The Metallicity Dependence of PAH Emission in Galaxies I: Insights from Deep Radial Spitzer Spectroscopy
arxiv(2024)
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 μm as metallicity decreases continuously from
solar (Z_⊙) to 0.2 Z_⊙. The total PAH to dust luminosity ratio
remains relatively constant until reaching a threshold of
∼2/3Z_⊙, 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_⊙,
especially in the 17 μm 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.
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