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# Reconstructing the Last Major Merger of the Milky Way with the H3 Survey

Abstract: Several lines of evidence suggest the Milky Way underwent a major merger at z~2 with a galaxy known as Gaia-Sausage-Enceladus (GSE). Here we use H3 Survey data to argue that GSE entered the Galaxy on a retrograde orbit based on a population of highly retrograde stars with chemistry similar to the largely radial GSE debris. We present the first tailored, high-resolution N-body simulations of the merger. From a grid of ~500 simulations we find a GSE with $M_{*}=5\times10^{8}\ M_{\odot}, M_{\rm{DM}}=2\times10^{11} M_{\odot}$ (a 2.5:1 total mass merger) best matches the H3 data. This simulation shows the retrograde GSE stars are stripped from its outer disk early in the merger before the orbit loses significant angular momentum. Despite being selected purely on angular momenta and radial distributions, this simulation reproduces and explains the following empirical phenomena: (i) the elongated, triaxial shape of the inner halo (axis ratios $10:7.9:4.5$), whose major axis is at ~35{\deg} to the plane and connects GSE's apocenters, (ii) the Hercules-Aquila Cloud & the Virgo Overdensity, which arise due to apocenter pile-up, (iii) the 2 Gyr lag between the quenching of GSE and the truncation of the age distribution of the in-situ halo, which tracks the 2 Gyr gap between the first and final GSE pericenters. We make the following predictions: (i) the inner halo has a "double-break" density profile with breaks at both ~15-18 kpc and 30 kpc, coincident with the GSE apocenters, (ii) the outer halo has retrograde streams containing ~10% of GSE stars awaiting discovery at >30 kpc. The retrograde (radial) GSE debris originates from its outer (inner) disk -- exploiting this trend we reconstruct the stellar metallicity gradient of GSE ($-0.04\pm0.01$ dex $r_{\rm{50}}^{-1}$). These simulations imply GSE delivered ~20% of the Milky Way's present-day dark matter and ~50% of its stellar halo. (ABRIDGED)

Topics: Milky Way (52%), Galaxy (51%), Population (50%)
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15 results found

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Abstract: We consider the orbital evolution of satellites in galaxy mergers, focusing on the evolution of eccentricity. Using a large suite of N-body simulations, we study a previously unexplored phenomenon of satellite orbital radialization, i.e., a profound increase in the eccentricity of satellite's orbit as it decays under dynamical friction. While radialization is detected in a variety of different setups, we find that it is most efficient in the cases of high satellite mass, moderately steep host density profiles and high initial eccentricity. To understand the origin of this phenomenon, we run additional simulations with various physical factors selectively turned off: satellite mass loss, reflex motion and distortion of the host, etc. We find that all these factors are important for radialization, since it does not occur for point-mass satellites or when the host potential is replaced with an unperturbed initial profile. The analysis of forces and torques acting on both galaxies confirms the major role of self-gravity of both host and satellite in the reduction of orbital angular momentum. The classical Chandrasekhar dynamical friction formula, which accounts only for the forces between the host and the satellite, but not for internal distortions of both galaxies, does not match the evolution of eccentricity observed in $N$-body simulations.

Topics: Galaxy merger (55%), , Orbit (dynamics) (52%)

5 Citations

Open accessJournal Article
Abstract: Identifying stars found in the Milky Way as having formed in situ or accreted can be a complex and uncertain undertaking. We use Gaia kinematics and APOGEE elemental abundances to select stars belonging to the Gaia-Sausage-Enceladus (GSE) and Sequoia accretion events. These samples are used to characterize the GSE and Sequoia population metallicity distribution functions, elemental abundance patterns, age distributions, and progenitor masses. We find that the GSE population has a mean [Fe/H] ∼-1.15 and a mean age of 10-12 Gyr. GSE has a single sequence in [Mg/Fe] versus [Fe/H] consistent with the onset of SN Ia Fe contributions and uniformly low [Al/Fe] of ∼-0.25 dex. The derived properties of the Sequoia population are strongly dependent on the kinematic selection. We argue the selection with the least contamination is Jφ/Jtot < -0.6 and (Jz - JR)/Jtot < 0.1. This results in a mean [Fe/H] ∼-1.3 and a mean age of 12-14 Gyr. The Sequoia population has a complex elemental abundance distribution with mainly high-[Mg/Fe] stars. We use the GSE [Al/Fe] versus [Mg/H] abundance distribution to inform a chemically based selection of accreted stars, which is used to remove possible contaminant stars from the GSE and Sequoia samples.

