Astronomers using the Isaac Newton Telescope report in a research paper published in the December issue of the Astronomical Journal the discovery of a new Local Group galaxy, never catalogued before, in the constellation Cetus. The Cetus dwarf galaxy, as it is called, is a very intriguing object. At a distance of 800 kiloparsec, it is only 1-2 kiloparsecs in diameter and it contains only a million or so stars, placing it firmly at the faint end of the galaxy luminosity function.

The observational universe is built mostly from galaxies. For obvious reasons, most of the known (detected and catalogued) galaxies are intrinsically the largest and brightest ones, those which can be seen from the greatest distance and are most easily studied. Dwarf galaxies, however, dominate numerically in any volume-limited sample, and were probably even more numerous in the cosmological past. Despite their unassuming appearance dwarf galaxies hold the key to many questions of galaxy formation, structure and evolution. They also provide important constraints on the distribution and nature of dark matter, and star formation in low density environments.

The need for more data in all these matters, together with the relatively few known dwarf galaxies, make a search for more of them very worthwhile. However, almost by definition dwarf galaxies are difficult to detect and observe. 

Searches for dwarf galaxies have been carried out in nearby galaxy groups with good results. However, owing to their small intrinsic size, dwarfs in external groups are difficult to characterize morphologically, and it is only within and near the Local Group that the resolved stellar photometry necessary for construction of detailed star formation histories can be obtained. Thus it appears most promising to limit a search to the Local Group and its immediate environs. 

To this end a visual examination of all 894 fields covered by the ESO-SRC and SERC Equatorial surveys of the southern sky was performed. Objects resembling the Andromeda dwarf spheroidals and the Tucana dwarf, that is of very low surface brightness (VLSB), diffuse and large (1 to a few minutes of arc), were noted. Some of the more northerly candidates were followed up using the Wide Field Camera (WFC) on the 2.5 m Isaac Newton Telescope. 

Candidates were initially examined by taking short exposures in the R band. With good seeing this enables stellar objects to R~23 to be detected. At this depth objects close to or within the Local Group should begin to resolve into stars, with the tip of the giant branch becoming readily visible. If a candidate appeared to resolve into stellar components, further broadband observations in V and I together with narrow-band H-alpha were obtained. The initial exposure of an uncataloged object in the constellation Cetus, at  RA = 00h26m11.0s,  DEC=-11º02´40" (J2000), showed a diffuse swarm of faint stars. Further exposures were taken in order to characterise the new object.

The Cetus dwarf has a smooth, diffuse appearance and appears to be a dwarf spheroidal of type dE3.5. A color-magnitude diagram in V, V-I shows a clear giant branch but no sign of recent star formation. From the position of the tip of the giant branch, a reddening-corrected distance modulus of 24.45 ± 0.15 and a metallicity of -1.9 ± 0.2 is derived. With an implied heliocentric distance of 775 ± 50 kpc, and a corresponding Local Group barycentric distance of 615 kpc, the Cetus dwarf lies well within the boundaries of the Local Group. Although the Cetus dwarf is unlikely to be directly associated with any other Local Group galaxy, it does lie in the general direction of the extension of the Local Group toward the Sculptor Group.

The Isaac Newton Group of Telescopes (ING) is an establishment of the Particle Physics and Astronomy Research Council (PPARC) of the United Kingdom and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands. The ING operates the 4.2 metre William Herschel Telescope, the 2.5 metre Isaac Newton Telescope, and the 1.0 metre Jacobus Kapteyn Telescope. The telescopes are located in the Spanish Observatorio del Roque de Los Muchachos on La Palma which is operated by the Instituto de Astrofísica de Canarias (IAC).

PICTURES

Caption:. Combined V-band image of the Cetus dwarf with a total exposure of 1800 s taken using the Wide Field Camera on the 2.5 m Isaac Newton Telescope. The obvious visible stars in the dwarf galaxy are all red giants. The area shown is approximately 11 × 11 arcminutes corresponding to one-half of the central CCD of the four WFC CCDs.
Picture credit: Alan B. Whiting, George K. T. Hau, Mike Irwin.
Available formats:
GIF (760×760 pixels, 591 K)
LZW compressed TIFF (1,772×1,772 pixels, 4,397 K)

Caption: The 2.5 metre Isaac Newton Telescope.
Picture credit: Nik Szymanek and Ian King.
Available formats:
GIF (600×390 pixels, 107 K)
LZW compressed TIFF (1820×1182 pixels, 6,309 K)

FOR FURTHER INFORMATION PLEASE CONTACT:

Alan B. Whiting
Physics Department, US Naval Academy
Annapolis, MD 21402, USA
E-mail: whiting@nadn.navy.mil

George K. T. Hau
Facultad de Física y Astronomía, Universidad Católica de Chile
Casilla 104, Santiago, 22, Chile
E-mail: ghau@astro.puc.pl

Mike Irwin
Institute of Astronomy
Madingley Road, University of Cambridge, Cambridge, England CB3 0HA
E-mail: mike@ast.cam.ac.uk

Javier Méndez
Public Relations Officer
Isaac Newton Group of Telescopes
E-mail: jma@ing.iac.es
Phone: +34 922 425464, +34 616 464111
Fax: +34 922 425401

More information:

A B Whiting, G K T Hau, M Irwin, 1999, "A New Local Group galaxy in Cetus", Astron J, 118, 2767.
Mike Irwin's web pages on the Cetus Dwarf Galaxy.

More information on ING:

http://www.ing.iac.es/PR/
http://www.ast.cam.ac.uk/ING/PR/