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The phenomenology of jet in astrophysics was studied. Analytical methods were used to obtain an equation for describing jet motion. From the analysis, we understood that βT  > 1, where βT is the apparent jet velocity along the observers line of sight. The observed motions of the components show curvatures and changes in velocity. Curved trajectories are due to observed perpendicular acceleration, while variations in velocity are due to changes in apparent parallel acceleration.


Title pagei

Certification         ii

Dedication iii



Table of Contents                                                                                       vii



1.1     Background of study                                                                                1

1.2     Astrophysical Jets                                                                                      2

1.3     The Family Tree of Astrophysical Jets                                                       4

1.4     Relativistic Jets                                                                                 6

1.5     Galaxies                                                                                            7

1.5.1  The Classification of Galaxies                                                          8

1.5.2  Galaxy Formation                                                                                      12

1.5.2. Evolution of the Galaxy                                                                    14

1.6     AGN jets                                                                                           14

1.7     Active galactic nuclei                                                                        18

1.7.1  Classification of AGN                                                                       18

1.8     Phenomenology of AGN                                                                            20

1.9     Black holes                                                                                       21

1.10   Accretion Disk                                                                                  23

1.10.1         Definition and Evidence                                                                         23

1.11   Aims and objectives                                                                26

1.11.1 Aim                                                                                                         26

1.11.2 Objectives                                                                                               26



2.1     A Brief History of Galaxy Formation                                                        27

2.1.1  Galaxies as Extragalactic Objects                                                     27

2.1.2  Cosmology                                                                                        29

2.1.3  Structure Formation                                                                         33

2.1.4  The Emergence of the Cold Dark Matter Paradigm                                    38

2.2     Galaxy formation                                                                              44

2.2.1  Monolithic Collapse and Merging                                                     44

2.2.2  The Role of Radiative Cooling                                                                   47

2.2.3  Galaxy Formation in Dark Matter Halos                                          48

2.3     The origin of the bright knots                                                           51

2.3.1  YSO jets                                                                                           51

2.3.2  AGN and microquasar jets                                                               54

2.4     AGN jets composition                                                                      56

2.5.    Black holes observation                                                                    57

2.5.1  Observational Evidence for Black Holes in AGN                             57 Optical and IR data                                                                        58 VLBI radio data                                                                             58

2.6     X–ray observations                                                                          60

2.7     History of Superluminal Motions                                                     63



3.1     superluminal motion                                                                                66

   3.2     Analysis of the formation of an astrophysical jet                                      70

   3.3     Conclusion                                                                         74


4.1     Phenomenology                                                        75

4.2     Explanation of the phenomenon                              78

4.3     Derivation of apparent velocity                                                                83

4.4     Some contrary evidence                                           86

4.5     Laser ranging                                                                                            87

4.6     Special relativity                                                      87

4.6.1 Observational effect                                                         87


5.1     Conclusion                                                                 91









Astrophysical jets are observed in the Universe in a large variety of environments and under a wide range of sizes and powers. They are generated in active galactic nuclei (AGNs) and YSOs, can travel up to a few thousands of Megaparsecs, and reach the largest powers observed in the Universe (up to ∼104748 erg s1), (Zanni et al., 2003; Godfrey and Shabala, 2013). Astrophysical jets can be found in giant molecular clouds, emanating in the vicinities of young stellar objects (YSOs), and reaching distances of some parsecs (Reipurth and Bally, 2001). They are also located near neutron stars in galactic X-ray binary star systems, such as GRS 1915 + 105 that behave as microquasars generating relativistic jets (Fender, 2004). Astrophysical jets can be found in the asymptotic giant branch (post-AGB) stars as well in pre-planetary and planetary nebulae. Opposite, precessing jets are observed in the SS433 binary source, leading to a peculiar phenomenology (Frank, 2011). A jet-like structure is observed, at X-ray energies, inside the Crab Nebula departing from the embedded pulsar (Hester, 2008). Finally, jets can be at the base of the phenomenology of gamma-ray bursts, observed at the highest radiation energies that are still elusive phenomena because of their extreme distances (Granot, 2007).




