Interarm Regions Research Articles

Measuring interstellar magnetic fields by radio synchrotron emission

AbstractRadio synchrotron emission, its polarization and its Faraday rotation are powerful tools to study the strength and structure of interstellar magnetic fields. The total intensity traces the strength and distribution of total magnetic fields. Total fields in gas-rich spiral arms and bars of nearby galaxies have strengths of 20–30 μGauss, due to the amplification of turbulent fields, and are dynamically important. In the Milky Way, the total field strength is about 6 μG near the Sun and several 100 μG in filaments near the Galactic Center. – The polarized intensity measures ordered fields with a preferred orientation, which can be regular or anisotropic fields. Ordered fields with spiral structure exist in grand-design, barred, flocculent and even in irregular galaxies. The strongest ordered fields are found in interarm regions, sometimes forming “magnetic spiral arms” between the optical arms. Halo fields are X-shaped, probably due to outflows. – The Faraday rotation of the polarization vectors traces coherent regular fields which have a preferred direction. In some galaxies Faraday rotation reveals large-scale patterns which are signatures of dynamo fields. However, in most galaxies the field has a complicated structure and interacts with local gas flows. In the Milky Way, diffuse polarized radio emission and Faraday rotation of the polarized emission from pulsars and background sources show many small-scale and large-scale magnetic features, but the overall field structure in our Galaxy is still under debate.

The ISM in spiral galaxies: can cooling in spiral shocks produce molecular clouds?

We investigate the thermodynamics of the ISM and the formation of molecular hydrogen through numerical simulations of spiral galaxies. The model follows the chemical, thermal and dynamical response of the disc to an external spiral potential. Self-gravity and magnetic fields are not included. The calculations demonstrate that gas can cool rapidly when subject to a spiral shock. Molecular clouds in the spiral arms arise through a combination of compression of the ISM by the spiral shock and orbit crowding. These results highlight that local self-gravity is not required to form molecular clouds. Self-shielding provides a sharp transition density, below which gas is essentially atomic, and above which the molecular gas fraction is >0.001. The timescale for gas to move between these regimes is very rapid ( 50 %). Although our calculations are unable to resolve turbulent motions on scales smaller than the spiral arm and do not include self-gravity. True cloud formation timescales are therefore expected to be even shorter. The mass budget of the disc is dominated by cold gas residing in the spiral arms. Between 50 and 75 % of this gas is in the atomic phase. When this gas leaves the spiral arm and drops below the self-shielding limit it is heated by the galactic radiation field. Consequently, most of the volume in the interarm regions is filled with warm atomic gas. However, some cold spurs and clumps can survive in interarm regions for periods comparable to the interarm passage timescale. Altogether between 7 and 40% of the gas in our disc is molecular, depending on the surface density of the calculation, with approximately 20% molecular for a surface density comparable to the solar neighbourhood.

Galactic Spiral Shocks with Thermal Instability

Using one-dimensional hydrodynamic simulations including interstellar heating, cooling, and thermal conduction, we investigate nonlinear evolution of gas flow across galactic spiral arms. We model the gas as a non-self-gravitating, unmagnetized fluid and follow its interaction with a stellar spiral potential in a local frame comoving with the stellar pattern. Initially uniform gas with density n0 in the range 0.5 cm −3 ≤ n0 ≤ 10 cm −3 rapidly separates into warm and cold phases as a result of thermal instability (TI) and also forms a quasi-steady shock that prompts phase transitions. After saturation, the flow follows a recurring cycle: warm and cold phases in the interarm region are shocked and immediately cool to become a denser cold medium in the arm; postshock expansion reduces the mean density to the unstable regime in the transition zone and TI subsequently mediates evolution back into warm and cold interarm phases. For our standard model with n0 = 2 cm −3, the gas resides in the dense arm, thermally unstable transition zone, and interarm region for 14%, 22%, and 64% of the arm-to-arm crossing time, respectively. These regions occupy 1%, 16%, and 83% of the arm-to-arm distance, respectively. Gas at intermediate temperatures (i.e., neither warm stable nor cold states) represents ~25%-30% of the total mass, similar to the fractions estimated from H I observations (larger interarm distances could reduce this mass fraction, whereas other physical processes associated with star formation could increase it). Despite transient features and multiphase structure, the time-averaged shock profiles can be matched to that of a diffusive isothermal medium with temperature 1000 K (which is < Twarm) and a particle mean free path of l0 = 100 pc . Finally, we quantify numerical conductivity associated with translational motion of phase-separated gas on the grid and show that convergence of numerical results requires the numerical conductivity to be comparable to or smaller than the physical conductivity.

