Galaxy formation and evolution

Abstract:

Polycyclic aromatic hydrocarbons (PAHs) are carbon-based molecules that are ubiquitous in a variety of astrophysical objects and environments. In this work we use JWST/MIRI MRS spectroscopy of three Seyferts to compare their nuclear PAH emission with that of star-forming (SF) regions. This study represents the first of its kind to use sub-arcsecond angular resolution data of local luminous Seyferts (Lbol >  1044.46 erg s−1) with a wide wavelength coverage (4.9–28.1 μm). We present an analysis of their nuclear PAH properties by comparing the observed ratios with PAH diagnostic model grids derived from theoretical spectra. Our results show that a suite of PAH features is present in the innermost parts of luminous Seyfert galaxies (∼0.45″ at 12 μm; in the inner ∼142–245 pc). We find that the nuclear regions of active galactic nuclei (AGN) lie at different positions of the PAH diagnostic diagrams, whereas the SF regions are concentrated around the average values of SF galaxies. In particular, we find that the nuclear PAH emission mainly originates in neutral PAHs. In contrast, PAH emission originating in the SF regions favours ionised PAH grains. The observed PAH ratios in the nuclear region of the AGN-dominated galaxy NGC 6552 indicate the presence of larger PAH molecules compared with those of the SF regions. Therefore, our results provide evidence that the AGN have a significant impact on the ionisation state (and probably the size) of the PAH grains on scales of ∼142–245 pc.

Abstract:

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 = 3.3x1022 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 ionizedgas mass (M_ion = (2.4-12)x105 Msun) is comparable to that of the warm H_2, but the former is more turbulent (sigma_ion~300 vs. sigma_H2~150km/s), so the mechanical energy of the ionized gas is ~1.3-10 times higher. From these estimates, we find that only 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