Infall Patterns in the Great Wall

The largest coherent structures discovered in recent surveys of the local Universe are large voids (empty regions) and sheets or walls of galaxies.  One of the largest of these nearby structures is the Great Wall, discovered in 1989 in the CfA Redshift Survey (right picture) This sheet contains several thousand galaxies, and stands out very clearly from the nearly empty regions surrounding it.  We know, however, that most of the Universe is made of Dark Matter.  The galaxies make up a very small part of the mass.  What is the distribution of mass in this picture like?  The positions of the galaxies only give us a partial picture.  We can, however, use the galaxies to help tell us where the matter is.  This is because over densities in the matter attract the galaxies, causing them to move.  The larger the over density, the faster the galaxies will move.

 

Therefore, by measuring the velocities of galaxies, we can measure the mass.  In particular, by measuring how fast the galaxies are falling onto the sheet (the infall velocity), we can measure the mass of the Wall.  The velocity of galaxies is not so easy to measure, however.  The redshift of the galaxy can be measured by obtaining a spectrum.  However, the redshift combines the effect of the expansion of the Universe with the velocity of the galaxy due to the gravitational pull of the over densities.  To remove the effect of the expansion, we need to know the distance to the galaxy.  This allows us to use Hubble's formula to measure: Vexpansion = HD where H is Hubble's constant. The velocity due to the effect of the over densities (the ``peculiar velocity'') is then, Vpeculiar = Vredshift - Vexpansion To measure the distance, we use the Tully-Fisher relation.  This is a relation between the Luminosity of a spiral galaxy (how bright it really is) and its rotation velocity. We can measure the rotation velocity of a galaxy by observing it in the radio.  In particular, the width of a special spectral line, the 21cm neutral hydrogen line, measures the rotation velocity of the spiral.   Measuring the width, then, we are measuring the true luminosity of the galaxy. By comparing that to how bright the galaxy appears in optical (or near-infrared) observations, we can measure its distance!  The radio observations are being carried out at the Arecibo radio telescope. The optical observations come from the Mount Hopkins 1.2m telescope and the Kitt Peak 0.9m telescope.