10,879 research outputs found
A Primeval Magellanic Stream and Others
The Magellanic Stream might have grown out of tidal interactions at high
redshift, when the young galaxies were close together, rather than from later
interactions among the Magellanic Clouds and Milky Way. This is illustrated in
solutions for the orbits of Local Group galaxies under the cosmological
condition of growing peculiar velocities at high redshift. Massless test
particles initially near and moving with the Large Magellanic Cloud in these
solutions end up with distributions in angular position and redshift similar to
the Magellanic Stream, though with the usual overly prominent leading component
that the Milky Way corona might have suppressed. Another possible example of
the effect of conditions at high redshift is a model primeval stream around the
Local Group galaxy NGC 6822. Depending on the solution for Local Group dynamics
this primeval stream can end up with position angle similar to the HI around
this galaxy, and a redshift gradient in the observed direction. The gradient is
much smaller than observed, but might have been increased by dissipative
contraction. Presented also is an even more speculative illustration of the
possible effect of initial conditions, primeval stellar streams around M31.Comment: 23 pages, 7 figure
Is the far border of the Local Void expanding?
According to models of evolution in the hierarchical structure formation
scenarios, voids of galaxies are expected to expand. The Local Void (LV) is the
closest large void, and it provides a unique opportunity to test
observationally such an expansion. It has been found that the Local Group,
which is on the border of the LV, is running away from the void center at ~260
km/s. In this study we investigate the motion of the galaxies at the far-side
border of the LV to examine the presence of a possible expansion. We selected
late-type, edge-on spiral galaxies with radial velocities between 3000 km/s and
5000 km/s, and carried out HI 21 cm line and H-band imaging observations. The
near-infrared Tully-Fisher relation was calibrated with a large sample of
galaxies and carefully corrected for Malmquist bias. It was used to compute the
distances and the peculiar velocities of the LV sample galaxies. Among the 36
sample LV galaxies with good quality HI line width measurements, only 15
galaxies were selected for measuring their distances and peculiar velocities,
in order to avoid the effect of Malmquist bias. The average peculiar velocity
of these 15 galaxies is found to be -419+208-251 km/s, which is not
significantly different from zero. Due to the intrinsically large scatter of
Tully-Fisher relation, we cannot conclude whether there is a systematic motion
against the center of the LV for the galaxies at the far-side boundary of the
void. However, our result is consistent with the hypothesis that those galaxies
at the far-side boundary have an average velocity of ~260 km/s equivalent to
what is found at the position of the Local Group.Comment: 15 pages, 6 figures, and 4 tables. Accepted for publication in A&
Patient safety
“If healthcare was an airline, only dedicated risk takers, thrill seekers and those tired of living would fly on it.”peer-reviewe
The NGC 5846 Group: Dynamics and the Luminosity Function to M_R=-12
We conduct a photometric and spectroscopic survey of a 10 sq. deg. region
surrounding the nearby NGC 5846 group of galaxies, using the
Canada-France-Hawaii and Keck I telescopes to study the population of dwarf
galaxies as faint as M_R=-10. Candidates are identified on the basis of
quantitative surface brightness and qualitative morphological criteria.
Spectroscopic follow up and a spatial correlation analysis provide the basis
for affirming group memberships. Altogether, 324 candidates are identified and
83 have spectroscopic membership confirmation. We argue on statistical grounds
that a total 251 +/- 10 galaxies in our sample are group members. The
observations, together with archival Sloan Digital Sky Survey, ROSAT,
XMM-Newton, and ASCA data, suggest that the giant ellipticals NGC 5846 and NGC
5813 are the dominant components of subgroups separated by 600 kpc in
projection and embedded in a 1.6 Mpc diameter dynamically evolved halo. The
galaxy population is overwhelmingly early type. The group velocity dispersion
is 322 km/s, its virial mass is 8.4 x 10^13 M_sun, and M/L_R = 320 M_sun/L_sun.
The ratio of dwarfs to giants is large compared with other environments in the
Local Supercluster studied and, correspondingly, the luminosity function is
relatively steep, with a faint end Schechter function slope of \alpha_d = -1.3
+/- 0.1 (statistical) +/- 0.1 (systematic) at our completeness limit of M_R =
-12.Comment: 17 pages; accepted for publication in the Astronomical Journa
On structure, entrainment, and transport in estuarine embayments
When fresh water enters the surface of an embayment it moves persistently seaward. Enroute it entrains seawater from below to form a halocline, in which the salinity increases with depth and to seaward. Wind mixing creates a nearly homogeneous zone in the upper part of the halocline. Below the halocline there is a nearly homogeneous lower zone in which sea water intrudes at a rate sufficient to supply the demand for entrainment...
On Estimating the Flux of the Brightest Cosmic Ray Source above 57x10^18 eV
The sources of ultra-high energy cosmic rays are not yet known. However, the
discovery of anisotropic cosmic rays above 57x10^18 eV by the Pierre Auger
Observatory suggests that a direct source detection may soon be possible. The
near-future prospects for such a measurement are heavily dependent on the flux
of the brightest source. In this work, we show that the flux of the brightest
source above 57x10^18 eV is expected to comprise 10% or more of the total flux
if two general conditions are true. The conditions are: 1.) the source objects
are associated with galaxies other than the Milky Way and its closest
neighbors, and 2.) the cosmic ray particles are protons or heavy nuclei such as
iron and the Greisen-Zatsepin-Kuz'min effect is occurring. The Pierre Auger
Observatory collects approximately 23 events above 57x10^18 eV per year.
Therefore, it is plausible that, over the course of several years, tens of
cosmic rays from a single source will be detected.Comment: 10 pages, 2 figures, submitted to Astrophysical Journal Letter
The Correlation Between Galaxy HI Linewidths and K' Luminosities
The relationship between galaxy luminosities and rotation rates is studied
with total luminosities in the K' band. Extinction problems are essentially
eliminated at this band centered at 2.1 micron. A template luminosity-linewidth
relation is derived based on 65 galaxies drawn from two magnitude-limited
cluster samples. The zero-point is determined using 4 galaxies with accurately
known distances. The calibration is applied to give the distance to the Pisces
Cluster (60 Mpc) at a redshift in the CMB frame of 4771 km/s. The resultant
value of the Hubble Constant is 81 km/s/Mpc. The largest sources of uncertainty
arises from the small number of zero-point calibrators at this time at K' and
present application to only one cluster.Comment: 13 pages including 5 figures and 2 tables. Accepted for publication
in Astrophysical Journa
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