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Register a new userModelling the Milky Ways globular cluster system
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We construct a model for the Galactic globular cluster system based on a realistic gravitational potential and a distribution function (DF) analytic in the action integrals. The DF comprises disc and halo components whose functional forms resemble those recently used to describe the stellar discs and stellar halo. We determine the posterior distribution of our model parameters using a Bayesian approach. This gives us an understanding of how well the globular cluster data constrain our model. The favoured parameter values of the disc and halo DFs are similar to values previously obtained from fits to the stellar disc and halo, although the cluster halo system shows clearer rotation than does the stellar halo. Our model reproduces the generic features of the globular cluster system, namely the density profile, the mean rotation velocity. The fraction of disc clusters coincides with the observed fraction of metal-rich clusters. However, the data indicate either incompatibility between catalogued cluster distances and current estimates of distance to the Galactic Centre, or failure to identify clusters behind the bulge. As the data for our Galaxys components increase in volume and precision over the next few years, it will be rewarding to revisit the present analysis.
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The nuclear stellar disc (NSD) is a flattened stellar structure that dominates the gravitational potential of the Milky Way at Galactocentric radii $30 lesssim R lesssim 300{, rm pc}$. In this paper, we construct axisymmetric Jeans dynamical models of the NSD based on previous photometric studies and we fit them to line-of-sight kinematic data of APOGEE and SiO maser stars. We find that (i) the NSD mass is lower but consistent with the mass independently determined from photometry by Launhardt et al. (2002). Our fiducial model has a mass contained within spherical radius $r=100{, rm pc}$ of $M(r<100{, rm pc}) = 3.9 pm 1 times 10^8 {, rm M_odot}$ and a total mass of $M_{rm NSD} = 6.9 pm 2 times 10^8 {, rm M_odot}$. (ii) The NSD might be the first example of a vertically biased disc, i.e. with ratio between the vertical and radial velocity dispersion $sigma_z/sigma_R>1$. Observations and theoretical models of the star-forming molecular gas in the central molecular zone suggest that large vertical oscillations may be already imprinted at stellar birth. However, the finding $sigma_z/sigma_R > 1$ depends on a drop in the velocity dispersion in the innermost few tens of parsecs, on our assumption that the NSD is axisymmetric, and that the available (extinction corrected) stellar samples broadly trace the underlying light and mass distributions, all of which need to be established by future observations and/or modelling. (iii) We provide the most accurate rotation curve to date for the innermost $500 {, rm pc}$ of our Galaxy.
We present spatially resolved imaging and integral field spectroscopy data for 450 cool giant stars within 1,pc from Sgr,A*. We use the prominent CO bandheads to derive effective temperatures of individual giants. Additionally we present the deepest spectroscopic observation of the Galactic Center so far, probing the number of B9/A0 main sequence stars ($2.2-2.8,M_odot$) in two deep fields. From spectro-photometry we construct a Hertzsprung-Russell diagram of the red giant population and fit the observed diagram with model populations to derive the star formation history of the nuclear cluster. We find that (1) the average nuclear star-formation rate dropped from an initial maximum $sim10$,Gyrs ago to a deep minimum 1-2,Gyrs ago and increased again during the last few hundred Myrs, and (2) that roughly 80% of the stellar mass formed more than 5,Gyrs ago; (3) mass estimates within $rm Rsim1,pc$ from Sgr,A* favor a dominant star formation mode with a normal Chabrier/Kroupa initial mass function for the majority of the past star formation in the Galactic Center. The bulk stellar mass seems to have formed under conditions significantly different from the young stellar disks, perhaps because at the time of the formation of the nuclear cluster the massive black hole and its sphere of influence was much smaller than today.
We investigate the kinematic and photometric properties of the Segue 3 Milky Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g and r-band imaging. Using maximum likelihood methods to analyze the photometry, we study the structure and stellar population of Segue 3. We find the half-light radius of Segue 3 is 26 +/- 5 (2.1 +/- 0.4 pc, for a distance of 17 kpc) and the absolute magnitude is a mere M_V = 0.0 +/- 0.8 mag, making Segue 3 the least luminous old stellar system known. We find Segue 3 to be consistent with a single stellar population, with an age of 12.0 +1.5/-0.4 Gyr and an [Fe/H] of -1.7 +0.07/-0.27. Line-of-sight velocities from the spectra are combined with the photometry to determine a sample of 32 stars which are likely associated with Segue 3. The member stars within three half-light radii have a velocity dispersion of 1.2 +/- 2.6 km/s. Photometry of the members indicates the stellar population has a spread in [Fe/H] of <0.3 dex. These facts, together with the small physical size of Segue 3, imply the object is likely an old, faint stellar cluster which contains no significant dark matter. We find tentative evidence for stellar mass loss in Segue 3 through the eleven candidate member stars outside of three half-light radii, as expected from dynamical arguments. Interpretation of the data outside of three half-light radii, is complicated by the objects spatial coincidence with a previously known halo substructure, which may enhance contamination of our member sample.
We present a new, high-resolution chronographic (age) map of the Milky Ways halo, based on the inferred ages of ~130,000 field blue horizontal-branch (BHB) stars with photometry from the Sloan Digital Sky Survey. Our map exhibits a strong central concentration of BHB stars with ages greater than 12 Gyr, extending up to ~15 kpc from the Galactic center (reaching close to the solar vicinity), and a decrease in the mean ages of field stars with distance by 1-1.5 Gyr out to ~45-50 kpc, along with an apparent increase of the dispersion of stellar ages, and numerous known (and previously unknown) resolved over-densities and debris streams, including the Sagittarius Stream. These results agree with expectations from modern LambdaCDM cosmological simulations, and support the existence of a dual (inner/outer) halo system, punctuated by the presence of over-densities and debris streams that have not yet completely phase-space mixed.
The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive >1:10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the Milky Ways thin and thick stellar disc. However, it is interesting because: (i) its associated `dark disc has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we develop a chemo-dynamical template to hunt for the accreted disc. We apply our template to the high-resolution spectroscopic sample from Ruchti et al. (2011), finding at present no evidence for accreted stars. Our results are consistent with a quiescent Milky Way with no >1:10 mergers since the disc formed and a correspondingly light `dark disc. However, we caution that while our method can robustly identify accreted stars, our incomplete stellar sample makes it more challenging to definitively rule them out. Larger unbiased stellar samples will be required for this.
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