arXiv subjects

A. Burkert

Publications and source records attributed to A. Burkert.

At least 127 records · Page 7Linked to original sources

arXiv2006

The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows

Molecular Cloud Complexes (MCCs) are highly structured and ``turbulent''. Observational evidence suggests that MCCs are dynamically dominated systems, rather than quasi-equilibrium entities. The observed structure is more likely a consequence of the formation process rather than something that is imprinted after the formation of the MCC. Converging flows provide a natural mechanism to generate MCC structure. We present a detailed numerical analysis of this scenario. Our study addresses the evolution of a MCC from its birth in colliding atomic hydrogen flows up until the point when H$_2$ may begin to form. A combination of dynamical and thermal instabilities breaks up coherent flows efficiently, seeding the small-scale non-linear density perturbations necessary for local gravitational collapse and thus allowing (close to) instantaneous star formation. Many observed properties of MCCs come as a natural consequence of this formation scenario. Since converging flows are omnipresent in the ISM, we discuss the general applicability of this mechanism, from local star formation regions to galaxy mergers.

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arXiv2006

The Fragmentation of Cores and the Initial Binary Population

Almost all young stars are found in multiple systems. This suggests that protostellar cores almost always fragment into multiple objects. The observed properties of multiple systems such as their separation distribution and mass ratios provide strong constraints on star formation theories. We review the observed properties of young and old multiple systems and find that the multiplicity of stars changes. Such an evolution is probably due to (a) the dynamical decay of small-N systems and/or (b) the destruction of multiple systems within dense clusters. We review simulations of the fragmentation of rotating and turbulent molecular cores. Such models almost always produce multiple systems, however the properties of those systems do not match observations at all well. Magnetic fields appear to supress fragmentation, prehaps suggesting that they are not dynamically important in the formation of multiple systems. We finish by discussing possible reasons why theory fails to match observation, and the future prospects for this field.

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arXiv2006

How galaxies lose their angular momentum

The processes are investigated by which gas loses its angular momentum during the protogalactic collapse phase, leading to disk galaxies that are too compact with respect to the observations. High-resolution N-body/SPH simulations in a cosmological context are presented including cold gas and dark matter. A halo with quiet merging activity since z~3.8 and with a high spin parameter is analysed that should be an ideal candidate for the formation of an extended galactic disk. We show that the gas and the dark matter have similar specific angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and falls quickly into the center. Dynamical friction plays a minor role,in contrast to previous claims. In fact, after this event a new extended disk begins to form from gas that was not involved in the 5:1 merger event and that falls in subsequently. We argue that the angular momentum problem of disk galaxy formation is a merger problem: in cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1 are expected to occur in almost all galaxies. We suggest that energetic feedback processes could in principle solve this problem, however only if the heating occurs at the time or shortly before the last substantial merger event. Good candidates for such a coordinated feedback would be a merger-triggered star burst or central black hole heating. If a large fraction of the low angular momentum gas would be ejected as a result of these processes, late-type galaxies could form with a dominant extended disk component, resulting from late infall, a small bulge-to-disk ratio and a low baryon fraction, in agreement with observations.

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arXiv2005

Dynamical properties of Ultraluminous Infrared Galaxies I: Mass ratio conditions for ULIRG activity in interacting pairs

We present first results from our Very Large Telescope large program to study the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs), which are the products of mergers of gas-rich galaxies. The full data set consists of high resolution, long-slit, H- and K-band spectra of 38 ULIRGs and 12 QSOs (between 0.042 3:1 typically do not force enough gas into the center to generate ULIRG luminosities.

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arXiv2005

The dwarf galaxy DDO 47: testing cusps hiding in triaxial halos

We present HI data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce non-circular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. We performed a harmonic decomposition of the velocity field in order to search for alleged non-circular motions needed to ``hide'' a cusp: in DDO 47 non-circular motions are globally at a level of 2-3 km s$^{-1}$, far from being sufficient to reconcile the observed rotation curve with the $Λ$CDM predictions. We conclude that the dark matter halo around DDO 47 is truly cored and that a cusp cannot be hidden by non-circular motions. More details are shown in Gentile et al. (2006, ApJL in press, astro-ph/0506538).

