66 research outputs found
The anatomy of Leo I: how tidal tails affect the kinematics
We model the recently published kinematic data set for Leo I dSph galaxy by
fitting the solutions of the Jeans equations to the velocity dispersion and
kurtosis profiles measured from the data. We demonstrate that when the sample
is cleaned of interlopers the data are consistent with the assumption that mass
follows light and isotropic stellar orbits with no need for an extended dark
matter halo. Our interloper removal scheme does not clean the data of
contamination completely, as demonstrated by the rotation curve of Leo I. When
moving away from the centre of the dwarf, the rotation appears to be reversed.
We interpret this behaviour using the results of an N-body simulation of a
dwarf galaxy possessing some intrinsic rotation, orbiting in the Milky Way
potential and show that it can be reproduced if the galaxy is viewed almost
along the tidal tails so that the leading (background) tail contaminates the
western part of Leo I while the trailing (foreground) tail the eastern one. We
show that this configuration leads to a symmetric and Gaussian distribution of
line-of-sight velocities. The simulation is also applied to test our modelling
method on mock data sets. We demonstrate that when the data are cleaned of
interlopers and the fourth velocity moment is used the true parameters of the
dwarf are typically within 1 \sigma errors of the best-fitting parameters.
Restricting the fitting to the inner part of Leo I our best estimate for the
anisotropy is \beta = -0.2^{+0.3}_{-0.4} and the total mass M = (4.5 +/- 0.7) x
10^7 M_sun. The mass-to-light ratio including the errors in mass, brightness
and distance is M/L_V = 8.2 +/- 4.5 solar units.Comment: 10 pages, 10 figures, revised version accepted for publication in
MNRA
Galaxies with kinematically distinct cores in Illustris
The growing amount of integral-field spectroscopic data creates an increased
demand for understanding kinematic peculiarities that carry valuable
information about the evolution of the host galaxies. For kinematically
distinct cores (KDCs), a number of formation mechanisms have been proposed, but
it is still unclear which of them commonly occur in the Universe. We aim to
address the KDC formation in the cosmological context. We used the publicly
available data of the large-scale hydrodynamic cosmological simulation
Illustris. We identify 134 KDCs, study their properties, and follow their
evolution back in time. Examples of four galaxies hosting KDCs are presented
and described in detail. The masses of the KDC hosts follow the general
distribution of the Illustris galaxies, with a possible slight preference
towards massive galaxies. KDCs can be long-lived features, with their formation
epochs roughly uniformly distributed in look-back times 0-11.4 Gyr, and they
can survive even major or multiple subsequent mergers. There is no single
channel of KDC formation, but mergers seem to be the formation mechanism for
about 60% of KDCs with a significant preference for major mergers and with the
percentage being higher among massive hosts. Other KDCs formed during a
pericentric passage or flyby of another galaxy, by precession of a previously
formed rapidly rotating core, or without an obvious external cause. The mean
mass-weighted stellar age inside the KDC radius is either about the same as the
look-back time of the KDC formation or older. Although the radii of our KDCs
are on average larger than observed, we find that younger stellar ages are
typically associated with smaller KDCs. A significant fraction of KDC hosts
possess stellar shells formed during mergers that led to KDCs within the last 5
Gyr, or double peaks in their velocity dispersion maps.Comment: 18 pages, 16 figures, 1 table; accepted for publication in A&
The anatomy of Leo I: how tidal tails affect the kinematics
We model the recently published kinematic data set for Leo I dwarf spheroidal (dSph) galaxy by fitting the solutions of the Jeans equations to the velocity dispersion and kurtosis profiles measured from the data. We demonstrate that when the sample is cleaned of interlopers the data are consistent with the assumption that mass follows light and isotropic stellar orbits with no need for an extended dark matter halo. Our interloper removal scheme does not clean the data of contamination completely, as demonstrated by the rotation curve of Leo I. When moving away from the centre of the dwarf, the rotation appears to be reversed. We interpret this behaviour using the results of an N-body simulation of a dwarf galaxy possessing some intrinsic rotation, orbiting in the Milky Way potential and show that it can be reproduced if the galaxy is viewed almost along the tidal tails so that the leading (background) tail contaminates the western part of Leo I while the trailing (foreground) tail the eastern one. We show that this configuration leads to a symmetric and Gaussian distribution of line-of-sight velocities. The simulation is also applied to test our modelling method on mock data sets. We demonstrate that when the data are cleaned of interlopers and the fourth velocity moment is used the true parameters of the dwarf are typically within 1σ errors of the best-fitting parameters. Restricting the fitting to the inner part of Leo I our best estimate for the anisotropy is β=−0.2+0.3−0.4 and the total mass M= (4.5 ± 0.7) × 107M⊙. The mass-to-light ratio (M/L) including the errors in mass, brightness and distance is M/LV= 8.2 ± 4.5 solar unit
Tidal evolution of discy dwarf galaxies in the Milky Way potential: the formation of dwarf spheroidals
We conduct high-resolution collisionless N-body simulations to investigate the tidal evolution of dwarf galaxies on an eccentric orbit in the Milky Way (MW) potential. The dwarfs originally consist of a low surface brightness stellar disc embedded in a cosmologically motivated dark matter halo. During 10 Gyr of dynamical evolution and after five pericentre passages, the dwarfs suffer substantial mass loss and their stellar component undergoes a major morphological transformation from a disc to a bar and finally to a spheroid. The bar is preserved for most of the time as the angular momentum is transferred outside the galaxy. A dwarf spheroidal (dSph) galaxy is formed via gradual shortening of the bar. This work thus provides a comprehensive quantitative explanation of a potentially crucial morphological transformation mechanism for dwarf galaxies that operates in groups as well as in clusters. We compare three cases with different initial inclinations of the disc and find that the evolution is fastest when the disc is coplanar with the orbit. Despite the strong tidal perturbations and mass loss, the dwarfs remain dark matter dominated. For most of the time, the one-dimensional stellar velocity dispersion, σ, follows the maximum circular velocity, Vmax, and they are both good tracers of the bound mass. Specifically, we find that Mbound∝V3.5max and in agreement with earlier studies based on pure dark matter simulations. The latter relation is based on directly measuring the stellar kinematics of the simulated dwarf, and may thus be reliably used to map the observed stellar velocity dispersions of dSphs to halo circular velocities when addressing the missing satellites proble
