The Magnetic Menagerie of NGC 1097

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Title: Extragalactic magnetism with SOFIA (Legacy Program) — II: The bimodal magnetic field in the starburst ring of NGC 1097
Authors: Enrique Lopez-Rodriguez et al.
First Author’s Institution: Kavli Institute for Particle Astrophysics & Cosmology, Stanford University
Status: Accepted to ApJ

Galaxies throughout the cosmos display a delightful diversity of morphologies, from elegant and complex spirals to bizarre irregular galaxies. More and more, it seems that the structure and evolution of galaxies are strongly influenced by magnetic fields. The invisible magnetic fields that permeate interstellar space have long been suspected to play a critical role in the formation of stars, buoying clouds against gravitational collapse, steering gas flows throughout galaxies, as well as feeding supermassive black holes in galactic centers. Despite their importance, these magnetic fields are notoriously difficult to measure and map. State-of-the-art instruments and techniques have opened a window into a golden age of magnetic field measurements from local molecular clouds within our Milky Way to the distant maelstroms of gas swirling around galactic nuclei.

How Do We Measure Magnetic Fields?

These magnetic fields cannot be detected directly, so we have to rely on measuring their effects on gas and dust within the environments we aim to study. One way to reveal the magnetic field in the interstellar medium is dust polarimetry. Individual grains of dust amidst the gas in molecular clouds tend to orient themselves relative to the magnetic field that is present, and these organized dust grains emit light with a certain polarization. One can measure the polarization of that light and then infer the orientation of the glowing dust grains to map out the magnetic field lines influencing the dust (Figure 1). This process requires sensitive measurements, but it’s possible using instruments like those aboard SOFIA (the Stratospheric Observatory For Infrared Astronomy).

Cartoon showing a gas cloud with a vertically oriented magnetic field. Elongated dust grains aligned parallel to the magnetic field (i.e., their long dimension is parallel to the magnetic field lines) are labeled “unlikely dust grain orientation.” Elongated dust grains aligned perpendicular to the magnetic field are labeled “likely dust grain orientation.” A sine curve representing the thermal emission from the dust grain emerges from the cloud, traveling toward a telescope.

Figure 1: A cartoon schematic demonstrating how a magnetic field in a molecular cloud influences the likely orientation of dust grains, which emit light with a polarization indicating their orientation. Measurements of this polarization can then be used to infer the original magnetic field. [H Perry Hatchfield]

The Magnetic Heart of NGC 1097

Today’s paper explores the magnetic-field structure of the center of NGC 1097, a barred spiral galaxy with an active galactic nucleus and a brilliant starburst ring fed by a pair of linear gas structures called dust lanes that are aligned with the bar. The ring of dense gas orbiting about 3,000 light-years from the galaxy’s nucleus is forming stars at more than twice the rate of the entire Milky Way! In addition to its spectacular spiral arms, prominent dust lanes, and glowing core, NGC 1097 is also known to host one of the strongest interstellar magnetic fields in a nuclear starburst ring. This galaxy provides an excellent testbed for studying the interactions of star formation, galactic-scale flows and structures, and powerful magnetic fields.

The authors use a combination of far-infrared (89-μm) polarimetry from SOFIA’s High-resolution Airborne Wideband Camera Plus (HAWC+) instrument and radio (3.5-cm and 6-cm) polarimetry from the Very Large Array (VLA) to trace the orientation of dust grains shifted by the magnetic field. They also explore the velocity structure of the gas using molecular line emission from carbon monoxide. By understanding both the gas motions and the magnetic field properties, they can see how the gas flows and magnetic structure of the galaxy might be related. The magnetic field traced by the far-infrared emission appears to have a different structure from the field revealed by the radio observations; the 89-μm dust emission seems to indicate a compressed field, while the radio observations clearly suggest a spiral structure to the field (Figure 2).

A central blob of carbon monoxide emission surrounded by a fragmented ring with a fainter spiral arm trailing off to either side. Yellow and red polarization vectors are scattered across the center of the image, as well as red and yellow contours.

Figure 2: The magnetic field structure traced by far-infrared observations from SOFIA’s HAWC+ instrument (in yellow) and 3.5-cm radio observations from the VLA (in red). In each case, the lines show the orientation of the magnetic field, and the contours show the intensity of the polarized light. The background color-scale image is the integrated carbon monoxide emission (J=2–1 transition) from ALMA. [Lopez-Rodriguez et al. 2021]

The authors interpret this apparent difference in magnetic-field morphology as the two observations tracing separate modes of the magnetic field associated with different phases of the gas: the far-infrared polarization reveals the magnetic field compressed by a shock wave crashing through the dense gas in the starburst ring, while the radio polarization shows the magnetic field being twisted in a spiral by shearing motions in the more diffuse gas. This suggests that the gas motions, whether diffuse or dense, may be guided by the strong magnetic fields. Unraveling the intricate dance of magnetic fields, kinematics, and gravity is no easy task, but multi-wavelength polarization studies like this provide an exciting window into the diversity of the magnetic structures of galaxies — and when it comes to magnetic fields, it seems like there is always more than meets the eye.

Original astrobite edited by Sasha Warren.

About the author, H Perry Hatchfield:

I’m a PhD candidate in Physics at the University of Connecticut, where I study star formation and gas structure in the Milky Way’s galactic center. I do this using radio observations of molecular clouds as well as hydrodynamic simulations, and I’m all about trying to find ways to compare these two exciting means of exploring the universe.