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It has long been discovered that environment plays a major role in galaxy evolution, with galaxies in dense regions of the cosmos more likely to have their star formation suppressed or \textquote{quenched} relative to their more isolated counterparts. Despite this remarkable discovery, our current understanding of the physical processes responsible for the suppression (or \textquote{quenching}) of star formation remains woefully incomplete. This is especially true for galaxies that are members of massive galaxy groups and clusters – i.e. \textquote{satellite} galaxies – given that our best cosmological models struggle to reproduce key observations, such as the satellite quenched fraction. This dissertation aims to advance our knowledge in the areas of environmental quenching that are poorly understood, focusing on the quenching of low-mass dwarf satellite galaxies and satellite populations at intermediate and high redshifts. Observational studies of low-mass satellite quenching have been limited by their inability to robustly characterize the local environment and star-formation activity of faint systems. This work overcomes these limitations by combining supervised machine learning and statistical background subtraction techniques to constrain the satellite quenched fraction of group populations ($\mhalo = 10^{13-14}~\msun$) down to a satellite stellar mass limit of ${\sim}10^7~\msun$. This approach successfully reproduces the established quenched fraction trends at high-masses while finding that ram-pressure stripping -- the rapid removal of cold gas from the interstellar medium of satellites as they move through the hot and dense intra-group or intracluster medium permeating the host halo -- is the likely dominant quenching mechanism responsible for shutting down star formation in the low-mass regime. The importance of investigating intermediate and high redshift is that the vast majority of environmental quenching studies are limited to the very local Universe ($z \lesssim 0.1$). This work addresses this shortcoming by combining multi-wavelength spectroscopic observations of massive clusters ($\mhalo = 10^{14-15}~\msun$) at $z\gtrsim1$ with the infall histories of analogs from $N$-body simulations to model satellite quenching at this poorly studied epoch. The quenching timescales derived from this study indicate that \textquote{starvation} -- the slow depletion of cold gas in the absence of cosmological accretion after a satellite becomes a member of a more massive host -- is the dominant driver of environmental quenching at this epoch. However, this study lends support to the scenario where ram-pressure stripping potentially acts as a secondary quenching mechanism in massive clusters at this epoch. Overall, this work provides valuable insights into the quenching timescales and underlying mechanisms of satellite galaxies in massive clusters at $z\gtrsim1$, while introducing an impactful methodology that combines supervised machine learning and statistical background subtraction to study the quenching of low-mass satellites beyond the Local Group.

We investigate the role of dense environments in suppressing star formation by studying $\rm \log_{10}(M_\star/M_\odot) > 9.7$ star-forming galaxies in nine clusters from the Local Cluster Survey ($0.0137 < z < 0.0433$) and a large comparison field sample drawn from the Sloan Digital Sky Survey. We compare the star-formation rate (SFR) versus stellar mass relation as a function of environment and morphology. After carefully controlling for mass, we find that in all environments, the degree of SFR suppression increases with increasing bulge-to-total (B/T) ratio. In addition, the SFRs of cluster and infall galaxies at a fixed mass are more suppressed than their field counterparts at all values of B/T. These results suggest a quenching mechanism that is linked to bulge growth that operates in all environments and an additional mechanism that further reduces the SFRs of galaxies in dense environments. We limit the sample to $B/T < 0.3$ galaxies to control for the trends with morphology and find that the excess population of cluster galaxies with suppressed SFRs persists. We model the timescale associated with the decline of SFRs in dense environments and find that the observed SFRs of the cluster core galaxies are consistent with a range of models including: a mechanism that acts slowly and continuously over a long (2-5 Gyr) timescale, and a more rapid ($, 17 pages, 12 figures

We present initial results of a survey of host $L_{*}$ galaxies environments in the Local Volume ($D, 17 pages, 12 figures, accepted to MNRAS

Recent observations have shown that the environmental quenching of galaxies at z & SIM; 1 is qualitatively different to that in the local Universe. However, the physical origin of these differences has not yet been elucidated. In addition, while low-redshift comparisons between observed environmental trends and the predictions of cosmological hydrodynamical simulations are now routine, there have been relatively few comparisons at higher redshifts to date. Here we confront three state-of-the-art suites of simulations (BAHAMAS+MACSIS, EAGLE+Hydrangea, IllustrisTNG) with state-of-the-art observations of the field and cluster environments from the COSMOS/UltraVISTA and GOGREEN surveys, respectively, at z & SIM; 1 to assess the realism of the simulations and gain insight into the evolution of environmental quenching. We show that while the simulations generally reproduce the stellar content and the stellar mass functions of quiescent and star-forming galaxies in the field, all the simulations struggle to capture the observed quenching of satellites in the cluster environment, in that they are overly efficient at quenching low-mass satellites. Furthermore, two of the suites do not sufficiently quench the highest mass galaxies in clusters, perhaps a result of insufficient feedback from AGN. The origin of the discrepancy at low stellar masses (M-* less than or similar to 10(10) M-circle dot), which is present in all the simulations in spite of large differences in resolution, feedback implementations, and hydrodynamical solvers, is unclear. The next generation of simulations, which will push to significantly higher resolution and also include explicit modelling of the cold interstellar medium, may help us to shed light on the low-mass tension.

One of the challenges in understanding the quenching processes for galaxies is connecting progenitor star-forming populations to their descendant quiescent populations over cosmic time. Here we attempt a novel approach to this challenge by assuming that the underlying stellar mass distribution of galaxies is not significantly altered during environmental quenching processes that solely affect the gas content of cluster galaxies, such as strangulation and ram-pressure stripping. Using the deep, high-resolution photometry of the Hubble Frontier Fields, we create resolved stellar mass maps for both cluster and field galaxies, from which we determine 2D Sérsic profiles, and obtain Sérsic indices and half-mass radii. We classify the quiescent cluster galaxies into disk-like and bulge-like populations based on their Sérsic indices, and find that bulge-like quiescent galaxies dominate the quiescent population at higher masses ($M_\star > 10^{9.5}M_\odot$), whereas disk-like quiescent galaxies dominate at lower masses ($10^{8.5}M_\odot< M_\star < 10^{9.5}M_\odot$). Using both the Sérsic indices and half-mass radii, we identify a population of quiescent galaxies in clusters that are "morphological analogues" of field star-forming galaxies. These analogues are interpreted to be star-forming galaxies that had been environmentally quenched. We use these morphological analogues to compute the environmental-quenching efficiency, and we find that the efficiency decreases with increasing stellar mass. This demonstrates that environmental quenching is more effective on less massive galaxies and that the effect of environment on quenching galaxies is not completely separable from the effect of mass on quenching galaxies., 25 pages, 14 figures, accepted for publication in ApJ Revision ver. 1: Replaced title, fixed link to section 3

