Origin, Evolution and Structure of Galaxies, Physics tutorial


By the discovery of nature of galaxies, the very first hypothesis developed to describe their existence was one of gravitational collapse in the primordial gas. As the forming galaxies grew smaller, the gas tended to fall to the flat plane, having fragmentation into stars taking place throughout both the collapse stage and continuing after the formation of final disk.

Origin and Evolution of Galaxies:

The formation of a galaxy was done when the mass distribution came into equilibrium between the motions and gravity. The differentiation between the kinds of galaxies was assumed to have been the outcome of the initial conditions. When lots of angular momentum was present, a disk galaxy would be generated. If primarily there is little angular momentum, all matter would become stars throughout the collapse stage, resultant in an elliptical galaxy. 

Observational and theoretical work in latest times has illustrated that the galaxy formation is a much more complex method. At first, the efficiency of star formation is low. As an outcome, elliptical galaxies can't be generated as was once thought; galaxy formation generates disk galaxies having significant interstellar material left over. Subsequently, interactions between the galaxies over the history of universe can be noteworthy. Galaxies do combine, and they cannibalize smaller companions. Violent interactions between the disk galaxies come out to randomize motions and as well to proficiently transform colliding interstellar gas into stars, leaving behind gas-free elliptical galaxies.

Galaxies that might have grown in size, however avoided main disruptive encounters, appear to have evolved to the spectrum of spiral galaxies which exist nowadays. As well, gentle encounters between the two gassy disk galaxies are possible and such encounters leave their basic stellar distributions unchanged however result in the gas being swept out, therefore generating the relatively rare, flat and gas-free galaxies.

This is now assumed that the early era of galaxies was much more confused than today's universe. The method of generating equilibrium galaxies was related with the growth of massive, non-stellar black holes in the nuclei. The release of tremendous energies throughout their formative phases is observed as quasars; however quasars died when galaxies accomplished their equilibrium structures and ended mass in-fall into the centers. If new mass falls into the centers of galaxies, the central black hole phenomena can be re-ignited, describing the active galactic nuclei of the present day.

Active Galactic Nuclei:

Our main vision toward cosmic world and in specific of the whole Universe has been changing dramatically in the last century. As we will observe much later, galaxies were repeatedly the major protagonist in the scene of such changes. This is around 80 years since E. Hubble recognized the nature of galaxies as gigantic self-bound stellar systems and employed their kinematics to illustrate that the Universe as a whole is expanding uniformly at the current time.

Galaxies are the fundamental building blocks of the Universe. By looking inside the galaxies we find out that there are the arena where stars form, evolve and collapse in constant interaction having the interstellar medium (ISM), a complex mixture of plasma and gas, dust, radiation, cosmic rays and magnetic fields. The centre of an important fraction of galaxies harbors super massive black holes. If they are fed by infalling material, the accretion disks around them discharge, mostly via powerful plasma jets, the huge amounts of energy known in astronomical objects. This concept of Active Galactic Nuclei (AGN) was much more common in the past than in the present, being the high-red shift quasars (QSO's) the most influential incarnation of the AGN phenomenon. However the most astonishing surprise of galaxies comes from the fact that the luminous matter (that is, stars, gas, AGN's and so on) is just a tiny fraction around 1 - 5% of all the mass measured in the galaxies and the huge halos around them.

Therefore, exploring and understanding the galaxies is of principal interest to cosmology, high-energy and particle physics, gravitation theories and, obviously, astronomy and astrophysics. Since astronomical objects, among other questions, we would like to identify how they take shape and develop, what is the cause of their diversity and scaling laws, why they cluster in space as noticed, following a sponge-like structure, finding out the dark component which predominates in their masses. By responding to such questions we would capable as well to use galaxies as a true link between the observed universe and the properties of the early universe, and as physical laboratories for testing the basic concepts and theories.

Galaxy Properties and Correlations:

Throughout several decades galaxies were considered fundamentally as self-gravitating stellar systems so that the study of their physics was a domain of Galactic Dynamics. Galaxies are mostly conglomerates of hundreds of millions to trillions of stars supported against the gravity either by rotation or by arbitrary motions. In the past case, the system consists of the shape of a flattened disk, where most of the material is on circular orbits at radii that are the minimal ones permitted by the specific angular momentum of the material. Alongside, disks are dynamically fragile systems, not stable to perturbations. Therefore, the mass distribution all along the disks is the outcome of the specific angular momentum distribution of the material from which the disks form and of the posterior dynamical (that is, internal and external) methods. In the later case, though, the shape of the galactic system is a concentrated spheroid or ellipsoid, with generally (disordered) radial orbits. The spheroid is dynamically hot and stable to perturbations.

