DOCSLIB.ORG

On the Difference of Quasars and BL Lac Objects

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
On the Difference of Quasars and BL Lac Objects Approaching Micro-Arcsecond Resolution with VSOP-2: Astrophysics and Technology ASP Conference Series, Vol. 402, c 2009 Y. Hagiwara, E. Fomalont, M. Tsuboi, and Y. Murata, eds. On the Difference of Quasars and BLLac Objects Xiang Liu Urumqi Observatory of NAOC, 40-5 South Beijing Rd, Urumqi 830011, China Abstract. We analyze the two differences between radio-loud quasars and BLLacs: the lack of emission lines in BLLacs, and the different position angles of the jets. We propose that BLLacs have much less gas and are possibly optically-thin near the central engine in nuclei. This could be attributed to the orbiting of binary black holes in a very late merging stage. 1. Introduction Blazars are one of the most extreme classes of active galactic nuclei (AGN), characterized by high luminosity, rapid variability, high polarization, radio core- dominance and apparent superluminal speeds. The two blazar classes, flat- spectrum radio quasars (FSRQ) and BLLacertae objects, are thought to be the beamed counterparts of high- and low-luminosity radio galaxies, respectively. The main difference between the two blazar classes is shown in their emission lines, which are strong and quasar-like for FSRQs and weak or in some cases absent in BLLacs (Padovani et al. 2007). BLLac objects are often divided into two classes according to the position of the location of the peak in their synchrotron distribution: low-energy peaked BLLacs (LBLs), with the peak located at IR/optical wavelengths, and high- energy peaked BLLacs (HBLs) with the synchrotron emission peaking in the UV/X-ray energy band (e.g. Giommi & Padovani 1994). It is also noted that BLLacs have cosmological properties different than those of FSRQs and other type of AGNs. Although based on a small sam- ple, LBLs have consistent properties of a non-evolving population (Stickel et al. 1991), while HBLs are less numerous and/or less luminous at high redshift (Piranomonte et al. 2007). The broad-band emission from blazars is dominated by non-thermal pro- cesses, emitted by a relativistic jet pointed close to our line of sight. The prop- erties of misdirected blazars are consistent with those of radio galaxies, which form the so-called ‘parent population’ within the unified schemes (e.g. Urry & Padovani 1995). It is generally agreed that BLLacs are radio galaxies with their jets oriented close to the line-of-sight. Our knowledge of BLLacs is based mainly with the studies of two samples: the radio selected 1-Jy sample and the X-ray selected Einstein Medium Sensitivity Survey (EMSS) sample. Recently, new samples of BLLacs have been found by cross-correlating existing radio and X-ray source catalogues, searching for compact sources which are then classified based on optical identification. These searches tend to find BLLacs with properties in- 307 308 Liu termediate between those of the 1-Jy and EMSS samples. Detailed studies of intermediate samples are expected to provide a more complete picture of the BLLacs class of objects and their relation to radio galaxies. One such sample was drawn from the Deep X-ray Radio Blazar Survey (DXRBS)(Padovani et al 2007, Bignall et al. 2007). Because of these extreme physical characteristics and their differences with quasars, BLLacs have been the subject of intense research activity and obser- vation campaigns from radio to TeV energies. In this early stage of study, it is first important to analyze the observational differences between BLLacs and quasars and to question the underline physics of them, especially in the low energy-peaked synchrotron emission regime. We have summarized the proper- ties of BLLacs and the differences with quasars, and propose possible underline physics. 