Black holes do the tango

Animation: Excerpts from the solo multimedia concert “Einstein’s Cosmic Messengers”, by Andrea Centazzo and Michele Vallisneri, illustrate a galaxy and black hole merger. Animation and audio courtesy of Andrea Centazzo and Michele Vallisneri, Caltech. Please send your comments on this issue of Cosmic Matters to .(JavaScript must be enabled to view this email address).

Two dancers waltzing in a darkened ballroom are invisible to an observer standing in the room. Similarly, two inspiralling black holes in a merger-remnant galaxy can be invisible to astronomers. Now imagine the two ballroom dancers picking up sparklers; suddenly their dance becomes very visible. A similar concept holds true for black holes—if a black hole is surrounded by gas from its host galaxy, this gas can fall onto the black hole and cause it to light up. In this way, the inspiralling of two black holes can become very visible to astronomers.                                                                                                                            —Julie Comerford, UC Berkeley astronomer

Despite the vastness of space, many galaxies interact and eventually merge with one another. Understanding how often this happens provides astronomers with information about how galaxies evolve in the Universe.

However, observing two galaxies as they merge is difficult. Images from the Hubble Space Telescope hint at past and future mergers, but these images have limitations.  From them, it is not clear how to accurately classify a galaxy merger.

Julie Comerford and her colleagues at the University of California Berkeley have developed a new technique to study galaxy mergers. Using the Keck II telescope and its DEIMOS instrument, the astronomers observe spectra from material accreting onto black holes as they inspiral in merger-remnant galaxies. The data shed light on the dynamics of galaxy mergers and black hole mergers, as well as how often the black holes do the tango.