1,594 research outputs found
Velocity Variations in the Phoenix-Hermus Star Stream
Measurements of velocity and density perturbations along stellar streams in the Milky Way provide a time-integrated measure of dark matter substructure at larger galactic radius than the complementary instantaneous inner-halo strong lensing detection of dark matter sub-halos in distant galaxies. An interesting case to consider is the proposed Phoenix–Hermus star stream, which is long, thin, and on a nearly circular orbit, making it a particular good target to study for velocity variations along its length. In the presence of dark matter sub-halos, the stream velocities are significantly perturbed in a manner that is readily understood with the impulse approximation. A set of simulations shows that only sub-halos above a few 10^7 M_⊙ lead to reasonably long-lived observationally detectable velocity variations of amplitude of order 1 km s^(−1), with an average of about one visible hit per (two-armed) stream over a 3 Gyr interval. An implication is that globular clusters themselves will not have a visible impact on the stream. Radial velocities have the benefit of being completely insensitive to distance errors. Distance errors scatter individual star velocities perpendicular and tangential to the mean orbit, but their mean values remain unbiased. Calculations like these help build the quantitative case to acquire large, fairly deep, precision velocity samples of stream stars
Gaps in the GD-1 Star Stream
GD-1 is a long, thin, Milky Way star stream that has readily visible density variations along its length. We quantify the locations, sizes, and statistical significance of the density structure, i.e., gaps, using a set of scaled filters. The shapes of the filters are based on the gaps that develop in simulations of dark matter sub-halos crossing a star stream. The high Galactic latitude 8.4 kpc long segment of GD-1 that we examine has 8 ± 3 gaps of 99% significance or greater, with the error estimated on the basis of tests of the gap-filtering technique. The cumulative distribution of gaps more than three times the width of the stream is in good agreement with predictions for dark matter sub-halo encounters with cold star streams. The number of gaps narrower than three times the width of the GD-1 stream falls well below the cold stream prediction which is taken into account for the gap creation rate integrated over all sizes. Simple warm stream simulations scaled to GD-1 show that the falloff in gaps is expected for sub-halos below a mass of 10^6 M_☉. The GD-1 gaps requires 100 sub-halos >10^6 M_☉ within 30 kpc, the apocenter of GD-1 orbit. These results are consistent with LCDM sub-halo predictions but further improvements in stream signal-to-noise and gap modeling will be welcome
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
A Hubble Space Telescope Snapshot Survey of Dynamically Close Galaxy Pairs in the CNOC2 Redshift Survey
We compare the structural properties of two classes of galaxies at
intermediate redshift: those in dynamically close galaxy pairs, and those which
are isolated. Both samples are selected from the CNOC2 Redshift Survey, and
have redshifts in the range 0.1 < z <0.6. Hubble Space Telescope WFPC2 images
were acquired as part of a snapshot survey, and were used to measure bulge
fraction and asymmetry for these galaxies. We find that paired and isolated
galaxies have identical distributions of bulge fractions. Conversely, we find
that paired galaxies are much more likely to be asymmetric (R_T+R_A >= 0.13)
than isolated galaxies. Assuming that half of these pairs are unlikely to be
close enough to merge, we estimate that 40% +/- 11% of merging galaxies are
asymmetric, compared with 9% +/- 3% of isolated galaxies. The difference is
even more striking for strongly asymmetric (R_T+R_A >= 0.16) galaxies: 25% +/-
8% for merging galaxies versus 1% +/- 1% for isolated galaxies. We find that
strongly asymmetric paired galaxies are very blue, with rest-frame B-R colors
close to 0.80, compared with a mean (B-R)_0 of 1.24 for all paired galaxies. In
addition, asymmetric galaxies in pairs have strong [OII]3727 emission lines. We
conclude that close to half of the galaxy pairs in our sample are in the
process of merging, and that most of these mergers are accompanied by triggered
star formation.Comment: Accepted for publication in the Astronomical Journal. 40 pages,
including 15 figures. For full resolution version, please see
http://www.trentu.ca/physics/dpatton/hstpairs
Catalog of Galaxy Morphology in Four Rich Clusters: Luminosity Evolution of Disk Galaxies at 0.33<z<0.83
Hubble Space Telescope (HST) imaging of four rich, X-ray luminous, galaxy
clusters (0.33<z<0.83) is used to produce quantitative morphological
measurements for galaxies in their fields. Catalogs of these measurements are
presented for 1642 galaxies brighter than F814W(AB)=23.0 . Galaxy luminosity
profiles are fitted with three models: exponential disk, de Vaucouleurs bulge,
and a disk-plus-bulge hybrid model. The best fit is selected and produces a
quantitative assessment of the morphology of each galaxy: the principal
parameters derived being B/T, the ratio of bulge to total luminosity, the scale
lengths and half-light radii, axial ratios, position angles and surface
brightnesses of each component. Cluster membership is determined using a
statistical correction for field galaxy contamination, and a mass normalization
factor (mass within boundaries of the observed fields) is derived for each
cluster. In the present paper, this catalog of measurements is used to
investigate the luminosity evolution of disk galaxies in the rich-cluster
environment. Examination of the relations between disk scale-length and central
surface brightness suggests, under the assumption that these clusters represent
a family who share a common evolutionary history and are simply observed at
different ages, that there is a dramatic change in the properties of the small
disks (h < 2 kpc). This change is best characterized as a change in surface
brightness by about 1.5 magnitude between z=0.3 and z=0.8 with brighter disks
at higher redshifts.Comment: 53 pages, including 13 figures and 7 tables. Accepted for publication
in the Astrophysical Journal Supplement Serie
New Techniques for Relating Dynamically Close Galaxy Pairs to Merger and Accretion Rates : Application to the SSRS2 Redshift Survey
We introduce two new pair statistics, which relate close galaxy pairs to the
merger and accretion rates. We demonstrate the importance of correcting these
(and other) pair statistics for selection effects related to sample depth and
completeness. In particular, we highlight the severe bias that can result from
the use of a flux-limited survey. The first statistic, denoted N_c, gives the
number of companions per galaxy, within a specified range in absolute
magnitude. N_c is directly related to the galaxy merger rate. The second
statistic, called L_c, gives the total luminosity in companions, per galaxy.
