The University of Birmingham

17th - 18th February 2015


Malcolm Bremer

Morphological evolution in cluster red sequence galaxies

Red sequence galaxies in galaxy clusters are thought of a "red and dead" objects where star formation appears to have completed at high redshift (typically z>1.4) and the galaxies evolve passively thereafter. By comparing measures of the morphology of cluster red sequence galaxies over a wide range in redshifts we show that while this is true, apart from the most massive galaxies, red sequence galaxies evolve significantly in morphology subsequent to the end of their star formation.

Christopher Collins

Build-up of stellar mass in cluster cores

It is becoming recognised that the growth of stellar mass at the centres of galaxy clusters is driven both by the build up of the Brightest Cluster Galaxy (BCG) and the Intra Cluster Light (ICL). We have been using X-ray selected clusters with deep optical and near infrared imaging (including the CLASH clusters) over a wide redshift range to examine the co-evolution of the BCG and ICL in rich clusters. I will review some of the recent results on the growth rate of BCGs from z~1 to the present day, examine the merger rates of nearby companions to the BCGs and estimate the build up of the ICL over the same cosmic period. These results will be compared with the latest simulations and I will discuss briefly implications for LSST preparation.

Judith Croston

Where and when does galaxy feedback via AGN jets matter?

What role do different AGN populations play in the evolution of cosmic over densities? The locations and epochs of energy input by AGN into groups and clusters can dramatically affect the evolution of their baryonic content and hence their observational properties. Future radio surveys will remove major unknowns about the low & intermediate luminosity AGN population providing mechanical feedback at the epochs of group and cluster formation, but finding out which environments host which types of source remains a crucial question. I will summarise new results from a comprehensive X-ray survey of radio-galaxy environments for two fixed redshift samples, which has allowed us to disentangle the effects of epoch and environment on jet luminosity for the first time, and has revealed important differences in the large-scale environments hosting radiatively efficient and inefficient radio-loud AGN.

Jessica Democles

Morphological study of the 100XXL galaxy group sample and prediction from the cosmological hydrodynamical simulation cosmo-OWLS

It is now acknowledged that in order to interpret large galaxy groups and clusters dataset, we need to understand how we select them. This selection isn't only a ratio of number of systems we can detect but also expresses itself through a bias towards a specific sub-population. Therefore, this can play a significant role when assessing the scaling relations linking galaxy systems properties with their mass. In particular, an important source of dispersion can be attributed to the difference in the systems relaxation degree. X-ray observations are widely used for this purpose, since the gas distribution is at a fair extent a good probe of the relaxation degree. We study the surface brightness profiles of the 100 brightest galaxy groups detected in the XXL survey together with similarly selected groups from the cosmo-OWLS hydrodynamical simulation. This group sample span a large range of redshift (0.04>z>1) and one decade in mass. We observe a similar trend of the outer slope of the profile to evolve with the mass of the systems while the core radius as defined by a simple beta model evolves with the redshift. Those results are in agreement with the literature. Moreover, the good agreement between the simulations and the observations is an encouragement to use those simulations to compute a reliable and accurate selection function for the survey.

Christopher Duncan

Cluster mass profiles with size and flux magnification

The use of gravitational lensing has provided the means of accurately probing the mass pro- files of foreground lenses on a large range of scales, from galaxy scales to super clusters and the large scale mass reconstruction of the CFHTLenS field. Most frequently, these studies are undertaken using galaxy shape as the observable, however recent studies have turned their attention to the various methods extracting information from the magnification signal, such as through galaxy clustering, source redshift enhancement or changes in source size and flux. Further, theoretical studies suggest there is significant gain from the combination of such measures with traditional shear analyses. In this talk, I will present a method of Bayesian mass profile determination using magnification information from measures of source size and magnitude, with an application to large clusters using HST data and will discuss how this method can be applied to different lens mass scales such as galaxy-galaxy lensing. I will show how the use of this information can be used to produce accurate constraints on cluster mass profile parameters, to accuracy comparable to shear analyses, detail the limitations of such an analysis, and discuss the combination of such a measure with shear data.

