58 research outputs found
On feathers, bifurcations and shells: the dynamics of tidal streams across the mass scale
I present an organic description of the regimes of collisionless tidal
streams and define the orderings between the physical quantities that shape
their morphology. Three fundamental dichotomies are identified in the form of
dimensionless inequalities. These govern i) the speed of the stream's growth,
ii) its internal coherence, iii) its thickness or opening angles. The
mechanisms that regulate such main properties are analysed. The slope of the
host's density profile influences the speed of the stream's growth, in both
length and width, as steeper profiles enhance differential streaming. Internal
coherence is the requirement for the appearance of substructure in tidal
debris, and I concentrate on the `feathering' typical of GC streams.
Overdensities are associated with minima in the relative streaming velocity of
the stream members. For streams with high circularity, these are caused by the
epicyclic oscillations of stars; however, for highly non-circular progenitor's
orbits, substructure is caused by the oscillating differences in energy and
actions with which material is shed at different orbital phases of the
progenitor. This modulation results in different streaming speeds: the
streakline of material shed between two successive apocentric passages is
folded along its length, pulled at its centre by the faster streaming of
particles released near pericenter, which are therefore more widely scattered.
When the stream is coherent enough, this mechanism is potentially capable of
generating a bimodal profile in the density distributions of the longer wraps
of more massive progenitors, which I dub `bifurcations'. The conditions for
internal coherence are explored and I comment on the cases of Palomar 5,
Willman 1, the Anticenter and Sagittarius' streams. Analytical methods are
accompanied by numerical experiments, performed using a purposely built
generative model, also presented here.Comment: 18 pages, submitted to MNRA
Contributions to the accreted stellar halo: an atlas of stellar deposition
The accreted component of stellar halos is composed of the contributions of
several satellites, falling onto their host with their different masses, at
different times, on different orbits. This work uses a suite of idealised,
collisionless N-body simulations of minor mergers and a particle tagging
technique to understand how these different ingredients shape each contribution
to the accreted halo, in both density and kinematics. I find that more massive
satellites deposit their stars deeper into the gravitational potential of the
host, with a clear segregation enforced by dynamical friction. Earlier
accretion events contribute more to the inner regions of the halo; more
concentrated subhaloes sink deeper through increased dynamical friction. The
orbital circularity of the progenitor at infall is only important for low-mass
satellites: dynamical friction efficiently radialises the most massive minor
mergers erasing the imprint of the infall orbit for satellite-to-host virial
mass ratios . The kinematics of the stars contributed by each
satellite is also ordered with satellite mass: low-mass satellites contribute
fast-moving populations, in both ordered rotation and radial velocity
dispersion. In turn, contributions by massive satellites have lower velocity
dispersion and lose their angular momentum to dynamical friction, resulting in
a strong radial anisotropy.Comment: 15 pages, accepted version, discussion extende
The core size of the Fornax dwarf Spheroidal
We exploit the detection of three distinct stellar subpopulations in the red
giant branch of the Fornax dwarf Spheroidal to probe its density distribution.
This allows us to resolve directly the evolution with radius of the dark matter
mass profile. We find that a cored dark matter halo provides a perfect fit to
the data, being consistent with all three stellar populations well within
1-sigma, and for the first time we are able to put constraints on the core size
of such a halo. With respect to previous work, we do not strengthen the
statistical exclusion of a dark matter cusp in Fornax, but we find that
Navarro-Frenk-White haloes would be required to have unrealistically large
scale radii in order to be compatible with the data, hence low values of the
concentration parameter. We are then forced to conclude that the Fornax dwarf
Spheroidal sits within a dark matter halo having a constant density core, with
a core size of between 0.6 and 1.8 kpc.Comment: MNRAS Letters, submitte
Chemodynamic subpopulations of the Carina dwarf galaxy
We study the chemodynamical properties of the Carina dwarf spheroidal by
combining an intermediate spectroscopic resolution dataset of more than 900 red
giant and red clump stars, with high-precision photometry to derive the
atmospheric parameters, metallicities and age estimates for our targets. Within
the red giant branch population, we find evidence for the presence of three
distinct stellar sub-populations with different metallicities, spatial
distributions, kinematics and ages. As in the Fornax and Sculptor dwarf
spheroidals, the subpopulation with the lowest average metallicity is more
extended and kinematically hotter than all other populations. However, we
identify an inversion in the parallel ordering of metallicity, kinematics and
