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Astron. Astrophys. 363, 843-850 (2000)

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4. Discussion

As shown above, there exists at least two different systems of emitting gas in the "spur" of NGC 1084. One of the gaseous systems shows nearly circular rotation, while the velocities of the other one differ by about [FORMULA] from those expected in the case of non-perturbed rotation, and in most regions the difference is positive.

The common explanation of double-peaked emission lines in star-forming galaxies, namely an expanding superbubble, is inapplicable in this particular case because the regions with velocity anomalies have an unusual location in the sky plane. As shown in the Sect. 2.2 the SE half of the galaxy disk is the nearest to the observer. In this case the region of redshifted residual velocities lies on the nearest side while the region of blueshifted velocities lies on the far side of the disk. This would rather suggest a shrinking superbuble, which is improbable.

If a sound velocity in the gas clouds of the "spur" is close to its mean value for the ISM in outer disk regions of spiral galaxies (solar neighbourhood as example), [FORMULA], then, the velocity of perturbed motions exceeds strongly the sound velocity (of neutral hydrogen). The observed enhancement of the [NII] emission line and the lower velocity difference measured from the [NII] profiles may be naturally explained if one considers the strong [NII] emission line as emitted by shock-excited gas slowed down by collisions with the unperturbed medium.

Such fast-moving gas cannot be retained by the galactic plane and must fill a volume with some filling factor, so there could be different gas velocities along the line-of-sight. This accounts for the complex shape of emission-line profiles on a scale of 200 pc (a pixel of 2"). So it is not surprising that the residual velocity field appears complex being projected onto the galactic plane. Bright HII regions, connected with massive gas clouds have quite normal velocities. The perturbed component, if it exists there, might be hidden by the bright background.

There are at least two possible interpretations of the observed velocity peculiarities.

One is an infall of high latitude gas clouds (intergalactic clouds or clouds expelled from the disk earlier) onto the galactic disk. Massive starforming clouds in the disk are too heavy to be pushed by gas inflow, so their velocities remain circular as observed, although the interaction with infalling gas may trigger star formation there. A similar phenomenon on smaller scales is occurring in our Galaxy, where high-velocity clouds (HVCs) are observed far from the galactic plane. Their infall onto the disk is confirmed by the detection of X-ray emission from heated gas spots (Kerp et al. 1994; Kerp et al. 1999). The existence of gas off the galactic plane has also been noticed in external spiral galaxies, for example in NGC 891 (Swaters et al. 1997).

However, invoking the HVCs explanation is in contradiction with the measure of the current star formation rate (SFR) in NGC 1084, as deduced from the integrated intensity of [FORMULA]: by using the total [FORMULA] flux emission from Kennicutt & Kent (1983) and [FORMULA] one gets [FORMULA] (absorption [FORMULA] was accepted to be [FORMULA]). The model dependence between [FORMULA] and the SFR (Kennicutt 1983) gives [FORMULA] for the stellar mass interval [FORMULA]. This value is normal (or mildly enhanced) for a late-type galaxy. The "spur" luminosity fraction is about of 18% of the total [FORMULA] flux, and corresponds to as [FORMULA] in the "spur" region. On the other hand, the present-day SFR may be more intense in this galaxy: three supernovae have been detected during the last 40 years. These are SN 1963P, SN 1996an and SN 1998dl. The latter two are Type II supernovae (Nakano 1996; Filippenko 1998) connected with recent star formation. The location of both SNe II are marked by white crosses in Fig. 2a. So it cannot be excluded that the observed off-disk gas may be expelled during the short and intense burst of starformation which took place in this region of the galaxy.

Moreover the central parts of the HII regions in the "spur" has a very blue color (the color index [FORMULA]). This may indicate a large fraction of young OB stars (see Fig. 3). The red features on the E-side of the "spur" (Region A) with [FORMULA]) can be explained by strong dust absorption due to the shock waves in the "spur" region.

The second possible interpretation is an interaction with a gas-rich dwarf galaxy accompanied by tidal disruptive merging. Indeed, on the opposite side of the galaxy, at [FORMULA] to the S of the nucleus, there is a small "island" of [FORMULA] emission (Fig. 2a). It does not distinguish itself dynamically, but the radio map in the non-thermal continuum at 1.49 GHz obtained at the VLA (Condon 1987) shows that a long radio tail begins here which connects NGC 1084 to another radio source located [FORMULA] (about two optical diameters) from the galaxy. As no HI map at 21 cm is available for NGC 1084, one cannot confirm whether this radio tail contains some expelled gas. But the configuration resembles a tidal tail as usually developing on the opposite side of a galaxy colliding with another one. Therefore, the gas flow twisted in the northern half of NGC 1084 might be accretion; if the initial rotation momentum of this gas is nearly orthogonal to the rotation momentum of NGC 1084, the gaseous flow could look like an off-center polar ring. This hypothesis would explain easily the different signs of velocity anomalies in the NW and SE ends of the "spur": we would be seeing the receding and approaching parts of the rotating "polar ring". Such a configuration is short-lived because all the gas must fall towards the center of the galaxy in some [FORMULA] years, but this time is not too short to prevent its detection.

To clarify further the possible mechanisms responsible for the peculiar velocity field of the "spur", high-resolution observations of neutral and molecular gas distributions and two-dimensional spectral investigations in various forbidden optical emission lines are required.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000