Magnetic fields in interacting galaxies

Galaxy mergers are an interesting tool to study the amplification and restructuring of small-scale magnetic fields within the scope of the global evolution of the cosmic magnetic field. Magnetic fields play an important role during galaxy mergers, as they lead to amplifications of the initial magnetic fields and are therefore efficient drivers for the cosmic magnetic field evolution.

We used the N-body/smoothed particle hydrodynamics (SPH) code GADGET [d1,d2] with implementations for magnetohydrodynamical aspects [d3] to perform 32 simulations of binary mergers of disc galaxies with mass ratios of 2:1 up to 100:1, whereby we have additionally varied the initial magnetic field strengths, disc orientations and resolutions [d4]. We investigate the amplification of a given initial magnetic field within the galaxies and an ambient intergalactic medium (IGM) during the interaction. We find that the magnetic field strengths of merger remnants with mass ratios up to 10:1 saturate at a common value of several muG. For higher mass ratios, the field strength saturates at lower values. The saturation values correspond to the equipartition value of magnetic and turbulent energy density. The initial magnetization, disc orientation and numerical resolution show only minor effects on the saturation value of the magnetic field. We demonstrate that a higher impact energy of the progenitor galaxies leads to a more efficient magnetic field amplification. The magnetic and turbulent energy densities are higher for larger companion galaxies, consistent with the higher impact energy supplied to the system. We present a detailed study of the evolution of the temperature and the bolometric X-ray luminosity within the merging systems. Thereby we find that magnetic fields cause a more efficient increase of the IGM temperature and the corresponding IGM X-ray luminosity after the first encounter. However, the presence of magnetic fields does not enhance the total X-ray luminosity. Generally, the final value of the X-ray luminosity is even clearly lower for higher initial magnetic fields.

Collision of two galaxies

Movie of density and magnetic field evolution of a minor merger simulation with mass ratio 3:1. Images processed from raw data with the SPLASH software by Daniel Price, Monash University, Australia (http://users.monash.edu.au/~dprice/splash/index.html).

Stephan's Quintet

In a further project, we performed simulations of the compact galaxy group Stephan's Quintet (SQ) [d5] including magnetic fields, also using the N-body/SPH code GADGET [d1,d2]. Thereby, we adapted two different initial models for SQ based on Renaud et al. [d6] and Hwang et al. [d7], both including four galaxies (NGC 7319, NGC 7320c, NGC 7318a and NGC 7318b). We investigate the morphology, regions of star formation, temperature, X-ray emission, magnetic field structure and radio emission within the two different SQ models.

Magnetic field models for the Milky Way

Magnetic fields are found in almost all structures found in the Universe. As already shown by the research projects “Magnetic fields in star formation” and “Magnetic fields in galaxies” they play an important role.

In our Galaxy magnetic fields permeate the interstellar medium (ISM) and can be divided into a large-scale Galactic magnetic field which remains poorly understood and a small-scale random part; both with strengths in the range of micro-Gauss.

We will use the numerical Hammurabi code by [d8] to calculate full-sky maps of Intensity (I), Polarized Intesity (PI) and Rotation Measure (RM) of synchrotron radiation. Therefore models of the spatial distribution of the thermal electrons ne, the cosmic ray electrons nCRE and the magnetic field B are needed. To constrain model parameters by the observational full-sky data, it is planed to use Monte Carlo (MC) techniques.

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b) Old publications

c) Completed work

  1. A. Geng, dissertation: Numerical Simulations of Magnetic Fields in Interacting Galaxies (2012)
  2. U. Steinwandel, master thesis: Magnetic driven outflow in disk galaxies (2017)

d) References

  1. V. Springel, N. Yoshida, S.D. White, New Astron., 6, 79 (2001).
  2. V. Springel, MNRAS 364, 1105 (2005).
  3. K. Dolag, F. Stasyszyn, MNRAS 398, 1678 (2009).
  4. A. Geng et al., MNRAS 419, 3571 (2012).
  5. A. Geng et al., MNRAS 426, 3160 (2012).
  6. F. Renaud et al., ApJ 724, 80 (2010).
  7. J.-S. Hwang et al., MNRAS 419, 1780 (2012).
  8. A. Waelkens et al., A&A, 495, 697 (2009).

e) External links (programs, potentials ...)

f) Books