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Register a new userLiving with Neighbors. II. Statistical Analysis of Flybys and Mergers of Dark Matter Halos in Cosmological Simulations
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We present a statistical analysis of flybys of dark matter halos compared to mergers using cosmological $N$-body simulations. We mainly focus on gravitationally interacting target halos with mass of $10^{10.8}-10^{13.0}h^{-1}M_{odot}$, and their neighbors are counted only when the mass ratio is 1:3$-$3:1 and the distance is less than the sum of the virial radii of target and neighbor. The neighbors are divided into the flyby or merger samples if the pairs total energy is greater or smaller, respectively, than the capture criterion with consideration of dynamical friction. The main results are as follows: (a) The flyby fraction increases by up to a factor of 50 with decreasing halo mass and by up to a factor of 400 with increasing large-scale density, while the merger fraction does not show any significant dependencies on these two parameters; (b) The redshift evolution of the flyby fraction is twofold, increasing with redshift at $0<z<1$ and remaining constant at $z>1$, while the merger fraction increases monotonically with redshift at $z=0sim4$; (c) The multiple interactions with two or more neighbors are on average flyby-dominated, and their fraction has a mass and environment dependence similar to that for the flyby fraction; (d) Given that flybys substantially outnumber mergers toward $z=0$ (by a factor of five) and the multiple interactions are flyby-dominated, the flybys contribution to galactic evolution is stronger than ever at the present epoch, especially for less massive halos and in the higher density environment. We propose a scenario that connects the evolution of the flyby and merger fractions to the hierarchical structure formation process.
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Spin$-$orbit alignment (SOA; i.e., the vector alignment between the halo spin and the orbital angular momentum of neighboring halos) provides an important clue to how galactic angular momenta develop. For this study, we extract virial-radius-wise contact halo pairs with mass ratios between 1/10 and 10 from a set of cosmological $N$-body simulations. In the spin--orbit angle distribution, we find a significant SOA in that 52.7%$pm$0.2% of neighbors are on the prograde orbit. The SOA of our sample is mainly driven by low-mass target halos ($<10^{11.5}h^{-1}M_{odot}$) with close merging neighbors, corroborating the notion that the tidal interaction is one of the physical origins of SOA. We also examine the correlation of SOA with the adjacent filament and find that halos closer to the filament show stronger SOA. Most interestingly, we discover for the first time that halos with the spin parallel to the filament experience most frequently the prograde-polar interaction (i.e., fairly perpendicular but still prograde interaction; spin--orbit angle $sim$ 70$^{circ}$). This instantly invokes the spin-flip event and the prograde-polar interaction will soon flip the spin of the halo to align it with the neighbors orbital angular momentum. We propose that the SOA originates from the local cosmic flow along the anisotropic large-scale structure, especially that along the filament, and grows further by interactions with neighbors.
We present that the spin$-$orbit alignment (SOA; i.e., the angular alignment between the spin vector of a halo and the orbital angular momentum vector of its neighbor) provides an important clue to how galactic angular momenta develop. In particular, we identify virial-radius-wise contact halo pairs with mass ratios from 1/3 to 3 in a set of cosmological $N$-body simulations, and divide them into merger and flyby subsamples according to their total (kinetic+potential) energy. In the spin$-$orbit angle distribution, we find a significant SOA in that $75.0pm0.6$ % of merging neighbors and $58.7pm0.6$ % of flybying neighbors are on the prograde orbit. The overall SOA of our sample is mainly driven by fast-rotating halos, corroborating that a well-aligned interaction spins a halo faster. More interestingly, we find for the first time a strong number excess of nearly perpendicular but still prograde interactions ($sim75^{circ}$) in the spin$-$orbit angle distribution for both the merger and flyby cases. Such prograde-polar interactions predominate for slow-rotating halos, testifying that misaligned interactions reduce the halos spin. The frequency of the prograde-polar interactions correlates with the halo mass, yet anticorrelates with the large-scale density. This instantly invokes the spin-flip phenomenon that is conditional on the mass and environment. The prograde-polar interaction will soon flip the spin of a slow-rotator to align with its neighbors orbital angular momentum. Finally, we propose a scenario that connects the SOA to the ambient large-scale structure based on the spin-flip argument.
We study the distribution of cold dark matter (CDM) in cosmological simulations from the FIRE (Feedback In Realistic Environments) project, for $M_{ast}sim10^{4-11},M_{odot}$ galaxies in $M_{rm h}sim10^{9-12},M_{odot}$ halos. FIRE incorporates explicit stellar feedback in the multi-phase ISM, with energetics from stellar population models. We find that stellar feedback, without fine-tuned parameters, greatly alleviates small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile ($alpha$) shows a strong mass dependence: profiles are shallow at $M_{rm h}sim10^{10}-10^{11},M_{odot}$ and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of $alpha$, and relatively late growth of cores. Because the star formation efficiency $M_{ast}/M_{rm h}$ is strongly halo mass dependent, a rapid change in $alpha$ occurs around $M_{rm h}sim 10^{10},M_{odot}$ ($M_{ast}sim10^{6}-10^{7},M_{odot}$), as sufficient feedback energy becomes available to perturb the DM. Large cores are not established during the period of rapid growth of halos because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid buildup has completed. Stellar feedback dramatically reduces circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the Too Big To Fail problem without invoking non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass halos produce DM profiles slightly shallower than the Navarro-Frenk-White profile, consistent with the normalization of the observed Tully-Fisher relation.
We investigate the ability of basis function expansions to reproduce the evolution of a Milky Way-like dark matter halo, extracted from a cosmological zoom-in simulation. For each snapshot, the density of the halo is reduced to a basis function expansion, with interpolation used to recreate the evolution between snapshots. The angular variation of the halo density is described by spherical harmonics, and the radial variation either by biorthonormal basis functions adapted to handle truncated haloes or by splines. High fidelity orbit reconstructions are attainable using either method with similar computational expense. We quantify how the error in the reconstructed orbits varies with expansion order and snapshot spacing. Despite the many possible biorthonormal expansions, it is hard to beat a conventional Hernquist-Ostriker expansion with a moderate number of terms ($gtrsim15$ radial and $gtrsim6$ angular). As two applications of the developed machinery, we assess the impact of the time-dependence of the potential on (i) the orbits of Milky Way satellites, and (ii) planes of satellites as observed in the Milky Way and other nearby galaxies. Time evolution over the last 5 Gyr introduces an uncertainty in the Milky Way satellites orbital parameters of $sim 15$ per cent, comparable to that induced by the observational errors or the uncertainty in the present-day Milky Way potential. On average, planes of satellites grow at similar rates in evolving and time-independent potentials. There can be more, or less, growth in the planes thickness, if the plane becomes less, or more, aligned with the major or minor axis of the evolving halo.
Narrow stellar streams in the Milky Way halo are uniquely sensitive to dark-matter subhalos, but many of these subhalos may be tidally disrupted. I calculate the interaction between stellar and dark-matter streams using analytical and $N$-body calculations, showing that disrupting objects can be detected as low-concentration subhalos. Through this effect, we can constrain the lumpiness of the halo as well as the orbit and present position of individual dark-matter streams. This will have profound implications for the formation of halos and for direct and indirect-detection dark-matter searches.
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