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CO in NGC 1530, aperture synthesis of a
barred spiral
By D. R e yn a ud & D. D o wn e s
e­mail: reynaud@iraux2.iram.fr
Institut de Radio Astronomie Millim'etrique, Grenoble, FRANCE
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 COJ = 1 ! 0 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 20 km s \Gamma1 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.
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 cos 2` func­
tion. Then in a frame rotating with the potential, the periodic orbits are ellipses either
parallel to the bar (x 1 ) or perpendicular (x 2 ), depending on the radius of the orbit. The
orientation (x 1 or x 2 ) of ellipses changes by 90 ffi 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 Lind­
blad 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 90 ffi between two resonances. A ring of stars or gas is often found at
the inner Lindblad resonance(s). Orbits x 2 inside the ILR and x 1 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
COJ = 1 ! 0 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 3:5 00 \Theta 3:9 00 . The primary
beam of each antenna is 43 00 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 105 00 \Theta 45 00 . We smoothed to a velocity resolution of 20 km s \Gamma1 , which gives
1

2 D. Reynaud & D. Downes: CO in NGC 1530
Right Ascension (J2000) 04 h 23 m 27:3 s
Declination (J2000) +75 ffi 17 0 45 00
Distance (for H0 = 75 km s \Gamma1 Mpc \Gamma1 ) (Mpc) 32
Diameter (arcmin) 4.8
Optical Bar length (arcmin) 1.5
Inclination (degrees) 60
Major Axis Position Angle (degrees) 19
Velocity­integrated intensity measured with
the IRAM 30m telescope, L 0
CO (10 8 K km s \Gamma1 pc 2 ) 23
Table 1. Characteristics of NGC 1530
a reasonable signal­to­noise ratio. The maps had useful data over a total velocity range
of 360 km s \Gamma1 . 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
NGC 1530, one of the largest and brightest barred galaxies in the northern sky, is relat­
ively nearby (Table 1). Its inclination of 60 ffi 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 non­axisymmetric 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.
COJ = 2 ! 1 images made with a 13 00 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
(a) 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 6 00 (about 1 kpc)
and in velocity by 230 km s \Gamma1 , 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.
For Table 2, linewidths and CO source sizes were measured at the half­power level. Cloud
masses were estimated with the standard conversion factor of 5 M fi (K km s \Gamma1 pc 2 ) \Gamma1 ;
e.g. Solomon & Barrett (1991).
(b) The S shaped molecular bar. Figure 2 shows a map in the +40 km s \Gamma1 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 45 ffi 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 B band images show

D. Reynaud & D. Downes: CO in NGC 1530 3
Figure 1. Central region contour maps, at \Gamma40 km s \Gamma1 (thick) and +160 km s \Gamma1 (thin)
North Cloud South Cloud
Velocity (km s \Gamma1 ) relative to 2410 km s \Gamma1 \Gamma40 +160
R.A. (J2000) 04 h 23 m 26:7 s 04 h 23 m 26:8 s
Dec. (J2000) +75 ffi 17 0 47 00 :5 +75 ffi 17 0 39 00 :8
Angular Size (arcsec 2 ) 9:5 \Theta 5:3 6:6 \Theta 5:3
Linewidth (km s \Gamma1 ) 60 40
Peak flux (mJy beam \Gamma1 ) 315 426
Peak temperature (K) 2.2 3.0
L 0
CO (K km s \Gamma1 pc 2 ) 1:7 10 8 1:1 10 8
Mass (M fi ) 8:7 10 8 5:7 10 8
Table 2. Characteristics of the two main central clouds
dust lanes parallel to this curved molecular bar. The northwest cloud in the map is near
the corotation point.
4. Conclusions
NGC 1530 shows strong CO emission in its center and along the bar. Non­circular
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 (x 1
orbits)
change over to perpendicular x 2
orbits. These orbits may be inside a ring traced in
individual channel maps (Figure 3). These points seem to illustrate the strength of the
non­axisymmetric perturbation in NGC 1530.
REFERENCES
Kenney J.D.P., Wilson C.D., Scoville N.Z., 1992, ApJ, 395, L79.

4 D. Reynaud & D. Downes: CO in NGC 1530
Figure 2. Extended bar contour map at +40 km s \Gamma1
Figure 3. NGC 1530 contour maps, between \Gamma340 and +340 km s \Gamma1 )
Combes F., 1988, in Galactic and Extragalactic Star Formation, eds R.E. Pudritz & M. Fich
p.475, (Kluwer, Dordrecht).
Solomon P.M. & Barrett J.W., 1991, in Dynamics of Galaxies and their Molecular Cloud Distri­
butions (IAU Symposium 146), eds Combes F. & F. Casoli F., p.235, (Kluwer, Dordrecht).