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This thesis is concerned with the detection of very high energy (VHE) 7-rays from microquasars, a recently discovered subclass of x-ray binaries that display jets of relativistic particles. A general introduction to ground based 7-ray astronomy is presented, which discusses the production of Cherenkov radiation and how VHE 7-rays are detected via extended air showers in the atmosphere. The important processes involved in the production and absorption of 7-rays are detailed, before the design of The High Energy Stereoscopic System (H.E.S.S.) Phase I telescope array, based in Namibia, is briefly described. There follows a general section on microquasars, describing x-ray binaries and the morphological analogy between microquasars and active galactic nuclei (AGN). Some important properties of jets are detailed before the spectral states of x-ray binaries and their importance in relation to jets are discussed. A synchrotron self-Compton (SSC) model designed for AGN is reviewed in some detail and is applied to the blazar 3C 279, showing a good fit to data. This SSC model is then applied to the microquasars GRS 1915+105, V4641 Sgr and GX 339-4 using a scale-invariant method to investigate if the jets from microquasars can be considered simply as scaled down AGN jets. Conclusions are drawn from the results. Data taken with H.E.S.S. on four microquasars are presented. The techniques used to take and analyse this data are described in some detail using data taken on the Crab nebula as an example of a VHE 7-ray emitter, before the analysis results for observations of GRS 1915+105, Circinus X-1, GX 339-4 and V4641 Sgr are presented. A novel method for manufacturing aluminum mirrors suitable for use on ground based 7-ray telescopes is described in detail along with the testing techniques used. The next generation telescope, H.E.S.S. Phase II is then described briefly. The thesis concludes with a brief outline of future work to be conducted, based on the SSC model, as applied to microquasars. A proposal for observations of IGR J19140+0951, which was tentatively detected in the field of GRS 1915+105, is outlined.

This thesis is composed of two parts; the first part deals with observations of the molecular gas towards an unlensed, obscured quasar AMS12, and the second part investigates radio undetected, optically selected quasi-stellar objects (QSOs) to determine the nature of the radio flux density distributions of these objects. AMS12 is an unlensed, obscured, z = 2.767 quasar which we observed with the Plateau de Bure Interferometer to detect carbon monoxide rotational transitions and atomic carbon fine structure lines in the molecular gas. We present new detections of the CO(5-4), CO(7-6), [CI]( ³P₁- ³P₀) and [CI](³P₂− ³P₁) molecular and atomic line transitions in this thesis. AMS12 is the first unlensed, high redshift source to have both atomic carbon ([CI]) transitions detected. The highly excited molecular gas probed by CO(3-2), (5-4) and (7-6), is modelled with large velocity gradient models. The gas kinetic temperature TG, density n(H₂), and the characteristic size r₀, are determined using the dust temperature from the far-infrared spectral energy distribution which had the following best-fitting parameters log₁₀[LFIR/L☉] = 13.5, dust temperature TD = 88 K and emissivity index β=0.6, as a prior for the gas temperature. The best fitting parameters are TG = 89.6 K, n(H₂) = 10 3.9 cm⁻³ and r₀ = 0.8 kpc. The ratio of the [CI] lines gives a [CI] excitation temperature of 43 ± 10 K, indicating the [CI] and the high-excitation CO are not in thermal equilibrium. The [CI] excitation temperature is below that of the dust temperature and the gas kinetic temperature of the high excitation CO, perhaps because [CI] lies at a larger radius where there may also be a large reservoir of CO at a cooler temperature, which may be detectable through the CO(1-0). Using the [CI]( ³P₁− ³P₀) line we can estimate the strength of the CO(1-0) line and hence the gas mass. This suggests that a significant fraction (~30%) of the molecular gas is missed from the high-excitation line analysis, giving a gas mass higher than that inferred from the assumption that the high-excitation gas is a good tracer of the low-excitation gas. The stellar mass was estimated from the mid-/near-infrared spectral energy distribution to be M* ~ 3 × 10¹¹M☉. The Eddington limited black hole mass is found from the bolometric luminosity to be M• ≳ 1.5×10⁹M☉. These give a black hole - bulge mass ratio of M•/M* ≳ 0.005. This is in agreement with studies on the evolution of the M•/M* relationship at high redshifts, which find a departure from the local value ~ 0.002. In the second half of the thesis we investigate the possible existence of a lower envelope in the radio luminosity versus optical luminosity plane. We select a population of QSOs from the Sloan Digital Sky Survey photometric quasar catalogue from Richards et al. The QSOs are within a narrow redshift band 0.3 < zphot < 0.5 and cross-matched with the 1.4 GHz National Radio Astronomy Observatory Very Large Array Sky Survey. The radio images extracted from the positions of the optical QSOs are retained if the flux integrated over the beam size of the radio survey is less than 3σIrms ≤ 1.35 mJy. The radio-undectected QSO population is split into eight samples depending on their optical magnitudes and stacked to determine the mean flux in each sample. The stacked mean flux is detected in all but the faintest optical magnitude sample. The radio versus optical luminosity relation from the stacked samples hint at a lower envelope in the radio luminosity which may be interpreted as there being a minimum radio jet power for a given accretion rate. Stacking assumes the underlying distribution of the property being measured is fairly represented by the stacked result. We investigate the underlying distribution of the radio flux density from the QSOs taking the noise of the sample into account. We find the distribution of the QSO flux density is modelled by a power-law with a negative index in all eight optical magnitude samples. This implies the mean stacked result is not a good representation of the distribution of the flux density of the QSOs and that there is no lower envelope. This highlights the danger of interpreting results from stacking without verifying the distribution is characterised by the mean stacked value. We appear to recover the quasar optical luminosity function when we model the distribution of radio loudness parameters suggesting that, since we are essentially holding the radio flux density fixed, the radio loudness is a function of the optical luminosity. This suggest that the radio loudness is not a fundamental property of the QSO but rather the ratio of two independent properties, the radio and optical luminosities. We convert the radio loudness parameter to jet efficiencies and find a minimum jet efficiency of ηmin = 4 × 10⁻⁴. We find there is no sign of a minimum jet efficiency as far as our data’s sensitivity limit allows, so we expect η<ηmin. Hence we provide an observational constraint for theoretical models of jet production in the minimum jet efficiency.