James Etherington

Measuring galaxy environment in large-scale photometric surveys

The next generation of large scale photometric surveys, such as the Dark Energy Survey and Euclid, provide the opportunity to study galaxy environment not only as a function of galaxy properties but also as a function of redshift. The DES will map 300 million galaxies over 5,000 deg^2. This enormous dataset comes at the cost of redshift precision. In preparation we have studied the impact of redshift precision on galaxy environment using data from the SDSS. We found that even at large redshift uncertainties (~0.1) there was a significant correlation between environment measurements and a set of spectroscopic benchmark measurements leading to the conclusion that it should be possible to make robust measurements of galaxy environment in photometric surveys. We are currently analysing the DES science verification data (~100 squared degrees), examining the evolution of galaxy properties with galaxy environment through cosmic time since z~1.

Tim Green

A Photometric Census of "Activity" in Brightest Cluster Galaxies

The details of the feedback cycle in Brightest Cluster Galaxies (BCGs) and its effect on the evolution of the galaxy and host cluster are uncertain. Identifying "active" BCG candidates, for detailed follow- up studies, is the first necessary step in addressing this problem. Using multi-wavelength photometric data we are investigating the properties of ~900 RASS X-ray selected clusters up to z=0.5, with a particular emphasis on BCGs anomalous to the bulk population. We aim to find what fraction of BCGs exhibit signs of star formation and/or AGN activity at each wavelength and combination of wavelengths. On comparing the Pan-STARRS 3pi colours of the BCG, with that of the host cluster's red sequence, we find that ~9% are anomalously blue relative to the red sequence galaxies. Of those, where spectroscopy is available, nearly all (~90%) exhibit optical emission-lines. We find a similar fraction, with a strong overlap, of BCGs with a FUV and NUV excess in GALEX and presenting outlying IR WISE colours (which we use to differentiate between star forming and AGN dominated BCGs). Across all wavelengths a positive relation between the anomalous BCGs and the X-ray luminosity of the host cluster is seen, consistent with cooling of the intra-cluster gas as the dominant source of fuel for this "activity". We are currently extending this work to include NVSS radio data for yet further comparison.

Will Hartley

Galactic conformity and central / satellite quenching at 0.4<z<1.9 in the UKIDSS UDS

A curious correlation exists between the star-formation activities of central galaxies and their satellites. The tendency for passive central galaxies to have an enhanced fraction of passive satellites (and vice-versa) has been dubbed 'galactic conformity', and has been demonstrated at group and sub-group halo masses. Theoretical models that include only halo-mass dependent quenching mechanisms are unable to reproduce this behaviour. Understanding how conformity arises may therefore lead us to new discoveries regarding the physical processes that govern the termination of star-formation in satellite galaxies. A vital, but currently unknown, aspect of the conformity effect is how it evolves with redshift. Thus far the correlation has only been identified at low redshift (z~0.1). In this contribution I use the UKIDSS Ultra Deep Survey to investigate the properties and radial distribution of satellite galaxies around typical mass (M*) galaxies over 0.4<z<1.9. I show that galactic conformity is present to at least z~2 and is not a simple manifestation of halo-mass-driven effects. Finally, I discuss some of the possible implications of conformity for the quenching of central and satellite galaxies.

Nina Hatch

Galaxy Protoclusters

I will present a review of galaxy protoclusters at z > 1. I will show how these cosmic overdensities can be linked across cosmic time to trace galaxy evolution in an evolving environment. Over the past few years both detailed studies of individual objects and large samples of protoclusters have greatly increased our understanding of these complex objects. Using a sample of over 200 protoclusters from the CARLA survey, I will reveal the mass growth and star formation histories of cluster galaxies during their most formative years. On the other end of the scale, I will reveal the detailed structure of a single galaxy protocluster at z~1.6. I will show how galaxy properties strongly depend on their local environment within this protocluster, indicating that nurturing processes greatly influence galaxy evolution, even at z > 1.5. Finally, I will discuss the prospects of finding galaxy protoclusters with LSST and Euclid.

Ryan Houghton

Morphology, angular momentum and environment

The SAURON & ATLAS3D surveys brought about a paradigm shift in our understanding of elliptical and lenticular galaxies: specific angular momentum and ellipticity better divide ETGs than visual morphology. Slow Rotators (33% of Es; 7% of S0s) are rounder, have misaligned photometry & kinematics and decoupled cores. Fast Rotators (66% of Es, 93% of S0s) span all ellipticities and are consistent with oblate axisymmetry. We extend the FR/SR classification to dense environments such as Coma and Abell 1689 and find that, while the SRs are found at the centres each cluster, the average SR:ETG ratio is fixed at 15%, across 5 orders of magnitude in projected density. Revisiting Dressler's original visual classification of galaxies in 55 clusters, we couple visual morphology to kinematic morphology using the statistics from ATLAS3D and show that analogous, and new, results are found using just visual morphologies.

