Although various cosmological observations congruously suggest that our universe is dominated by two dark components , the cold dark matter without pressure and the dark energy with negative pressure , the nature and origin of these components is yet unknow .
The generalized Chaplygin gas ( gCg ) , parametrized by an equation of state , p = - A / \rho _ { gCg } ^ { \alpha } , was recently proposed to be a candidate of the unified dark matter/energy ( UDME ) scenarios .
In this work , we investigate some observational constraints on it .
We mainly focus our attention on the constraints from recent measurements of the X-ray gas mass fractions in clusters of galaxies published by Allen et al .
( 2002,2003 ) and the dimensionless coordinate distances to type Ia supernovae and Fanaroff-Riley type IIb radio galaxies compiled by Daly and Djorgovski ( 2003 ) .
We obtain the confidence region on the two parameters fully characterizing gCg , A _ { s } \equiv A / \rho _ { { gCg } 0 } ^ { ( 1 + \alpha ) } and \alpha , from a combined analysis of these databases , where \rho _ { { gCg } 0 } is the energy density of gCg at present .
It is found that A _ { s } = 0.70 ^ { +0.16 } _ { -0.17 } and \alpha = -0.09 ^ { +0.54 } _ { -0.33 } , at a 95 % confidence level , which is consistent within the errors with the standard dark matter + dark energy model , i.e. , the case of \alpha = 0 .
Particularly , the standard Chaplygin gas ( \alpha = 1 ) is ruled out as a feasible UDME by the data at a 99 % confidence level .