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Post Info TOPIC: Milky Way Companions
Blobrana


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Posts: 131433
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RE: Milky Way Companions
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Title: Are the Magellanic Clouds on their First Passage about the Milky Way?
Authors: Gurtina Besla (1), Nitya Kallivayalil (1), Lars Hernquist (1), Brant Robertson (2,3,4), T.J. Cox (1), Roeland P. van der Marel (5), Charles Al**** (1) ((1) Harvard-Smithsonian CfA, (2) KICP, (3) Enrico Fermi Institute, (4) Spitzer Fellow, (5) STScI)

Recent proper motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil et al (2006a,b) suggest that the 3D velocities of the Clouds are substantially higher (~100 km/s) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modelled as an isothermal sphere to large distances. Here we re-examine the orbital history of the Clouds using the new velocities and a LCDM-motivated MW model with virial mass Mvir = 1e12 Msun (e.g. Klypin et al 2002). We conclude that the Clouds are either currently on their first passage about the MW or, if the MW can be accurately modelled by an isothermal sphere to distances >200 kpc (i.e., Mvir > 2e12 Msun), that their orbital period and apogalacticon distance must be a factor of two larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of HI gas trailing the LMC and SMC that extends roughly 100 degrees across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.

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Blobrana


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Posts: 131433
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Dwarf Galaxies Tucana and LGS3
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Title: Short-Period Variables in the Local Group Dwarf Galaxies Tucana and LGS3
Authors: E. J. Bernard, M. Monelli, C. Gallart, A. Aparicio, G. Bertelli, S. Cassisi, A. A. Cole, P. Demarque, A. E. Dolphin, I. Drozdovsky, H. C. Ferguson, S. Hidalgo, M. Mateo, L. Mayer, J. Navarro, F. Pont, E. D. Skillman, P. B. Stetson, E. Tolstoy (the LCID team)

We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the fraction of the observed field we studied in each galaxy, a total of 133 and 30 variables were found, respectively. For Tucana, we identified 76 of them as RR Lyrae (RRL) stars pulsating in the fundamental mode (RRab) and 32 in the first-overtone mode (RRc), as well as 2 anomalous Cepheids (AC). The mean period of the RRab and RRc is 0.59 and 0.35 days, respectively. In the case of LGS3, we found 24 RRab and 4 RRc, with mean periods 0.61and 0.39 days, respectively, plus two candidate ACs. These values place both galaxies in the Oosterhoff gap.

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Blobrana


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Posts: 131433
Date:
RE: Milky Way Companions
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An international team of scientists, which includes a Penn State astronomer, has discovered seven -- and perhaps eight -- dwarf galaxies orbiting Earth's home galaxy, the Milky Way.
The discovery will be announced today, Jan. 9, during a press conference at the American Astronomical Society's meeting in Seattle. The galaxies were discovered as part of the Sloan Digital Sky Survey (SDSS-II), the most ambitious survey of the sky ever undertaken.

"These dwarf galaxies have been captured by the gravity of the Milky Way and most eventually will merge with our own galaxy. The discovery of these dwarf galaxies demonstrates the unexpected power of large astronomical surveys like the Sloan Digital Sky Survey, which originally was designed to investigate objects millions and billions of light years from Earth, not to discover faint companions of our own galaxy" - Donald Schneider, Penn State professor of astronomy and astrophysics, a coauthor of the investigation.

Schneider is the chairman of the Sloan Digital Sky Survey (SDSS) Quasar Science Group and the SDSS scientific publications coordinator.
The seven new Milky Way satellites all lie in the area of sky surveyed by the SDSS-II around the North Galactic Pole. There are two new dwarfs in the constellation of Canes Venatici (the Hunting Dogs), one in Bootes (the Herdsman), one in Leo (the Lion), one in Coma Berenices (Bernice's Hair), one in Ursa Major (the Great Bear) and one in Hercules.

