Geschichte unserer Milchstraße

K2.3   Where are the Missing Galactic Baryons?

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	Author: J. Sommer-Larsen
	Journal-ref: ApJ 644 (2006) L1 [astro-ph/0602595 ]
	Title: Where are the Missing Galactic Baryons?
Abstract: Based on 19 high-resolution N-body/gas-dynamical galaxy formation simulations in the LCDM cosmology it is shown, that for a galaxy like the Milky Way, in addition to the baryonic mass of the galaxy itself, about 70% extra baryonic mass should reside around the galaxy (inside of the virial radius), chiefly in the form of hot gas. Averaging over the entire field galaxy population, this external component amounts to 64-85% of the baryonic mass of the population itself. These results are supported by the recent detection of very extended, soft X-ray emission from the halo of the quiescent, massive disk galaxy NGC 5746. Some of the hot gas may, by thermal instability, have condensed into mainly pressure supported, warm clouds, similar to the Galactic High Velocity Clouds (HVCs). Based on an ultra-high resolution cosmological test simulation of a Milky Way like galaxy (with a gas particle mass and gravity softening length of only 7600 h-1 M and 83 h-1 pc, respectively), it is argued, that the hot gas phase dominates over the warm gas phase, in the halo. Finally, an origin of HVCs as leftovers from filamentary, cold accretion events, mainly occurring early in the history of galaxies, is proposed.

K3  Milky Way Tomography with SDSS

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	Authors: M. Juric, Z. Ivezic, A. Brooks, R.H. Lupton, D. Schlegel, D. Finkbeiner, Ni. Padmanabhan, N. Bond, B. Sesar, C.M. Rockosi, G.R. Knapp, J.E. Gunn, T. Sumi, D. Schneider, J.C. Barentine, H.J. Brewington, J. Brinkmann, M. Fukugita, M. Harvanek, S.J. Kleinman, J. Krzesinski, D. Long, E.H. Neilsen, Jr., A. Nitta, S.A. Snedden, D.G. York
	Journal-ref: ApJ (2008) [astro-ph/0510520 ]
	Title: The Milky Way Tomography with SDSS: I. Stellar Number Density Distribution
Abstract: Using the photometric parallax method we estimate the distances to ~48 million stars detected by the Sloan Digital Sky Survey (SDSS) and map their three-dimensional number density distribution in the Galaxy. The currently available data sample the distance range from 100 pc to 20 kpc and cover 6,500 deg2 of sky, mostly at high galactic latitudes (|b| > 25). These stellar number density maps allow an investigation of the Galactic structure with no a priori assumptions about the functional form of its components. The data show strong evidence for a Galaxy consisting of an oblate halo, a disk component, and a number of localized overdensities. The number density distribution of stars as traced by M dwarfs in the Solar neighborhood (D < 2 kpc) is well fit by two exponential disks (the thin and thick disk) with scale heights and lengths, bias-corrected for an assumed 35% binary fraction, of H1 = 300 pc and L1 = 2600 pc, and H2 = 900 pc and L2 = 3600 pc, and local thick-to-thin disk density normalization rthick/rthin (R) = 12%. We use the stars near main-sequence turnoff to measure the shape of the Galactic halo. We find a strong preference for oblate halo models, with best-fit axis ratio c/a = 0.64, rH ~ r-2.8 power-law profile, and the local halo-to-thin disk normalization of 0.5%. Based on a series of Monte-Carlo simulations, we estimate the errors of derived model parameters not to be larger than ~ 20% for the disk scales and ~ 10% for the density normalization, with largest contributions to error coming from the uncertainty in calibration of the photometric parallax relation and poorly constrained binary fraction. While generally consistent with the above model, the measured density distribution shows a number of statistically significant localized deviations. In addition to known features, such as the Monoceros stream, we detect two overdensities in the thick disk region at cylindrical galactocentric radii and heights (R,Z) ~ (6.5, 1.5) kpc and (R,Z) ~ (9.5, 0.8) kpc, and a remarkable density enhancement in the halo covering over a thousand square degrees of sky towards the constellation of Virgo, at distances of ~6-20 kpc. Compared to counts in a region symmetric with respect to the l = 0° line and with the same Galactic latitude, the Virgo overdensity is responsible for a factor of 2 number density excess, and may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging with the Milky Way. The u - g color distribution of stars associated with it implies metallicity lower than that of thick disk stars, and consistent with the halo metallicity distribution. After removal of the resolved overdensities, the remaining data are consistent with a smooth density distribution; we detect no evidence of further unresolved clumpy substructure at scales ranging from ~ 50 pc in the disk, to ~ 1-2 kpc in the halo. 1. Introduction In the canonical model of Milky Way formation (Eggen, Lynden-Bell, & Sandage 1962) the Galaxy began with a relatively rapid (~ 100 Myr) radial collapse of the initial protogalactic cloud, followed by an equally rapid settling of gas into a rotating disk. This model readily explained the origin and general structural, kinematic and metallicity correlations of observationally identified populations of field stars (Baade 1944; O’Connell 1958): low metallicity Population II stars formed during the initial collapse and populate the extended stellar halo; younger Population I and Intermediate Population II stars formed after the gas has settled into the Galactic plane and constitute the disk. The observationally determined distribution of disk stars is commonly described by exponential density laws (Bahcall & Soneira 1980), while power-laws or flattened de Vaucouleurs spheroids are usually used to describe the halo (e.g., Larsen 1996). In both disk and the halo, the distribution of stars is expected to be a smooth function of position, perturbed only slightly by localized bursts of star formation or spiral structure induced shocks. Most recently with the data from modern large-scale sky surveys (e.g., the Sloan Digital Sky Survey, York et al. 2000; The Two Micron All Sky Survey, 2MASS, Majewski et al. 2003; and the QUEST survey Vivas et al. 2001) evidence has been mounting for a more complex picture of the Galaxy and its formation. Unlike the smooth distribution easily captured by analytic density laws, new data argue for much more irregular substructure, especially in the stellar halo. Examples include the Sgr dwarf tidal stream in the halo (Ivezic et al. 2000; Yanny et al. 2000; Vivas et al. 2001; Majewski et al. 2003), or the Monoceros stream closer to the Galactic plane (Newberg et al. 2002; Rocha-Pinto et al. 2003). The existence of ongoing merging points to a likely significant role of accretion events in the early formation of the Milky Way’s components, making the understanding of both the distribution of merger remnants, and of overall Milky Way’s stellar content, of considerable theoretical interest. References Baade, W. 1944, ApJ, 100, 137 Bahcall, J. N., & Soneira, R. M. 1980, ApJS, 44, 73 Eggen, O.J., Lynden-Bell, D., & Sandage, A.R. 1962, ApJ, 136, 748 Larsen, J.A., & Humphreys, R.M. 1996, ApJ, 468, L99 Majewski, S.R., Skrutskie, M.F., Weinberg, M.D., & Ostheimer, J.C. 2003, ApJ, 599, 1082 O’Connell, D.J.K. 1958, Ricerche Astronomiche, 5 Vivas, A.K., Zinn, R., Andrews, P., et al. 2001, ApJ, 554, L33 York, D. G., Adelman, J., Anderson, Jr., et al. 2000, AJ 120, 1579