Driven by increasingly complete observational knowledge of systems of satellite galaxies, mutual spatial alignments and relations in velocities among satellites belonging to a common host have become a productive field of research. Numerous studies have investigated different types of such phase-space correlations, and were met with varying degrees of attention by the community. The Planes of Satellite Galaxies issue is maybe the best-known example, with a rich field of research literature and an ongoing, controversial debate on how much of a challenge it poses to the LCDM model of cosmology. Another type of correlation, the apparent excess of close pairs of dwarf galaxies, has received considerably less attention despite its reported tension with LCDM expectations. With the expansion of proper motion measurements, largely driven by Gaia, other peculiar phase-space correlations were uncovered among the Milky Way satellites. Examples are the tangential velocity excess of satellites compared to cosmological expectations, and the unexpected preference of satellites to be close to pericenter. At the same time, other kinds of correlations have been found to be more in line with cosmological expectations, specifically lopsided satellite galaxy systems and the accretion of groups of satellite galaxies. The latter has mostly been studied in cosmological simulations thus far, but offers the potential to address some of the other issues by providing a way to produce correlations among the orbits of a group's satellite galaxy members. This review is the first to provide an introduction to the highly active field of phase-space correlations among satellite galaxy systems. It summarizes existing, recent research and highlights interdependencies between the different, currently almost exclusively individually considered types of correlations. Future prospects are also briefly discussed. (abridged), Invited review, 33 pages, 10 figures, 2 tables, 155 references. Accepted for publication in the special issue Challenges in Our Understanding of Dwarf Galaxies, edited by Yanbin Yang and Behnam Javanmardi, in Galaxies

The nearby elliptical galaxy Cen A is surrounded by a flattened system of dwarf satellite galaxies with coherent motions. Using a novel Bayesian approach, we measure the mean rotation velocity $v_{\rm rot}$ and velocity dispersion $\sigma_{\rm int}$ of the satellite system. We find $v_{\rm rot}/\sigma_{\rm int} \simeq 0.7$ indicating that the satellite system has non-negligible rotational support. Using Jeans' equations, we measure a circular velocity of 258 km s$^{-1}$ and a dynamical mass of $1.2\times 10^{13}$ M$_\odot$ within 800 kpc. In a $\Lambda$CDM cosmological context, we find that the Cen A group has a baryon fraction $M_{\rm b}/M_{200}\simeq0.035$ and is missing $\sim$77$\%$ of the cosmologically available baryons. Consequently, Cen A should have a hot intergalactic medium with a mass of $\sim$8$\times$10$^{11}$ M$_\odot$, which is more than $\sim$20 times larger than current X-ray estimates. Intriguingly, The whole Cen A group lies on the baryonic Tully-Fisher relation defined by individual rotationally supported galaxies, as expected in Milgromian dynamics (MOND) with no need of missing baryons., Comment: Accepted for publication in A&A, 7 pages, 3 figures, 1 table

The abundance of low mass halos is one of the key predictions of LCDM, which remains at apparent odds with observations of luminous structure. We present new measurements of the spatial distribution and the cumulative luminosity function of satellite galaxies up to a thousand times fainter than their hosts, as a function of host stellar mass and morphology between redshifts 0.1 and 0.8, using imaging from the GOODS and COSMOS fields in conjunction with a rigorous statistical analysis. We demonstrate how these measurements provide powerful new constraints for abundance matching and cosmological simulations in the context of both warm and cold dark matter, and how future measurements of faint satellite colors using CANDELS, will provide important distinguishing power between warm and cold dark matter models.In addition, we present results from a complementary gravitational lens modeling project in which we use strongly lensed AGN narrow-line emission in order to detect dark matter subhalos, demonstrating a promising new method for measuring the subhalo mass function in thousands of lensed systems which will be discovered in ongoing and future optical surveys.

With the advent of modern all-sky surveys, the number of known Milky Way (MW) satellite galaxies has expanded well beyond the original eleven classical satellites. One key use of this catalog of objects has been to test predictions of cosmological cold dark matter simulations on the smallest scales in the universe. However, these tests rely on modeling of the density profiles of the satellite galaxies. To build the most accurate model, three dimensional motions of the stars in the satellite are required, a measurement only now becoming possible with the new era of precision astrometry, led by work with the Hubble Space Telescope (HST) and the Gaia Space Telescope. We use data from both of these observatories to study one of the brightest nearby satellites, the Small Magellanic Cloud (SMC), to both understand its own history with the MW and its companion the Large Magellanic Cloud (LMC) and develop a novel kinematic modeling technique for application to the broader MW satellite population. Given the SMC's irregular nature and large spatial extent across the sky, we require broad spatial sampling to thoroughly study its kinematics. To provide this, we observe 30 new fields in the SMC using HST, producing a new proper motion catalog for analysis. Using this catalog, we improve the known systemic motion of the SMC and constrain the minimum separation distance in its last interaction with the LMC to roughly 7 kpc. This places the center of the SMC passing directly through the disk of the LMC, indicating a highly turbulent interaction. Internal kinematics reveal coherent outward motion in the southeastern side of the SMC in the direction of the Magellanic Bridge, consistent with the scenario of ongoing tidal disruption. The Gaia Data Release 2 (DR2) expanded our proper motion catalogs to well over a billion stars across the MW with proper motions, including many thousands in the direction of the Magellanic Clouds. We use this database to present the first kinematic characterization of the stellar component of the Magellinc Bridge. This analysis reveals in the young stars a roughly linearly increasing relative motion from the SMC towards the LMC, at velocities above 100 km/s, suggesting an active outflow of stars (and gas) from the SMC towards the LMC. We compare these kinematics against numerical simulations of the interactions between the SMC and LMC and find good agreement for a recent direct collision scenario, consistent with our original HST work. However, significant uncertainties remain in our understanding of the full internal kinematics of the SMC. We present a new analysis of this system using the large DR2 catalog, attempting to account for both possible coherent rotation within the SMC and a tidal expansion component due to the LMC, as suggested by both our Bridge and earlier HST results. To capture the full 3D information present in the observations (as the proper motions are measured for stars at varying depths along the line of sight), we generate a toy 3D model of the SMC and create mock data for comparisons to the DR2 catalog. We find a need for an updated center of mass location and systemic motion for the older stellar population compared to earlier averaged measurements for the SMC, with the older stars located further from the LMC and moving away from the LMC faster than the younger stars. Taken together, we can understand this as a Bullet Cluster-like scenario where existent old stellar populations in the SMC and LMC passed through relatively unscathed while the gas violently collided, imparting different kinematics on the stars formed post-interaction. Intriguingly, we find a need for a non-zero rotation throughout much of the SMC, at a relatively high inclination angle, in addition to accounting for the tidal expansion in the SMC RGs caused by recent interactions with the LMC. We present a similar analysis for the red supergiant (RSG) population in the SMC, whose age closely coincides with the time of last interaction between the SMC and LMC. We find compelling evidence for the existence of coherent rotation in a subpopulation of the RSGs, potentially mapping onto a previously measured age bimodality in the SMC Classical Cepheid population. The rotation signal appears when the internal kinematics are studied assuming the systemic properties for the SMC RG population, suggesting that the gas within the SMC may have been relatively unperturbed before the most recent interaction. The small area of rotation may also provide new leverage on improving the constraints on the mass ratios of the Magellanic system as it could place a hard boundary on where the gas remained fully bound to the SMC. Taken as a whole, the analysis of the SMC has led to the creation of a modeling framework capable of accounting for compounding kinematic mechanisms (like rotation and tidal expansion) and developing intuition for understanding the broader MW satellite population in a truly 3D manner.

