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We numerically study axion-U(1) inflation, focusing on the regime where the coupling between axions and gauge fields results in significant backreaction from the amplified gauge fields during inflation. These amplified gauge fields not only generate high-frequency gravitational waves (GWs) but also induce spatial inhomogeneities in the axion field, which can lead to the formation of primordial black holes (PBHs). Both GWs and PBHs serve as key probes for constraining the coupling strength between the axion and gauge fields. We find that, when backreaction is important during inflation, the constraints on the coupling strength due to GW overproduction are relaxed compared to previous studies, in which backreaction matters only after inflation. For PBH formation, understanding the probability density function (PDF) of axion field fluctuations is crucial. While earlier analytical studies assumed that these fluctuations followed a $\chi^2$-distribution, our results suggest that the PDF tends toward a Gaussian distribution in cases where gauge field backreaction is important, regardless whether during or after inflation. We also calculate the spectrum of the produced magnetic fields in this model and find that their strength is compatible with the observed lower limits., Comment: 28 pages, 12 figures

We explore the redshift evolution of the radio luminosity function (RLF) of star-forming galaxies using GALFORM, a semi-analytic model of galaxy formation and a dynamo model of the magnetic field evolving in a galaxy. Assuming energy equipartition between the magnetic field and cosmic rays, we derive the synchrotron luminosity of each sample galaxy. In a model where the turbulent speed is correlated with the star formation rate, the RLF is in fair agreement with observations in the redshift range $0 \leq z \leq 2$. At larger redshifts, the structure of galaxies, their interstellar matter and turbulence appear to be rather different from those at $z\lesssim2$, so that the turbulence and magnetic field models applicable at low redshifts become inadequate. The strong redshift evolution of the RLF at $0 \leq z \leq 2$ can be attributed to an increased number, at high redshift, of galaxies with large disc volumes and strong magnetic fields. On the other hand, in models where the turbulent speed is a constant or an explicit function of $z$, the observed redshift evolution of the RLF is poorly captured. The evolution of the interstellar turbulence and outflow parameters appear to be major (but not the only) drivers of the RLF changes. We find that both the small- and large-scale magnetic fields contribute to the RLF but the small-scale field dominates at high redshifts. Polarisation observations will therefore be important to distinguish these two components and understand better the evolution of galaxies and their nonthermal constituents., Comment: 19 pages, 8 figures. MNRAS, in press

The structure of magnetic fields in galaxies remains poorly constrained, despite the importance of magnetism in the evolution of galaxies. Radio synchrotron and far-infrared (FIR) polarization and polarimetric observations are the best methods to measure galactic scale properties of magnetic fields in galaxies beyond the Milky Way. We use synthetic polarimetric observations of a simulated galaxy to identify and quantify the regions, scales, and interstellar medium (ISM) phases probed at FIR and radio wavelengths. Our studied suite of magnetohydrodynamical cosmological zoom-in simulations features high-resolutions (10 pc full-cell size) and multiple magnetization models. Our synthetic observations have a striking resemblance to those of observed galaxies. We find that the total and polarized radio emission extends to approximately double the altitude above the galactic disk (half-intensity disk thickness of $h_\text{I radio} \sim h_\text{PI radio} = 0.23 \pm 0.03$ kpc) relative to the total FIR and polarized emission that are concentrated in the disk midplane ($h_\text{I FIR} \sim h_\text{PI FIR} = 0.11 \pm 0.01$ kpc). Radio emission traces magnetic fields at scales of $\gtrsim 300$ pc, whereas FIR emission probes magnetic fields at the smallest scales of our simulations. These scales are comparable to our spatial resolution and well below the spatial resolution ($<300$ pc) of existing FIR polarimetric measurements. Finally, we confirm that synchrotron emission traces a combination of the warm neutral and cold neutral gas phases, whereas FIR emission follows the densest gas in the cold neutral phase in the simulation. These results are independent of the ISM magnetic field strength. The complementarity we measure between radio and FIR wavelengths motivates future multiwavelength polarimetric observations to advance our knowledge of extragalactic magnetism., Comment: Accepted for publication in the ApJ. Replaced to match final version. 35 pages, 17 figures, 4 tables

We study the magneto-rotational instability (MRI) driven dynamo in a geometrically thin disc ($H/R\ll 1$) using stratified zero net flux (ZNF) shearing box simulations. We find that mean fields and EMFs oscillate with a primary frequency $f_{\rm dyn} = 0.017$ ($\approx 9$ orbital period), but also have higher harmonics at $3f_{\rm dyn}$. Correspondingly, the current helicity, has two frequencies $2f_{\rm dyn}$ and $4f_{\rm dyn}$ respectively, which appear to be the beat frequencies of mean fields and EMFs as expected from the magnetic helicity density evolution equation. Further, we adopt a novel inversion algorithm called the `Iterative Removal Of Sources' (IROS), to extract the turbulent dynamo coefficients in the mean-field closure using the mean magnetic fields and EMFs obtained from the shearing box simulation. We show that an $\alpha-$effect ($\alpha_{yy}$) is predominantly responsible for the creation of the poloidal field from the toroidal field, while shear generates back a toroidal field from the poloidal field; indicating that an $\alpha-\Omega$-type dynamo is operative in MRI-driven accretion discs. We also find that both strong outflow ($\bar{v}_z$) and turbulent pumping ($\gamma_z$ ) transport mean fields away from the mid-plane. Instead of turbulent diffusivity, they are the principal sink terms in the mean magnetic energy evolution equation. We find encouraging evidence that a generative helicity flux is responsible for the effective $\alpha$-effect. Finally, we point out potential limitations of horizontal ($x-y$) averaging in defining the `mean' on the extraction of dynamo coefficients and their physical interpretations., Comment: 16 pages, 13 figures, published in MNRAS

We describe a novel method to compute the components of dynamo tensors from direct magnetohydrodynamic (MHD) simulations. Our method relies upon an extension and generalisation of the standard H\"ogbom CLEAN algorithm widely used in radio astronomy to systematically remove the impact of the strongest beams onto the corresponding image. This generalisation, called the Iterative Removal of Sources (IROS) method, has been adopted here to model the turbulent electromotive force (EMF) in terms of the mean magnetic fields and currents. Analogous to the CLEAN algorithm, IROS treats the time series of the mean magnetic field and current as beams that convolve with the dynamo coefficients which are treated as (clean) images to produce the EMF time series (the dirty image). We apply this method to MHD simulations of galactic dynamos, to which we have previously employed other methods of computing dynamo coefficients such as the test-field method, the regression method, as well as local and non-local versions of the singular value decomposition (SVD) method. We show that our new method reliably recovers the dynamo coefficients from the MHD simulations. It also allows priors on the dynamo coefficients to be incorporated easily during the inversion, unlike in earlier methods. Moreover, using synthetic data, we demonstrate that it may serve as a viable post-processing tool in determining the dynamo coefficients, even when the power of additive noise to the EMF is twice as much the actual EMF., Comment: 11 pages, 9 figures

