The observations were made at a mean frequency of 1368 MHz, in the 1350-1400 MHz frequency band allocated on a co-primary basis to the Radio Astronomy Service, where its protection from harmful interference is limited. Though care was taken to make the renovated Arecibo telescope more robust against radio frequency interference (RFI), and to coordinate its operation as well as possible with the frequency plan and emission periods of local radar installations, RFI signals with strengths that hamper the detection of faint H I line signals were present during the first half of the observations. After the main terrestrial RFI source was identified halfway through the observing run, a blanker could be implemented that effectively removed these signals from our spectra.
The data were reduced using IDL routines developed at Arecibo Observatory. The two
polarizations were averaged and corrections were applied for the variation in gain and
system temperature of the telescope as function of azimuth and zenith angle, using the
most recent calibration data available. A first-order baseline was then fitted to the
data, excluding those velocity ranges with H I line emission or RFI. Once the
baselines were subtracted, the velocities were corrected to the heliocentric system,
using the optical convention. All data were boxcar smoothed to a velocity resolution
of 19.5 km s-1 for further analysis. If the average of all data on an object contained
a signal with a peak level exceeding 1 mJy, it was checked if that feature
corresponded to RFI signals that did not occur in all spectra - if so, the
contaminated spectra were not used for the final analysis. The rms noise levels of the
averaged spectra were determined in channels 300-1800, avoiding lines with a maximum
exceeding about 1 mJy peak line flux density.
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Figure 1: H I column density contours from the Dickey (1997) VLA survey superimposed on our V-band CCD images (from Iglesias-Páramo et al. 2002). The H I clouds without optical counterparts in these images, reported as tentative detections in Dickey (1997), have been indicated. |
Copyright ESO 2003