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Magnetic Fields Reduce Galactic Star Formation Rates

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Posted December 1, 2014

It may seem very obvious at a first glance: the more gas there is in the interstellar medium, the more stars form there and the larger their formation rate is. But there are at least several factors potentially impacting the star formation rate, and magnetic field is one of them.

A molecular cloud of Cepheus B is a region containing cool interstellar gas and dust left over from the formation of the galaxy and mostly contains molecular hydrogen. Image credit: X-ray: NASA/CXC/PSU/K. Getman et al.; IRL NASA/JPL-Caltech/CfA/J. Wang et al.

A molecular cloud of Cepheus B is a region containing cool interstellar gas and dust left over from the formation of the galaxy and mostly contains molecular hydrogen. Image credit: X-ray: NASA/CXC/PSU/K. Getman et al.; IRL NASA/JPL-Caltech/CfA/J. Wang et al.

Certainly, scientists have determined different-scale correlations between star formation activity, available gas content, and galactic dynamical properties. It appears, that in regions of normal galaxies rich with molecular gas overall star formation rates are relatively slow and inefficient: only relatively small percentage of total gas gets converted to stars every local galactic orbital time. The rate at which the newborn stars appear is very similar even within the giant molecular space clouds, where no lack of star-building material could be expected. Several physical factors or their combinations (including gas turbulences, magnetic fields and star formation feedback) are considered to be the cause of this phenomenon.

Last week, a team of scientists from the UK and the USA published a paper on arXiv.org where they describe their study of how the presence of strong magnetic fields could suppress galactic star formation rates (SFRs).

The authors of this research adapted the model of purely hydrodynamic galactic evolution with no feedback from star formation, proposed by Van Loo et al. (2013). “Observations show that clouds such as, e.g., Taurus and ρOph, are threaded by strong magnetic fields, <…> and that the magnetic field is connected on a larger scale to the Galactic field”, the scientists write, arguing that the relationship between magnetic field amplitude and interstellar gas dynamics should exist.

To test this hypothesis, the scientists performed a numerical simulation, where they compared the results of star formation simulations using different scenarios: one that included a uniform magnetic field in the domain of analysis , and another without magnetic field. Also, the influence of different magnetic field strength values was investigated in detail.

Mass surface densities of two different regions of simulation - Region 1 (top) and 2 (bottom) - for 0μG (left), 10μG (middle) and 80μG (right). Black dots show star cluster particles, each representing 100 M⊙ of stars, while lines indicate direction of mass-weighted magnetic field. Image courtesy of the researchers.

Mass surface densities of two different regions of simulation – Region 1 (top) and 2 (bottom) – for 0μG (left), 10μG (middle) and 80μG (right). Black dots show star cluster particles, each representing 100 M⊙ of stars, while lines indicate direction of mass-weighted magnetic field. Image courtesy of the researchers.

The results indicated, that the pure hydrodynamical model (i.e. without magnetic field) produced a very fragmented gas structure, which became smoother with the increase of magnetic field strength. As the authors explain, the fragmentation was quantified as the number of separate clouds that had formed in the end of simulated galactic evolution. For comparison, 287 separate gas clouds were formed in the absence of magnetic field, while only 28 clouds were produced under the impact of 80μG magnetic field.

When investigating resulting star formation rates, the presence of magnetic field also yielded lower rate of emerging stars. In some cases 80μG produced star formation rates reduced approximately by a factor of 3, the authors note.

The results of simulations overlapped with observational results from real galactic giant molecular clouds, established by previous studies, the team wrote. They also point to the fact that stronger magnetic fields almost always lead to lower star formation rates. “Other effects of local star formation feedback, such as stellar winds, ionization and supernovae, have not yet been included in these simulations, but are expected to act to further reduce SFRs”, the authors conclude.

Written by Alius Noreika

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