Lorentz Center - Distribution of Mass in the Milky Way Galaxy from 13 Jul 2009 through 17 Jul 2009
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    Distribution of Mass in the Milky Way Galaxy
    from 13 Jul 2009 through 17 Jul 2009

 
Lopez-Corredoira:

T. Antoja, O. Valenzuela, F. Figueras, B. Pichardo, E. Moreno

Title:

Moving groups: Stellar Kinematic Constraints on Galactic Structure Models

 

Abstract

Recent studies suggest that moving groups have a dynamic or “resonant” origin. Under this hypothesis, these kinematic structures become a powerful tool for studying the large-scale structure and dynamics of the Milky Way. We study the local stellar distribution both from observational data in the U-V-age-[Fe/H] space and through test particle orbit integration using the Galactic potential from Pichardo et al. 2003 & 2004. We find that the spiral arm contribution to the kinematic structure in the solar neighborhood may be as important as the one produced by the Galactic bar. We suggest that structures such as Hercules and the kinematic branches can be created by the dynamical resonances of self-gravitating spiral arms and not exclusively by the Galactic bar. A structure coincident with the Arcturus kinematic group is developed when a hot stellar disk population is considered, which introduces a new perspective on the interpretation of its extragalactic origin. We show that particles in the dark matter disk-like structure predicted by recent LCDM galaxy formation experiments, with similar kinematics to the thick disk, are affected by the same resonances, developing phase space structures or dark kinematic groups that are independent of the Galaxy assembly history and substructure abundance.

 

 

Battaglia

Title:

Mass determinations for Milky Way dwarf spheroidal galaxies from the DART sample

 

Abstract:

T.b.c.

 

 

Benjamin:

Title:

Spitzer/GLIMPSE constraints on the spiral structure of the Galaxy

 

Abstract:

Abstract:  Decades of work have led to a provisional picture of spiral structure for the Milky Way. Much of this work has been based on mapping the distribution of gas (HI and CO) or star formation (HII regions) using kinematic distances.  We present new information on the spiral structure of the Milky Way based on analysis of mid-infrared star counts. We identify the Centaurus tangency at l=308 degrees, but do not detect the Sagittarius arm tangency at l~50 degrees which was expected to be at the same distance.  A comparison of gas, stellar, and star formation tracers of spiral structure is given, and potential kinematic effects of the stellar spiral structure of the Milky Way are suggested.

 

 

Binney:

Title:

Reconstructing streams and using them as probes of stellar and Galactic physics

 

Abstract:

If streams delineated single orbits, dynamics could be used to determine their full phase-space coordinates from readily measureable subsets. Moreover, in the process we could assess the accuracy of the the Galactic potential employed. In detail streams differ from orbits and some subtlety is required in doing the dynamics.  The talk will explain the exciting potential of this area and reoprt progress towards realising it.

 


Leo Blitz and Amber Bauermeister (UC Berkeley)

 

Title:

The Gas Consumption History of the Universe to z = 5

 

Abstract

We use the measured star formation history of the Universe embodied in the “Madau” plot, the mean density of molecules and the measured efficiency of star formation locally to infer quantitatively how the universe has consumed its gas to produce stars over cosmic timescales. We show that closed box models of molecular gas consumption are at odds with observations, and that the reservoir of atomic gas is insufficient to make up for the dearth of molecular gas at all epochs in the past.  We show that conversion of the warm-hot ionized medium into neutral gas is necessary and derive quantitatively the rates of conversion from one phase into another.  Using cooling time arguments, we derive how much ionized gas is needed at particular densities to satisfy the mass flow rates.

 

 

Bovy:

Title:

Inferring dynamics from a kinematical snapshot: The importance of the distribution function

 

Abstract:

The problem of inferring the dynamics of the Galaxy from a snapshot of its kinematics is fundamentally ill-posed: kinematics—positions and velocities—provides merely initial conditions; only accelerations are set by the gravitational potential, but these are, in general, unavailable to us. One basis for this non-trivial inference is the assumption that the potential is (close to) integrable and that (a large part of) the Galaxy is fully mixed. In this talk I will focus on the latter assumption. Using a few examples I discuss the importance of assumptions about the distribution function (DF) for the inference of the mass distribution of the Milky Way as well as methods to test and go beyond these assumptions: The velocity distribution of stars in the Solar neighborhood manifestly breaks the simple Gaussian velocity ellipsoid assumption; I show how this leads to a systematic uncertainty, many times larger than the statistical uncertainty, on the Solar motion with respect to the local standard of rest. On larger scales, trigonometric parallaxes of masers found in massive star forming regions have recently been proposed as a promising new method for probing Galactic structure: I discuss how one can simultaneously infer the DF of the masers as well as the fundamental parameters of Galactic structure by going beyond maximum likelihood estimates, and how one can integrate out (part of) the assumptions about the DF. With the large data sets becoming available in the next decade, detailed inferences of the DF will become increasingly important.