Topics: Population (53%)

3 Citations

Open accessPosted Content
Abstract: Since the advent of $Gaia$ astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, $Gaia$-Sausage-Enceladus (GSE), appears to be an early "building block" given its virial mass $> 10^{10}\,\mathrm{M_\odot}$ at infall ($z\sim1-3$). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low-$\alpha$ abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via [Na/Fe] vs. [Mg/Mn] in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including $30 < \sqrt{J_R~/~\mathrm{kpc\,km\,s^{-1}}} < 55$, we can characterise an unprecedented 24 abundances of this structure with GALAH+ DR3. Our chemical selection allows us to prevent circular reasoning and characterise the dynamical properties of the GSE, for example mean $\sqrt{J_R~/~\mathrm{kpc\,km\,s^{-1}}} = 26_{-14}^{+9}$. We find only $(29\pm1)\%$ of the GSE stars within the clean dynamical selection region. We thus discuss chemodynamic selections (such as eccentricity and upper limits on [Na/Fe]).

Topics: Order (ring theory) (51%), Population (50%), Stars (50%)

3 Citations

Open accessPosted Content
Abstract: In the $Gaia$ era stellar kinematics are extensively used to study Galactic halo stellar populations, to search for halo structures, and to characterize the interface between the halo and hot disc populations. We use distribution function-based models of modern datasets with 6D phase space data to qualitatively describe a variety of kinematic spaces commonly used in the study of the Galactic halo. Furthermore, we quantitatively assess how well each kinematic space can separate radially anisotropic from isotropic halo populations. We find that scaled action space (the action diamond'') is superior to other commonly used kinematic spaces at this task. We present a new, easy to implement selection criterion for members of the radially-anisotropic $Gaia$-Enceladus merger remnant, which we find achieves a sample purity of 82 per cent in our models with respect to contamination from the more isotropic halo. We compare this criterion to literature criteria, finding that it produces the highest purity in the resulting samples, at the expense of a modest reduction in completeness. We also show that selection biases that underlie nearly all contemporary spectroscopic datasets can noticeably impact the $E-L_{z}$ distribution of samples in a manner that may be confused for real substructure. We conclude by providing recommendations for how authors should use stellar kinematics in the future to study the Galactic stellar halo.

Topics: Galactic halo (68%), Halo (61%), Stellar kinematics (56%) ... read more

1 Citations

Open accessPosted Content
Abstract: We examine whether the Large Magellanic Cloud (LMC) is currently losing its stellar halo to Milky Way (MW) tides. We present a live $N$-body model for the ongoing MW-LMC interaction that predicts a prominent stream of stars tidally stripped from the progenitor LMC. We use this model to define a strategy to search for stripped material in kinematic space. Of the available stellar tracers, we conclude that samples of RR Lyrae stars provide the highest density of kinematic tracers at present. Using a sample of RR Lyrae stars with Gaia EDR3 astrometry we show that the LMC stellar halo in the Southern Galactic hemisphere extends at least out to $\sim 30^\circ$ from the galaxy centre. In addition, several leading arm candidates are found in the Northern hemisphere as far above the disc plane as $b=+34^\circ$ (at 68$^\circ$ from the LMC).