Stellar Populations

In the year 1940, W. Baade introduced that according to the ages, metallicities, kinematics and spatial distribution of the stars in our Galaxy is separable into two major groups namely:

a) Population I star that populate the plane of the disk; their ages don't go beyond 10 Giga years and a fraction of them however are young; being less than 106 yr.

b) Population II stars that are positioned in the spheroid component of the Galaxy (that is, stellar halo and partially in the bulge), where velocity dispersion (or random motion) is higher than the rotation velocity (or ordered motion); they are old stars greater than 10 Giga years having very low metallicities, on the average lower via two orders of magnitude than Population I stars. In between Population's I and II there are some stellar subsystems.

The Stellar populations are proper fossils of the galaxy assembling method. The differences between them are proved in the formation and evolution of the galaxy components. The Population II stars, being old, of low metallicities, and dominated through random motions (that is, dynamically hot), had to form early in the assembling history of galaxies and via instead violent methods. There is a big range of ages of Population I stars, however on average younger than the Population II stars. This point out a slow star formation method which carries on even nowadays in the disk plane.

The general astronomical wisdom states that spheroids form early in a violent collapse (that is, monolithic or major merger), whereas disks assemble via continuous in fall of cosmic gas rich in angular momentum.

Interstellar Medium (Ism):

Galaxies are not just conglomerates of stars. The study of galaxies is not complete if it doesn't take into account the ISM, which for late-type galaxies accounts for more mass than that of the stars. Alongside, it is expected that in the deep past, galaxies were gas-dominated and by the passing of time the cold gas was being converted into stars.

The complex structure of the ISM is associated to:  

a) Its peculiar thermo-dynamical properties (that is, in specific the heating and cooling methods).

b) Its hydro-dynamical and magnetic properties that imply the growth of turbulence.

c) The different energy input sources.

The star formation unities (that is, molecular clouds) appear to form throughout the large-scale compression of the diffuse ISM driven by the supernovae (SN), magneto-rotational instability and disk gravitational instability. At similar time, the energy input through stars affects the hydro-dynamical conditions of the ISM: the star formation outcomes self-regulated via a delicate energy (or turbulent) balance.

Classification of Galaxies:

Galaxies are the correct 'ecosystems' where stars form, evolve and collapse in the constant interaction by the complex ISM. Following a pedagogical analogy by biological sciences, we might state that the study of galaxies proceeded via taxonomical and anatomical approaches.


As it occurs in any science, as soon as galaxies were discovered, the subsequent step was to try to categorize these news objects. This endeavor was taken on by E. Hubble. The showiest features of galaxies are the bright shapes generated by their stars, in particular those most luminous. Hubble observed that by their external look (that is, morphology), galaxies can be categorized into three principal kinds namely:

a) Ellipticals (E, from round to the flattened elliptical shapes).

b) Spirals (S, characterized through spiral arms emanating from their central areas where a spheroid structure termed as bulge is present)

c) Irregulars (Irr, clumpy devoid of any defined shape)

However, the last two classes of galaxies are disk-dominated and rotating structures. The spirals are categorized into Sa, Sb, Sc types according to the size of the bulge in relation to the disk, the openness of the winding of the spiral arms, and the degree of resolution of the arms into stars (in between the arms there are as well stars however less luminous than in the arms). Around 40% of S galaxies present an extended rectangular structure (known as bar) further from the bulge; these are the barred Spirals (SB), here the bar is proof of disk gravitational instability.


The morphological categorization of galaxies is based on their external feature and it implies fairly subjective criteria. Alongside, the 'showy' characteristics that distinguish this categorization might change by the color band: in blue bands that trace young luminous stellar populations, the arms, bar and other features might look dissimilar to what it is seen in infrared bands that trace less massive, older stellar populations. This is interesting to discover deeper into the internal physical properties of galaxies and observe whether such properties correlate all along the Hubble sequence. Luckily, this seems to be the case in general in such a way that, despite of the complexity of galaxies, a few clear and sequential trends in their properties encourage us to think regarding regularity and the possibility to determine driving parameters and factors beyond this intricacy.