Using magnetohydrodynamic simulations of fluctuation dynamos in turbulent flows with rms Mach numbers $\mathcal{M}_{\rm rms} = 0.2, 1.1$ and $3$, we show that magnetic pressure forces play a crucial role in dynamo saturation in supersonic flows. First, as expected when pressure forces oppose compression, an increase in anticorrelation between density and magnetic field strengths obtains even in subsonic flows with the anti-correlation arising from the intense but rarer magnetic structures. In supersonic flows, due to stronger compressive motions density and magnetic field strength continue to maintain a positive correlation. However, the degree of positive correlation decreases as the dynamo saturates. Secondly, we find that the unit vectors of $\nabla\rho$ and $\nabla B^{2}$ are preferentially antiparallel to each other in subsonic flows. This is indicative of magnetic pressure opposing compression. This antiparallel alignment persists in transonic and supersonic flows at dynamo saturation. However, compressive motions also lead to the emergence of a parallel alignment in these flows. Finally, we consider the work done against the components of the Lorentz force and the different sources of magnetic energy growth and dissipation. We show that while in subsonic flows, suppression of field line stretching is dominant in saturating the dynamo, the picture is different in supersonic flows. Both field line stretching and compression initially amplifies the field. However, growing magnetic pressure opposes further compression of magnetic flux which tends to reduce the compressive motions. Simultaneously, field line stretching also reduces. But, suppression of compressive amplification dominates the saturation of the dynamo., Comment: Accepted for publication in MNRAS

We present the analysis of the magnetic field ($B$-field) structure of galaxies measured with far-infrared (FIR) and radio (3 and 6 cm) polarimetric observations. We use the first data release of the Survey on extragALactic magnetiSm with SOFIA (SALSA) of 14 nearby ($<20$ Mpc) galaxies with resolved (5 arcsec-18 arcsec; $90$ pc--$1$ kpc) imaging polarimetric observations using HAWC+/SOFIA from $53$ to $214$ \um. We compute the magnetic pitch angle ($\Psi_{B}$) profiles as a function of the galactrocentric radius. We introduce a new magnetic alignment parameter ($\zeta$) to estimate the disordered-to-ordered $B$-field ratio in spiral $B$-fields. We find FIR and radio wavelengths to not generally trace the same $B$-field morphology in galaxies. The $\Psi_{B}$ profiles tend to be more ordered with galactocentric radius in radio ($\zeta_{\rm{6cm}} = 0.93\pm0.03$) than in FIR ($\zeta_{\rm{154\mu m}} = 0.84\pm0.14$). For spiral galaxies, FIR $B$-fields are $2-75$\% more turbulent than the radio $B$-fields. For starburst galaxies, we find that FIR polarization is a better tracer of the $B$-fields along the galactic outflows than radio polarization. Our results suggest that the $B$-fields associated with dense, dusty, turbulent star-forming regions, those traced at FIR, are less ordered than warmer, less-dense regions, those traced at radio, of the interstellar medium. The FIR $B$-fields seem to be more sensitive to the activity of the star-forming regions and the morphology of the molecular clouds within a vertical height of few hundred pc in the disk of spiral galaxies than the radio $B$-fields., Comment: 28 pages, 14 figures. Accepted for publication in The Astrophysical Journal

Fluxes of the magnetic helicity density play an important role in large-scale turbulent dynamos, allowing the growth of large-scale magnetic fields while overcoming catastrophic quenching. We show here, analytically, how several important types of magnetic helicity fluxes can arise from terms involving triple correlators of fluctuating fields in the helicity density evolution equation. For this, we assume incompressibility and weak inhomogeneity, and use a quasinormal closure approximation: fourth-order correlators are replaced by products of second-order ones, and the effect of the fourth-order cumulants on the evolution of the third moments is modelled by a strong damping term. First, we show how a diffusive helicity flux, till now only measured in simulations, arises from the triple correlation term. This is accompanied by what we refer to as a `random advective flux', which predominantly transports magnetic helicity along the gradients of the random fields. We also find that a new helicity flux contribution, in some aspects similar to that first proposed by Vishniac, can arise from the triple correlator. This contribution depends on the gradients of the random magnetic and kinetic energies along the large-scale vorticity, and thus arises in any rotating, stratified system, even if the turbulence is predominantly nonhelical. It can source a large-scale dynamo by itself while spatially transporting magnetic helicity within the system., Comment: 19 pages. Version accepted by ApJ. Changes from v1: More prominent discussion of prior work in sec 4.2. Full expression for the helicity fluxes (earlier equation 12) has now been moved to appendix E. Clarified the closure used. Corrected some typos in the explanation of the VC approximation (no change in the presented results)

We present the first data release of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program) with a set of 14 nearby ($<20$ Mpc) galaxies with resolved imaging polarimetric observations using HAWC+ from $53$ to $214$ $\mu$m at a resolution of $5-18$" ($90$ pc $-$ $1$ kpc). We introduce the definitions and background on extragalactic magnetism, and present the scientific motivation and sample selection of the program. Here, we focus on the general trends in the emissive polarization fraction. Far-infrared polarimetric observations trace the thermal polarized emission of magnetically aligned dust grains across the galaxy disks with polarization fractions of $P=0-15$% in the cold, $T_{\rm d} = [19,48]$ K, and dense, $\log_{10}(N_{\rm HI+H_{2}}) = [19.96,22.91]$, interstellar medium. The spiral galaxies show a median $\langle P_{154\mu m} \rangle = 3.3\pm0.9 $% across the disks. We report the first polarized spectrum of starburst galaxies showing a minimum within $89-154$ $\mu$m. The falling $53-154$ $\mu$m polarized spectrum may be due to a decrease in the dust grain alignment efficiency produced by variations in dust temperatures along the line-of-sight in the galactic outflow. We find that the starburst galaxies and the star-forming regions within normal galaxies have the lowest polarization fractions. We find that 50% (7 out of 14) of the galaxies require a broken power-law in the $P-N_{HI+H_{2}}$ and $P-T_{d}$ relations with three different trends. Group 1 has a relative increase of anisotropic random B-fields produced by compression or shear of B-fields in the galactic outflows, starburst rings, and inner-bar of galaxies; and Groups 2 and 3 have a relative increase of isotropic random B-fields driven by star-forming regions in the spiral arms, and/or an increase of dust grain alignment efficiency caused by shock-driven regions or evolutionary stages of a galaxy., Comment: 43 pages, 28 figures. Accepted for publication to ApJ

The generation of large-scale magnetic fields ($\overline{\mathbf{B}}$) in astrophysical systems is driven by the mean turbulent electromotive force ($\overline{{\cal E}}$), the cross correlation between local fluctuations of velocity and magnetic fields. This can depend non-locally on $\overline{\mathbf{B}}$ through a convolution kernel $K_{ij}$. In a new approach to find $K_{ij}$, we directly fit the time series data of $\overline{{\cal E}}$ versus $\overline{\mathbf{B}}$ from a galactic dynamo simulation using singular value decomposition. We calculate the usual turbulent transport coefficients as moments of $K_{ij}$, show the importance of including non-locality over eddy length scales to fully capture their amplitudes and that higher order corrections to the standard transport coefficients are small in the present case., Comment: 11 pages, 13 figures

Galactic bars are frequent in disk galaxies and they may support the transfer of matter towards the central engine of active nuclei. The barred galaxy NGC 1097 has magnetic forces controlling the gas flow at several kpc scales, which suggest that magnetic fields (B-fields) are dynamically important along the bar and nuclear ring. However, the effect of the B-field on the gas flows in the central kpc scale has not been characterized. Using thermal polarized emission at $89$ $\mu$m with HAWC+/SOFIA, here, we measure that the polarized flux is spatially located at the contact regions of the outer-bar with the starburst ring. The linear polarization decomposition analysis shows that the $89$ $\mu$m and radio ($3.5$ and $6.2$ cm) polarization traces two different modes, $m$, of the B-field: a constant B-field orientation and dominated by $m=0$ at $89$ $\mu$m, and a spiral B-field dominated by $m=2$ at radio. We show that the B-field at 89 $\mu$m is concentrated in the warmest region of a shock driven by the galactic-bar dynamics in the contact regions between the outer-bar with the starburst ring. Radio polarization traces a superposition of the spiral B-field outside and within the starburst ring. According to Faraday rotation measures between $3.5$ and $6.2$ cm, the radial component of the B-field along the contact regions points toward the galaxy's center on both sides. We conclude that gas streams outside and within the starburst ring follow the B-field, which feeds the black hole with matter from the host galaxy., Comment: 17 pages, 10 figures, Accepted for publication to ApJ

