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Astron. Astrophys. 322, 86-88 (1997)

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1. Introduction

Whether warps of different spiral galaxies are coherently aligned is an open and interesting question. Battaner et al. (1991) used the catalogue of warped edge-on northen spiral by Sánchez-Saavedra et al. (1990) to study the orientations of warps in the [FORMULA] Mpc Milky Way neighbourhood . They found typical cells of about 25 Mpc of coherent alignment. In particular, they identified a Virgo group, containing the Virgo cluster [FORMULA] and an adjacent group [FORMULA] with a relatively small standard deviation in the distribution of orientations. The mean orientations of the Virgo group and that of the adjacent one were nearly perpendicular. The orientation vector is precisely defined below.

Together with a statistical study of as large a sample as possible, an inspection of the closer and better studied warps would be an interesting complement. Even if we reduce the sample to three galaxies, the Milky Way, M 31 and M 33, it is convenient to study the topic of coherently orientated warps "at home" in the Local Group.

We will only use 21 cm data. The M 33 warp has been studied by Wright (1979), Deul and van der Hulst (1987), Corbelli, Schneider and Salpeter (1989) and others. For our purposes, we adopted the geometrical description in the early work by Rogstad, Wright and Lockhart (1976). The M 31 warp has been described by Newton and Emerson (1977), Emerson and Newton (1978), Henderson (1979), Bajaja and Shane (1982), Brinks and Shane (1984), Brinks and Burton (1985), Braun (1991) and others. Here we have adopted the detailed analysis by Brinks and Burton (1984) in which it is shown that the M 31 warp is orientated almost directly towards us, near the line-of-sight, in contrast with early works.

The Milky Way warp has been studied in 21 cm by Burke (1957), Kerr (1957), Burton (1978), Henderson et al. (1982), Kerr (1983), Burton and te Lintel Hekkert (1986), Burton and Deul (1987), Diplas and Savage (1991) and others. The discussion by Burton (1992) contains the most up-to-date summary of the warp situation in the Milky Way, summarizing also the M 31 and M 33 warps. The interpetration of the 21 cm Milky Way data is somewhat ambiguous and subject to a possible reinterpetration. In some galaxies the warp rises first from the mean plane and then turns back to the plane and moves in the opposite direction. One example is NGC 4012 as observed by combining the data in the optical (Florido et al., 1991) and in 21 cm (Bottema, 1995). The Milky Way warp probably belongs to this kind with an "elbow" or initial counterwarp. It was already known that the south warp changes its direction in this way. New measurements carried out by Burton (1996) which extend to much larger galactocentric distances, reveal the same behaviour for the north warp. This trend was already appreciable in the early measurements which are now confirmed. We therefore conclude that our own galaxy is warped in this way.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998