Hubble photographed galaxies with ‘forbidden’ light

This Hubble image shows MCG-01-24-014, an active spiral galaxy some 275 million light-years away in the constellation of Hydra. Image credit: NASA / ESA / Hubble / C. Kilpatrick

Active galactic nuclei (AGNs) are one of the most amazing and mysterious phenomena in the universe. These are regions at the center of galaxies that experience huge bursts of energy that exceed the brightness of other parts of the galaxy. What is the cause of this activity? And how can we study it? This image taken by NASA’s Hubble Space Telescope shows a bright spiral galaxy called MCG-01-24-014, located about 275 million light-years from Earth. Two bright stars are visible against the background of dark space. One is red and the other is blue. These are actually within our galaxy and have nothing to do with MCG-01-24-014.

An example of an AGN is the spiral galaxy MCG-01-24-014, located in the constellation Hydra, about 275 million light-years from Earth. This galaxy features a unique spiral structure with two branches emerging from a bright core. MCG-01-24-014 is a type 2 Seyfert galaxy, named after the American astronomer Karl Seyfert, who first discovered its features. Seyfert galaxies are classified into his two main subtypes: type 1 and type 2. They differ in their spectra, that is, the distribution of light intensity over wavelengths.

Spectra can be used to determine the chemical composition and physical state of a light source. When light passes through a material, it is partially absorbed at certain wavelengths. This corresponds to the energy of electronic transitions between different levels of atoms and molecules. This creates dark lines in the spectrum, so-called absorption lines. On the other hand, when light is emitted from a substance, bright lines with the same wavelength appear, called emission lines. Type 1 Seyfert galaxies have broad emission lines, indicating that the central material is moving rapidly.

Type 1 Seyfert galaxies have broad emission lines, indicating that the central material is moving rapidly. Type 2 Seyfert galaxies have thin emission lines, suggesting that matter moves more slowly. In addition, Type 2 Seyfert galaxies have special emission lines called “forbidden”. Forbidden emission lines are emission lines that should not occur according to quantum physics. According to quantum laws, electrons can only exist at certain energy levels, and transitions between them occur with a certain probability.

Forbidden emission lines indicate that type 2 Seyfert galaxies have very thin regions of gas at their centers that are strongly influenced by energy from the center. This energy ionizes atoms. That is, it removes an electron from an atom and excites the atom to a higher level. When the electron returns to a lower level, light is emitted, including the forbidden light. Where does this energy at the center of a Seyfert galaxy come from? The most likely source is a supermassive black hole that absorbs surrounding matter and converts it into radiation. A black hole is surrounded by an accretion disk, a flat structure of gas and dust that orbits around it.

But not all the light from the accretion disk reaches us. In some cases, it is overlaid by a dusty torus, a thick ring of dust surrounding the center of a galaxy. A dusty torus transmits only certain wavelengths, absorbing or scattering others. This explains the difference between type 1 and type 2 Seyfert galaxies. In type 1, the dust torus is at an angle to our field of view, so the accretion disk and broad emission line are directly visible. In type 2, only a thin emission line coming from the thin gas outside the dust torus is visible because it obscures the view of the accretion disk.

Therefore, type 1 and type 2 Seyfert galaxies are the same phenomenon, but at different angles. This is called the integrated AGN model, and it is assumed that the differences between different types of she-AGNs are due to the orientation of the she-AGN structure with respect to the observer. Although the unified AGN model is supported by many observations, there are also many problems and unclear details. Therefore, AGN research continues and remains one of the most relevant and interesting areas in astronomy.