Kate Husband

The Environments of z=2.2 Radio Galaxies as Traced by H-alpha Emitters

Radio galaxies are the most massive galaxies in the high redshift universe and are known to lie in protocluster environments. Feedback from the central AGN of the radio galaxy may affect the protocluster galaxies, perhaps suppressing star formation within these galaxies. We have studied seven z=2.2 radio galaxies with HAWKI narrow band and broad band imaging in order to map out their environment using Halpha emitters and to explore the effect the radio galaxy may be having on these companion galaxies. The results can be compared to the blank field HAE survey HiZELS (Sobral+12 etc.). We find that 60% (or 4 out of the 7) radio galaxy fields are over dense compared to the field, in agreement with Venemans et al. (2007) who used Lyman alpha emitters to map out the environment of radio galaxies. We also find that while the star formation rate (SFR) of the HAEs in the radio galaxy fields is higher the specific SFR is lower than for field galaxies of the same mass. This suggests that the radio galaxy fields have undergone rapid evolution that is now haltering, perhaps because the radio galaxy is heating the surrounding gas, stopping it accreting onto the companion galaxies and fuelling their star formation.

Holger Israel

Reconciling Planck cluster counts and cosmology? Chandra/XMM instrumental calibration and hydrostatic mass bias.

The mass of galaxy clusters can be inferred from the temperature of their X-ray emitting gas, their masses may be underestimated if it is assumed that the gas is in hydrostatic equilibrium, by an amount suggested by simulations. We have previously found consistency between a sample of observed Chandra X-ray masses and independent weak lensing measurements. Unfortunately, uncertainties in the instrumental calibration of Chandra and XMM-Newton observatories mean that they measure different temperatures for the same gas. We translate that relative instrumental bias into mass bias, and infer that XMM-Newton masses of clusters are unbiased compared to WL masses. For massive clusters, Chandra's calibration may thus be more accurate. The opposite appears to be true at the low mass end. We observe the mass bias to increase with cluster mass, but presence of Eddington bias precludes firm conclusions at this stage. Nevertheless, the systematic Chandra XMM- Newton difference is important because Planck's detections of massive clusters via the Sunyaev-Zeldovich (SZ) effects are calibrated via XMM-Newton observations. The number of detected SZ clusters are inconsistent with Planck's cosmological measurements of the primary Cosmic Microwave Background (CMB). Given the Planck cluster masses, if an (unlikely) uncorrected calibration bias existed, this tension would be eased, but not resolved.

Mathilde Jauzac

Hubble Frontier Fields : A New Era for Gravitational Lensing

The Hubble Frontier Fields (HFF) initiative constitutes the largest commitment ever of HST time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. This program devotes 140 orbits of HST time to deep imaging observations of each of six cluster lenses reaching m~29 (AB) uniformly in all pass-bands (10-30 orbits per filter - 3 ACS and 4 WFC3 pass- bands). The full set of data on Abell 2744 (z=0.308) has been taken in October-November 2013 with WFC3, and May-July 2014 with ACS. The second target, MACSJ0416.1-2403 (z=0.397) has been observed with ACS in January-February 2014, and with WFC3 in July-August 2014. I will present the new gravitational lensing pictures of these two complex systems using this exquisite set of data coming from the HFF program. We have demonstrated that we are now able to "weight" these clusters' cores down to the percent level precision (recently published works), serving our quest for the high-redshift Universe. However, while the depth of these dataset makes these clusters amazing Cosmic Telescopes, it also enables us to get an unprecedented understanding of the cluster physics. Therefore, presenting the case of MACSJ0416, I will demonstrate the importance of such high-quality data to analyse the merging/dynamical history of the cluster itself while comparing dark matter, light and gas distributions.

Scott Kay

Cosmological simulations of galaxy clusters: a powerful tool for next-generation surveys

In this talk I will present our latest work on cosmological simulations of galaxy clusters, arguing that they are now becoming sufficiently realistic to enable their use as a powerful, quantitative tool for the design and interpretation of next-generation surveys. Using results from the Millennium Gas simulations, I will first show how the inclusion of radiative cooling and star formation; supernova feedback; and AGN feedback shape the properties of the gas and stars. Together, these processes allow them to be brought into good agreement with many observational properties at low redshift. I will then present initial results from the MACSIS project where clusters and super-clusters are being re-simulated from a very large (3200 Mpc) simulation. By extending the cosmo-OWLS simulations to the most massive clusters, we will produce very wide and deep lightcones, ideal for upcoming and future cluster surveys. At the same time, we are also using the Eagle code to produce ultra-high resolution simulations of individual clusters in the MACSIS simulation. This allows us to start resolving the internal structure of the cluster galaxies and study cluster physics with unprecedented detail.

