Resolved neutral carbon emission in nearby galaxies: [CI] Lines as Total
  Molecular Gas Tracers
  We present maps of atomic carbon [CI](1-0) and [CI](2-1) at a linear
resolution ~1kpc scale for a sample of one HII, six LINER, three Seyfert and
five starburst galaxies observed with Herschel. We compare spatial
distributions of two [CI] lines with that of CO(1-0) emission, and find that
both [CI] lines distribute similarly to CO(1-0) emission in most galaxies. We
present luminosity ratio maps of L'_[CI](1-0)/L'_CO(1-0),
L'_[CI](2-1)/L'_CO(1-0), L'_[CI](2-1)/L'_[CI](1-0) (hereafter R_[CI]) and
f_70/f_160. L'_[CI](2-1)/L'_CO(1-0), R_[CI] and f_70/f_160 are centrally peaked
in starbursts; whereas remain relatively constant in LINERs, indicating that
star-forming activity can enhance carbon emission, especially for [CI](2-1). We
explore the correlations between the luminosities of CO(1-0) and [CI] lines,
and find that L'_CO(1-0) correlates tightly and almost linearly with both
L'_[CI](1-0) and L'_[CI](2-1), suggesting that [CI] lines, similar as CO(1-0),
can trace total molecular gas in our resolved galaxies on kpc scales. We
investigate the dependence of L'_[CI](1-0)/L'_CO(1-0), L'_[CI](2-1)/L'_CO(1-0)
and [CI] excitation temperature T_ex on dust temperature T_dust, and find
non-correlation, a weak and modest correlation, respectively. The ratio of
L'_[CI](1-0)/L'_CO(1-0) stays smooth distribution in most galaxies, indicating
that the conversion factor of [CI](1-0) luminosity to H_2 mass (X_[CI](1-0))
changes with CO(1-0) conversion factor (\alpha_CO) proportionally. Under
optically thin and LTE assumptions, we derive a galaxy-wide average carbon
excitation temperature T_ex ~ 19.7 \pm 0.5K and an average neutral carbon
abundance X[CI]/X[H_2] ~2.5 \pm 1.0 * 10^{-5} in our resolved sample, which is
comparable to the usually adopted value of 3*10^{-5}, but ~3 times lower than
the carbon abundance in local (U)LIRGs. We conclude that the carbon abundance
varies in different galaxy types.
