Other
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...210.0221l&link_type=abstract
American Astronomical Society Meeting 210, #02.21; Bulletin of the American Astronomical Society, Vol. 39, p.96
Other
Scientific paper
Broad-line region (BLR) emission should be sensitive to the illuminating spectral energy distribution (SED). Gas illuminated by a soft, UV-dominant SED responds in an interesting way. A lack of high-energy photons means high-ionization line-emission is weak. The HII region is dominated by intermediate-ionization states (Si+2, Fe+2, Al+2). However, the soft SED produces a low electron temperature, resulting in weak emission from these predominately collisionally-excited ions. In the partially-ionized zone, collisionally-excited lines such as MgII λ2800 are also weak, again because of the low temperature. Yet the number of hydrogen ionizations is high, and the energy is deposited primarily in excited states of hydrogen and continuum emission.
The principal density diagnostics for BLR gas are provided by semiforbidden lines such as CIIII] λ 1909 from ions that are collisionally deexcited at high density. However, these lines are predicted also to be weak in gas illuminated by a soft SED because of the low electron temperature. These similarities are a consequence of both types of gas being unable to cool ("cooling challenged") by the usual BLR emission. These two cases can be differentiated by the strength of the low-ionization line MgII. This permitted line is not very sensitive to density, but will be weaker in gas illuminated by the soft SED. Other similaries and distinguishing characteristics will be discussed.
We show how the unusual optical and UV spectrum of PHL 1181, a luminous, intrinsically X-ray weak, narrow-line Seyfert 1 galaxy, is qualitatively explained by its soft SED, and although its spectrum lacks semiforbidden lines, a high density is not required. We also show that SDSS spectra selected by their MgII equivalent widths show some similarities to PHL 1811. Finally, we demonstrate that transmission of a hard continuum through ionized gas can sometimes soften it sufficiently to produce the characteristic properties outlined above.
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