3. Observations & results

The observations were made during the period 1996 May 21 to May 26 using the 15m diameter SEST telescope, situated at the European Southern Observatory Cerro La Silla site (altitude 2300m). A review of the SEST and its capabilities can be found in Booth et al. (1989). We observed using the 2mm and 3mm SIS receivers simultaneously (single sideband), each coupled to an acousto-optical spectrometer (LR1 and LR2 respectively). A description of these spectrometers can be found in Schieder et al. (1989). The 2mm/LR1 configuration produced 1440 channels (694kHz separation), centered on 146.969GHz (the rest frequency of the CS(3-2) transition). The 3mm/LR2 configuration produced 1600 channels (681kHz spacing), centered on 89.188GHz (rest frequency of transition). These frequencies were chosen to allow us to search for Galactic CS or HCO+ absorption towards any strongly-detected sources. The receiver temperatures were typically . The primary beam of the SEST telescope has a full-width at half-maximum (FWHM) of 33  and 55  at 2 and 3mm respectively.

Calibration of the system was performed using the chopper-wheel method (Ulich 1980), which automatically corrects for atmospheric opacity effects. To convert the measured antenna temperatures into flux densities, standard sensitivities of 30 Jy/K (2mm) and 25 Jy/K (3mm) were assumed; these values agreed to within 20% and 10% (respectively) with values determined from a single low-elevation observation of Saturn during the run. One 30minute observation of each source was performed, consisting of 30 cycles of a dual beam-switch mode, where the target is first placed in a signal beam from which a reference beam is subtracted at 6Hz, then in the reference beam (to remove any residuals due to differences in gain between the beams). The sky separation of the signal and reference beams was . Each half of the cycle produces a spectrum which is the sum of 10s of integration on source; these are averaged to produce a flat baseline, and the spectral channels averaged (channels 100-1300 for LR1, 100-1500 LR2). The resultant observing bandwidths were 834 (2mm) and 953 MHz (3mm). The 30 beamswitch cycles (consisting of approximately 10min integration on source) were then averaged; a error was estimated as the standard error of the mean of the individual cycle results. One source (0454-810) was observed on five separate occasions, with consistent results; the value given in Table 1 (click here) is the observation with the lowest noise. The zenith opacities at 2mm ranged from 0.2 to <0.02, with a median value of . The majority of sources were observed at elevations greater than , therefore the small corrections for variations in the telescope gain with elevation at 2 and 3mm have not been applied.

In Tables 1 (click here) and 2 (click here), the results of our observations for the Parkes and USNO samples are presented. In Col. 1, the J2000 source name is given (B1950 names are used in Table 2 (click here)). Columns 2 and 3 are the source type and redshift (where known) from the NASA/IPAC Extragalactic Database (Q = quasar, G = galaxy, B = BL Lac), Cols. 4 and 5 are the observed J2000 coordinates, Cols. 6 and 7 are the 2mm flux density and error (), and Cols. 8 and 9 are the 3mm flux density and error (); all flux densities are given in Jy. Column 10 contains character codes indicating cross-references between the Parkes (P), USNO (U) and (T) Tornikoski et al. (1996) samples. Flux densities preceded by < indicate non-detections at a level. Our overall detection rate for the combined sample is 66% above typical limits of ; 37 of 47 sources in the Parkes sample were detected at at least one frequency, and 15 of 32 sources in the USNO sample. We have chosen as a conservative detection limit for these observations, although given the variations in the noise level due to changing atmospheric conditions, it is likely we actually detected sources at lower significance levels, e.g. . Inspection of the final summed spectra for each detected source showed no unambiguous examples of Galactic CS or HCO+ absorption.

 

