Galaxies - PowerPoint PPT Presentation

Galaxies

Chapter Twenty-Six

• I posted as a way to help you a list of topics covered in Exam 2 (they are all in my class notes).The link is at:
• http://physics.gmu.edu/~mopher/ASTR113/topics_exam2.html

• How did astronomers first discover other galaxies?
• How did astronomers first determine the distances to galaxies?
• Do all galaxies have spiral arms, like the Milky Way?
• How do modern astronomers tell how far away galaxies are?
• How do the spectra of galaxies tell astronomers that the universe is expanding?
• Are galaxies isolated in space, or are they found near other galaxies?
• What happens when galaxies collide with each other?
• Is dark matter found in galaxies beyond the Milky Way?
• How do astronomers think galaxies formed?

spirals

elliptical

irregulares

• Most galaxies are in Groups and in Clusters
• Remote clusters of galaxies are receding from us…what is called the Hubble Law
• (this law reveals that the Universe is expanding)

A century ago the astronomers thought that the entire universe was only thousand light-year and nothing was beyondthe Milky Way Galaxy!

M31: Andromeda and its

satellites

Look at :

“The Local Group of Galaxies”

http://seds.lpl.arizona.edu/

messier/more/local.html

Kant in 1755 suggested that vas collection of stars lie outside of the Milky Way…

What he called “island universes”

In 1845, William Parsons built the largest telescope of the 19th century

1.8 meter in diameter (with no

Photographic equipment)

A modern view of the Sprial Galaxy M51

(its 8.5Mpc away frokm us; has glowing HII regions (sites of star formation))

And one arm extends toward the companion galaxy

Many astronomers of the 19th century disagreed with this notion of

Islands universes-they thought that the “spiral nebulae” were components

of our galaxy

In 1920 two opposing ideas were presented in the National Academy of Sciences

In DC:

Harlow Shapley from Mount Wilson Observatiory (was renowed from determining the

Size of our galaxy)…he thought that the spiral nevulae were relatively small objects

Like the globular clusters that he studied…

Vs

Heber D. Curtis from U. of California Lick Observatory (thought that each spiral nebulae

Were a rotating system much like MW)

…who solved the problem was a young guy

called Edwin Hubble…

In 1929 Hubble took a historic picture ofAndromeda…he used Cepheids:Given their period you can get their luminosity (Period-Luminosity relationship)(you also have to determine if it’s a Type I or Type II Cepheid metal poor or rich; different period-luminosity relationships

• Edwin Hubble used Cepheid variables to show that the “nebula” were actually immense star systems far beyond our Galaxy

Cepheids were 104LS so

For them to be so dim-they had

To be distant!

Lenticular galaxies are intermediate between spiral and elliptical galaxies

Spiral Galaxies: They contain young, hot, blue stars and associated HII regions indicating ongoing star formation…therefore these galaxies will be rich in metals…Population I…indeed the visible-light spectrum of the disk of a spiral galaxy has strong metal absorption lines….

By contrast there is little star formation in the central bulges of spiral galaxies…dominated by Population II that has low metal content…(central bulges has a yellowish or reddish color)

…Hubble classification of spirals…

Classification: Sa (smooth broad sprial arms and fat central bulge); Sb (moderate central size bulge); Sc (well defined spiral arms and a tiny central bulge)

The difference between Sa, Sb and Sc may be related

To the relative amounts of gas and dust that they contain

4% of the mass of a Sa galaxy is in gas & dust

8% for a Sb

And 25% for Sc

Sc has a greater proportion of its mass involed in star formation

(tiny bulge)

In barred spiral galaxies:

SBa has a large central bulge and thin tightly wound spiral armsSBb has a moderate central bulge and moderately wound spiral armsSBc has lumpy, loosely wound spiral arms and tiny central bulge(the difference might be related to the amount of gas and dust)

Bars appear to form

naturally in many spiral galaxies-they

Outnumber ordinary

Spirals by about two to one

J. Ostriker and P. Peebles a bar will not develop if a galaxy is surrounded by a sufficiently massive halo of dark matter …the difference between barred spirals and ordinary spirals may thus lie in

the amount of dark matter

Elliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibitedThey are mostly composed of old, red, Population II with little metals

Hubble classified them according to how round or flattened they look

E0: The roundest…and E7 the flattest…be aware that E1 and E2 might actually

Be a flattened disk that we just happen to view face-on….

Giant galaxies in Virgo Cluster

Two giant elliptical galaxies (about 20 times larger than an average

Galaxy)

You also have dwarf elliptical galaxies that are only fraction of the size

Of normal elliptical and contain so few stars that are completely transparent

Hubble first thought that the tuning fork diagram was an evolutionary sequence -> this is not the case!Elliptical galaxies have little or no overall rotation, while spiral and barred spiral have a substantial amount of overall rotationA more modern interpretation is that the Hubble tunning fork diagram is an arrangement of galaxies according to their overall rotationSa and Sba have enough rotation to form a disk altough their central bulge are still dominant; the galaxies with the greatest amount of rotation are Sc and SBcGalaxies that do not fit are the irregular galaxies. Irr I have many OB associations and HII regionsIrr II had asymmetrical distorted shapes that seem to have been caused by collisions

They are often found associated with other galaxies

For a Standard candle you want:1. Luminous Object (so you can see it at great distances)2. Be certain of it’s luminosity3. Easily identifiable4. Should be relatively common

For nearby galaxies:Cepheid (they can be seen out to 30Mpc and we have Period-Luminosity relationship)RR Lyrae can be used in the same way but because they are less luminous they can only be seen until 100KpcFor distant stars astronomers are using Type Ia supernovae (that occurs when a white dward in a close binary system accretes enough matter) A Type Ia supernovae can reach a luminosity of 3x109Ls

Brent Tully and Richard Fisher discover a new way of determining distances: by the width of 21-cm emission line of a spiral galaxy is related to the galaxy luminosity(Tully-Fisher relation)[Why: radiation from approaching side of a rotating galaxy is blueshifted while the one receding is redshifted. So the broadening is related to the rotation of the galaxy that is related to the mass that is related to the luminosity.]

For elliptical galaxies (that do not rotate) Marc Davis and George Djorgovski pointed out a relation between the size of the elliptical galaxy, the average motion and how the galaxy brightness will appear distributed: this is calledfundamental planeMeasuring 2 quantities we can get the actual size of the galaxy and by the apparent size get the distance

Standard candles, such as Cepheid variables and the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances

• Each technique can be used to callibrate the other…so a change oin the distance-measuring techniques for nearby objects can also have substantial effects on distances to remote galaxies!

• One distance-measuring technique that has broken free of the distance ladder uses observations of molecular clouds called masers
• “Maser” is an acronym for “microwave amplification by stimulated emission of radiation” (nearby luminous stars can stimulate water molecules in a maser to emit intensely at microwave wavelengths)…this maser technique is still in the infancy

There is a simple linear relationship between the distance from the Earth to a remote galaxy and the redshift of that galaxy (which is a measure of the speed with which it is receding from us)

• The redshift of a receding object:
• z = - 0/ 0

The value of the Hubble constant, H0, is not known with certainty but is close to 71 km/s/Mpc

• The value of H0? Depends on the determination of the distance (vary depending on the technique between 40-100km/s/Mpc)
• Because the value of H0 is somewhat uncertain usually
• We express the distance in terms of redshift z (that can be measured very accurately)
• The greater the redshift of a distant galaxy the greater its distance