47 research outputs found
Stellar orbits in cosmological galaxy simulations: the connection to formation history and line-of-sight kinematics
We analyze orbits of stars and dark matter out to three effective radii for
42 galaxies formed in cosmological zoom simulations. Box orbits always dominate
at the centers and -tubes become important at larger radii. We connect the
orbital structure to the formation histories and specific features (e.g. disk,
counter-rotating core, minor axis rotation) in two-dimensional kinematic maps.
Globally, fast rotating galaxies with significant recent in situ star formation
are dominated by -tubes. Slow rotators with recent mergers have significant
box orbit and -tube components. Rotation, quantified by the
-parameter often originates from streaming motion of stars on
-tubes but sometimes from figure rotation. The observed anti-correlation of
and in rotating galaxies can be connected to a dissipative
formation history leading to high -tube fractions. For galaxies with recent
mergers in situ formed stars, accreted stars and dark matter particles populate
similar orbits. Dark matter particles have isotropic velocity dispersions.
Accreted stars are typically radially biased (). In
situ stars become tangentially biased (as low as ) if
dissipation was relevant during the late assembly of the galaxy. We discuss the
relevance of our analysis for integral field surveys and for constraining
galaxy formation models.Comment: 21 pages, 19 figure
Galacto-forensic of LMC's orbital history as a probe for the dark matter potential in the outskirt of the Galaxy
The 3D observed velocities of the Large and Small Magellanic Clouds(LMC and
SMC) provide an opportunity to probe the Galactic potential in the outskirt of
the Galactic halo. Based on a canonical NFW model of the Galactic potential,
Besla et al.(2007) reconstructed LMC and SMC's orbits and suggested that they
are currently on their first perigalacticon passage about the Galaxy. Motivated
by several recent revisions of the Sun's motion around the Galactic center, we
re-examine the LMC's orbital history and show that it depends sensitively on
the dark-matter's mass distribution beyond its present Galactic distance. We
utilize results of numerical simulations to consider a range of possible
structural and evolutionary models for the Galactic potentials. We find that
within the theoretical and observational uncertainties, it is possible for the
LMC to have had multiple perigalacticon passages on the Hubble time scale,
especially if the Galactic circular velocity at the location of the Sun is
greater than km s. Based on these models, a more accurate
determination of the LMC's motion may be used to determine the dark matter
distribution in the outskirt of the Galactic halo.Comment: 9 pages, 10 figures. Accepted for publication in Ap
The impact of mechanical AGN feedback on the formation of massive early-type galaxies
We employ cosmological hydrodynamical simulations to investigate the effects
of AGN feedback on the formation of massive galaxies with present-day stellar
masses of . Using
smoothed particle hydrodynamics simulations with a pressure-entropy formulation
that allows an improved treatment of contact discontinuities and fluid mixing,
we run three sets of simulations of 20 halos with different AGN feedback
models: (1) no feedback, (2) thermal feedback, and (3) mechanical and radiation
feedback. We assume that seed black holes are present at early cosmic epochs at
the centre of emerging dark matter halos and trace their mass growth via gas
accretion and mergers with other black holes. Both feedback models successfully
recover the observed M_BH - sigma relation and black hole-to-stellar mass ratio
for simulated central early-type galaxies. The baryonic conversion efficiencies
are reduced by a factor of two compared to models without any AGN feedback at
all halo masses. However, massive galaxies simulated with thermal AGN feedback
show a factor of ~10-100 higher X-ray luminosities than observed. The
mechanical/radiation feedback model reproduces the observed correlation between
X-ray luminosities and velocity dispersion, e.g. for galaxies with sigma = 200
km/s, the X-ray luminosity is reduced from erg/s to erg/s.
It also efficiently suppresses late time star formation, reducing the specific
star formation rate from to on
average and resulting in quiescent galaxies since z=2, whereas the thermal
feedback model shows higher late time in-situ star formation rates than
observed.Comment: 13 pages, 11 figures, accepted for the publication in MNRA
Why stellar feedback promotes disc formation in simulated galaxies
We study how feedback influences baryon infall onto galaxies using
cosmological, zoom-in simulations of haloes with present mass
to . Starting
at z=4 from identical initial conditions, implementations of weak and strong
stellar feedback produce bulge- and disc-dominated galaxies, respectively.
Strong feedback favours disc formation: (1) because conversion of gas into
stars is suppressed at early times, as required by abundance matching
arguments, resulting in flat star formation histories and higher gas fractions;
(2) because 50% of the stars form in situ from recycled disc gas with angular
momentum only weakly related to that of the z=0 dark halo; (3) because
late-time gas accretion is typically an order of magnitude stronger and has
higher specific angular momentum, with recycled gas dominating over primordial
infall; (4) because 25-30% of the total accreted gas is ejected entirely before
z~1, removing primarily low angular momentum material which enriches the nearby
inter-galactic medium. Most recycled gas roughly conserves its angular
momentum, but material ejected for long times and to large radii can gain
significant angular momentum before re-accretion. These processes lower galaxy
formation efficiency in addition to promoting disc formation.Comment: 23 pages, 29 figures, accepted for publication in MNRA
The Dark Halo - Spheroid Conspiracy and the Origin of Elliptical Galaxies
Dynamical modeling and strong lensing data indicate that the total density
profiles of early-type galaxies are close to isothermal, i.e., rho_tot ~
r^gamma with gamma approx -2. To understand the origin of this universal slope
we study a set of simulated spheroids formed in isolated binary mergers as well
as the formation within the cosmological framework. The total stellar plus dark
matter density profiles can always be described by a power law with an index of
gamma approx -2.1 with a tendency toward steeper slopes for more compact,
lower-mass ellipticals. In the binary mergers the amount of gas involved in the
merger determines the precise steepness of the slope. This agrees with results
from the cosmological simulations where ellipticals with steeper slopes have a
higher fraction of stars formed in situ. Each gas-poor merger event evolves the
slope toward gamma ~ -2, once this slope is reached further merger events do
not change it anymore. All our ellipticals have flat intrinsic combined stellar
and dark matter velocity dispersion profiles. We conclude that flat velocity
dispersion profiles and total density distributions with a slope of gamma ~ -2
for the combined system of stars and dark matter act as a natural attractor.
