Calibrating Photometric Mid-Infrared Star Formation Rates for JWST

Abstract

The mid-infrared (IR) spectrum of galaxies has a long history as a valuable proxy for the dust-obscured star formation rate (SFR) in massive galaxies. Now, with JWST, we can explore the mid-IR's full potential as a SFR tracer over four orders of magnitude in total infrared luminosity (9<~log LIR/Lo<~13). First, combining the SMILES and FRESCO surveys, we evaluate MIRI photometry against the Pa-alpha emission line - a gold standard SFR indicator - in Main Sequence (MS) galaxies at cosmic noon. We find the rest-frame 6-8um luminosity has a steeply superlinear relation with SFR(Pa-alpha) below ~8 Mo/yr, in contrast with the unity slope seen in coeval massive galaxies. We derive broken power-law SFR indicators from single-band MIRI photometry plus a representative dust template, with a scatter typical of IR SFRs (~0.2-0.3 dex). Despite the break in the mid-IR behavior and our simplifying assumption of a single dust SED, we next successfully formulate a UV+IR composite relation (scatter ~0.15 dex) under the usual assumption of energy balance. This implies that the rest-frame 6-8um primarily tracks the global dust-obscuration fraction - which decreases rapidly at log M*/Mo<~10 - rather than reflecting a deficit in PAH abundances at low mass. Our results thus support MIRI photometry as a robust SFR proxy at log M*/Mo>~9 up to z~3. Finally, extending to local and z>~1 ultraluminous infrared galaxies not represented in SMILES, we examine when Pa-alpha and the IR reliably track SFR in the bright regime.

Figures (6)

• Figure 1: An example of a $\mathrm{Pa\alpha}$ emitter at $z=1.3895$. The top row shows image cutouts of the $\mathrm{Pa\alpha}$ line map and the F1280W, F1500W, F1800W, and F2100W bands, which contain PAH features at this redshift. The middle and bottom rows show the NIRCam grism F444W 2D and 1D spectra, respectively. The best fit is shown via the purple line.
• Figure 2: The rest wavelengths and bandwidths of four MIRI filters (F1280W, F1500W, F1800W, F2100W) over the redshift range $1<z<1.75$. The F1800W and F2100W filters are dominated by the 7.7$\,\mu$m PAH emission complex (top panel). The narrower 6.2$\,\mu$m PAH emission line (top panel) falls mostly in the F1500W, with partial coverage in the F1280W. The width of the PAHs features draine2021 is shown via the hatched regions. (top) The mid-infrared regions of a representative log $L_{\rm IR}/{L_{\odot}}=11.25$ SFG template from rieke2009. (right) A histogram of the redshifts of the sources used in this study.
• Figure 3: The difference in A$_{\mathrm{Pa\alpha}}$ derived from the $\mathrm{Pa\alpha}$/$\mathrm{H\alpha}$ line ratio and from SED fitting as a function of stellar mass. The two measurements are in good agreement within 0.1 mag (dotted lines), corresponding to a difference in the final $\mathrm{Pa\alpha}$ flux of $\lesssim10\%$.
• Figure 4: A$_{\rm Pa\alpha}$ and the correction factor 10$^{0.4 A_{\rm Pa\alpha}}$ (right axis) as a function of A$_{V}$ for values derived from SED fitting (blue squares) and from the $\mathrm{Pa\alpha}$/$\mathrm{H\alpha}$ line ratios (purple circles). Open symbols denote AGN. The dotted line and shaded region show the median value 0.03 with a scatter of 0.04 dex. The dashed line shows the relation A$_{V}$/A$_{\rm Pa\alpha}=6$calzetti2007a.
• Figure 5: SFR versus stellar mass for the $\mathrm{Pa\alpha}$ emitters in our sample. Solid (open) circles show $\mathrm{Pa\alpha}$ emitters on the MS detected (undetected) in MIRI long wavelength filters (F1800W or F2100W). The dashed line and shaded region shows the MS at $z\sim1.3$, the median redshift in our sample. $\mathrm{Pa\alpha}$ emitters above the MS ($\Delta$MS$\,>0.6$), below the MS ($\Delta$MS$\,<-0.6$), and hosting AGN are shown as red squares, blue diamonds, and purple stars, respectively. The MIPS 24$\mu$m confusion limit at $z\sim1.3$ is shown as a dotted line.