We study the galactic bulges in the Auriga simulations , a suite of thirty cosmological magneto-hydrodynamical zoom-in simulations of late-type galaxies in Milky Way-sized dark matter haloes performed with the moving-mesh code AREPO .
We aim to characterize bulge formation mechanisms in this large suite of galaxies simulated at high resolution in a fully cosmological context .
The bulges of the Auriga galaxies show a large variety in their shapes , sizes and formation histories .
According to observational classification criteria , such as Sérsic index and degree of ordered rotation , the majority of the Auriga bulges can be classified as pseudo-bulges , while some of them can be seen as composite bulges with a classical component ; however , none can be classified as a classical bulge .
Auriga bulges show mostly an in-situ origin , 21 \% of them with a negligible accreted fraction ( f _ { acc } < 0.01 ) .
In general , their in-situ component was centrally formed , with \sim 75 \% of the bulges forming most of their stars inside the bulge region at z = 0 .
Part of their in-situ mass growth is rapid and is associated with the effects of mergers , while another part is more secular in origin .
In 90 \% of the Auriga bulges , the accreted bulge component originates from less than four satellites .
We investigate the relation between the accreted stellar haloes and the bulges of the Auriga simulations .
The total bulge mass shows no correlation with the accreted stellar halo mass , as in observations .
However , the accreted mass of bulges tends to correlate with their respective accreted stellar halo mass .