arXiv subjects

S. Nasiri

Publications and source records attributed to S. Nasiri.

17 recordsLinked to original sources

arXiv2023

Influence of Magnetic Fields on the Gas Rotation in the Galaxy $NGC\;6946$

Magnetic fields can play an important role in the energy balance and formation of gas structures in galaxies. However, their dynamical effect on the rotation curve of galaxies is immensely unexplored. We investigate the dynamical effect of the known magnetic arms of $NGC\;6946$ on its circular gas rotation traced in HI, considering two dark matter mass density models, ISO, and the universal NFW profile. We used a three-dimensional model for the magnetic field structure to fit the modeled rotation curve to the observed data via an $χ$-squared minimization method. The shape of the HI gas rotation curve is reproduced better including the effect of the magnetic field, especially in the outer part, where the dynamical effect of the magnetic field could become important. The typical amplitude of the regular magnetic field contribution in the rotation curve is about $ 6 - 14 \; km s^{-1}$ in the outer gaseous disk of the galaxy $NGC\;6946$. The contribution ratio of the regular magnetic field to the observed circular velocity and to dark matter increases with the galactocentric radius. Its ratio to the observed rotational velocity is about five percent and to dark matter is about 10 percent in the outer regions of the galaxy $NGC\;6946$. Therefore, the large-scale magnetic fields cannot be completely ignored in the large-scale dynamics of spiral galaxies, especially in the outer parts of galaxies.

astro-ph.GA↗

arXiv2021

Role of Thermal and Non-thermal Processes in the ISM of Magellanic Clouds

The radio continuum emission is a dust-unbiased tracer of both the thermal and non-thermal processes in the interstellar medium. We present new maps of the free-free and synchrotron emission in the Magellanic Clouds (MCs) at 0.166, 1.4, and 4.8GHz with no prior assumption about the radio non-thermal spectrum. The maps were constructed using a de-reddened H$α$ map as a template for the thermal radio emission, which we subtract from maps of the total radio continuum emission. To de-redden the H$α$ emission, it is important to know the fraction of dust surface density that attenuates the H$α$ emission along the line-of-sight, $f_\text{d}$. This fraction is obtained by comparing the dust opacity obtained through the infrared emission spectrum and the Balmer decrement method. In star-forming regions, the median $f_\text{d}$ is about 0.1 which is by a factor of 3 lower than that in diffuse regions. We obtain a global thermal radio fraction, $f_\text{th}$, of 30 per cent (35 per cent) in the LMC (SMC) at 1.4GHz. Furthermore, we present maps of the equipartition magnetic field strength with average values of $\simeq10.1μ$G in the LMC and $\simeq\,5.5\,μ$G in the SMC. The magnetic field is proportional to the star formation rate to a power of 0.24 and 0.20 for the LMC and SMC, respectively. This study shows that the non-thermal processes control the interstellar medium in the MCs.

astro-ph.GA↗

arXiv2021

Kinematical asymmetry in the dwarf irregular galaxy WLM and a perturbed halo potential

WLM is a dwarf irregular that is seen almost edge-on that has prompted a number of kinematical studies investigating its rotation curve and its dark matter content. In this paper, we investigate the origin of the strong asymmetry of the rotation curve, which shows a significant discrepancy between the approaching and the receding side. We first examine whether an $m = 1$ perturbation (lopsidedness) in the halo potential could be a mechanism creating such kinematical asymmetry. To do so, we fit a theoretical rotational velocity associated with an $m = 1$ perturbation in the halo potential model to the observed data via a $χ-$squared minimization method. We show that a lopsided halo potential model can explain the asymmetry in the kinematic data reasonably well. We then verify that the kinematical classification of WLM shows that its velocity field is significantly perturbed due to both its asymmetrical rotation curve and also its peculiar velocity dispersion map. In addition, based on a kinemetry analysis, we find that it is possible for WLM to lie in the transition region, where the disk and merger coexist. In conclusion, it appears that the rotation curve of WLM diverges significantly from that of an ideal rotating disk, which may significantly affect investigations of its dark matter content.

astro-ph.GA↗

arXiv2019

Oscillations of coronal loops using Rayleigh-Ritz technique

In this paper, we apply the Rayleigh-Ritz Variational Scheme for studying the transverse oscillations of a magnetic flux tube. The flux tube is considered in low-$β$ solar coronal condition. The perturbations decomposed into irrotational and solenoidal components and MHD equations are reduced as a matrix eigenvalue problem. In the case of longitudinally stratified thin flux tube, the fundamental and higher order kink frequencies are computed. The results are in good agreements with previous studies.

