12,979 research outputs found
The galaxy counterpart of the high-metallicity and 16 kpc impact parameter DLA towards Q0918+1636 - a challenge to galaxy formation models?
The quasar Q0918+1636 (z=3.07) has an intervening high-metallicity Damped
Lyman-alpha Absorber (DLA) along the line of sight, at a redshift of z=2.58.
The DLA is located at a large impact parameter of 16.2 kpc, and has an almost
solar metallicity. It is shown, that a novel type of cosmological galaxy
formation models, invoking a new SNII feedback prescription, the Haardt & Madau
(2012) UVB field and explicit treatment of UVB self-shielding, can reproduce
the observed characteristics of the DLA. UV radiation from young stellar
populations in the galaxy, in particular in the photon energy range 10.36-13.61
eV (relating to Sulfur II abundance), are also considered in the analysis. It
is found that a) for L~L* galaxies (at z=2.58), about 10% of the sight-lines
through the galaxies at impact parameter 16.2 kpc will display a Sulfur II
column density N(SII) 10 cm (the observed value for the
DLA), and b) considering only cases where a near-solar metallicity will be
detected at 16.2 kpc impact parameter, the probability distribution of galaxy
SFR peaks near the value observed for the DLA galaxy counterpart of ~27
Msun/yr. It is argued, that the bulk of the alpha-elements, like Sulfur, traced
by the high metal column density, b=16.2 kpc absorption lines, were produced by
evolving young stars in the inner galaxy, and later transported outward by
galactic winds.Comment: 22 pages, 24 figures, MNRAS in pres
CDM, Feedback and the Hubble Sequence
We have performed TreeSPH simulations of galaxy formation in a standard LCDM
cosmology, including effects of star formation, energetic stellar feedback
processes and a meta-galactic UV field, and obtain a mix of disk, lenticular
and elliptical galaxies. The disk galaxies are deficient in angular momentum by
only about a factor of two compared to observed disk galaxies. The stellar
disks have approximately exponential surface density profiles, and those of the
bulges range from exponential to r^{1/4}, as observed. The bulge-to-disk ratios
of the disk galaxies are consistent with observations and likewise are their
integrated B-V colours, which have been calculated using stellar population
synthesis techniques. Furthermore, we can match the observed I-band
Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk
galaxies, (M/L_I), is about 0.8. The ellipticals and lenticulars have
approximately r^{1/4} stellar surface density profiles, are dominated by
non-disklike kinematics and flattened due to non-isotropic stellar velocity
distributions, again consistent with observations.Comment: 6 pages, incl. 4 figs. To appear in the proceedings of the
EuroConference "The Evolution of Galaxies: II - Basic Building Blocks", Ile
de La Reunion (France), 16-21 October 2001 (Slightly updated version). A much
more comprehensive paper about this work with links to pictures of some of
the galaxies can be found at http://babbage.sissa.it/abs/astro-ph/020436
INCOMPOSITA: a MADS-box gene controlling prophyll development and floral meristem identity in Antirrhinum
Spectral density of the Dirac operator in two-flavour QCD
We compute the spectral density of the (Hermitean) Dirac operator in Quantum
Chromodynamics with two light degenerate quarks near the origin. We use
CLS/ALPHA lattices generated with two flavours of O(a)-improved Wilson fermions
corresponding to pseudoscalar meson masses down to 190 MeV, and with spacings
in the range 0.05-0.08 fm. Thanks to the coverage of parameter space, we can
extrapolate our data to the chiral and continuum limits with confidence. The
results show that the spectral density at the origin is non-zero because the
low modes of the Dirac operator do condense as expected in the Banks-Casher
mechanism. Within errors, the spectral density turns out to be a constant
function up to eigenvalues of approximately 80 MeV. Its value agrees with the
one extracted from the Gell-Mann-Oakes-Renner relation
Chiral symmetry breaking in QCD Lite
A distinctive feature of the presence of spontaneous chiral symmetry breaking
in QCD is the condensation of low modes of the Dirac operator near the origin.
The rate of condensation must be equal to the slope of (Mpi^2 Fpi^2)/2 with
respect to the quark mass m in the chiral limit, where Mpi and Fpi are the mass
and the decay constant of the Nambu-Goldstone bosons. We compute the spectral
density of the (Hermitian) Dirac operator, the quark mass, the pseudoscalar
meson mass and decay constant by numerical simulations of lattice QCD with two
light degenerate Wilson quarks. We use CLS lattices at three values of the
lattice spacing in the range 0.05-0.08 fm, and for several quark masses
corresponding to pseudoscalar mesons masses down to 190 MeV. Thanks to this
coverage of parameters space, we can extrapolate all quantities to the chiral
and continuum limits with confidence. The results show that the low quark modes
do condense in the continuum as expected by the Banks-Casher mechanism, and the
rate of condensation agrees with the Gell-Mann-Oakes-Renner (GMOR) relation.
