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 a nuclear accretion disk or torus viewed close to edge-on. 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 x 10(41) ergs 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 similar or equal to 5 x 10(23) 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 integral n(e)(2) dl/integral n(H)dl may be determined. If the masing molecular gas is responsible for both absorptions, the required ionization fraction is greater than or similar to 1.3 x 10(-5), which is comparable to the theoretical upper limit derived by Neufeld, Maloney, and Conger for X-ray heated molecular gas. The two values may be reconciled if the molecular gas is very dense: n(H2) greater than or similar to 10(9) cm(-3). The measured ionization fraction is also consistent with the idea that both absorptions occur in a hot (similar to 6000 K), weakly ionized (ionization fraction a few times 10(-2)) atomic region that may coexist with the warm molecular gas. If this is the case, the absorbing gas is similar to 1 pc from the nucleus. We rule out the possibility that both absorptions occur in a fully ionized gas near 10(4) K. 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.