  We present JWST/MIRI MRS spectroscopy of NGC7319, the largest galaxy in the
Stephan's Quintet, observed as part of the Early Release Observations (ERO).
NGC7319 hosts a type 2 active galactic nucleus (AGN) and a low-power radio jet
(L_1.4GHz=3.3x10^22 W Hz^-1) with two asymmetric radio hotspots at 430 pc (N2)
and 1.5 kpc (S2) projected distances from the unresolved radio core. The MRS
data suggest that the molecular material in the disk of the galaxy decelerates
the jet and causes this length asymmetry. We find enhanced emission from warm
and hot H_2 (T_w=330+-40 K, T_h=900+-60 K) and ionized gas at the intersection
between the jet axis and dust lanes in the disk. This emission is coincident
with the radio hotspot N2, the hotspot closer to the core, suggesting that the
jet-interstellar medium (ISM) interaction decelerates the jet. Conversely, the
mid-infrared emission at the more distant hotspot is fainter, more highly
ionized, and with lower H_2 excitation, suggesting a more diffuse atomic
environment where the jet can progress to farther distances. At the N2 radio
hotspot, the ionized gas mass (M_ion=(2.4-12)x10^5 Msun) is comparable to that
of the warm H_2, but the former is more turbulent (sigma_ion~300 vs.
sigma_H2~150 km/s), so the mechanical energy of the ionized gas is ~1.3-10
times higher. From these estimates, we find that only <1% of the jet energy
remains as mechanical energy in these two ISM phases at N2. We also find
extended (r>0.3-1.5 kpc) high-ionization emission ([MgV], [NeVI], and [NeV])
close to the radio hotspots. This initial analysis of NGC7319 shows the
potential of MIRI/MRS to investigate the AGN feedback mechanisms due to radio
jets and their radiation field in the, often heavily dust-enshrouded, central
regions of galaxies. Understanding these mechanisms is an essential ingredient
in the development of cosmological simulations of galaxy evolution.
