Thesis (Ph. D.)--University of Chicago, Dept. of Astronomy and Astrophysics, August 1997. / Includes bibliographical references. Also available on the Internet.

Not available / text

This DPhil dissertation presents two new and independent samples of faint radio sources. The first sample is the 37 SXDS radio sources with flux densities at 1.4-GHz above 2 mJy, a spectroscopic completeness of 65% and a median redshift z_med ≈ 1.1. The second sample is the 47 TOOT00 radio sources with flux densities at 151 MHz above 100 mJy, a spectroscopic completeness of 85% and z_med ∼ 1.25. Optical, near- and mid-IR photometry, optical spectroscopy, and radio observations are used in the analysis and comparison of the two samples. The quasar fraction in the TOOT00 radio sources is 0.13 < f_q < 0.25 above the FRI/FRII break in radio luminosity, while use of 24 μm data reveals objects with significant but sometimes obscured accretion and gives quasar-mode fraction of 0.5 → 0.9 above the FRI/FRII break. The FRI/FRII divide seen at z < ∼ 0.5 is also observed at z ∼ 1 for FRII objects in the TOOT00 and SXDS samples, but examples of FRI radio sources above the FRI/FRII break do exist. The total number of the TOOT00 objects and their distribution are consistent with simulations based on extrapolations from previous work, while for the SXDS objects the results are only broadly similar. Based on that comparison, the redshift spikes seen at z ∼ 1.3 in TOOT00 and at z ∼ 0.65 & 2.7 in SXDS appear to be significant, and might be due to Large-Scale Structure. A V/Vmax test suggests the cosmic evolution of the TOOT00 and SXDS samples, is different. The TOOT00 radio sources are 2-times more luminous in host-galaxy starlight than the SXDS radio sources. The almost proportionality between radio luminosity at 1.4 GHz and 24 μm luminosity suggests that L_rad traces accretion luminosity and L[OII] ∝ L_rad^0.7 may reflect imperfections in the L[OII]-accretion luminosity scaling. Mid-IR 24 μm observations in the SXDS sample suggest that 30% of the light from the nucleus is absorbed by the torus and re-emitted in the mid-IR, while ∼ 1% of the light is scattered above and below the torus.

Not available / text

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.

Thesis (Ph. D.)--Ohio State University, 2008. / Title from first page of PDF file. Includes bibliographical references (p. 195-199).

Thesis (Ph. D.)--University of Florida, 1982. / Description based on print version record. Typescript. Vita. Includes bibliographical references (leaves 95-97).