K4   SgrA*: The centre of our Milky Way

Using the motions of these stars to probe the gravitational field, observations over the last decade have shown that a mass of about 3 million times that of the Sun is concentrated within a radius of only 10 light-days of the compact radio and X-ray source SgrA* ("Sagittarius A") at the centre of the star cluster.

SIGMA telescope view of the Galactic plane. SIGMA is the coded-mask telescope capable to get images and spectra in the 30-1000 keV energy range with ~10' angluar resolutiuon and the field of view (fully coded) of 4.3X4.7 degrees. The main results from SIGMA include the VERY deep (more than 5 million seconds) imaging of the galactic center region, discovery of the electron-positron annihilation lines from the Galactic "micro-quasar" 1E1740-294, and X-ray Nova Muscae, study of spectra and time variability of the balck-hole candidates.

This means that SgrA* is the most likely counterpart of the black hole believed to exist at the centre of our Galaxy. The new unique data collected by NACO on the VLT and illustrated here show unambiguously that S2, which is the one currently closest to SgrA*, is moving along an elliptical orbit with SgrA* at one focus, i.e. S2 orbits SgrA* like the Earth orbits the Sun.

The superb data also allow a precise determination of the orbital parameters (shape, size, etc.). It turns out that S2 reached its closest distance to SgrA* in the spring of 2002, at which moment it was only 17 light-hours away from the radio source, or just 3 times the Sun-Pluto distance. It was then moving at more than 5000 km/s, or nearly two hundred times the speed of the Earth in its orbit around the Sun. The orbital period is 15.2 years.

