galaxies Flashcards
What is a galaxy?
An enormous collection of stars held
together by their common gravity.
some details on galaxies
Galaxies have a wide range of masses: from 100 m stars (dwarf galaxies) to >1 t stars (giant galaxies). Lower mass galaxies are more common. Galaxies have a wide range of ages, stellar populations (the mix of stars in a galaxy), and gas content.
Anatomy of the Milky Way
The Milky Way (like any spiral galaxy) consists of:
A central bulge
A disk: extremely wide, but very thin
Disk diameter: 100,000 l.y. = 100 kl.y. (30 kpc )
Disk thickness: 1,000 l.y. = 1 kl.y. (300 pc) A large halo which surrounds almost completely the entire disk
how many stars in milky way
~100 b stars (up to 1 t )
where is milky located in the galaxy
Our solar system is located in the disk, 28,000 light years (=28 kl.y.) from centre Being situated in the plane of the disk we cannot observe clearly the structure of the Milky Way. Moreover, dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium that makes new star systems.
arms
Previously, our galaxy was thought to have 4 major arms.
In 2008, it was announced that IR images from NASA’s Spitzer Space Telescope have shown that the Milky Way’s elegant spiral structure is dominated by just 2 major arms wrapping off the ends of a central bar. It is orbited by a few small and very small galaxies (among them the Magellanic clouds are the most representative)
Contents of the Milky Way
disk
bulge
halo
Disk:
Filled with interstellar gas & dust ≡ interstellar medium (ISM), made up of H gas (atomic & molecular) & dust Younger stars Open clusters
Bulge:
Filled with denser gas & dust Young & older stars @ high density A few globular clusters may also be present here
halo
No gas/dust, or very rarified (tenuous) Old stars Globular clusters
Milky Way = typical . . .
all spiral galaxies have the
same structure
small galaxy companions
Milky Way’s strong gravity influences smaller galaxies
in its vicinity
small galaxy companions, The most important
the Large Magellanic Cloud (LMC), and the Small Magellanic Cloud (SMC), Both are irregular galaxies and much smaller than the Milky Way
Other small galaxy companions:
The Sagittarius Dwarf Elliptical Galaxy (SagDEG) → a small elliptical galaxy Canis Major Dwarf Galaxy: the closest known satellite galaxy but well hidden behind banks of dust in the plane of the Milky Way
Our galaxy’s tidal forces will ultimately
rip apart & ‘cannibalize’ these 2 small companions in ~1 b yrs
Stars in disk
- relatively young.
Plenty of high- & low-mass stars, blue & red Fraction of heavy elements same as or greater than the Sun
Stars in halo are old
→
formed early in Milky Way’s history
Mostly low mass, red stars
Fraction of heavy elements much less than the Sun → Formed when few heavy elements existed
Star formation stopped long ago when all gas flattened into disk No (or extremely little) ISM in the halo The fact that the stars in the disk have very different origins than those in the bulge & halo is also reflected in great differences between their galactic orbits .
All stars in the disk orbit around the galactic centre.
Circular motion of all stars in disk orbits → arises from gravitational attraction towards galactic centre
It is always in the same direction & in the same plane (roughly)
There is “bobbing” up & down due to the localized disk gravity → defines disk thickness
All stars in bulge &
halo also orbit around
the galactic centre, but…
They have random orientations: different directions (even opposite!) at various inclinations to disk Higher velocities & orbits are elliptical, sometimes at great distances from center
Orbital motions of mass in spiral arms of galaxies indicate that most of their mass is
NOT concentrated near the galactic centre, but the opposite is true . If most mass were concentrated in the center, the stars closer to the centre would orbit very fast, those more distant would orbit slower Measurements indicate that orbital speeds remain constant even up to great distances from the centre
Most of the galaxy’s mass resides far from the center
is distributed throughout the halo
There is little gas or dust, very few stars and no emitted light from the halo → Most of a galaxy’s mass is due to the presence of dark matter
it has a large mass-to-luminosity (M/L) ratio
Galactic recycling is essential for the formation of stars
.
