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There is no doubt that the Milky Way is a barred galaxy; however, factors that establish its prominent morphology remain largely elusive. In this work, we attempt to constrain the history of the MW by tracing the present-day parameters and evolution of a set of MW and M31 analogues from the TNG50 simulations. We find that the strength of bars at |$z=0$| correlates well not only with the stellar mass build-up but also, more crucially, with the time of onset of stellar discs. Discs of strongly barred galaxies form early (⁠|$z \gtrsim 2-3$|⁠), compared to weakly and non-barred galaxies (⁠|$z \approx 1-1.5$|⁠). Although we are cautious to draw ultimate conclusions about the governing factor of discs formation due to the complexity and correlations between different phenomena, the observed morphological diversity can be tentatively explained by a substantial variation in the gas angular momentum around proto-galaxies already at |$z\approx 3-5$|⁠ ; in such a way, early discs formed from gas with larger angular momentum. By comparing the formation time-scales of discs of barred galaxies in the TNG50 sample, we infer that the MW has a strong bar (⁠|$0.35\lt A_2\lt 0.6$|⁠), and that its stellar disc started to dominate over the spheroidal component already at |$z \approx 2$|⁠ , with a mass of |$\approx 1 \pm 0.5 \times 10^{10} M_\odot$|⁠. We conclude that the presence of a strong bar in the MW is a natural manifestation of the early formation of the stellar disc, which made possible bursty but highly efficient star formation at high redshift. [ABSTRACT FROM AUTHOR]

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]

The relative enrichment of s -process to α-elements ([ s /α]) has been linked with age, providing a potentially useful avenue in exploring the Milky Way's chemical evolution. However, the age–[ s /α] relationship is non-universal, with dependencies on metallicity and current location in the Galaxy. In this work, we examine these chemical clock tracers across birth radii (⁠|${R}_\text{birth}$|⁠), recovering the inherent trends between the variables. We derive |${R}_\text{birth}$| and explore the [ s /α]–age– |${R}_\text{birth}$| relationship for 36 652 APOGEE DR17 red giant and 24 467 GALAH DR3 main-sequence turn-off and subgiant branch disc stars using [Ce/Mg], [Ba/Mg], and [Y/Mg]. We discover that the age– |$\rm [{\it s}/Mg]$| relation is strongly dependent on birth location in the Milky Way, with stars born in the inner disc having the weakest correlation. This is congruent with the Galaxy's initially weak, negative |$\rm [{\it s}/Mg]$| radial gradient, which becomes positive and steep with time. We show that the non-universal relations of chemical clocks is caused by their fundamental trends with |${R}_\text{birth}$| over time, and suggest that the tight age– |$\rm [{\it s}/Mg]$| relation obtained with solar-like stars is due to similar |${R}_\text{birth}$| for a given age. Our results are put into context with a Galactic chemical evolution model, where we demonstrate the need for data-driven nucleosynthetic yields. [ABSTRACT FROM AUTHOR]

Chemical abundances are an essential tool in untangling the Milky Way's enrichment history. However, the evolution of the interstellar medium abundance gradient with cosmic time is lost as a result of radial mixing processes. For the first time, we quantify the evolution of many observational abundances across the Galactic disc as a function of lookback time and birth radius, |$\rm \text{R}_\text{birth}$|⁠. Using an empirical approach, we derive |$\rm \text{R}_\text{birth}$| estimates for 145 447 APOGEE DR17 red giant disc stars, based solely on their ages and |$\rm [Fe/H]$|⁠. We explore the detailed evolution of six abundances [Mg, Ca (α), Mn (iron-peak), Al, C (light), Ce (s-process)] across the Milky Way disc using 87 426 APOGEE DR17 red giant stars. We discover that the interstellar medium had three fluctuations in the metallicity gradient ∼9, ∼6, and ∼4 Gyr ago. The first coincides with the end of high-α sequence formation around the time of the Gaia–Sausage–Enceladus disruption, while the others are likely related to passages of the Sagittarius dwarf galaxy. A clear distinction is found between present-day observed radial gradients with age and the evolution with lookback time for both [X/Fe] and [X/H], resulting from the significant flattening and inversion in old populations due to radial migration. We find the |$\rm [Fe/H]$| – |$\rm [\alpha /Fe]$| bimodality is also seen as a separation in the |$\rm \text{R}_\text{birth}$| – |$\rm [X/Fe]$| plane for the light and α-elements. Our results recover the chemical enrichment of the Galactic disc over the past 12 Gyr, providing tight constraints on Galactic disc chemical evolution models. [ABSTRACT FROM AUTHOR]

