GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114

Abstract

The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 μm, we find highly excited CO (T ex ≈ 700–800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex ≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.

On the Interpretation of Mid-infrared Absorption Lines of Gas-phase H2O as Observed by JWST/MIRI

“…JWST/Mid-Infrared Instrument (MIRI; Rieke et al 2015;Labiano et al 2021) can detect the ν 2 rovibrational band of gasphase H 2 O from 5 to 7.5 μm with a spectral resolving power of R ∼ 3500 (85 km s −1 ). MIRI has detected several tens to hundreds of gaseous water lines in a broad range of systems from merging galaxies (e.g., VV 114 in Buiten et al 2024;González-Alfonso et al 2024) to hot cores associated with massive protostars (Beuther et al 2023), to brown dwarfs (e.g., VHS 1256b in Miles et al 2023). Depending on the astrophysical environment, the spectral resolution of MIRI may or may not be sufficient to resolve the lines individually (Figure 1).…”

“…This paper aims to provide guidance to recover the physical conditions and abundance of the absorbing water molecules from JWST/MIRI spectra. While we recognize that the observed spectra with absorption lines can contain more complicated features such as emission due to the MIR pumping (e.g., Buiten et al 2024;García-Bernete et al 2024), this paper deals with pure absorption features. We present in Section 2 the construction of modeled absorption spectra of gaseous water, and in Section 3 the coupled relation of physical parameters in the crowded water lines based on the modeled spectra.…”