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Supermassive black holes (SMBH) are now believed to be present in the centers of most, if not all, large galaxies. During growth phases, SMBH are observed as active galactic nuclei (AGN); the SMBH grows by accreting interstellar gas while releasing vast amounts of energy in the form of electromagnetic radiation, high speed gas outflows and jets of plasma ejected at relativistic speeds. This energy release is thought to have a major impact on galaxy evolution, clearing away or superheating gas that would otherwise form stars.

The most luminous AGN (known as quasars) are thought to be triggered during mergers between gas-rich spiral galaxies. Gravitational tides generated by the merger drive gas into the galaxy core(s), causing intense star-formation and also fueling the SMBH. Theory predicts that a galaxy merger also results in the formation of an SMBH binary (a bound pair of SMBH orbiting their center of mass) at the center of the merged galaxy. The SMBH binary will itself eventually coalesce with an enormous outburst of gravitational waves and electromagnetic radiation.

ASPIRE Astrophysicists study several aspects of AGN and the interplay between SMBHs and their host galaxies using a combination of targeted multiwavelength observations with space and ground-based telescopes as well as computer simulations.

Multiwavelength observations of IRAS16399-0937, an advanced galaxy merger hosting an OH megamaser. Top row, left to right: Hubble Space Telescope images at 0.4, 0.6 and 1.6 µm. Bottow row: HST image of ionized gas emission in Hydrogen (Ha) and Nitrogen ([NII]6548,83); Spitzer IR image at 8 µm and Very Large Array radio image at 1.49 GHz. From Sales et al. 2015, ApJ, 799, 25).