A JWST MIRI LRS Survey of 37 Massive Star-Forming Galaxies and AGN at Cosmic Noon -- Overview and First Results

TL;DR

This study presents a JWST/MIRI LRS survey of 37 infrared-bright galaxies at cosmic noon (z ~ 0.65–2.46), combining JWST data with archival Spitzer IRS spectra to achieve rest-frame 2–15 micron coverage. It details sample selection from a Spitzer/Herschel parent set, the JWST observations with a fixed-slit LRS mode, and a tailored data reduction workflow including empirical background subtraction and robust slit-loss corrections. A joint spectral decomposition framework separates stellar, dust, PAH, and extinction components to yield extinction-corrected PAH luminosities (notably the 3.3, 7.7, and 11.3 micron features) and to quantify ice and aliphatic absorptions; redshifts are derived from both emission lines and PAH features when lines are weak. The key result is that the 11.3/3.3 micron PAH ratio in these z~1–2 galaxies is about three times higher than in local LIRGs, interpreted as evidence for larger PAH grains due to coagulation in dense, cold ISM conditions, supported by strong 3.05 micron ice absorption; the 3.3 micron PAH can contribute up to ~1% of L_IR, providing important constraints on dust processing and obscured star formation at cosmic noon.

Abstract

We present a large spectroscopic survey with \textit{JWST}'s Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS) targeting $37$ infrared-bright galaxies between $z=0.65-2.46$ with infrared luminosities $\log L_{\rm IR}/L_\odot>11.5$ and $\log M_*/M_\odot=10-11.5$. Targets were taken from a \textit{Spitzer} $24\,μ$m-selected sample with archival spectroscopy from the Infrared Spectrograph (IRS) and include a mix of star-forming galaxies and dust-obscured AGN. By combining IRS with the increased sensitivity of LRS, we expand the range of spectral features observed between $5-30\,μ$m for every galaxy in our sample. In this paper, we outline the sample selection, \textit{JWST} data reduction, 1D spectral extraction, and polycyclic aromatic hydrocarbon (PAH) feature measurements from $λ_{rest}=3.3-11.2\,μ$m. In the \textit{JWST} spectra, we detect PAH emission features at $3.3-5.3\,μ$m, as well as Paschen and Brackett lines. The $3.3\,μ$m feature can be as bright as $1\%$ of the $8-1000\,μ$m infrared luminosity and exhibits a tight correlation with the dust-obscured star-formation rate. We detect absorption features from CO gas, CO$_2$ ice, H$_2$O ice, and aliphatic dust. From the joint \textit{JWST} and \textit{Spitzer} analysis we find that the $11.3/3.3\,μ$m PAH ratios are on-average three times higher than that of local luminous, infrared galaxies. This is interpreted as evidence that the PAH grains are larger at $z\sim1-2$. The size distribution may be affected by coagulation of grains due to high gas densities and low temperatures. These conditions are supported by the observation of strong water ice absorption at $3.05\,μ$m, and can lower stellar radiative feedback as large PAHs transmit less energy per photon into the interstellar medium.

Figures (13)

• Figure 1: Observed 8$\,\mu$m/$3.6\,\mu$m ratio to 250$\,\mu$m/$24\,\mu$m ratio from Spitzer and Herschel observations of our sample (red), which is drawn from the larger sample of Kirkpatrick2015 shown in circles . The parent sample is colored by their Spitzer-derived spectroscopic redshifts. Histograms on each axis show the flux ratio distribution for our sample (red) and the parent sample (gray). We are biased towards larger $S_{250}/S_{24}$ ratios because of our redshift selection, but our sample spans 16th and 84th percentiles of the parent distribution along both axes.
• Figure 2: Demonstration of our background subtraction procedure for the 2D spectrum of one of the targets in our sample. The top panel shows the 2D spectrum extracted from the pipeline with no background subtraction. The middle panel shows result of the pipeline's default background subtraction routine, and the bottom panel shows the final product after our two-stage approach as described in Section \ref{['sec:data']}. Pixel values are in MJy/sr. On the bottom we show the observed wavelength corresponding to pixels along the spectral axis. We note that the wavelength calibration is not reliable below $4.7\,\mu$m and we do not make use of those data. With this custom background subtraction routine we are able to extract spectra between $4.7-14\,\mu$m.
• Figure 3: JWST MIRI LRS $\lambda_{\rm obs}=5-14\,\mu$m spectra of $z\sim1-2$ mid-infrared bright galaxies. We highlight three star-forming galaxies at $z_{\rm spec}=0.776$ (Top), $z_{\rm spec}=1.515$ (Middle), and $z_{\rm spec}=1.998$ (Bottom). Prominent $3.05\,\mu$m H$_2$O ice absorption can be seen in all three spectra, including other PAH features as well as atomic lines like Pa$\alpha$ and Br$\alpha$.
• Figure 4: JWST MIRI LRS spectra of six mid-infrared bright AGN at $z=1.8-2.4$. These AGN spectra are dominated by a rising continuum originating from hot dust. The $3.3\,\mu$m PAH, marked with a vertical dashed line, is detected in 4/6 of these power-law AGN. Aliphatic absorption is seen at $3.4\,\mu$m (dot-dashed line) in FLS-IRS-22722, FLS-IRS-16122, and GN-IRS-12.
• Figure 5: Combined JWST/MIRI LRS (Red) and Spitzer/IRS (grey) spectra spanning $\lambda_{rest}\sim2-15\,\mu$m ($\lambda_{obs}\sim5-30\,\mu$m) for the galaxies in our sample with $f_{\rm AGN}<10\%$. Black diamonds show the Spitzer photometry between $5.6-24\,\mu$m from IRAC, MIPS and IRS Peak Up imaging (where available). We detect the $3.3\,\mu$m PAH feature between $z=0.8-2$ in massive, star-forming galaxies with longer wavelength PAHs from the Spitzer spectra. The $3.05\,\mu$m ice absorption feature is always present blue-ward of the $3.3\,\mu$m PAH.