The recent availability of high-resolution far-infrared (FIR) polarization observations of galaxies using HAWC+/SOFIA has facilitated studies of extragalactic magnetic fields in the cold and dense molecular disks.We investigate if any significant structural differences are detectable in the kpc-scale magnetic field of the grand design face-on spiral galaxy M51 when traced within the diffuse (radio) and the dense and cold (FIR) interstellar medium (ISM). Our analysis reveals a complex scenario where radio and FIR polarization observations do not necessarily trace the same magnetic field structure. We find that the magnetic field in the arms is wrapped tighter at 154um than at 3 and 6 cm; statistically significant lower values for the magnetic pitch angle are measured at FIR in the outskirts (R > 7 kpc) of the galaxy. This difference is not detected in the interarm region. We find strong correlations of the polarization fraction and total intensity at FIR and radio with the gas column density and 12CO(1-0) velocity dispersion. We conclude that the arms show a relative increase of small-scale turbulent B-fields at regions with increasing column density and dispersion velocities of the molecular gas. No correlations are found with HI neutral gas. The star formation rate shows a clear correlation with the radio polarized intensity, which is not found in FIR, pointing to a small-scale dynamo-driven B-field amplification scenario. This work shows that multi-wavelength polarization observations are key to disentangling the interlocked relation between star formation, magnetic fields, and gas kinematics in the multi-phase ISM., Comment: 35 pages, 25 figures, 5 tables. Accepted for publication in ApJ

Using magnetohydrodynamic simulations of fluctuation dynamos, we perform broad-bandwidth synthetic observations to investigate the properties of polarized synchrotron emission and the role that Faraday rotation plays in inferring the polarized structures in the intracluster medium (ICM) of galaxy clusters. In the saturated state of the dynamo, we find a Faraday depth (FD) dispersion $\sigma_{\rm FD} \approx 100$ rad m$^{-2}$, in agreement with observed values in the ICM. Remarkably, the FD power spectrum is qualitatively similar to $M(k)/k$, where $M(k)$ is the magnetic spectrum and $k$ the wavenumber. However, this similarity is broken at high $k$ when FD is obtained by applying RM synthesis to polarized emission from the ICM due to poor resolution and complexities of spectrum in FD space. Unlike the Gaussian probability distribution function (PDF) obtained for FD, the PDF of the synchrotron intensity is lognormal. A relatively large $\sigma_{\rm FD}$ in the ICM gives rise to strong frequency-dependent variations of the pixel-wise mean and peak polarized intensities at low frequencies ($\lesssim 1.5\,{\rm GHz}$). The mean fractional polarization $\langle p \rangle$ obtained at the resolution of the simulations increases from $<0.1$ at 0.5 GHz to its intrinsic value of $\sim0.3$ at 6 GHz. Beam smoothing significantly affects the polarization properties below $\lesssim 1.5\,{\rm GHz}$, reducing $\langle p \rangle$ to $\lesssim 0.01$ at 0.5 GHz. At frequencies $\gtrsim 5\,{\rm GHz}$, polarization remains largely unaffected, even when recovered using RM synthesis. Thus, our results underline the need for high-frequency ($\gtrsim 5\,{\rm GHz}$) observations with future radio telescopes to effectively probe the properties of polarized emission in the ICM., Comment: Published in MNRAS

Generation of magnetic fields during inflation is a promising mechanism for the origin of the observed large scale magnetic fields in the universe. Among several attempts, a popular model is one where the inflaton and the electromagnetic field are coupled through a coupling function $f$ leading to a term in the Lagrangian density of the form, $f^2 F^{\mu \nu} F_{\mu\nu}$. A number of potential difficulties with such models have been raised in the literature. In our earlier work, we have suggested viable models of inflationary magnetogenesis which avoid these problems and at the same time can lead to either nonhelical or helical magnetic fields of astrophysical interest. Our models require a low energy scale for inflation and reheating (reheating temperature, $T_R < 10^4$ GeV) and generate a blue spectrum of electromagnetic (EM) field which peaks around the horizon scale of reheating. We show here that the anisotropic stress associated with these EM fields naturally source the production of a stochastic background of Gravitational waves (GW) with frequencies in the range of tens of nano Hertz to milli Hertz. These two extremes of the range can be probed respectively by pulsar timing arrays (PTA) experiments and the upcoming Laser Interferometric Space Array (LISA). The peak value of the GW spectrum energy represented by $d \Omega_{GW}/d \ln k$ is $ 10^{-6} $ for the models which lead to nonhelical primordial fields and $2 \times 10^{-6} $ for the helical case for $T_R=100$ GeV. In this case the spectrum peaks at a frequency $30 \mu$Hz for non helical case and at $40 \mu$Hz for helical case. These values are obtained when the ratio of EM energy density to the cosmological density at reheating $\epsilon \sim 1$ and decrease approximately as $\epsilon^2$ for smaller values., Comment: 25 pages, 5 figures, comments are welcome

We explore the MRI driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least squares fitting dynamo coefficients $\alpha$ and $\gamma$ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient $\gamma$ which transports the mean magnetic flux radially outward. This would have implications for the launching of magnetised jets which are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large scale dynamo-generated poloidal magnetic field can aid jet-launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of $\alpha_{\phi \phi}$, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the `test field' (TF) method. By estimating the relative importance of $\alpha$-effect and shear, we conclude that the MRI driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the $\alpha-\Omega$ type., Comment: 13 pages, 11 figures, published in MNRAS

The origin of large-scale magnetic fields, detected in some low-mass (dwarf and irregular) galaxies via polarised synchrotron emission and Faraday rotation, remained unexplained for a long time. We suggest that mean-field dynamo can be active in galaxies of this class despite their slow rotation because their discs are relatively thick. Earlier assessments of the possibility of the mean-field dynamo action in low-mass galaxies relied on estimates applicable to $\textit{thin}$ discs, such as those in massive spiral galaxies. Using both order-of-magnitude estimates and numerical solutions, we show that the strength of differential rotation required to amplify magnetic field reduces as the aspect ratio of the galactic gas layer increases. Thus, the puzzle of the origin of large-scale magnetic fields appears to be solved. As a result, this class of galaxies provides a new ground for testing our understanding of galactic magnetism., Comment: Article submitted to Galaxies for a special issue "New Perspectives on Galactic Magnetism"

Coherent magnetic fields in disc galaxies are thought to be generated by a large-scale (or mean-field) dynamo operating in their interstellar medium. A key driver of mean magnetic field growth is the turbulent electromotive force (EMF), which represents the influence of correlated small-scale (or fluctuating) velocity and magnetic fields on the mean field. The EMF is usually expressed as a linear expansion in the mean magnetic field and its derivatives, with the dynamo tensors as expansion coefficients. Here, we adopt the singular value decomposition (SVD) method to directly measure these turbulent transport coefficients in a simulation of the turbulent interstellar medium that realizes a large-scale dynamo. Specifically, the SVD is used to least-square fit the time series data of the EMF with that of the mean field and its derivatives, to determine these coefficients. We demonstrate that the spatial profiles of the EMF reconstructed from the SVD coefficients match well with that taken directly from the simulation. Also, as a direct test, we use the coefficients to simulate a 1-D mean-field dynamo model and find an overall similarity in the evolution of the mean magnetic field between the dynamo model and the direct simulation. We also compare the results with those which arise using simple regression and the ones obtained previously using the test-field (TF) method, to find reasonable qualitative agreement. Overall, the SVD method provides an effective post-processing tool to determine turbulent transport coefficients from simulations., Comment: 14 pages, 14 figures

