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Nanyao Lu (IPAC/SSC/Caltech)

Global Relation between Aromatic Features in Emission (AFEs) and Star Formation in Galaxies ( Does the Former Trace the Latter?). Nanyao Lu (IPAC/SSC/Caltech). AFEs (or PAH Emission features) Nearly Ubiquitous in Disk Galaxies. (Lu et al. 2003). Average spectrum for spiral galaxies.

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Nanyao Lu (IPAC/SSC/Caltech)

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  1. Global Relation between Aromatic Features in Emission (AFEs) and Star Formation in Galaxies ( Does the Former Trace the Latter?) Nanyao Lu (IPAC/SSC/Caltech)

  2. AFEs (or PAH Emission features) Nearly Ubiquitous in Disk Galaxies (Lu et al. 2003) Average spectrum for spiral galaxies Flux density (Jy) Two elliptical galaxies λ(um)

  3. Exceptions: Some Low-Metallicity Starburst Dwarf Galaxies SBS0335-052 (12+log[O/H] = 7.30) (Houck et al. 2004) Why? PAHs are more servely destroyed by far-UV photons, or PAHs have not been produced enough in such a young galaxy (i.e., an intrinsic low PAH abundance w.r.t. large dust grains).

  4. Controversy on AFEs as a Tracer of Star Formation Pro Evidences Anti Evidences • Global surface brightness of AFEs correlates • with that of Hα emission (e.g. Roussel et al. • 2001). • AFEs peak spatially near HII regions (i.e., • PDRs) • Surface brightness of AFEs correlates with • that of cold dust emission at 850um (e.g., • Haas et al. 2002). • AFEs detected in diffuse ISM in our own • Galaxy by COBE (Dwek et al. 1997), ISO • (e.g., Lemke et al 1998), and Spitzer (e.g., • Lu 2004). • A possible way out of this controversy is that AFEs arise from both warm • star-forming regions and the colder, diffuse ISM; and along the line-of-sight • through high surface density regions, there is always a mix of these two • components. • (for an example, see Dale & Helou (2002) who assumed a power law distribution • of dust mass over heating intensity and predicted a correlation between AFEs and • the 850um fluxes)

  5. IRAS/ISO Data Suggest a Two Component Scenario Lu et al 2003 Green line – a Galactic reflection nebula; red line – two Galactic HII regions (from Werner, Gautier & Cawlfeild 1994)

  6. A Simplified, Two-Component Model for AFEs FPAH = aFw + bFc Flux of AFE Flux of large grain emssion from star-forming regions (i.e., the warm component). Flux of large grain emission from diffuse ISM (i.e., the cold component). where b is a universal constant, while a could have a dependence on some characteristics (e.g., the hardness) of the mean radiation field in star-forming regions. FPAH = νfνat some wavelength where AFEs are dominant (e.g, 8um or 12um).

  7. Modelling the Far-Infrared Dust Emission fν= Nw νβB(ν, Tw) + Nc νβB(ν, Tc) We assume: β = 2 Tc = 20K. Solve Tw, Nw, and Ncusing IRAS flux densities at 60 and 100μm and SCUBA flux density at 850μm. Then the integrated fluxes are: Fw = Nw∫νβB(ν, Tw) dν, and Fc = Nw∫νβB(ν, Tc) dν. Searched the literature for IRAS galaxies with available 850um fluxes:  106 galaxies, all in IRAS Bright Galaxy Catalog (Soifer et al. 1987) with the 850um fluxes mostly from the SCUBA Local Universe Galaxy Survey (Dunne et al. 2000).

  8. Modelling the Far-Infrared Dust Emission

  9. AFEs and Cold Dust Component where FPAH = νfν(12um). Least-squares fit on 97 galaxies: slopeb= 0.295 (+/–0.028); Y-intercept a = 0.127(+/–0.016); C. Coef. = 0.74

  10. AFEs vs. FIR Flux for the Warm Dust Component Least-squares fit on 97 galaxies by forcing the fit through (0, 0): Slope a = 0.116 (+/– 0.010).

  11. AFEs-to-FIR Flux Ratio for the Warm Dust Component

  12. Fractional AFEs from the Warm Dust Component

  13. Cold-Component AFEs in Dwarf Irregular Galaxies Filled squares: Dwarfs. Open squares: Spirals. N1569 Recall FPAH / Fw = a + b Fc / Fw, and The slope “b” depends on R, the abundance ratio of PAHs to large dust grains. If lower-metallicity dwarf irregulars have a lower R, one should expect to see a shallower slope for these galaxies. This is not obviously the case here though a definitive confirmation needs more dwarf galaxies.

  14. Predictions for Spitzer For the Tc=20K cold dust component: νfν(12μm) ≈ 0.29 Fc. For the warm, star-forming component at Tw: νfν(12μm) ≈ 0.12 Fw. Assuming fν(8μm)/fν(12μm) ~ 1.1 (i.e, cirrus-like color), we have the following ratios for Spitzer fluxes:

  15. A Spitzer Test Case: NGC6946 70um 8um (stars subtracted) Gray: 24um; Contours: ratio 8um/70um Contour step = 0.2; red contours: ratios > 1; blue contours: ratios < 1 Credit: Data taken from SINGS observations released in the Spitzer Public Archive

  16. A Spitzer Test Case: NGC6946 70um 8um (stars subtracted) Gray: 24um; Contours: ratio 8um/70um Contour step = 0.2; red contours: ratios > 1; blue contours: ratios < 1 Credit: Data taken from SINGS observations released in the Spitzer Public Archive

  17. A Spitzer Test Case: NGC6946 Model prediction for diffuse ISM

  18. Conclusions • AFEs or PAH emissions arise from both star-forming regions as well as diffuse ISM. • Only in more actively star-forming galaxies, is the global PAH emission dominated by star-forming regions. • The PAH/FIR luminosity ratio in star-forming regions is lower than that in diffuse ISM, and may vary significantly as functions of factors such as the hardness of the radiation field. • We showed some preliminary indication that lower PAH feature strengths in lower-metallicity dwarf irregulars is not a result of an intrinsic deficiency of its carriers.

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