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arXiv2005

The dwarf galaxy DDO 47 as a dark matter laboratory: testing cusps hiding in triaxial halos

We present an analysis of high resolution HI data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce non-circular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. This hypothesis could be invoked in order to reconcile the predictions of the standard LambdaCDM theory with the rotation curves of disk galaxies. DDO 47 is an ideal case to test this hypothesis because it has a very regular velocity field, its rotation curve is best fitted by a cored halo and a NFW halo is inconsistent with the data. We analysed the velocity field through the higher-order harmonic terms in order to search for kinematical signatures of alleged non-circular motions needed to "hide" a cusp: the result is that globally non-circular motions are at a level of 2-3 km/s, and they are more likely to be associated to the presence of some spiral structure than to a global elongated potential (e.g. a triaxial halo). These non-circular motions are far from being sufficient to account for the discrepancy with the LambdaCDM predictions. We therefore conclude that the dark matter halo around the dwarf galaxy DDO 47 is truly cored and that a cusp cannot be hidden by non-circular motions.

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arXiv2005

Evolution of local Ultraluminous mergers from NIR spectroscopy

We present results from our VLT Large Program to study the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs). We have so far obtained near-infrared high-resolution ISAAC spectra of 53 local ULIRGs at several merger timescales and 12 Palomar-Green QSOs (more than half of which are IR-bright sources). We have extracted the stellar velocity dispersion and rotational velocity along our slits to derive the kinematics of the merging galaxies. These quantities enable us to answer the following questions about the evolution of ULIRGs: 1) What are the progenitor mass ratios?, 2) How do the stellar kinematics evolve?, and, 3) Is there a connection between ULIRGs and QSOs? We find that the Ultraluminous phase is mainly triggered by mergers of approximately equal mass galaxies, however, less violent minor mergers (of progenitor mass ratio ~3:1) also exist in our sample. Dynamical heating of the merging hosts is observed as the stellar systematic rotation decreases with time in favour of the increase of random motions. The merger remnants, being dispersion-dominated systems with non-negligible rotation, resemble elliptical galaxies. Placing ULIRGs on the fundamental plane of early-type galaxies shows that they resemble intermediate-mass ellipticals. After the nuclear coalescence, the black hole masses of ULIRGs, calculated from their relation to the host dispersions, are of the order 10^7-10^8 M_sun. To investigate whether ULIRGs go through a QSO phase during their evolution, we perform a similar (preliminary) analysis of the IR-bright-QSO kinematics. We find that the average dispersion of the IR-bright QSOs is similar to that of the ULIRG remnants, indicating that evolutionary links between the two populations may exist.

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arXiv2005

Orbital Parameters of Merging Dark Matter Halos

In order to specify cosmologically motivated initial conditions for major galaxy mergers (mass ratios $\leq$ 4:1) that are supposed to explain the formation of elliptical galaxies we study the orbital parameters of major mergers of cold dark matter halos using a high-resolution cosmological simulation. Almost half of all encounters are nearly parabolic with eccentricities $e \approx 1$ and no correlations between the halo spin planes or the orbital planes. The pericentric argument $ω$ shows no correlation with the other orbital parameters and is distributed randomly. In addition we find that 50 % of typical pericenter distances are larger than half the halo's virial radii which is much larger than typically assumed in numerical simulations of galaxy mergers. In contrast to the usual assumption made in semi-analytic models of galaxy formation the circularities of major mergers are found to be not randomly distributed but to peak around a value of $ε\approx 0.5$. Additionally all results are independent of the minimum progenitor mass and major merger definitions (i.e. mass ratios $\leq$ 4:1; 3:1; 2:1).

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arXiv2005

The Formation of Fossil Galaxy Groups in the hierarchical Universe

We use a set of twelve high-resolution N-body/hydrodynamical simulations in the $Λ$CDM cosmology to investigate the origin and formation rate of fossil groups (FGs), which are X-ray bright galaxy groups dominated by a large elliptical galaxy, with the second brightest galaxy being at least two magnitudes fainter. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star burst driven galactic super winds, effects of a meta-galactic UV field and full stellar population synthesis. We find an interesting correlation between the magnitude gap between the first and second brightest galaxy and the formation time of the group. It is found that FGs have assembled half of their final dark matter mass already at $z\ga1$, and subsequently typically grow by minor merging only, wheras non-FGs on average form later. The early assembly of FGs leaves sufficient time for galaxies of $L \sim L_*$ to merge into the central one by dynamical friction, resulting in the large magnitude gap at $z=0$. A fraction of 33$\pm$16% of the groups simulated are found to be fossil, whereas the observational estimate is $\sim$10-20%. The FGs are found to be X-ray over-luminous relative to non-FGs of the same optical luminosity, in qualitative agreement with observations. Finally, from a dynamical friction analysis is found that only because infall of $L \sim L_*$ galaxies happens along filaments with small impact parameters do FGs exist at all.

