Kavli Affiliate: Claudio Ricci
| First 5 Authors: Jeffrey D. McKaig, Shobita Satyapal, Ari Laor, Nicholas P. Abel, Sara M. Doan
| Summary:
Forbidden collisionally excited optical atomic transitions from high
ionization potential (IP$geq$54.8,eV) ions, such as Ca$^{mathrm{4+}}$,
Ne$^{mathrm{4+}}$, Fe$^{mathrm{6+}}$, Fe$^{mathrm{10+}}$,
Fe$^{mathrm{13+}}$, Ar$^{mathrm{9+}}$, and S$^{mathrm{11+}}$, are known as
optical coronal lines (CLs). The spectral energy distribution (SED) of active
galactic nuclei (AGN) typically extends to hundreds of electron volts and
above, which should be able to produce such highly ionized gas. However,
optical CLs are often not detected in AGN. Here we use photoionization
calculations with the textsc{Cloudy} spectral synthesis code to determine
possible reasons for the rarity of these optical CLs. We calculate CL
luminosities and equivalent widths from radiation pressure confined
photoionized gas slabs exposed to an AGN continuum. We consider the role of
dust, metallicity, and ionizing SED in the formation of optical CLs. We find
that (1) dust reduces the strength of most CLs by $sim$three orders of
magnitude, primarily as a result of depletion of metals onto the dust grains.
(2) In contrast to the CLs, the more widely observed lower IP optical lines
such as [O, III] 5007,AA, are less affected by depletion and some are
actually enhanced in dusty gas. (3) In dustless gas many optical CLs become
detectable, and are particularly strong for a hard ionizing SED. This implies
that prominent CL emission likely originate in dustless gas. Our calculations
also suggest optical CL emission is enhanced in galaxies with low mass black
holes characterized by a harder radiation field and a low dust to metal ratio.
The fact that optical CLs are not widely observed in the early universe with
JWST may point to rapid dust formation at high redshift.
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