The X-Ray Spectral Properties of SCUBA Galaxies

Deep SCUBA surveys have uncovered a large population of massive submillimeter-emitting galaxies (SMGs; f850μm ≳ 4 mJy) at z ≳ 1. Although it is generally believed that these galaxies host intense star formation activity, there is growing evidence that a substantial fraction also harbor an active galactic nucleus (AGN; i.e., an accreting super-massive black hole [SMBH]). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultradeep X-ray observations (the 2 Ms Chandra Deep Field-North) and deep Keck spectroscopic data of SMGs with radio counterparts. We find that the majority (≈75%) of these radio-selected spectroscopically identified SMGs host AGN activity; the other ≈25% have X-ray properties consistent with star formation (X-ray-derived star formation rates of ≈1300-2700 M☉ yr-1). The AGNs have properties generally consistent with those of nearby luminous AGNs (Γ ≈ 1.8 ± 0.5, NH ≈ 1020-1024 cm-2, and LX ≈ 1043-1044.5 ergs s-1), and the majority (≈80%) are heavily obscured (NH ≳ 1023 cm-2). We construct composite rest-frame 2-20 keV spectra for three different obscuration classes [NH < 1023 cm-2, NH = (1-5) × 1023 cm-2, and NH > 5 × 1023 cm-2], which reveal features not seen in the individual X-ray spectra. An ≈1 keV equivalent width Fe Kα emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs, suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray to far-IR luminosity ratio of the AGN-classified SMGs (LX/LFIR = 0.004) is approximately 1 order of magnitude below that found for typical quasars. This result suggests that intense star formation activity (of order ≈1000 M☉ yr-1) dominates the bolometric output of these SMGs. However, we also explore the possibility that the X-ray to far-IR luminosity ratio of the AGN components is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the implications of our results for the growth of massive black holes, discuss the prospects for deeper X-ray observations, and explore the scientific potential offered by the next generation of X-ray observatories.

[1]  Jean-Luc Starck,et al.  Astronomical Data Analysis , 2007 .

[2]  C. Crawford,et al.  X-rays from the Type II quasar in the hyperluminous infrared galaxy IRAS F15307+3252★ , 2005, astro-ph/0506305.

[3]  A. Sternberg,et al.  Mid-Infrared Spectroscopy of Two Luminous Submillimeter Galaxies at z ~ 2.8 , 2005, astro-ph/0504431.

[4]  W. Brandt,et al.  Rapid growth of black holes in massive star-forming galaxies , 2005, Nature.

[5]  Edinburgh,et al.  An interferometric CO survey of luminous submillimetre galaxies , 2005, astro-ph/0503055.

[6]  M. Nonino,et al.  The Chandra deep field South/GOODS survey. Optically faint X-ray sources , 2005, astro-ph/0502542.

[7]  T. D. Matteo,et al.  A Physical Model for the Origin of Quasar Lifetimes , 2005, astro-ph/0502241.

[8]  T. D. Matteo,et al.  Energy input from quasars regulates the growth and activity of black holes and their host galaxies , 2005, Nature.

[9]  W. Keel,et al.  Optical and near-infrared integral field spectroscopy of the SCUBA galaxy N2 850.4 , 2005, astro-ph/0502096.

[10]  W. Brandt,et al.  DEEP EXTRAGALACTIC X-RAY SURVEYS , 2005, astro-ph/0501058.

[11]  I. Smail,et al.  A Redshift Survey of the Submillimeter Galaxy Population , 2004, astro-ph/0412573.

[12]  A. Evans,et al.  Fe K emission in the ultraluminous infrared galaxy Arp 220 , 2004, astro-ph/0411562.

[13]  D. M. Alexander,et al.  A Chandra observation of the z= 2.285 galaxy FSC 10214+4724: evidence for a Compton-thick quasar? , 2004, astro-ph/0411289.

[14]  J. Dunlop,et al.  Submillimetre detection of a high-redshift type 2 QSO , 2004, astro-ph/0410632.

[15]  R. Bender,et al.  Looking for obscured QSOs in the X-ray emitting ERO population , 2004, astro-ph/0410434.

[16]  Caltech,et al.  The Hubble Deep Field North SCUBA Super-map - III. Optical and near-infrared properties of submillimetre galaxies , 2005, astro-ph/0502526.

