Abstracts

• Matteo Bianconi
  LoCuSS: Pre-processing in galaxy groups falling in to massive galaxy clusters at z=0.2⇩
  We report direct evidence of pre-processing of the galaxies residing in galaxy groups falling in to galaxy clusters drawn from the Local Cluster Substructure Survey (LoCuSS). 34 groups have been identified via their X-ray emission in the infall regions of 23 massive (⟨M200⟩ = 1015 M⊙) clusters at 0.15 < z < 0.3. Highly complete spectroscopic coverage combined with 24 μm imaging from Spitzer allows us to make a consistent and robust selection of cluster and group members including star forming galaxies down to a stellar mass limit of M⋆ = 2 × 1010 M⊙. The fraction fSF of star forming galaxies in infalling groups is lower and with a flatter trend with respect to clustercentric radius when compared to the rest of the cluster galaxy population. At 1.3R/r200 the fraction of star forming galaxies in infalling groups is half that in the cluster galaxy population. This is direct evidence that star formation quenching is effective in galaxies already prior to them settling in the cluster potential, and that groups are favourable locations for this process.
  
  
• Chris Collins
  Recovering the Intracluster light from LSST Data⇩
  I shall address the question of how quickly stellar mass is built up in the centres of rich X-ray clusters since z=1. Contributions from both galaxy mergers and the build up of intra cluster light need to be included to form a consistent picture. We find that major mergers dominate the growth at z=1 compared to lower redshifts when minor merging is more significant. The ICL grows quickly since a redshift z=1 to the present time, dominating the cluster stellar mass budget, in contrast to the BCG which evolves more slowly. New results from the SPIDERS data (part of SDSS-IV) indicate little correlation between BCG structural parameters and environment. Finally, I will briefly review the prospects of ICL measurement with LSST.
  
  
• Chris Conselice
  Using Deep NIR Imaging to Probe the History of Galaxy Mergers⇩
  Mass assembly of galaxies is one of the most important questions in extragalactic astronomy which relates to many other issues including: environment, feedback, star formation, gas accretion and merging. In fact, all of these processes are related and must be accounted for and understood to paint a full picture of galaxy assembly. Perhaps the most straightforward of these processes to measure are the merging and star formation histories. I will present results of combining in a new reanalysis of the three deepest and large NIR surveys take to date: UDS, Ultra-VISTA and VIDEO. Using consistently measured stellar masses and photometric redshifts for galaxies in these fields up to z =3, I will show how the major and minor merger rate can consistently be measured across these fields. Our new method involves a full use of the PDF for photo-zs and stellar masses. We show how the merger fraction and rate are lower than previous results and the implications for this for other methods of galaxy assembly and feedback mechanisms.
  
  
• Jacob Crossett
  Do large groups cause a more rapid quenching of star formation?
  Post starburst galaxies are galaxies which have had a significant episode of recent star formation, which has since been rapidly quenched. The reasons for this rapid quench however, remain uncertain. We have compared the environments, and morphologies of post starburst galaxies in the Galaxy And Mass Assembly (GAMA) survey, to mass matched transitioning galaxies, to determine the effect of environment in causing a rapid quench of star formation. We find that at low stellar masses, post starburst galaxies are more likely to reside in groups than other transitioning galaxies. Additionally, the mean group mass, radius, and velocity is also significantly larger for groups with post starbursts, compared to groups with other transitioning galaxies, suggesting that halo mass could be responsible for a rapid quenching of star formation. At higher stellar masses however, there are no significant environmental differences between post starbursts and other transitioning galaxies. We therefore explore other potential causes for a high stellar mass galaxies to undergo a rapid quench in star formation.
  
  
• Boris Deshev
  The passive core of the "Train Wreck" cluster, Abell 520⇩
  We analyse the galaxy populations in the core and near field of the merging cluster of galaxies Abell 520 at z=0.2. This is a merger of two subclusters of near-equal mass observed ~0.4 Gyr after the core passage, accompanied by few other smaller substructures. We use multi-fibre spectroscopy to map the star formation activity of the cluster members and look for signs of past star formation events that can be associated with the core passage. Our comparison sample consists of 10 non-merging clusters at 0.15<z<0.25 selected from the ACReS survey. The core of A520 shows lower fraction of star forming galaxies than the comparison clusters. This "passive" region is elongated along the axis along which the merger progresses. The galaxies in the core of A520 show no signs of past star formation on the time scales of the cluster merger. We identify a population of recently quenched galaxies, possibly associated with a recent infall, located at cluster-centric radii r~R_200.
  
