Dead Galaxy Discovered By Hubble Leads To More Questions Than Answers

What they saw was totally unexpected. It's enough to challenge a widely accepted assumption about the evolution of galaxies.

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As it continues with its relentless task of gazing up at the sky and the stars, the Hubble Space Telescope has made another intriguing discovery. And it might call for rewriting of a well-known theory about the way massive galaxies form and evolve.

As reported in the June 22 issue of the journal Nature, Hubble was able to photograph a dead disk-shaped galaxy that stopped forming stars just a few billion years after the Big Bang. What’s special about the find is that it revealed something unexpected. Instead of seeing a chaotic star formation resulting from the merging of galaxies, what the astronomers saw were stars formed in a pancake-shaped disk. Apparently, this is the first evidence showing that dead galaxies could evolve from a Milky Way-shaped disk into a giant elliptical galaxy.

Elliptical galaxies have older stars while spiral-shaped galaxies have young blue stars. The theory now is that older dead disk galaxies must have undergone major changes that eventually resulted in forming an elliptical galaxy.

Previously, it was widely believed that dead galaxies had a structure that was similar with the elliptical galaxies they would later evolve into. Proving this ideally requires more powerful telescopes than what we currently have. With the help of gravitational lensing, however, it became possible to see into the core of the dead galaxy being studied.

Gravitational lensing is one of the predictions of Einstein’s General Theory of Relativity. It takes off from the concept that massive objects can curve or warp the space surrounding it. As light travels along this curvature, it is deflected toward the massive object. For instance, light from a far-off object like a dead galaxy, could be deflected as it passes through a cluster of galaxies, focusing the dead galaxy’s light in our direction — magnifying it so that it appears larger, brighter, and easier to observe.

By combining all available data (including those from the Hubble and what gravitational lensing provided), astronomers studying the dead galaxy in focus were able to determine that it’s about half the size of our galaxy, is three times as massive, and is spinning more than double the rate at which our own galaxy is spinning. They were also able to figure out the ages of its stars, the rate at which the stars were formed, and the stellar mass.

What remains to be determined now are the following: Why did the galaxy stop forming stars? And how do such galaxies evolve into the elliptical galaxies we have today? According to lead study author Sune Toft from the University of Copenhagen’s Niels Bohr Institute, it’s probably through mergers.

As he described it in a press statement: “If these galaxies grow through merging with minor companions, and these minor companions come in large numbers and from all sorts of different angles onto the galaxy, this would eventually randomize the orbits of stars in the galaxies. You could also imagine major mergers. This would definitely also destroy the ordered motion of the stars.”

Like we mentioned, it’s an amazing discovery. But unfortunately, now there are more questions than answers.

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