arXiv subjects

Andreas Burkert

Publications and source records attributed to Andreas Burkert.

At least 235 records · Page 13Linked to original sources

arXiv2001

Gas dynamics and disk formation in 3:1 mergers

We performed N-body/SPH simulations of merging gas rich disk-galaxies with a mass ratio of 3:1. A stellar disk and bulge component and a dark halo is realized with collisionless particles, the gas is simulated using SPH particles. In this paper we focus on the gas dynamics in mergers and its influence on the structure of the final merger remnant, neglecting star formation. During the merger around $8\times 10^9 M_{\odot}$ of gas accumulate in the central region of the merger remnant (inside 300 pc). Part of the gas falls to the center, channeled by a tidally induced bar in the more massive galaxy. A peak accretion rate of 150 solar masses/year is reached when the two galaxy centers merge. It is likely that the gas will experience a central starburst and/or fuel a central black hole. The gas in the outer regions accumulates in dense knots within tidal tails which could lead to the formation of open clusters or dwarf satellites. Later on, the gas knots loose angular momentum by dynamical friction and also sink to the center of the remnant, thereby increasing the central gas content. In the end, the gas settles in a central power-law disk with surface density $Σ\propto r^{-2}$ surrounded by dense, orbiting gas clumps. The stellar profile is of de Vaucouleurs-type, like in simulations of pure stellar mergers

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arXiv2001

Formation of ellipticals by unequal mass mergers

Collisionless N-body simulations of merging disk-galaxies with mass ratios ($η$) of 1:1, 2:1, 3:1, and 4:1 have been performed using direct summation with the special purpose hardware GRAPE. The simulations are used to examine whether the formation of elliptical galaxies can be explained in the context of the merger scenario. The photometric, kinematical and isophotal properties of the merger remnants are investigated and turn out to be in very good agreement with observations of giant elliptical galaxies. We conclude that equal mass mergers lead to slowly rotating, anisotropic remnants having predominantly boxy isophotes. Mergers with a mass ratio of 3:1 and 4:1, on the other hand, are fast isotropic rotators with disky isophotes. Projection effects can explain the observed scatter in the kinematical and isophotal properties of elliptical galaxies.

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arXiv2001

Evidence for Large Stellar Disks in Elliptical Galaxies

High-resolution numerical simulations of galaxy mergers are analysed. The global structure and isophotal shapes of the merger remnants are in good agreement with the observations. Whereas equal-mass mergers lead to anisotropic, boxy ellipticals, unequal-mass mergers result in disky and isotropic systems. The line-of-sight velocity distributions show small deviations from a Gaussian distribution. In all cases we find that the retrograde wings are steeper and that the prograde wings are broader than a Gaussian distribution. This is in contradiction with the observations which show broader retrograde and steeper prograde wings in all ellipticals. This fundamental difference between observation and theory can be explained if all ellipticals, even anistropic boxy ones, contain extended stellar disk components with luminosities of order 10 to 20 percent the total luminosity and scale radii of order the effective radii of the spheroids.

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arXiv2000

Self-Interacting Cold Dark Matter Halos

The evolution of halos consisting of weakly self-interacting dark matter particles is summarized. The halos initially contain a central density cusp as predicted by cosmological models. Weak self-interaction leads to the formation of an isothermal, low-density core which is in agreement with published data on the rotation curves of dwarf galaxies. However, subsequently, core collapse leads to an even steeper central density cusp. In order to explain the observed dark matter rotation curves by weak interaction, the cross section must be in a narrow range around sigma/mp= 0.1 cm*cm/g.

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arXiv2000

Star Formation-Regulated Growth of Black Holes in Protogalactic Spheroids

The observed relation between central black hole mass and spheroid velocity dispersion is interpreted in terms of a self-regulation model that incorporates a viscous Keplerian accretion disk to feed the black hole, embedded in a massive, self-gravitating star forming disk that eventually populates the spheroid. The model leads to a constant ratio between black hole mass and spheroid mass which is equal to the inverse of the critical Reynolds number for the onset of turbulence in the accretion disk surrounding the central black hole. Applying the fundamental plane correlation for spheroids, we find that the black hole mass has a power-law dependence on the spheroid velocity dispersion with a slope in the range of 4-5. We explain the larger scatter in the Magorrian relation with respect to the black hole mass-spheroid velocity dispersion relationship as a result of secular evolution of the spheroid that primarily affects its luminosity and to a much lesser extent its velocity dispersion.

