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Astron. Astrophys. 325, 124-134 (1997)

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3. Molecular gas in NGC 5907

3.1. Observational results and kinematics

The spectra along the major axis of the galaxy obtained during the molecular line observations are shown in Fig. 1. The maximum peak temperatures of [FORMULA] K and [FORMULA] K for the [FORMULA] and the [FORMULA] line respectively are reached near the center. CO is detected up to radii of more than [FORMULA] ([FORMULA]).

[FIGURE] Fig. 1. Maps of the observed [FORMULA] (left) and [FORMULA] (right) major axis spectra of NGC 5907, smoothed to a velocity resolution of 10.4 km/s. The scale of the spectra is indicated by the small box at the bottom. Offsets are along the major axis, north is negative

A striking feature of many of the spectra ([FORMULA]) is that the observed lines contain at least two components, and that the central spectrum shows a clear asymmetry. A natural explanation for the latter is that the location of the dynamical centre of the galaxy is not at [FORMULA] (the centre of the [FORMULA] emission), but shifted by a few arcseconds to the northwest. This idea is also supported by the small asymmetry visible in the position-velocity diagrams (Fig. 2).

[FIGURE] Fig. 2. Position-velocity diagram of the [FORMULA] (left) and the [FORMULA] (right) observations parallel to the major axis at [FORMULA]. Contour levels are -0.04 (dashed), 0.04, 0.08, 0.12, ..., 0.4 K for both transitions. The velocity resolution is 20 km/s. The rms noise is variable along the major axis with a typical value of about 30 mK for both transitions. The thick lines indicate the rotation curve as described in the text

García-Burillo et al. (1997), who observed the central region of NGC 5907 using the Plateau de Bure interferometer, found the dynamical centre of the galaxy at [FORMULA], [FORMULA]. This corresponds to [FORMULA] in our coordinates. They also found a small offset between the dynamical centre of the galaxy and the position of the maximum CO intensity, which is in agreement with our results.

The kinematics of NGC 5907 can basically be described by rigid rotation up to a radius of [FORMULA] (which corresponds to [FORMULA]), followed by differential rotation with a rotational velocity of [FORMULA]. These values are relative to the systemic velocity of [FORMULA] and the dynamical centre given in Tab. 1. The rotation curve follows from this work and the HI data (Casertano 1983) and is plotted as a thick line on the position-velocity diagrams (Fig. 2). There are, however, a few deviations from this simple behaviour. A second line component is visible in the spectra near the central region (at [FORMULA]). Here the rigid ("normal") rotation of the inner disk is accompanied by a high-velocity wing, with a much larger velocity gradient. This component is visible in the individual spectra as well as in the p -v -diagrams, and leads to a total line width of about 350 km/s at the assumed central position. It most probably results from non-circular motions due to a bar, as already suggested by García-Burillo & Guélin (1995) and García-Burillo et al. (1997).

3.2. Radial gas distribution

The observed major axis distribution of the CO intensity is the sum of the radial distribution and a projection effect (by which the emission at several radii is projected onto a certain position on the major axis). In order to deproject this distribution, we assumed a radial model function, consisting of a central Gaussian peak and two Gaussian rings, and fitted the resulting major axis distribution to the data. The existence of two ring-like structures is suggested by two intensity maxima (at [FORMULA] and [FORMULA]) on either side of the centre in the p -v -diagrams. Additionally this distribution with two rings fits the data better than a distribution with just one ring (at [FORMULA]).

The result can be seen in Fig. 3. The best fit was obtained for ring radii of [FORMULA] and [FORMULA] and widths (FWHM) of [FORMULA], [FORMULA], and [FORMULA] for the central peak and the inner and outer ring respectively. However, the outer "ring" may in fact be spiral arms seen more or less tangentially.

[FIGURE] Fig. 3. a Observed (dashed line and filled black circles) and modelled (solid line) [FORMULA] intensity distribution along the major axis of NGC 5907. The modelled distribution is obtained by a least-squares-fit and results from the line-of-sight-integrated radial distribution shown in b. b Adopted radial CO profile which leads to the major axis distribution as shown in a
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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998