1,552 research outputs found
Anatomy of Ursa Majoris
A nearby friable cloud in Ursa Majoris contains 270 galaxies with radial
velocities 500 < VLG < 1500 km s^-1 inside the area of RA= [11h; 13h] and DEC=
[+40deg; +60deg]. At present, 97 galaxies of them have individual distance
estimates. We use these data to clarify the structure and kinematics of the UMa
complex. According to Makarov & Karachentsev (2011), most of the UMa galaxies
belong to seven bound groups, which have the following median parameters:
velocity dispersion of 58 km s^-1, harmonic projected radius of 300 kpc, virial
mass of 2.10^12 Msol, and virial- mass-to-K-band-luminosity of 27Msol/Lsol.
Almost a half of the UMa cloud population are gas-rich dwarfs (Ir, Im, BCD)
with active star formation seen in the GALEX UV-survey. The UMa groups reside
within 15-19 Mpc from us, being just at the same distance as Virgo cluster. The
total virial mass of the UMa groups is 4.10^13 Msol, yielding the average
density of dark matter in the UMa cloud to be Omega_m = 0.08, i.e. a factor
three lower than the cosmic average. This is despite the fact that the UMa
cloud resides in a region of the Universe that is an apparent overdensity. A
possible explanation for this is that most mass in the Universe lies in the
empty space between clusters. Herewith, the mean distances and velocities of
the UMa groups follow nearly undisturbed Hubble flow without a sign of the
'Z-wave" effect caused by infall toward a massive attractor. This constrains
the total amount of dark matter between the UMa groups within the cloud volume.Comment: correction of a typo in the abstract, 18 pages, 2 figures. accepted
for MNRAS, nov 26, 201
Filaments from the galaxy distribution and from the velocity field in the local universe
The cosmic web that characterizes the large-scale structure of the Universe
can be quantified by a variety of methods. For example, large redshift surveys
can be used in combination with point process algorithms to extract long
curvilinear filaments in the galaxy distribution. Alternatively, given a full
3D reconstruction of the velocity field, kinematic techniques can be used to
decompose the web into voids, sheets, filaments and knots. In this paper we
look at how two such algorithms - the Bisous model and the velocity shear web -
compare with each other in the local Universe (within 100 Mpc), finding good
agreement. This is both remarkable and comforting, given that the two methods
are radically different in ideology and applied to completely independent and
different data sets. Unsurprisingly, the methods are in better agreement when
applied to unbiased and complete data sets, like cosmological simulations, than
when applied to observational samples. We conclude that more observational data
is needed to improve on these methods, but that both methods are most likely
properly tracing the underlying distribution of matter in the Universe.Comment: 6 Pages, 2 figures, Submitted to MNRAS Letter
Star Formation in Nearby Isolated Galaxies
We use the FUV fluxes measured with the GALEX to study the star formation
properties of galaxies collected in the "Local Orphan Galaxies" catalog (LOG).
Among 517 LOG galaxies having radial velocities V(LG) < 3500 km/s and Galactic
latitudes |b|> 15 degr, 428 objects have been detected in FUV. We briefly
discuss some scaling relations between the specific star formation rate (SSFR)
and stellar mass, HI-mass, morphology, and surface brightness of galaxies
situated in extremely low density regions of the Local Supercluster. Our sample
is populated with predominantly late-type, gas-rich objects with the median
morphological type of Sdm. Only 5% of LOG galaxies are classified as early
types: E, S0, S0/a, however, they systematically differ from normal E and S0
galaxies by lower luminosity and presence of gas and dust. We find that almost
all galaxies in our sample have their SSFR below 0.4 [Gyr^{-1}]. This limit is
also true even for a sample of 260 active star-burst Markarian galaxies
situated in the same volume. The existence of such a quasi-Eddington limit for
galaxies seems to be a key factor which characterizes the transformation of gas
into stars at the current epoch.Comment: 10 pages, 8 figures, 3 table
Local spectroscopy of a proximity superconductor at very low temperature
We performed the local spectroscopy of a Normal-metal--Superconductor (N-S)
junction with the help of a very low temperature (60 mK) Scanning Tunneling
Microscope (STM). The spatial dependence of the local density of states was
probed locally in the vicinity of the N-S interface. We observed spectra with a
fully-developed gap in the regions where a thin normal metal layer caps the
superconductor dot. Close to the S metal edge, a clear pseudo-gap shows up,
which is characteristic of the superconducting proximity effect in the case of
a long normal metal. The experimental results are compared to the predictions
of the quasiclassical theory.Comment: 7 pages, 3 figure
Planes of satellite galaxies and the cosmic web
Recent observational studies have demonstrated that the majority of satellite
galaxies tend to orbit their hosts on highly flattened, vast, possibly
co-rotating planes. Two nearly parallel planes of satellites have been
confirmed around the M31 galaxy and around the Centaurus A galaxy, while the
Milky Way also sports a plane of satellites. It has been argued that such an
alignment of satellites on vast planes is unexpected in the standard
({\Lambda}CDM) model of cosmology if not even in contradiction to its generic
predictions. Guided by {\Lambda}CDM numerical simulations, which suggest that
satellites are channeled towards hosts along the axis of the slowest collapse