2. A model for BLLacs 37 of the 518 radio sources of 1-Jy sample are BLLACs (Kuhr et al. 1981, Stickel et al. 1991), about 7%, and quasars consist of 52% in this sample. The majority of BLLacs in the 1-Jy sample are LBLs (Bignall et al. 2007), while BLLacs in X-ray selected are mostly HBLs (Padovani et al. 2007). As mentioned in the introduction, the main difference of BLLacs and quasars is that BLLacs have weak or absent emission lines, and they are thought to be the beamed counterparts of low-luminosity radio galaxies. We conclude that BLLacs lack much gas in their nuclei. Two possibilities: that BLLacs have much less gas than FSRQs in their nuclei, and that the featureless of emission lines occur because they are extremely face-on, as suggested in the framework of unified scheme. In the first case, BL Lacs in the radio selected samples are FR-1 radio sources which have a featureless spectrum and low accretion rate. In the second case, the BLLac jets are pointed nearly toward us so that the gas would be pushed away with little line emission, in contrast to FSRQs whose jets are pointed over a large range of viewing angles so that more intensive emission lines would be observable from the gas outside of the jet cone. For case 1, why is there much less gas in BLLacs? We propose that the evolution of binary black holes (BBH) system has reduced the amount of gas in BLLacs. In the merging scenario for the formation of blazars, they have a binary black hole system, and we suggest that these BLLacs may be at an evolutionary phase of a close pair of BBH at an orbital separation less than 1 pc. The gas in both the narrow and broad line regions are absorbed in the process of BBH evolution, and the fueling rate is also reduced. In contrast, FSRQs may be at a phase of a wide pair of BBH with an orbital separation of a few pc, so the interaction of BBH with stars and gas would increase the felling rate, and the BBH at this stage would produce pc scale, FR-2 scale jets and strong broad line region (Lobanov 2004). For case 2, the jets of BLLacs are pointed closer to the line-of-sight than that for FSRQs, and this can partly account for the differences of BLLacs and FSRQs. Quasars and BLLac Objects 309 Hence, we propose the combined model from the case 1 and case 2: BLLacs are the subclass of extremely beamed blazars with a close pair of BBH which has decreased the amount of gas in both the narrow and broad line regions. FSRQs are beamed blazars associated with a wider pair of BBH and significant gas has remained. In this model, BLLacs are at a very late stage of galaxy-galaxy merging since they have a close pair of BBH, and this is consistent with a non-evolving population. Furthermore, the secondary BH of the close BBH in some BLLacs, could be interacting (even impacting) with the accretion disk of the primary BH, and producing the HBLs which show a synchrotron emission peak in the UV/X-ray band. In contrast, the wide pair of BBH in FSRQs produces a low- energy peaked synchrotron emission. Also, BLLacs may have an optically-thin broad line region (BLR) due to the lack of gas, and this enhances the probability of detections of Tev gamma-ray from BLR. That 12 out of 13 extragalactic Tev sources detected so far are BLLacs (Padovani 2006) supports this assertion. On the contrary, FSRQs may have an optically-thick BLR, so that the ‘blazar sequence’ from the ‘FSRQs to BLLacs’ is still valid. 3. Outlook Taylor et al. (2006) reported a BBH binary system in the radio galaxy 0402+379 that has a projected separation of 7.3 pc. This may be an example of the wide pair of BBH. A BLLacs associated with a close pair of BBH could be the source OJ287 (Shi et al.). If it contained close double radio core associated with a BBH, it may be detectable with the VSOP-2 program at 22 and 43 GHz. Acknowledgments. I wish to thank Leonid Gurvits for useful comment. This work was supported by the National Natural Science Foundation of China under grant No.10773019. References Bignall, H., et al., in Proceedings of the 8th EVN Symposium, 2006, eds. Baan W. et al., PoS(8thEVN)026 Giommi, P., Padovani, P. 1994, MNRAS, 268, L51 Lobanov, A., astro-ph/0412416, in Proceedings of the Conference “Growing Black Holes”, Garching, Germany June 21-25, 2004, edited by A.Merloni, S.Nayakshin, R.Sunyaev (Springer-Verlag series of ESO Astrophysics) Kuhr, H., et al. 1981, A&AS, 45, 367 Padovani, P., et al. 2007, ApJ, 662, 182 Padovani, P. 2007, ApSS, 309, 63 Piranomonte, S., et al. 2007, A&A, 470, 787 Shi, W., Liu, X., Song, H. 2007, ApSS, 310, 59 Stickel, M., et al. 1991, ApJ, 374, 431 Taylor, G. B., et al. 2007, IAUS, 238, 269 Urry, C. M., Padovani, P. 1995, PASP, 107, 803.