Galactic dynamos and galactic winds

Spiral galaxies host dynamically important magnetic fields which can affect gas flows in the disks and halos. Total magnetic fields in spiral galaxies are strongest (up to 30 μG) in the spiral arms where they are mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to 15 μG). Faraday rotation measures of radio polarization vectors in the disks of several spiral galaxies reveal large-scale patterns which are signatures of coherent fields generated by a mean-field dynamo. Magnetic fields are also observed in radio halos around edge-on galaxies at heights of a few kpc above the disk. Cosmic-ray driven galactic winds transport gas and magnetic fields from the disk into the halo. The halo scale height and the electron lifetime allow to estimate the wind speed. The magnetic energy density is larger than the thermal energy density, but smaller than the kinetic energy density of the outflow. There is no observation yet of a halo with a large-scale coherent dynamo pattern. A global wind outflow may prevent the operation of a dynamo in the halo. Halo regions with high degrees of radio polarization at very large distances from the disk are excellent tracers of interaction between galaxies or ram pressure of the intergalactic medium. The observed extent of radio halos is limited by energy losses of the cosmic-ray electrons. Future low-frequency radio telescopes like LOFAR and the SKA will allow to trace halo outflows and their interaction with the intergalactic medium to much larger distances.

Magnetic fields and gas in the cluster-influenced spiral galaxy NGC 4254

Aims. The origin of asymmetric radio polarized emission in the Virgo Cluster spiral NGC 4254 is investigated and the influence of cluster environment on the properties of magnetic fields is explored. Methods. Structures of magnetic fields are analyzed with the concept of “magnetic maps”, presenting distributions of different magnetic field components (total, regular, and random) over the entire galaxy, free of Faraday rotation and projection effects. A number of different physical phenomena influencing the magnetic field are modeled analytically and confronted with the galaxy's depolarization pattern and distribution of magnetic field strength obtained from multifrequency polarimetric radio observations. Results. The study of orientation of intrinsic magnetic field vectors in NGC 4254 indicates that their dramatic variation (from 0° to more than 40°) throughout the galaxy cannot arise from the dynamo process alone, but must be dominated by effects such as density waves and local gas flows. We determine within the galaxy the relation between the strength of total magnetic field and the local star-formation rate (SFR) as a power-law with an index of +0.18 ± 0.01. We find the opposite sense of the relation between magnetic field regularity and SFR (-0.32 ± 0.03), and suggest that it results from efficient production of random field with rising turbulence in the regions with actively-forming stars. The distribution of Faraday rotation measures in NGC 4254 indicates a perturbed axisymmetrical mean-field dynamo mode or a mixture of axisymmetrical and bisymmetrical ones with regular field directed outwards from the disk, which is contrary to most observed galaxies. The galaxy's northern magnetic arm, located on the upstream side of the local density wave, with regular field strength of about 8 μ G and the total one of 17 μ G, much resembles those observed in other galaxies. But the magnetic field within two outer arms (shifted downstream of a density wave) is much stronger, up to 13 μ G in the regular field component and 20 μ G in the total field. Our modeling of cluster influence on different magnetic field components indicates that within the outer magnetic arms the dynamo-induced magnetic fields are modified by stretching and shearing forces rather than by cluster ram pressure. Those forces, which are likely triggered by the galaxy's gravitational interaction, produce an anisotropic component of the regular field and enhance the polarized emission. We also show that the magnetic energy within the large interarm regions and the galaxy's outskirts exceeds the gas thermal and turbulent energy, likely becoming dynamically important.

