Browsing by Author "Subramanian, Kandaswamy"

Now showing 1 - 20 of 49

Aspects of Zeldovich approximation
(American Astronomical Society, 1993-04-15) Padmanabhan, T.; Subramanian, Kandaswamy
A generalized version of the Zel'dovich approximation which is applicable in both the radiation-dominated and the matter dominated epochs is presented. This approximation allows one to follow the growth of inhomogeneities from the time the mode enters the hubble radius until it turns around. Comparison of the results with the standard spherical model shows that the analytical approximation is quite good even in the nonlinear regime. Detailed application to cold dark matter models and seeded models are given.
Astrophysical magnetic fields and nonlinear dynamo theory
(2006-01-10) Brandenburg, Axel; Subramanian, Kandaswamy
The current understanding of astrophysical magnetic ﬁelds is reviewed, focusing on their generation and maintenance by turbulence. In the astrophysical context this generation is usually explained by a self-excited dynamo, which involves ﬂows that can amplify a weak ‘seed’ magnetic ﬁeld exponentially fast. Particular emphasis is placed on the nonlinear saturation of the dynamo. Analytic and numerical results are discussed both for small scale dynamos, which are completely isotropic, and for large scale dynamos, where some form of parity breaking is crucial. Central to the discussion of large scale dynamos is the so-called alpha effect which explains the generation of a mean ﬁeld if the turbulence lacks mirror symmetry, i.e. if the ﬂow has kinetic helicity. Large scale dynamos produce small scale helical ﬁelds as a waste product that quench the large scale dynamo and hence the alpha effect. With this in mind, the microscopic theory of the alpha effect is revisited in full detail and recent results for the loss of helical magnetic ﬁelds are reviewed.
CMB Anisotropy Due to Tangled magnetic fields in re-ionized models
(2005-04-01) Seshadri, T. R.; Subramanian, Kandaswamy
Primordial tangled cosmological Magnetic Fields source rotational velocity perturbations of the baryon ﬂuid, even in the post-recombination universe. These vortical modes inturn leave a characteristic imprint on the temperature anisotropy of the Cosmic Microwave Background (CMB), if the CMB photons can be re-scatterred after recombination. Observations from WMAP in- dicate that the Universe underwent a relatively early re-ionization (zri ∼ 15), which does indeed lead to a signiﬁcant optical depth for re-scattering of CMB photons after the re-ionization epoch. We compute the resulting additional temperature anisotropies, induced by primordial magnetic ﬁelds in the postrecombination universe. We show that in models with early re-ionization, a nearly scale-invariant spectrum of tangled magnetic ﬁelds which redshift to a present value of B0 ∼ 3 × 10−9 Gauss, produce vector mode perturbations which in turn induce additional temperature anisotropy of about 0.3 to 0.4 µK over very small angular scales, with l upto ∼ 10000 or so.98.62.En, 98.70.Vc, 98.80.Cq, 95.30.Qd
Constrained semi-analytical models of Galactic outflows
(2008-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
We present semi-analytic models of galactic outﬂows that are constrained by available ob- servations on high redshift star formation and reionization. Galactic outﬂows are modeled in a manner akin to models of stellar wind blown bubbles. Large scale outﬂows can generically escape from low mass halos (M . 109 M ) for a wide range of model parameters while this is not the case in high mass halos (M & 1011 M ). The ﬂow generically accelerates within the halo virial radius, then starts to decelerate, and traverses well into the intergalactic medium (IGM), before freezing to the Hubble ﬂow. The acceleration phase can result in shell fragmentation due to the Rayleigh-Taylor instability, although the ﬁnal outﬂow radius is not signiﬁcantly altered. The gas phase metallicity of the outﬂow and within the galaxy are computed assuming uniform instantaneous mixing. Ionization states of different metal species are calculated and used to examine the detectability of metal lines from the outﬂows. The global inﬂuence of galactic outﬂows is also investigated using porosity weighted averages and probability density functions of various physical quantities. Models with only atomic cooled halos signiﬁcantly ﬁll the IGM at z 3 with metals (with 2:5 & [Z=Z ] & 3:7), the actual extent depending on the efﬁciency of winds, the initial mass function (IMF) and the fractional mass that goes through star formation. The reionization history has a signiﬁcant effect on the volume ﬁlling factor, due to radiative feedback. In these models, a large fraction of outﬂows at z 3 are supersonic, hot (T 105K) and have low density, making metal lines difﬁcult to detect. They may also result in signiﬁcant perturbations in the IGM gas on scales probed by the Lyman- forest. On the contrary, models including molecular cooled halos with a normal mode of star formation can potentially volume ﬁll the universe at z 8 without drastic dynamic effects on the IGM, thereby setting up a possible metallicity ﬂoor ( 4:0 [Z=Z ] 3:6). The order unity ﬂuctuations at z 8 that becomes the mildly non-linear ﬂuctuations traced by Lyman- forest at z < 4 will then have this metallicity. Interestingly, molecular cooled halos with a “top-heavy” mode of star formation are not very successful in establishing the metallicity ﬂoor because of the additional radiative feedback, that they induce.
