392 research outputs found
Addressing the missing matter problem in galaxies through a new fundamental gravitational radius
We demonstrate that the existence of a Noether symmetry in theories of
gravity gives rise to a further gravitational radius, besides the standard
Schwarzschild one, determining the dynamics at galactic scales. By this
feature, it is possible to explain the baryonic Tully-Fisher relation and the
rotation curve of gas-rich galaxies without the dark matter hypothesis.Comment: 9 pages, 2 figures, to be published in JCA
Recovering the fundamental plane of galaxies by gravity
The fundamental plane (FP) of galaxies can be recovered in the framework of
gravity avoiding the issues related to dark matter to fit the
observations. In particular, the power-law version , resulting
from the existence of Noether symmetries for , is sufficient to implement
the approach. In fact, relations between the FP parameters and the corrected
Newtonian potential, coming from , can be found and justified from a
physical point of view. Specifically, we analyze the velocity distribution of
elliptical galaxies and obtain that , the scale-length depending on the
gravitational system properties, is proportional to , the galaxy effective
radius. This fact points out that the gravitational corrections induced by
can lead photometry and dynamics of the system. Furthermore, the main
byproduct of such an approach is that gravity could work in different ways
depending on the scales of self-gravitating systems.Comment: 18 pages, 3 tables, 8 figures. Accepted for publication in Phys. Dark
Univers
Constraining Extended Gravity Models by S2 star orbits around the Galactic Centre
We investigate the possibility to explain theoretically the observed
deviations of S2 star orbit around the Galactic Centre using gravitational
potentials derived from modified gravity models in absence of dark matter. To
this aim, an analytic fourth-order theory of gravity, non-minimally coupled
with a massive scalar field is considered. Specifically, the interaction term
is given by analytic functions and where is the Ricci
scalar and is a scalar field whose meaning can be related to further
gravitational degrees of freedom. We simulate the orbit of S2 star around the
Galactic Centre in (Yukawa-like) and (Sanders-like) gravity
potentials and compare it with NTT/VLT observations. Our simulations result in
strong constraints on the range of gravity interaction. In the case of analytic
functions , we are not able to obtain reliable constraints on the
derivative constants and , because the current observations of S2
star indicated that they may be highly mutually correlated. In the case of
analytic functions , we are able to obtain reliable constraints on
the derivative constants , , , , and
. The approach we are proposing seems to be sufficiently reliable
to constrain the modified gravity models from stellar orbits around Galactic
Centre.Comment: 9 pages, 6 figure to appear in Phys. Rev.
Constraining the range of Yukawa gravity interaction from S2 star orbits
We consider possible signatures for Yukawa gravity within the Galactic
Central Parsec, based on our analysis of the S2 star orbital precession around
the massive compact dark object at the Galactic Centre, and on the comparisons
between the simulated orbits in Yukawa gravity and two independent sets of
observations. Our simulations resulted in strong constraints on the range of
Yukawa interaction and showed that its most probable value in the
case of S2 star is around 5000 - 7000 AU. At the same time, we were not able to
obtain reliable constrains on the universal constant of Yukawa
gravity, because the current observations of S2 star indicated that it may be
highly correlated with parameter in the range . For
they are not correlated. However, the same universal constant
which was successfully applied to clusters of galaxies and rotation curves of
spiral galaxies () also gives a satisfactory agreement with the
observed orbital precession of the S2 star, and in that case the most probable
value for the scale parameter is AU. Also, the
Yukawa gravity potential induces precession of S2 star orbit in the same
direction as General Relativity for and for , and in
the opposite direction for . The future observations with
advanced facilities, such as GRAVITY or/and European Extremely Large Telescope,
are needed in order to verify these claims.Comment: 16 pages, 8 figures, accepted for publication in JCA
Constraints on gravity from precession of orbits of S2-like stars
We study some possible observational signatures of gravity at Galactic
scales and how these signatures could be used for constraining this type of
gravity. For that purpose, we performed two-body simulations in
gravity potential and analyzed the obtained trajectories of S2-like stars
around Galactic center, as well as resulting parameter space of gravity
potential. Here, we discuss the constraints on the gravity which can be
obtained from the observations of orbits of S2-like stars with the present and
next generations of large telescopes. We make comparison between the
theoretical results and observations. Our results show that the most probable
value for the parameter in gravity potential in the case of S2-like
stars is 100 AU, while the universal parameter is close to 0.01.
Also, the gravity potential induces the precession of S2-like stars orbit
in opposite direction with respect to General Relativity, therefore, such a
behavior of orbits qualitatively is similar to a behavior of Newtonian orbits
with a bulk distribution of matter (including a stellar cluster and dark matter
distributions).Comment: 12 pages, 12 figures, accepted in Phys. Rev.
