Observational cosmology: what can we observe?
Electromagnetic waves Radio, submillimeter, IR, optical, UV, X, gamma
Particles Neutrinos, cosmic rays
Gravitational waves? .....
Observational cosmology: where can we observe?
From Earth (radio & optical: almost everywhere; close IR, sub-mm: some high places like Atakama desert)
From satellites (microwaves, IR, UV, X, gamma)
Observational cosmology: what kind of objects do we observe?
galaxies individual surveys
Clusters of galaxies CMBR IGM AGNs Lensing effects Transient phenomena:
GRBs SNIa
Cosmic Microwave Background Radiation
Temperature maps Polarisation maps 5-year WMAP data Measurements:
Temperature fluctuations (2.7 K +/- 10^(-5))
Power spectrum Photon polarization
Galaxies: individual
Many types of galaxies in a local and distant Universe
Investigating individual galaxies we can understand their evolution, the relation between their properties and their position in the LSS and so on
Galaxies: individual
An example of a “cosmologically interesting” local galaxy: NGC 1705
Low-metallicity local irregular dwarf galaxy with a recent outburst of star formation: a model of the first galaxies?
Galaxies: surveys
Angular (only positions on the sky) 3-dimensional (with a redshift measurement)
Different wavelengths
Sky looks differently in different wavelengths
+ identification problems
Galaxies: local surveys In optical: local: 2dFGRS: spectra for 245 591
objects (mainly galaxies) in 1500 square degrees
Galaxies: local surveys
In optical: local: SDSS
230 million celestial objects detected in 8,400 square degrees of imaging and spectra of 930,000 galaxies, 120,000 quasars, and 225,000 stars.
Galaxies: local surveys
In total, the local Universe in ~2 billion light year radius is pretty well known in the optical light
More than 1M galaxies LSS “special” objects (quasars, satellites of Milky
Way)
Galaxies: deep redshift surveys
DEEP2, VVDS, zCOSMOS
In total ~a few tens of thousands of galaxies at z>0.5
Galaxies: next generation of deep redshift surveys
VIPERS >100 000 galaxies at z~1 at 24 square degrees Statistical counterpart of SDSS or 2dF but at z~1 Volume (comoving) ~ 5 x 107 h-3 Mpc3
Galaxies: dedicated surveys to search for particular objects
If we want to increase the chance that our survey contains “interesting” (e.g. EROs) objects, we make a preselection, most often based on color-color diagrams
Galaxies: surveys in other wavelengths (IR)
IR: IRAS, 2MASS, Spitzer, AKARI...
350 000 objects, many of them still not identified!
Low resolution: 30”-2'
Galaxies: surveys in other wavelengths (UV)
GALEX: all-sky map in FUV and NUV
Unlike in IR, almost every UV source has an optical counterpart
Also a poor resolution
(Credit: Mark Seibert, OCIW)
Surveys: basic tools
For all the objects: Redshifts: how to measure and recognize Colors/morphological types/sizes Luminosities: apparent and absolute Stellar masses...
Statistics: Number/magnitude counts (how many objects brighter
than...) Luminosity function Angular/spatial distribution Correlation function(s) ...
Clusters of galaxies
The largest gravitationally bound structures in the Universe
Their distribution is “the closest” to the primordial matter distribution
Observed in X-rays, because they are filled with hot intergalactic gas (10 – 100 mln K)
Often with a massive central elliptical galaxy
Virgo: the closest massive cluster of galaxies in optical
light and X-rays (1 Mpc)
Credit: Ray White, University of Alabama
Intergalactic medium
Diffuse intergalactic gas in clusters (in some cases of a mass ~ mass of stars in galaxies)
Between clusters: Ly-alpha absorption systems
A powerful tool to study Metal production and feedback processes in the
history of galaxy formation Thermal and ionization history of the Universe
during and after the epoch of reionisation
Lensing
An effect of the change of the direction of photons in the gravitational field, due to the bending of the spacetime (relativistic effect) Strong (multiple images of the same object,
typically a quasar or an “Einstein ring”) Weak (distorted images of objects e.g. behind a
rich cluster) Microlensing (change of luminosity, in case of
lensing on small objects like planets or small stars)
Gravitational lensing is used to “map” mass distribution in galaxy clusters
Cosmological lensing:
An example of lensing on a rich galaxy cluster – fragments of Einstein rings, distorted images...
Transient sources: SN Ia
Thought to be white dwarfs from binary systems which collected too much matter from a companion through accretion processes and exceeded the Chandrasekhar mass
All ~ of the same mass -> the same absolute luminosity -> standard candles
Visible until z~1.3-1.4 Supernova Cosmology Project, Supernova
Legacy Survey (SNLS)
Transient sources: SN Ia
Hubble diagram from SN Ia led to “rediscovery” of the cosmological constant