Astro 105: The Milky Way
Lecture XV:
Our Galaxy
Other Galaxies
The Large Scale Struture of the Universe
Our Galaxy, The Milky Way
The Milky way is a collection of more than
100 billion stars arranged in a disk-like pattern.
There are actually 3 main components of the
Milky Way
1) The disk - relatively younger (metal rich
stars)
2) The Halo - relatively older (metal poor stars)
3) The nuclear bulge
The disk has a diamter of about 25,000 pc.
(25 kpc)
This is where most of the gas and dust is.
The sun is located about 2/3 out from center
(bulge) in the disk.
The bulge is only about 3 kpc arcross.
Little gas and dust, lots and lots of stars.
Black hole at center?
Halo is, perhaps a few times larger than
disk.
No gas and dust, just stars and star clusters.
Disk has spiral arms.
Stars in disk move in circular orbits in
disk-plane around center.
Stars in halo move on elliptical orbits without any one orbital plane.
Note, if we measure the speed of a stars
orbit about the galactic center, and we know how far it is from the center we
can use
From chapter 16
M = a2/P3
Where a is radus and P is the period which can be found using a and the velocity v.
At the Sun we find M = .9x10 11 solar masses for the galaxy mass. But there is more mass out there...
How can we tell?
Rotation curves! Rotation speed vs distance from center
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Galactic Nucleus
The center of the galaxy (in the bulge) is a
strange and amazing region.
In the region of constellation Sagittarius, highly
obscured.
Dense region, stars are only 1000 AU apart!
Using radio and IR maps we see very fast
rotation rates indicating presence of Black Hole
(Sagittarius A*)
MBH = 3 x106
Solar Masses
Other Galaxies
Much of our knowledge of our own galaxy comes from studying other galaxies.
There are estimated to be 100 billion other galaxies in the universe.
Some are larger than the milky way, most are smaller.
Even a modest size galaxy contains more stars than all the people who have ever lived.
The most distant galaxies are so far away that the light we see them by left before the earth existed.
Mapping the galaxies astronomers have discovered the order and structure inherent to the Universe at the largest scales.
It also reminds of us of "Kepler's fourth law" 'The Universe doesn't revolve around you, ya know'
This is actually something called the Copernican Principle
We do not live in a special place or time in the Universe.
This is also called the principle of Cosmic Mediocrity
Galaxy Classification
Galaxies come in a variety of shapes and sizes.
The Milky Way and Andromeda are Spirals but that is not the rule.
Even when we take into account the different possible viewing angles it is clear that galaxies do not all look a like.
It was Edwin Hubble, the Telescope's Namesake, who classified the galaxy types.
4 types
Spirals, Barred Spirals, Ellipicals, Irregulars
Coma Cluster Of Galaxies - (a) A collection of many galaxies, each consisting of hundreds of billions of stars. Called the Coma Cluster, this group of galaxies lies over 100 million pc from Earth. (The blue spiked object at top right is a nearby star; virtually every other object visible is a genuine galaxy.) (b) A recent Hubble image of part of the cluster.
Spirals
Like the Milky Way all Spirals have a flattened disk and a central bulge.
There is a correlation between how tighly wound the arms are and how big the bulge is.
Tightly wrapped Arms = Large central bulge
Hubble Classified the spirals as Sa,Sb,Sc depending on the arm/bulge pattern.
Spirals show a lot of gas and dust and the bright hot blue stars in the arms show that star formation is occuring "now".
Shape Variations Among Spiral
Galaxies - Variation in shape among different spiral galaxies. As we progress
from type Sa to Sb to Sc, the bulges tend to get smaller, while the spiral arms
become less tightly wound.
You don't have to see the spiral arms to classify something as a spiral galaxy. The presence of a disk and a central bulge in a galaxy seen edge on is enough.
Barred Spirals
These are a variation of Spirals in Hubble's classification scheme.
Spiral arms project from the end of a central bar rather than directly from the bulge.
SBa, SBb, SBc
Some astronomers think Spirals and barred Spirals are similar enough to ignore them as two seperate groups.
Others say the bar must result from something which distinquishes the Barred and normal Spirals in terms of their origin and evolution
Shape Variations Among Barred-Spiral
Galaxies - Variation in shape among different barred-spiral galaxies. The
variation from SBa to SBc is similar to that for the spirals, except that now
the spiral arms begin at either end of a bar through the galactic center.
Ellipicals
Ellipical galaxies have no spiral arms.
They exhibit little internal structure at all!
They do show increaseing stellar density from the edge inward (a bright central nucleus)
Range in shape from circular to highly elongated
(circular) E0, E1, ..., E7 (elongated)
Shape Variations Among Elliptical Galaxies - Variation in shape among different elliptical galaxies. (a) The E1 galaxy M49 is nearly circular in appearance. (b) M84 is a slightly more elongated elliptical galaxy. It is classified as E3. Both these galaxies lack spiral structure, and neither shows evidence of interstellar matter.