Cosmic Microwave Background Bispectrum from Primordial Magnetic Fields on Large Angular Scales
(2009-09-01) Seshadri, T. R.; Subramanian, Kandaswamy
Primordial magnetic ﬁelds lead to non-Gaussian signals in the Cosmic Microwave Background (CMB) even at the lowest order, as magnetic stresses, and the temperature anisotropy they induce, depend quadratically on the magnetic ﬁeld. In contrast, CMB non-Gaussianity due to inﬂationary scalar perturbations arise only as a higher order effect. We propose here a novel probe of stochastic primordial magnetic ﬁelds that exploits the characteristic CMB non-Gaussianity that they induce. In particular, we compute the CMB bispectrum (bl 1 l2 l3)induced by stochastic primordial ﬁelds on large angular scales. We ﬁnd a typical value of l1(l1 + 1)l3(l3 + 1)bl1 l2 l3∼ 10−22, for magnetic ﬁelds of strength B0 ∼ 3 nano Gauss and with a nearly scale invariantmagnetic spectrum. Current observational limits on the bispectrum allow us to set upper limits on B0 ∼ 35 nano Gauss, which can be improved by including other magnetically induced contributions to the bispectrum
Cosmic Microwave Background Trispectrum and Primordial Magnetic Field Limits
(2012-06-08) Trivedi, Pranjal; Seshadri, T. R.; Subramanian, Kandaswamy
Primordial magnetic fields will generate non-Gaussian signals in the cosmic microwave background (CMB) as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field. We compute a new measure of magnetic non-Gaussianity, the CMB trispectrum, on large angular scales, sourced via the Sachs-Wolfe effect. The trispectra induced by magnetic energy density and by magnetic scalar anisotropic stress are found to have typical magnitudes of approximately a few times 10 29 and 10 19, respectively. Observational limits on CMB non-Gaussianity from WMAP data allow us to conservatively set upper limits of a nG, and plausibly sub-nG, on the present value of the primordial cosmic magnetic field. This represents the tightest limit so far on the strength of primordial magnetic fields, on Mpc scales, and is better than limits from the CMB bispectrum and all modes in the CMB power spectrum. Thus, the CMB trispectrum is a new and more sensitive probe of primordial magnetic fields on large scales.
Cosmic ray driven outflows from high redshift galaxies
(2010-01-10) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
We study winds in high redshift galaxies driven by a relativistic cosmic ray (proton) component in addition to the hot thermal gas component. Cosmic rays (CRs) are likely to be efﬁciently generated in supernova(SNe)shocks inside galaxies. We obtain solutions of such CR driven free winds in a gravitational potential of the Navarro-Frenk-White (NFW) form, relevant to galaxies. Cosmic rays naturally provide the extra energy and/or momentum input to the system, needed for a transonic wind solution in a gas with adiabatic index = 5=3.We show that cosmic rays can effectively drive winds even when the thermal energy of the gas is lost due to radiative cooling. These wind solutions predict an asymptotic wind speed closely related to the circular velocity of the galaxy. Furthermore, the mass outﬂow rate per unit star formation rate ( w) is predicted to be ~ 0:2 0:5 for massive galaxies, with masses M ~ 1011 1012M .We show w to be inversely proportional to the square of the circular velocity. Magnetic ﬁelds at the G levels are also required in these galaxies to have a signiﬁcant mass loss. A large w for small mass galaxies implies that cosmic ray driven outﬂows could provide a strong negative feedback to the star formation in dwarf galaxies. Further, our results will also have important implications to the metal enrichment of the intergalactic medium. These conclusions are applicable to the class of free wind models where the source region is conﬁned to be within the sonic point.
