616 research outputs found
Lopsided spiral galaxies: evidence for gas accretion
We quantify the degree of lopsidedness for a sample of 149 galaxies observed
in the near-infrared from the OSUBGS sample, and try to explain the physical
origin for the observed disk lopsidedness. We confirm previous studies, but now
for a larger sample, that a large fraction of galaxies show significant
lopsidedness in their stellar disks, measured as the Fourier amplitude of the
m=1 component, normalised to the average or m=0 component, in the surface
density. Late-type galaxies are found to be more lopsided, while the presence
of m=2 spiral arms and bars is correlated. The m=1 amplitude is found to be
uncorrelated with the tidal forces acting on a galaxy via nearby companions.
Numerical simulations are carried out to study the generation of m=1 via
different processes: galaxy tidal encounters, galaxy mergers, and external gas
accretion and subsequent star formation. The simulations show that galaxy
interactions and mergers can trigger strong lopsidedness, but do not explain
several independent statistical properties of observed galaxies. To explain all
the observational results, it is required that a large fraction of lopsidedness
results from cosmological accretion of gas on galactic disks, which can create
strongly lopsided disks when this accretion is asymmetrical enough.Comment: accepted for publication in Astronomy and Astrophysics - Final
version after language editio
Modelling CO emission from hydrodynamic simulations of nearby spirals, starbursting mergers, and high-redshift galaxies
We model the intensity of emission lines from the CO molecule, based on
hydrodynamic simulations of spirals, mergers, and high-redshift galaxies with
very high resolutions (3pc and 10^3 Msun) and detailed models for the
phase-space structure of the interstellar gas including shock heating, stellar
feedback processes and galactic winds. The simulations are analyzed with a
Large Velocity Gradient (LVG) model to compute the local emission in various
molecular lines in each resolution element, radiation transfer and opacity
effects, and the intensity emerging from galaxies, to generate synthetic
spectra for various transitions of the CO molecule. This model reproduces the
known properties of CO spectra and CO-to-H2 conversion factors in nearby
spirals and starbursting major mergers. The high excitation of CO lines in
mergers is dominated by an excess of high-density gas, and the high turbulent
velocities and compression that create this dense gas excess result in broad
linewidths and low CO intensity-to-H2 mass ratios. When applied to
high-redshift gas-rich disks galaxies, the same model predicts that their
CO-to-H2 conversion factor is almost as high as in nearby spirals, and much
higher than in starbursting mergers. High-redshift disk galaxies contain giant
star-forming clumps that host a high-excitation component associated to gas
warmed by the spatially-concentrated stellar feedback sources, although CO(1-0)
to CO(3-2) emission is overall dominated by low-excitation gas around the
densest clumps. These results overall highlight a strong dependence of CO
excitation and the CO-to-H2 conversion factor on galaxy type, even at similar
star formation rates or densities. The underlying processes are driven by the
interstellar medium structure and turbulence and its response to stellar
feedback, which depend on global galaxy structure and in turn impact the CO
emission properties.Comment: A&A in pres
Multiple minor mergers: formation of elliptical galaxies and constraints for the growth of spiral disks
Multiple, sequential mergers are unavoidable in the hierarchical build-up
picture of galaxies, in particular for the minor mergers that are frequent and
highly likely to have occured several times for most present-day galaxies.
However, the effect of repeated minor mergers on galactic structure and
evolution has not been studied systematically so far. We present a numerical
study of multiple, subsequent, minor galaxy mergers, with various mass ratios
ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star
formation. We study the morphological and kinematical properties of the
remnants, and show that several so-called "minor" mergers can lead to the
formation of elliptical-like galaxies that have global morphological and
kinematical properties similar to that observed in real elliptical galaxies.