  
• Dominique Eckert
  Building a galaxy cluster through continuous accretion of group-scale systems
  According to the hierarchical scenario for the formation of structures, massive galaxy clusters should have acquired most of their mass through the continuous accretion of smaller structures (galaxies, groups) throughout cosmic time. This process can be currently observed in the outer regions of local clusters, where individual infalling structures can be observed. I will present recent results from the XMM cluster outskirts project, where we discovered several such structures in their first interaction with the main halo. The detected groups show clear signs of ram-pressure stripping already at a large distance from the core of the main cluster, which has implications on the virialization of the infalling gas and on the quenching of the member galaxies.
  
  
• Arya Farahi
  Constraining the shape of Mass Function⇩
  From BAHAMAS and MACSIS simulations produced with a consistent astrophysics treatment that includes AGN feedback, we demonstrate that a simple multi-property statistical model describes the stellar and hot gas mass behavior in halos hosting groups and clusters of galaxies. We derive mass--observable relations (MORs) by performing local linear regression (LLR) on individual halo stellar and hot gas masses as a function of total halo mass. We find that: 1) both the local slope and variance of the MORs run with mass (primarily) and redshift (secondarily); 2) the MPR likelihood forms of stellar and hot gas mass at fixed halo mass are accurately described by log-normal distributions, and; 3) the correlation coefficient between stellar and hot gas mass at fixed halo mass is generally negative, reflective of a closed box model, but exhibits non-monotonic behavior with halo mass. Using the ensemble at z=0, we test the validity of the analytical population model of Evrard et al. (2014). LLR-based expectations of the log-mean halo mass selected at fixed property are accurate at the one percent level, a significant improvement over simple MOR models that assume a fixed slope and scatter. We finally illustrate the application of this model on constraining the shape of Mass Function.
  
  
• Emanuele Paolo Farina
  The birth of the giants: Where do the first quasars form?⇩
  Luminous quasars have been detected well into the epoch of reionization, probing the existence of super-massive black holes (SMBH) less than a Gyr after the Big Bang, and setting strong constraints on the the first crucial phases of the build-up of SMBHs and their host galaxies. To form a SMBH in such a short time scale, the first quasars need to be embedded in rich galactic environments with abundant cold gas reservoirs able to continuously feed the growth of the first black holes. This key ingredient of the galaxy formation could be mapped at z>6 through the detection of an extended and diffuse LyAlpha emission that appears when the cold gas is fluorescently illuminated by the emission of a bright quasar and/or by the UV-photons generated by an intense star formation event. I will present our effort to directly observe this feature and thus to probe the CGM of massive galaxies in the early Universe.
  
  
• Alexis Finoguenov
  eROSITA and Influence on mergers on cosmological modeling of galaxy clusters
  I will provide a description of the eROSITA experiment and discuss the influence of mergers on the cosmological modelling of galaxy clusters.
  
  
• Philip Grylls
  Modelling the growth of massive galaxies via mergers: A Semi Empirical approach⇩
  We present a Semi Empirical Model (SEM) that models the size, mass and morphology evolution from redshift 0-3 in a cosmological (100 Mpc) volume. We initialise stellar masses into Dark Matter (DM) halos from the Bolshoi simulation using abundance matching relations obtained from the latest stellar mass functions. Our galaxies then follow a merging history as predicted by the DM merger trees, using observations to inform the physical processes wherever possible. We present early results showing the size growth and morphological change of galaxies, and discuss the direction of our model and its potential for collaborative work.
  
