Surveying the Universe

Russell Johnston

Dept of Physics and AstronomyUniversity of Glasgow

Edwin Hubble

Hubble measured the shift in colour, or wavelength, of the light from distant galaxies.

Galaxy

Hubble measured the shift in colour, or wavelength, of the light from distant galaxies.

Galaxy

Laboratory

Wavelength

Energy

Spectrum of a nearby galaxy

Spectrum of a Distant Galaxy

Hubble’s Law: 1929Hubble’s Law: 1929

Distant galaxies are receding from us with a speed proportional to their distance

Spacetime is expanding like the surface of a balloon.

As the balloon expands, galaxies are carried farther apart

Although Hubble got the expansion law correct, his measurement of the current rate of expansion was quite wrong, and took many decades to correct.

Measuring the Hubble constant was a key project of the Hubble Space Telescope

More recently we have extended the Hubble diagram to great distances, using e.g. Supernovae….

Region probed by

Hubble’s data

redshift

‘Speeding up’ model

‘Slowing down’ model

Models with different shapes

Hubble’s law for nearby supernovae

mea

sure

of

dist

ance

….This has led to a remarkable discovery:The expansion of the Universe is speeding up!

What is driving the cosmic acceleration?…What is driving the cosmic acceleration?…

Around Galaxies

0

50

100

150

200

250

300

0 20 40 60 80

Distance from the Galaxy Centre (kpc)

Orb

ital v

eloc

ity (

km/s

)

Typical size of galaxy disk

What we seeWhat we see

What is really there.

We can also measure the redshifts of many galaxies.

We call this a redshift survey.

Redshift surveys can tell us many useful things:

• How galaxies cluster in space

• How galaxies evolve in time

• Different types of galaxy and where (and when) they are found

• How galaxies formed in the first place

• How much dark matter and dark energy…

And

The First Redshift Surveys• CfA Survey #1 : 1977 - 1982

CfA # 1

• Surveyed a total of 1100 galaxies

• Marc Davis,

John Tonry

Dave Latham,

John Huchra,

• Redshift range: out to z

0.05

Our own Galaxy

de Lapparent, Geller, and Huchra (1986), ApJ, 302, L1

de Lapparent, Geller, and Huchra (1986), ApJ, 302, L1

Filament

Rich cluster

Void?

CfA # 2

The First Redshift Surveys• CfA Survey #2 : 1985 -1995• John Huchra &• Surveyed a total of 18,000 galaxies

Margaret Geller

• Redshift range: out to z

0.05 208 Mpc

Redshift surveys (mid-

1980s)

1 Mpc = 3.26 milion light years

1 Mpc = 3.26 milion light years

The largest structures in LCRS are much smaller thanthe survey size

The size of thestructures issimilar in both samples

LCRS

1995 (LAS CAMPANAS)

The First Redshift Surveys• IRAS PSCz : 1992 – 1996, 15,000 galaxies• Team originally consisted of around 24 members including:

• Catalogued over 83% of the sky -

Will Sutherland,

Steve Maddox,

Largest full sky survey.

Will Saunders,Carlos Frenk &

Seb Oliver,

Luis Teodoro.

Surveys….

The Next Generation

• Ran from 1998 to 2003.• Used the multifibre spectrograph on the Anglo Australian Telescope. • The survey covered two strips : NGP -

75 10

80 15 SGP -

• Photometry was taken from the APM galaxy catalogue. • Galaxies brighter than

19.45b

m

• Recovered a total of 245,591 redshifts, 220,000 of which were galaxies out to 0.2z

The Two Degree Field Galaxy Redshift Survey

(2dFGRS)


(2dFGRS)

• 35 collaborators fro UK, Australia and the US.• including: Carlos Frenk, Matthew Colles, Richard Ellis, Ofer Lahav, John Peacock, Will Sutherland…. and these guys:

Keith Taylor

Simon Driver

Karl Glazebrook Nick

Cross

Shaun Cole

Peder Norberg

Warrick Couch


(2dFGRS)

= 100 Mpc diameter

The Sloan Digital Sky Survey

(SDSS)• Most ambitious ongoing

survey to date.• Began in early nineties and was due to complete in 2008 …. ish

• Uses a dedicated 2.5m telescope on Apache Point, new Mexico and a pair of spectrographs that measure more than 600 galaxy spectra in a single observation.