Recycles material from old stars into new ones: the birth of the Sun & the Solar System could not have occurred without it. Takes place within the disk of the galaxy & its ISM
Galactic recycling gradually changes the chemical composition of
the ISM.
With each cycle more heavy elements are made by fusion in stars
This is the process due to which all heavier elements than H & He have been produced → after 10 b years of recycling, the chemical composition of the ISM is: 70%H2 , 28%He & 2% heavier elements. Different galactic regions change composition at different rates
How does it take place, and how these chemical riches
produced by stars remained in our galaxy?
It is a complex and long process, taking place in several stages
hot bubbles atomic hydrogen clouds molecular clouds star formation stellar burning/heavy-element formation supernovae and stellar winds
All stars return much of their material into ISM in 2 ways:
Through stellar winds, and Death events.
Low mass stars return most of their material into ISM via:
Gas ejection through mild stellar winds, and Mass loss through planetary nebulae
High mass stars return most of their material into ISM via:
Massive gas ejection through strong stellar winds,
Mass loss through supernovae
Heavy elements forged in (large) stars are thus returned in the ISM & contribute to formation of new stars, planets & appearance of Life
Consequently, low mass stars have much
less effect
on the ISM than high mass stars which influence & contribute to it much more significantly
Supernovae eject high-speed gas in a shock wave
Sweeps up ISM
Bubble of hot gas ( > 1 m K) excavated
Gas is strongly compressed, heated, ionized & excited → emits mainly X-rays, but also visible & IR radiation as it expands & cools
Some e– accelerated to nearly speed of light as they interact with the shock wave As these fast e– spiral around mgn. field lines threading the supernova remnant (SNR) → they emit radio waves (this radio emission is sometimes called synchrotron radiation )
Supernovae also generate cosmic rays
highly energetic particles, mainly protons (p +) & atomic nuclei -including some heavy ones- and a few e–, travelling at close to the speed of light
Our solar system is also in a huge
supernova bubble created long
time ago!
Bubbles fill
20~50% of Milky Way’s disk
Multiple supernovae can create huge bubbles of hot gas
which blow out of the galactic disk.
In many places in our galactic disk elongated superbubbles
are observed
Multiple supernovae can create huge bubbles of
hot gas part 2
The hot gas has very low densities and temperatures as high as a few m K, hence it is hot enough to emit Xrays → can be observed
The hot gas breaks out of the disk a blowout like a volcanic eruption
Part of the disk’s own material is also ejected
Hot gas bubbles/gas clouds cool
in the halo
Energy shared with swept-up ISM matter & also radiated from all shocked gas Also lose angular momentum
Only gravity slows down gradually the bubble’s expansion & eventually reverses the rise of the gas from the blowout → The galactic fountain model
Cooled gas clouds rain back down onto the disk → merge & impact the ISM in a large region of the disk
These collisions may trigger future star formation .
Heavy elements also returned & merge into disk’s ISM
What happens As ISM cools
p + recombine with e– neutral atomic H formed (neutral because is cool enough)
Atomic H emits at λ = 21 cm
Radio emission line emitted when spin state of e– flips Used to map atomic H distribution in disk with radio telescopes
Milky Way has 5 b MSun of atomic H.
Large, tenuous, 10,000 K warm clouds (1 atom/cm 3)
Small, dense, 100 K cool clouds (100 atoms/cm 3)
Warm atomic H clouds slowly cool & contract into denser clouds over m of years
Gravity slowly draws gas together into tighter clumps Energy radiated more efficiently as cloud grows denser
Molecular H 2 forms as atomic H cools from 100 K to 10…30 K.
Molecular clouds created: 70% H2 , 28% He, ~1% CO + many other substances
No emission from H2 !
Cold, heavy & dense molecular clouds settle in the central layers of the Milky Way’s disk
Radio emission of other molecules can be observed.
H2O, CO (3 mm emission line), NH3 , -OH, alcohol
Supernova explosions disturb the cold molecular clouds
→
stir them up & create turbulences
Gravitational push triggers
cloud core formation & their collapse
forms new stars, thereby completing the star-gas-star cycle.