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arXiv2000

The Formation of Stellar Clusters: Gaussian Initial Conditions II

Using hydrodynamic simulations we investigate the time evolution and fragmentation of regions within molecular clouds which have lost their turbulent support leading to gravitational contraction. The initial density distributions are described by random Gaussian fluctuations with varying slopes x of the power spectrum P(k) ~ k^-x, covering the range from flat (x=0) to very steep spectra (x=3). We consider molecular cloud volumes containing different masses relative to the average Jeans mass M_J, from 1 M_J to 222 M_J. This parameter study extends the detailed analysis of systems with initially P(k) ~ k^-2 and mass 222 M_J presented by Klessen & Burkert (2000). The dynamical evolution of the simulated molecular cloud regions is insensitive to the slope of the initial density fluctuation spectrum. The system evolves into a complicated network of intersecting filaments and collapsing clumps leading to the formation of a compact cluster of accreting and interacting embedded protostellar cores. The cluster builds up as bound entity, but dissolves later due to collisional effects. In all simulations, the mass spectrum of collapsed cores is very broad, has approximately log-normal shape and peaks roughly at the average Jeans mass. This supports the hypothesis that the average Jeans mass is the main parameter determining the peak in the stellar spectrum, and suggests that the interplay between self-gravity on the one side and thermal and turbulent pressure on the other side is the dominant process that regulates the formation of stellar clusters.

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arXiv2000

Star Formation in Viscous Galaxy Disks

The Lin and Pringle model (1987) of galactic disk formation postulates that if star formation proceeds on the same timescale as the viscous redistribution of mass and angular momentum in disk galaxies, then the stars attain an exponential density profile. Their claim is that this result holds generally: regardless of the disk galaxy's initial gas and dark matter distribution and independent of the nature of the viscous processes acting in the disk. We present new results from a set of 2D hydro-simulations which investigate their analytic result.

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arXiv2000

Inhomogeneous Chemical Evolution of the Galactic Halo: Abundance of r-process Elements

We present a stochastic model based on the Monte Carlo technique to study the inhomogeneous chemical evolution of the Galactic halo. In particular, we consider the local enrichment and subsequent mixing of the interstellar gas resulting from bursts of star formation, following explicitly the fragmentation and coalescence of interstellar gas clouds. The model takes into account the mixing of halo gas through cloud-cloud collisions and the delayed mixing of supernova ejecta into the interstellar medium. The consequences of the infall of halo gas onto the Galactic disk are also discussed. The mass spectrum of clouds, the age-metallicity relation, and the G-dwarf distribution are investigated at different times. We analyze in detail the predictions of our model for the abundance of elements like Eu, Ba, and Sr in the Galactic halo, following their production by $r$-process nucleosynthesis for different assumptions on the supernova mass range. Finally, we compare our results with spectroscopic data for the chemical composition of metal-poor halo stars.

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arXiv2000

The Effect of Gas Loss on the Formation of Bound Stellar Clusters

The effect of gas ejection on the structure and binding energy of newly formed stellar clusters is investigated. The star formation efficiency (SFE), necessary for forming a gravitationally bound stellar cluster, is determined. Two sets of numerical N-body simulations are presented: As a first simplified approach we treat the residual gas as an external potential. The gas expulsion is approximated by reducing the gas mass to zero on a given timescale, which is treated as a free parameter. In a second set of simulations we use smoothed particle hydrodynamics (SPH) to follow the dynamics of the outflowing residual gas self-consistently. We investigate cases where gas outflow is induced by an outwards propagating shock front and where the whole gas cloud is heated homogeneously, leading to ejection. If the stars are in virial equilibrium with the gaseous environment initially, bound clusters only form in regions where the local SFE is larger than 50% or where the gas expulsion timescale is long compared to the dynamical timescale. A small initial velocity dispersion of the stars leads to a compaction of the cluster during the expulsion phase and reduces the SFE needed to form bound clusters to less than 10%.

