A Method to Extract Spatially Resolved Polycyclic Aromatic Hydrocarbon Emission from Spitzer Spectra: Application to M51

TL;DR

This study develops a new method to extract spatially resolved PAH emission from Spitzer spectra, demonstrating its effectiveness as a star formation rate indicator across different regions of galaxy M51.

Contribution

The paper introduces a novel spectral analysis approach that maximizes spatial resolution and coverage, enabling reliable PAH emission measurements even with partial spectral data.

Findings

PAH emission correlates tightly with UV and Hα indicators of star formation.

PAH emission is suppressed near the galaxy nucleus.

Spiral arms show stronger PAH features than inter-arm regions.

Abstract

The mid-infrared spectrum contains rich diagnostics to probe the physical properties of galaxies, among which the pervasive emission features from polycyclic aromatic hydrocarbons (PAHs) offer a promising means of estimating the star formation rate (SFR) relatively immune from dust extinction. This paper investigates the effectiveness of PAH emission as a SFR indicator on sub-kpc scales by studying the Spitzer/IRS mapping-mode observations of the nearby grand-design spiral galaxy M51. We present a new approach of analyzing the spatial elements of the spectral datacube that simultaneously maximizes spatial resolution and spatial coverage, while yielding reliable measurements of the total, integrated $5-20\,\mu$m PAH emission. We devise a strategy of extracting robust PAH emission using spectra with only partial spectral coverage, complementing missing spectral regions with properly combined mid-infrared photometry. We find that in M51 the PAH emission correlates tightly with the extinction-corrected far-ultraviolet, near-ultraviolet, and H$\alpha$ emission, from scales$\sim 0.4$ kpc close to the nucleus to 6 kpc out in the disk of the galaxy, indicating that PAH serves as an excellent tracer of SFR over a wide range of galactic environments. But regional differences exist. Close to the active nucleus of M51 the 6.2 $\mu$m feature is weaker, and the overall level of PAH emission is suppressed. The spiral arms and the central star-forming region of the galaxy emit stronger 7.7 and 8.6 $\mu$m PAH features than the inter-arm regions.