60 research outputs found
Dynamical models with a general anisotropy profile
Both numerical simulations and observational evidence indicate that the outer
regions of galaxies and dark matter haloes are typically mildly to
significantly radially anisotropic. The inner regions can be significantly
non-isotropic, depending on the dynamical formation and evolution processes. In
an attempt to break the lack of simple dynamical models that can reproduce this
behaviour, we explore a technique to construct dynamical models with an
arbitrary density and an arbitrary anisotropy profile. We outline a general
construction method and propose a more practical approach based on a
parameterized anisotropy profile. This approach consists of fitting the density
of the model with a set of dynamical components, each of which have the same
anisotropy profile. Using this approach we avoid the delicate fine-tuning
difficulties other fitting techniques typically encounter when constructing
radially anisotropic models. We present a model anisotropy profile that
generalizes the Osipkov-Merritt profile, and that can represent any smooth
monotonic anisotropy profile. Based on this model anisotropy profile, we
construct a very general seven-parameter set of dynamical components for which
the most important dynamical properties can be calculated analytically. We use
the results to look for simple one-component dynamical models that generate
simple potential-density pairs while still supporting a flexible anisotropy
profile. We present families of Plummer and Hernquist models in which the
anisotropy at small and large radii can be chosen as free parameters. We also
generalize these two families to a three-parameter family that
self-consistently generates the set of Veltmann potential-density pairs.
(Abridged...)Comment: 18 pages, accepted for publication in A&
Clues on the Physical Origin of the Fundamental Plane from Self-consistent Hydrodynamical Simulations
We report on a study of the parameters characterizing the mass and velocity
distributions of two samples of relaxed elliptical-like objects (ELOs)
identified, at z=0, in a set of self-consistent hydrodynamical simulations
operating in the context of a concordance cosmological model. Star formation
(SF) has been phenomenologically implemented in the simulations in the
framework of the turbulent sequential scenario through a threshold gas density
and an efficiency parameter. Each ELO sample is characterized by the values
these parameters take. We have found that the (logarithms of the) ELO stellar
masses, projected stellar half-mass radii, and stellar central line-of-sight
(LOS) velocity dispersions define dynamical fundamental planes (FPs). Zero
points depend on the particular values that the SF parameters take, while
slopes do not change. The ELO samples have been found to show systematic trends
with the mass scale in both the relative content and the relative distributions
of the baryonic and the dark mass ELO components. The physical origin of these
trends lies in the systematic decrease, with increasing ELO mass, of the
relative dissipation experienced by the baryonic mass component along ELO mass
assembly, resulting in a tilt of the dynamical FP relative to the virial plane.
The dynamical FPs shown by the two ELO samples are consistent with that shown
by the SDSS elliptical sample in the same variables, with no further need for
any relevant contribution from stellar population effects to explain the
observed tilt. These effects could, however, have contributed to the scatter of
the observed FP, as the dynamical FPs have been found to be thinner than the
observed one. The results we report on hint, for the first time, at a possible
way to understand the tilt of the observed FP in a cosmological context.Comment: 12 pages, 1 figure. Accepted to Astrophysical Journal Letter
The impact of baryonic physics on the structure of dark matter haloes: the view from the FIRE cosmological simulations
We study the distribution of cold dark matter (CDM) in cosmological simulations from the FIRE (Feedback In Realistic Environments) project, for M_* ⌠10^(4â11)âM_â galaxies in M_h ⌠10^(9â12)âM_â haloes. FIRE incorporates explicit stellar feedback in the multiphase interstellar medium, with energetics from stellar population models. We find that stellar feedback, without âfine-tunedâ parameters, greatly alleviates small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile (α) shows a strong mass dependence: profiles are shallow at M_h ⌠10^(10)â10^(11)âM_â and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of α, and relatively late growth of cores. Because the star formation efficiency M_*/M_h is strongly halo mass dependent, a rapid change in α occurs around M_h ⌠10^(10)âM_â (M_* ⌠10^6â10^7âM_â), as sufficient feedback energy becomes available to perturb the DM. Large cores are not established during the period of rapid growth of haloes because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid build-up has completed. Stellar feedback dramatically reduces circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the âToo Big To Failâ problem without invoking non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass haloes produce DM profiles slightly shallower than the NavarroâFrenkâWhite profile, consistent with the normalization of the observed TullyâFisher relation
Two-dimensional kinematics of SLACS lenses: III. Mass structure and dynamics of early-type lens galaxies beyond z ~ 0.1
We combine in a self-consistent way the constraints from both gravitational
lensing and stellar kinematics to perform a detailed investigation of the
internal mass distribution, amount of dark matter, and dynamical structure of
the 16 early-type lens galaxies from the SLACS Survey, at z = 0.08 - 0.33, for
which both HST/ACS and NICMOS high-resolution imaging and VLT VIMOS IFU