Há evidências contundentes de que u ma parcela significativa das galáxias no universo presente hospedam buracos negros supermassivos que acretam gás a baixas taxas. Tal acreção origina os núcleos ativos de baixa luminosidade (LLAGNs), que constituem a população dominante de AGNs nas galáxias próximas. O objetivo deste trabalho é elucidar a natureza da fonte central dos LLAGNs, comparando diversos vínculos observacionais com as previsões de modelos para o processo de acreção. Para este fim, adotamos o modelo de escoamento acretivo favorecido para buracos negros "subalimentados", o ADAF (advectiondominated accretion fiow), que possui baixa eficiência radiativa ao contrário do disco de acreção fino que ocorre nos quasares distantes. Analisamos dois modelos físicos para a produção dos jatos nos ADAFs, nos quais a potência dos jatos depende do spin do buraco negro - o modelo de Blandford-Znajek e o modelo híbrido proposto por Meier - incorporando efeitos de relatividade geral. A nossa análise indica que a potência do jato é uma fração significativa da energia de repouso associada à massa acretada, quando o spin do buraço negro é alto. Aplicamos tais modelos a galáxias elípticas gigantes que hospedam LLAGNs e obtemos que os modelos explicam a natureza da correlação observada entre a taxa de acreção de Bondi e a potência do jato. Nossos resultados indicam que os buracos negros centrais nas galáxias elípticas próximas devem estar rotando rapidamente (a/M > 0.9). Nós modelamos as distribuições espectrais de energia (SEDs) de um conjunto de 17 LINERs (low ionization emission-line region). A partir da nossa modelagem, concluímos que a natureza das fontes centrais dos LINERs pode ser interpretada com sucesso no contexto do modelo ADAF, com a contribuição da emissão de um jato relativístico. Nossos resultados indicam que os LLAGNs produzem jatos intensos, nos quais. a potência cinética é maior que a luminosidade bolométrica. O seguinte cenário físico é favorecido para a origem da emissão nuclear dos LINERs. A emissão rádio (λ>1 mm) origina-se da radiação síncrotron do jato; no intervalo 1 mm - 100 µm, a emissão é dominada pela radiação síncrotron do ADAF; no intervalo 10µm - 1 µm,a radiação provém da emissão térmica do disco fino truncado. Dada a incerteza atual no conhecimento sobre a física dos plasmas nos ADAFs e jatos, diferentes cenários físicos são possíveis para explicar a origem da emissão raios X em LLAGNs: o ADAF, o jato ou uma combinação de ambos. Apresentamos também uma nova implementação do feedback devido aos jatos dos AGNs em simulações da formação de galáxias. Consideramos dois regimes de acreção dependendo da taxa de acreção: disco fino padrão e ADAF; apenas os ADAFs são responsáveis pela produção de jatos intensos. A nossa metodologia para o feedback foi implementada para simular a coevolução entre o buraco negro central e a sua galáxia hospedeira. Obtemos que para z > 1 o AGN é relativamente brilhante (acreção via disco fino) em relação à galáxia e produz jatos fracos. Para z < 1, a falta de gás para alimentar o buraco negro central faz com que o AGN torne-se praticamente "invisível" (acreção via ADAF) e a galáxia torna-se urna galáxia disco hospedando um LLAGN. Neste estágio, a produção de jatos intensos suprime parte da formação estelar na galáxia e regula a acreção de gás ao centro, criando ciclos de atividade do buraco negro central, onde há períodos em que a galáxia é inativa como a Via Láctea. As propriedades da galáxia simulada e da sua atividade nuclear estão em amplo acordo com as propriedades inferidas de observações. / There is compellíng evidence that a significant fraction of the present-day galaxies host supermassive black holes, which are accretíng gas at low rates. The accretion onto these black holes originates the low-Iuminosity active galactic nudei (LLAGNs), which constitute the dominant population of AGNs in nearby galaxies. Our goal in this thesis is to explain the nature of the central engines of LLAGNs, by comparing models for the black hole accretion with different observations. To trus end, we adopt the accretion model favored for "underfed" black holes, the advection-dominated accretion flow (ADAF), which has a low radiative efficíency unlike the standard thin accretion disks that explain distant quasars. We investigate two physical models for the jet production from ADAFs, in which the jet power is a function of the black hole spin - the dassical Blandford-Znajek model and a hybrid model developed by Meier - accoímting for relativistic effects. Our analysis indicates that when the black hole spin is high, the jet power is a significant fraction of the energy associated with the rest mass of material accreting onto the black hole. We apply these models to giant ellíptical galaxies hosting LLAGNs, and obtain that the models can explain the observed correlation between the Bondi accretion rates and the jet powers. Our results indicate that the central black holes in ellíptical galaxies are rapidly spinning (a/M > 0.9). We model the spectral energy distributions (SEDs) of a sample of 17 LINERs (low ionization emission-line regions). Our modeling shows that the observed SEDs are explained by ADAFs assocíated with relativistic jets, and that LLAGNs produce powerful jets for which the kinetic power exceeds the bolometric luminosity. The following physical scenario is favored to explain the nature of the nuclear emission of LINERs. The radio emission (λ>1 mm) is originated in the jet synchrotron radiation; in the wavelenght range 1 mm - 100 µm,the emission is dominated by the ADAF synchrotron radiation; over the range 10 µm- 1 µm,the main source of light is the thermal emission of the truncated thin disk. Given the uncertainties currently affecting our knowledge of the plasma physics of ADAFs and jets, different scenarios are possible to explain the origin of X-rays in LLAGNs: the ADAF, the jet or a combination of both. Finally, we present a new implementation of feedback due to black holes in simulations of galaxy formation. The novelty of our approach is that v/e consider two distinct accretion modes depending on the black hole accretion rate: thin accretion disks and ADAFs, the latter mode being able to produce powerful jets, which are a source of feedback. This prescription for the AGN feedback was incorporated in a cosmological simulation to investigate the interplay between galaxies and their central black holes during the formation of a disc galaxy. We find that for z > 1 the AGN is relatively bright (thin disk regime) compared to the galaxy and produces weak jets. For z < 1 the accretion rate onto the central black hole drops, the AGN is almost invisible (ADAF regime) and the galaxy becomes a disc galaxy hosting a LLAGN. At this point, the AGN feedback becomes efficient and suppresses star forination, regulating the mass accretion onto the black hole. As a result, the LLAGN has an "on-off" cycle of activity, in which the "off" phase corresponds to an inactive galaxy as our Milky Way. Several properties of the simulated galaxy and its nuclear activity are in broad agreement with observations.

