© sierra college astronomy department1 the diverse galaxies

© Sierra College Astronomy Department© Sierra College Astronomy Department 11

The Diverse GalaxiesThe Diverse Galaxies



Galaxy ClassificationGalaxy Classification Until the 1920s, we thought of our own galaxy Until the 1920s, we thought of our own galaxy

as the “Island Universe” and that everything we as the “Island Universe” and that everything we saw lay in our galaxysaw lay in our galaxy

In 1924, Edwin Hubble found Cepheid variables In 1924, Edwin Hubble found Cepheid variables in three spiral nebulae, including one in in three spiral nebulae, including one in Andromeda, proving that they were actually Andromeda, proving that they were actually spiral galaxies.spiral galaxies.

The proof that galaxies existed outside the Milky The proof that galaxies existed outside the Milky Way expanded the scope of the universe.Way expanded the scope of the universe. Today it is estimated that over 100 billion galaxies exist in

the visible Universe.

The Hubble ClassificationThe Hubble Classification Hubble divided galaxies into three basic types: Hubble divided galaxies into three basic types:

spiralspiral, , ellipticalelliptical, , irregularirregular..



Galaxy ClassificationGalaxy Classification

A A spiral galaxyspiral galaxy is one that is characterized by is one that is characterized by bright spiral arms and a central bulge. They have bright spiral arms and a central bulge. They have both a both a spheroidalspheroidal and and diskdisk component component

An An elliptical galaxyelliptical galaxy is one of a class of galaxies is one of a class of galaxies that have smooth spheroidal shapes. They have that have smooth spheroidal shapes. They have only a only a spheroidalspheroidal component component

An An irregular galaxyirregular galaxy is a galaxy of irregular shape is a galaxy of irregular shape that cannot be classified as spiral or elliptical.that cannot be classified as spiral or elliptical.

Each major classification contains subdivisions Each major classification contains subdivisions and come in variety of sizesand come in variety of sizes Dwarf galaxiesDwarf galaxies: as few as 100 million stars: as few as 100 million stars Giant galaxiesGiant galaxies: as great as 1 trillion stars: as great as 1 trillion stars


The Diverse Galaxies

Galaxy Characteristics

Spiral Galaxies Have relatively thin and flat whitish disks

with disk component stars (all ages and masses with circular orbits in the disk).

Have yellowish central bulges with spheroidal component stars (old and low mass with orbits of all inclinations).

Bulges merge smoothly into halos that can extend to a radius beyond 100,000 light-years.




Spiral Galaxies (continued)

Disks are filled with cool gas and dust interspersed with hotter gas, and usually display spiral arms, while bulges/halos exhibit very little cool gas or dust.

Large bulge galaxies generally have less gas and dust than small bulge galaxies.

Among the large galaxies in the Universe, 75-85% are spiral or lenticular (“spiral” disks with no arms).




Elliptical GalaxiesElliptical GalaxiesMost of the galaxies in existence are Most of the galaxies in existence are

ellipticals, but most of these are smaller ellipticals, but most of these are smaller than spiral galaxies.than spiral galaxies.

A few giant elliptical galaxies have 10A few giant elliptical galaxies have 101313 stars and are thus larger than any spiral stars and are thus larger than any spiral galaxy.galaxy.




Elliptical Galaxies Lack a significant disk component of stars. Look like the bulges and halos of spiral galaxies

(and hence sometimes referred to as spheroidal galaxies).

Compared to spirals, ellipticals appear redder, rounder, and often longer in one direction than in the other two.

Redder color infers ellipticals do not have hot, young, blue stars.

Have very little cool gas or dust, though often contain very hot, ionized gas.




Elliptical Galaxies (continued)

Come in wide ranges of sizes.Some of the most massive galaxies are

giant elliptical galaxies.Majority are small and are the most

common in the Universe.Very small dwarf elliptical galaxies (one

billion solar masses) are often found near larger spirals.



Galaxy Classification

The Hubble ClassificationSpiral Galaxies Hubble divided spiral galaxies into two

groups: normal spirals and barred spirals. A barred spiral galaxy is a spiral galaxy in

which the spiral arms come from the ends of a bar through the nucleus rather than from the nucleus itself.

Spirals are designated with an S; barred spirals are designated with an SB.



Galaxy ClassificationSpiral Galaxies (continued)

Each type of spiral galaxy is then further subdivided into categories a, b, and c depending on how tightly the spiral arms are wound around the nucleus. Galaxies with the most tightly wound arms are type a. The size of a spiral galaxy’s bulge and the “dustiness”

of the disk can also be used to determine the subdivision.

