Neighbors of the Milky Way are Methuselahs

Dwarf galaxies around the Milky Way are among the oldest galaxies in the cosmos

Simulated Milky Way with lighter (blue) and fainter, old dwarf galaxies (white). © Durham University, Max Planck Institute for Astrophysics
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Ancient Neighbors: If you are looking for the oldest galaxies in the universe, you do not have to wander into the distance. Because some dwarf galaxies in the milky way belong to these cosmic Methuselahs, as astronomers have now discovered. These very faint galaxies were created more than 13 billion years ago - in the dark age of the cosmos. They are one of the first galaxies in our universe.

A few hundred million years after the Big Bang, the universe underwent a major transformation: The first stars and galaxies were born. They ended the "dark age" of the cosmos and ushered in the epoch of reionization: the intense radiation of the young stars ionized the hitherto neutral interstellar gas clouds.

Surprisingly old Milky Way companions

So far, astronomers have observed the oldest stars and galaxies, especially at very great distances. But Sownak Bose of the Harvard-Smithsonian Center for Astrophysics at Cambridge and his team have targeted some potential Methuselahs right outside our cosmic door: dwarf galaxies orbiting the Milky Way as satellites. Some of them have less than a thousand stars and are therefore difficult to track despite their close proximity. Only about 50 of these satellites are known so far.

For their study, astronomers have studied the light curves of these dwarf galaxies. For as they explain, their brightness and number of stars hide their age and the time of their creation. It turned out that some of these very feeble Milky Way satellites, including the dwarf galaxies Segue-1, Bootes-1, Tucana II and Ursa Major I, must already be more than 13 billion years old.

Some satellite galaxies of the Milky Way. Only 59 dwarf galaxies in their environment are known so far. Ursa Major I is one of the most ancient representatives of these satellites. © Richard Powell / Public domain

Emergence in the dark age of the cosmos

"Finding some of the very first galaxies in the universe in the backyard of our Milky Way is very exciting, " says co-author Carlos Frenk of Durham University. "This is the astronomical equivalent to the discovery of the remains of the very first humans on Earth." This discovery is also due to the powerful modern telescopes: "Just a decade ago, these faint galaxies would be complete escaped, "says Bose. display

This discovery is exciting, but not only because of the proximity of this Methusalemsem to our Milky Way. The existence of these ancient galaxies also confirms an astronomical theory of early galaxy formation in the universe, as the researchers explain. After this, the first galaxies were formed in the dark age, when neutral hydrogen clouds were cooled and concentrated by halos of dark matter, causing star formation in them.

Stunted by reionization

"The UV light of the first stars, however, ionized these hydrogen clouds and heated them up to 10, 000 Kelvin, " explain Bose and his colleagues. This stopped the formation of stars and thus the growth of these galaxies. As a result, these first galaxies remained rather small and faint and are still preserved as dwarf galaxies. They are therefore important witnesses of the cosmic transition from the dark age to the era of reionization.

A part of the Milky Way dwarf galaxies belongs to this first generation of galaxies, astronomers report. But there is a second group in our galactic backyard. These dwarf galaxies come from a later phase and are hundreds of millions of years younger. They were created when the halos of dark matter became larger and the ionized gas gradually cooled down again. As a result, the formation of stars could start again and now larger, brighter galaxies as well as our Milky Way formed.

"Our results thus support the current model for the evolution of our universe, " says Frenk. "In this Lambda Cold Dark Matter model (ΛCDM), the elementary particles that compose the dark matter drive cosmic evolution." Even though it is still puzzling today, what are these particles and what does it produce? Dark matter, it seems clear that it played a crucial role in making the universe what it is today. (Astrophysical Journal, 2018; doi: 10.3847 / 1538-4357 / aacbc4)

(Durham University, 17.08.2018 - NPO)