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arXiv2005

Formation of Structure in Molecular Clouds: A Case Study

Molecular clouds (MCs) are highly structured and ``turbulent''. Colliding gas streams of atomic hydrogen have been suggested as a possible source of MCs, imprinting the filamentary structure as a consequence of dynamical and thermal instabilities. We present a 2D numerical analysis of MC formation via converging HI flows. Even with modest flow speeds and completely uniform inflows, non-linear density perturbations as possible precursors of MCs arise. Thus, we suggest that MCs are inevitably formed with substantial structure, e.g., strong density and velocity fluctuations, which provide the initial conditions for subsequent gravitational collapse and star formation in a variety of galactic and extragalactic environments.

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arXiv2005

Orbital Structure of Collisionless Merger Remnants: On the Origin of Photometric and Kinematic Properties of Elliptical and S0 Galaxies

We present a detailed investigation of the relation between the orbital content of merger remnants and observable properties of elliptical and S0 galaxies. Our analysis is based on the statistical sample of collisionless mergers of disk galaxies with different mass ratios and orbital parameters, published by Naab & Burkert. We use the spectral method by Carpintero & Aguilar to determine the orbital content of every remnant and correlate it with its intrinsic shape, and its projected kinematic and photometric properties. We discuss the influence of the bulge component and varying pericenter distances. The two most abundant orbit classes are the minor axis tubes and the box orbits. Their ratio seems to determine the basic properties of a remnant. On average, the fraction of minor axis tubes increases by a factor of two from a merger mass ratio of 1:1 to 4:1, whereas the fraction of box orbits decreases by 10%. At a given mass the central velocity dispersion of a remnant scales with the ratio of minor axis tubes to box orbits. Interestingly, the division line between rotational supported systems and pressure supported objects, $(v_{maj}/σ_0)^*=0.7$, turns out to coincide with a box to minor axis tube ratio of unity. The observed $h_3$-$v/σ$ anti-correlation for ellipticals can not be reproduced by collisionless merger remnants. We propose that this can only be reconciled by an additional physical process that significantly reduces the box orbit content. Remnants which are dominated by minor axis tube orbits have predominantly disky projections. Boxy remnants have always a box to minor axis tube ratio larger than one. This study will enable to identify observed ellipticals that could have formed, in the collisionless limit, by gas-poor disk mergers.

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arXiv2004

Three-dimensional Continuum Radiative Transfer Images of a Molecular Cloud Core Evolution

We analyze a three-dimensional smoothed particle hydrodynamics simulation of an evolving and later collapsing pre-stellar core. Using a three-dimensional continuum radiative transfer program, we generate images at 7 micron, 15 micron, 175 micron, and 1.3 mm for different evolutionary times and viewing angles. We discuss the observability of the properties of pre-stellar cores for the different wavelengths. For examples of non-symmetric fragments, it is shown that, misleadingly, the density profiles derived from a one-dimensional analysis of the corresponding images are consistent with one-dimensional core evolution models. We conclude that one-dimensional modeling based on column density interpretation of images does not produce reliable structural information and that multidimensional modeling is required.

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arXiv2004

Monte Carlo simulations of the halo white dwarf population

The interpretation of microlensing results towards the Large Magellanic Cloud (LMC) still remains controversial. Whereas white dwarfs have been proposed to explain these results and, hence, to contribute significantly to the mass budget of our Galaxy, there are as well several constraints on the role played by white dwarfs. In this paper we analyze self-consistently and simultaneously four different results, namely, the local halo white dwarf luminosity function, the microlensing results reported by the MACHO team towards the LMC, the results of Hubble Deep Field (HDF) and the results of the EROS experiment, for several initial mass functions and halo ages. We find that the proposed log-normal initial mass functions do not contribute to solve the problem posed by the observed microlensing events and, moreover, they overproduce white dwarfs when compared to the results of the HDF and of the EROS survey. We also find that the contribution of hydrogen-rich white dwarfs to the dynamical mass of the halo of the Galaxy cannot be more than $\sim 4%$.

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arXiv2004

The Tilt of the Fundamental Plane: Three-quarters Structural Nonhomology, One-quarter Stellar Population

The variation of the mass-to-light ratios M/L of early type galaxies as function of their luminosities L is investigated. It is shown that the tilt beta=0.27 (in the B--band) of the fundamental plane relation M/L ~ L^{beta} can be understood as a combination of two effects: about one-quarter (i.e. dbeta =0.07) is a result of systematic variations of the stellar population properties with increasing luminosity. The remaining three-quarters (i.e. dbeta =0.2) can be completely attributed to nonhomology effects that lead to a systematic change of the surface brightness profiles with increasing luminosity. Consequently, the observed tilt in the K-band (beta=0.17) where stellar population effects are negligible, is explained by nonhomology effects alone. After correcting for nonhomology, the mean value of the mass-to-light ratio of elliptical galaxies (M/L_B) is 7.1+-2.8 (1 sigma scatter).