Topics: , Milky Way (58%), RR Lyrae variable (56%) ... read more

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20 results found

Open accessJournal Article
Nabila Aghanim1, Yashar Akrami2, Yashar Akrami3, Yashar Akrami4  +229 moreInstitutions (70)
Abstract: We present cosmological parameter results from the final full-mission Planck measurements of the CMB anisotropies. We find good consistency with the standard spatially-flat 6-parameter $\Lambda$CDM cosmology having a power-law spectrum of adiabatic scalar perturbations (denoted "base $\Lambda$CDM" in this paper), from polarization, temperature, and lensing, separately and in combination. A combined analysis gives dark matter density $\Omega_c h^2 = 0.120\pm 0.001$, baryon density $\Omega_b h^2 = 0.0224\pm 0.0001$, scalar spectral index $n_s = 0.965\pm 0.004$, and optical depth $\tau = 0.054\pm 0.007$ (in this abstract we quote $68\,\%$ confidence regions on measured parameters and $95\,\%$ on upper limits). The angular acoustic scale is measured to $0.03\,\%$ precision, with $100\theta_*=1.0411\pm 0.0003$. These results are only weakly dependent on the cosmological model and remain stable, with somewhat increased errors, in many commonly considered extensions. Assuming the base-$\Lambda$CDM cosmology, the inferred late-Universe parameters are: Hubble constant $H_0 = (67.4\pm 0.5)$km/s/Mpc; matter density parameter $\Omega_m = 0.315\pm 0.007$; and matter fluctuation amplitude $\sigma_8 = 0.811\pm 0.006$. We find no compelling evidence for extensions to the base-$\Lambda$CDM model. Combining with BAO we constrain the effective extra relativistic degrees of freedom to be $N_{\rm eff} = 2.99\pm 0.17$, and the neutrino mass is tightly constrained to $\sum m_ u< 0.12$eV. The CMB spectra continue to prefer higher lensing amplitudes than predicted in base -$\Lambda$CDM at over $2\,\sigma$, which pulls some parameters that affect the lensing amplitude away from the base-$\Lambda$CDM model; however, this is not supported by the lensing reconstruction or (in models that also change the background geometry) BAO data. (Abridged)

Topics: Spectral index (60%)

1,810 Citations

Open accessJournal Article
Abstract: Globular clusters (GCs) formed when the Milky Way experienced a phase of rapid assembly. We use the wealth of information contained in the Galactic GC population to quantify the properties of the satellite galaxies from which the Milky Way assembled. To achieve this, we train an artificial neural network on the E-MOSAICS cosmological simulations of the co-formation and co-evolution of GCs and their host galaxies. The network uses the ages, metallicities, and orbital properties of GCs that formed in the same progenitor galaxies to predict the stellar masses and accretion redshifts of these progenitors. We apply the network to Galactic GCs associated with five progenitors: {\it Gaia}-Enceladus, the Helmi streams, Sequoia, Sagittarius, and the recently discovered, low-energy' GCs, which provide an excellent match to the predicted properties of the enigmatic galaxy Kraken'. The five galaxies cover a narrow stellar mass range [$M_\star=(0.6{-}4.6)\times10^8~{\rm M}_\odot$], but have widely different accretion redshifts ($z_{\rm acc}=0.57{-}2.65$). All accretion events represent minor mergers, but Kraken likely represents the most major merger ever experienced by the Milky Way, with stellar and virial mass ratios of $r_{M_\star}=1$:$31^{+34}_{-16}$ and $r_{M_{\rm h}}=1$:$7^{+4}_{-2}$, respectively. The progenitors match the $z=0$ relation between GC number and halo virial mass, but have elevated specific frequencies, suggesting an evolution with redshift. Even though these progenitors likely were the Milky Way's most massive accretion events, they contributed a total mass of only $\log{(M_{\rm \star,tot}/{\rm M}_\odot)}=9.0\pm0.1$, similar to the stellar halo. This implies that the Milky Way grew its stellar mass mostly by in-situ star formation. We conclude by organising these accretion events into the most detailed reconstruction to date of the Milky Way's merger tree.

