An Introduction toGalaxies and Cosmology

Section 2.5O.Koizumi

2.5 The formation and evolution galaxies

How do galaxies form?

When did galaxies form?

What are the important factors that determine a galaxy's morphological type?

What is the relationship between the formation of stars and the formation of a galaxy?

Definitive answers to these questions do not exist.

2.5 The formation and evolution galaxiestwo approaches

"work backwards"

Using observations of the properties of galaxiesHow galaxies have evolved?How they ware formed?

"looking backwards"

How conditions in the early universe are likely to have given rise to the structures we observe?

alternative

2.5.1 The early universehot big bang theory (summary)

finite age ~ 140 billion yearscosmic expansion has persisted since the earliest timesphysical conditions - extremely high temperatures and densities.As the Universe expanded and cooled, protons and neutrons formed.nuclear reaction - formation of helium nucleiThe matter also contained particles of non-baryonic matter.baryon and non-baryon was subject to slight density fluctuations.

2.5.2 The origin of galaxiesGravitational instability

Fig. 2.31 The effect of gravitational instabilities in a region of the expanding Universe dominated by dark matter.

expand

highly uniform

seeds

2.5.2 The origin of galaxiesmonolithic collapse scenario

the collapse of a single over-dense region gives rise to a single galaxy

the mass mainly due to dark matter.

baryonic matter would radiate away energy

the baryonic matter settles into the centre of the dark matter halo

galaxy

2.5.2 The origin of galaxiesCold Dark Matter (CDM)

dark matter consists of slow moving, massive particles.

Computer Simulation

106 solar mass

time

smaller entities

1011 solar mass

galaxy

hierarchical scenario

orbottom-up scenario

2.5.2 The origin of galaxiesHot Dark Matter (HDM)

the dark matter particles have speeds that are comparable to the speed of light

time

Simulation

top-down scenario

Some problems

2.5.2 The origin of galaxiesfrom simulation

merger makes elliptical

How spiral forms?monolithic collapse scenario

an overdense region aquired angular momentum from interactions with its neighbors

central part

form a spheroidal leaving a thin rotating disc of gas

outer part

2.5.2 The origin of galaxiesA merger treehierarchical growth

not create a disc

One solution

hierarchical+

monolithic

a plausible scenario rather than a fully accepted theory

Fig. 2.32 A 'merger tree'

low-mass elliptical

giant elliptical

orangestellar componentgreenneutral gas

2.5.3 The evolution of isolated galaxies

galaxy evolutiondue to interactionsintrinsic to the galaxy itself

Here, we consider an isolated galaxy

2.5.3 The evolution of isolated galaxiesEvolution of luminosity and spectra

galaxy's luminous output = sum of the stars output

1 The star formation rate (SFR).2 The luminosity evolution of the stars that are

formed within the galaxy.

depends on two factors

evolution of stars - well known

If SFR was known, we were able to calculate luminous output of galaxy.

2.5.3 The evolution of isolated galaxies

Fig. 2.33 SFR as a function of age.

elliptical's and bulge's SFR is initially very high, but that it declined very rapidly.

in the discs of spiral galaxies declines much more slowly

Except for the discs of Sc galaxies, young galaxies might well have been highly luminous and were probably rather blue.

Star formation rate of each galaxy type

Butcher - Oemler effect

2.5.3 The evolution of isolated galaxiesChemical evolution of galaxies

Cosmic Recycling

hydrogen slowly decrease

helium and heavy elements

gradually increase

ex.) in spiral galaxySFR varies from one region to another.

The study of chemical evolution is another highly technical field.

2.5.4 The role of interactions and mergers in galaxy evolution

Observational interestThey tend to be sites of intense star formation.

The numbers of galaxies that currently seem to be undergoing interactions is rather low - only a few per cent of bright ( L > 1010 Lo ) galaxies.

Theoretical work 1970s ~simulations of interacting galaxies

2.5.4 The role of interactions and mergers in galaxy evolution

Fig. 2.34 (b) Numerical simulationA. Toomre (MIT) and J. Toomre(University of Colorado) (c) STScI

2.5.4 The role of interactions and mergers in galaxy evolution

Fig 2.35 A schematic diagram of HII regions and dust in a starburst galaxy.

many galaxies shine more brightly at infrared wavelength than they do at visible wave length.(IRAS 1983)

Enhanced infrared emission is frequentry associated with interecting galaxies.

Galaxies that shows evidence for very high current rates of star formation

starburst galaxies

2.5.4 The role of interactions and mergers in galaxy evolution

Fig. 2.36 NGC 4038 and NGC 4039

infrared @ 15um (ISO)

the peak of the infrared emission occurs at a different location from the maximum optical emission.

about half of the entire luminosity of this system is radiated in the infrared from the region that appears dark on the optical image.

2.5.4 The role of interactions and mergers in galaxy evolution

The dust absorbs UV and visible light from stars but it re-radiates all this energy as far-infrared radiation.

observations of interacting galaxy

very strong far-infrared source, and may not be particularly strong sources in the UV and visible bands

starburst galaxy (?)

2.5.4 The role of interactions and mergers in galaxy evolution

Fig. 2.37 The merger galaxy NGC 7252

short exposure longer exposure

peculiar

Detailed study of NGC 7252 reveal that the collision was between two spiral galaxies, and suggest that it is in the process of becoming an elliptical galaxy.

2.5.4 The role of interactions and mergers in galaxy evolution

Fig. 2.38 Observations of the galaxy NGC 4365

An example of galaxy that has undergone a merger event, but shows no structural trace of interaction.

blue area - moving towards usred area - moving away from us

outer part - rotating around a long axis

inner part - rotating around a short axis

merger

2.5.5 Observations of galaxy evolution by deep surveys

Deep Surveybeing free from any bright foreground stars or nearby galaxiessuch a survey should detect galaxies over a range of redshiftssensitive survey of this kind that include galaxies with large redshifts

Fig. 2.39The relationship between the redshift and the time t(z)

2.5.5 Observations of galaxy evolution by deep surveys

Hubble Deep Field (HDF)The HDF consists of two fields.

HDF North (HDF-N)HDF South (HDF-S)

Fig. 2.40 HDF-N 2.5 arcmin across

HDF surveys include many galaxies with redshifts between 1 and 3, and there are a few galaxies detected in the field that have redshifts greater than 5.

2.5.5 Observations of galaxy evolution by deep surveys

Hubble Deep Field (HDF)Some difficulties

many of the observed sources have very low flux densities

many galaxies are only just resolveddetermining morphological class is very difficult

band shifting

em=obs

1z We see UV fluxes.

2.5.5 Observations of galaxy evolution by deep surveys

Hubble Deep Field (HDF)Morphological change with redshift

irregular or peculiar was much higher in the past than it is in the present-day Universe.

this result has arisen because of the problem of band-shifting?

or

we are observing the Universe at a time when mergers and interactions were far more frequent than they are at present?

hierarchical merging

2.5.5 Observations of galaxy evolution by deep surveys

Hubble Deep Field (HDF)The star formation history of the Universe

Fig. 2.41 The star formation rate (as determined by UV luminosities of galaxies) against redshift .

current rate is lower than it was in past. max.@z=1~3

This diagram potentially gives information about the formation rate of galaxies with cosmic history.

It seems to support the hierarchical model of galaxy formation.