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Blobrana


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Posts: 131433
Date:
Magellanic Clouds
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Speeding dwarfs upset galactic family picture
The Milky Way’s two best-known companions may be nothing more than strangers passing by. Recent observations of the Magellanic Clouds, a pair of nearby dwarf galaxies, reveal that they are moving too fast to be satellites of the Milky Way – unless our galaxy contains twice as much dark matter as thought.
Astronomers at the Harvard-Smithsonian Centre for Astrophysics (CfA) in Cambridge, Massachusetts, and the Space Telescope Science Institute (STScI) in Baltimore, Maryland, both in the US, compared Hubble Space Telescope images taken two years apart to make the most accurate measurements to date of the velocities of the Magellanic Clouds.
In those two years, both galaxies had shifted a minuscule amount, about one hundredth the width of one pixel in Hubble’s field of view. The astronomers detected these tiny shifts by carefully aligning the clouds in each pair of images and then measuring the apparent change in the positions of several background quasars, bright objects powered by black holes devouring their surroundings.
The astronomers used this information to calculate the clouds’ true 3D velocities through space, not just across our line of sight. This yielded figures of 378 kilometres per second and 302 km/s for the Large and Small Magellanic clouds respectively.

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Blobrana


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Posts: 131433
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Leo T dwarf galaxy
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Title: Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way
Authors: M.J. Irwin (1), V. Belokurov (1), N.W. Evans (1), E.V. Ryan-Weber (1), J.T.A. de Jong (2), S. Koposov (2), D.B. Zucker (1), S.T. Hodgkin (1), G. Gilmore (1), P. Prema (1), L. Hebb (3), A. Begum (1), M. Fellhauer (1), P.C. Hewett (1), R.C. Kennicutt, Jr. (1), M.I. Wilkinson (1), D.M. Bramich (1), S. Vidrih (1), H.-W. Rix (2), T.C. Beers (4), J.C. Barentine (5), H. Brewington (5), M. Harvanek (5), J. Krzesinski (5,6), D. Long (5), A. Nitta (7), S.A. Snedden (5) ((1) Cambridge, (2) MPIA, (3) St Andrews, (4) MSU, (5) APO, (6) Cracow, (7) Gemini)

In this Letter, we announce the discovery of a new dwarf galaxy, Leo T, in the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The colour-magnitude diagram of Leo T shows two well-defined features, which we interpret as a red giant branch and a sequence of young, massive stars. As judged from fits to the colour-magnitude diagram, it lies at a distance of about 420 kpc and has an intermediate-age stellar population with a metallicity of (Fe/H)= -1.6, together with a young population of blue stars of age of 200 Myr. There is a compact cloud of neutral hydrogen with mass roughly 10^5 solar masses and radial velocity 35 km/s coincident with the object visible in the HIPASS channel maps. Leo T is the smallest, lowest luminosity galaxy found to date with recent star-formation. It appears to be a transition object similar to, but much lower luminosity than, the Phoenix dwarf.

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Position(J2000): RA = 09:34:53.4 Dec = +17:03:05

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Blobrana


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Posts: 131433
Date:
The Canes Venatici II galaxy
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A dying galaxy near the Milky Way appears to be sowing the seeds of its own rebirth and may hold the secret to the apparent reincarnation of several other similar galaxies. The phoenix-like process may also help resolve a longstanding mystery about missing dark matter clumps near the Milky Way.
About 20 small galaxies are known to exist around our galaxy. Most fall into two main categories: dwarf spheroidals, which are dead because they lack the gas needed for making new stars, and dwarf irregulars, which have plenty of gas and show signs of ongoing star formation.
Some astronomers have suggested that dwarf galaxies can switch back and forth between the two types by dying and being reborn repeatedly. In the dwarf irregular phase, abundant gas fuels rapid star formation. But the newly formed stars then sterilise the galaxy when some of them explode as supernovae and blow away its gas. The galaxy's gravity later pulls the gas back in to fuel a new cycle of star formation.
The Canes Venatici II galaxy is one of several new dwarf galaxies discovered by the Sloan Digital Sky Survey in recent years.

