On unveiling Buried Nuclei with JWST: a technique for hunting the most obscured galaxy nuclei from local to high redshift

TL;DR

This paper develops a new infrared diagnostic technique using JWST observations to identify deeply obscured galaxy nuclei across local and high redshift ranges, enhancing our ability to study hidden galactic centers.

Contribution

The study introduces a novel diagnostic diagram based on PAH features and continuum ratios, extending obscuration detection methods to higher redshifts with JWST's MIRI filters.

Findings

PAH EW ratios are affected by nuclear obscuration.

Buried nuclei cluster in a specific region of the diagnostic diagram.

Method enables identification of obscured nuclei up to z~3.

Abstract

We analyze JWST NIRSpec+MIRI/MRS observations of the infrared (IR) Polycyclic Aromatic Hydrocarbon (PAH) features in the central regions ($\sim$0.26'' at 6 micron; $\sim$50-440 pc depending on the source) of local luminous IR galaxies. In this work, we examine the effect of nuclear obscuration on the PAH features of deeply obscured nuclei, predominantly found in local luminous IR galaxies, and we compare these nuclei with ``normal'' star-forming regions. We extend previous work to include shorter wavelength PAH ratios now available with the NIRSpec+MIRI/MRS spectral range. We introduce a new diagnostic diagram for selecting deeply obscured nuclei based on the 3.3 and 6.2 micron PAH features and/or mid-IR continuum ratios at $\sim$3 and 5 micron. We find that the PAH equivalent width (EW) ratio of the brightest PAH features at shorter wavelengths (at 3.3 and 6.2 micron) is impacted by nuclear obscuration. Although the sample of luminous IR galaxies used in this analysis is relatively small, we find that sources exhibiting a high silicate absorption feature cluster tightly in a specific region of the diagram, whereas star-forming regions experiencing lower extinction levels occupy a different area in the diagram. This demonstrates the potential of this technique to identify buried nuclei. To leverage the excellent sensitivity of the MIRI imager onboard JWST, we extend our method of identifying deeply obscured nuclei at higher redshifts using a selection of MIRI filters. Specifically, the combination of various MIRI JWST filters enables the identification of buried sources beyond the local Universe and up to z$\sim$3, where other commonly used obscuration tracers such as the 9.7 micron silicate band, are out of the spectral range of MRS. Our results pave the way for identifying distant deeply obscured nuclei with JWST.