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Discovery of PAHs in the Halo of NGC 5907

Discovery of PAHs in the Halo of NGC 5907. Judith Irwin Queen’s University, Kingston Suzanne Madden CEA/Saclay, France. Polycyclic Aromatic Hydrocarbons. Aromatic Infra-red Features. Large ~ 50 C-atom molecules. HII region type nuclear spectrum – no compact radio core.

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Discovery of PAHs in the Halo of NGC 5907

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  1. Discovery of PAHs in the Halo of NGC 5907 Judith Irwin Queen’s University, Kingston Suzanne Madden CEA/Saclay, France Polycyclic Aromatic Hydrocarbons Aromatic Infra-red Features Large ~ 50 C-atom molecules HII region type nuclear spectrum – no compact radio core SFR ~ 2 M_sun/yr, f_60/f_100 = 0.3, L_fir = 7.5E9 L_sun, L_fir/(2 pi R^2) ~ 2E40 ergs/s/kpc^2, D = 11 Mpc CASCA 2005

  2. Observing bands shown Typically little variation in spectra Typical (ISO) MIR Spectrum Very consistent spectrum galaxy to galaxy and within galaxies,Excitation by single-photon far-UV Our line ratios in different bands are consistent with that seen in other quiescent SF regions

  3. 3.2 kpc LW6 LW8 (res 7”) LW6 (smoothed) over DSS

  4. WENSS 327 MHz Previous Searches for Gas/Dust in Halo: B fields, CRs in halo • Ionized gas: NO, Rand 1996 • CO: NO, Dumke et al. 1997 • HI: MAYBE, FWHM=1.5 kpc? Dumke et al. 1997 (warp) • Radio continuum: YES, z_e = 1.5 kpc, Dumke et al. 2000 (res 45”) • Dust in absorption: NO, Howk & Savage 1999 • Dust in emission: Large (~ 0.1 micron) cool grains Rms = 0.03 mJy/arcsec^2 1.2 mm, Dumke et al. 1997 SCUBA 850 micron, Alton et al. 2005

  5. Major Axis Slices 7.7 micron PAH, CO,HI => PAHs follow molecular gas dist’n globally, suggestingorigin in PDRs, excitation by far UV But there is a real excess beyond the CO, also seen at 1.2 mm,suggest PAHs more widespread, some excitation by far-UV in the ISRF => 7.7 micron PAH, SCUBA 850 micron Extremely good correlation between PAHs and 850 micron emission,f_(7.7)/f(850) ~ 2 Edge of SCUBA field -> PAHs correlated with cold dust,Need a minimal radiation field for excitation (cf. Haas et al. 2002)

  6. LW2 + 2MASS K LW2: 6.2, 7.7, 8.6 micron PAHs Beam-switching observations,Dynamic Range = 700/1 3.2 kpc Excitation: Possibly far-UV photons leaking from disk

  7. Mean z Slice LW2/LW10 (PAHs / VSGs) LW2 -- PAHs LW6 (smoothed) / 850 micron7.7 micron PAH / cold large grains 2.7 kpc The halo appears to be selectively PAHs, in comparison to VSGs or large grains=> PAHs are selectively escaping or are selectively excited?-- large grains (graphite) escape more easily (Davies 1998, modeled, no B fields), but note morphology

  8. CONCLUSIONS • Sensitive observations in 6 MIR ISO bands shows emission from NGC 5907 consistent with what is expected in quiescent SF galaxies or quiescent regions within galaxies • PAH emission is detected throughout the optical disk • PAH emission is detected to typical scale height of 3.5 kpc and higher in places – the morphology is sometimes arc-like • The PAH distribution follows the molecular gas, not the HI; this could be a statement of excitation rather than distribution • There is a correlation between PAHs and 850 micron emission in the disk (PAHs are found wherever cold dust is found) • Ratio of PAHs to both VSGs and cold large grains seems to increase with z – reason unclear • Morphology of the high z emission and presence of halo synchrotron emission suggest that B fields may be important in transporting the PAH carriers to high z. • Starburst galaxies may not be the place to look because high SFRs destroy PAHs

  9. Optical high latitude emission Shang et al. 1998, Zheng et al. 1999 Deeper observations: No optical halo but remnants of interaction. Ring luminosity < 1% of galaxy, < 0.5% of mass. Johnson et al. 2001: modelled as destruction of 2E8 Msun satellite 0.8 Gyr ago. => No Stellar Halo. Sackett et al. 1994 Subtraction of thin disk model, stellar residuals extending 3 to 7 kpc from the plane.

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