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Post Info TOPIC: Milky Way Companions


Posts: 131433
RE: Milky Way Companions

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

The first was found in the direction of the constellation Canes Venatici (the Hunting Dog) by SDSS-II researcher Daniel Zucker at Cambridge University (UK). His colleague Vasily Belokurov discovered the second in the constellation Bootes (the Herdsman).

"I was poring over the survey's map of distant stars in the Northern Galactic sky - what we call a Field of Streams -- and noticed an overdensity in Canes Venatici. Looking further, it proved to be a previously unknown dwarf galaxy. It's about 640,000 light years (200 kiloparsecs) from the Sun. This makes it one of the most remote of the Milky Way's companion galaxies" - Daniel Zucker.

Zucker emailed Belokurov with the news, and, just as discoveries often build upon one another, Belokurov excitedly emailed back a few hours later with the discovery of a new, even fainter dwarf galaxy. The new galaxy in Bootes, which Belokurov called 'Boo,' shows a distorted structure that suggests it is being disrupted by the Milky Way's gravitational tides.

"Something really bashed Boo about" - Vasily Belokurov .

Credit: Vasily Belokurov, Cambridge University, The Sloan Digital Sky Survey-II collaboration

Although the dwarf galaxies are in our own cosmic backyard, they are hard to discover because they are so dim. In fact, the new galaxy in Bootes is the faintest galaxy so far discovered, with a total luminosity of only about 100,000 Suns. But because of its distance (640,000 light years) it appears almost invisible to most telescopes. The previous dimness record holder was discovered last year in Ursa Major using SDSS-II data.
New galactic neighbours are exciting in their own right, but the stakes in searches for ultra-faint dwarfs are especially high because of a long-standing conflict between theory and observations. The leading theory of galaxy formation predicts that hundreds of clumps of "cold dark matter" should be orbiting the Milky Way, each one massive enough in principle to host a visible dwarf galaxy. But only about ten dwarf companions have been found to date.
One possibility is that the galaxies in the smaller dark matter clumps are too faint to have appeared in previous searches, but might be detectable in deep surveys like SDSS-II.

"It's like panning for gold. Our view of the sky is enormous, and we're looking for very small clumps of stars" - Wyn Evans, Cambridge University astronomer , a member of the SDSS-II research team.

"Finding and studying these small galaxies is really important. From their structure and their motions, we can learn about the properties of dark matter, as well as measure the mass and the gravity field of the Milky Way" - Mark Wilkinson, collaborator.

The new discoveries are part of the SEGUE project (Sloan Extension for Galactic Understanding and Exploration), one of the three component surveys of SDSS-II. SEGUE will probe the structure and stellar make-up of the Milky Way Galaxy in unprecedented detail.

"I'm confident there are more dwarf galaxies out there and SEGUE will find them" - Heidi Newberg of Rensselaer Polytechnic Institute, co-chair of SEGUE.

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Posts: 131433
Bootes Dwarf

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)

Astronomers have announced 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 -5.7, 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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Posts: 131433
Canes Venatici Dwarf

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

Astronomers have announced 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 colour-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, they estimate an absolute magnitude of Mv ~ -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.

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Posts: 131433
Missing Dwarf Galaxies

A team of astronomers from the University of Pittsburgh and the Universitäts-Sternwarte München in Munich, Germany, announced today in a paper presented at the meeting of the American Astronomical Society in Washington, D.C., that their search for dwarf galaxies in fast-moving clouds of gas has yielded no results, leading them to suggest alternative avenues of research to find the supposedly “missing” galaxies.

The team, which includes Regina Schulte-Ladbeck, associate dean for undergraduate studies and professor of physics and astronomy in Pitt's School of Arts and Sciences, and Ulrich Hopp of the Universitäts-Sternwarte München, has been searching for stars in high-velocity clouds.

Our searches have come up empty” - Schulte-Ladbeck.

The mathematical simulations that astronomers use to establish how galaxies were formed predict that every giant galaxy should have a few hundred “dwarf” galaxy companions. But in our own neighbourhood, the Milky Way Galaxy, there are only 50 or so such dwarves.

One simple way to explain the difference would be if the missing dwarf galaxies were located in high-velocity clouds, astronomer Leo Blitz of the University of California, Berkeley, and his colleagues had suggested. Schulte-Ladbeck and Hopp hoped to measure the distances between the clouds and the Milky Way to obtain proof that the clouds indeed held additional satellite galaxies of our Milky Way.

To search for stars in the clouds, the researchers took a two-pronged approach. First, they used the Two Micron All Sky Survey, a survey conducted by the University of Massachusetts and funded primarily by NASA and the National Science Foundation, to look for bright stars in circular patches of sky two degrees across, the area typically covered by the gas clouds that make the most promising dwarf galaxy candidates.