Large-scale magnetic fields in stars and galaxies are thought to arise by mean-field dynamo action due to the combined influence of both helical turbulence and shear. Those systems are also highly conducting and the turbulence therein leads to a fluctuation (or small-scale) dynamo which more rapidly amplifies magnetic field fluctuations on the eddy scales and smaller. Will this then interfere with and suppress the mean (or large-scale) field growth? Using direct numerical simulations of helical turbulence (with and without shear), we identify a novel quasi-kinematic large-scale dynamo which operates as the small-scale dynamo saturates. Thus both dynamos operate efficiently, one after the other, and lead to the generation of significant large-scale fields., Comment: 8 pages, 11 figures. Comments/feedback/discussions are welcome

The origin and maintenance of coherent magnetic fields in the Universe is reviewed with an emphasis on the possible challenges that arise in their theoretical understanding. We begin with the interesting possibility that magnetic fields originated at some level from the early universe. This could be during inflation, the electroweak or the quark-hadron phase transitions. These mechanisms can give rise to fields which could be strong, but often with much smaller coherence scales than galactic scales. Their subsequent turbulent decay decreases their strength but increases their coherence. We then turn to astrophysical batteries which can generate seed magnetic fields. Here the coherence scale can be large, but the field strength is generally very small. These seed fields need to be further amplified and maintained by a dynamo to explain observed magnetic fields in galaxies. Basic ideas behind both small and large-scale turbulent dynamos are outlined. The small-scale dynamo may help understand the first magnetization of young galaxies, while the large-scale dynamo is important for the generation of fields with scales larger than the stirring scale, as observed in nearby disk galaxies. The current theoretical challenges that turbulent dynamos encounter and their possible resolution are discussed., Comment: Invited Review Article for "New perspectives on galactic magnetism", Accepted in Galaxies

The origin and maintenance of large scale magnetic fields in the Universe is discussed. A popular idea is that cosmic batteries generate seed magnetic fields which were amplified by magnetic induction due to turbulent motions, at times combined with differential rotation. We outline a seed field mechanism, the Biermann battery and then consider some basic ideas behind both small and large-scale turbulent dynamos. The small-scale dynamo may help understand magnetism in galaxy clusters and young galaxies, while the large-scale dynamo is important for the generation of fields with scales larger than stirring, as observed in nearby disk galaxies. Another intriguing possibility is that magnetic fields originated at some level from the early universe., Comment: To be published in the M.N. Saha 125th anniversary issue of "Science and Culture"

We fit the recently updated UV luminosity functions (LF) of high-$z$ ($1.5\le z\le8.0$) galaxies using our semi-analytical models of galaxy formation that take into account various feedback processes. In order to reproduce the overall redshift evolution we require the efficiency of converting gas into stars to decrease with decreasing redshift. Even for $z\ge 6$, our models require supernovae (SNe) feedback to reproduce the observed LF suggesting the prevalence of galactic winds that could have polluted the inter-galactic medium even at very high redshifts. The observed LF in the low luminosity end for $z<2.5$ shows an upward turn. In our models we reproduce this trend using passively evolving population of galaxies. Measuring stellar mass, age and metallicity of these galaxies using multi-band spectral energy distribution (SED) fitting will place strong constraints on the existence of such galaxies. While Active galactic Nuclei (AGN) feedback is essential to reproduce the LF at high luminosity end for $z<4$, it may not needed in the case of $z\ge 6$. We show that the expected turn around in the LF due to cooling criteria occurs at luminosity much lower than what has been probed with present day observations. With future deep observations, that can measure the LF more accurately, we will be able to distinguish between different modes of SNe feedback and get insights into the physical processes that drive the galaxy evolution., Comment: 9 pages, 6 figures, 1 table, submitted to MNRAS

We study the generation of helical magnetic fields in a model of inflationary magnetogenesis which is free from the strong coupling and back-reaction problems. To generate helical magnetic fields, we add an $f^2 \tilde{F}^{\mu\nu} F_{\mu\nu}$ term to the lagrangian of Ratra model. The strong coupling and back-reaction problems are avoided if we take a particular behaviour of coupling function $f$, in which $f$ increases during inflation and decreases post inflation to reheating. The generated magnetic field is fully helical and has a blue spectrum, $d\rho_B/d\ln k \propto k^4$. This spectrum is obtained when coupling function $f\propto a^2$ during inflation. The scale of reheating in our model has to be lower than $4000$ GeV to avoid back-reaction post inflation. The generated magnetic field spectrum satisfies the $\gamma$-ray bound for all the possible scales of reheating. The comoving magnetic field strength and its correlation length are $\sim 4 \times 10^{-11} $ G and $70$ kpc respectively, if reheating takes place at 100 GeV. For reheating at the QCD scales of $150$ MeV, the field strength increases to $\sim$ nano gauss, with coherence scale of $0.6$ Mpc., Comment: 11 pages, Submitted to PRD

Observations of Faraday rotation through high-redshift galaxies have revealed that they host coherent magnetic fields that are of comparable strengths to those observed in nearby galaxies. These fields could be generated by fluctuation dynamos. We use idealized numerical simulations of such dynamos in forced compressible turbulence up to rms Mach number of 2.4 to probe the resulting rotation measure (RM) and the degree of coherence of the magnetic field. We obtain rms values of RM at dynamo saturation of the order of 45 - 55 per cent of the value expected in a model where fields are assumed to be coherent on the forcing scale of turbulence. We show that the dominant contribution to the RM in subsonic and transonic cases comes from the general sea of volume filling fields, rather than from the rarer structures. However, in the supersonic case, strong field regions as well as moderately overdense regions contribute significantly. Our results can account for the observed RMs in young galaxies., Comment: Published in MNRAS Letters

Models of inflationary magnetogenesis with a coupling to the electromagnetic action of the form $f^2 F_{\mu\nu}F^{\mu\nu}$, are known to suffer from several problems. These include the strong coupling problem, the back reaction problem and also strong constraints due to Schwinger effect. We propose a model which resolves all these issues. In our model, the coupling function, $f$, grows during inflation and transits to a decaying phase post inflation. This evolutionary behaviour is chosen so as to avoid the problem of strong coupling. By assuming a suitable power law form of the coupling function, we can also neglect back reaction effects during inflation. To avoid back reaction post-inflation, we find that the reheating temperature is restricted to be below $ \approx 1.7 \times 10^4$ GeV. The magnetic energy spectrum is predicted to be non-helical and generically blue. The estimated present day magnetic field strength and the corresponding coherence length taking reheating at the QCD epoch(150 MeV) are $ 1.4 \times 10^{-12}$ G and $6.1 \times 10^{-4}$ Mpc, respectively. This is obtained after taking account of nonlinear processing over and above the flux freezing evolution after reheating. If we consider also the possibility of a non-helical inverse cascade, as indicated in direct numerical simulations, the coherence length and the magnetic field strength are even larger. In all cases mentioned above, the magnetic fields generated in our models satisfy the $\gamma$-ray bound below a certain reheating temperature., Comment: 13 pages, 4 figures, Published in PRD

