Massive Substructures Research Articles

The torsion of stellar streams and the overall shape of galactic gravity’s source

Context. Flat rotation curves, v(r), are naturally explained by elongated (prolate) dark matter (DM) distributions, and we have provided competitive fits to the SPARC database. To further probe the geometry of the halo, or the equivalent source of gravity in other formulations, one needs observables outside the galactic plane. Stellar streams, poetically analogous to airplane contrails, but caused by tidal dispersion of massive substructures such as satellite dwarf galaxies, would lie on their own plane (consistently with angular momentum conservation) should the DM-halo gravitational field be spherically symmetric. Tracks resembling entire orbits are seldom available because their periods are commensurable with Hubble time, with streams often presenting themselves as short segments. Aims. Therefore, we aim to establish stellar stream torsion, a local observable that measures the deviation from planarity in differential curve geometry, as a diagnostic providing sensitivity to aspherical DM distributions and ensuring the use of even relatively short streams. Methods. We performed small-scale simulations of tidally distorted star clusters to check that indeed a central force center produces negligible torsion, while distorted halos can generate it. Turning to observational data, we identified among the known streams those that are at the largest distance from the Galactic center, and that are likely not affected by the Magellanic clouds, as the most promising for the study, and by means of polynomial fits we extracted their differential torsion. Results. We find that the torsion of the few known streams that should be sensitive to most of the Milky Way’s DM halo is much larger than expected for a central spherical bulb alone. This is consistent with the nonsphericity of the halo. Conclusions. Future studies of stellar stream torsion with larger samples and further out of the galactic plane should be able to extract the ellipticity of the halo to see whether it is just a slight distortion of a spherical shape or whether it rather resembles a more elongated cigar.

Gravitational imaging through a triple source plane lens: revisiting the ΛCDM-defying dark subhalo in SDSSJ0946+1006

ABSTRACT The ΛCDM paradigm successfully explains the large-scale structure of the Universe, but is less well constrained on subgalactic scales. Gravitational lens modelling has been used to measure the imprints of dark substructures on lensed arcs, testing the small-scale predictions of ΛCDM. However, the methods required for these tests are subject to degeneracies among the lens mass model and the source light profile. We present a case study of the unique compound gravitational lens SDSSJ0946+1006, wherein a dark, massive substructure has been detected, whose reported high concentration would be unlikely in a ΛCDM universe. For the first time, we model the first two background sources in both I- and U-band HST imaging, as well as VLT-MUSE emission line data for the most distant source. We recover a lensing perturber at a 5.9σ confidence level with mass $\log _{10}(M_\mathrm{sub}/{\rm M}_{\odot })=9.2^{+0.4}_{-0.1}$ and concentration $\log _{10}c=2.4^{+0.5}_{-0.3}$. The concentration is more consistent with CDM subhaloes than previously reported, and the mass is compatible with that of a dwarf satellite galaxy whose flux is undetectable in the data at the location of the perturber. A wandering black hole with mass $\log _{10}(M_\mathrm{BH}/{\rm M}_{\odot })=8.9^{+0.2}_{-0.1}$ is a viable alternative model. We systematically investigate alternative assumptions about the complexity of the mass distribution and source reconstruction; in all cases the subhalo is detected at around the ≥5σ level. However, the detection significance can be altered substantially (up to 11.3σ) by alternative choices for the source regularization scheme.

Effect of Zr addition on the microstructure evolution and mechanical properties of extruded Al-Cu-Li-Mn alloys

