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Star formation in the early Universe has left its imprint on the chemistry of observable stars in galaxies. We derive elemental abundances and the slope of the low-mass end of the initial mass function (IMF) for a sample of 25 very massive galaxies, separated into brightest cluster galaxies (BCGs) and their massive satellites. The elemental abundances of BGCs and their satellites are similar, but for some elements, satellite galaxies show a correlation with the global velocity dispersion. Using a subset of derived elemental abundances, we model the star formation histories of these galaxies with chemical evolution models, and predict the high-mass end slope of the IMF and star formation time-scales. The high-mass end IMF slope of the satellite galaxies correlates with the global velocity dispersion. The low- and the high-mass end IMF slopes are weakly correlated in a general sense that top heavy IMFs are paired with bottom heavy IMFs. Our results do not necessarily imply that the IMF was simultaneously bottom and top heavy. Instead, our findings can be considered consistent with a temporal variation in the IMF, where, for massive galaxies, the high-mass end IMF slope is representative of the very early age and the low-mass end slope of the later star formation. The small but noticeable differences between the BCGs and the satellites in terms of their elemental abundances and IMF slopes, together with their stellar kinematical properties, suggest somewhat different formation pathways, where BCGs experience more major, gas-free mergers. [ABSTRACT FROM AUTHOR]

With the advent of large spectroscopic surveys the amount of high quality chemodynamical data in the Milky Way (MW) increased tremendously. Accurately and correctly capturing and explaining the detailed features in the high-quality observational data is notoriously difficult for state-of-the-art numerical models. In order to keep up with the quantity and quality of observational data sets, improved prescriptions for galactic chemical evolution need to be incorporated into the simulations. Here we present a new, flexible, time-resolved chemical enrichment model for cosmological simulations. Our model allows us to easily change a number of stellar physics parameters such as the shape of the initial mass function (IMF), stellar lifetimes, chemical yields, or SN Ia delay times. We implement our model into the Gasoline2 code and perform a series of cosmological simulations varying a number of key parameters, foremost evaluating different stellar yield sets for massive stars from the literature. We find that total metallicity, total iron abundance, and gas phase oxygen abundance are robust predictions from different yield sets and in agreement with observational relations. On the other hand, individual element abundances, especially alpha-elements show significant differences across different yield sets and none of our models can simultaneously match constraints on the dwarf and MW mass scale. This offers a unique way of observationally constraining model parameters. For MW mass galaxies we find for most yield tables tested in this work a bimodality in the [α/Fe] versus [Fe/H] plane of rather low intrinsic scatter potentially in tension with the observed abundance scatter. [ABSTRACT FROM AUTHOR]

To date at least 10 highly flattened planes of dwarf galaxies are claimed to have been discovered in the Local Universe. The origin of these planes of galaxies remains unknown. One suggestion is that they are related to the large-scale structure of the cosmic web. A recent study found that the normal of a number of these dwarf galaxy planes is very closely aligned with the eigenvector of the shear tensor corresponding to the direction of greatest collapse obtained by reconstructing the full velocity field in the linear regime. Here we extend that work by both considering an additional 5 planes beyond the five examined previously and examining the alignment with respect to the quasi-linear field, a more sophisticated reconstruction, which is a better approximation on smaller (quasi-linear) scales. Our analysis recovers the previous result while not finding a significantly tight alignment with the additional five planes. However, the additional five plane normals also do not appear to be randomly oriented. We conclude that this could be due either to the normals of the new planes being poorly defined and described; the quasi-linear field at those locations being poorly constrained; or different formation mechanisms for the orientation of planes of dwarf galaxies. [ABSTRACT FROM AUTHOR]

