The Behavior of the Aromatic Features in M101 H II Regions: Evidence for Dust Processing

By K. Gordon, C. Engelbracht, G. Rieke, K. Misselt, J.-D. T. Smith, Robert C. Kennicutt Jr.. 2008

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The aromatic features in M101 were studied spectroscopically and photometrically using observations from all three instruments on the Spitzer Space Telescope. The global SED of M101 shows strong aromatic feature (commonly called PAH feature) emission. The spatially resolved spectral and photometric measurements of the aromatic feature emission show strong variations with significantly weaker emission at larger radii. We compare these variations with changes in the ionization index (as measured by [Ne III]/[Ne II] and [S IV/S III], which range from 0.03 to 20 and 0.044 to 15, respectively) and metallicity [expressed as log (O/H) + 12, which ranges from 8.1 to 8.8]. Over these ranges, the spectroscopic equivalent widths of the aromatic features from seven H II regions and the nucleus were found to correlate better with ionization index than metallicity. This implies that the weakening of the aromatic emission in massive star-forming regions is due primarily to processing of the dust grains in these environments, not to differences in how they form. The behavior of the aromatic feature versus ionization index correlation can be described as a constant equivalent width until a threshold in ionization index is reached ([Ne III]/[Ne II] ∼ 1), above which the equivalent widths decrease with a power-law dependence. This behavior is also seen for the starburst galaxy sample presented in the companion study by Engelbracht and coworkers, which expands the range of [Ne III]/[Ne II] ratios to 0.03-25 and log (O/H) + 12 values to 7.1-8.8. The form of the correlation explains seemingly contradictory results present in the literature. The behavior of the ratios of different aromatic features versus ionization index does not follow the predictions of existing PAH models of the aromatic features, implying a more complex origin of the aromatic emission in massive star-forming regions.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/cf31368b75302f600b835de94bf52397bf5c52eb

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Citations in the corpus

RADIAL DISTRIBUTION OF STARS, GAS, AND DUST IN SINGS GALAXIES. II. DERIVED DUST PROPERTIES (2009, methodology): “In their analysis of individual HII regions in M 101, Gordon et al. (2008) find that the equivalent width of the aromatic features is better correlated with the level of ionization than with metallicity, and this seems to hold for the starburst galaxies studied by Engelbracht et al. (2008).”
Investigations of dust heating in M81, M83 and NGC 2403 with the Herschel Space Observatory (2011, methodology): “To correct for this, we created convolution kernels following the procedure given by Gordon et al. (2008) (and also see Bendo et al. (2010a)).”
The applicability of far-infrared fine-structure lines as star formation rate tracers over wide ranges of metallicities and galaxy types (2014, background): “The weak PAH emission toward lower metallicities is not directly related to the lower metal abundance but rather emanates from the generally strong and hard radiation fields in low-metallicity systems destroying and/or ionizing PAHs (e.g., Gordon et al. 2008; Sandstrom et al. 2012).”
CALIBRATING EXTINCTION-FREE STAR FORMATION RATE DIAGNOSTICS WITH 33 GHz FREE–FREE EMISSION IN NGC 6946 (2011, methodology): “Each of the Herschel images were first convolved to the resolution of the 350µm data using custom smoothing kernels as described in Gordon et al. (2008) and updated by those authors for Herschel, CLEANed to suppress side-lobe structure in the PSF, and then restored using a Gaussian beam having a…”
THE SPITZER LOCAL VOLUME LEGACY: SURVEY DESCRIPTION AND INFRARED PHOTOMETRY (2009, background): “…or the relatively fragile PAHs are photo-dissociated in the hard radiation fields typically associated with low-metallicity environments (Engelbracht et al. 2005; Madden et al. 2006; Wu et al. 2006; Jackson et al. 2006; Draine et al. 2007; Sloan et al. 2008; Gordon et al. 2008; Dale et al. 2009).”
Common-Resolution Convolution Kernels for Space- and Ground-Based Telescopes (2011, methodology): “Using techniques similar to those used here, Gordon et al. (2008) created kernels for the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS) cameras of Spitzer, and Sandstrom et al. (2009) created kernels for the Infrared Spectrograph (IRS) of Spitzer.”
Dust spectral energy distributions of nearby galaxies: an insight from the Herschel Reference Survey (2014, background): “Indeed, previous works found that the PAH emission is inhibited within star-forming regions relative to other star formation tracers (e.g. Helou et al. 2004; Calzetti et al. 2005; Bendo et al. 2006; Pérez-González et al. 2006; Bendo et al. 2008; Gordon et al. 2008).”
RECONSTRUCTING THE STELLAR MASS DISTRIBUTIONS OF GALAXIES USING S4G IRAC 3.6 AND 4.5 μm IMAGES. I. CORRECTING FOR CONTAMINATION BY POLYCYCLIC AROMATIC HYDROCARBONS, HOT DUST, AND INTERMEDIATE-AGE STARS (2011, methodology): “The choice of kernel (i.e. substituting the Gordon et al. 2008 kernel) or input PSFs (using the S4G “super”-PSFs; Sheth et al. 2010) also does not significantly impact the results.”
WHAT DRIVES THE EXPANSION OF GIANT H ii REGIONS?: A STUDY OF STELLAR FEEDBACK IN 30 DORADUS (2010, methodology): “For this analysis, we employed the convolution kernels and method described in §2.3 of Gordon et al. (2008).”
SURVEYING THE AGENTS OF GALAXY EVOLUTION IN THE TIDALLY STRIPPED, LOW METALLICITY SMALL MAGELLANIC CLOUD (SAGE-SMC). I. OVERVIEW (2011, methodology): “20 of Draine et al. 2007) which is not surprising given the SMC’s low metallicity [12+log(O/H) ∼ 8.0 and, thus, high radiation field hardness (Gordon et al. 2008).”
EXTENDING THE NEARBY GALAXY HERITAGE WITH WISE: FIRST RESULTS FROM THE WISE ENHANCED RESOLUTION GALAXY ATLAS (2012, background): “Analysis for many of these galaxies have been published, including NGC 777 (IRAC Instrument Handbook), NGC 4486 (Shi et al. 2007), M 83 (Dong et al. 2008), M 101 (Gordon et al. 2008), and IC 342 (IRS spectra from Brandl et al. 2006).”