Spiral Galaxy MCG-01-24-014 in Hydra | Hubble

Spiral Galaxy MCG-01-24-014 in Hydra | Hubble

This whirling image features a bright spiral galaxy known as MCG-01-24-014. It is located about 275 million light-years from Earth. In addition to being a well-defined spiral galaxy, MCG-01-24-014 has an extremely energetic core, known as an active galactic nucleus (AGN), so it is referred to as an active galaxy. Even more specifically, it is categorized as a Type-2 Seyfert galaxy. Seyfert galaxies host one of the most common subclasses of AGN, alongside quasars. While the precise categorization of AGNs is nuanced, Seyfert galaxies tend to be relatively nearby ones where the host galaxy remains plainly detectable alongside its central AGN, while quasars are invariably very distant AGNs whose incredible luminosities outshine their host galaxies.

Image Description: A spiral galaxy. It appears to be almost circular and seen face-on, with two prominent spiral arms winding out from a glowing core. It is centered in the frame as if a portrait. Most of the background is black, with only tiny, distant galaxies, but there are two large bright stars in the foreground, one blue and one red, directly above the galaxy.

There are further subclasses of both Seyfert galaxies and quasars. In the case of Seyfert galaxies, the predominant subcategories are Type-1 and Type-2. These are differentiated from one another by their spectra—the pattern that results when light is split into its constituent wavelengths—where the spectral lines that Type-2 Seyfert galaxies emit are particularly associated with specific so-called ‘forbidden’ emission. To understand why emitted light from a galaxy could be considered forbidden, it helps to understand why spectra exist in the first place. Spectra look the way they do because certain atoms and molecules will absorb and emit light very reliably at very specific wavelengths. The reason for this is quantum physics: electrons (the tiny particles that orbit the nuclei of atoms and molecules) can only exist at very specific energies, and therefore electrons can only lose or gain very specific amounts of energy. These very specific amounts of energy correspond to certain light wavelengths being absorbed or emitted.

Forbidden emission lines, therefore, are spectral emission lines that should not exist according to certain rules of quantum physics. Nevertheless, quantum physics is complex, and the rules adopted to predict it use assumptions that suit laboratory conditions here on Earth. Under those rules, this emission is ‘forbidden’—so improbable that it’s disregarded. However, in space, in the midst of an incredibly energetic galactic core, those assumptions do not hold anymore, and the ‘forbidden’ light gets a chance to shine out towards us.

Credit: ESA/Hubble & NASA, C. Kilpatrick

Release Date: Dec. 18, 2023

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