SKY & TELESCOPE NEWS BULLETIN
APRIL 11, 1997

NEW LOCAL GALAXY

Galaxies keep turning up where astronomers thought there was only blank sky, and a new one object has just been added to the 30 members of what's called the Local Group. Alan Whiting and George Hau (University of Cambridge) detected a large, extremely dim glow in the constellation Antlia while examining Schmidt survey photographs. This newly discovered "Antlia Dwarf" is about 3.3 million light-years away, half again as far as the Andromeda Galaxy, and it contains only about a million stars -- enough to make a dim, 16th-magnitude glow. The Antlia galaxy is centered at right ascension 10h 0.41m, declination -27d 20'. It's only the second dwarf elliptical found to lie outside the gravitational dominance of a larger galaxy in the Local Group.

Royal Astronomical Society Press Notices
Date: 7 April 1997

Discovery of New Galaxy in the Local Group: The Antlia Dwarf Galaxy

Astronomers in Cambridge have discovered a new near-by galaxy belonging to the Local Group of Galaxies. It was found in the little-know southern constellation of Antlia (the Air Pump) and so has been named the 'Antlia Galaxy' by its discoverers. Astronomers had previously overlooked the Antlia Galaxy because it appears so dim relative to the background of the night sky. It is in a region of space previously thought to be devoid of nearby galaxies and is important for understanding our Local Group because there are very few members that are isolated in this way. Most of the smaller galaxies in the Local Group are satellites of our own Milky Way and the Andromeda Galaxy, which are much larger spiral galaxies. Being on its own, the Antlia Galaxy has not been disturbed and distorted by the gravity of a much more massive neighbour. It will help astronomers understand more about the nature of undisturbed dwarf galaxies, and how the Local Group formed and has developed over cosmic history.

In the course of the same study, the researchers also found a second previously unknown dwarf galaxy, lying just beyond the Local Group. They have called this one the 'Argo' Galaxy. It lies in the constellation Carina (the Keel). Since there is already a Carina Galaxy, to avoid confusion, the discoverers invoked the name of the obsolete constellation Argo Navis (the Argonauts' ship), which was divided up into several smaller constellations including Carina.

The discoveries were made by two research students, Alan Whiting and George Hau, working at the Institute of Astronomy, University of Cambridge, with Dr Mike Irwin of the Royal Greenwich Observatory, and were made public at the National Astronomy Meeting in Southampton on 10th April 1997.

Dr Irwin commented, 'Most galaxy surveys have been biased towards finding galaxies with high surface brightness. Extreme dwarf and low surface brightness galaxies are difficult to find and are generally found only locally (astronomically speaking). Most of the extreme examples are in the Local Group. But despite their unassuming appearance, dwarf galaxies hold the key to many questions about the formation, structure and evolution of galaxies. Dwarf galaxies also tell us important facts about the distribution and nature of dark matter, and about star formation in regions of space where the distribution of material is not very dense.'

The researchers had started by making a visual inspection of 894 large survey photographs of the southern sky taken by the UK Schmidt Telescope in Australia. They searched the photographs for large, dim, diffuse objects that no-one had catalogued before, which could possibly be overlooked galaxies. Using an instrument at the Royal Greenwich Observatory (the PDS microdensitometer), they digitised the images of the candidate galaxies and studied them more closely to confirm their potentially interesting nature. Then in March 1997, they took their final list to the Cerro Tololo Inter-American Observatory in Chile and obtained CCD images on the 1.5-meter (60-inch) telescope. The results showed two of the objects clearly resolved into stars.

From the red giant stars visible in the Antlia Galaxy, Whiting, Hau and Irwin realised they had identified a new member of the Local Group. It seems to be similar to other dwarf spheroidal galaxies. The brightest stars are easily resolved and the galaxy appears quite smooth and devoid of any obvious concentrations of stars or possible clusters. There are no obvious star-forming regions and no young hot blue stars apparent in any of the deep CCD images taken. The range of colours and magnitudes of the stars in the Antlia Galaxy is typical of other known dwarf spheroidal galaxies which are satellites of the Milky Way.