The variety of complex formation histories as present in cosmological
simulations, including major as well as minor merger events, is essential to
generate the full range of observed density slopes seen for present-day
elliptical galaxies.Comment: Accepted by the Astrophysical Journal, 17 pages, 12 figure
The mass and angular momentum distribution of simulated massive early-type galaxies to large radii
We study the dark and luminous mass distributions, circular velocity curves
(CVC), line-of-sight kinematics, and angular momenta for a sample of 42
cosmological zoom simulations of massive galaxies. Using a temporal smoothing
technique, we are able to reach large radii. We find that: (i)The dark matter
halo density profiles outside a few kpc follow simple power-law models, with
flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass
haloes. The projected stellar density distributions at large radii can be
fitted by Sersic functions with n>10, larger than for typical ETGs. (ii)The
massive systems have nearly flat total CVCs at large radii, while the less
massive systems have mildly decreasing CVCs. The slope of the CVC at large
radii correlates with v_circ itself. (iii)The dark matter fractions within Re
are in the range 15-30% and increase to 40-65% at 5Re. Larger and more massive
galaxies have higher dark matter fractions. (iv)The short axes of simulated
galaxies and their host dark matter haloes are well aligned and their
short-to-long axis ratios are correlated. (v)The stellar vrms(R) profiles are
slowly declining, in agreement with planetary nebulae observations in the outer
haloes of most ETGs. (vi)The line-of-sight velocity fields v show that rotation
properties at small and large radii are correlated. Most radial profiles for
the cumulative specific angular momentum parameter lambda(R) are nearly flat or
slightly rising, with values in [0.06,0.75] from 2Re to 5Re. (vii)Stellar mass,
ellipticity at 5Re, and lambda(5Re) are correlated: the more massive systems
have less angular momentum and are rounder, as for observed ETGs. (viii)More
massive galaxies with a large fraction of accreted stars have radially
anisotropic velocity distributions outside Re. Tangential anisotropy is seen
only for galaxies with high fraction of in-situ stars. (Full abstract in PDF)Comment: 17 pages, 15 figures, 2 tables, accepted by MNRA
SPHGal: Smoothed Particle Hydrodynamics with improved accuracy for Galaxy simulations
We present the smoothed-particle hydrodynamics implementation SPHGal, which
combines some recently proposed improvements in GADGET. This includes a
pressure-entropy formulation with a Wendland kernel, a higher order estimate of
velocity gradients, a modified artificial viscosity switch with a modified
strong limiter, and artificial conduction of thermal energy. With a series of
idealized hydrodynamic tests we show that the pressure-entropy formulation is
ideal for resolving fluid mixing at contact discontinuities but performs
conspicuously worse at strong shocks due to the large entropy discontinuities.
Including artificial conduction at shocks greatly improves the results. In
simulations of Milky Way like disk galaxies a feedback-induced instability
develops if too much artificial viscosity is introduced. Our modified
artificial viscosity scheme prevents this instability and shows efficient shock
capturing capability. We also investigate the star formation rate and the
galactic outflow. The star formation rates vary slightly for different SPH
schemes while the mass loading is sensitive to the SPH scheme and significantly
reduced in our favored implementation. We compare the accretion behavior of the
hot halo gas. The formation of cold blobs, an artifact of simple SPH
implementations, can be eliminated efficiently with proper fluid mixing, either
by conduction and/or by using a pressure-entropy formulation.Comment: Replaced with the version accepted by MNRA
Short-lived star-forming giant clumps in cosmological simulations of z~2 disks
Many observed massive star-forming z\approx2 galaxies are large disks that
exhibit irregular morphologies, with \sim1kpc, \sim10^(8-10)Msun clumps. We
present the largest sample to date of high-resolution cosmological SPH
simulations that zoom-in on the formation of individual M*\sim10^(10.5)Msun
galaxies in \sim10^(12)Msun halos at z\approx2. Our code includes strong
stellar feedback parameterized as momentum-driven galactic winds. This model
reproduces many characteristic features of this observed class of galaxies,
such as their clumpy morphologies, smooth and monotonic velocity gradients,
high gas fractions (f_g\sim50%) and high specific star-formation rates
(\gtrsim1Gyr^(-1)). In accord with recent models, giant clumps
(Mclump\sim(5x10^8-10^9)Msun) form in-situ via gravitational instabilities.
However, the galactic winds are critical for their subsequent evolution. The
giant clumps we obtain are short-lived and are disrupted by wind-driven mass
loss. They do not virialise or migrate to the galaxy centers as suggested in
recent work neglecting strong winds. By phenomenologically implementing the
winds that are observed from high-redshift galaxies and in particular from
individual clumps, our simulations reproduce well new observational constraints
on clump kinematics and clump ages. In particular, the observation that older
clumps appear closer to their galaxy centers is reproduced in our simulations,
as a result of inside-out formation of the disks rather than inward clump
migration.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in the
Astrophysical Journa