[17]  Mark Swinbank,et al.  The Rest-Frame Optical Spectra of SCUBA Galaxies , 2004, astro-ph/0412050.

[18]  C. Megan Urry,et al.  Obscured Active Galactic Nuclei and the X-Ray, Optical, and Far-Infrared Number Counts of Active Galactic Nuclei in the GOODS Fields , 2004 .

[19]  R. J. Ivison,et al.  The Rest-Frame Optical Properties of SCUBA Galaxies , 2004, astro-ph/0412574.

[20]  T. D. Matteo,et al.  Modelling feedback from stars and black holes in galaxy mergers , 2004, astro-ph/0411108.

[21]  R. Maiolino,et al.  Multiwavelength AGN Surveys , 2004 .

[22]  J. Mittaz,et al.  Soft x-ray emission from clusters of galaxies and related phenomena , 2004 .

[23]  R. J. Ivison,et al.  Evidence for Extended, Obscured Starbursts in Submillimeter Galaxies , 2004, astro-ph/0412051.

[24]  M. Halpern,et al.  The Hubble Deep Field North SCUBA Super-map – II. Multiwavelength properties , 2004, astro-ph/0408376.

[25]  Cambridge,et al.  Infrared Number Counts of AGN in the GOODS Fields , 2004, astro-ph/0408099.

[26]  R. Ivison,et al.  The Evolutionary Sequence of Active Galactic Nuclei and Galaxy Formation Revealed , 2004, astro-ph/0407171.

[27]  S. Serjeant,et al.  SPITZER OBSERVATIONS OF THE SCUBA/VLA SOURCES IN THE LOCKMAN HOLE: STAR FORMATION HISTORY OF INFRARED-LUMINOUS GALAXIES , 2004, astro-ph/0406359.

[28]  F. Masci,et al.  Infrared Properties of Radio-selected Submillimeter Galaxies in the Spitzer First Look Survey Verification Field , 2004, astro-ph/0406351.

[29]  D. O. Astronomy,et al.  An 850 Micron SCUBA Survey of the Hubble Deep Field-North GOODS Region , 2004, astro-ph/0406261.

[30]  I. Smail,et al.  Spitzer Observations of MAMBO Galaxies: Weeding Out Active Nuclei in Starbursting Protoellipticals , 2004, astro-ph/0406158.

[31]  J. Dunlop,et al.  Deep near-infrared spectroscopy of submillimetre-selected galaxies , 2004, astro-ph/0405390.

[32]  I. Smail,et al.  Submitted to ApJ Preprint typeset using L ATEX style emulateapj v. 6/22/04 CLUSTERING OF SUBMILLIMETER-SELECTED GALAXIES , 2004 .

[33]  Scott C. Chapman,et al.  Accurate Spectral Energy Distributions and Selection Effects for High-Redshift Dusty Galaxies: A New Hot Population to Discover with the Spitzer Space Telescope? , 2004, astro-ph/0404438.

[34]  Baltimore,et al.  Prevalence of X-Ray Variability in the Chandra Deep Field-South , 2004, astro-ph/0404418.

[35]  Oxford,et al.  The 2dF QSO Redshift Survey – XII. The spectroscopic catalogue and luminosity function , 2004, astro-ph/0403040.

[36]  A. Franceschini,et al.  2-10 KeV luminosity of high - mass binaries as a gauge of ongoing star - formation rate , 2004, astro-ph/0402568.

[37]  G. Palumbo,et al.  The XMM-Newton and BeppoSAX view of the Ultra Luminous Infrared Galaxy MKN 231 , 2004, astro-ph/0402226.

[38]  P. Capak,et al.  The Evolution of the Ultraluminous Infrared Galaxy Population from Redshift 0 to 1.5 , 2004, astro-ph/0402235.

[39]  I. Smail,et al.  A Filamentary Structure of Massive Star-forming Galaxies Associated with an X-Ray-absorbed QSO at z = 1.8 , 2004, astro-ph/0402098.

[40]  M. Pettini,et al.  A Survey of Star-forming Galaxies in the 1.4 ≲ z ≲ 2.5 Redshift Desert: Overview , 2004, astro-ph/0401439.

