The Milky Way's new neighbour

December 22, 2014 by Robert Massey, Royal Astronomical Society
A negative image of KKs 3, made using the Advanced Camera for Surveys on the Hubble Space Telescope. 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.) Credit: D. Makarov

The Milky Way, the galaxy we live in, is part of a cluster of more than 50 galaxies that make up the 'Local Group', a collection that includes the famous Andromeda galaxy and many other far smaller objects. Now a Russian-American team have added to the canon, finding a tiny and isolated dwarf galaxy almost 7 million light years away. Their results appear in Monthly Notices of the Royal Astronomical Society.

The team, led by Prof Igor Karachentsev of the Special Astrophysical Observatory in Karachai-Cherkessia, Russia, found the new galaxy, named KKs3, using the Hubble Space Telescope Advanced Camera for Surveys (ACS) in August 2014. Kks3 is located 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. These systems also have an absence of the raw materials (gas and dust) needed for new generations of stars to form, leaving behind older and fainter relics. In almost every case, this 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.

Isolated objects must have formed in a different way, with one possibility being that they had an early burst of star formation that used up the available gas resources. Astronomers are particularly interested in finding dSph objects to understand galaxy formation in the universe in general, as even HST struggles to see them beyond the Local Group. The absence of clouds of hydrogen gas in nebulae also makes them harder to pick out in surveys, so scientists instead try to find them by picking out individual stars.

For that reason, only one other isolated dwarf spheroidal, KKR 25, has been found in the Local Group, a discovery made by the same group back in 1999.

Team member Prof Dimitry Makarov, also of the Special Astrophysical Observatory, commented: "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."

The team will continue to look for more dSph , a task that will become a little easier in the next few years, once instruments like the James Webb Space Telescope and the European Extremely Large Telescope begin service.

Explore further: Milky Way ransacks nearby dwarf galaxies, stripping all traces of star-forming gas

More information: "A new isolated dSph galaxy near the Local Group." MNRAS (February 11, 2015) Vol. 447 L85-L89 DOI: 10.1093/mnrasl/slu181 First published online December 21, 2014

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not rated yet Dec 23, 2014
Kind of a contrived title. There are almost 100 dwarf galaxies withing 3 million light years, so 7 is pretty far away. With new imaging technology I'm wondering if there needs to be something below "galaxy", like "star streams", or "extended intergalactic open clusters", EIOCs?

To whit, some local dwarf galaxies have less luminosity, all stars combined, than a star like Rigel. Some even get the qualification, "and have been stripped of all gas and dust".'s just some faint stars still travelling in the same direction, no? And in between you have captured ellipticals like Omega Centauri...that even though much more massive aren't galaxies because they've been captured. Point being the nomenclature should have more to do with their composition than the gravity of their neighbors. That's important, but...a galaxy, this?
4.5 / 5 (2) Dec 23, 2014
Just to clear up a couple of misstatements in the picture caption, the globular cluster near the center of the galaxy IS associated with KKs 3 and is not a member of the Milky Way's retinue of globular clusters (nor an intergalactic 'tramp'). The cluster is well seen (as inset) in Fig 1 of the relevant paper (link below). Also, according to the 'discovery' paper "The bright spot at the center of the galaxy, just to the right of the globular caused by a tight pair of red foreground stars" and is not the "core of the galaxy" as stated in the caption.

I phrased this as a 'discovery' based on the fact the galaxy was first seen by researchers combing the just released southern extension of the Palomar Sky Survey back in 1985, then rediscovered in targeted surveys in 2000 and 2002. The recent Hubble observations were the first to pin down the distance (2.1 Mpc) and mass (23 million solar masses) of this dwarf galaxy:

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