More More on on GalaxieGalaxies & s & HubbleHubbleJane Turner Physics 316

UMBC-JCA/NASA-GSFC

First - an HST Press Release

The coldest known object in the universe comes into sharp focus in a newly released image from the Hubble Space Telescope.

The Boomerang Nebula, a shell of glowing gas around a fading star, hovers just above absolute zero, the lowest possible temperature

At -272 degrees Celsius, it is even chillier than background radiation from the Big Bang, which is a "balmy" -270 degrees Celsius

Absolute zero is -273 degrees Celsius (-459 degrees Fahrenheit), a temperature at which point the movement of atomic particles comes nearly to a standstill

Galaxies come in many shapes... Hubble’s Tuning Fork Diagram

E0 E6 S0

SB0

Sa Sb Sc Sd

SBa SBb SBc SBd

Irr

Not an evolutionary sequence (rotation) although early ideas led to early-type/late-type nomenclature

Useful because appearance reflects internal conditions

-mergers possibly transform spiral-> elliptical/lenticular

Last Time:Detection of Galaxy Redshifts

All galaxies have spectral features (from their constituent stars) that appear Doppler-shifted wrt their laboratory/theoretical values. - relative motion between the galaxy and us

Commonly known as redshift (z) because

spectral shift is more often towards lower frequencies (longer (ie the red end of the optical spectrum) indicating recession between the galaxy and the observer.

In 1914 Vesto Melvin Slipher first noticed this characteristic amongst the more "obvious” island universe (36/41 objects)although it was unclear what the implication was, -at first

Cepheids solve the debate

-Breakthrough in 1924 with discovery by Hubble of faint Cepheid variable stars in Andromeda

Davison Soper(Univ. of Oregon)

Very important type of star for measuring distance.

Luminous variable stars “Breath” in and out Periodicities in range 3-30 days Period and luminosity closely

related! Good standard candles

Have to calibrate the luminosity relation of Cepheids with parallax of nearby examples.

Cepheids solve the debate

-Breakthrough in 1924 with discovery by Hubble of faint Cepheid variable stars in Andromeda -Established they were not faint due to extinction (not red)-They must be very distant, inverse square law gave a distance estimate of 300 kpc (actually 600 kpc)

This distance lies beyond the edge of the Milky Way, settling the debate - Andromeda is another galaxy In general, spiral nebulae were separate & distant galaxies, like the Milky Way

Cepheids solve the debate

-Expanded humanities concept of the Universe-opened up a new field of study-galaxies

LAST TIME: We looked at the properties & constituents of different types of galaxies and the Hubble classification scheme

-~100 billion galaxies thought accessible to observation

Redshift & the Local Group

By the mid-1920's Hubble had detected Cepheid Variables in many other of the brighter galaxies in the Local Group (and hence derived distances).

However the results were still not clear - the complex gravitation interaction within Local Group (hence motion) hinders the detection of any trends. The Local Group is weakly gravitationally bound

He also had redshifts for these galaxies (from spectroscopy).

The galaxies were extragalactic & moving

Indeed M31 is approaching us at 300 km/s (“blueshifted”)

Hubble goes Fainter

Hubble and Humason used the Mount Wilson 100” telescope to get dozens more galaxy spectra

They took it further, by combining spectral information with distance indicators from independent methods

Hubble noticed a trend : fainter spiral galaxies appear to have higher redshifts

Hubble goes Fainter

1) Using Cepheids to measure distance2) For cases too faint to see individual Cepheids they used some

assumption that they knew the brightness of the brightest star in any galaxy (natural upper limit to stellar brightness)

3) For the faintest cases they had to assume they knew the absolute luminosity of the galaxy based on its type

Then they plotted redshift (from spectra) versus distance (from 1,2, or 3)

flux = luminosity4 x (distance)2

The luminosity-distance formula

apparent brightness = absolute brightness

4 x (distance)2

Some real data

Hubble’s Plot

Hubble’s original value was H=500 km/s/Mpc… way off the real value. He had mistakenly identified bright nebulae in other galaxies with bright stars, thereby making them seem closer. Also, there are two types of Cepheids

v = HO r

Plotting the redshift vs distance showed that (despite large scatter) there is indeed a clear trend: Larger distance, higher redshift

Hubble’s Law

Hubble’s relation between velocity (v) and redshift (z) is well approximated by a straight line

Hubble's data only goes out to a redshift z=3x10-3 Since confirmed to be linear out redshifts z=0.5 (using >104 galaxies)

This is the famous Hubble Law v = H0 r

with H0 known as the Hubble parameter or Hubble constant. (most often expressed in terms of km s-1 Mpc-1), r is the distance Cosmological importance : reveals the expansion of the universe.

Value of H0 required to constrain cosmological models.

Hubble’s Law

Note the subscript H0 is used to denote the current epoch since the

velocity/distance relation can change with time.

