New analyses from the PAHSPECS program use spatially and spectrally resolved JWST/MIRI Mid-Infrared Spectrometer (MIRI MRS) observations of star-forming galaxies at “cosmic noon” (redshift around z ≈ 1.1). One study presents a method for extracting polycyclic aromatic hydrocarbon (PAH) emission features from MIRI data cubes by forward-modeling the observations with non-parametric spatial distributions while accounting for the point-spread function. Using this approach, it recovers spatial profiles and maps for major PAH bands at 3.3, 6.2, 7.7, and 11.3 microns, as well as [ArII] at 6.98 microns, and derives PAH ratio maps across galaxy radii. It finds that PAHs become larger and more neutral with increasing galactocentric distance, a trend opposite to that often seen in local galaxies. The second study analyzes integrated spectra for five galaxies and compares their PAH band ratios and luminosities with local luminous infrared galaxies. It reports, relative to local LIRGs, higher 6.2/7.7 ratios and lower 11.3/7.7 ratios in most PAHSPECS sources, consistent with PAH emission weighted toward smaller grains. It also examines correlations with specific star-formation rate and star-formation surface density, and notes that an AGN-hosting galaxy shows the lowest 6.2/7.7 ratio.
JWST MIRI MRS maps PAH emission in z~1.1 galaxies to probe cosmic-noon ISM conditions
New analyses from the PAHSPECS program use spatially and spectrally resolved JWST/MIRI Mid-Infrared Spectrometer (MIRI MRS) observations of star-forming galaxies at “cosmic noon” (redshift around z ≈...
- PAHSPECS uses JWST/MIRI MRS to observe star-forming galaxies at z ~ 1.1 (cosmic noon).
- The studies measure multiple PAH features (3.3, 6.2, 7.7, 11.3 μm) and use their band ratios to infer PAH properties and radiation-field conditions.
- Spatially resolved results show PAHs become larger and more neutral at larger galactocentric radii, opposite to typical local-galaxy trends.
- Compared with local luminous infrared galaxies, most PAHSPECS sources show PAH ratio differences suggesting different ISM processing at cosmic noon.
- Correlations with star-formation intensity and an AGN-hosting source indicate variations in the contribution of smaller/ionized PAH carriers.
arXiv:2606.18244v1 Announce Type: new Abstract: We present spatially resolved spectroscopy with JWST/MIRI MRS of a representative sample of normal star-forming galaxies at $z\sim1.1$ as part of the PAHSPECS program. To extract emission from Polycyclic Aromatic Hydrocarbon (PAH) features, we forward model the data cubes with non-parametric spatial distributions, accounting for convolution with the PSF. With this method we are able to recover accurate spatial profiles of the 3.3 $\mu$m, 6.2 $\mu$m, 7.7 $\mu$m, 11.3 $\mu$m PAHs and [ArII] (6.98 $\mu$m) emission and produce PAH ratio maps at cosmic noon. From the PAH ratio maps we find that PAHs become larger and more neutral with increasing galactocentric radius, which is the opposite of trends in local galaxies, indicating radial ISM gradients in normal star-forming galaxies are different at cosmic noon. Through spatially resolved SED fitting of HST and JWST photometry we measure the UV radiation field hardness through the intrinsic ratio of UV to optical flux and find the 3.3/11.3 PAH ratio to decrease with increasing hardness and the 11.3/7.7 to increase. This may suggest photo-destruction of small/ionized PAHs is driving the observed PAH ratio trends and may explain the overall lower 3.3/11.3 PAH at cosmic noon compared to the local universe. This work demonstrates that PAH properties hold crucial information on the resolved ISM physics of galaxies at cosmic noon.
4 days agoarXiv:2606.18230v1 Announce Type: new Abstract: Context. Cosmic noon (z ~ 1-3) marks the peak of the cosmic star-formation rate density, when dust-obscured star formation dominated galaxy growth. Mid-infrared spectroscopy probes the interstellar medium through PAH emission, whose band ratios trace PAH charge, size, and local radiation-field conditions. Aims. We characterize the PAH properties of five z ~ 1.1 star-forming galaxies from the PAHSPECS survey and investigate how their PAH luminosities and band ratios relate to global galaxy properties. We compare them with local luminous infrared galaxies (LIRGs) to assess whether PAH emission at cosmic noon differs from that nearby. Methods. We analyze JWST/MIRI MRS observations of five ASPECS galaxies in the HUDF. Integrated spectra are extracted with wavelength-dependent apertures and modeled with CAFE, including ancillary photometry to constrain the dust emission. Stellar masses and SFRs are derived with Prospector. Results. Compared to local LIRGs, most PAHSPECS sources show higher 6.2/7.7 and lower 11.3/7.7 ratios, suggesting an ionized PAH component weighted toward smaller grains. The 3.3/11.3 ratio is less constrained, since the 3.3 micron feature is detected in only two sources. Within the sample, 11.3/7.7 increases with sSFR and star-formation surface density, while 6.2/7.7 decreases with sSFR, consistent with preferential processing of small ionized PAH carriers. ASPECS-15, the AGN-hosting source, has the lowest 6.2/7.7 ratio and highest sSFR, suggesting a reduced contribution from small PAHs, potentially due to AGN activity. The 7.7 micron luminosity follows the local L7.7-SFR relation, supporting its use as a star-formation tracer at z ~ 1. Conclusions. PAH emission at cosmic noon appears shaped by different ISM conditions than in nearby starburst galaxies, likely reflecting more intense radiation fields, while the 7.7 micron feature remains a robust SFR tracer.
4 days ago
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