Open AccessSub‐arcsecond atomic hydrogen absorption in the Seyfert galaxies NGC 7674 and NGC 7469
Author(s) -
Beswick R. J.,
Pedlar A.,
McDonald A. R.
Publication year - 2002
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2002.05699.x
Subject(s) - physics , astrophysics , galaxy , hydrogen line , absorption (acoustics) , torus , giant metrewave radio telescope , equivalent width , green bank telescope , astronomy , radio galaxy , emission spectrum , spectral line , optics , geometry , mathematics
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 × 10 21 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 ∼4 × 10 5 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 ± 0.4 × 10 21 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.