The HST Spectrum of I Zw 1: Implications of the C III^* λ1176 Emission Line

Details The HST Spectrum of I Zw 1: Implications of the C III^* λ1176 Emission Line The HST Spectrum of I Zw 1: Implications of the C III^* λ1176 Emission Line

1996-09-24

arxiv.org/abs/astro-ph/9609161v1

Astronomy and Astrophysics

Astrophysics

2 pages of text only, uses aspconf.sty file, To appear in the proceedings of IAU Colloquium 159, Shanghai, June 1996. "Emissio

Scientific paper

I Zw 1 is a well known narrow line quasar with very strong Fe II emission. High S/N spectra obtained with the HST FOS show a remarkably rich emission line spectrum. The C III$^* \lambda 1176$ line is clearly detected in emission for the first time in AGNs. This line arises from radiative decay to the $2s2p^3P^o_{0,1,2}$ metastable levels of C III. The observed flux is 50 larger than expected from collisional excitation, or dielectronic recombination, in photoionized gas. The most plausible mechanism for the large enhancement in the C III^* \lambda 1176 flux is resonance scattering of continuum photons by C III^* ions. This mechanism requires large velocity gradients 1000 km/s within each emitting cloud in the BLR. Such large velocity gradients can be induced by forces external to the gas in the BLR clouds, such as tidal disruption, or radiation pressure.

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