127 research outputs found
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
Sub-Halo Spreading of Thin Tidal Star Streams
Dark matter sub-halos that pass near or through a thin tidal star stream
locally increase its velocity dispersion. Subsequent orbital evolution further
increases the velocity dispersion and stream width, lowering the surface
density of a stream. The kinematic properties of streams are measured in
cosmological Milky Way-like halo simulations. The distance along a stream is a
proxy for the time a star has spent in the stream, although there are a range
of ages at any distance. Power law fits to the velocity dispersion with angular
distance for the average of the streams in the 10-60 kpc range finds
sigma_theta=6 phi^{0.25} km/s, sigma_phi=8 phi^{0.39} km/s, and sigma_r=10
phi^{0.44} km/s for |phi|< 34 degrees, for stars within theta=+/-5 degrees of
the stream equator. The errors of the coefficients are about 10% for these
streams, with comparable systematic errors depending on exactly which streams
are selected and the stream width and length fitted. The stream velocity
dispersions close to the clusters generally increase with the sub-halo numbers
What a Tangled Web We Weave: Hermus as the Northern Extension of the Phoenix Stream
We investigate whether the recently discovered Phoenix stream may be part of a much longer stream that includes the previously discovered Hermus stream. Using a simple model of the Galaxy with a disk, bulge, and a spherical dark matter halo, we show that a nearly circular orbit, highly inclined with respect to the disk, can be found that fits the positions, orientations, and distances of both streams. While the two streams are somewhat misaligned in the sense that they do not occupy the same plane, nodal precession due to the Milky Way disk potential naturally brings the orbit into line with each stream in the course of half an orbit. We consequently consider a common origin for the two streams as plausible. Based on our best-fitting orbit, we make predictions for the positions, distances, radial velocities, and proper motions along each stream. If our hypothesis is borne out by measurements, then at ≈183° (≈235° with respect to the Galactic center) and ≈76 kpc in length, Phoenix–Hermus would become the longest cold stream yet found. This would make it a particularly valuable new probe of the shape and mass of the Galactic halo out to ≈20 kpc
Simulating Globular Clusters in Dark Matter Subhalos
A cosmological zoom-in simulation that develops into a Milky Way-like halo begins at redshift 7. The initial dark matter distribution is seeded with dense star clusters of median mass 5 × 105 M o˙, placed in the largest subhalos present, which have a median peak circular velocity of 25 km s-1. Three simulations are initialized using the same dark matter distribution with the star clusters starting on approximately circular orbits having initial median radii 6.8, 0.14 kpc, and, at the exact center of the subhalos. The simulations are evolved to the current epoch at which time the median galactic orbital radii of the three sets of clusters are 30, 5, and 16 kpc, with the clusters losing about 2%, 50%, and 15% of their mass, respectively. Clusters beginning at small orbital radii have so much tidal forcing that they are often not in equilibrium. Clusters that start at larger subhalo radii have a velocity dispersion that declines smoothly to ≃20% of the central value at ≃20 half-mass radii. The clusters that begin in the subhalo centers can show a rise in velocity dispersion beyond 3-5 half-mass radii. That is, the clusters that form without local dark matter always have stellar-mass-dominated kinematics at all radii, whereas about 25% of the clusters that begin in subhalo centers have remnant local dark matter
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