The Local Group is a unique environment in which to study the astrophysics of galaxy formation. The proximity of the Milky Way and M31 enhances the frequency of interactions of the low-mass halo population with more massive dark matter haloes, which increases their concentrations and strips them of gas and other material. Some low-mass haloes pass through the haloes of the Milky Way or M31 and are either ejected into the field or exchanged between the two primary hosts. We use high resolution gas-dynamical simulations to describe a new class of field haloes that passed through the haloes of both the Milky Way and M31 at early times and are almost twice as concentrated as field haloes that do not interact with the primary pair. These 'Hermeian' haloes are distributed anisotropically at larger distances from the Local Group barycentre than the primary haloes and appear to cluster along the line connecting the Milky Way and M31. Hermeian haloes facilitate the exchange of dark matter, gas, and stars between the Milky Way and M31 and can enhance the star formation rates of the gas in the primary haloes during their interactions with them. We also show that some Hermeian haloes can host galaxies that, because they are embedded in haloes that are more concentrated than regular field haloes, are promising targets for indirect dark matter searches beyond the Milky Way virial radius and can produce signals that are competitive with those of some dwarf galaxies. Hermeian galaxies in the Local Group should be detectable by forthcoming wide-field imaging surveys. [ABSTRACT FROM AUTHOR]

Recent observations have revealed remarkable insights into the gas reservoir in the circumgalactic medium (CGM) of galaxy haloes. In this paper, we characterize the gas in the vicinity of Milky Way and Andromeda analogues in the hestia (High resolution Environmental Simulations of The Immediate Area) suite of constrained Local Group (LG) simulations. The hestia suite comprise of a set of three high-resolution arepo -based simulations of the LG, run using the Auriga galaxy formation model. For this paper, we focus only on the z = 0 simulation data sets and generate mock skymaps along with a power spectrum analysis to show that the distributions of ions tracing low-temperature gas (H  i and Si  iii) are more clumpy in comparison to warmer gas tracers (O  vi , O  vii , and O  viii). We compare to the spectroscopic CGM observations of M31 and low-redshift galaxies. hestia underproduces the column densities of the M31 observations, but the simulations are consistent with the observations of low-redshift galaxies. A possible explanation for these findings is that the spectroscopic observations of M31 are contaminated by gas residing in the CGM of the Milky Way. [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]

We present the hestia simulation suite: High-resolutions Environmental Simulations of The Immediate Area, a set of cosmological simulations of the Local Group. Initial conditions constrained by the observed peculiar velocity of nearby galaxies are employed to accurately simulate the local cosmography. Halo pairs that resemble the Local Group are found in low resolutions constrained, dark matter only simulations, and selected for higher resolution magneto hydrodynamic simulation using the arepo code. Baryonic physics follows the auriga model of galaxy formation. The simulations contain a high-resolution region of 3–5 Mpc in radius from the Local Group mid-point embedded in the correct cosmographic landscape. Within this region, a simulated Local Group consisting of a Milky Way and Andromeda like galaxy forms, whose description is in excellent agreement with observations. The simulated Local Group galaxies resemble the Milky Way and Andromeda in terms of their halo mass, mass ratio, stellar disc mass, morphology separation, relative velocity, rotation curves, bulge-disc morphology, satellite galaxy stellar mass function, satellite radial distribution, and in some cases, the presence of a Magellanic cloud like object. Because these simulations properly model the Local Group in their cosmographic context, they provide a testing ground for questions where environment is thought to play an important role. [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]

The solar neighbourhood contains disc stars that have recently crossed spiral arms in the Galaxy. We propose that boundaries in local velocity distributions separate stars that have recently crossed and been more strongly perturbed by a particular arm from those that haven't. Ridges in the stellar velocity distributions constructed from the second Gaia data release trace orbits that could have touched nearby spiral arms at apocentre or pericentre. The multiple ridges and arcs seen in local velocity distributions are consistent with the presence of multiple spiral features and different pattern speeds and imply that the outer Galaxy is flocculent rather than grand design. [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]

