Image of a spiral galaxy. You are probably imagining something like Messier 34: long, spiral arms that move away from a brilliantly bright center, dotted with sparkling star clusters and dotted with dark streaks of dust.
But from our point of view, galaxies can be oriented in any way, including NGC 5907, also known as Knife Edge Galaxy, the magnificent subject of a Hubble Space Telescope image recently released and processed by citizen scientist Judy Schmidt.
As you can see in the image above, the spiral arms cannot be seen from this angle. Even most star clusters are obscured by thick galactic dust. However, we can infer that the galaxy is a spiral because we can still see a bright glow at its center.
This is the galactic bulk: a group of dense, compact spheroidal stars found at the center of most spiral galaxies. Even the Milky Way has one. We can’t see what it looks like from the outside because of course we can’t get out of our galaxy, but we can see what it looks like from the inside and put the rest together based on our observations of other spiral galaxies.
NGC 5907, located about 50 million light-years away in the northern Draco constellation, is actually a rare bug in some other ways. It has very low metallicity; Analyzes suggest that it is primarily made up of hydrogen and helium, and not much else. Furthermore, very few giant stars have been detected in it.
Since the heaviest elements are produced over stellar generations, forged in supernovae whose dust is absorbed by subsequent generations of stars, this suggests that NGC 5907 is populated primarily by very old, long-lived dwarf stars.
(Van Dokkum et al., ApJL, 2019)
The other cool thing about NGC 5907 can only be seen when you zoom out. There is a single giant stream of stars spinning around it, more than 700,000 light-years long, according to a recent study, and confirmed by independent observations.
Before these studies, the stellar current of NGC 5907 was thought to have a complex shape of corkscrew with two loops.
There were two competing explanations for this. The first was that the galaxy subsumed a smaller dwarf galaxy orbiting in it, just as the Milky Way is believed to be doing with the Sagittarius Dwarf Spheroidal Galaxy. The second was a major merger with a galaxy the same size as NGC 5907, about 8 to 9 billion years ago.
The latest observations suggest a third scenario: a close encounter with a “tidal surge satellite” that drew a stream of stars around NGC 5907.
However, a more complete analysis will be necessary to identify the best match for the recently observed configuration.
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