Milky Way Sidelined in Galactic Tug of War


From a Harvard-Smithsonian Center for Astrophysics press release:

The Magellanic Stream is an arc of hydrogen gas spanning more than 100 degrees of the sky as it trails behind the Milky Way’s neighbor galaxies, the Large and Small Magellanic Clouds. Our home galaxy, the Milky Way, has long been thought to be the dominant gravitational force in forming the Stream by pulling gas from the Clouds. A new computer simulation by Gurtina Besla and her colleagues from the Harvard-Smithsonian Center for Astrophysics now shows, however, that the Magellanic Stream resulted from a past close encounter between these dwarf galaxies rather than effects of the Milky Way.

“The traditional models required the Magellanic Clouds to complete an orbit about the Milky Way in less than 2 billion years in order for the Stream to form,” says Besla. Other work by Besla and her colleagues, and measurements from the Hubble Space Telescope by colleague Nitya Kallivaylil, rule out such an orbit, however, suggesting the Magellanic Clouds are new arrivals and not long-time satellites of the Milky Way.

This creates a problem: How can the Stream have formed without a complete orbit about the Milky Way?

To address this, Besla and her team set up a simulation assuming the Clouds were a stable binary system on their first passage about the Milky Way in order to show how the Stream could form without relying on a close encounter with the Milky Way.

The team postulated that the Magellanic Stream and Bridge are similar to bridge and tail structures seen in other interacting galaxies and, importantly, formed before the Clouds were captured by the Milky Way.

“While the Clouds didn’t actually collide,” says Besla, “they came close enough that the Large Cloud pulled large amounts of hydrogen gas away from the Small Cloud. This tidal interaction gave rise to the Bridge we see between the Clouds, as well as the Stream.”

“We believe our model illustrates that dwarf-dwarf galaxy tidal interactions are a powerful mechanism to change the shape of dwarf galaxies without the need for repeated interactions with a massive host galaxy like the Milky Way.”

While the Milky Way may not have drawn the Stream material out of the Clouds, the Milky Way’s gravity now shapes the orbit of the Clouds and thereby controls the appearance of the tail.

“We can tell this from the line-of-sight velocities and spatial location of the tail observed in the Stream today,” says team member Lars Hernquist of the Center.

The paper describing this work has been accepted for publication in the October 1 issue of the Astrophysical Journal Letters and is available online: Simulations of the Magenllanic Stream in a First Infall Scenario.

2 Replies to “Milky Way Sidelined in Galactic Tug of War”

  1. It is worth pointing out these tidal interactions attentuate the kinentic energy of bodies. Usually tidal interactions result in friction and energy loss. This is how large galaxies are able to gobble up smaller ones.


  2. “the large cloud galaxy pulls hydrogen gas streams away from the small cloud galaxy without a collision”. This is because of the black hole mathematical motion force of Newton-Hooke gravity, that increases gravity the farther the distance from the black hole the star orbits all do at the same constant velocity. Newtonian gravity is from the entire mass of the galaxy. Hooke’s law transfers the kinetic energy of the galaxy, into gravity of black holes when the gravitational forces are strong enough to pull away an outer star from its orbit. This too is how globular clusters hop between galaxies. When the black holes eat up all the hydrogen, stars can no longer form, which ultimately the smaller star-stripped black hole galaxy collides with the larger.

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