Há evidências contundentes de que u ma parcela significativa das galáxias no universo presente hospedam buracos negros supermassivos que acretam gás a baixas taxas. Tal acreção origina os núcleos ativos de baixa luminosidade (LLAGNs), que constituem a população dominante de AGNs nas galáxias próximas. O objetivo deste trabalho é elucidar a natureza da fonte central dos LLAGNs, comparando diversos vínculos observacionais com as previsões de modelos para o processo de acreção. Para este fim, adotamos o modelo de escoamento acretivo favorecido para buracos negros "subalimentados", o ADAF (advectiondominated accretion fiow), que possui baixa eficiência radiativa ao contrário do disco de acreção fino que ocorre nos quasares distantes. Analisamos dois modelos físicos para a produção dos jatos nos ADAFs, nos quais a potência dos jatos depende do spin do buraco negro - o modelo de Blandford-Znajek e o modelo híbrido proposto por Meier - incorporando efeitos de relatividade geral. A nossa análise indica que a potência do jato é uma fração significativa da energia de repouso associada à massa acretada, quando o spin do buraço negro é alto. Aplicamos tais modelos a galáxias elípticas gigantes que hospedam LLAGNs e obtemos que os modelos explicam a natureza da correlação observada entre a taxa de acreção de Bondi e a potência do jato. Nossos resultados indicam que os buracos negros centrais nas galáxias elípticas próximas devem estar rotando rapidamente (a/M > 0.9). Nós modelamos as distribuições espectrais de energia (SEDs) de um conjunto de 17 LINERs (low ionization emission-line region). A partir da nossa modelagem, concluímos que a natureza das fontes centrais dos LINERs pode ser interpretada com sucesso no contexto do modelo ADAF, com a contribuição da emissão de um jato relativístico. Nossos resultados indicam que os LLAGNs produzem jatos intensos, nos quais. a potência cinética é maior que a luminosidade bolométrica. O seguinte cenário físico é favorecido para a origem da emissão nuclear dos LINERs. A emissão rádio (λ>1 mm) origina-se da radiação síncrotron do jato; no intervalo 1 mm - 100 µm, a emissão é dominada pela radiação síncrotron do ADAF; no intervalo 10µm - 1 µm,a radiação provém da emissão térmica do disco fino truncado. Dada a incerteza atual no conhecimento sobre a física dos plasmas nos ADAFs e jatos, diferentes cenários físicos são possíveis para explicar a origem da emissão raios X em LLAGNs: o ADAF, o jato ou uma combinação de ambos. Apresentamos também uma nova implementação do feedback devido aos jatos dos AGNs em simulações da formação de galáxias. Consideramos dois regimes de acreção dependendo da taxa de acreção: disco fino padrão e ADAF; apenas os ADAFs são responsáveis pela produção de jatos intensos. A nossa metodologia para o feedback foi implementada para simular a coevolução entre o buraco negro central e a sua galáxia hospedeira. Obtemos que para z > 1 o AGN é relativamente brilhante (acreção via disco fino) em relação à galáxia e produz jatos fracos. Para z < 1, a falta de gás para alimentar o buraco negro central faz com que o AGN torne-se praticamente "invisível" (acreção via ADAF) e a galáxia torna-se urna galáxia disco hospedando um LLAGN. Neste estágio, a produção de jatos intensos suprime parte da formação estelar na galáxia e regula a acreção de gás ao centro, criando ciclos de atividade do buraco negro central, onde há períodos em que a galáxia é inativa como a Via Láctea. As propriedades da galáxia simulada e da sua atividade nuclear estão em amplo acordo com as propriedades inferidas de observações. / There is compellíng evidence that a significant fraction of the present-day galaxies host supermassive black holes, which are accretíng gas at low rates. The accretion onto these black holes originates the low-Iuminosity active galactic nudei (LLAGNs), which constitute the dominant population of AGNs in nearby galaxies. Our goal in this thesis is to explain the nature of the central engines of LLAGNs, by comparing models for the black hole accretion with different observations. To trus end, we adopt the accretion model favored for "underfed" black holes, the advection-dominated accretion flow (ADAF), which has a low radiative efficíency unlike the standard thin accretion disks that explain distant quasars. We investigate two physical models for the jet production from ADAFs, in which the jet power is a function of the black hole spin - the dassical Blandford-Znajek model and a hybrid model developed by Meier - accoímting for relativistic effects. Our analysis indicates that when the black hole spin is high, the jet power is a significant fraction of the energy associated with the rest mass of material accreting onto the black hole. We apply these models to giant ellíptical galaxies hosting LLAGNs, and obtain that the models can explain the observed correlation between the Bondi accretion rates and the jet powers. Our results indicate that the central black holes in ellíptical galaxies are rapidly spinning (a/M > 0.9). We model the spectral energy distributions (SEDs) of a sample of 17 LINERs (low ionization emission-line regions). Our modeling shows that the observed SEDs are explained by ADAFs assocíated with relativistic jets, and that LLAGNs produce powerful jets for which the kinetic power exceeds the bolometric luminosity. The following physical scenario is favored to explain the nature of the nuclear emission of LINERs. The radio emission (λ>1 mm) is originated in the jet synchrotron radiation; in the wavelenght range 1 mm - 100 µm,the emission is dominated by the ADAF synchrotron radiation; over the range 10 µm- 1 µm,the main source of light is the thermal emission of the truncated thin disk. Given the uncertainties currently affecting our knowledge of the plasma physics of ADAFs and jets, different scenarios are possible to explain the origin of X-rays in LLAGNs: the ADAF, the jet or a combination of both. Finally, we present a new implementation of feedback due to black holes in simulations of galaxy formation. The novelty of our approach is that v/e consider two distinct accretion modes depending on the black hole accretion rate: thin accretion disks and ADAFs, the latter mode being able to produce powerful jets, which are a source of feedback. This prescription for the AGN feedback was incorporated in a cosmological simulation to investigate the interplay between galaxies and their central black holes during the formation of a disc galaxy. We find that for z > 1 the AGN is relatively bright (thin disk regime) compared to the galaxy and produces weak jets. For z < 1 the accretion rate onto the central black hole drops, the AGN is almost invisible (ADAF regime) and the galaxy becomes a disc galaxy hosting a LLAGN. At this point, the AGN feedback becomes efficient and suppresses star forination, regulating the mass accretion onto the black hole. As a result, the LLAGN has an "on-off" cycle of activity, in which the "off" phase corresponds to an inactive galaxy as our Milky Way. Several properties of the simulated galaxy and its nuclear activity are in broad agreement with observations.

