SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl -- Data
  Description & Release
  We present the full data release for the Southern Parkes Large-Area Survey in
Hydroxyl (SPLASH), a sensitive, unbiased single-dish survey of the Southern
Galactic Plane in all four ground-state transitions of the OH radical at 1612,
1665, 1667 and 1720 MHz. The survey covers the inner Galactic Plane, Central
Molecular Zone and Galactic Centre over the range $|b|<$ 2$^{\circ}$,
332$^{\circ}$ $< l <$ 10$^{\circ}$, with a small extension between 2$^{\circ}$
$< b <$ 6$^{\circ}$, 358$^{\circ}$ $< l <$ 4$^{\circ}$. SPLASH is the most
sensitive large-scale survey of OH to-date, reaching a characteristic
root-mean-square sensitivity of $\sim15$ mK for an effective velocity
resolution of $\sim0.9$ km/s. The spectral line datacubes are optimised for the
analysis of extended, quasi-thermal OH, but also contain numerous maser
sources, which have been confirmed interferometrically and published elsewhere.
We also present radio continuum images at 1612, 1666 and 1720 MHz. Based on
initial comparisons with $^{12}$CO(J=1-0), we find that OH rarely extends
outside CO cloud boundaries in our data, but suggest that large variations in
CO-to-OH brightness temperature ratios may reflect differences in the total gas
column density traced by each. Column density estimation in the complex,
continuum-bright Inner Galaxy is a challenge, and we demonstrate how failure to
appropriately model sub-beam structure and the line-of-sight source
distribution can lead to order-of-magnitude errors. Anomalous excitation of the
1612 and 1720 MHz satellite lines is ubiquitous in the inner Galaxy, but is
disabled by line overlap in and around the Central Molecular Zone.
