We carried out the largest ( > 3.5 \times 10 ^ { 5 } Mpc ^ { 3 } , 26 deg ^ { 2 } ) H \alpha narrow band survey to date at z \sim 0.2 in the SA22 , W2 and XMMLSS extragalactic fields .
Our survey covers a large enough volume to overcome cosmic variance and to sample bright and rare H \alpha emitters up to an observed luminosity of \sim 10 ^ { 42.4 } erg s ^ { -1 } , equivalent to \sim 11 M _ { \odot } yr ^ { -1 } .
Using our sample of 220 sources brighter than > 10 ^ { 41.4 } erg s ^ { -1 } ( > 1 M _ { \odot } yr ^ { -1 } ) , we derive H \alpha luminosity functions , which are well described by a Schechter function with \phi ^ { * } = 10 ^ { -2.85 \pm 0.03 } Mpc ^ { -3 } and L ^ { * } _ { \mathrm { H \alpha } } = 10 ^ { 41.71 \pm 0.02 } erg s ^ { -1 } ( with a fixed faint end slope \alpha = -1.35 ) .
We find that surveys probing smaller volumes ( \sim 3 \times 10 ^ { 4 } Mpc ^ { 3 } ) are heavily affected by cosmic variance , which can lead to errors of over 100 per cent in the characteristic density and luminosity of the H \alpha luminosity function .
We derive a star formation rate density of \rho _ { \mathrm { SFRD } } = 0.0094 \pm 0.0008 M _ { \odot } yr ^ { -1 } , in agreement with the redshift-dependent H \alpha parametrisation from \citet 2013MNRAS.428.1128S .
The two-point correlation function is described by a single power law \omega ( \theta ) = ( 0.159 \pm 0.012 ) \theta ^ { ( -0.75 \pm 0.05 ) } , corresponding to a clustering length of r _ { 0 } = 3.3 \pm 0.8 Mpc/h .
We find that the most luminous H \alpha emitters at z \sim 0.2 are more strongly clustered than the relatively fainter ones .
The L ^ { * } _ { \mathrm { H \alpha } } H \alpha emitters at z \sim 0.2 in our sample reside in \sim 10 ^ { 12.5 - 13.5 } M _ { \odot } dark matter haloes .
This implies that the most star forming galaxies always reside in relatively massive haloes or group-like environments and that the typical host halo mass of star-forming galaxies is independent of redshift if scaled by L _ { \mathrm { H \alpha } } / L ^ { * } _ { \mathrm { H \alpha } } ( z ) , as proposed by \citet 2010MNRAS.404.1551S .