Structure, kinematics and chemical enrichment patterns after major gas-rich disc-disc mergers

We used an N-body smoothed particle hydrodynamics algorithm, with a detailed treatment of star formation, supernovae feedback and chemical enrichment, to perform eight simulations of mergers between gas-rich disc galaxies. We vary the mass ratio of the progenitors, their rotation axes and their orbital parameters and analyse the kinematic, structural and chemical properties of the remnants. Six of these simulations result in the formation of a merger remnant with a disc morphology as a result of the large gas fraction of the remnants. We show that stars formed during the merger (a sudden starburst occurs in our simulation and lasts for 0.2-0.3 Gyr) and those formed after the merger have different kinematical and chemical properties. The first ones are located in the thick disc or the halo. They are partially supported by velocity dispersion and have high [alpha/Fe] ratios even at metallicities as high as [Fe/H] = -0.5. The former ones - the young component - are located in a thin disc rotationally supported and have lower [alpha/Fe] ratios. The difference in the rotational support of both components results in the rotation of the thick disc lagging that of the thin disc by as much as a factor of 2, as recently observed. We also find counter-rotating stars in both the old and young populations. A variety of structures are formed during the merger, i.e. most simulations form a ring of young stars and two simulations formed a bar. The scalelength of the thick disc is either equal to that of the thin disc or larger by factors of up to 1.60 and in six out of the eight simulations, the thin and thick discs both have exponential luminosity profiles and are nearly coplanar. We find that, while the kinematic and structural properties of the merger remnant depend strongly upon the orbital parameters of the mergers, there is a remarkable uniformity in the chemical properties of the mergers. This suggests that general conclusions about the chemical signature of gas-rich mergers can be drawn.