Dynamo Models: where do we stand?
(2014-12-09) Subramanian, Kandaswamy; Bhat, Pallavi; Brandenburg, Axel
Effects of curvature and interactions on the dynamics of the deconfinement phase transition
(2011-07-06) Subramanian, Kandaswamy; Seshadri, T. R.; Barrow, John D.
Evolving turbulence and magnetic fields in galaxy clusters
(2006-01-10) Subramanian, Kandaswamy; Shukurov, A.; Haugen, N. E. L.
We discuss, using simple analytical models and MHD simulations, the origin and parameters of turbulence and magnetic ﬁelds in galaxy clusters. Any pre-existing tangled magnetic ﬁeld must decay in a few hundred million years by generating gas motions even if the electric conductivity of the intracluster gas is high. We argue that tur- bulent motions can be maintained in the intracluster gas and its dynamo action can prevent such a decay and amplify a random seed magnetic ﬁeld by a net factor typically 10⁴ in 5Gyr. Three physically distinct regimes can be identiﬁed in the evolution of turbulence and magnetic ﬁeld in galaxy clusters. Firstly, the ﬂuctuation dynamo will produce microgauss-strong, random magnetic ﬁelds during the epoch of cluster formation and major mergers. At this stage pervasive turbulent ﬂows with r.m.s. velocity of about 300 kms−ᶥ can be maintained at scales 100–200 kpc. The magnetic ﬁeld is intermittent, has a smaller scale of 20–30 kpc and average strength of 2 G. Secondly, turbulence will decay after the end of the major merger epoch; we discuss the dynamics of the decaying turbulence and the behavior of magnetic ﬁeld in it. Magnetic ﬁeld and turbulent speed undergo a power-law decay, decreasing by a factor of two during this stage, whereas their scales increase by about the same factor. Thirdly, smaller-mass subclusters and cluster galaxies will produce turbulent wakes where magnetic ﬁelds will be generated as well. Although the wakes plausibly occupy only a small fraction of the cluster volume, we show that their area covering factor can be close to unity, and thus they can produce some of the signatures of turbulence along virtually all lines of sight. The latter could potentially allow one to reconcile the possibility of turbulence with ordered ﬁlamentary gas structures, as in the Perseus cluster. The turbulent speeds and magnetic ﬁelds in the wakes are estimated to be of order 300 kms−ᶥ and 2 G, respectively, whereas the turbulent scales are of order 200 kpc for wakes behind subclusters of a mass 3 × 10ᶥᶟM⊙ and about 10 kpc in the galactic wakes. Magnetic ﬁeld in the wakes is intermittent and has the scale of about 30 kpc and 1 kpc in the subcluster and galactic wakes, respectively. Random Faraday rotation measure is estimated to be typically 100–200 radm−², in agreement with observations. We predict detectable polarization of synchrotron emission from cluster radio halos at wavelengths 3–6 cm, if observed at suﬃciently high resolution.
Focusing equations, caustics and the condition for multiple imaging by thick gravitational lenses
(Royal Astronomical Society, 1987-12-27) Padmanabhan, T.; Subramanian, Kandaswamy
The condition for the production of multiple images by an arbitrary (thick or thin) gravitational lens are studied. We show that the necessary and sufficient condition for the production of multiple images by a lens is the following: The lens should produce a point conjugate to the observer, along some null geodesic, at an affine distance smaller than that of the source. It is shown that previous results on multiple imaging by thin lenses can be obtained as a special case. We also show that a thick lens cannot be more efficient than a suitably designed thin lens for the production of multiple images.