Kimmo Kettula

Weak lensing calibrated scaling relations for low mass clusters in the CFHTLS field

The outstanding problem in galaxy cluster count cosmology is to determine accurate scaling relations between survey observables and cluster mass. As indicated by the tension between Planck CMB and Sunya'ev-Zeldovich cluster count cosmology, the robustness of the cluster count results are dominated by the mass measurements used to calibrate the scaling relations. Here we present our work on the scaling of X-ray observables obtained with XMM-Newton to weak lensing masses in the CFHTLS fields. We study the mass dependence of the scaling relations, the effects of substructure and X-ray cross-calibration.

Ben Maughan

Cluster scaling relations from XXL and beyond

I will present new results on galaxy cluster scaling relations and their evolution from the XXL survey. I will emphasise the importance of correctly modelling selection effects, and discuss the constraints on ICM feedback physics from current data and the prospects for future missions.

Ian McCarthy

A simple model for galaxies and galaxy clusters

Recent studies have highlighted the sensitivity of the outcome of cosmological simulations to the details of subgrid modelling, calling into question the predictive power of such simulations. Here I will show (using a new set of simulations) that with only a minimal degree of calibration of stellar and AGN feedback (on judiciously chosen observables) it is possible to reproduce an extraordinarily wide range of observations, spanning the X-ray, SZ, and optical scaling relations of clusters, the galaxy stellar mass function, the projected 2-point correlation function of galaxies, the various black hole scaling relations, the quasar luminosity function and its evolution, etc. The passing of these basic tests strongly indicates the general framework is correct and it opens up the door to much more thorough tests of LambdaCDM and our picture of galaxy formation. With this in mind, I will briefly comment on the case for massive neutrinos from astrophysical observations.

Sean McGee

The baryon cycle and the quenching of satellite galaxies

The baryon cycle of galaxies is a dynamic process involving the intake, consumption and ejection of vast quantities of gas. In contrast, the conventional picture of satellite galaxies has them methodically turning a large gas reservoir into stars until this reservoir is forcibly removed due to external ram pressure. This picture needs revision. Our modern understanding of the baryon cycle suggests that in some regimes the simple interruption of the fresh gas supply may quench satellite galaxies long before stripping events occur, a process we call overconsumption. I will use observed satellite quenching times at a range of redshifts to determine if satellites are principally quenched through orbit-based gas stripping events – either direct stripping of the disc (ram pressure stripping) or the extended gas halo (strangulation) – or from internally driven star formation outflows via overconsumption. These time-scales show significant deviations from the evolution expected for gas stripping mechanisms and suggest that either ram pressure stripping is much more efficient at high redshift, or that secular outflows quench satellites before orbit-based stripping occurs. Further, we will use low redshift surveys of HI gas to probe the ongoing balancing of inflows and outflows in satellite galaxies.

Alex Merson

Abundance and colours of high redshift cluster galaxies in hierarchical models

ESA's Euclid mission is expected to observe many thousands of galaxy clusters at z > 1, which is thought to be a crucial epoch in the evolution of the galaxy cluster population. I present predictions from one of the latest semi-analytical models of galaxy formation for the abundance and colours of galaxies in clusters at this epoch. The model gives a good match to the location of the cluster red sequence and is able to match the cluster galaxy luminosity function in the faintest and brightest magnitude bins, but under-estimates the counts around L*. This discrepancy between the observed and predicted counts can be reduced by adopting an alternative treatment for dust attenuation, or by adjusting the strength of feedback due to AGN, but at the expense of worsening the match to the cluster red sequence or to the z = 0 galaxy luminosity function. The discrepancy suggests that the model is predicting a population of dusty star forming galaxies in clusters, in addition to the quiescent galaxies with old stellar populations, and highlights the need to examine further the dust extinction applied to galaxies at high and low redshift, as well as the need to consider observations beyond those obtained for the local Universe when constraining the parameters of galaxy formation models.

Stuart Muldrew

What are Protoclusters?

