The opening episode of star formation in the Milky Way occurs in long, dense filaments of gas and dust. These massive galactic endoskeletons seen in other galaxies, dubbed “bones,” to use an anatomical term, stretch along the spiral arms. These filaments delineate the galaxy’s densest skeletal spiral structures –characterized by being at least fifty times longer than they are wide and having coherent internal motions along their lengths.
Magnetic Fields of the “Bones”
While most of the key physical properties of these bones are known, we know little of their magnetic field properties. The magnetic fields are created when charged ions and electrons flow in preferential directions. These fields can play a critical role either in supporting the gas and dust against gravitational collapse into new stars, or alternatively, in assisting the flow of mass along the bone into cores making new stars.
Magnetic Fields are Notoriously Difficult to Measure
Magnetic fields are notoriously difficult to measure in space. The most common method relies on the emission from non-spherical dust grains that align their short axes with the direction of the field, resulting in infrared radiation that is preferentially polarized perpendicular to the field. Measuring this faint polarization signal, and inferring the field strength and direction, has only recently become easier to do with the HAWC+ instrument on SOFIA, NASA’s Stratospheric Observatory for Infrared Astronomy, and its 2.5-m telescope. SOFIA flies as high as 45,000 feet, above most of the atmospheric water vapor that absorbs far infrared infrared signals from space.
“Of the three main forms of energy in interstellar clouds, magnetic fields are the hardest to measure,” Philp Myers, Senior Astrophysicist at the Smithsonian Astrophysical Observatory wrote in an email to The Daily Galaxy. “In an interstellar cloud,” he explains, “we estimate the gravitational mass by measuring the intensity and extent of its radiation from dust grains and molecules. We infer the kinetic energy of its gas from the Doppler shift of its molecular spectral lines. Magnetic fields are believed to be important in channeling gas flows in clouds and in limiting their star formation, but estimating their strength has been elusive.”
“Until recently we have been limited to measuring extremely small frequency shifts of a few weak spectral lines, through the Zeeman effect at centimeter and millimeter wavelengths,” Myers added in his email. “Now there is increasing reliance on spatial variations in the polarization of radiation from dust grains. These are most useful to observe at far infrared and submillimeter wavelengths. The increasing use of the HAWC+ camera on the SOFIA airborne observatory, as in our paper on G47, promises to make significant advances in our understanding of interstellar magnetic fields.”
190 Light-years Long, Five Light-years Across
CfA astronomers Ian Stephens, Phil Myers, Catherine Zucker, and Howard Smith led a team that used HAWC+ polarization to map the detailed magnetic field along the bone G47.06+0.26. This filament is about 190 light-years long, five light-years across, and has a mass of 28,000 solar-masses with a typical dust temperature of 18 kelvin. The IRAC camera on Spitzer had previously mapped the bone to identify the regions of young star formation along its length.
The astronomers determined where along the bone the magnetic field is capable of supporting the gas against collapse into stars, and those regions where it is too weak. They also mapped low density regions where the field is more complex in shape.
So far, astronomers have discovered 18 bones on our side of the Milky Way, most of them identified by the infrared Spitzer Space Telescope. G47.06+0.26 is just the first object studied in a larger program to map the magnetic fields in ten of the eighteen known Milky Way bones. Once an analysis of this larger statistical sample has been completed, the scientists expect to be able to quantify more precisely how the strength and orientation of fields influence the evolution of the bones and their pockets of star formation.
Source: Ian W. Stephens et al, The Magnetic Field in the Milky Way Filamentary Bone G47. arXiv:2201.11933v1 [astro-ph.GA], arxiv.org/abs/2201.11933