Load more

Recommended publications
  • Polarimetric Properties of Blazars Caught by the WEBT
    galaxies Review Polarimetric Properties of Blazars Caught by the WEBT Claudia M. Raiteri * and Massimo Villata INAF, Osservatorio Astrofisico di Torino, via Osservatorio 20, I-10025 Pino Torinese, Italy; [email protected] * Correspondence: [email protected] Abstract: Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called “blazars”. The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the g-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies. Keywords: active galactic nuclei; blazars; jets; polarimetry Citation: Raiteri, C.M.; Villata, M.
    [Show full text]
  • Quantum Detection of Wormholes Carlos Sabín
    www.nature.com/scientificreports OPEN Quantum detection of wormholes Carlos Sabín We show how to use quantum metrology to detect a wormhole. A coherent state of the electromagnetic field experiences a phase shift with a slight dependence on the throat radius of a possible distant wormhole. We show that this tiny correction is, in principle, detectable by homodyne measurements Received: 16 February 2017 after long propagation lengths for a wide range of throat radii and distances to the wormhole, even if the detection takes place very far away from the throat, where the spacetime is very close to a flat Accepted: 15 March 2017 geometry. We use realistic parameters from state-of-the-art long-baseline laser interferometry, both Published: xx xx xxxx Earth-based and space-borne. The scheme is, in principle, robust to optical losses and initial mixedness. We have not observed any wormhole in our Universe, although observational-based bounds on their abundance have been established1. The motivation of the search of these objects is twofold. On one hand, the theoretical implications of the existence of topological spacetime shortcuts would entail a challenge to our understanding of deep physical principles such as causality2–5. On the other hand, typical phenomena attributed to black holes can be mimicked by wormholes. Therefore, if wormholes exist the identity of the objects in the center of the galaxies might be questioned6 as well as the origin of the already observed gravitational waves7, 8. For these reasons, there is a renewed interest in the characterization of wormholes9–11 and in their detection by classical means such as gravitational lensing12, 13, among others14.
    [Show full text]
  • Active Galactic Nuclei: a Brief Introduction
    Active Galactic Nuclei: a brief introduction Manel Errando Washington University in St. Louis The discovery of quasars 3C 273: The first AGN z=0.158 2 <latexit sha1_base64="4D0JDPO4VKf1BWj0/SwyHGTHSAM=">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</latexit> <latexit sha1_base64="H7Rv+ZHksM7/70841dw/vasasCQ=">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</latexit> The power source of quasars • The luminosity (L) of quasars, i.e. how bright they are, can be as high as Lquasar ~ 1012 Lsun ~ 1040 W. • The energy source of quasars is accretion power: - Nuclear fusion: 2 11 1 ∆E =0.007 mc =6 10 W s g−
    [Show full text]
  • A Multimessenger View of Galaxies and Quasars from Now to Mid-Century M
    A multimessenger view of galaxies and quasars from now to mid-century M. D’Onofrio 1;∗, P. Marziani 2;∗ 1 Department of Physics & Astronomy, University of Padova, Padova, Italia 2 National Institute for Astrophysics (INAF), Padua Astronomical Observatory, Italy Correspondence*: Mauro D’Onofrio [email protected] ABSTRACT In the next 30 years, a new generation of space and ground-based telescopes will permit to obtain multi-frequency observations of faint sources and, for the first time in human history, to achieve a deep, almost synoptical monitoring of the whole sky. Gravitational wave observatories will detect a Universe of unseen black holes in the merging process over a broad spectrum of mass. Computing facilities will permit new high-resolution simulations with a deeper physical analysis of the main phenomena occurring at different scales. Given these development lines, we first sketch a panorama of the main instrumental developments expected in the next thirty years, dealing not only with electromagnetic radiation, but also from a multi-messenger perspective that includes gravitational waves, neutrinos, and cosmic rays. We then present how the new instrumentation will make it possible to foster advances in our present understanding of galaxies and quasars. We focus on selected scientific themes that are hotly debated today, in some cases advancing conjectures on the solution of major problems that may become solved in the next 30 years. Keywords: galaxy evolution – quasars – cosmology – supermassive black holes – black hole physics 1 INTRODUCTION: TOWARD MULTIMESSENGER ASTRONOMY The development of astronomy in the second half of the XXth century followed two major lines of improvement: the increase in light gathering power (i.e., the ability to detect fainter objects), and the extension of the frequency domain in the electromagnetic spectrum beyond the traditional optical domain.