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arXiv2003

About the morphology of dwarf spheroidal galaxies and their dark matter content

The morphological properties of the Carina, Sculptor and Fornax dwarfs are investigated using new wide field data with a total area of 29 square degrees. The stellar density maps are derived, hinting that Sculptor possesses tidal tails indicating interaction with the Milky Way. Contrary to previous studies we cannot find any sign of breaks in the density profiles for the Carina and Fornax dwarfs. The possible existence of tidal tails in Sculptor and of King limiting radii in Fornax and Carina are used to derive global M/L ratios, without using kinematic data. By matching those M/L ratios to kinematically derived values we are able to constrain the orbital parameters of the three dwarfs. Fornax cannot have M/L smaller than 3 and must be close to its perigalacticon now. The other extreme is Sculptor that needs to be on an orbit with an eccentricity bigger than 0.5 to be able to form tidal tails despite its kinematic M/L.

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arXiv2003

Role of clusters of galaxies in the evolution of the metal budget in the Universe

Using the guidelines on SN element production provided by XMM-Newton, we summarize the results of ASCA observations on the element abundance in groups and clusters of galaxies. We show that while the metal production in groups could be described by a stellar population with a standard local IMF, clusters of galaxies require a more top-heavy IMF. We attribute an excess heavy element production to an IMF evolution with redshift. Dating the galaxy formation in clusters by observations of the star-formation rate, we conclude that the IMF variations have occurred preferentially at z>~4. We further combine our metallicity measurements with the mass function of clusters to estimate the role of clusters in the evolution of the metal content of the Universe. We argue that at no epoch stars are a major container of metals, unless groups of galaxies are not representative for the star-formation. This lends further support for the reduced (0.6 solar) mass-averaged oxygen abundance in the stellar population.

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arXiv2003

A model for the jet-disk connection in BH accreting systems

Based on theoretical arguments and quasi-stationary radiative MHD calculations, a model for an accretion-powered jet is presented. It is argued that accretion disks around BHs consist of 1) a cold, Keplerian-rotating and weakly magnetized mediumin the outer part, 2) a highly advective and turbulent-free plasma inside $r_\mm{tr}=10-20$ Schwarzschild radii, where magnetic fields are predominantly of large scale topology and in excess of thermal equipartition, and 3) an ion-dominated torus in the vicinity of the hole, where magnetic fields undergo a topological change into a monopole like-configuration. The action of magnetic fields interior to $r_\mm{tr}$ is to initiate torsional {\Alfven} waves that extract angular momentum from the disk-plasma and deposit it into the transition layer (-TL) above the disk, where the plasma is dissipative and tenuous. A significant fraction of the shear-generated toroidal magnetic field reconnects in the TL, thereby heating the plasma up to the virial-temperature and forming a super-Keplerian rotating, and hence centrifugally accelerated outflow. The strong magnetic field in the TL forces the electrons to cool rapidly yielding thereby an ion-dominated outflow. The toroidal magnetic field in the TL is in thermal equipartition with the ions, whereas the poloidal component is in equipartition with the electrons. Such a strong toroidal magnetic field is essential for increasing the jet-disk luminosity in the radio regime.

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arXiv2002

Constraints of the Clumpyness of Dark Matter Halos Through Heating of the Disk Galaxies

Motivated by the presence of numerous dark matter clumps in the Milky Way's halo as expected from the cold dark matter cosmological model, we conduct numerical simulations to examine the heating of the disk. We construct an initial galaxy model in equilibrium, with a stable thin disk. The disk interacts with dark matter clumps for about 5 Gyr. Three physical effects are examined : first the mass spectrum of the dark matter clumps, second the initial thickness of the galactic disk, and third the spatial distribution of the clumps. We find that the massive end of the mass spectrum determines the amount of disk heating. Thicker disks suffer less heating. There is a certain thickness at which the heating owing to the interaction with the clumps becomes saturates. We also find that the heating produced by the model which mimics the distribution found in Standard CDM cosmology is significant and too high to explain the observational constraints. On the other hand, our model that corresponds to the clump distribution in a $Λ$CDM cosmology produces no significant heating. This result suggests that the $Λ$CDM cosmology is preferable with respect to the Standard CDM cosmology in explaining the thickness of the Milky Way.

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