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Paper text:
the development of tidal tails during galaxy encounters is predominantly a gravitational effect ; as a result , both gas and stars should have similar kinematics and spatial morphology on large scales .
however , in a significant number of systems the hi and stellar tidal debris do in fact show marked spatial offsets , including the tidal tails of ngc 520 ( hibbard , vacca , & yun 2000 ) , 7714/5 ( smith etal 1997 ) , and , most dramatically , ngc 3690 ( hibbard etal 2000 ) . at face value , these observations suggest that hydrodynamic forces may also play a role in the global evolution of the tidal tails , via effects such as an interaction between the tidal gas and a hot gaseous halo or starburst wind ( hibbard etal 2000 ) . here
i show that the offsets arise naturally from the differing radial distributions of gas and stars , coupled with interaction - driven dissipation and inflow in the tidal gas .
no hot gas component is needed to explain these offsets .
previous dynamical modeling of merging galaxies ( e.g. , barnes & hernquist 1991 , 1996 ; mihos & hernquist 1996 ) showed no evidence for segregation of gas and stars in tidal debris .
however , in those models , both the gas and stars had identical exponential spatial distributions , whereas the gas distribution in disk galaxies is typically much more spatially extended than the starlight .
i have rerun the fiducial merger model of mh96 , using an gas distribution which is exponential within @xmath0 ( where @xmath1 is the stellar disk scale length ) , then falls as @xmath2 until a truncation radius of @xmath3 .
figure 1 shows the model just prior to the final merging , and a prominent gas / star offset can be seen in the prograde tail .
the fact that we see an offset at all argues that these offsets need not be the result of an interaction between the tail gas and a hot gas component , since no such hot gas component is present in our model .
is the offset due solely to the differing initial spatial distribution of the gas and stars ?
a simulation was run identical to the first , but with the gas particles evolved collisionlessly . in this case , no gas / star offset developed , indicating that hydrodynamic forces are at work in shaping the tails . by analyzing a subset of tail particles in the full hydrodynamical model
, we find that both the gas and star particles seen in the tail are following a largely collisionless evolution on large scales .
the gas in the tail comes from larger initial radius than the stars ; its systematically larger angular momentum , results in a leading gas tail .
so what happened to the gas that would have been cospatial with the stars ?    to answer this question , we followed the evolution of with gas particles that came from the same region of initial ( i.e. , pre - encounter ) phase space as the stars which ended up in the tidal tail .
because of the exponential distribution of stars , these gaseous `` phase space partners '' come from small radius in the disk and are more subject to collisional dissipation and inflow during the encounter . as a result ,
this gas decouples from the collisionless evolution that the stars follow , flowing inwards and leaving the stellar tail gas - poor .
in essence , it is not the hydrodynamic evolution of the gas _ observed _ in the tail which leads to the offset , but rather the hydrodynamic decoupling and inflow of the low angular momentum gas which would have otherwise filled in the stellar tail .
we use numerical simulations to study the development of gas / star offsets in the tidal tails of merging galaxies . 
 these offsets are shown to be a natural consequence of the radially extended hi spatial distribution in disk galaxies , coupled with internal dissipation in the gaseous component driven by the interaction . 
 this mechanism explains the observed gas / star offsets in interacting galaxies without invoking interactions with a hot ( unseen ) gaseous component .