Galactic conveyor belts feed star formation
How magnetic fields push the formation of stars
September 12, 2019
Stars form from compressed clouds of the interstellar medium (ISM). The ISM consists of gas (mostly hydrogen) and tiny particles of carbon and silicates, which the astrophysicist calls dust. If the ISM reaches a sufficiently high density, the self-gravity leads to a collapse of the initially cold matter down to hot stars. How such clouds form and condense, however, is not yet fully understood. Magnetic fields are a major component of ISM  in the Milky Way and other galaxies. They contribute significantly to the total pressure, which balances the ISM against gravity. Still, their exact role in the process of star formation is the subject of lively discussions.
In order to approach this puzzle, Juan Diego Soler of the Max Planck Institute for Astronomy (MPIA) in Heidelberg investigated the orientation of magnetic fields with respect to the density distribution towards the most nearby regions of star formation at distances of up to 450 parsecs (1450 lightyears) from the Sun. "The idea is that if the magnetic field has a strong influence on the ISM, it should shape its density structures," Soler explained.
In fact, in all cases he found a parallel alignment of the magnetic fields to the diffuse, i.e. less dense, component of the ISM . However, at higher densities of the ISM there was a gradual shift in alignment towards larger angles. In the densest zones, the magnetic field was even perpendicular to the structures of the ISM. This finding is shown in Figure 1.
The magnetic field guides the ISM
These results confirm a scenario illustrated in Figure 2. The partially ionised, diffuse ISM is coupled to the magnetic field via electromagnetism and can only move along the field lines (a) . Collisions with the electrically neutral components, such as the dust, carry them along. Therefore, the less dense zones appear to be aligned with the magnetic field. The turbulence in the clouds helps them to expand along the field lines into filaments.
When triggered by external influences, such as expanding bubbles from supernovae explosions or the passage of the matter through a spiral arm, different clouds move towards each other as if on conveyor belts. When they converge, they continuously form an accumulation of ISM, which then has a preferred direction rather perpendicular to the magnetic field lines (b). The conveyor belt transports additional ISM and increases the density until it becomes so high that the cloud (or parts of it) collapses under its own gravity (c). During this phase, the magnetic field is not strong enough to prevent collapse. The field retains its orientation with respect to the density profile during the collapse, which in turn distorts the magnetic field.
ESA space telescopes make the difference
Soler has been investigating the relationship between magnetic fields and the structure of star formation regions for several years. This time he used data from the Planck all-sky observations and the “Herschel Gould Belt Survey” (HGBS) project for his analysis. Both Herschel and Planck went into service in mid-2009. They measured the radiation of the cold ISM at different wavelengths.
The Herschel data are particularly suitable for determining the density distribution of the ISM from the radiation emitted with high spatial resolution. From the Planck data, Soler measured the polarisation of the radiation, which provides information about the magnetic field. The elongated dust particles of the ISM align themselves with the magnetic field and therefore function similarly to antennas. The planes of oscillation of the electric and magnetic fields of the emitted radiation thus have preferred directions, i.e. it is polarised. Astronomers have known for decades that the ISM emits partially polarised radiation. However, they have not yet been able to quantify the large-scale orientation to the structures in the ISM.
Image recognition techniques help investigating ISM structures
Soler adapted a technique that is used in a modified form for image recognition – for example, in internet image searches or when creating panoramic images. It is based on the mathematical treatment of gradients, i.e. the strength and direction of changes, e.g. in the brightness of the images. Figure 3 shows how patterns in two images are identified by equal brightness gradients. The gradients used in the Planck and Herschel data relate to the magnetic field and density distribution of the ISM. Thus, Soler was able to calculate with statistical methods under which conditions both components are rather parallel or perpendicular to each other.
“The Planck satellite polarisation observations have revealed unprecedented details on the interstellar magnetic fields. They are the cornerstone for our future understanding of the magnetised ISM, which will be improved with forthcoming satellite and balloon-borne missions," Soler summarised.
 Charged particles moving in a magnetic field experience a sideways force. It is proportional to the strength of the magnetic field, the component of the velocity perpendicular to the magnetic field, and the charge of the particle. This force is known as the Lorentz force.
The data were recorded with the Planck satellite and the Herschel space telescope. Both missions were mainly developed and operated by the European Space Agency (ESA), with NASA also making important contributions. Planck was mainly built for cosmic background radiation research and covered a wavelength range between 300 µm and 11.1 mm. Herschel was a versatile observatory covering the electromagnetic spectrum between 55 µm and 672 µm.
The development of Planck has been supported by: ESA; CNES and CNRS/INSU-IN2P3-INP (France); ASI, CNR, and INAF (Italy); NASA and DoE (USA); STFC and UKSA (UK); CSIC, MICINN, JA, and RES (Spain); Tekes, AoF, and CSC (Finland); DLR and MPG (Germany); CSA (Canada); DTU Space (Denmark); SER/SSO (Switzerland); RCN (Norway); SFI (Ireland); FCT/MCTES (Portugal); and PRACE (EU).
This research has made use of data from the “Herschel Gould Belt Survey” (HGBS) project (http://gouldbelt-herschel.cea.fr) led by Dr. Philippe André at the CEA/Saclay (France). The HGBS is a Herschel Key Programme jointly carried out by SPIRE Specialist Astronomy Group 3 (SAG 3), scientists of several institutes in the PACS Consortium (CEA Saclay, INAF-IFSI Rome and INAF-Arcetri, KU Leuven, MPIA Heidelberg), and scientists of the Herschel Science Center (HSC).
All the data from the Planck and the Herschel missions are available through public archives for the use of the astronomical community.