We present the stellar velocity maps of 25 massive early-type galaxies located in dense environments observed with MUSE. Galaxies are selected to be brighter than MK = -25.7 mag, reside in the core of the Shapley Super Cluster or be the brightest galaxy in clusters richer than the Virgo Cluster.We thus targeted galaxies more massive than 1012 M... and larger than 10 kpc (half-light radius). The velocity maps show a large variety of kinematic features: oblate-like regular rotation, kinematically distinct cores, and various types of non-regular rotation. The kinematic misalignment angles show that massive galaxies can be divided into two categories: those with small or negligible misalignment and those with misalignment consistent with being 90°. Galaxies in this latter group, comprising just under half of our galaxies, have prolate-like rotation (rotation around the major axis). Among the brightest cluster galaxies the incidence of prolate-like rotation is 50 per cent, while for a magnitude limited sub-sample of objects within the Shapley Super Cluster (mostly satellites), 35 per cent of galaxies show prolate-like rotation. Placing our galaxies on the mass--size diagram, we show that they all fall on a branch extending almost an order of magnitude in mass and a factor of 5 in size from the massive end of galaxies, previously recognized as associated with major dissipation-less mergers. The presence of galaxies with complex kinematics and, particularly, prolate-like rotators suggests, according to current numerical simulations, that the most massive galaxies grow predominantly through dissipation-less equal-mass mergers. [ABSTRACT FROM AUTHOR]

Tidal torque theory suggests that galaxies gain angular momentum in the linear stage of structure formation. Such a theory predicts alignments between the spin of haloes and tidal shear field. However, non-linear evolution and angular momentum acquisition may alter this prediction significantly. In this paper, we use a reconstruction of the cosmic shear field from observed peculiar velocities combined with spin axes extracted from galaxies within 115 Mpc (~8000 km s-1) from 2MASS Redshift Survey (2MRS) catalogue to test whether or not galaxies appear aligned with principal axes of shear field. Although linear reconstructions of the tidal field have looked at similar issues, this is the first such study to examine galaxy alignments with velocity shear field. Ellipticals in the 2MRS sample show a statistically significant alignment with two of the principal axes of the shear field. In general, elliptical galaxies have their short axis aligned with the axis of greatest compression and perpendicular to the axis of slowest compression. Spiral galaxies show no signal. Such an alignment is significantly strengthened when considering only those galaxies that are used in velocity field reconstruction. When examining such a subsample, a weak alignment with the axis of greatest compression emerges for spiral galaxies as well. This result indicates that although velocity field reconstructions still rely on fairly noisy and sparse data, the underlying alignment with shear field is strong enough to be visible even when small numbers of galaxies are considered -especially if those galaxies are used as constraints in the reconstruction. [ABSTRACT FROM AUTHOR]

Multi-Unit Spectroscopic Explorer (MUSE) observations of NGC 5813 reveal a complex structure in the velocity dispersion map, previously hinted at by SAURON observations. The structure is reminiscent of velocity dispersion maps of galaxies comprising two counter- rotating discs, and might explain the existence of the kinematically distinct core (KDC). Further evidence for two counter-rotating components comes from the analysis of the higher moments of the stellar line-of-sight velocity distributions and fitting MUSE spectra with two separate Gaussian line-of-sight velocity distributions. The emission-line kinematics show evidence of being linked to the present cooling flows and the buoyant cavities seen in X-rays. We detect ionized gas in a nuclear disc-like structure, oriented like the KDC, which is, however, not directly related to the KDC. We build an axisymmetric Schwarzschild dynamical model, which shows that the MUSE kinematics can be reproduced well with two counter-rotating orbit families, characterized by relatively low angular momentum components, but clearly separated in integral phase space and with radially varying contributions. The model indicates that the counter-rotating components in NGC 5813 are not thin discs, but dynamically hot structures. Our findings give further evidence that KDCs in massive galaxies should not necessarily be considered as structurally or dynamically decoupled regions, but as the outcomes of the mixing of different orbital families, where the balance in the distribution of mass of the orbital families is crucial. We discuss the formation of the KDC in NGC 5813 within the framework of gas accretion, binary mergers and formation of turbulent thick discs from cold streams at high redshift. [ABSTRACT FROM AUTHOR]

