The Metallicity Dependence of PAH Emission in Galaxies II: Insights from JWST/NIRCam Imaging of the Smallest Dust Grains in M101

Abstract

We explore the physical origins of the observed deficit of polycyclic aromatic hydrocarbons (PAHs) at sub-solar metallicity using JWST/NIRCam imaging of the nearby galaxy M101, covering regions from solar metallicity (Z$_{\odot}$) down to 0.4 Z$_{\odot}$. These maps are used to trace the radial evolution of the shortest-wavelength PAH feature at 3.3 $μ$m, which is emitted preferentially by the smallest PAHs ($<100$ carbon atoms). The fractional contribution of PAH 3.3 $μ$m to the total PAH luminosity ($Σ$PAH) increases by 3x as metallicity declines, rising from $\sim$1$\%$ to $\sim$3$\%$ over the observed range, consistent with prior predictions from the inhibited grain growth model based on Spitzer spectroscopy. We explore model refinements including photon effects and alternative size evolution prescriptions, and find that a modest amount of small grain photo-destruction remains possible, provided the grain size cutoff does not exceed $\sim55$ carbon atoms. The best-fit models predict 3.3 $μ$m/$Σ$PAH will rise to $\sim5.6-7.7\%$ at 10$\%$ Z$_{\odot}$. Surprisingly, even as $Σ$PAH drops significantly relative to the total infrared luminosity (TIR) as metallicity declines, 3.3 $μ$m/TIR alone rises, potentially indicating the mass fraction of the smallest PAH grains increases as the total dust content in galaxies drops. The current model cannot fully reproduce this trend even if the unusually strong effects of changing radiation field hardness on 3.3 $μ$m/TIR are included. This may be evidence that the smallest PAHs are uniquely robust against destruction and inhibited growth effects. These results highlight the pivotal role that short-wavelength PAH emission can play in studies of low-metallicity and high-redshift galaxies.

Figures (5)

• Figure 1: Top: F335M$_{\rm PAH}$ as a function of metallicity, as defined in each labeled work. Center: Corrected PAH 3.3/$\Sigma$PAH as a function of metallicity for the spectroscopic-equivalent 3.3 PAH feature alone (see § \ref{['sec:pah33_Z']}). Bottom: fractional change in PAH 3.3/$\Sigma$PAH with respect to the value at $Z_{\odot}$. The dashed gray line indicates the threshold metallicity where $\Sigma$PAH/TIR begins to drop. Hexagons represent the median value in six equally-spaced bins from 1.0 to 0.4 $Z_{\odot}$ with errorbars indicating the standard deviation. Note the S23 and L20 trends in the bottom two panels are identical since the methods vary only by a fixed factor.
• Figure 2: (Left) modeled metallicity-evolution of the characteristic grain size $a_{\rm 01}$ (black) and the minimum grain size $a_{\rm min}$ (blue) for each model (see § \ref{['sec:dvresults']}). (Right) fractional change in all major PAH features with predictions from various inhibited growth models indicated by solid lines: linear inhibited growth from Paper I with fixed standard $a_{\rm min}$ = 4$\rm \AA$ ($N_{\mathrm{C}}$ = 26) (top), linear inhibited growth with $a_{\rm min}$ increasing linearly up to 4.6$\rm \AA$ ($N_{\mathrm{C}}$ = 40) (center), and a modified inhibited growth model with fixed standard $a_{\rm min}$(bottom). The dashed gray line indicates the threshold metallicity from Paper I ($\sim$0.63 $Z_{\odot}$).
• Figure 3: Top left: PAH to Total PAH ratios as a function of metallicity, top right: PAH to TIR ratios as a function of metallicity, bottom left: fractional change in PAH to Total PAH ratios relative to the value at $Z_{\odot}$, bottom right: fractional change in PAH to TIR ratios relative to the value at $Z_{\odot}$. The dashed gray line indicates the threshold metallicity where $\Sigma$PAH/TIR begins to drop. Hexagons represent the median value in bins of width 0.1 $Z_{\odot}$ with errorbars indicating the standard deviation. Note the trend for all PAH features except 3.3 is taken from Paper I and these include data from NGC 628 and NGC 2403 in addition to M101.
• Figure 4: Fractional change in the ratio of PAH 3.3 to TIR relative to the value at $Z_{\odot}$ with D21 model lines overplotted. Dotted lines indicate models where only the incident radiation field changes with metallicity, and solid lines indicate these models combined with the inhibited growth model from § \ref{['sec:IG2']} shown in solid gray. The dashed gray line indicates the threshold metallicity where $\Sigma$PAH/TIR begins to drop.
• Figure 5: Radial variations in brightness near the edge of M101's disk, normalized to the brightness of the innermost aperture (cyan in Figure \ref{['figure:prettypics']}). The dashed blue line indicates the galactocentric radius where galaxy structure in the PAH 3.3 map appears to end abruptly ($\sim$13.8 kpc). The propagated uncertainty in the 3.3 map, including the uncertainty in the background subtraction, is indicated by the error bar on the rightmost point.