Once a few stars form in a newborn cluster, they begin to hamper the creation of new stars
The molecular cloud is eroded & pushed away by stellar winds & radiation pressure It is also heated/excited & ionized by UV photons from high-mass stars
Supernovae are crucial for both star & planet formation.
Tremendous amounts of matter blasted into intergalactic space
Heavy elements created → the production and dispersal of even a small amount could have had a major effect on star & planets formation
Enable creation of stars from protostellar disks.
When the abundance of ‘metals’ in star-forming clouds rises above one thousandth of the metal abundance in the Sun, the metals rapidly cool the gas to the temperature of the cosmic background radiation
‘Metals’ are much more effective than hydrogen in cooling starforming clouds → faster & easier collapse into stars
Next generation stars begin life with more heavy
elements
→ numerous advantages:
Nuclear fusion in the star core is less efficient without ‘metals’ otherwise it would have to be hotter and more compact to produce enough energy to counteract gravity
Because of the more compact structure, the surface layers of the star would also be much hotter: Tsurf ~100,000 K (17× higher than the Sun’s surface temperature)
The heavy elements also enable the creation of planets & appearance of life
Star-gas-star cycle cannot go on forever.
Stellar formation & evolution -> matter gradually “locked” in red dwarfs, brown dwarfs & stellar corpses
Other material → irretrievably spread out as ISM or intergalactic gas
Star formation rate will taper off over next 50 b years Eventually, star formation will cease!
Galactic recycling in
the Milky Way
The star-gas-star cycle is observed in the Milky Way using many different wavelengths of light
Galactic recycling: SUMMARY
Stars make new heavy elements by fusion.
Dying stars expel gas and new elements , producing hot bubbles of gas (~1m K) which emit X-rays .
This hot gas cools, allowing atomic H clouds to form (~100,000-10,000 K).
Has a 21 cm wavelength emission line. Further cooling molecules (CO, etc.) form, making molecular clouds (~30 K).
Observed using CO emission line at 3 mm.
Gravity forms new stars (and planets) in molecular clouds.
The process starts over again!
Where will our Galaxy’s gas be in 1 trillion years from
now?
A. Scattered in ISM and intergalactic space
B. Locked into white dwarfs and low mass stars
C. Both of the above
D. Blown out of galaxy
E. Still recycling just like now
Galactic recycling is an imperfect process. More and more gas gets locked up into low-mass stars and white dwarfs, which never return their material to the ISM, and some material will remain scattered in space and will never have the possibility to collapse & form stars again
Where do stars form in our galaxy?
In any galaxy, stars -and also regions of new star formation- are not spread evenly (more dense in gas&dust clouds-rich areas) Much of the star formation in the galactic disk happens in the spiral arms.
Spiral arms are enormous & prolific & star formation waves propagating through the gaseous disk
Stars & clouds more densely packed → Gas clouds get squeezed as they move into spiral arms
Squeezing of clouds triggers star formation.
Young stars flow out of spiral arms.
Supernovae of massive stars compress further the clouds, triggering more star formation.
The disk does not appear solid.
Has spiral arms The arms are not fixed strings of stars
Spiral arms are high density waves propagating through the gaseous disk
Stars & clouds more densely packed
Prolific star formation activity
Massive stars, formed as gas clouds pass through spiral arms, die out quickly before completing one galactic orbit → spiral arms appear bluer than the bulge or gaps between arms
Long-lived yellow & red stars survive many galactic orbits & pass through many spiral arms → more evenly distributed throughout the galactic disk
Halo:
Star formation started first in halo, then stopped
NO ionization nebulae, NO blue stars
NO star formation (and hence NO recycling ) →
Only old stars, and with fewer heavy elements (0.02…0.2%)
Galactic disk
Ionization nebulae, blue stars
Star formation active →
Stars of many different ages with ~2% heavy elements
Disk stars formed later & keep forming!
Milky Way’s star formation rate is about 1 MSun/yr.
