We present the methodology for ``blind millimetre-wave surveys for redshifted molecular absorption in the CO/HCO$^+$ rotational lines. The frequency range $30-50$ GHz appears optimal for such surveys, providing sensitivity to absorbers at $z gtrsim 0
.85$. It is critical that the survey is ``blind, i.e. based on a radio-selected sample, including sources without known redshifts. We also report results from the first large survey of this kind, using the Q-band receiver on the Green Bank Telescope (GBT) to search for molecular absorption towards 36 sources, 3 without known redshifts, over the frequency range $39.6 - 49.5$ GHz. The GBT survey has a total redshift path of $Delta z approx 24$, mostly at $0.81 < z < 1.91$, and a sensitivity sufficient to detect equivalent ${rm H_2}$ column densities $gtrsim 3 times 10^{21}$ cm$^{-2}$ in absorption at $5sigma$ significance (using the CO-to-${rm H_2}$ and HCO$^+$-to-${rm H_2}$ conversion factors of the Milky Way). The survey yielded no confirmed detections of molecular absorption, yielding the $2sigma$ upper limit $n(z=1.2) < 0.15$ on the redshift number density of molecular gas at column densities $N({rm H_2}) gtrsim 3 times 10^{21}$ cm$^{-2}$.
The Fermi-LAT survey provides a large sample of blazars selected on the strength of their inverse Compton emission. We cross-correlate the first Fermi-LAT catalogue with the CRATES radio catalogue and use this sample to investigate whether blazar gam
ma-ray luminosities are influenced by the availability of external photons to be up-scattered. Using the 8.4 GHz flux densities of their compact radio cores as a proxy for their jet power, we calculate their Compton Efficiency parameters, which measure the ability of jets to convert power in the form of ultra-relativistic electrons into Compton gamma-rays. We find no clear differences in Compton efficiencies between BL Lac objects and FSRQs and no anti-correlation between Compton efficiency and synchrotron peak frequency. This suggests that the scattering of external photons is energetically unimportant compared to the synchrotron self-Compton process. These results contradict the predictions of the blazar sequence.
We present the first results from the new Survey of Extragalactic Nuclear Spectral Energies (SENSE) sample of blazars. The sample has been chosen with minimal selection effects and is therefore ideal to probe the intrinsic properties of the blazar po
pulation. We report evidence for negative cosmological evolution in this radio selected sample and give an outline of future work related to the SENSE sample.
