Orphan dwarf galaxy could shed light on history of galaxy formation

The space surrounding our local group of galaxies – a collection of more than 50 galaxies in the Milky Way's neighborhood – may be a bit more densely populated than previously thought.

Astronomers mapping the group have found that a spherical dwarf galaxy once thought to be gravitationally tied to a galaxy some 12 million light-years away is actually a stand-alone object only about 7 million light-years away – a rare find that hints at more, according to members of the team reporting the measurements in the Monthly Notices of the Royal Astronomical Society in Britain.

The space immediately surrounding the local group "turns out to be less empty than we thought," said Dimitry Makarov, a researcher at the Special Astrophysical Observatory in Karachai-Cherkessia, Russia, and a member of the team mapping the group, in a prepared statement. "It may be that are a huge number of dwarf spheroidal galaxies out there, something that would have profound consequences for our ideas about the evolution of the cosmos.”

If the properties of this spheroidal dwarf, labeled KKs 3, are any indication, these objects could open a window on the types of small galaxies that formed early in the universe and through collisions and mergers grew into the more-massive galaxies astronomers see today. Since galaxies exist within a halo of dark matter, freestanding dwarf spheroids could help build a more detailed look at the distribution of dark matter in and around clusters such as the local group.

Dwarf spheroidal galaxies are dim collections of stars virtually bereft of gas and dust that form stars and planets. They typically appear as satellites to vastly larger galaxies or in areas around a galaxy cluster where galaxies are most densely packed, the researchers note. Dwarf spheroidal galaxies thought to have formed from so-called gas-rich, irregular dwarf galaxies that larger galaxies captured, then cannibalized of gas and dust.

If this is the dominant process for spawning spheroidal dwarfs, then they shouldn't show up in the middle of nowhere, the team adds.

If, however, a gas-rich, irregular dwarf avoided capture, it could produce stars until it ran out of gas. This would turn an isolated, undisturbed gas-rich irregular dwarf into a spheroidal one, dwelling in isolation.

The research team, led by University of Hawaii astronomer Brent Tully, discovered the KKs 3 during a project designed to map in three dimensions all of the galaxies within 23 million light-years of the Milky Way.

The team was interested in studying the process of galaxy clustering and the velocities of the galaxies within the cluster. The velocities hold clues about the distribution of dark matter and dark energy in the volume of space the researchers are exploring.

Dark matter is a form of matter detectable in the cosmos only by its gravitational influence on matter astronomers can observe. Dark energy is a mysterious component of the cosmos discovered in 1990s. It is accelerating the universe's expansion at a time when by dint of gravity alone, the expansion of the universe should be slowing.

According to current estimates, dark energy makes up some 68 percent of all the matter and energy in the universe, dark matter makes up another 27 percent, and ordinary matter brings up the rear at 5 percent.

The researchers used the Hubble Space Telescope to find the brightest red-giant stars in each galaxy. Red giants are low-mass stars, such as the sun, that have nearly exhausted their hydrogen fuel. They can be used as beacons with common, known attributes.

They made their measurements of KKs 3 last August and found it to be a stand-alone galaxy whose stars collectively have a mass some 23 million times the mass of the sun. By contrast, the Milky Way's star collection is roughly 1,000 times more massive.

After analyzing a sample of stars within KKs 3, the team concluded that the galaxy underwent three bursts of star formation. Some 74 percent of the stars formed within the first 1.8 billion years after the universe formed some 13.8 billion years ago. Between 4 billion and 6 billion years ago, the dwarf added another 14 percent to its total observed stellar mass. It added a final 12 percent to the total between 800 million and 2 billion years ago.

And there is little question that it is isolated. The team reported that KKs 3 is nearly 7 million light-years from the nearest large galaxy and 3 million light-years from the nearest dwarf galaxy.

This is only the second time in 15 years that anyone has found one of these isolated dwarf spheroid galaxies, according to the team. Another, KKR 25, was first discovered in 1999 and is some 6 million light-years away. Two years ago, a team of US and Russian astronomers published a study of KKR 25 that set out its history of star formation.

Its stars collectively tipped the cosmic scales at 3 million solar masses. KKR 25 generated some 62 percent of its stellar mass between 13.8 and 12.6 billion years ago. Over the past 1 billion years, it has been forming new stars at a pace of about 1 solar-mass star every 10,000 years. By contrast, some estimates hold that the Milky Way is producing one new star each year on average.

The dwarf spheroid galaxies are dim, making them hard to spot, researchers note. But with a new generation of space-and ground-based telescopes under construction, they anticipate finding more.