Advective and diffusive cosmic ray transport in galactic haloes

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

doi:10.1093/mnras/stw360

Cited by 61 publications

(87 citation statements)

References 72 publications

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“…If streaming dominated the cosmic ray electron transport, the convolution kernel is expected to be an exponential. This is justified by the results of Heesen et al (2016), who found that within their 1D cosmic ray transport model advection leads to approximately exponential radio continuum intensity profiles, whereas diffusion leads to profiles that can be better approximated by Gaussian functions. Since advection and streaming are formally similar (the outflow velocity has to be replaced by the Alfvén velocity), the effect of streaming can be approximated by a convolution with an exponential kernel.…”

Section: Methodsmentioning

confidence: 76%

Deciphering the radio star formation correlation on kpc scales

Vollmer

Soida

Beck

et al. 2020

A&A

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One of the tightest correlations in astronomy is the relation between the integrated radio continuum and the far-infrared emission. Within nearby galaxies, variations in the radio-FIR correlation have been observed, mainly because the cosmic ray electrons migrate before they lose their energy via synchrotron emission or escape. The major cosmic ray electron transport mechanisms within the plane of galactic disks are diffusion and streaming. A predicted radio continuum map can be obtained by convolving the map of comic ray electron sources, represented by that of the star formation, with adaptive Gaussian and exponential kernels. The ratio between the smoothing lengthscales at 6 cm and 20 cm can be used to distinguish between diffusion and streaming as the dominant transport mechanism. The dependence of the smoothing lengthscale on the star formation rate bares information on dependence of the magnetic field strength or the ratio between the ordered and turbulent magnetic field strengths on star formation. Star formation maps of eight rather face-on local and Virgo cluster spiral galaxies were constructed from Spitzer and Herschel infrared and GALEX UV observations. These maps were convolved with adaptive Gaussian and exponential smoothing kernels to obtain model radio continuum emission maps. It is found that in asymmetric ridges of polarized radio continuum emission the total power emission is enhanced with respect to the star formation rate. At a characteristic star formation rate ofΣ * = 8 × 10 −3 M yr −1 kpc −2 the typical lengthscale for the transport of cosmic ray electrons is l = 0.9 ± 0.3 kpc at 6 cm and l = 1.8 ± 0.5 kpc at 20 cm. Perturbed spiral galaxies tend to have smaller lengthscales. This is a natural consequence of the enhancement of the magnetic field caused by the interaction. The discrimination between the two cosmic ray electron transport mechanisms, diffusion and streaming, is based on (i) the convolution kernel (Gaussian or exponential), (ii) the dependence of the smoothing kernel on the local magnetic field and hence on the local star formation rate, (iii) the ratio between the two smoothing lengthscales via the frequency-dependence of the smoothing kernel, and (iv) the dependence of the smoothing kernel on the ratio between the ordered and the turbulent magnetic field. Based on our empirical results, methods (i) and (ii) cannot be used to determine the cosmic ray transport mechanism. Important asymmetric large-scale residuals and a local dependence of the smoothing length on B ord /B turb are most probably responsible for the failure of method (i) and (ii), respectively. On the other hand, the classifications based on l6cm/l20cm (method iii) and B ord /B turb (method iv) are well consistent and complementary. We argue that in the six Virgo spiral galaxies the turbulent magnetic field is globally enhanced in the disk. Therefore, the regions where the magnetic field is independent of the star formation rate are more common. In addition, B ord /B turb decreases leading to a diffusion lengths...

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Section: Methodsmentioning

confidence: 76%

Deciphering the radio star formation correlation on kpc scales

Vollmer

Soida

Beck

et al. 2020

A&A

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“…We fitted the intensity profiles with 1D cosmic ray transport models for advection and diffusion using SPINNAKER (Heesen et al 2016). In order to calculate the radio spectral indices that are required as input, we used a 1570 MHz map from L-band observations with the Karl G. Jansky Very Large Array (JVLA), combined from the B, C, and D array (Schmidt et al 2019).…”