Galaxy spectral energy distributions (SEDs) remain among the most challenging yet informative quantities to reproduce in simulations due to the large and complex mixture of physical processes that shape the radiation output of a galaxy. With the increasing number of surveys utilising broadband colours as part of their target selection criteria, the production of realistic SEDs in simulations is necessary for assisting in survey design and interpretation of observations. The recent success in reproducing the observed luminosity functions (LF) from far-UV to far-IR, using the state-of-the-art semi-analytic model \shark\ and the SED generator \prospect, represents a critical step towards better galaxy colour predictions. We show that with \shark\ and \prospect\ we can closely reproduce the optical colour distributions observed in the panchromatic GAMA survey. The treatment of feedback, star formation, central-satellite interactions and radiation re-processing by dust are critical for this achievement. The first three processes create a bimodal distribution, while dust attenuation defines the location and shape of the blue and red populations. While a naive comparison between observation and simulations displays the known issue of over-quenching of satellite galaxies, the introduction of empirically-motivated observational errors and classification from the same group finder used in GAMA greatly reduces this tension. The introduction of random re-assignment of $\sim 15\%$ of centrals/satellites as satellites/centrals on the simulation classification closely resembles the outcome of the group finder, providing a computationally less intensive method to compare simulations with observations., Accepted for publication in MNRAS after minor corrections (20 pages, 19 figures)

The magnifications of compact-source lenses are extremely sensitive to the presence of low mass dark matter halos along the entire sight line from the source to the observer. Traditionally, the study of dark matter structure in compact-source strong gravitational lenses has been limited to radio-loud systems, as the radio emission is extended and thus unaffected by microlensing which can mimic the signal of dark matter structure. An alternate approach is to measure quasar nuclear-narrow line emission, which is free from microlensing and present in virtually all quasar lenses. In this paper, we double the number of systems which can be used for gravitational lensing analyses by presenting measurements of narrow-line emission from a sample of 8 quadruply imaged quasar lens systems, WGD J0405-3308, HS 0810+2554, RX J0911+0551, SDSS J1330+1810, PS J1606-2333, WFI 2026-4536, WFI 2033-4723 and WGD J2038-4008. We describe our updated grism spectral modelling pipeline, which we use to measure narrow-line fluxes with uncertainties of 2-10\%, presented here. We fit the lensed image positions with smooth mass models and demonstrate that these models fail to produce the observed distribution of image fluxes over the entire sample of lenses. Furthermore, typical deviations are larger than those expected from macromodel uncertainties. This discrepancy indicates the presence of perturbations caused by small-scale dark matter structure. The interpretation of this result in terms of dark matter models is presented in a companion paper., Accepted for publication in MNRAS

Understanding galaxies calls for the understanding of their appearance, which results from their stellar content, and stars form out of cold gas. Feedback processes can impede star formation: by heating the gas or preventing it from cooling and forming stars; by triggering galactic outflows that eject the gas; and by preventing the inflow of gas. In the lowest mass galaxies – dwarf galaxies –powerful and luminous stellar explosions, supernovae (SNe), are thought to be the predominant feedback process. However, recent work suggests that SN feedback is too weak. Moreover, several puzzles surrounding dwarf galaxies challenge our understanding of dark matter – of which the exact nature that is still unknown –, and these discrepancies might be solved by feedback processes. Besides, Active Galactic Nuclei (AGN) – i.e. growing massive black holes –, are another source of feedback in galaxies. The effect of AGN in dwarf galaxies is often thought to be minimal, because their growth is suppressed. But there is growing evidence that their presence in dwarf galaxies might be stronger than once thought. First, I show, using analytical methods, that the capacity of AGN to trigger outflows in dwarf galaxies is stronger than that of SNe in most of the parameter space. I then show numerically, that the AGN of a galaxy can influence the evolution of a neighboring galaxy. Finally, I assess the impact of the injection of cosmic rays by SNe: I find that it reduces the rate of star formation and increases the efficiency of SNe at driving winds, bringing simulated wind properties of dwarf galaxies much closer to observations, but the effect is not sufficient to fully match the observations.; Comprendre l’évolution des galaxies suppose d’expliquer leur apparence, qui résulte de leur contenu stellaire. Or les étoiles se forment à partir de gaz froid. Les processus de rétroaction baryonique peuvent empêcher la formation d’étoiles en chauffant le gaz, en générant des vents galactiques qui expulsent le gaz ou l’empêchent de parvenir dans la galaxie. Sans ces processus, les modèles peinent à reproduire les observations. Certaines étoiles, lorsqu’elles explosent en supernovae (SNe), sont considérées comme la source principale de rétroaction dans les plus petites galaxies – les galaxies naines. Cependant, selon des travaux récents, leur capacité de rétroaction est insuffisante. Par ailleurs, les galaxies naines font l'objet de divergences entre théorie et observations concernant la nature de la matière noire, qui reste méconnue. Il se peut que ces divergences soient résolues en invoquant des processus de rétroaction. En parallèle, les noyaux actifs – des trous noirs massifs qui croissent –, constituent une autre source de rétroaction. Des observations récentes suggèrent qu’ils pourraient avoir de l’importance dans les galaxies naines. Premièrement, je montre analytiquement que la capacité des noyaux actifs à générer des vents galactiques dépasse souvent celle des SNe. Ensuite, je montre numériquement que le noyau actif d’une galaxie peut impacter l’évolution galaxie voisine. Enfin, je montre par une série de simulations numériques que l’injection de rayonnement cosmique par les SNe réduit la formation d’étoiles et augmente la génération de vents, qui sont alors en bien meilleur accord avec les observations, bien qu'encore en deçà.

