La Civiltà Cattolica, the Jesuit periodical that has been in publication since since 1850, sports a special section on the front page of its website, dedicated to the Vatican Observatory. The special section is titled “A Riveder le Stelle: Novità dall’osservatorio astronomico vaticano”. That translates to “A Review of the Stars: News from the Vatican astronomical observatory”. More poetically, it is “To see the stars again: News from the Vatican astronomical observatory”.
However, it is all in Italian. Here you can read in English its articles for July and August, which feature galaxies and dark matter. The tagline for La Civiltà Cattolica says “Reflecting the Mind of the Vatican since 1850”. It is good to see that astronomy is on the mind!
Click here for other articles from La Civiltà Cattolica.
An astronomer with the Vatican Observatory is helping to piece together the history of our galaxy and nearby galaxies in order to understand how their present-day properties are shaped by their pasts.
Galaxies like our Milky Way were formed roughly 12 billion years ago. They grew over time, each enduring a series of “mergers” with other smaller galaxies because of the forces of gravity. Like a corporation growing by gobbling up other businesses, galaxies can dramatically increase their masses by “acquisitions” of other galaxies. Rarely can one business absorb another and remain unaltered. Likewise, the properties of these galaxies like the Milky Way have been altered by the other galaxies they have absorbed.
To understand how these Milky Way-like galaxies have been shaped by these mergers, Fr. Richard D’Souza, S.J. of the Vatican Observatory has been engaged in “galactic archaeology”. He is involved in various efforts to decipher the merger history of these galaxies.
In 2018, Fr. Richard led efforts to piece together how the Andromeda galaxy, our largest close neighbor, endured a collision nearly 2 billion years ago with a galaxy that was half the size of the Milky Way. The Andromeda galaxy is a large spiral galaxy that is visible to the naked eye. It is a favorite among amateur astro-photographers. Astronomers had long thought that Andromeda had a very quiet history, because of its picturesque spiral disk-like nature. However, deep observations around Andromeda revealed a series of stars left behind from the debris of a large galaxy that it had recently “gobbled up”. These “breadcrumbs” pointed to the violent history of the Andromeda galaxy, which left it in a ruffled state that has caused a burst of star formation and left its disk agitated and thickened.
In contrast, the Milky Way has had a very quiet acquisition history. Precise measurements from the European Space Agency’s Gaia spacecraft of the positions and movements of more than a billion nearby stars finally made it possible to uncover this quiet history. The largest galaxy that the Milky Way devoured was a galaxy not even one-tenth its size. This happened about 10 billion years ago. This long-ago destroyed galaxy was given the name the “Gaia-Enceladus” galaxy. “Enceladus” refers to a mythological giant, supposedly buried under Mount Etna. This former galaxy is buried in the Milky Way, puffing up its disk. With more precise data for the Milky Way available, scientists have also been able to uncover 5 other smaller mergers between 8 and 11 billion years ago.
More recently, Fr. Richard contributed to a work titled, “The Global Dynamical Atlas of the Milky Way Mergers” (The Astrophysical Journal 20 February 2022). This effort was led by Khyati Malhan of the Max-Planck-Institute for Astronomy in Germany. The Malhan team tried to summarize and confirm a series of recent discoveries of past mergers of the Milky Way. They used multiple tracers rather than focusing on one piece of evidence at a time, trying to consider everything together. In the process, they discovered possible evidence of a sixth smaller merger, happening around 7 billion years ago.
Astronomers have now begun to take the next steps in examining the chemical compositions of the stars in the various detected groups. Stars coming from the same destroyed galaxy will have similar compositions. Stars from different galaxies will be different. This work is still ongoing. Much remains to be learned about the history of the Milky Way.
A central question of Astronomy is historical: “where do we come from?” Central to that question is mapping and understanding the history of the Milky Way. With the Gaia satellite, “galactic archaeology” is coming of age, and the Vatican Observatory is contributing to that.
Studying the universe remains an exhilarating and humbling experience. All the particles and elements ever discovered, all the atoms and neutrinos, comprise a very small fraction of our universe’s content. This is one of the great scientific surprises of recent decades. Indeed, today astronomers believe that the overwhelming majority of the universe consists of “Dark Matter” and “Dark Energy”, entities that have never been detected on Earth. We look up into a universe made of substances utterly foreign to our world.
But scientists are searching for Dark Matter here on Earth. One Vatican Observatory cosmologist is part of that search. Maria Elena Monzani, a lead scientist at the SLAC National Accelerator Laboratory, in Menlo Park, California and the Kavli Institute for Particle Astrophysics and Cosmology of Stanford University—and also an adjunct scholar at the Vatican Observatory—is part of a team of over 200 co-authors whose paper “First Dark Matter Search Results from the LUX-ZEPLIN (LZ) Experiment” was recently accepted for publication in Physical Review Letters, one of the most prestigious journals in the field of physics.
Hints of Dark Matter were first observed by the astronomer Fritz Zwicky in the 1930s. He noticed that galaxies within a galaxy cluster seemed to be moving as though there was more matter in the cluster than was visible. Forty years later, Vera Rubin found that the orbits of stars within a galaxy are governed by an invisible mass. (Rubin is the namesake of the new Vera Rubin Telescope, expected to significantly advance what we know about the dark universe; she taught at the first Vatican Observatory Summer School in 1986.) Over the following decades, work by other cosmologists has indicated that Dark Matter is five times more abundant in mass than the ordinary atoms in the universe.
The LZ experiment hunts for hypothetical particles known as “weakly interacting massive particles” (or WIMPs) that have been proposed to account for Dark Matter. The name involves some humor, because “wimp” is the English equivalent of “fifone”. As befits the humorous name, WIMPs are supposed to have mass, but to do little else, interacting rarely and weakly with the normal matter we see. They are therefore “dark”—almost but not quite, impossible to see.
LZ uses a ten-ton tank of liquid xenon to try to detect rare interactions between WIMPS and xenon atoms that might produce light flashes or loose electrons which could then be recorded. To reduce interference from other sources in the Earth’s atmosphere, the experiment is located 1.5 km underground, in a former gold mine in South Dakota in the USA.
Monzani is the deputy operations manager for software and computing of LZ. She led simulated “dress rehearsals”, or “Mock Data Challenges”, to prepare for the early science phase of the experiment. Her team developed powerful analysis software to search out signals from a handful of possible dark matter interactions from petabytes of data collected by the detector. The LZ computing infrastructure employs supercomputers hosted at the National Energy Research Center (NERSC) in Berkeley, California.
The authors of “First Dark Matter Search Results” report a non-detection, while establishing the world record sensitivity for a search of WIMP dark matter particles. “Unfortunately,” says Monzani, “we did not discover the Dark Matter particle in this search.” However, the work so far has been oriented toward testing the experiment as much as toward detecting particles. “We knew that we would not have sufficient exposure for a discovery,” she says. “We pivoted immediately to a longer data taking campaign, which will take us into 2028 and will accumulate 20 times more exposure, allowing us to probe interesting scenarios describing the nature of the Dark Matter particle.” The search for the unknown in the universe continues.