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29th of April 2022
The Southern Parkes Large-Area Survey in Hydroxyl (SPLASH)
by Dawson et al.
Dawson et al. have presented the full data release for the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) -- the most sensitive large-scale survey of hydroxyl (OH) to-date. The survey of the Southern Galactic Plane was undertaken in all four ground-state transitions of the OH radical at 1612, 1665, 1667 and 1720 MHz with the Parkes 64m radio-telescope, "Murriyang". The major challenge for SPLASH data is spectral baseline correction. The OH lines are weak; broad/blended spectral features may occupy a significant fraction of the spectral channels and may (particularly in the case of the satellite lines) ‘flip’ between emission and absorption over the range of a feature; and residual baseline structure may have broad, irregular bumps and humps or comparable widths and heights to real signal. The CMZ and Galactic Centre region are particularly problematic in this regard, due to the extreme velocity widths of the spectral features. These factors necessitated a careful, iterative approach to line-finding and baseline correction.

The figure above shows examples of baseline fitting and subtraction for single integrations of one map. These plots show the full 8MHz bandwidth of the 1666 MHz band, which contains both the 1667.359 line (channels ∼1500–3000) and the 1665.402 MHz line (channels ∼3500–5000). The top panels show the bandpass- and flux-density-calibrated data prior to baselining, and the bottom panels show the baseline-subtracted data, both smoothed with a 10-channel boxcar kernel. The solid dark pink lines on the upper panels show the baseline models – a combination of polynomial fits and low-pass filtering. Signal identified in the 3D datacubes by the Duchamp source finder is shown as pink bands on the lower plots, and the equivalent spectral ranges greyed out on the upper plots; these ranges are excluded from the baseline model fitting. Both example positions shown here are continuum-bright, meaning that the OH absorption signal is strong enough to be seen even in these short integrations, and that residual baseline structure is fairly severe. The paper has been published in Monthly Notices of the Royal Astronomical Society.

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