Understanding galaxies calls for the understanding of their appearance, which results from their stellar content, and stars form out of cold gas. Feedback processes can impede star formation: by heating the gas or preventing it from cooling and forming stars; by triggering galactic outflows that eject the gas; and by preventing the inflow of gas. In the lowest mass galaxies – dwarf galaxies –powerful and luminous stellar explosions, supernovae (SNe), are thought to be the predominant feedback process. However, recent work suggests that SN feedback is too weak. Moreover, several puzzles surrounding dwarf galaxies challenge our understanding of dark matter – of which the exact nature that is still unknown –, and these discrepancies might be solved by feedback processes. Besides, Active Galactic Nuclei (AGN) – i.e. growing massive black holes –, are another source of feedback in galaxies. The effect of AGN in dwarf galaxies is often thought to be minimal, because their growth is suppressed. But there is growing evidence that their presence in dwarf galaxies might be stronger than once thought. First, I show, using analytical methods, that the capacity of AGN to trigger outflows in dwarf galaxies is stronger than that of SNe in most of the parameter space. I then show numerically, that the AGN of a galaxy can influence the evolution of a neighboring galaxy. Finally, I assess the impact of the injection of cosmic rays by SNe: I find that it reduces the rate of star formation and increases the efficiency of SNe at driving winds, bringing simulated wind properties of dwarf galaxies much closer to observations, but the effect is not sufficient to fully match the observations.; Comprendre l’évolution des galaxies suppose d’expliquer leur apparence, qui résulte de leur contenu stellaire. Or les étoiles se forment à partir de gaz froid. Les processus de rétroaction baryonique peuvent empêcher la formation d’étoiles en chauffant le gaz, en générant des vents galactiques qui expulsent le gaz ou l’empêchent de parvenir dans la galaxie. Sans ces processus, les modèles peinent à reproduire les observations. Certaines étoiles, lorsqu’elles explosent en supernovae (SNe), sont considérées comme la source principale de rétroaction dans les plus petites galaxies – les galaxies naines. Cependant, selon des travaux récents, leur capacité de rétroaction est insuffisante. Par ailleurs, les galaxies naines font l'objet de divergences entre théorie et observations concernant la nature de la matière noire, qui reste méconnue. Il se peut que ces divergences soient résolues en invoquant des processus de rétroaction. En parallèle, les noyaux actifs – des trous noirs massifs qui croissent –, constituent une autre source de rétroaction. Des observations récentes suggèrent qu’ils pourraient avoir de l’importance dans les galaxies naines. Premièrement, je montre analytiquement que la capacité des noyaux actifs à générer des vents galactiques dépasse souvent celle des SNe. Ensuite, je montre numériquement que le noyau actif d’une galaxie peut impacter l’évolution galaxie voisine. Enfin, je montre par une série de simulations numériques que l’injection de rayonnement cosmique par les SNe réduit la formation d’étoiles et augmente la génération de vents, qui sont alors en bien meilleur accord avec les observations, bien qu'encore en deçà.

We use a sample of 17 strong gravitational lens systems from the BELLS GALLERY survey to quantify the amount of low-mass dark matter haloes within the lensing galaxies and along their lines of sight, and to constrain the properties of dark matter. Based on a detection criterion of 10$\sigma$, we report no significant detection in any of the lenses. Using the sensitivity function at the 10-$\sigma$ level, we have calculated the predicted number of detectable cold dark matter (CDM) line-of-sight haloes to be $\mu_{l} = 1.17\pm1.08$, in agreement with our null detection. Assuming a detection sensitivity that improved to the level implied by a 5-$\sigma$ threshold, the expected number of detectable line-of-sight haloes rises to $\mu_l = 9.0\pm3.0$. Whilst the current data find zero detections at this sensitivity level (which has a probability of P$^{{\rm5}\sigma}_{{\rm CDM}}(n_{\rm det}=0)$=0.0001 and would be in strong tension with the CDM framework), we find that such a low detection threshold leads to many spurious detections and non-detections and therefore the current lack of detections is unreliable and requires data with improved sensitivity. Combining this sample with a subsample of 11 SLACS lenses, we constrain the half-mode mass to be $\log$(M$_{\rm hm}) < 12.26$ at the 2-$\sigma$ level. The latter is consistent with resonantly produced sterile neutrino masses m$_{\rm s} < 0.8$ keV at any value of the lepton asymmetry at the 2-$\sigma$ level., Comment: 14 pages, 7 figures, submitted for publication on MNRAS

The predicted abundance and properties of the low-mass substructures embedded inside larger dark matter haloes differ sharply among alternative dark matter models. Too small to host galaxies themselves, these subhaloes may still be detected via gravitational lensing, or via perturbations of the Milky Way's globular cluster streams and its stellar disk. Here we use the Apostle cosmological simulations to predict the abundance and the spatial and velocity distributions of subhaloes in the range 10^6.5-10^8.5 solar masses inside haloes of mass ~ 10^12 solar masses in LCDM. Although these subhaloes are themselves devoid of baryons, we find that baryonic effects are important. Compared to corresponding dark matter only simulations, the loss of baryons from subhaloes and stronger tidal disruption due to the presence of baryons near the centre of the main halo, reduce the number of subhaloes by ~ 1/4 to 1/2, independently of subhalo mass, but increasingly towards the host halo centre. We also find that subhaloes have non-Maxwellian orbital velocity distributions, with centrally rising velocity anisotropy and positive velocity bias which reduces the number of low-velocity subhaloes, particularly near the halo centre. We parameterise the predicted population of subhaloes in terms of mass, galactocentric distance, and velocities. We discuss implications of our results for the prospects of detecting dark matter substructures and for possible inferences about the nature of dark matter., Comment: 19 pages, submitted to MNRAS, comments welcome

Aims. The goal of this paper is to demonstrate the outstanding quality of the second data release of the Gaia mission and its power for constraining many di erent aspects of the dynamics of the satellites of the Milky Way. We focus here on determining the proper motions of 75 Galactic globular clusters, nine dwarf spheroidal galaxies, one ultra-faint system, and the Large and Small Magellanic Clouds. Methods. Using data extracted from the Gaia archive, we derived the proper motions and parallaxes for these systems, as well as their uncertainties. We demonstrate that the errors, statistical and systematic, are relatively well understood. We integrated the orbits of these objects in three different Galactic potentials, and characterised their properties. We present the derived proper motions, space velocities, and characteristic orbital parameters in various tables to facilitate their use by the astronomical community. Results. Our limited and straightforward analyses have allowed us for example to (i) determine absolute and very precise proper motions for globular clusters; (ii) detect clear rotation signatures in the proper motions of at least five globular clusters; (iii) show that the satellites of the MilkyWay are all on high-inclination orbits, but that they do not share a single plane of motion; (iv) derive a lower limit for the mass of the Milky Way of 9:8$^{+6:7}_{2:7}$ x 10$6{11}$$\odot$ based on the assumption that the Leo I dwarf spheroidal is bound; (v) derive a rotation curve for the Large Magellanic Cloud based solely on proper motions that is competitive with line-of-sight velocity curves, now using many orders of magnitude more sources; and (vi) unveil the dynamical effect of the bar on the motions of stars in the Large Magellanic Cloud. Conclusions. All these results highlight the incredible power of the Gaia astrometric mission, and in particular of its second data release., Includes STFC and ERC.

