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TOPIC: Milky Way
Blobrana


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Title: Dynamical effects of the long bar in the Milky Way
Authors: Esko Gardner, Kimmo A. Innanen, Chris Flynn

We examine the dynamical effects on disk stars of a "long bar" in the Milky Way by inserting a triaxial rotating bar into an axisymmetric disk+bulge+dark halo potential and integrating 3-D orbits of 10^4 tracer stars over a period of 5 Gyr. The long bar has been detected via "clump giants" in the IR by Lopez-Corredoira et al. (2007), and is estimated to have semi-major axes of (3.9:0.6:0.1) kpc and a mass of 6 10^9 Msun. We find such a structure has striking effects on disk orbits. The long bar is able to move considerable inner disk material into prominent stellar streams in the solar neighbourhood, apparently inconsistently with observations; furthermore it can eject inner disk material to considerable heights (several kpc) above the Galactic plane in a manner which may be inconsistent with observations. The effects of the long bar can be greatly ameliorated by reducing its density and/or making it considerably thicker vertically. These effects are under continuing study.

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Blobrana


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Title: A New Spiral Arm of the Galaxy: The Far 3-Kpc Arm
Authors: T. M. Dame, P. Thaddeus

We report the detection in CO of the far-side counterpart of the well-known expanding 3-Kpc Arm in the central region of the Galaxy. In a CO longitude-velocity map at b = 0 deg the Far 3-Kpc Arm can be followed over at least 20 deg of Galactic longitude as a faint lane at positive velocities running parallel to the Near Arm. The Far Arm crosses l = 0 deg at +56 km/s, quite symmetric with the -53 km/s expansion velocity of the Near Arm. In addition to their symmetry in longitude and velocity, we find that the two arms have linewidths (~21 km/s), linear scale heights (~103 pc FWHM), and H2 masses per unit length (~4.3 x 10^6 Mo/kpc) that agree to 26% or better. Guided by the CO, we have also identified the Far Arm in high-resolution 21 cm data and find, subject to the poorly known CO-to-H2 ratio in these objects, that both arms are predominately molecular by a factor of 3-4. The detection of these symmetric expanding arms provides strong support for the existence of a bar at the centre of our Galaxy and should allow better determination of the bar's physical properties.

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Blobrana


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Astronomers find new Milky Way arm
Astronomers describe our Milky Way as a barred spiral galaxy, with a central bar-shaped region loosely cradled by spiral arms. Whats more, it appears symmetrical, meaning it looks the same from the top if you were to give it a 180 degree turn. But one of the arms, expanding near the galaxys centre 10,000 light years from Earth, had no known partner until now.

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Blobrana


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Astronomers have long believed that our galaxy possesses four spiral arms, since radio observations show concentrations of gas that trace such a spiral structure.
But now, two of the Milky Way's arms have failed to turn up in a sensitive new survey that used the Spitzer Space Telescope to map the distribution of millions of stars. Spitzer is well-suited to mapping the galaxy's stars because its infrared vision can pierce through the dust that obscures stars at optical wavelengths of light.
Astronomer Robert Benjamin of the University of Wisconsin in Whitewater, US, says these two arms, called Sagittarius and Norma, may be mostly concentrations of gas, perhaps sprinkled with pockets of young stars.
By contrast, the other two arms, called Scutum-Centaurus and Perseus, appear rich not only in gas, but in stars both young and old.

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ssc2008-11a_200.jpgMore than 800,000 snapshots from NASA's Spitzer Space Telescope have been stitched together to create a new "coming of age" portrait of stars in our inner Milky Way galaxy.
The image depicts an area of sky 120 degrees wide by two degrees tall. It was unveiled today at the 212th meeting of the American Astronomical Society in St. Louis, Mo.

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GLIMPSEII Version 2.0 Data Release (April 2008)
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Blobrana


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glimpGalaxy
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Credit NASA/GLIMPSE

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Blobrana


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Milky Way Spiral Arms



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Blobrana


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Milky Way Spiral Arms
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For decades, astronomers have been blind to what our galaxy, the Milky Way, really looks like. After all, we sit in the midst of it and can't step outside for a bird's eye view.
Now, new images from NASA's Spitzer Space Telescope are shedding light on the true structure of the Milky Way, revealing that it has just two major arms of stars instead of the four it was previously thought to possess.

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Blobrana


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Title: The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
Authors: X.-X. Xue, H.-W. Rix, G. Zhao, P. Re Fiorentin, T. Naab, M. Steinmetz, F. C. van den Bosch, T. C. Beers, Y. S. Lee, E. F. Bell, C. Rockosi, B. Yanny, H. Newberg, R. Wilhelm, X. Kang, M. C. Smith, D. P. Schneider
(Version v5)

We derive new constraints on the mass of the Milky Way's dark matter halo, based on a set of halo stars from SDSS as kinematic tracers. Our sample comprises 2401 rigorously selected Blue Horizontal-Branch (BHB) halo stars drawn from SDSS DR-6. To interpret these distributions, we compare them to matched mock observations drawn from two different cosmological galaxy formation simulations designed to resemble the Milky Way, which we presume to have an appropriate orbital distribution of halo stars. We then determine which value of
m V_{cir}(r) brings the observed distribution into agreement with the corresponding distributions from the simulations. This procedure results in an estimate of the Milky Way's circular velocity curve to ~ 60 kpc, which is found to be slightly falling from the adopted value of
m 220 km s^{-1} at the Sun's location, and implies M(<60
m kpc) = 4.0 ± 0.7 x  10^{11}solar masses. The radial dependence of
m V_{cir}(r), derived in statistically independent bins, is found to be consistent with the expectations from an NFW dark matter halo with the established stellar mass components at its centre. If we assume an NFW halo profile of characteristic concentration holds, we can use the observations to estimate the virial mass of the Milky Way's dark matter halo, M_{
m vir}=1.0^{+0.3}_{-0.2} x 10^{12} solar masses, which is lower than many previous estimates. This estimate implies that nearly 40% of the baryons within the virial radius of the Milky Way's dark matter halo reside in the stellar components of our Galaxy. A value for M_{
m vir} of only ~ 1 x 10^{12}solar masses also (re-)opens the question of whether all of the Milky Way's satellite galaxies are on bound orbits.

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An international team of astronomers from the Sloan Digital Sky Survey (SDSS-II) has unveiled the most complete and detailed map yet of the metals present in more than 2.5 million stars in the Milky Way.
Previous chemical composition maps were based on much smaller samples of stars and didn't go as far from a region extending from near the Sun to about 30,000 light years away.

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