Conclusion: The halo lacked the gas for new star formation for a very long time because it has settled into the disk
The initial most basic model for Milky Way’s formation
(Monolithic Collapse Model):
It was first thought that the galaxy began as a giant protogalactic cloud containing all the H & He gas that eventually turned into stars or is now present in the ISM.
The protogalactic cloud collapsed and halo stars began to form
Conservation of angular momentum ensured that the remaining gas flattened into a spinning disk. Spiral arms then formed and the star-gas-star cycle started supporting ongoing star formation in the disk.
Multiple merger model: Recent data show that our galaxy formed from a few different gas clouds, not just one.
The earliest stars formed in relatively small clouds, each with a few globular clusters.
These clouds later collided ( rapid star formation in what is now the halo) & created a combined full protogalactic cloud that became the Milky Way.
Once the protogalactic cloud was in place, it collapsed into the disk and the formation of stars & heavy element enrichment proceeded in a more orderly fashion
The evidence for the multiple merger model
the heavy element content of stars in the intermediate layer between the disk & halo depends on their distance from the galactic center
These stars are almost as old as halo stars but formed just before the spinning protogalactic cloud finished flattening into a disk → it had only 10% of the present heavy element content
Old stars in the bulge have a
composition similar to that of the Sun, even though their ages exceed 10 b years
The centre of Milky Way lies in the direction of Sagittarius
.
Bulge obscured by ISM IR & radio views reveals swirling gas clouds & a cluster of several million stars
Bright radio source (with vast threads of emission tracing mgn. field lines) in the centre with occasional Xray flares Sgr A*.
Hundreds of stars crowded within 1 light-year of the region
Mass in Sgr A*
Determined from orbits of fast-moving stars near galactic centre.
Kepler’s Law gives a mass of ~3.7 m Msun
This mass is packed into a space a little larger than our solar system
What is so small, yet so massive?
A supermassive black hole
The black hole at the centre of our
galaxy has an unusually faint X-ray emission
most probably lacks an accretion disk (already gobbled up all material around it)
Life cycles of galaxies are studied the same way as
those of stars:
Impossible to observe continuously from start to end
Observe galaxies at various stages of their lives
Life cycles of galaxies are more difficult to study
Studying the lives & evolution of galaxies is impossible without considering the evolution of the Universe
The study of galaxies is intimately connected with cosmology = the study of the overall structure & evolution of the Universe
Hubble Space Telescope’s long-distance (far field & ultra far field) observations even in regions apparently
utterly empty, devoid of any planets or stars revealed amazing & humbling images of thousands of galaxies.
The images: the farthest we’ve ever seen into the Universe
Over 100 b galaxies estimated in the entire Universe! What did the Deep Field observations tell us?
There is a wealth of distant galaxies to be studied!
moon
1 s
Sun
8 minutes
Pluto
8 hours
Nearest star (α Centauri)
4 years
Sirius
8 years
Nearest large galaxy (Andromeda)
2.5 m years
Nearest cluster of galaxies
50m years
Most distant known galaxy
13.4 b years (formed when Universe was 3% of its age, only 407 m y. old. Taking into account the expansion of the Universe, it is now 32.1 b l.y away!)
Galaxies classified into 3 types:
spiral, elliptical, irregular
Spiral
Flat white disk, yellowish bulge
Cool gas & dust, hot ionized gas
Elliptical
Redder, more rounded
Very little cool gas or dust
May have very hot ionized gas
Irregular:
Neither disk-like nor rounded
Dwarf galaxies
100 m stars
Giant Galaxies
1 t stars
Spiral galaxies Two primary components:
Disk = a thin flat disk (extending outward from the central region, the bulge ) in which stars follow orderly, nearly circular orbits around the galactic centre
Stars of all ages (including many young ones) & masses
Appears white, as it contains stars of all spectral types
Filled with interstellar medium (ISM), made up of H gas (atomic & molecular) & dust
Active star formation
Open clusters
Spheroidal components
(bulge
& halo)
Spheroidal components (bulge & halo)
the stars within 10 kl.y. of the center belong to the bulge , those outside this radius are members of the halo .