We present a large-scale galaxy structure Cl J021734-0513 at z~0.65 discovered in the UKIDSS UDS field, made of ~20 galaxy groups and clusters, spreading over 10 Mpc. We report on a VLT/VIMOS spectroscopic follow-up program that, combined with past spectroscopy, allowed us to confirm four galaxy clusters (M200 ~ 10^14 Msol) and a dozen associated groups and star-forming galaxy overdensities. Two additional filamentary structures at z ~ 0.62 and z ~ 0.69 and foreground and background clusters at 0.6 < z < 0.7 were also confirmed along the line-of-sight. The structure subcomponents are at different formation stages. The clusters have a core dominated by passive galaxies and an established red sequence. The remaining structures are a mix of star-forming galaxy overdensities and forming groups. The presence of quiescent galaxies in the core of the latter shows that `preprocessing' has already happened before the groups fall into their more massive neighbours. Our spectroscopy allows us to derive spectral index measurements e.g. emission/absorption line equivalent widths, strength of the 4000A break, valuable to investigate the star formation history of structure members. Based on these line measurements, we select a population of `poststarburst' galaxies. These galaxies are preferentially found within the virial radius of clusters, supporting a scenario in which their recent quenching could be prompted by gas stripping by the dense intracluster medium. We derive stellar age estimates using MCMC-based spectral fitting for quiescent galaxies and find a correlation between ages and colours/stellar masses which favours a top-down formation scenario of the red sequence. A catalogue of ~650 redshifts in UDS will be released alongside the paper., 23 pages, 19 figures, 4 tables, accepted for publication in MNRAS. A catalogue of ~650 VIMOS spectroscopic redshifts in UKIDSS UDS will be released alongside the paper. Feel free to contact the authors for early access to the catalogue

Context. The great majority of early-type dwarf galaxies, in the Local Group as well as in other galaxy groups, are found in the vicinity of much larger galaxies, making it hard to disentangle the role of internal versus external effects in driving their evolution., Aims. In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of these systems, we study one of the few dwarf spheroidal galaxies of the Local Group found in isolation: Cetus. This system is of particular interest since it does not follow the Local Group morphology-density relation., Methods. We obtained Very Large Telescope (VLT) FORS2 spectra (R similar to 2600) in the region of the nIR CaII triplet lines for 80 candidate red giant branch (RGB) stars. The analysis yielded line-of-sight velocities and metallicities ([Fe/H]) for 54 bona fide member stars. Results. The kinematic analysis shows that Cetus is a mainly pressure-supported (sigma(v) = 11.0(-1.3)(+1.6) km s(-1)), dark-matter-dominated system (M-1/2/L-V = 23.9(-8.9)(+9.)7 M-circle dot/L-circle dot) with no significant signs of internal rotation. We find Cetus to be a metal-poor system with a significant metallicity spread (median [Fe/H] = -1.71 dex, median-absolute-deviation = 0.49 dex), as expected for its stellar mass. We report the presence of a mild metallicity gradient compatible with those found in other dwarf spheroidals of the same luminosity; we trace the presence of a stellar population gradient also in the spatial distribution of stars in different evolutionary phases in ancillary SuprimeCam photometric data. There are tentative indications of two chemo-kinematically distinct sub-populations, with the more metal-poor stars showing a hotter kinematics than the metal-richer ones. Furthermore, the photometric dataset reveals the presence of a foreground population that most likely belongs to the Sagittarius stream., Conclusions. This study represents an important step forward in assessing the internal kinematics of the Cetus dwarf spheroidal galaxy as well as the first wide-area spectroscopic determination of its metallicity properties. With our analysis, Cetus adds to the growing scatter in stellar-dark matter halo properties in low-mass galactic systems. The presence of a metallicity gradient akin to those found in similarly luminous and pressure-supported systems inhabiting very different environments may hint at metallicity gradients in Local Group early-type dwarfs being driven by internal mechanisms.

We describe a new Large Program in progress on the Gemini North and South telescopes: Gemini Observations of Galaxies in Rich Early Environments (GOGREEN). This is an imaging and deep spectroscopic survey of 21 galaxy systems at $110$ in halo mass. The scientific objectives include measuring the role of environment in the evolution of low-mass galaxies, and measuring the dynamics and stellar contents of their host haloes. The targets are selected from the SpARCS, SPT, COSMOS and SXDS surveys, to be the evolutionary counterparts of today's clusters and groups. The new red-sensitive Hamamatsu detectors on GMOS, coupled with the nod-and-shuffle sky subtraction, allow simultaneous wavelength coverage over $\lambda\sim 0.6$--$1.05\mu$m, and this enables a homogeneous and statistically complete redshift survey of galaxies of all types. The spectroscopic sample targets galaxies with AB magnitudes $z^{\prime}, Comment: Accepted for publication in MNRAS May 26, 2017

The cosmic web is one of the most striking features of the distribution of galaxies and dark matter on the largest scales in the Universe. It is composed of dense regions packed full of galaxies, long filamentary bridges, flattened sheets and vast low density voids. The study of the cosmic web has focused primarily on the identification of such features, and on understanding the environmental effects on galaxy formation and halo assembly. As such, a variety of different methods have been devised to classify the cosmic web -- depending on the data at hand, be it numerical simulations, large sky surveys or other. In this paper we bring twelve of these methods together and apply them to the same data set in order to understand how they compare. In general these cosmic web classifiers have been designed with different cosmological goals in mind, and to study different questions. Therefore one would not {\it a priori} expect agreement between different techniques however, many of these methods do converge on the identification of specific features. In this paper we study the agreements and disparities of the different methods. For example, each method finds that knots inhabit higher density regions than filaments, etc. and that voids have the lowest densities. For a given web environment, we find substantial overlap in the density range assigned by each web classification scheme. We also compare classifications on a halo-by-halo basis; for example, we find that 9 of 12 methods classify around a third of group-mass haloes (i.e. $M_{\rm halo}\sim10^{13.5}h^{-1}M_{\odot}$) as being in filaments. Lastly, so that any future cosmic web classification scheme can be compared to the 12 methods used here, we have made all the data used in this paper public., Comment: 24 pages, 8 figures, 2 tables. Submitted to MN. Comments Welcome

The shallow faint-end slope of the galaxy mass function is usually reproduced in $\Lambda$CDM galaxy formation models by assuming that the fraction of baryons that turns into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities $V_{\rm max}, Comment: replaced to match the accepted version. 2 new figures added compared to the previous version

