First WALLABY Early Science papers

The first two papers from the WALLABY Early Science observations, Lee-Waddell et al. 2019 and Reynolds et al. 2019, are now publicly available. They show the potential for the full WALLABY survey to make detailed studies of the commonest ‘homes’ for galaxies in the nearby universe – galaxy groups.

Most nearby galaxies live in groups containing a few tens of neighbouring galaxies, all kept huddled together by gravity. These are relatively low-density environments; nevertheless, the galaxies may jostle each other, sometimes colliding or making near misses.

Through these interactions, galaxies can gain or lose their neutral hydrogen gas – a major component of most galaxies, and one that ultimately fuels star formation. So a galaxy’s neighbourhood sets it up for more or fewer interactions with its fellows, influencing how that galaxy changes over time.

Although most local galaxies live in these small groups, the groups are relatively little studied: more studies have been done of high-density galaxy clusters, which contain a thousand or so galaxies. This is because more telescope time is needed to observe a large number of galaxies if they live in these small, low-density groups.

With its wide field of view, ASKAP is set to change that. Its WALLABY survey will let astronomers make detailed studies of large numbers of galaxy groups and draw general conclusions about how such environments shape galaxy evolution.

The two first WALLABY Early Science papers arise from a single set of ASKAP observations, led by Karen Lee-Waddell (CSIRO). They were intended to test ASKAP and validate its data-reduction pipeline, ASKAPsoft, as well as produce useful science.

The observations cover a field centred on the NGC 7232 galaxy group, a triplet of spiral galaxies – NGC 7232, NGC 7232B and NGC 7233. The ~180 hours of observations were made over 16 nights and used 12 antennas (ASKAP–12) and limited bandwidth (48, 192, and 240 MHz rather than the full ASKAP bandwidth of 304 MHz). Most of the data were taken shortly after WALLABY Early Science began in October 2016. For both papers, the Source Finding Application (SoFiA) was used to locate significant concentrations of neutral hydrogen (HI) in the ASKAP datacube.

The NGC 7232 galaxy group

When galaxies within a group interact, they often produce tidal bridges and tails: over time, these tidal streams can accrete material and form tidal dwarf galaxies (TDGs). Such bridges, tails and TDGs can provide evidence that lets us reconstruct, or at least constrain, the galaxies’ likely interactions.

The ASKAP observations of the NGC 7232 galaxy group detected 17 HI sources close to the NGC 7232/3 triplet that are members of the group. Six of these detections are well-known, HI-rich galaxies; five are galaxies with identifiable stellar counterparts that have been resolved in HI for the first time. The other six HI detections are probably tidal debris associated with the NGC 7232/3 triplet: Lee-Waddell et al. comment on their possible origins. The full WALLABY survey will examine many similar galaxy groups, allowing findings on tidal structures to be generalised.

 

The NGC 7232 galaxy group Contours show total HI intensity (moment 0) contours from the present ASKAP observations (white) and the earlier HI Parkes All-Sky Survey (HIPASS) (yellow), superimposed on an archival DSS2 Blue optical image. Beam sizes are at bottom left; the physical scale bar (upper right) is based on a group distance of ~24 Mpc. Blue diamonds mark new ASKAP HI detections that appear to have stellar counterparts; green circles indicate probable tidal debris with no detectable stellar counterparts. Red contours represent the sensitivity pattern of the 12-beam footprint, at 15, 50 and 90% of the peak sensitivity. (From Lee-Waddell et al. 2019)

 

The NGC 7162 galaxy group

In a second paper, Reynolds et al. present the first ASKAP observations of the NGC 7162 galaxy group, which lies in the same ASKAP field ∼3.8 deg to the northwest of the NGC 7232 triplet.

The researchers detected HI in three of the group’s galaxies but not in the fourth (NGC 7166): this was consistent with previous ATCA observations. They also detected HI in ESO288–G033, which is within ATCA’s field of view in archival observations of the field; and in two nearby dwarf galaxies, AM 2159–434 and J220338–431131, both of which were beyond ATCA’s field of view and not previously detected in HI.

ASKAP’s high resolution and circular beam let Reynolds et al. derive dark-matter masses for all four spiral galaxies (NGC 7162, NGC 7162A, ESO 288–G025 and ESO 288–G033) and dynamical masses for the two dwarfs (AM2159-434 and GALEXASC J220338.65–431128.7). The spiral galaxies they found to be dark-matter dominated, with dark-matter fractions of ∼0.81−0.95.

Publications

Lee-Waddell, K. et al., 2019, MNRAS, in press, WALLABY Early Science – II. The NGC 7232 galaxy group

Reynolds, T.N., et al., 2019, MNRAS, 482, 3591, WALLABY early science – I. The NGC 7162 galaxy group

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