Stars here have orbits of random orientations: different directions (even opposite!) & shapes (even highly elliptical) at various inclinations
Bulge:
Filled with denser gas & dust Both young & older stars @ high density
Halo
No gas or dust or extremely rarified/tenuous & hot
Mostly low mass, red, old stars
Appears reddish/orange
Globular clusters
75~85% of large galaxies are
spiral or lenticular.
Very active galactic recycling
Variations of spiral galaxies
Some have a bar of stars cutting
through their centres:
barred spiral galaxies Spiral arms attached to ends of bar
Latest data suggested Milky Way is a barred type
Its bulge appears elongated
Others have NO spiral arms: Lenticular galaxies
Uniform disk → looks like a lens seen edge-on
Contain less cool gas than normal spiral galaxies
Only spheroidal components, NO surrounding disk component.
They are NOT globular clusters!
Much more random star motion than orderly rotational one
Elliptical galaxies Very little ISM, mostly low-density, hot
& ionized.
Most ellipticals lack cool gas → little or no star formation
Some have some cold gas & dust, e.g. as disks rotating around their centers
Some large ellipticals have a lot of very hot gas X-ray emission
Mostly cool stars, NO hot blue stars look reddish/orange.
Hence very little and slow galactic recycling, or none at all!!
Elliptical galaxies Very numerous, and have a very wide range of sizes
The most massive galaxies are giant elliptical galaxies
~15% of all large galaxies are ellipticals Vast majority of ellipticals are small → small elliptical galaxies are the most common type of galaxy in the Universe!
Irregular galaxies
Miscellaneous class of galaxies. Appear white & dusty from ISM, like the disk of spirals Most are small & faint Contain young massive stars Stars & gas clouds in random patches Hence very active galactic recycling
More likely to be distant galaxies
Indicate they were more common when universe was young
Milky Way’s close companions, the LMC & SMC are irregular galaxies
Edwin Hubble invented a system for classifying galaxies
The so-called “Hubble Tuning Fork”
Classification made according to shape: It is NOT an evolutionary
sequence
The number after “E” is
= 10((D-d)/D)
In 1959 de Vaucouleurs modified it into a more detailed classification system
Many galaxies are not alone
→
gravitationally bound to other neighbours
Galaxies often stay in loose collections of up to a few dozen called
groups
The Milky Way is a relatively large galaxy, part of our
Local Group of galaxies
3 m l.y. in size; ~40 galaxies Within our Local Group → 2 large spirals: The Milky Way & the Andromeda galaxy (M31), the only one comparable in size
Some galaxies associate in tightly bound clusters
.
Contain 100s to 1000s galaxies over >10 m l.y. Half of the large galaxies in clusters are elliptical
The Local Group is part of the
Virgo supercluster
Distance can be measured if we know the
apparent brightness & luminosity.
From parallax method on nearby stars
Any astronomical object of known luminosity
is a standard candle. IF we can find out its luminosity without first knowing its distance ! → e.g. any Sun-like star!
Distance to standard candle can be determined by the inverse square law
We only need to measure its apparent brightness (A.B.)
However, Sun-like stars are dim at distances >1 kl.y.
Key challenge to measure larger distances:
find better
(brighter) standard candles.
Luminosities of all main sequence stars are known
use bright main sequence stars as standard candles:
Distance to a close reference star cluster determined from parallax
Plot its stars on the H-R diagram
Measure relative brightness of unknown cluster
Determine distance to unknown cluster by comparing A.B. to known cluster
Cepheid Variables & white dwarf supernovae
Cepheid variable stars → more luminous standard candles
Bright giants → luminous enough to see at great distances
Follow well-defined period-luminosity relationships.
Measuring period of variability tells us the luminosity (within 10%) With them we can measure distances up to 100 m l.y.! Very bright standard candles needed to measure intergalactic distances beyond the Milky Way.
White dwarf supernovae (Type Ia) are distant standard candles.
