1,688 research outputs found
Testing the Least Action Principle in an Omega_0=1 Universe
The least action principle (LAP) is a dynamically rigorous method for
deriving the history of galaxy orbits. In particular it is an Omega_0 test,
predicting current epoch galaxy velocities as a function of position and of the
cosmological background. It is most usefully applied to in--falling structures,
such as the local group, where its application indicates that the preferred
cosmological model is Omega_0 = 0.1 and h=0.75 (h is the Hubble parameter in
units of 100 Km s^-1 Mpc^-1). The method assumes that all the mass acts as if
it were distributed as the visible galaxies. We test the reliability of the LAP
to Local Group-like systems extracted from Omega_0=1 N--body simulations. While
the orbits of the galaxies are qualitatively well reconstructed, the LAP
systematically underestimates the mass of the system. This failure is
attributed to the presence of extended halos weakly clustered around visible
galaxies which prevent a large fraction of the group mass from being detected
by the LAP technique. We conclude that the LAP method cannot rule out an
Omega_0=1 value on the Local Group scale. Better constraints on Omega_0 may be
obtained by applying this technique to in--falling systems, such as clusters,
containing objects with separations large compared to galaxy sizes.Comment: accepted by APJ, uuencoded-compressed-tarred PostScript file
including figures. SISSA Ref. 56/94/
The CFHTLS Deep Catalog of Interacting Galaxies I. Merger Rate Evolution to z=1.2
We present the rest-frame optical galaxy merger fraction between 0.2<z<1.2,
as a function of stellar mass and optical luminosity, as observed by the
Canada-France-Hawaii Telescope Legacy Deep Survey (CFHTLS-Deep). We developed a
new classification scheme to identify major galaxy-galaxy mergers based on the
presence of tidal tails and bridges. These morphological features are signposts
of recent and ongoing merger activity. Through the visual classification of all
galaxies, down to i_vega<22.2 (~27,000 galaxies) over 2 square degrees, we have
compiled the CFHTLS Deep Catalog of Interacting Galaxies, with ~1600 merging
galaxies. We find the merger fraction to be 4.3% +/-0.3% at z~0.3 and 19.0%
+/-2.5% at z~1, implying evolution of the merger fraction going as (1+z)^m,
with m=2.25 +/-0.24. This result is inconsistent with a mild or non-evolving
(m4sigma level of confidence. A mild trend, where massive
galaxies with M>10^10.7 M_sun are undergoing fewer mergers than less massive
systems M~10^10 M_sun), consistent with the expectations of galaxy assembly
downsizing is observed. Our results also show that interacting galaxies have on
average SFRs double that found in non-interacting field galaxies. We conclude
that (1) the optical galaxy merger fraction does evolve with redshift, (2) the
merger fraction depends mildly on stellar mass, with lower mass galaxies having
higher merger fractions at z<1, and (3) star formation is triggered at all
phases of a merger, with larger enhancements at later stages, consistent with
N-body simulations.Comment: e.g.: 17 pages, 14 figures, accepted for publication in Ap
Dark Matter Sub-Halo Counts via Star Stream Crossings
Dark matter sub-halos create gaps in the stellar streams orbiting in the
halos of galaxies. We evaluate the sub-halo stream crossing integral with the
guidance of simulations to find that the linear rate of gap creation, R_gap, in
a typical Cold Dark Matter (CDM) galactic halo at 100 kpc is R_gap = 0.0066
M_8^-0.35 kpc^-1 Gyr^-1$, where M_8 = M/10^8 M_sun, is the minimum mass halo
that creates a visible gap. The relation can be recast entirely in terms of
observables, as R_gap= 0.059 w^-0.85 kpc^-1 Gyr^-1, for w in kpc, normalized at
100 kpc. Using published data, the density of gaps is estimated for M31's NW
stream and the Milky Way Pal 5 stream, Orphan stream, and Eastern Banded
Structure. The estimated rates of gap creation all have errors of 50% or more
due to uncertain dynamical ages and the relatively noisy stream density
measurements. The gap rate-width data are in good agreement with the CDM
predicted relation. The high density of gaps in the narrow streams require a
total halo population of 10^5 sub-halos above a minimum mass of 10^5 M_sun.Comment: ApJ accepted and copy-edite
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