The orbit is rather elongated - the eccentricity is 0.87 - indicating that S2 is about 10 light-days away from the central mass at the most distant orbital point.

The best estimate of the mass of the Black Hole at the centre of the Milky Way is 2.6 ± 0.2 million times the mass of the Sun.

K5  M31 and the Origin of the Local Group of Galaxies

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	Authors: T. Sawa M. Fujimoto
	Journal-ref: PASJ 57 (2005) 429 [astro-ph/0404547 ]
	Title: A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies
Abstract: We propose a new model for the origin and evolution of the Local Group of Galaxies (LGG) that naturally explains the formation of the Magellanic Clouds and their large orbital angular momenta around the Galaxy. The basic idea is that an off-center hydrodynamical collision occurred some 10Gyr ago between the primordial Andromeda galaxy (M31) and a similar Galaxy, and compressed the halo gas to form the LGG dwarf galaxies, including the Magellanic Clouds. New-born dwarf galaxies can be expected to locate on the orbital plane of these two massive galaxies. We reexamined the two-dimensional sky distribution of the LGG members, and confirmed an early idea that they align along two similar great circles. The planes of these circles are approximately normal to the line joining the present position of the Sun and the galactic center. We made a distribution map of these objects, and found a well-defined plane of finite thickness. Thus we could determine the orbital elements of M31 relative to the Galaxy by reproducing the well-studied dynamics of the LMC and the SMC around the Galaxy. The expected proper motion of M31 is (µl, µb) = (38 ± 16 µas yr-1, -49 ± 5 µas yr-1). 1. Introduction References

Stoß MWG - M31 Lupe: the Local Group of Galaxies (LGG)   Seitenansicht der Stoßgeometrie

Bild mit 3 snapshots LGG: Local Group of Galaxies

A huge gaseous halo was hydrodynamically compressed at their closest approach and driven to form a number of dwarf members, including the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), of the Local Group of Galaxies (LGG) and scatter them on the orbital plane of these two mass-dominant galaxies.

In order to see the reality of this model, we reexamine the two-dimensional sky distribution of the LGG members and the Magellanic Stream, we confirm an earlier and widely-discussed idea that they align along two similar great circles, each with an angular width of ~ 30°, and the planes of these circles are approximately normal to the line joining the present position of the sun and the Galactic center. Further we make a three-dimensional distribution map of these objects, and observe it from various directions.

A well-defined plane of finite thickness is found, within which most of the member galaxies are confined,
Bild
supporting the existence of the above circles on the sky. Thus we could determined the orbital elements of M31 relative to the Galaxy through reproducing the well-studied dynamics of the LMC and SMC around the Galaxy.
Probable orbital motions of the other dwarfs are also determined, and the expected proper motion for each object is given to compare with observations in near future.

The Magellanic Stream is a narrow band of diffuse atomic hydrogen gas emerging from the SMC region of the Magellanic Clouds, and passing by the south Galactic pole on an overhead great circle spanning over 100°. A tidal model has been successfully introduced to the dynamics of the Galaxy-LMC-SMC system for reproducing the geometrical as well as dynamical structures of the Magellanic Stream.

We note that the orbits of the LMC and SMC can be traced back in time over the entire past period of ~ 10 Gyr: The orbital plane is approximately perpendicular to the line joining the present position of the sun and the Galactic center, and they are viewed to move counterclockwise along a nearly great circle centered on (l, b)=(0,0) or the Galactic center.

K6   A Magellanic Origin for the Warp of the Galaxy

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	Authors: M.D. Weinberg, L. Blitz
	Journal-ref: ApJ 641 (2006) L33 [astro-ph/0601694 ]
	Title: A Magellanic Origin for the Warp of the Galaxy
Abstract: We show that a Magellanic Cloud origin for the warp of the Milky Way can explain most quantitative features of the outer HI layer recently identified by Levine, Blitz & Heiles (2005). We construct a model similar to that of Weinberg (1998) that produces distortions in the dark matter halo, and we calculate the combined effect of these dark-halo distortions and the direct tidal forcing by the Magellanic Clouds on the disk warp in the linear regime. The interaction of the dark matter halo with the disk and resonances between the orbit of the Clouds and the disk account for the large amplitudes observed for the vertical m=0,1,2 harmonics. The observations lead to six constraints on warp forcing mechanisms and our model reasonably approximates all six. The disk is shown to be very dynamic, constantly changing its shape as the Clouds proceed along their orbit. We discuss the challenges to MOND placed by the observations.

