The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission

By John-David T. Smith, B. Draine, D. Dale, John Moustakas, Robert C. Kennicutt Jr., G. Helou, Lee Armus, H. Roussel, K. Sheth, G. Bendo and 9 more. 2006

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We present a sample of low-resolution 5-38 μm Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for decomposing mid-infrared galaxy spectra is described and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in ~ of the sample, at strengths that indicate that most normal galaxies undergo AV ≲ 3 mag averaged over their centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to ~20% and is suppressed at metallicities Z ≲ Z☉/4, as well as in low-luminosity AGN environments. Strong interband PAH feature strength variations (2-5 times) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8 μm features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGNs. The AGNs may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGNs may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled and points to more than a factor of 2 uncertainty in results that assume a fixed PAH emission spectrum, for redshifts z = 0-2.5.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/a091cde9dbb6e7fa30997db0d1e2ac5946bc0a5b

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Citations in the corpus

• A simple model to interpret the ultraviolet, optical and infrared emission from galaxies (2008, background): “…In reality, changes in PAH molecule size and ionisation state, variations in metallicity and contamination by an AGN could cause variations of up to 40 per cent in the relative strengths of some PAH emission features (Helou et al. 2000; Brandl et al. 2006; Smith et al. 2007; Galliano et al. 2007).”
• A TWO-PARAMETER MODEL FOR THE INFRARED/SUBMILLIMETER/RADIO SPECTRAL ENERGY DISTRIBUTIONS OF GALAXIES AND ACTIVE GALACTIC NUCLEI (2014, methodology): “…the inclusion of prominent fine-structure lines (e.g., [Ne III]15.6 µm, [S III]18.7 µm, and [S III]33.5 µm) and the 17 µm PAH complex, the latter of which accounts for up to 10% of the total PAH emission in normal star-forming galaxies (see Figure 2 of this work and Table 7 of Smith et al. 2007).”
• DUST-CORRECTED STAR FORMATION RATES OF GALAXIES. I. COMBINATIONS OF Hα AND INFRARED TRACERS (2009, background): “…longer wavelength tracers such as the 24 μm emission, with systematic dependences of the emissivity on metal abundance, and local radiation field strength and hardness (e.g., Madden 2000; Peeters et al. 2004; Engelbracht et al. 2005, 2008; Dale et al. 2005; Wu et al. 2006; Smith et al. 2007; C07).”
• The Behavior of the Aromatic Features in M101 H II Regions: Evidence for Dust Processing (2008, methodology): “Using the HII classified galaxies in the SINGS (Spitzer Nearby Galaxies Survey) sample Smith et al. (2007b) found variations in the summed strength of the aromatic features when ratioed to the total IR flux with some indication of a threshold at log(O/H)+12=8.1.”
• Variations of the Mid-IR Aromatic Features inside and among Galaxies (2008, methodology): “…interstellar medium (ISM; e.g., Abergel et al. 2005; Flagey et al. 2006; Povich et al. 2007), as well as whole galaxies (e.g., Verma et al. 2005; Smith et al. 2007) that have been extensively observed by the Infrared Space Observatory (ISO) and are currently investigated with a higher…”
• THE EMISSION BY DUST AND STARS OF NEARBY GALAXIES IN THE HERSCHEL KINGFISH SURVEY (2011, background): “For NGC 1266, we believe this could be due to an active nucleus (see Dale et al. 2007; Smith et al. 2007) which could be heating dust in the central region; however, this galaxy is likely an exception, as galaxies in the sample were selected not to have luminous AGN (Kennicutt et al. 2003).”
• What makes a planet habitable? (2009, background): “PAHs are ubiquitous in galactic and extragalactic regions and are the most abundant carbonaceous gas phase molecules in space (Allamandola et al. 1999; Smith et al. 2007).”
• Multiwavelength Study of Massive Galaxies at z~2. I. Star Formation and Galaxy Growth (2007, background): “Spitzer observations show a substantial scatter in the local correlation of mid-IR luminosity versus the total infrared luminosity LIR (Dale et al. 2005; Smith et al. 2007; Armus et al. 2007).”
• MID-INFRARED SPECTROSCOPY OF SUBMILLIMETER GALAXIES: EXTENDED STAR FORMATION IN MASSIVE HIGH-REDSHIFT GALAXIES (2009, background): “At this S/N we found that intricate spectral fitting programs (e.g. pahfit; Smith et al. 2007) are not appropriate.”
• Deriving Physical Properties from Broadband Photometry with Prospector: Description of the Model and a Demonstration of its Accuracy Using 129 Galaxies in the Local Universe (2016, methodology): “The PAH ionized fractions and absorption cross-sections in Draine & Li (2007) are updated from the Li & Draine (2001) model by using observations of SINGS galaxies with 5–38 μm Spitzer IRS spectra (Smith et al. 2007).”
• Dust spectral energy distributions of nearby galaxies: an insight from the Herschel Reference Survey (2014, background): “…sample the results of Engelbracht et al. (2005) who found a weakening of the PAH features at low metallicity from 8 to 24 µm flux density ratios, and Smith et al. (2007) who found that the strength of the PAH bands is directly linked to the metallicity from the Spitzer/IRS spectrum of 59 nearby…”
• Warm Molecular Hydrogen in the Spitzer SINGS Galaxy Sample (2007, methodology): “Nuclear classifications derived from optical spectroscopy, published by Smith et al. (2007a), were modified for the following targets.”
• EFFICIENT THREE-DIMENSIONAL NLTE DUST RADIATIVE TRANSFER WITH SKIRT (2011, background): “The ISO and particularly the Spitzer mission have been instrumental in uncovering the mid-infrared emission of nearby galaxies (Helou et al. 2000; Smith et al. 2007; Draine et al. 2007).”
• The evolution of the dust temperatures of galaxies in the SFR–M∗ plane up to z ~ 2 (2013, background): “In local star-forming galaxies and in the metallicity range of our sample (μ > 8.1), there is only a weak correlation between the metallicity and the PAH-to-LIR ratio (Smith et al. 2007; Engelbracht et al. 2008; Hunt et al. 2010).”
• CALIBRATING EXTINCTION-FREE STAR FORMATION RATE DIAGNOSTICS WITH 33 GHz FREE–FREE EMISSION IN NGC 6946 (2011, methodology): “Similar values have been estimated using the 9.7µm silicate absorption feature in the central kpc (AV ∼ 5.0 mag; Smith et al. 2007), while much larger estimates have been quoted for the central ∼60 pc using total gas masses (AV ∼ 100 mag; Schinnerer et al. 2006), indicating a highly obscured,…”
• DETERMINING STAR FORMATION RATES FOR INFRARED GALAXIES (2008, methodology): “Previous work (e.g., Papovich & Bell 2002; Dale et al. 2005; Smith et al. 2007) has emphasized the broad variety of infrared spectral energy distributions (SEDs) and the resulting uncertainties in the bolometric luminosities and thus SFRs extrapolated from 24µm measurements.”