We present semi-analytical models of galactic outflows in high redshift galaxies driven by both hot thermal gas and non-thermal cosmic rays. Thermal pressure alone may not sustain a large scale outflow in low mass galaxies (i.e $M\sim 10^8$~M$_\odot$), in the presence of supernovae (SNe) feedback with large mass loading. We show that inclusion of cosmic ray pressure allows outflow solutions even in these galaxies. In massive galaxies for the same energy efficiency, cosmic ray driven winds can propagate to larger distances compared to pure thermally driven winds. On an average gas in the cosmic ray driven winds has a lower temperature which could aid detecting it through absorption lines in the spectra of background sources. Using our constrained semi-analytical models of galaxy formation (that explains the observed UV luminosity functions of galaxies) we study the influence of cosmic ray driven winds on the properties of the intergalactic medium (IGM) at different redshifts. In particular, we study the volume filling factor, average metallicity, cosmic ray and magnetic field energy densities for models invoking atomic cooled and molecular cooled halos. We show that the cosmic rays in the IGM could have enough energy that can be transferred to the thermal gas in presence of magnetic fields to influence the thermal history of the intergalactic medium. The significant volume filling and resulting strength of IGM magnetic fields can also account for recent $\gamma$-ray observations of blazars., Comment: 16 Pages, 9 figures, Revised version, Accepted for publication in MNRAS

We incorporate the non-linear clustering of dark matter halos, as modelled by Jose et al. (2016) into the halo model to better understand the clustering of Lyman break galaxies (LBGs) in the redshift range $z=3-5$. We find that, with this change, the predicted LBG clustering increases significantly on quasi-linear scales ($0.1 \leq r\,/\,h^{-1} \,{\rm Mpc} \leq 10$) compared to that in the linear halo bias model. This in turn results in an increase in the clustering of LBGs by an order of magnitude on angular scales $5" \leq \theta \leq 100"$. Remarkably, the predictions of our new model on the whole remove the systematic discrepancy between the linear halo bias predictions and the observations. The correlation length and large scale galaxy bias of LBGs are found to be significantly higher in the non-linear halo bias model than in the linear halo bias model. The resulting two-point correlation function retains an approximate power-law form in contrast with that computed using the linear halo bias theory. We also find that the non-linear clustering of LBGs increases with increasing luminosity and redshift. Our work emphasizes the importance of using non-linear halo bias in order to model the clustering of high-z galaxies to probe the physics of galaxy formation and extract cosmological parameters reliably., Comment: Accepted for publication in MNRAS

Origin of magnetic fields, its structure and effects on dynamical processes in stars to galaxies are not well understood. Lack of a direct probe has hampered its study. The first phase of Square Kilometre Array (SKA-I), will have more than an order of magnitude higher sensitivity than existing radio telescopes. In this contribution, we discuss specific science cases that are of interest to the Indian community concerned with astrophysical turbulence and magnetic fields. The SKA-I will allow observations of a large number of background sources with detectable polarisation and measure their Faraday depths (FDs) through the Milky Way, other galaxies and their circum-galactic medium. This will probe line-of-sight magnetic fields in these objects well and provide field configurations. Detailed comparison of observational data with models which consider various processes giving rise to field amplification and maintenance will then be possible. Such observations will also provide the coherence scale of the fields and measure its random component. Measuring the random component is important to characterise turbulence in the medium. Observations of FDs with redshift will provide important information on magnetic field evolution as a function of redshift. The background sources could also be used to probe magnetic fields and its coherent scale in galaxy clusters and in bridges formed between interacting galaxies. Other than FDs, sensitive observations of synchrotron emission from galaxies will provide complimentary information on their magnetic field strengths in the sky plane. The core shift measurements of AGNs can provide more precise measurements of magnetic field very close (

The initial magnetic field of previous magnetorotational instability (MRI) simulations has always included a significant system-scale component, even if stochastic. However, it is of conceptual and practical interest to assess whether the MRI can grow when the initial field is turbulent. The ubiquitous presence of turbulent or random flows in astrophysical plasmas generically leads to a small-scale dynamo (SSD), which would provide initial seed turbulent velocity and magnetic fields in the plasma that becomes an accretion disc. Can the MRI grow from these more realistic initial conditions? To address this we supply a standard shearing box with isotropically forced SSD generated magnetic and velocity fields as initial conditions, and remove the forcing. We find that if the initially supplied fields are too weak or too incoherent, they decay from the initial turbulent cascade faster than they can grow via the MRI. When the initially supplied fields are sufficient to allow MRI growth and sustenance, the saturated stresses, large-scale fields, and power spectra match those of the standard zero net flux MRI simulation with an initial large scale vertical field., Comment: Published in MNRAS, 6 pages, 5 figures

An elegant model for passive scalar mixing was given by Kraichnan assuming the velocity to be delta-correlated in time. We generalize this model to include the effects of a finite correlation time, $\tau$, using renewing flows. The resulting equation for the 3-D passive scalar spectrum $\hat{M}(k,t)$ or its correlation function $M(r,t)$, gives the Kraichnan equation when $\tau \to 0$, and extends it to the next order in $\tau$. It involves third and fourth order derivatives of $M$ or $\hat{M}$ (in the high $k$ limit). For small-$\tau$, it can be recast using the Landau-Lifshitz approach, to one with at most second derivatives of $\hat{M}$. We present both a scaling solution to this equation neglecting diffusion and a more exact solution including diffusive effects. We show that the steady state 1-D passive scalar spectrum, preserves the Batchelor form, $E_\theta(k) \propto k^{-1}$, in the viscous-convective limit, independent of $\tau$. This result can also be obtained in a general manner using Lagrangian methods. Interestingly, in the absence of sources, when passive scalar fluctuations decay, we show that the spectrum in the Batchelor regime and at late times, is of the form $E_\theta(k) \propto k^{1/2}$ and also independent of $\tau$. More generally, finite $\tau$ does not qualitatively change the shape of the spectrum during decay, although the decay rate is reduced. From high resolution ($1024^3$) direct numerical simulations of passive scalar mixing and decay, We find reasonable agreement with predictions of the Batchelor spectrum, during steady state. The scalar spectrum during decay agrees qualitatively with analytic predictions when power is dominantly in wavenumbers corresponding to the Batchelor regime, but is however shallower when box scale fluctuations dominate during decay (abridged)., Comment: 32 pages, 7 figures, Journal Of Fluid Mechanics (In Press)

We use high resolution direct numerical simulations (DNS) to show that helical turbulence can generate significant large-scale fields even in the presence of strong small-scale dynamo action. During the kinematic stage, the unified large/small-scale dynamo grows fields with a shape-invariant eigenfunction, with most power peaked at small scales or large $k$, as in \citet{SB14}. Nevertheless, the large-scale field can be clearly detected as an excess power at small $k$ in the negatively polarized component of the energy spectrum for a forcing with positively polarized waves. Its strength $bar{B}$, relative to the total rms field $B_{rms}$, decreases with increasing magnetic Reynolds number, $R_m$. However, as the Lorentz force becomes important, the field generated by the unified dynamo orders itself by saturating on successively larger scales. The magnetic integral scale for the positively polarized waves, characterizing the small-scale field, increases significantly from the kinematic stage to saturation. This implies that the small-scale field becomes as coherent as possible for a given forcing scale, which averts the $R_m$-dependent quenching of $\bar{B}/B_rms$. These results are obtained for $1024^3$ DNS with magnetic Prandtl numbers of $P_m=0.1$ and $10$. For $P_m=0.1$, $\bar{B}/B_{rms}$ grows from about $0.04$ to about $0.4$ at saturation, aided in the final stages by helicity dissipation. For $P_m=10$, $\bar{B}/B_{rms}$ grows from much less than 0.01 to values of the order the $0.2$. Our results confirm that there is a unified large/small-scale dynamo in helical turbulence., Comment: 8 pages, 7 figures. Published in mnras