Elliptical and Irregular Galaxies Elliptical galaxies are classified from round (E0)

to very elongated (E7). Irregulars are designated as Irr




Hubble’s Tuning Fork DiagramHubble’s Tuning Fork Diagram Hubble’s Hubble’s tuning fork diagramtuning fork diagram relates the relates the

various types of galaxies.various types of galaxies. Astronomers once also thought the Astronomers once also thought the

diagram represented an evolutionary diagram represented an evolutionary sequence, but this interpretation has been sequence, but this interpretation has been discarded as old stars have been found in discarded as old stars have been found in all three types.all three types.



Type Designation Description

Elliptical E0–E7 Galaxies that appear circular (E0) to very elongated (E7).

Spiral Sa–Sc Sa: large nuclei and tightly wound arms. Sc: small nuclei and open arms.

Barred spiral SBa–SBc Spirals with elongated nuclei.

S0 S0 Disklike; no spiral structure.

Irregular Irr Do not fit into any other category.



Measuring Galaxies

The most important properties of a galaxy that we can measure are its distance, mass, and motion.Distances Measured by a chain of interlocking techniques: Radar Ranging (Solar System) Parallax (about 2000 ly max) Main-Sequence Fitting (about 1 Mly max) Cepheid Variables (about 100 Mly max) Distant Standards (billions of light-years)



Measuring Galaxies

Some Near-by Standard Candles (to a few 100 Mly) Bright stars (giants, supergiants, novae) can be used as

distance indicators. Large globular clusters and supernovae are of consistent

brightness so they, too, can be used to determine distances to more distant galaxies.

Standard Candles and Indicators Beyond 100 Mly Brightest galaxies Brightest galaxies in a galaxy cluster Supernovae, especially Type Ia Tully-Fisher relation Photometric “graininess”



Hubble Law

Some History In 1921, Slipher found that spiral

nebulae had redshifted spectra indicating that they were moving away from us at tremendous velocity.

In 1929, Hubble showed that there is a linear relationship between the recessional velocities of galaxies and their distances.



Hubble LawHubble Law The Hubble law:The Hubble law:

vv = = HH00ddwhere where vv is radial velocity, is radial velocity, dd is distance, and is distance, and HH00 is the is the Hubble constantHubble constant (the 0-subscript refers to its value (the 0-subscript refers to its value today, and not the past).today, and not the past).

The The Hubble constantHubble constant is the proportionality constant is the proportionality constant in the Hubble law; the ratio of recessional velocities of in the Hubble law; the ratio of recessional velocities of galaxies to their distances.galaxies to their distances.

Modern day measurements of the Hubble constant place it about 73 km/s per megaparsec (Mpc) or 22 km/s per Mly.

The Hubble law is not ideal: The Hubble law is not ideal: It does not apply to nearby galaxies where gravity dominatesIt does not apply to nearby galaxies where gravity dominates It relies on a accurate measurement of Hubble’s constantIt relies on a accurate measurement of Hubble’s constant



Hubble Law Practical Use of Hubble Law

The spectral shift seen in distant galaxy spectra can be translated into the recession velocities of the galaxies.

For these distant galaxies, the Hubble law can then be used to determine their distances.

Cosmological Implications The Hubble Law shows that the universe is

expanding, and it is the foundation for today’s theories of cosmology - the study of the nature and evolution of the universe as a whole

Order of magnitude age of the Universe using Hubble Law is 1/Ho and this gives a range of 12-15 billion years.



Into The Past

Look-Back Time Since the distances between galaxies are always

changing, it is actually easier to speak in terms of the amount of time it takes an object’s light to reach us. This travel time is called the look-back time. The look-back time is the difference between the current

age of the Universe and the age of the Universe when it left the object.

If galaxies did not move, the look-back times would give a direct measure of the distances to the galaxies (say in light-years).

The maximum look-back time is called the cosmic horizon and is related to the age of the Universe.



Galaxy EvolutionGalaxy Evolution Now turn how galaxies change over time Now turn how galaxies change over time

– – galaxy evolutiongalaxy evolution

Galaxies near us are 13 billion years old, Galaxies near us are 13 billion years old, while galaxies seen with a lookback time while galaxies seen with a lookback time of 10 billion years, are 3 billion years oldof 10 billion years, are 3 billion years old

This allows to see what has changed This allows to see what has changed about galaxies over the many years, about galaxies over the many years, though looking at distant galaxies is though looking at distant galaxies is more difficult because they are so far more difficult because they are so far away.away.



Galaxy EvolutionGalaxy EvolutionHow did galaxies form?How did galaxies form? The most successful models assume the The most successful models assume the

following:following:Hydrogen and helium gas filled all of Hydrogen and helium gas filled all of

space nearly uniformly space nearly uniformly The distribution of matter was not perfectly The distribution of matter was not perfectly

uniform, as certain areas of the universe uniform, as certain areas of the universe were slightly more dense than others.were slightly more dense than others.

