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Defining the Local Group of Galaxies

written by: •edited by: Rebecca Scudder•updated: 5/13/2011

The Milky Way, our home galaxy, is not alone in the cosmos but is accompanied by a collection of galaxies gravitationally bound together, called the Local Group. Read on to find out about galactic clusters and superclusters.

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    Galaxy Clusters

    The Universe is expanding and galaxies are racing away from each other, as indicated by their redshifts. The further they are, the faster they are receding. But, on a more regional scale we find that galaxies tend to cluster together, drawn toward each other through the relentless pull of gravity. The spectra of these galaxies measured from within the cluster will show a blueshift, indicating that they are moving toward each other.

    There are “rich clusters” made up of 1000 or more galaxies, and there are “poor clusters”, which have less than 1000 members. Our group falls into the latter category since it consists of about 30 - 40 galaxies, depending on what reference you use. There is some evidence to indicate that in rich clusters there are more elliptical (type - E) and lenticular (type - S0 - spirals with no arms) galaxies, whereas in the poor clusters, spiral type galaxies tend to dominate. This may be because there is a larger number of collisions in the crowded rich clusters, as opposed to the sparsely populated poor clusters. Although, in our group, spirals seem to be in the minority.

    1.6 Million Local Galaxies in IR - Image courtesy of NASA, JPL, Caltech 

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    Our Group of Galaxies

    Edwin Hubble first used the term “Local Group” to describe the isolated group of about a dozen nebulae, or island universes that could be observed around our galaxy. These galaxies, as we know them today, showed a blueshift in their spectra as compared to the other galaxies that were further away and whose spectra exhibited a redshift.

    Our group is defined by its two largest members, the Milky Way and Andromeda (M31) galaxies. The center of mass for the group lies between these two galaxies.

    Each galaxy has a collection of satellite galaxies that accompany it. The more famous ones for the Milky Way are the Large and Small Magellanic Clouds, which are easily seen in the Southern Hemisphere, and Barnard’s Galaxy, NGC 6822, a barred irregular galaxy.

    Andromeda has M32, M110, and the spiral galaxy Triangulum (M33) also known as the Pinwheel galaxy, which is the third largest galaxy in the group and about half the size of the Milky Way. M33 is about 3 million light-years from our galaxy.

    The Local Group of Galaxies - Image courtesy of Review of the Universe, 

    The galactic cluster is loosely bound within a volume that extends over a 10 million light year diameter. Contained in this volume of space are:

    • 3 spiral galaxies
    • 2 elliptical galaxies
    • 12 irregular galaxies
    • 4 dwarf elliptical galaxies
    • 22 dwarf spheroidal galaxies
    • 5 galaxies of unidentified class.

    In time, Andromeda and the Milky Way will close the 2 million light-year gap that separates them and eventually collide. It will take about 3 billion years before this will happen, so it’s not something we have to worry about today. For an idea of what we are going to miss, take a look at this short video at the Hayden Planetarium’s web site.

    For a review of the some of the galaxies in the our group, check out the article "Survey of Our Nearest Galaxies."

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    Beyond Our Cluster of Galaxies

    Our cluster is not unique in the cosmos. It is part of a super-group called the Virgo Supercluster, which contains over 100 galaxy groups and clusters. This supercluster extends over a volume of space 110 million light-years across. There are millions of superclusters in the universe!. Local Supercluster from the 2MASS survey - Image courtesy of NASA and JPL/ Caltech The Local Supercluster image above contains approximately 6600 galaxies in a volume of 296,000 Mpc3 and shows galaxies which have spectral shifts (z) in the range:

    0< z < 0.01 ; 0 to 3000 km/s ; 0 to 41 Mpc

    The image includes our Local Group and nearby clusters - the Virgo Galaxy Cluster (D = 16 Mpc) and the Fornax Cluster. This image was produced with the data from the 2MASS survey.