Regular Magnetic Field Strengths Research Articles

Disentangling Magnetic Fields in NGC 6946 with Wide-band Polarimetry

We present λ13 cm polarization observations of the nearby spiral galaxy NGC 6946 with the Westerbork Synthesis Radio Telescope (WSRT) to examine the nearside halo magnetic fields. Despite λ13 cm exhibiting similar two-dimensional morphology as observed at longer (λ18–22 cm) or shorter (λ3 and λ6 cm) wavelengths, more complete frequency coverage will be required to explain the gap in polarization in the southwest quadrant of the galaxy. We fit models of the turbulent and coherent line-of-sight magnetic fields to the fractional degree of linearly polarized emission at λ3, λ6, λ13, λ18, and λ22 cm from observations taken with the WSRT, Karl G. Jansky Very Large Array, and Effelsberg telescopes. The results favor a multilayer turbulent magneto-ionized medium consistent with current observations of edge-on galaxies. We constrain the physical properties of the synchrotron-emitting thin and thick disks (scale heights of 300 pc and 1.4 kpc, respectively) along with the thermal thick disk and halo (scale heights of 1 and 5 kpc, respectively). Our preferred model indicates a clumpy and highly turbulent medium within 1 kpc of the midplane, and a diffuse extraplanar layer with a substantially lower degree of Faraday depolarization. In the halo, we estimate a regular magnetic field strength of 0.4–2.2 μG and that turbulence and a total magnetic field strength of ∼6 μG result in a Faraday dispersion of σ RM = 4–48 rad m−2. This work is an example of how the advanced capabilities of modern radio telescopes are opening a new frontier for the study of cosmic magnetism.

The effect of magnetic field on the inner Galactic rotation curve

ABSTRACT In the past few decades, some studies pointed out that magnetic field might affect the rotation curves in galaxies. However, the impact is relatively small compared with the effects of dark matter and the baryonic components. In this letter, we revisit the impact of magnetic field on the rotation curve of our Galaxy. We show that the inner Galactic rotation curve could be affected significantly by the magnetic field. The addition of the inner bulge component, which has been proposed previously to account for the inner rotation curve data, is not necessary. The magnetic field contribution can fully account for the excess of the inner rotation velocity between 5 to 50 pc from the Galactic Centre. Our analysis can also constrain the azimuthal component of the central regular magnetic field strength to $B_0 \sim 50-60\, \mu$G, which is consistent with the observed range.

The Magnetized Disk-Halo Transition Region of M51

AbstractAn excellent laboratory for studying large scale magnetic fields is the grand design face-on spiral galaxy M51. Due to wavelength-dependent Faraday depolarization, linearly polarized synchrotron emission at different radio frequencies gives a picture of the galaxy at different depths: Observations at L-band (1 – 2 GHz) probe the halo region while at C- and X-band (4 – 8 GHz) the linearly polarized emission probe the disk region of M51. We present new observations of M51 using the Karl G. Jansky Very Large Array (VLA) at S-band (2 – 4 GHz), where previously no polarization observations existed, to shed new light on the transition region between the disk and the halo. We discuss a model of the depolarization of synchrotron radiation in a multilayer magneto-ionic medium and compare the model predictions to the multi-frequency polarization data of M51 between 1 – 8 GHz. The new S-band data are essential to distinguish between different models. Our study shows that the initial model parameters, i.e. the total regular and turbulent magnetic field strengths in the disk and halo of M51, need to be adjusted to successfully fit the models to the data.

Constraining regular and turbulent magnetic field strengths in M 51 via Faraday depolarization

We employ an analytical model that incorporates both wavelength-dependent and wavelength-independent depolarization to describe radio polarimetric observations of polarization at $\lambda \lambda \lambda \, 3.5, 6.2, 20.5$ cm in M51 (NGC 5194). The aim is to constrain both the regular and turbulent magnetic field strengths in the disk and halo, modeled as a two- or three-layer magneto-ionic medium, via differential Faraday rotation and internal Faraday dispersion, along with wavelength-independent depolarization arising from turbulent magnetic fields. A reduced chi-squared analysis is used for the statistical comparison of predicted to observed polarization maps to determine the best-fit magnetic field configuration at each of four radial rings spanning $2.4 - 7.2$ kpc in $1.2$ kpc increments. We find that a two-layer modeling approach provides a better fit to the observations than a three-layer model, where the near and far sides of the halo are taken to be identical, although the resulting best-fit magnetic field strengths are comparable. This implies that all of the signal from the far halo is depolarized at these wavelengths. We find a total magnetic field in the disk of approximately $18~\mu$G and a total magnetic field strength in the halo of $\sim 4-6~\mu$G. Both turbulent and regular magnetic field strengths in the disk exceed those in the halo by a factor of a few. About half of the turbulent magnetic field in the disk is anisotropic, but in the halo all turbulence is only isotropic.

