6,238 research outputs found
Viking '75 spacecraft design and test summary. Volume 1: Lander design
The Viking Mars program is summarized. The design of the Viking lander spacecraft is described
Viking '75 spacecraft design and test summary. Volume 3: Engineering test summary
The engineering test program for the lander and the orbiter are presented. The engineering program was developed to achieve confidence that the design was adequate to survive the expected mission environments and to accomplish the mission objective
The Properties of Satellite Galaxies in External Systems. I. Morphology and Structural Parameters
We present the first results of an ongoing project to study the
morphological, kinematical, dynamical, and chemical properties of satellite
galaxies of external giant spiral galaxies. The sample of objects has been
selected from the catalogue by Zaritsky et al. (1997). The paper analyzes the
morphology and structural parameters of a subsample of 60 such objects. The
satellites span a great variety of morphologies and surface brightness
profiles. About two thirds of the sample are spirals and irregulars, the
remaining third being early-types. Some cases showing interaction between pairs
of satellites are presented and briefly discussed.Comment: Accepted for publication in Astrophys. Journal Supp. Se
Tweets as impact indicators: Examining the implications of automated bot accounts on Twitter
This brief communication presents preliminary findings on automated Twitter
accounts distributing links to scientific papers deposited on the preprint
repository arXiv. It discusses the implication of the presence of such bots
from the perspective of social media metrics (altmetrics), where mentions of
scholarly documents on Twitter have been suggested as a means of measuring
impact that is both broader and timelier than citations. We present preliminary
findings that automated Twitter accounts create a considerable amount of tweets
to scientific papers and that they behave differently than common social bots,
which has critical implications for the use of raw tweet counts in research
evaluation and assessment. We discuss some definitions of Twitter cyborgs and
bots in scholarly communication and propose differentiating between different
levels of engagement from tweeting only bibliographic information to discussing
or commenting on the content of a paper.Comment: 9 pages, 4 figures, 1 tabl
Anisotropic Distribution of SDSS Satellite Galaxies: Planar (not Polar) Alignment
The distribution of satellite galaxies relative to isolated host galaxies in
the Sloan Digital Sky Survey (SDSS) is investigated. Host-satellite systems are
selected using three different methods, yielding samples of ~3300, ~1600, and
\~950 satellites. In the plane of the sky, the distributions of all three
samples show highly significant deviations from circular symmetry (> 99.99%, >
99.99%, and 99.79% confidence levels, respectively), and the degree of
anisotropy is a strong function of the projected radius, r_p, at which the
satellites are found. For r_p < 100 kpc, the SDSS satellites are aligned
preferentially with the major axes of the hosts. This is in stark contrast to
the Holmberg effect, in which satellites are aligned with the minor axes of
host galaxies. The degree of anisotropy in the distribution of the SDSS
satellites decreases with r_p and is consistent with an isotropic distribution
at of order the 1-sigma level for 250 kpc < r_p < 500 kpc.Comment: ApJ Letters (in press); Discussion section substantially revised,
SDSS DR3 included in the analysis, no significant changes to the result
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
Substructure around M31 : Evolution and Effects
We investigate the evolution of a population of 100 dark matter satellites
orbiting in the gravitational potential of a realistic model of M31. We find
that after 10 Gyr, seven subhalos are completely disrupted by the tidal field
of the host galaxy. The remaining satellites suffer heavy mass loss and
overall, 75% of the mass initially in the subhalo system is tidally stripped.
Not surprisingly, satellites with pericentric radius less than 30 kpc suffer
the greatest stripping and leave a complex structure of tails and streams of
debris around the host galaxy. Assuming that the most bound particles in each
subhalo are kinematic tracers of stars, we find that the halo stellar
population resulting from the tidal debris follows an r^{-3.5} density profile
at large radii. We construct B-band photometric maps of stars coming from
disrupted satellites and find conspicuous features similar both in morphology
and brightness to the observed Giant Stream around Andromeda. An assumed star
formation efficiency of 5-10% in the simulated satellite galaxies results in
good agreement with the number of M31 satellites, the V-band surface brightness
distribution, and the brightness of the Giant Stream. During the first 5 Gyr,
the bombardment of the satellites heats and thickens the disk by a small
amount. At about 5 Gyr, satellite interations induce the formation of a strong
bar which, in turn, leads to a significant increase in the velocity dispersion
of the disk.Comment: 45 pages, 18 figures. To be submitted to the Astrophysical Journal,
version 2.0 : scale height value corrected, references added, and some
figures have been modifie
The Structure of the Outer Halo of the Galaxy and its Relationship to Nearby Large-Scale Structure
We present evidence to support an earlier indication that the Galaxy is
embedded in an extended, highly inclined, triaxial halo outlined by the spatial
distribution of companion galaxies to the Milky Way. Signatures of this spatial
distribution are seen in 1) the angular variation of the radial-velocity
dispersion of the companion galaxies, 2) the spatial distribution of the M~31
sub-group of galaxies, 3) the spatial distribution of the isolated, mainly
dwarf irregular, galaxies of the Local Group, 4) the velocity anisotropy
quadrupole of a sub-group of high-velocity clouds, and 5) the spatial
distribution of galaxies in the Coma-Sculptor cloud. Tidal effects of M~31 and
surrounding galaxies on the Galaxy are not strong enough to have affected the
observed structure. We conclude that this distribution is a reflection of
initial conditions. A simple galaxy formation scenario is proposed which ties
together the results found here with those of Holmberg (1969) and Zaritsky et
al. (1997) on the peculiar distribution of satellites around a large sample of
spiral galaxies.Comment: Accepted for publication in the Astron J., March 2000, 12 pages with
1 figur
Plant Community Assessment and Management Recommendations for Minneapolis Park Natural Areas
In 2017 the Minneapolis Park and Recreation Board (MPRB) began a two-phase study to collect quantitative and qualitative data for urban park natural areas in Minneapolis, MN parks to inform management activities. The first phase took existing GIS data and quality ranking systems and tailored them to the Minneapolis park system. The second phase, which is still in process, involves field checking the data, applying the quality ranking system, and writing a management plan
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