If a tree falls in a forest and no one is around to hear it, does it make a sound? This is an age-old question in philosophy about the distinction between reality and perception. In a timely context, addicts of the Metaverse (or the multiverse) should ponder whether their actions in virtual reality are of significance without a traditional footprint in the reality we all share.
A similar question can be phrased in an astronomical context: If a supermassive black hole forms at the center of a galaxy and there is no gas around to feed it, does it make light? This is a timely question for the nearby dwarf spheroidal galaxy, Leo I, one of the most distant satellites of our Milky Way galaxy, nearly a million light years away.
In 2021, a team of astronomers identified an increase in the characteristic speed of stars near the center of Leo I and inferred the existence of a supermassive black hole with three million times the mass of the Sun. This is a tantalizing suggestion, since the spheroid of stars in Leo I contains only twenty million solar masses, a thousand times less than the spheroid of the Milky Way. Yet, the two spheroids host a black hole of roughly the same mass.
For several decades, statistical data on numerous galaxies implied a tight relation between the mass of the black hole and the mass of the spheroid of stars that hosts it. As I showed in a 2002 paper with my former postdoc, Stuart Wyithe, this relation is expected theoretically since black holes are efficient converters of accreted mass to radiation and fast outflows of gas. The outpouring of energy removes the fuel from the parent galaxy that feeds the black hole — once it grows big enough. It sets the valve that caps the black hole growth in the belly of its parent galaxy. One therefore expects the limiting black hole mass to be correlated with the depth of the gravitational potential well or mass in stars that maintains its fuel.
A supermassive black hole at the nucleus of a galaxy behaves just like a baby that shoves the food off the dining table once it eats too much. The energetic episodes of shoving the food off the table are observed as brilliant quasars and active galactic nuclei, which are seen all the way back to when the Universe was less than a billion years old. The characteristic accretion episodes are short-lived, lasting for tens of millions of years or less than a percent of the age of the Universe. In a film about cosmic history, they would appear as nearly instantaneous explosions at the centers of galaxies, after which there is almost no gas left.
Consequently, most black holes in the present-day Universe are dormant with negligible fuel to light them up. This is also true for our own Milky Way galaxy. The black hole at the Galactic center, Sagittarius A* (Sgr A*), was initially discovered as a faint radio source and later confirmed through the rapid acceleration of individual stars in its vicinity — for which the Physics Nobel Prize was awarded in 2020 to Reinhard Genzel and Andrea Ghez. In 2004, I wrote a paper suggesting that this faint emission from Sgr A* is powered by winds from the stars in its vicinity. This idea was recently supported by detailed computer simulations.
Could winds from stars also reveal the existence of the black hole in Leo I? Yes, indeed.
In a new paper that I wrote with my postdoc, Fabio Pacucci, we showed that winds from a population of a hundred old stars in the core of Leo I produce enough fuel to power its supermassive black hole at a detectable level. Our model calculations predict that existing astronomical observatories could detect the black hole in the radio and X-ray bands. We are currently analyzing preliminary X-ray and radio data that we obtained from the Chandra X-ray Observatory and the Very Large Array.
One way to look at this endeavor is that Fabio and I chose to visit the forest in order to hear the sound of the tree falling. We are currently analyzing the data in preparation of a follow-up paper that will compare our observational findings to theoretical expectations. It appears that this invisible baby, which we labeled Leo I*, has some bread crumbs around it so we could see its footprint.
Why was this giant baby born out of such a slim parent? One possibility is that the Leo I galaxy used to be much more massive, but its outer envelope was stripped away during an early passage near the center of the Milky Way.
Science is a learning process. Finding an answer to one question raises new questions. Although one never settles to a final destination, the journey is fascinating. To keep expanding our scientific knowledge, we just need to maintain our childhood curiosity and not pretend that we already know everything. In other words, just stay forever young.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021.