2011MNRAS.415L..74S

The nature of the dominant component of galaxies and clusters remains unknown. While the astrophysics community supports the cold dark matter (CDM) paradigm as a clue factor in the current cosmological model, no direct CDM detections have been performed. Faber & Visser have suggested a simple method for measuring the dark matter equation of state that combines kinematic and gravitational lensing data to test the widely adopted assumption of pressureless dark matter. Following this formalism, we have measured the dark matter equation of state for the first time using improved techniques. We have found that the value of the equation-of-state parameter is consistent with pressureless dark matter within the errors. Nevertheless, the measured value is lower than expected because, typically, the masses determined with lensing are larger than those obtained through kinematic methods. We have tested our techniques using simulations and we have also analysed possible sources of error that could invalidate or mimic our results. In light of this result, we can now suggest that understanding the nature of requires a complete general relativistic analysis.