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Aromatic infrared bands (AIBs) are a set of broad emission bands at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 mu m, seen in the infrared spectra of most galaxies. With the James Webb space telescope (JWST), the 3.3 mu m AIB can in principle be detected up to a redshift of $ 7$. Relating the evolution of the 3.3 mu m AIB to local physical properties of the interstellar medium (ISM) is thus of paramount importance. By applying a dedicated machine learning algorithm to JWST NIRSpec observations of the Orion Bar photodissociation region obtained as part of the PDRs4All Early Release Science (ERS) program, we extracted two template spectra capturing the evolution of the AIB-related emission in the 3.2-3.6 range, which includes the AIB at 3.3 and its main satellite band at 3.4 um. In the Orion Bar we analyzed the spatial distribution of the templates and their relationship with the ro-vibrational H$_2$ line at 2.12 the pure rotational line of H$_2$ at 4.69 and the Pfund delta line at 3.29 um. We find that one template (AIB$_ Irrad $) traces regions of neutral atomic gas with strong far-UV fields, while the other template (AIB$_ Shielded $) corresponds to shielded regions with lower FUV fields and a higher molecular gas fraction. We then show that these two templates can be used to fit the NIRSpec AIB-related spectra of nearby galaxies. The relative weight of the two templates (AIB$_ Irrad/Shielded $) is a tracer of the radiative feedback from massive stars on the ISM. We derive an estimate of AIB$_ Irrad/Shielded $ in a $z=4.22$ lensed galaxy and find that it has a lower value than for local galaxies. This pilot study illustrates how a detailed analysis of AIB emission in nearby regions can be used to probe the physical conditions of the extragalactic ISM.
Aromatic infrared bands (AIBs) are a set of broad emission bands at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 mu m, seen in the infrared spectra of most galaxies. With the James Webb space telescope (JWST), the 3.3 mu m AIB can in principle be detected up to a redshift of $ 7$. Relating the evolution of the 3.3 mu m AIB to local physical properties of the interstellar medium (ISM) is thus of paramount importance. By applying a dedicated machine learning algorithm to JWST NIRSpec observations of the Orion Bar photodissociation region obtained as part of the PDRs4All Early Release Science (ERS) program, we extracted two template spectra capturing the evolution of the AIB-related emission in the 3.2-3.6 range, which includes the AIB at 3.3 and its main satellite band at 3.4 um. In the Orion Bar we analyzed the spatial distribution of the templates and their relationship with the ro-vibrational H$_2$ line at 2.12 the pure rotational line of H$_2$ at 4.69 and the Pfund delta line at 3.29 um. We find that one template (AIB$_ Irrad $) traces regions of neutral atomic gas with strong far-UV fields, while the other template (AIB$_ Shielded $) corresponds to shielded regions with lower FUV fields and a higher molecular gas fraction. We then show that these two templates can be used to fit the NIRSpec AIB-related spectra of nearby galaxies. The relative weight of the two templates (AIB$_ Irrad/Shielded $) is a tracer of the radiative feedback from massive stars on the ISM. We derive an estimate of AIB$_ Irrad/Shielded $ in a $z=4.22$ lensed galaxy and find that it has a lower value than for local galaxies. This pilot study illustrates how a detailed analysis of AIB emission in nearby regions can be used to probe the physical conditions of the extragalactic ISM.
We report the detection of extended ($>$0.5--1\,kpc) high-ionization (80\,eV) emission in four local luminous infrared galaxies observed with JWST NIRSpec. Excluding the nucleus and outflow of the Type 1 active galactic nucleus (AGN) in the sample, we find that the luminosity is well correlated with that of H recombination lines, which mainly trace star-forming clumps in these objects, and that the (75\,eV), usually seen in AGN, is undetected. On 100--400\,pc scales, the line profiles are broader ($ $) and shifted ($ v$ up to pm $) compared to those of the H recombination lines and lower ionization transitions (e.g., $ $). The kinematics follow the large-scale rotating velocity field of these galaxies, and the broad profiles are compatible with the broad wings detected in the H recombination lines. Based on these observational results, extended highly ionized gas more turbulent than the ambient interstellar medium, possibly a result of ionizing shocks associated with star formation, is the most likely origin of the emission. We also computed new grids of photoionization and shock models to investigate where the line originates. Shocks with velocities of $ reproduce the observed line ratios and the luminosity agrees with that expected from the mechanical energy released by supernove (SNe) in these regions. Therefore, these models support shocks induced by SNe as the origin of the line. Future studies on the stellar feedback from SNe will benefit from the line that is little affected by obscuration and, in the absence of an AGN, can only be produced by shocks due to its high ionization-potential.
We present a systematic study of mid-infrared spectra of galaxies including star-forming galaxies and active galactic nuclei observed with JWST MIRI-MRS and NIRSpec-IFU. We focus on the relative variations of the 3.3, 6.2, 7.7, 11.3, 12.7, and 17 $\mu$m polycyclic aromatic hydrocarbon (PAH) features within spatially resolved regions of galaxies including NGC 3256, NGC 7469, VV 114, II Zw96, and NGC 5728. Using theoretical PAH models and extending our earlier work, we introduce a new PAH diagnostic involving the 17 $\mu$m PAH feature. To determine the drivers of PAH band variations in galaxies, we compare observed PAH spectral bands to predictions from theoretical PAH models. We consider extinction, dehydrogenation, and PAH size and charge as possible drivers of PAH band variations. We find a surprising uniformity in PAH size distribution among the spatially resolved regions of the galaxies studied here, with no evidence for preferential destruction of the smallest grains, contrary to earlier findings. Neither extinction nor dehydrogenation play a crucial role in setting the observed PAH bands. Instead, we find that PAH charge plays a significant role in PAH inter-band variations. We find a tight relation between PAH charge and the intensity of the radiation field as traced by the [Ne iii]$/$[Ne ii] maps. In agreement with recent JWST results, we find a predominance of neutral PAH molecules in the nuclei of active galaxies and their outflows. Ionized PAHs are the dominant population in star-forming galaxies. We discuss the implications of our findings for the use of PAHs as ISM tracers in high redshift galaxies.
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