Enhancement of magnetic fields arising from galactic encounters

Galactic encounters are usually marked by a substantial increase of synchrotron emission of the interacting galaxies compared to the typical emission from similar isolated galaxies. This is believed to be associated with an increase of the star formation rate and the associated turbulent magnetic fields. The regular magnetic field is usually believed to decrease. We consider a simple, however rather realistic, mean-field galactic dynamo model where the effects of small-scale generation are represented by random injections of magnetic field from star forming regions. We represent an encounter by the introduction of large-scale streaming velocities and by an increase in small-scale magnetic field injections. The latter describes the effect of an increase of the star formation rate caused by the encounter. We demonstrate that large-scale streaming, with associated deviations in the rotation curve, can result in an enhancement of the anisotropic turbulent (ordered) magnetic field strength, mainly along the azimuthal direction, leading to a significant temporary increase of the total magnetic energy during the encounter; the representation of an increase in star formation rate has an additional strong effect. In contrast to expectations, the large-scale (regular) magnetic field structure is not significantly destroyed by the encounter. It may be somewhat weakened for a relatively short period, and its direction after the encounter may be reversed. The encounter causes enhanced total and polarized emission without increase of the regular magnetic field strength. The increase of synchrotron emission caused by the large-scale streaming can be comparable to the effect of the increase of the star formation rate, depending on the choice of parameters.The effects of the encounter on the total magnetic field energy last only slightly longer than the duration of the encounter (ca. 1 Gyr).

THE INTEGRATED POLARIZATION OF SPIRAL GALAXY DISKS

We present integrated polarization properties of nearby spiral galaxies at 4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a function of inclination. Spiral galaxies in our sample have observed integrated fractional polarization in the range < 1% to 17.6%. At inclinations less than 50 degrees, the fractional polarization depends mostly on the ratio of random to regular magnetic field strength. At higher inclinations, Faraday depolarization associated with the regular magnetic field becomes more important. The observed degree of polarization is lower (<4%) for more luminous galaxies, in particular those with L_{4.8} > 2 x 10^{21} W/Hz. The polarization angle of the integrated emission is aligned with the apparent minor axis of the disk for galaxies without a bar. In our axially symmetric models, the polarization angle of the integrated emission is independent of wavelength. Simulated distributions of fractional polarization for randomly oriented spiral galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization measurements, e.g. with the SKA, of unresolved spiral galaxies allow statistical studies of the magnetic field in disk galaxies using large samples in the local universe and at high redshift. As these galaxies behave as idealized background sources without internal Faraday rotation, they can be used to detect large-scale magnetic fields in the intergalactic medium.

Magnetism in the nearby galaxy M 33

AbstractUsing high-resolution data of the linearly polarized intensity and polarization angle at 3.6, 6.2, and 20 cm together with a 3-D model of the regular magnetic field, we study variations of the structure, strength, and energy density of the magnetic field in the Scd galaxy M 33. The regular magnetic field consists of a horizontal component (represented by an axisymmetric mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric modes from 3 to 5 kpc radius) and a vertical component. However, the inferred ‘vertical field’ may be partly due to a galactic warp. We estimate the average total and regular magnetic field strengths as ≃6.4 and 2.5 μG, respectively. Generation of interstellar magnetic fields by turbulent gas motion in M 33 is indicated as the turbulent and magnetic energy densities are about equal.

High-resolution radio continuum survey of M 33

Aims. We study the magnetic field structure, strength, and energy density in the Scd galaxy M 33. Methods. Using the linearly polarized intensity and polarization angle data at 3.6, 6.2 and 20 cm, we determine variations of Faraday rotation and depolarization across M 33. We fit a 3D model of the regular magnetic field to the observed azimuthal distribution of polarization angles. We also analyze the spatial variation of depolarization across the galaxy. Results. Faraday rotation, measured between 3.6 and 6.2 cm at an angular resolution of 3 � (0.7 kpc), shows more variation in the south than in the north of the galaxy. About 10% of the nonthermal emission from M 33 at 3.6 cm is polarized. High degrees of polarization of the synchrotron emission (>20%) and strong regular magnetic fields in the sky plane (� 6.6 μG) exist in-between two northern spiral arms. We estimate the average total and regular magnetic field strengths in M 33 as � 6.4 and 2.5 μG, respectively. Under the assumption that the disk of M 33 is flat, the regular magnetic field consists of horizontal and vertical components: however the inferred vertical field may be partly due to a galactic warp. The horizontal field is represented by an axisymmetric (m = 0) mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric (m = 0 + 1) modes from 3 to 5 kpc radius. Conclusions. An excess of differential Faraday rotation in the southern half together with strong Faraday dispersion in the southern spiral arms seem to be responsible for the north-south asymmetry in the observed wavelength dependent depolarization. The presence of an axisymmetric m = 0 mode of the regular magnetic field in each ring suggests that a galactic dynamo is operating in M 33. The pitch angles of the spiral regular magnetic field are generally smaller than the pitch angles of the optical spiral arms but are twice as big as simple estimates based on the mean-field dynamo theory and M 33’s rotation curve. Generation of interstellar magnetic fields from turbulent gas motion in M 33 is indicated by the equipartition of turbulent and magnetic energy densities.

