The Cetus Dwarf Galaxy

The Cetus Dwarf Galaxy On the original sky survey plate Cetus looks like a fuzzy low surface brightness patch some 4mm (= 4 arcmin) in diameter. This appearance is characteristic of potential new Local Group galaxies and a whole host of interlopers such as: planetary nebulae, Galactic reflection nebulosity and distant low surface brightness galaxies.

The Cetus dwarf galaxy 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. Multicolour photometry reveals no obvious star forming regions and there are no young hot blue stars apparent in any of the deep CCD images taken. The distribution of the colours and magnitudes of stars visible is typical of the Milky Way satellite dwarf spheroidals and comparison with these and other nearby galaxies lead us to conclude that the distance of Cetus is close to 800 Kpc. It is most similar in appearance to the Tucana dwarf, the most isolated Local Group dwarf spheroidal galaxy known. At a distance of 800 Kpc the apparent size of Cetus on the deep CCD images suggests that it is only 1-2 kpc in diameter and probably only contains a million or so stars placing it firmly at the faint end of the galaxy luminosity function.

The Cetus 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 two large Local Group galaxies, Cetus will provide interesting constraints 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 with sizeable relative velocities. Knowing their present distances and velocities and the masses of the larger members makes it possible to work backwards and decide how old the Universe is independently of knowledge of Ho.

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