This quantity can be used to investigate the mass accretion rate. Both N_c and
L_c are related to the galaxy correlation function and luminosity function in a
straightforward manner. We outline techniques which account for various
selection effects, and demonstrate the success of this approach using Monte
Carlo simulations. If one assumes that clustering is independent of luminosity
(which is appropriate for reasonable ranges in luminosity), then these
statistics may be applied to flux-limited surveys.
These techniques are applied to a sample of 5426 galaxies in the SSRS2
redshift survey. Using close dynamical pairs, we find N_c(-21<M_B<-18) =
0.0226+/-0.0052 and L_c(-21<M_B<-18) = 0.0216+/-0.0055 10^{10} h^2 L_sun at
z=0.015. These are the first secure estimates of low-z close pair statistics.
If N_c remains fixed with redshift, simple assumptions imply that ~ 6.6% of
present day galaxies with -21<M_B<-18 have undergone mergers since z=1. When
applied to redshift surveys of more distant galaxies, these techniques will
yield the first robust estimates of evolution in the galaxy merger and
accretion rates. [Abridged]Comment: 26 pages (including 10 postscript figures) plus 3 gif figures.
Accepted for publication in ApJ. Paper (including full resolution images)
also available at http://www.astro.utoronto.ca/~patton/ssrs2, along with
associated pair classification experiment (clickable version of Figure 5
Evolution of galactic disks in clusters and the field at in the CNOC survey
Two-dimensional surface photometry is presented for a sample of 351 late-type
galaxies with . These objects are drawn from the Canadian
Network for Observational Cosmology (CNOC) cluster survey and are either
spectroscopically confirmed members of clusters at (64 galaxies),
(45), and (36) or field galaxies with similar redshifts. Galaxies
in the rich cluster Abell 2256 at were also analyzed with the same
methods to provide a local reference point. At redshifts of (0.23, 0.43, 0.55)
the disk surface brightness in cluster late-type galaxies is higher in the
-band by )
mag, respectively, relative to the Freeman (1970) constant surface-brightness
relation; whereas disks in cluster galaxies at are consistent with
that relation. Field galaxies show a progressive disk-brightening with redshift
that is consistent with that seen in the cluster population. Taken together
with similar measurements of early-type galaxies (Schade et al. 1996a), these
results suggest that the evolution of the field and cluster galaxy populations
are similar, although we emphasize that our sample of cluster galaxies is
dominated by objects at large distances (up to 3 Mpc) from the dense cluster
core, so that the implications of these findings with respect to the
Butcher-Oemler effect and the morphology-density relation will not be clear
until an analysis of galaxy properties as a function of cluster-centric
distance is completed.Comment: accepted for ApJ Letters, also available at
http://manaslu.astro.utoronto.ca/~carlberg/cnoc/disk
The Luminosity Function of Field Galaxies in the CNOC1 Redshift Survey
We have computed the luminosity function for 389 field galaxies from the
Canadian Network for Observational Cosmology cluster redshift survey (CNOC1),
over redshifts z = 0.2-0.6. We find Schechter parameters M^* - 5 log h = -19.6
\pm 0.3 and \alpha = -0.9 \pm 0.2 in rest-frame B_{AB}. We have also split our
sample at the color of a redshifted but nonevolving Sbc galaxy, and find
distinctly different luminosity functions for red and blue galaxies. Red
galaxies have a shallow slope \alpha \approx -0.4 and dominate the bright end
of the luminosity function, while blue galaxies have a steep \alpha \approx
-1.4 and prevail at the faint end. Comparisons of the CNOC1 results to those
from the Canada-France (CFRS) and Autofib redshift surveys show broad agreement
among these independent samples, but there are also significant differences
which will require larger samples to resolve. Also, in CNOC1 the red galaxy
luminosity density stays about the same over the range z = 0.2-0.6, while the
blue galaxy luminosity density increases steadily with redshift. These results
are consistent with the trend of the luminosity density vs. redshift relations
seen in the CFRS, though the normalizations of the luminosity densities appear
to differ for blue galaxies. Comparison to the local luminosity function from
the Las Campanas redshift survey (LCRS) shows that the luminosity density at z
\approx 0.1 is only about half that seen at z \approx 0.4. A change in the
luminosity function shape, particularly at the faint end, appears to be
required to match the CNOC1 and LCRS luminosity functions, if galaxy evolution
is the sole cause of the differences seen. However, it should be noted that the
specific details of the construction of different surveys may complicate the
comparison of results and so may need to be considered carefully.Comment: 22 pages, including 6 postscript figures, uses AASTEX v4.0 style
files. Corrected minor typos and updated references. Results and conclusions
unchanged. Final version to appear in the Astrophysical Journa
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