Galaxy formation and evolution

Abstract:

We analyze JWST NIRSpec+MIRI/MRS observations of the infrared (IR) polycyclic aromatic hydrocarbon (PAH) features in the central regions (a at 6 μm; a 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 astar-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 μm PAH features and/or mid-IR continuum ratios at a3 and 5 μm. We find that the PAH equivalent width ratio of the brightest PAH features at shorter wavelengths (at 3.3 and 6.2 μm) 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 on board 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 za 3, where other commonly used obscuration tracers such as the 9.7 μm silicate band, are out of the spectral range of MRS. Our results pave the way for identifying distant deeply obscured nuclei with JWST.

Abstract:

JWST observations, when combined with Hubble Space Telescope (HST) data, promise to improve age estimates of star clusters in nearby spiral galaxies. However, feedback from young cluster stars pushes out the natal gas and dust, making cluster formation and evolution a challenge to model. Here, we use JWST+ HST observations of the nearby spiral galaxy NGC 628 to produce spectral energy distribution (SED) templates of compact star clusters spanning 275 nm through 21 μm. These preliminary SEDs capture the cluster stars and associated gas and dust within radii of ≈0 .″ 12–0 .″ 67 (corresponding to ≈6–33 pc at the distance of NGC 628). One important finding is that the SEDs of 1, 2, 3, and 4 Myr clusters can be differentiated in the infrared. Another is that, in 80%–90% of the cases we study, the polycyclic aromatic hydrocarbon (PAH) and Hα emission track one another, with the dust responsible for the 3.3 μm PAH emission largely removed by 4 Myr, consistent with pre-supernova stellar feedback acting quickly on the surrounding gas and dust. Nearly embedded cluster candidates have infrared SEDs that are quite similar to optically visible 1–3 Myr clusters. In nearly all cases, we find there is a young star cluster within a few tenths of an arcsec (10–30 pc) of the nearly embedded cluster, suggesting the formation of the cluster was triggered by its presence. The resulting age estimates from the empirical templates are compatible both with dynamical estimates based on CO superbubble expansion velocities, as well as the TODDLERS models, which track spherical evolution of homogeneous gas clouds around young stellar clusters.