  
• Chris Haines
  A survey of X-ray groups being accreted by massive galaxy clusters⇩
  Galaxy clusters assemble hierarchically, growing primarily through the continual accretion of lower mass clusters and groups. Clusters also assemble late, doubling their masses since z~0.5 and so their outer regions should be replete with infalling group-mass systems. We present an XMM survey to search for X-ray groups in the vicinity of 23 massive galaxy clusters at z~0.2 from the LoCuSS sample, identifying 39 X-ray groups that have been spectroscopically confirmed to lie at the cluster redshift. We examine the mass function of these infalling groups, and show that it is top-heavy in comparison to isolated groups from the XXL survey, consistent with groups being a biased tracer of the underlying density distribution. We discuss the contribution of these groups to the expected mass growth rate of clusters at late epochs.
  
  
• Mike Hudson
  Does size matter? The link between stellar mass, galaxy size and dark matter halo mass from weak gravitation lensing⇩
  The correlation between stellar mass and halo mass has been well established by abundance matching and weak gravitational lensing. We have recently shown that, at fixed stellar mass, there is a correlation between galaxy size and dark matter halo mass. Simulations suggests that, for most stellar masses, this correlation is driven by tidal stripping of satellite galaxies as they fall into groups and clusters. For the most massive early type centrals, however, we show that a minor merger driven model can explain the observed correlation.
  
  
• Mathilde Jauzac
  Hubble Frontier Fields : An Extraordinary View into Galaxy Cluster Evolution⇩
  The outskirts of clusters make the best, and most efficient locations to observe and trace the mass assembly processes of the Cosmic Web. Residing at the vertices of this Cosmic Web (Bond et al. 1996), galaxy clusters grow by steady accretion of matter from the surroundings, as well as by discrete mergers with nearby groups and clusters. Supported by simulations, this scenario predictions regarding the total mass content and distribution in filaments themselves remain largely untested. Filaments are vital elements of the cosmic census, containing up to half the baryonic mass of the Universe as a ‘warm hot intergalactic medium’ but also the majority of the dark matter. Recently, some of the most massive and disturbed clusters have been the centre of attention thanks to the Hubble Frontier Fields (HFF) initiative, which constitutes the largest commitment ever of HST time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. These clusters were chosen for their strong lens properties, and are all highly disturbed objects, showing major and minor merging on-going processes, making them ideal target to trace the Cosmic Web assembly. While combining strong and weak-lensing regimes to map the total mass with X-rays observations of the hot gas and spectroscopy of cluster galaxies to look at their direction of motion, we can thus study the dynamical scenarios in place within these massive galaxy clusters, and trace the sub-structures engaged in these processes. The Hubble Frontier Fields campaign ended in September 2016 and provided us with the deepest HST imaging ever obtained of galaxy cluster cores with the Hubble Space Telescope. The depth of these dataset makes these clusters amazing Cosmic Telescopes, but also enables us to get an unprecedented understanding of the cluster physics. I will present the latest results we obtained on the HFF clusters, and discuss the different caveats present on both the observing and simulation sides.
  
  
• Sugata Kaviraj
  Unveiling the low surface brightness Universe: the fundamental but unexplored role of minor mergers in galaxy evolution⇩
  The advent of extra-galactic surveys that are both deep and wide (e.g. LSST) is poised to revolutionise the study of galaxy evolution, by revealing aspects of galaxies and their environments that are invisible in past datasets. For example, these surveys will reveal low-surface-brightness tidal features that encode galaxy merger histories, enable studies of the inter-galactic medium and the faint outskirts of local systems, and truly open up the realm of dwarf galaxies. In this talk I will focus on the role of ‘minor’ mergers - i.e. mergers with unequal mass ratios - in driving galaxy evolution. Minor mergers produce faint tidal features that are invisible in past surveys like the SDSS. However, using data from the SDSS Stripe 82, I will argue that the persistence and ubiquity of this process over cosmic time makes it a fundamental (but largely unexplored) driver of galaxy evolution. I will show that around half of all the stellar mass growth in the local Universe likely takes place when galaxies are undergoing a minor merger, and show that this process promotes nuclear accretion and therefore the growth of BHs. Surveys like LSST (and precursors like DECaLS and the Hyper Suprime Cam Survey) will enable us, for the first time, to quantify the influence of this key process in the evolution of the observable Universe.
  
