A previously known dwarf spheroidal galaxy of similar type to the new discovery, and also a satellite of our Milky Way, lying in the constellation of Fornax. Image credit: ESO/Digital Sky Survey 2

A previously known dwarf spheroidal galaxy of similar type to the new discovery, and also a satellite of our Milky Way, lying in the constellation of Fornax. Image credit: ESO/Digital Sky Survey 2

Sen—A team of astronomers using the Hubble Space Telescope have found a new neighbouring galaxy to our own Milky Way, a tiny and isolated dwarf galaxy almost 7 million light years away.

The Milky Way and Andromeda (M31) are the most massive members of what is known as the Local Group, a cluster of more than 50 galaxies. The group itself is a part of the larger Virgo Supercluster.

Russian and American astronomers, led by Professor Igor Karachentsev of the Special Astrophysical Observatory in Russia, found the new galaxy using the Hubble Space Telescope Advanced Camera for Surveys (ACS) in August 2014.

Named KKs3, the new galaxy lies in the southern sky in the direction of the constellation of Hydrus and its stars have only one ten-thousandth of the mass of the Milky Way. KKs3 is a dwarf spheroidal (or dSph) galaxy, lacking features like the spiral arms found in our own galaxy. It does not resemble the classic image of a galaxy, being very strung out and shapeless, which helps explain why it has remained unnoticed for so long.

Dwarf spheroidal galaxies also have an absence of the raw materials (gas and dust) needed for new generations of stars to form, so are home only to older and fainter stars. The raw material seems to have been stripped out by nearby massive galaxies like Andromeda, so the vast majority of dSph objects are found near much bigger companions.

A negative image of KKs 3, made using Hubble's Advanced Camera for Surveys. The core of the galaxy is the right hand dark object at the top centre of the image, with its stars spreading out over a large section around it. The left hand of the two dark objects is a much nearer globular star cluster. Image credit: D. Makarov.

Isolated galaxies like KKs3 may have had an early burst of star formation that depleted the available gas resources. The absence of clouds of hydrogen gas in nebulae makes them harder to pick out in surveys. Even Hubble, which is operated jointly by NASA and ESA, struggles to see them beyond the Local Group, so scientists instead try to find them by picking out individual stars.

In fact only one other isolated dwarf spheroidal has been found in the Local Group. KKR 25 was discovered by the same team back in 1999.

Hubble's ACS sees in wavelengths from the far ultraviolet to visible light. It is responsible for many of Hubble’s most impressive images of deep space. With its wide field of view, sharp image quality and enhanced sensitivity, the camera doubled Hubble’s field of view and expanded its capabilities significantly when it was installed in March 2002.

Team member Prof Dimitry Makarov, also of the Special Astrophysical Observatory, said: “Finding objects like Kks3 is painstaking work, even with observatories like the Hubble Space Telescope. But with persistence, we’re slowly building up a map of our local neighbourhood, which turns out to be less empty than we thought. 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.”

These dSph objects can help astronomers understand galaxy formation in the Universe. In the next few years, once instruments like the James Webb Space Telescope and the European Extremely Large Telescope begin service, the task of finding more dSph galaxies, should become a little easier. The team's results appear in the journal Monthly Notices of the Royal Astronomical Society.