Instabilities of Spiral Shocks. II. A Quasi–Steady State in the Multiphase Inhomogeneous ISM

Galactic (Fujimoto; Roberts) are investigated using full three-dimensional hydrodynamic simulations that take into account the self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential improvement over previous numerical models, in which two-dimensional isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks are unstable and transient, and they shift to a globally quasi-steady, inhomogeneous pattern due to the nonlinear development of instabilities in the disk. The spiral patterns consist of many giant molecular cloud-like dense condensations, but those local structures are not steady, and they evolve into irregular spurs in the interarm regions. Energy feedback from supernovae does not destroy the quasi-steady spiral arms; rather, it mainly contributes to the vertical motion and the structures of the ISM. The results and methods presented here are a starting point for a more consistent treatment of the ISM in spiral galaxies, in which the effects of magnetic fields, radiative transfer, chemistry, and dynamical evolution of a stellar disk are taken into account.

Warm Molecular Gas in M51: Mapping the Excitation Temperature and Mass of H2with theSpitzerInfrared Spectrograph

We have mapped the warm molecular gas traced by the H2 S(0)-H2 S(5) pure rotational mid-infrared emission lines over a radial strip across the nucleus and disk of M51 (NGC 5194) using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The six H2 lines have markedly different emission distributions. We obtained the H2 temperature and surface density distributions by assuming a two-temperature model: a warm (T = 100–300 K) phase traced by the low J [S(0)-S(2)] lines and a hot phase (T = 400–1000 K) traced by the high J [S(2)-S(5)] lines. The lowest molecular gas temperatures are found within the spiral arms (T ∼ 155 K), while the highest temperatures are found in the inter-arm regions (T > 700 K). The warm gas surface density reaches a maximum of 11 M☉ pc−2 in the northwest spiral arm, whereas the hot gas surface density peaks at 0.24 M☉ pc−2 at the nucleus. The spatial offset between the peaks in the different phases suggests that the warm phase is more efficiently heated by star formation activity and the hot phase is more efficiently heated by nuclear activity. The warm H2 is found in the dust lanes of M51 and is generally spatially coincident with the cold molecular gas traced by CO emission, consistent with excitation of the warm phase in dense photodissociation regions. The hot H2 is most prominent in the nuclear region. Here, the hot H2 coincides with [O IV] (25.89 μm) and X-ray emission indicating that shocks and/or X-rays are responsible for exciting this phase.

The Outer Scale of Turbulence in the Magnetoionized Galactic Interstellar Medium

We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of the partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's spiral arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in spiral arms are present, and show power-law behavior with a shallow spectrum. These results suggest that in the spiral arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and superbubble explosions are the main sources of energy on scales on the order of 100 pc.

New knowledge of the Galactic magnetic fields

The magnetic fields of our Milky Way galaxy are the main agent for cosmic rays to transport. In the last decade, much new knowledge has been gained from measurements of the Galactic magnetic fields. In the Galactic disk, from the RMs of a large number of newly discovered pulsars, the large-scale magnetic fields along the spiral arms have been delineated in a much larger region than ever before, with alternating directions in the arm and interarm regions. The toroidal fields in the Galactic halo were revealed to have opposite directions below and above the Galactic plane, which is an indication of an A0 mode dynamo operating in the halo. The strength of large-scale fields obtained from pulsar RM data has been found to increase exponentially towards the Galactic center. Compared to the steep Kolmogorov spectrum of magnetic energy at small scales, the large-scale magnetic fields show a shallow broken spatial magnetic energy spectrum.

CO(1-0), CO(2-1), and Neutral Gas in NGC 6946: Molecular Gas in a Late-Type, Gas-Rich, Spiral Galaxy

We present On The Fly maps of the CO(1-0) and CO(2-1) emission covering a 10' X 10' region of the NGC 6946. Using our CO maps and archival VLA HI observations we create a total gas surface density map, Sigma_gas, for NGC 6946. The predominantly molecular inner gas disk transitions smoothly into an atomic outer gas disk, with equivalent atomic and molecular gas surface densities at R = 3.5' (6 kpc). We estimate that the total H2 mass is 3 X 10^9 Mo, roughly 1/3 of the interstellar hydrogen gas mass, and about 2% of the dynamical mass of the galaxy at our assumed distance of 6 Mpc. The value of the CO(2-1)/CO(1-0) line ratio ranges from 0.35 to 2; 50% of the map is covered by very high ratio, >1, gas. The very high ratios are predominantly from interarm regions and appear to indicate the presence of wide-spread optically thin gas. Star formation tracers are better correlated with the total neutral gas disk than with the molecular gas by itself implying SFR is proportional to Sigma_gas. Using the 100 FIR and 21 cm continuum from NGC 6946 as star formation tracers, we arrive at a gas consumption timescale of 2.8 Gyr, which is relatively uniform across the disk. The high star formation rate at the nucleus appears to be due to a large accumulation of molecular gas rather than a large increase in the star formation efficiency. The mid-plane gas pressure in the outer (R > 10 kpc) HI arms of NGC 6946 is close to the value at the radial limit (10 kpc) of our observed CO disk. If the mid-plane gas pressure is a factor for the formation of molecular clouds, these outer HI gas arms should contain molecular gas which we do not see because they are beyond our detection limit.

