Ringed by resonance — the barred spiral galaxy NGC 7140 in Indus
NGC 7140 is a barred spiral galaxy in the constellation Indus, about 130 million light-years from Earth.
It is classified as (R′₂)SB(rs)b, showing a central bar, loosely wound spiral arms, and a faint inner ring-like structure where resonances gather gas into arcs and knots of star formation. With an apparent size of 4.1′ × 3.0′, the galaxy spans roughly 160,000 light-years across, making it larger than the Milky Way.
Dust lanes cut across the bar, channeling gas inward, while scattered blue knots highlight regions of active star formation in the arms. The ring encircling the bar is not a closed loop, but the result of spiral arms curving back and piling up material at resonance zones. In these regions, the bar’s rotation and the orbital motion of stars and gas fall into step, causing matter to crowd together. Gas clouds collide and compress, sparking bursts of new star formation that trace the arcs of the ring.
Beyond the main structure, an extended faint halo surrounds the disk, hinting at older interactions or accreted material. The spiral arms show slight asymmetries and patchiness, giving further evidence that NGC 7140 is not only shaped by internal bar dynamics but also by its wider environment.
Together these features make NGC 7140 an elegant yet evolving barred spiral, where resonances, dust, and past encounters combine to sculpt its luminous form in the southern sky.
Imaged in LRGB on my Planewave CDK 1000 at Observatorio El Sauce, Chile.
Image acquisition and processing: Mike Selby