NGC 4921

NGC 4921

The Coma Galaxy Cluster in the northern constellation of Coma Berenices, the hair of Queen Berenice, is one of the closest very rich collections of galaxies in the nearby Universe. NGC 4921, also known as Abell 1656 is one of the rare spirals in Coma, about 320 million light-years from Earth, and contains more than 1000 members, discovered back in the late 18th century by William Herschel.

The galaxy has a nucleus with a bar structure that is surrounded by a distinct delicate swirl of dust, accompanied by some bright young blue stars. Much of the pale spiral structure in the outer parts of the galaxy is unusually smooth and gives the whole galaxy the ghostly look of a vast translucent jellyfish. In 1976, Canadian astronomer Sidney Van den Bergh categorized this galaxy as ‘anemic’ because of the low rate at which stars are being formed. He noted that it has “an unusually low surface brightness and exhibits remarkably diffuse spiral arms”. Nonetheless, it is the brightest spiral galaxy in the coma cluster. This galaxy is located near the center of the cluster and has a high relative velocity of 7,560 km/s compared to the mean cluster velocity.

Jeffrey Kenney of Yale University and colleagues recently captured this wind’s erosion in Hubble Space Telescope and Very Large Array (VLA) images, as reported in the August Astronomical Journal. The VLA’s observations reveal the large reservoir of neutral hydrogen gas in which NGC 4921 — like other galaxies — sits. But the hydrogen disk isn’t circular, as it would be if the face-on galaxy lived alone. Instead, it’s compressed on the northwest side, crushed inward by the intracluster wind. Hubble close-ups of the galaxy’s northwestern side confirm that the wind that compresses the neutral hydrogen is also eroding all but the densest dust clouds in the spiral disk; much like the erosion of the Eagle Nebula’s Pillars of Creation, which are 5 light-years long. The wind-eroded pillars of NGC 4921, on the other hand, are 1,000 times larger. The green contours show the neutral hydrogen reservoir superimposed on Hubble’s image of the galaxy in visible light. The contours aren’t circular in this face-on galaxy – they’re crushed inward in the northwest corner.

Another force is likely at play in NGC 4921, too. The nearly linear dust pillars on the northwestern side are connected to a dusty front that runs about 65,000 light-years long, like an embattled line holding against the wind’s onslaught. The fact that the densest globules are still connected perpendicularly to this filament, rather than breaking off like the globules in the Carina Nebula, suggests that something is helping to hold this gas together. Kenney’s team ran simulations that showed that the structures seen in NGC 4921 could only be reproduced if magnetic fields are affecting gas dynamics. The wind faced by NGC 4921, technically known as “ram pressure,” is a force felt by most cluster galaxies, and it’s instrumental to their evolution. The wind strips away gas and dust, eventually quenching star formation and ushering galaxies from youth into old age. Just how this transformation happens still hasn’t been deciphered, but these high-resolution images will guide more detailed simulations in revealing the underlying process.

Credits: Hubble, NASA, Sky & Telescope, Wikipedia.