For the renormalisation-group-invariant ratios we obtain [\Sigma^RGI]^(1/3)/F
=2.77(2)(4) and Lambda^MSbar/F = 3.6(2), which correspond to [\Sigma^\MSbar(2
GeV)]^(1/3) =263(3)(4) MeV and F=85.8(7)(20) MeV if FK is used to set the scale
by supplementing the theory with a quenched strange quark.Comment: 4 pages, 3 figures, 1 tabl
Reactive Atom Plasma (RAP) figuring machine for meter class optical surfaces
A new surface figuring machine called Helios 1200 is presented in this paper. It is designed for the figuring of meter sized optical surfaces with form accuracy correction capability better than 20 nm rms within a reduced number of iterations. Unlike other large figuring facilities using energy beams, Helios 1200 operates a plasma torch at atmospheric pressure, offers a high material removal rate, and a relatively low running cost. This facility is ideal to process large optical components, lightweight optics, silicon based and difficult to machine materials, aspheric, and free form surfaces. Also, the surfaces processed by the reactive atom plasma (RAP) are easy to fine polish through hand conventional sub-aperture polishing techniques. These unique combined features lead to a new capability for the fabrication of optical components opening up novel design possibilities for optical engineers. The key technical features of this large RAP machine are fast figuring capabilities, non-contact material removal tool, the use of a near Gaussian footprint energy beam, and a proven tool path strategy for the management of the heat transfer. Helios 1200 complies with the European machine safety standard and can be used with different types of reactive gases using either fluorine or chlorine compounds. In this paper, first the need for large optical component is discussed. Then, the RAP facility is described: radio frequency R.F generator, plasma torch, and 3 axis computer numerically controlled motion system. Both the machine design and the performance of the RAP tool is assessed under specific production conditions and in the context of meter class mirror and lens fabrication
Lyman alpha Resonant Scattering in Young Galaxies - Predictions from Cosmological Simulations
We present results obtained with a 3D, Ly alpha radiative transfer code,
applied to a fully cosmological galaxy formation simulation. The developed
Monte Carlo code is capable of treating an arbitrary distribution of source Ly
alpha emission, neutral hydrogen density, temperature, and peculiar velocity of
the interstellar medium. We investigate the influence of resonant scattering on
the appearance and properties of young galaxies by applying the code to a
simulated "Lyman Break Galaxy" at redshift z = 3.6, and of star formation rate
22 M_sun/yr and total Ly alpha luminosity 2.0 X 10^43 erg/s. It is found that
resonant scattering of Ly alpha radiation can explain that young galaxies
frequently are observed to be more extended on the sky in Ly alpha than in the
optical. Moreover, it is shown that, for the system investigated, due to the
anisotropic escape of the photons, the appearent maximum surface brightness can
differ by a factor of ~15, and the total derived luminosity by a factor of ~4,
depending on the orientation of the system relative to the observer.Comment: Letter updated to match version published in Ap
A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios
We re-analyse the kinematics of the system of blue horizontal branch field
(BHBF) stars in the Galactic halo (in particular the outer halo), fitting the
kinematics with the model of radial and tangential velocity dispersions in the
halo as a function of galactocentric distance r proposed by Sommer-Larsen,
Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF
stars. The basic result is that the character of the stellar halo velocity
ellipsoid changes markedly from radial anisotropy at the sun to tangential
anisotropy in the outer parts of the Galactic halo (r greater than approx 20
kpc). Specifically, the radial component of the stellar halo's velocity
ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/-
10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The
rapid decrease in the radial velocity dispersion is matched by an increase in
the tangential velocity dispersion, with increasing r.
Our results may indicate that the Galaxy formed hierarchically (partly or
fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation
scenario, which for quite a while has been favoured by most theorists and
recently also has been given some observational credibility by HST observations
of a potential group of small galaxies, at high redshift, possibly in the
process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical
Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm
Supersonic flow calculation using a Reynolds-stress and an eddy thermal diffusivity turbulence model
A second-order model for the velocity field and a two-equation model for the temperature field are used to calculate supersonic boundary layers assuming negligible real gas effects. The modeled equations are formulated on the basis of an incompressible assumption and then extended to supersonic flows by invoking Morkovin's hypothesis, which proposes that compressibility effects are completely accounted for by mean density variations alone. In order to calculate the near-wall flow accurately, correction functions are proposed to render the modeled equations asymptotically consistent with the behavior of the exact equations near a wall and, at the same time, display the proper dependence on the molecular Prandtl number. Thus formulated, the near-wall second order turbulence model for heat transfer is applicable to supersonic flows with different Prandtl numbers. The model is validated against flows with different Prandtl numbers and supersonic flows with free-stream Mach numbers as high as 10 and wall temperature ratios as low as 0.3. Among the flow cases considered, the momentum thickness Reynolds number varies from approximately 4,000 to approximately 21,000. Good correlation with measurements of mean velocity, temperature, and its variance is obtained. Discernible improvements in the law-of-the-wall are observed, especially in the range where the big-law applies
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