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The First Stars and Galaxies

 

There is a veil of mystery in gradual dissipation that still prevents cosmologists from getting a clear idea about how the small temperature fluctuations resolved by COBE (Cosmic Background Explorer) in 1992 evolved in order to form the first galaxies.

 

 

Galactic Genesis

 

In an earlier stage there would have been a gravitational collapse of regions whose densities assumed values above the average.

 

Galactic Aggregation

Concerning to what happened later, among the theories that were proposed to explain the origin of the observed structure, the so-called "down-up model" ended up prevailing: sub-galactic masses or mini-galaxies merged into galaxies that, in the end, accreted to form galaxy clusters. This model is more consonant with a cold and slow dark matter scenario, which favours an initial aggregation of matter in minor objects that later aggregate in larger massive bodies. However, this model may also be consistent with a hot dark matter scenario, since we assume the possibility of the existence of cosmic strings.

The aggregation process would have been easy in a smaller Universe, where galaxies were tighter than they are today. The frequency of the collisions between them can explain their predominantly irregular shapes.

 

Stone Age Galaxies

In recent observations were detected the most primitive galaxies, formed only 600 million years after the Big Bang, under the form of future elliptical galaxies cores and components (namely central bulges) of the future spiral galaxies.

These ancient galaxies are characterized for being surrounded by hydrogen envelopes that absorb ultraviolet radiation produced by stars and, therefore, making them invisible in this segment of the spectrum.

 

The first galaxies of the Universe (HST - NASA)

 

 

The First Stars

 

The ignition of the earliest stars - Population III - was the responsible factor for a period of re-heating of the Universe, during which the dust clouds previously expelled by the explosions of similar bodies absorbed and re-emitted a huge quantity of the radiation produced by those enormous stars. This period is known as the "re-ionization age". These stars would be ignited in clouds holding more than 1 million solar masses, inside completely black proto-galaxies whose diameter wouldn't be larger than 2000 light-years.

The Population III stars would be systematically heavier than the current ones (holding between 100 and 1000 solar masses), since they would have been formed exclusively by helium and hot atomic hydrogen (not molecular, given the absence of agglutinating metals). For that reason, they would have had a very quick evolution and none of them would have survived until today.

A small metallic content (carbon and other elements heavier than helium) fused inside these stars would have been scattered by the gigantic supernovae (called hypernovae) that put an end to their lifes. This content would be further incorporated in a second generation of stars - the Population II, which includes the stars of the galactic bulges and halos. These hypernovae were responsible for the incredibly energetic gamma-ray bursts observed in very distant galaxies.

 

Two surprisingly near remnants of gamma-ray bursts, close to the M101 galaxy (HST-NASA)

 

Who Came First: Stars vs. Black Holes

Two possible scenarios are considered to enlighten what may have been the galactic childhood:

    1. The outside in theory: According to this model, the earlier black holes could have been arisen from the final collapse of the first generation stars (Population III), which later would be converted into gravitational trappers. In consequence, a cocoon containing a growing amount of gas and dust, responsible for the subsequent appearance of new stars, enshrouded these gravitational whirlpools. Are the huge black holes thought to exist in the centre of the galaxies the result of these primitive supernovae?
    2. The inside out theory: According to this model, the collapse of the dark matter existing in the proto-galaxies would have given origin to black holes before any first generation stars had been ignited around them. If a black hole was formed early enough it could prevent the formation of a galactic disk, because the energy emitted by its accretion ring would power a wind that would expel the matter surrounding it. The timing for its birth could determine what kind of galaxy would be formed: with a disk (spirals) or with no disk (elliptical). The observation of a big quantity of quasars existing before the appearance of the first visible galaxies may support this model, as this objects would be solely the result of the copious radiation emitted by the material present around the primordial black holes. A complementary model proposes that these swallowing bodies are the offspring of the mini-black holes that arose during the Big Bang.