We explore the connections between stellar age, chemistry, and kinematics across a Galacto- centric distance of 7.5 < R(kpc) < 9.0, using a sample of ~12 000 intermediate-mass (FGK) turn-off stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho--Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young (0 < τ < 3 Gyr) and old (8 < τ < 13 Gyr) populations, and then consider different metallicity bins for each of these age groups.We ?nd signi?cant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we ?nd a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius (∂VR/∂R) for young stars, with metal-rich stars having a much steeper gradient thanmetal-poor stars. For ∂Vφ/∂R, young, metal-rich stars signi?cantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR.We interpret these ?ndings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits. [ABSTRACT FROM AUTHOR]

We combine parallaxes from the first Gaia data release with the spectrophotometric distance estimation framework for stars in the fifth RAVE survey data release. The combined distance estimates aremore accurate than either determination in isolation -- uncertainties are on average two times smaller than for RAVE-only distances (three times smaller for dwarfs), and 1.4 times smaller than TGAS parallax uncertainties (two times smaller for giants). We are also able to compare the estimates from spectrophotometry to those from Gaia, and use this to assess the reliability of both catalogues and improve our distance estimates. We find that the distances to the lowest log g stars are, on average, overestimated and caution that they may not be reliable. We also find that it is likely that the Gaia random uncertainties are smaller than the reported values. As a by-product we derive ages for the RAVE stars, many with relative uncertainties less than 20 per cent. These results for 219 566 RAVE sources have been made publicly available, and we encourage their use for studies that combine the radial velocities provided by RAVE with the proper motions provided by Gaia. A sample that we believe to be reliable can be found by taking only the stars with the flag notification 'flag any=0'. [ABSTRACT FROM AUTHOR]

Cosmological simulations suggest that most of the matter in the Universe is distributed along filaments connecting galaxies. Illuminated by the cosmic UV background (UVB), these structures are expected to glow in fluorescent Ly α emission with a surface brightness (SB) that is well below current limits for individual detections. Here, we perform a stacking analysis of the deepest MUSE/VLT data using three-dimensional regions (subcubes) with orientations determined by the position of neighbouring Ly α galaxies at 3 < z < 4. Our method increase the probability of detecting filamentary Ly α emission, provided that these structures are Lyman-limit systems (LLSs). By stacking 390 oriented subcubes we reach a 2σ sensitivity level of SB ≈ 0.44 × 10-20 erg s-1 cm-2 arcsec-2 in an aperture of 1 arcsec2 × 6.25 Å, three times below the expected fluorescent Ly α signal from the Haardt & Madau UVB at z ~ 3.5. No detectable emission is found on intergalactic scales, implying that at least two thirds of our subcubes do not contain oriented LLSs. On the other hand, significant emission is detected in the circumgalactic medium (CGM) in the direction of the neighbours. The signal is stronger for galaxies with a larger number of neighbours and appears to be independent of any other galaxy properties. We estimate that preferentially oriented satellite galaxies cannot contribute significantly to this signal, suggesting instead that gas densities in theCGMare typically larger in the direction of neighbouring galaxies on cosmological scales. [ABSTRACT FROM AUTHOR]

Cosmological simulations suggest that most of the matter in the Universe is distributed along filaments connecting galaxies. Illuminated by the cosmic UV background (UVB), these structures are expected to glow in fluorescent Ly α emission with a surface brightness (SB) that is well below current limits for individual detections. Here, we perform a stacking analysis of the deepest MUSE/VLT data using three-dimensional regions (subcubes) with orientations determined by the position of neighbouring Ly α galaxies at 3 < z < 4. Our method increase the probability of detecting filamentary Ly α emission, provided that these structures are Lyman-limit systems (LLSs). By stacking 390 oriented subcubes we reach a 2σ sensitivity level of SB ≈ 0.44 x 10-20 erg s-1 cm-2 arcsec-2 in an aperture of 1 arcsec2 × 6.25 Å, three times below the expected fluorescent Ly α signal from the Haardt & Madau UVB at z ∼ 3.5. No detectable emission is found on intergalactic scales, implying that at least two thirds of our subcubes do not contain oriented LLSs. On the other hand, significant emission is detected in the circumgalactic medium (CGM) in the direction of the neighbours. The signal is stronger for galaxies with a larger number of neighbours and appears to be independent of any other galaxy properties. We estimate that preferentially oriented satellite galaxies cannot contribute significantly to this signal, suggesting instead that gas densities in theCGMare typically larger in the direction of neighbouring galaxies on cosmological scales. [ABSTRACT FROM AUTHOR]