    [Show full text]
  • A Supernova Origin for Dust in a High-Redshift Quasar
    A SUPERNOVA ORIGIN FOR DUST IN A HIGH-REDSHIFT QUASAR R. Maiolino ∗, R. Schneider †∗, E. Oliva ‡∗, S. Bianchi §, A. Ferrara ¶, F. Mannucci§, M. Pedani‡, M. Roca Sogorb k ∗ INAF - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy † “Enrico Fermi” Center, Via Panisperna 89/A, 00184 Roma, Italy ‡ Telescopio Nazionale Galileo, C. Alvarez de Abreu, 70, 38700 S.ta Cruz de La Palma, Spain § CNR-IRA, Sezione di Firenze, Largo Enrico Fermi 5, 50125 Firenze, Italy ¶ SISSA/International School for Advanced Studies, Via Beirut 4, 34100 Trieste, Italy k Astrofisico Fco. S`anchez, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Interstellar dust plays a crucial role in the evolution of the Universe by assisting the formation of molecules1, by triggering the formation of the first low-mass stars2, and by absorbing stellar ultraviolet-optical light and subsequently re-emitting it at infrared/millimetre wavelengths. Dust is thought to be produced predominantly in the envelopes of evolved (age >1 Gyr), low-mass stars3. This picture has, however, recently been brought into question by the discovery of large masses of dust in the host galaxies of quasars4,5 at redshift z > 6, when the age of the Universe was less than 1 Gyr. Theoretical studies6,7,8, corroborated by observations of nearby supernova remnants9,10,11, have suggested that supernovae provide a fast and efficient dust formation environment in the early Universe. Here we report infrared observations of a quasar at redshift 6.2, which are used to obtain directly its dust extinction curve. We then show that such a curve is in excellent agreement with supernova dust models.
    [Show full text]
  • Stellar Death: White Dwarfs, Neutron Stars, and Black Holes
    Stellar death: White dwarfs, Neutron stars & Black Holes Content Expectaions What happens when fusion stops? Stars are in balance (hydrostatic equilibrium) by radiation pushing outwards and gravity pulling in What will happen once fusion stops? The core of the star collapses spectacularly, leaving behind a dead star (compact object) What is left depends on the mass of the original star: <8 M⦿: white dwarf 8 M⦿ < M < 20 M⦿: neutron star > 20 M⦿: black hole Forming a white dwarf Powerful wind pushes ejects outer layers of star forming a planetary nebula, and exposing the small, dense core (white dwarf) The core is about the radius of Earth Very hot when formed, but no source of energy – will slowly fade away Prevented from collapsing by degenerate electron gas (stiff as a solid) Planetary nebulae (nothing to do with planets!) THE RING NEBULA Planetary nebulae (nothing to do with planets!) THE CAT’S EYE NEBULA Death of massive stars When the core of a massive star collapses, it can overcome electron degeneracy Huge amount of energy BAADE ZWICKY released - big supernova explosion Neutron star: collapse halted by neutron degeneracy (1934: Baade & Zwicky) Black Hole: star so massive, collapse cannot be halted SN1006 1967: Pulsars discovered! Jocelyn Bell and her supervisor Antony Hewish studying radio signals from quasars Discovered recurrent signal every 1.337 seconds! Nicknamed LGM-1 now called PSR B1919+21 BRIGHTNESS TIME NATURE, FEBRUARY 1968 1967: Pulsars discovered! Beams of radiation from spinning neutron star Like a lighthouse Neutron
    [Show full text]
  • HUBBLE SPACE TELESCOPE ULTRAVIOLET SPECTROSCOPY of 14 LOW-REDSHIFT QUASARS1 Rajib Ganguly,2 Michael S
