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Starts With A Bang

JWST reveals dusty secrets inside spiral galaxies

How does star-formation, occurring in small regions within galaxies, affect the entire host galaxy that contains it? JWST holds the answers.
spiral galaxies MIRI PHANGS JWST
These are six of the 19 spiral galaxies imaged by the PHANGS collaboration with JWST in the mid-infrared. JWST's MIRI instrument reveals high-resolution features such as heated dust, polycyclic aromatic hydrocarbons, and future star-forming material.
Credit: NASA, ESA, CSA, STScI and the JWST/PHANGS team
Key Takeaways
  • Matter within galaxies starts off diffuse, contracts to become dense, then forms stars, which in turn feeds back to affect the matter that surrounds it.
  • Using four separate observatories, including ALMA and the Very Large Telescope on Earth, plus Hubble and JWST in space, the PHANGS collaboration aims to untangle this cosmic mystery.
  • By looking at six galaxies through the eyes of both Hubble and JWST, a number of important cosmic secrets have been revealed at last. You’ll never see dust the same way again.

Dust is, simultaneously, both an astronomer’s dream and nightmare.

molecular dust cloud
Dark, dusty molecular clouds, like this image of Barnard 59, part of the Pipe Nebula, found within our Milky Way, will collapse over time and give rise to new stars, with the densest regions within forming the most massive stars. However, even though there are a great many stars behind it, the starlight cannot break through the dust; it gets absorbed until more of the nebula itself becomes ionized. Only with longer-wavelength light, like in the mid-infrared, will this dust appear luminous when heated, rather than dark.
Credit: ESO

Lining galactic spiral arms, this dense material leads directly to new star-formation.

Hubble NGC 1566
This Hubble space telescope image, taken with Hubble WFC3 camera in ultraviolet, optical, and near-infrared light, showcases the bright star-forming arms, lined with dust, of nearby spiral galaxy NGC 1566. The galaxy is 40 million light-years away, with a small but extremely bright galactic nucleus.
Credit: ESA/Hubble & NASA; Acknowledgement: Det58

But it’s also opaque: blocking our view of luminous sources inside.

spiral galaxy NGC 1433 Hubble
This nearby, dust-rich spiral galaxy, NGC 1433, is only 32 million light-years away. The luminous, bright core makes this a Seyfert galaxy, containing a small molecular outflow and a spiral structure in its central molecular gas. Outside of the nucleus, dust dominates Hubble’s view of this galaxy.
Credit: ESA/Hubble & NASA; Acknowledgements: D. Calzetti (UMass) and the LEGUS Team

To truly understand how dust impacts a galaxy’s evolution, multiwavelength views are necessary.

multiwavelength andromeda
The Andromeda galaxy, the closest large galaxy to Earth, displays a tremendous variety of details depending on which wavelength or set of wavelengths of light it’s viewed in. Even the optical view, at top left, is a composite of numerous different filters. Shown together, they reveal an incredible set of phenomena present in this spiral galaxy. Multiwavelength astronomy can shed unexpected views on almost any astronomical object or phenomenon, revealing details in one wavelength that are wholly invisible in another.
Credit: infrared: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent; X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, MPE; optical: R. Gendler

ALMA, at very long wavelengths, reveals individual molecules and ions within galaxies.

astrophysics ALMA
The Atacama Large Millimetre/Submillimetre Array (ALMA) consists of an array of radio telescopes. The array has the light-gathering power of the sum total of the individual dishes’ collecting areas, but has the resolution of the distance separating the dishes. It can be used to identify molecular signatures such as neutral and ionized hydrogen, carbon monoxide, and other common molecules and ions found in space.
Credit: ESO/B. Tafreshi (

Hubble and ground-based observatories can measure stars, starlight, and energized regions directly.

hubble phantom galaxy m74 ngc 628
This view of the Phantom galaxy, also known as Messier 74/NGC 628, combined blue, visible, and near-infrared images from Hubble, along with a particular emission line of hydrogen to create this composite. While this was previously our best view of the Phantom galaxy, revealing many interesting features, JWST’s views of it have already revealed so much more.
Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration; Acknowledgment: R. Chandar (University of Toledo) and J. Miller (University of Michigan)

But only with JWST, and specifically its MIRI instrument, can dust be revealed directly.

Spiral galaxy IC 5332 Hubble JWST
This top-and-bottom view shows the same face-on spiral galaxy, IC 5332, in visible and near-infrared light (top) from Hubble, along with a mid-infrared view (bottom) from JWST. The intricate, filament-like structure of the dust, heated by nearby star-forming activity, shines prominently in the JWST view.
Credit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST and PHANGS-HST Teams

The Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey aims to explain matter’s entire galactic life cycle.