 

 

Quasars and Active Galaxies

 

Among the most ancient and brightest objects of the whole Universe there are the quasars, 100 times more luminous than a normal galaxy. It's thought that they are the cores of young and extremely active galaxies that are appeased today. There are two kinds of quasars: those that are identified with strong radio emissions and those where those emissions are relatively absent.

 

The Origin of the Radiations

It's estimated that the very powerful light emissions in most of the spectrum segments (reaching up to 10 trillion times the solar emission) are originated by an enormous black hole containing about 100 million solar masses and absorbing big quantities of matter (1 to 2 solar masses each year). 10% of this matter is converted into energy during the process.

The radio waves are proceeding from a synchrotron, which is a whirlwind created by the orbit of charged particles (mainly electrons) around a magnetic field, swirling it at a velocity compared to the light speed. The process is similar to what happens in neutron stars or in other active galaxies. These plasma jets would be headed to the directions of the magnetic fields, approximately perpendicular to the disk plane. These magnetic fields accelerate the jets, which this way avoid the strong resistance of the thick galactic disks.

The absorption of the energy generated in the centre of the quasar by the dust clouds existing in the host galaxy and subsequent re-emission in infrared will be, on its turn, the responsible mechanism for the radiation that are observed in that segment.

On the other hand, the radiation in the visible and ultraviolet segments would be emitted by a disk of matter (called accretion disk) spiraling at high speeds around the central object and freeing big quantities of energy as this disk would be sucked.

The x-rays are thought to be proceeding from a region close to the central black hole (possibly the synchrotron or the zone of production of electron/positron virtual pairs), being absorbed and re-emitted by the accretion disk.

 

Electromagnetic spectrum, from the gamma radiation to the radio waves (MoonRunner Design UK)

 

Life Expectancy of a Quasar

It's estimated that the average life expectancy of a quasar is between 10 and 100 million years. It's also conjectured that in the primitive Universe, which was wealthier in gases and a stage for more frequent galactic collisions (given the higher density of the matter), the arising of quasars would have been very much eased. For this reason, the big majority of the quasars are observed at big distances, at the early Universe.

 

Other Active Galaxies

It's still worth to mention 3 objects that are analog to quasars, which are believed to also result from high activity levels in the core of the galaxy:

    1. The blazars, including the BL Lacertæ objects and the OVV (optically violently variable) quasars, are extremely luminous and variable (the light emitted by them may vary in a factor of 100 in a few days or hours) and their spectra are plain, not showing emission or absorption lines. It's thought that these objects are similar to the quasars, but observed exactly along the axis of their plasma jets (a 0º angle). According to the theory of relativity, the high speed at which the jet moves pushes this one's radiation toward the direction of the jet itself, thus strongly amplifying its intensity. Given the spectral characteristics, it's difficult to calculate the age and distance of these objects.
    2. The radio-galaxies, on the other hand, are mostly gigantic elliptical galaxies exhibiting jets that provide energy to 2 opposite lobules located at big distances from the core. The jets as the lobules are powerful radio sources, emitting the equivalent to 1 million times what a normal galaxy like ours does. It's believed that radio-galaxies are active galaxies seen from angles above 45º relatively to the axis of the jets, contrarily to the quasars (0-45º) and the blazars (0º). M87, an elliptical galaxy in the centre of the Virgo cluster (at 50 million light-years) is a radio-galaxy.
    3. The Seyfert galaxies are mostly spiral galaxies that emit big quantities of radiation from a central point. Based on spectral studies, they were detected two kinds of Seyfert galaxies - type 1 and 2, and it's thought nowadays that both represent no more than the same kind of object seen from different angles. Thus, it's conjectured that dense and fast clouds located at short distances and sparse and slow clouds located farther away enshroud the luminous central point (which is thought to be a black hole). In a similar way to what happens with other kind of active galaxies, the Seyfert are also known for having two ionization cones pointing to opposite directions from their core. The Seyfert galaxies are located at smaller distances than the quasars and it's believed that they represent an intermediate level between these ones and the normal galaxies (activated by less massive black holes).

 

Radio source 3C 449 (Ediciones Orbis - Astronomia)

 

 

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