Particularly efficient star formation in M 33

The Star Formation (SF) rate in galaxies is an important parameter at all redshifts and evolutionary stages of galaxies. In order to understand the increased SF rates in intermediate redshift galaxies one possibility is to study star formation in local galaxies with properties frequently found at this earlier epoch like low metallicity and small size. We present sensitive observations of the molecular gas in M 33, a small Local Group spiral at a distance of 840 kpc which shares many of the characteristics of the intermediate redshift galaxies. The observations were carried out in the CO(2–1) line with the HERA heterodyne array on the IRAM 30 m telescope. A region in the northern part of M 33 was observed, reaching a detection threshold of a few 103 . The correlation in this field between the CO emission and tracers of SF (8 μm, 24 μm, Hα, FUV) is excellent and CO is detected very far North, showing that molecular gas forms far out in the disk even in a small spiral with a subsolar metallicity. One major molecular cloud was discovered in an interarm region with no HI peak and little if any signs of SF – without a complete survey this cloud would never have been found. The radial dependence of the CO emission has a scale length similar to the dust emission, less extended than the Hα or FUV. If, however, the ratio varies inversely with metallicity, then the scale length of the H2 becomes similar to that of the Hα or FUV. Comparing the SF rate to the H2 mass shows that M 33, like the intermediate redshift galaxies it resembles, has a significantly higher SF efficiency than large local universe spirals. The data presented here also provide an ideal test for theories of molecular cloud formation and cover a new region in parameter space, where . We find that a simple pressure-based prescription for estimating the molecular to atomic gas fraction does not perform well for M 33, at least in the outer parts. On the other hand, we show that the molecular gas fraction is influenced by (i) the total Hydrogen column density, dominated in M 33 by the HI, and (ii) the galactocentric distance.

Spiral shocks and the formation of molecular clouds in a two-phase medium

We extend recent numerical results (Dobbs et. al. 2006) on molecular cloud formation in spiral galaxies by including a multi-phase medium. The addition of a hot phase of gas enhances the structure in the cold gas, and significantly increases the fraction of molecular hydrogen that is formed when the cold gas passes through a spiral shock. The difference in structure is reflected in the mass power spectrum of the molecular clouds, which is steeper for the multi-phase calculations. The increase in molecular gas occurs as the addition of a hot phase leads to higher densities in the cold gas. In particular, cold gas is confined in clumps between the spiral arms and retains a higher molecular fraction. Unlike the single phase results, molecular clouds are present in the inter-arm regions for the multi-phase medium. However the density of the inter-arm molecular hydrogen is generally below that which can be reliably determined from CO measurements. We therefore predict that for a multi-phase medium, there will be low density clouds containing cold atomic and molecular hydrogen, which are potentially entering the spiral arms.

High‐Resolution Molecular Gas Maps of M33

New observations of CO (J = 1 → 0) line emission from M33, using the 25 element BEARS focal plane array at the Nobeyama Radio Observatory 45 m telescope, in conjunction with existing maps from the BIMA interferometer and the FCRAO 14 m telescope, give the highest resolution (13'') and most sensitive (σrms ~ 60 mK) maps to date of the distribution of molecular gas in the central 5.5 kpc of the galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit of 1.3 × 105 M☉. The fraction of molecular gas found in GMCs is a strong function of radius in the galaxy, declining from 60% in the center to 20% at galactocentric radius Rgal ≈ 4 kpc. Beyond that radius, GMCs are nearly absent, although molecular gas exists. Most (90%) of the emission from low-mass clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1 kpc < Rgal < 4.1 kpc, GMC masses follow a power-law distribution with index -2.1. Inside that radius, the mass distribution is truncated, and clouds more massive than 8 × 105 M☉ are absent. The cloud mass distribution shows no significant difference in the grand-design spiral arms versus the interarm region. The CO surface brightness ratio for the arm to interarm regions is 1.5, typical of other flocculent galaxies.

