Transport of cosmic rays in the nearby starburst galaxy NGC 253

Heesen, V.; Beck, Rainer; Krause, M.; Dettmar, R.‐J.

doi:10.1002/asna.200911279

Cited by 10 publications

(13 citation statements)

References 23 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…It is apparent that the bulk speeds of the wind, driven by the vertical gradient of CR pressure, reach 65 ± 15 km s −1 at z = 2 kpc. Vertical systematic winds of bulk speeds, comparable to rotational galactic velocities, influence large-scale structures of galactic magnetic fields and are observed in external starburst galaxies such as NGC 253 (see Heesen et al 2007Heesen et al , 2009.…”

Section: Resultsmentioning

confidence: 99%

Cosmic-ray-driven dynamo in galactic disks

Hanasz¹,

Otmianowska-Mazur

Kowal

et al. 2009

A&A

View full text Add to dashboard Cite

Aims. We present a parameter study of the magnetohydrodynamical-dynamo driven by cosmic rays in the interstellar medium (ISM), focusing on the efficiency of magnetic-field amplification and the issue of energy equipartition between magnetic, kinetic, and cosmicray (CR) energies. Methods. We perform numerical CR-MHD simulations of the ISM using an extended version of ZEUS-3D code in the shearingbox approximation and taking into account the presence of Ohmic resistivity, tidal forces, and vertical disk gravity. CRs are supplied in randomly-distributed supernova (SN) remnants and are described by the diffusion-advection equation, which incorporates an anisotropic diffusion tensor. Results. The azimuthal magnetic flux and total magnetic energy are amplified in the majority of models depending on a particular choice of model parameters. We find that the most favorable conditions for magnetic-field amplification correspond to magnetic diffusivity of the order of 3 × 10 25 cm 2 s −1 , SN rates close to those observed in the Milky Way, periodic SN activity corresponding to spiral arms, and highly anisotropic and field-aligned CR diffusion. The rate of magnetic-field amplification is relatively insensitive to the magnitude of SN rates spanning a range of 10% to 100% of realistic values. The timescale of magnetic-field amplification in the most favorable conditions is 150 Myr, at a galactocentric radius equal to 5 kpc, which is close to the timescale of galactic rotation. The final magnetic-field energies reached in the efficient amplification cases fluctuate near equipartition with the gas kinetic energy. In all models CR energy exceeds the equipartition values by a least an order of magnitude, in contrast to the commonly expected equipartition. We suggest that the excess of cosmic rays in numerical models can be attributed to the fact that the shearing box does not permit cosmic rays to leave the system along the horizontal magnetic field, as may be the case for true galaxies.

show abstract

Section: Resultsmentioning

confidence: 99%

Cosmic-ray-driven dynamo in galactic disks

Hanasz¹,

Otmianowska-Mazur

Kowal

et al. 2009

A&A

View full text Add to dashboard Cite

show abstract

“…In particular, the cosmic rays will be depleted through diffusion processes in the interstellar medium. Typical values of the diffusion coefficients range from 3 × 10 27 cm 2 s −1 (Mulcahy et al 2014) to 2 × 10 29 cm 2 s −1 (Heesen et al 2009), and may depend both on the magnetic field strength and the alignment of the magnetic fields. We adopt here a typical diffusion coefficient of D E = 2×10 28 cm 2 s −1 , which is characteristic for cosmic ray energies of 1 GeV and which allows us to assess the potential relevance of the diffusion (Murphy 2009).…”

