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Astron. Astrophys. 343, 367-372 (1999)

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2. Millimetre and submillimetre observations with SEST

Observations of NGC 1365 were carried out in August 1995 using the 15-m Swedish ESO-Submillimetre Telescope (SEST) on La Silla in Chile. During the first four nights of the observing run, the atmospheric conditions were excellent, permitting submillimetre observations of the [FORMULA] CO line. The weather then deteriorated somewhat, forcing a change to the millimetre wavelength region. The telescope properties at the observed frequencies are presented in Table 1.


Table 1. SEST parameters and observational results

A dual beam switch mode, with a beam separation of 11:06, was used placing the source alternatively in the two beams to eliminate asymmetries in the signal paths. Three different SIS receivers were used in conjunction with two low-resolution acousto-optical spectrometers, each with a total bandwidth of about 1 GHz (channel resolution of 1.4 MHz). The average receiver temperatures ([FORMULA]) are given in Table 1, as are the channel velocity resolutions after smoothing operations. Only linear baseline subtractions were performed on the profiles. All profile temperatures have been converted to main beam brightness temperatures [FORMULA] by dividing the antenna temperatures [FORMULA] by the respective main beam efficiencies ([FORMULA]. The velocities are heliocentric radial velocities.

Four new molecular species have been detected in NGC 1365, namely HCN, HCO+, H2CO and CS. These four profiles are presented in Fig. 1 together with that of [FORMULA] 13CO. All five species are tracers of high density gas. The integrated main beam brightness line intensities, [FORMULA], are HCN: 4.91, HCO+: 4.25, 13CO: 7.61, H2CO: 0.93 and CS: 2.21 K km s-1. The [FORMULA] rms noise levels of the profiles are of the order of 0.002 K. Submillimetre [FORMULA] CO line profiles observed towards three positions in the centre of NGC 1365 are also presented in Fig. 1; the equatorial offsets from the optical nucleus for these observations are given in square brackets. The total integrated CO line intensities, [FORMULA], are 276 and 277 K km s-1 at the southwest and northeast maxima, respectively; at the centre, this value is 236 K km s-1.

[FIGURE] Fig. 1. Left : Millimetre molecular line profiles observed towards the central position in NGC 1365. Right : Submillimetre [FORMULA] 12CO molecular line profiles observed towards three positions in the centre of NGC 1365. The equatorial offsets from the optical nucleus (inside square brackets) are given in arcseconds

The mapping in the [FORMULA] CO line was done over an approximately [FORMULA] region, centered on the optical nucleus [FORMULA] and covering the bar region. A total number of 133 positions were observed. A grid spacing of [FORMULA] was used for the outer parts of this region. For the inner part (approximately [FORMULA]) a grid spacing of [FORMULA] was used, i.e. a sampling rate of three points per HPBW. Great care was applied to frequent pointing checks. This included using the central profile of NGC 1365 as a pointing check, since an error of a few arcseconds would be immediately noticeable in the relative amplitudes of the two main components in the central profile. Furthermore, observations of the inner part were made only during night time, after midnight and before sunrise, which is the time of maximum atmospheric stability. In addition, observations were only made at elevations between [FORMULA] and [FORMULA] in order to minimize beam distortion and maximize aperture efficiency. A profile map including all the [FORMULA] CO observations is presented in Fig. 2.

[FIGURE] Fig. 2. [FORMULA] CO profile map of NGC 1365. The equatorial offsets are in units of arcseconds and are measured from the optical nucleus

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999