characteristic length scale in the two most metal rich subpopulations, which
therefore do not contribute to a global negative chemical gradient. Contrary to
common trends in the chemical properties with radius, the metal richest
population is more extended and mildly kinematically hotter than the main
component of intermediate metallicity. More investigations are required to
ascertain the nature of this inversion, but we comment on the mechanisms that
might have caused it.Comment: 9 pages, 9 figures, accepted for publication in MNRA
Line Profiles from Discrete Kinematic Data
We develop a method to extract the shape information of line profiles from
discrete kinematic data. The Gauss-Hermite expansion, which is widely used to
describe the line of sight velocity distributions extracted from absorption
spectra of elliptical galaxies, is not readily applicable to samples of
discrete stellar velocity measurements, accompanied by individual measurement
errors and probabilities of membership. We introduce two parameter families of
probability distributions describing symmetric and asymmetric distortions of
the line profiles from Gaussianity. These are used as the basis of a maximum
likelihood estimator to quantify the shape of the line profiles. Tests show
that the method outperforms a Gauss-Hermite expansion for discrete data, with a
lower limit for the relative gain of approx 2 for sample sizes N approx 800. To
ensure that our methods can give reliable descriptions of the shape, we develop
an efficient test to assess the statistical quality of the obtained fit. As an
application, we turn our attention to the discrete velocity datasets of the
dwarf spheroidals of the Milky Way. In Sculptor, Carina and Sextans the
symmetric deviations are consistent with velocity distributions more peaked
than Gaussian. In Fornax, instead, there is an evolution in the symmetric
deviations of the line profile from a peakier to more flat-topped distribution
on moving outwards. These results suggest a radially biased orbital structure
for the outer parts of Sculptor, Carina and Sextans. On the other hand,
tangential anisotropy is favoured in Fornax. This is all consistent with a
picture in which Fornax may have had a different evolutionary history to
Sculptor, Carina and Sextans.Comment: MNRAS, accepted for publication, minor change
Self-consistent nonspherical isothermal halos embedding zero-thickness disks
Disk-halo decompositions of galaxy rotation curves are generally performed in
a parametric way. We construct self-consistent models of nonspherical
isothermal halos embedding a zero-thickness disk, by assuming that the halo
distribution function is a Maxwellian. The method developed here can be used to
study other physically-based choices for the halo distribution function and the
case of a disk accompanied by a bulge. In a preliminary investigation we note
the existence of a fine tuning between the scalelengths R_{\Omega} and h,
respectively characterizing the rise of the rotation curve and the luminosity
profile of the disk, which surprisingly applies to both high surface brightness
and low surface brightness galaxies. This empirical correlation identifies a
much stronger conspiracy than the one required by the smoothness and flatness
of the rotation curve (disk-halo conspiracy). The self-consistent models are
characterized by smooth and flat rotation curves for very different
disk-to-halo mass ratios, hence suggesting that conspiracy is not as dramatic
as often imagined. For a typical rotation curve, with asymptotically flat
rotation curve at V_{\infty} (the precise value of which can also be treated as
a free parameter), and a typical density profile of the disk, self-consistent
models are characterized by two dimensionless parameters, which correspond to
the dimensional scales (the disk mass-to-light ratio M/L and the halo central
density) of standard disk-halo decompositions. We show that if the rotation
curve is decomposed by means of our self-consistent models, the disk-halo
degeneracy is removed and typical rotation curves are fitted by models that are
below the maximum-disk prescription. Similar results are obtained from a study
of NGC 3198. Finally, we quantify the flattening of the spheroidal halo, which
is significant, especially on the scale of the visible disk.Comment: accepted for publication in A&
Constraints on Decaying Dark Matter from Fermi Observations of Nearby Galaxies and Clusters
We analyze the impact of Fermi gamma-ray observations (primarily
non-detections) of selected nearby galaxies, including dwarf spheroidals, and
of clusters of galaxies on decaying dark matter models. We show that the fact
that galaxy clusters do not shine in gamma rays puts the most stringent limits
available to-date on the lifetime of dark matter particles for a wide range of
particle masses and decay final states. In particular, our results put strong
constraints on the possibility of ascribing to decaying dark matter both the
increasing positron fraction reported by PAMELA and the high-energy feature in
the electron-positron spectrum measured by Fermi. Observations of nearby dwarf
galaxies and of the Andromeda Galaxy (M31) do not provide as strong limits as
those from galaxy clusters, while still improving on previous constraints in
some cases.Comment: 27 pages, 5 figures, submitted to JCAP, revised version with some
additions and correction
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