Comparison of the new discovery with these and other nearby galaxies led the team to conclude that the distance of Antlia is close to 3 million light years (1 megaparsec). It is most similar in appearance to the Tucana dwarf galaxy, the only other isolated dwarf spheroidal galaxy known in the Local Group. The apparent size of the Antlia Galaxy on the deep CCD images suggests that its diameter is only 4000 - 6000 light years (1-2 kiloparsecs). It probably only contains a million or so stars placing it firmly at the faint end of brightness range covered by galaxies. (By comparison, the Milky Way Galaxy is about 100,000 light years across and contains some two hundred billion stars.)

The Antlia dwarf is a very intriguing object. It is similar to the extreme dwarf spheroidals orbiting the Milky Way and Andromeda, and yet is relatively isolated in the Local Group. Being far away from the large Local Group galaxies, Antlia will help to put limits on the age and total mass of the Local Group by means of the 'timing argument'. According to Big-Bang cosmologies, all the members of the Local Group were born close together in space but moving relative to each other. Knowing their present distances and velocities, and the masses of the larger member galaxies, makes it possible to work backwards and decide how old the universe. This method does not involve knowing the Hubble parameter, H0, which relates to the expansion of the universe as a whole.

Note: The Local Group of Galaxies

The Local Group was first recognised by Edwin Hubble in the early days of extragalactic research when distances to galaxies were first being measured. There was a distinct difference between those galaxies that resolved easily into stars and those that did not, implying that the Milky Way Galaxy is part of a small local cluster of galaxies that condensed out of the general expansion of the Universe. We now recognise approximately 30 members of this condensation, which we refer to as the Local Group. The Local Group is dominated by two large spiral galaxies, the Milky Way and the Andromeda Galaxy, which between them contain most of the group's mass and give out most of the light. Nearly all the known smaller members of the Local Group are satellite galaxies orbiting these two.

The members of the Local Group discovered most recently are: Tucana -- an unusual isolated dwarf galaxy discovered in 1990 by Lavery on the outer fringes of the Local Group; Sextans -- the 8th dwarf satellite galaxy to be found orbiting the Milky Way, discovered in 1990 by Irwin; Sagittarius dwarf -- the 9th dwarf satellite galaxy of the Milky Way, caught in the act of being tidally destroyed and incorporated into the Milky Way, discovered by Ibata, Irwin and Gilmore in 1994.

Images

Colour images of the Antlia Galaxy and the Argo Galaxy will be available at the following WWW site:

http://www.ast.cam.ac.uk/~mike

Captions to the images...

NEW GALAXY IN THE LOCAL GROUP

The Antlia dwarf galaxy, a member of the Local Group of galaxies, discovered in 1997. This image was made by combining three separate images made through colour filters with a CCD camera to give an effect close to real colour. The telescope used was the 1.5-metre at the Cerro Tololo Inter-American Observatory in Chile. The Galaxy is about 3 million light years away, around 5000 light years across, and contains roughly a million stars. Individual bright stars are clearly resolved.

Credit: Mike Irwin (Royal Greenwich Observatory), Alan Whiting and George Hau (University of Cambridge).

NEWLY DISCOVERED 'ARGO' DWARF GALAXY

The Argo dwarf galaxy, located just beyond the Local Group at a distance of about 13 million light years, discovered in 1997. This image was made by combining three separate images made through colour filters with a CCD camera to give an effect close to real colour. The telescope used was the 1.5-metre at the Cerro Tololo Inter-American Observatory in Chile. Argo is a typical irregular dwarf galaxy. Hot blue star-forming regions and red supergiant stars are visible in the image.

Credit: Mike Irwin (Royal Greenwich Observatory), Alan Whiting and George Hau (University of Cambridge).

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