    A The Astronomical Journal, 133:479Y486, 2007 February # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. HUBBLE SPACE TELESCOPE ULTRAVIOLET SPECTROSCOPY OF 14 LOW-REDSHIFT QUASARS1 Rajib Ganguly,2 Michael S. Brotherton,2 Nahum Arav,3 Sara R. Heap,4 Lutz Wisotzki,5 Thomas L. Aldcroft,6 Danielle Alloin,7,8 Ehud Behar,9 Gabriela Canalizo,10 D. Michael Crenshaw,11 Martijn de Kool,12 Kenneth Chambers,13 Gerald Cecil,14 Eleni Chatzichristou,15 John Everett,16,17 Jack Gabel,3 C. Martin Gaskell,18 Emmanuel Galliano,19 Richard F. Green,20 Patrick B. Hall,21 Dean C. Hines,22 Vesa T. Junkkarinen,23 Jelle S. Kaastra,24 Mary Elizabeth Kaiser,25 Demosthenes Kazanas,4 Arieh Konigl,26 Kirk T. Korista,27 Gerard A. Kriss,28 Ari Laor,9 Karen M. Leighly,29 Smita Mathur,30 Patrick Ogle,31 Daniel Proga,32 Bassem Sabra,33 Ran Sivron,34 Stephanie Snedden,35 Randal Telfer,36 and Marianne Vestergaard37 Received 2006 June 27; accepted 2006 October 4 ABSTRACT We present low-resolution ultraviolet spectra of 14 low-redshift (zem P 0:8) quasars observed with the Hubble Space Telescope STIS as part of a Snapshot project to understand the relationship between quasar outflows and luminosity. By design, all observations cover the C iv emission line. Ten of the quasars are from the Hamburg-ESO catalog, three are from the Palomar-Green catalog, and one is from the Parkes catalog. The sample contains a few interesting quasars, including two broad absorption line (BAL) quasars (HE 0143À3535 and HE 0436À2614), one quasar with a mini-BAL (HE 1105À0746), and one quasar with associated narrow absorption (HE 0409À5004).
    [Show full text]
  • OJ 287: NEW TESTING GROUND for GENERAL RELATIVITY and BEYOND C Sivaram Indian Institute of Astrophysics, Bangalore
    OJ 287: NEW TESTING GROUND FOR GENERAL RELATIVITY AND BEYOND C Sivaram Indian Institute of Astrophysics, Bangalore Abstract: The supermassive short period black hole binary OJ287 is discussed as a new precision testing ground for general relativity and alternate gravity theories. Like in the case of binary pulsars, the relativistic gravity effects are considerably larger than in the solar system. For instance the observed orbital precession is forty degrees per period. The gravitational radiation energy losses are comparable to typical blazar electromagnetic radiation emission and it is about ten orders larger than that of the binary pulsar with significant orbit shrinking already apparent in the light curves. This already tests Einstein gravity to a few percent for objects at cosmological distances. Constraints on alternate gravity theories as well as possible detection of long term effects of dark matter and dark energy on this system are described. 1 For more than fifty years after Einstein proposed the general theory of relativity in 1915, observational tests to verify some of the predictions were confined to within the solar system; where the effects are quite small. This situation changed with the discovery of the binary pulsar in 1975 where the relativistic periastron shift was more than four degrees per year, a whopping thirty thousand times more than the paltry well known correction of 43 arc seconds/century for mercury.1, 2 The recently discovered 2.4 hour period binary pulsar has a periastron shift of sixteen degrees per year!3 Other relativistic effects like the time delay of the signals and time dilation and frequency shifts are also much larger for these binary systems.
