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

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

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.Comment: 9 pages, 7 figures, accepted by A&

QCD radiative and power corrections and Generalized GDH sum rules

We extend the earlier suggested QCD-motivated model for the $Q^2$-dependence of the generalized Gerasimov-Drell-Hearn (GDH) sum rule which assumes the smooth dependence of the structure function $g_T$, while the sharp dependence is due to the $g_2$ contribution and is described by the elastic part of the Burkhardt-Cottingham sum rule. The model successfully predicts the low crossing point for the proton GDH integral, but is at variance with the recent very accurate JLAB data. We show that, at this level of accuracy, one should include the previously neglected radiative and power QCD corrections, as boundary values for the model. We stress that the GDH integral, when measured with such a high accuracy achieved by the recent JLAB data, is very sensitive to QCD power corrections. We estimate the value of these power corrections from the JLAB data at $Q^2 \sim 1 {GeV}^2$. The inclusion of all QCD corrections leads to a good description of proton, neutron and deuteron data at all $Q^2$.Comment: 10 pages, 4 figures (to be published in Physical Review D

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.Comment: 12 pages, no figures, submitted to ApJ Letter

Photometric Variability Among the Brightest Asymptotic Giant Branch Stars Near the Center of M32

Deep K' images with 0.1 arcsec angular resolution, obtained with ALTAIR+NIRI on Gemini North, are used to investigate photometric variablity among the brightest asymptotic giant branch (AGB) stars in the central regions of M32. Based on a comparison with brightnesses obtained from the K-band data discussed by Davidge et al. (2000, ApJ, 545, L89), it is concluded that (1) at least 60% of bright AGB stars near the center of M32 are photometrically variable, and (2) the amplitudes of the light variations are similar to those of long period variables in the Galactic bulge. We do not find evidence for a population of large amplitude variables, like those detected by IRAS in the Galactic bulge. The technique discussed here may prove useful for conducting an initial reconnaisance of photometric variability among AGB stars in spheroids in the Virgo cluster and beyond, where the required long exposure times may restrict observations to only a few epochs.Comment: 8 pages of text, 3 postscript figures. ApJ (letters) in pres

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

The Validity of the Adiabatic Contraction Approximation for Dark Matter Halos

We use high resolution numerical simulations to investigate the adiabatic contraction of dark matter halos with a Hernquist density profile. We test the response of the halos to the growth of additional axisymmetric disk potentials with various central concentrations and the spherically symmetric potential of a softened point mass. Adding the potentials on timescales that are long compared to the dynamical time scale of the halo, the contracted halos have density profiles that are in excellent agreement with analytical predictions based on the conservation of the adiabatic invariant $M(r)r$. This is surprising as this quantity is strictly conserved only for particles on circular orbits and in spherically symmetric potentials. If the same potentials are added on timescales that are short compared to the dynamical timescale, the result depends strongly on the adopted potential. The adiabatic approximation still works for disk potentials. It does, however, fail for the central potential.Comment: 7 pages, 3 figures, 1 table. Added reference. Accepted for publication in ApJ