Aims. We study the two main constituent galaxies of a constrained simulation of the Local Group as candidates for the Milky Way (MW) and Andromeda (M31). We focus on the formation of the stellar discs and its relation to the formation of the group as a rich system with two massive galaxies, and investigate the effects of mergers and accretion as drivers of morphological transformations. We also assess the effects of varying the assumed feedback model on our results by running two different simulations, a first one where only supernova feedback is included and a second where we also model radiation pressure from stars. Methods. We use a state-of-the-art hydrodynamical code which includes star formation, feedback, and chemical enrichment to carry out our study. We use our two simulations, where we include or neglect the effects of radiation pressure from stars, to investigate the impact of this process on the morphologies and star formation rates of the simulated galaxies. Results. We find that the simulated M31 and MW have different formation histories, even though both inhabit, at z=0, the same environment. These differences directly translate into and explain variations in their star formation rates, in situ fractions, and final morphologies. The simulated M31 candidate has an active merger history, as a result of which its stellar disc is unable to survive unaffected until the present time. In contrast, the MW candidate has a smoother history with no major mergers at late times, and forms a disc that grows steadily; at z=0 the simulated MW has an extended, rotationally-supported disc that is dominant over the bulge. Our two feedback implementations predict similar evolutions of the galaxies and their discs, although some variations are detected, the most important of which is the formation time of the discs: in the model with weaker (stronger) feedback the discs form earlier (later). In summary, by comparing the formation histories of the two simulated galaxies, we conclude that the particular merger and accretion history of a galaxy rather than its environment at the LG-scales is the main driver of the formation and subsequent growth or destruction of galaxy discs. [ABSTRACT FROM AUTHOR]

We perform a set of non-radiative hydrodynamical simulations of merging spherical halos in order to understand the angular momentum (AM) properties of the galactic halos seen in cosmological simulations. The universal shape of AM distributions seen in simulations is found to be generically produced as a result of mergers. The universal shape is such that it has an excess of low AM material and hence cannot explain the exponential structure of disk galaxies. A resolution to this is suggested by the spatial distribution of low AM material which is found to be in the center and a conical region close to the axis of rotation. A mechanism that preferentially discards the material in the center and prevents the material along the poles from falling onto the disk is proposed as a solution. We implement a simple geometric criterion for the selective removal of low AM material and show that in order for 90% of halos to host exponential disks one has to reject at least 40% of material. Next, we explore the physical mechanisms responsible for distributing the AM within the halo during a merger. For dark matter there is an inside--out transfer of AM, whereas for gas there is an outside--in transfer, which is due to differences between collision less and gas dynamics. This is responsible for the spin parameter λ and the shape parameter α of AM distributions being higher for gas compared to dark matter. We also explain the apparent high spin of dark matter halos undergoing mergers and show that a criterion stricter than what is currently used would be required to detect such unrelaxed halos. Finally, we demonstrate that the misalignment of AM between gas and dark matter only occurs when the intrinsic spins of the merging halos are not aligned with the orbital AM of the system. The self-misalignment (orientation of AM when measured in radial shells not being constant), which could be the cause of warps and anomalous rotation in disks galaxies, also occurs under similar conditions. The frequency and amplitude of this misalignment are roughly consistent with the properties of warps seen in disk galaxies. [ABSTRACT FROM AUTHOR]

We study the orbital properties of stars in four (published) simulations of thick discs formed by (i) accretion from disrupted satellites, (ii) heating of a pre-existing thin disc by a minor merger, (iii) radial migration and (iv) gas-rich mergers. We find that the distribution of orbital eccentricities is predicted to be different for each model: a prominent peak at low eccentricity is expected for the heating, migration and gas-rich merging scenarios, while the eccentricity distribution is broader and shifted towards higher values for the accretion model. These differences can be traced back to whether the bulk of the stars in each case is formed in situ or is accreted, and is robust to the peculiarities of each model. A simple test based on the eccentricity distribution of nearby thick-disc stars may thus help elucidate the dominant formation mechanism of the Galactic thick disc. [ABSTRACT FROM AUTHOR]

