1,417 research outputs found
Constraints on the Proper Motion of the Andromeda Galaxy Based on the Survival of Its Satellite M33
A major uncertainty in the dynamical history of the local group of galaxies
originates from the unknown transverse speed of the Andromeda galaxy (M31)
relative to the Milky Way. We show that the recent VLBA measurement of the
proper motion of Andromeda's satellite, M33, severely constrains the possible
values of M31's proper motion. The condition that M33's stellar disk will not
be tidally disrupted by either M31 or the Milky Way over the past 10 billion
years, favors a proper motion amplitude of 100+-20km/s for M31 with the
quadrant of a negative velocity component along Right Ascension and a positive
component along Declination strongly ruled-out. This inference can be tested by
future astrometric measurements with SIM, GAIA, or the SKA. Our results imply
that the dark halos of Andromeda and the Milky Way will pass through each other
within the next 5-10 billion years.Comment: Accepted for publication in Ap
Adaptive and plastic variation in growth and maturation of the cutlassfish Trichiurus japonicus in the subtropical Pacific Ocean
We investigate d va ria- t ion in growth and maturation in response to 1) spatial variation in climate and 2) exploitation of the subtropical cutlassfish Trichiurus japonicus , an important fishery species whose population ecology i s virtually unknown . Individuals of this cutlassfish species were sampled monthly at 2 primary landing sites: Kengfang, in northeast (NE) Taiwan, and Tsukuan, in southwest (SW ) Taiwan, during 2013â2015. Habitat temperatures were about 1â4°C lower at the NE site t han at the SW site, and the length at age of adult fish had a n inverse pattern with temperature (NE lengths > SW lengths at age). The probabilistic maturation reaction norms did not differ significantly between the 2 areas, but ages and lengths a t maturation were higher for males from NE than for males from SW. Differences in asymptotic lengths (NE>SW) and growth coefficients (NE<SW), tog eth- er with narrower length distribution at the SW site than at the NE site, indicate potentially different mortality between the areas (NE<SW). These results indicate plastic variation in maturation and potentially adaptive variation in growth for this species that are related to the differences in temperature and mortality between the areas, although reliable proxies for mortality are lacking. The observed life-history variation was consistent with the tem peratureâsize rule that organisms tend to grow faster, mature earlier, and reach smal ler asymptotic sizes at warmer temperatures and may indicate an adaptive divergence of cutlassfish populat
The Rotation Measure and 3.5mm Polarization of Sgr A*
We report the detection of variable linear polarization from Sgr A* at a
wavelength of 3.5mm, the longest wavelength yet at which a detection has been
made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg
with rms scatters of 0.4% and 9 deg over the five epochs. We also detect
polarization variability on a timescale of days. Combined with previous
detections over the range 150-400GHz (750-2000 microns), the average
polarization position angles are all found to be consistent with a rotation
measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation
occurs external to the polarized source at all wavelengths. This implies an
accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles
expected of ADAF, jet and CDAF models and assuming that the region at which
electrons in the accretion flow become relativistic is within 10 R_S. The
inferred accretion rate is inconsistent with ADAF/Bondi accretion. The
stability of the mean polarization position angle between disparate
polarization observations over the frequency range limits fluctuations in the
accretion rate to less than 5%. The flat frequency dependence of the inter-day
polarization position angle variations also makes them difficult to attribute
to rotation measure fluctuations, and suggests that both the magnitude and
position angle variations are intrinsic to the emission.Comment: Ap.J.Lett. accepte
Temporal stability of the maturation schedule of capelin (Mallotus villosus Mueller) in the Barents Sea
