Abstract
We report the detection of far-IR CO rotational emission from the
prototypical Seyfert 2 galaxy NGC 1068. Using Herschel-PACS, we have detected
10 transitions in the J_upper=14-24 (E_upper/k_B = 580-1656 K) range, all of
which are consistent with arising from within the central 10" (700 pc). The
detected transitions are modeled as arising from 2 different components: a
moderate excitation (ME) component close to the galaxy systemic velocity, and a
high excitation (HE) component that is blueshifted by ~70 km s^-1. We employ
a large velocity gradient (LVG) model and derive n_H2~10^5.7 cm^-3,
T_kin~150 K, and M_H2~10^6.9 M_sun for the ME component, and n_H2~10^6.3
cm^-3, T_kin~440 K, and M_H2~10^5.8 M_sun for the HE component, although
for both components the uncertainties in the density and mass are ~plus/minus 1
dex. Both components arise from denser and possibly warmer gas than traced by
low-J CO transitions, and the ME component likely makes a significant
contribution to the mass budget in the nuclear region. We compare the CO line
profiles with those of other molecular tracers observed at higher spatial and
spectral resolution, and find that the ME transitions are consistent with these
lines arising in the ~200 pc diameter ring of material traced by H_2 1-0 S(1)
observations. The blueshift of the HE lines may also be consistent with the
bluest regions of this H_2 ring, but a better kinematic match is found with a
clump of infalling gas ~40 pc north of the AGN. We consider potential heating
mechanisms, and conclude that X-ray or shock heating of both components is
viable, while far-UV heating is unlikely. We also report sensitive upper limits
extending up to J_upper=50, which place constraints on the emission from the
X-ray obscuring medium.
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