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We perform the first self-consistent measurement of the rate of interactions between stellar tidal streams created by disrupting satellites and dark subhalos in a cosmological simulation of a Milky-Way-mass galaxy. Using a retagged version of the Aquarius A dark-matter-only simulation, we selected 18 streams of tagged star particles that appear thin at the present day and followed them from the point their progenitors accrete onto the main halo, recording in each snapshot the characteristics of all dark-matter subhalos passing within several distance thresholds of any tagged star particle in each stream. We considered distance thresholds corresponding to constant impact parameters (1, 2, and 5 kpc), as well as those proportional to the region of influence of each subhalo (one and two times its half-mass radius $r_{1/2}$). We then measured the age and present-day, phase-unwrapped length of each stream in order to compute the interaction rate in different mass bins and for different thresholds, and compared these to analytic predictions from the literature. We measure a median rate of $1.5^{+3.0}_{-1.1} (9.1^{+17.5}_{-7.1}, 61.8^{+211}_{-40.6})$ interactions within 1 (2, 5) kpc of the stream per 10 kpc of stream length per 10 Gyr. Resolution effects (both time and particle number) affect these estimated rates by lowering them.
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We identify a new, nearby (0.5 < d < 10 kpc) stream in data from the RAdial Velocity Experiment (RAVE). As the majority of stars in the stream lie in the constellation of Aquarius we name it the Aquarius Stream. We identify 15 members of the stream lying between 30 < l < 75 and -70< b <-50, with heliocentric line-of-sight velocities V_los~-200 km/s. The members are outliers in the radial velocity distribution, and the overdensity is statistically significant when compared to mock samples created with both the Besanc{c}on Galaxy model and newly-developed code Galaxia. The metallicity distribution function and isochrone fit in the log g - T_eff plane suggest the stream consists of a 10 Gyr old population with [m/H]~-1.0. We explore relations to other streams and substructures, finding the stream cannot be identified with known structures: it is a new, nearby substructure in the Galaxys halo. Using a simple dynamical model of a dissolving satellite galaxy we account for the localization of the stream. We find that the stream is dynamically young and therefore likely the debris of a recently disrupted dwarf galaxy or globular cluster. The Aquarius stream is thus a specimen of ongoing hierarchical Galaxy formation, rare for being right in the solar suburb.
We present an abundance analysis of six member stars of the recently discovered Aquarius stream, in an attempt to ascertain whether this halo stream is real and, if so, to understand its origin. The mean metallicities of the six stars have a dispersion of only 0.10 dex, indicating that they are part of a chemically coherent structure. We then investigate whether the stream represents the debris of a disrupted dwarf galaxy or a disrupted globular cluster. The [Ni/Fe] - [Na/Fe] plane provides a good diagnostic: globular cluster stars and dwarf spheroidal galaxy stars are well separated in this plane, and the Aquarius stream stars lie unambiguously in the globular cluster region. The Aquarius stream stars also lie on the distinct [Na/Fe] - [O/Fe] and [Mg/Fe] - [Al/Fe] relations delineated by Galactic globular cluster stars. Spectroscopic parameters for the six Aquarius stars show that they are tightly confined to a 12 Gyr, [Fe/H] = -1.0, alpha-enhanced isochrone, consistent with their identification as globular cluster debris. We present evidence that the Aquarius stream may continue through the disk and out into the northern halo. Our results indicate a globular cluster origin for the Aquarius stream, and demonstrate the potential for chemical tagging to identify the origins of Galactic substructures.
When a star passes close to a supermassive black hole (BH), the BHs tidal forces rip it apart into a thin stream, leading to a tidal disruption event (TDE). In this work, we study the post-disruption phase of TDEs in general relativistic hydrodynamics (GRHD) using our GPU-accelerated code H-AMR. We carry out the first grid-based simulation of a deep-penetration TDE ($beta$=7) with realistic system parameters: a black-hole-to-star mass ratio of $10^6$, a parabolic stellar trajectory, and a nonzero BH spin. We also carry out a simulation of a tilted TDE whose stellar orbit is inclined relative to the BH midplane. We show that for our aligned TDE, an accretion disk forms due to the dissipation of orbital energy with $sim$20 percent of the infalling material reaching the BH. The dissipation is initially dominated by violent self-intersections and later by stream-disk interactions near the pericenter. The self-intersections completely disrupt the incoming stream, resulting in five distinct self-intersection events separated by approximately 12 hours and a flaring in the accretion rate. We also find that the disk is eccentric with mean eccentricity e$approx$0.88. For our tilted TDE, we find only partial self-intersections due to nodal precession near pericenter. Although these partial intersections eject gas out of the orbital plane, an accretion disk still forms with a similar accreted fraction of the material to the aligned case. These results have important implications for disk formation in realistic tidal disruptions. For instance, the periodicity in accretion rate induced by the complete stream disruption may explain the flaring events from Swift J1644+57.
Recent high precision proper motions from the Hubble Space Telescope (HST) suggest that the Large and Small Magellanic Clouds (LMC and SMC, respectively) are either on their first passage or on an eccentric long period (>6 Gyr) orbit about the Milky Way (MW). This differs markedly from the canonical picture in which the Clouds travel on a quasi-periodic orbit about the MW (period of ~2 Gyr). Without a short period orbit about the MW, the origin of the Magellanic Stream, a young (1-2 Gyr old) coherent stream of HI gas that trails the Clouds ~150 degrees across the sky, can no longer be attributed to stripping by MW tides and/or ram pressure stripping by MW halo gas. We propose an alternative formation mechanism in which material is removed by LMC tides acting on the SMC before the system is accreted by the MW. We demonstrate the feasibility and generality of this scenario using an N-body/SPH simulation with cosmologically motivated initial conditions constrained by the observations. Under these conditions we demonstrate that it is possible to explain the origin of the Magellanic Stream in a first infall scenario. This picture is generically applicable to any gas-rich dwarf galaxy pair infalling towards a massive host or interacting in isolation.
Stellar tidal streams are sensitive tracers of the properties of the gravitational potential in which they orbit and detailed observations of their density structure can be used to place stringent constraints on fluctuations in the potential caused by, e.g., the expected populations of dark matter subhalos in the standard cold dark matter paradigm (CDM). Simulations of the evolution of stellar streams in live $N$-body halos without low-mass dark-matter subhalos, however, indicate that streams exhibit significant perturbations on small scales even in the absence of substructure. Here we demonstrate, using high-resolution $N$-body simulations combined with sophisticated semi-analytic and simple analytic models, that the mass resolutions of $10^4$--$10^5,rm{M}_{odot}$ commonly used to perform such simulations cause spurious stream density variations with a similar magnitude on large scales as those expected from a CDM-like subhalo population and an order of magnitude larger on small, yet observable, scales. We estimate that mass resolutions of $approx100,rm{M}_{odot}$ ($approx1,rm{M}_{odot}$) are necessary for spurious, numerical density variations to be well below the CDM subhalo expectation on large (small) scales. That streams are sensitive to a simulations particle mass down to such small masses indicates that streams are sensitive to dark matter clustering down to these low masses if a significant fraction of the dark matter is clustered or concentrated in this way, for example, in MACHO models with masses of $10$--$100,rm{M}_{odot}$.
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