We study the formation of early-type galaxies ( ETGs ) through mergers with a sample of 70 high-resolution ( softening length < 60 pc and 12 \times 10 ^ { 6 } particles ) numerical simulations of binary mergers of disc galaxies ( with 10 per cent of gas ) and 16 simulations of ETG remergers .
These simulations , designed to accompany observations and models conducted within the ATLAS ^ { 3 D } project , encompass various mass ratios ( from 1:1 to 6:1 ) , initial conditions and orbital parameters .
The progenitor disc galaxies are spiral-like with bulge to disc ratios typical of Sb and Sc galaxies and high central baryonic angular momentum .
We find that binary mergers of disc galaxies with mass ratios of 3:1 and 6:1 are nearly always classified as Fast Rotators according to the ATLAS ^ { 3 D } criterion ( based on the \lambda _ { R } parameter , see Emsellem et al .
2011 – ATLAS ^ { 3 D } Paper III ) : they preserve the structure of the input fast rotating spiral progenitors .
They have intrinsic ellipticities larger than 0.5 , cover intrinsic \lambda _ { R } values between 0.2 and 0.6 , within the range of observed Fast Rotators .
The distribution of the observed Fastest Rotators does in fact coincide with the distribution of our disc progenitors .
Major disc mergers ( mass ratios of 2:1 and 1:1 ) lead to both Fast and Slow Rotators .
Most of the Fast Rotators produced in major mergers have intermediate flattening , with ellipticities \epsilon between 0.4 and 0.6 .
Most Slow Rotators formed in these binary disc mergers hold a stellar Kinematically Distinct Core ( KDC ) in their \sim 1 -3 central kilo-parsec : these KDCs are built from the stellar components of the progenitors .
However , these remnants are still very flat with \epsilon often larger than 0.45 and sometimes as high as 0.65 .
Besides a handful of specific observed systems – the counter-rotating discs ( 2- \sigma galaxies , see Krajnović et al .
2011 , ATLAS ^ { 3 D } Paper II ) – these therefore can not reproduce the observed population of Slow Rotators in the nearby Universe .
This sample of simulations supports the notion of Slow and Fast Rotators : these two families of ETGs present distinct characteristics in term of their angular momentum content ( at all radii ) and intrinsic properties : the Slow Rotators are not simply velocity-scaled down versions of Fast Rotators .
The mass ratio of the progenitors is a fundamental parameter for the formation of Slow Rotators in these binary mergers , but it also requires a retrograde spin for the earlier-type ( Sb ) progenitor galaxy with respect to the orbital angular momentum .
We also study re-mergers of these merger remnants : these produce relatively round Fast Rotators or systems near the threshold for Slow Rotators .
In such cases , the orbital angular momentum dominates the central region , and these systems no longer exhibit a KDC , as KDCs are destroyed during the remergers and do not reform in these relatively dry events .