We characterize the selection function of RAdial Velocity Experiment (RAVE) using 2 Micron All Sky Survey (2MASS) as our underlying population, which we assume represents all stars that could have potentially been observed.We evaluate the completeness fraction as a function of position, magnitude and colour in two ways: first, on a field-by-field basis, and second, in equal-size areas on the sky. Then, we consider the effect of the RAVE stellar parameter pipeline on the final resulting catalogue, which in principle limits the parameter space over which our selection function is valid. Our final selection function is the product of the completeness fraction and the selection function of the pipeline. We then test if the application of the selection function introduces biases in the derived parameters. To do this, we compare a parent mock catalogue generated using GALAXIA with a mock-RAVE catalogue where the selection function of RAVE has been applied. We conclude that for stars brighter than I = 12, between 4000 < Teff < 8000K and 0.5 < log g < 5.0, RAVE is kinematically and chemically unbiased with respect to expectations from GALAXIA. [ABSTRACT FROM AUTHOR]

Near-field cosmology is practised by studying the Local Group (LG) and its neighbourhood. This paper describes a framework for simulating the 'near field' on the computer. Assuming the Λ cold dark matter (ΛCDM) model as a prior and applying the Bayesian tools of the Wiener filter and constrained realizations of Gaussian fields to the Cosmicflows-2 (CF2) survey of peculiar velocities, constrained simulations of our cosmic environment are performed. The aim of these simulations is to reproduce the LG and its local environment. Our main result is that the LG is likely a robust outcome of the ΛCDMscenario when subjected to the constraint derived from CF2 data, emerging in an environment akin to the observed one. Three levels of criteria are used to define the simulated LGs. At the base level, pairs of haloes must obey specific isolation, mass and separation criteria. At the second level, the orbital angular momentum and energy are constrained, and on the third one the phase of the orbit is constrained. Out of the 300 constrained simulations, 146 LGs obey the first set of criteria, 51 the second and 6 the third. The robustness of our LG 'factory' enables the construction of a large ensemble of simulated LGs. Suitable candidates for high-resolution hydrodynamical simulations of the LG can be drawn from this ensemble, which can be used to perform comprehensive studies of the formation of the LG. [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]

This paper combines observational data sets and cosmological simulations to generate realistic numerical replicas of the nearby Universe. The latter are excellent laboratories for studies of the non-linear process of structure formation in our neighbourhood. With measurements of radial peculiar velocities in the local Universe (cosmicflows-2) and a newly developed technique, we produce Constrained Local UniversE Simulations (CLUES). To assess the quality of these constrained simulations, we compare them with random simulations as well as with local observations. The cosmic variance, defined as the mean one-sigma scatter of cell-to-cell comparison between two fields, is significantly smaller for the constrained simulations than for the random simulations. Within the inner part of the box where most of the constraints are, the scatter is smaller by a factor of 2 to 3 on a 5 h-1 Mpc scale with respect to that found for random simulations. This one-sigma scatter obtained when comparing the simulated and the observation-reconstructed velocity fields is only 104 ± 4 km s-1, i.e. the linear theory threshold. These two results demonstrate that these simulations are in agreement with each other and with the observations of our neighbourhood. For the first time, simulations constrained with observational radial peculiar velocities resemble the local Universe up to a distance of 150 h-1 Mpc on a scale of a few tens of megaparsecs. When focusing on the inner part of the box, the resemblance with our cosmic neighbourhood extends to a few megaparsecs (<5 h-1 Mpc). The simulations provide a proper large-scale environment for studies of the formation of nearby objects. [ABSTRACT FROM AUTHOR]

