2,366 research outputs found
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
Tidal Torques and the Orientation of Nearby Disk Galaxies
We use numerical simulations to investigate the orientation of the angular
momentum axis of disk galaxies relative to their surrounding large scale
structure. We find that this is closely related to the spatial configuration at
turnaround of the material destined to form the galaxy, which is often part of
a coherent two-dimensional slab criss-crossed by filaments. The rotation axis
is found to align very well with the intermediate principal axis of the inertia
momentum tensor at this time. This orientation is approximately preserved
during the ensuing collapse, so that the rotation axis of the resulting disk
ends up lying on the plane traced by the protogalactic material at turnaround.
This suggests a tendency for disks to align themselves so that their rotation
axis is perpendicular to the minor axis of the structure defined by surrounding
matter. One example of this trend is provided by our own Galaxy, where the
Galactic plane is almost at right angles with the supergalactic plane (SGP)
drawn by nearby galaxies; indeed, the SGP latitude of the North Galactic Pole
is just 6 degrees. We have searched for a similar signature in catalogs of
nearby disk galaxies, and find a significant excess of edge-on spirals (for
which the orientation of the disk rotation axis may be determined
unambiguously) highly inclined relative to the SGP. This result supports the
view that disk galaxies acquire their angular momentum as a consequence of
early tidal torques acting during the expansion phase of the protogalactic
material.Comment: 5 pages, 2 figures, accepted for publication in ApJ
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
-anomalies in a twin Pati-Salam theory of flavour including the 2022 LHCb analysis
We perform a comprehensive phenomenological analysis of the twin Pati-Salam
theory of flavour, focussing on the parameter space relevant for interpreting
the -anomalies via vector leptoquark exchange. This model provides a
very predictive framework in which the couplings and the Yukawa couplings
find a common origin via mixing of chiral quarks and leptons with vector-like
fermions, providing a direct link between the -anomalies and the fermion
masses and mixing. We propose and study a simplified model with three
vector-like fermion families, in the massless first family approximation, and
show that the second and third family charged fermion masses and mixings and
the -anomalies can be simultaneously explained and related. The model has
the proper flavour structure to be compatible with all low-energy observables,
and leads to predictions in promising observables such as
, and at Belle II and LHCb. The model also predicts a rich
spectrum of TeV scale gauge bosons and vector-like fermions, all accessible to
the LHC. In this updated version we have included an extended analysis
considering the new 2022 LHCb data on , which has slightly shifted
the preferred parameter space with respect to the 2021 case. The model can
still explain the anomalies at 1 in a narrow window,
however we expect small deviations from the SM on the ratios, to
be tested in the future via more precise measurements by the LHCb
collaboration. We also predict , with future
measurements shifting the world averages to slightly smaller central values.Comment: 44 pages + appendices and references, 14 figures. v5: Typos correcte
Simulations of galaxy formation in a Λ cold dark matter universe : I : dynamical and photometric properties of a simulated disk galaxy.
We present a detailed analysis of the dynamical and photometric properties of a disk galaxy simulated in the cold dark matter (CDM) cosmogony. The galaxy is assembled through a number of high-redshift mergers followed by a period of quiescent accretion after z1 that lead to the formation of two distinct dynamical components: a spheroid of mostly old stars and a rotationally supported disk of younger stars. The surface brightness profile is very well approximated by the superposition of an R1/4 spheroid and an exponential disk. Each photometric component contributes a similar fraction of the total luminosity of the system, although less than a quarter of the stars form after the last merger episode at z1. In the optical bands the surface brightness profile is remarkably similar to that of Sab galaxy UGC 615, but the simulated galaxy rotates significantly faster and has a declining rotation curve dominated by the spheroid near the center. The decline in circular velocity is at odds with observation and results from the high concentration of the dark matter and baryonic components, as well as from the relatively high mass-to-light ratio of the stars in the simulation. The simulated galaxy lies 1 mag off the I-band Tully-Fisher relation of late-type spirals but seems to be in reasonable agreement with Tully-Fisher data on S0 galaxies. In agreement with previous simulation work, the angular momentum of the luminous component is an order of magnitude lower than that of late-type spirals of similar rotation speed. This again reflects the dominance of the slowly rotating, dense spheroidal component, to which most discrepancies with observation may be traced. On its own, the disk component has properties rather similar to those of late-type spirals: its luminosity, its exponential scale length, and its colors are all comparable to those of galaxy disks of similar rotation speed. This suggests that a different form of feedback than adopted here is required to inhibit the efficient collapse and cooling of gas at high redshift that leads to the formation of the spheroid. Reconciling, without fine-tuning, the properties of disk galaxies with the early collapse and high merging rates characteristic of hierarchical scenarios such as CDM remains a challenging, yet so far elusive, proposition
Counterrotating Stars in Simulated Galaxy Disks
Counterrotating stars in disk galaxies are a puzzling dynamical feature whose
origin has been ascribed to either satellite accretion events or to disk
instabilities triggered by deviations from axisymmetry. We use a cosmological
simulation of the formation of a disk galaxy to show that counterrotating
stellar disk components may arise naturally in hierarchically-clustering
scenarios even in the absence of merging. The simulated disk galaxy consists of
two coplanar, overlapping stellar components with opposite spins: an inner
counterrotating bar-like structure made up mostly of old stars surrounded by an
extended, rotationally-supported disk of younger stars. The opposite-spin
components originate from material accreted from two distinct filamentary
structures which at turn around, when their net spin is acquired, intersect
delineating a "V"-like structure. Each filament torques the other in opposite
directions; the filament that first drains into the galaxy forms the inner
counterrotating bar, while material accreted from the other filament forms the
outer disk. Mergers do not play a substantial role and most stars in the galaxy
are formed in situ; only 9% of all stars are contributed by accretion events.
