Andreas Burkert Max-Planck Institute (MPE, Garching) Observatory University (Munich) Elena D’Onghia Observatory University (Munich) with

Understanding the formation of disk galaxies is linked to understanding the origin of its angular momentum General Theory (Fall & Efstathiou 1980): Gas had the same specific angular momentum of dark matter halos today Infalling gas conserves its specific angular momentum (Mestel 1963) Angular momentum origins from tidal torques Galactic disks with correct sizes!!!!!

Tidal torques impart angular momentum to dark matter halos Spin parameter Energy Mass hierarchical structure formation + General Theory Galactic disks with correct sizes!!!!! SEMI-ANALYTIC MODELS

However hydrodynamical N-body simulations of disk Formation in CDM yield disks too small The Angular Momentum Catastrophe of the gas !! Navarro & Steinmetz 2000 Abadi et al. 2003

D´Onghia, Murante & Burkert 2004, in preparation

Simulations show that galaxies are built up by merging of baryonic subclump rather than smooth accretion of gas. Much of the gas cools at the centre of subhalos and spirals toward the centre of parent halo, tranferring its orbital angular momentum to the dark matter Better fits to real disks if Stellar FEEDBACK is included (Sommer-Larsen et al. 2003; Abadi et al. 2003) Too dense and too massive bulges than real galaxies!!

Haloes that have had recent major mergers have larger Spin values! Vitvitska, Klypin et al. 2002, ApJ, 581, 799

We focus on this question: Is the halo merging history playing a role in building small disks?

STRATEGY We build up a set of CDM only simulations exploring the spin parameter of halos formed through various Merging history (we use Gadget code) We identify the last major merger Bulge/(Bulge+Disk) Spin parameter at z=0 For each halo:

D´Onghia & Burkert, ApJL submitted astro-ph/

in BBS sample of bulgeless galaxies estimated for disks based on 1000 Sb-Sdm galaxies (de Jong & Lacey 2000)

Net Result: halos dominated by minor merger accretion from z=3 to z=0 acquired by tidal torques only 2% of the angular momentum for their rotational support, whereas bulgeless galaxies acquired 6-7% POSSIBLE SOLUTIONS? Modification to the nature of the dark matter? Warm dark matter or Self-Interacting dark matter ? NO Feedback from early structure formation, e.g. from small sublumps before they merge? However, feedback processes are unlikely to provide the additional specific angular momentum for bulgeless galaxies Hard to form bulgeless galaxies