Well, it may not make it into the news, but it seems worthy of note to me. Two new papers showed up on the preprint list today, both submitted to the Astrophysical Journal Letters, and announcing the discovery of two new Milky Way dwarf satellite galaxies. Both were discovered as overdensities of stars in the Sloan Digital Sky Survey, and I suspect there will be more to follow as time goes on. And note, for those who don't already know, the "~" symbol in the abstracts means "approximately" or "approximately equal to".

A New Milky Way Dwarf Satellite in Canes Venatici
Abstract: In this Letter, we announce the discovery of a new dwarf satellite of the Milky Way, located in the constellation Canes Venatici. It was found as a stellar overdensity in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitude diagram shows a well-defined red giant branch, as well as a horizontal branch. As judged from the tip of the red giant branch, it lies at a distance of ~220 kpc. Based on the SDSS data, we estimate an absolute magnitude of M_v ~ -7.9, a central surface brightness of mu_0,V ~ 28 mag arcsecond^-2, and a half-light radius of ~8.5' (~550 pc at the measured distance). The outer regions of Canes Venatici appear extended and distorted. The discovery of such a faint galaxy in proximity to the Milky Way strongly suggests that more such objects remain to be found.

A Faint New Milky Way Satellite in Bootes
Abstract: In this Letter, we announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 60 kpc. It was found in a systematic search for stellar overdensities in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram shows a well-defined turn-off, red giant branch, and extended horizontal branch. Its absolute magnitude is -5.7, which makes it fainter than the faintest galaxy known. The half-light radius is 220 pc. The isodensity contours are elongated and have an irregular shape.

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Thanks Tim,

It's interesting to keep finding more about the galactic companions. I'm sure that eventually we'll learn whether these were originally indepentant mini-galaxies, or the evolved debris from a previous interaction with M31, or other such galaxy.

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Thank you Tim Thompson!

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This seems like the place to ask:
Several years ago I read a mention (probably in Sky & Telescope) about what was believed to be a severely disrupted former globular cluster that had been stretch or smeared into a trail of stars, and that this trail was orbiting the Milky Way, possibly intersecting the halo and into a spiral arm. Sorry to be so vague. Does anybody recall this? I searched and found nothing so far.

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Nice find, thanks!

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Sky and Telescope, volume 105, number 4, page 22 (April 2003).
You are remembering the Monoceros Ring ...

One ring to encompass them all: a giant stellar structure that surrounds the Galaxy
R.A. Ibata, et al., Monthly Notice of the Royal Astronomical Society 340(3): L21-L27
Abstract: We present evidence that the curious stellar population found by the Sloan Digital Sky Survey in the Galactic anticentre direction extends to other distant fields that skirt the plane of the Milky Way. New data, taken with the Isaac Newton Telescope Wide Field Camera, show a similar population, narrowly aligned along the line of sight, but with a galactocentric distance that changes from ~15 to ~20 kpc (over ~100° on the sky). Despite being narrowly concentrated along the line of sight, the structure is fairly extended vertically out of the plane of the disc, with a vertical scaleheight of 0.75 +/- 0.04 kpc. This finding suggests that the outer rim of the Galaxy ends in a low surface brightness stellar ring. Presently available data do not allow us to ascertain the origin of the structure. One possibility is that it is the wraith of a satellite galaxy devoured long ago by the Milky Way, although our favoured interpretation is that it is a perturbation of the disc, possibly the result of ancient warps. Assuming that the ring is smooth and axisymmetric, the total stellar mass in the structure may amount to ~2 × 10⁸ M_solar up to~10⁹ M_solar.

Follow the link to the ADS and you can download a PDF of the paper from there (from the Cornell archive). There has been a lot of observational work since then on the Monoceros Ring, which you can follow via the link "Citations to the article", of which there are 74 so far.

It is proposed that the Monoceros Ring really is the tidal debris from a disrupted dwarf galaxy, specifically the Canis Major Dwarf Galaxy, which was discovered after the Monoceros Ring (i.e., Martin, et al., 2005, Conn, et al., 2005). However, the very existence of the Canis Major Dwarf Galaxy is not at all certain. Other astronomers argue that it is only a local overdensity of stars in the natural spiral structure of the Milky Way (i.e., Momany, et al., 2004, Moitinho, et al., 2006, and the debate is in S&T again, in the Oct 2004 issue). But other astronomers argue that the spiral overdensity argument is not very good, and it really is a real dwarf galaxy after all (i.e., Martínez-Delgado, et al., 2005).