astro-ph.SR↗

arXiv2012

Slow Mode Oscillations and Damping of Hot Solar Coronal Loops

The effect of temperature inhomogeneity on the periods, their ratios (fundamental vs. first overtone), and the damping times of the standing slow modes in gravitationally stratified solar coronal loops are studied. The effects of optically thin radiation, compressive viscosity, and thermal conduction are considered. The linearized one-dimensional magnetohydrodynamic (MHD) equations (under low-$β$ condition) were reduced to a fourth--order ordinary differential equation for the perturbed velocity. The numerical results indicate that the periods of non-isothermal loops (i.e. temperature increases from the loop base to apex) are smaller compared to those of isothermal loops. In the presence of radiation, viscosity, and thermal conduction, an increase in the temperature gradient is followed by a monotonic decrease in the periods (compared with the isothermal case), while the period ratio turns out to be a sensitive function of the gradient of the temperature and the loop lengths. We verify that radiative dissipation is not a main cooling mechanism of both isothermal and non-isothermal hot coronal loops and has a small effect on the periods. Thermal conduction and compressive viscosity are primary mechanisms in the damping of slow modes of the hot coronal loops. The periods and damping times in the presence of compressive viscosity and/or thermal conduction dissipation are consistent with the observed data in specific cases. By tuning the dissipation parameters, the periods and the damping times could be made consistent with the observations in more general cases.

astro-ph.SR↗

arXiv2011

Resonant absorption in dissipative flux tubes

We study the resonant absorption of MHD waves in magnetized flux tubes with a radial density inhomogeneity. Within the approximation that resistive and viscous processes are operative in thin layers surrounding the singularities of the MHD equations, we give the full spectrum of the eigenfrequencies and damping rates of the MHD quasi modes of the tube. Both surface and body modes are analyzed. \keywords{Sun -- corona: magnetohydrodynamics (MHD) -- Sun: magnetic fields -- Sun: oscillations

astro-ph↗

arXiv2010

The effect of density stratification on the resonant absorption of MHD waves in coronal loops

The standing quasi-modes of the ideal MHD in a zero-$β$ cylindrical magnetic flux tube that undergoes a longitudinal density stratification and radial density structuring is considered. The radial structuring is assumed to be a linearly varying density profile. Using the relevant connection formulae of the resonant absorption, the dispersion relation for the fast MHD body waves is derived and solved numerically to obtain both the frequencies and damping rates of the fundamental and first-overtone, $k=1,2$ modes of both the kink ($m=1$), and fluting ($m=2$) waves. Where $k$ and $m$ are the longitudinal and azimuthal mode numbers, respectively.

astro-ph.SR↗

arXiv2008

Exact solutions for standing kink modes of the longitudinally stratified coronal loops

The influence of longitudinal structuring on the fast kink modes of coronal loops is investigated. Analytical dispersion relations and mode profiles are derived for the second-order ordinary differential equation governing the z- component of the perturbation in the magnetic field, $δB_z$. All other components are given in terms of $δB_z$. Deviations from the frequencies and mode profiles of homogenous loops are given as functions of the density scale height. The effects of the fixed and variable column masses, negative scale heights, and density contrasts inside and outside of the loops are studied. The frequency ratios, mode profiles, and the antinode shifts from those of the sine profiles of the homogenous loops, are tools to estimate solar photospheric parameters. To this end, we have expanded the relevant factors up to the second order in the stratification parameter. In particular, we verify that the first overtone antinode shifts are in the Mm range and are within the reach of the resolutions of the present day observations.

astro-ph↗

arXiv2007

Oscillations of longitudinally density stratified coronal loops

We investigate the standing kink modes of a cylindrical model of coronal loops. The density is stratified along the loop axis and changes discontinuously at the surface of the cylinder. The periods and mode profiles are studies with their deviation from those of the unstratified loops. The aim is to extract information on the density scale heights prevailing in the solar corona. The problem is reduced to solving a single second-order partial differential equation for $δB_z(r,z)$, the longitudinal component of the Eulerian perturbations of the magnetic field. This equation, in turn, is separated into two second-order ordinary, differential equations in $r$ and $z$ that are, however, connected through a dispersion relation between the frequencies and the longitudinal wave numbers. In the thin tube approximation, the eigensolutions are obtained by a perturbation technique, where the perturbation parameter is the density stratification parameter. Otherwise the problem is solved numerically.

