The Hubble Space Telescope has taken a new gorgeous picture of NGC 4861, a galaxy located 30 million light-years away that defies classification. It appears to be a small messy galaxy, almost comet-like in appearance, but a study published a few years ago in Astronomy & Astrophysics has shown that things are not as simple as they look.
When researchers estimated the bulk properties of NGC 4861, they discovered that they are consistent with a spiral galaxy. And yet, the Hubble image shows a bright, dense region full of new young stars, with a dimmer stellar trail stretching away from it.
The curious appearance was actually the reason the galaxy was studied in the first place. Irregular dwarf galaxies tend to have a lower gravitational potential, which makes it easier for material to escape their clutches. The material is propelled by galactic winds, a stream of charged particles produced by stars, and both young and exploding stars can generate particularly strong winds.
Between the colorful "comet head" region and its diminutive look, NGC 4861 was on paper the ideal candidate for the study of stellar winds in dwarf galaxies. The researchers studied outflows for this object and found something very interesting.
The outflows moved at about 30 kilometers per second (18.6 miles per second), and while that’s fast, it’s quite low compared to the escape velocity of the galaxy. This discovery implies that although NGC 4861 has all the right characteristics (strong star formation, small size, etc.), it doesn’t show any sign of galactic-wide winds created by stars.
This defiant galaxy might be mysterious for now, but the more we study these unusual objects, the better we get at discovering new patterns.
Galaxies are usually classified as either spiral, elliptical, or irregular – three simple words that have other meanings attached to them already. Elliptical galaxies tend to be older and larger, spirals are young and dusty, and irregulars are messy. Placing a galaxy in a box helps with our understanding of the universe, but we can allow some stubborn objects to ignore the way we see the world.