The dark matter conspiracy

Computer simulation of a galaxy, with the dark matter colorized to enhance visibility. Dark matter envelops and pervades the galaxy, playing a crucial role in its cohesion and the formation of stars and planets. Credit: Springel et al., Virgo Consortium, Max-Planck-Institute for Astrophysics. An international team of astronomers, led by Michele Cappellari from the University of Oxford, utilized data from the W. M. Keck Observatory in Hawaii to study the movements of stars in the outer regions of elliptical galaxies. This groundbreaking survey, the first of its kind capturing a large number of these galaxies, revealed unexpected gravitational similarities between spiral and elliptical galaxies, hinting at the presence of hidden forces. The findings of this study are set to be published in The Astrophysical Journal Letters. The research involved mapping and analyzing the velocities of stars in an elliptical galaxy. Blues indicate areas where stars are approaching the Earth observer rapidly, while reds signify regions moving away, forming a coherent rotational pattern. The top panel displays raw data collected using the DEIMOS spectrograph at the W.M. Keck Observatory, while the bottom panel showcases a numerical model that aligns closely with the data, accounting for the combined gravitational effects of luminous and dark matter. Credit: M. Cappellari and the SLUGGS team. Scientists from the USA, Australia, and Europe leveraged the DEIMOS spectrograph on the Keck Observatory's largest optical telescope to conduct a comprehensive survey of nearby galaxies known as SLUGGS, charting the velocities of their stars. They then applied Newton's law of gravity to translate these velocity measurements into the distribution of matter within the galaxies. According to Aaron Romanowsky from San Jose State University, "The DEIMOS spectrograph played a pivotal role in this discovery by capturing data from an entire massive galaxy simultaneously, while accurately sampling the velocities of its stars at a hundred distinct locations." One of the most significant scientific revelations of the 20th century was the realization that spiral galaxies, like our Milky Way, rotate at much higher speeds than anticipated, driven by an additional gravitational force from invisible "dark matter." This enigmatic substance, likely an exotic elementary particle, constitutes roughly 85 percent of the universe's mass, leaving only 15 percent for familiar matter encountered in daily life. Dark matter is fundamental to our comprehension of galaxy formation and evolution, playing a crucial role in the existence of life on Earth, yet its nature remains largely elusive. The velocities of stars on circular orbits have been scrutinized around both spiral and elliptical galaxies. In the absence of dark matter, velocities should decrease with distance from the galaxy, exhibiting varying rates for each galaxy type. However, dark matter seemingly conspires to maintain consistent speeds, a phenomenon depicted in the images. Credit: M. Cappellari and the Sloan Digital Sky Survey. Michele Cappellari remarked, "Our study's surprising revelation was the constancy of circular speeds in elliptical galaxies extending far from their centers, mirroring the behavior already observed in spiral galaxies." This alignment suggests a redistribution of stars and dark matter within these galaxies, with stars prevailing in the inner regions and a gradual transition to dark matter dominance in the outer regions. While this alignment challenges conventional dark matter models and necessitates some fine-tuning to explain the observations, it raises the possibility that deviations in Newton's law of gravity at large distances could offer an alternative explanation. Despite being proposed decades ago, this alternative theory excluding dark matter remains inconclusive. Spiral galaxies represent less than half of the universe's stellar mass, with elliptical and lenticular galaxies dominating and exhibiting denser star configurations devoid of the flat gas disks characteristic of spirals. The technical challenges in measuring the masses and dark matter distribution in these galaxies have been formidable until now. Due to their distinct shapes and formation histories compared to spirals, the newfound alignment in elliptical galaxies poses profound implications, prompting experts in dark matter and galaxy formation to reevaluate the mysteries of the "dark sector" of the universe. Professor Jean Brodie, principal investigator of the SLUGGS survey, highlighted the relevance of this inquiry in the current period, as physicists at CERN gear up to resume operations of the Large Hadron Collider in a bid to directly detect the elusive dark matter particle, which, if proven to exist, underpins the rapid rotation of galaxies.