Star formation in massive clusters

Galaxy clusters are the largest structures in the Universe. The evolution of their constituent galaxies is a product of both nature (e.g. individual galaxy mass) and nurture (the local environment). The purpose of my ongoing research is to improve constraints on the relative importance of these effects.

The influence of cluster mergers

The core passage phase of a massive cluster merger may produce a shock front moving through the intracluster medium. This density wave could affect cluster galaxies via induced starbursts or extreme ram-pressure stripping.

The Frontier Field cluster Abell 2744 is a complex massive merger with a supersonic bullet-like subcomponent. Within the halo populatio behind this sub-cluster, I identified a trail of star-forming and dust-stripped sources including several extreme jellyfish-type galaxies. The orientation of the trail is consistent with a significant influence from the passing shock front.

-- Rawle et al., 2014, MNRAS, 442, 196

Trail of transformation behind the shock front in A2744
Several starburst and jellyfish galaxies (such as the stripped source shown to the right) lie along the estimated dynamic axis of the bullet-like subcomponent

Further investigation of the significance of cluster-scale dynamics on galaxy evolution requires a large cluster sample. I am currently working on a UV-IR analysis of 65 clusters at z~0.2-0.8 which includes both massive mergers and more virialised systems (in a sample including LoCuSS, CLASH and HST Frontier Fields).

The origin of S0 galaxies

With their spheroidal bulge and flat, mostly gas-free disc, S0 (lenticular) galaxies have long been postulated as a transitional stage between spiral and elliptical galaxies. Within the Coma cluster, the offset of S0s from the spiral galaxies Tully-Fisher relation (luminosity vs maximum rotational velocity), exhibits a correlation with projected cluster-centric radius. Since local environment is correlated with time of accretion into the cluster, the results support a scenario in which transformation of spirals to S0s is triggered by cluster infall.

-- Rawle et al., 2013, MNRAS, 433, 2667

S0 TFR offset in Coma
S0s in the Coma cluster are systematically offset from the spiral Tully-Fisher relation, corresponding to their distance from the cluster centre

Clusters as gravitational telescopes

Recent studies suggest that high-redshift galaxies harbour different modes of star formation, or physical conditions, compared to local galaxies. However, even the deepest far-infrared blank-field surveys are limited by the inherent confusion noise to ULIRGs at z>2.

Gravitational lensing offers a powerful tool to explore intrinsically fainter systems, yet the discovery of galaxy-galaxy lenses in field surveys is serendipitous. Lensing by galaxy clusters offers a known target with an increased probability of significant amplification. The "Herschel Lensing Survey" (HLS) offers a large sample of 65 cluster telescopes through which to observe the high redshift Universe.

Spatially resolved maps of a merging system at z=5.2

Behind the massive cluster Abell 773 lies a high-redshift system of three merging galaxies (HLS0918). The gravitational lensing effect magnifies the system at z~5.2, allowing us to detect many atomic and molecular emission lines (including [CII], CO, [NII] and water), as well as create map with sub-Kpc resolution.

-- Rawle et al., 2014, ApJ, 783, 59

HLS0918, originally discovered in the Herschel far-infrared data (left), is gravitationally lensed by the periphery of A773. The magnification allows kinematic components to be associated with distinct spatial regions in the image plane

A large sample of well-constrained HLS sources

Analysis of strongly lensed sources has now reached the era of statistical samples. Using fully band-merged (UV to submm) source catalogues for the 65 HLS clusters, I am currently compiling a large sample of intrinsically faint (cluster-lensed) galaxies at redshift z>1. With this sample, I intend to explore evolution via star formation of "normal" galaxies through cosmic time.