The universe is magnetized on all scales probed so far. On the largest scales, galaxies and galaxy clusters host magnetic fields at the micro Gauss level coherent on scales up to ten kpc. Recent observational evidence suggests that even the intergalactic medium in voids could host a weak $\sim 10^{-16}$ Gauss magnetic field, coherent on Mpc scales. An intriguing possibility is that these observed magnetic fields are a relic from the early universe, albeit one which has been subsequently amplified and maintained by a dynamo in collapsed objects. We review here the origin, evolution and signatures of primordial magnetic fields. After a brief summary of magnetohydrodynamics in the expanding universe, we turn to magnetic field generation during inflation and other phase transitions. We trace the linear and nonlinear evolution of the generated primordial fields through the radiation era, including viscous effects. Sensitive observational signatures of primordial magnetic fields on the cosmic microwave background, including current constraints from Planck, are discussed. After recombination, primordial magnetic fields could strongly influence structure formation, especially on dwarf galaxy scales. The resulting signatures on reionization, the redshifted 21 cm line, weak lensing and the Lyman-$\alpha$ forest are outlined. Constraints from radio and $\gamma$-ray astronomy are summarized. Astrophysical batteries and the role of dynamos in reshaping the primordial field are briefly considered. The review ends with some final thoughts on primordial magnetic fields., Comment: 57 pages, 12 figures, Invited review for Reports on Progress in Physics, in Press

Fluctuation dynamos are generic to turbulent astrophysical systems. The only analytical model of the fluctuation dynamo, due to Kazantsev, assumes the velocity to be delta-correlated in time. This assumption breaks down for any realistic turbulent flow. We generalize the analytic model of fluctuation dynamo to include the effects of a finite correlation time, $\tau$, using renewing flows. The generalized evolution equation for the longitudinal correlation function $M_L$ leads to the standard Kazantsev equation in the $\tau \to 0$ limit, and extends it to the next order in $\tau$. We find that this evolution equation involves also third and fourth spatial derivatives of $M_L$, indicating that the evolution for finite $\tau$ will be non-local in general. In the perturbative case of small-$\tau$ (or small Strouhl number), it can be recast using the Landau-Lifschitz approach, to one with at most second derivatives of $M_L$. Using both a scaling solution and the WKBJ approximation, we show that the dynamo growth rate is reduced when the correlation time is finite. Interestingly, to leading order in $\tau$, we show that the magnetic power spectrum, preserves the Kazantsev form, $M(k) \propto k^{3/2}$, in the large $k$ limit, independent of $\tau$., Comment: 20 pages, Submitted to JPP

Zeldovich's stretch-twist fold (STF) dynamo provided a breakthrough in conceptual understanding of fast dynamos, including fluctuation or small scale dynamos. We study the evolution and saturation behaviour of two types of Baker's map dynamos, which have been used to model Zeldovich's STF dynamo process. Using such maps allows one to analyze dynamos at much higher magnetic Reynolds numbers $R_M$ as compared to direct numerical simulations. In the 2-strip map dynamo there is constant constructive folding while the 4-strip map dynamo also allows the possibility of field reversal. Incorporating a diffusive step parameterised by $R_M$, we find that the magnetic field $B(x)$ is amplified only above a critical $R_M=R_{crit} \sim 4$ for both types of dynamos. We explore the saturation of these dynamos in 3 ways; by a renormalized decrease of the effective $R_M$ (Case I) or due to a decrease in the efficiency of field amplification by stretching (Case II), or a combination of both effects (Case III). For Case I, we show that $B(x)$ in the saturated state, for both types of maps, goes back to the marginal eigenfunction, which is obtained for the critical $R_M=R_{crit}$. This is independent of the initial $R_M=R_{M0}$. On the other hand in Case II, for the 2-strip map, we show that $B(x)$ now saturates preserving the structure of the kinematic eigenfunction. Thus the energy is transferred to larger scales in Case I but remains at the smallest resistive scales in Case II. For the 4-strip map, the $B(x)$ oscillates with time, although with a structure similar to the kinematic eigenfunction. Interestingly, the saturated state for Case III shows an intermediate behaviour, with $B(x)$ now similar to the kinematic eigenfunction for an intermediate $R_M=R_{sat}$, with $R_{M0}>R_{sat}>R_{crit}$. These saturation properties are akin to the ones discussed in the context of fluctuation dynamos., Comment: 17 pages, 14 figures, Accepted in JPP (Special Issue - Zeldovich)

We study the inflationary generation of helical cosmological magnetic fields in a higher-dimensional generalization of the electromagnetic theory. For this purpose, we also include a parity breaking piece to the electromagnetic action. The evolution of extra-dimensional scale factor allows the breaking of conformal invariance of the effective electromagnetic action in $1+3$ dimensions required for such generation. Analytical solutions for the vector potential can be obtained in terms of Coulomb wave-functions for some special cases. We also present numerical solutions for the vector potential evolution in more general cases. In the presence of a higher-dimensional cosmological constant there exist solutions for the scale factors in which both normal and extra dimensional space either inflate or deflate simultaneously with the same rate. In such a scenario, with the number of extra dimensions $D=4$, a scale invariant spectrum of helical magnetic field is obtained. The net helicity arises, as one helical mode comes to dominate over the other at the superhorizon scales. A magnetic field strength of the order of $10^{-9}$ $G$ can be obtained for the inflationary scale $H\simeq 10^{-3}$ $M_{pl}$. Weaker fields will be generated for lower scales of inflation. Magnetic fields generated in this model respects the bounds on magnetic fields by Planck and $\gamma$-ray observations (i.e. $10^{-16}$ $G$ $<$ $B_{obs}<3.4\times 10^{-9}$ $G$)., Comment: 4 postscript figures, version submitted to Physical Review D

Using direct numerical simulations (DNS) we verify that in the kinematic regime, a turbulent helical dynamo grows in such a way that the magnetic energy spectrum remains to high precision shape-invariant, i.e., at each wavenumber $k$ the spectrum grows with the same growth rate. Signatures of large-scale dynamo action can be identified through the excess of magnetic energy at small $k$, of one of the two oppositely polarized constituents. Also a suitably defined planar average of the magnetic field can be chosen such that its rms value isolates the strength of the mean field. However, these different means of analysis suggest that the strength of the large-scale field diminishes with increasing magnetic Reynolds number ${\rm Re}_{\rm M}$ like ${\rm Re}_{\rm M}^{-1/2}$ for intermediate values and like ${\rm Re}_{\rm M}^{-3/4}$ for larger ones. Both an analysis from the Kazantsev model including helicity and the DNS show that this arises due to the magnetic energy spectrum still peaking at resistive scales, even when helicity is present. As expected, the amplitude of the large-scale field increases with increasing fractional helicity, enabling us to determine the onset of large scale dynamo action and distinguishing it from that of the small-scale dynamo. Our DNS show that, contrary to earlier results for smaller scale separation (only 1.5 instead of now 4), the small-scale dynamo can still be excited at magnetic Prandtl numbers of 0.1 and only moderate values of the magnetic Reynolds numbers ($\sim 160$)., Comment: 12 pages, 10 figures, Version accepted in MNRAS