  
• Charutha Krishnan
  Enhancement of AGN activity in a protocluster at z=1.6
  I will present our recent study of the prevalence of X-ray AGN in the high-redshift protocluster Cl 0218.3-0510 at z=1.62, and review the implications for our understanding of galaxy evolution. There has long been a consensus that X-ray AGN avoid clusters in the local universe, particularly their cores. The high-redshift universe appears to not follow these trends, as there is a reversal in the local anti-correlation between galaxy density and AGN activity. In this z=1.62 protocluster, we find a large overdensity of AGN by a factor of ~23, and an enhancement in the AGN fraction among massive galaxies relative to the field by a factor of ~2. I will discuss the comparison of the properties of AGN in the protocluster to the field, and explain how our results point towards similar triggering mechanisms in the two environments. I will also describe how our study of the morphologies of these galaxies provide tentative evidence towards galaxy mergers and interactions being responsible for triggering AGN, and explain the reversal of the local anti-correlation between galaxy density and AGN activity.
  
  
• Lucio Mayer
  Merger timescales of massive black holes in hierarchical galaxy formation
  I will review the current understanding of coalescence timescales for massive BHs stressing the important connection with physical processes shaping the formation and evolution of galaxies. I will show how merger timescales can be very different in gas poor versus gas rich galaxies, with differences starting already at large scales where the BHs still have to form a binary. The gas-rich host case, which has long believed to be a more favourable case for fast MBH binary formation and merger, is indeed quite complex and can lead to very different pathways depending on the clumpiness of the interstellar medium. I will also show the importance of the epoch of galaxy mergers, as at high redshift the much higher density of galaxy cores allows a much faster evolution and coalescence of a MBH binary. Indeed new multi-scale simulations starting from cosmological initial conditions indicate that timescales can differ by more than 2 orders of magnitude between z=3 and z=0. All these effects need to be quantified in a statistical manner before we can construct reliable forecasts of MBH merger rates as a function of redshift, thus making robust predictions for LISA.
  
  
• Stuart McAlpine
  Major mergers and the triggering of black hole growth
  I will present some recent results looking at the link between major mergers and central supermassive black hole activity in the EAGLE simulations. I will discuss the role major mergers play in triggering a black holes rapid growth 'non-linear phase' and the more general influence of major mergers upon black hole growth through a wide dynamic range of systems over the course of their lifetimes. I will also discus if there remain any signatures today in the black hole population linked to major mergers and some observational predictions.
  
  
• Hannah Middleton
  Inference on properties of massive black hole binaries with pulsar timing array searches for gravitational waves⇩
  Pulsar timing arrays are searching for the stochastic gravitational wave background from the merging population of massive black hole binaries. Upper limits on the background are beginning to reach astrophysically interesting sensitivities, however no detection has been reported so far. The recent upper limit from the Parkes Pulsar Timing Array has been interpreted as casting into doubt the standard model of binary assembly through galaxy mergers and hardening via stellar interactions, suggesting that their evolution must be accelerated or stalled. We use a Bayesian hierarchical analysis to consider the implications of the upper limit for a range of astrophysical scenarios. Weak constraints can be placed on the population parameters, however we find that these astrophysical scenarios are as yet fully consistent with the current observations.
  
  
• Debora Sijacki
  Galaxy formation simulations: "sub-grid" vs. physics⇩
  Cosmological simulations of structure formation have undergone rapid development over the last decade and evolved from purely gravitational computations of large scale structure to full hydrodynamical simulations which include a plethora of complex baryonic physics. Recent successes of these simulations have been impressive, with several independent groups obtaining models of present day galaxies with a morphological mix in broad accord with observation, which has been a long standing goal for more than 20 years. Yet, due to many unknowns and the prohibitively large mass and spatial resolutions needed to solve the problem ab initio, cosmological simulations have adopted so-called "sub-grid" physics models, which are employed on small, unresolved scales. These invariably contain a number of free or poorly constrained parameters which can crucially affect the main simulation results. In this talk I will critically review how much cosmological simulations are subject to fine tuning due to these "sub-grid" models, which consequences this has for our understanding of the underlying physics and what are the future prospects of moving towards ab initio simulations.
  