The effects of Zr addition on microstructure evolution and mechanical properties of extruded Al-Cu-Li-Mn alloy were investigated during heat treatment processing. The recrystallization and grain growth were profoundly inhibited in solid solution-treated (SST) Al-Cu-Li-Mn-Zr alloy (Zr-containing), and massive substructures were retained, which were attributed to the higher Zener pinning pressure provided by Al3Zr dispersoids. But, complete recrystallization and considerable grain growth occurred in SST Al-Cu-Li-Mn alloy (Zr-free). Besides, the deformation textures containing Brass and S components were predominant in Zr-containing alloy, which contributed to a higher average Talyor factor (M) value. The higher strength behavior in SST Zr-containing alloy mainly originated from the extra Al3Zr dispersoids apart from the higher M value and dislocation density. The higher dislocation density and Al3Zr dispersoids in Zr-containing alloy promoted the precipitation of T1 phases and narrowed the width of precipitate-free zones (PFZs). Compared with Zr-free alloy, the enhanced combination of good strength and ductility shown in T6-aged Zr-containing alloy was mostly due to the higher T1 phase density and M value, higher dislocation density, and extra Al3Zr dispersoids.

Beyond the ultradeep frontier fields and legacy observations (BUFFALO): a high-resolution strong+weak-lensing view of Abell 370

ABSTRACT The HST treasury programme BUFFALO provides extended wide-field imaging of the six Hubble Frontier Fields galaxy clusters. Here we present the combined strong and weak-lensing analysis of Abell 370, a massive cluster at z = 0.375. From the reconstructed total projected mass distribution in the 6 arcmin × 6 arcmin BUFFALO field-of-view, we obtain the distribution of massive substructures outside the cluster core and report the presence of a total of seven candidates, each with mass ∼5 × 1013 M⊙. Combining the total mass distribution derived from lensing with multiwavelength data, we evaluate the physical significance of each candidate substructure, and conclude that five out of the seven substructure candidates seem reliable, and that the mass distribution in Abell 370 is extended along the north-west and south-east directions. While this finding is in general agreement with previous studies, our detailed spatial reconstruction provides new insights into the complex mass distribution at large cluster-centric radius. We explore the impact of the extended mass reconstruction on the model of the cluster core and in particular, we attempt to physically explain the presence of an important external shear component, necessary to obtain a low root-mean-square separation between the model-predicted and observed positions of the multiple images in the cluster core. The substructures can only account for up to half the amplitude of the external shear, suggesting that more effort is needed to fully replace it by more physically motivated mass components. We provide public access to all the lensing data used as well as the different lens models.

The Hateful Eight: Connecting Massive Substructures in Galaxy Clusters like A2744 to Their Dynamical Assembly State Using the Magneticum Simulations

Substructures are known to be good tracers for the dynamical states and recent accretion histories of the most massive collapsed structures in the universe, galaxy clusters. Observations find extremely massive substructures in some clusters, especially Abell 2744 (A2744), which are potentially in tension with the ΛCDM paradigm because they are not found in simulations directly. However, the methods to measure substructure masses strongly differ between observations and simulations. Using the fully hydrodynamical cosmological simulation suite Magneticum Pathfinder, we develop a method to measure substructure masses in projection from simulations, similarly to the observational approach. We identify a simulated A2744 counterpart that not only has eight substructures of similar mass fractions but also exhibits similar features in the hot gas component. This cluster formed only recently through a major merger together with at least six massive minor merger events since z = 1, where previously the most massive component had a mass of less than 1 × 1014 M ⊙. We show that the mass fraction of all substructures and of the eighth substructure separately are excellent tracers for the dynamical state and assembly history for all galaxy cluster mass ranges, with high fractions indicating merger events within the last 2 Gyr. Finally, we demonstrate that the differences between subhalo masses measured directly from simulations as bound and those measured in projection are due to methodology, with the latter generally 2–3 times larger than the former. We provide a predictor function to estimate projected substructure masses from SubFind masses for future comparison studies between simulations and observations.

