DOUBLE-NUCLEI GALAXIES (Syllabus: GS Paper 1 - Geography)

Context: Researchers have developed a new algorithm which has led to the discovery of thousands of double-nuclei galaxies. 

• Out of these 159 were confirmed to have pairs of accreting supermassive black holes or active galactic nuclei (AGN) as they are usually called.

Key Points

• Supermassive black holes (SMBHs) have masses of a million to several billion times the mass of the sun. 
• Accreting SMBH or AGN gives out enormous amounts of radiation over the entire electromagnetic spectrum. Detecting this radiation is one of the only ways of observing SMBHs in distant galaxies. 
• So detecting AGN pairs is one of the best ways of tracing the mergers of massive black holes. However, it is extremely rare to find galaxies that have double-nuclei and dual AGN.
• In fact, detection of dual AGN is often only by accident, and before this study less than 50 were known. 
• With time, such AGN pairs finally result in two supermassive black holes spinning around each other and giving out nano-Hz gravitational waves.  The final merger of the SMBHs will give out a burst of gravitational waves.
• Many of these double nuclei galaxies are at separations similar to the separation of the Milky Way and Andromeda galaxies. 
• Their existence shows us how merging galaxies appear to change with time and how their SMBHs will appear before finally merging to form more massive black holes. 
• Thus, studying a large sample of double nuclei galaxies can help predict how the Milky Way and Andromeda will appear as they merge in the future. This study also shows that galactic collisions can signal “the beginning of the end” for the individual AGN as they merge to form a more massive SMBH.
• The investigators also found that the merging galaxies have a tendency to look red - indicating the presence of old stars and the decreased formation of new stars. 
• Since star formation is the lifeblood of galaxies, red galaxies represent the old, evolved population of galaxies. Thus the study reveals how AGN can extinguish the star formation in galaxies leading to the formation of red and “dead” elliptical galaxies.