Αντικείμενο της εργασίας είναι η φυσική των κβάζαρς με έμφαση στη μελέτη του φαινομένου των πιδάκων. Στο πρώτο κεφάλαιο γίνεται μια εισαγωγή στους γαλαξίες και στις μελανές οπές. Το δεύτερο κεφάλαιο αναφέρεται στην φυσική των γαλαξιών. Με τη χρήση μαθηματικών μοντέλων εξηγείται η δημιουργία τους, ο σχηματισμός τους, οι κινήσεις και η ταξινόμηση τους. Επίσης, γίνεται μια εισαγωγή στους ενεργούς γαλαξιακούς πυρήνες. Το τρίτο κεφάλαιο είναι αφιερωμένο στις μελανές οπές. Η περιγραφή τους γίνεται στα πλαίσια της Γενικής Θεωρίας της Σχετικότητας. Περιγράφεται ο σχηματισμός τους, τα διαφορετικά είδη μελανών οπών καθώς και πώς αυτές αλληλεπιδρούν με το περιβάλλον στο οποίο βρίσκονται. Το τέταρτο κεφάλαιο εστιάζει στο αστρικό σύστημα που επιλέξαμε να μελετήσουμε. Περιγράφονται κβάζαρς, υπερμαζικές μελανές οπές, δίσκοι προσαύξησης και πίδακες. Το πέμπτο και τελευταίο κεφάλαιο περιέχει τα αποτελέσματα και συμπεράσματά μας, καθώς και τα μαθηματικά εργαλεία που χρησιμοποιήσαμε. Επιλέξαμε να εστιάσουμε στο πρόβλημα της καμπύλωσης των πιδάκων. Ερευνήθηκαν οι περιπτώσεις για 1) καμπύλωση λόγω ύπαρξης νέφους στο γαλαξιακό κέντρο, και 2) καμπύλωση λόγω σχετικής κίνησης του γαλαξία. Για την πρώτη περίπτωση, επιλέξαμε να προσεγγίσουμε ένα μοντέλο από την βιβλιογραφία, υποθέτοντας ότι η ταχύτητα του πίδακα μεταβάλλεται. Παρουσιάζονται τα αποτελέσματα και γίνεται σύγκριση με παρατηρησιακά δεδομένα. Για την δεύτερη περίπτωση, μελετήθηκαν πραγματικά συστήματα στα σμήνη γαλαξιών Abell 2220 και Abell 1446 και ακολουθήθηκε η προηγούμενη διαδικασία. Για την κατασκευή του κώδικα χρησιμοποιήθηκαν οι Fortran G95 και η βιβλιοθήκη Slatec, ενώ, επιπλέον, για τους αριθμητικούς υπολογισμούς τα προγράμματα Wolfram-Alpha, Maxima και Maple-Calculator. Οι υποθέσεις και τα συμπεράσματά μας βρίσκονται σε συμφωνία με την σχετική βιβλιογραφία. / The aim of this work is to study the physics of quasars with emphasis on the associated jets. In the rst chapter, there is an introduction on galaxies and black holes. The second chapter deals with the physics of galaxies. Using mathematical models, we discuss how they are created, shaped, moving and classi ed. There is also an introduction to active galactic nuclei. In the third chapter we discuss on black holes. With the using the general theory of relativity , we describe how they are created, their di erent kinds, and how black holes interact with their environment. The fourth chapter focuses on the system particular of our study. There is a discussion on quasars, super massive black holes, accretion disks, and jets. The fth and nal chapter presents our results and conclusions, as well as the mathematical tools used. We focus on the problem of the curvature of the jets. We study the cases of 1) curvature due to the existence of a nebula within the galactic center, and 2) curvature due to the relative motion of the galaxy. For the rst case, we chose to treat a model from the bibliography, assuming that the speed of the jet is not constant. We present our results and compare them with respective observational data. For the second case, we study two systems in the galaxy clusters of Abell 2220 and Abell 1446 and follow the procedure discussed above. For the construction of our code we use Fortran G95 and the library Slatec, and for auxiliary calculations the programs: Wolfram-Alpha, Maxima and Maple Calculator.

