Keywords

We present a new total intensity image of M31 at 1.248 GHz, observed with the Five-hundred-meter Aperture Spherical radio telescope (FAST) with an angular resolution of $4^{\prime}$ and a sensitivity of about 16 mK. The new FAST image clearly reveals weak emission outside the ring due to its high sensitivity on large-scale structures. We derive a scale length of 2.7 kpc for the cosmic ray electrons and find that the cosmic ray electrons propagate mainly through diffusion by comparing the scale length at 4.8 GHz. The spectral index of the total intensity varies along the ring, which can be attributed to the variation of the spectra of synchrotron emission. This variation is likely caused by the change of star formation rates along the ring. We find that the azimuthal profile of the non-thermal emission can be interpreted by an axisymmetric large-scale magnetic field with varying pitch angle along the ring, indicating a complicated magnetic field configuration in M31.

In this work, we study the magnetic field morphology of selected star-forming clouds spread over the galactic latitude (b) range −10° to 10°. The polarimetric observations of clouds CB24, CB27 and CB188 are conducted to study the magnetic field geometry of those clouds using the 104 cm Sampurnanand Telescope (ST) located at ARIES, Manora Peak, Nainital, India. These observations are combined with those of 14 further low latitude clouds available in the literature. Most of these clouds are located within a distance range 140–500 pc except for CB3 (∼2500 pc), CB34 (∼1500 pc), CB39 (∼1500 pc) and CB60 (∼1500 pc). Analyzing the polarimetric data of 17 clouds, we find that the alignment between the envelope magnetic field (${\theta }_{B}^{\mathrm{env}}$) and galactic plane (GP) (θ_GP) of the low-latitude clouds varies with their galactic longitudes (l). We observe a strong correlation between the longitude (l) and the offset (${\theta }_{\mathrm{off}}=| {\theta }_{B}^{\mathrm{env}}-{\theta }_{\mathrm{GP}}|$) which shows that ${\theta }_{B}^{\mathrm{env}}$ is parallel to the GP when the clouds are situated in the region 115° < l < 250°. However, ${\theta }_{B}^{\mathrm{env}}$ has its own local deflection irrespective of the orientation of θ_GP when the clouds are at l < 100° and l > 250°. To check the consistency of our results, the stellar polarization data available in the Heiles catalog are overlaid on the DSS image of the clouds having mean polarization vector of field stars. The results are almost consistent with the Heiles data. A systematic discussion is presented in the paper. The effect of turbulence in the cloud is also studied which may play an important role in causing the misalignment phenomenon observed between ${\theta }_{B}^{\mathrm{env}}$ and θ_GP. We have used Herschel (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.) SPIRE 500 μm and SCUBA 850 μm dust continuum emission maps in our work to understand the density structure of the clouds.

We present an updated version of POLARIS, a well-established code designated for dust polarization and line radiative transfer (RT) in arbitrary astrophysical environments. We extend the already available capabilities with a synchrotron feature for polarized emission. Here, we combine state-of-the-art solutions of the synchrotron RT coefficients with numerical methods for solving the complete system of equations of the RT problem, including Faraday rotation (FR) as well as Faraday conversion (FC). We validate the code against Galactic and extragalactic observations by performing a statistical analysis of synthetic all-sky synchrotron maps for positions within the Galaxy and for extragalactic observations. For these test scenarios we apply a model of the Milky Way based on sophisticated magnetohydrodynamic simulations and population synthesis post-processing techniques. We explore different parameters for modeling the distribution of free electrons and for a turbulent magnetic field component. We find that a strongly fluctuating field is necessary for simulating synthetic synchrotron observations on small scales, we argue that FR alone can account for the depolarization of the synchrotron signal, and we discuss the importance of the observer position within the Milky Way. Altogether, we conclude that POLARIS is a highly reliable tool for predicting synchrotron emission and polarization, including FR in a realistic galactic context. It can thus contribute to a better understanding of the results from current and future observational missions.

