ALMA imaging of C2H emission in the disk of NGC1068
  We study the feedback of star formation and nuclear activity on the chemistry
of molecular gas in NGC1068, a nearby (D=14Mpc) Seyfert 2 barred galaxy, by
analyzing if the abundances of key molecular species like ethynyl (C2H), a
classical tracer of PDR, change in the different environments of the disk of
the galaxy. We have used ALMA to map the emission of the hyperfine multiplet of
C2H(N=1-0) and its underlying continuum emission in the central
r~35"(2.5kpc)-region of the disk of NGC1068 with a spatial resolution
1.0"x0.7"(50-70pc). We have developed a set of time-dependent chemical models
to determine the origin of the C2H gas. A sizeable fraction of the total C2H
line emission is detected from the r~1.3kpc starburst (SB) ring. However, the
brightest C2H emission originates from a r~200pc off-centered circumnuclear
disk (CND), where evidence of a molecular outflow has been previously found in
other molecular tracers imaged by ALMA. We also detect significant emission
that connects the CND with the outer disk. We derived the fractional abundances
of C2H (X(C2H)) assuming LTE conditions. Our estimates range from X(C2H)~a few
10^-8 in the SB ring up to X(C2H)~ a few 10^-7 in the outflow region. PDR
models that incorporate gas-grain chemistry are able to account for X(C2H) in
the SB ring for moderately dense (n(H2)>10^4 cm^-3) and moderately
UV-irradiated gas (UV-field<10xDraine field) in a steady-state regime. However,
the high fractional abundances estimated for C2H in the outflow region can only
be reached at very early times (T< 10^2-10^3 yr) in models of UV/X-ray
irradiated dense gas (n(H2)>10^4-10^5) cm^-3). We interpret that the transient
conditions required to fit the high values of X(C2H) in the outflow are likely
due to UV/X-ray irradiated non-dissociative shocks associated with the highly
turbulent interface between the outflow and the molecular gas in NGC1068.
