* Astronomy

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RE: Milky Way

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The first stars of the Milky Way and the death of satellite galaxies

Two researchers at the Observatoire de Strasbourg (CNRS-INSU, Université de Strasbourg) showed for the first time the existence of a new signature of the appearance of the first stars in our Galaxy. For more than 12 billion years, these stars would have dispersed the gas from satellite galaxies of the Milky Way through their intense radiation. It is by studying the observable consequences of this process, that Peter Ocvirk (CNRS) and Dominique Aubert (University of Strasbourg) have highlighted their role. This result confirms the place of the reionisation process as most of the standard cosmological model that describes the evolution of the universe and the formation of its structures. The study, carried out in collaboration LIDAU (Light In the Dark Ages of the Universe), published in October in the letters of the journal Monthly Notices of the Royal Astronomical Society.
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Title: A signature of the internal reionisation of the Milky Way?
Authors: Pierre Ocvirk & Dominique Aubert

We present a new semi-analytical model of the population of satellite galaxies of the Milky Way, aimed at estimating the effect of the spatial structure of reionisation at galaxy scale on the properties of the satellites. In this model reionisation can be either: (A) externally-driven and uniform, or (B) internally-driven, by the most massive progenitor of the Milky-Way. In the latter scenario the propagation of the ionisation front and photon dilution introduce a delay in the photo-evaporation of the outer satellites' gas with respect to the inner satellites. As a consequence, outer satellites experience a longer period of star formation than those in the inner halo. We use simple models to account for star formation, the propagation of the ionisation front, photo-evaporation and observational biases. Both scenarios yield a model satellite population at z=0 that matches the observed luminosity function and mass-to-light ratios. However, the predicted population for scenario (B) is significantly more extended spatially than for scenario (A), by about 0.3 dex in distance, resulting in a much better match to the observations. The survival of this structural signature imprinted by the local UV field during reionisation on the radial distribution of satellites makes it a promising tool for studying the reionisation epoch at galaxy scale in the Milky Way and nearby galaxies resolved in stars with forthcoming large surveys. However, more work is needed to determine how the effect reported here can be disentangled from that of cosmic variance between different realisations of Milky Way haloes.

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Title: Structure and Evolution of the Milky Way
Authors: Ken Freeman

This review discusses the structure and evolution of the Milky Way, in the context of opportunities provided by asteroseismology of red giants. The review is structured according to the main Galactic components: the thin disk, thick disk, stellar halo, and the Galactic bar/bulge. The review concludes with an overview of Galactic archaeology and chemical tagging, and a brief account of the upcoming HERMES survey with the AAT.

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Fast Falling Clouds Fuel Milky Way Star Formation

The long-term forecast for the Milky Way is cloudy with gaseous rain. A study by Nicolas Lehner and Christopher Howk of the University of Notre Dame concludes that massive clouds of ionised gas are raining down from our galaxy's halo and intergalactic space and will continue to provide fuel for the Milky Way to keep forming stars. Using the Hubble Space Telescope's Cosmic Origins Spectrograph they measured for the first time the distances to huge, fast-moving clouds of ionised gas previously seen covering a large fraction of the sky.
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First glimpse into birth of the Milky Way

For almost 20 years astrophysicists have been trying to recreate the formation of spiral galaxies such as our Milky Way realistically. Now astrophysicists from the University of Zurich present the world's first realistic simulation of the formation of our home galaxy together with astronomers from the University of California at Santa Cruz. The new results were partly calculated on the computer of the Swiss National Supercomputing Centre (CSCS) and show, for instance, that there has to be stars on the outer edge of the Milky Way.
The aim of astrophysical simulations is to model reality in due consideration of the physical laws and processes. Astronomical sky observations and astrophysical simulations have to match up exactly. Being able to simulate a complex system like the formation of the Milky Way realistically is the ultimate proof that the underlying theories of astrophysics are correct. All previous attempts to recreate the formation of spiral galaxies like the Milky Way faltered on one of two points: Either the simulated spiral galaxies displayed too many stars at the center or the overall stellar mass was several times too big. A research group jointly run by Lucio Mayer, an astrophysicist at the University of Zurich, and Piero Madau, an astronomer at University of California at Santa Cruz, is now publishing the first realistic simulation of the formation of the Milky Way in the Astrophysical Journal. Javiera Guedes and Simone Callegari, who are PhD students at Santa Cruz and the University of Zurich respectively, performed the simulation and analysed the data. Guedes will be working on the formation of galaxies as a postdoc in Zurich from the autumn.

