2009 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/332
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Item Magnetic quenching of alpha and diffusity tensors in helical turbulence(2011-07-06) Brandenburg, Axel; Biman, B.; Subramanian, Kandaswamy; et al.We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissipation of primordial magnetic fields owing to ambipolar diffusion and decaying turbulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hydrogen.We show that, for magnetic field strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of first structures in the universeItem Primordial magnetic fields and formation of molecular hydrogen(2009-04-01) Sethi, Shiv K.; Nath, B. B.; Subramanian, Kandaswamy; et al.We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissi- pation of primordial magnetic fields owing to ambipolar diffusion and decaying tur- bulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hy- drogen.We show that, for magnetic field strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of first structures in the universeItem Understanding the redshift evolution of the luminosity functions of Lyman-alpha emitters(2009-06-01) Samui, Saumyadip; Srianand, R.; Subramanian, KandaswamyWe present a semi-analytical model of star formation which explains simultaneously the observed UV luminosity function of high redshift Lyman break galaxies (LBGs) and luminosity functions of Lyman-α emitters. We consider both models that use the Press-Schechter (PS) and Sheth-Tormen (ST) halo mass functions to calculate the abundances of dark matter halos. The Lyman-α luminosity functions at z . 4 are well reproduced with only . 10% of the LBGs emitting Lyman-α lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed luminosity function at z > 5 can be reproduced only when we assume that nearly all LBGs are Lyman-α emitters. Thus it appears that 4 < z < 5 marks the epoch when a clear change occurs in the physical properties of the high redshift galaxies. As Lyman-α escape depends on dust and gas kinematics of the inter stellar medium (ISM), this could mean that on an average the ISM at z > 5 could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman-α photons. At z > 5 the observed Lyman-α luminosity function are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high redshift galaxies. In particular, upto z = 6.5, we do not see the effect of evolving inter galactic medium (IGM) opacity on the Lyman-α escape from these galaxiesItem Shear dynamo problem: Quasilinear kinematic theory(2009-04-01) Sridhar, S.; Subramanian, KandaswamyLarge–scale dynamo action due to turbulence in the presence of a linear shear flow is studied. Our treatment is quasilinear and kinematic but is non perturbative in the shear strength. We derive the integro–differential equation for the evolution of the mean magnetic field, by systematic use of the shearing coordinate transformation and the Galilean invariance of the linear shear flow. For non helical turbulence the time evolution of the cross–shear components of the mean field do not depend on any other components excepting themselves. This is valid for any Galilean–invariant velocity field, independent of its dynamics. Hence the shear–current assisted dynamo is essentially absent, although large–scale non helical dynamo action is not ruled out.Item Reconnecting Flux Rope Dynamo(2009-10-01) Subramanian, Kandaswamy; Baggaley, Andrew W; Barenghi, Carlo F.; et al.We develop a new model of the fluctuation dynamo in which the magnetic field is confined to thin flux ropes advected by a multi-scale model of turbulence. Magnetic dissipation occurs only via reconnection of the flux ropes. This model can be viewed as an implementation of the asymptotic limit Rm → ∞ for a continuous magnetic field, where magnetic dissipation is strongly localized to small regions of strong field gradients. We investigate the kinetic energy release into heat, mediated by the dynamo action, both in our model and by solving the induction equation with the same flow. We find that a flux rope dynamo is an order of magnitude more efficient at converting mechanical energy into heat. The probability density of the magnetic energy release in reconnections has a power-law form with the slope −3, consistent with the Solar corona heating by nanoflares.Item Non-perturbative quasilinear approach to the shear dynamo problem(2009-10-01) Sridhar, S.; Subramanian, KandaswamyWe study large–scale dynamo action due to turbulence in the presence of a linear shear flow. Our treatment is quasilinear and equivalent to the standard ‘first order smoothing approximation’. However it is non perturbative in the shear strength. We first derive an integro–differential equation for the evolution of the mean magnetic field, by systematic use of the shearing coordinate transformation and the Galilean invariance of the linear shear flow. We show that, for non helical turbulence, the time evolution of the cross–shear components of the mean field do not depend on any other components excepting themselves; this is valid for any Galilean–invariant velocity field, independent of its dynamics. Hence, to all orders in the shear parameter, there is no shear–current type effect for non helical turbulence in a linear shear flow, in quasilinear theory in the limit of zero resistivity. We then develop a systematic approximation of the integro–differential equation for the case when the mean magnetic field varies slowly compared to the turbulence correlation time. For non-helical turbulence, the resulting partial differential equations can again be solved by making a shearing coordinate transformation in Fourier space. The resulting solutions are in the form of shearing waves, labeled by the wavenumber in the sheared coordinates. These shearing waves can grow at early and intermediate times but are expected to decay in the long time limit. small regions of strong field gradients. We investigate the kinetic energy release into heat, mediated by the dynamo action, both in our model and by solving the induction equation with the same flow. We find that a flux rope dynamo is an order of magnitude more efficient at converting mechanical energy into heat. The probability density of the magnetic energy release in reconnections has a power-law form with the slope −3, consistent with the Solar corona heating by nanoflares.Item Models of high redshift luminosity functions and galactic outflows: The dependence on halo mass function(2009-02-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.The form of the halo mass function is a basic ingredient in any semi-analytical galaxy formation model. We study the existing forms of the mass functions in the literature and compare their predictions for semi-analytical galaxy formation models. Two methods are used in the literature to compute the net formation rate of halos, one by simply taking the derivative of the halo mass function and the other using the prescription due to Sasaki (1994). For the historically used Press-Schechter (PS) mass function, we compare various model predictions, using these two methods. However, as the Sasaki formalism cannot be easily generalized for other mass functions, we use the derivative while comparing model predictions of di erent mass functions. We show that the reionization history and UV luminosity function of Lyman break galaxies (LBGs) predicted by the PS mass function di ers from those using any other existing mass function, like Sheth-Tormen (ST) mass func- tion. In particular the reionization e ciency of molecular cooled halos has to be substantially reduced when one uses the ST and other mass functions obtained from the simulation instead of the PS mass function. Using 2 - minimization, we nd that the observed UV luminosity functions of LBGs at 3:0 z 7:4 are better reproduced by models using the ST mass function compared to models that use the PS mass function. On the other hand, the volume lling factor of the metals expelled from the galaxies through super- novae driven out ows di ers very little between models with di erent mass functions. It depends on the way we treat merging out ows. We also show that the porosity weighted average quantities related to the out ow are not very sensitive to the di erences in the halo mass functionItem Cosmic Microwave Background Bispectrum from Primordial Magnetic Fields on Large Angular Scales(2009-09-01) Seshadri, T. R.; Subramanian, KandaswamyPrimordial 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