Why are the bulge particles coming out in the Gadget 2 simulation?

[bijuastro]

I am facing a strange error while evolving an isolated galaxy using GADGET-2. The IC condition contains halo, disc, bulge and gas. On evolution, I notice (when viewed using gadgetreader) particles of type 3 (i.e., bulge particles) in the system are escaping outwards. Soon after the initial stage, the bulge particles form a spherical shell whose radius keeps increasing indefinitely as the time progresses. I have played around with various ICs taken from papers. But I observe the same behaviour. What does it imply? How can it be solved?
I checked the smoothning lengths as well. I attached the figure for reference.

 

[mfb]

Are you sure you have suitable velocities for these particles? If they are way too fast, they'll spread out like a sphere.