2MASS Tully-Fisher Survey maps the mass of the local Universe
26 June 2015
In the expanding Universe, measurements of a source’s redshift refer to its relative velocity away from the observer, known as Hubble’s law, and its distance. Those galaxy redshifts that have been found to deviate from Hubble’s law are known as ‘peculiar velocities’: they are induced by the gravitational attraction of all surrounding matter. Peculiar velocities thus provide a tool to trace the overall matter density field and detect all gravitating matter, both visible and dark. They can be measured using redshift-independent distance indicators (‘standard candles’), including Type Ia Supernovae, the Tully-Fisher relation of spiral galaxies and the Fundamental Plane relation of elliptical galaxies.
The 2MASS Tully-Fisher Survey (2MTF) is an all-sky Tully-Fisher survey aiming to measure the redshift-independent distances of nearby bright spiral galaxies. To provide a uniform sky-coverage, the 2MTF team (including CAASTRO members Dr Tao Hong, Dr Chris Springob and Prof Lister Staveley-Smith at ICRAR-UWA) used the Green Bank Telescope and the Parkes radio telescope to make new high-accuracy HI 21-cm line observations of galaxies in the Northern and Southern hemisphere, respectively. Combining these data with the ALFALFA survey data and archived HI data, 2MTF provides uniform sky coverage down to low Galactic latitudes of up to 5º.
The bulk flow is the dipole component of the peculiar velocity field which traces the matter over-densities outside the survey region. In past measurements of the bulk flow in the local Universe, astronomers have largely agreed on the direction of the flow, but disagree on its amplitude. Some research showed a large bulk flow velocity which cannot be explained by the standard cosmological model, Lambda Cold Dark Matter (ΛCDM). Dr Hong and his colleagues measured the bulk flow of the local Universe at three different depths using the 2MTF sample and found that the bulk flow velocity is consistent with the ΛCDM model at the 1σ level.