Recent observational studies have demonstrated that the majority of satellite galaxies tend to orbit their hosts on highly flattened, vast, possibly corotating planes. Two nearly parallel planes of satellites have been confirmed around the M31 galaxy and around the Centaurus A galaxy, while the Milky Way also sports a plane of satellites. It has been argued that such an alignment of satellites on vast planes is unexpected in the standard Λ cold dark matter (ΛCDM) model of cosmology if not even in contradiction to its generic predictions. Guided by ΛCDM numerical simulations, which suggest that satellites are channelled towards hosts along the axis of the slowest collapse as dictated by the ambient velocity shear tensor, we re-examine the planes of local satellites systems within the framework of the local shear tensor derived from the Cosmicflows-2 data set. The analysis reveals that the Local Group and Centaurus A reside in a filament stretched by the Virgo cluster and compressed by the expansion of the Local Void. Four out of five thin planes of satellite galaxies are indeed closely aligned with the axis of compression induced by the Local Void. Being the less massive system, the moderate misalignment of the Milky Way's satellite plane can likely be ascribed to its greater susceptibility to tidal torques, as suggested by numerical simulations. The alignment of satellite systems in the local Universe with the ambient shear field is thus in general agreement with predictions of the ΛCDM model. [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]

We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at z = 0 for three different phases: ‘cold’, ‘hot’ and H i, and compare our results with observations. The amount of material in the hot halo (Mhot ≈ 4–5 × 1010 M⊙), and the cold (Mcold(r ≲ 10 kpc) ≈ 108 M⊙) and H i ($M_{\rm H\,{\small i}}(r\lesssim 50\,{\rm kpc})\approx 3{-}4\times 10^8\, {M_{\odot}}$) components displays reasonable agreement with observations. We also compute the accretion/ejection rates together with the H i (radial and all-sky) covering fractions. The integrated H i accretion rate within r = 50 kpc gives ∼0.2–0.3 M⊙ yr−1, i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with T ≲ 105 K can contribute significantly too. The net accretion rates of all material at the virial radii are 6–8 M⊙ yr−1. At z = 0, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at z ∼ 1, as a result of the kinematical evolution of the LG. [ABSTRACT FROM AUTHOR]

We present results on the clustering of 282 068 galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) sample of massive galaxies with redshifts 0.4 < z < 0.7 which is part of the Sloan Digital Sky Survey III project. Our results cover a large range of scales from ∼500 to ∼90 h−1 Mpc. We compare these estimates with the expectations of the flat Λ cold dark matter (ΛCDM) standard cosmological model with parameters compatible with Wilkinson Microwave Anisotropy Probe 7 data. We use the MultiDark cosmological simulation, one of the largest N-body runs presently available, together with a simple halo abundance matching technique, to estimate galaxy correlation functions, power spectra, abundance of subhaloes and galaxy biases. We find that the ΛCDM model gives a reasonable description to the observed correlation functions at z ≈ 0.5, which is remarkably good agreement considering that the model, once matched to the observed abundance of BOSS galaxies, does not have any free parameters. However, we find a ≳10 per cent deviation in the correlation functions for scales ≲ 1 and ∼10–40 h−1 Mpc. A more realistic abundance matching model and better statistics from upcoming observations are needed to clarify the situation. We also estimate that about 12 per cent of the ‘galaxies’ in the abundance-matched sample are satellites inhabiting central haloes with mass M ≳ 1014 h−1 M⊙. Using the MultiDark simulation, we also study the real-space halo bias b of the matched catalogue finding that b = 2.00 ± 0.07 at large scales, consistent with the one obtained using the measured BOSS-projected correlation function. Furthermore, the linear large-scale bias, defined using the extrapolated linear matter power spectrum, depends on the number density n of the abundance-matched sample as b = −0.048 − (0.594 ± 0.02)log10(n/ h3 Mpc−3). Extrapolating these results to baryon acoustic oscillation scales, we measure a scale-dependent damping of the acoustic signal produced by non-linear evolution that leads to ∼2–4 per cent dips at ≳ 3σ level for wavenumbers k ≳ 0.1 h Mpc−1 in the linear large-scale bias. [ABSTRACT FROM AUTHOR]

ABSTRACT We use a 64 h−1 Mpc dark-matter-only cosmological simulation to examine the large-scale orientation of haloes and substructures with respect to the cosmic web. A web classification scheme based on the velocity shear tensor is used to assign to each halo in the simulation a web type: knot, filament, sheet or void. Using ∼106 haloes that span ∼3 orders of magnitude in mass, the orientation of the halo's spin and the orbital angular momentum of subhaloes with respect to the eigenvectors of the shear tensor is examined. We find that the orbital angular momentum of subhaloes tends to align with the intermediate eigenvector of the velocity shear tensor for all haloes in knots, filaments and sheets. This result indicates that the kinematics of substructures located deep within the virialized regions of a halo is determined by its infall which in turn is determined by the large-scale velocity shear, a surprising result given the virialized nature of haloes. The non-random nature of subhalo accretion is thus imprinted on the angular momentum measured at z= 0. We also find that the haloes' spin axis is aligned with the third eigenvector of the velocity shear tensor in filaments and sheets: the halo spin axis points along filaments and lies in the plane of cosmic sheets. [ABSTRACT FROM AUTHOR]

