Galaxy Dynamics

Lab 11

The areas of the sky covered by various surveys

Redshift Surveys• Redshift surveys are ways of mapping the distribution of galaxies

around us• We use redshift as the measure of the radial coordinate in a

spherical coordinate system centered on the Milky Way• These are spectroscopic observations of about 1100 galaxies in a

strip on the sky 6 degrees wide and about 130 degrees long• We are at the apex of the wedge• The radial coordinate is redshift measured in kilometers per second

with the current best Hubble constant conversion of about 20 kilometers per second per million light years.

• That means the outer arc of the plot is at a distance of about 700 million light years

• The angular coordinate is the right ascension, or the celestial equivalent of longitude

Distribution of Galaxies

• This initial map showed that the distribution of galaxies in space was NOT random

• That with galaxies actually appear to be distributed on surfaces, almost bubble like, surrounding large empty regions, or ``voids''

a plot of the sky distribution of the galaxies, each point represents a galaxy in the northern celestial hemisphere that is brighter than an apparent blue magnitude of 15.5 and with a measured redshift inside 15,000 km/s. The nearest galaxies are shown in red, followed by blue, magenta, cyan and green.

Red V < 3000 km/s Blue V 3000- 6000 km/s Magenta 6000-9000 km/s Cyan 9000-12000 km/s Green >12000 km/s

CfA Redshift Survey Map

• The large red area at the center of the map (12 hours, +10 degrees) is the dense central region of the Virgo Cluster of galaxies which is the core of the Local Supercluster

• The dark blue points which dominate the RHS of the map (0-4 hours +30 to +40 degrees) show the location of the Pisces-Perseus Supercluster

• The cyan points at 15 hours running almost from the equator to +40 degrees declination are the Hercules Supercluster.

• The blank areas running N-S and looping over the north celestial pole are the regions of the sky inaccessible to optical surveys due to dust extinction when looking through the Milky Way, aka the "Zone of Avoidance"

The Great Wall

• perhaps the largest single structure yet detected in any redshift survey

• dimensions are ~ 600x250x30 million light years, sort of like a giant quilt of galaxies across the sky

Details of The Great Wall

• The Great Wall itself can be viewed in this CfA Survey with velocities between 6,500 and 11,500 km/s in the north galactic cap

• Galaxies with velocities of 6,500-8,500 km/s are plotted as red points and those 8,500-11,500 km/s are blue

• Because the Great Wall is slightly tilted in velocity space, the higher velocity end dominates the eastern portion of the figure

• The Great Wall is a surface with both low and high density regions with considerable 2-D structure --- embedded clusters and groups of galaxies as well as some filaments and other more amorphous structures

• The Coma Cluster is the dense region right near the center of the plot (13h and +29d)

The Local Group

• The Local Group of galaxies consists of:– 2 large spiral galaxies (Milky Way and M31)– the small Sc spiral M33– 2 large satellites of M31 (the E2 galaxy M32

and the dE NGC 205)– 2 large satellites of the Milky Way (the Large

and Small Magellanic Clouds)– And ~30 dIrr, dE and dSph dwarf galaxies,

mostly satellites of the Milky Way or M31

Differences in Galaxies

• The individual galaxies seem to have widely differing star formation histories

• The main difference in galaxy properties seems to be between large and dwarf galaxies

• In both spiral and elliptical galaxies, there is a difference in properties like total mass, surface brightness (both ↓ in dwarfs), and gas content of spirals (↑ in dwarfs)

Elliptical galaxies

• In large elliptical galaxies the main distinction is between “boxy” (blunt) and “disky” (pointed) ellipticals

• boxy ellipticals are larger and more massive, pressure-supported, and sometimes radio-loud

• disky ellipticals are smaller, more flattened, rotationally supported and always radio-quiet

• Disky ellipticals have much in common with S0 galaxies.

Hubble Constant

• The constant H is the rate of recession of distant astronomical objects per unit distance away

• This single number describes the rate of the cosmic expansion, relating the apparent recession velocities of external galaxies to their distance

• The more distant objects are receding more rapidly than closer ones seems to imply expansion of the universe, and is the main observation which led to the Big Bang theory

Not constant????

• The Hubble constant changes as a function of time depending on the precise cosmological models as the expansion of the universe slows due to gravitational attraction of the matter within it

• The current value of the Hubble constant is hotly debated, with two opposing camps generally getting values near the high and low ends of 50-100 km s-1/Mpc

• Using observations of Cepheid variables, Hubble constant has been reported as 83±13 km s-1/Mpc or 81±8 km s-1/Mpc

• Using supernovae, results favor a relatively small Hubble constant (slow expansion rate) of ~ 55 kilometers per second per megaparsec, which means that galaxies one megaparsec (3 million lightyears) distant appear to recede from us at a speed of 55 kilometers per second

Kinematic density waves

(a) nested orbits forming a bar(b) orbits offset to form two-armed spiral(c) 3:2 resonance (instead of 2:1) makes a 3-armed spiral(d) 4:1 resonance makes a 4-armed spiral.