Astron. Astrophys. 347, 37-46 (1999)

3. Observational results

3.1. Molecular maps

Fig. 1 shows the integrated ¹³CO(1-0) and ¹²CO(2-1) of the inner of NGC 1530. In the same figure are also displayed for comparison the integrated ¹²CO(1-0) and HCN(1-0) maps from RD97. The integration ranges are respectively (¹²CO(1-0)), (¹²CO(2-1)), (¹³CO(1-0)), and (HCN(1-0)). Each map is corrected for the primary beam attenuation. The primary beams (FWHM) are respectively (¹²CO(1-0)), (¹²CO(2-1)), (¹³CO(1-0)), and (HCN(1-0)). These maps are centered on the dynamical center of the galaxy, which has coordinates , (J2000) (from RD97), i.e. from the tracking center of the interferometer.

Fig. 1. False color maps of four velocity integrated transitions. a) upper left: 12CO(1-0). Labels indicate the regions described in Sect. 4.1. A sketch of the arcs and the nuclear feature (ring or unresolved spiral arms displayed as an ellipse) is also displayed (full black line). b) upper right: 12CO(2-1). c) lower left: 13CO(1-0). d) lower right: HCN(1-0). For each map the X sign indicates the position of the dynamical center, and the clean beam is indicated at lower left. Each map is corrected for the primary beam attenuation (see text). The primary beams (FWHM) are shown as red circles. The color scale is indicated at the right of the diagram. The minimum displayed flux are 0 for all transitions, except for 12CO(2-1) where it is 3 Jy beam. The maxima correspond to the value 1 on the color scale, and are 10.0 Jy beam-1 (12CO(1-0)), 25.0 (12CO(2-1)), 2.7 (13CO(1-0)) and 1.7 (HCN(1-0)) respectively. The bottom panel shows an optical image of the bar and a part of the spiral arms of NGC 1530 (Optical image NOAO). The red square indicates the region shown in the four integrated transitions.

The ¹²CO(2-1) map is truncated beyond a diameter of . Structures visible at the truncation limit are probably real molecular clouds deformed by the high noise level, since the noise is amplified by the primary beam correction. The two transitions of ¹²CO give similar maps, with two arcs, and inside them, a central structure which is a ring or unresolved nuclear spiral arms. These two maps have similar resolutions, and for ¹²CO(1-0) and ¹²CO(2-1) respectively. The ¹³CO(1-0) map is grossly similar to the ¹²CO maps, with a beam twice as large (). However the arcs seem dimmer in ¹³CO than in ¹²CO. There is a real difference between the brightness of the arcs and the brightness of the nuclear feature, a difference which had already been detected in HCN (see Fig. 1 and Fig. 3 of RD97). The difference is confirmed on the ratio map obtained by smoothing ¹²CO(1-0) to the resolution of ¹³CO(1-0) (Fig. 5).

Fig. 2 shows the channel maps of the ¹³CO(1-0) emission. Fig. 3 shows the channel maps of the ¹²CO(2-1) emission. These maps are not corrected for the primary beam attenuation. The kinematic pattern shown by these maps is the same as that found by RD97 in the ¹²CO(1-0) transition. That is, the kinematics of the gas in the arcs shows large () infall motions (due to the orbits along the bar) and in the central feature shows mainly circular rotation or weakly elliptical orbits (the orbits normal to the bar). Fig. 4 shows position-velocity diagrams in the CO(2-1) (left panel) and the CO(1-0) (right panel) transitions. These diagrams are cuts in the data cube, along the line of nodes passing through the dynamical center. The circular component is therefore the only component of the velocity field detected on these diagrams. The maximal radius of the emission is 1.4 kpc. The diagrams are very similar in the two transitions of CO, with a steep rising part in the central region and a flattening of the rotation curve at the crossing of the nuclear feature (incomplete ring or spiral within a diameter of the nucleus). Outside this region, the rotation curve is steep (see Fig. 4 at radii of ).

Fig. 2. 13CO(1-0) maps of the central of NGC 1530 in wide channels. Radial velocities (, upper left of each box) are relative to . The contour intervals are -6, -3, 3, 6, 9, 12, 21, 30, 39, 48 mJy beam-1 (mJy beam-1). The cross indicates the position of the tracking center of the interferometer (, ; J2000). The clean beam is shown in the lower right box.

Fig. 3. 12CO(2-1) maps of the central of NGC 1530 in wide channels. Radial velocities (, upper left of each box) are relative to . The contour intervals are -60, -30, 30, 60, 90, 120, 180, 240, 300 mJy beam-1 (mJy beam-1). The cross indicates the position of the phase tracking center of the interferometer. The clean beam is shown in the lower right box.

Fig. 4. Position velocity diagrams in 12CO(2-1) (left, contour levels 0.03 Jy beam-1) and 12CO(1-0) (right, contour levels 0.015 Jy beam-1). The horizontal coordinate is a distance offset (in arcsec) from the dynamical center along the line of nodes (p.a. 5o). The vertical coordinate is a radial velocity offset relative to 2470 km s-1.