ABSTRACT We make detailed theoretical predictions for the assembly properties of the Local Group (LG) in the standard Λ cold dark matter cosmological model. We use three cosmological N-body dark matter simulations from the Constrained Local Universe Simulations project, which are designed to reproduce the main dynamical features of the matter distribution down to the scale of a few Mpc around the LG. Additionally, we use the results of an unconstrained simulation with a 60 times larger volume to calibrate the influence of cosmic variance. We characterize the mass aggregation history (MAH) for each halo by three characteristic times: the formation, assembly and last major merger times. A major merger is defined by a minimal mass ratio of 10: 1. We find that the three LGs share a similar MAH with formation and last major merger epochs placed on average ≈10-12 Gyr ago. Between 12 and 17 per cent of the haloes in the mass range 5 × 1011 < Mh < 5 × 1012 h−1 M⊙ have a similar MAH. In a set of pairs of haloes within the same mass range, a fraction of 1-3 per cent share similar formation properties as both haloes in the simulated LG. An unsolved question posed by our results is the dynamical origin of the MAH of the LGs. The isolation criteria commonly used to define LG-like haloes in unconstrained simulations do not narrow down the halo population into a set with quiet MAHs, nor does a further constraint to reside in a low-density environment. The quiet MAH of the LGs provides a favourable environment for the formation of disc galaxies like the Milky Way and M31. The timing for the beginning of the last major merger in the Milky Way dark matter halo matches with the gas-rich merger origin for the thick component in the galactic disc. Our results support the view that the specific large- and mid-scale environments around the LG play a critical role in shaping its MAH and hence its baryonic structure at present. [ABSTRACT FROM AUTHOR]

The Monoceros ring is a collection of stars in nearly circular orbits at roughly 18 kpc from the Galactic Centre. It may have originated (i) as the response of the disc to perturbations excited by satellite companions or (ii) from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros stars differs from that of disc stars at comparable Galactocentric distances, an observation that disfavours the first scenario. On the other hand, circular orbits are difficult to accommodate in the tidal-disruption scenario, since it requires a satellite which at the time of disruption was itself in a nearly circular orbit. Such satellite could not have formed at the location of the ring and, given its low mass, dynamical friction is unlikely to have played a major role in its orbital evolution. We search cosmological simulations for low-mass satellites in nearly circular orbits and find that they result, almost invariably, from orbital changes induced by collisions with more massive satellites: the radius of the circular orbit thus traces the galactocentric distance of the collision. Interestingly, the Sagittarius dwarf, one of the most luminous satellites of the Milky Way, is in a polar orbit that crosses the Galactic plane at roughly the same Galactocentric distance as Monoceros. We use idealized simulations to demonstrate that an encounter with Sagittarius might well have led to the circularization and subsequent tidal demise of the progenitor of the Monoceros ring. [ABSTRACT FROM AUTHOR]

Using a high-resolution dark matter (DM) simulation of the Local Group, conducted within the framework of the Constrained Local UniversE Simulation (CLUES) project, we investigate the nature of how satellites of the Milky Way (MW) and M31 are accreted. Satellites of these two galaxies are accreted anisotropically on to the main haloes, entering the virial radius of their hosts, from specific 'spots' with respect to the large-scale structure. Furthermore, the material which is tidally stripped from these accreted satellites is also, at , distributed anisotropically and is characterized by an ellipsoidal subvolume embedded in the halo. The angular pattern created by the locus of satellite infall points and the projected stripped dark matter is investigated within a coordinate system determined by the location of the Local Group companion and the simulated Virgo cluster across concentric shells ranging from 0.1 to . Remarkably, the principal axis of the ellipsoidal subvolume shows a coherent alignment extending from well within the halo to a few r. A spherical harmonics transform applied to the angular distributions confirms the visual impression: namely, the angular distributions of both the satellites entry points and stripped DM for both haloes are dominated by the quadrupole term, whose major principal axis is approximately aligned across the shells considered. It follows that the outer structure of the main haloes of the Local Group composed of stripped material is closely related to the cosmic web, within which it is embedded. Given the very plausible hypothesis that an important fraction of the stellar halo of the MW has been accreted from satellite galaxies, the present results can be directly applied to the stellar halo of the MW and M31. We predict that the remnants of tidally stripped satellites should be embedded in streams of material composed of dark matter and stars. The present results can therefore shed light on the existence of satellites embedded within larger streams of matter, such as the Segue 2 satellite. [ABSTRACT FROM AUTHOR]