So, we know the Monoceros Ring is there, it's an observed fact. But whether or not the Canis Major Dwarf Galaxy is really there, and really being eaten by the Milky Way, is not certain. Maybe the ring is really a gobbled dwarf, or maybe Ibata, et al., were right the first time, and the ring is a consequence of the spiral structure of the Milky Way. The fight is being fought on the field of science even as we speak.

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Very cool observations!

Anyone taken a look at IRAS, radio, ... data to see what else these new dwarfs contain (other than stars)?

Are any of the brightest stars in either on POSS plates? If so, would they be worthy of a study of their proper motions, via differential plate analyses?

Thank you Tim Thompson for a welcomed wealth of information. This will keep me busy for a while!

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Footnote: (older news from 2004 I came across recently.)

There might be an ultra-faint dwarf galaxy or weird globular cluster (termed SDSSJ1049+5103) on the outskirts of the Milky Way in the direction of in Ursa Major. It was detected by Beth Willman of New York University (NYU) and colleagues using the Sloan Digital Sky Survey (SDSS).

http://arxiv.org/abs/astro-ph/0410416

http://cosmo.nyu.edu/~bw427/SDSSJ1049+5103.html

I'm not close to what professional astronomers are doing but I reckon they're still studying this.

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Two new, very faint companion galaxies to the Milky Way has been announced by The Sloan Digital Sky Survey (SDSS-II).

Images of Canes Venatici clump found by SDSS-II researcher Daniel Zucker at Cambridge University (UK), and the Bootes clump found by his colleague Vasily Belokurov can be found HERE.

A large image of newly found star streams can be found HERE.

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The fork in the Sagittarius stream of stars suggests the Milky Way's dark matter halo is spherical. Astronomers are still trying to identify the source of the newly discovered Orphan stream


The "Field of Streams"
This image is a map of stars in the outer regions of the Milky Way covering about one-quarter of the night sky, as observed by the Sloan Digital Sky Survey (SDSS-II). The trails and streams that cross the image are stars torn from disrupted Milky Way satellites. The colour corresponds to distance, with red being the most distant and blue being the closest. The large, forked feature is the Sagittarius stream, further away from us (lower left) and closer to us (middle right). Other features marked are the Monoceros ring and the as yet unidentified stream.
^{Credit: Vasily Belokurov, The SDSS-II Collaboration}

Read more

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Two more spheroidal dwarf galaxy companions to the Milky Way have been discovered by the SDSS, bringing the total to at least 14.

http://www.cnn.com/2006/TECH/space/0...eut/index.html

Dave Mitsky

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Pictures here.

Is this the tip of the iceberg in terms of little companions, or are these some of the last few to be discovered? Would these microgalaxies be visible if they were orbiting M31?

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Threads merged.

That's one of the key questions the SDSS II work is setting out to answer!Well, the brigthtest of the individual stars would likely be detected by deep HST images (I think - I've not done the calculations), but I don't know whether the areal coverage would be great enough to tease out overdensities (of course, if radial velocities were measured, via spectra, such microgalaxies would likely be detected).

According to the galaxy formation models, there should be as many as hundreds of tiny galaxies around the large ones like the Milky Way.

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Very interesting discovery. It looks like there are more galaxies orbiting the Milky Way waiting for discovery.

However, what are the differences between such small galaxies and globular clusters of the same mass/number of stars? Different stellar populations? Dust?

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Title: Detection of a New, 60 Degree-Long Dwarf Galaxy Debris Stream
Authors: C. J. Grillmair

Researchers report on the discovery in Sloan Digital Sky Survey data of a 60 degree-long stream of stars, extending from Ursa Major to Sextans. The stream is approximately 2 degrees wide and is clearly distinct from the northern tidal arm of the Sagittarius dwarf galaxy.
The apparent width of the stream suggests a progenitor with a size and mass similar to that of a dwarf galaxy. The stream is about 21 kpc distant and appears to be oriented almost perpendicular to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies, though they cannot rule out that the stream may form the inner part of a known dwarf galaxy's orbit.
The most likely explanation is that the stream constitutes the remains a dwarf galaxy that has been completely disrupted at some point in the past. The researchers also briefly report on the discovery of a diminutive Galactic satellite which lies near the projected path of the new stream, but is unlikely to be related to it.

The colour-magnitude distribution of stars in the stream closely matches that of the globular cluster M 13, indicating that the stars making up the stream are old and metal poor.

Read more (628kb, PDF)

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yes, and this is not the only challenge facing current models.

here is a site that shows, what are IMHO, much more realistic #'s.


http://anzwers.org/free/universe/universe.html

Enjoy..................................

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RussT
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