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Paper text:
the understanding of galaxy formation remains a primary goal in astrophysics . within the current accepted scenario of structure formation
, small galaxies are built up first and larger objects are assembled through the merging of less massive building blocks , in a hierarchical way . in this context , mergers and interactions play a crucial role modifying the properties of galaxies such as their mass distribution and star formation activity and affecting the subsequent evolution of the systems .
the formation of disc - like systems is still a matter of debate and several scenarios have been proposed in order to explain the growth of these two components and their interactions .
the simplest scenarios currently accepted for the formation of the bulges are two : the first one establishes that the bulge and the disc are formed independently ( andredakis et al .
1995 ) , and the second one assumes that the disc forms first and the bulge emerges from it as a consequence of gas inflow during a period of secular evolution ( courteau et al .
1996 ) .
numerical simulations are a powerful tool to tackle galaxy formation since they can account for the joint evolution of baryons and dark matter in a cosmological framework .
we use smooth particle hydrodynamical ( sph ) simulations to investigate the effects of mergers and interactions on the determination of the structural properties of galaxies .
we analize galactic objects identified from numerical sph simulations that take into account the gravitational and hydrodynamical evolution of the matter in cold dark matter universes .
we run two simulations with @xmath0 ( s1 ) and @xmath1 ( s2 ) total number of particles and the following cosmological parameters : @xmath2 , @xmath3 , @xmath4 ( s1)and @xmath5 , @xmath6 , @xmath7 ( s2 ) , where ( @xmath8 ) .
the simulated boxes represent cubic volumes of @xmath9 ( s1 ) and @xmath10 ( s2 ) mpc length .
we select galaxy - like objects ( glos ) at @xmath11 by using a density - contrast criterium and follow their evolution with look - back time , constructing their star formation and merger histories ( see scannapieco & tissera 2003 for details ) . with the aim at investigating the effects of mergers on the structural properties of the systems we perform bulge - disc decompositions to the projected mass surface density of the glos at @xmath11 , and of the progenitors during merger events , assuming an exponential profile for the discs and the srsic law for the bulges .
galaxy - like objects identified in our simulations are found to have structural parameters similar to those of observed spiral galaxies ( siz et al .
2001 ) .
tissera et al .
( 2002 ) showed that during merger events , early gas inflows can be triggered depending on the stability properties of the systems .
those with shallower potential wells are more likely to suffer gas inflows since they can be more strongly perturbed from the incoming satellite .
secular evolution phases feed the bulge components which significantly contribute to the stability of the discs .
the actual fusion of the baryonic cores increases the star formation activity which in turn affect the subsequent properties of the systems .
understanding the effects of these two processes ( i.e. secular evolution and fusion ) on the growth of the disc and bulge components in galaxies can help us to enlighten the picture of galaxy formation . to this end
we analize the evolution of the mass distribution of galaxy - like objects during merger events by studying the variation of their structural parameters .
we find that secular evolution during the orbital decay phase produces the largest changes in the structural parameters of the systems : bulges get larger with respect to the discs and with more exponential profiles .
these changes are found to be more important in unstable systems with shallower potential wells .
fusions introduce further modifications in the opposite direction although they are less important .
we also find that secular evolution phases contribute to establish the observed fundamental relations : after secular evolution , bulges tend to satisfy the observed fundamental plane and the disc components the tully - fisher relation .
fusions may add some dispersion but do not significantly change them ( smith castelli et al .
departures from the fundamental relations could be indicating a lack of secular phases .
our findings suggest that interactions play a fundamental role being able to transform discs into spheroids .
discs can also survive violent events , being either partially destroyed or just feeded by the colliding object .
the stability properties of the initial system seem to be a fundamental point in the response of the disc to the incoming satellite .
other factors such as orbital parameters and relative masses of the colliding objects would also affect the determination of the structural properties of the outcome systems .
the different possible histories of evolution of galactic systems in hierarchical universes could potentially explain the diversity of galaxies in our universe .
we use cosmological sph simulations to investigate the effects of mergers and interactions on the formation of the bulge and disc components of galactic systems . we find that secular evolution during mergers seems to be a key process in the formation of stable disc - bulge systems with observational counterparts and contributes to establish the fundamental relations observed in galaxies . 
 our findings suggest that the secular evolution phase couples the formation mechanisms of the bulge and disc components . according to our results , depending on the particular stability properties and merger parameters 
 , violents events could drive a morphological loop in which the outcome could be a disc or a spheroid .