We present a systematic study of stellar feedback processes in simulations of disc galaxy formation. Using a dark matter halo with properties similar to the ones for the Milky Way's stellar halo, we perform a comparison of different methods of distributing energy related to feedback processes to the surrounding gas. A most promising standard model is applied to haloes spanning a range of masses in order to compare the results to disc galaxy scaling relations. With few exceptions we find little or no angular momentum deficiency for our galaxies and a good agreement with the angular momentum-size relation. Our galaxies are in good agreement with the baryonic Tully-Fisher relation and the slope of the photometric Tully-Fisher relation is reproduced. We find a zero-point offset of 0.7-1 mag, depending on the employed IMF. Applying the standard model to Milky Way-type haloes with different assembly histories, we show that a quiet assembly history does not guarantee the formation of a disc. We also study our standard feedback model in combination with additional physical processes like a UV background, kinetic feedback, a delayed energy deposition as expected for type Ia supernovae, mass return and metal-dependent cooling. Only a combination of effects yields a real improvement of the resulting galaxy by reducing the bulge, while including metal-dependent cooling increases the bulge again. We find that in general the stellar mass fraction of our galaxies is too high. In an ad hoc experiment we show that an removal of the bulge could reconcile this. However, the fit of the Tully-Fisher relation can only be improved by delaying the star formation, but not suppressing it completely. Our models do not seem to be efficient enough to achieve either effect. A mechanism to create bulge-less disc galaxies in simulations therefore remains elusive. We conclude that disc formation is a complex, highly interconnected problem that is difficult to handle in a controlled manner. We expect a solution to be coming from a combination of small effects rather than one large breakthrough. [ABSTRACT FROM AUTHOR]

We use N-body/gasdynamical cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its surrounding dark matter halo. Two series of simulations are compared; one that follows only the evolution of the dark matter component of individual haloes in the proper Λcold dark matter (ΛCDM) cosmological context, and a second series where a baryonic component is added and followed hydrodynamically. The simulations with baryons include radiative cooling but neglect the formation of stars and their feedback. The efficient, unimpeded cooling that results leads most baryons to collect at the halo centre in a centrifugally supported disc which, due to angular momentum losses, is too small and too massive when compared with typical spiral galaxies. This admittedly unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their surrounding dark haloes. We find, in agreement with earlier work, that the shape of the halo becomes more axisymmetric: post galaxy assembly, haloes are transformed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening . Haloes always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the traditional ‘adiabatic-contraction’ hypothesis. The reduced contraction helps to reconcile ΛCDM haloes with constraints on the dark matter content inside the solar circle and should alleviate the longstanding difficulty of matching simultaneously the scaling properties of galaxy discs and the galaxy luminosity function. The halo contraction we report is also less pronounced than found in earlier simulations, a disagreement which suggests that halo contraction is not solely a function of the initial and final distribution of baryons. Not only how much baryonic mass has been deposited at the centre of a halo matters, but also the mode of its deposition. Although simple formulae might work in particular cases where galaxies form nearly adiabatically, in general it might prove impossible to predict the halo response to galaxy formation without a detailed understanding of a galaxy's detailed assembly history. [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 present a method to numerically estimate the densities of a discretely sampled data based on a binary space partitioning tree. We start with a root node containing all the particles and then recursively divide each node into two nodes each containing roughly equal number of particles, until each of the nodes contains only one particle. The volume of such a leaf node provides an estimate of the local density and its shape provides an estimate of the variance. We implement an entropy-based node splitting criterion that results in a significant improvement in the estimation of densities compared to earlier work. The method is completely metric free and can be applied to arbitrary number of dimensions. We use this method to determine the appropriate metric at each point in space and then use kernel-based methods for calculating the density. The kernel-smoothed estimates were found to be more accurate and have lower dispersion. We apply this method to determine the phase-space densities of dark matter haloes obtained from cosmological N-body simulations. We find that contrary to earlier studies, the volume distribution function v( f) of phase-space density f does not have a constant slope but rather a small hump at high phase-space densities. We demonstrate that a model in which a halo is made up by a superposition of Hernquist spheres is not capable in explaining the shape of v( f) versus f relation, whereas a model which takes into account the contribution of the main halo separately roughly reproduces the behaviour as seen in simulations. The use of the presented method is not limited to calculation of phase-space densities, but can be used as a general purpose data-mining tool and due to its speed and accuracy it is ideally suited for analysis of large multidimensional data sets. [ABSTRACT FROM AUTHOR]