Galactic magnetic arms have been observed between the gaseous arms of some spiral galaxies; their origin remains unclear. We suggest that magnetic spiral arms can be naturally generated in the interarm regions because the galactic fountain flow or wind is likely to be weaker there than in the arms. Galactic outflows lead to two countervailing effects: removal of small-scale magnetic helicity, which helps to avert catastrophic quenching of the dynamo, and advection of the large-scale magnetic field, which suppresses dynamo action. For realistic galactic parameters, the net consequence of outflows being stronger in the gaseous arms is higher saturation large-scale field strengths in the interarm regions as compared to in the arms. By incorporating rather realistic models of spiral structure and evolution into our dynamo models, an interlaced pattern of magnetic and gaseous arms can be produced., Comment: 5 pages, 4 figures, version accepted by MNRAS Letters

Fluctuation dynamos are generic to astrophysical systems. The only analytical model of the fluctuation dynamo is Kazantsev model which assumes a delta-correlated in time velocity field. We derive a generalized model of fluctuation dynamo with finite correlation time, $\tau$, using renovating flows. For $\tau \to 0$, we recover the standard Kazantsev equation for the evolution of longitudinal magnetic correlation, $M_L$. To the next order in $\tau$, the generalized equation involves third and fourth spatial derivatives of $M_L$. It can be recast using the Landau-Lifschitz approach, to one with at most second derivatives of $M_L$. Remarkably, we then find that the magnetic power spectrum, remains the Kazantsev spectrum of $M(k) \propto k^{3/2}$, in the large $k$ limit, independent of $\tau$., Comment: 5 pages, Published in ApJL

We compare various models and approximations for non-linear mean-field dynamos in disc galaxies to assess their applicability and accuracy, and thus to suggest a set of simple solutions suitable to model the large-scale galactic magnetic fields in various contexts. The dynamo saturation mechanisms considered are the magnetic helicity balance involving helicity fluxes (the dynamical $\alpha$-quenching) and an algebraic $\alpha$-quenching. The non-linear solutions are then compared with the marginal kinematic and asymptotic solutions. We also discuss the accuracy of the no-$z$ approximation. Although these tools are very different in the degree of approximation and hence complexity, they all lead to remarkably similar solutions for the mean magnetic field. In particular, we show that the algebraic $\alpha$-quenching non-linearity can be obtained from a more physical dynamical $\alpha$-quenching model in the limit of nearly azimuthal magnetic field. This suggests, for instance, that earlier results on galactic disc dynamos based on the simple algebraic non-linearity are likely to be reliable, and that estimates based on simple, even linear models are often a good starting point. We suggest improved no-$z$ and algebraic $\alpha$-quenching models, and also incorporate galactic outflows into a simple analytical dynamo model to show that the outflow can produce leading magnetic spirals near the disc surface. The simple dynamo models developed are applied to estimate the magnetic pitch angle and the arm-interarm contrast in the saturated magnetic field strength for realistic parameter values., Comment: 15 pages, 10 figures, 2 tables, version accepted by MNRAS

Generation of primordial magnetic fields during inflation typically requires the breaking of conformal invariance of the Electromagnetic action. In this paper this has been achieved naturally in a higher dimensional cosmological model with a Gauss-Bonnet term in the action. The evolution of the scale factor of the extra dimension (whose dynamics is influenced by the Gauss-Bonnet term) acts as the cause for the breaking of conformal invariance. Different cases have been investigated, each of which is characterized by the number of higher dimensions, the value of the Gauss-Bonnet parameter, and the cosmological constant. Many of the scenarios considered are highly constrained by the requirements that the cosmic evolution is stable, that the normal dimensions expand and that there is no back reaction due to growing electric fields. However there do exist scenarios which satisfy the above requirements and are well suited for magnetogenesis. In particular, a scenario where the number of extra dimensions $D=4$ and the cosmological constant is non-zero, turns out to be best suited for generating primordial magnetic fields. It is shown that for these values of parameters, a scale invariant magnetic field of the order of $10^{-10}-10^{-9}$ $G$ can be produced. Even in these most favorable scenarios, the higher dimensional space expands during inflation at the same rate as the normal dimension. Hence if a mechanism could freeze the evolution of the higher dimension, this seems to be a viable mechanism to produce acceptable primordial magnetic fields., Comment: 13 pages, 2 tables

Cosmic magnetic fields are observed to be coherent on large scales and could have a primordial origin. Non-Gaussian signals in the cosmic microwave background (CMB) are generated by primordial magnetic fields as the magnetic stresses and temperature anisotropy they induce depend quadratically on the magnetic field. We compute the CMB scalar trispectrum on large angular scales, for nearly scale-invariant magnetic fields, 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 $10^{-29}$ and $10^{-19}$, respectively. The scalar anisotropic stress trispectrum is also calculated in the flat-sky approximation and yields a similar result. Observational limits on CMB non-Gaussianity from the Planck mission data allow us to set upper limits of $B_0 \lesssim 0.6 $ nG on the present value of the primordial cosmic magnetic field. Considering the inflationary magnetic curvature mode in the trispectrum can further tighten the magnetic field upper limit to $B_0 \lesssim 0.05 $ nG. These sub-nanoGauss constraints from the magnetic trispectrum are the most stringent limits so far on the strength of primordial magnetic fields, on megaparsec scales, significantly better than the limits obtained from the CMB bispectrum and the CMB power spectrum., Comment: 20 pages, 3 figures. arXiv admin note: text overlap with arXiv:1111.0744

We present a galaxy formation model to understand the evolution of stellar mass (M*) - UV luminosity relations, stellar mass functions and specific star formation rate (sSFR) of Lyman Break Galaxies (LBGs) along with their UV luminosity functions in the redshift range 3 < z < 8. Our models assume a physically motivated form for star formation in galaxies and model parameters are calibrated by fitting the observed UV luminosity functions (LFs) of LBGs. We find the fraction of baryons that gets converted into stars remains nearly constant for z < 4 but shows an increase for z < 4. However, the rate of converting baryons into stars does not evolve significantly in the redshift range 3 < z < 8. Our model further successfully explains the M* - UV luminosity (M_AB) correlations of LBGs. While our model predictions of stellar mass functions compare well with the inferred data from observations at the low mass end, we need to invoke the Eddington bias to fit the high mass end. At any given redshift, we find the sSFR to be constant over the stellar mass range 5 \times 10^8 -5 \times 10^9 M_\odot and the redshift evolution of sSFR is well approximated by a form (1+z)^2.4 for 3 < z < 8 which is consistent with observations. Thus we find that dark matter halo build up in the LCDM model is sufficient to explain the evolution of UV LFs of LBGs along with their M* - M_AB relations, the stellar mass functions and the sSFR for 3 < z < 8., Comment: 12 pages, 8 figures, Accepted for publication in MNRAS

The recent study of Blackman and Subramanian (Paper I) indicates that large scale helical magnetic fields are resilient to turbulent diffusion in the sense that helical fields stronger than a critical value, decay on slow (~resistively mediated), rather than fast ($\sim$ turbulent) time scales. This gives more credence to potential fossil field origin models of the magnetic fields in stars, galaxies and compact objects. We analyze a suite of direct numerical simulations (DNS) of decaying large scale helical magnetic fields in the presence of non-helical turbulence to further study the physics of helical field decay. We study two separate cases: (1) the initial field is large enough to decay resistively, is tracked until it transitions to decay fast, and the critical large scale helical field at that transition is sought; (2) the case of Paper I, wherein there is a critical initial helical field strength below which the field undergoes fast decay right from the beginning. In case (1), both DNS and solutions of the two scale model (from Paper 1), reveal that the transition energy, $E_{c1}$, is independent of the scale of the turbulent forcing, within a small range of $\Rm$. We also find that the kinetic alpha, $\alpha_K$, is subdominant to magnetic alpha, $\alpha_M$, in the DNS, justifying an assumption in the two scale model. For case (2), we show exact solutions of two scale model in the limit of $\eta \rightarrow 0$ in fully helical case, leading to the transition energy, $E_{c2} = (k_1/\kf)^2 M_{eq}$, where $k_1$ and $\kf$ are the large scale and small turbulent forcing scale respectively and $M_{eq}$ is the equipartition magnetic energy. The DNS in this case agree qualitatively with the two scale model but the $R_M$ currently achievable, is too small to satisfy a condition $3/R_M << (k_1/k_f)^2$, necessary to robustly reveal the transition, $E_{c2}$ (Abridged)., Comment: 15 pages, 16 figures, submitted to MNRAS

