OUR PLACE IN THE UNIVERSE: ZOOMING INTO THE LOCAL GROUP
Most of this course will focus on the Local Group. Let's place the Local Group within its context in the
We have seen that CDM models of the universe predict a "soap-sudsy" universe, with matter concentrated into
What do we actually observe?
The earliest redshift surveys revealed the types of stringy structures now obtained
in the models.
Map of galaxies from the original CfA redshift survey out to about 230 Mpc. The famous
"stick man" is the Coma galaxy cluster.
Cluster of galaxies in Coma Berenices, near the North Galactic Pole. Kitt Peak image.
Modern surveys, like the Sloan Digital Sky Survey, reveal the filamentary large scale structure --
in this case to 2000 Mpc.
Map of 50,000 galaxies from the Sloan Digital Sky Survey Early Release Data to about 2 Gpc
for luminous red galaxies and 1 Gpc for the main galaxy survey.
Zooming by a factor of two makes the filaments and "voids" more obvious.
Map of 24,000 galaxies from the Las Campanas Redshift Survey out to about 900 Mpc.
And still 4X closer, here is a more recent version of the CfA redshift survey, with two hemispheric
Map of galaxies from the CfA-SSRS2 database out to about 230 Mpc.
The filamentary structure is obvious even in the sky distribution of galaxies:
All sky galaxy distribution in the 2MASS survey. The blue swaths represent the Milky Way disk.
From antwrp.gsfc.nasa.gov/ apod/ap030917.html.
Here is a more complete, albeit schematic view of local superstructures.
Cartoon map of the local universe on similar scale as above, and showing
nearest superclusters. From http://homepages.wmich.edu/~korista/phys325.html.
The nearest 125 Mpc shows that there are vast voids and several large mass concentrations.
From Hudson, 1993, MNRAS, 265, 43.
The largest nearby large cluster of several thousand galaxies is Virgo, which covers some 100 deg2.
The Virgo Cluster, with bright galaxies named (left; www.star.ucl.ac.uk/ ~apod/apod/ap010126.html)
and a zoomed view of the Leo Group in the
Virgo cluster (right; from www.randybrewer.net/ virgocluster.htm). Sixteen Messier objects are
Virgo, at about 17 +/- 5 Mpc, is at about the distance limit where some individual "normal"
stars (bright Cepheid variables)
can still be resolved with HST, and thus is an important step in the cosmological distance ladder.
Obviously, supernovae are stars that can be seen much farther and are keys to
the distance ladder, but these are rare.
Supernovae in Virgo are important for calibration of the distance scale against Cepheids.
The enormous mass of Virgo gives rise to a Virgocentric flow, seen as filaments of galaxies
falling in towards Virgo:
The Virgo Cluster has filaments of galaxies apparently connecting
to it. On the left is a map of the density of galaxies
from Tully (1982, ApJ, 257, 389). On the right is a naming scheme for
the filaments from Elmegreen, Galaxies & Galactic Structure.
The Virgocentric flow results in significant deviations in the local Hubble flow, especially visible in the
(a) The Hubble diagram for galaxies to 25 Mpc in the direction of Virgo showing deviation from Hubble flow.
(b) After correction for a Virgo infall of about 250 km/s.
From John Tonry's page, http://msowww.anu.edu.au/~heron/Tonry/tonry_p3.html.
The Milky Way and Local Group live on the outskirts of the Virgo supercluster.
That the Local Group is influenced by Virgo is seen by the Cosmic Microwave Background dipole anisotropy:
The CMB dipole anisotropy as seen by COBE. The color scale shows a spread of +/-3.5 mK.
The anisotrpy is actually not pointed at Virgo and is larger than our measured motion
with respect to Virgo galaxies. This indicates that more is going on than our infall into the Virgo cluster.
From http://zebu.uoregon.edu/1997/ph410/l12.html .
A breakdown of possible motions leading to the net motion of the Local Group, including possible
motion of the entire Local Supercluster toward the Hydra-Centaurus Supercluster.
From http://zebu.uoregon.edu/1997/ph410/l12.html .
CENSUS OF THE LOCAL GROUP
From Sparke & Gallagher, Galaxies in the Universe.
A view of the Local Group in 3-D by Eva Grebel. The dashed ellipses are centered on the
LG barycenter (462 kpc away, towards Andromeda). Distances of galaxies from the the arbitrarily chosen plane through the
Milky Way are indicated by solid lines (above the plane) and dotted lines (below). The small ellipses are large
spirals. The dEs are in yellow, dSphs in orange, the dIrrs are in blue, dSph/dIrr tranisition types are
in green. Note how most dIrrs are in fairly isolated regions away from large mass concentrations, and
dSphs cluster around the spirals.
If our detailed analysis of stellar populations in resolved galaxies is
presently limited to Local Group, how well does the LG represent
the universe of galaxies?
First, the dwarf to giant ratio of galaxies greatly varies
For dwarf defined as (MB < -16) and
giant as (-16 < MB < -11) the difference
in dwarf/giant is very different in the Local Group
(D/G ~ 3) that Virgo (~4-5) and Coma (~12) clusters.
The luminosity functions of galaxy groups and clusters .
From Trentham & Hodgkin 2002, MNRASm, 333, 423.
But it is not just the relative numbers of bright and faint
galaxies that varies by environment, but also the morphologies:
The distribution of galaxy types by their environmental density.
From Dressler, 1980, ApJ, 236, 351.
What kinds of galaxies in the universe are not well represented in the LG?