spectroscopy are available. Based on this data set, we analyze the inner
regions of the galaxies, i.e. typically within one (3D) effective radius r_e,
under the assumption of axial symmetry and by constructing dynamical models
supported by two-integral stellar DFs. For all systems, the total mass density
distribution is found to be well approximated by a simple power-law: this
profile is on average slightly super-isothermal, with a logarithmic slope
= 2.074^{+0.043}_{-0.041} (68% CL) and an intrinsic scatter
0.144^{+0.055}_{-0.014}, and is fairly round, with an average axial ratio =
0.77+/-0.04. The lower limit for the dark matter fraction (fDM) inside r_e
ranges, in individual systems, from nearly zero to almost a half, with a median
value of 12%. By including stellar masses derived from SPS models with a
Salpeter IMF, we obtain an average fDM = 31%. The fDM rises to 61% if, instead,
a Chabrier IMF is assumed. For both IMFs, the dark matter fraction increases
with the total mass of the galaxy (3-sigma correlation). Based on the intrinsic
angular momentum parameter calculated from our models, we find that the
galaxies can be divided into two dynamically distinct groups, which are shown
to correspond to the usual classes of the slow and fast rotators. Overall, the
SLACS systems are structurally and dynamically very similar to their nearby
counterparts, indicating that the inner regions of early-type galaxies have
undergone little, if any, evolution since redshift z ~ 0.35. (Abridged)Comment: 27 pages, 34 figures. MNRAS, in pres
Kinematic properties of early-type galaxy haloes using planetary nebulae
We present new planetary nebulae (PNe) positions, radial velocities, and
magnitudes for 6 early-type galaxies obtained with the Planetary Nebulae
Spectrograph, their two-dimensional velocity and velocity dispersion fields. We
extend this study to include an additional 10 early-type galaxies with PNe
radial velocity measurements available from the literature, to obtain a broader
description of the outer-halo kinematics in early-type galaxies. These data
extend the information derived from stellar kinematics to typically up to ~8
Re. The combination of photometry, stellar and PNe kinematics shows: i) good
agreement between the PNe number density and the stellar surface brightness in
the region where the two data sets overlap; ii) good agreement between PNe and
stellar kinematics; iii) that the mean rms velocity profiles fall into two
groups: with of the galaxies characterized by slowly decreasing profiles and
the remainder having steeply falling profiles; iv) a larger variety of velocity
dispersion profiles; v) that twists and misalignments in the velocity fields
are more frequent at large radii, including some fast rotators; vi) that outer
haloes are characterised by more complex radial profiles of the specific
angular momentum-related lambda_R parameter than observed within 1Re; vii) that
many objects are more rotationally dominated at large radii than in their
central parts; and viii) that the halo kinematics are correlated with other
galaxy properties, such as total luminosity, isophotal shape, total stellar
mass, V/sigma, and alpha parameter, with a clear separation between fast and
slow rotators.Comment: 36 pages, 21 figures, revised version for MNRA
The PN.S Elliptical Galaxy Survey: the dark matter in NGC 4494
We present new Planetary Nebula Spectrograph observations of the ordinary
elliptical galaxy NGC 4494, resulting in positions and velocities of 255 PNe
out to 7 effective radii (25 kpc). We also present new wide-field surface
photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2.
The spatial and kinematical distributions of the PNe agree with the field stars
in the region of overlap. The mean rotation is relatively low, with a possible
kinematic axis twist outside 1 Re. The velocity dispersion profile declines
with radius, though not very steeply, down to ~70 km/s at the last data point.
We have constructed spherical dynamical models of the system, including Jeans
analyses with multi-component LCDM-motivated galaxies as well as logarithmic
potentials. These models include special attention to orbital anisotropy, which
we constrain using fourth-order velocity moments. Given several different sets
of modelling methods and assumptions, we find consistent results for the mass
profile within the radial range constrained by the data. Some dark matter (DM)
is required by the data; our best-fit solution has a radially anisotropic
stellar halo, a plausible stellar mass-to-light ratio, and a DM halo with an
unexpectedly low central density. We find that this result does not
substantially change with a flattened axisymmetric model.
Taken together with other results for galaxy halo masses, we find suggestions
for a puzzling pattern wherein most intermediate-luminosity galaxies have very
low concentration halos, while some high-mass ellipticals have very high
concentrations. We discuss some possible implications of these results for DM
and galaxy formation.Comment: 29 pages, 17 figures. MNRAS, accepte
Corrosion behaviour of MgâZnâYâMischmetal alloys in phosphate buffer saline solution
The influence of the processing route and chemical composition in the corrosion behaviour of two Mg-Zn-Y-Mischmetal alloys has been evaluated in phosphate buffer saline solution. The corrosion resistance of the alloy processed by conventional techniques was substantially higher than that found for the same alloy processed from atomised powders. Fine homogeneous distribution of the second-phase particles promoted severe attack due to the enhanced number of galvanic microcells. A higher concentration of zinc and a lower content of rare earth additions improved the corrosion resistance of the alloys due to the lower volume fraction of second-phase particles. © 2012 Elsevier Ltd.Peer Reviewe
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