Aims.We perform clustering measurements of 800 X-ray selectedChandraCOSMOS Legacy (CCL) Type 2 active galactic nuclei (AGN) with known spectroscopic redshift to probe the halo mass dependence on AGN host galaxy properties, such as galaxy stellar massMstar, star formation rate (SFR), and specific black hole accretion rate (BHAR;λBHAR) in the redshift rangez = [0−3].Methods.We split the sample of AGN with known spectroscopic redshits according toMstar, SFR andλBHAR, while matching the distributions in terms of the other parameters, including redshift. We measured the projected two-point correlation functionwp(rp) and modeled the clustering signal, for the different subsamples, with the two-halo term to derive the large-scale biasband corresponding typical mass of the hosting halo.Results.We find no significant dependence of the large-scale bias and typical halo mass on galaxy stellar mass and specific BHAR for CCL Type 2 AGN at meanz ∼ 1, while a negative dependence on SFR is observed, i.e. lower SFR AGN reside in richer environment. Mock catalogs of AGN, matched to have the same X-ray luminosity, stellar mass,λBHAR, and SFR of CCL Type 2 AGN, almost reproduce the observedMstar − Mh,λBHAR − Mhand SFR–Mhrelations, when assuming a fraction of satellite AGNfAGNsat∼ 0.15. This corresponds to a ratio of the probabilities of satellite to central AGN of being activeQ ∼ 2. Mock matched normal galaxies follow a slightly steeperMstar − Mhrelation, in which low mass mock galaxies reside in less massive halos than mock AGN of similar mass. Moreover, matched mock normal galaxies are less biased than mock AGN with similar specific BHAR and SFR, at least forQ > 1.

We examine the spatial distribution of the oldest and most metal poor stellar populations of Milky Way-sized galaxies using the APOSTLE cosmological hydrodynamical simulations of the Local Group. In agreement with earlier work, we find strong radial gradients in the fraction of the oldest (tform < 0.8 Gyr) and most metal poor ([Fe/H]< -2.5) stars, both of which increase outwards. The most metal poor stars form over an extended period of time; half of them form after z = 5.3, and the last 10% after z = 2.8. The age of the metal poor stellar population also shows significant variation with environment; a high fraction of them are old in the galaxy's central regions and an even higher fraction in some individual dwarf galaxies, with substantial scatter from dwarf to dwarf. Overall, over half of the stars that belong to both the oldest and most metal-poor population are found outside the solar circle. Somewhat counter-intuitively, we find that dwarf galaxies with a large fraction of metal poor stars that are very old are systems where metal poor stars are relatively rare, but where a substantial old population is present. Our results provide guidance for interpreting the results of surveys designed to hunt for the earliest and most pristine stellar component of our Milky Way., 14 pages, 9 figures. Submitted to MNRAS, revised after referee comments

We use cosmological simulations from the Aquarius Project to study the orbital history of the Large Magellanic Cloud (LMC) and its potential association with other satellites of the Milky Way (MW). We search for dynamical analogues to the LMC and find a subhalo that matches the LMC position and velocity at either of its two most recent pericentric passages. This suggests that the LMC is not necessarily on its first approach to the MW, provided that the virial mass of the MW is as high as that of the parent Aquarius halo; M200= 1.8 × 1012 M⊙. The simulation results yield specific predictions for the position and velocity of systems associated with the LMC prior to infall. If on the first approach, most should lie close to the LMC because the Galactic tidal field has not yet had enough time to disperse them. If on the second approach, the list of potential associates increases substantially because of the greater sky footprint and velocity range of LMC-associated debris. Interestingly, our analysis rules out an LMC association for Draco and Ursa Minor, two of the dwarf spheroidals suggested by Lynden-Bell & Lynden-Bell to form part of the 'Magellanic Ghostly Stream'. Our results also indicate that the direction of the orbital angular momentum is a powerful test of LMC association. This test, however, requires precise proper motions, which are unavailable for most MW satellites. Of the four satellites with published proper motions, only the Small Magellanic Cloud is clearly associated with the LMC. Taken at the face value, the proper motions of Carina, Fornax and Sculptor rule them out as potential associates, but this conclusion should be revisited when better data become available. The dearth of satellites clearly associated with the Clouds might be solved by wide-field imaging surveys that target its surroundings, a region that may prove a fertile hunting ground for faint, previously unnoticed MW satellites.

We use the high-resolution cosmological N-body simulations from the Aquarius project to investigate in detail the mechanisms that determine the shape of Milky Way type dark matter haloes. We find that, when measured at the instantaneous virial radius, the shape of individual haloes changes with time, evolving from a typically prolate configuration at early stages to a more triaxial/oblate geometry at the present day. This evolution in halo shape correlates well with the distribution of the infalling material: prolate configurations arise when haloes are fed through narrow filaments, which characterizes the early epochs of halo assembly, whereas triaxial/oblate configurations result as the accretion turns more isotropic at later times. Interestingly, at redshift z = 0, clear imprints of the past history of each halo are recorded in their shapes at different radii, which also exhibit a variation from prolate in the inner regions to triaxial/oblate in the outskirts. Provided that the Aquarius haloes are fair representatives of Milky Way like 10(12) M-circle dot objects, we conclude that the shape of such dark matter haloes is a complex, time-dependent property, with each radial shell retaining memory of the conditions at the time of collapse.

Semi-analytic models (SAMs) are currently one of the primary tools with which we model statistically significant ensembles of galaxies. The underlying physical prescriptions inherent to each SAM are, in many cases, different from one another. Several SAMs have been applied to the dark matter merger trees extracted from the Millennium Run, including those associated with the well-known Munich and Durham lineages. We compare the predicted luminosity distributions of galaxy groups using four publicly available SAMs, in order to explore a galactic environment in which the models have not been explored to the same degree as they have in the field or in rich clusters. We identify a characteristic 'wiggle' in the group galaxy luminosity function generated using the De Lucia et al. SAM, which is not present in the Durham-based models, consistent to some degree with observations. However, a comparison between conditional luminosity functions of groups between the models and observations suggests that neither model is a particularly good match. The luminosity function wiggle is interpreted as the result of the two-mode active galactic nucleus feedback implementation used in the Munich models, which itself results in flattened magnitude gap distribution. An associated analysis of the magnitude gap distribution between first-and second-ranked group galaxies shows that while the Durham models yield distributions with approximately equal luminosity first-and second-ranked galaxies, in agreement with observations, the De Lucia et al. models favour the scenario in which the second-ranked galaxy is approximately 1 mag fainter than the primary, especially when the dynamic range of the mock data is limited to 3 mag.