Section: Resultsmentioning

confidence: 99%

Warped diffusive radio halo around the quiescent spiral edge-on galaxy NGC 4565

Heesen

Whitler²,

Schmidt³

et al. 2019

A&A

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Context. Cosmic rays play a pivotal role in launching galactic winds, particularly in quiescently star-forming galaxies where the hot gas alone is not sufficient to drive a wind. Except for the Milky Way, not much is known about the transport of cosmic rays in galaxies. Aims. In this Letter, we present low-frequency observations of the nearby edge-on spiral galaxy NGC 4565 using the LOw-Frequency ARray (LOFAR). With our deep 144-MHz observations, we obtain a clean estimate of the emission originating from old cosmic-ray electrons (CRe), which is almost free from contamination by thermal emission. Methods. We measured vertical profiles of the non-thermal radio continuum emission that we fitted with Gaussian and exponential functions. The different profile shapes correspond to 1D cosmic-ray transport models of pure diffusion and advection, respectively. Results. We detect a warp in the radio continuum that is reminiscent of the previously known H i warp. Because the warp is not seen at GHz-frequencies in the radio continuum, its minimum age must be about 100 Myr. The warp also explains the slight flaring of the thick radio disc that can otherwise be well described by a Gaussian profile with an FWHM of 65 arcsec (3.7 kpc). Conclusions. The diffusive radio halo together with the extra-planar X-ray emission may be remnants of enhanced star-forming activity in the past where the galaxy had a galactic wind, as GHz-observations indicate only a weak outflow in the last 40 Myr. NGC 4565 could be in transition from an outflow-to an inflow-dominated phase.

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“…It has been shown observationally (Dahlem et al 1995;Heesen et al 2009) that the diffusion coefficient in the z direction is several times lower than the radial component. Heesen et al (2016), using 1D cosmic ray electron transport models and Australia Telescope Compact Array observations of two edge-on galaxies, found that the cosmic ray transport in the halo of NGC 7090 is advection dominated with a velocity of approximately 150 km s −1 , while NGC 7462 is diffusion dominated with a diffusion coefficient of D = 3.0 × 10 28 E 0.5…”

Section: Discussionmentioning

confidence: 99%

Modelling the cosmic ray electron propagation in M 51

Mulcahy

Fletcher

Beck

et al. 2016

A&A

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Context. Cosmic ray electrons (CREs) are a crucial part of the interstellar medium and are observed via synchrotron emission. While much modelling has been carried out on the CRE distribution and propagation of the Milky Way, little has been done on normal external star-forming galaxies. Recent spectral data from a new generation of radio telescopes enable us to find more robust estimations of the CRE propagation. Aims. To model the synchrotron spectral index of M 51 using the diffusion energy-loss equation and to compare the model results with the observed spectral index determined from recent low-frequency observations with LOFAR. Methods. We solve the time-dependent diffusion energy-loss equation for CREs in M 51. This is the first time that this model for CRE propagation has been solved for a realistic distribution of CRE sources, which we derive from the observed star formation rate, in an external galaxy. The radial variation of the synchrotron spectral index and scale-length produced by the model are compared to recent LOFAR and older VLA observational data and also to new observations of M 51 at 325 MHz obtained with the GMRT. Results. We find that propagation of CREs by diffusion alone is sufficient to reproduce the observed spectral index distribution in M 51. An isotropic diffusion coefficient with a value of 6.6 ± 0.2 × 10 28 cm 2 s −1 is found to fit best and is similar to what is seen in the Milky Way. We estimate an escape time of 11 Myr from the central galaxy to 88 Myr in the extended disk. It is found that an energy dependence of the diffusion coefficient is not important for CRE energies in the range 0.01 GeV-3 GeV. We are able to reproduce the dependence of the observed synchrotron scale-lengths on frequency, with l ∝ ν −1/4 in the outer disk and l ∝ ν −1/8 in the inner disk.

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Advective and diffusive cosmic ray transport in galactic haloes

Cited by 61 publications

References 72 publications

Deciphering the radio star formation correlation on kpc scales

Deciphering the radio star formation correlation on kpc scales

Warped diffusive radio halo around the quiescent spiral edge-on galaxy NGC 4565

Modelling the cosmic ray electron propagation in M 51

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