We present a new algorithm to estimate quasar photometric redshifts (photo-zs), by considering the asymmetries in the relative flux distributions of quasars. The relative flux models are built with multivariate Skew-t distributions in the multidimensional space of relative fluxes as a function of redshift and magnitude. For 151,392 quasars in the SDSS, we achieve a photo-z accuracy, defined as the fraction of quasars with the difference between the photo-z z(p) and the spectroscopic redshift z(s), vertical bar Delta z vertical bar=vertical bar z(s)-z(p)vertical bar/(1 + z(s)) within 0.1, of 74%. Combining the WISE W1 and W2 infrared data with the SDSS data, the photo-z accuracy is enhanced to 87%. Using the Pan-STARRS1 or DECaLS photometry with WISE W1 and W2 data, the photo-z accuracies are 79% and 72%, respectively. The prior probabilities as a function of magnitude for quasars, stars, and galaxies are calculated, respectively, based on (1) the quasar luminosity function, (2) the Milky Way synthetic simulation with the Besancon model, and (3) the Bayesian Galaxy Photometric Redshift estimation. The relative fluxes of stars are obtained with the Padova isochrones, and the relative fluxes of galaxies are modeled through galaxy templates. We test our classification method to select quasars using the DECaLS g, r, z, and WISE W1 and W2 photometry. The quasar selection completeness is higher than 70% for a wide redshift range 0.5 < z < 4.5, and a wide magnitude range 18 < r < 21.5 mag. Our photo-z regression and classification method has the potential to extend to future surveys. The photo-z code will be publicly available.