Cosmic Structure Formation:

We are now familiar that the formation and evolution of galaxy are definitively associated to the cosmological conditions. Cosmology gives the theoretical frame-work for the initial and boundary conditions of the cosmic structure formation models. At similar time, the confrontation of model predictions all along with astronomical observations became the most powerful test bed for cosmology. As an outcome of this fruitful convergence between cosmology and astronomy, there emerged the present paradigmatic scenario of cosmic structure formation and evolution of the Universe termed as A Cold Dark Matter (CDM). The CDM scenario integrates i.e.: 

  • Cosmological theories (such as, Big Bang and Inflation)
  • Physical models (that is, standard and extensions of the particle physics models)
  • Astrophysical models (that is, gravitational cosmic structure growth, hierarchical clustering and astrophysics)
  • Phenomenology

Gravitational Evolution of Fluctuations:

The CDM scenario supposes the gravitational instability paradigm: the cosmic structures in the Universe were made as a result of the growth of primordial tiny fluctuations (for illustration seeded in the inflationary epochs) through gravitational instability in an expanding frame. The fluctuation or perturbation is featured by its density contrast:

δ = δρ/ρ = (ρ - ρ‾)/ρ‾

Here, ρ‾ is the average density of the Universe and ρ is the perturbation density. At early epochs, δ << 1 for perturbation of all scales, or else the homogeneity condition in the Big Bang theory is not followed. If δ << 1, the perturbation is in the linear regime and its physical size expands with the expansion proportional to a(t). The perturbation analysis in the linear approximation illustrates whether a given perturbation is stable (δ ≈ const or even → 0) or unstable (δ grows) In the subsequent case, if δ → 1, the linear approximation is not any longer valid, and the perturbation 'separates' from the expansion, collapses and becomes a self-gravitating structure. The gravitational evolution in the non-linear regime is complex for the realistic cases and is studied by numerical N-body simulations.

Dark Matter

In cosmology and astronomy, dark matter is matter which neither emits nor scatters light or other electromagnetic radiation, and therefore can't be directly detected through optical or radio astronomy. Dark matter is assumed to comprise of 83% of the matter in the universe and 23% of the mass-energy.

Dark matter was hypothesized by Fritz Zwicky in the year 1934 to account for proof of 'missing mass' in the orbital velocities of galaxies in clusters. Afterward, other observations have pointed the presence of dark matter in the universe; such observations comprise the rotational speeds of galaxies, gravitational lensing of background objects via galaxy clusters like the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. However, the existence of dark matter is usually accepted through the mainstream scientific community, some alternative theories to describe the anomalies that dark matter is intended to resolve have been introduced.

The existence of Dark matter is inferred from the gravitational effects on visible matter and gravitational lensing of background radiation, and was formerly hypothesized to account for discrepancies between computations of the mass of galaxies, clusters of galaxies and the whole universe made via dynamical and general relativistic means, and computations based on the mass of the visible 'luminous' matter these objects include: stars and the gas and dust of the  interstellar and intergalactic medium. The most broadly accepted description for these phenomena is that dark matter exists and that it is most probable comprised of heavy particles which interact only via the weak force and gravity; though, alternate descriptions have been introduced, and there is not yet adequate experimental proof to find out which is right. Most of the experiments to detect proposed dark matter particles via non-gravitational means are ongoing.

According to the observations of structures larger than solar systems and also galaxies, Big Bang dark matter accounts for 23% of the mass-energy density of the apparent universe. In comparison, ordinary matter accounts for just 4.6% of the mass-energy density of the observable universe, having the remainder being attributable to dark energy. From such figures, dark matter comprises 83%, (23/(23+4.6)), of the matter in the universe, while ordinary matter makes up just 17%.

Dark matter plays a key role in state-of-the-art modeling of structure formation and galaxy evolution, and consists of measurable effects on the anisotropies noticed in the cosmic microwave background. All such lines of evidence recommend that galaxies, clusters of galaxies, and the universe as an entire include far more matter than that which interacts by electromagnetic radiation. The largest portion of dark matter, which by definition doesn't interact by electromagnetic radiation, is not just 'dark' however as well by definition, completely transparent.

