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RE: Milky Way Companions
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Four new satellite galaxies of the Milky Way have been discovered, bringing the total known to about 20. The pace of new discoveries suggests that many more such satellites remain unknown, which would present a serious challenge to models of dark matter as "warm", fast-moving particles.

The satellites are dwarf galaxies a few hundred to a few thousand light years across. The tiny galaxies are thought to be the building blocks of large galaxies, such as our own Milky Way which is about 100,000 light years wide.
The four new discoveries were made by a team led by Vasily Belokurov of the University of Cambridge, UK. Named after the constellations in which they were found Coma Berenices, Canes Venatici II, Hercules, and Leo IV, all of them lie between roughly 100,000 and 500,000 light years from Earth.
The largest and smallest are Hercules and Coma Berenices, which are about 1000 and 200 light years across, respectively. Like most of the other dwarfs discovered by SDSS, the new finds are much smaller and fainter than the 10 dwarfs that were known previously.

"They should not really be called dwarfs they are more like hobbits" - Vasily Belokurov.

Based on the number of dwarf galaxies SDSS has found so far, the researchers estimate that there may be a total of about 50 small satellites around the Milky Way.

Astronomers try to study them to understand more about how this merger process took place. Their exact numbers could also tell astronomers how easily dark matter forms clumps, which might offer clues about the nature of this mysterious substance, whose presence is detected only by its gravitational effect on ordinary matter and light.
But the small galaxies are faint and difficult to observe they tend to get lost behind the throngs of foreground stars from the Milky Way. Just two years ago, only 10 satellite galaxies were known, with the exact classification of some objects in dispute.

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Sagittarius Stream
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Title: Detection of a population gradient in the Sagittarius Stream
Authors: M. Bellazzini (INAF - OA Bologna), H.J. Newberg (Rennselaer Inst.), M. Correnti (INAF - OA Bologna), F.R. Ferraro (Bologna Univ.), L. Monaco (ESO)

We present a quantitative comparison between the Horizontal Branch morphology in the core of the Sagittarius dwarf spheroidal galaxy (Sgr) and in a wide field sampling a portion of its tidal stream (Sgr Stream), located tens of kpc away from the centre of the parent galaxy. We find that the Blue Horizontal Branch (BHB) stars in that part of the Stream are five times more abundant than in the Sgr core, relative to Red Clump stars. The difference in the ratio of BHB to RC stars between the two fields is significant at the 4.8 sigma level. This indicates that the old and metal-poor population of Sgr was preferentially stripped from the galaxy in past peri-Galactic passages with respect to the intermediate-age metal rich population that presently dominates the bound core of Sgr, probably due to a strong radial gradient that was settled within the galaxy before its disruption. The technique adopted in the present study allows to trace population gradients along the whole extension of the Stream.

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Title: The Dwarf Satellites of M31 and the Galaxy
Authors: Sidney van den Bergh

The satellite systems of M31 and the Galaxy are compared. It is noted that all five of the suspected stripped dSph cores of M31 companions are located within a projected distance of 40 kpc of from the nucleus of this galaxy, whereas the normal dSph companions to this object have distances > 40 kpc from the center of M31. All companions within 200 kpc < D(M31) < 600 kpc are late-type objects. The companions to the Galaxy appear to exhibit different systematics with the irregular LMC and SMC being located at small R_{gc}. It is speculated that this difference might be accounted for by assuming that the Magellanic Clouds are interlopers that were originally formed in the outer reaches of the Local Group. The radial distribution of the total sample of 40 companions of M31 and the Galaxy, which is shown in Figure 1, may hint at the possibility that these objects contain distinct populations of core (R < 25 kpc) and halo (R > 25 kpc) satellites.)

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Dwarf companions to M31

B327 GC 00h 41m 35s +41o 1455 785 -11.7 3 ...
M32 E2,N 00 42 42 +40 51 55 785 -16.5 6 -1.1
Hux C1 GC 00 38 20 +41 47 15 785 -7.1 13 ...
Hux C3 GC 00 38 05 +40 44 39 785 -7.1 14 ...
G1 GC 00 32 47 +39 34 40 785 -10.9 35 -1.0
Hux C2 GC 00 42 55 +43 57 28 785 -7.7 37 ...
NGC 205 E5pec 00 40 22 +41 41 07 824 -16.4 40 -0.5
And IX dSph 00 52 53 +43 12 00 765 -8.3 42 -2.2
And I dSph 00 45 40 +38 02 28 745 -11.8 59 -1.4
And III dSph 00 35 34 +36 29 52 749 -10.2 76 -1.7
And V dSph 01 10 16 +47 37 52 774 -9.1 110 -1.9
And X dSph 01 06 34 +44 48 16 783 -8.1 112 -2.0
NGC 147 Sph 00 33 12 +48 30 32 675 -15.1 145 -1.1
And II dSph 01 16 30 +33 25 09 652 -11.8 185 -1.5
NGC 185 Sph 00 38 58 +48 20 15 616 -15.6 190 -0.8
M33 Sc 01 33 51 +30 39 37 809 -18.9 208 -0.3
And VII dSph 23 26 31 +50 41 31 763 -12.0 219 -1.5
IC 10 Ir 00 20 17 +59 18 14 825 -16.0 260 -1.3
And VI dSph 23 51 47 +24 34 57 783 -11.3 269 -1.7
Pisces dIr/Sph 01 03 53 +21 53 05 769 -9.8 269 -1.7
Pegasus Ir(?) 23 28 36 +14 44 35 919 -12.3 474 -1.5
IC 1613 Ir 01 04 47 +02 08 14 700 -15.3 508 -1.3



Companions to the Galaxy.



