A velocity field of interstellar gas in spiral galaxies is not only a good indicator of mass distribution and structural properties of their disks, but can also reveal perturbations in diffuse matter related to local sources of energy. Non-circular motions of gas can locally trigger an active star formation, and, in turn, may be the result of the collective action of massive young stars on a short time scale.
Actually, a model of circular gas motion in the disk of any galaxy is no more than a first approximation to the real kinematic picture. Apart from the evident cases of tidal forces or of an active nuclear region (which will not be considered here), responsible for peculiar gas motions, the most common reason of deviation from circular motion is a presence of a spiral density wave or a bar. In these cases, non-circular velocity components have well-ordered systematic character related with the optically observed structure.
However in some galaxies local velocity perturbations exceeding have been discovered, which cover large regions from a few hundred pc up to a few kpc size. Good examples are two regions observed in M 101 in the HI line (van der Hulst & Sancisi 1988) which show peculiar velocity components reaching , as well as giant HI supershells expanding with velocities of in NGC 4631 (Rand & van der Hulst 1993), NGC 1313 (Ryder et al. 1995) and IC 2574 (Walter et al. 1998). The typical kinetic energy of perturbed gas motions in these cases is about ergs, and their connection with sites of star formation is obvious. Although in different galaxies the nature of local velocity peculiarities may not be the same, there are two ways to explain it: local bursts of star formation (stellar winds, explosions of supernova or hypernova) or accretion of intergalactic gas clouds and dwarf gas-rich galaxies (see references and discussion in Rand & van der Hulst 1993).
In this paper we describe the discovery of an extended region of unusual strong non-circular motion of gas in the spiral galaxy NGC 1084 from optical observations in the and [NII] emission lines.
NGC 1084 is a late-type spiral galaxy classified as SA(s)c in the Reference catalog of bright galaxies (RC3). The distance adopted in this paper is 18.5 Mpc (). At first glance, NGC 1084 is a normal galaxy with mildly inclined disk, a regular two-armed grand design spiral structure, and without close optical companions or any morphological peculiarities. The rotation of the gas in this galaxy was measured on several occasions. Burbidge et al. (1963) obtained three long-slit spectra in the spectral range near , making two cuts along the major axis and one cut along the minor axis. Yet, the accuracy of their measurements was low, and the obtained velocity curve was unreliable. Kyazumov (1981) has performed a long-slit study of NGC 1084. He has obtained improved line-of-sight velocity distributions, although the shape of the rotation curve remains uncertain. The maximum rotation velocity of which he finds, has been confirmed later by Afanasiev et al. (1988). These authors have obtained long-slit spectroscopy with a digital detector (IPCS ) at the 6m telescope. The rotation curve of the ionized gas has been of a higher accuracy than previous determination. In particular, it is found that the velocity curve reaches its maximum very close to the center - at a radius of . Two long-slit cross-sections - along the major axis and under an angle of to it - indicate flat velocity distributions up to from the center. Besides, the higher spatial resolution enables them to detect some non-circular phenomena. Firstly, in the central region () a difference in velocities measured from and from the forbidden emission lines ([NII] and [SII]) is found. The authors have interpreted it as a possible signature of two differently rotating gaseous systems, where non-circular velocities associated to the forbidden lines would be caused by a low-contrast nuclear minibar. Secondly, at an extended region located to the N has been localized, which shows a negative excess of line-of-sight velocity up to . No explanation is proposed for this feature.
In this paper, new observations of NGC 1084 with a Fabry-Perot interferometer at the SAO 6m telescope are presented. The main goal of the observations was the study of the velocity field of the ionized gas in the galaxy as a whole. We focus on investigating strong non-circular gas motions in the northern part of the galaxy. The analysis of gas grand-design motions in the spiral structure will be given in a forthcoming papers.
The paper is structured as follow: In the next section (Sect. 2), we describe the observations and data reduction; the ionized gas kinematics is described in Sect. 3; possible explanations of nom-circular gaseous motions are discussed in Sect. 4; conclusions are drawn in Sect. 5.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000