• Currently on data release 5 which contains 674749 galaxies.

• On completion will have surveyed over 1 million galaxies.• The Survey has over 150 collaborators at 26 institutions

The Sloan Digital Sky Survey

(SDSS)

SDSS

CfA

Sloan Digital Sky Survey: The Footprint of the Survey

Area and Size of Redshift Surveys

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10 1.00E+11

Volume in Mpc 3

No

of

ob

jec

ts

LCRS

SDSSmain

SDSSred

SDSSabs line

SDSSphoto-z

2dFRCfA+SSRS

SAPMQDOT

2dF

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10 1.00E+11

Volume in Mpc 3

No

of

ob

jec

ts

LCRS

SDSSmain

SDSSred

SDSSabs line

SDSSphoto-z

2dFRCfA+SSRS

SAPMQDOT

2dF

CMBR fluctuations, 380000 years after the Big Bang, are the seeds of today’s galaxies

The pattern of CMBR temperature fluctuations can be used to constrain the background cosmological model and its parameters

Galaxies and Cosmology: the Basic Paradigm



Both the CMBR and present-day galaxy clustering favour :


CDM

Cold dark matter + non-zero cosmological constant



Both the CMBR and present-day galaxy clustering favour :


CDM

Cold dark matter + non-zero cosmological constant

The Concordance ModelThe Concordance Model

CDM

From Lineweaver (1998)

The cosmological constant now dominates over CDM and

baryonic dark matter (i.e. atoms).

It is not yet clear if is constant, or perhaps evolves with

time.

More generally, is referred to as

‘Dark Energy’.

Dark Energy

Cold Dark Matter

Ato

ms




time.


‘Dark Energy’.

Dark Energy

Cold Dark Matter

Ato

ms




time.


‘Dark Energy’.

Unlike ‘normal’ matter, dark

energy is gravitationally repulsive :

it is causing the expansion of the

Universe to accelerate.

Dark Energy

Cold Dark Matter

Ato

ms




time.


‘Dark Energy’.

Unlike ‘normal’ matter, dark

energy is gravitationally repulsive :

it is causing the expansion of the

Universe to accelerate.

This affects the rate of growth of

cosmic structure, which we can model

via computer simulations

Hierarchical clustering:

Galaxies form out of the mergers of fragments: CDM halos at high redshift.

Clusters form where filaments and sheets of matter intersect

140 Mpc

11 Gyr ago




140 Mpc

8 Gyr ago




140 Mpc

Present day




20 Mpc

11 Gyr ago




8 Gyr ago

20 Mpc




Present day

20 Mpc




11 Gyr ago

20 Mpc




8 Gyr ago

20 Mpc




Present day

20 Mpc

Which simulation model matches the observations?...

Hubble’s tuning fork classification

We see spiral and elliptical galaxies…

Morphological Segregation

Nowadays we find few spiral galaxies in rich clusters. This is thought to be because the spiral disks are disrupted by tidal forces…

Morphological Segregation

Nowadays we find few spiral galaxies in rich clusters. This is thought to be because the spiral disks are disrupted by tidal forces…

…Conversely, many ellipticals (and some spirals) may have formed from galaxy mergers.

See talk by Bonnie Steves

A long time ago,

in a galaxy far, far away…

z = 2.0

Light travel time =10.3 billion years

z = 2.1


z = 2.2


z = 2.3


z = 2.4


z = 2.5


z = 2.6


z = 2.7


z = 2.8


z = 2.9


z = 3.0


z = 3.1


z = 3.2


z = 3.3


z = 3.4


z = 3.6


z = 3.7


z = 3.8


z = 4.0


z = 4.1


z = 4.3


z = 4.4


z = 4.5


z = 4.6


z = 5.0


Surveying the Universe

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