Mid-Infrared Properties of Low-Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared Spectrograph

By Yanling Wu, Vassilis Charmandaris, L. Hao, B. Brandl, Jeronimo Bernard-Salas, H. Spoon, J. Houck. 2005

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We present a Spitzer-based mid-infrared (MIR) study of a large sample of blue compact dwarfs (BCDs) using the Infrared Spectrograph (IRS), including the first MIR spectrum of I Zw 18, the archetype for the BCD class and among the most metal-poor galaxies known. We show the spectra of polycyclic aromatic hydrocarbon (PAH) emission in a low-metallicity environment. We find that the equivalent widths (EWs) of PAHs at 6.2, 7.7, 8.6, and 11.2 μm are generally weaker in BCDs than in typical starburst galaxies and that the fine-structure line ratio, [Ne III]/[Ne II], has a weak anticorrelation with the PAH EW. A much stronger anticorrelation is shown between the PAH EW and the product of the [Ne III]/[Ne II] ratio and the UV luminosity density divided by the metallicity. We conclude that the PAH EW in metal-poor high-excitation environments is determined by a combination of PAH formation and destruction effects.

Subfields: Astronomy and Astrophysics

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CASSIS: THE CORNELL ATLAS OF SPITZER/INFRARED SPECTROGRAPH SOURCES (2011, background): “…seen that objects with a large 6.2/11.3 ratio show a steeper dust continuum while also having a larger [S iv]/[Ne ii] ratio, suggesting that the PAH band ratio correlates with the radiation field hardness (e.g., Lebouteiller et al. 2011; Madden et al. 2006; Wu et al. 2006; Engelbracht et al. 2005).”
Metallicity Effects on Dust Properties in Starbursting Galaxies (2008, background): “Observations with Spitzer have confirmed this trend and indicated that there is a fairly sharp transition in the relative aromatic feature strength near a metallicity of 12 + log(O/H) = 8 (Houck et al. 2004; Engelbracht et al. 2005; Wu et al. 2006; Madden et al. 2006).”
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).”
The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wave Bands (2006, background): “PAHs in the SMC Among the recent Spitzer results is the disappearance of PAH emission in galaxies of low metallicity (Engelbracht et al. 2005; Hogg et al. 2005; Wu et al. 2006; O’Halloran et al. 2006).”
Deep Spitzer 24 μm COSMOS Imaging. I. The Evolution of Luminous Dusty Galaxies—Confronting the Models (2009, background): “Given the variety of starburst and AGN SEDs in the mid-IR (e.g., Laurent et al. 2000; Brandl et al. 2006; Weedman et al. 2005; Wu et al. 2006; Armus et al. 2007), observations at these wavelengths are affected by quite complex k-correction effects and their sensitivity decreases rapidly beyond z ∼…”
Modeling the Pan-Spectral Energy Distribution of Starburst Galaxies. IV. The Controlling Parameters of the Starburst SED (2007, background): “Such a decrease in the PAH strength with decreasing metallicity has been observed with both the Spitzer and ISO Space Observatories (Engelbracht et al. 2005; Rosenberg et al. 2006; Wu et al. 2006; Madden et al. 2006; Jackson et al. 2007).”
RADIAL DISTRIBUTION OF STARS, GAS, AND DUST IN SINGS GALAXIES. II. DERIVED DUST PROPERTIES (2009, background): “Wu et al. (2006) found that the equivalent width of the aromatic features correlates better with a parameter simultaneously taking into account both the starlight hardness and the metallicity than with any of those parameters alone.”
Stellar Evolutionary Effects on the Abundances of Polycyclic Aromatic Hydrocarbons and Supernova-Condensed Dust in Galaxies (2007, methodology): “Among these galaxies, the spectra of SBS 0335 052, NGC 7714, VII Zw 403, Haro 11, and I Zw 18 are described in detail in Houck et al. (2004b), Brandl et al. (2004), and Wu et al. (2006).”
The Behavior of the Aromatic Features in M101 H II Regions: Evidence for Dust Processing (2008, methodology): “Wu et al. (2006) studied high ionizations (−0.3 < II < 1.3) and found large variations.”
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).”
Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample (2007, background): “In a spectroscopic study of blue compact dwarf galaxies, Wu et al. (2006) found that the equivalent width of the PAH 6.2 and 11.2 m features appeared to be suppressed for Z/Z P 0:2, or AOP 7:9.”
THE CALIBRATION OF MONOCHROMATIC FAR-INFRARED STAR FORMATION RATE INDICATORS (2010, background): “…Lequeux & Gavazzi 2004; Bendo et al. 2008), variations in the dust effective temperature (for L(24), see Calzetti et al. 2007), and, especially for L(8), gas metallicity (Engelbracht et al. 2005; Rosenberg et al. 2006; Wu et al. 2006; Madden et al. 2006; Jackson et al. 2006; Draine et al. 2007).”
An Overview of the Dwarf Galaxy Survey (2013, background): “ISO and Spitzer spectroscopy confirmed the relatively weak PAH bands in dwarf galaxies (e.g., Madden et al. 2006; O’Halloran et al. 2006; Wu et al. 2006; Hunter & Kaufman 2007; Smith et al. 2007; Hunt et al. 2010) and are sometimes completely absent as metallicities drop to a threshold level of…”
The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission (2006, background): “…the absolute strength of PAH emission can depend on metallicity has recently been well established (Engelbracht et al. 2005; Madden et al. 2006; Wu et al. 2006; O’Halloran et al. 2006; Rosenberg et al. 2006), with indications of a threshold 12+log(O/H) ∼ 8.1, or approximately Z⊙/4, below which…”
AN ATLAS OF GALAXY SPECTRAL ENERGY DISTRIBUTIONS FROM THE ULTRAVIOLET TO THE MID-INFRARED (2013, background): “…and Great Observatories All-Sky LIRG Survey (GOALS; Armus et al. 2009; U et al. 2012; Inami et al. 2013; Stierwalt et al. 2013), supplemented by other programs targeting specific galaxy types such as starbursts, low metallicity galaxies and ellipticals (e.g., Brandl et al. 2006; Wu et al. 2006).”
Variations of the Mid-IR Aromatic Features inside and among Galaxies (2008, background): “There is a general correlation between the PAH-to-continuum intensity ratio and the ISMmetallicity (Madden et al. 2006; Wu et al. 2006; O’Halloran et al. 2006; Smith et al. 2007), and consequently between the IRAC8 m/MIPS24 m broadband ratio and the metallicity (Engelbracht et al. 2005).”
The Calibration of Mid-Infrared Star Formation Rate Indicators (2007, background): “A possible mechanism to reduce the mass fraction of PAH molecules in the low–metallicity galaxies is destruction by the hard radiation field in those galaxies (e.g., Madden et al. 2006; Wu et al. 2006), although these molecules have proven surprisingly robust (Engelbracht et al. 2006).”