 

 

Fuchs (Astronomisches Rechen-Institut Heidelberg):

Title:
The kinematics of late type stars in the solar cylinder, studied with SDSS data

Abstract:
I report on a study of the velocity distribution of Milky Way disk stars in a kiloparsec-sized region

around the Sun, based on  ~ 2 million M-type stars from DR7 of SDSS, which have newly re-calibrated absolute proper motions from combining SDSS positions with the USNO-B catalogue. We (Fuchs et al.~2009) estimate photometric distances to all stars, accurate to ~ 20\%, and combine them with the proper motions to derive tangential velocities for this kinematically unbiased sample of stars. Based on a statistical de-projection method we then derive the vertical profiles (to heights of Z = 800 pc above the disk plane) for the first and second moments of the three dimensional stellar velocity distribution.  Since we can expect M dwarfs to represent a well relaxed population of the disk of the Milky Way the U and W components of the mean streaming motion of the stars reflect simply the motion of the Sun relative to the standard of rest. The mean V velocity component is more negative than the solar V velocity because of the asymmetry of the V velocity distribution of the stars. We find that all three velocity dispersions rise significantly with height above the midplane. Due to the asymmetric drift affect the rotational lag in the V velocity component of the stars increases with height above the midplane. We determine the orientation of the velocity ellipsoid, and find a significant vertex deviation of 20 to 25 degrees, which decreases only slightly to heights of Z = 800 pc. Away from the mid-plane, our sample exhibits a remarkably large tilt of the velocity ellipsoid towards the Galactic plane, which reaches 20 degrees at Z = 800 pc and which is not easily explained. Finally, I use these kinematical measurements for Jeans modelling of the Milky Way disk.

 

 

Gan:

Title:

A semi-analytical model for substructural halo evolution

 

Abstract:

A typical model that coupling the merger trees with subhalo dynamical evolution in host halo has been developed in the past years(e.g. Taylor \& Babul 2001; 2004; 2005a; 2005b; Zentner et al. 2003; 2005). The former one gives the subhalos accretion history and the latter one includes some important physical processes such as dynamical friction, tidal stripping and tidal heating. It was sucessful but still have some problem remain unclear. For example, what is an appropriate tidal mass loss rate, when should a subhalo suffer tidal disruption and how subhalo evolves at its late stage? We revisit this model and combine the subhalos mass function, merger time scale and radial distribution to constrain the model. We find the mass loss rate can be restricted by subhalos mass function, which has been well measured in simulation. We will talk the dynamical properties of merger time and how the merger time correlate with other scaling relation. We show the dependence of radial distribution on subhalos mass and accretion time. By comparing with observation, we will also discuss how the baryonic component affect the merger time and radial distribution.

 

 

Gilessen:

Title:

The mass in the central parsec

 

Abstract:

T.b.c.

 

 

Peter Hammersley:

Title:

The inner Galaxy, Bulge and Bar

 

Abstract:

In this talk I show the increasing evidence for there being two triaxial features in the inner galaxy. The triaxial bulge with a position angle of 23 degrees and extending many degrees from the plane is now widely accepted. The second feature is the more controversial long in plane bar which has a position angle of about 40 degrees and half length of 4 kpc.

 

 

Kawata:

Title:

Chemodynamical Modeling of Disc Galaxies

 

Abstract:

T.b.c.

 

 

Libeskind:

Title:

The satellites of the Milky Way

 

Abstract:

T.b.c.

