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

Observations

The Mopra 22-m Telescope

The data presented here were obtained using the Mopra 22-m radio-telescope in New South Wales, Australia. It is located at a southern latitude (31 S) and 850 m above sea-level. The Mopra telescope consists of a 22 metre Cassegrain dish, identical to that used in the Australia Telescope Compact Array (ATCA) (see the special edition of the Journal for Electrical and Electronic Engineering, Australia IREE (1992) Vol 12, No. 2). The dish sits on a fixed base altitude-azimuth mount, unlike the moveable mounts of the ATCA. Only the central 15 m is surfaced for 115 GHz observing, providing a resolution of 45 (900 pc at 4 Mpc). The front end was connected to a receiver of the SiS type covering the 86 - 115 GHz range with a 600 MHz bandwidth. The receiver and system temperatures were 150 K and 280 K respectively. The correlator was used in a 1-bit mode, providing 512 channels over a bandwidth of 256 MHz. At the observing frequency of 115 GHz, this provides a velocity spread of 660 kms with a resolution of 1.6 kms . Table 2 summarises the system set-up used for the observations.

 
Table 2: System Specifications

Observing programme

The weather conditions during the week allocated for observing were quite poor (only 2 of the 7 days allocated were suitable for 115 GHz observing). The observing programme for these 2 days is summarised in Table 3. We used a position switching mode for observing as opposed to beam switching (unavailable at the time) and frequency switching (unsuitable for broad band sources). The pointing accuracy of the telescope was checked by observing the SiO maser source, W Hydra. We estimate a pointing accuracy of about 15 in both the altitude and azimuth axes for our observations. Figure 1 shows the 5 pointing positions superimposed on a = 20 cm radio continuum map of the Circinus galaxy.

 
Table 3: Observing programme

 
Figure 1: A total power radio continuum map of the Circinus galaxy at = 20 cm using the ATCA. The major axis of the galaxy runs from top left to bottom right. Contours are 1.41, 2, 2.82, 4, 5.7, 8, 11.3, 16 x 1 mJy/beam. The 5 pointing centres are marked as open circles (representing the Mopra radio-telescope beam size). The filled circle in the bottom left corner shows the ATCA beam.

To estimate the atmospheric opacity at the observing site, we measured total power at several different zenith angles and calculated the opacity by fitting an exponential to the data (opacities ranged from 0.25 - 0.27). For the purpose of amplitude calibration, the source M17SW was observed once each day.

Data reduction

The data were reduced using the Spectral Line Reduction Package (SPC). Baseline polynomials of order 1 were subtracted from individual quotient spectra (source spectrum divided by reference spectrum). The spectra were then binned so that a frequency resolution of 5 MHz (or 13 kms ) was acheived.

The amplitude data were calibrated to correct for the variation of atmospheric gain with elevation. Absolute amplitude calibration to the Rayleigh-Jeans main brightness temperature scale, T , was acheived by scaling the elevation adjusted fluxes to those of the CO calibrator, M17SW ( T = 40.4 K).

The velocity data were transformed to the local standard of rest (LSR) frame. Thus the spectra presented here show CO (1-0) emission over a velocity range of 50 - 750 kms with a resolution of 13 kms , with RMS noise levels ranging from 0.05 - 0.07 K.

© Copyright Astronomical Society of Australia 1997