Understanding the formation of clusters is important for cosmology and studying the evolution of galaxies. At high redshift, z>1.5, we witness the early stages of cluster formation, and these structures are referred to as protoclusters. The terms cluster and protocluster, while often used interchangeably, do not denote the same thing. Clusters are typically single virialised haloes, while protoclusters are the diffuse collection of haloes that will merge to make up the final low redshift cluster. Using a semi-analytic model applied to the Millennium Simulation, I will demonstrate that protoclusters are very extended objects that are not necessarily dominated by a single dark matter halo. Additionally, I will explain the difficulties in obtaining a complete and clean sample of galaxies which is fundamental for estimating the size of the over density, and ultimately the final z=0 mass.

James Nightingale

Density Profile Decomposition of Massive Early-Type Galaxies

I present the decomposition of the density profile of strong lens 0047-2808, a massive early-type galaxy lensing a double source, forming a complete Einstein ring system. Strong lens modelling is performed with our new adaptive semi-linear inversion method, which reconstructs the unlensed source light distribution on a pixelized grid derived from the lens model magnification. Light-profile fitting of the lens galaxy is performed simultaneously, allowing decomposition of the total density profile into its constituent light and dark components and therefore offering robust determination of the stellar mass-to-light ratio and dark matter fraction. I conclude by considering the prospects density profile decomposition offers to test models of structure formation and galaxy evolution, given that the strong lenses observed both in current and future surveys like HATLAS, DES and LSST act as biased tracers of cosmic over densities.

Electra Panagoulia

Chemodynamical effects of AGN feedback in nearby groups and clusters

In my talk, I will be exploring the effect that AGN feedback has on the dynamics and chemical make- up of a sample of nearby groups and clusters of galaxies. The previously reported flattening of the entropy profile in the core of clusters, which may be due to resolution effects, is seen to a far lesser extent here. This can impact the importance of hot accretion in brightest cluster galaxies. In addition, AGN feedback in the form of radio bubbles, where it is present, is found to be able to offset cooling in cluster cores. Drops in the iron abundance in the cores of groups and clusters are also seen, and could be due to AGN feedback. The detection rate of both X-ray cavities and these central abundance drops is found to be dependent on data quality.

Helen Russell

Massive molecular gas flows and AGN feedback in galaxy clusters

Powerful radio jets launched by a central supermassive black hole pump a substantial amount of energy into their host galaxies and cluster environment. This feedback from the central AGN is thought to regulate galaxy growth and cooling of the surrounding hot atmosphere. But many key questions remain, including how the black hole is fuelled, how the heating can be distributed over large scales yet closely coupled to the gas cooling rate and the role of the cold molecular gas apparently cooling from cluster atmospheres. I will present ALMA Early Science observations of molecular gas in central cluster galaxies which show massive outflows driven out by the radio jets and inflows that could be feeding gas disks around the central AGN.

Matthieu Schaller

The effect of baryons on the inner structure of rich clusters

We use the Evolution and assembly of galaxies and their environment (eagle) cosmological simulation to investigate the effect of baryons on the density profiles of rich galaxy clusters. We focus on eagle clusters with M 200 > 10 14 M of which we have six examples. The central brightest cluster galaxies (BCGs) in the simulation have steep stellar density profiles, ρ(r) α r ^-3 . Stars dominate the mass density for r <10 kpc, and, as a result, the total mass density profiles are steeper than the Navarro-Frenk- White (NFW) profile. However, the dark matter halo itself closely follows the NFW form at all resolved radii (r <3.0 kpc).The central slopes of the total mass profiles are also consistent with the observed clusters The eagle BCGs have similar surface brightness and line-of- sight velocity dispersion profiles as the BCGs observed by Newman et al.(2013), which represent the best cluster profile data currently available. However, after subtracting the contribution of the stars to the central density, Newman et al. find significantly shallower slopes than NFW, in contradiction with the eagle results. We discuss possible reasons for this discrepancy, and conclude that an inconsistency between the kinematical model adopted by Newman et al. for their BCGs, which assumes isotropic stellar orbits, and the kinematical structure of the eagle BCGs, in which the orbital stellar anisotropy varies with radius and tends to be radially biased, could explain at least part of the discrepancy.,

Graham Smith

Weighing galaxy groups and clusters: new results from LoCuSS and XXL

I will present the new weak-lensing mass calibration of massive galaxy clusters from the Local Cluster Substructure Survey, highlighting our control of systematic uncertainties at the ~5% level. Combining these measurements with our own hydrostatic masses and the Planck SZ analysis gives new constraints on the hydrostatic mass bias parameter (1-b). I will discuss our results in the context of the tension between Planck CMB and cluster cosmology, and consider the next steps in cluster weak-lensing studies, including poor clusters and groups from XXL.

