7 research outputs found
The impact of two massive early accretion events in a Milky Way-like galaxy: repercussions for the buildup of the stellar disc and halo
We identify and characterize a Milky Way-like realization from the Auriga simulations with two consecutive massive mergers âŒ2
âGyr apart at high redshift, comparable to the reported Kraken and Gaia-Sausage-Enceladus. The Kraken-like merger (z = 1.6, MTot=8Ă1010Mââ ) is gas-rich, deposits most of its mass in the inner 10âkpc, and is largely isotropic. The Sausage-like merger (z = 1.14, MTot=1Ă1011Mâ) leaves a more extended mass distribution at higher energies, and has a radially anisotropic distribution. For the higher-redshift merger, the stellar mass ratio of the satellite to host galaxy is high (1:3). As a result, the chemistry of the remnant is indistinguishable from contemporaneous in situ populations, making it challenging to identify through chemical abundances. This naturally explains why all abundance patterns attributed so far to Kraken are in fact fully consistent with the metal-poor in situ so-called Aurora population and thick disc. However, our model makes a falsifiable prediction: if the Milky Way underwent a gas-rich double merger at high redshift, then this should be imprinted on its star formation history with bursts about âŒ2 s apart. This may offer constraining power on the highest-redshift massive mergers
Kinematics with Gaia DR2: the force of a dwarf
We use Gaia DR2 astrometric and line-of-sight velocity information combined with two sets of distances obtained with a Bayesian inference method to study the 3D velocity distribution in the Milky Way disc. We search for variations in all Galactocentric cylindrical velocity components (VÏ, VR, and Vz) with Galactic radius, azimuth, and distance from the disc mid-plane. We confirm recent work showing that bulk vertical motions in the Râz plane are consistent with a combination of breathing and bending modes. In the xây plane, we show that, although the amplitudes change, the structure produced by these modes is mostly invariant as a function of distance from the plane. Comparing to two different Galactic disc models, we demonstrate that the observed patterns can drastically change in short time intervals, showing the complexity of understanding the origin of vertical perturbations. A strong radial VR gradient was identified in the inner disc, transitioning smoothly from 16âkmâsâ1âkpcâ1 at an azimuth of 30° < Ï < 45° ahead of the Sun-Galactic centre line to â16âkmâsâ1âkpcâ1 at an azimuth of â45° < Ï < â30° lagging the solar azimuth. We use a simulation with no significant recent mergers to show that exactly the opposite trend is expected from a barred potential, but overestimated distances can flip this trend to match the data. Alternatively, using an N-body simulation of the Sagittarius dwarfâMilky Way interaction, we demonstrate that a major recent perturbation is necessary to reproduce the observations. Such an impact may have strongly perturbed the existing bar or even triggered its formation in the last 1â2âGyr
Fluctuations in galactic bar parameters due to bar-spiral interaction
We study the late-time evolution of the central regions of two Milky Way (MW)-like simulations of galaxies formed in a cosmological context, one hosting a fast bar and the other a slow one. We find that bar length, Rb, measurements fluctuate on a dynamical time-scale by up to 100 perâcent, depending on the spiral structure strength and measurement threshold. The bar amplitude oscillates by about 15 perâcent, correlating with Rb. The TremaineâWeinberg method estimates of the barsâ instantaneous pattern speeds show variations around the mean of up to âŒ20 per centâ , typically anticorrelating with the bar length and strength. Through power spectrum analyses, we establish that these bar pulsations, with a period in the range âŒ60â200 Myr, result from its interaction with multiple spiral modes, which are coupled with the bar. Because of the presence of odd spiral modes, the two bar halves typically do not connect at exactly the same time to a spiral arm, and their individual lengths can be significantly offset. We estimated that in about 50 perâcent of bar measurements in MW-mass external galaxies, the bar lengths of SBab-type galaxies are overestimated by âŒ15 per cent and those of SBbc types by âŒ55 per centâ . Consequently, bars longer than their corotation radius reported in the literature, dubbed âultrafast barsâ, may simply correspond to the largest biases. Given that the ScutumâCentaurus arm is likely connected to the near half of the MW bar, recent direct measurements may be overestimating its length by 1â1.5 kpc, while its present pattern speed may be 5â10 km sâ1 kpcâ1 smaller than its time-averaged value
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On the Hunt for the Origins of the Orphan-Chenab Stream: Detailed Element Abundances with APOGEE and Gaia
Abstract
Stellar streams in the Galactic halo are useful probes of the assembly of galaxies like the Milky Way. Many tidal stellar streams that have been found in recent years are accompanied by a known progenitor globular cluster or dwarf galaxy. However, the OrphanâChenab (OC) stream is one case where a relatively narrow stream of stars has been found without a known progenitor. In an effort to find the parent of the OC stream, we use astrometry from the early third data release of ESAâs Gaia mission (Gaia EDR3) and radial velocity information from the Sloan Digital Sky Survey (SDSS)-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to find up to 13 stars that are likely members of the OC stream. We use the APOGEE survey to study the chemical nature (for up to 10 stars) of the OC stream in the α (O, Mg, Ca, Si, Ti, and S), odd-Z (Al, K, and V), Fe-peak (Fe, Ni, Mn, Co, and Cr), and neutron-capture (Ce) elemental groups. We find that the stars that make up the OC stream are not consistent with a monometallic population and have a median metallicity of â1.92 dex with a dispersion of 0.28 dex. Our results also indicate that the α elements are depleted compared to the known Milky Way populations and that its [Mg/Al] abundance ratio is not consistent with second-generation stars from globular clusters. The detailed chemical pattern of these stars, namely the [α/Fe]â[Fe/H] plane and the metallicity distribution, indicates that the OC stream progenitor is very likely to be a dwarf spheroidal galaxy with a mass of âŒ106
M
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Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk
Our Galaxy is thought to have undergone an active evolutionary history dominated by star formation, the accretion of cold gas, and, in particular, mergers up to 10 gigayear ago. The stellar halo reveals rich fossil evidence of these interactions in the form of stellar streams, substructures, and chemically distinct stellar components. The impact of dwarf galaxy mergers on the content and morphology of the Galactic disk is still being explored. Recent studies have identified kinematically distinct stellar substructures and moving groups, which may have extragalactic origin. However, there is mounting evidence that stellar overdensities at the outer disk/halo interface could have been caused by the interaction of a dwarf galaxy with the disk. Here we report detailed spectroscopic analysis of 14 stars drawn from two stellar overdensities, each lying about 5 kiloparsecs above or below the Galactic plane â locations suggestive of association with the stellar halo. However, we find that the chemical compositions of these stars are almost identical, both within and between these groups, and closely match the abundance patterns of the Milky Way disk stars. This study hence provides compelling evidence that these stars originate from the disk and the overdensities they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies</p