“Long Shrouded in Mystery” –Dwarf Galaxies of the Milky Way

Large Magellanic Cloud



In the fall of 2018, astronomers reported the detection of 39 dwarf galaxies orbiting the Milky Way, with many yet to be discovered that are hiding at large distances from the galactic center, one of which was a strangely large and dim ‘Ghost’ dwarf hiding at the edge of the Milky Way. The object is about the same size as the 7,000 light-year-wide Large Magellanic Cloud (LMC), which is the Milky Way’s largest satellite galaxy, yet 10,000 times fainter. The discovery suggests other nearby “ghost” galaxies could exist undetected.

There are around two thousand billion galaxies in our Universe and, while our own Milky Way galaxy encompasses between two to four hundred billion stars, small dwarf galaxies reports an international research team led by Lund University, Sweden contain only tens of thousands to a few billion stars and can be tens of thousands of times or even millions of times less luminous than the Milky Way. The largest globular cluster in the Milky Way, Omega Centauri, is believed to be the core of a dwarf galaxy with a black hole at its center, which was absorbed by the Milky Way in the distant past.

What has long been shrouded in mystery, is how stars are formed in these dwarf objects. Dwarf galaxies are capable of lying dormant for several billion years before starting to form stars again according to the Lund University team that has found that dormant small galaxies can slowly accumulate gas over many billions of years. When this gas suddenly collapses under its own weight, new stars are able to arise.

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“It is estimated that these dwarf galaxies stopped forming stars around 12 billion years ago. Our study shows that this can be a temporary hiatus,” says Martin Rey, an astrophysicist at Lund University and the leader of the study.

Through high-resolution computer simulations, the researchers demonstrate that star formation in dwarf galaxies ends as a result of heating and ionization from the strong light of newborn stars across the Universe. Explosions of so-called white dwarfs—small faint stars made of the core that remains when normal-sized stars die—further contribute in preventing the star formation process in dwarf galaxies.

“Our simulations show that dwarf galaxies are able to accumulate fuel in the form of gas, which eventually condenses and gives birth to stars. This explains the observed star formation in existing faint dwarf galaxies, which has long puzzled astronomers,” states Rey.

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The computer simulations used by the researchers in the study are extremely time-intensive: each simulation takes as long as two months and requires the equivalent of 40 laptop computers operating around the clock. The work is continuing with the development of methods to better explain the processes behind star formation in our Universe’s smallest galaxies.

“By deepening our understanding of this subject, we gain new insights into the modeling of astrophysical processes such as star explosions, as well as the heating and cooling of cosmic gas. In addition, further work is underway to predict how many such star-forming dwarfs exist in our Universe, and could be discovered by astronomical telescopes,” concludes Rey.

Source: Martin P Rey et al. EDGE: from quiescent to gas-rich to star-forming low-mass dwarf galaxies, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa1640

The Daily Galaxy, Sam Cabot, via Lund University

Image at the top of the page: a simulated Large Magellanic Cloud-like galaxy with stars and gas, and several smaller companion galaxies. Ethan Jahn, University of California, Riverside.

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