A diagram depicting the baryon life cycle in spiral galaxies.
A spiral galaxy typically consists of four main gaseous regions within the disk: diffuse atomic gas, dense molecular gas, stars and star clusters, and ionized regions of matter arising from energy injections from star-forming regions, young stars, and stellar cataclysms. JWST, along with the other PHANGS data sources, helps reveal different aspects of this life cycle, but once a galaxy’s gas is gone and no new gas reservoirs fall inside, star-formation ends permanently.
Credit: PHANGS collaboration, Design: Daniela Leitner

They sampled 19 nearby spiral galaxies, imaging their insides in unprecedented detail.

galaxy ngc 1566 PHANGS MIRI JWST
This MIRI view, from JWST and the PHANGS collaboration, of spiral galaxy NGC 1566 shows heated, dusty and nuclear features that are entirely invisible to other observatories observing in optical/UV and even radio wavelengths. This dust filament network is ubiquitous in spirals, but the spirals themselves do not follow the pattern one might expect from the golden ratio or the Fibonacci sequence.
Credit: NASA/ESA/CSA/Judy Schmidt

Dust traces the spiral arms, where new stars form, nearly perfectly.

NGC 1433, shown earlier with primarily optical (Hubble) views, shows off its rich dust structures to JWST’s MIRI instrument. The central bright region contains a spiral structure, an energetic outflow, and is shrouded by an external dust spiral that extends in two directions into the outer arms. These features not only trace out the “dark” regions found in optical views, but reveal heating due to nearby young stars as well.
Credit: NASA, ESA, CSA, Janice Lee (NSF’s NOIRLab); Processing: Alyssa Pagan (STScI)

Galaxies with rich central bulges house a hotbed of activity inside.

This top/bottom comparison shows nearby, dusty, barred spiral galaxy NGC 7496 in optical (top) and mid-infrared (bottom) views. The bright spikes in infrared are a hint of nuclear, central activity, potentially revealing an active supermassive black hole. The dark features in the top image, caused by light-blocking dust, are what’s illuminated in the lower image.
Credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team; Acknowledgement: J. Schmidt

Young, newly-formed stars heat the dust, causing it to radiate.

great barred spiral NGC 1365
This optical view of barred spiral galaxy NGC 1365 showcases a variety of features: star formation, a central supermassive black hole, extended spiral arms, a massive central bar, and dust-rich features that block the light from stars behind them. Our Milky Way is a barred spiral, making NGC 1365 of particular relevance to our home galaxy.
Credit: Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA; Processing: T. Rector, Jen Miller, Mahdi Zamani & Davide de Martin

Glowing cavities of dust result from overlapping shells/bubbles, where stars inject energy.

An image of a spiral galaxy in space captured by JWST.
In this MIRI image of galaxy NGC 1365, clumps of dust and gas in the interstellar medium have absorbed the light from forming stars and emitted it back out as infrared light. This illuminates an intricate network of cavernous bubbles and filamentary shells. The center of the galaxy has an active supermassive black hole and many overlapping shells of ionized material, while the central bar shows off its extremely dust-rich nature as well.
Credit: NASA, ESA, CSA, and J. Lee (NOIRLab); Processing: A. Pagan (STScI)

MIRI reveals polycyclic aromatic hydrocarbons: organic, carbon-rich molecules.

spiral galaxy M74 phangs phantom galaxy jwst miri
This mid-infrared view of the heart of the phantom galaxy, Messier 74, showcases dust warmed by nearby star-forming regions. At the center, the gas-free core allows the material there to shine brightly at mid-infrared wavelengths, while “bubbles” blown along the spiral arms showcase cavities carved by energetic phenomena.
Credit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST Team; Acknowledgement: J. Schmidt

The dust filament network shows a huge diversity of features across galaxies.

JWST phantom galaxy M74
This three-panel animation shows three different views of the center of the Phantom Galaxy, M74 (NGC 628). The familiar color image is the Hubble (optical) view, the second panel showcases near-infrared views from both Hubble and Webb, while the mid-infrared panel shows the warm dust that will eventually form new stars at a later time, containing data from JWST alone.
Credit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST Team; ESA/Hubble & NASA, R. Chandar; Acknowledgement: J. Schmidt; Animation: E. Siegel

JWST’s views give us indirect glimpses into the earliest stages of stellar life cycles.

A diagram depicting the stages of star formation and the influence of dust in spiral galaxies.
Star forming regions are driven by gravitational collapse, turbulent flows, and energy arising from radiation, carving small-scale and large-scale features into spiral galaxies. On stellar/star-forming region scales up to galactic and galaxy group/cluster scales, its effects can be felt and seen by cosmic structures.
Credit: PHANGS collaboration, Design: Daniela Leitner

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