation
field strength. Thus, the results prefer a PDR+XDR origin for the CO SLED. The
excitation of the high-J CO lines in both objects is likely dominated by the
central AGN. We then check the CO (9-8)-to-(6-5) line luminosity ratio $r_{96}$
for all $z sim 6$ quasars with available CO SLEDs (seven in total) and find
that there are no clear correlations between $r_{96}$ and both fir and the AGN
UV luminosities. This further demonstrates the complexity of the CO excitation
powered by both the AGN and nuclear star formation in these young quasar host
galaxies.
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