Line shifts in accretion disks - the case of Fe K
Here we present a short overview and main results of our investigations of
several effects which can induce shifts in the broad Fe K line emitted
from relativistic accretion disks around single and binary supermassive black
holes. We used numerical simulations based on ray-tracing method in the Kerr
metric to study the role of classical Doppler shift, special relativistic
transverse Doppler shift and Doppler beaming, general relativistic
gravitational redshift, and perturbations of the disk emissivity in the
formation of the observed Fe K line profiles. Besides, we also
investigated whether the observed line profiles from the binary systems of
supermassive black holes could be affected by the Doppler shifts due to
dynamics of such systems. The presented results demonstrate that all these
effects could have a significant influence on the observed profiles of the
broad Fe K line emitted from relativistic accretion disks around single
and binary supermassive black holes.Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Astrophysics
and Space Scienc
Masses of constituent quarks confined in open bottom hadrons
We apply color-spin and flavor-spin quark-quark interactions to the meson and
baryon constituent quarks, and calculate constituent quark masses, as well as
the coupling constants of these interactions. The main goal of this paper was
to determine constituent quark masses from light and open bottom hadron masses,
using the fitting method we have developed and clustering of hadron groups. We
use color-spin Fermi-Breit (FB) and flavor-spin Glozman-Riska (GR) hyperfine
interaction (HFI) to determine constituent quark masses (especially quark
mass). Another aim was to discern between the FB and GR HFI because our
previous findings had indicated that both interactions were satisfactory. Our
improved fitting procedure of constituent quark masses showed that on average
color-spin (Fermi-Breit) hyperfine interaction yields better fits. The method
also shows the way how the constituent quark masses and the strength of the
interaction constants appear in different hadron environments.Comment: 15 pages, 6 tables, 1 figure. Accepted for publication in Mod. Phys.
Lett.
Constraining Non-local Gravity by S2 star orbits
Non-local theories of gravity have recently gained a lot of interest because
they can suitably represent the behavior of gravitational interaction in the
ultraviolet regime. Furthermore, at infrared scales, they give rise to notable
cosmological effects which could be important to describe the dark energy
behavior. In particular, exponential forms of the distortion function seem
particularly useful for this purpose. Using Noether Symmetries, it can be shown
that the only non-trivial form of the distortion function is the exponential
one, which is working not only for cosmological mini-superspaces, but also in a
spherically symmetric spacetime. Taking this result into account, we study the
weak field approximation of this type of non-local gravity, and comparing with
the orbits of S2 stars around the Galactic center (NTT/VLT data), we set
constraints on the parameters of the theory. Non-local effects do not play a
significant role on the orbits of S2 stars around Sgr A*, but give richer
phenomenology at cosmological scales than the CDM model. Also, we show
that non-local gravity model gives better agreement between theory and
astronomical observations than Keplerian orbits.Comment: 11 pages, 6 figures. Accepted for publication in Phys. Rev.
Study of spectral index of giant radio galaxy from Leahy's Atlas: DA 240
Here we investigate the giant radio galaxy DA 240, which is a FR II source.
Specifically, we investigate its flux density, as well as the spectral index
distribution. For that purpose, we used publicly available data for the source:
Leahy's atlas of double radio-sources and NASA/IPAC Extragalactic Database
(NED). We used observations at 326 MHz (92 cm) and at 608 MHz (49 cm) and
obtained spectral index distributions between 326 and 608 MHz. For the first
time we give spectral index map for these frequencies. We found that the
synchrotron radiation is the dominant radiation mechanism over most of the area
of DA 240, and also investigated the mechanism of radiation at some
characteristic points, namely its core and the hotspots. The results of this
study will be helpful for understanding the evolutionary process of the DA 240
radio source.Comment: 9 pages, 5 figures. Accepted for publication in Contrib. Astron. Obs.
Skalnate Pleso (Invited lecture at V Meeting on Astrophysical Spectroscopy -
A&M DATA 2023
Anisotropy of the in-plane angular magnetoresistance of electron-doed Sr1-xLaxCuO2 thin films
Signatures of antiferromagnetism (AF) in the underdoped Ln2-xCexCuO4 (Ln =
Nd, Pr,...) family are observed even for doping levels for which
superconductivity exists. We have looked for a similar property in a different
electron-doped cuprate family, Sr1-xLaxCuO2, which consists of CuO2 planes
separated by Sr/La atoms, and is exempt of the possible influence of magnetic
rare earth ions. We report in-plane magnetoresistance measurements in the
normal state of underdoped, superconducting, c-axis oriented, epitaxial
Sr1-xLaxCuO2 thin films. This probe is sensitive to spin arrangement and we
find that the in-plane magnetoresistance, which is negative and does not
saturate for T, exhibits an angular dependence when measured upon rotating a
magnetic field within the CuO2 planes. The analysis reveals a superposition of
fourfold and twofold angular oscillations. Both of these increase in amplitude
with increasing field and decreasing and appear below a temperature, which gets
higher with decreasing doping levels. Our results demonstrate that these
magnetoresistance oscillations, also observed for the Ln2-xCexCuO4 (Ln = Nd,
Pr,...) family and attributed to an AF signature, are, without ambiguity, a
property of CuO2 planes. Besides, these oscillations vary with doping in an
unusual way compared to previous results: fourfold oscillations are essentially
present in the more underdoped samples while only twofold oscillations are
visible in the less underdoped ones. This intriguing observation appears to be
a consequence of spin dilution with increasing doping level.Comment: 25 pages, 6 figure
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