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Galaxies’ missing matter may be found – but now there’s too much of it

Most galaxies seemed to be missing a huge proportion of the matter we expected them to have – now researchers may have found its hiding spot, but the discovery contradicts accepted models of galaxy formation

By Leah Crane

6 March 2023

The magnificent spiral galaxy M99 fills the frame in this image from the NASA/ESA Hubble Space Telescope. M99 ??? which lies roughly 42 million light-years from Earth in the constellation Coma Berenices ??? is a ???grand design??? spiral galaxy, so-called because of the well-defined, prominent spiral arms visible in this image. M99 was captured by Hubble???s Wide Field Camera 3 on two separate occasions, helping astronomers study two entirely different astronomical phenomena.?? The first set of observations aimed to explore a gap between two different varieties of cosmic explosions; novae and supernovae. Novae, which are caused by the interactions between white dwarfs and larger stars in binary systems, are far less bright than the supernovae which mark the catastrophically violent deaths of massive stars. However, current astronomical theories predict that sudden, fleeting events could occur that shine with brightnesses between those of novae and supernovae. Despite being described by astronomers as being shrouded in mystery and controversy, just such an event was observed in M99. Astronomers turned to Hubble???s keen vision to take a closer look and precisely locate the fading source. The second set of observations were part of a large Hubble project which aims to chart the connections between young stars and the clouds of cold gas from which they form. Hubble inspected 38 nearby galaxies, identifying clusters of hot, young stars. These galaxies were also observed by the Atacama Large Millimeter/submillimeter Array (ALMA), a colossal radio telescope consisting of 66 individual dishes perched high in the Chilean Andes. The combination of Hubble???s observations of young stars and ALMA???s insight into clouds of cold gas will allow astronomers to delve into the details of star formation, and paves the way for future science with the NASA/ESA/CSA James Webb Space Telescope.??

The spiral galaxy M99, imaged by the Hubble Space Telescope

ESA/Hubble & NASA, M. Kasliwal

Many large galaxies were long thought to be missing a huge proportion of matter – but it has now been found, and that is an even bigger problem for our understanding of the universe.

Observations over the course of the last decade or so have shown that galaxies about the size of the Milky Way appear to have far less baryonic matter – meaning normal matter, not dark matter – than we expect. This expectation is based on the ratio of dark …

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