2008 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/333
Browse
3 results
Search Results
Item Kinematic alpha effect in isotropic turbulence simulations(2008-01) Sur, Sharanya; Brandenburg, Axel; Subramanian, KandaswamyUsing 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.Item Galactic dynamo action in presence of stochastic alpha and shear(2008-10) Sur, Sharanya; Subramanian, KandaswamyUsing 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.Item 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.