astro-ph↗

arXiv2006

Phase Space Quantum Mechanics - Direct

Conventional approach to quantum mechanics in phase space, (q,p), is to take the operator based quantum mechanics of Schrodinger, or and equivalent, and assign a c-number function in phase space to it. We propose to begin with a higher level of abstraction, in which the independence and the symmetric role of q and p is maintained throughout, and at once arrive at phase space state functions. Upon reduction to the q- or p-space the proposed formalism gives the conventional quantum mechanics, however, with a definite rule for ordering of factors of non commuting observables. Further conceptual and practical merits of the formalism are demonstrated throughout the text.

quant-ph↗

arXiv2005

Gauge Invariant Quantization of Dissipative Systems of Charged Particles in Extended Phase Space

Recently, it is shown that the extended phase space formulation of quantum mechanics is a suitable technique for studying the quantum dissipative systems. Here, as a further application of this formalism, we consider a dissipative system of charged particles interacting with an external time dependent electric field. Such a system has been investigated by Buch and Denman, and two distinct solutions with completely different structure have been obtained for Schrödinger's equation in two different gauges. However, by generalizing the gauge transformations to the phase space and using the extended phase space technique to study the same system, we demonstrate how both gauges lead to the same conductivity, suggesting the recovery of gauge invariance for this physical quantity within the extended phase space approach.

quant-ph↗

arXiv2005

Study of Dissipative System of Charged Particles by Wigner's Functions

Recently, it is shown that the extended phase space formulation of quantum mechanics is a suitable technique for studying the quantum dissipative system. Here, as an application of this formalism, we consider a dissipative system of charged particles interacting with an external time dependent electric field. Such a system has been investigating by Buch and Denman, and two solutions with completely different structure have been obtained for Schrödinger's equation in two different gauges. We demonstrate how both gauges lead to the same conductivity by generalizing the gauge transformations to the phase space and using the extended phase space technique.

quant-ph↗

arXiv2005

A Generalized Rule For Non-Commuting Operators in Extended Phase Space

A generalized quantum distribution function is introduced. The corresponding ordering rule for non-commuting operators is given in terms of a single parameter. The origin of this parameter is in the extended canonical transformations that guarantees the equivalence of different distribution functions obtained by assuming appropriate values for this parameter.

quant-ph↗

arXiv2005

A symmetric treatment of damped harmonic oscillator in extended phase space

Extended phase space (EPS) formulation of quantum statistical mechanics treats the ordinary phase space coordinates on the same footing and thereby permits the definite the canonical momenta conjugate to these coordinates . The extended lagrangian and extended hamiltonian are defined in EPS by the same procedure as one does for ordinary lagrangian and hamiltonian. The combination of ordinary phase space and their conjugate momenta exhibits the evolution of particles and their mirror images together. The resultant evolution equation in EPS for a damped harmonic oscillator, is such that the energy dissipated by the actual oscillator is absorbed in the same rate by the image oscillator leaving the whole system as a conservative system. We use the EPS formalism to obtain the dual hamiltonian of a damped harmonic oscillator, first proposed by Batemann, by a simple extended canonical transformations in the extended phase space. The extended canonical transformations are capable of converting the damped system of actual and image oscillators to an undamped one, and transform the evolution equation into a simple form. The resultant equation is solved and the eigenvalues and eigenfunctions for damped oscillator and its mirror image are obtained. The results are in agreement with those obtained by Bateman. At last, the uncertainty relation are examined for above system.

quant-ph↗

arXiv2003

A Phenomenological Model for QSOs

Using LBQS data, it seems that the predictions of the model is confirmed by observations. The behavior of comoving density versus redshift is consistent with LDDE for QSQs. Considering the cosmic evolution of SFR, a unified aspect for origin and evolution of QSOs and ordinary galaxies is implied by this model.

astro-ph↗

arXiv2003

A Phenomenological Model for the Evolution of Proto-Galaxies

The contraction model of Field and Colgate for proto-galaxies, first proposed to describe the observed properties of quasars, is generalized and used to investigate the evolution of galaxies. The LEDA data base for elliptical, spiral, compact and diffuse galaxies is employed and it is shown that the above model is consistent with observational evidences regarding their dynamical evolution, star formation rate and different morphologies.

astro-ph↗

arXiv1999

Quasars as Extreme Case of Galaxies

We introduce a phenomenological investigation of the evolution and large scale distribution of quasars using a modified version of the Field and Colgate gravitational contraction model for proto-galaxies. By studying the distribution of about 7000 quasars in 5 luminosity classes, it seems that, such a model is capable of solving the energy problem and discussing some of the observational properties of these objects. A sketch of luminosity function of the quasars and the normal galaxies shows a unified aspect for these objects. The large scale distribution of the quasars in the galactic coordinate shows the existence of filamentary structures and voids in the same sence that have been resolved by exploring the clusters of galaxies.

astro-ph↗