JWST and ALMA Imaging of Dust-obscured, Massive Substructures in a Typical z ∼ 3 Star-forming Disk Galaxy

We present an identification of dust-attenuated star-forming galactic-disk substructures in a typical star-forming galaxy (SFG), UDF2, at z = 2.696. To date, substructures containing significant buildup of stellar mass and actively forming stars have yet to be found in typical (i.e., main-sequence) SFGs at z > 2. This is due to the strong dust attenuation common in massive galaxies at the epoch and the scarcity of high-resolution, high-sensitivity extinction-independent imaging. To search for disk substructures, we subtracted the central stellar-mass disk from the JWST/NIRCam rest-frame 1.2 μm image (0.″13 resolution) and subtracted, in the visibility plane, the central starburst disk from Atacama Large Millimeter/submillimeter Array (ALMA) rest-frame 240 μm observations (0.″03 resolution). The residual images revealed substructures at rest-frame 1.2 μm colocated with those found at rest-frame 240 μm, ≃2 kpc away from the galactic center. The largest substructure contains ≃20% of the total stellar mass and ≃5% of the total star formation rate of the galaxy. While UDF2 exhibits a kinematically ordered velocity field of molecular gas consistent with a secularly evolving disk, more sensitive observations are required to characterize the nature and the origin of this substructure (spiral arms, minor merger, or other types of disk instabilities). UDF2 resides in an overdense region (N ≥ 4 massive galaxies within 70 kpc projected distance at z = 2.690–2.697) and the substructures may be associated with interaction-induced instabilities. Importantly, a statistical sample of such substructures identified with JWST and ALMA could play a key role in bridging the gap between the bulge-forming starburst and the rest of the galaxy.

Determination of Distance, Extinction, Mass, and Age for Stars in LAMOST DR7

Large scale spectroscopic surveys such as that using Large-sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) have collected spectra of millions stars in the Milky Way. Utilizing this huge sample of stars to study the assembling history and structure of our Galaxy requires accurate estimates of distance, extinction, age, and mass for individual stars. Combining the parallax constraint from Gaia EDR3 with Bayesian inference, we have estimated the distance and extinction for stars observed in LAMOST DR7, as well as the stellar mass and age for evolved stars in this data release. We validated the accuracies of the stellar parameters by comparing our results against various measurements, including the star-pair technique, asteroseismology, globular clusters, and isochrone fits to main sequence stars and subgiants. This is a valuable catalog of stellar parameters under a Bayesian framework estimated using the data from Gaia EDR3 and LAMOST spectroscopic data. With this data set we explored the stellar population of the Galactic massive substructure Gaia-Sausage-Enceladus (GSE). The kinematically selected members of GSE have a median metallicity of [Fe/H] = −1.29 and a median age of 11.6 Gyr.

Seismic Performance Analysis of Tuned Mass Rocking Wall (TMRW)-Frame Building Structures

A tuned mass rocking wall (TMRW) is a passive control device that combines the merits of a traditional tuned mass damper (TMD) and a traditional rocking wall (RW). TMRWs not only help avoid weak story failure of the host structure but can also be regarded as a largely tuned mass substructure in the building structure. Through the appropriate design of the frequency ratio, the host structure can dissipate much more energy under earthquake excitations. In this paper, the basic equations of motion for the mechanical model of an SDOF structure-rigid rocking wall are established, and the optimization formulas of frequency ratio and damping ratio of TMRW are derived. Through the dynamic elastoplastic analysis of a six-story TMRW-frame model, the applicability of the derived parameter optimization formulas and the effectiveness of the TMRW in seismic performance control are investigated. The results demonstrate that the TMRW can coordinate the uneven displacement angle between stories of the host structure. Additionally, the TMRW is found to possess the merit of reducing both the peak and root-mean-square (RMS) structural responses when subjected to different types of earthquake excitations.

