Full Survey Results

Observations have been complete since 2015 and DECRA funding has been awarded to analyse the project. We will be releasing the full data in late 2017 or early 2018. Stay tuned!


Key Results from the SPLASH Pilot Region

First results from the pilot region were published in Dawson et al. 2014, MNRAS. Some highlights are given below.

The figure below shows combined peak emission and peak absorption maps for all four ground state OH lines in the SPLASH pilot region, created by plotting the most extreme value of the brightness temperature at each spatial position. Only detections significant at the 4σ level are shown. The panels are ordered with the main lines (1667 and 1665 MHz) first and satellite lines (1720 and 1612 MHz) second to highlight the corresponding or conjugate nature of the four transitions.

Small circles overlaid on the left-hand panels show the peak positions of masers and maser candidates. Contours overlaid on the right-hand panels show the continuum brightness temperature at the line rest frequency. Contour levels run from 10.0 to 20.0 K at intervals of 2.5 K, then from 20.0 to 50.0 K at intervals of 5.0 K. (Contour colours are chosen to aid visibility and have no physical meaning.)


Key findings:

  • Strong detection of diffuse OH in emission and absorption throughout the survey region
  • Detection of 196 masers and maser candidates, of which ~50% were previously unknown
  • Mapping of variation in satellite line anomalies throughout the Galactic Disk
  • Demonstration that diffuse OH optical depths are always small (τ < 1) when averaged over the Parkes beam
  • Finding that the OH main lines are not in general in local thermodynamic equilibrium (LTE)
  • No evidence for OH outside the CO-bright regions of molecular clouds






The lack of evidence for OH outside the CO-bright regions of molecular clouds is one of the most surprising results from SPLASH so far. It was unexpected, given the high sensitivity of the survey, and the growing body of literature demonstrating the effectiveness of OH as a tracer of "dark" molecular gas. We believe that excitation effects are at least partially responsible: the similarity of OH excitation temperatures to the diffuse Galactic continuum background renders low column density OH particularly difficult to detect in the inner Galactic Plane.




Details can be found in Dawson et al. 2014, MNRAS, in press. Follow-up work is now underway to test this hypothesis further.



Last modified 7th June 2017.