Source Type z RA(J2000) Dec(J2000) ID

J0004-4736 Q 0 4 35.65 -47 36 19.6 < 0.93 0.22 1.40 0.21 P

J0051-4226 Q 1.74 0 51 9.50 -42 26 33.2 < 0.18 0.07 0.38 0.06 P

J0106-4034 Q 0.58 1 6 45.10 -40 34 19.9 2.06 0.14 3.63 0.10 PU

J0210-5101 Q 1.00 2 10 46.38 -51 1 1.0 2.65 0.13 3.53 0.07 PTU

J0245-4459 Q 0.28 2 45 54.12 -44 59 39.7 0.49 0.09 0.45 0.05 P

J0246-4651 Q 2 46 0.00 -46 51 16.0 < 0.23 0.13 0.62 0.06 P

J0253-5441 Q 0.53 2 53 29.15 -54 41 51.3 0.63 0.12 0.94 0.07 PU

J0303-6211 Q 3 3 50.58 -62 11 25.0 0.60 0.09 0.83 0.10 PU

J0309-6058 Q 3 9 56.03 -60 58 38.6 < 0.46 0.13 0.88 0.08 PU

J0311-7651 Q 0.22 3 11 55.50 -76 51 50.1 < 0.29 0.13 < 0.37 0.12 PU

J0455-4616 Q 0.85 4 55 51.11 -46 16 4.0 0.79 0.07 1.32 0.06 PT

J0506-6109 Q 1.09 5 6 43.90 -61 9 41.1 0.81 0.13 1.19 0.10 PTU

J0515-4556 Q 0.19 5 15 45.27 -45 56 42.8 < 0.36 0.09 0.32 0.05 P

J0522-6107 Q 1.40 5 22 34.27 -61 7 58.5 < 0.43 0.10 0.84 0.11 PU

J0525-4557 Q 1.47 5 25 31.40 -45 57 54.7 < 0.20 0.12 0.31 0.06 P

J0538-4405 Q 0.89 5 38 50.36 -44 5 8.9 3.19 0.12 4.58 0.05 PTU

J0635-7516 Q 0.15 6 35 46.54 -75 16 16.8 1.01 0.10 1.76 0.16 PTU

J0743-6726 Q 1.51 7 43 31.52 -67 26 26.0 <-0.13 0.09 < 0.31 0.08 PTU

J0757-7353 7 57 14.13 -73 53 9.5 < 0.03 0.11 <-0.01 0.13 P

J0904-5735 Q 0.69 9 4 53.21 -57 35 3.6 < 0.30 0.10 < 0.20 0.10 P

J1041-4739 10 41 44.66 -47 39 59.6 < 0.22 0.10 0.32 0.06

J1058-8003 Q 10 58 43.69 -80 3 54.3 0.41 0.07 1.05 0.10 PU

J1103-5356 Q 11 3 52.27 -53 56 59.8 0.47 0.07 1.01 0.09 PU

J1107-4449 Q 1.59 11 7 8.69 -44 49 7.6 0.79 0.10 1.58 0.07 PU

J1118-4634 Q 0.71 11 18 26.92 -46 34 15.0 < 0.29 0.09 < 0.30 0.11 PU

J1147-6753 Q 11 47 33.69 -67 53 41.8 1.67 0.06 2.62 0.07 P

J1224-8313 12 24 54.49 -83 13 10.4 < 0.49 0.12 0.62 0.10 P

J1255-7138 Q 12 54 59.99 -71 38 20.5 < 0.15 0.08 0.45 0.08 PU

J1424-6807 Q 14 24 55.65 -68 7 59.2 0.48 0.07 0.75 0.09 P

J1427-4206 Q 1.52 14 27 56.28 -42 6 18.5 3.34 0.09 4.83 0.12 PTU

J1454-4012 Q 1.81 14 54 32.89 -40 12 31.7 0.49 0.08 0.70 0.14 PU

J1624-6809 Q 1.36 16 24 18.56 -68 9 12.8 < 0.05 0.07 < 0.08 0.12 PU

J1723-6500 G 0.01 17 23 40.92 -65 0 35.9 < 0.34 0.10 0.61 0.06 PU

J1744-5144 G 17 44 25.41 -51 44 43.9 < 0.10 0.20 < 0.20 0.04 P

J1803-6507 G 18 3 23.86 -65 7 39.4 <-0.13 0.09 0.50 0.05 PU

J1809-4552 18 9 57.80 -45 52 41.0 < 0.94 0.20 1.67 0.18 P

J1819-5521 Q 18 19 45.44 -55 21 21.4 < 0.26 0.05 0.32 0.06 PU

J1837-7108 Q 1.35 18 37 28.78 -71 8 41.4 0.73 0.07 1.17 0.10 PU

J1932-4536 Q 0.65 19 32 44.90 -45 36 37.8 < 0.24 0.07 0.42 0.04 P

J1937-3958 Q 0.96 19 37 16.21 -39 58 1.5 0.77 0.06 < 0.39 0.17 PT

J2009-4849 Q 0.07 20 9 25.40 -48 49 53.7 < 0.41 0.10 0.52 0.10 PTU

J2207-5346 Q 1.20 22 7 43.73 -53 46 33.8 <-0.04 0.21 < 0.21 0.18 PTU

J2229-4051 Q 0.44 22 29 18.61 -40 51 31.7 < 0.02 0.18 <-0.22 0.17 P

J2235-4835 Q 0.51 22 35 13.24 -48 35 58.5 0.73 0.14 1.20 0.10 PU

J2329-4730 Q 1.29 23 29 17.71 -47 30 19.2 0.49 0.06 0.86 0.06 PTU

J2336-5236 Q 23 36 11.88 -52 36 12.9 < 0.15 0.07 0.37 0.07 P

J2357-5311 Q 1.00 23 57 53.18 -53 11 13.8 < 0.58 0.18 < 0.48 0.24 PTU

 