The mass and anisotropy profiles of galaxy clusters from the projected phase space density: testing the method on simulated data
We present a new method of constraining the mass and velocity anisotropy
profiles of galaxy clusters from kinematic data. The method is based on a model
of the phase space density which allows the anisotropy to vary with radius
between two asymptotic values. The characteristic scale of transition between
these asymptotes is fixed and tuned to a typical anisotropy profile resulting
from cosmological simulations. The model is parametrized by two values of
anisotropy, at the centre of the cluster and at infinity, and two parameters of
the NFW density profile, the scale radius and the scale mass. In order to test
the performance of the method in reconstructing the true cluster parameters we
analyze mock kinematic data for 20 relaxed galaxy clusters generated from a
cosmological simulation of the standard LCDM model. We use Bayesian methods of
inference and the analysis is carried out following the Markov Chain Monte
Carlo approach. The parameters of the mass profile are reproduced quite well,
but we note that the mass is typically underestimated by 15 percent, probably
due to the presence of small velocity substructures. The constraints on the
anisotropy profile for a single cluster are in general barely conclusive.
Although the central asymptotic value is determined accurately, the outer one
is subject to significant systematic errors caused by substructures at large
clustercentric distance. The anisotropy profile is much better constrained if
one performs joint analysis of at least a few clusters. In this case it is
possible to reproduce the radial variation of the anisotropy over two decades
in radius inside the virial sphere.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Mass modelling of dwarf spheroidal galaxies: the effect of unbound stars from tidal tails and the Milky Way
We study the origin and properties of the population of unbound stars in the kinematic samples of dwarf spheroidal (dSph) galaxies. For this purpose we have run a high-resolution N-body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. In agreement with the tidal stirring scenario of Mayer et al., the dwarf is placed on a highly eccentric orbit, its initial stellar component is in the form of an exponential disc and it has a NFW-like dark matter (DM) halo. After 10 Gyr of evolution the dwarf produces a spheroidal stellar component and is strongly tidally stripped so that mass follows light and the stars are on almost isotropic orbits. From this final state, we create mock kinematic data sets for 200 stars by observing the dwarf in different directions. We find that when the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. We also study another source of possible contamination by adding stars from the Milky Way. We demonstrate that most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert and recently tested on simulated DM haloes. We model the cleaned-up kinematic samples using solutions of the Jeans equation with constant mass-to-light ratio (M/L) and velocity anisotropy parameter. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and M/L of the dwarf with accuracy typically better than 25 per cent and almost exactly in the case when the line of sight is perpendicular to the tidal tails. The analysis was applied to the new data for the Fornax dSph galaxy. We show that after careful removal of interlopers the velocity dispersion profile of Fornax can be reproduced by a model in which mass traces light with a M/L of 11 solar units and isotropic orbits. We demonstrate that most of the contamination in the kinematic sample of Fornax probably originates from the Milky Wa
Mass Modelling of dwarf Spheroidal Galaxies
We study the origin and properties of unbound stars in the kinematic samples of dwarf spheroidal galaxies. For this purpose we have run a high resolution N-body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. We create mock kinematic data sets by observing the dwarf in different directions. When the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. However, most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert. We model the velocity dispersion profiles of the cleaned-up kinematic samples using solutions of the Jeans equation. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and mass-to-light ratio of the dwarf with accuracy typically better than 25
The orientation and kinematics of inner tidal tails around dwarf galaxies orbiting the Milky Way
Using high-resolution collisionless N-body simulations, we study the properties of tidal tails formed in the immediate vicinity of a two-component dwarf galaxy evolving in a static potential of the Milky Way (MW). The stellar component of the dwarf is initially in the form of a disc and the galaxy is placed on an eccentric orbit motivated by cold dark matter based cosmological simulations. We measure the orientation, density and velocity distribution of the stars in the tails. Due to the geometry of the orbit, in the vicinity of the dwarf, where the tails are densest and therefore most likely to be detectable, they are typically oriented towards the MW and not along the orbit. We report on an interesting phenomenon of ‘tidal tail flipping': on the way from the pericentre to the apocentre, the old tails following the orbit are dissolved and new ones pointing towards the MW are formed over a short time-scale. We also find a tight linear relation between the velocity of stars in the tidal tails and their distance from the dwarf. Using mock data sets, we demonstrate that if dwarf spheroidal (dSph) galaxies in the vicinity of the MW are tidally affected their kinematic samples are very likely contaminated by tidally stripped stars which tend to artificially inflate the measured velocity dispersion. The effect is stronger for dwarfs on their way from the pericentre to the apocentre due to the formation of new tidal tails after pericentre. Realistic mass estimates of dSph galaxies thus require removal of these stars from kinematic sample
- …