The properties of these systems are compared with those of elliptical galaxies
produced by the standard scenario of one single major merger. We thus show that
repeated minor mergers can theoretically form elliptical galaxies without major
mergers, and can be more frequent than major mergers, in particular at moderate
redshift. This process must then have formed some elliptical galaxies seen
today, and could in particular explain the high boxiness of massive
ellipticals, and some fundamental relations observed in ellipticals. In
addition, because repeated minor mergers, even at high mass ratios, destroy
disks into spheroids, these results indicate that spiral galaxies cannot have
grown only by a succession of minor mergers.Comment: A&A in press, final version to be published with minor corrections
and updated reference lis
Unequal-mass galaxy merger remnants: spiral-like morphology but elliptical-like kinematics
It is generally believed that major galaxy mergers with mass ratios in the
range 1:1-3:1 result in remnants that have properties similar to elliptical
galaxies, and minor mergers below 10:1 result in disturbed spiral galaxies. The
intermediate range of mass ratios 4:1-10:1 has not been studied so far. Using
N-body simulations, we show that such mergers can result in very peculiar
systems, that have the morphology of a disk galaxy with an exponential profile,
but whose kinematics is closer to that of elliptical systems. These objects are
similar to those recently observed by Jog & Chitre (2002). We present two cases
with mass ratios 4.5:1 and 7:1, and show that the merging causes major heating
and results in the appearance of elliptical-type kinematics, while surprisingly
the initial spiral-like mass profile is conserved.Comment: 4 pages, 3 figures, Accepted for publication in A&A Letter
Hydrodynamics of high-redshift galaxy collisions: From gas-rich disks to dispersion-dominated mergers and compact spheroids
Disk galaxies at high redshift (z~2) are characterized by high fractions of
cold gas, strong turbulence, and giant star-forming clumps. Major mergers of
disk galaxies at high redshift should then generally involve such turbulent
clumpy disks. Merger simulations, however, model the ISM as a stable,
homogeneous, and thermally pressurized medium. We present the first merger
simulations with high fractions of cold, turbulent, and clumpy gas. We discuss
the major new features of these models compared to models where the gas is
artificially stabilized and warmed. Gas turbulence, which is already strong in
high-redshift disks, is further enhanced in mergers. Some phases are
dispersion-dominated, with most of the gas kinetic energy in the form of
velocity dispersion and very chaotic velocity fields, unlike merger models
using a thermally stabilized gas. These mergers can reach very high star
formation rates, and have multi-component gas spectra consistent with
SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas
are also characterized by highly dissipative gas collapse to the center of
mass, with the stellar component following in a global contraction. The final
galaxies are early-type with relatively small radii and high Sersic indices,
like high-redshift compact spheroids. The mass fraction in a disk component
that survives or re-forms after a merger is severely reduced compared to models
with stabilized gas, and the formation of a massive disk component would
require significant accretion of external baryons afterwards. Mergers thus
appear to destroy extended disks even when the gas fraction is high, and this
lends further support to smooth infall as the main formation mechanism for
massive disk galaxies.Comment: ApJ accepte
Galactic star formation in parsec-scale resolution simulations
The interstellar medium (ISM) in galaxies is multiphase and cloudy, with
stars forming in the very dense, cold gas found in Giant Molecular Clouds
(GMCs). Simulating the evolution of an entire galaxy, however, is a
computational problem which covers many orders of magnitude, so many
simulations cannot reach densities high enough or temperatures low enough to
resolve this multiphase nature. Therefore, the formation of GMCs is not
captured and the resulting gas distribution is smooth, contrary to
observations. We investigate how star formation (SF) proceeds in simulated
galaxies when we obtain parsec-scale resolution and more successfully capture
the multiphase ISM. Both major mergers and the accretion of cold gas via
filaments are dominant contributors to a galaxy's total stellar budget and we
examine SF at high resolution in both of these contexts.Comment: 4 pages, 4 figures. To appear in the proceedings for IAU Symposium
270: Computational Star Formation (eds. Alves, Elmegreen, Girart, Trimble
The Global Star-Formation Law by Supernova Feedback
We address a simple model where the Kennicutt-Schmidt (KS) relation between
the macroscopic densities of star-formation rate (SFR, ) and
gas () in galactic discs emerges from self-regulation of the SFR via
supernova feedback. It arises from the physics of supernova bubbles,
insensitive to the microscopic SFR recipe and not explicitly dependent on
gravity. The key is that the filling factor of SFR-suppressed supernova bubbles
self-regulates to a constant, . Expressing the bubble fading radius
and time in terms of , the filling factor is with
, where is the supernova rate density. A constant thus
refers to , with a density-independent SFR
efficiency per free-fall time . The self-regulation to
and the convergence to a KS relation independent of the local SFR recipe are
demonstrated in cosmological and isolated-galaxy simulations using different
codes and recipes. In parallel, the spherical analysis of bubble evolution is
generalized to clustered supernovae, analytically and via simulations, yielding
. An analysis of photo-ionized bubbles about
pre-supernova stars yields a range of KS slopes but the KS relation is
dominated by the supernova bubbles. Superbubble blowouts may lead to an
alternative self-regulation by outflows and recycling. While the model is
over-simplified, its simplicity and validity in the simulations may argue that
it captures the origin of the KS relation
Evolution of the mass, size, and star formation rate in high-redshift merging galaxies MIRAGE - A new sample of simulations with detailed stellar feedback
We aim at addressing the questions related to galaxy mass assembly through
major and minor wet merging processes in the redshift range 1<z<2. A consequent
fraction of Milky Way like galaxies are thought to have undergone an unstable
clumpy phase at this early stage. Using the adaptive mesh refinement code
RAMSES, with a recent physically-motivated implementation of stellar feedback,
we build the Merging and Isolated high-Redshift Adaptive mesh refinement
Galaxies (MIRAGE) sample. It is composed of 20 mergers and 3 isolated idealized
disks simulations with global physical properties in accordance with the 1<z<2
mass complete sample MASSIV. The numerical hydrodynamical resolution reaches 7
parsecs in the smallest Eulerian cells. Our simulations include: star
formation, metal line cooling, metallicity advection, and a recent
implementation of stellar feedback which encompasses OB-type stars radiative
pressure, photo-ionization heating, and supernovae. The initial conditions are
set to match the z~2 observations, thanks to a new public code DICE. The
numerical resolution allows us to follow the formation and evolution of giant
clumps formed in-situ from Jeans instabilities triggered by high initial gas
fraction. The star formation history of isolated disks shows stochastic star
formation rate, which proceeds from the complex behavior of the giant clumps.
