2002MNRAS.335.1091B

Neutral hydrogen has been studied on sub-arcsecond scales in two Seyfert galaxies, NGC 7469 and NGC 7674. The 1.4-GHz continuum emission from these sources agree with higher frequency measurements and shows linear, triple source in NGC 7674 and a compact source surrounded by a weak ring of radio emission in NGC 7469. In both of these sources we detect deep localized H i absorption in front of a single continuum component.

In the case of NGC 7674 the H i absorption is only seen against the central component and has a peak optical depth of 0.21±0.03, a linewidth of 114±7 km.s^–1 and a heliocentric velocity of 8623±5 km.s^–1. The integrated H i absorption against the central component corresponds to a column density of 5.0±0.7 x10²¹ atom cm ^–2. We interpret the lack of H i absorption against the east and west components ( τ < 0.07 and 0.06) as evidence that the H i is localized within the central few hundred pc, similar to the H i observed against the nucleus of NGC 4151, and has an upper mass limit of ∼4x10⁵ M_☉. We speculate that this H i may be associated with a dusty torus. Giant Meterwavelength Radio Telescope (GMRT) results show extended emission and give a rest velocity of 8699±20 km.s^–1.

The H i absorption against NGC 7469 shows a single deep feature against the strong central component. The line has a peak optical depth 0.25, a linewidth of 65±6 km.s^–1 and a heliocentric velocity of 4843±5 km.s^–1. The integrated column density is 3.6±0.4x10²¹ atom cm ^–2. Unfortunately the other components in NGC 7469 are too weak to set significant limits to the H i optical depth, and hence, unlike NGC 7674, it is not possible to constrain the size of the absorbing gas.