The warp of the outer Milky Way, known since 1957 (Kerr et al. 1957), has been quantitatively determined for the first time by Levine et al. (2005). It can be described as a superposition of three and only three of the lowest order vertical harmonics of a disk:
a dish-shaped m=0,
an integral-sign-shaped m=1, and
a saddle-shaped harmonic} m=2.
The lines of nodes for each are close to coincident and nearly radial. The amplitudes of each reaches 7--10% of the radius of the disk. A number of possible warp producing mechanisms have been suggested including long-lived eigenmodes, forcing by halo triaxiality, persistent cold-gas accretion, and tidal excitation.

We show here that the origin of this warp can be well-described as the tidal interaction of the Magellanic Clouds with the disk and dark matter halo of the Milky Way. The interaction of the dark matter halo with the disk and resonances between the orbit of the Clouds and the disk account for the large amplitudes of the three harmonics and their approximate shape and orientation.

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	Authors: E.S. Levine, L. Blitz, C. Heiles
	Journal-ref: ApJ 643 (2006) 881 [astro-ph/0601697 ]
	Title: The Vertical Structure of the Outer Milky Way HI Disk
Abstract: We examine the outer Galactic HI disk for deviations from the b=0 plane by constructing maps of disk surface density, mean height, and thickness. We find that the Galactic warp is well described by a vertical offset plus two Fourier modes of frequency 1 and 2, all of which grow with Galactocentric radius. Adding the m=2 mode accounts for the large asymmetry between the northern and southern warps. We use a Morlet wavelet transform to investigate the spatial and frequency localization of higher frequency modes; these modes are often referred to as "scalloping." We find that the m=10 and 15 scalloping modes are well above the noise, but localized; this suggests that the scalloping does not pervade the whole disk, but only local regions.

Disk warping by the Magellanic Clouds
The Magellanic Clouds roam the Milky Way in a halo around the galaxy’s flattened disk.
Dumbo the Elephant
The following movie shows the time evolution of the disk "flapping". The scaling for each harmonic and the total is the same as in the previous tab. The vertical motion of the satellite continuously excites the m=0 "bobbing" (with a little glitch during the disk plane passage). Notice that the m=1 asymmetry "turns" to face the orbital plane of the satellite; its pattern speed is close to the radial frequency of the satellite. The m=2 pattern speed is faster than the orbit but also close to a resonance (looks like 3:2 to me). Because the m=2 pattern speed is faster, one gets a large change in m=2 amplitude as one gradually changes the satellite orbital frequency (as I described earlier). My point here is that the amplitude m=2 component can be either large or small depending serendipitously on the satellite. Not surprising given the ampltidue of the responses seen previously, the m=1 dominates and m=0 and m=2 modulates.
LMC + Galaxy coordinate system
cf.: NGC 6231 - a globular cluster roams the Milky Way producing stars

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	Authors: E.S. Levine, L. Blitz, C. Heiles
	Journal-ref: Science 312 (2006) 1773-1777 [astro-ph/0605728 ]
	Title: The Spiral Structure of the Outer Milky Way in Hydrogen
Abstract: We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk, demonstrating that the Galaxy is a non-axisymmetric multiarmed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kiloparsec, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20° to 25°.

Literatur zu "Our Milky Way"
Kerr, F.J., Hindman, J.V., & Carpenter, M.S.	1957	Nature 180, 677	"The large-scale structure of the galaxy"
B. Nordström, M. Mayor, J. Andersen, et al.	2004	A&A 418, 989	"Geneva-Copenhagen survey: Ages, metallicities, properties of 14,000 F and G dwarfs"
T. Sawa M. Fujimoto	2005	PASJ 57, 429	"A Dynamical Model for the Orbit of the Andromeda Galaxy M31"
E.S. Levine, L. Blitz, C. Heiles	2006	ApJ 643, 881	"The Vertical Structure of the Outer Milky Way HI Disk"
E.S. Levine, L. Blitz, C. Heiles	2006	Science 312, 1773	"The Spiral Structure of the Outer Milky Way in Hydrogen"
M.D. Weinberg, L. Blitz	2006	ApJ 641, L33	"A Magellanic Origin for the Warp of the Galaxy"
J. Sommer-Larsen	2006	ApJ 644, L1	"Where are the Missing Galactic Baryons?"

H. Heintzmann

( Eintrag vom 16.4.2008)

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