Astrophysical jets are physical conduits along which mass, momentum; energy and magnetic flux are channeled from stellar, galactic and extragalactic objects to the outer medium. Geometrically, these jets are narrow (small opening angle) conical or cylindrical/semi-cylindrical protrusions (Das, 1999). Jets are a ubiquitous phenomenon in the universe. They span a large range of luminosity and degree of collimation, from the most powerful examples observed to emerge from the nuclei of active galaxies (or AGNs) to the jets associated to low-mass young stellar objects (YSOs) within our own Galaxy. In the intermediate scales between these two extremes one finds evidences of outflows associated to neutron stars, massive X-ray binary systems (with SS433 being the best example of this class), symbiotic stars, and galactic stellar mass black holes (or microquasars). Figs. 1 and 2 show some of the finest examples of YSO and AGN jets. Recent reviews of the observational and structural properties of the YSO jets can be found in Reipurth and Bally (2001).

In this work, instead of discussing all the current knowledge about the various classes of astrophysical jets (which would be impossible to cover just in few pages), I focus mainly on the YSO jets, which are the closest and for this reason, excellent laboratories for cosmic jet investigation, and on AGNs jets. Some specific issues about the theory of microquasar jets will be also addressed.

Figure 1.1 Hubble Space telescope images of three YSO (also called Herbig-Haro, or simply HH) jets. (Extracted from the HST website).


Figure 1.2 Images obtained with the Very Large Array (VLA) of jets in a FRI I source (top: 3C31 at the radio frequencies 1.4 GHz and 8.4 GHz), and a FR II source (bottom: 3C175 at 4.9 GHz); extracted from Bridle (1998).



The association of accretion disks and jet production is probably fundamental, the notable exception being the conical jets emitted from rapidly rotating neutron stars. The disks provide both a mechanism to generate the jets and the mass and angular momentum necessary to keep the jet stable. In fact, the example of giant radio galaxies argues that even the jets that appear to be precessing do so in a consistent fashion over time scales on the order of the life time of the jets (i.e., 105 years).

Figure 1.1 shows a possible “family tree” of jet sources. Variants of this figure have been presented in the past (Brinkmann and Siebert, 1999) with regard to AGN, and a number of authors have constructed similar tables. However, Fig. 1.1 includes microquasars; bipolar outflows in the star forming regions of giant molecular clouds; and other jet-like structures. This is done to remark on the association posited above. The fundamental physical principals implicit in the figure are the ubiquity of angular momentum, the enormous mass of compact astrophysical objects, and their energetic and inertial properties.

Figure1.3: The family tree of astrophysical jets (Brinkmann and Siebert, 1999).



Figure 1.4: Accretion disc and jets (Meier, 2003).


Relativistic jets are very powerful jets (Wehrle et al., 2009) of plasma, with speeds close to the speed of light, that are emitted near the central black holes of some active galaxies, notably radio galaxies and quasars, and stellar black holes and neutron stars. Their lengths can reach several thousand (Biretta, 1999) or even hundreds of thousands of light years (Meier, 2003). Because the jet speed is close to the speed of light, the effects of the Special Theory of Relativity are important; in particular, relativistic beaming will change the apparent brightness. The mechanics behind both the creation of the jets (Semenov et al., 2004) and the composition of the jets (Georganopoulos et al., 2005) are still a matter of much debate in the scientific community. Jet composition might vary; some studies favor a model in which the jets are composed of an electrically neutral mixture of nucleielectrons, and positrons, while others are consistent with a jet primarily of positron-electron plasma (Wardle, 1998).

Massive galactic central black holes have the most powerful jets. Similar jets on a much smaller scale develop from neutron stars and stellar black holes. These systems are often called microquasars. An example is SS433, whose well-observed jet has a velocity of 0.23c, although other microquasars appear to have much higher (but less well measured) jet velocities. Even weaker and less relativistic jets may be associated with many binary systems; the acceleration mechanism for these jets may be similar to the magnetic reconnection processes observed in the Earth’s magnetosphereand the solar wind.

The general hypothesis among astrophysicists is that the formation of relativistic jets is the key to explaining the production of gamma-ray bursts. These jets have Lorentz factors of ~100 or greater (that is, speeds over roughly 0.99995c), making them some of the swiftest celestial objects currently known.


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