Supermassive Pockets in the Cosmic Neighborhood: Searching for Active Galactic Nuclei BY RUJUTA PUROHIT '24
Cover Image: The spiral galaxy NGC 3393 hosts an obscured active galactic nucleus at its center. In the sky, NGC 3393 is located in the constellation of Hydra and is about 56 mega parsecs away from us. The galaxy is characterized as a Seyfert galaxy and actually has two supermassive black holes at its core. Image Source: Wikimedia Commons
Introduction An active galactic nucleus (AGN) is the physical manifestation of gas accreting onto a supermassive black hole (SMBH). SMBHs lie at the centers of most, if not all, large spiral and elliptical galaxies and are believed to be responsible for powering the galaxy. Sagittarius A* (Sgr A*) is the SMBH at the center of the Milky Way Galaxy. Although not producing large amounts of radiation (thus, often called dormant) today, the SMBH is truly giant, with a mass of 4.3 million times that of the Sun (Abuter, 2019). In 2020, Andrea Ghez of the University of California, Los Angeles, and Reinhard Genzel of the Max Planck Institute received the Nobel Prize in Physics for proving that Sgr A* indeed hosts a supermassive black hole at its center.
galaxies. However, a small fraction is known to emit significant amounts of light and energy. This is due to an interesting combination of many factors, including the masses of the black holes and their host galaxy, the rate of star formation in the galaxy, and the amount of dust and gas present in the vicinity of the SMBH. These black holes then outshine the entire host galaxy across the electromagnetic spectrum, resulting in an active galactic nucleus (AGN). AGN are powered by the release of gravitational energy from all of the matter that is accreted onto the SMBHs. The large energy released by an AGN can effectively influence its surrounding environment, eventually impacting the whole nuclear region, as well as the star formation and the evolution of the galaxy as a whole (Masini et al., 2020).
SMBHs have masses on the order of millions of times that of the Sun and are very strong sources of radiation. Their strong gravitational force means that no physical matter can escape their event horizons, including light. Event horizons are the last optically visible regions of the black holes themselves; they are large, compact discs of gas and dust surrounding the center of a black hole. The event horizon is what could theoretically be “seen” when looking at a black hole.
Physical Characteristics of AGN
At any given time, the majority of SMBHs are mostly dormant at the centers of their host 54
The Eddington luminosity (also called the Eddington limit) is the maximum luminosity an object can ever have such that the inward gravitational force is balanced by the outward radiation force. If the luminosity exceeds the Eddington limit, the radiation pressure drives jets, which are high-energy beams of matter travelling out of the center of the black hole at relativistic speeds (speeds very close to that of light) (Bassani & Sembay, 1983). They are extremely energetic and thus very bright. The jets DARTMOUTH UNDERGRADUATE JOURNAL OF SCIENCE