We present a radio continuum study of a population of extremely young and starburst galaxies, termed as blueberries at ∼1 GHz using the upgraded Giant Metrewave Radio Telescope. We find that their radio-based star formation rate (SFR) is suppressed by a factor of ∼3.4 compared to the SFR based on optical emission lines. This might be due to (i) the young ages of these galaxies as a result of which a stable equilibrium via feedback from supernovae has not yet been established; (ii) escape of cosmic-ray electrons via diffusion or galactic-scale outflows. The estimated nonthermal fraction in these galaxies has a median value of ∼0.49, which is relatively lower than that in normal star-forming galaxies at such low frequencies. Their inferred equipartition magnetic field has a median value of 27 μG, which is higher than those in more evolved systems like spiral galaxies. Such high magnetic fields suggest that small-scale dynamo rather than large-scale dynamo mechanisms might be playing a major role in amplifying magnetic fields in these galaxies.

Faraday rotation measures (RMs) of extragalactic radio sources provide information on line-of-sight magnetic fields, including contributions from our Galaxy, source environments, and the intergalactic medium (IGM). Looking at differences in RMs, ΔRM, between adjacent sources on the sky can help isolate these different components. In this work, we classify adjacent polarized sources in the NRAO VLA Sky Survey (NVSS) as random or physical pairs. We recompute and correct the uncertainties in the NVSS RM catalog, since these were significantly overestimated. Our sample contains 317 physical and 5111 random pairs, all with Galactic latitudes $| b| \geqslant 20^\circ$, polarization fractions ≥2%, and angular separations between 15 and 20'. We find an rms ΔRM of 14.9 ± 0.4 and 4.6 ± 1.1 rad m⁻² for the random and physical pairs, respectively. This means that polarized extragalactic sources that are close on the sky but at different redshifts have larger differences in RM than two components of one source. This difference of ∼10 rad m⁻² is significant at 5σ and persists in different data subsamples. While there have been other statistical studies of ΔRM between adjacent polarized sources, this is the first unambiguous demonstration that some of this RM difference must be extragalactic, thereby providing a firm upper limit on the RM contribution of the IGM. If the ΔRMs originate local to the sources, then the local magnetic field difference between random sources is a factor of 2 larger than that between components of one source. Alternatively, attributing the difference in ΔRMs to the intervening IGM yields an upper limit on the IGM magnetic field strength of 40 nG.

The Galactic halo contains a complex ecosystem of multiphase intermediate-velocity and high-velocity gas clouds whose origin has defied clear explanation. They are generally believed to be involved in a Galaxy-wide recycling process, either through an accretion flow or a large-scale fountain flow, or both. We examine the evolution of these clouds in light of recent claims that they may trigger condensation of gas from the Galactic corona as they move through it. We measure condensation along a cloud's wake, with and without the presence of an ambient magnetic field, using two- (2D) and three-dimensional (3D), high-resolution simulations. We find that 3D simulations are essential to correctly capture the condensation in all cases. Magnetic fields significantly inhibit condensation in the wake of clouds at t ≳ 25 Myr, preventing the sharp upturn in cold gas mass seen in previous non-magnetic studies. The magnetic field suppresses the Kelvin–Helmholtz instability responsible for the ablation and consequent mixing of a cloud with halo gas which drives the condensation. This effect is universal across different cloud properties (density, metallicity, velocity) and magnetic field properties (strength and orientation). Simple convergence tests demonstrate that resolving the gas on progressively smaller scales leads to even less condensation. While condensation still occurs in all cases, our results show that an ambient magnetic field drastically lowers the efficiency of fountain-driven accretion and likely also accretion from condensation around high-velocity clouds. These lower specific accretion rates are in better agreement with observational constraints compared to 3D, non-magnetic simulations.

Radiative transfer models were developed to understand the optical polarizations in edge-on galaxies, which are observed to occur even outside the geometrically thin dust disk, with a scale height of ≈0.2 kpc. In order to reproduce the vertically extended polarization structure, we find that it is essential to include a geometrically thick dust layer in the radiative transfer model, in addition to the commonly known thin dust layer. The models include polarizations due to both dust scattering and dichroic extinction, which are responsible for the observed interstellar polarization in the Milky Way. We also find that the polarization level is enhanced if the clumpiness of the interstellar medium, and the dichroic extinction by vertical magnetic fields in the outer regions of the dust lane are included in the radiative transfer model. The predicted degree of polarization outside the dust lane was found to be consistent with that (ranging from 1% to 4%) observed in NGC 891.

