The Potential of the SPHEREx Mission for Characterizing Polycyclic Aromatic Hydrocarbon 3.3 μm Emission in Nearby Galaxies

TL;DR

The paper evaluates SPHEREx's capability to measure the PAH $3.3\,\mu$m emission in galaxies out to $z\sim0.4$ by building calibrated CIGALE-based spectral templates and simulating SPHEREx observations. Using local PAH constraints and AKARI/SPICY data, the authors fix $q_{\rm PAH}=1.12\%$ while exploring realistic ISM and SFH variations, then generate a dense grid of galaxy spectra. The SPHEREx QuickCatalog Simulator is used to create realistic wide-field and deep-field observations, enabling a quantitative assessment of detection limits and the number of galaxies with robust $3.3\,\mu$m PAH measurements: $\lesssim30\%$ flux accuracy down to $\sim2\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ (wide) and $\sim3\times10^{-16}$ erg s$^{-1}$ cm$^{-2}$ (deep). The results indicate SPHEREx will measure this PAH feature for hundreds of thousands of galaxies, providing population-level insights into nano grain properties and ISM radiation fields across a broad stellar-mass and SFR range, with significant gains from the deep-field observations. This work establishes SPHEREx as a powerful tool for expanding PAH-based dust studies beyond the capabilities of prior infrared surveys and into a statistically robust extragalactic context.

Abstract

Together with gas, stars, and supermassive black holes, dust is crucial in stellar and galaxy evolution. Hence, understanding galaxies' dust properties across cosmic time is critical to studying their evolution. In addition to photometric constraints on the absorption of blue light and its reemission at infrared wavelengths, dust grain properties can be explored spectroscopically via polycyclic aromatic hydrocarbon (PAH) emission bands in the mid-IR. The new SPHEREx space telescope conducts an all-sky spectrophotometric survey of stars and galaxies at wavelengths of 0.75-5$\,μ$m, making it ideal for studying the widespread presence of the 3.3$\,μ$m PAH emission across galaxy populations out to z ~ 0.4. In this paper, we simulated galaxy spectra to investigate SPHEREx's capability to study PAH emission in such galaxies. We find that for the all-sky survey the PAH 3.3$\,μ$m emission band flux can be measured to 30% accuracy at $\log(\rm M/{\rm M_\odot})>9.5$ and star formation rate (SFR) $> 1\,{\rm M_\odot\,yr^{-1}}$ at $z=0.1$, $\log(\rm M/{\rm M_\odot}) > 10.5$ and ${\rm SFR} > 10\,{\rm M_\odot\,yr^{-1}}$ at $z=0.2-0.3$, and $\log(\rm M/{\rm M_\odot})>11$ and ${\rm SFR} > 100\,{\rm M_\odot\,yr^{-1}}$ at $z=0.4$. For deep SPHEREx fields, a factor of ~10 deeper sensitivity limits can be reached. Overall, SPHEREx will enable the measurement of the 3.3$\,μ$m PAH band emission in several hundred thousand galaxies across the sky, providing a population study of the smallest dust grains ("nano grains") and radiation properties in massive galaxies in the nearby Universe.

Figures (5)

• Figure 1: Top: calibration of our CIGALE models to observations from gregg24 (gregg24; individual star forming regions in local galaxies) and lai20 (lai20; low-redshift star forming galaxies). The models are shown for different $q_{\rm PAH}$ (acting as free parameter, indicated by colors) and a range of E(B$-$V) from $0.1$ to $0.5\,{\rm mag}$ (indicated by the width of the bands). We find that a value of $q_{\rm PAH} = 1.12\%$ fits well the observations across more than 7 orders of magnitude in total SFRs. Bottom: comparison of our CIGALE models (red) to observed AKARI SPICY spectra (data and best fit) from ohyama18. Stellar mass is fixed to the one measured from photometric SED fitting, and a $q_{\rm PAH} = 1.12\%$ is assumed. Note that the AKARI spectra cover a wavelength range redder than SPHEREx (specifically the 6.2, 7.7, 8.6, 11.3, and $12.7\,{\rm \mu m}$ PAH bands) and therefore miss the $3.3\,{\rm \mu m}$ band (indicated by the arrow). We also show photometric data from WISE (filled squares) and AKARI (open circles) for comparison.
• Figure 2: Examples of simulated spectra for sources in the wide-field area ( top four panels) and the deep-field area ( bottom four panels). The two areas differ in the number of scans per pixel (see Section \ref{['sec:spherexsim']}) and thus number of observations (gray points). The sources are chosen to have a detected PAH $3.3\,{\rm \mu m}$ band emission at $z=0.1$ and $z=0.3$ bracketing E(B$-$V) values of $0.1$ and $0.5\,{\rm mag}$. The input model is shown as a thick blue line, and intrinsic measurements are shown as gray points. We also show the measurements binned at $0.1\,{\rm \mu m}$ (wide field) and $600\,{\rm \AA}$ (deep field) in red (errors computed from bootstrapping)
• Figure 3: Comparison of input and measured PAH $3.3\,{\rm \mu m}$ total flux for the wide-field ( top panel) and deep-field ( bottom panel) survey configuration. The gray areas show an error of $30\%$ (dark gray) and a factor of two (light gray) around the 1-to-1 line (dashed). The dashed horizontal lines show the median $1\sigma$ detection limits.
• Figure 4: Summary of SFR vs. observed $K$--band magnitude (proxy for stellar mass; contours) parameter space for three different redshift bins in which SPHEREx is able to measure the PAH $3.3\,{\rm \mu m}$ emission band (indicated by the light-red hatched region). The hexagonal background shows galaxies in the COSMOS field and from SDSS in the given redshift bins color-coded by average stellar mass.
• Figure 5: Impact of different model assumptions (SFH, dust, and AGN) on the PAH $3.3\,{\rm \mu m}$ emission flux (resulting change with respect to the basis model in dex).