Dynamical Relics of the Ancient Galactic Halo

Abstract We search for dynamical substructures in the LAMOST DR3 very metal-poor (VMP) star catalog. After cross-matching with Gaia DR2, there are ∼3300 VMP stars with available high-quality astrometric information that have halo-like kinematics. We apply a method based on the self-organizing map StarGO to find groups clustered in the 4D space of orbital energy and angular momentum. We identify 57 dynamically tagged groups (DTGs), which we label DTG-1 to DTG-57. Most of them belong to existing massive substructures in the nearby halo, such as the Gaia Sausage or Sequoia. The stream identified by Helmi et al. is recovered, but the two disjointed portions of the substructure appear to have distinct dynamical properties. The very retrograde substructure Rg5 found previously by Myeong et al. is also retrieved. We report six new DTGs with highly retrograde orbits, two with very prograde orbits, and 12 with polar orbits. By mapping other data sets (APOGEE halo stars, and catalogs of r-process-enhanced and carbon-enhanced metal-poor [CEMP] stars) onto the trained neuron map, we can associate stars with detailed chemical abundances with the DTGs and look for associations with chemically peculiar stars. The highly eccentric Gaia Sausage groups contain representatives of both debris from the satellite itself (which is α-poor) and the Splashed Disk, sent up into eccentric halo orbits from the encounter (and which is α-rich). The new prograde substructures also appear to be associated with the Splashed Disk. The DTGs belonging to the Gaia Sausage host two relatively metal-rich r-II stars and six CEMP stars in different subclasses, consistent with the idea that the Gaia Sausage progenitor is a massive dwarf galaxy. Rg5 is dynamically associated with two highly r-process-enhanced stars with [Fe/H] ∼ −3. This finding indicates that its progenitor might be an ultrafaint dwarf galaxy that has experienced r-process enrichment from neutron star mergers.

Characterization of Ni3Al Alloy Fabricated by Thermal Explosion and Hot Extrusion Process

In the present paper, the trace B doped Ni3Al alloy was prepared by the thermal explosion and hot extrusion (TH/HE) technique. Its micro-structure characterization and mechanical properties were carried out by OM, XRD, TEM, compression and micro-hardness tests. Micro-structure examinations exhibit that the TH/HE technology has transformed the powders into Ni3Al alloy with less porosity. In the synthesized part, coarse and ultrafine Ni3Al grains comprise the dual-scale grain structure with segregated Al2O3 particles along grain boundary. TEM observations reveal that γ-Ni, Al2O3 and Ni3Al phases coexist in the synthesized part. Moreover, the Al2O3 particles in the synthesized part have α-Al2O3 and γ-Al2O3 two crystal structures. In addition, there is a transition region along the interface of Al2O3 and Ni3Al grain. The subsequent hot extrusion optimize the micro-structure of the extruded part by homogenizing the Ni3Al grain, uniformly distributing Al2O3 and generating massive substructures. The great deformation results in the pile-up of dislocations, intersected dislocations and substructures. Moreover, the movement of dislocation promotes the formation of micro-twinning in the Ni3Al grain, which is accompanied with the stacking faults and nano-structure. The mechanical properties exhibit that the micro-structure optimization caused by the TE/HE technology improves the compressive ductility and strength of the Ni3Al alloy obviously. DOI: http://dx.doi.org/10.5755/j01.ms.24.4.19460

Spatial Planning and Architectural Innovation in the Roman Town of Ocriculum.

