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Astron. Astrophys. 345, 787-812 (1999)

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2. Main observed features

In the region [FORMULA] several features stand out from the observed longitude-velocity distribution of both atomic and molecular gas (see Figs. 1 and 2). The nomenclature of these features principally refers to Rougoor (1964) and we omit here the historical but confusing "expanding" qualification, since local radial motions do not necessarily imply a net flux when averaged over azimuth. Other more exhaustive inventories of observed [FORMULA] features can be found in van der Kruit (1970) and Cohen (1975) for the HI, and in Bania (1977), Bally et al. (1988) and the review of Combes (1991) for the CO.

  • The 3-kpc arm . This arm is the largest apparent feature, covering more than [FORMULA] in Galactic longitude, and with a "forbidden" radial velocity of -53 km s-1 at [FORMULA]. It was discovered by van Woerden et al. (1957) and its name is related to the location of its tangent point near [FORMULA], corresponding to [FORMULA] kpc (at this time the galactocentric distance of the Sun was assumed to be 8.2 kpc). The same authors also noticed that this arm must lie in front of the Galactic centre because of absorbing the continuum spectrum of radio sources close to the centre (see the HI [FORMULA] diagram at [FORMULA] in Fig. 2), which is consistent with its rather large latitudinal width. The 3-kpc arm cannot be represented by a uniformly rotating and expanding circular arc over its whole longitude range (Burke & Tuve 1964). It was successfully modeled by Mulder & Liem (1986) as a stationary density wave in a rotating barred potential.

  • The 135-km s-1 arm . This feature shows no absorption lines against Galactic centre radio sources and is suspected to be the far-side counterpart of the 3-kpc arm (e.g. Oort 1977). However it crosses the zero longitude axis at 135 km s-1 (hence his name), i.e. more than twice the absolute velocity of the 3-kpc arm, which is among the strongest evidence for an asymmetric spiral structure in the central few kpc (velocity asymmetries at [FORMULA] cannot result from perspective effects). The 135-km s-1 arm extends down to [FORMULA], where its radial velocity still reaches about 100 km s-1, and lies slightly above the Galactic plane, at [FORMULA]. It also involves less HI mass than the 3-kpc arm and is more lumpy.

  • The connecting arm . This is a very rarely discussed feature in the recent literature, despite its substantial brightness. It was probably mentioned the first time by Rougoor & Oort (1960). The origin of its name comes from the fact that it seems to link the nuclear ring/disc to the outer disc. It becomes easily identifiable in the HI and 12CO data near [FORMULA] and [FORMULA] km s-1, where it is located far below the plane at [FORMULA], and passes through the peak of the positive terminal velocity curve. In Rougoor's (1964) description, this feature is doubtfully combined with another distinct feature extending at roughly constant velocity out to [FORMULA]. The connecting arm was soon recognized as a very inclined spiral arm, i.e. with a high pitch angle, although its situation in front or behind the Galactic centre remained unclear (Rougoor 1964; Cohen & Davies 1976). Kerr (1967) adopted the latter alternative and interpreted this arm as part of a central gaseous bar.

  • The nuclear ring/disc , or central molecular zone . Concerns the offcentred dense molecular gas located within [FORMULA], i.e. somewhat inside the positive and negative peaks of the terminal velocity curves (see for instance the high resolution CO [FORMULA] maps in Oort 1977; Liszt & Burton 1978; Bally et al. 1988; Combes 1991; Morris & Serabyn 1996 and Oka et al. 1998b). The parallelogram bounding this structure is also called "expanding molecular ring" owing to its original interpretation (Scoville 1972; Kaifu et al. 1972), and more recently "180-pc ring" according to its size. The molecular complex enclosed by this ring forms a kind of disc rotating with a maximum velocity of order 100 km s-1 and is speculated to form two mini spirals (Sofue 1995). Almost the entire CS emission comes from this disc. Binney et al. (1991) have interpreted the parallelogram and the disc respectively as gas on the cusped [FORMULA] orbit and on [FORMULA] orbits in the bar potential (see Sect. 6.5). An extensive review on this pattern is given by Morris & Serabyn (1996).

  • The molecular ring . Meant to represent a ring-like gas concentration with a radius of roughly 4 kpc, the spatial distribution of this structure is in fact poorly known and could as well involve imbricated spiral arms. Close to the positive terminal velocity curve, it forms two branches with tangent points at [FORMULA] and [FORMULA], corresponding to [FORMULA] kpc and 4 kpc. The molecular ring probably compares to the inner (pseudo-) rings seen in external spiral galaxies (Buta 1996).

All features listed here appear both in the CO and HI data with a rigorous coincidence in position, although the central molecular zone is much weaker in HI. It is not yet clear whether these features are transient or represent a permanent gas flow on closed orbits.

[FIGURE] Fig. 1. Grey scale longitude-velocity diagram of 12CO [FORMULA] emission within [FORMULA] and over a [FORMULA] latitude strip centred on the Galactic plane (Dame et al. 1999), with the dominant features indicated by dashed lines and Bania's (1977) molecular clumps.

[FIGURE] Fig. 2. (right column ) Longitude-velocity diagrams of HI 21 cm (left ) and 12CO [FORMULA] (right ) emission as a function of latitude in the range [FORMULA]. The HI data are from Hartmann & Burton (1997), Burton & Liszt (1978) and Kerr et al. (1986), and the CO data from Dame et al. (1987).

Bania (1977) has also isolated two particular "clumps" in the 12CO data (see Fig. 1), corresponding to massive molecular cloud complexes.

  • Clump 1 is located at the negative longitude end of the 135-km s-1 arm, at [FORMULA] and [FORMULA], and represents the most extreme case of non-circular motion in the Milky Way's dense gas kinematics. It is located at slightly lower latitude than the nearby emission from the 135-km s-1 arm. The clump 1 complex has been resolved into three distinct sub-complexes each of about [FORMULA] [FORMULA] H2 mass. More details on its physical properties are presented in Bania et al. (1986).

  • Clump 2 is confined near [FORMULA] and [FORMULA] and spans an extraordinary large velocity range of over 150 km s-1. It has been resolved into 16 emitting cores of [FORMULA] [FORMULA] by Stark & Bania (1986), who also interpret this clump as a dustlane or an inner spiral arm in a barred potential formed by molecular clouds distributed along the line of sight.

The CO is usually considered as a tracer of H2, although it is now well established that the density ratio between these two molecules is not homogeneous in galaxies. Beside the 2.6 mm 12CO [FORMULA] emission line, other more transparent molecular tracer like 12CO [FORMULA] (Oka et al. 1998a), 13CO and CS (Bally et al. 1987), as well as HCN (Jackson et al. 1996; Lee 1996), have also been used to map dense gas like in the Galactic centre region.

The interpretation of features in the [FORMULA] diagrams is subject to many difficulties. First, since only the radial component of the velocity is measured, the in-plane velocity field can be recovered directly only under very restrictive symmetry assumptions like axisymmetry. Second, some features may not trace real spiral arms but be artifact of velocity crowding along the line of sight (Burton 1973; Mulder & Liem 1986). Third, features elongated in the velocity direction can be understood either by real structures elongated along the line of sight or by spatially localised emission with a very large velocity spread, as would result for example in a supernova explosion or in violent shocks. Finally, some regions in the [FORMULA] space overlay the emission from several distinct sources, in particular the low velocity emission from the Galactic centre region is partly hidden by the emission from the surrounding disc spiral arms. This problem can however be addressed considering the latitudinal gas distribution or resorting to very dense gas tracers. Optically thick regions are also strongly affected by absorption.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999