Starting with these assumptions, after Starting with these assumptions, after one billion years after the Big Bang, one billion years after the Big Bang, protogalactic clouds started to form and protogalactic clouds started to form and contract via gravity.contract via gravity.



Galaxy EvolutionGalaxy Evolution As the protogalatic clouds formed the coolest and As the protogalatic clouds formed the coolest and

densest material formed into the first stardensest material formed into the first star These tended to massive stars which went supernovae with These tended to massive stars which went supernovae with

10 million years10 million years This in turn lead to more star formation from the shock This in turn lead to more star formation from the shock

waves of the supernovaewaves of the supernovae

The The disk populationdisk population is a fairly flat plane with uniform is a fairly flat plane with uniform revolutionrevolution

The The spheroidal populationspheroidal population of stars (including the of stars (including the globular clusters) was formed before the galaxy globular clusters) was formed before the galaxy collapsed into a disk and has more randomly collapsed into a disk and has more randomly oriented orbits.oriented orbits.

This explains the basic structure of spiral galaxiesThis explains the basic structure of spiral galaxies



Galaxy EvolutionGalaxy EvolutionWhy are galaxies different?Why are galaxies different? If a galaxy did have If a galaxy did have notnot a great deal of a great deal of net net

angular momentum (which causes it to angular momentum (which causes it to spin) then it would not flatten out and spin) then it would not flatten out and would form an would form an elliptical galaxyelliptical galaxy

An irregular galaxy is thought to be An irregular galaxy is thought to be caused by a collision of two or more caused by a collision of two or more galaxies, though collisions may form an galaxies, though collisions may form an elliptical galaxyelliptical galaxy as well. as well.Some collisions can cause an incredible Some collisions can cause an incredible

increase of star formation. These galaxies increase of star formation. These galaxies are called are called starburst galaxiesstarburst galaxies. .



Active Galaxies

Active GalaxiesAn active galaxy is a galaxy with an

unusually luminous nucleus.

Three main types of active galaxies:Radio galaxies

Have greatest luminosity at radio wavelengths with a double-lobed radio source.

Radio galaxies often exhibit unusual jets in visible light.

Generally, they are elliptical galaxies.



Active Galaxies

Seyfert galaxies A class of spiral galaxies having abnormally luminous nuclei. The immense luminosity is spread over all wavelengths are

fluctuates rapidly. Contain very fast moving gas clouds in some instances being

ejected in small jets.

BL Lacertae objects is another type of active galaxy which have their jets point right at us

Quasars (Quasi-stellar objects or QSO) A small, intense celestial source of radiation with a very large

redshift (implying speeds close to c and at very large cosmological distances).

Some are powerful radio sources and others eject hot gas from their centers.

Often appear to lie within ordinary galaxies.



Active Galaxies - QuasarsActive Galaxies - Quasars

From the Hubble Law, From the Hubble Law, a large redshift a large redshift implies large distances and existence in the implies large distances and existence in the past (era of the quasars).past (era of the quasars).

Due to their large distances, quasars Due to their large distances, quasars provide an excellent testing ground for provide an excellent testing ground for general relativity through observations of general relativity through observations of gravitational lensing and microlensing.gravitational lensing and microlensing.


What Makes Some Galaxies Active? Current explanation: An accretion disk feeding

material into a supermassive black hole at the galactic center.

A supermassive black hole is created in the early years of a galaxy’s growth.

As long as there is enough material in the disk to feed to black hole, the galaxy remains active.

All galaxies appear to have supermassive black holes at their centers.

The different types of active galaxies may be the same The different types of active galaxies may be the same basic object simply seen from different vantage points.basic object simply seen from different vantage points.


The Nature of Active Galaxies


Galaxy EvolutionGalaxy Evolution

More on Supermassive Black HolesMore on Supermassive Black Holes

Evidence for the Existence of Supermassive Black Evidence for the Existence of Supermassive Black HolesHoles

Most convincing evidence is the speed at which gas Most convincing evidence is the speed at which gas rotates in a very small region in the center of the galaxy rotates in a very small region in the center of the galaxy (e.g., data from M87).(e.g., data from M87).

Similar results from Similar results from masermaser emissions in NGC 4258 emissions in NGC 4258 indicate the existence of a 36 million solar mass object indicate the existence of a 36 million solar mass object inside a radius of 1 ly.inside a radius of 1 ly.

The relationship between a galaxy’s central bulge mass The relationship between a galaxy’s central bulge mass (M(Mcbcb) and its central black hole mass (M) and its central black hole mass (Mbhbh) (M) (Mcbcb is about is about 500M500Mbhbh) for a large variety of galaxies, indicates that the ) for a large variety of galaxies, indicates that the growth of the central black hole must be closely linked to growth of the central black hole must be closely linked to the process of galaxy formation.the process of galaxy formation.

© sierra college astronomy department1 the diverse galaxies

Documents

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giant elliptical galaxies

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