SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation

The intrinsic anisotropy delta and flattening epsilon of simulated merger remnants is compared with elliptical galaxies that have been observed by the SAURON collaboration, and that were analysed using axisymmetric Schwarzschild models. Collisionless binary mergers of stellar disks and disk mergers with an additional isothermal gas component, neglecting star formation cannot reproduce the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is however found for merger simulations of disks with gas fractions >= 20%, including star formation and stellar energy feedback. Massive black hole feedback does not strongly affect this result. Subsequent dry merging of merger remnants however does not generate the slowly-rotating SAURON ellipticals which are characterized by low ellipticities epsilon < 0.25 and low anisotropies. This indicates that at least some ellipticals on the red galaxy sequence did not form by binary mergers of disks or early-type galaxies. We show that stellar spheroids resulting from multiple, hierarchical mergers of star-bursting subunits in a cosmological context are in excellent agreement with the low ellipticities and anisotropies of the slowly rotating SAURON ellipticals and their observed trend of delta with epsilon. The numerical simulations indicate that the SAURON relation might be a result of strong violent relaxation and phase mixing of multiple, kinematically cold stellar subunits with the angular momentum of the system determining its location on the relation.Comment: 13 pages, 3 figures, submitted to Ap

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.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution figures to the author

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 $\sim 4$.Comment: 17 pages, 10 figures; accepted for publication in Astronomy and Astrophysic

Have Pentaquark States Been seen?

The status of the search for pentaquark baryons is reviewed in light of new results from the first two dedicated experiments from CLAS at Jefferson Lab and of new analyses from several laboratories on the $Theta^+(1540)$. Evidence for and against two heavier pentaquark states is also discussed.Comment: Added some references, corrected typo

Cloud Dispersal in Turbulent Flows

Cold clouds embedded in warm media are very common objects in astrophysics. Their disruption timescale depends strongly on the dynamical configuration. We discuss the evolution of an initially homogeneous cold cloud embedded in warm turbulent gas. Within a couple of dynamical timescales, the filling factor of the cold gas within the original cloud radius drops below 50%. Turbulent diffusivities estimated from the time evolution of radial filling factor profiles are not constant with time. Cold and warm gas are bodily transported by turbulence and mixed. This is only mildly indicated by column density maps. The radiation field within the cloud, however, increases by several orders of magnitudes due to the mixing, with possible consequences for cloud chemistry and evolution within a few dynamical timescales.Comment: 11 pages, 12 figures, accepted by MNRA

A light-front description of electromagnetic form factors for J≤3/2J \leq {3/2} hadrons

A review of the hadron electromagnetic form factors obtained in a light-front constituent quark model, based on the eigenfunctions of a mass operator, is presented. The relevance of different components in the q-q interaction for the description of hadron experimental form factors is analysed.Comment: 6 pages, Latex, 3 Postscript figures included. Proceedings of "Nucleon 99", Frascati, June 1999. To appear in Nucl. Phys.

Comment on The Evidence for a Pentaquark and Kinematic Reflections

The Regge exchange model used by Dzierba et al. is shown to be questionable, since the pion pole term is not allowed. Hence the Regge amplitudes in their calculation are exaggerated. The amount of kinematic reflection in the mass spectrum of the (nK+) system, which is one decay channel of a possible pentaquark, is not well justified in the fitting procedure used by Dzierba et al., as shown by comparison with the (K+K-) invariant mass spectrum, which is one decay channel of the a_2 and f_2 tensor mesons. While kinematic reflections are still a concern in some papers that have presented evidence for the pentaquark, better quantitative calculations are needed to demonstrate the significance of this effect.Comment: Comment submitted to Phys. Rev. D (no figures

Magnetized Non-linear Thin Shell Instability: Numerical Studies in 2D

We revisit the analysis of the Non-linear Thin Shell Instability (NTSI) numerically, including magnetic fields. The magnetic tension force is expected to work against the main driver of the NTSI -- namely transverse momentum transport. However, depending on the field strength and orientation, the instability may grow. For fields aligned with the inflow, we find that the NTSI is suppressed only when the Alfv\'en speed surpasses the (supersonic) velocities generated along the collision interface. Even for fields perpendicular to the inflow, which are the most effective at preventing the NTSI from developing, internal structures form within the expanding slab interface, probably leading to fragmentation in the presence of self-gravity or thermal instabilities. High Reynolds numbers result in local turbulence within the perturbed slab, which in turn triggers reconnection and dissipation of the excess magnetic flux. We find that when the magnetic field is initially aligned with the flow, there exists a (weak) correlation between field strength and gas density. However, for transverse fields, this correlation essentially vanishes. In light of these results, our general conclusion is that instabilities are unlikely to be erased unless the magnetic energy in clouds is much larger than the turbulent energy. Finally, while our study is motivated by the scenario of molecular cloud formation in colliding flows, our results span a larger range of applicability, from supernovae shells to colliding stellar winds.Comment: 12 pages, 17 figures, some of them at low resolution. Submitted to ApJ, comments welcom