    [Show full text]
  • Safeye Quasar 900 the New Safeye Quasar 900 Is an Open Path Detection System Which Provides Continuous Monitoring for Combustible Hydrocarbon Gases
    SafEye Quasar 900 The New SafEye Quasar 900 is an open path detection system which provides continuous monitoring for combustible hydrocarbon gases. It employs "spectral fingerprint" analysis of the atmosphere using the Differential Optical Absorption Spectroscopy (DOAS) technique. FEATURES & BENEFITS • Detects Hydrocarbon gases including methane, ethylene, propylene • Detection range: 4-200m in three different models (same detector different sources) • Built in event recorder – real time record of the last 100 events • Fast connection to Hand-Held for prognostic and diagnostic maintenance • Heated optics • Design to meet SIL2, per IEC61508 • Outputs: - 0-20mA - HART protocol for maintenance and asset managements - RS-485, Modbus Compatible The Quasar 900 consists of a Xenon Flash infrared transmitter and infrared receiver, separated over a • High reliability – MTBF minimum 100,000 hours line of sight from 13 ft (4m) up to 650 ft (200m) • User programmable via HART or RS-485 in extremely harsh environments where dust, fog, • 3 years warranty (10 years for the flash source) rain, snow or vibration can cause a high reduction of signal. • Ex approval: - Design to ATEX, IECEx per The Quasar 900 transmitter and receiver are both Ex II 2 (1) GD housed in a rugged, stainless steel, ATEX and IECEx approved enclosure. The main enclosure is EExd EEx de ia II C T5 flameproof with an integral, segregated, EExe - Design to FM, CSA per increased safety terminal section. Class I Div 1 Group B, C and D Class I, II Div 1 Group E, F and D The hand-held communication unit can be connected - Performance test: design to FM6325 and in-situ via the intrinsically safe approved data port EN50241-1 and 2 for prognostic and diagnostic maintenance.
    [Show full text]
  • 8. Active Galactic Nuclei. Radio Galaxies and Quasars
    8. ACTIVE GALACTIC NUCLEI. RADIO GALAXIES AND QUASARS This Section starts analyzing the phenomenon of extragalactic Active Galactic Nuclei. Here we will essentially discuss their low-photon-energy emissions, while in Sect. 9 we will expand on the high-photon-energy ones. 8.1 Active Galactic Nuclei: Generalities We know that normal galaxies constitute the majority population of cosmic sources in 12 the local universe. Although they may be quite massive (up to 10 M), they are not 11 44 typically very luminous (never more luminous than 10 Lʘ10 erg/sec). For this reason normal galaxies have remained hardly detectable at very large cosmic distances for long time, with recent advances mostly thanks to technological observational improvements. For several decades, since the ’60, the only detectable objects at large cosmic distances belonged to a completely different class of sources, which did not derive their energy from thermonuclear burning in stars. These are the Active Galactic Nuclei, for which we use the acronym AGN. The galaxies hosting them are called active galaxies. For their luminosity, Active Galactic Nuclei and Active Galaxies have plaid a key role in the development of cosmology, further to the fact that they make one of the most important themes of extragalactic astrophysics and High Energy Astrophysics. Apparently, the interest for the Active Galaxies as a population might seem modest, as in the local universe they make a small fraction of normal galaxies, as summarized in the following. There are four main categories of Active Galaxies and AGN: 1. galaxies with an excess of infrared emission and violent activity of star formation (starburst), selected in particular in the far-IR, the luminous (LIRG), ultra-luminous (ULIRG) and hyper-luminous (HYLIRG) galaxies: the 3 classes 11 12 13 have bolometric luminosities >10 L (LIRG), >10 L (ULIRG), >10 L (HYLIRG); 2.