The formation scenario we describe here implies a significant age difference
between the co- and counterrotating components, which may be used to
discriminate between competing scenarios for the origin of counterrotating
stars in disk galaxies.Comment: 7 pages, 7 figures. Accepted for publication in MNRA
Tri-unification: a separate for each fermion family
In this paper we discuss with cyclic symmetry as a possible grand
unified theory (GUT). The basic idea of such a tri-unification is that there is
a separate for each fermion family, with the light Higgs doublet(s)
arising from the third family , providing a basis for charged fermion
mass hierarchies. tri-unification reconciles the idea of gauge
non-universality with the idea of gauge coupling unification, opening the
possibility to build consistent non-universal descriptions of Nature that are
valid all the way up to the scale of grand unification. As a concrete example,
we propose a grand unified embedding of the tri-hypercharge model
based on an framework with cyclic symmetry. We discuss a minimal
tri-hypercharge example which can account for all the quark and lepton
(including neutrino) masses and mixing parameters. We show that it is possible
to unify the gauge couplings into a single gauge coupling associated with the
cyclic gauge group, by assuming minimal multiplet splitting,
together with a set of relatively light colour octet scalars. We also study
proton decay in this example, and present the predictions for the proton
lifetime in the dominant channel.Comment: 18 pages + Appendix, 7 figures. v2: citations added, conclusions
unchange
Mergers and the outside-in formation of dwarf spheroidals
We use a cosmological simulation of the formation of the Local Group to
explore the origin of age and metallicity gradients in dwarf spheroidal
galaxies. We find that a number of simulated dwarfs form "outside-in", with an
old, metal-poor population that surrounds a younger, more concentrated
metal-rich component, reminiscent of dwarf spheroidals like Sculptor or
Sextans. We focus on a few examples where stars form in two populations
distinct in age in order to elucidate the origin of these gradients. The
spatial distributions of the two components reflect their diverse origin; the
old stellar component is assembled through mergers, but the young population
forms largely in situ. The older component results from a first episode of star
formation that begins early but is quickly shut off by the combined effects of
stellar feedback and reionization. The younger component forms when a late
accretion event adds gas and reignites star formation. The effect of mergers is
to disperse the old stellar population, increasing their radius and decreasing
their central density relative to the young population. We argue that
dwarf-dwarf mergers offer a plausible scenario for the formation of systems
with multiple distinct populations and, more generally, for the origin of age
and metallicity gradients in dwarf spheroidals.Comment: 10 pages, 8 figures, Accepted for publication in MNRA
Dwarf Galaxies and the Cosmic Web
We use a cosmological simulation of the formation of the Local Group of
Galaxies to identify a mechanism that enables the removal of baryons from
low-mass halos without appealing to feedback or reionization. As the Local
Group forms, matter bound to it develops a network of filaments and pancakes.
This moving web of gas and dark matter drifts and sweeps a large volume,
overtaking many halos in the process. The dark matter content of these halos is
unaffected but their gas can be efficiently removed by ram-pressure. The loss
of gas is especially pronounced in low-mass halos due to their lower binding
energy and has a dramatic effect on the star formation history of affected
systems. This "cosmic web stripping" may help to explain the scarcity of dwarf
galaxies compared with the numerous low-mass halos expected in \Lambda CDM and
the large diversity of star formation histories and morphologies characteristic
of faint galaxies. Although our results are based on a single high-resolution
simulation, it is likely that the hydrodynamical interaction of dwarf galaxies
with the cosmic web is a crucial ingredient so far missing from galaxy
formation models.Comment: Submitted to ApJL. 6 pages, 4 figures. A set of movies showing the
interaction between dwarf galaxies and the Cosmic Web can be found at mirror
1 http://www.astro.uvic.ca/~mario/dwarf-web/ or at mirror 2
http://www.iate.oac.uncor.edu/~alejandro/dwarf-web/ . Comments are welcome
- …