# Refiloe KekanaRoger Deane

Abstract

The purpose of this report is to investigate the merging of galaxies paying particular attention to the evolution of their nuclei. It is to study how these mergers lead to increased star formation and accretion of matter into the nuclear black-hole using radio wave observations. Radio observations on these mergers are not easily compiled as radio waves are not affected by frequent dust obscuration within galactic nuclei - which biases the statistical inferences of surveys at other wavelengths. It is important to note that galaxy mergers are postulated to provide a prime source for the detection of gravitational waves. This report will aim to learn more about galaxy mergers using observations that are not detected in the visible and infra-red bands. Through the use of statistical methods, a better understanding of galaxy-galaxy mergers can be obtained when these are studied by making observations across the electromagnetic spectrum. Observations are more easily obtained in the near- infra-red, visible and x-ray wavelengths. This means that our current knowledge is only limited to observations made in these bands, leaving much undiscovered. We observe an increase in quasar activity as the separation between paired quasars decreases. Through this observation and results obtained in studies at the X-ray band, we observe that indeed AGN merging events lead to enhanced radio jet activity.

# Introduction

## Galaxy Evolution: from the Big Bang to present day

The Big Bang is the birth of our Universe. The early state of the Universe was hot and dense. In this era, matter was coupled to radiation, according to early models of structure formation which assumed adiabatic initial conditions. The Universe expanded and temperature decreased, this occurred approximately 380 000 years after the Big Bang at a redshift of z = 1100 and is depicted in Figure 1. It is also theorized that matter and radiation became decoupled at this same time as well. Redshift (z) is the increase of apparent wavelength of light coming toward an observer as the result of an object moving away from the observer where $$1 + z = \frac{\lambda_{\rm obs}}{\lambda_{emit}}$$. The radiation is a relic of the big-bang that is still observed today; it is called the Cosmic Microwave Background (CMB). The CMB is nearly isotropic blackbody radiation which expanded and cooled and fills the universe and is now at $$T_{0}=2.725 \pm 0.002 K$$. Burke et al. (1997) state that the isotropy of the CMB implies that sections of the Universe that were never in communication with one another have similar properties at the time of observation.

Observations of the CMB communicate that the post Big Bang universe is a homogeneous, isotropic expanding or contracting universe, however this is not the reality of the Universe. It has been further theorized that current structure formation originated from quantum fluctuations. These fluctuations give rise to the measured temperature and density contrast seen in the CMB and large scale structure in a homogeneous isotropic universe (Mo 2009). Burke et al. (1997) further state that the same fluctuations would have been imprinted on the radiation that we now see as the CMB. The quantum fluctuations resulted from regions whose density was slightly higher than the mean density of the universe. These regions of higher density attracted surrounding matter through gravity. As a result, slightly-denser regions attracted matter towards them and become even denser. Low-density regions on the other hand become even less dense because matter flows away from them. This ampliﬁcation of quantum fluctuations is referred to as gravitational instability (Mo 2009). The initial fluctuations were the epicenters of the newly formed clumps of matter mainly made from hydrogen helium and ionized plasma. Thus initial perturbations are the building blocks of atomic nuclei and ionized plasma which combined via recombination to form neutral atoms, there is; however, gas clouds, stars, galaxies, and other astrophysical structures.

There is a significant amount of matter in the universe, which is not directly detectable. Fritz Zwicky observed using the Doppler shift and the virial theorem that the velocity spread within a cluster of galaxies implied that there was more mass than the luminous matter accounted for. Through further experimentation it was later concluded that baryons are set in matter which does not interact with light, but whose effects are observed through gravitational interactions. This non-luminous matter is called dark matter. The nature of dark matter is still unknown; however, dark matter is largely expected to be a collision-less particle (Mo 2009). Dark matter influences how galaxies form and even their rotation as it is the frame upon which the visible matter is embedded. The distribution of galaxies also depends on the distribution of dark matter in the universe. Burke et al. (1997) state that dark matter may be hot or cold - allowing for different levels of clustering in the Universe. The Hot Dak Matter (HDM) theory allows for a hierarchy in structure formation. The theory states that large scale structure such as the distribution of galaxies form first due to the presence of hot dark matter. Cold dark matter is theorized to be the seed for galaxy formation.

Galaxies are formed from the mergers of stellar material which result in the formation of systems of billions of stars, gas and dust, held together by gravitational attraction. It has been shown that under appropriate conditions, the process of gravitational collapse within a gaseous cloud, the cloud, or some part of it becomes unstable and begins to collapse if it lacks sufficient gaseous pressure support to balance the force of gravity. The cloud is stable for sufficiently small mass (at a given temperature and radius), but once the Jeans mass (a critical mass) is exceeded, the cloud begins a process of runaway contraction until an external unbalanced force can impede the collapse. Loeb et al. (2010) states that when an object above the Jeans mass collapses, the dark matter forms a halo inside of which the collapsed matter cools, condenses to the center of the dark matter halo, and eventually form stars. Dark matter is weakly interacting and is thus unable to cool. Consequently, the emergent structure of a galaxy becomes one in which a central core that is occupied by stars and cold gas which is enclosed by dark matter. Loeb et al. (2010) further states that a centrifugal force associated with the rotation of the galaxy’s center prevents the gas from collapsing into the center and forming a black hole. The gas later forms stars and a galaxy is born.

Dark matter halos grow in hierarchy in the sense that larger halos are formed through the merging of smaller predecessors. Figure 2 illustrates the formation of a dark matter halo. From this hierarchical model, it can been seen that present formation and evolution of galaxies has its roots in the Big Bang.

Radiative cooling, star formation and supernova explosions processes that are also integral to the formation of a galaxy while processes such as accretion of gas, and galaxy mergers, govern the galaxy’s structure. These sets of processes together drive the formation and evolution of galaxies (Ceverino 2009).

Fanaroff et al. (1974) classification of radio galaxies groups them into two major categories, FR-I, and FR-II. The two categories are based on whether radio-galaxies have edge-darkened (FR-I) morphologies or edge-brightened (FR-II) morphologies. Saripalli et al. (2012) believes that these morphologies arose from the interaction of jets (as depicted in Figure 4) and the material in their surrounding environment. Spectroscopy observations further reveal that FR-I radio galaxy hosts exhibit optical spectra with only absorption lines, while FR-II hosts display mixed characteristics. Some FR-II hosts are similar to FR-Is in that they only exhibit absorption lines ,but some others have spectra with strong high ionization emission lines.

Singal et al. (2014) states that the morphology of galaxies is closely related to their luminosities. It is said that Fanaroff et al. (1974) noted that the morphology of radio galaxies is dependent on their luminosities. Since luminosity is in turn related to redshift, it is easy to confuse the effects due to luminosity as those due to redshift. Singal et al. (2014) further discovered that FR-I and FR-II morphology is dependent only on luminosity and not redshift. Fanaroff et al. (1974) observed that the luminosity of FR-I galaxies falls below a threshold luminosity at 1.4 GHz $$L_{1.4} = 2 \times 10^{25} W Hz^{-1}sr^{−1}$$ . Studies have also shown that through optical observations we find most FR-II sources at find that at redshift ∼ 0.2 − 0.3 FR-II sources ((Belsole 2007) , and references therein).