More “general” Hubble Law v = H r (time-dependence of H is implicit)

H quoted in units km s-1 Mpc-1 (estimates fall between 50-100)

Hubble's contribution was to find ways of measuring distances that did not depend on the redshift (scaling by what seemed to be bright stars in other galaxies), thus could discover relation

H = v / r = r_dot / r

(where r can be any measure of scale-size)

Hubble’s Law

Hubble Law v = H r

So, suppose we take a value of 100 km s-1 Mpc-1 for H, then a galaxy located at a distance of 10 Mpc (32.6 light years) would be expected to be moving away from us at an apparent velocity

v =100 km s-1 Mpc-1 x 10 (Mpc) = 1000 km s-1

The Universe is not Static

Hubble's result - universe is expanding.

Let's assume this "local" expansion is not a "local" perturbation, representative of the universe as a whole, (for now) ignoring the Steady-State Cosmological Model

then clearly

the universe is not static

Hubble’s Law

Note:Hubble’s law does not mean that we are at the center of the expansion pattern!

Galaxies do move slightly (by up to 1000 km/s) relative to the Hubble Flow.

Why is that?

Hubble’s Law

Hubble Law v = H r shows the universe is expanding

Every galaxy getting further from every other, rather than us at center of expansion pattern

Understanding Universal Expansion

Since galaxies are moving away from each other (Hubble’s Law), they must have been closer together in the past

This is the observational basis for the big bang - if you trace this expansion back, you can find a timewhen the universe had zero size

Ever since then the universe has been expanding, and ever since then gravity has been trying to reverse that expansion

Cosmological Principle: matter in the universe is evenly distributed on large scales expansion is uniform and looks the same everywhere

Note - the universe is not expanding into anything, it is space & time itself that are expanding

Cosmological Redshifts

BUT, waves of light get stretched as the universe expands

thus the expansion of space itself naturally produces a redshift of photons from distant galaxies

can be thought of as photon wavelengths being stretched

Galaxy redshifts are thus called Cosmological

Conceptually different to galaxies zipping into a void - distinction between redshift being due to velocity or expansion of space

The expanding pattern of galaxies is known as the Hubble Flow.

Cosmological Redshift

Although local gravity means some galaxies are moving towards us (peculiar motions), at large distances the cosmological redshift dominates

So, while nearby objects have velocity shifts which can be measured accurately, the velocities due to gravity of their local environment can be comparable to cosmological redshift - have to look at distant galaxies where cosmological redshift really dominates

“Lookback" Time

Lookback Time (tL) is difference between present (expansion) age of the universe (texp), and age (tz) when light was emitted by an object with a redshift z. ie.

tL = texp - tz

Picture of furthest galaxies show us how they looked when only a few billion yrs old

Pictures of the closest show galaxies almost as old as the universe

“Lookback" Time

Photons from galaxy 400 million ly away took 400 million yrs to reach us. OK, in that time, the galaxy has moved further from us because space expanded between us.

Have to consider the picture in space and time!

So, the lookback time to that galaxy is the difference between the age of the universe now, & when the photons left the galaxy

Some Implications

Note that the observable universe does have a horizon, a boundary at which the lookback time would exceed the age of the universe

This doesn’t necessarily mean the universe has finite size, just that we can’t see beyond a given distance! 

A Problem with Age ?

Recap: Galaxies (generally) receding, in the past clearly the galaxies must have been closer -basis of ‘Big Bang’ models

H has units of 1/ time

If one ignores the effect of gravity (& inflation).. (no acceleration or deceleration in the Hubble Flow) then the 1/H gives a crude estimate of the “age” of the flow thus of the universe

1/H is known as the Hubble time or Hubble age

Age Estimations

Initially estimates of age from H0 suggested the universe to be too young (younger than some rocks)- result of Hubble’s underestimate of distance- partly responsible for the Steady-State Cosmological Theory

H0 is relatively poorly determined at present: between 50 and 100 km s-1 Mpc-1

and hence

texp is between 7 and 13 Gyr

So quick review some independent estimates of age of the universe.

Obviously these provide rigid limits

- the model must not predict an age for the universe shorter than anything in it !

Earth/Moon Rocks & Meteorites

The "easiest" method - based on the radio-active decay of U238 (half-life 4.5 Gyr). Earth & Moon Rocks > 3.9 Gyr Meteorites > 4.6 Gyr

...of interest regarding the formation of the Solar system, but not for constraining for cosmology

Recent Results: based on rocks and crystals from various sites Greenland, Canada, Australia & Africa place the age of the Earth at 4.5 Gyr

Harper, C.L., Jacobsen, S.B., 1992, Nature, 360, 728 Scherer, E, et al., 2001, Science, 293, 683

Ages beyond the Solar System

Harder requires ... accurate observations of some characteristic, & often theoretical modeling of the time evolution ("aging") of the body.

