The Dual Origin of a

Simulated Milky Way Halo

Adi Zolotov (N.Y.U.), Beth Willman (Haverford), Fabio Governato, Chris Brook (University of Washington, Seattle), Alyson Brooks

(CalTech)We investigate the origin of stellar halos using high resolution cosmological SPH + N-Body simulations that include all the major physical processes involved in galaxy formation self-consistently (Governato et al 2007). This simulation of a Milky Way mass galaxy (MW1hr) allows us to study the competing importance of dissipative in-situ star formation and accretion of of subhalos in the building of a stellar halo in a lambda CDM universe. We find that in-situ stars compose 20% of an SDSS-like observational sample of this galaxy’s observed stellar halo.

In-Situ vs. Accreted Halo StarsRecent observational work has revealed substantial streamy substructure in our Galaxy, showing that the Milky Way’s halo has been built at least partly from the accretion and disruption of dwarf galaxies. The possibility remains that the halo is in part composed of an “in-situ” population of stars which were formed during the initial collapse of the Galaxy. Identifying such an in-situ component in the Milky Way’s halo has proven to be non-trivial, but is an important piece in understanding just how much of the stellar halo is accreted through mergers predicted by CDM models.

A kinematic analysis of the stars in MW1hr was done to decompose it into disk, thick disk, bulge, and halo components (Brook in prep). All particles in the simulated box are traced back to z=6. We find that this galaxy is surrounded by a kinematically defined halo with more than one distinct population. “Accreted” stars are those which formed inside of halos other than MW1hr’s dark matter halo, but have since become unbound to their progenitor and now belong to MW1hr’s halo. “In-situ” stars are those which were originally brought into the primary’s halo as gas particles and then formed within the primary’s potential well. Unlike other numerical approaches to investigate the makeup of stellar halos through pure accretion (e.g. Johnston et al, 2008), we are able to study both the in-situ and accreted component of a stellar halo. In the SDSS-like sample shown in Figure 1, 20% of the stars in MW1hr’s observed halo were formed in-situ.

Fig. 1. Observing our simulation like the Sloan Digital Sky Survey. Black points represent all stellar particles in the simulation which are 30 degrees above the plane of the baryonic disk for this observer. Grey points represent stars satisfying a distance cut for proper motions.

Fig 3. Accretion time of halo stars. The majority of the inner halo is composed of stars accreted more than 10 Gyr ago, while the outer halo is dominated by late accretions.

Observational Tracer of the Dual Origin?

A simulated SDSS-like observational sample of inner halo stars contains stars which have been both accreted, and those which formed in-situ. These “observed” stars are 8% of the overall stellar halo of MW1hr. The accreted stars in this sample were stripped mainly from a few large progenitors accreted early in the formation of the galaxy. Despite their different formation mechanisms, accreted stars and in-situ stars in the inner halo were both in place in the halo more than 10 Gyr ago. Since the dynamical timescale of the inner halo is quite short in comparison to that of the outer halo, both populations of stars have time for phase space information revealing their origins to be erased. This can be seen in Figure 4, which shows the U, V, W velocities of MW1hr’s halo stars. The mean rotation and velocity dispersions of both the accreted and in-situ populations are comparable to each other, making them indistinguishable through kinematics for an SDSS observer whose velocity measurement errors are of order 10 km/sec.

Fig. 2. The relative contribution of each kinematic galactic component of MW1hr for the simulated observational sample. The hatched region represents in-situ stars located in the halo.

On-going Investigations• Resolution testing

• Comparing different realizations of MW-mass galaxies

• Look at energy - angular momentum distribution of halo stars

• Simulating metal distribution with metal diffusion

• Look for [Fe/H] and [O/Fe] trends between in-situ and accreted halo stars

Fig. 5. The MDF of all halo stars in a run with metal diffusion. Halo stars in the outer regions of the galaxy are more metal poor, as observed in the Milky Way. The in-situ halo stars, formed extremely early, are overall more metal poor than their early accreted counterparts in the halo. This result hints that chemical abundances may prove to be a more reliable method in differentiating these two populations than phase space mapping.

Fig. 4. The space velocities of all stars in the SDSS-like observed sample of MW1hr.

ReferencesBrook, C. et al, in prep

Brooks A. M., Governato F., Booth C.M., Willman B., Gardner J. P., Wadsley J., Stinson G., Quinn T., 2007, ApJ, 655, L17

Governato F., Willman B., Mayer L., Brooks A., Stinson G., Valenzuela O. , Wadsley J., & Quinn T. 2007, MNRAS, 374, 1479

Governato F., Mayer L., Brook C., 2008, preprint (arXiv:0801.1707)

Johnston K., Bullock J., Sharma S., Font A., Roberston, B., Leitner S, 2008, preprint (arXiv:0807.3911v1)

Accreted Halo Stars In-situ Halo Stars

The Simulation

The Milky Way mass galaxy (MW1hr) analyzed here was run to z=0 as part of a larger study (Governato et al 2007, Governato et al 2008, Brooks et al 2007, Brook in prep), and has Mvirial of 1.1 x 1012 M , Rvirial of 270 kpc, and Nparticles, virial ~ 4.8 x 106 (dark matter, gas and stellar particles).