Second, using accurate positions of where most of the hydrogen gas in several clouds is located-supplied to them by radio astronomer Jürgen Kerp of the University of Bonn-the researchers also trained one of the 8-meter telescopes of the European Southern Observatory's Very Large Telescope, located in northern Chile's Atacama Desert, on small regions within the clouds to search if any faint stars had formed there. However, neither of these methods turned up any stars.

In their paper, Schulte-Ladbeck and Hopp conclude that it is unlikely that hundreds of additional dwarf satellites of the Milky Way have been somehow “hiding” from observers, and they encourage astronomers to pursue other solutions to the discrepancy.




Posts: 131433
RE: Milky Way Companions

The Sloan Digital Sky Survey Reveals A New Milky Way Neighbour

A huge but very faint structure, containing hundreds of thousands of stars spread over an area nearly 5,000 times the size of a full moon, has been discovered and mapped by astronomers of the Sloan Digital Sky Survey (SDSS-II).

At an estimated distance of 30,000 light years (10 kiloparsecs) from Earth, the structure lies well within the confines of the Milky Way Galaxy. However, it does not follow any of Milky Way's three main components: a flattened disk of stars in which the sun resides, a bulge of stars at the centre of the Galaxy and an extended, roughly spherical, stellar halo. Instead, the researchers believe that the most likely interpretation of the new structure is a dwarf galaxy that is merging into the Milky Way.

The new dwarf galaxy is found toward the constellation Virgo.

"Some of the stars in this Milky Way companion have been seen with telescopes for centuries. But because the galaxy is so close, its stars are spread over a huge swath of the sky, and they always used to be lost in the sea of more numerous Milky Way stars. This galaxy is so big, we couldn't see it before'" - Princeton University graduate student Mario Juric, principal author of the findings describing what may well be our closest galactic neighbour.

The result was presented today in a session on The Milky Way at the American Astronomical Society meeting in Washington, D.C.
The discovery was made possible by the unprecedented depth and photometric accuracy of the SDSS, which to date has imaged roughly one-quarter of the northern sky.

"We used the SDSS data to measure distances to 48 million stars and build a 3-d map of the Milky Way" - Zeljko Ivezic of the University of Washington, a co-author of the study.

Details of this "photometric parallax" method use the colours and apparent brightnesses of stars to infer their distances.

"It's like looking at the Milky Way with a pair of 3-d glasses. This structure that used to be lost in the background suddenly snapped into view" - Princeton University co-author Robert Lupton.

The new result is reminiscent of the 1994 discovery of the Sagittarius dwarf galaxy, by Rodrigo Ibata and collaborators from Cambridge University. They used photographic images of the sky to identify an excess of stars on the far side of the Milky Way, some 75,000 light years from Earth. The Sagittarius dwarf is slowly dissolving, trailing streams of stars behind it as it orbits the Milky Way and sinks into the Galactic disk.

In the last decade a new generation of sky surveys using large digital cameras identified a number of streams and lumps of stars in the outer Milky Way. Some of these lumps are probably new Milky Way companions; others may be shreds of the Sagittarius dwarf or of other dissolving dwarf galaxies. Earlier SDSS discoveries include an apparent ring of stars encircling the Milky Way disk that may be the remnant of another disrupted galaxy; and the Ursa Major dwarf, the faintest known neighbour of the Milky Way.
The first hints of an unusually high density of stars in the direction of Virgo were made in 2001 by the QUEST survey, which used a 1-meter telescope in Venezuela to study a class of variable stars called RR Lyrae variables.

"We found a clump of 5 RR Lyrae stars, and speculated that they were they belonged to a small galaxy being cannibalised by the Milky Way. In light of the new SDSS results, it appears that the stellar stream we detected is itself part of the larger structure identified by Juric and collaborators" - Kathy Vivas, Centro de Investigaciones de Astronomia in Venezuela, who (as a Yale graduate student) was the author of the QUEST discovery paper.

In the January 10, 2006 Astrophysical Journal Letter (astro-ph/0510589), the QUEST team led by Universidad de Chile graduate student Sonia Duffau presented further evidence for this interpretation by measuring the motions and chemical compositions of stars in the region.

"With so much irregular structure in the outer Galaxy, it looks as though the Milky Way is still growing, by cannibalising smaller galaxies that fall into it" - Mario Juric.