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arXiv2000

The Cosmological Angular Momentum Problem of Low-Mass Disk Galaxies

The rotational properties of the visible and dark components of low-mass disk galaxies (vrot<=100 km/s) are investigated using the Swaters sample. The rotational parameter lambda'=lambda_DM*(j_d/m_d) is determined, where lambda_DM is the dark halo spin parameter and j_d/m_d is the ratio between the specific angular momentum of the disk and that of the dark halo. The distribution of lambda' is in excellent agreement with cosmological predictions of hierarchical clustering if j_d/m_d=1, that is if the protogalactic gas did not loose a significant amount of specific angular momentum due to dynamical friction. This result is in disagreement with current cosmological Nbody/SPH simulations where the baryonic component looses 90% of its specific angular momentum while settling into the equatorial plane. The Swaters sample also shows a surprisingly strong correlation between lambda' and the baryonic mass fraction m_d: lambda'=0.4 m_d**0.6. This correlation can be explained if the total amount of gas in protogalaxies is a universal fraction of their dark matter mass, of order 10%, and if the variation in m_d is a result of the fact that only the inner parts of the primordial gas distribution managed to form a visible disk component. In this case the specific angular momentum of the gas out of which the disk formed is a factor of 2.75 larger than that of the dark halo, which would require a yet unknown spin-up process for the visible baryonic component.

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arXiv2000

Turbulent Molecular Cloud Cores: Rotational Properties

The rotational properties of centrally condensed, turbulent molecular cloud cores with velocity fields that are characterized by Gaussian random fields are investigated. It is shown that the observed line width-size relationship can be reproduced if the velocity power spectrum is a power-law with P(k)=k**n and n=-3 to -4. The line-of-sight velocity maps of these cores show velocity gradients that can be interpreted as rotation. For n=-4, both, the deduced values of the angular velocity Omega=1.6 km/s/pc * (R/0.1 pc)**0.5 and the scaling relations between Omega and the core radius R are in very good agreement with the observations. As a result of the dominance of long wavelength modes, the cores also have a net specific angular momentum with an average value of j=7*(10**20)*(R/0.1 pc)**(1.5) cm**2/s with a large spread. Their internal dimensionless rotational parameter is beta=0.03, independent of the scale radius R. In general, the line-of-sight velocity gradient of an individual turbulent core does not provide a good estimate of its internal specific angular momentum. We find however that the distribution of the specific angular momenta of a large sample of cores which are described by the same power spectrum can be determined very accurately from the distribution of their line-of-sight velocity gradients Omega using the simple formula j=p*Omega*R*R where p depends on the density distribution of the core and has to be determined from a Monte-Carlo study. Our results show that for centrally condensed cores the intrinsic angular momentum is overestimated by a factor of 2-3 if p=0.4 is used.

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arXiv2000

Dark Matter Scaling Relations

We establish the presence of a dark matter core radius, for the first time in a very large number of spiral galaxies of all luminosities. Contrary to common opinion we find that the sizes of these cores and the " DM core problem" are bigger for more massive spirals. As a result the Burkert profile provides an excellent mass model for dark halos around disk galaxies. Moreover, we find that the spiral dark matter core densities $ρ_{0}$ and core radii $r_{0}$ lie in the same scaling relation $ρ_{0}=4.5\times 10^-2 (r_{0}/kpc)^{-2/3} M_{\odot}pc^{-3}$ of dwarf galaxies with core radii upto ten times more smaller.

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arXiv2000

The structure and evolution of weakly self-interacting cold dark matter halos

The evolution of halos consisting of weakly self-interacting dark matter particles is investigated using a new numerical Monte-Carlo N-body method. The halos initially contain kinematically cold, dense 1/r-power-law cores. For interaction cross sections (sigma/mp) >= 10-100 cm**2/g weak self-interaction leads to the formation of isothermal, constant density cores within a Hubble time as a result of heat transfer into the cold inner regions. This core structure is in good agreement with the observations of dark matter rotation curves in dwarf galaxies. The isothermal core radii and core densities are a function of the halo scale radii and scale masses which depend on the cosmological model. Adopting the currently popular LCDM model, the predicted core radii and core densities are on good agreement with the observations. For large interaction cross sections, massive dark halos with scale radii rs >= 14000 cm**2/g/(sigma/mp) kpc could experience core collapse during their lifetime, leading to cores with singular isothermal density profiles.

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arXiv2000

Disks and Planets in Binary Systems

The star formation process in molecular clouds usually leads to the formation of multiple stellar systems, mostly binaries. Remaining disks around those stars may be located around individual stars (circumstellar disks) or around the entire binary system (circumbinary disk). We shall briefly review the present observational evidence for both types of disks in binary stars, in particular the properties of circumbinary disks. We then present recent results of the theoretical modeling of the collapse and fragmentation of gravitationally unstable molecular cloud cores and their implications for binary and disk formation, and discuss the dynamical influence of the binary companions on disk truncation and gap formation. The presence of binaries may have profound influence on the process of planet formation as well. We present results on the stability and evolution of orbits of planets in disks around binaries.

