PAH and Emission Line Ratios in AGN and Starburst Galaxies
msra(2010)
摘要
We study the Polycyclic Aromatic Hydrocarbons (PAH) bands, ionic emission
lines, and Mid-infrared continuum properties, in a sample of 171 emission line
galaxies taken from literature plus 15 new active galactic nuclei (AGN) Spitzer
spectra. The continuum shape steeply rises for longer wavelengths and can be
fitted with a warm blackbody distribution of T=150-300K. The brightest PAH
spectral bands (6.2, 7.7, 8.6, 11.3, and 12.7\,$\mu$m) and the forbidden
emission lines of [Si{\sc\,ii]} 34.8\,$\mu$m, [Ar{\sc\,ii]} 6.9, [S{\sc\,iii]}
18.7 and 33.4 were detected in all the Starbursts and in ~80% of the Seyfert~2.
Taking under consideration only the PAH bands at 7.7$\mu$m, 11.3$\mu$m, and
12.7$\mu$m we find they are present in ~80% of the Seyfert 1, while only half
of this type of activity show the 6.2$\mu$m and 8.6 PAH bands. The observed
intensities ratios for neutral and ionized PAHs (6.2/7.7 x 11.3/7.7) were
compared to theoretical intensity ratios, showing that AGNs have higher
ionization fraction and larger PAH (> 180 carbon atoms) than SB galaxies. The
ratio between the ionized (7.7) and the neutral PAH bands (8.6 and 11.3) are
distributed over different ranges for AGNs and SB galaxies, suggesting that
these ratios could depend on the ionization fraction, as well as on the
hardness of the radiation field. The ratio between the 7.7 and 11.3 bands is
nearly constant with the increase of [Ne{\sc\,iii]}15.5/[Ne{\sc\,ii]},
indicating that the fraction of ionized to neutral PAH bands does not depend on
the hardness of the radiation field. The equivalent width of both PAH features
show the same dependence with [Ne{\sc\,iii]}/[Ne{\sc\,ii]}, suggesting that the
PAH, emitting either ionized (7.7) or neutral (11.3) bands, may be destroyed
with the increase of the hardness of the radiation field.
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关键词
active galactic nuclei,equivalent width,emission line
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