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Tracing the Growth of Galaxies

Hubble’s Ultra Deep Field is one of the most distant looks into space ever (detail). The cumulative exposure time needed to capture the image was about a million seconds (11 days). (PhotoNASA, ESA, S. Beckwith (STScI) and the HUDF Team)

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By Staff Middle Land

June 3, 2024
6:45 pm
Los Angeles, CA

Like documenting a child’s development in a scrapbook, astronomers use Hubble to capture the appearance of many developing galaxies throughout cosmic time. This is possible because of the mathematical relationship between cosmic distance and time: the deeper Hubble peers into space, the farther back it looks in time. This occurs because light travels at a fast, but finite speed, and it takes time to travel the incredible distance from galaxies far, far away. When this light finally reaches Earth, it shows us the object as it appeared at the time its light was emitted. As it happens, the most distant and earliest galaxies spied by Hubble are smaller and more irregularly shaped than today’s grand spiral and elliptical galaxies. This is evidence that galaxies grew over time through mergers

Because the universe was smaller in the past, galaxies were more likely to interact with one another gravitationally. Some of Hubble’s cosmic “snapshots” show fantastic stellar streamers pulled out and flung across space by colliding galaxies. They apparently settled over time into the more familiarly shaped galaxies seen closer to Earth and hence nearer to the present time. By carefully studying galaxies at different epochs, astronomers can see how galaxies changed and evolved over time. Among the things they investigate are the relative amounts of stars and gas in galaxies, the types and amounts of identifiable chemical elements they hold, and star-formation rates.

A sample of the faintest and farthest galaxies in the Hubble Ultra Deep Field shows that they were irregularly shaped and frequently interacting in the distant past.
(Photo: NASA, ESA, R. Bouwens and G. Illingworth (University of California, Santa Cruz)

Hubble observations of our neighboring galaxy, the Andromeda galaxy (M31), show that the galaxy is moving ever closer to an inevitable collision with our own Milky Way galaxy.

Andromeda is currently 2.5 million light-years away, but it and the Milky Way are moving toward each other under the mutual pull of gravity between the two galaxies and the invisible dark matter that surrounds them both. The merger will begin about four billion years from now and will likely result in the creation of a giant elliptical galaxy.