UMa I dSph (Wim van Berlo)

UMa I dSph

UMa I is a dwarf galaxy of the Milky Way, in the constellation Ursa Major. This galaxy was discovered in 2005 by studying images from the Sloan Deep Sky Survey. It is also supposedly the dimmest dwarf galaxy ever found. UMa I is located approximately 330 000 ly from the sun, and has a radius of 11.3 arcminutes. The galaxy consists of very old stars, red giants mostly, created in the early days of the history of the universe. As most other dwarf galaxies surrounding the Milky Way (some 100+ in total), this galaxy is slowly being ripped apart and absorbed.

"This object was detected as an overdensity of red, resolved stars in Sloan Digital Sky Survey data.

The color-magnitude diagram ofthe Ursa Major dwarf looks remarkably similar to that of Sextans,

the lowest surface brightness Milky Way companion known, but with approximately an order of

magnitude fewer stars. Deeper follow-up imaging confirms this object has an old and metal-poor

stellar population and is ~ 100 kpc away.

We roughly estimate MV = -6.75 and r1/2 = 250 pc for this dwarf. Its luminosity is several times

fainter than the faintest known Milky Way dwarf. However, its physical size is typical for dSphs. Even

though its absolute magnitude and size are presently quite uncertain, Ursa Major is likely the lowest

luminosity and lowest surface brightness galaxy yet known."

(From B Willman et al. ApJL, 2005, 626(2) L86 - L 88)

This is one of very few images of this galaxy, and I believe it's a first on Astrobin.

Final
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B

B

Description: in red colour: very few member stars (very faint red giants) of UMa I (only 50 - 60 are catalogued to date)
in yellow colour:marked V1 and V5, two RR-Lyrae variables in UMa I
in gold colour: two quasars
in pink colour: several PGC/LEDA galaxies at distances up to 500 Mly
in gray colour: a galaxy cluster which I haven't been able to identify yet. Simbad reports a cluster ([SPD2011] 47308) in this area that is 6 600 Mly distant, but I very much doubt that this is it.

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UMa I dSph

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