  
• Chris Skipper
  Identifying galaxy mergers through spatially-offset radio AGN⇩
  The spatial offset of a supermassive black hole from the centre of its host galaxy is a strong indicator of a recent, or ongoing, merger event. However, the fraction of AGN that have become displaced in this way is currently not well constrained. We have constructed a sample of 345 galaxies (z < 0.2) from the Sloan Digital Sky Survey galaxy catalog for which we find compact radio sources listed in the Cosmic-Lens All-Sky Survey (CLASS) which are offset from the optical centroids of their host galaxies. We highlight 38 galaxies with strong nuclear radio sources (> 8 mJy) and spatial offsets of more than 150 milliarcsec. Some of these galaxies are well-known mergers (i.e. Arp 220, Arp 243, IC 4553, NGC 2623), or dual-AGN candidates (i.e. Mrk 273), but most are relatively anonymous, and a few show potential signs of disturbance. We place a strong upper limit on the fraction of offset-AGN candidates in our sample of 17 per cent.
  
  
• Clare Wethers
  A Merger History? The Star-Forming Hosts of Luminous, Dust-Obscured Quasars at z~2⇩
  Quasars are thought to govern many fundamental processes within galaxies, from quenching star formation to shaping galactic morphology. Whilst secular processes appear sufficient to fuel low luminosity quasars, recent evidence suggests the most luminous quasars at z>1.5 may require major mergers to accrete the bulk of their mass. In an evolutionary picture of quasars, the most luminous systems are thought to evolve from merger-driven starbursts, appearing heavily obscured during their transition to UV-luminous quasars as dust from the decaying starburst is being cleared out of the galaxy. Understanding the connection between dust obscuration, BH accretion and star formation in these luminous quasars is therefore an important test of both evolutionary quasar models and the role of mergers in galaxy formation. Unfortunately, the extreme nature of quasars means host galaxy studies of the most massive and luminous quasars (MBH~10^10M0, Lbol~10^47erg) at z>1.5 remain challenging, particularly in the rest-frame UV where the quasar typically outshines the galaxy by several orders of magnitude. I will present the first rest-frame UV study for a population of obscured type-1 quasars at z~2 - a peak epoch in both star formation and BH accretion. By combining deep ground-based imaging from DES with NIR observations from VHS and ULAS, I exploit dust obscuration towards the quasar to isolate host galaxy emission, finding obscured quasars to reside in prodigiously star-forming hosts at z~2, with the most actively star-forming galaxies appearing to host the most luminous quasars.
  
  

Posters

• Kate Furnell
  The Influence of Environment on the Evolution of Brightest Cluster Galaxies⇩
  Brightest Cluster Galaxies constitute the most massive population of galaxies in the Universe. They are widely thought to have gained the majority of their characteristics from their extensive merger histories, due to their locus at the bottom of the cluster potential well. Using SPIDERS-clusters survey data (SDSS photometry/spectroscopy + RASS X-ray data, Clerc et al., 2016), we identify a sample of 199 BCGs in massive clusters (median ~10^14 Msolar) from 0 < z < 0.3. We highlight 4 environmental parameters of interest: richness, X-ray luminosity, dynamical cluster mass and luminosity-weighted environmental density. We model our BCGs in 3 bands (SDSS gri) with Sersic profiles, testing the reliability of our fitting pipeline using 10^4 simulated galaxies injected into real SDSS fields. We then derive stellar mass estimates for our BCGs using our model fits. We find that, for our sample: BCGs are more massive and extended than the general population of SDSS n > 2.5 galaxies, BCG mass is correlated with all of the environmental characteristics outlined in this work (with no strong trends arising with either BCG profile slope or scale size) and there appears to be little overall environmental influence on the mass-size relation of our BCGs. Our results support the scenario of BCGs undergoing the bulk of their growth at earlier epochs, with little evidence for much environmental influence on morphology in massive clusters up to z ~ 0.3.
  
