Messier 77 (MIRI + NIRCam)
This latest Picture of the Month from the NASA/ESA/CSA James Webb Space Telescope features Messier 77 (M77), a barred spiral galaxy famous among astronomers for its combination of relative proximity and spectacular features to study. It is located 45 million light-years away in the constellation Cetus (The Whale). This new image from Webb highlights its swirling spiral arms, the dust in its disc and its piercingly bright core like never before.
At the heart of M77 is a compact region filled with hot gas that handily outshines the rest of the galaxy put together, even overcoming the light-gathering capacity of Webb’s cameras. This is an active galactic nucleus (AGN), and it’s powered by M77’s central supermassive black hole, which is eight million times as massive as our Sun. Gas in the galaxy’s central regions is pulled by the strong gravity into a tight and rapid orbit around the black hole, where it crashes together and heats up, releasing tremendous amounts of radiation.
The bright orange lines appearing to radiate out from the centre of M77 are not actually a feature of the galaxy: they are a type of distortion that arises from the optical design of the telescope. Called diffraction spikes, they are created because the intense light from the unresolved AGN is bent ('diffracted') very slightly at the edges of Webb’s hexagonal mirror panels and around one of the struts that hold up its secondary mirror. This distinctive six-plus-two-pointed pattern is the same for any image taken by Webb. For diffraction spikes to appear, the light source has to be very bright and very concentrated, so they’re most often seen on stars. But in some galaxies, as here, the nucleus is bright and compact enough to make diffraction spikes appear as well.
M77 is not just known for its easily visible AGN, but also as a prolific star-forming galaxy. Data in this image from Webb’s Near-Infrared Camera (NIRCam) reveals a bar spanning across the central region, which doesn’t appear in visible-light images of the galaxy. The bar is enclosed by a bright ring, called a starburst ring, formed by the inner ends of M77’s two spiral arms. Starburst regions in galaxies are typified by extremely high star-formation rates. This ring is more than 6000 light-years across and displays intense and widespread starbursts, visible in this image by the densely concentrated orange bubbles all around the ring. Since M77 is relatively close to Earth, this starburst ring is a very well-studied example of the phenomenon.
Beyond the ring and bar, M77’s spiral arms spin lazily out into the disc of the galaxy and beyond. The arms are the location of much of this new star birth, with dense clumps of gas collapsing to form tightly-packed clusters of stars. NIRCam pinpoints the light from these stars along the spiral arms, as well as capturing the glow that suffuses the galaxy from the billions of stars in its disc. Particularly along the southern spiral arm, NIRCam also traces infrared emission at slightly longer wavelengths from complex molecules including polycyclic aromatic hydrocarbons (PAHs).
As an active spiral galaxy, M77’s disc is filled with gas and dust which is both a product of and fuel for future star formation. NIRCam picks out the glitter of countless stars spread across the disc, and Webb’s Mid-Infrared Instrument (MIRI) fills out the view with the glow of interstellar dust grains emitted at longer wavelengths, shown here in dark red. The dust forms a huge vortex of smoky, swirling filaments with cavities in between.
The data used to create this image are from an observing programme (#3707) that surveyed massive, nearby, star-forming galaxies to create a rich dataset useful for many scientific investigations. As can be seen here, the stunning resolution of Webb’s instruments reveals star clusters and rich reservoirs of gas, which can be used to explore the cycle of star formation, life and death in these and other galaxies.
[Image Description: A spiral galaxy shown in infrared light. Six long and two smaller rays of light emit from the centre, which are diffraction spikes created by the telescope’s optics. A glowing bar spans across the centre. A glittering ring of stars and dust surrounds the bar; at each side, the ring splits off into a spiral arm that winds outwards. Faint, dark red dust clouds swirl throughout the rest of the disc, backed by a pale glow from all the galaxy’s stars.]
