How did our Milky Way galaxy take this shape?


A question that has long puzzled scientists is how our Milky Way galaxy which has an elegant spiral shape with long arms, took this form.

Universities Space Research Association today announced that new observations of another galaxy are shedding light on how spiral-shaped galaxies like our own get their iconic shape.

According to research from the Stratospheric Observatory for Infrared Astronomy (SOFIA), magnetic fields play a strong role in shaping these galaxies.

“Magnetic fields are invisible, but they may influence the evolution of a galaxy,” said Dr. Enrique Lopez-Rodriguez, a Universities Space Research Association scientist at the SOFIA Science Center at NASA’s Ames Research Center in California’s Silicon Valley.

“We have a pretty good understanding of how gravity affects galactic structures, but we’re just starting to learn the role magnetic fields play.”

Magnetic fields in the spiral galaxy are aligned with the spiral arms across the entire galaxy—more than 24,000 light years across.

The magnetic field alignment with the star formation implies that the gravitational forces that created the galaxy’s spiral shape is also compressing the magnetic field.

The alignment supports the leading theory of how the arms are forced into their spiral shape known as “density wave theory.”

Scientists measured magnetic fields along the spiral arms of the galaxy called NGC 1068, or M77. The fields are shown as streamlines that closely follow the circling arms.

The M77 galaxy is located 47 million light years away in the constellation Cetus.

It has a supermassive active black hole at its center that is twice as massive as the black hole at the heart of our Milky Way galaxy. The swirling arms are filled with dust, gas and areas of intense star formation called starbursts.

SOFIA’s infrared observations reveal what human eyes cannot: magnetic fields that closely follow the newborn-star-filled spiral arms.

This supports the leading theory of how these arms are forced into their iconic shape known as “density wave theory.” It states that dust, gas and stars in the arms are not fixed in place like blades on a fan. Instead, the material moves along the arms as gravity compresses it, like items on a conveyor belt.

The magnetic field alignment stretches across the entire length of the massive, arms—approximately 24,000 light years across.

This implies that the gravitational forces that created the galaxy’s spiral shape are also compressing its magnetic field, supporting the density wave theory. The results are published in the Astrophysical Journal.

“This is the first time we’ve seen magnetic fields aligned at such large scales with current star birth in the spiral arms,” said Lopez-Rodriquez. “It’s always exciting to have observational evidence like this from SOFIA that supports theories.”

Celestial magnetic fields are notoriously difficult to observe. SOFIA’s newest instrument, the High-resolution Airborne Wideband Camera-Plus, or HAWC+, uses far-infrared light to observe celestial dust grains, which align perpendicular to magnetic field lines.

From these results, astronomers can infer the shape and direction of the otherwise invisible magnetic field.

Far-infrared light provides key information about magnetic fields because the signal is not contaminated by emission from other mechanisms, such as scattered visible light and radiation from high-energy particles. SOFIA’s ability to study the galaxy with far infrared light , specifically at the 89 micron wavelength, revealed previously unknown facets of its magnetic fields.

Further observations such as these from SOFIA are necessary to understand how magnetic fields influence the formation and evolution of other types of galaxies, such as those with irregular shapes.

During its 10-year design lifetime this new astronomical instrument, called MOONS, is expected to observe in the order of ten million objects.

By viewing objects up to 40,000 light-years from the Earth, astronomers will be able to see in unprecedented detail the innermost regions of our galaxy, the Milky Way. Not only that, MOONS will allow astronomers to see across even more vast distances so that they will be able to study the formation and evolution of galaxies over the entire history of the Universe.

MOONS is a unique astronomical instrument which is being designed, built and assembled at the UK Astronomy Technology Centre (UK ATC) in Edinburgh, in collaboration with an international consortium of institutions and commercial partners.

The next generation Multi-Object Optical and Near-infrared Spectrograph, MOONS, will be operational in 2021 on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) at the Paranal Observatory in Northern Chile.

Dr Oscar Gonzalez, Instrument Scientist at UK ATC, says “To explore galaxy formation and evolution we need to investigate the properties of millions of stars from the very centre of our own galaxy to as far away as the other millions of galaxies in the early Universe. For example, to understand how our galaxy reached its current form, we need to map in detail its innermost regions.

This is tremendously challenging because of the large amounts of dust between us, and the stellar populations we need to target.

“MOONS is able to observe in the Infrared, and so we will finally be able to ‘see through’ the dust.”

In a marriage of precision and scale, two significant technical milestones in the design and build of MOONS have now been achieved.

These technical milestones, one in the development of robotic arms to assist in the alignment of the telescope with celestial objects in the sky, and one in optics, are important because the cutting edge capability of MOONS is in turn demanding cutting edge design to push the boundaries of technical innovation, and blaze a trail for future spectrographs.

Precision-engineered robotic arms

“One thousand small robotic arms are at the heart of the MOONS instrument.” says Dr William Taylor, Instrument Scientist at UK ATC. “Called Fibre Positioning Units (FPUs), these robotic arms move quickly and with an accuracy of about the width of human hair (25 microns), to allow the telescope to align with about 1000 celestial objects, at the same time! It is now all systems go on delivering a production run of 1000 of these robotic arms”

“It’s been a complex design challenge,” continues William. “The FPUs sit on a large focal plate measuring over 1m in diameter and are monitored as they move around each other by 12 cameras. 

Interspersed throughout the FPUs are a further 20 cameras which are responsible for the fine alignment of the instrument with objects in the sky. With the design now tested and validated, 1000 FPUs are currently being manufactured and will soon be delivered to us here in Edinburgh for assembly.”

Very large optics

At the magnitude of celestial observing required of MOONS, the new instrument needs very fast, and very large optics to capture the light from as many astronomical targets as possible in a single shot.

The optical design of the MOONS cameras, which will sit within the incredibly low temperatures of a 7-tonne cryostat, is a ground-breaking never-been-done-before technical innovation.

“It’s the green light for the alignment of the cameras after the successful mounting of the incredibly large and tricky-to-handle 40cm lenses into the camera housings.” says Dr David Lee, Optical Engineer at UK ATC.

“The design and construction of the cameras is a multi-disciplinary, multi-organisation achievement involving colleagues in Italy, France and England,” continues David.

“What makes the camera so novel,” adds David, “is that it has been specifically designed to have fewer optical components, with the aim of making the alignment easier.

Essentially, two lenses have been glued together – a smaller lens inside a larger lens, which is an alarming idea, because of the constraints this puts on the glass during the cooling process. But the beauty in the idea is that there are only two optical elements to align – so whilst one is held still, the other can tilt to focus.”

More information: “SOFIA/HAWC+ Traces the Magnetic Fields in NGC 1068,” E. Lopez-Rodriguez et al., 2019, to appear in the Astrophysical


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