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.
Highlights
Understanding how the interstellar medium (ISM) is linked to stellar birth and death in our Galaxy is a major problem in astrophysics today
We have completed a survey of 100 square degrees of the southern Galaxy in multiple spectral lines, using the Mopra radiotelescope in the 12 mm band (HOPS - The H2O southern Galactic Plane Survey)
We smoothed the data with a 90 × 90 arcsecond two-dimensional Gaussian kernel. We found that this size effectively reduced the noise by about 15%, compared to the maser emission. This kernel is slightly smaller than the size of the beam (132′′), we found that a smoothing kernel the size of the beam produced maps that were not as sensitive to identifying weak masers close to the noise level
Summary
Understanding how the interstellar medium (ISM) is linked to stellar birth and death in our Galaxy is a major problem in astrophysics today. In order to understand the processes involved, it is often useful to conduct large-scale Galactic plane surveys. H2O masers are an important signpost of unusual astrophysical conditions such as outflows and shocked gas. They are known to occur in both high and low-mass star forming regions An untargeted survey is required to determine the relative occurrence of bright H2O masers with other types of astrophysical objects. There is some evidence that within star forming regions, H2O masers may be observable at very early stages The relative occurrence of these two masers can be assessed through the two untargeted surveys: the Methanol Multibeam Survey (Green et al 2009) and HOPS, described here. It is hoped that the bright H2O masers discovered in HOPS may be used for such distance determinations
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