2025ApJ...982..149O

While molecular outflows have been studied in detail with radio interferometry, observations of the hotter gas in protostellar outflows at a comparable physical scale are often challenging. Combined with the Atacama Large Millimeter/submillimeter Array (ALMA), JWST allows us to investigate the cold and hot gas with unprecedented spatial resolution and sensitivity. We present a detailed comparison between the gas distributions probed with ALMA and JWST in the primary outflow of IRAS 15398-3359. At 2000 au scale, the southwestern outflow shows four shell structures in 5-10 μm continuum, whereas the submillimeter H₂CO emission traces two of the four shells closest to the protostar. Submillimeter emission from CS, CCH, c-C₃H₂, and CH₃OH shows the same two shells, and the ¹²CO emission covers most of the outflow region. SO and SiO only trace a condensation at the edge of the shell closest to the protostar. None of these lines observed with ALMA show the outermost shell. At 500 au scale, we find hot H₂ gas inside the outflow cavity with JWST. The derived temperature of H₂ is 1147 ± 198 K within a 0$\mathop{.}\limits^{^{\prime\prime} }$5 aperture at the protostar. The foreground mass column density of dust is (1.4-2.0) × 10^–3 g · cm^–2 (A_V = 47-66 mag) in the outflow, using the dust model from J. C. Weingartner & B. T. Draine (). We also find an 8^∘ difference between the directions toward the [Fe II] knot and the outermost shell in the MIRI image, which may be interpreted as the precession of the [Fe II] jet. The dynamical timescale of the [Fe II] knot is 10 yr, suggesting a current event.