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Abstract

To study the origin of far-IR emission in starburst nuclei (SBNs) and active galactic nuclei (AGNs), the 25-60 micron spectral index [α(25, 60)] has been compared with the intensity ratios of [O I] λ6300 to Hα ([O I])/Hα) and H_2_ v = 1-0 S(1) 2.12 microns to Brγ [S(1)/Brγ) for 48 galaxies (16 SBNs, 27 AGNs, and five LINERs). In SBNs, α(25, 60) correlates tightly with [O I]/Hα and with S(1)/Brγ. To explain the correlations, we adopt the following excitation mechanisms: (1)25 micron emission is emitted by dust in ionized gas heated by Lyα and ionizing photons; (2)60 micron emission is emitted by dust in molecular clouds heated by nonionizing photons; and (3) [O I] and S(1) are excited by supernova-driven shocks. Nonionizing photons are from stars with M >= 5 M_sun_ (supernova progenitors), while ionizing photons are from stars with M >= 20 M_sun_. Hence, with an increase in the number of stars with M < 20 M_sun_ relative to stars with M >= 20 M_sun_, the 25-60 micron slope becomes steeper, and the ratios of [O I]/Hα and S(1)/Brγ become larger. On the other hand, AGNs scatter widely in the plots of α(25, 60) versus [O I]/Hα and of α(25, 60) versus S(1)/Brγ. In these plots, AGNs with starburst activity tend to have steep 25-60 micron slopes, and obey the α(25, 60)-[O I]/Hα and α(25, 60)-S(1)/Brγ correlation obtained for SBNs. Hence far-IR emission in AGNs is a mixture of the flat component from a nonthermal source and the steep component from dust heated by young stars, with their fractions varying from one to another. Notably, some AGNs have steeper 25-60 micron slopes than SBNs, indicating the predominance of stars with M < 20 M_sun_. This trend may suggest the difference in star-forming activity between SBNs and AGNs.

Origin of far-infrared emission in Seyfert and starburst galaxies

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