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

Abstract

We present high-resolution (FWHM≃20 km s−1), high signal-to-noise observations of the interstellar lines of CaII and NaI 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 Galactic interstellar lines are strong, with CaII showing at least four components in the velocity range |$-65\leqslant\upsilon_\text{LSR}\leqslant+100 \enspace \text {km s}^{-1}$|⁠, two of which are also found in the NaID lines. In contrast, no CaII or NaI absorption is detected at velocities appropriate to material associated with NGC 3783 (υH=+2930 km s−1). We 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 (14 kpc from the Sun), if they share in the differential rotation of the disc. The high Ca+/Na0 ratios deduced from these components are a further indication of the reduced depletion of Ca in the halo relative to diffuse interstellar clouds in the plane. In addition, we detect a weak CaII component at υLSR=+241 km s−1, in excellent agreement with the velocity of an intervening neutral hydrogen high-velocity complex (HVC) which has been known from its 21-cm emission for over 13 years, but whose distance and origin are still controversial. This is the first reported detection of metal-enriched gas in a 21-cm HVC. We use the low Ca abundance deduced for the HVC, together with its velocity of +50 km s−1 relative to the Galaxy, to argue that this cloud is not local, but more likely is an independent extragalactic object, possibly associated with the Magellanic Stream. If this interpretation is correct, the mass of the cloud probably exceeds 107 solar masses. The implications of our findings for the interpretation of QSO absorption line systems are briefly discussed.