We consider stochastic magnetic reconnection in high-β plasmas with large magnetic Prandtl numbers, Pr_m > 1. For large Pr_m, field line stochasticity is suppressed at very small scales, impeding diffusion. In addition, viscosity suppresses very small-scale differential motions and therefore also the local reconnection. Here we consider the effect of high magnetic Prandtl numbers on the global reconnection rate in a turbulent medium and provide a diffusion equation for the magnetic field lines considering both resistive and viscous dissipation. We find that the width of the outflow region is unaffected unless Pr_m is exponentially larger than the Reynolds number Re. The ejection velocity of matter from the reconnection region is also unaffected by viscosity unless Re ∼ 1. By these criteria the reconnection rate in typical astrophysical systems is almost independent of viscosity. This remains true for reconnection in quiet environments where current sheet instabilities drive reconnection. However, if Pr_m > 1, viscosity can suppress small-scale reconnection events near and below the Kolmogorov or viscous damping scale. This will produce a threshold for the suppression of large-scale reconnection by viscosity when ${\Pr }_{m}\gt \sqrt{\mathrm{Re}}$. In any case, for Pr_m > 1 this leads to a flattening of the magnetic fluctuation power spectrum, so that its spectral index is ∼−4/3 for length scales between the viscous dissipation scale and eddies larger by roughly ${{\Pr }}_{m}^{3/2}$. Current numerical simulations are insensitive to this effect. We suggest that the dependence of reconnection on viscosity in these simulations may be due to insufficient resolution for the turbulent inertial range rather than a guide to the large Re limit.

We present observations and analysis of the polarized radio emission from the nearby radio galaxy Fornax A over 1.28–3.1 GHz, using data from the Australia Telescope Compact Array. In this, the first of two associated papers, we use modern broadband polarimetric techniques to examine the nature and origin of conspicuous low-polarization (low-p) patches in the lobes. We resolve the (low-p) patches and find that their low fractional polarization is associated with complicated frequency-dependent interference in the polarized signal generated by Faraday effects along the line of sight (LOS). The low-p patches are spatially correlated with interfaces in the magnetic structure of the lobe, across which the LOS-projected magnetic field changes direction. Spatial correlations with the sky-projected magnetic field orientation and structure in total intensity are also identified and discussed. We argue that the (low-p) patches, along with associated reversals in the LOS magnetic field and other related phenomena, are best explained by the presence of ${ \mathcal O }({10}^{9})\,{M}_{\odot }$ of magnetized thermal plasma in the lobes, structured in shells or filaments, and likely advected from the interstellar medium of NCG 1316 or its surrounding intracluster medium. Our study underscores the power and utility of spatially resolved, broadband, full-polarization radio observations to reveal new facets of flow behaviors and magneto-ionic structure in radio lobes and their interplay with the surrounding environment.

In this paper, we examine to what extent the radio continuum can be used as an extinction-free probe of star formation in dwarf galaxies. To that aim, we observe 40 nearby dwarf galaxies with the Very Large Array at 6 cm (4–8 GHz) in C-configuration. We obtained images with 3''–8'' resolution and noise levels of 3–15 μJy beam⁻¹. We detected emission associated with 22 of the 40 dwarf galaxies, eight of which are new detections. The general picture is that of an interstellar medium largely devoid of radio continuum emission, interspersed by isolated pockets of emission associated with star formation. We find an average thermal fraction of ∼50%–70% and an average magnetic field strength of ∼5–8 μG, only slightly lower than that found in larger, spiral galaxies. At 100 pc scales, we find surprisingly high values for the average magnetic field strength of up to 50 μG. We find that dwarf galaxies follow the theoretical predictions of the radio continuum–star formation rate relation within regions of significant radio continuum emission but that the nonthermal radio continuum is suppressed relative to the star formation rate when considering the entire optical disk. We examine the far-infrared–star formation rate relation for our sample and find that the far-infrared is suppressed compared to the expected star formation rate. We discuss explanations for these observed relations and the impact of our findings on the radio continuum–far-infrared relation. We conclude that radio continuum emission at centimeter wavelengths has the promise of being a largely extinction-free star formation rate indicator. We find that star formation rates of gas-rich, low-mass galaxies can be estimated with an uncertainty of ±0.2 dex between the values of 2 × 10⁻⁴ and 0.1 M_⊙ yr⁻¹.