Wide range in size and number of stars in ellipicals
Giant Ellipicals - few Megaparsecs accross, trillions of stars
Dwarf Ellipicals - a kiloparsec accross, a million stars.
(1 parsec (pc) = 3.2 lightyears)
Ellipticals contain little or no dust and gas.
No star formation!
Comparing them with the milky way - they seem to be all "Halo"
The stars in Ellipical galaxies have have random orbits (plunging orbits)
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There are also irregulars which are, ... well, irregular.
Midway between E and S/SB are the S0/SB0 galaxies which have a disk and a bulge and perhaps a bar but no arms.
Shape Variations Among S0 Galaxies - (a) S0 galaxies contain a disk and a bulge but no interstellar gas or spiral arms. They are in many respects intermediate between E7 ellipticals and Sa spirals in their properties. (b) SB0 galaxies are similar, except for a bar of stellar material extending beyond the central bulge.
Distribution of Galaxies in Space
We want to map out the loaction of galaxies in space.
Space is three 3-D. Location on th sky gives 2 of the 3 coordinates.
We also need distance - a hard thing to get in astronomy.
Getting distances requires finding an object whose intrinsic luminosity is known.
Remember the fomula
If you know how bright the object actually is, Bo, you can compare that to what you bright it appears, B, and this way get the distance, r, between you and the object.
Any object for which Bo is known is called a STANDARD CANDLE.
Standard Candles are the holy grails of astronomy.
Remember the Cepheid variables - Period-Luminosity relation.
Using standard candles astronomers have slowly extended their ability to get distances to ever more distant objects.
This is achieved using the so-called "distance ladder"
At each rung we have some technique (or standard candle) which is good out to some distance.
Then we think we find a new technique - a new standard candle - and we calibrate it using the old technique at nearby distances.
Distance Ladder For Various Cosmic Scale Measurement Techniques - The inverted pyramid summarizes the various distance techniques used to study different realms of the universe. Radar-ranging, stellar parallax, spectroscopic parallax, and variable stars take us as far as the nearest galaxies in our study of the universe. To go farther, new techniques must be employed, each based on distances known by techniques at lower levels.
Cepheid Variable Stars In Galaxy M100 - This sequence of six snapshots chronicles the rhythmic changes in a Cepheid variable star in the spiral galaxy M100. The Cepheid appears at the center of each inset, taken at the different times indicated in 1994. The star looks like a square because of the high magnification by the digital CCD camera-we are seeing individual pixels of the image. The 24th-magnitude star periodically doubles in brightness every 7 weeks.
The use of cepheids is good out to 15 Mpc.
This gives us a good idea of the "Local Group" the nearby collection of galaxies ( 1 Mpc of the Milky Way)
This is the next highest level of struture in the Universe after the Milky Way itself.
It really consists of two large spirals (us and andromeda) and a collection of dwarf ellipical and irregulars (M33 is a spiral)
These are all held together by their mutual gravitional attraction.
They are a Galaxy Cluster
The Local Group Of Galaxies - Diagram of the Local Group of some 20 galaxies within approximately 1 Mpc of our Milky Way Galaxy. Only a few are spirals; most of the rest are dwarf elliptical or irregular galaxies. Spirals and irregulars are shown in blue, ellipticals in red.
The next step up the ladder (to larger distance scales) requires new standard candles.
It has turned out that type I supernova can act as standard candles. This is good because they are soooooo bright they can be seen across the universe.
Another good technique is the Tully-Fisher relation.
It links the rotation speed of a spiral galaxy to its intrinstic luminosity (brightness).
The faster a galaxy rotates the brighter it is. (Why would this be?)
By looking at specral line broadening it is possible to find the rotation speed.
Doppler Effect From Galaxy's Rotation - A galaxy's rotation causes some of the radiation it emits to be blueshifted and some to be redshifted (relative to what the emission would be from an unmoving source). From a distance, when the radiation from the galaxy is combined into a single beam and analyzed spectroscopically, the redshifted and blueshifted components combine to produce a broadening of its spectral lines. The amount of broadening is a direct measure of the rotation speed of the galaxy.
The use of the Tully-Fisher relation is good out to 200 Mpc.
After the "Local Group" comes the "Virgo Cluster" in size
It is huge, containing more than 2500 galaxies.
All these galaxies are contained in a space only 3 Mpc across.
Portion Of Virgo Cluster - Part of the Virgo Cluster of galaxies, about 20 Mpc from Earth. Several large spiral and elliptical galaxies can be seen. The galaxy near the center is a giant elliptical known as M86.
There are other well defined clusters within 30 Mpc of us.
Thus there is a heirachy of scales and galaxies are not spread out evenly over space.
Several Clusters Of Galaxies In Local Universe - Schematic diagram of the locations of several galaxy clusters in our part of the universe. Our Milky Way is only one of these dots and our Local Group only one of the clusters of dots.
There are a few wandering loner galaxies but for the most part the Intercluster Medium is pretty empty.