The Roman settlement of Ocriculum (Otricoli, TR - Umbria), built on a tufa slope between the Tiber’s valley to the north and the San Vittore stream valley to the south, was established on massive substructures, which allowed the exploitation of a larger area. Albeit being relatively neglected by modern scholarship, these preliminary constructions are none the less crucial to a thorough analysis of the urban planning of Ocriculum. There are two buildings mutually linked inside the city, although built not at the same time: the bath complex, erected in the middle of the II cent. AD and restored until IV-V cent. AD, and the underlying culvert in which still flows the San Vittore stream, built surely before the baths and as much certainly together with all the urban substructures. This artificial terrace, on which the baths tand and under which the channel runs, has been the first regularisation of the slope, starting indeed from the little but deep valley of the San Vittore stream. The bath complex, additionally, although not entirely preserved, features several interesting architectural innovations. Thanks to modern digital technologies but even to traditional methods, it was possible to study in a greater detail the aforementioned buildings. This allowed to rebuild by 3D-modeling programs these structures and to deepen knowledge about the urban planning and history of Ocriculum, on one side, and about its architectural influences, on the other side. The culvert, indeed, belongs to the first regularisation work of the settlement: on the overlying terrace should stand not only the bath complex, but also the theatre scene and its rear porticus (both no longer visible). Thus, also the theatre is later than the culvert and not previous (Hay-Keay-Millet 2013). Consequently, the near “Great Substructures” belong to the same construction period of the theatre, because they together support the thrust of the upper terrace, on which, most probably, there was the political and religious centre of Ocriculum. Furthermore, the octagonal hall of the baths and the smaller round one (the only two preserved of the entire complex) are an important proof of the richness of this city. They were roofed by a segmental dome consisted of 41 nails (indeed it is so called “shell-shaped”) and by a dodecagonal cross-vault, consisted of six larger convex wedges alternate with other six smaller ones: in this way it seems to be a hexagonal segmental dome. The first one, built as pluri-composed cross-vault, surely had to work as hemispherical dome: indeed, it lies on a circular springing, that is linked with the underlying octagonal hall through masonry convex triangles, covered with plaster. Both the “shell-shaped” dome and the angular links are an innovation and also an unicum in Roman architectural context. This allows to identify Ocriculum as a very rich town, inhabited by wealthy people enriched certainly thanks to the trades on the Tiber and the Via Flaminia, but also thanks to the imperial frequentations about which the ancient authors talk. Furthermore we have to consider that this town near the Tiber’s banks kept to be occupied during Barbarian Invasions and further to the end of Roman Empire. The baths mostly kept to be restored until IV-V century: this testifies the importance of this building for the citizenry. In the end, the absence of defensive walls could be explained with the identification of this settlement near the port as a monumental detachment of the city, never abandoned, on the top of the hill. DOI: http://dx.doi.org/10.5755/j01.sace.22.1.21086

The origin of the Gaia phase-plane spiral

A simple model is presented of the formation of the spiral the (z,v_z) phase plane of solar-neighbourhood stars that was recently discovered in Gaia data. The key is that the frequency Omega_z at which stars oscillate vertically depends on angular momentum about the z axis in addition to the amplitude of the star's vertical oscillations. Spirals should form in both <v_phi> and <v_R> whenever a massive substructure, such as the Sgr dwarf galaxy, passes through the Galactic plane. The model yields similar spirals to those observed in both <v_phi> and <v_R>. The primary driver is the component of the tidal force that lies in the plane. We investigate the longevity of the spirals and the mass of the substructure, but the approximations inherent in the model make quantitative results unreliable. The work relies heavily on a self-consistent, multi-component model of our Galaxy produced by the AGAMA package for f(J) modelling.

Resolution of the apparent discrepancy between the number of massive subhaloes in Abell 2744 and ΛCDM

ABSTRACT Schwinn et al. have recently compared the abundance and distribution of massive substructures identified in a gravitational lensing analysis of Abell 2744 by Jauzac et al. and N-body simulation, and found no cluster in Lambda cold dark matter (ΛCDM) simulation that is similar to Abell 2744. Schwinn et al. identified the measured projected aperture masses with the actual masses associated with subhaloes in the Millenium XXL N-body simulation. We have used the high-resolution Phoenix cluster simulations to show that such an identification is incorrect: the aperture mass is dominated by mass in the body of the cluster that happens to be projected along the line of sight to the subhalo. This enhancement varies from factors of a few to factors of more than 100, particularly for subhaloes projected near the centre of the cluster. We calculate aperture masses for subhaloes in our simulation and compare them to the measurements for Abell 2744. We find that the data for Abell 2744 are in excellent agreement with the matched predictions from ΛCDM. We provide further predictions for aperture mass functions of subhaloes in idealized surveys with varying mass detection thresholds.

Abell 2744: too much substructure for ΛCDM?