Expand (419kb, 804 x 841)
The figure by Mario Juric of Princeton university shows the counts of faint blue stars selected from a narrow magnitude and colour range, and corresponding to a distance from Earth of about 10 kpc. The map shows the view from far away above the galactic plane.
According to the standard Milky Way models, the top and bottom halves should be symmetric with respect to the horizontal line in the middle, because the Milky Way is believed to be symmetric around its rotation axis that passes through the Galactic Centre and is perpendicular to this map.
The discovery of the large over density of stars at the longitudes of about 300 degrees and latitudes of about 60 degrees breaks the expected symmetry and is at odds with the standard models.
The dashed line shows the position of the plane containing debris from the Sagittarius dwarf galaxy that is being cannibalised by the Milky Way. While its proximity to the new structure suggests that perhaps the two may be related, the known Sagittarius debris is located about four times further away.
(The map is based on upcoming SDSS-II Data Release 5, and is shown in Lambert projection of galactic coordinates: radial rays are lines of constant longitude, circles are lines of constant latitude, the North
Galactic Pole is in the centre, and the Galactic Centre is towards the left. The counts are shown on a logarithmic stretch, with a dynamic range of 10 increasing from blue to red).
CREDIT - M. Juric/SDSS-II Collaboration

Another group of SDSS astronomers used the data to find the two faintest-known companions of the Andromeda Galaxy, the closest giant spiral galaxy similar in size to the Milky Way. Daniel Zucker of the Max Planck Institute of Astronomy in Heidelberg and Cambridge University's Institute of Astronomy led that team.

"These new Andromeda companions, alongside the new Milky Way neighbours, suggest that faint satellite galaxies may be plentiful in the Local Group" - Mario Juric .

While the SDSS was originally designed to study the distant universe, its wide area, high precision maps of faint stars have made it an invaluable tool for studying the Milky Way and its immediate neighbourhood. The 3-d map created by Juric and his collaborators also provides strong new constraints on the shape and extent of the Milky Way's disk and stellar halo.
Another Princeton graduate student, Nick Bond, is using the subtle motions of stars detected during the five-year span of the SDSS-I observations to limit the amount of dark matter in the solar neighbourhood.
University of Washington graduate student Jillian Meyer is mapping the distribution of interstellar dust, by carefully studying the colours of stars found in both the SDSS and the infrared 2MASS survey.

Building on these many successes, the SEGUE project (Sloan Extension for Galactic Understanding and Exploration) will use the SDSS-II telescope, its 120-megapixel digital camera and its 640-fiber optical spectrograph to carry out detailed studies of the structure and chemical evolution of the Milky Way.
SEGUE is one of three components of SDSS-II, the three-year extension of the Sloan Survey continuing through mid-2008.
Fermilab scientist Brian Yanny, one of the SEGUE team leaders, is excited at the prospect of examining its just completed, first season of observations.

"The SDSS has already told us surprising things about the Milky Way, but the most exciting discoveries should lie just ahead"

Mario Juric Princeton University Observatory, Princeton, NJ 08544
Zeljko Ivezi University of Washington, Dept. of Astronomy, Box 351580, Seattle, WA 98195
Alyson Brooks, University of Washington
Robert H. Lupton, Princeton University Observatory
David Schlegel, Princeton University Observatory
Douglas Finkbeiner, Princeton University Observatory
Nikhil Padmanabhan Princeton University, Dept. of Physics, Princeton, NJ 08544
Nicholas Bond, Princeton University Observatory
Constance M. Rockosi, University of Washington
Gillian R. Knapp, Princeton University Observatory
James E. Gunn, Princeton University Observatory
Takahiro Sumi, Princeton University Observatory
Donald Schneider Department of Astronomy and Astrophysics, Pennsylvania State University,
University Park, PA 16802
J.C. Barentine Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349, U.S.A.
Howard J. Brewington, Apache Point Observatory
J. Brinkmann, Apache Point Observatory
Masataka Fukugita 6 University of Tokyo, Institute for Cosmic Ray Research
Michael Harvanek, Apache Point Observatory
S.J. Kleinman, Apache Point Observatory
Jurek Krzesinski Apache Point Observatory and Mt. Suhora Observatory, Cracow Pedagogical University, ul. Podchorazych 2, 30-084 Cracow, Poland
Dan Long, Apache Point Observatory
Eric H. Neilsen, Jr. Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510, U.S.A.
Atsuko Nitta, Apache Point Observatory
Stephanie A. Snedden, Apache Point Observatory
Donald G. York, Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60037 USA

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Posts: 131433

Title: Two New Milky Way Companions
Beth Willman

The researchers discuss the detection limits and current status of a uniform survey of
SDSS I for ultra-faint Milky Way dwarf galaxies; and present the properties of
two new, low surface brightness Milky Way companions discovered as a result of
this survey.
One of these companions is the Ursa Major dwarf, the newest dwarf spheroidal companion to the Milky Way and the lowest luminosity galaxy yet known.
Ursa Major is about 100 kpc away and is similar to Sextans, but with roughly an order of magnitude fewer stars. The other companion, SDSSJ1049+5103,
lies about 50 kpc away. Its stellar distribution suggests that it may be undergoing tidal stripping. This companion is extremely faint, but has a large half-light size for its luminosity. It is therefore unclear whether it is a globular cluster or a dwarf galaxy.

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