The Ionization Fraction in the Obscuring ‘Torus’ of an Active Galactic Nucleus

The LINER galaxy NGC 2639 contains a water vapor megamaser, suggesting the presence of an edge-on nuclear accretion disk or torus. This galaxy is thus a good candidate for revealing absorption by the torus of any compact nuclear continuum emission. In this paper, we report VLBA radio maps at three frequencies and an ASCA X-ray spectrum obtained to search for free-free and photoelectric absorptions, respectively. The radio observations reveal a compact (< 0.2 pc) nuclear source with a spectrum that turns over sharply near 5 GHz. This turnover may reflect either synchrotron self-absorption or free-free absorption. The galaxy is detected by ASCA with an observed luminosity of 1.4× 1041 erg s−1 in the 0.6 – 10 keV band. The X-ray spectrum shows emission in excess of a power-law model at energies greater than 4 keV; we interpret this excess as compact, nuclear, hard X-ray emission with the lower energies photoelectrically absorbed by an equivalent hydrogen column of ≃ 5 × 1023 cm−2. If we assume that the turnover in the radio spectrum is caused by free-free absorption and that both the free-free and photoelectric absorptions are produced by the same gaseous component, the ratio ∫ nedl/ ∫ nHdl may be determined. If the masing molecular gas is responsible for both absorptions, the required ionization fraction is ∼> 1.3× 10 −5, which is comparable to the theoretical upper limit derived by Neufeld, Maloney & Conger (1994) for X-ray heated molecular gas. The two values may be reconciled if the molecular gas is very dense – nH2 ∼> 10 9 cm−3. The measured ionization fraction is also consistent with the idea that both absorptions occur in a hot (∼ 6,000K), weakly ionized (ionization fraction a few times 10−2) atomic region that may co-exist with the warm molecular gas. If this is the case, the absorbing gas is ∼ 1 pc from the nucleus. We rule out the possibility that both absorptions occur in a fully ionized gas near 104K. If our line of sight passes through more than one phase, the atomic gas probably dominates the free-free absorption while the molecular gas may dominate the photoelectric absorption. Subject headings: accretion disks – galaxies: active – galaxies: individual (NGC 2639) – galaxies: nuclei – galaxies: Seyfert – radio continuum: galaxies