All have same peak luminosity of 10 b Suns → Can be seen in galaxies b of l.y. away
Calibrate those in nearby galaxies with Cepheids
Must observe & measure when one explodes
Not easily applicable: a white dwarf supernova (Type Ia) occurs only once every few hundred years in a typical galaxy
The Tully-Fisher relation
Mass of spiral galaxy determines its rotation rate & luminosity → Luminosity-Rotation rate = a proportional dependence: L ∝ Vrot , with γ ≅ 4 but depends on wavelength λ thus Faster rotating spirals are more luminous
Measure the 21 cm emission line of H gas in spiral disk with radio telescopes
Compare Doppler shifts of portions of disk rotation towards us & away from us
solar system
-4 ly (im gonna be saying in powers of 10)
nearby stars
2
milky way
5
nearby galaxies
7
galaxy clusters
10
least to most accurate measuring methods
radar ranging > parallax > main-sequence-fitting > cepheids >distance standards
Chain of methods to measure size of universe.
Edwin Hubble
Developed galaxy classification scheme. Studied stars in Andromeda galaxy and determined they were not in the Milky Way.
Also measured distances to other galaxies
Measured the redshifts of distant galaxies
discovered that more distant galaxies are moving faster away The Universe is expanding
Hubble’s Law:
The more distant a galaxy, the greater its redshift, and hence the faster it moves away from us
Velocity-distance relation given by:
v = H0 . d
H0 = Hubble’s constant
Often used to estimate galaxy’s distance from its redshift
Hubble’s constant
Hubble’s constant is extracted from the velocity-distance plot of many galaxies.
2 difficulties when trying to use practically Hubble’s law:
Galaxies do not obey Hubble’s law perfectly . Gravitational influences from other neighbours alter their speeds.
Distance determined from Hubble’s Law only as accurate as our best measurement of H0
20~24 km/s per m l.y. estimated by HST (~70…80 km/s/Mpc).
What does it mean actually that the Universe is expanding
The expansion of the Universe is the expansion of the SPACE ITSELF
Galaxies move apart with it but do not extend → gravity holds them together
On very large scales, the distribution of galaxies is relatively uniform
The overall appearance of the Universe is about the same no matter where you look or where you are located No centre or edge
COSMOLOGICAL PRINCIPLE =
the Universe is
uniformly distributed, without a center or an edge
From any galaxy’s view,
other galaxies are all moving away from it. Galaxies must have been closer together in the past The Universe must have a starting point!
When did the Universe begin?
Run clock backward to starting point to determine age.
Expansion rate assumed to be constant Ho gives the rate galaxies are moving away from each another 1/Ho tells us how long it took to expand to current size → AGE!
H0 = 22 km/s per m l.y.
Age of universe, 1/H0 = 13.6 b years
H0 = 21.8 km/s per m l.y.
Age of universe, 1/H0 = 13.75 b years Best available evidence indicates the Universe is ~14 b years old
Ho changes with time but stays roughly equal to
1/(AGE of the Universe)
Speed of light is finite
Light from galaxy 400 m l.y. away took 400 m years to arriv
The relationship between distance, expansion & look-back tim
spacetime diagram
How do we define the distance to a faraway galaxy?
Is it distance when photons were emitted or are received?
Makes more sense to call this ‘the look-back time’.
No ambiguity that photons took 400 m years to get here!
Cosmological redshif
The shift to longer, redder wavelengths due to the expansion of the Universe which stretches out all the photons within it
It tells us how much space has expanded during the time since light from the galaxy left on its journey to us
Horizon of the Universe
aUniverse has no edge, but it does have a horizon.
A place beyond which we cannot see!
Cosmological horizon = the place where the look-back
time equals the AGE of the Universe.
Boundary in time, not in space
Beyond this horizon you will be trying to see a time before
universe even existed!
Observable universe grows
1 l.y. in size every year
How did galaxies form?
