The chemical evolution of the peculiar Globular Cluster Omega CentauriAndrea Marcolini (Uclan, Central Lancashire) Antonio Sollima (Bologna University) Annibale DErcole (Bologna Observatory) Brad Gibson (Uclan) Francesco Ferraro (Bologna University) Fabrizio Brighenti (Bologna University) Simone Recchi (Trieste Observatory)

Dwarf spheroidals galaxies of the local group were originally thought to be very similar in their metallicity and star formation histories to the galactic globular clusters, but their star formation history is now known to be much more complex.

Star Formation History in Dwarf Spheroidal Galaxies:DracoSculptorTime (Gyr)Time (Gyr)Time (Gyr)Time (Gyr)Relative SFRRelative SFRRelative SFRRelative SFRMateo 1998 (Review)

The small dynamical mass of dSphs means that their binding energy is small compared to the energy released by several SNe, which leads the high metallicity spread and relatively high mean metallicity derived for these galaxies puzzling: how did the gas stay bound enough to have an extended star formation and gas enrichment? Babusiaux, Gilmore & Irwin 2005 It is not currently understood how low-mass dSphs managed to hold on to enoigh gas to form stars over an extended period of time.Simon & Geha 2007

Goals of the simulationsFind a galaxy model and a SFH able to reproduce the observed stellar content in a consistent way (e.g. without ejecting the whole ISM too soon).Reproduce the range of the observed metallicity and chemical properties of dSphs.

Type II Supernovae: 1 SN II every 100 Msol of formed stars, uniformally distributed in time for 30 Myr after each istantaneous burst. Stochastically distributed in space proportionally to the stellar density.Dark Matter Halo:Modified isothermal Halo obtained following Burkert 1995 so that M/Lv agrees with observations (e.g. Peterson & Caldwell 1992)Star formation historyWe choose several sequences of instantaneous bursts differing in number and intensity in such a way that the stellar mass formed after 3 Gyr is always the same.Stellar Component: King Model following Peterson & Caldwell 1992 and Mateo 1998Gas Component: Mgas = 0.18*Mdark, in hydrostatic equilibrium with T=TvirType Ia Supernovae: The same as before but following Matteucci & Recchi 2001 in time.Assumptions for the dSph Model:Marcolini et al 2006, MNRAS 371, 643

Draco:

If the explosion of a SNIa occurres during the re-collapse phase its ejecta results to be much more localized.ISM densitySNII ejecta densitySNIa ejecta densityMarcolini et al 2006, MNRAS 371, 643At the beginning of the simulation there is a high spread in metallicity, while at later times the SNII ejecta becomes more uniform.

15 % inside the star forming region (~600 pc) 60 % inside the galaxy region17 % inside the star forming region (~600 pc)70 % inside the galaxy regionInside dark matter halo Inside stellar region

SNIISNIa homogeneousInhomogeneous!!Shetrone et al. 2003

Draco, Fornax, Leo I, Sextans, Sculptor and Ursa Minor

Marcolini et al in preparation (Maybe)

How is the metal enrichment in the central region of the dSph?ISMSNII ejectaSNIa ejecta

Time~1Gyr7%30%W Cen!!!=Star with [alpha/Fe]

Time~1GyrSNe Ia produce a lot of Fe but expel relatively few metals (few percent) compared to the total number of SNe II.=Star with [alpha/Fe]

The Z is not proportional to the [Fe/H] content of the star

Consistent with a model in which the ancient dwarf galaxy lost most but not all of its gas with the first interaction with the Milky Way and its Halo (e.g. Mayer et al 2006).Marcolini et al 2007 astroph-07083445 MNRAS acceptedDid you miss it?

Simulated and observed [Fe/H] and [Ca/H] distributions.[Fe/H]=-1.3[Fe/H]=-0.6Sollima et al 2005Norris et al. 1996

Ref: Francois et al 1988; Brown & Wallerstein 1993; Norris & Da Costa 1995; Smith et al 1995 and 2000; Pancino et al 2002, Vanture et al. 2002; Origlia et al. 2003; Villanova et al. 2007

Age-metallicity relation

Isochrones:Cassisi et al 2004Pietrinferni et al. 2006

Extra Helium??AGB pollution is fundamental!!!

Main sequence[Fe/H]=-1.7[Fe/H]=-1.3ZThe need for Helium enrichment is reduced of 40%.

dSph nucleusConclusion:

Conclusion:Open problem and critical points:dSph: At the end of the simulation (3 Gyr) quite all the gas is still inside the dark matter potential well, we need a mechanism to remove it: ram pressure stripping (e.g. Marcolini at al 2003) plus tidal interaction (Mayer et al 2006). We need more models to take such an interaction into account.Omega Cen: One of the strong constrain of the model is the presence of a non negligible amount of alpha-depleted stars. Even if this is consistent with preliminary results (which I did NOT show you), these results must be confirmed. Even if the spread of the SGB-a is quite satisfactory its morphology is not. We fail in reproducing the double main sequence even if due to the inhomogeneous pollution the enigmatic extra Helium required is halved. We need more models taking the AGB pollution into account (plus MW interaction .... again)

Preliminary results form Pancino et al. (in preparation).

Conclusions: Under the hypothesis that Omega Cen is the survivor nucleus of a dSph we are able to reproduce the main features of this peculiar Globular Cluster. To fit better the observation we must assume that the SFH is constant till 1Gyr after that it drops to 0. after further 600Myr due to the interaction with the Milky Way. The total SF lasted ~1.6 Gyr. With this SFH, we are able to reproduce the [Fe/H] and [Ca/H] distribution as well as the general trend observed in the [alpha/Fe]-[Fe/H] diagram. In both these diagram the peculiar features are due to the inhomogeneous pollution of SNe Ia. Due to this inhomogeneous pollution the metal mass fraction content of the stars is not proportional to the Iron. The general properties of CMD diagram are reproduce quite satisfactory as well as the peculiar SGB-a and RGB-a, even if the morphology of the SGB-a is not satisfactory. We fail in reproducing the double main sequence but in our model the He content to reproduce it is halved.

Brown et al.

Small Iron Gradient