The H_2O southern Galactic Plane Survey (HOPS): I. Techniques and H_2O maser data

TL;DR

The HOPS survey mapped 100 square degrees of the southern Galactic plane, detecting 540 water masers with many new discoveries, providing insights into their properties and distribution in the Galaxy.

Contribution

This paper introduces the HOPS survey techniques and presents the first extensive water maser data set in the southern Galactic plane, including new detections and analysis of maser characteristics.

Findings

Detected 540 water masers, 334 of which are new.

Maser velocities can exceed 100 km/s, with some separated by over 350 km/s.

Estimated 800-1500 water masers are detectable in the Galaxy.

Abstract

We present first results of the H_2O Southern Galactic Plane Survey (HOPS), using the Mopra radiotelescope with a broad band backend and a beam size of about 2'. We have observed 100 square degrees of the southern Galactic plane at 12mm (19.5 to 27.5GHz), including spectral line emission from water masers, multiple metastable transitions of ammonia, cyanoacetylene, methanol and radio recombination lines. In this paper, we report on the characteristics of the survey and water maser emission. We find 540 water masers, of which 334 are new detections. The strongest maser is 3933Jy and the weakest is 0.7Jy, with 62 masers over 100Jy. In 14 maser sites, the spread in velocity of the water maser emission exceeds 100km/s. In one region, the water maser velocities are separated by 351.3km/s. The rms noise levels are typically between 1-2Jy, with 95% of the survey under 2Jy. We estimate completeness limits of 98% at around 8.4Jy and 50% at around 5.5Jy. We estimate that there are between 800 and 1500 water masers in the Galaxy that are detectable in a survey with similar completeness limits to HOPS. We report possible masers in NH_3 (11,9) and (8,6) emission towards G19.61-0.23 and in the NH_3 (3,3) line towards G23.33-0.30.