1,446 research outputs found
An Alternative Origin for Hypervelocity Stars
Halo stars with unusually high radial velocity ("hypervelocity" stars, or
HVS) are thought to be stars unbound to the Milky Way that originate from the
gravitational interaction of stellar systems with the supermassive black hole
at the Galactic center. We examine the latest HVS compilation and find
peculiarities that are unexpected in this black hole-ejection scenario. For
example, a large fraction of HVS cluster around the constellation of Leo and
share a common travel time of -200 Myr. Furthermore, their velocities
are not really extreme if, as suggested by recent galaxy formation models, the
Milky Way is embedded within a dark halo
with virial velocity of km/s. In this case, the escape velocity at
kpc would be km/s and very few HVS would be truly unbound.
We use numerical simulations to show that disrupting dwarf galaxies may
contribute halo stars with velocities up to and sometimes exceeding the nominal
escape speed of the system. These stars are arranged in a thinly-collimated
outgoing ``tidal tail'' stripped from the dwarf during its latest pericentric
passage. We speculate that some HVS may therefore be tidal debris from a dwarf
recently disrupted near the center of the Galaxy. In this interpretation, the
angular clustering of HVS results because from our perspective the tail is seen
nearly ``end on'', whereas the common travel time simply reflects the fact that
these stars were stripped simultaneously from the dwarf during a single
pericentric passage. This proposal is eminently falsifiable, since it makes a
number of predictions that are distinct from the black-hole ejection mechanism
and that should be testable with improved HVS datasets.Comment: 4 pages, 4 figures. Replacement to match version accepted to ApJ
Stars beyond Galaxies: The Origin of Extended Luminous Halos around Galaxies
(Abridged) We use numerical simulations to investigate the origin and
structure of the luminous halos that surround isolated galaxies. These stellar
structures extend out to several hundred kpc away from a galaxy, and consist of
stars shed by merging subunits during the many accretion events that
characterize the hierarchical assembly of galaxies. Such origin suggests that
outer luminous halos are ubiquitous and that they should appear as an excess of
light over extrapolations of the galaxy's inner profile beyond its traditional
luminous radius. The mass profile of the accreted stellar component is well
approximated by a model where the logarithmic slope steepens monotonically with
radius; from -3 at the luminous edge of the galaxy to -4 or steeper near the
virial radius of the system. Such spatial distribution is consistent with that
of Galactic and M31 globular clusters, suggesting that many of the globulars
were brought in by accretion events, in a manner akin to the classic
Searle-Zinn scenario. The outer stellar spheroid is supported by a velocity
dispersion tensor with a substantial and radially increasing radial anisotropy.
These properties distinguish the stellar halo from the dark matter component,
which is more isotropic in velocity space, as well as from some tracers of the
outer spheroid such as satellite galaxies. Most stars in the outer halo formed
in progenitors that have since merged with the central galaxy; very few stars
in the halo are contributed by satellites that survive as self-bound entities
at the present. These features are in reasonable agreement with recent
observations of the outer halo of the MW, of M31, and of other isolated
spirals, and suggest that all of these systems underwent an early period of
active merging, as envisioned in hierarchical models of galaxy formation.Comment: Submitted to MNRAS, 13 pages, 12 figure
The Hierarchical Formation of the Galactic Disk
I review the results of recent cosmological simulations of galaxy formation
that highlight the importance of satellite accretion in the formation of
galactic disks. Tidal debris of disrupted satellites may contribute to the disk
component if they are compact enough to survive the decay and circularization
of the orbit as dynamical friction brings the satellite into the disk plane.
This process may add a small but non-negligible fraction of stars to the thin
and thick disks, and reconcile the presence of very old stars with the
protracted merging history expected in a hierarchically clustering universe. I
discuss various lines of evidence which suggest that this process may have been
important during the formation of the Galactic disk.Comment: paper to be read at the "Penetrating Bars through Masks of Cosmic
Dust" conference in South Afric
Exponential distribution for the occurrence of rare patterns in Gibbsian random fields
We study the distribution of the occurrence of rare patterns in sufficiently
mixing Gibbs random fields on the lattice , . A typical
example is the high temperature Ising model. This distribution is shown to
converge to an exponential law as the size of the pattern diverges. Our
analysis not only provides this convergence but also establishes a precise
estimate of the distance between the exponential law and the distribution of
the occurrence of finite patterns. A similar result holds for the repetition of
a rare pattern. We apply these results to the fluctuation properties of
occurrence and repetition of patterns: We prove a central limit theorem and a
large deviation principle.Comment: To appear in Commun. Math. Phy
A Sagittarius-Induced Origin for the Monoceros Ring
The Monoceros ring is a collection of stars in nearly-circular orbits at
roughly 18 kpc from the Galactic center. It may have originated (i) as the
response of the disc to perturbations excited by satellite companions or (ii)
from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros
stars differs from that of disc stars at comparable Galactocentric distances,
an observation that disfavours the first scenario. On the other hand, circular
orbits are difficult to accommodate in the tidal-disruption scenario, since it
requires a satellite which at the time of disruption was itself in a nearly
circular orbit. Such satellite could not have formed at the location of the
ring and, given its low mass, dynamical friction is unlikely to have played a
major role in its orbital evolution. We search cosmological simulations for
low-mass satellites in nearly-circular orbits and find that they result, almost
invariably, from orbital changes induced by collisions with more massive
satellites: the radius of the circular orbit thus traces the galactocentric
distance of the collision. Interestingly, the Sagittarius dwarf, one of the
most luminous satellites of the Milky Way, is in a polar orbit that crosses the
Galactic plane at roughly the same Galactocentric distance as Monoceros. We use
idealized simulations to demonstrate that an encounter with Sagittarius might
well have led to the circularization and subsequent tidal demise of the
progenitor of the Monoceros ring.Comment: 6 pages, 4 figures, to match version published in MNRAS Letters
(http://onlinelibrary.wiley.com/doi/10.1111/j.1745-3933.2011.01035.x/abstract
Satellites of Simulated Galaxies: survival, merging, and their relation to the dark and stellar halos
We study the population of satellite galaxies formed in a suite of
N-body/gasdynamical simulations of galaxy formation in a LCDM universe. We find
little spatial or kinematic bias between the dark matter and the satellite
population. The velocity dispersion of the satellites is a good indicator of
the virial velocity of the halo: \sigma_{sat}/V_{vir}=0.9 +/- 0.2. Applied to
the Milky Way and M31 this gives V_{vir}^{MW}=109 +/- 22$ km/s and
V_{vir}^{M31} = 138 +/- 35 km/s, respectively, substantially lower than the
rotation speed of their disk components. The detailed kinematics of simulated
satellites and dark matter are also in good agreement. By contrast, the stellar
halo of the simulated galaxies is kinematically and spatially distinct from the
population of surviving satellites. This is because the survival of a satellite
depends on mass and on time of accretion; surviving satellites are biased
toward low-mass systems that have been recently accreted by the galaxy. Our
results support recent proposals for the origin of the systematic differences
between stars in the Galactic halo and in Galactic satellites: the elusive
``building blocks'' of the Milky Way stellar halo were on average more massive,
and were accreted (and disrupted) earlier than the population of dwarfs that
has survived self-bound until the present.Comment: 13 pages, 11 figures, MNRAS in press. Accepted version with minor
changes. Version with high resolution figures available at:
http://www.astro.uvic.ca/~lsales/SatPapers/SatPapers.htm
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