Observational constraints on generalized Chaplygin gas model

The generalized Chaplygin gas model with parameter space $\alpha>-1$ is studied in this paper. Some reasonable physical constraints are added to justify the use of the larger parameter space. The Type Ia supernova data and age data of some clusters are then used to fit the model. We find that the parameters have bimodal distributions. For the generalized Chaplygin gas model, we also find that less free parameters fit the data better. The best fit model is the spatially flat model with baryons. The best fit parameters are: $\Omega_{m0}=0.044$, $w_{c0}=-0.881$ and $\alpha=1.57$. The transition redshift is $z_T=0.395$.