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arXiv2000

The Formation of Stellar Clusters: Gaussian Cloud Conditions I

The isothermal dynamical evolution of a clumpy and turbulent molecular cloud region and its fragmentation into a protostellar cluster is investigated numerically. The effect of different initial density and velocity distributions, generated from different realizations of a Gaussian random field with power spectrum P(k) ~ k^{-2} is investigated. As a result of the interplay between gas pressure and gravitational forces, a quasi-equilibrium clump mass spectrum emerges with a power-law distribution dN/dM ~ M^{-1.5}, independent of the initial conditions. Being part of a complex network of filaments, individual clumps are elongated, centrally condensed objects with 2:1 to 4:1 axis ratios with outer r^{-2} density distributions. Dense, Jeans-unstable gas clumps collapse and form protostellar cores which evolve through competitive accretion and $N$-body interactions with other cores. In contrast to the clumps, the core mass spectrum is best described by a log-normal distribution which peaks approximately at the average Jeans mass of the system. Scaled to physical conditions typical for star-forming molecular clouds, the mass function is in good agreement with the IMF of multiple stellar systems. The final dynamical state of the newly formed stellar cluster closely resembles observed young stellar clusters. It has a core/halo structure which is typical for collision dominated N-body systems. The 2-point correlation function of the spatial stellar distribution can be described by two power-laws with a break in the slope at the transition point from the binary to the large-scale clustering regime. The protostellar cluster is marginally bound.

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arXiv2000

Gas Stripping of Dwarf Galaxies in Clusters of Galaxies

Many clusters of galaxies contain an appreciable amount of hot gas, the intracluster medium. As a consequence, gas will be stripped from galaxies that move through the inter cluster medium, if the ram pressure exceeds the internal gravitational force. We study the interaction between the intracluster medium and an extended gas component of dwarf galaxies confined by a surrounding cold dark matter halo analytically and numerically, using axisymmetric two-dimensional hydrodynamical simulations at high resolution. The results show that the gas within the dark matter halo is totally stripped in a typical galactic cluster. The process of ram pressure stripping therefore must have played an important role during the chemo-dynamical evolution of dwarf galaxies in galactic clusters. Our results predict that most of the chemical evolution of dwarf galaxies must have occurred at high redshift, before the virialized cluster had formed.

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arXiv1999

On the Formation of Helium Double Degenerate Stars and Pre-Cataclysmic Variables

The evolution of low mass (M < 2.5 Msun) binaries through the common envelope phase has been studied for systems in which one member is on its first ascent of the red giant branch. Three-dimensional hydrodynamical simulations have been carried out for a range of red giant masses (1 - 2 Msun) with degenerate helium cores (0.28 - 0.45 Msun) and companions (0.1 - 0.45 Msun) for initial orbital periods ranging from about 15 - 1000 days. The results suggest that these low mass binary systems can survive the common envelope phase provided that the helium degenerate core is more massive than about 0.2 - 0.25 Msun and that the mass of the red giant progenitor is less than about 2 Msun. For the observed short period double degenerate systems, it is found that evolutionary scenarios involving two phases of common envelope evolution are not likely and that a scenario involving an Algol-like phase of mass transfer followed by a common envelope phase is viable, suggesting that the first-formed white dwarf is often reheated by nuclear burning on its surface. A formation mechanism for two subdwarf B stars observed in eclipsing short period binaries with low mass main sequence stars is also described.

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arXiv1999

On the Formation of Boxy and Disky Elliptical Galaxies

The origin of boxy and disky elliptical galaxies is investigated. The results of two collisionless N-body simulations of spiral-spiral mergers with mass ratios of 1:1 and 3:1 are discussed and the projected properties of the merger remnants are investigated. It is shown that the equal-mass merger leads to an anisotropic, slowly rotating system with preferentially boxy isophotes and significant minor axis rotation. The unequal-mass merger results in the formation of a rotationally supported elliptical with disky isophotes and small minor axis rotation. The observed scatter in the kinematical and isophotal properties of both classes of elliptical galaxies can be explained by projection effects.

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