 

 

Lopez-Corredoira:

Title:

Canis Major overdensity and Monoceros ring explained in terms of pure Milky Way structure

 

Abstract:

Some authors have suggested the existence of a nearby dwarf galaxy associated to the Canis Major overdensity, which was tidally disrupted leaving remnants throughout the Monoceros ring observed along several positions within ~20 degrees from the Galactic plane. Here, we present arguments to think that the Canis Major overdensity is just a part of the Galactic warp+flare, instead of a dwarf galaxy or a new substructure in the Galaxy, and the Monoceros ring overdensities and its features on color-magnitude diagrams are mainly populations of the most external non-truncated flared thin+thick stellar disc. Failures to fit these features with the Milky structure stem mainly from the use of Galactic models which do not represent appropriately the southern warp, the disc flare, and that set an unrealistic truncation of the stellar populations of the disc at R=14 kpc, like the Besançon model. Suggested sections: 1) "Satellites, LMC/SMC: velocities, distributions"; or 2) "Mass distribution of the stellar component"

 

 

Maccio:

Title:

The luminosity function of Milky Way Satellites in a LCDM Universe

 

Abstract:

We study the luminosity function and the radial distribution of satellite galaxies within Milky Way size haloes as predicted in LCDM model, making use of hydrodynamic and N-body

simulations as well as of three different semi-analytic models (SAMs) of galaxy formation.

We extract merger trees from high-resolution N-body simulations of four Galaxy-sized DM haloes, and use these as common input for the SAMs. We present a detailed comparison of our predictions with the recent observational data on the Milky Way satellite luminosity function (LF).  We find that semi-analytic models with rather standard astrophysical ingredients are able to reproduce the observed luminosity function over six orders of magnitude in luminosity, down to magnitudes as faint as Mv=-2. We also perform a comparison with the actual observed number of satellites as a function of luminosity, by applying the selection criteria of the SSDS to our simulations. We find good agreement for both the luminosity and radial distributions of MW satellites. We investigate which physical processes in our models are responsible for shaping the satellite LF, and find that tidal destruction, suppression of gas infall by a photo-ionizing background, and supernova feedback all make important contributions. We conclude that the number and luminosity of Milky Way satellites can be naturally accounted for within the LCDM paradigm, and this should no longer be considered a problem.

 

 

Moster (MPIA, Heidelberg, Germany):

 

Title:

Can Gas prevent the Destruction of Thin Stellar Discs by Minor Mergers?

 

Abstract:

We study the effect of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. We produce a suite of minor merger simulations for Milky Way-like galaxies. This suite consists of collision-less simulations as well as hydrodynamical runs including a gaseous component in the galactic disc. We find that in dissipationless simulations minor mergers cause the scale height of the disc to increase by up to a factor of ~2. When the presence of gas in the disc is taken into account this thickening is reduced by 25% (50%) for an initial disc gas fraction of 20% (40%), leading to a final scale height z0 between 0.6 and 0.7 kpc, regardless of the initial scale height. We argue that the presence of gas reduces disc heating via two mechanisms: absorption of kinetic impact energy by the gas and/or formation of a new thin stellar disc that can cause heated stars to recontract towards the disc plane. Final disc scale heights found in our simulations are in good agreement with studies of the vertical structure of spiral galaxies. We also found no tension between recent measurements of the scale height of the Milky Way thin disc and results coming from our hydrodynamical simulations. We conclude that the existence of a thin disc in the Milky Way and in external galaxies is not in obvious conflict with the predictions of the CDM model.

 

 

Read (University of Zurich, Switzerland / University of Leicester, UK):

Title:

Heated versus accreted thick discs in LCDM

Abstract:
Using a suite of high resolution N-body simulations, we show that the thick disc of the Milky Way must comprise a mix of stars heated from an early thin disc, and stars accreted from infalling satellites. For the cosmological mean merger history, it is difficult for more than half of the thick disc stars to be accreted. I show that the most massive satellites merging at high redshift provide enough heating to form the Milky Way thick disc, and that the Milky Way thin disc must then reform from fresh gas. I discuss how these results relate to recent solar neighborhood observations.

 

 

Reid (Harvard-Smithsonian CfA):

 

Title:

Galactic Structure from Parallaxes and Proper Motions of High Mass Star Forming Regions

 

Abstract:

We are conducting a large program with the Very Long Baseline Array (VLBA) to measure trigonometric parallaxes and proper motions of high-mass star-forming regions across the Milky Way.  These measurements give the full 3-dimension locations and space velocities of the star forming regions.  Initial results locate several spiral arms and start to yield direct measurements of arm pitch angles.  We find that star forming regions have large departures from circular orbits in the Galaxy and, on average, orbit the Galaxy 15 km/s slower than for circular rotation.  By modeling the spatial and kinematic data, we can estimate the size and rotation speed of the Milky Way.  We find the rotation speed is 15% greater than the IAU recommended value.  The Milky Way and the Andromeda galaxy appear to have similar rotation curves, suggesting that the dark matter halos of the the two galaxies may be similarly massive.

