Spectral line surveys with Jodrell Bank telescopes

• Main Author: D'Cruze, Michael
• Other Authors: Dickinson, Clive
• Published: University of Manchester 2018
• Subjects: 500
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.764723

Spectral line observations are of vital importance in astrophysics. Hydrogen recombination lines are a uniquely powerful tracer of ionised gas in the Galaxy, and of star formation and gas kinematics in other galaxies. Improvements in receiver performance at ~GHz frequencies has generated renewed interest in recombination lines from other galaxies. The 76m Lovell Telescope should be sensitive enough to detect recombination lines from new galaxies in 10-100 hrs. Jodrell Bank Observatory has been without a dedicated spectral line backend for more than 15 years. The advancement of digital technology means a step-change in bandwidth and resolution capability is now possible. A ROACH2 board was configured as a spectrometer with up to ≈15kHz resolution in two polarisations across 512MHz bandwidth and up to ≈62.5kHz resolution across 2GHz bandwidth in one polarisation. Signal transport systems were developed to allow observation of signals from the MkII and Lovell telescopes by the spectrometer. The 2GHz primary system was found to be dominated by correlated noise, introduced by the e-MERLIN frequency conversion system. Initial observations of the H96-103α recombination line were found to be dominated by the band shape, which will limit calibration accuracy. The MkII telescope was commissioned for use in spectral line observations. The C-band receiver showed good performance. f_knee≈17mHz across 250kHz was found, scaling with bandwidth as expected. The telescope pointing accuracy was found to be a limiting factor, with offsets up to ~50% of the half-power beam width observed. Nevertheless hourly corrections demonstrated that sources could be tracked with ≲10% incremental offsets required. A 1hr observation of the calibrator NGC 7027 indicated ≈6% increase in the effective system noise, indicating long observations are well-represented by the radiometer equation. The RFI environment at Jodrell Bank was examined. C-band was found to be largely available for observation. The range of H96-103αRRLs were reliably observable. L-band is densely populated with ~10⁹ Jy signals. The protected band was observed over a four-week period using the Lovell Telescope. A strong correlation is found between human activity and the presence of RFI in the protected band, indicating that even where parts of L-band appear free from coherent RFI, sensitive total-power observations must take place at night. The future of RRL studies from Jodrell Bank is in C-band Galactic plane surveys using the MkII; extragalactic RRL work is unfeasible. The Galactic anti-centre is sparsely observed. The MkII is capable of a fully-sampled survey of 10-20 square degrees with T_rms ~1mK sensitivity in ~ 100 days observing time.