 

Source Type z RA(J2000) Dec(J2000) ID

B0047-579 Q 1.79 0 49 59.47 -57 38 27.3 < 0.34 0.10 0.60 0.06 U

B0056-572 Q 0.01 0 58 46.58 -56 59 11.4 < 0.45 0.15 0.56 0.08 U

B0131-522 Q 0.01 1 33 5.76 -52 0 3.9 < 0.28 0.06 < 0.23 0.08 U

B0230-790 Q 1.07 2 29 34.94 -78 47 45.6 < 0.03 0.10 < 0.08 0.09 U

B0332-403 Q 1.44 3 34 13.65 -40 8 25.3 0.73 0.12 1.21 0.07 UT

B0437-454 4 39 0.85 -45 22 22.5 < 0.29 0.07 < 0.58 0.15 U

B0438-436 Q 2.85 4 40 17.17 -43 33 8.6 < 0.49 0.11 1.49 0.08 UT

B0454-810 Q 0.44 4 50 5.44 -81 1 2.2 0.72 0.06 1.63 0.05 U

B0516-621 Q 5 16 44.92 -62 7 5.3 < 0.28 0.29 < 0.50 0.45 U

B0530-727 5 29 30.04 -72 45 28.5 < 0.13 0.15 < 0.24 0.07 U

B0629-418 Q 1.41 6 31 11.99 -41 54 26.9 < 0.20 0.10 0.33 0.04 U

B0738-674 Q 1.66 7 38 56.49 -67 35 50.8 < 0.11 0.08 < 0.40 0.10 U

B0823-500 8 25 26.86 -50 10 38.4 <-0.13 0.08 < 0.12 0.04 U

B1105-680 Q 0.58 11 7 12.69 -68 20 50.7 < 0.19 0.08 0.53 0.06 U

B1148-671 Q 11 51 13.42 -67 28 11.0 <-0.02 0.06 < 0.08 0.09 U

B1236-684 Q 12 39 46.65 -68 45 30.8 < 0.16 0.09 <-0.05 0.10 U

B1349-439 B 0.05 13 52 56.53 -44 12 40.3 0.50 0.07 0.42 0.07 UT

B1549-790 G 0.15 15 56 58.86 -79 14 4.2 < 0.23 0.09 0.63 0.08 UT

B1610-771 Q 1.71 16 17 49.27 -77 17 18.4 0.60 0.09 1.10 0.11 UT

B1903-802 Q 0.50 19 12 40.01 -80 10 5.9 < 0.05 0.12 0.51 0.09 U

B1925-610 Q 19 30 6.15 -60 56 9.1 < 0.28 0.08 < 0.23 0.07 U

B1935-692 Q 3.15 19 40 25.52 -69 7 56.9 < 0.26 0.08 <-0.20 0.07 U

B1950-613 Q 19 55 10.77 -61 15 19.1 < 0.24 0.08 <-0.05 0.08 U

B2052-474 Q 1.48 20 56 16.35 -47 14 47.6 0.63 0.09 1.19 0.12 UT

B2059-786 Q 21 5 44.96 -78 25 34.5 < 0.11 0.10 < 0.20 0.06 U

B2106-413 Q 1.05 21 9 33.18 -41 10 20.6 1.03 0.09 1.55 0.06 U

B2109-811 G 21 16 30.84 -80 53 55.2 < 0.07 0.14 0.33 0.06 U

B2142-758 Q 1.13 21 47 12.73 -75 36 13.2 < 0.11 0.10 < 0.06 0.08 U

B2146-783 Q 21 52 3.15 -78 7 6.6 < 0.08 0.08 < 0.08 0.05 U

B2152-699 G 0.02 21 57 5.98 -69 41 23.6 0.65 0.09 1.07 0.04 U

B2311-452 Q 2.88 23 14 9.38 -44 55 49.2 <-0.18 0.14 < 0.00 0.07 U

B2353-686 Q 1.71 23 56 0.68 -68 20 3.4 < 0.64 0.16 < 0.08 0.14 U

 

The expected theoretical thermal noise for our observations was of order , while in practice the observed noise varied from about 40mJy to 200mJy, and was roughly correlated with the zenith opacity during the integrations. This discrepancy is most likely due to slight systematic differences in gain between the target and reference beams, combined with temporal and spatial variations in the atmosphere on timescales of the beam switching. The planned replacement of the present chopper wheel system with a rapidly-nutating subreflector with a variable target-reference beam separation may improve the performance of the telescope for continuum surveys.