The virgo radial merger: a recent and substantial contribution to the local stellar halo

Donlon, Thomas, Joseph
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Brown, Ethan
Meunier, Vincent
Christian, John, A
Newberg, Heidi, J
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The Milky Way's (MW) stellar halo is built up from the debris of many disrupted dwarf galaxies. It is widely known that the MW's inner stellar halo contains a [Fe/H]-rich, radialized component, which contributes the majority of halo stars near the Sun, and is often referred to as the MW's "last major merger." The prevailing hypothesis for the origin of this component is that a sizeable dwarf galaxy collided with the MW proto-disk 8-11 Gyr ago, which is known as the \textit{Gaia}-Sausage/Enceladus (GSE) merger event. In this work we consider several collections of observed chemical and kinematic data for MW halo stars; our analyses of these data collectively come to the conclusion that the observed properties and structures in the MW stellar halo are inconsistent with a collision between the MW and a large dwarf galaxy 8-11 Gyr ago (as is proposed to explain the GSE). Instead, these observations are better explained as the result of a collision between a dwarf galaxy and the MW disk that occurred definitely less than 5 Gyr ago, and probably less than 3 Gyr ago, which we call the Virgo Radial Merger (VRM). We start by re-analyzing existing datasets of RR Lyrae (RRL) stars in the Virgo Overdensity (VOD) in the MW stellar halo. These stars originally had line-of-sight velocity measurements, but we are able to add proper motions to obtain their 3-dimensional velocities. The VOD contains a multitude of previously identified overlapping stellar streams and moving groups; we show that it is possible to generate the majority of the known structures using a simulation of a single dwarf galaxy passing through the center of the MW on a radial orbit (the VRM). This radial orbit gives the VRM debris a large range of energies and nearly zero Lz angular momentum. We potentially link the VRM with the Hercules-Aquila Cloud (HAC) and the Eridanus-Phoenix Overdensity (EPO) halo structures. The simulation of the VRM produces debris in the Solar neighborhood similar to the material associated with the Gaia-Enceladus/Gaia Sausage (GSE) merger event, despite only being integrated for 2 Gyr. As the GSE supposedly requires an age of 8-11 Gyr, resolving this discrepancy becomes the major focus of the remainder of this work. Radial merger events generate shells, which are spherically concentric distributions of stars that have been observed in other galaxies. Because we believe that the progenitor dwarf galaxy of the VRM collided with the MW on a radial orbit, we looked for the shells this collision was expected to produce and were able to identify the first shell structures every found in the MW. There are 2 shells in the VOD region and 2 shells in the HAC region, and we associate these structures with the VRM. The morphology of shell structure depends on how long it has been since the progenitor dwarf became unbound; eventually, the debris becomes indistinguishable from background stars. We analyze phase mixing in a collection of radial merger $N$-body simulations, and find that shell structure similar to that observed in the MW disappears by 5 Gyr after collision with the Galactic center; this provides an upper limit on the time since the VRM progenitor collided with the MW disk. By placing test particles at the location of the MW shells and integrating their orbits backwards in time, we are able to identify the time when their progenitor dwarf galaxy became unbound. Our result indicates that the progenitor of the VRM passed through the Galactic center 2.7 +/- 0.2 Gyr ago. Based on the time of collision, it is possible that the VRM is related to the phenomenon that created phase-space spirals in the vertical motion of the disk, and could have caused a burst of star formation in the inner disk. Shells are formed from a subset of stars in a radial merger; however, if one looks at all stars in a radial merger in phase space (vr vs. r), the merger event is described by one or more parabolic ``caustics'', which can be thought of as a generalization of shells. We analyze the local phase space distribution in Gaia DR3 data, and recover the recently-identified phase-space caustics, which agree well with analytical models. The observed caustics have positive caustic velocities (the velocity of the structure at maximum Galactocentric radius), which support a more recent merger time, because phase-mixed debris produces chevron patterns with zero caustic velocity. Other works have not always noted the significance of this difference when comparing observed and simulated distributions of phase-space folds. We compare the observed phase space data to an analog GSE merger from the FIRE-2 Latte simulations. This simulated analog assembles a density profile similar to that observed in the MW much earlier after collision than previous simulations; a late assembly of a smooth density profile had previously been an argument for the large age of the GSE. We utilize a quantitative metric (causticality) that measures how phase-mixed a given distribution is, and find that the observed local phase-space distribution most closely matches the simulated data 1.5 Gyr after collision, and certainly not later than 3 Gyr after collision. This is further evidence that the progenitor of the ``last major merger'' did not collide with the MW proto-disk at early times, as is thought for the GSE, but instead collided with the MW disk within the last few Gyr. This is consistent with our previous collision time estimate for the VRM. After our assessments of the dynamics of radial merger events, we begin analyses of the chemical abundances of halo stars near the Sun. Because stars that are formed in different environments will have different chemical abundances, in principle these chemical abundances can separate the MW stellar halo into its distinct progenitor merger events. First, we use halo dwarf stars with photometrically determined metallicities that are located within 2 kpc of the Sun to identify local halo substructure. The kinematic properties of these stars are inconsistent with arising from a single, dominant radial merger event, such as the GSE. We find a number of distinct components in the stellar halo, including the VRM, and two new components which we name Nereus and Cronus. Nereus is non-rotating and has similar enegies to the VRM, but has lower [Fe/H]. Cronus has higher [Fe/H] than the VRM, and is co-rotating with the disk. We also identify the Nyx Stream, one or two smooth halo ``background'' components, and a disk contamination and/or in-situ halo component. We then analyze the chemical and kinematic properties of giant stars near the Sun, and we find that the chemical abundances and dynamics of this data are also inconsistent with a scenario in which the inner halo is primarily composed of debris from a single, massive, ancient merger event. The trends of chemical composition with energy in the data are opposite to expectations for a single massive, ancient merger event. Also, multiple chemical evolution paths with distinct dynamics are present. We find that the data is best fit by a model with four components. These components are the same as before: the VRM, Nereus, and Cronus, plus a previously-identified component named Thamnos. Nereus and Thamnos likely represent more than one accretion event because the chemical abundance distributions of their member stars contain many peaks. Because the local stellar halo contains multiple substructures, different popular methods of selecting GSE stars will actually select different mixtures of these substructures. We conclude with a novel picture of the MW's accretion history. We find it likely that the MW experienced one or more sizeable merger events early on in its history, which are located predominantly inside the Solar circle. The disequilibrium features in the MW stellar halo, including cloud overdensities, shells, and caustics in phase space, are all due to a much more recent merger event: the VRM. The progenitor of the VRM collided with the MW disk 2-3 Gyr ago, and produced a population of [Fe/H]-rich, [Alpha/Fe]-poor stars in the local stellar halo. These stars have a double-lobed velocity distribution that is responsible for the high-|vr| portions of the Gaia-Sausage velocity structure.
School of Science
Dept. of Physics, Applied Physics, and Astronomy
Rensselaer Polytechnic Institute, Troy, NY
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