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Blobrana


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Posts: 131433
Date:
RE: Milky Way Companions
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The Milky Way: a Growing Galactic Family
Like a Goddess, the Milky Way is truly something to be awed by. We live in a disk of stars so vast, that if our Sun was the size of the dot of an "i" on this page, the Milky Way would stretch from Los Angeles to New York City. Hundreds of billions of stars, our Sun among them, orbit around the dense centre of our galaxy, passing in and out of giant dust clouds, the debris of the lives of the stars. Residing in the outskirts of the Milky Way, our Sun takes 226 million years to make one orbit around the dense centre of the galaxy. Even so, our motion is far from leisurely. The Sun, along with all our planets, is moving at well over 400,000 miles per hour around a bulging central swarm of millions of stars concealing a supermassive black hole in their midst. And we are part of all this. Some of the stars we see in the sky, although we don't know which ones, formed out of the same birth cloud the Sun did. Other stars, long dead, seeded our solar system with the rich chemicals needed to form life.

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Blobrana


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Title: Is Ursa Major II the Progenitor of the Orphan Stream?
Authors: M. Fellhauer (1), N.W. Evans (1), V. Belokurov (1), D.B. Zucker (1), B. Yanny (2), M.I. Wilkinson (1), G. Gilmore (1), M.J. Irwin (1), D.M. Bramich (1), S. Vidrih (1), P. Hewett (1), T. Beers (3) ((1) Cambridge, (2) FNAL, (3) Michigan)
(revised v2)

Prominent in the 'Field of Streams' -- the Sloan Digital Sky Survey map of substructure in the Galactic halo -- is an 'Orphan Stream' without obvious progenitor. In this numerical study, we show a possible connection between the newly found dwarf satellite Ursa Major II (UMa II) and the Orphan Stream. We provide numerical simulations of the disruption of UMa II that match the observational data on the position, distance and morphology of the Orphan Stream. We predict the radial velocity of UMa II as -100 km/s as well as the existence of strong velocity gradients along the Orphan Stream. The velocity dispersion of UMa II is expected to be high, though this can be caused both by a high dark matter content or by the presence of unbound stars in a disrupted remnant. However, the existence of a gradient in the mean radial velocity across UMa II provides a clear-cut distinction between these possibilities. The simulations support the idea that some of the anomalous, young halo globular clusters like Palomar 1 or Arp 2 or Ruprecht 106 may be physically associated with the Orphan Stream.

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Blobrana


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Posts: 131433
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Title: Draco, a flawless dwarf galaxy
Authors: Mathieu Segall, Rodrigo Ibata, Michael Irwin, Nicolas Martin, Scott Chapman

The Draco dwarf spheroidal galaxy (dSph), with its apparent immense mass to light ratio and compact size, holds many clues to the nature of the enigmatic dark matter. Here we present deep photometric studies of this dwarf galaxy, undertaken with the MegaCam Camera at the Canada-France-Hawaii Telescope, the Wide Field Camera at the Isaac Newton Telescope and the Wide-Field and Planetary Camera on board the Hubble Space Telescope. The new photometric data cover the entirety of the galaxy, and reach i=24.5 at 50% completeness, significantly deeper than previous panoramic studies, allowing searches for tidal disturbances of much lower surface brightness than has been possible before. With these improved statistics, we find no evidence for asymmetric disturbances or tidal tails that possess more than 3% of the stars found within the centre of the galaxy. We find that the central stellar density, as probed by the HST data, rises into the central 0.5'. Uncertainties in the position of the centroid of the galaxy do not allow us to determine whether the apparent flattening of the profile interior to 0.5' is reliable or not. Draco is therefore a flawless dwarf galaxy, featureless and apparently unaffected by Galactic tides.

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Blobrana


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Title: Possible Streams of the Globular Clusters in the Galaxy
Authors: Shuang Gao, Bi-Wei Jiang, Yong-Heng Zhao

This paper aims to retrieve the ghost streams under the pre-assumption that all the globular clusters in our Galaxy were formed in the very early merge events. The results are based on two speculations: that the specific energy and angular momentum of the globular clusters after merge are not changed in process of evolution and that the globular clusters with common origin would stay in the same orbit plane as parent galaxy. In addition, taking into account the apo-galacticum distance of the orbits, five possible streams were suggested with a significant confidence. The number of streams is consistent with previous results. Three of the four well established members of the Sagittarius stream were found to be in one of our streams. Several other globular clusters in our result were also thought to come from accretion by previous research. Furthermore, the orbital parameters of the streams are derived, which provide a way to testify whether these streams are true with the help of the accurate measurement of proper motions of the globular clusters.

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