[41]  L. Kewley,et al.  The Chandra Deep Field-South: Optical Spectroscopy. I. , 2003, astro-ph/0312324.

[42]  F. Lockman,et al.  PEERING THROUGH THE MUCK: NOTES ON THE INFLUENCE OF THE GALACTIC INTERSTELLAR MEDIUM ON EXTRAGALACTIC OBSERVATIONS , 2003, astro-ph/0311386.

[43]  G. Lewis,et al.  Further Multiwavelength Observations of the SSA 22 Lyα-Emitting Blob , 2003, astro-ph/0310670.

[44]  J. Dunlop,et al.  The cosmological evolution of quasar black hole masses , 2003, astro-ph/0405393.

[45]  G. Granato,et al.  A Physical Model for the Coevolution of QSOs and Their Spheroidal Hosts , 2003, astro-ph/0307202.

[46]  Edward Colbert,et al.  Old and Young X-Ray Point Source Populations in Nearby Galaxies , 2003, astro-ph/0305476.

[47]  F. Bertoldi,et al.  Interferometric Observations of Powerful CO Emission from Three Submillimeter Galaxies at z = 2.39, 2.51, and 3.35 , 2003 .

[48]  L. Silva,et al.  A Physical Model for the Joint Evolution of QSOS and Spheroids , 2003, astro-ph/0309420.

[49]  W. Percival,et al.  The formation of cluster elliptical galaxies as revealed by extensive star formation , 2003, Nature.

[50]  Loretta Dunne,et al.  The Canada-UK Deep Submillimeter Survey. VII. Optical and Near-Infrared Identifications for the 14 Hour Field , 2003, astro-ph/0308439.

[51]  G. Palumbo,et al.  An XMM-Newton hard X-ray survey of ultraluminous infrared galaxies , 2003 .

[52]  C. Conselice,et al.  Hubble Space Telescope Images of Submillimeter Sources: Large Irregular Galaxies at High Redshift , 2003, astro-ph/0308197.

[53]  Scott C. Chapman,et al.  Evidence for a Major Merger Origin of High-Redshift Submillimeter Galaxies , 2003, astro-ph/0308198.

[54]  Takamitsu Miyaji,et al.  Cosmological Evolution of the Hard X-Ray Active Galactic Nucleus Luminosity Function and the Origin of the Hard X-Ray Background , 2003, astro-ph/0308140.

[55]  A. Sternberg,et al.  ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240 , 2003, astro-ph/0307552.

[56]  P. Ciliegi,et al.  The HELLAS2XMM survey IV. Optical identifications and the evolution of the accretion luminosity in the Universe , 2003, astro-ph/0306556.

[57]  J. Surace,et al.  The IRAS Revised Bright Galaxy Sample , 2003, astro-ph/0306263.

[58]  S. Lilly,et al.  XMM-Newton surveys of the Canada-France Redshift Survey fields - I. The submillimetre/X-ray relation , 2003 .

[59]  M. Halpern,et al.  The Hubble Deep Field North SCUBA Super-map—I. Submillimetre maps, sources and number counts , 2003, astro-ph/0305444.

[60]  G. Palumbo,et al.  An XMM-Newton hard X-ray survey of ULIRGs , 2003, astro-ph/0304529.

[61]  D. M. Alexander,et al.  The Chandra Deep Field North Survey. XIII. 2 Ms Point-Source Catalogs , 2003, astro-ph/0304392.

[62]  I. Smail,et al.  A median redshift of 2.4 for galaxies bright at submillimetre wavelengths , 2003, Nature.

[63]  D. Clements,et al.  Starburst and AGN activity in ultraluminous infrared galaxies , 2003, astro-ph/0304154.

[64]  J. Stephen,et al.  BeppoSAX Average Spectra of Seyfert Galaxies , 2003, astro-ph/0304133.

[65]  J. Cuby,et al.  Hα Spectroscopy of Galaxies at z > 2: Kinematics and Star Formation , 2003, astro-ph/0303392.

[66]  A. W. Blain,et al.  A vigorous starburst in the SCUBA galaxy N2 850.4 , 2003, astro-ph/0303128.