(Abridged) We use cosmological N-body simulations to study the properties of substructure halos in galaxy-sized dark matter halos. We extend prior work on the subject by considering the whole population of subhalos physically associated with the main system. These are defined as subhalos that have at some time in the past been within the virial radius of the halo's main progenitor and that have survived as self-bound entities to $z=0$. We find that this population extends beyond {\it three times} the virial radius, and contains objects on extreme orbits. We trace the origin of these unorthodox orbits to the tidal dissociation of bound groups of subhalos, which results in the ejection of some subhalos along tidal streams. Ejected subhalos are primarily low-mass systems, leading to mass-dependent biases in their spatial distribution and kinematics: the lower the subhalo mass at accretion time, the less centrally concentrated and kinematically hotter their descendant population. The bias is strongest amongst the most massive subhalos, but disappears at the low-mass end. Our findings imply that subhalos identified within the virial radius represent an incomplete census of the substructure physically related to a halo: only about {\it one half} of all associated subhalos are found today within the virial radius of a halo. These results may explain the age dependence of the clustering of low-mass halos, and has implications for (i) the interpretation of the structural parameters and assembly histories of halos neighboring massive systems; (ii) the existence of low-mass dynamical outliers in the Local Group; and (iii) the presence of evidence for evolutionary effects well outside the traditional virial boundary of a galaxy system., Comment: 12 pages, 8 figures, ApJ in press

We use ZENS data at low redshift to study the dependence of the quenched satellite fraction and of the morphological mix of these quenched satellites, on three different environmental parameters: group halo mass, halo-centric distance and large-scale structure over-density. The fraction of quenched satellites is independent of halo mass and the surrounding large-scale overdensity, but increases towards the centres of the haloes, as previously found. The morphological mix is, however, constant with radial position, indicating that the well-known morphology-density relation results from the increasing fraction of quenched galaxies towards the centres of haloes. The constancy of the morphological outcome suggests that mass-quenching and satellite quenching have the same effect on the morphologies of the galaxies. The quenched satellites have larger B/T and smaller half-light radii than the star-forming satellites. These are mostly due to differences in the disks. The bulges in quenched satellites have very similar luminosities and surface brightness profiles, and any mass growth of the bulges associated with quenching cannot greatly change these quantities. The quenched disks are fainter and have smaller scale lengths than in star-forming satellites. This can be explained either by a differential fading of the disks or if disks were generally smaller in the past, both of which are expected in an inside-out growth of disks. At least at low redshifts, the structure of massive quenched satellites is produced by processes that operate before quenching takes place. A comparison with semi-analytic models argues for a reduction in the efficiency of group halos in quenching their disk satellites and for mechanisms to increase the B/T of low mass quenched satellites. [abridged], Submitted to ApJ, 23 pages, 13 figures; Comments welcome

Stellar halos and globular cluster (GC) systems contain valuable information regarding the assembly history of their host galaxies. Motivated by the detection of a significant rotation signal in the outer halo GC system of M31, we investigate the likelihood of detecting such a rotation signal in projection, using cosmological simulations. To this end we select subsets of tagged particles in the halos of the Aquarius simulations to represent mock GC systems, and analyse their kinematics. We find that GC systems can exhibit a non-negligible rotation signal provided the associated stellar halo also has a net angular momentum. The ability to detect this rotation signal is highly dependent on the viewing perspective, and the probability of seeing a signal larger than that measured in M31 ranges from 10% to 90% for the different halos in the Aquarius suite. High values are found from a perspective such that the projected angular momentum of the GC system is within 40 deg of the rotation axis determined via the projected positions and line-of-sight velocities of the GCs. Furthermore, the true 3D angular momentum of the outer stellar halo is relatively well aligned, within 35 deg, with that of the mock GC systems. We argue that the net angular momentum in the mock GC systems arises naturally when the majority of the material is accreted from a preferred direction, namely along the dominant dark matter filament of the large-scale structure that the halos are embedded in. This, together with the favourable edge-on view of M31's disk suggests that it is not a coincidence that a large rotation signal has been measured for its outer halo GC system., Comment: 18 pages, 13 figures, 1 table. Accepted by Astronomy & Astrophysics

We generate mock galaxy catalogues for a grid of different cosmologies, using rescaled N-body simulations in tandem with a semi-analytic model run using consistent parameters. Because we predict the galaxy bias, rather than fitting it as a nuisance parameter, we obtain an almost pure constraint on sigma(8) by comparing the projected two-point correlation function we obtain to that from the Sloan Digital Sky Survey (SDSS). A systematic error arises because different semi-analytic modelling assumptions allow us to fit the r-band luminosity function equally well. Combining our estimate of the error from this source with the statistical error, we find sigma(8) = 0.97 +/- 0.06. We obtain consistent results if we use galaxy samples with a different magnitude threshold, or if we select galaxies by b(J)-band rather than r-band luminosity and compare to data from the 2dF Galaxy Redshift Survey (2dFGRS). Our estimate for sigma(8) is higher than that obtained for other analyses of galaxy data alone, and we attempt to find the source of this difference. We note that in any case, galaxy clustering data provide a very stringent constraint on galaxy formation models.

The thin, extended planes of satellite galaxies detected around both the Milky Way and Andromeda are not a natural prediction of the LCDM paradigm. Galaxies in these distinct planes may have formed and evolved in a different way (e.g., tidally) to their off-plane neighbours. If this were the case, one would expect the on- and off-plane dwarf galaxies in Andromeda to have experienced different evolutionary histories, which should be reflected by the chemistries, dynamics, and star formation histories of the two populations. In this work, we present new, robust kinematic observations for 2 on-plane M31 dSphs (And XVI and XVII) and compile and compare all available observational metrics for the on- and off-plane dwarfs to search for a signal that would corroborate such a hypothesis. We find that, barring their spatial alignment, the on- and off-plane Andromeda dwarf galaxies are indistinguishable from one another, arguing against vastly different formative and evolutionary histories for these two populations., 7 pages, 3 figures, accepted for publication in ApJ Letters

We identify 42 "candidate groups" lying between 1.8

We identify 42 "candidate groups" lying between 1.8

We identify 42 "candidate groups" lying between 1.8

We present deep photometry in the B,V and I filters from CTIO/MOSAIC for about 270.000 stars in the Fornax dwarf Spheroidal galaxy, out to a radius of r_ell\sim0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual Red Giant Branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations, but also includes ancient (10-14 Gyr), and young (-1.5 dex, with a clear trend in age., Comment: 18 pages, 20 figures

We use cosmological simulations from the Aquarius Project to study the orbital history of the Large Magellanic Cloud (LMC) and its potential association with other satellites of the Milky Way (MW). We search for dynamical analogs to the LMC and find a subhalo that matches the LMC position and velocity at either of its two most recent pericentric passages. This suggests that the LMC is not necessarily on its first approach to the MW, provided that the virial mass of the Milky Way is M_200 ~1.8e12 M_sun. The simulation results yield specific predictions for the position and velocity of systems associated with the LMC prior to infall. If on first approach, most should lie close to the LMC because the Galactic tidal field has not yet had enough time to disperse them. If on second approach, the list of potential associates increases substantially. Interestingly, our analysis rules out an LMC association for Draco and Ursa Minor, two of the dwarf spheroidals suggested by Lynden-Bell & Lynden-Bell to form part of the "Magellanic Ghostly Stream". Our results also indicate that the direction of the orbital angular momentum is a powerful test of LMC association. This test, however, requires precise proper motions, which are unavailable for most MW satellites. Of the 4 satellites with published proper motions, only the Small Magellanic Cloud is clearly associated with the LMC. Taken at face value, the proper motions of Carina, Fornax and Sculptor rule them out as potential associates, but this conclusion should be revisited when better data become available. The dearth of satellites clearly associated with the Clouds might be solved by wide-field imaging surveys that target its surroundings, a region that may prove a fertile hunting ground for faint, previously unnoticed MW satellites., 13 pages, 8 figures, MNRAS in press. Minor corrections added to match published version

