Interstellar and Circumgalactic Properties of an Unseen $z=6.84$ Galaxy:
  Abundances, Ionization, and Heating in the Earliest Known Quasar Absorber
  We analyze relative abundances and ionization conditions in a strong
absorption system at z=6.84, seen in the spectrum of the z=7.54 background
quasar ULAS J134208.10+092838.61. Singly ionized C, Si, Fe, Mg, and Al
measurements are consistent with a warm neutral medium that is metal-poor but
not chemically pristine. Firm non-detections of C IV and Si IV imply that any
warm ionized phase of the IGM or CGM has not yet been enriched past the
ultra-metal-poor regime (<0.001Z_{solar}), unlike lower redshift DLAs where
these lines are nearly ubiquitous. Relative abundances of the heavy elements
794 Myr after the Big Bang resemble those of metal-poor damped Lyman Alpha
systems at intermediate redshift and Milky Way halo stars, and show no evidence
of enhanced [alpha/Fe], [C/Fe] or other signatures of yields dominated by
massive stars. A detection of the CII* fine structure line reveals local
sources of excitation from heating, beyond the level of photo-excitation
supplied by the CMB. We estimate the total and [CII] cooling rates, balancing
against ISM heating sources to develop an heuristic two-phase model of the
neutral medium. The implied heating requires a surface density of star
formation slightly exceeding that of the Milky Way but not at the level of a
strong starburst. For a typical (assumed) NHI=10^{20.6}, an abundance of
[Fe/H]=-2.2 matches the columns of species in the neutral phase. To remain
undetected in C IV, a warm ionized phase would either need much lower
[C/H]<-4.2 over an absorption path of 1 kpc, or else a very small absorption
path (a few pc). While still speculative, these results suggest a significant
reduction in heavy element enrichment outside of neutral star forming regions
of the ISM, as would be expected in early stages of galactic chemical
evolution.