The characteristic polarized radio continuum distribution of cluster spiral galaxies

Deep observations are presented of the 6 cm polarized radio continuum emission of 8 Virgo spiral galaxies. All galaxies show strongly asymmetric distributions of polarized intensity with elongated ridges located in the outer galactic disk. These features are not found in existing observations of polarized radio continuum emission of field spiral galaxies, where the distribution of 6 cm polarized intensity is generally relatively symmetric and strongest in the interarm regions. We therefore conclude that most Virgo spiral galaxies and, most probably, the majority of cluster spiral galaxies show asymmetric distributions of polarized radio continuum emission due to their interaction with the cluster environment. The polarized continuum emission is sensitive to compression and shear motions in the plane of the sky and thus contains important information about the velocity distortions caused by these interactions.

Magnetic Field Structure from Synchrotron Polarization

Total magnetic fields in spiral galaxies, as observed through their total synchrotron emission, are strongest (up to ≲30 µ G) in the spiral arms. The degree of radio polarization is low; the field in the arms must be mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to ≲15 µ G), sometimes forming “magnetic arms” parallel to the optical arms. The field structure is spiral in almost every galaxy, even in flocculent and bright irregular types which lack spiral arms. The observed large-scale patterns of Faraday rotation in several massive spiral galaxies reveal coherent regular fields, as predicted by dynamo models. However, in most galaxies observed so far no simple patterns of Faraday rotation could be found. Either many dynamo modes are superimposed and cannot be resolved by present-day telescopes, or most of the apparently regular field is in fact anisotropic random , with frequent reversals, due to shearing and compressing gas flows. In galaxies with massive bars, the polarization pattern follows the gas flow. However, around strong shocks in bars, the compression of the regular field is much lower than that of the gas; the regular field decouples from the cold gas and is strong enough to affect the flow of the diffuse warm gas. – The average strength of the total magnetic field in the Milky Way is 6µ G near the sun and increases to 20–40 μ G in the Galactic center region. The Galactic field is mostly parallel to the plane, except in the center region. Rotation measure data from pulsars indicate several field reversals, unlike external galaxies, but some reversals could be due to distortions of the nearby field.

Morphological Analysis of HiFeatures. II. Wavelet‐based Multifractal Formalism

The 2D wavelet transform modulus maxima method is used to obtain quantitative information on the fractal/multifractal nature and anisotropic structure of Galactic atomic hydrogen (H I) from the Canadian Galactic Plane Survey. Five mosaics were analyzed in the second quadrant of the Milky Way, corresponding to the Local, Perseus, and Outer spiral arms, as well as two regions located between these three spiral arms. A monofractal signature is found for all five mosaics. An anisotropic signature is detected: the rms roughness fluctuations of the mosaics in the latitude direction differ from those in the longitude direction. This anisotropy is scale-independent for the interarm regions while it is scale-dependent for the spiral arms. The longitudinal matter distribution of H I structure is similar for all five mosaics while the latitudinal distribution is smoother in the spiral arms. These results hold for all physical length scales studied, from ~2 pc in the Local arm to ~44 pc in the Outer arm. Several hypotheses are investigated to provide a physical explanation: the scale-height gradient, the density wave, star formation activity, photolevitation of dusty clouds, random motion of H I clouds, corrugation, and turbulence. The slopes of the power spectra for an increasing number of velocity channels were compared for 11 sections of the Local arm column density mosaic. All slopes are identical within the uncertainties (-3.0) and we do not detect for the Galactic plane the change in the power law index predicted by Lazarian and Pogosyan.