Section: Timescales For Radio Emissionmentioning

confidence: 99%

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Schleicher

Beck

2016

A&A

View full text Add to dashboard Cite

The far-infrared -radio correlation connects star formation and magnetic fields in galaxies, and has been confirmed over a large range of far-infrared / radio luminosities, both in the local Universe and even at redshifts of z ∼ 2. Recent investigations indicate that it may even hold in the regime of local dwarf galaxies, and we therefore explore here the expected behavior in the regime of star formation surface densities below 0.1 M⊙ kpc −2 yr −1 . We derive two conditions that can be particularly relevant for inducing a change in the expected correlation: a critical star formation surface density to maintain the correlation between star formation rate and the magnetic field, and a critical star formation surface density below which cosmic ray diffusion losses dominate over their injection via supernova explosions. For rotation periods shorter than 1.5 × 10 7 (H/kpc) 2 yrs, with H the scale height of the disk, the first correlation will break down before diffusion losses are relevant, as higher star formation rates are required to maintain the correlation between star formation rate and magnetic field strength. For high star formation surface densities ΣSFR, we derive a characteristic scaling of the non-thermal radio to the far-infrared / infrared emission with Σ 1/3 SFR , corresponding to a scaling of the non-thermal radio luminosity Ls with the infrared luminosity L th as L 4/3 th . The latter is expected to change when the above processes are no longer steadily maintained. In the regime of long rotation periods, we expect a transition towards a steeper scaling with Σ 2/3 SFR , implying Ls ∝ L 5/3th , while the regime of fast rotation is expected to show a considerably enhanced scatter, as a well-defined relation between star formation and magnetic field strength is not maintained. The scaling relations above explain the increasing thermal fraction of the radio emission observed within local dwarfs, and can be tested with future observations by LOFAR as well as the SKA and its precursor radio telescopes.

show abstract

“…They are about one order of magnitude weaker than the toroidal fields in the disk (Ruzmaikin et al 1988) and hence are irrelevant for our models. Polarization observations of nearby galaxies seen edge-on generally show a magnetic field parallel to the disk near the disk plane, but recent high-sensitivity observations of several edge-on galaxies like M104, NGC 891, NGC 253, and NGC 5775 show vertical field components that increase with increasing height z above and below the galactic plane and also with increasing radius, socalled X-shaped magnetic fields (Heesen et al 2005(Heesen et al , 2007Soida 2005;Krause et al 2006;Krause 2007). Such a vertical field can be due to a galactic wind that may transport the disk field into the halo.…”

Section: Models For the Magnetic Field In The Halomentioning

confidence: 99%

Magnetic field structures of galaxies derived from analysis of Faraday rotation measures, and perspectives for the SKA

Stepanov¹,

Arshakian²,

Beck³

et al. 2008

A&A

Self Cite

106

View full text Add to dashboard Cite

Context. The forthcoming new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Aims. Our analysis looks at what can be learned from these sources concerning the structure and evolution of magnetic fields of external galaxies. Methods. Recognition of magnetic structures is possible from Faraday rotation measures (RM) towards background sources behind galaxies or a continuous RM map obtained from the diffuse polarized emission from the galaxy itself. We constructed models for the ionized gas and magnetic field patterns of different azimuthal symmetries (axisymmetric, bisymmetric and quadrisymmetric spirals, and superpositions) plus a halo magnetic field. RM fluctuations with a Kolmogorov spectrum due to turbulent fields and/or fluctuations in ionized gas density are superimposed. Assuming extrapolated number density counts of polarized sources, we generated a sample of RM values within the solid angle of the galaxy. Applying various templates, we derived the minimum number of background sources and the minimum quality of the observations. For a large number of sources, reconstruction of the field structure without precognition becomes possible. Results. Any large-scale regular component of the magnetic field can be clearly recognized from RM data with the help of the χ 2 criterium. Under favorite conditions, a few dozen polarized sources are enough for a reliable result. A halo field with a vertical component does not affect the results of recognition. The required source number increases for small inclinations of the galaxy's disk and for larger RM turbulence. A flat number density distribution of the sources can be overcome by more sensitive observations. Application of the recognition method to the available RM data in the region around M 31 indicates that there are significant RM contributions intrinsic to the background sources or due to the foreground of the Milky Way. A reliable reconstruction of the field structure needs at least 20 RM values on a cut along the projected minor axis. Conclusions. Recognition or reconstruction of regular field structures from the RM data of polarized background sources is a powerful tool for future radio telescopes. Measuring RM at frequencies around 1 GHz with the SKA, simple field structures can be recognized in galaxies up to about 100 Mpc distance and will allow to test dynamo against primordial or other models of field origin. The low-frequency SKA array and low-frequency precursor telescopes like LOFAR may also have good RM sensitivity if background sources are still significantly polarized at low frequencies.

show abstract

Transport of cosmic rays in the nearby starburst galaxy NGC 253

Cited by 10 publications

References 23 publications

Cosmic-ray-driven dynamo in galactic disks

Cosmic-ray-driven dynamo in galactic disks

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Magnetic field structures of galaxies derived from analysis of Faraday rotation measures, and perspectives for the SKA

Contact Info

Product

Resources

About