The massive substructures found in Abell 2744 by Jauzac et al. (2016) present a challenge to the cold dark matter paradigm due to their number and proximity to the cluster centre. We use one of the biggest N-body simulations, the Millennium XXL, to investigate the substructure in a large sample of massive dark matter haloes. A range of effects which influence the comparison with the observations is considered, extending the preliminary evaluation carried out by Jauzac et al. (2016). There are many tens of haloes in the simulation with a total mass comparable with or larger than that of Abell 2744. However, we find no haloes with a number and distribution of massive substructures (> 5 10^13 Msun) that is close to that inferred from the observations of Abell 2744. The application of extreme value statistics suggests that we would need a simulation of at least ten times the volume of the Millennium XXL to find a single dark matter halo with a similar internal structure to Abell 2744. Explaining the distribution of massive substructures in clusters is a new hurdle for hierarchical models to negotiate, which is not weakened by appeals to baryonic physics or uncertainty over the nature of the dark matter particle.

The Clouds are breaking: tracing the Magellanic system with Gaia DR1 Mira variables

We exploit the first data release from the Gaia mission to identify candidate Mira variables in the outskirts of the Magellanic Clouds. The repeated observations of sources during the initial phase of the Gaia mission is used to identify stars that show signs of variability. This variability information, combined with infrared photometry from 2MASS and WISE, allows us to select a clean sample of giants in the periphery of the LMC. We find evidence for Miras surrounding the LMC out to ~20 deg in all directions, apart from the North-West quadrant. Our sample does not generally follow the gas distribution of the Magellanic system; Miras are notably absent in the gaseous bridge between the LMC and SMC, but they are likely related to the stellar RR Lyrae bridge reported by Belokurov et al. (2016). The stellar stream discovered by Mackey et al. (2016) to the North of the LMC is almost perfectly delineated by our Mira variables, and likely extends further East toward the Galactic plane. The presence of an intermediate-age population in this stream advocates an LMC disc origin. We also find a significant excess of Miras to the East of the LMC; these more diffusely distributed stars are likely stripped SMC stars due to interactions with the LMC. Miras are also identified in regions of the sky away from the Clouds; we locate stars likely associated with known massive substructures, and also find potential associations with stripped SMC debris above the Galactic plane.

The extraordinary amount of substructure in theHubble Frontier Fieldscluster Abell 2744

We present a joint optical/X-ray analysis of the massive galaxy cluster Abell 2744 (z=0.308). Our strong- and weak-lensing analysis within the central region of the cluster, i.e., at R<1Mpc from the brightest cluster galaxy, reveals eight substructures, including the main core. All of these dark-matter halos are detected with a significance of at least 5sigma and feature masses ranging from 0.5 to 1.4x10^{14}Msun within R<150kpc. Merten et al. (2011) and Medezinski et al. (2016) substructures are also detected by us. We measure a slightly higher mass for the main core component than reported previously and attribute the discrepancy to the inclusion of our tightly constrained strong-lensing mass model built on Hubble Frontier Fields data. X-ray data obtained by XMM-Newton reveal four remnant cores, one of them a new detection, and three shocks. Unlike Merten et al. (2011), we find all cores to have both dark and luminous counterparts. A comparison with clusters of similar mass in the MXXL simulations yields no objects with as many massive substructures as observed in Abell 2744, confirming that Abell 2744 is an extreme system. We stress that these properties still do not constitute a challenge to $\Lambda$CDM, as caveats apply to both the simulation and the observations: for instance, the projected mass measurements from gravitational lensing and the limited resolution of the sub-haloes finders. We discuss implications of Abell 2744 for the plausibility of different dark-matter candidates and, finally, measure a new upper limit on the self-interaction cross-section of dark matter of sigma_{DM}<1.28cm2/g(68\% CL), in good agreement with previous results from Harvey et al. (2015).

Dark census: Statistically detecting the satellite populations of distant galaxies