  
• Yjan Gordon
  Are Low Excitation Radio Galaxies Fuelled by Minor-Mergers?⇩
  Low Excitation Radio Galaxies (LERGs) are radio-loud active galactic nuclei with an absence of strong ionisation lines. LERGs are thought to be the result of weak, inefficient accretion onto the supermassive black hole (e.g. Narayan & Yi 1994; Hardcastle et al. 2007). Such inefficient accretion may be the result of a limited fuel supply to the central engine, either in the form of hot gas from an ageing stellar population or from a drip-feeding external process (Ellison, Patton & Hickox 2015). Should minor-mergers provide such an external source, then one might expect to see an excess of low mass satellites in the vicinity of LERGs relative to radio-quiet galaxies of similar mass and redshift. We exploit the depth (r less than 19.8) and high spectroscopic completeness (greater than 98%) of the Galaxy And Mass Assembly (GAMA) survey to test this hypothesis for LERGs detected by the Faint Images of the Radio Sky at Twenty cm (FIRST) survey. GAMA is greater than 80% complete for stellar masses down to 10^9.25 solar masses out to a redshift of 0.12. At this redshift there are 18 LERGs in the GAMA footprint and we compare these to 1114 radio-quiet galaxies of similar stellar mass, redshift and group or field environment to the LERGs. We find no evidence of an excess of low mass satellites down to 10^9.25 solar masses, suggesting that if minor-mergers play a role in LERG fuelling, then the accreted satellites are less than 10^9.25 solar masses in stellar mass.
  
  
• Tim Hewlett
  Supermassive black holes: the role of galaxy mergers through cosmic time⇩
  Supermassive Black Holes (SMBHs) exist in every large galaxy. Accreting supermassive black holes (Active Galactic Nuclei, AGN) can be amongst the most luminous objects in the universe, brighter than a trillion stars. How is gas, in the vacuum of space, funnelled in such huge quantities toward these billion-Solar-mass monsters to be accreted? Theoretical work suggests galaxy mergers may provide the necessary torque, driving the highest accretion rates whilst transforming galaxy morphologies. Observational evidence for this scenario has been mixed. We use high resolution Hubble Space Telescope (HST) imaging to identify galaxy mergers, quantify the role they may play in feeding the central beast, and see how that varies through time (0.5<z<2.2). In addition, we use data from a state of the art simulation (Illustris) to exclude some models of merger-driven accretion and make sense of the existing data.
  
  
• Jordan Penney
  The Environments of Luminous Infra-Red Radio-WISE Selected Galaxies⇩
  A population of heavily obscured, luminous radio-WISE selected galaxies has recently been observed using the Spitzer Space Telescope. These galaxies are expected to trace periods of intense star formation and interactions between galaxies within a cluster. Using comparisons to blank-fields, we measure the level of overdensity of these fields using color cuts to understand these overdensities within specific redshift ranges and find an overdensity of sources with IRAC color [3.6]-[4.5]>0.4 potentially linked to the central Radio-WISE selected source, with 33% of fields containing densities >5 sigma with respect to blank fields.
  
  
• Jonny Pierce
  Morphological signatures of merger-based triggering in intermediate radio powered AGNs⇩
  The effects of an AGN on key galaxy properties are apparent from both simulations and observations. The consequences are particularly prominent in the case of radio AGNs, where powerful jets interact with the interstellar medium in a way that could suppress, or even promote, star formation activity. Determining how, why and when such AGNs are triggered is hence vital for our understanding of galaxy evolution. One way of driving the requisite gas inwards is through a galaxy merger, be it major or minor. Previous studies have shown that this may be the dominant triggering mechanism for the most powerful radio-loud AGNs, but it is not yet clear whether this is applicable to all types of radio AGNs. It is therefore interesting to explore how the importance of merger-based triggering varies with radio power. Mergers can be traced using deep optical imaging through identification of signs of morphological disturbance within the host galaxies (e.g. tidal tails, fans, shells, double nuclei). Here I present new results based on deep INT/WFC data that consider the detailed morphologies of a complete sample of local intermediate power (22.5 < log P1.4GHz (W/Hz) < 24) radio AGNs.