Followed by a theoretical understanding of when (& how) in the history of the universe such a body formed

Before we can use them to set a minimum age of the universe

(latter also applies to Terrestrial/Moon Rocks and Meteorites)

Age Estimates (non-Solar system)

Globular Clusters 13 to 18 Gyr (model dependent)

Radioactive Isotopes 10 to 21 Gyr

Cool White Dwarfs 9 to 10 Gyr (model dependent)

High-z Quasars > 16 Gyr (model dep, obs limited)

All require (at least) a knowledge of time for Galaxy (or galaxies) to form star formation/evolution timescales within Galaxy (galaxies)

… still very much a subject of active research.

Age Estimate Summary

Most of the age estimates in an interesting range for cosmology 8 to 18 Gyr

ie H0 (using 3texp/2) 80 to 40 km s-1 Mpc-1

With modern values approx (!) 13 Gyr (Giga years)(WMAP gives us 13.7 Gyr of course!)

However current observational & theoretical limitations conspire

such that we do not have particularly rigorous/reliable/consistent constraints on the cosmological parameters.

Hubble length & sphere

Related concept is the Hubble length-the distance light travels in one Hubble time

ie DH = c tH = c/H

For H0 50 km s-1 Mpc-1 DH = 1.5 trillion pc

Can be thought of as a sphere around us with radius equal to the Hubble length -the Hubble Sphere

This sphere is the volume of the universe which we could observe because the light has had time to reach us…defines our observable universe to date

Every point in the universe has its own Hubble sphere

Galaxy Evolution

• How did galaxies form?

• How did we get the properties we observe today in terms of:• Galaxy dynamics• Stellar characteristics• ISM properties

• What happens to galaxies as the universeages and how does this affect their properties?

How do spiral galaxies form?

CLUES: Flattened, rotating disk , non-rotating spherical component, stellar populations differ with radius

RECAP: Population II stars formed early in bulge & have random orbits Population I stars formed later, in the rotating disk, have ordered orbits

Spiral Galaxy formation...

Cloud starts to collapse under the attraction of its own gravity.

First stars to form are globular cluster stars (Pop II) on disordered, noncircular orbits, since the cloud from which they formed, in its early state, was rotating only very slowly and was nearly spherical in shape.

The low-mass Population II stars are still around today -- they are still on the main sequence, and are happily fusing H into He within their cores. The high-mass Population II stars, by contrast, have exploded as supernovae, enriching the collapsing cloud with heavy elements

Initial state: Galaxy starts out as a slowly rotating cloud of H & He, ~100 kiloparsecs across.

Spiral Galaxy formation...

.

Cloud completes its collapse to a rotationally supported disk, contaminated with heavy elements (from supernovae)

The gas in the disk fragments to form disk stars (Population I). These stars are rich in heavy elements (produced in an earlier generation of halo stars). The stars will also be on orderly, circular orbits within the rotating disk.

Second stage: Formation of disk.

Spiral galaxies…

Spiral galaxies…

How did Elliptical Galaxies form?Competing theories: 1. Clue from clusters? Giant ellipticals sit in center of clusters -> ellipticals form in dense cluster environments which strip extended clouds that otherwise would have become disks

2. Spiral mergers destroy both disks, cold gas is used up in star formation --> elliptical merger remnant, gas splatters but stars don't come close to touching

How did Elliptical Galaxies form?

When Galaxies Collide

Simulations show tidal forces tear apart disks and randomize stellar orbits

Gas sinks to center of colliding pair, it is dense & shocked & quickly forms stars. Galaxies where this is happening are known as starburst galaxies, the phenomenon is short-lived (~100 million yr)

Subsequent supernovae blow off gas & system settles to elliptical remnant

Some surely form this way, ellipticals in centers of clusters

Elliptical Galaxy Formation...

More Ideas:

3. Did ellipticals form from gas with little angular momentum?

Doesn’t explain their lack of ISM gas

4. Did ellipticals form from dense regions which cooled fast & stars formed fast, preventing disk formation Would predict all ellipticals to be massive - maybe but unclear ??

Galaxy Evolution

HST deep images found 18 small galaxies 11 billion light years away, formed when the universe was young. Building blocks of regular galaxies?

At least x 10 smaller than present epoch galaxies-do mergers of these form ‘normal’ size galaxies?

Galaxy Evolution

Ellipticals: old ones resemble todays….formed early?

Spirals: Structure more disrupted in old spirals….spiral galaxies took longer to form than ellipticals?

Conflict with one formation scenario for ellipticals…

Hubble Space Telescope ResultsGalaxies seen out to redshifts ~ 3-6 (lookback time 10.2-11.3 billion yrs) - z up to 6 confirmed with spectroscopy

Galaxies at high redshift, i.e those in the early universe DO look different to nearby galaxies

Lots of irregularsHigher rates of star formation

Difficult to observe high-z galaxies - @z=6.5 Ly a passes out of the visible@z>20 it passes out of IR

Understanding galaxy evolution is work in progress