We study the population of satellite galaxies formed in a suite of N-body/gasdynamical simulations of galaxy formation in a Λ cold dark matter universe. The simulations resolve nearly 10 most luminous satellites around each host, and probe systems up to six or seven magnitudes fainter than the primary. We find little spatial or kinematic bias between the dark matter and the satellite population. The radius containing half of all satellites is comparable to the half-mass radius of the dark matter component, and the velocity dispersion of the satellites is a good indicator of the virial velocity of the halo; . Applied to the Local Group, this result suggests that the virial velocity of the Milky Way and M31 might be substantially lower than the rotation speed of their disc components; we find and , respectively, compared to and .Although the uncertainties are large, it is intriguing that both estimates are significantly lower than expected from some semi-analytic models, which predict a smaller difference between Vvir and Vrot. The detailed kinematics of simulated satellites and dark matter are also in good agreement: both components show a steadily decreasing velocity dispersion profile and a mild radial anisotropy in their velocity distribution. By contrast, the stellar halo of the simulated galaxies, which consists predominantly of stellar debris from disrupted satellites, is kinematically and spatially distinct from the population of surviving satellites. This is because the survival of a satellite as a self-bound entity depends sensitively on mass and on time of accretion and surviving satellites are significantly biased toward low-mass systems that have been recently accreted by the galaxy. Our results support recent proposals for the origin of the systematic differences between stars in the Galactic halo and in Galactic satellites: the elusive ‘building blocks’ of the Milky Way stellar halo were on average more massive, and were accreted (and disrupted) earlier than the population of dwarfs that has survived self-bound until thepresent. [ABSTRACT FROM AUTHOR]

We examine the orbits of satellite galaxies identified in a suite of N-body/gasdynamical simulations of the formation of L* galaxies in a Lambda cold dark matter universe. The numerical resolution of the simulations allows us to track in detail the orbits of the ∼ 10 brightest satellites around each primary. Most satellites follow conventional orbits; after turning around, they accrete into their host halo and settle on orbits whose apocentric radii are steadily eroded by dynamical friction. As a result, satellites associated with the primary are typically found within its virial radius, rvir, and have velocities consistent with a Gaussian distribution with mild radial anisotropy. However, a number of outliers are also present. We find that a surprising number (about one-third) of satellites identified at are on unorthodox orbits, with apocentres that exceed their turnaround radii. These include a number of objects with extreme velocities and apocentric radii at times exceeding (or, e.g. ≳1 Mpc when scaled to the Milky Way). This population of satellites on extreme orbits consists typically of the faint member of a satellite pair whose kinship is severed by the tidal field of the primary during first approach. Under the right circumstances, the heavier member of the pair remains bound to the primary, whilst the lighter companion is ejected on to a highly energetic orbit. Since the concurrent accretion of multiple satellite systems is a defining feature of hierarchical models of galaxy formation, a fairly robust prediction of this scenario is that at least some of these extreme objects should be present in the Local Group. We speculate that this three-body ejection mechanism may be the origin of (i) some of the newly discovered high-speed satellites around M31 (such as Andromeda XIV); (ii) some of the distant fast-receding Local Group members, such as Leo I and (iii) the oddly isolated dwarf spheroidals Cetus and Tucana in the outskirts of the Local Group. Our results suggest that care must be exercised when using the orbits of the most weakly bound satellites to place constraints on the total mass of the Local Group. [ABSTRACT FROM AUTHOR]

We report first results from a series of N-body/gasdynamical simulations designed to study the origin of galaxy morphologies in a cold dark matter-dominated universe. The simulations include star formation and feedback and have numerical resolution sufficiently high to allow for a direct investigation of the morphology of simulated galaxies. We find, in agreement with previous theoretical work, that the presence of the main morphological components of galaxies—disks, spheroids, bars—is regulated by the mode of gas accretion and intimately linked to discrete accretion events. In the case we present, disks arise from the smooth deposition of cooled gas at the center of dark halos, spheroids result from the stirring of preexisting disks during mergers, and bars are triggered by tides generated by satellites. This demonstrates that morphology is a transient phenomenon within the lifetime of a galaxy and that the Hubble sequence reflects the varied accretion histories of galaxies in hierarchical formation scenarios. In particular, we demonstrate directly that disk/bulge systems can be built and rebuilt by the smooth accretion of gas onto the remnant of a major merger and that the present-day remnants of late dissipative mergers between disks are spheroidal stellar systems with structure resembling that of field ellipticals. The perplexing variety of galaxy morphologies is thus highly suggestive of—and may actually even demand—a universe where structures have evolved hierarchically. [Copyright &y& Elsevier]