N 5139 GC 13 26 46 -47 28 37 06 -10.3 -1.6
Sgr dSph 18 55 03 -30 28 42 19 -15.0 -0.5
LMC Ir 05 23 35 -69 45 22 50 -18.5 -0.6
SMC Ir 00 52 49 -72 49 43 63 -17.1 -1.2
UMi dSph 15 09 10 +67 12 52 69 -8.9 -1.9
Dra dSph 17 20 12 +57 54 55 79 -9.4 -2.0
Sex dSph 10 13 03 -01 36 53 86 -9.5 -1.9
Scl dSph 01 00 09 -33 42 33 88 -9.8 -1.5
N 2419 GC 07 38 08 +38 52 55 92 -9.6 -2.1
Car dSph 06 41 37 -50 57 58 94 -9.4 -1.8
UMa dSph 10 34 53 +51 55 12 105 -6.8 -2
For dSph 02 39 59 -34 26 57 138 -13.1 -1.2
Leo II dSph 11 13 29 +22 9 17 205 -10.1 -1.6
Leo I dSph 10 08 27 +12 18 27 270 -11.9 -1.4
Phe dIr/dSph 01 51 06 -44 26 41 405 -9.8 -1.9
NGC 6822 Ir 19 44 56 -14 52 11 500 -16.0 -1.2



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Title: A Faint New Milky Way Satellite in Bootes
Authors: V. Belokurov (1), D. B. Zucker (1), N. W. Evans (1), M. I. Wilkinson (1), M. J. Irwin (1), S. Hodgkin (1), D. M. Bramich (1), J. M. Irwin (1), G. Gilmore (1), B. Willman (2), S. Vidrih (1), H. J. Newberg (3), R. F. G. Wyse (4), M. Fellhauer (1), P. C. Hewett (1), N. Cole (3), E. F. Bell (5), T. C. Beers (6), C. M. Rockosi (7), B. Yanny (8), E. K. Grebel (9), D. P. Schneider (10), R. Lupton (11), J. C. Barentine (12), H. Brewington (12), J. Brinkmann (12), M. Harvanek (12), S. J.Kleinman (12), J. Krzesinski (12,13), D. Long (12), A. Nitta (12), J. A. Smith (14), S. A. Snedden (12) ((1) Cambridge University, (2) New York University, (3) Rensselaer Polytechnical Institute, (4) JHU, (5) MPIA, Heidelberg, (6) Michigan State University, (7) Lick Observatory, UCSC, (8) FNAL, (9) University of Basel, (10) Pennsylvania State University, (11) Princeton University, (12) Apache Point Observatory, (13) Cracow Pedagogical University, (14) LANL)
UPDATE

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 colour-magnitude diagram shows a well-defined turn-off, red giant branch, and extended horizontal branch. Its absolute magnitude is -5.8, which makes it one of the faintest galaxies known. The half-light radius is 220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.

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Ursa Major II
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Title: A Curious New Milky Way Satellite in Ursa Major
Authors: D.B. Zucker (1), V. Belokurov (1), N.W. Evans (1), M.J. Irwin (1), J.T. Kleyna (2), M.I. Wilkinson (1), M. Fellhauer (1), D.M. Bramich (1), G. Gilmore (1), H.J. Newberg (3), B. Yanny (4), J.A. Smith (5,6), P.C. Hewett (1), E.F. Bell (7), H.-W. Rix (7), O.Y. Gnedin (8), S. Vidrih (1), R.F.G. Wyse (9), B. Willman (10), E.K. Grebel (11), D.P. Schneider (12), T.C. Beers (13), A.Y. Kniazev (7,14), J.C. Barentine (15), H. Brewington (15), J. Brinkmann (15), M. Harvanek (15), S.J. Kleinman (16), J. Krzesinski (15,17) D. Long (15), A. Nitta (18), S.A. Snedden (15) ((1) Cambridge, (2) Hawaii, (3) RPI, (4) FNAL, (5) LANL, (6) Austin Peay State University, (7) MPIA, (8) OSU, (9) JHU, (10) NYU, (11) Basel, (12) Penn State University, (13) Michigan State University, (14) SAAO, (15) Apache Point, (16) Subaru Telescope, (17) Cracow, (18) Gemini Observatory)

In this Letter, rResearcherse study a localised stellar overdensity in the constellation of Ursa Major, first identified in the Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. Its colour-magnitude diagram shows a well-defined sub-giant branch, main sequence and turn-off, from which they estimate a distance of about 30 kpc and a projected size of about 250 pc.
Based on its extent and its stellar population, they argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named after its constellation as Ursa Major II (UMa II). Using SDSS data, they find an absolute magnitude of M_V = -3.8, which would make it the faintest known satellite galaxy. UMa II's isophotes are irregular and distorted with evidence for multiple concentrations; this suggests that the satellite may be in the process of disruption.