Movie

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Twisted Tale of our Galaxy's Ring

New observations from the Herschel Space Observatory show a bizarre, twisted ring of dense gas at the center of our Milky Way galaxy. Only a few portions of the ring, which stretches across more than 600 light-years, were known before. Herschel's view reveals the entire ring for the first time, and a strange kink that has astronomers scratching their heads.
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Title: A 100-parsec elliptical and twisted ring of cold and dense molecular clouds revealed by Herschel around the Galactic Center
Authors: S. Molinari, J. Bally, A. Noriega-Crespo, M. Compiègne, J.P. Bernard, D. Paradis, P. Martin, L. Testi, M. Barlow, T. Moore, R. Plume, B. Swinyard, A. Zavagno, L. Calzoletti, A.M. Di Giorgio, D. Elia, F. Faustini, P. Natoli, M. Pestalozzi, S. Pezzuto, F. Piacentini, G. Polenta, D. Polychroni, E. Schisano, A. Traficante, M. Veneziani, C. Battersby, M. Burton, S. Carey, Y. Fukui, J.Z. Li, S.D. Lord, L. Morgan, F. Motte, F. Schuller, G.S. Stringfellow, J.C. Tan, M. A. Thompson, D. Ward-Thompson, G. White, G. Umana

Thermal images of cold dust in the Central Molecular Zone of the Milky Way, obtained with the far-infrared cameras on-board the Herschel satellite, reveal a 3x10^7 solar masses ring of dense and cold clouds orbiting the Galactic Center. Using a simple toy-model, an elliptical shape having semi-major axes of 100 and 60 parsecs is deduced. The major axis of this 100-pc ring is inclined by about 40 degrees with respect to the plane-of-the-sky and is oriented perpendicular to the major axes of the Galactic Bar. The 100-pc ring appears to trace the system of stable x_2 orbits predicted for the barred Galactic potential. Sgr A* is displaced with respect to the geometrical center of symmetry of the ring. The ring is twisted and its morphology suggests a flattening-ratio of 2 for the Galactic potential, which is in good agreement with the bulge flattening ratio derived from the 2MASS data.

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Milky Way a galactic cannibal

The Milky Way has a history of devouring smaller neighbouring galaxies that get too close. Now it has been found that one such incident could be responsible for the shape of our galaxy, according to two international astronomers.
Dr Kenji Bekki, from the International Centre for Radio Astronomy Research in Perth, worked with Takuji Tsujimoto from the National Astronomical Observatory of Japan to simulate a merger between a smaller galaxy and an infant Milky Way nine billion years ago.
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Title: Origin of chemical and dynamical properties of the Galactic thick disk
Authors: Kenji Bekki, Takuji Tsujimoto

We adopt a scenario in which the Galactic thick disk was formed by minor merging between the first generation of the Galactic thin disk (FGTD) and a dwarf galaxy about 9 Gyr ago and thereby investigate chemical and dynamical properties of the Galactic thick disk. In this scenario, the dynamical properties of the thick disk have long been influenced both by the mass growth of the second generation of the Galactic thin disk (i.e., the present thin disk) and by its non-axisymmetric structures. On the other hand, the early star formation history and chemical evolution of the thin disk was influenced by the remaining gas of the thick disk. Based on N-body simulations and chemical evolution models, we investigate the radial metallicity gradient, structural and kinematical properties, and detailed chemical abundance patterns of the thick disk. Our numerical simulations show that the ancient minor merger event can significantly flatten the original radial metallicity gradient of the FGTD, in particular, in the outer part, and also can be responsible for migration of inner metal-rich stars into the outer part (R>10kpc). The simulations show that the central region of the thick disk can develop a bar due to dynamical effects of a separate bar in the thin disk. The simulated orbital eccentricity distributions in the thick disk for models with higher mass-ratios (~0.2) and lower orbital eccentricities (~ 0.5) of minor mergers are in good agreement with the corresponding observations. The simulated V_{phi}-|z| relation of the thick disk in models with low orbital inclination angles of mergers are also in good agreement with the latest observational results. Our Galactic chemical evolution models can explain both the observed metallicity distribution functions (MDFs) and correlations between [Mg/Fe] and [Fe/H] for the two disks in a self-consistent manner.

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