1,028 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
"Filmisches Schreiben" als Grundlage einer "Literaturgeschichte des Films"
Das übergeordnete Ziel von Franz-Josef Albersmeiers Monographie zu Theater, Literatur und Film in Spanien ist es, eine "Literaturgeschichte des Films" zu begründen. Mit der Analyse von ausgewählten lyrischen, dramatischen sowie epischen Texten auf ihre "filmische Faktur" (vgl. S. 13) hin begibt sich der Medienwissenschaftler auf den bisher verkannten Pfad einer "vernetzten Medienbetrachtung". Albersmeier gelingt es, trotz der fast unüberschaubaren Fülle an Material die konstante Nähe des Mediums Film zu den drei literarischen Gattungen, auf die er seit seiner Entstehung am Anfang des 20. Jahrhunderts stets Einfluss ausgeübt hat, nachzuweisen
Theatralität zwischen Avantgarde und Medienzeitalter
Ziel des Tagungsbandes ist die Überprüfung der im Titel gestellten Frage, ob (und inwiefern) Theater als Paradigma der Moderne gelten soll/kann. In neun Kapiteln, die thematisch auf der Einteilung der Tagung in Sektionen basieren, spüren Theater-, Tanz-, Film- und Fernsehwissenschaftler der paradigmatischen Funktion des Theaters nach. Die Aufsätze zeigen auf, wie dringend der Begriff der Theatralität, unabhängig aus welcher Forschungsperspektive betrachtet, heute neu zu verhandeln ist
The inner structure and kinematics of the Sagittarius dwarf galaxy as a product of tidal stirring
The tidal stirring model envisions the formation of dwarf spheroidal (dSph)
galaxies in the Local Group via the tidal interaction of disky dwarf systems
with a larger host galaxy like the Milky Way. These progenitor disks are
embedded in extended dark halos and during the evolution both components suffer
strong mass loss. In addition, the disks undergo the morphological
transformation into spheroids and the transition from ordered to random motion
of their stars. Using collisionless N-body simulations we construct a model for
the nearby and highly elongated Sagittarius (Sgr) dSph galaxy within the
framework of the tidal stirring scenario. Constrained by the present known
orbit of the dwarf, the model suggests that in order to produce the majority of
tidal debris observed as the Sgr stream, but not yet transform the core of the
dwarf into a spherical shape, Sgr must have just passed the second pericenter
of its current orbit around the Milky Way. In the model, the stellar component
of Sgr is still very elongated after the second pericenter and morphologically
intermediate between the strong bar formed at the first pericenter and the
almost spherical shape existing after the third pericenter. This is thus the
first model of the evolution of the Sgr dwarf that accounts for its observed
very elliptical shape. At the present time there is very little intrinsic
rotation left and the velocity gradient detected along the major axis is almost
entirely of tidal origin. We model the recently measured velocity dispersion
profile for Sgr assuming that mass traces light and estimate its current total
mass within 5 kpc to be 5.2 x 10^8 M_sun. To have this mass at present, the
model requires that the initial virial mass of Sgr must have been as high as
1.6 x 10^10 M_sun, comparable to that of the Large Magellanic Cloud, which may
serve as a suitable analog for the pre-interaction, Sgr progenitor.Comment: 14 pages, 14 figures, minor changes to match the version published in
Ap
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 shapes of Milky Way satellites: looking for signatures of tidal stirring
We study the shapes of Milky Way satellites in the context of the tidal
stirring scenario for the formation of dwarf spheroidal galaxies. The standard
procedures used to measure shapes involve smoothing and binning of data and
thus may not be sufficient to detect structural properties like bars, which are
usually subtle in low surface brightness systems. Taking advantage of the fact
that in nearby dwarfs photometry of individual stars is available we introduce
discrete measures of shape based on the two-dimensional inertia tensor and the
Fourier bar mode. We apply these measures of shape first to a variety of
simulated dwarf galaxies formed via tidal stirring of disks embedded in dark
matter halos and orbiting the Milky Way. In addition to strong mass loss and
randomization of stellar orbits, the disks undergo morphological transformation
that typically involves the formation of a triaxial bar after the first
pericenter passage. These tidally induced bars persist for a few Gyr before
being shortened towards a more spherical shape if the tidal force is strong
enough. We test this prediction by measuring in a similar way the shape of
nearby dwarf galaxies, satellites of the Milky Way. We detect inner bars in
Ursa Minor, Sagittarius, LMC and possibly Carina. In addition, six out of
eleven studied dwarfs show elongated stellar distributions in the outer parts
that may signify transition to tidal tails. We thus find the shapes of Milky
Way satellites to be consistent with the predictions of the tidal stirring
model.Comment: 14 pages, 11 figures, 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
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
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
- …