On even larger scales there are Clusters of Clusters (Superclusters ).
The local supercluster contains more than 10,000 galaxies distributed in a patchy filimentary network (froth).
Plot Of The Local Supercluster - The Local Supercluster. Each of the 2200 points shown represents a galaxy, and the Sun is at the center of the diagram. The Virgo Cluster and the plane of our own Galaxy are marked. (Our Galaxy is seen edge-on. Its dust obscures our views to the top and the bottom, and two empty V-shaped regions on the map result.) The circle shown here is about 100 Mpc across.
Beyond the local superclusters are even richer collections of clusters of galaxies.
This pattern of structure emerging on ever larger scales contines up and up and has really challenged everyones ideas of the Universe.
Why - well just the shear size but also the time needed for galaxy to make these structures.
It poses a challenge to cosmology - a theory of the origin of the Universe.
The Expansion of Galaxies and The Universe
Relation Between Galaxy's Redshift And Distance - Optical spectra (on the left) of several different galaxies (on the right). The extent of the redshift (denoted by the horizontal yellow arrows) and the distance to each galaxy (in the center) increase from top to bottom.
Plot Of Recession Velocity Against Distance - Plots of recessional velocity against distance (a) for the galaxies shown in Figure 24.22, and (b) for numerous other galaxies within about 1 billion parsecs of the Earth.
Hubble Law On Cosmic Distance Ladder - Hubble's law tops the inverted pyramid of distance techniques. This last method is used to find the distances of astronomical objects all the way out to the limits of the observable universe.
Slice Survey Of Universe - The first slice of a survey of the universe, covering 1057 galaxies out to an approximate distance of 200 Mpc, clearly shows that galaxies and clusters are not randomly distributed on large scales. Instead, they appear to have a filamentary structure, surrounding vast, nearly empty voids. The distances shown assume H0 = 75 km/s/Mpc.
Plot Of 4500 Galaxies On Sky - Combination of data from several redshift surveys of the universe reveal the extent of large-scale structure within 200-300 Mpc of the Sun. The arc on the left is the Great Wall. The empty regions are mostly areas obscured by our Galaxy. Positions for more than 4500 galaxies are plotted here. We assume a Hubble constant of 75 km/s/Mpc.
Deepest View Of Heavens - (a) The present view of the formation of galaxies holds that large systems were built up from smaller ones through collisions and mergers, as shown schematically in this drawing. (b) This photograph, one of the deepest ever taken of the universe, shows objects down to 29th magnitude. It provides "fossil evidence" for hundreds of galaxy shards and fragments, most about 3000 Mpc distant.
Galaxy Masses
The use of rotation curves for determining galaxy masses works for other galaxies as well as our own.
Looking at the inner parts of the curves allows us to determine that spiral galaxies have masses between 1 - 5x1011 solar masses within 25 Kpc.
All the galaxies also show the flat curves at large distances which indicate the presence of dark matter.
Dark matter is material which only interacts with luminous matter through gravity
Rotation Curves For Nearby Spiral
Galaxies - Rotation curves for some nearby spiral galaxies indicate masses of a
few hundred billion times the mass of the Sun. The corresponding curve for our
own Galaxy is marked in red for comparison.
From Rotation Curves we find
that 99% of Universe is Dark Matter!!
What is Dark Matter?
Rocks? Exotic Particles?
Binary
Galaxies can also be used to determine combined mass using
Of couse you don't get a full orbit, just two velocities.
Lots of uncertainty with this method. If you have many different binary galaxies you can gather statistics.
From this method the Ellipical Galaxies have had their mass measured at 1011 to 1012 solar masses
In a similar way the mass of a gravitationally bound cluster can be determined.
Measurements of velocity are associated with the escape velocity of the cluster which yeilds M(Cluster)
M(Cluster) ~ 1013 - 1014 Solar Masses.
Intra-cluster Gas
For a long time it was debated if there was material between the galaxies in a cluster.
X-ray satillites were the first to detect the material.
The X-ray emission turns out show that there are vast quantities of material at very high temperatures in the clusters.
More
matter in emiting the X-rays then emits the other wavelengths (stars)
X-Ray Images Of Distant Galaxy Cluster Abel 85 - (a) X-ray image of Abell 85, an old, distant cluster of galaxies, taken by the Einstein X-ray satellite observatory. The cluster's X-ray emission is shown in orange. The green graphs display a smooth, peaked intensity profile centered on the cluster but not associated with individual galaxies. (b) The contour map of X rays is superimposed on an optical photo, showing its X rays peaked on Abell 85's central supergiant galaxy. Images like these demonstrated for the first time that the space between the galaxies within galaxy clusters is filled with superheated gas. (c) A ROSAT X-ray image of hot gas within another cluster of galaxies (called Abell 2256). The cluster is nearly a billion pc from Earth and measures about 3 Mpc in diameter.
The presence of so much gravitationally bound hot gas argues that the clusters posess a lot of dark matter (not just the dark matter haloes of the galaxies)