Galactic dynamo action in presence of stochastic alpha and shear
(2008-10) Sur, Sharanya; Subramanian, Kandaswamy
Using a one-dimensional αω-dynamo model appropriate to galaxies, we study the possibility of dynamo action driven by a stochastic alpha effect and shear. To determine the ﬁeld evolution, one needs to examine a large number of different realizations of the stochastic component of α. The net growth or decay of the ﬁeld depends not only on the dynamo parameters but also on the particular realization, the correlation time of the stochastic α compared to turbulent diffusion timescale and the time over which the system is evolved. For dynamos where both a coherent and ﬂuctuating α are present, the stochasticity of α can help alleviate catastrophic dynamo quenching, even in the absence of helicity ﬂuxes. One can obtain ﬁnal ﬁeld strengths up to a fraction ∼ 0.01 of the equipartition ﬁeld Beq for dynamo numbers |D| ∼ 40, while ﬁelds comparable to Beq require much larger degree of α ﬂuctuations or shear. This type of dynamomay be particularly useful for amplifying ﬁelds in the central regions of disk galaxies.
Galactic dynamo and helicity losses through fountain flow
(2006-01-19) Shukurov, A.; Sokoloff, Dmitry; Subramanian, Kandaswamy
Nonlinear behaviour of galactic dynamos is studied, allowing for magnetic helicity removal by the galactic fountain ﬂow. Methods. A suitable advection speed is estimated, and a one-dimensional mean-ﬁeld dynamo model with dynamic α-effect is explored. Results. It is shown that the galactic fountain ﬂow is eﬃcient in removing magnetic helicity from galactic discs. This alleviates the constraint on the galactic mean-ﬁeld dynamo resulting from magnetic helicity conservation and thereby allows the mean magnetic ﬁeld to saturate at a strength comparable to equipartition with the turbulent kinetic energy.
Galactic dynamos supported by magnetic helicity fluxes
(2007-03-08) Sur, Sharanya; Shukurov, A.; Subramanian, Kandaswamy
We present a simple semi-analytical model of nonlinear, mean-ﬁeld galactic dynamos and use it to study the effects of various magnetic helicity ﬂuxes. The dynamo equations are reduced using the ‘no-z’ approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity ﬂux. We discuss the implications and interplay of two types of magnetic helicity ﬂux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising fromanisotropy of turbulence as suggested by Vishniac & Cho (2001). We argue that the latter is signiﬁcant if the galactic differential rotation is strong enough: in ourmodel, forRω . −10 in terms of the corresponding turbulent magnetic Reynolds number. We conﬁrm that the intensity of gas outﬂow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic ﬁeld supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity ﬂux can boost magnetic ﬁeld further to achieve energy equipartition with turbulence. For stronger outﬂows that may occur in starburst galaxies, the Vishniac-Cho ﬂux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic ﬁeld of at least≃ 1 Gto be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.
Galaxy Formation
(Astronomical Society of India, 1992-10-15) Padmanabhan, T.; Subramanian, Kandaswamy
Hyperdiffusion in non-linear, large and small-scale turbulent dynamos
(2011-07-05) Subramanian, Kandaswamy
The generation of large-scale magnetic ﬁelds is generically accompanied by the more rapid growth of small-scale ﬁelds. The growing Lorentz force due to these ﬁelds back reacts on the turbulence to saturate the mean-ﬁeld and small-scale dynamos. For the mean-ﬁeld dynamo, in a quasi-linear treatment of this saturation, it is generally thought that, while the alpha-eﬀect gets renormalised and suppressed by non-linear eﬀects, the turbulent diﬀusion is left unchanged. We show here that this is not true and the eﬀect of the Lorentz forces, is also to generate additional non-linear hyperdiﬀusion of the mean ﬁeld. A combination of such non-linear hyperdiﬀusion with diﬀusion at small scales, also arises in a similar treatment of small-scale dynamos, and is crucial to understand its saturation.
