The New Cosmos: Answering Astronomy's Big Questions by David J. Eicher

Author:David J. Eicher [Eicher, David J.]
Language: eng
Format: epub
Tags: Nature, Sky Observation, Science, Astronomy, Cosmology
ISBN: 9781107068858
Google: lMvZCgAAQBAJ
Amazon: 1107068851
Publisher: Cambridge University Press
Published: 2015-12-03T13:34:36.603261+00:00

Figure 10.2 Galaxy mergers like Milkomeda are common in the universe. The Hubble Space Telescope has uncovered a swarm of interacting galaxies in the cluster Abell 1689.

NASA, ESA, the Hubble Heritage Team (STScI/AURA), J. Blakeslee (NRC Herzberg Astrophysics Program, Dominion Astrophysical Observatory), and H. Ford (JHU)

Loeb and Cox used a model of the Local Group that assumes normal, baryonic matter is limited to the disks and bulges of the two galaxies, and that a vast dark matter halo around each galaxy has about 20 times the mass of the normal matter. They used total masses for the galaxies of 1 trillion solar masses for the Milky Way and 1.6 trillion solar masses for the Andromeda Galaxy. Unlike the early thoughts about the Local Group's mass, however – that nearly all of it existed between these two giant galaxies – Loeb and Cox argue for a diffuse intragroup medium, a soup of particles (80 percent dark matter and 20 percent gas) that contains another 2.6 trillion solar masses over the whole of the Local Group.

And the fact that this soup of particles exists between the two galaxies means understanding their orbits over time is far more complicated than simply harking back to Kepler's laws. The astronomers settled on a best-fit model that begins by simulating the two galaxies separated by 4.2 million light-years and on an eccentric orbit that brings them to a perigalacticon – a closest separation at initial approach – of 1.5 million light-years.

The astronomers modeled the dynamics between the two galaxies from 5 billion years ago, a short time before the formation of our solar system, to a point 10 billion years in the future, long after the Milky Way–Andromeda Galaxy merger. Not only did they take the overall status of the two galaxies into consideration, but also the fate of our own Sun.

The Loeb‒Cox analysis produced some very intriguing results. They found the two galaxies make a first, close passage in less than 2 billion years. The collision and merger itself will take place on a timescale of something less than 5 billion years. The relatively short times involved in the passage and then merger – given the vast future of the universe as a whole – is due to the gravitational effects of the intragroup medium. The merger accelerates and completes pretty quickly once the dark matter halos of the two galaxies penetrate each other, when the galaxies are separated by about 300,000 light-years.

Of course, amateur astronomers can only dream of the night sky that will exist from Earth's viewpoint in the distant future, with the form of the Andromeda Galaxy becoming closer and closer, larger and larger in our sky. It will eventually rival the disk of the Milky Way in size and brightness across our sky, and the resulting show that any living beings will witness will be absolutely incredible. We know from Chapter 3 that by the time these two galaxies merge, Earth will have been inhospitable for a very long time. But

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