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved, Semi-analytic models (SAMs) are currently one of the primary tools with which we model statistically significant ensembles of galaxies. The underlying physical prescriptions inherent to each SAM are, in many cases, different from one another. Several SAMs have been applied to the dark matter merger trees extracted from the Millennium Run, including those associated with the well-known Munich and Durham lineages. We compare the predicted luminosity distributions of galaxy groups using four publicly available SAMs, in order to explore a galactic environment in which the models have not been explored to the same degree as they have in the field or in rich clusters. We identify a characteristic 'wiggle' in the group galaxy luminosity function generated using the De Lucia et al. SAM, which is not present in the Durham-based models, consistent to some degree with observations. However, a comparison between conditional luminosity functions of groups between the models and observations suggests that neither model is a particularly good match. The luminosity function wiggle is interpreted as the result of the two-mode active galactic nucleus feedback implementation used in the Munich models, which itself results in flattened magnitude gap distribution. An associated analysis of the magnitude gap distribution between first- and second-ranked group galaxies shows that while the Durham models yield distributions with approximately equal luminosity first- and second-ranked galaxies, in agreement with observations, the De Lucia et al. models favour the scenario in which the second-ranked galaxy is approximately 1 mag fainter than the primary, especially when the dynamic range of the mock data is limited to 3 mag., ONS acknowledges the support of the STFC through its PhD Studentship Programme. BKG and CBB acknowledge the support of the UK Science & Technology Facilities Council (STFC Grant ST/F002432/1) and the Commonwealth Cosmology Initiative; visitor support (PS-B, DK, AK and LVS) from the STFC (ST/G003025/1) is similarly acknowledged. PS-B acknowledges the support of a Marie Curie Intra-European Fellowship within the 6th European Community Framework Programme. AK and PS-B are supported by the Ministerio de Ciencia e Innovación (MICINN) in Spain through the Ramón y Cajal programme. The Millennium Simulation data bases used in this paper and the web application providing online access to them were constructed as part of the activities of the German Astrophysical Virtual Observatory. Access to the University of Central Lancashire’s High Performance Computing Facility is gratefully acknowledged. We acknowledge the computational support provided by the UK National Cosmology Supercomputer, COSMOS. We thank the DEISA consortium, cofunded through EU FP6 project RI-031513 and the FP7 project RI-222919, for support within the DEISA Extreme Computing Initiative

We use the high resolution cosmological N-body simulations from the Aquarius project to investigate in detail the mechanisms that determine the shape of Milky Way-type dark matter haloes. We find that, when measured at the instantaneous virial radius, the shape of individual haloes changes with time, evolving from a typically prolate configuration at early stages to a more triaxial/oblate geometry at the present day. This evolution in halo shape correlates well with the distribution of the infalling material: prolate configurations arise when haloes are fed through narrow filaments, which characterizes the early epochs of halo assembly, whereas triaxial/oblate configurations result as the accretion turns more isotropic at later times. Interestingly, at redshift z=0, clear imprints of the past history of each halo are recorded in their shapes at different radii, which also exhibit a variation from prolate in the inner regions to triaxial/oblate in the outskirts. Provided that the Aquarius haloes are fair representatives of Milky Way-like 10^12 Msun objects, we conclude that the shape of such dark matter haloes is a complex, time-dependent property, with each radial shell retaining memory of the conditions at the time of collapse., Comment: 16 pages, 14 figures. Accepted for publication in MNRAS. Minor changes to match published version

We provide a physical interpretation and explanation of the morphology-density relation for galaxies, drawing on stellar masses, star formation rates, axis ratios and group halo masses from the Sloan Digital Sky Survey. We first re-cast the classical morphology-density relation in more quantitative terms, using low star formation rate (quiescence) as a proxy for early-type morphology and dark matter halo mass from a group catalog as a proxy for environmental density: for galaxies of a given stellar mass the quiescent fraction is found to increase with increasing dark matter halo mass. Our novel result is that - at a given stellar mass - quiescent galaxies are significantly flatter in dense environments, implying a higher fraction of disk galaxies. Supposing that the denser environments differ simply by a higher incidence of quiescent disk galaxies that are structurally similar to star-forming disk galaxies of similar mass, explains simultaneously and quantitatively these quiescence -nvironment and shape-environment relations. Our findings add considerable weight to the slow removal of gas as the main physical driver of the morphology-density relation, at the expense of other explanations.

We report the discovery that substructures/subhaloes of a galaxy-size halo tend to fall in together in groups in cosmological simulations, something that may explain the oddity of the MW satellite distribution. The original clustering at the time of infall is still discernible in the angular momenta of the subhaloes even for events which took place up to eight Gyrs ago, $z \sim 1$. This phenomenon appears to be rather common since at least 1/3 of the present-day subhaloes have fallen in groups in our simulations. Hence, this may well explain the Lynden-Bell & Lynden-Bell ghostly streams. We have also found that the probability of building up a flattened distribution similar to the MW satellites is as high as $\sim 80%$ if the MW satellites were from only one group and $\sim 20%$ when five groups are involved. Therefore, we conclude that the `peculiar' distribution of satellites around the MW can be expected with the CDM structure formation theory. This non-random assignment of satellites to subhaloes implies an environmental dependence on whether these low-mass objects are able to form stars, possibly related to the nature of reionization in the early Universe., To appear in the proceedings of IAU Symposium 254 "The Galaxy disk in a cosmological context", Copenhagen, June 2008, 6 pages, 2 figures

We analyse the dynamical properties of substructures in a high-resolution dark matter simulation of the formation of a Milky Way-like halo in a $\Lambda$CDM cosmology. Our goal is to shed light on the dynamical peculiarities of the Milky Way satellites. Our simulations show that about 1/3 of the subhalos have been accreted in groups. We quantify this clustering by measuring the alignment of the angular momentum of subhalos in a group. We find that this signal is visible even for objects accreted up to $z \sim 1$, i.e. 8 Gyr ago, and long after the spatial coherence of the groups has been lost due the host tidal field. This group infall may well explain the ghostly streams proposed by Lynden-Bell & Lynden-Bell to orbit the Milky Way. Our analyses also show that if most satellites originate in a few groups, the disk-like distribution of the Milky Way satellites would be almost inevitable. This non-random assignment of satellites to subhalos implies an environmental dependence on whether these low-mass objects are able to form stars, possibly related to the nature of reionization in the early Universe. With this picture, both the ``ghostly streams'' and the ``disk-like configuration'' are manifestations of the same phenomenon: the hierarchical growth of structure down to the smallest scales., Comment: 9 pages, 10 figures, MNRAS in press