Kinematic alpha effect in isotropic turbulence simulations
(2008-01) Sur, Sharanya; Brandenburg, Axel; Subramanian, Kandaswamy
Using numerical simulations at moderate magnetic Reynolds numbers up to 220 it is shown that in the kinematic regime, isotropic helical turbulence leads to an alpha effect and a turbulent diffusivitywhose values are independent of the magnetic Reynolds number,Rm, provided Rm exceeds unity. These turbulent coefﬁcients are also consistent with expectations from the ﬁrst order smoothing approximation. For small values of Rm, alpha and turbulent diffusivity are proportional to Rm. Over ﬁnite time intervals meaningful values of alpha and turbulent diffusivity can be obtained even when there is small-scale dynamo action that produces strong magnetic ﬂuctuations. This suggests that small-scale dynamo-generated ﬁelds do not make a correlated contribution to the mean electromotive force.
Kinetic and magnetic alpha effects in nonlinear dynamo theory
(2007-01-19) Sur, Sharanya; Subramanian, Kandaswamy; Brandenburg, Axel
The backreaction of the Lorentz force on the α-effect is studied in the limit of small magnetic and ﬂuid Reynolds numbers, using the ﬁrst order smoothing approximation (FOSA) to solve both the induction and momentum equations. Both steady and time dependent forcings are considered. In the low Reynolds number limit, the velocity and magnetic ﬁelds can be expressed explicitly in terms of the forcing function. The nonlinear α-effect is then shown to be expressible in several equivalent forms in agreement with formalisms that are used in various closure schemes. On the one hand, one can express α completely in terms of the helical properties of the velocity ﬁeld as in traditional FOSA, or, alternatively, as the sum of two terms, a so-called kinetic α-effect and an oppositely signed term proportional to the helical part of the small scale magnetic ﬁeld. These results hold for both steady and time dependent forcing at arbitrary strength of the mean ﬁeld. In addition, the τ-approximation is considered in the limit of small ﬂuid and magnetic Reynolds numbers. In this limit, the τ closure term is absent and the viscous and resistive terms must be fully included. The underlying equations are then identical to those used under FOSA, but they reveal interesting differences between the steady and time dependent forcing. For steady forcing, the correlation between the forcing function and the small-scale magnetic ﬁeld turns out to contribute in a crucial manner to determine the net α-effect. However for delta-correlated time-dependent forcing, this force–ﬁeld correlation vanishes, enabling one to write α exactly as the sum of kinetic and magnetic α-effects, similar to what one obtains also in the large Reynolds number regime in theτ-approximation closure hypothesis. In the limit of strong imposed ﬁelds, B0, we ﬁnd α ∝ B−2 0 for delta-correlated forcing, in contrast to the well-known α ∝ B−3 0 behaviour for the case of a steady forcing. The analysis presented here is also shown to be in agreement with numerical simulations of steady as well as random helical ﬂows.
Magnetic helicity in galactic dynamos
(2002-07-06) Subramanian, Kandaswamy
Magnetic ﬁelds correlated on kiloparsec scales are seen in spiral galaxies. Their origin could be due to ampliﬁcation of a small seed ﬁeld by a turbulent galactic dynamo. We review the current status of the galactic dynamo, especially the constraints imposed by magnetic helicity conservation. We estimate the minimal strength of the large-scale magnetic ﬁeld which could arise inspite of the helicity constraint
Magnetic helicity in stellar dynamos : new numerical experiments
(2001-01-04) Axel, Brandenburg; Wolfgang, Dobler; Subramanian, Kandaswamy
The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation has grown to the maximum possible value. A resistively limited evolution towards saturation is also found at intermediate scales before the largest scale of the system is reached. Larger aspect ratios can give rise to different structures of the mean ﬁeld which are obtained at early times, but the ﬁnal saturation ﬁeld strength is still decreasing with decreasing resistivity. In the presence of shear, cyclic magnetic ﬁelds are found whose period is increasing with decreasing resistivity, but the saturation energy of the mean ﬁeld is in strong super-equipartition with the turbulent energy. It is shown that artiﬁcially induced losses of small scale ﬁeld of opposite sign of magnetic helicity as the large scale ﬁeld can, at least in principle, accelerate the production of large scale (poloidal) ﬁeld. Based on mean ﬁeld models with an outer potential ﬁeld boundary condition in spherical geometry, we verify that the sign of the magnetic helicity ﬂux from the large scale ﬁeld agrees with the sign of α. For solar parameters, typical magnetic helicity ﬂuxes lie around 1047 Mx2 per cycle.