Submillimetre Observations of Clusters of Galaxies

Abstract

Advances in submillimetre astronomy has opened the doors to carrying out surveys and high-resolution imaging of intense star forming galaxies out to and beyond the cosmic star-formation peak (ɀ ≈ 1- 4). The Submillimetre Galaxy (SMG) population may well be the progenitors to the massive elliptical galaxies seen today, but the detailed nature of their assembly is still not clear. Gravitational lensing provides a route to highly resolved and detailed studies, but the current sample of lensed SMGs is relatively small. Capitalising on the lensing potential of a galaxy cluster observed were 202 massive clusters at 850µm in relatively poor submillimetre observing conditions (t225GHz > 0.1) with the SCUBA-2 instrument mounted on the JCMT. The SCUBA-2 Cluster Snapshot Survey (S2CSS) aim was to identify rare examples of bright (tens of mJy) gravitationally lensed submillimeter galaxies. The S2CSS covered over 0.33 deg2 to an average depth of σ850 ≈ 12 mJy beam-1. In this thesis a sample of 97 bright 850µm point sources selected from the S2CSS which are potentially gravitationally lensed and, eight sources where the strong submillimetre emission is co-located with the central Brightest Cluster Galaxy (BCG) are presented. Constructed are far-infrared spectral energy distributions for each that have additional Herschel SPIRE coverage; used in conjunction to estimate the redshifts and luminosities of the sources. The bright submillimetre flux density of the sources make them excellent targets for detailed follow-up work that will allow for detection of spectral features in the submillimetre/millimetre that would otherwise be too faint to detect. Already an extension of the original S2CSS aimed at confirming a number of potentially lensed galaxies which fell beneath a coarse selection criterion but formally were > 4σ or sources exceeding 80% completeness in the central deep regions of the SCUBA-2 maps has been successfully been observed. Higher signal-to-noise maps will confirm or rule out candidates as a spurious detection. The additional observations will allow for the lensed source counts to be refined sufficiently that they can be directly compared with lensed sources found with ALMA in clusters.

It is well known that BCGs harbour AGN, possibly reserves of cold gas and some on-going star formation. All of these might cause (directly or indirectly) submillimeter emission. This has yet to have been well-explored in a large sample like the one presented in this thesis. The individual maps are too shallow in general to detect a lot of this sub-mm emission, but since we observed a large number, and know where the BCG is, we can stack - average the maps together, co-aligned on the BCG - to detect statistical (average) emission. Matching sources from the SCUBA-2 Cluster Snapshot Survey with radio observations from the NRAO Very Large Array and the TGSS Alternative Data Release we constructed SEDs comprised of radio-binned clusters stacked on their Brightest Cluster Galaxy (BCG) location. In doing so we found that the radio and 850µm emissions correlate and suggest that in the strongest radio sources AGN are dominating the submillimeter excess whereas for galaxies with weak radio emission, star formation is the driving mechanism. Furthermore, measurements of the Sunyaev Zel’dovich effect (due to the Compton scattering of CMB photos by the hot intracluster medium) is contaminated from the sub-mm emission present in clusters which may well detriment mass measurements, this work finds an average contribution from BCGs S850µm = 2:16±0.58 mJy

Finally, the serendipitous detection of a bright 3-mm continuum source in the brightest galaxy cluster in Abell 0851 (ɀ = 0.4). Collating decades of multi-frequency observations across 21cm–100µm including new Arcminute Microkelvin Imager 15 GHz observations, evidence for a relatively flat, variable (Mean of 53% increase in flux density between 1.9 to 21 cm) core source associated with the BCG. The radio power and amplitude of variability observed in this galaxy is consistent with the cores in lower redshift BCGs in X-ray–selected clusters, and the flat mm–cm spectrum is suggestive of the BCG being a low luminosity AGN archetype due to the relatively low X-ray and Optical flux compared to the radio flux. The discovery of this system could provide a basis for a long-term study of the role of low luminosity radio mode ‘regulatory’ feedback in massive clusters.