as dictated by the ambient velocity shear tensor, we re-examine the planes of
local satellites systems within the framework of the local shear tensor derived
from the Cosmicflows-2 dataset. The analysis reveals that the Local Group and
Centaurus A reside in a filament stretched by the Virgo cluster and compressed
by the expansion of the Local Void. Four out of five thin planes of satellite
galaxies are indeed closely aligned with the axis of compression induced by the
Local Void. Being the less massive system, the moderate misalignment of the
Milky Way's satellite plane can likely be ascribed to its greater
susceptibility to tidal torques, as suggested by numerical simulations. The
alignment of satellite systems in the local universe with the ambient shear
field is thus in general agreement with predictions of the {\Lambda}CDM model.Comment: 9 pages, 3 figures, 3 tables. Accepted by MNRAS, 9 June 201
Predicting Outcomes of ElimLin Attack on Lightweight Block Cipher Simon
There are two major families in cryptanalytic attacks on symmetric ciphers: statistical attacks and algebraic attacks. In this position paper we argue that algebraic cryptanalysis has not yet been developed properly due to the weakness of the theory which has substantial difficulty to prove most basic results on the number of linearly independent equations in algebraic attacks. Consequently most authors present a restricted range of attacks which are shown experimentally to work with their computer but refrain from claiming results which would work on a larger computer but have not yet been tested. For example in recent 2015 work of Raddum we discover that (experimentally) ElimLin attack breaks up to 16 rounds of Simon block cipher however it is hard to know what happens for 17 rounds. In this paper we argue that one CAN predict and model the behavior of such attacks and evaluate complexity of the attacks which we cannot yet execute. To the best of our knowledge this has never been done before
First DENIS I-band extragalactic catalog
This paper presents the first I-band photometric catalog of the brightest
galaxies extracted from the Deep Near Infrared Survey of the Southern Sky
(DENIS) An automatic galaxy recognition program has been developed to build
this provisional catalog. The method is based on a discriminating analysis. The
most discriminant parameter to separate galaxies from stars is proved to be the
peak intensity of an object divided by its array. Its efficiency is better than
99%. The nominal accuracy for galaxy coordinates calculated with the Guide Star
Catalog is about 6 arcseconds. The cross-identification with galaxies available
in the Lyon-Meudon Extragalactic DAtabase (LEDA) allows a calibraton of the
I-band photometry with the sample of Mathewson et Al. Thus, the catalog
contains total I-band magnitude, isophotal diameter, axis ratio, position angle
and a rough estimate of the morphological type code for 20260 galaxies. The
internal completeness of this catalog reaches magnitude , with a
photometric accuracy of . 25% of the Southern sky has been
processed in this study.
This quick look analysis allows us to start a radio and spectrographic
follow-up long before the end of the survey.Comment: 13 pages, 17 figures, to appear A&A Supl.
The alignment of galaxy spin with the shear field in observations
Tidal torque theory suggests that galaxies gain angular momentum in the
linear stage of structure formation. Such a theory predicts alignments between
the spin of haloes and tidal shear field. However, non-linear evolution and
angular momentum acquisition may alter this prediction significantly. In this
paper, we use a reconstruction of the cosmic shear field from observed peculiar
velocities combined with spin axes extracted from galaxies within () from 2MRS
catalog, to test whether or not galaxies appear aligned with principal axes of
shear field. Although linear reconstructions of the tidal field have looked at
similar issues, this is the first such study to examine galaxy alignments with
velocity-shear field. Ellipticals in the 2MRS sample, show a statistically
significant alignment with two of the principal axes of the shear field. In
general, elliptical galaxies have their short axis aligned with the axis of
greatest compression and perpendicular to the axis of slowest compression.
Spiral galaxies show no signal. Such an alignment is significantly strengthened
when considering only those galaxies that are used in velocity field
reconstruction. When examining such a subsample, a weak alignment with the axis
of greatest compression emerges for spiral galaxies as well. This result
indicates that although velocity field reconstructions still rely on fairly
noisy and sparse data, the underlying alignment with shear field is strong
enough to be visible even when small numbers of galaxies are considered -
especially if those galaxies are used as constraints in the reconstruction.Comment: 9 pages, 3 figures, accepted in MNRA
Brillouin propagation modes in optical lattices: Interpretation in terms of nonconventional stochastic resonance
We report the first direct observation of Brillouin-like propagation modes in a dissipative periodic optical lattice. This has been done by observing a resonant behavior of the spatial diffusion coefficient in the direction corresponding to the propagation mode with the phase velocity of the moving intensity modulation used to excite these propagation modes. Furthermore, we show theoretically that the amplitude of the Brillouin mode is a nonmonotonic function of the strength of the noise corresponding to the optical pumping, and discuss this behavior in terms of nonconventional stochastic resonance
- …