    [Show full text]
  • 16.1 Quasars and Active Galactic Nuclei
    Quasars and Active Galactic Nuclei Ay1 – Lecture 16 16.1 Quasars and Active Galactic Nuclei: General Properties and Classification Quasars and Active Galactic Nuclei • Highly energetic manifestations in the nuclei of galaxies, powered by accretion onto supermassive massive black holes • Empirical classification schemes have been developed, on the basis of the spectra; but recently, various unification schemes have been developed (~ the same underlying phenomenon) • Evolve strongly in time, with the comoving densities of luminous ones increasing by ~ 103 from z ~ 0 to z ~ 2 • At z ~ 0, at least 30% of all galaxies show some sign of a nuclear activity; ~ 1% can be classified as Seyferts, and ~ 10-6 contain luminous quasars • We think that most or all non-dwarf galaxies contain SMBHs, and thus probably underwent at least one AGN phase AGN, an artist’s view Relativistic jet / illumination cone Central black hole Accretion disk Obscuring dusty torus Observable Properties of AGN • Emission over a broad range of frequencies, radio to γ-rays – Nonthermal radio or X-ray emission is a good way to find AGN – Generally bluer spectra than stars: “UV excess” – Colors unlike those of stars, especially when modified by the intergalactic absorption • Presence of strong, broad emission lines in their spectra 15 • Can reach large luminosities, up to ~ 10 L • Strong variability at all time scales – Implies small physical size of the emission region • Central engines unresolved • Undetectable proper motions due to a large distances All of these have been used to devise methods to discover AGN, and each method has its own limitations and selection effects Broad-Band Spectra of Quasars Radio Loud Radio Quiet UV-Optical Spectra of Quasars Strong, broad emission lines: Balmer lines of hydrogen Prominent lines of abundant ions Quasar Counts For the unobscured, Type 1 QSOs; they may be outnumbered by the obscured ones.
    [Show full text]
  • Quasar Catalogue for the Astrometric Calibration of the Forthcoming ILMT Survey
    J. Astrophys. Astr. (2020) 41:22 Ó Indian Academy of Sciences https://doi.org/10.1007/s12036-020-09642-xSadhana(0123456789().,-volV)FT3](0123456789().,-volV) Quasar catalogue for the astrometric calibration of the forthcoming ILMT survey AMIT KUMAR MANDAL1,4,*, BIKRAM PRADHAN2, JEAN SURDEJ3, C. S. STALIN4, RAM SAGAR4 and BLESSON MATHEW1 1Department of Physics, CHRIST (Deemed to be University), Hosur Road, Bangalore 560 029, India. 2Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 001, India. 3Space Science, Technologies and Astrophysics Research (STAR) Institute, Universite´ de Lie`ge, Alle´edu6 Aouˆt 19c, 4000 Lie`ge, Belgium. 4Indian Institute of Astrophysics, Block II, Koramangala, Bangalore 560 034, India. E-mail: [email protected] MS received 27 May 2020; accepted 13 July 2020 Abstract. Quasars are ideal targets to use for astrometric calibration of large scale astronomical surveys as they have negligible proper motion and parallax. The forthcoming 4-m International Liquid Mirror Tele- scope (ILMT) will survey the sky that covers a width of about 270. To carry out astrometric calibration of the ILMT observations, we aimed to compile a list of quasars with accurate equatorial coordinates and falling in the ILMT stripe. Towards this, we cross-correlated all the quasars that are known till the present date with the sources in the Gaia-DR2 catalogue, as the Gaia-DR2 sources have position uncertainties as small as a few milli arcsec (mas). We present here the results of this cross-correlation which is a catalogue of 6738 quasars that is suitable for astrometric calibration of the ILMT fields.
    [Show full text]