We investigate the formation and evolution of the pseudobulge in “ Eris ” , a high-resolution N -body + smoothed particle hydrodynamics ( SPH ) cosmological simulation that successfully reproduces a Milky Way-like massive late-type spiral in a cold dark matter ( \Lambda CDM ) universe .
At the present epoch , Eris has a virial mass M _ { vir } \simeq 8 \times 10 ^ { 11 } { M _ { \odot } } , a photometric stellar mass M _ { * } = 3.2 \times 10 ^ { 10 } { M _ { \odot } } , a bulge-to-total ratio B / T = 0.26 , and a weak nuclear bar .
We find that the bulk of the pseudobulge forms quickly at high redshift via a combination of non-axisymmetric disk instabilities and tidal interactions or mergers both occurring on dynamical timescales , not through slow secular processes at lower redshift .
Its subsequent evolution is not strictly secular either , and is closely intertwined with the evolution of the stellar bar .
In fact , the structure that we recognize as a pseudobulge today evolves from a stellar bar that formed at high redshift , was destroyed by minor mergers at z \sim 3 , reformed shortly after , and weakened again following a steady gas inflow at z \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 316 $ } } 1 .
The gradual dissolution of the bar ensues at z \sim 1 and continues until the present without increasing the stellar velocity dispersion in the inner regions .
In this scenario the pseudobulge is not a separate component from the inner disk in terms of formation path , rather it is the first step in the inside-out formation of the baryonic disk , in agreement with the fact that pseudobulges of massive spiral galaxies have typically a dominant old stellar population .
If our simulations do indeed reproduce the formation mechanisms of massive spirals , then the progenitors of late-type galaxies should have strong bars and small photometric pseudobulges at high redshift .