Shallowed cusp slope of dark matter in disc galaxy formation through
  clump clusters
  Cusp-core problem is a controversial problem on galactic dark matter haloes.
Cosmological N-body simulations has demonstrated that galactic dark matter
haloes have a cuspy density profile at the centre. However, baryonic physics
may affect the dark matter density profile. For example, it was suggested that
adiabatic contraction of baryonic gas makes the dark matter cusp steeper.
However, it is still an open question if the gas falls into the galactic centre
in smooth adiabatic manner. Recent numerical studies suggested that disc
galaxies might experience clumpy phase in their early stage of the disc
formation, which could also explain clump clusters and chain galaxies observed
in high redshift Universe. In this letter, using numerical simulations with an
isolated model, we study how the dark matter halo responds to these clumpy
nature of baryon component in disc galaxy formation through the clump cluster
phase. Our simulation demonstrates that such clumpy phase leads to a shallower
density profile of the dark matter halo in the central region while clumps fall
into the centre due to dynamical friction. This mechanism helps to make the
central dark matter density profile shallower in the galaxies whose virial mass
is as large as 5.0*10^11 solar masses. This phenomenon is caused by reaction to
dynamical friction of the stellar clumps against the dark matter halo. The halo
draws the clumps into the galactic centre, while kinematically heated by the
clumps. We additionally run a dark matter only simulation excluding baryonic
component and confirm that the resultant shallower density profile is not due
to numerical artifact in the simulation, such as two-body relaxation.
