The interstellar spectrum of the bright Seyfert galaxy NGC 3783 : evidence for an extragalactic origin of high-velocity clouds.

The authors present high-resolution (FWHM ≅ 20 km s^-1), high signal-to-noise observations of the interstellar lines of Ca II and Na I in the spectrum of the bright Seyfert galaxy NGC 3783. These observations probe the interstellar medium over a very extended path length through the disc and halo of our Galaxy and of NGC 3783, as well as any intergalactic gas. The authors interpret the strong and complex interstellar absorption as evidence for parcels of gas in the Galactic halo which could be as far as 5.5 kpc above the disc, if they share in the differential rotation of the disc. In addition, a weak Ca II component at v_LSR = +241 km s^-1 is detected, in excellent agreement with the velocity of an intervening neutral hydrogen high-velocity complex. It is argued that this cloud is not local, but more likely is an independent extragalactic object, possibly associated with the Magellanic Stream. The mass of the cloud probably exceeds 10⁷ solar masses. The implications of the findings for the interpretation of QSO absorption line systems are briefly discussed.