Thesis (M.Sc.)--Chulalongkorn University, 2010

We investigate the effects of chameleon scalar field on the effective density and effective pressure of a dark matter halo. Since the density from the scalar field is far smaller than the mass density of dark matter, we cannot see the effects of the chameleon scalar field on the deflection angle of light in the galactic scale. However, the fifth force from the chameleon experienced by the dark matter could modify the rotation curve of galaxies. We found that the fifth force makes the slope of rotation curves steeper around the center of galaxy for any dark matter profiles starting from the Navarro-Frenk-White (NFW) to the pseudo-isothermal (ISO) profile. The coupling constants of matter-chameleon from the rotation curves of low surface brightness (LSB) galaxies are constrained by 95% C.L. where the strongest constraint comes from ISO profile (≈10⁻³). Moreover, we demonstrate that the thick-shell non-singular boundary condition which forbids singular point leads to extremely stringent constraint on the matter-chameleon coupling (≲10⁻⁷) when applied to galaxy.