Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253

Abstract

Context. Nearby edge-on galaxies showing a synchrotron halo are nearly ideal objects for studying the transport of cosmic rays (CRs) in galaxies. Among them, the nearby starburst galaxy NGC 253 hosts a galactic wind indicated by various ISM phases in its halo. Aims. The diffusive and convective CR transport from the disk into the halo is investigated using the local CR bulk speed. The connection between the CR transport and the galactic wind is outlined. Methods. We observed NGC 253 with the VLA at λ6.2 cm in a mosaic with 15 pointings. The missing zero-spacing flux density of the VLA mosaic was filled in using observations with the 100-m Effelsberg telescope. We also obtained a new λ3.6 cm map from Effelsberg observations and reproduced VLA maps at λ20 cm and λ90 cm. The high dynamic range needed due to the strong nuclear point-like source was addressed with a special data calibration scheme for both the single-dish and the interferometric observations. Results. We find a thin and a thick radio disk with exponential scaleheights of 0.3 kpc and 1.7 kpc at λ6.2 cm. The equipartition total magnetic field strength between 7 μG and 18 μG in the disk is remarkably high. We use the spectral aging of the cosmic ray electrons (CREs) seen in the vertical profiles of the spectral index to determine a lower limit for the global CR bulk speed as 170 ± 70 km s −1 . The linear correlation between the scaleheights and the CRE lifetimes, as evident from the dumbbell shaped halo, requires a vertical CR transport with a bulk speed of 300 ± 30 km s −1 in the northeastern halo, similar to the escape velocity of 280 km s −1 . This shows the presence of a "disk wind" in NGC 253. In the southwestern halo, the transport is mainly diffusive with a diffusion coefficient of 2.0 ± 0.2 × 10 29 cm 2 s −1 . Conclusions. Measuring the radio synchrotron scaleheight and estimating the CRE lifetime allow us to determine the bulk speed of the CR transport into the halo. The transport is convective and more efficient in the northeastern halo, while it is diffusive in the southwestern halo. The luminous material is transported by the disk wind, which can explain the different amounts of extra-planar H i, Hα, and soft X-ray emission in the two halo parts. Future low-frequency radio observations will provide the data to analyze the vertical velocity profile of galactic winds.

Cosmic-ray-driven dynamo in galactic disks

“…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.…”

Modelling the cosmic ray electron propagation in M 51

“…If CRE diffusivity is much lower A123, page 12 of 14 in the z direction, then a wind could possibly exist. 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…”

“…Not only do relativistic electrons leave via diffusion but in starburst galaxies, large-scale galactic winds can also advect CREs out of the disk into the halo on a shorter timescale than the diffusion observed in M 82 (Adebahr et al 2013) and NGC 253 (Heesen et al 2009). There is no observational evidence for a global outflow or a galactic-scale wind in M 51 (Mao et al 2015).…”

Galactic winds and the symmetry properties of galactic magnetic fields

“…We recall that a straightforward estimate of the wind velocity is ≥150 km −1 (Heesen et al 2009a), but this flow is associated with only a small part of the interstellar medium. In contrast, conventional mean-field galactic dynamos deal with an averaged wind velocity u.…”

“…The key ingredient of the model is the galactic wind which is traced by the cosmic-ray bulk flow, which is as large as u = 300 ± 30 km s −1 (Heesen et al 2009a). Heesen et al argue that the cosmic-ray bulk flow is the sum of the wind velocity of the gas and Alfvén velocity, which may reduce the estimate by up to 150 km s −1 .…”