As significant as dark matter is assumed to be in the cosmos, direct proof of its existence and concrete understanding of its nature have both remained elusive. However, the theory of dark matter remains the most broadly accepted theory to describe the anomalies in observed galactic rotation, a few alternative theoretical approaches have been made which generally fall into the classes of modified gravitational laws, and quantum gravitational laws.

Baryonic and Non Baryonic Dark Matter:

A small amount of dark matter might be baryonic dark matter: astronomical bodies, like massive compact halo objects, which are composed of ordinary matter however which emit little or no electromagnetic radiation. The huge majority of dark matter in the universe is assumed to be nonbaryonic, and therefore not formed out of atoms. This is as well assumed that it doesn't interact by ordinary matter through electromagnetic forces; in specific, dark matter particles don't carry any electric charge.

The nonbaryonic dark matter comprises neutrinos, and possibly hypothetical entities like axions, or super-symmetric particles. Dissimilar baryonic dark matter, nonbaryonic dark matter doesn't contribute to the creation of the elements in the early universe (that is, Big Bang nucleosynthesis) and therefore its presence is revealed only through its gravitational attraction. Moreover, if the particles of which it is comprised are supersymmetric, they can experience annihilation interactions with themselves resultants in observable by-products like photons and neutrinos (that is, indirect detection).

Nonbaryonic dark matter is categorized in terms of the mass of the particle(s) which is supposed to make it up, and/or the typical velocity dispersion of such particles (as more massive particles move more slowly). There are three well-known hypotheses on nonbaryonic dark matter, termed as Hot Dark Matter (HDM), Warm Dark Matter (WDM) and Cold Dark Matter (CDM); some combination of such is as well possible. The most broadly illustrated models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most generally supposed to be a neutralino. Hot dark matter might comprise of (massive) neutrinos. Cold dark matter would lead to the bottom-up formation of structure in the universe whereas hot dark matter would outcome in a top-down formation scenario.

One prospect is that cold dark matter could comprise of primordial black holes in the range of 1014 kg to 1023 kg. Being in the range of an asteroid's mass, they would be small adequate to pass via objects such as stars, having minimal impact on the star itself. Such black holes might have formed shortly after the big bang if the energy density was great adequate to make black holes directly from density variations, rather than from star collapse. In huge numbers they could account for the missing mass essential to describe star motions in galaxies and gravitational lensing effects.

Evidence of Dark Matter:

The first person to give proof and infer the presence of dark matter was Swiss astrophysicist Fritz Zwicky in the year 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained proof of unseen mass. Zwicky estimated the cluster's net mass based on the motions of galaxies close to its edge and compared that estimate to one based on the number of galaxies and total brightness of the cluster. He discovered that there was around 400 times more estimated mass than was visually observable. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, thus somewhat extra was needed. This is termed as the missing mass problem. Based on such conclusions, Zwicky inferred that there should be some non-visible form of matter that would give enough of the mass and gravity to hold the cluster altogether.

A lot proof for dark matter comes from the study of the motions of galaxies and most of these appear to be quite uniform, therefore by the virial theorem the total kinetic energy must be half the total gravitational binding energy of the galaxies. Experimentally, though, the total kinetic energy is found to be much greater: in specific, supposing the gravitational mass is due to only the visible matter of the galaxy; stars far from the centre of galaxies have much higher velocities than predicted through the virial theorem.

Galactic rotation curves that describe the velocity of rotation versus the distance from the galactic centre can't be illustrated by only the visible matter. Supposing that the visible material makes up just a small portion of the cluster is the most straightforward means of accounting for this. Galaxies represents signs of being composed largely of a roughly spherically symmetric, centrally concentrated halo of dark matter having the visible matter concentrated in a disc at the centre. Low surface brightness dwarf galaxies are significant sources of information for studying dark matter, as they encompass an unusually low ratio of visible matter to dark matter, and encompass few bright stars at the centre that would or else impair observations of the rotation curve of outlying stars.

Gravitational lensing examinations of galaxy clusters let direct estimates of the gravitational mass based on its effect on light from the background galaxies, as large collections of matter (that is, dark or otherwise) will gravitationally deflect light. In clusters like Abell in the year 1689, lensing observations confirm the presence of considerably more mass than is pointed out by the clusters' light alone. In the Bullet Cluster, lensing observations represent that much of the lensing mass is separated from the X-ray-emitting baryonic mass.

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