We use numerical simulations to investigate the origin and structure of the luminous haloes that surround isolated galaxies. These stellar structures extend out to several hundred kpc away from a galaxy, and consist of stars shed by merging subunits during the many accretion events that characterize the hierarchical assembly of galaxies. Such an origin suggests that outer luminous haloes are ubiquitous and that they should appear as an excess of light over extrapolations of the galaxy's inner profile beyond its traditional luminous radius. The mass profile of the accreted stellar component is well approximated by a model where the logarithmic slope steepens monotonically with radius; from at the luminous edge of the galaxy to or steeper near the virial radius of the system. Such spatial distribution is consistent with that of Galactic and M31 globular clusters, suggesting that many of the globulars were brought in by accretion events, in a manner akin to the classic Searle–Zinn scenario. Luminous haloes are similar in shape to their dark matter counterparts, which are only mildly triaxial and much rounder than dark haloes formed in simulations that do not include a dissipative luminous component. The outer stellar spheroid is supported by a velocity dispersion tensor with a substantial and radially increasing radial anisotropy: from at the edge of the central galaxy to ∼5 at the virial radius. These properties distinguish the stellar halo from the dark matter component, which is more isotropic in velocity space, as well as from some tracers of the outer spheroid such as satellite galaxies. Most stars in the outer halo formed in progenitors that have since merged with the central galaxy or have been substantially disrupted in its immediate surroundings; very few stars in the halo are contributed by satellites that survive as self-bound entities at the present. Although the stellar spheroid in the simulations is more prominent than in disc-dominated galaxies, many of these features are in reasonable agreement with recent observations of the outer halo of the Milky Way, of M31, and of other isolated spirals, and suggest that all of these systems underwent an early period of active merging, as envisioned in hierarchical models of galaxy formation. [ABSTRACT FROM AUTHOR]

We use numerical simulations to investigate the orbital characteristics of tidal debris from satellites whose orbits are dragged into the plane of galactic discs by dynamical friction before disruption. We find that these satellites may deposit a significant fraction of their stars into the disc components of a galaxy, and use our results to motivate the search for accretion relics in samples of metal-poor disc stars in the vicinity of the Sun. Satellites disrupted on very eccentric orbits coplanar with the disc are expected to shed stars in‘trails’ of distinct orbital energy and angular momentum during each pericentric passage. To an observer located between the pericentre and apocentre of such orbits, these trails would show as distinct groupings of stars with low vertical velocity and a broad, symmetric, often double-peaked distribution of Galactocentric radial velocities. One group of stars with these characteristics stands out in available compilations of nearby metal-poor stars. These stars have specific angular momenta similar to that of the globular clusterωCen, long hypothesized to be the nucleus of a dwarf galaxy disrupted by the Milky Way tidal field. In addition to their kindred kinematics, stars in theωCen group share distinct chemical abundance characteristics, and trace a well-defined track in the[α/Fe] versus[Fe/H] plane, consistent with simple closed-box enrichment models and a protracted star formation history. The dynamical and chemical coherence of this group suggests that it consists of stars that once belonged to the dwarf that broughtωCen into the Galaxy. The presence of this and other‘tidal relics’ in the solar neighbourhood suggest an extra-Galactic origin for the presence of nearby stars with odd kinematics and chemistry, and implies that accounting for stars contributed by distinct satellite galaxies may be crucial to the success of models of Galactic chemical enrichment. [ABSTRACT FROM AUTHOR]