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Catch of the day: UMa I dSph


wimvb

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While waiting for my main target to reach its best position, I decided to test my rig on a faint target.

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)

I couldn't find any amateur images of this galaxy online, understandably. The data for this image was collected 27 March 2020. Setup details are in my signature. Total integration time was only 1 hour (30 x 120 s exposures) using an L filter, calibrated and stacked and annotated in PixInsight.

UMa_1_30_L_annot.thumb.png.7479756d6ccb4e0aa1c26e8760d0fed6.png

What is in this image:

  • Most of the stars are Milky Way stars
  • 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 PCG 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.

An inverted image, showing the extent of the dwarf galaxy

UMaI_inverted.png.cebc464955858cb9b0b5e6897d672911.png

Edited by wimvb
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Wow, that's a subtle one. The labeled image is very informative, thanks! Amazing to imagine how many more of these elusive dwarf galaxies are still to be discovered. I suppose analysis of GAIA data will reveal more members of UMa I?

Edited by Waddensky
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25 minutes ago, wimvb said:

Vizier & Simbad, so far. I haven't really looked at GAIA yet. 

I understand that. I meant science in general, new datasets like GAIA will provide much more data on the structure and evolution of these dwarf galaxies, I assume.

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6 minutes ago, gorann said:

So a dying galaxy. Very intresting even if it must be the least impressive galaxy around😉

Beauty is in the eyes of the beholder. There is some very interesting physics going on here.

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19 hours ago, Owmuchonomy said:

Excellent post. Do you know the distance (or z) of the two quasars?

J103559.74+520235.9: z=0.741221

J103600.4+520218: z=0.66256

Z for QSO's doesn't always translate directly to distance, as is the case with galaxies. I've seen QSO z values in excess of 5.

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