This thesis presents a study of high-redshift obscured (type-2) quasars, selected at mid-infrared and radio wavelengths. This population had remained elusive, even to hard X-ray surveys, and in Chapter 2 I compare the selection of type-2 quasars in X-ray and mid-infrared surveys, as well as explaining the criteria I will use to search for these objects at z ~ 2, around the peak in the unobscured (type-1) quasar activity. Chapter 3, presents a sample of radio-intermediate type-2 quasars selected from the criteria de- scribed in Chapter 2. Optical spectroscopy shows indeed that at least half of the objects have the characteristic narrow emission lines, and lie around the expected redshift of z = 2. The other half of the objects are consistent with also being type-2 quasars at similar redshifts, although no emission lines are visible. In Chapter 4,1 discuss the possibility of two types of obscured quasars, some obscured by a dusty torus and some by a dusty host galaxy, to explain the lack of emission lines in half of the sample. I model the number of type-1 quasars expected to follow similar selection criteria and at the same redshifts as our type-2 quasars, and find that the obscured quasars outnumber the unobscured by a ~2-3:1 ratio. I conclude that most supermassive black hole growth is obscured by dust. When comparing this to predictions from unified schemes, I find that this result is only consistent with the schemes provided host-obscuration is indeed happening. The lower ratio of type-2 to type-1 quasars inferred from X-ray surveys (~1:1) suggests that some of the type-2 quasars in this sample might be Compton thick. Radio data taken at three frequencies, are presented in Chapter 5, to study the spectral properties and intrinsic luminosities of our sample. I show that some type-2 quasars have flat radio spectra, which is inconsistent with obscuration by the torus, but consistent with host-obscuration. Some gigahertz-peaked spectra, characteristic of young radio jets, are present, but the majority of the sources have very steep spectra. These steep spectral indices can be explained by active developed jets in which continuous injection of electrons is accompanied by inverse-Compton losses against the cosmic microwave background. In Chapter 6, I select a similar sample of type-2 quasars in a different field, where X-ray data are available. The selection criteria are kept identical, except for the radio flux density cut, which is lowered. This is expected to introduce significant numbers of starburst contaminants. To filter these out, and due to a lack of spectroscopy, I use a bayesian method to fit the spectral energy distributions, obtain photometric redshifts, and select between a quasar and a starburst model. I measure the X-ray properties for the resultant sample of type-2 quasars. The entire sample is found to be Compton-thick, and repeating the modelling of Chapter 4, I find that the population of Compton-thick quasars is at least comparable to the population of unobscured quasars, and probably larger.

All massive nearby galaxies, including our own, host supermassive black holes. Active galactic nuclei (AGN) are seen when such black holes accrete, and when they produce powerful jets of synchrotron-emitting plasma, they are termed radio-loud AGN. The close correlation between black hole mass and galaxy bulge mass in elliptical galaxies indicates that AGN feedback may be the key to the regulation of galaxy formation. It is thus necessary to fully understand the structure of AGN, the way that they are fuelled, and their duty cycle, in order to study the feedback processes and get a clear picture of galaxy formation. In this thesis, independent methods are developed to constrain the accretion disk and radio jet angles to the line of sight. H IX emission from a sub-sample of high-redshift quasars is measured from near-infrared spectroscopy and modelled as sums of different components, including the characteristic double-peaked profile which results from a thin, rotating accretion disk. Comparing the models using Bayesian evidence, almost all quasars were found to have infrared spectra consistent with the presence of a disk. The jet inclination angles of the same set of quasars were constrained by fitting a model, including the effect of Doppler boosting and the receding torus model for dust obscuration, to the radio \ spectral energy distribution. The fitted disk and jet angles correlate strongly, and are consistent with a model in which the radio jets are launched orthogonally to the plane of the accretion disk, as expected if the jet is powered by energy drawn from the spin of the black hole. Both disk and jet angles correlate with the observed linear source size, which is a projection effect; when deprojected using the fitted angles, the distribution of source sizes agrees with a scenario in which the sources expand into the surrounding medium at a constant rate up to ~ 1 Mpc and then shut off, probably as the nuclei become quiescent. The accretion disk angle was found to correlate weakly with the low-frequency radio luminosity, which provides direct, albeit tenuous, evidence for the receding torus model.