Ursa Major II
The UMa II Dwarf as seen by SDSS: Upper left: Combined SDSS g, r, i images of a 1.2. x 1.2. field centred on the overdensity (J2000 08:51:30 +63:07:48).

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RE: Milky Way Companions
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A long, slender stream of ancient stars racing across the northern sky.

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Title: Sub-Structures in the Halo of the Milky Way
Authors: A. Katherina Vivas (CIDA), R. Zinn (Yale), Y. Subero (CIDA), J. Hernandez (CIDA)

The latest results of the QUEST survey for RR Lyrae stars are described. This survey, which is designed to find and characterise sub-structures in the halo of our Galaxy, has covered about 700 square degrees of the sky and has detected 693 RR Lyrae stars, most of which are new discoveries.
The spatial distribution of the RR Lyrae stars reveals several interesting groups in the halo. Some of them appear to be related to previous detections of the destruction of dwarf spheroidal galaxies (the Sagittarius tidal streams, the Virgo stellar stream, the Monoceros ring) or globular clusters (Palomar 5), while others still have an unknown origin.

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Sagittarius Debris Stream
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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.

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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.

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-- Edited by Blobrana at 17:40, 2006-05-09

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A new map of stars in the Milky Way Galaxy, constructed with data from the Sloan Digital Sky Survey (SDSS-II), reveals a night sky criss-crossed with streams of stars, left behind by satellite galaxies and star clusters spiraling to their deaths.

Analysing five years of data spanning nearly one-quarter of the sky, Cambridge University (UK) researchers Vasily Belokurov and Daniel Zucker created a dramatic new image of the outer Milky Way, using stellar colors eliminating the redder, nearby stars that would otherwise swamp the view of background structures. They found so many trails of stars in their high contrast image that they named the area the "Field of Streams."
Satellite galaxies orbiting the Milky Way are literally ripped apart by the tidal forces of our galaxy. As these satellites sink in gravitational quicksand, their stars are torn from them in giant streams that trace their orbital paths -- just like meteor streams lie along the paths of defunct comets in the Solar system.


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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


Dominating the Field of Streams image is the enormous, arching stream of the Sagittarius dwarf galaxy. The Sagittarius dwarf was discovered more than a decade ago and other researchers have previously mapped its long tidal stream in other regions of the sky.
But the new SDSS-II data had a remarkable surprise in store.

"The stream appears forked. We are seeing different wraps superimposed on the sky, as the stream goes around the galaxy two or three times" - Vasily Belokurov.

Because of the multiple wraps, the observations provide strong new constraints on the dark matter halo of the Milky Way.

"The leading theories of dark matter predict that the Galaxy's halo should be flattened, like a rugby football. But our simulations only match the forked Sagittarius stream if the inner halo is round, like a soccer ball" - Mike Fellhauer, Cambridge University.

In addition to the Sagittarius arches, the Field shows faint trails of stars torn from globular clusters, and other rings, trails, and lumps that appear to be the remains of disrupted dwarf galaxies.
Prominent among these is the Monoceros stream, discovered previously by SDSS-II scientists Heidi Jo Newberg of Rensselaer Polytechnic Institute and Brian Yanny of the Fermi National Accelerator Laboratory. The multiple rings of stars are all that remain from a dwarf satellite that was absorbed by the Milky Way long ago.
Crossing the Field is an enigmatic, new stream of stars extending over 70 degrees on the sky, whose original source remains unknown.

"Some of these 'murdered' galaxies have been named; but this galactic corpse hasn't been identified yet. We're looking for it right now" - Wyn Evans, Cambridge, SDSS-II team member.

These new discoveries add weight to a picture in which galaxies like the Milky Way are built up from the merging and accretion of smaller galaxies.

"We've known about merging for some time; but the Field of Streams gives us a striking demonstration of multiple merger events going on the Milky Way galaxy right now. This is happening all over the Universe, as big galaxies grow by tearing up smaller ones into streams" - Brian Yanny.

These streams also provide new tests of the nature of dark matter itself, according to theorist James Bullock of University of California at Irvine; Bullock was not part of the SDSS team.

"The fact that we can see a 'Field of Streams' like this suggests that dark matter particles are very 'cold', or slow moving. If the dark matter was made up of 'warm,' fast moving particles, we wouldn't expect these thin streams to hang around long enough for us to find them" - James Bullock.

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