29 research outputs found
Creation/destruction of ultra-wide binaries in tidal streams
This paper uses statistical and -body methods to explore a new mechanism
to form binary stars with extremely large separations (),
whose origin is poorly understood. Here, ultra-wide binaries arise via chance
entrapment of unrelated stars in tidal streams of disrupting clusters. It is
shown that (i) the formation of ultra-wide binaries is not limited to the
lifetime of a cluster, but continues after the progenitor is fully disrupted,
(ii) the formation rate is proportional to the local phase-space density of the
tidal tails, (iii) the semimajor axis distribution scales as at , where is the mean interstellar distance, and (vi)
the eccentricity distribution is close to thermal, . Owing to
their low binding energies, ultra-wide binaries can be disrupted by both the
smooth tidal field and passing substructures. The time-scale on which tidal
fluctuations dominate over the mean field is inversely proportional to the
local density of clumps. Monte-Carlo experiments show that binaries subject to
tidal evaporation follow at ,
known as \"Opik's law, with a peak semi-major axis that contracts with time as
. In contrast, a smooth Galactic potential
introduces a sharp truncation at the tidal radius, at . The scaling relations of young clusters suggest that most ultra-wide
binaries arise from the disruption of low-mass systems. Streams of globular
clusters may be the birthplace of hundreds of ultra-wide binaries, making them
ideal laboratories to probe clumpiness in the Galactic halo.Comment: 17 pages, 11 figures. Accepted to MNRA
Are stellar over-densities in dwarf galaxies the "smoking gun" of triaxial dark matter haloes?
We use N-body simulations to study the tidal evolution of globular clusters
(GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically
motivated scenario in which the dSph is approximated by a static NFW halo with
a triaxial shape. For a large set of orbits and projection angles we examine
the spatial and velocity distribution of stellar debris deposited during the
complete disruption of stellar clusters. Our simulations show that such debris
appears as shells, isolated clumps and elongated over-densities at low surface
brightness (>26 mag/arcsec^2), reminiscent of substructure observed in several
MW dSphs. Such features arise from the triaxiality of the galaxy potential and
do not dissolve in time. Stellar over-densities reported in several MW dSphs
may thus be the telltale evidence of dark matter haloes being triaxial in
shape.
We explore a number of kinematic signatures that would help to validate (or
falsify) this scenario. The mean angular momentum of the cluster debris
associated with box and resonant orbits, which are absent in spherical
potentials, is null. As a result, we show that the line-of-sight velocity
distribution may exhibit a characteristic "double-peak" depending on the
oriention of the viewing angle with respect to the progenitor's orbital plane.
Kinematic surveys of dSphs may help to detect and identify substructures
associated with the disruption of stellar clusters, as well as to address the
shape of the dark matter haloes in which dSphs are embedded.Comment: 4 pages, 2 figures, to be published in the proceedings of "Hunting
for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009,
eds. V.P. Debattista & C.C. Popescu, AIP Conf. Ser., in pres
Multiple dynamical components in Local Group dwarf spheroidals
The dwarf spheroidal (dSph) satellites of the Local Group have long been
thought to be simple spheroids of stars embedded within extended dark matter
halos. Recently, however, evidence for the presence of spatially and
kinematically distinct stellar populations has been accumulating. Here, we
examine the influence of such components on dynamical models of dwarf galaxies
embedded in cold dark matter halos. We begin by constructing a model of
Andromeda II, a dSph satellite of M31 which shows evidence for spatially
distinct stellar components. We find that the two-component model predicts an
overall velocity dispersion profile that remains approximately constant at
km s out to kpc from the center; this is despite
wide kinematic and spatial differences between the two individual components.
The presence of two components may also help to explain oddities in the
velocity dispersion profiles of other dSphs; we show that velocity dispersion
profiles which appear to rise from the center outwards before leveling
off--such as those of Leo I, Draco, and Fornax--can result from the gradual
transition from a dynamically cold, concentrated component to a second, hotter,
and more spatially extended one, both in equilibrium within the same dark halo.
