Polycyclic Aromatic Hydrocarbons in Galaxies at z~0.1: the Effect of Star Formation and AGN

TL;DR

This study analyzes PAH spectra in 92 galaxies at z~0.1, revealing how star formation and AGN activity influence PAH properties and molecule sizes, with implications for understanding galaxy evolution.

Contribution

It provides new insights into how PAH emission features correlate with star formation and AGN activity, highlighting the impact of radiation fields on PAH molecule sizes.

Findings

PAH ratios correlate with star formation indicators.

Active galaxies show weaker 7.7 micron PAH emission.

Radiation fields from AGN and starbursts differently affect PAH molecules.

Abstract

We present the analysis of the Polycyclic Aromatic Hydrocarbon (PAH) spectra of a sample of 92 typical star forming galaxies at 0.03 < z < 0.2 observed with the Spitzer IRS. We compare the relative strengths of PAH emission features with SDSS optical diagnostics to probe the relationship between PAH grain properties and star formation and AGN activity. Short-to-long wavelength PAH ratios, and in particular the 7.7-to-11.3 micron feature ratio, are strongly correlated with the star formation diagnostics D_n(4000) and H-alpha equivalent width, increasing with younger stellar populations. This ratio also shows a significant difference between active and non-active galaxies, with the active galaxies exhibiting weaker 7.7 micron emission. A hard radiation field as measured by [OIII]/H-beta and [NeIII]_15.6/[NeII]_12.8 affects PAH ratios differently depending on whether this field results from starburst activity or an AGN. Our results are consistent with a picture in which larger PAH molecules grow more efficiently in richer media and in which smaller PAH molecules are preferentially destroyed by AGN.