Topics: Globular cluster (56%), Galaxy (55%), Milky Way (54%) ... read more

66 Citations

Open accessJournal Article
Amina Helmi1Institutions (1)
Abstract: The advent of Gaia's 2nd data release in combination with large spectroscopic surveys are revolutionizing our understanding of the Galaxy. Thanks to these and the knowledge accumulated thus far, a more mature picture of the evolution of the early Milky Way is emerging: * Two of the traditional Galactic components, i.e. the stellar halo and the thick disk, appear to be intimately linked: stars with halo-like kinematics originate in similar proportions, from a "heated" (thick) disk and from debris from a system named Gaia-Enceladus. Gaia-Enceladus was the last big merger event experienced by the Milky Way and probably completed around 10 Gyr ago. The puffed-up stars now present in the halo as a consequence of the merger have thus exposed the existence of a disk component at z ~ 1.8. * The Helmi streams, Sequoia, and Thamnos are amongst the newly uncovered or better characterized merger events. Knowledge of their progenitor's properties, star formation and chemical histories is still incomplete. * Debris' from different objects often overlap in phase-space. A task for the next years will be to use spectroscopic surveys for chemical labelling and to disentangle events from one another using dimensions other than only phase-space, metallicity or [alpha/Fe]. * These surveys will also provide line-of-sight velocities missing for faint stars and more accurate distance determinations for distant objects. The resulting samples of stars will cover a much wider volume of the Galaxy allowing, for example, linking kinematic substructures in the inner halo to spatial overdensities in the outer halo. * All the results obtained so far are in-line with expectations of current cosmological models. Yet, tailored hydrodynamical simulations as well as "constrained" cosmological simulations are needed to push our knowledge of the assembly of the Milky Way back to the earliest times. [abridged]

Topics: Thick disk (66%), Milky Way (58%), Galaxy (57%) ... read more

41 Citations

Open accessJournal Article
Abstract: Numerical methods have become a powerful tool for research in astrophysics, but their utility depends critically on the availability of suitable simulation codes. This calls for continuous efforts in code development, which is necessitated also by the rapidly evolving technology underlying today's computing hardware. Here we discuss recent methodological progress in the GADGET code, which has been widely applied in cosmic structure formation over the past two decades. The new version offers improvements in force accuracy, in time-stepping, in adaptivity to a large dynamic range in timescales, in computational efficiency, and in parallel scalability through a special MPI/shared-memory parallelization and communication strategy, and a more-sophisticated domain decomposition algorithm. A manifestly momentum conserving fast multipole method (FMM) can be employed as an alternative to the one-sided TreePM gravity solver introduced in earlier versions. Two different flavours of smoothed particle hydrodynamics, a classic entropy-conserving formulation and a pressure-based approach, are supported for dealing with gaseous flows. The code is able to cope with very large problem sizes, thus allowing accurate predictions for cosmic structure formation in support of future precision tests of cosmology, and at the same time is well adapted to high dynamic range zoom-calculations with extreme variability of the particle number density in the simulated volume. The GADGET-4 code is publicly released to the community and contains infrastructure for on-the-fly group and substructure finding and tracking, as well as merger tree building, a simple model for radiative cooling and star formation, a high dynamic range power spectrum estimator, and an initial conditions generator based on second-order Lagrangian perturbation theory.

Topics: , , Solver (52%)

37 Citations

Open accessPosted Content
Abstract: Now that conventional weakly interacting massive particle (WIMP) dark matter searches are approaching the neutrino floor, there has been a resurgence of interest in detectors with sensitivity to nuclear recoil directions. A large-scale directional detector is attractive in that it would have sensitivity below the neutrino floor, be capable of unambiguously establishing the galactic origin of a purported dark matter signal, and could serve a dual purpose as a neutrino observatory. We present the first detailed analysis of a 1000~m$^3$-scale detector capable of measuring a directional nuclear recoil signal at low energies. We propose a modular and multi-site observatory consisting of time projection chambers (TPCs) filled with helium and SF$_6$ at atmospheric pressure. Depending on the TPC readout technology, 10-20 helium recoils above 6 kevr or only 3-4 recoils above 20~\kevr would suffice to distinguish a 10~GeV WIMP signal from the solar neutrino background. High-resolution charge readout also enables powerful electron background rejection capabilities well below 10~keV. We detail background and site requirements at the 1000~m$^3$-scale, and identify materials that require improved radiopurity. The final experiment, which we name CYGNUS-1000, will be able to observe 10-40 neutrinos from the Sun, depending on the final energy threshold. With the same exposure, the sensitivity to spin independent cross sections will extend into presently unexplored sub-10 GeV parameter space. For spin dependent interactions, already a 10~m$^3$-scale experiment could compete with upcoming generation-two detectors, but CYGNUS-1000 would improve upon this considerably. Larger volumes would bring sensitivity to neutrinos from an even wider range of sources, including galactic supernovae, nuclear reactors, and geological processes.