Capelin in the Barents Sea are primarily harvested in a terminal fishery that targets maturing indiviuals. Theory predicts that, in a semelparous population (i.e., one in which reproduction is seasonal, synchronous, and followed by parental mortality), an unselective, terinal fishery (i.e., one in which most of the fish that are not caught will not have a new spawning opportunity) does not generate strong selection for changed age and size at mturation. The probabilistic maturation reaction norm (PMRN) method was applied to test this prediction and to detect possible temporal changes in length at maturation o Barents Sea capelin between 1978 and 2008. Maturation reaction norms suggest that maturation is age-independent in capelin, but that males require a larger size to attainthe same maturation probability as females. No temporal trends in length at maturation could be detected, thus confirming the theoretical prediction. Furthermore, none of thecandidate environmental variables tested to explain the temporal variability in length at maturation (water temperature and capelin biomass) consistently shoed a significant correlation with the PMRN midpoints
Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow
We report the discovery of variability in the linear polarization from the
Galactic Center black hole source, Sagittarius A*. New polarimetry obtained
with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm
shows a position angle that differs by 28 +/- 5 degrees from observations 6
months prior and then remains stable for 15 months. This difference may be due
to a change in the source emission region on a scale of 10 Schwarzschild radii
or due to a change of 3 x 10^5 rad m^-2 in the rotation measure. We consider a
change in the source physics unlikely, however, since we see no corresponding
change in the total intensity or polarized intensity fraction. On the other
hand, turbulence in the accretion region at a radius ~ 10 to 1000 R_s could
readily account for the magnitude and time scale of the position angle change.Comment: accepted for publication in ApJ
Detection of Circular Polarization in the Galactic Center Black Hole Candidate Sagittarius A*
We report here the detection of circular polarization in the Galactic Center
black hole candidate, Sagittarius A*. The detection was made at 4.8 GHz and 8.4
GHz with the Very Large Array. We find that the fractional circular
polarization at 4.8 GHz is and that the spectral index of
the circular polarization is (). The systematic error in is less than 0.04% at both
frequencies. In light of our recent lower limits on the linear polarization in
Sgr A*, this detection is difficult to interpret with standard models. We
consider briefly whether scattering mechanisms could produce the observed
polarization. Detailed modeling of the source and the scattering medium is
necessary. We propose a simple model in which low energy electrons reduce
linear polarization through Faraday depolarization and convert linear
polarization into circular polarization. Circular polarization may represent a
significant new parameter for studying the obscured centimeter wavelength radio
source in Sgr A*.Comment: ApJL accepted, 11 pages including 1 figur
The Linear Polarization of Sagittarius A* II. VLA and BIMA Polarimetry at 22, 43 and 86 GHz
We present a search for linear polarization at 22 GHz, 43 GHz and 86 GHz from
the nearest super massive black hole candidate, Sagittarius A*. We find upper
limits to the linear polarization of 0.2%, 0.4% and 1%, respectively. These
results strongly support the conclusion of our centimeter wavelength
spectro-polarimetry that Sgr A* is not depolarized by the interstellar medium
but is in fact intrinsically depolarized.Comment: Accepted for publication in ApJ, 13 pages, 2 figure
The Linear Polarization of Sagittarius A* I. VLA Spectro-polarimetry at 4.8 and 8.4 GHz
Synchrotron radiation from active galactic nuclei (AGN) is often highly
polarized. We present a search for linear polarization with the Very Large
Array (VLA) at 4.8 GHz and 8.4 GHz from the nearest AGN, Sagittarius A*. As a
part of this study we used spectro-polarimetric data that were sensitive to a
rotation measure (RM) as large as 3.5 x 10^6 rad m^-2 at 4.8 GHz and 1.5 x 10^7
rad m^-2 at 8.4 GHz. The upper limit to the linear polarization of Sgr A* over
a broad range of RM is 0.2% at both frequencies. We also present continuum
observations with the VLA at 4.8 GHz which give an upper limit of 0.1% for RMs
less than 10^4 rad m^-2. We conclude that depolarization is unlikely to occur
in the Galacter Center scattering medium. However, it is possible for
depolarization to occur in the accretion region of Sgr A* if the outer scale of
turbulence is small enough. We also consider the implications of a very low
intrinsic polarization for Sgr A*.Comment: 16 pages, 3 figures, accepted for publication in the Astrophysical
Journal, August 20, 1999, Vol 521 #
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