The Metallicity Dependence of PAH Emission in Galaxies I: Insights from Deep Radial Spitzer Spectroscopy

TL;DR

This study investigates how PAH emission varies with galaxy metallicity, revealing that smaller PAH grains dominate at low metallicity due to inhibited grain growth, challenging previous destruction-based explanations.

Contribution

The paper introduces a new model with an evolving grain size distribution that explains PAH emission trends across metallicities, emphasizing grain growth processes.

Findings

PAH to dust luminosity ratio declines below 2/3 solar metallicity

Long wavelength PAH features weaken significantly at low metallicity

Grain size distribution shifts to smaller sizes as metallicity decreases

Abstract

We use deep Spitzer mid-infrared spectroscopic maps of radial strips across three nearby galaxies with well-studied metallicity gradients (M101, NGC 628, and NGC 2403) to explore the physical origins of the observed deficit of polycyclic aromatic hydrocarbons (PAHs) at sub-solar metallicity (i.e. the PAH-metallicity relation or PZR). These maps allow us to trace the evolution of all PAH features from 5-18 $\mu$m as metallicity decreases continuously from solar ($Z_\odot$) to 0.2 $Z_\odot$. The total PAH to dust luminosity ratio remains relatively constant until reaching a threshold of $\sim$$\frac{2}{3}$$Z_\odot$, below which it declines smoothly but rapidly. The PZR has been attributed to preferential destruction of the smallest grains in the hard radiation environments found at low metallicity. In this scenario, a decrease in emission from the shortest wavelength PAH features is expected. In contrast, we find a steep decline in long wavelength power below $Z_\odot$, especially in the 17 $\mu$m feature, with the shorter wavelength PAH bands carrying an increasingly large fraction of power at low metallicity. We use newly developed grain models to reproduce the observed PZR trends, including these variations in fractional PAH feature strengths. The model that best reproduces the data employs an evolving grain size distribution that shifts to smaller sizes as metallicity declines. We interpret this as a result of inhibited grain growth at low metallicity, suggesting continuous replenishment in the interstellar medium is the dominant process shaping the PAH grain population in galaxies.