CO J = 10 observations of the Circinus galaxy using the Mopra 22-m radio-telescope

Results

Figure 2 displays the spectra of the observed CO (1-0) emission and Table 4 summarises the results of our observations. The spectrum observed at the centre of the Circinus galaxy shows CO (1-0) has been detected with a signal to noise ratio (S/N) of about 10. The spectrum shows CO emission over a 400 kms range, beyond which the emission ceases sharply. The maximum intensity of CO from the central position is T = 0.62 K, the average intensity being T = 0.47 K. This spectrum compares well with that observed in 1988 using the SEST (see Figure 5 in Israel 1992).

 
Figure 2: The Circinus galaxy CO spectra observed at the 5 pointing centres. Pointings 1 to 5 from top to bottom.

 
Table 4: Spectra statistics

The full width at half maximum (FWHM) of the central CO spectrum is 320 15 kms and is centred on a systemic LSR velocity of kms (or a heliocentric velocity of =435 15 kms ). We note that this agrees well with the value determined using a HI study performed by Freeman et al (1977). The full width at zero level of the central spectrum is 445 15 kms .

The emission observed at points 2 and 3 peak at T = 0.39 K and T = 0.44 K respectively. However, the velocities of the emission peaks vary markedly from 535 kms for point 2 to 290 kms for point 3. The points are located on the major axis of the galaxy and so this velocity differential is probably the result of solid body rotation of the central bulge.

Points 4 and 5 were chosen as observing centres based on the published radio data of Elmouttie et al (1995). These points are located near the minor axis of the galaxy, along the directions of the previously detected anomalous radio lobes. The emission at point 5 shows weak, broadband emission of similar velocity distribution to the central point (200 - 600 kms ). This probably indicates that this point is near outer edge of the galactic disk. No broad band emission is seen at point 4, which is slightly further away from the nucleus than point 5 and probably away from the disk emission. However, the emission at point 4 shows a weak peak at around 565 kms and the possible significance of this emission feature will be discussed later.

Fitting a gaussian to the data suggests the intensity profile of the nuclear CO emission has a full width at half maximum (FWHM) of 53 arc, the peak emission being within 20 of the optical centre (see Table 1). Deconvolving the 45 primary beam from this profile yields a source FWHM of about 28 or 550 pc at a distance of 4 Mpc. This can be compared with the dimensions of the radio nuclear bulge which measures about 15 or 290 pc (Elmouttie et al, 1995). We clearly lack the resolution required to determine whether a molecular counterpart which more closely resembles the radio source exists at the centre of the Circinus galaxy, as seen in other galaxies (Dahlem et al, 1993; Adler et al, 1991).

 
Table 5: Summary of results

© Copyright Astronomical Society of Australia 1997