In the standard structure formation scenario based on the cold dark matter paradigm, galactic halos are predicted to contain a large population of dark matter subhalos. While the most massive members of the subhalo population can appear as luminous satellites and be detected in optical surveys, establishing the existence of the low mass and mostly dark subhalos has proven to be a daunting task. Galaxy-scale strong gravitational lenses have been successfully used to study mass substructures lying close to lensed images of bright background sources. However, in typical galaxy-scale lenses, the strong lensing region only covers a small projected area of the lens's dark matter halo, implying that the vast majority of subhalos cannot be directly detected in lensing observations. In this paper, we point out that this large population of dark satellites can collectively affect gravitational lensing observables, hence possibly allowing their statistical detection. Focusing on the region of the galactic halo outside the strong lensing area, we compute from first principles the statistical properties of perturbations to the gravitational time delay and position of lensed images in the presence of a mass substructure population. We find that in the standard cosmological scenario, the statistics of these lensing observables are well approximated by Gaussian distributions. The formalism developed as part of this calculation is very general and can be applied to any halo geometry and choice of subhalo mass function. Our results significantly reduce the computational cost of including a large substructure population in lens models and enable the use of Bayesian inference techniques to detect and characterize the distributed satellite population of distant lens galaxies.

A BARYONIC EFFECT ON THE MERGER TIMESCALE OF GALAXY CLUSTERS

ABSTRACT Accurate estimation of the merger timescales of galaxy clusters is important for understanding the cluster merger process and further understanding the formation and evolution of the large-scale structure of the universe. In this paper, we explore a baryonic effect on the merger timescale of galaxy clusters by using hydrodynamical simulations. We find that the baryons play an important role in accelerating the merger process. The merger timescale decreases upon increasing the gas fraction of galaxy clusters. For example, the merger timescale is shortened by a factor of up to 3 for merging clusters with gas fractions of 0.15, compared with the timescale obtained with 0 gas fractions. The baryonic effect is significant for a wide range of merger parameters and is particularly more significant for nearly head-on mergers and high merging velocities. The baryonic effect on the merger timescale of galaxy clusters is expected to have an impact on the structure formation in the universe, such as the cluster mass function and massive substructures in galaxy clusters, and a bias of “no-gas” may exist in the results obtained from the dark matter-only cosmological simulations.

The APOSTLE simulations: solutions to the Local Group's cosmic puzzles

The Local Group of galaxies offer some of the most discriminating tests of models of cosmic structure formation. For example, observations of the Milky Way (MW) and Andromeda satellite populations appear to be in disagreement with N-body simulations of the "Lambda Cold Dark Matter" ({\Lambda}CDM) model: there are far fewer satellite galaxies than substructures in cold dark matter halos (the "missing satellites" problem); dwarf galaxies seem to avoid the most massive substructures (the "too-big-to-fail" problem); and the brightest satellites appear to orbit their host galaxies on a thin plane (the "planes of satellites" problem). Here we present results from APOSTLE (A Project Of Simulating The Local Environment), a suite of cosmological hydrodynamic simulations of twelve volumes selected to match the kinematics of the Local Group (LG) members. Applying the Eagle code to the LG environment, we find that our simulations match the observed abundance of LG galaxies, including the satellite galaxies of the MW and Andromeda. Due to changes to the structure of halos and the evolution in the LG environment, the simulations reproduce the observed relation between stellar mass and velocity dispersion of individual dwarf spheroidal galaxies without necessitating the formation of cores in their dark matter profiles. Satellite systems form with a range of spatial anisotropies, including one similar to that of the MW, confirming that such a configuration is not unexpected in {\Lambda}CDM. Finally, based on the observed velocity dispersion, size, and stellar mass, we provide new estimates of the maximum circular velocity for the halos of nine MW dwarf spheroidals.

The transverse velocity of the Andromeda system, derived from the M31 satellite population

We present a dynamical measurement of the tangential motion of the Andromeda system, the ensemble consisting of the Andromeda Galaxy (M31) and its satellites. The system is modelled as a structure with cosmologically-motivated velocity dispersion and density profiles, and we show that our method works well when tested using the most massive substructures in high-resolution Lambda Cold Dark Matter simulations. Applied to the sample of 40 currently-known galaxies of this system, we find a value for the transverse velocity of 164.4 +/- 61.8 km/s (v_East = -111.5 +/- 70.2 km/s and v_North = 99.4 +/- 60.0 km/s), significantly higher than previous estimates of the proper motion of M31 itself. This result has significant implications on estimates of the mass of the Local Group, as well as on its past and future history.