The Pisces-Perseus Supercluster

The Pisces-Perseus supercluster is one of the most prominent structures in the local extragalactic sky. We can examine this structure by looking at the two dimensional distributions of galaxies in the region of the supercluster.

The top diagram shows the distribution on the plane of the sky of galaxies in a 90 by 30 degree slice across the supercluster. The main ridge of the supercluster is roughly outlined by straight lines. This is the same feature seen in the Aitoff equal area projection of the nearest 15,000 galaxies. Only galaxies with measured redshifts are shown here.

Note the clumpiness of the galaxies, particularly the continuous structure within the outlined region. Notice also the absence of galaxies in other portions of the map. Some of the empty regions to the extreme left (east) and at the top (north) are partly caused by obscuration within the Milky Way. However, most of the structure seen in this map is real.

The lower diagram shows the distribution in redshift space of all of the galaxies contained within the area outlined in the top one. This representation does not give adequate comparison of nearby and distant structures (see below), but it is clear that the majority of galaxies seen in the enhanced region in the upper plot all lie at approximately the same redshift.

The main ridge of the Pisces-Perseus supercluster can be traced over 90 degrees across the sky. It lies at a mean redshift of about 5500 km/s and is is best described as a linear "filament" with an axial ratio of at least 8:1, inclined by less than 12 degrees to the plane of the sky. The supercluster extends over 45 Mpc in length before it disappears into the Zone of Avoidance on the east.

This diagram shows the redshift distribution of the same galaxies shown in the middle one, but in this case, the spatial coordinate is shown as a true angle. This representation is called a cone diagram and gives a more accurate comparison between nearby and distant structures.

Velocity crowding into narrow lanes of width about 250 to 500 km/s shows that most of the structures seen confined on the plane of the sky are also confined in the redshift dimension. They are therefore two-dimensional linear structures. Occasional larger spreads in velocity are seen in the regions of rich clusters, where orbital velocities within the cluster potential are added to the Hubble expansion velocity.

The relative isolation of the supercluster from the Local Supercluster in the foreground and from other structures at larger distances is emphasized in the cone diagram. The empty "voids" are non-spherical regions of true galaxy underdensity.


[back to the topics page]	[back to astro 201 home page]