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CO in NGC 1530, aperture synthesis of a barred spiral

D. Reynaud & D. Downes
e-mail: reynaud@iraux2.iram.fr

Institut de Radio Astronomie Millimétrique, Grenoble, FRANCE

Abstract:

NGC 1530 is one of the largest barred spiral galaxies in the northern sky, with bright CO emission. It offers a chance to understand the basics of the kinematics of barred spirals. To trace molecular gas, we observed the line, with the IRAM Interferometer. After calibration of the data, we obtained mosaic images, with a synthesized beam of 4 arcsec, covering the entire bar, i.e. about 1.5 arcmin, in velocity channels wide, with a good signal to noise ratio. These maps agree with predictions of barred potential simulations, especially the fact that the strongest emission arises from twin peaks oriented perpendicular to the large scale stellar bar. This phenomenon can be explained by the crowding of gas streamlines near the inner Lindblad resonance.

Contents

1. Introduction

Barred spirals have become interesting subjects of study with high resolution millimeter interferometers. The bar and the center of these galaxies is often rich in molecular gas, which is detectable at millimeter wavelengths. Gas may be concentrated in clouds, streaming towards the galactic center because of dissipative processes.

In the usual description of the dynamics of molecular gas in a barred potential, e.g. Combes (1988), the azimuthal part of the barred potential is described by a function. Then in a frame rotating with the potential, the periodic orbits are ellipses either parallel to the bar () or perpendicular (), depending on the radius of the orbit. The orientation ( or ) of ellipses changes by at radii corresponding to each Lindblad resonance. (A Lindblad resonance in a potential occurs at a radius where epicyclic and circular orbital periods are in a rational fraction, so that the orbits are closed in the frame rotating with the potential. The existence and location of these resonances are only functions of the galactic gravitational potential. Usually there are two inner Lindblad resonances (ILR) near the galactic center, a corotation close to the ends of the bar, and an outer Lindblad resonance (OLR) near the ends of the spiral arms.) Because of dissipative processes, gas streamlines following these elliptical orbits gradually change their orientation by between two resonances. A ring of stars or gas is often found at the inner Lindblad resonance(s). Orbits inside the ILR and outside may induce a torsion of the bar at the radius of the ILR.

2. Observations

We observed the molecular gas in the bar of NGC 1530, a barred SBb spiral in the line. We used the IRAM Interferometer on Plateau de Bure between 1993 October and 1994 March. Each of the four 15 m antennas had an SIS receiver with about 80 K noise temperature. The synthesized beam was . The primary beam of each antenna is at 115 GHz, so to cover the 1.5 arcmin bar of NGC 1530, we made a mosaic of five partially overlapping fields along the bar, resulting in a total coverage of . We smoothed to a velocity resolution of , which gives a reasonable signal-to-noise ratio. The maps had useful data over a total velocity range of . The deconvolution of the images was done with a CLEAN algorithm, especially adapted for mosaics.

3. Properties of NGC 1530 and observational results

3.1. Description of NGC 1530

  
Table 1: Characteristics of NGC 1530

NGC 1530, one of the largest and brightest barred galaxies in the northern sky, is relatively nearby (Table 1). Its inclination of to the plane of the sky allows us to observe the structures in the galactic disk and to get good radial velocity information. The bar is long and strong, indicating a substantial non-axisymmetric perturbation of the inner gravitational potential. Most of the CO emission comes from the nuclear region and from the bar, indicating that most of the gas in these parts of the galaxy is molecular.

images made with a beam with the IRAM 30m telescope show a bright feature near the nucleus, elongated perpendicular to the bar. The interferometer resolves this feature, as described below.

3.2. Selected CO maps and their interpretation

  
Figure 1: Central region contour maps, at (thick) and (thin)

  1. Twin peaks in the central region. The strongest emission in CO comes from a region near the nucleus, from two molecular clouds separated in space by (about 1 kpc) and in velocity by , corrected for inclination. These two clouds are displaces almost perpendicularly with respect to the bar. Table 2 lists the clouds' characteristics and Figure 1 shows a superposition of the channel maps in which the two clouds are most prominent.

    Such double features have already been detected in other SB galaxies with the Owens Valley millimeter interferometer (Kenney et al. (1992)). The twin peaks are interpreted as the traces of two shocks that form when gas streamlines cross the inner Lindblad resonance.

      
    Table 2: Characteristics of the two main central clouds

    For Table 2, linewidths and CO source sizes were measured at the half-power level. Cloud masses were estimated with the standard conversion factor of 5 ; e.g. Solomon & Barrett (1991).

      
    Figure 2: Extended bar contour map at

  2. The S shaped molecular bar. Figure 2 shows a map in the channel of the molecular gas along the bar. The gas extends along two long straight arms which twist into the central region into an S shape, and are joined by a central bridge at about to the bar. This torsion of the bar could be interpreted, as mentioned in the introduction, in terms of elliptical orbits at the ILR. Some optical band images show dust lanes parallel to this curved molecular bar. The northwest cloud in the map is near the corotation point.

4. Conclusions

  
Figure 3: NGC 1530 contour maps, between and )

NGC 1530 shows strong CO emission in its center and along the bar. Non-circular orbits may be the cause of the observed double peak structures near the nucleus. This double structure is observed in the place where rectilinear parts of the bar ( orbits) change over to perpendicular orbits. These orbits may be inside a ring traced in individual channel maps (Figure 3). These points seem to illustrate the strength of the non-axisymmetric perturbation in NGC 1530.

References



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Wed Feb 22 21:41:32 GMT 1995