Our minor and major gas-rich merger simulations do not trigger starbursts,
suggesting a saturation of the star formation in a turbulent and clumpy
interstellar medium fed by substantial accretion from the circum-galactic
medium. Our simulations are close to the normal regime of the disk-like star
formation on a Schmidt-Kennicutt diagram. The mass-size relation and its rate
of evolution matches observations, suggesting that the inside-out growth
mechanisms of the stellar disk do not necessarily require to be achieved
through a cold accretion.Comment: 18 pages, 12 figures. Accepted in A&
Stars and gas in the very large interacting galaxy NGC 6872
The dynamical evolution of the large (> 100 kpc), barred spiral galaxy NGC
6872 and its small companion IC 4970 in the southern group Pavo is
investigated. We present N-body simulations with stars and gas and 21 cm HI
observations carried out with the Australia Telescope Compact Array of the
large-scale distribution and kinematics of atomic gas. HI is detected toward
the companion, corresponding to a gas mass of ca 1.3 10^9 Msun. NGC 6872
contains ca 1.4 10^{10} Msun of HI gas, distributed in an extended rotating
disk. Massive concentrations of gas (10^9 Msun) are found at the tip of both
tidal tails and towards the break seen in the optical northern arm near the
companion. We detect no HI counterpart to the X-ray trail between NGC 6872 and
NGC 6876, the dominant elliptical galaxy in the Pavo group located 8 arcmin to
the southeast. At the sensitivity and the resolution of the observations, there
is no sign in the overall HI distribution that NGC 6876 has affected the
evolution of NGC 6872. There is no evidence of ram pressure stripping either.
The X-ray trail could be due to gravitational focusing of the hot gas in the
Pavo group behind NGC 6872 as the galaxy moves supersonically through the hot
medium. The simulations of a gravitational interaction with a small nearby
companion on a low-inclination prograde passage are able to reproduce most of
the observed features of NGC 6872, including the general morphology of the
galaxy, the inner bar, the extent of the tidal tails and the thinness of the
southern tail.Comment: 12 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysics. The resolution of the figures has been greatly reduced. The
paper with the original figures can found at
http://www.oso.chalmers.se/~horellou/PAPERS/2006n6872.pd
Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
By means of N-body simulations we investigate the impact of minor mergers on
the angular momentum and dynamical properties of the merger remnant. Our
simulations cover a range of initial orbital characteristics and gas-to-stellar
mass fractions (from 0 to 20%), and include star formation and supernova
feedback. We confirm and extend previous results by showing that the specific
angular momentum of the stellar component always decreases independently of the
orbital parameters or morphology of the satellite, and that the decrease in the
rotation velocity of the primary galaxy is accompanied by a change in the
anisotropy of the orbits. However, the decrease affects only the old stellar
population, and not the new population formed from gas during the merging
process. This means that the merging process induces an increasing difference
in the rotational support of the old and young stellar components, with the old
one lagging with respect to the new. Even if our models are not intended
specifically to reproduce the Milky Way and its accretion history, we find
that, under certain conditions, the modeled rotational lag found is compatible
with that observed in the Milky Way disk, thus indicating that minor mergers
can be a viable way to produce it. The lag can increase with the vertical
distance from the disk midplane, but only if the satellite is accreted along a
direct orbit, and in all cases the main contribution to the lag comes from
stars originally in the primary disk rather than from stars in the satellite
galaxy. We also discuss the possibility of creating counter-rotating stars in
the remnant disk, their fraction as a function of the vertical distance from
the galaxy midplane, and the cumulative effect of multiple mergers on their
creation.Comment: 14 pages, 19 figures, in press at A&
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