The obscured nucleus and shocked environment of VV 114E revealed by JWST/MIRI spectroscopy

Abstract

Compact Obscured Nuclei (CONs) potentially hide extreme supermassive black hole (SMBH) growth behind large column densities of gas/dust. We present a spectroscopic analysis of the heavily obscured nucleus and the surrounding environment of the eastern region of the nearby (z = 0.02007) interacting galaxy VV 114 with the JWST Mid-InfraRed Instrument (MIRI). We model the spectrum from 4.9 - 28 μm to extract Polycyclic Aromatic Hydrocarbon (PAH) emission and the underlying obscured continuum. We find that the NE nucleus (A) is highly obscured where the low PAH equivalent width (EW) ratio, EW(12.7)/EW(11.3), reveals a dust enshrouded continuum source. This is confirmed by decomposing the continuum into nuclear and star-forming where the nuclear component is found to be typical of CONs. The 11.3/6.2 PAH flux ratio is consistent with originating in star-forming regions rather than typical AGN. The second nucleus (B) is much less obscured, with PAH flux ratios also typical of star-forming regions. We do not detect any high ionisation lines such as [Ne V] or [Ne VI] which suggests that if an AGN is present it must be highly obscured. Additionally, we detect a shock front south of the secondary nucleus (B) in the [Fe II] (5.34 μm) line and in warm molecular hydrogen. The 6.2 PAH emission does not spatially coincide with the low-J transitions of H2 but rather appears strong at the shock front which may suggest destruction of the ionised PAHs in the post-shock gas behind the shock front.

GOALS-JWST: Pulling Back the Curtain on the AGN and Star Formation in VV 114

“…Although Iono et al (2013) identified the NE core as potentially harboring an AGN due to the enhanced HCN/HCO + ratio, an analysis of X-ray and millimeter data by Privon et al (2020) showed that in fact the HCN/HCO+ is not a robust indicator of total AGN luminosity or its fractional contribution to IR luminosity. Our findings regarding the nature of the two bright cores in VV 114E are not in agreement with the analysis of JWST MIRI data by Donnan et al (2023) who Values measured with Spitzer/IRS for ULIRGs and LIRGs from the GOALS sample are plotted as "+" symbols, with VV 114E and three comparative ULIRGs denoted with red diamonds (Stierwalt et al 2013). The starburst galaxy M82 is shown as a blue square (Spoon et al 2007).…”

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

“…This number exceeds the value derived from the depth of the 9.7 μm silicate absorption feature (Donnan et al 2023, their Figures 4 and A1) by less than a factor of 2. With the corrected column density for SF-NE, the relative depths of the 9.7 μm silicate feature for SF-NE and SF-SW are also reproduced.…”

“…However, even then, collisional excitation will be irrelevant, since at such high densities the gas temperature will equal the dust temperature, which is set by the local radiation field. Mapping of the [Fe II] 5.34 μm emission line has revealed the presence of a large-scale bow shock covering the SF-SW region (Donnan et al 2023). It is of interest to consider the possible role of this shock for the excitation of the molecular gas in SF-SW, since C-type shocks in molecular clouds can easily produce postshock temperatures of several times 10 3 K without dissociating the molecules (Draine et al 1983).…”

Dissecting the Mid-infrared Heart of M83 with JWST

“…In Figure 10 we show the excitation diagrams for the different regions in the nucleus of M83. As noted by Donnan et al (2023), the S(3) transition at 9.665 μm is found at the peak of the silicate absorption, and therefore highly sensitive to extinction. In the analysis by Donnan et al (2023), they include this line with We show with a gray line the best model fit adopting the parameters listed in Table 4.…”