Russell Smith

The initial mass function in massive galaxies

The IMF plays a crucial role in extragalactic astronomy, e.g: [1] governing the translation between light (observed) and stellar mass (modelled); [2] affecting chemical evolution in galaxies and groups, via SN rates, locked mass fraction, etc; [3] complicating the determination of dark-matter halo profiles in massive clusters (separation of stellar and dark components). Recent studies have suggested that the IMF in massive ellipticals may differ substantially from that observed in any environment within the Milky Way. My talk will highlight new results in three areas: (a) IR spectroscopy of nearby elliptical galaxies, exploiting new spectral features to disentangle IMF and abundance effects (b) IMF constraints from several new low-redshift strong lensing ellipticals discovered through a novel IR-IFU search (c) comparison between spectroscopic and gravitational (lensing, dynamics) evidence for unusual IMFs in massive galaxies.

Dongyao Zhao

The relationship between morphology and structure of Brightest Cluster Galaxies and their environment

cD galaxies are a unique type of BCGs physically differing from the normal elliptical BCGs. By using a large sample of 625 morphologically-classified BCGs, we firstly quantify the structural differences between cD and elliptical BCGs, and then with taking into account more BCG intrinsic properties along with environments we explore any unique evolutionary history of cD galaxies. By carefully fitting the galaxies' 2D light profile we find that morphologically-classified cDs possess an unique extended envelope clearly distinct from their central bulge while morphologically-classified elliptical BCGs only possess the bulge structure. cD galaxies also have larger sizes compared with the elliptical ones. Based on this quantitative analysis, we propose an objective diagnostic which can automatically separate cDs from non-cD BCGs using structural information only, and thus avoiding the need for subjective visual classification. Moreover, besides the difference in structure, we also find that cD galaxies are more massive than elliptical BCGs, and they tend to reside in denser clusters being hosted by more massive dark matter halos. More interestingly, even at similar densities, cD galaxies are statistically more massive and larger than elliptical ones. This suggests that cDs galaxies originated as elliptical BCGs in early time and then grew in mass and size by developing their unique stellar envelopes from dry minor mergers.

Irina Zhuravleva

Turbulent heating in galaxy clusters and its role in AGN feedback

Radiative energy losses of the hot gas in galaxy clusters occur on timescales significantly shorter than the Hubble time, leading to massive accumulation of cold gas and vigorous star formation, in contradiction to observations. Several sources of heat have been discussed, most promising being heating by the SMBH in central galaxies through inflation of bubbles of relativistic plasma. The missing link in this scenario is the mechanisms, by which energy from bubbles is transported to the hot gas. Dissipation of gas turbulence, induced by bubbles during their buoyant rise and expansion, is a possible mechanism. However, direct measurements of gas velocities will be possible only with future X-ray calorimeter on board the Astro-H observatory. We recently overcame this problem, by analyzing long Chandra observations of the X-ray brightest clusters of galaxies and measuring statistical properties of density fluctuations, which allowed us to constrain the velocity power spectrum of gas motions in the ICM indirectly. I will show that the heating rate due to dissipation of turbulence is indeed sufficient to balance the radiative cooling locally at each radius within the cores. Turbulent dissipation, therefore, might be the key element in resolving the gas cooling problem in cluster cores and other X-ray gas-rich systems.

Felicia Ziparo

The XXL survey: weak-lensing mass - K band luminosity relation and implication on cluster galaxies

We present the weak-lensing mass and K-band luminosity relation, M-L_K, for a sub-sample of the 100 brightest clusters in the XXL survey. The clusters in our sample span a wide range in mass (M =0.54-11.83 10^14 M_SUN) at 0<z<0.6. We find that the weak lensing mass scales linearly with the luminosity, with a slope consistent with unity and an intrinsic scatter of 0.30. Combining our sample with some clusters in the LoCuSS survey present in the literature, we obtain a scatter of 0.12 with fully consistent slope and intercept. Our results suggest that star formation efficiency is independent from halo mass, at odds with what suggested by previous results based on X-ray or dynamical mass measurements. We also study the richness-mass relation finding that group-sized halos have more galaxies compared to their massive counterpart. However, the brightest cluster galaxies (BCG) in low-mass systems contribute more to the cluster total light that in massive cluster, where bright galaxies have similar luminosities. This can be reconciled with a hierarchical growth of structures: BCG would form and mature in groups and then become BCGs or bright galaxies in massive clusters.


