Dwarf galaxy mergers generate star formation

Dwarf galaxies have abundant levels of hydrogen gas, the fuel that drives star birth.
By Laurel Kornfeld | Aug 11, 2018
Dwarf galaxies such as the Large and Small Magellanic Clouds have abundant hydrogen gas, which plays a key role in star formation.

When two dwarf galaxies merge with one another or one dwarf galaxy merges with a larger, parent galaxy, this hydrogen gas is dispersed within the new combined galaxy, where it acts as fuel to generate the birth of new stars.

"You have this enormous reserve of star formation fuel sitting there ready to be stripped by another system," explained Mary Putnam of Columbia University, who took part in a study on the role of hydrogen gas in galaxy mergers.

Both the Large and Small Magellanic Clouds are dwarf galaxy satellites of the much larger Milky Way. The two were in the process of merging when they were gravitationally pulled into the Milky Way's orbit. Between them, there is enough hydrogen gas to replenish around half of the Milky Way's star-formation fuel.

Much dimmer than their larger spiral counterparts, dwarf galaxies are filled with swirling hydrogen gas.

Led by then-Columbia graduate student Sarah Pearson, now at the Flatiron Institute's Center for Computational Astrophysics, a team of researchers observed two distant dwarf galaxies, NGC 4490 and NGC 4485, both located approximately 23 million light years away, to learn more about the role hydrogen gas in dwarf galaxies plays in creating new stars. NGC 4490 is several times larger than its partner.

Unlike the Magellanic Clouds, these two dwarf galaxies are not bound to a larger spiral galaxy like the Milky Way, so scientists can observe their merging without the influence of a larger parent galaxy.

By inputting data about the two dwarf galaxies into a computer simulation, the researchers modeled their merger, focusing on the subsequent expansion of their hydrogen gas over five billion years. By that time, "tails" of hydrogen gas stretched more than a million light years.

"After five billion years, 10 percent of the gas envelope still resides more than 260,000 light years from the merged remnant, suggesting it takes a very long time before all the gas falls back to the merged remnant," Pearson said.

Over time, the gas clouds became more and more extended, thinning them out and making it easier for any nearby large galaxies to absorb them. This phenomenon likely makes it easy for the Milky Way to absorb gas from the Magellanic Clouds.

To better understand these dynamics, the researchers plan to study other pairs of dwarf galaxies.

A paper on the study has been published in Monthly Notices of the Royal Astronomical Society.

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