Faraday screens associated with local molecular clouds

Polarization observations at 21cm and 18cm towards the local Taurus molecular cloud complex were made with the Effelsberg 100-m telescope. Highly structured, frequency-dependent polarized emission features were detected. We discuss polarization minima with excessive rotation measures located at the boundaries of molecular clouds. The multi-frequency polarization data have been successfully modeled by considering magneto-ionic Faraday screens at the surface of the molecular clouds. Faraday rotated background emission adds to foreground emission towards these screens in a different way than in its surroundings. The physical size of the Faraday screens is of the order of 2 pc for 140 pc distance to the Taurus clouds. Intrinsic rotation measures between about -18 rad/m2 to -30 rad/m2 are required to model the observations. Depolarization of the background emission is quite small (compatible with zero), indicating a regular magnetic field structure with little turbulence within the Faraday screens. With observational constraints for the thermal electron density from H-alpha observations of less than 0.8 cm^-3 we conclude that the regular magnetic field strength along the line of sight exceeds 20 muG. We discuss some possibilities for the origin of such strong and well ordered magnetic fields. The modeling also predicts a large-scale, regularly polarized emission in the foreground of the Taurus clouds which is of the order of 0.24 K at 21cm. This amount of synchrotron emission is clearly excessive when compared to previous estimates of the local synchrotron emissivity.

Properties of the warm magnetized ISM, as inferred from WSRT polarimetric imaging

We describe a first attempt to derive properties of the regular and turbulent Galactic magnetic field from multi-frequency polarimetric observations of the diffuse Galactic synchrotron background. A single-cell-size model of the thin Galactic disk is constructed which includes random and regular magnetic fields and thermal and relativistic electrons. The disk is irradiated from behind with a uniform partially polarized background. Radiation from the background and from the thin disk is Faraday rotated and depolarized while propagating through the medium. The model parameters are estimated from a comparison with 350 MHz observations in two regions at intermediate latitudes done with the Westerbork Synthesis Radio Telescope. We obtain good agreement between the estimates for the random and regular magnetic field strengths and typical scales of structure in the two regions. The regular magnetic field strength found is a few μG, and the ratio of random to regular magnetic field strength is , for a typical scale of the random component of pc. Furthermore, the regular magnetic field is directed almost perpendicular to the line of sight. This modeling is a potentially powerful method to estimate the structure of the Galactic magnetic field, especially when more polarimetric observations of the diffuse synchrotron background at intermediate latitudes become available.

Hydrostatic equilibrium in a magnetized, warped Galactic disc

Hydrostatic equilibrium of the multiphase interstellar medium in the solar vicinity is reconsidered, with the regular and turbulent magnetic fields treated separately. The regular magnetic field strength required to support the gas is consistent with independent estimates, provided that energy equipartition is maintained between turbulence and random magnetic fields. Our results indicate that a mid-plane value of B0 = 4 μG for the regular magnetic field near the Sun leads to more attractive models than B0 = 2 μG. The vertical profiles of both the regular and random magnetic fields contain disc and halo components, the parameters of which we have determined. The layer at 1≲|z|≲4 kpc can be overpressured and an outflow at a speed of about 50 km s−1 may occur there, presumably associated with a Galactic fountain flow, if B0≃2 μG. We show that hydrostatic equilibrium in a warped disc must produce asymmetric density distributions in z, in rough agreement with H i observations in the outer Galaxy. This asymmetry may be a useful diagnostic of the details of the warping mechanism in the Milky Way and other galaxies. We find indications that gas and magnetic field pressures are different above and below the warped midplane in the outer Galaxy, and quantify the difference in terms of turbulent velocity and/or magnetic field strength.

Magnetic and optical spiral arms in the galaxy NGC 6946

The spiral pattern in the nearby spiral galaxy NGC 6946 has been studied using the wavelet transformation technique, applied to galaxy images in polarized and total non-thermal radio emission at lambda lambda3.5 and 6.2 cm, in broadband red light, in the lambda 21.1 cm H I line and in the optical H alpha line. Well-defined, continuous spiral arms are visible in polarized radio emission and red light, where we can isolate a multi-armed pattern in the range of galactocentric distances 1.5-12 kpc, consisting of four long arms and one short spiral segment. The 'magnetic arms' (visible in polarized radio emission) are localized almost precisely between the optical arms. Each magnetic arm is similar in length and pitch angle to the preceding optical arm (in the sense of galactic rotation) and can be regarded as its phase-shifted image. Even details like a bifurcation of an optical arm have their phase-shifted counterparts in the magnetic arms. The average relative amplitude of the optical spiral arms (the stellar density excess over the azimuthal average) grows with galactocentric radius up to 0.3-0.7 at r similar or equal to5 kpc, decreases by a factor of two at r=5-6 kpc and remains low at 0.2-0.3 in the outer parts of the galaxy. By contrast, the magnetic arms have a constant average relative amplitude (the excess in the regular magnetic field strength over the azimuthal average) of 0.3-0.6 in a wide radial range r=1.5-12 kpc. We briefly discuss implications of our findings for theories of galactic magnetic fields.