Cristina Furlanetto

The SOAR Gravitational Arc Survey

We present the SOAR Gravitational Arc Survey (SOGRAS) sample, comprising 47 galaxy clusters among the richest structures in SDSS Stripe 82, whose redshifts span around z=0.3 or z=0.5. The main motivation of this survey is to discover new gravitational lens systems and to constrain the variation of strong lensing efficiency as a function of cluster redshift. We found arcs candidates in 8 clusters of our sample, suggesting an overall efficiency for arcs consistent with previous studies. Deeper follow-up imaging with Gemini and results from lens modelling strengthen the case for the strong lensing nature of most of candidates found in this survey. From dynamical analysis using spectroscopic data and lens modelling we were able to obtain accurate measurements for the cluster masses. We found a good agreement between our strong lensing based mass estimations and those from our dynamical analysis measurements. We also found that the SOGRAS main sample is homogeneous in terms of mass.

Paul Giles

The Luminosity-Mass Relation of a Statistically Complete Sample of Luminous Clusters

We present the results of work involving a statistically complete sample of 35 galaxy clusters spanning the redshift range 0.15<z<0.3, observed with Chandra. We use the Mass-Temperature relation for the most dynamically relaxed clusters, utilising hydrostatic mass estimates, as a mass proxy for the cluster sample. We present the Luminosity-Mass relation, using a Bayesian approach to fully take into account selection effects, demonstrating the importance of modelling the selection effects to recover the true relation of the population.

Nancy Hine

Investigating the acceleration of galaxy growth in a z=3 protocluster

One route to accelerated galaxy growth in the very densest environments in the Universe at early epochs (protoclusters) is via an earlier onset / higher frequency of major mergers which trigger star formation and black hole growth compared to galaxies in the average density field. If this is the case, one would expect the relative fraction of normal galaxies exhibiting on-going merger and tidal interactions in protocluster regions to be different to that of an identically (mass) selected field population. We have examined the rest-frame UV morphologies of Lyman-break Galaxies (LBGs) in the SSA22 protocluster at z=3.1 compared to LBGs at z~3 in the HDF-N. Our results indicate that the merger fraction for the protocluster is significantly larger than that for the field suggesting that dense environments may indeed have higher merger fractions than the field at z~3.

David Maltby

The structure of galactic discs with environment

Various correlations are known to exist between the properties of galaxies and their local environment (e.g. the morphology-density relation). However, at present it is uncertain as to whether these correlations are due to the direct effect of an environmental process, or the consequence of cluster galaxies being older (more evolved) than those in the general field due to an earlier formation (Hierachical Theory). To explore this issue, we compare the structure of galactic discs in the field and cluster environment using spiral and S0 galaxies from the STAGES survey. In these comparisons, we show whether strong environmental processes (i.e. those that can disrupt a galaxy's stellar distribution) are affecting the structure of disc galaxies and therefore driving the environmental correlations we see in the Universe today. In related work, we also compare disc structure between our spiral and S0 galaxies and high-light some interesting results which shed light on the process by which spiral galaxies transform into S0s.

Clément Martinache

Spitzer analysis of z>2 proto-clusters candidates found by Planck and Herschel.

Few galaxy clusters have been discovered at z>2 . At these epochs, cluster members are thought to be forming most of their stars in synchronous, gigantic bursts of star formation, yet unseen. This observational difficulty can be explained first by the intrinsic rarity of objects on the sky (few 100- 1000s), then by the fact that local clusters analysis points to star formation episodes lasting less than a Gyr, making these objects yet rarer, and finally by methods used. Indeed, clusters at z~2 are not expected to resemble local, mature clusters and are often called proto-clusters to highlight these differences. Here we present a systematic search for these proto-clusters by tracing their dust emission via a multi-wavelength approach. Our z >2, highly star forming cluster candidates are first selected over the whole sky using Planck, taking benefit of the redshifted far-infrared peak into the Planck submillimetre channels and a clean component separation (among which Galactic cirrus & CMB). About 200 hundreds of these candidates were followed-up by Herschel/SPIRE, and shown to be significant over densities of red sources, confirming their high-z spectral energy distribution and high star formation rates (typically 700 Msun/yr per SPIRE source, and >5000 Msun/yr for each structure). These over densities could be protoclusters in their intense star formation phase. I will present further Spitzer/IRAC data which was obtained on 40 fields, in the 3.6 microns and 4.5 microns channels. The angular resolution of IRAC allows us to separate individual galaxies, which could not be done with Hesrchel/SPIRE. The data were obtained down to 1uJy (5 sigmas), and show unambiguous clustering of red galaxies, compatible with z~2 based on previous studies. I will focus on one particular candidate for which ground based observations in the Near Infrared were granted, which allows for SED estimates and colour diagnosis. This new window on the high-z (z>2) protocluster may yield powerful constraints on structure formation (e.g., SFR vs environment at high-z, z>2 mass assembly in clusters, bias).

