71 research outputs found
Biases in mass estimates of dSph galaxies
Using a high resolution N-body simulation of a two-component dwarf galaxy
orbiting in the potential of the Milky Way, we study two effects that lead to
significant biases in mass estimates of dwarf spheroidal galaxies. Both are due
to the strong tidal interaction of initially disky dwarfs with their host. The
tidal stripping of dwarf stars leads to the formation of strong tidal tails
that are typically aligned with the line of sight of an observer positioned
close to the host center. The stars from the tails contaminate the kinematic
samples leading to a velocity dispersion profile increasing with the projected
radius and resulting in an overestimate of mass. The tidal stirring of the
dwarf leads to the morphological transformation of the initial stellar disk
into a bar and then a spheroid. The distribution of stars in the dwarf remains
non-spherical for a long time leading to an overestimate of its mass if it is
observed along the major axis and an underestimate if it seen in the
perpendicular direction.Comment: 5 pages, 3 figures, contribution to 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 Conference Series, in pres
The stellar structure and kinematics of dwarf spheroidal galaxies formed by tidal stirring
Using high-resolution N-body simulations we study the stellar properties of
dwarf spheroidal galaxies resulting from the tidally induced morphological
transformation of disky dwarfs on a cosmologically motivated eccentric orbit
around the Milky Way. Dwarf galaxy models initially consist of an exponential
stellar disk embedded in an extended spherical dark matter halo. Depending on
the initial orientation of the disk with respect to the orbital plane,
different final configurations are obtained. The least evolved dwarf is
triaxial and retains a significant amount of rotation. The more evolved dwarfs
are prolate spheroids with little rotation. We show that the final density
distribution of stars can be approximated by a simple modification of the
Plummer law. The kinematics of the dwarfs is significantly different depending
on the line of sight which has important implications for mapping the observed
stellar velocity dispersions of dwarfs to subhalo circular velocities. When the
dwarfs are observed along the long axis, the measured velocity dispersion is
higher and decreases faster with radius. In the case where rotation is
significant, when viewed perpendicular to the long axis, the effect of minor
axis rotation is detected, as expected for triaxial systems. We model the
velocity dispersion profiles and rotation curves of the dwarfs by solving the
Jeans equations for spherical and axisymmetric systems and adjusting different
sets of free parameters. We find that the mass is typically overestimated when
the dwarf is seen along the long axis and underestimated when the observation
is along the short or intermediate axis. The effect of non-sphericity cannot
however bias the inferred mass by more than 60 percent in either direction,
even for the most strongly stripped dwarf which is close to disruption.Comment: 17 pages, 15 figures, revised version accepted for publication in Ap
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
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 disk and the galaxy is
placed on an eccentric orbit motivated by CDM-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 timescale. 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 peri- 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 samples.Comment: 8 pages, 7 figures, accepted for publication in MNRA
Tidal stirring of Milky Way satellites: a simple picture with the integrated tidal force
Most of dwarf spheroidal galaxies in the Local Group were probably formed via
environmental processes like the tidal interaction with the Milky Way. We study
this process via N-body simulations of dwarf galaxies evolving on seven
different orbits around the Galaxy. The dwarf galaxy is initially composed of a
rotating stellar disk and a dark matter halo. Due to the action of tidal forces
it loses mass and the disk gradually transforms into a spheroid while stellar
motions become increasingly random. We measure the characteristic scale-length
of the dwarf, its maximum circular velocity, mass, shape and kinematics as a
function of the integrated tidal force along the orbit. The final properties of
the evolved dwarfs are remarkably similar if the total tidal force they
experienced was the same, independently of the actual size and eccentricity of
the orbit.Comment: 5 pages, 2 figures, contribution to the proceedings of JENAM 2010 in
Lisbon, Symposium 2 "Environment and the formation of galaxies: 30 years
later", comments welcom
Formation and evolution of dwarf galaxies in the CDM Universe
We first review the results of the tidal stirring model for the
transformation of gas-rich dwarf irregulars into dwarf spheroidals, which turns
rotationally supported stellar systems into pressure supported ones. We
emphasize the importance of the combined effect of ram pressure stripping and
heating from the cosmic ultraviolet background in removing the gas and
converting the object into a gas poor system as dSphs. We discuss how the
timing of infall of dwarfs into the primary halo determines the final
mass-to-light ratio and star formation history. Secondly we review the results
of recent cosmological simulations of the formation of gas-rich dwarfs. These
simulations are finally capable to produce a realistic object with no bulge, an
exponential profile and a slowly rising rotation curve. The result owes to the
inclusion of an inhomogeneous ISM and a star formation scheme based on regions
having the typical density of molecular cloud complexes. Supernovae-driven
winds become more effective in such mode, driving low angular momentum baryons
outside the virial radius at high redshift and turning the dark matter cusp
into a core. Finally we show the first tidal stirring experiments adopting
dwarfs formed in cosmological simulations as initial conditions. The latter are
gas dominated and have have turbulent thick gaseous and stellar disks disks
that cannot develop strong bars, yet they are efficiently heated into spheroids
by tidal shocks.Comment: 14 pages, 4 Figures, o appear in the proceedings of the CRAL
conference, Lyon, June 2010, "A Universe of Dwarf Galaxies", eds. Philippe
Prugniel & Mina Koleva; EDP Sciences in the European Astronomical Society
Publications Series. (invited talk
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
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
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
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