Magnetic fields in the Southern Galactic Plane Survey

AbstractThe Southern Galactic Plane Survey (SGPS) is a 1.4 GHz radio polarization and H I survey in a large part of the inner Galactic plane at a resolution of about an arcmin. Depolarization and Faraday rotation of polarized radiation from diffuse Galactic synchrotron emission, pulsars, and extragalactic sources can be used to infer information about the strength and structure of the Galactic magnetic field. Here, we discuss early results of the polarization data from the SGPS. We show from statistical analysis of rotation measures of polarized extragalactic sources that fluctuations in the magnetoionized medium of the spiral arms are probably mainly caused by H II regions, while the rotation measure fluctuations in the interarm regions may be connected to the interstellar turbulent cascade. Furthermore, the variations of rotation measure with Galactic longitude enable modeling of the large‐scale component of the Galactic magnetic field, including determination of the number and location of magnetic field reversals. Finally, the SGPS is an excellent way to study subparsec‐scale structure in the ionized interstellar medium by way of depolarization studies in H II regions. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Pulsar Rotation Measures and the Large‐Scale Structure of the Galactic Magnetic Field

The large-scale magnetic field of our Galaxy can be probed in three dimensions using Faraday rotation of pulsar signals. We report on the determination of 223 rotation measures from polarization observations of relatively distant southern pulsars made using the Parkes radio telescope. Combined with previously published observations these data give clear evidence for large-scale counterclockwise fields (viewed from the north Galactic pole) in the spiral arms interior to the Sun and weaker evidence for a counterclockwise field in the Perseus arm. However, in interarm regions, including the Solar neighbourhood, we present evidence that suggests that large-scale fields are clockwise. We propose that the large-scale Galactic magnetic field has a bisymmetric structure with reversals on the boundaries of the spiral arms. Streaming motions associated with spiral density waves can directly generate such a structure from an initial inwardly directed radial field. Large-scale fields increase toward the Galactic Center, with a mean value of about 2~$\mu$G in the Solar neighbourhood and 4~$\mu$G at a Galactocentric radius of 3 kpc.

Enhanced Small-Scale Faraday Rotation in the Galactic Spiral Arms

We present an analysis of the rotation measures (RMs) of polarized extragalactic point sources in the Southern Galactic Plane Survey. This work demonstrates that the statistics of fluctuations in RM differ for the spiral arms and the interarm regions. Structure functions of RM are flat in the spiral arms, while they increase in the interarms. This indicates that there are no correlated RM fluctuations in the magnetoionized interstellar medium in the spiral arms on scales larger than ~05, corresponding to ~17 pc in the nearest spiral arm probed. The nonzero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magnetoionic medium in the Milky Way spiral arms are not dominated by the mainly supernova-driven turbulent cascade in the global ISM but are probably due to a different source, most likely H II regions.

Dissecting the spiral galaxy M 83: mid-infrared emission and comparison with other tracers of star formation

We present a detailed mid-infrared study of the nearby, face-on spiral galaxy M83 based on ISOCAM data. M83 is a unique case study, since a wide variety of MIR broad-band filters as well as spectra, covering the wavelength range of 4 to 18\mu m, were observed and are presented here. Emission maxima trace the nuclear and bulge area, star-formation regions at the end of the bar, as well as the inner spiral arms. The fainter outer spiral arms and interarm regions are also evident in the MIR map. Spectral imaging of the central 3'x3' (4 kpc x 4 kpc) field allows us to investigate five regions of different environments. The various MIR components (very small grains, polycyclic aromatic hydrocarbon (PAH) molecules, ionic lines) are analyzed for different regions throughout the galaxy. In the total 4\mu m to 18\mu m wavelength range, the PAHs dominate the luminosity, contributing between 60% in the nuclear and bulge regions and 90% in the less active, interarm regions. Throughout the galaxy, the underlying continuum emission from the small grains is always a smaller contribution in the total MIR wavelength regime, peaking in the nuclear and bulge components. The implications of using broad-band filters only to characterize the mid-infrared emission of galaxies, a commonly used ISOCAM observation mode, are discussed. We present the first quantitative analysis of new H-alpha and 6cm VLA+Effelsberg radio continuum maps of M83. The distribution of the MIR emission is compared with that of the CO, HI, R band, H-alpha and 6cm radio. A striking correlation is found between the intensities in the two mid-infrared filter bands and the 6cm radio continuum. To explain the tight mid-infrared-radio correlation we propose the anchoring of magnetic field lines in the photoionized shells of gas clouds.