Justyna Modzelewska

Quasars photometric reverberation study. Determination of the dark energy from LSST data.

High redshift quasars can be used as a complementary tool to SN Ia (Watson et al. 2011) to probe the distribution of dark energy in the Universe by measuring the time delay of the emission line with respect to the continuum. Determination of each quasar absolute monochromatic luminosity can be done at the basis of the size of the Broad Line Region (BLR). It is well known observationally for the nearby active galaxies that the BLR size and the monochromatic luminosity are well correlated, with small dispersion. This relation can be easily extended towards high redshift quasars since, as shown by Czerny & Hryniewicz (2011), the size of the BLR is determined by the conditions of the dust formation in accretion disk temperature. The method is currently applied to the spectroscopic data. However, as argued by Chelouche & Daniel (2012), photometric data also can be used for the time delay measurement, under the condition of the appropriate assessment of the continuum and line contribution to specific photometric channels. We perform simulations, including starlight, to show the expected accuracy of the method, when applied to many measurements expected from LSST.

Carl Mundy

Connecting the most massive galaxies in the universe across cosmic time

Understanding how galaxies become so massive and how and why their properties evolve are important questions in astrophysics. Ideally, to answer these questions one would prefer to track the same galaxies over time, however this is not possible observationally and various sample selection methods are employed instead. Using a suite of publicly available semi-analytical models applied to the output of the Millennium Simulation, I will quantify how well galaxy sample selections at a constant cumulative number density recover the evolution (out to z=3) in the physical properties of progenitors of an initial z=0 selection of the most massive galaxies.

Laura Prichard

The KMOS GTO cluster program: absorption line spectroscopy of cluster galaxies at z¡1.5

As part of the GTO KMOS cluster program (P.I.s Davies & Bender) we are investigating the absorption line spectra of individual cluster galaxies during the peak epoch of star formation at 1<z<2. The multiplexed nature of KMOS increases the observing efficiency by more than an order of magnitude compared to single integral field units, which is essential for obtaining deep spectra of such faint targets. Furthermore, the NIR capabilities of KMOS produce observations of the well-understood rest-frame V-band indices at these redshifts, providing reliable measures of age and composition for the stellar populations. The kinematics coupled with archival HST photometry allow us to construct the fundamental plane and study evolution in size and mass-to-light of galaxies when the Universe was less than 5 Gyrs old. The program has already obtained spectra of >40 galaxies in two clusters with on-source exposure times of 15-20 hrs per galaxy. We present early results from these data and provide an overview of the project.

Jose M. Rodriguez-Espinosa

A possible proto-cluster at z=6.5

Brief description of talk: We have searched for low-luminosity Lyman-alpha sources around two z=6.5 high star forming objects detected by Ouchi (2010) in the SXDS field. We have employed three narrow band filters with the OSIRIS instrument at the GTC. We have found 35 candidate sources. In the absence of spectroscopic velocities, we have used the width of the central narrow band filter as an uncertainty for their radial distances. We have then run Montecarlo iterations and a Friend of Friends algorithm to find that most sources are concentrated in one (26%) or two (44%) groups. We have therefore found what we think is one of the most distant proto-clusters ever seen. We have observing time (MOS) with GTC to confirm the velocities, which is in the queue.

Timothy Weinzirl

Introducing OMEGA, an Emission Line Galaxy Survey in A901/902

OMEGA is the OSIRIS Mapping of Emission-line Galaxies survey in the multi-cluster system A901/2 at z~0.165 (PI Aragón-Salamanca). The H-alpha and [NII] emission lines are targeted with the tunable filter mode of the OSIRIS instrument on GRANTECAN. This talk will provide an overview of the observing programme and science goals, along with preliminary results concerning star forming and active galaxy populations distinguished with galaxy emission line properties.