 

 

Rich:

Title:

The Bulge Radial Velocity Assay

 

Abstract:

A survey of 10,000 M giants in the Galactic bulge, BRAVA has now completed observations at b=-4, -6, and -8 deg, as well as a strip at +8 deg.  We report on the rotation and velocity dispersion profiles at these latitudes.  We find the first evidence of cylindrical rotation in the bulge/bar and we find no evidence of a dynamical "classical bulge" component at b=-8 deg.  The findings are consistent with a picture in which the bar population is a single dynamical structure.

 

 

Rix:

Title:

Measuring the Mass of the Milky Way's Halo (using 6000 distant halo stars from SDSS)

Abstract:

T.b.c.

 

 

Romero-Gomez:

Title:

Modeling spiral arms and rings with invariant manifolds. Application to the Galaxy.

 

Abstract:

T.b.c.

 

 

Ruzicka:

Title

Spatial motion of the Magellanic Clouds. Models is a dead end street?

 

Abstract:

I will introduce the current serious disagreement between the observational measurements of the proper motions of the Magellanic Clouds and their estimates based on the theoretical modeling of the evolution of the Clouds. The models devised so far seem to fail reproducing the large scale structures associated with the Clouds unless their spatial velocities are significantly lower than the measured ones. Recently, Kallivayalil et al. derived new values of the proper motion for the Large and Small Magellanic Clouds (LMC and SMC, respectively). The spatial velocities of both Clouds are unexpectedly higher than their previous values resulting from agreement between the available theoretical models of the Magellanic System and the observations of neutral hydrogen (HI) associated with the LMC and the SMC. Such proper motion estimates are likely to be at odds with the scenarios for creation of the large-scale structures in the Magellanic System suggested so far. We investigated this hypothesis for the pure tidal models, as they were the first ones devised to explain the evolution of the Magellanic System, and the tidal stripping is intrinsically involved in every model assuming the gravitational interaction. The parameter space for the Milky Way (MW)-LMC-SMC interaction was analyzed by a robust search algorithm (genetic algorithm) combined with a fast restricted N-body model of the interaction. Our method extended the known variety of evolutionary scenarios satisfying the observed kinematics and morphology of the Magellanic large-scale structures. Nevertheless, assuming the tidal interaction, no satisfactory reproduction of the HI data available for the Magellanic Clouds was achieved with the new proper motions. We conclude that for the proper motion data by Kallivayalil et al., within their 1-sigma errors, the dynamical evolution of the Magellanic System with the currently accepted total mass of the MW cannot be explained in the framework of pure tidal models. The optimal value for the western component of the LMC proper motion was found to be pm_w(LMC) > -1.3 mas/yr in case of tidal models. It corresponds to the reduction of the Kallivayalil et al. value for pm_w(LMC) by approx. 40% in its magnitude.

 

 

Sales:

Title:

On the formation mechanism of the Galactic thick disk

Abstract:

We compare the orbital properties of stars in four (published) simulations of thick disks formed by: i) accretion from disrupted satellites, ii) heating of a pre-existing thin disk by a minor merger, iii) radial migration and iv) gas rich mergers. We analyze the kinematics of local stars obtained for the “solar neighborhood'' in each model, finding characteristic imprints for each scenario. Based on these simulations we conclude that simple tests based on the orbits of local stars could shed some light on the formation path of the thick disk of our own Galaxy.

 

 

Sanchez-Conde:

Title:

Gamma-ray dark matter searches in the Milky Way

 

Abstract:

In the last few years, Imaging Atmospheric Cherenkov Telescopes (IACTs) have carried out gamma-ray observations of several astrophysical sources in the context of dark matter searches. Although without success for the time being, these searches have involved a large effort and still represent one of the main hot scientific objectives of these experiments.

Recent theoretical developments, in both particle physics and astrophysics, as well as the upcoming Fermi all-sky observations, invite us to keep optimistic for the near future. In this talk, I will review the present status of gamma-ray dark matter searches in the Milky Way system.

 

 

Zhao:

Title:

Dark Matter with Fifth Force

 

Abstract:

Dark Matter particles could interact with each other non-gravitationally by so-called fifth force.  I will show cosmological simulations of a class of such models.  Some of these models have implications on Dark Matter cusp, substructures, and Dark Energy.



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