Our Galaxy and its stellar halo

While being only one galaxy among many, our own Galaxy - the Milky Way - is the one we can study in unique detail. Scientists at the Max-Planck-Institute for Astrophysics (MPA) have studied its formation and evolution by coupling high-resolution N-body simulations to semi-analytic methods to study the evolution of the baryonic components within dark matter haloes.

Fig. 1: Age-metallicity distribution for all (left panel) and spheroid (right panel) stars of the model Milky Way.

Fig. 2: Projected distribution of the star particles that end up in the stellar halo at different redshifts.

Fig. 3: Projected density profile of the stellar halo (solid black line) and of the dark matter halo (dashed back line). The solid green and orange lines show the projected density profiles for star particles with metallicity smaller and larger than 0.4 times the solar value respectively.

Our own Galaxy - the Milky Way - is a fairly large spiral galaxy consisting of four main stellar components: (1) the thin disk, that contains most of the stars in the galaxy, with a wide range of ages and on high angular momentum orbits; (2) the thick disk, that contains about 10-20 % of the mass in the thin disk and whose stars are on average older and contain less metals than those in the thin disk; (3) the bulge, which contains old and metal rich stars on low angular momentum orbits; and (4) the stellar halo which contains only a few % of the total stellar mass and whose stars are old and metal poor and reside on low angular momentum orbits.

Researchers Gabriella De Lucia (MPA) and Amina Helmi (Kapteyn Astronomical Institute) have studied the formation of the Milky Way and of its stellar halo by combining high-resolution N-body simulations with semi-analytic models of galaxy formation (linkPfeil.gifResearch Highlight May 2004). In these models, galaxies are assumed to form when gas condenses at the centre of dark matter haloes whose location and evolution is followed by taking advantage of the N-body simulation. The evolution of the baryonic components of galaxies (e.g. gas, stars, metals) is then "painted" on top of the history of dark matter haloes by adopting simple analytic prescriptions that are supported by observational data and/or theoretical arguments.

The physical properties of our model Milky Way are in quite nice agreement with observational measurements for our Galaxy. Fig.1 shows, for example, the distribution of ages and metallicities for all stars (left panel) and for stars in the spheroidal component (right panel) of the model Milky Way. The Figure shows that the Galaxy contains stars of all ages - which means they were produced on a long time-scale - but covering a limited range in metal content. In contrast, the stars in the spheroidal component all have old ages and a few of them have relatively low metallicities, in qualitative agreement with observational measurements.

In order to study the structure and metallicity distribution of the stellar halo, we assume that it builds up from the cores of the satellite galaxies that merged with the Milky Way over its lifetime. Fig.2 shows the projected distribution of the star particles that end up in the stellar halo, at different cosmic epochs. The star particles are colour coded as a function of their metal content, as indicated in the leftmost panel. The Figure shows that the star particles that end up in the stellar halo extend over a projected region of ~1 Mpc2 at z ~10. At z ~1, the star particles are already assembled in a single relatively elongated component which becomes progressively more spherical with decreasing redshift.

The rightmost panel of Fig.2 shows that there is no clear correlation between metallicity and distance from the centre of the stellar halo (i.e. no clear metallicity gradient), with low and high metallicity stars distributed at various distances. High metallicity stars are, however, more centrally concentrated than stars of low metallicity. This is shown more explicitly in Fig.3 which shows the projected density profile of the stellar halo (black), and of stars with metallicity larger (orange) and lower (green) than 0.4 times the solar value respectively. The probability of observing low-metallicity stars therefore increases at larger distances from the Galactic centre (> ~10-20 kpc) where the contribution from the inner more metal-rich stars is less dominant. The stronger concentration of high metallicity stars is due to the fact that the building blocks of the stellar halo lie on a well defined mass-metallicity relation, and that they are dragged closer to the inner regions of the halo by dynamical friction.

The numerical resolution of the simulations used in our study is too low for studies of spatially and kinematically coherent stellar streams in the present day stellar halo. Higher resolution simulations (linkPfeil.gifPress Release November 2008) are needed for these studies. These are all much needed steps to interpret the outcome of ongoing and future large surveys such as SEGUE, RAVE and ultimately GAIA, with the goal of unrevealing the evolutionary history of our Galaxy.

Gabriella De Lucia

Further information:

Gabriella De Lucia and Amina Helmi, The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach,
MNRAS in press, linkPfeilExtern.gifarXiv0804.2465