Interfering two- and three-arm spiral patterns have previously been inferred to exist in many galaxies and also in numerical simulations, and invoked to explain important dynamical properties, such as lack of symmetry, kinks in spiral arms, and star formation in armlets. The non-axisymmetric galactic mean-field dynamo model of Chamandy et al. is generalized to allow for such multiple co-existing spiral patterns in the kinetic alpha_k effect, leading to the existence of magnetic spiral arms in the large-scale magnetic field with several new properties. The large-scale magnetic field produced by an evolving superposition of two- and three-arm (or two- and four-arm) patterns evolves with time along with the superposition. Magnetic arms can be stronger and more extended in radius and in azimuth when produced by two interfering patterns rather than by one pattern acting alone. Transient morphological features arise in the magnetic arms, including bifurcations, disconnected armlets, and temporal and spatial variation in arm strength and pitch angles. Pitch angles of the large-scale magnetic field and magnetic arm structures (ridges) are smaller than those typically inferred from observations of spiral galaxies for model parameters of Chamandy et al., but can become comparable to typically inferred values for certain (still realistic) parameters. The magnetic field is sometimes strongest in between the alpha_k-arms, unlike in standard models with a single pattern, where it is strongest within the alpha_k-arms. Moreover, for models with a two- and three-arm pattern, some amount of m=1 azimuthal symmetry is found to be present in the magnetic field, which is generally not the case for forcing by single two- or three-arm patterns. (abridged), Comment: 14 pages, 9 figures, version accepted by MNRAS

Understanding large scale magnetic field growth in turbulent plasmas in the magnetohydrodynamic limit is a goal of magnetic dynamo theory. In particular, assessing how well large scale helical field growth and saturation in simulations matches that predicted by existing theories is important for progress. Using numerical simulations of isotropically forced turbulence without large scale shear with the implications, we focus on several aspects of this comparison that have not been previously tested: (1) Leading mean field dynamo theories which break the field into large and small scales predict that large scale helical field growth rates are determined by the difference between kinetic helicity and current helicity with no dependence on the non-helical energy in small scale magnetic fields. Our simulations show that the growth rate of the large scale field from fully helical forcing is indeed unaffected by the presence or absence of small scale magnetic fields amplified in a precursor non-helical dynamo. However, because the precursor non helical dynamo in our simulations produced fields that were strongly sub-equipartition with respect to the kinetic energy, we cannot yet rule out the potential influence of stronger non- helical small scale fields. (2) We have identified two features in our simulations which cannot be explained by the most minimalist versions of two-scale mean field theory: (i) fully helical small scale forcing produces significant non-helical large scale magnetic energy and (ii) the saturation of the large scale field growth is time-delayed with respect to what minimalist theory predicts. We comment on desirable generalizations to the theory in this context and future desired work., Comment: 8 pages, 9 figures. This will be published soon(Phys. Rev. E)

We present a physically motivated semi-analytic model to understand the clustering of high redshift Lyman Alpha Emitters (LAEs). We show that the model parameters constrained by the observed luminosity functions, can be used to predict large scale bias and angular correlation function of LAEs. These predictions are shown to reproduce the observations remarkably well. We find that average masses of dark matter halos hosting LAEs brighter than threshold narrow band magnitude ~ 25 are ~ 10^11 M_\odot. These are smaller than that of typical Lyman Break Galaxies (LBGs) brighter than similar threshold continuum magnitude by a factor ~ 10. This results in a smaller clustering strength of LAEs compared to LBGs. However, using the observed relationship between UV continuum and Lyman-alpha luminosity of LAEs, we show that both LAEs and LBGs belong to the same parent galaxy population with narrow band techniques having greater efficiency in picking up galaxies with low UV luminosity. We also show that the lack of evidence for the presence of the one halo term in the observed LAE angular correlation functions can be attributed to sub-Poisson distribution of LAEs in dark matter halos as a result of their low halo occupations., Comment: Accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:1208.2097

The exploration of mean-field galactic dynamos affected by a galactic spiral pattern, begun in Chamandy et al. (2013, hereafter Paper I) with numerical simulations, is continued here with an asymptotic solution. The mean-field dynamo model used generalizes the standard theory to include the delayed response of the mean electromotive force to variations of the mean magnetic field and turbulence parameters (the temporal non-locality, or {\tau} effect). The effect of the spiral pattern on the dynamo considered is the enhancement of the {\alpha}-effect in spiral-shaped regions (which may overlap the gaseous spiral arms or be located in the interarm regions). The axisymmetric and enslaved non-axisymmetric modes of the mean magnetic field are studied semi-analytically to clarify and strengthen the numerical results. Good qualitative agreement is obtained between the asymptotic solution and numerical solutions of Paper I for a global, rigidly rotating material spiral (density wave). At all galactocentric distances except for the co-rotation radius, we find magnetic arms displaced in azimuth from the {\alpha}-arms, so that the ridges of magnetic field strength are more tightly wound than the {\alpha}-arms. Moreover, the effect of a finite dynamo relaxation time {\tau} (related to the turbulence correlation time) is to phase-shift the magnetic arms in the direction opposite to the galactic rotation even at the co-rotation radius. This mechanism can be used to explain the phase shifts between magnetic and material arms observed in some spiral galaxies., Comment: 17 pages, 10 figures, version accepted by MNRAS

Turbulence is ubiquitous in many astrophysical systems like galaxies, galaxy clusters and possibly even the IGM filaments. We study fluctuation dynamo action in turbulent systems focusing on one observational signature; the Faraday rotation measure (RM) from background radio sources seen through the magnetic field generated by such a dynamo. We simulate the fluctuation dynamo (FD) in periodic boxes up to resolutions of 512^3, with varying fluid and magnetic Reynolds numbers, and measure the resulting random RMs. We show that, even though the magnetic field generated is intermittent, it still allows for contributions to the RM to be significant. When the dynamo saturates, it is of order 40%-50% of the value expected in a model where fields of strength B_rms uniformly fill cells of the largest turbulent eddy but are randomly oriented from one cell to another. This level of RM dispersion obtains across different values of magnetic Reynolds number and Prandtl number explored. We also use the random RMs to probe the structure of the generated fields to distinguish the contribution from intense and diffuse field regions. We find that the strong field regions (say with B > 2B_rms) contribute only of order 15%-20% to the RM. Thus rare structures do not dominate the RM; rather the general 'sea' of volume filling fluctuating fields are the dominant contributors. We also show that the magnetic integral scale, L_{int}, which is directly related to the RM dispersion, increases in all the runs, as Lorentz forces become important to saturate the dynamo. It appears that due to the ordering effect of the Lorentz forces, L_{int} of the saturated field tends to a modest fraction, 1/2-1/3 of the integral scale of the velocity field, for all our runs. These results are then applied to discuss the RM signatures of FD generated fields in young galaxies, galaxy clusters and intergalactic filaments., Comment: 14 pages, 12 figures, version accepted to MNRAS