Dwarf galaxies with two stellar components generally have a leptokurtic
line-of-sight velocity distribution which is well described by a double
Maxwellian. Interestingly, we find that multiple equilibrium components could
also provide a potential alternative origin for ``extra-tidal'' stars (normally
ascribed to tidal effects) in situations where corroborating evidence for tides
may be lacking.Comment: Accepted by MNRAS Letters. Revised version, with addition of new
section and expanded discussio
Insights from the Outskirts: Chemical and Dynamical Properties in the outer Parts of the Fornax Dwarf Spheroidal Galaxy
We present radial velocities and [Fe/H] abundances for 340 stars in the
Fornax dwarf spheroidal from R~16,000 spectra. The targets have been obtained
in the outer parts of the galaxy, a region which has been poorly studied
before. Our sample shows a wide range in [Fe/H], between -0.5 and -3.0 dex, in
which we detect three subgroups. Removal of stars belonging to the most
metal-rich population produces a truncated metallicity distribution function
that is identical to Sculptor, indicating that these systems have shared a
similar early evolution, only that Fornax experienced a late, intense period of
star formation (SF). The derived age-metallicity relation shows a fast increase
in [Fe/H] at early ages, after which the enrichment flattens significantly for
stars younger than ~8 Gyr. Additionally, the data indicate a strong population
of stars around 4 Gyr, followed by a second rapid enrichment in [Fe/H]. A
leaky-box chemical enrichment model generally matches the observed relation but
does not predict a significant population of young stars nor the strong
enrichment at late times. The young population in Fornax may therefore
originate from an externally triggered SF event. Our dynamical analysis reveals
an increasing velocity dispersion with decreasing [Fe/H] from sigma_sys 7.5
km/s to >14 km/s, indicating an outside-in star formation history in a dark
matter dominated halo. The large velocity dispersion at low metallicities is
possibly the result of a non-Gaussian velocity distribution amongst stars older
than ~8 Gyr. Our sample also includes members from the Fornax GCs H2 and H5. In
agreement with past studies we find [Fe/H]=-2.04+-0.04 and a mean radial
velocity RV=59.36+-0.31 km/s for H2 and [Fe/H]=-2.02+-0.11 and RV=59.39+-0.44
km/s for H5. Overall, we find large complexity in the chemical and dynamical
properties, with signatures that additionally vary with galactocentric
distance.Comment: 21 pages, 18 figures, 4 tables, accepted for publication in A&
The metal-poor Knee in the Fornax Dwarf Spheroidal Galaxy
We present alpha-element abundances of Mg, Si, and Ti for a large sample of
field stars in two outer fields of the Fornax dwarf spheroidal galaxy (dSph),
obtained with VLT/GIRAFFE (R~16,000). Due to the large fraction of metal-poor
stars in our sample, we are able to follow the alpha-element evolution from
[Fe/H]=-2.5 continuously to [Fe/H]=-0.7 dex. For the first time we are able to
resolve the turnover from the Type II supernovae (SNe) dominated,
alpha-enhanced plateau down to subsolar [alpha/Fe] values due to the onset of
SNe Ia, and thus to trace the chemical enrichment efficiency of the galaxy. Our
data support the general concept of an alpha-enhanced plateau at early epochs,
followed by a well-defined "knee", caused by the onset of SNe Ia, and finally a
second plateau with sub-solar [alpha/Fe] values. We find the position of this
knee to be at [Fe/H]=-1.9 and therefore significantly more metal-poor than
expected from comparison with other dSphs and standard evolutionary models.
Surprisingly, this value is rather comparable to the knee in Sculptor, a dSph
about 10 times less luminous than Fornax. Using chemical evolution models, we
find that both the position of the knee as well as the subsequent plateau at
sub-solar level can hardly be explained unless the galaxy experienced several
discrete star formation events with a drastic variation in star formation
efficiency, while a uniform star formation can be ruled out. One possible
evolutionary scenario is that Fornax experienced one or several major accretion
events from gas-rich systems in the past, so that its current stellar mass is
not indicative of the chemical evolution environment at ancient times. If
Fornax is the product of several smaller building blocks, this may also have
implications of the understanding on the formation process of dSphs in general.Comment: 10 pages, 6 Figures, accepted for publication in Ap