1,315 research outputs found
Where are the ``Missing'' Galactic Baryons?
Based on 19 high-resolution N-body/gas-dynamical galaxy formation simulations
in the LCDM cosmology it is shown, that for a galaxy like the Milky Way, in
addition to the baryonic mass of the galaxy itself, about 70% extra baryonic
mass should reside around the galaxy (inside of the virial radius), chiefly in
the form of hot gas. Averaging over the entire field galaxy population, this
``external'' component amounts to 64-85% of the baryonic mass of the population
itself. These results are supported by the recent detection of very extended,
soft X-ray emission from the halo of the quiescent, massive disk galaxy NGC
5746.
Some of the hot gas may, by thermal instability, have condensed into mainly
pressure supported, warm clouds, similar to the Galactic High Velocity Clouds
(HVCs). Based on an ultra-high resolution cosmological test simulation of a
Milky Way like galaxy (with a gas particle mass and gravity softening length of
only 7600 h^-1 Msun and 83 h^-1 pc,respectively), it is argued, that the hot
gas phase dominates over the warm gas phase, in the halo. Finally, an origin of
HVCs as ``leftovers'' from filamentary, ``cold'' accretion events, mainly
occurring early in the history of galaxies, is proposed.Comment: 4 Pages, 4 Figures, Submitted to ApJL, Printing in colour recommende
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
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
The chemical evolution of gas-rich dwarf galaxies
A numerical double burst model of the chemical evolution of gas-rich dwarf
galaxies has been developed. The model is fitted to a sample of N/O, O/H, Y and
gas fraction observations, where N/O and O/H are the relative abundances by
number of nitrogen to oxygen and oxygen to hydrogen, respectively. Y is the
abundance by mass of helium. Closed models as well as models including enriched
outflow, ordinary outflow and ordinary outflow combined with inflow are
considered.The bursts are assumed to be instantaneous but ordered in pairs to
explain the scatter in N/O-O/H. The method of gas fraction fitting is revised,
and it is found that it is very important to specify whether dwarf irregulars
(dIrrs) or blue compact galaxies (BCGs) are considered. Effective enriched
winds fail when fitting N/O, whereas closed models, models with ordinary winds
or a combination of ordinary winds and inflow are all viable.Comment: 22 pages, 25 figures, MNRAS LaTeX forma
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