Peculiar velocity measurements are the only tool available in the low-redshift Universe for mapping the large-scale distribution of matter and can thus be used to constrain cosmology .
Using redshifts from the 2M++ redshift compilation , we reconstruct the density of galaxies within 200 h \mbox { $ { } ^ { -1 } $ } Mpc , allowing for the first time good sampling of important superclusters such as the Shapley Concentration .
We compare the predicted peculiar velocities from 2M++ to Tully-Fisher and SNe peculiar velocities .
We find a value of \beta ^ { * } \equiv \Omega _ { m } ^ { 0.55 } / b ^ { * } = 0.431 \pm 0.021 , suggesting \Omega _ { m } ^ { 0.55 } \sigma _ { 8 ,lin } = 0.401 \pm 0.024 , in good agreement with other probes .
The predicted peculiar velocity of the Local Group arising from the 2M++ volume alone is 540 \pm 40 km s ^ { -1 } , towards l = 268 \hbox { $ { } ^ { \circ } $ } \pm 4 \hbox { $ { } ^ { \circ } $ } , b = 38 \hbox { $ { } ^ { \circ } $ } \pm 6 \hbox { $ { } ^ { \circ } $ } , only 10 ^ { \circ } out of alignment with the Cosmic Microwave Background dipole .
To account for velocity contributions arising from sources outside the 2M++ volume , we fit simultaneously for \beta ^ { * } and an external bulk flow in our analysis .
We find that an external bulk flow is preferred at the 5.1 \sigma level , and the best fit has a velocity of 159 \pm 23 \ > \mbox { km~ { } s \mbox { $ { } ^ { -1 } $ } } towards l = 304 \hbox { $ { } ^ { \circ } $ } \pm 11 \hbox { $ { } ^ { \circ } $ } , b = 6 \hbox { $ { } ^ { \circ } $ } \pm 13 \hbox { $ { } ^ { \circ } $ } .
Finally , the predicted bulk flow of a 50 h \mbox { $ { } ^ { -1 } $ } Mpc Gaussian-weighted volume centred on the Local Group is 230 \pm 30 km s ^ { -1 } , in the direction l = 293 \hbox { $ { } ^ { \circ } $ } \pm 8 \hbox { $ { } ^ { \circ } $ } , b = 14 \hbox { $ { } ^ { \circ } $ } \pm 10 \hbox { $ { } ^ { \circ } $ } , in agreement with predictions from \Lambda CDM .