Theoretical modeling must be used to study the earliest stage of galaxy life & evolution → assumptions:
H & He filled the space in the early Universe ( ≤ 1 m years )
Matter was not uniformly distributed
We can see galaxies 13 b l.y. away (same age as oldest stars in Milky Way)
most galaxies began to form at that time & have same age
How did galaxies form? 2
The denser regions in the early Universe formed protogalactic clouds → cooled, contracted & collapsed to form galaxies. The origin of the density enhancements in the early Universe is still a major unknown
First-generation stars = massive Supernovae → provided heavy elements and generated shock waves which heated up ISM Slowed down collapse of protogalactic clouds & rate of star formation Allowed gas to settle into a rotating DISK
Why do galaxies differ?Differences between population of stars between the
galactic regions:
Disk population → born AFTER the gas settled in a rotating disk (in a flat plane) Spheroidal population → born BEFORE the gas settled in a rotating disk, hence with randomly oriented orbits around the center
Distinct differences between S, E & Irr types of galaxies
→ their evolutions/lives are different: WHY? → 2 possibilities:
Different ‘birth’ conditions in their protogalactic clouds Similar formation but different evolutions due to interactions with other galaxies
Galaxy formation 1) Conditions in the protogalactic cloud
1.1- Protogalactic spin:
Significant amount of angular momentum = fast spin
Spiral galaxy Little or no angular momentum = slow (or no) spin Elliptical galaxy
Galaxy formation 1) Conditions in the protogalactic cloud 1.2- Protogalactic density
Significant initial density → efficient energy radiation → quick cooling → fast star formation
Elliptical galaxy Low initial density → slow energy radiation → slow cooling → slow star formation Spiral galaxy
Galaxy evolution
Galaxies rarely evolve in perfect isolation Matter in Universe is constantly moving; even the Universe itself is constantly expanding
Average distances between galaxies are not much larger than the sizes of galaxies
COLLISIONS between galaxies are inevitable!
Galaxy collisions were much more frequent in the past:
Universe was smaller, matter denser, galaxies closer
Protogalactic clouds collisions also must have happened
Very spectacular but very slow events → 100s of m of years!
Distorted galaxies were more common in the early Universe
When 2 spiral galaxies collide Elliptical galaxy :
Tremendous tidal forces tear apart the disks → randomized orbits of the stars in long tails during first pass
A large volume of their ISMs collapses at the center of collision
rapid formation of new stars → Supernovae & stellar winds blow away remaining gas
Ultimately, they form a single elliptical galaxy
Our galaxy will most likely merge with the Andromeda galaxy in 3 b years.
Galaxies in dense clusters
At least some elliptical galaxies result from collisions & mergers
Ellipticals dominate the populations at the cores of dense clusters of galaxies → frequent collisions
Central dominant galaxies = giant ellipticals at the center of many dense clusters :
Grew to huge sizes by consuming other galaxies Frequently contain tightly bound clumps of stars → centers of the consumed galaxies
Dense Clusters Very hot gas in the center of such dense clusters:
Exerts a drag force on a spiral galaxy that may collide with it
The spiral’s gas is slowed down, but the stars keep moving
If the spiral’s disk had NOT previously formed many stars → the galaxy becomes an elliptical
If the disk had already produced many stars BEFORE its gas was stripped out during the merger → becomes lenticular
What are starbursts? L11 Starburst galaxies = galaxies in present-day Universe in which stars are forming at very rapid rates
Milky Way → ~1 star per year will not exhaust its ISM until long after the Sun died
Starburst galaxies → >100 stars per year ! will consume ALL their ISM in a few 100s m years!
Starburst galaxies
very bright in IR because of their ISM excited from UV & X-rays emitted from the many hot & young stars in it = so-called Ultraluminous Infrared Galaxies (ULRIGs)
Starburst could be a phase in the evolution of a galaxy
Starbursts explain why ellipticals lack
young stars & cool gas
Causes of small-scale starbursts are
not clear
Recent studies → collisions & mergers of gas-rich galaxies could
also cause starbursts
What are starbursts?
A star formation 100 × faster than that of Milky Way Supernovae will also occur 100 × more often!
