Neutral hydrogen (21 centimeter) absorption in Seyfert galaxies: Evidence for free-free absorption and subkiloparsec gaseous disks

Active galaxies are thought to be both fueled and obscured by neutral gas r emoved from the host galaxy and funneled into a central accretion disk. We performed a VLA imaging survey of 21 cm absorption in Seyfert and starburst nuclei to study the neutral gas in the near-nuclear environment. With the exception of NGC 4151, the absorbing gas traces 100 pc-scale, rotating disk s aligned with the outer galaxy disk. These disks appear to be rich in atom ic gas relative to nuclear disks in nonactive spirals. We find no strong ev idence for rapid infall or outflow of neutral hydrogen, but our limits on t he mass infall rates are compatible with that required to feed a Seyfert nu cleus. Among the galaxies surveyed here, neutral hydrogen absorption traces parsec-scale gas only in NGC 4151. Based on the kinematics of the absorpti on line, the disk symmetry axis appears to align with the radio jet axis ra ther than the outer galaxy axis. The most surprising result is that we dete ct no 21 cm absorption toward the central radio sources of the hidden Seyfe rt 1 nuclei Mrk 3, Mrk 348, and NGC 1068. Moreover, 21 cm absorption is com monly observed toward extended radio jet structure but appears to avoid cen tral, compact radio sources in Seyfert nuclei. To explain these results, we propose that 21 cm absorption toward the nucleus is suppressed by either f ree-free absorption, excitation effects (i.e., enhanced spin temperature), or rapid motion in the obscuring gas. Ironically, the implications of these effects is that the obscuring disks must be small, typically not larger th an a few tens of parsecs.