3.2. Line ratios

For a quantitative analysis of the previous maps, we made maps of the ratios of the various integrated intensities, corrected for their respective primary beams. This correction makes the noise non-uniform through the ratio maps, especially in the ¹²CO(2-1) transition. Figs. 5 and 6 show the ratios ¹²CO(1-0)/¹³CO(1-0) and ¹²CO(2-1)/¹²CO(1-0) respectively. Each ratio map was made by smoothing the map with the higher resolution to that of the lower-resolution map.

Fig. 5. Ratio of integrated intensity 12CO(1-0)/13CO(1-0)) in the central of NGC 1530. The ratio was calculated with a threshold for each transition. The greyscale runs from 4 to 20 (white to black). The contour levels are from 5 to 20 by 3. Labels indicate levels 8 and 11. The beam is indicated by an ellipse in the lower left corner. The X sign indicates the position of the dynamical center.

Fig. 6. Ratio of integrated intensity 12CO(2-1)/12CO(1-0) in the central of NGC 1530. The ratio was calculated with a threshold for each transition. The greyscale runs from 0.3 to 1.2 (white to black). The contour levels are from 0.4 to 1.3 by 0.3. Labels indicate levels 0.7 and 1.0 (in white contour). The beam is indicated by an ellipse in the lower left corner. The X sign indicates the position of the dynamical center.

Ratio map CO(1-0)/¹³CO(1-0): The resolution is . ¹³CO(1-0) is detected in the same places as ¹²CO(1-0), so the ratio can be studied in the entire CO nuclear disk. The average value is . The ratio is about 6 to 8 in the central zone (inside the two CO arcs), with the lowest value () near the center of NGC 1530. The value is 11 to 15 in the arcs, with a maximum value of 15. The spatial distribution of dense gas ( cm^-3) is best shown in the HCN map (see Fig. 1). The ratio CO/HCN is 7 to 10 in the central ring of NGC 1530 (between the two arcs), while in the arcs this ratio is larger, in the range 14 to 30. The ¹²CO(1-0)/¹³CO(1-0) ratio thus seems to have the same characteristics as the CO/HCN ratio.

Ratio map CO(2-1)/¹²CO(1-0): The resolution is . The ratio can be studied in the entire disk with a high signal-to-noise ratio. The average ratio is . The ratio is over a large region wide, with the dynamical center of the galaxy on the eastern edge of this region (see Fig. 6). The maximum value is 1.2, at a position west of the dynamical center. Between the two arcs, the ratio is generally . In the northern arc, the ratio is 0.4 to 0.7 while in the southern arc it is 0.5 to 1.1.

3.3. Radio continuum maps

NGC 1530 was observed with the Very Large Array ¹ (VLA) in a snapshot mode. Saikia et al. (1994) show the 20 cm emission map made with uniform weighting. Fig. 7 shows the same data, at 20 and 6 cm, in maps obtained with natural weighting, which allows maximum sensitivity. Superposed on the cm maps are a few contours of the ¹²CO(2-1) distribution. The beam sizes are (p.a. ) at 20 cm and (p.a. ) at 6 cm, both similar to the beam at CO(2-1). The emission peaks are at 20 cm and at 6 cm, so the detected features are not prominent in the maps. The 20 cm distribution has two types of emission: a weak () emission over most of the molecular disk, and a number of compact components, unresolved by the interferometer. These components are mainly distributed along a ring, around a central cavity. The radius of this ring is about , i.e. 500 pc. This ring coincides with the one obtained in the ¹²CO(2-1) and ¹²CO(1-0) lines. A strong component is detected south of the dynamical center. It coincides well with a CO compact component. The 6 cm map shows with a lower signal-to-noise ratio the same distribution as the 20 cm map, i.e. a strong ring distribution.

Fig. 7. Left: 12CO(2-1) superposed on a grey scale map of the radio continuum at cm. The grey scale runs from to 1.0 mJy beam-1). Right: 12CO(2-1) superposed on a grey scale map of the radio continuum at cm. The grey scale runs from to 0.42 mJy beam-1. The contours show the 6, 14 (black) and 18 mJy beam-1 (white) levels of the integrated intensity of 12CO(2-1). The cm-radio continuum lobes are indicated by ellipses in the lower left corners. The dynamical center is indicated by a X sign. The cm-radio continuum maps are partially presented in Saikia et al. (1994), and were made kindly available by Dr. A. Pedlar.

For the central of NGC 1530, integrated fluxes are 30 mJy at 20 cm and 8.6 mJy at 6 cm (integration threshold: ). The galaxy emits a total of 80 mJy at 20 cm and 37 mJy at 6 cm (Wunderlich et al. 1987, Condon et al. 1996). Thus and are emitted in the central (i.e. 5 kpc along the major axis) of NGC 1530. The 500 pc ring shares about of the central centimeter continuum emission, which is more than the ¹²CO(1-0) share of this ring, about 1/3 (RD97f). From the fluxes at 20 cm and 6 cm, we computed a spectral index of -0.93, which indicates that the synchrotron emission is predominant in these maps. There is a high star formation rate in the central part of NGC 1530, giving rise to radio continuum emission via synchrotron emission from supernova remnants. These supernova remnants give rise to most of the compact sources in the cm maps.

© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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