CO Line Observing Strategy

While the primary science driver for HASHTAG is the dust continuum science, how this releates to the gas is important, and the CO(J=3-2) line can also be a significant contaminant of the 850µm band. To make sure we cover a range of conditions across M31 we use pointings across a range of locations and environments to ensure we measure a range of conditions (these are shown in the Figure below).

For all but one regions we observed using HARP 2' × 2' Jiggle maps to create small fully sampled maps, and a 4' × 4' raster map. The fields were selected to include:

  • The five regions covered by Herschel spectroscopy of [CII] 158µm, [OI] 63µm and [NII] 122µm and optical IFU spectroscopy (Kapala et al. 2015, see the green squares on the Figure below).
  • The two regions where it has been suggested that there is a component of very cold gas (Allen & Lequeux, 1993; Loinard & Allen, 1998). These are shown by the cyan crosses on the figure.
  • For 2' × 2' fields in the area observed by PHAT, CARMA and IRAM CO surveys, chosen to cover the bulge, inner ring, and the outer ring and a range of CO(J=1-0) and 250µ intensities.
  • A 4' × 4' raster map on the main 10 kpc ring in order to carry out a complete survey of a segement of the ring.

CO Survey Strategy
The proposed CO(J=3-2) observations with HARP overlaid on the SPIRE 250µm and IRAM CO(J=1-0) image. The white squares are our observed proposed observations with HARP, the green are the Herschel spectroscopy and the optical IFU regions covered by the SLIMS survey (Kapala et al. 2015) and the red is a CO(J=3-2) survey of gas regions Allen & Lequeux (1993); Loinard et al. (1996); Loinard & Allen (1998).

In total these 12 regions will cover an area of 60 square arcminutes, and several memeners of the team have a complimentary survey to observe the bulge. Our observations are designed to reach an antenna temperature of 13 mK with a 2.6 km/s channel width. This requres a total of 55 hours of Band 3 time.