Three-dimensional hydrodynamical simulations of ISM pollution by type Ia and II supernovae in forming dwarf spheroidal galaxies Andrea Marcolini (Bologna University) Fabrizio Brighenti (Bologna University) Annibale D’Ercole (Bologna Observatory) with the enormous contribution of: Simone Recchi (Wien University) Francesca Matteucci (Trieste University) Vienna 12/04/06

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: Draco Sculptor Time (Gyr) Relative SFR

“The small dynamical mass of dwarf spheroidals means that their binding energy is small compared to the energy released by several supernovae, 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

Goals of the simulations  Find a galaxy model and a star formation history reproducing the amount of stellar content in a consistent way (without ejecting the whole ISM too soon).  Reproduce the range of the observed metallicity.  Reproduce the mass-metallicity relation.  See if a supernovae feedback model (Dekel & Silk) is able to explain the dwarf spheroidal (elliptical) properties.

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) One parameter family We construct a one parameter family models of galaxies ranging from 10^6- 10^10 solar masses. The only parameter is the luminosity (stellar mass) of the galaxy. Star formation history We 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 1998 Gas Component: Mgas = 0.2*Mdark, in hydrostatic equilibrium with T=Tvir Type Ia Supernovae: The same as before but following Greggio & Renzini 1983 in time. Assumptions for the Model:

Just a case: Draco g cm^-3 X axes (pc) Density profile of the three component for both models = Dark matter = Stellar component = Gas component

So high value of M/L are consistent with recent observations… Kleyna et al 2002 based on stellar radial velocity dispersion found a mass for Draco of 8 x 10 7 M O within 3 cores radii (=700 pc) Wilkinson 2004 based on 207 discrete stellar velocities inferred a halo mass of 1 x 10 8 M O for Draco and 2 x 10 8 M O for Ursa Minor. Lokas 2002 find a M/L V =100 inside two Sersic radii and a total mass typically of the order of 10 9 M O (M/L V =3500) Walker et al 2005 analyzing radial velocity of 178 stars in the Fornax dwarf found that Fornax has a more massive dark matter halo than found by previous studies (M/L V =10-40 in the sampled region) and proposed models with M= M O

Kazantzidis 2004 with high resolution N-body simulations showed that models of dwarf galaxies with isotropic and tangetially anisotropic velocity distributions for the stellar component fit the data only if the surrounding dark matter halos has maximum circular velocities in the range km s -1 Mayer et al made simulations of ram pressure stripping and tidal interaction and find a best model for Draco with V=42 km s -1. Mashchenko et al 2005 made N-body simulations of the formation of dwarf spheroidal galaxies and found that the best fitting models for Draco have a large range of DM halo virial masses ( M O ) Mashchenko et al 2006 made simulations of the interaction of Draco with our Galaxy and ruled out the “tidal dwarf” hypothesis and find that Draco is a cosmological halo with DM mass between 7 x 10 7 and 3 x 10 9 M O and the fraction of tidally stripped stars is < 3%....and theorical works:

Different star formation history:

= 1 burst = 10 burst = 25 burst = 50 burst (Myr)

results for model with a total M/L=80 Time evolution of the gas mass inside the dark matter halo Independent of the star formation history the galaxy loses its gas in a short period compared to the observed one (3 Gyr) and a few percent of stars are formed. = 50 bursts = 25 bursts = 10 bursts = instantaneous starburst Time (Myr)

Time evolution of the gas mass inside the dark matter halo More that 60% of the gas is still inside the dark matter halo, and is bound to the galaxy for this model Inside dark matter halo (1.2 kpc) Inside stellar region (600 pc)

SN II ejecta evolutionSN I ejecta evolution = period with SN II = total mass ejecta = mass ejecta inside time (Myr)

15 % inside the star forming region 60 % inside the galaxy region 17 % inside the star forming region 70 % inside the galaxy region 69 % of the total amount of Fe ejected after 3 Gyr come from SNIIs 31 % of the total amount of Fe ejected after 3 Gyr come from SNIs Inside dark matter halo Inside stellar region

Time evolution of the metallicity of the stars formed during the various bursts of star formation

e.g. Mateo (1998), Shetrone, Côtè & Sargent (2001) Aparicio, Carrara & Martinez-Delgado (2001) Only SN II SN II plus SN I Bellazzini, Ferraro & Origlia (2002)

model with 50 bursts model with 25 bursts Shetrone et al (2001): Our models

If the explosion of a SNIa occurres during the re-collapse phase its ejecta results to be much more localized. ISM density SNII ejecta density SNIa ejecta density

A high density region is formed, where the conditions for a new star formation episode is recovered, which has the dimension of the stellar region At the beginning of the simulation there is a high spread in metallicity, while at later times the SNII ejecta becomes more uniform.

.... Ram pressure stripping and tidal interaction? Marcolini, Brighenti & D’Ercole made simulations of disk-like dwarf in poor groups and found that ram pressure can remove most of the gas in dwarf galaxies with halo circular velocities lower than 30 km s -1. Mayer et at 2006 simulated the interaction of dwarf spheroidal galaxies with the Milky Way halo and found that a galaxy similar to Draco can be stripped completely of its gas in a time scale 2-3 Gyr if the gas is maintained at 10 4 K by some mechanism, a timescale comparable to our assumptions for the star formation. We are running now 2D simulation of the interaction of our model with the Milky Way Halo and find similar results.

Conclusions and Future work If a “low” M/L (~80) is assumed the gas is blown away after few star formation episodes, it doesn’t matter how weak they are. If we assume an extended dark matter halo and weak star formation a plausible scenario is possible, where the energy of the supernovae is radiated away and the galaxy is able to retain its gas for a period compatible with the observed star formation history. With a model M/L =280 (within 1.2 Kpc) we are able to simulate in the typical enrichment and the metallicity spread observed for this galaxy (Draco). We will run simulations in which the interaction of the galaxy with the Milky Way halo is taken consistently into account and with a self consistent star formation history. We’d like to extend this model at higher masses ( Msol) and try to simulate the mass-metallicity relation. We’d like to run models for galaxies like Sculptor, Fornax, NGC147, NGC5206.