Modelling the cosmic ray electron propagation in M 51

Mulcahy, D. D.; Fletcher, Andrew; Beck, Rainer; Mitra, Dipanjan; Scaife, Anna M. M.

doi:10.1051/0004-6361/201628446

Cited by 29 publications

(68 citation statements)

References 69 publications

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“…Our best-fitting diffusion coefficients for µ = 0.5 are at least 2 orders of magnitude smaller than the values of a few 10 28 cm 2 s −1 that are found in the Milky Way and in other external galaxies for CREs with a few GeV, either from a spatial correlation analysis between the radio continuum emission and various star-formation tracers (e.g. Berkhuijsen et al 2013) or from a spectral index analysis as in this work Mulcahy et al 2016). The larger diffusion coefficients in spiral galaxies could be explained by their more ordered magnetic field structure, where the cosmic rays diffuse along magnetic field lines.…”

Section: Discussioncontrasting

confidence: 56%

Exploring the making of a galactic wind in the starbursting dwarf irregular galaxy IC 10 with LOFAR

Heesen

Rafferty

Horneffer

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Low-mass galaxies are subject to strong galactic outflows, in which cosmic rays may play an important role; they can be best traced with low-frequency radio continuum observations, which are less affected by spectral ageing. We present a study of the nearby star burst dwarf irregular galaxy IC 10 using observations at 140 MHz with the LOw-Frequency ARray (LOFAR), at 1580 MHz with the Very Large Array (VLA) and at 6200 MHz with the VLA and the 100-m Effelsberg telescope. We find that IC 10 has a low-frequency radio halo, which manifests itself as a second component (thick disc) in the minor axis profiles of the non-thermal radio continuum emission at 140 and 1580 MHz. These profiles are then fitted with 1D cosmic-ray transport models for pure diffusion and advection. We find that a diffusion model fits best, with a diffusion coefficient of D = (0.4-0.8) × 10 26 (E/GeV) 0.5 cm 2 s −1 , which is at least an order of magnitude smaller than estimates both from anisotropic diffusion and the diffusion length. In contrast, advection models, which cannot be ruled out due to the mild inclination, while providing poorer fits, result in advection speeds close to the escape velocity of ≈50 km s −1 , as expected for a cosmic-ray driven wind. Our favoured model with an accelerating wind provides a self-consistent solution, where the magnetic field is in energy equipartition with both the warm neutral and warm ionized medium with an important contribution from cosmic rays. Consequently, cosmic rays can play a vital role for the launching of galactic winds in the disc-halo interface.

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Section: Discussioncontrasting

confidence: 56%

Exploring the making of a galactic wind in the starbursting dwarf irregular galaxy IC 10 with LOFAR

Heesen

Rafferty

Horneffer

et al. 2018

Monthly Notices of the Royal Astronomical Society

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“…The CRE diffusion length l diff ∝ (Dt CRE ) 1/2 , where D is the diffusion coefficient. Mulcahy et al (2016) found no evidence for an energy dependence of D in M 51 in a similar CRE energy range of our observations, hence we obtain for a synchrotron energy loss-dominated halo…”