Supernovae generate a strong shock wave that creates a bubble of gas
Shock waves from nearby supernovae overlap superbubble
Further supernovae add more gas and thermal & kinetic energy
When the superbubble breaks out of the galactic disk →
it will expand even faster & erupt into intergalactic space galactic wind It can blow the gas out of small galaxies shut down star formation for many b of years in those galaxies
Seyfert galaxies = one of the two largest groups of active galaxies, along with quasars
First described by Carl Seyfert in 1943. 10% of all galaxies; ~1% of all spirals. Very few Seyfert galaxies are ellipticals.
In visible, most Seyfert galaxies look like normal
normal spiral galaxies. In other wavelength ranges, the luminosity of their cores is comparable to that of a whole galaxy the size of the Milky Way!
Seyfert
They have quasar-like nuclei.
Active galaxy
Active galaxy = A galaxy hosting an AGN
AGN = An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly ALL, of the EM spectrum.
The radiation from AGN is believed to be a result of accretion of mass by a supermassive black hole (SMBH) at the centre of its host galaxy
The brightest AGNs are known as quasars
“ Quasi-stellar radio sources” = quasars
The most powerful produce more light than 1,000 galaxies like the Milky Way!
Observed only for very distant galaxies
Another temporary stage in the EARLY evolutions of galaxies → were most common b of years ago Not seen nearby objects that shine as quasars become dormant as galaxies age
The incredible luminosities of quasars & AGNs are generated in a
volume not much bigger than our Solar system!
We do NOT know much about their formation & lives,
and their link to the galaxies evolution
Certain galaxies emit unusually strong radio waves called radio galaxies
Much of the radiation does not come from the galaxy itself but from pairs of huge lobes, on each side of the galaxy A strong AGN is hosted → drives 2 jets of particles streaming out at nearly the speed of light
The jets hit ISM & intergalactic gas & excite it radio hotspots
The particles are then deflected & scattered → form the lobes
What is the power source for quasars & AGNs ?L
Accretion of mass by a supermassive black hole (SMBH) at the centre of its host galaxy
Accretion of mass by a supermassive black hole (SMBH) at the
centre of its host galaxy
Matter falling towards BH (up to relativistic velocities!) → Gravitational potential energy is converted into kinetic energy → Collisions & friction convert it into thermal energy excitation of matter intense radiation in a broad spectrum
10…40% of the mass is converted into energy (Fusion: only 1% conversion efficiency!)
What is the power source for quasars & AGNs ?
Accretion of mass by a supermassive black hole (SMBH) at the
centre of its host galaxy
Powerful jets due to twisted magnetic field
Same objects but viewed differently ! → Quasars & radio galaxies are THE SAME objects viewed in different ways
power source for quasars & AGNs ?
Powerful jets due to twisted magnetic field
Magnetic field lines twisted as accretion disk spins Charged particles fly out along field lines into space
power source for quasars & AGNs ?
Same objects but viewed differently ! → Quasars & radio galaxies are THE SAME objects viewed in different ways
When we can see directly the bright AGN → quasars When dust obscures the bright AGN → radio galaxies
We do NOT know why quasars eventually stop
shining so brightly or how they formed in the first place → linked to the SMBH formation
We infer the existence of SMBHs from their influence on their surroundings
Present evidence indicate that SMBHs are quite common
The central supermassive black hole (SMBH) → closely related to the:
Galaxy’s spheroidal component: its mass MSMBH = MBulge/500
Spread σ of orbital velocities of the galaxy’s stars: MSMBH ∝ σ 4
The central BH growth is intrinsically linked with the
evolution of the galaxy
Quasars & the study of intergalactic gas
The formation & development of protogalactic clouds and their evolution into galaxies is not yet observed directly Quasar spectra contains valuable info about the hydrogen clouds in the early Universe
Youngest galaxies observed to be made mostly of gas:
same mass in gas clouds as older galaxies have in the form of stars Absorption lines from heavy elements seen only in mature galaxies
We do NOT fully know yet how this process works! (supermassive black holes)
→ hints:
SMBHs could result from the collisions of galaxies/protogalactic clouds
Galaxies in the past were closer together/denser → more common collisions
Galaxies in the past had more gas (not yet incorporated into stars) feeding frenzy for the central BH!