Section: Frequency Dependence Of Radio Scale Heightssupporting

confidence: 65%

Chang-Es

Krause

Irwin

Wiegert

et al. 2018

A&A

169

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Aim. The vertical halo scale height is a crucial parameter to understand the transport of cosmic-ray electrons (CRE) and their energy loss mechanisms in spiral galaxies. Until now, the radio scale height could only be determined for a few edge-on galaxies because of missing sensitivity at high resolution.Methods. We developed a sophisticated method for the scale height determination of edge-on galaxies. With this we determined the scale heights and radial scale lengths for a sample of 13 galaxies from the CHANG-ES radio continuum survey in two frequency bands.Results. The sample average values for the radio scale heights of the halo are 1.1 ± 0.3 kpc in C-band and 1.4 ± 0.7 kpc in L-band. From the frequency dependence analysis of the halo scale heights we found that the wind velocities (estimated using the adiabatic loss time) are above the escape velocity. We found that the halo scale heights increase linearly with the radio diameters. In order to exclude the diameter dependence, we defined a normalized scale height h˜ which is quite similar for all sample galaxies at both frequency bands and does not depend on the star formation rate or the magnetic field strength. However, h˜ shows a tight anticorrelation with the mass surface density.Conclusions. The sample galaxies with smaller scale lengths are more spherical in the radio emission, while those with larger scale lengths are flatter. The radio scale height depends mainly on the radio diameter of the galaxy. The sample galaxies are consistent with an escape-dominated radio halo with convective cosmic ray propagation, indicating that galactic winds are a widespread phenomenon in spiral galaxies. While a higher star formation rate or star formation surface density does not lead to a higher wind velocity, we found for the first time observational evidence of a gravitational deceleration of CRE outflow, e.g. a lowering of the wind velocity from the galactic disk.

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“…where the diffusion coefficient D depends on the energy as Strong et al (2007) found 0.3 µ 0.6 from modelling CRE propagation in the Milky Way, and Murphy et al (2012) found (for a star-forming region in the Large Magellanic Cloud) a strong energy dependence of D for CRs between ≈3 and 70 GeV, there are recent indications that the energy dependence may not be significant for CRE energies 10 GeV (Recchia et al 2016a;Mulcahy et al 2016). We therefore fitted for D 0 using different choices of µ, as stated below.…”

Section: Parametrisation Of Advection and Diffusion Modelsmentioning

confidence: 98%

Chang-Es

Schmidt

Krause

Heesen

et al. 2019

A&A

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Context. Cosmic-ray electrons (CREs) originating from the star-forming discs of spiral galaxies frequently form extended radio haloes that are best observable in edge-on galaxies, where their properties can be directly investigated as a function of vertical height above the disc. Aims. For the present study, we selected two nearby edge-on galaxies from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES), NGC 891 and 4565, which differ largely in their detectable halo extent and their star-formation rates (SFRs). Our aim is to figure out how such differences are related to the (advective and/or diffusive) CRE transport in the disc and in the halo. Methods. We use wide-band 1.5 and 6 GHz Very Large Array (VLA) observations obtained in the B, C, and D configurations, and combine the 6 GHz images with Effelsberg observations to correct for missing short spacings. After subtraction of the thermal emission, we investigate the spatially resolved synchrotron spectral index distribution in terms of CRE spectral ageing. We further compute total magnetic field strengths assuming equipartition between the cosmic-ray (CR) energy density and the magnetic field, and measure synchrotron scale heights at both frequencies. Based on the fitted vertical profiles of the synchrotron intensity and on the spectral index profile between 1.5 and 6 GHz, we create purely advective and purely diffusive CRE transport models by numerically solving the 1D diffusion–loss equation. In particular, we investigate for the first time the radial dependence of synchrotron and magnetic field scale heights, advection speeds, and diffusion coefficients, whereas previous studies of these two galaxies only determined global values of these quantities. Results. We find that the overall spectral index distribution of NGC 891 is mostly consistent with continuous CRE injection. In NGC 4565, many of the local synchrotron spectra (even in the disc) feature a break between 1.5 and 6 GHz and are thus more in line with discrete-epoch CRE injection (Jaffe–Perola (JP) or Kardashev–Pacholczyk (KP) models). This implies that CRE injection time-scales are lower than the synchrotron cooling time-scales. The synchrotron scale height of NGC 891 increases with radius, indicating that synchrotron losses are significant. NGC 891 is probably dominated by advective CRE transport at a velocity of ≳150 km s−1. In contrast, NGC 4565 is diffusion-dominated up to z = 1 kpc or higher, with a diffusion coefficient of ≥2 × 1028 cm2 s−1.

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Modelling the cosmic ray electron propagation in M 51

Cited by 29 publications

References 69 publications

Exploring the making of a galactic wind in the starbursting dwarf irregular galaxy IC 10 with LOFAR

Exploring the making of a galactic wind in the starbursting dwarf irregular galaxy IC 10 with LOFAR

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