[67]  T. Courvoisier,et al.  The intrinsic emission of Seyfert galaxies observed with BeppoSAX/PDS - I. Comparison of the average spectra of the three classes of Seyfert galaxies , 2003, astro-ph/0303085.

[68]  C. Crawford,et al.  A deep Chandra observation of the cluster environment of the z=1.786 radio galaxy 3C 294 , 2003, astro-ph/0301468.

[69]  Marat Gilfanov,et al.  Lx-SFR relation in star-forming galaxies , 2003 .

[70]  J. Lee,et al.  Testing the Seyfert unification theory: Chandra HETGS observations of NGC 1068 , 2002, astro-ph/0211406.

[71]  W. Brandt,et al.  The Chandra Deep Field North Survey. XIV. X-Ray-detected Obscured AGNs and Starburst Galaxies in the Bright Submillimeter Source Population , 2002, astro-ph/0211267.

[72]  G. Palumbo,et al.  XMM-Newton observations of ULIRGs: A Compton-thick AGN in IRAS 19254–7245 , 2002, astro-ph/0211237.

[73]  R. Cutri,et al.  XMM—Newton observations of two hyperluminous IRAS galaxies: Compton-thick quasars with obscuring starbursts , 2002, astro-ph/0211166.

[74]  P. Capak,et al.  The Properties of Microjansky Radio Sources in the Hubble Deep Field-North, SSA 13, and SSA 22 Fields , 2002, astro-ph/0211075.

[75]  M. Mori,et al.  Protoquasars: Physical States and Observable Properties , 2002, astro-ph/0209452.

[76]  A. Comastri,et al.  The 2-10 keV luminosity as a Star Formation Rate indicator , 2002, astro-ph/0202241.

[77]  S. Lilly,et al.  The Canada-UK Deep Submillimeter Survey. VI. The 3 Hour Field , 2002, astro-ph/0201180.

[78]  J. Dunlop,et al.  The coincidence and angular clustering of Chandra and SCUBA sources , 2001, astro-ph/0108400.

[79]  Belinda J. Wilkes,et al.  The ISO view of Palomar-Green quasars , 2003 .

[80]  S. Ravindranath,et al.  X-Ray Properties of Lyman Break Galaxies in the Great Observatories Origins Deep Survey , 2002, astro-ph/0409600.

[81]  O. Almaini.  Do sub-mm sources and quasars form an evolutionary sequence? , 2002, astro-ph/0211353.

[82]  Reinhard Genzel,et al.  Spatially Resolved Millimeter Interferometry of SMM J02399–0136: A Very Massive Galaxy at z = 2.8 , 2002, astro-ph/0210449.

[83]  Frazer N. Owen,et al.  Gemini Multi-Object Spectrograph Observations of SCUBA Galaxies behind A851 , 2002 .

[84]  W. Brandt,et al.  The Chandra Deep Field North Survey. XII. The Link between Faint X-Ray and Radio Source Populations , 2002, astro-ph/0207433.

[85]  E. Oliva,et al.  Mid-Infrared line diagnostics of active galaxies - A spectroscopic AGN survey with ISO-SWS , 2002, astro-ph/0207381.

[86]  W. Brandt,et al.  CHANDRA Detects Relativistic Broad Absorption Lines from APM 08279+5255 , 2002, astro-ph/0207196.

[87]  European Southern Observatory,et al.  XMM-Newton observation of the Lockman Hole - II. Spectral analysis , 2002, astro-ph/0207166.

[88]  J. Dunlop,et al.  Deep radio imaging of the SCUBA 8-mJy survey fields: submillimetre source identifications and redshift distribution , 2002, astro-ph/0206432.

[89]  W. Brandt,et al.  Submitted to the Astronomical Journal , 2002 .

[90]  Marat Gilfanov,et al.  High mass x-ray binaries as a star formation rate indicator in distant galaxies , 2002 .

[91]  R. McMahon,et al.  Observations of hyperluminous infrared galaxies with the Infrared Space Observatory: implications for the origin of their extreme luminosities , 2002, astro-ph/0204524.

[92]  W. Brandt,et al.  The Ionized Gas and Nuclear Environment in NGC 3783. I. Time-averaged 900 Kilosecond Chandra Grating Spectroscopy , 2002, astro-ph/0203263.

[93]  G. Risaliti,et al.  The BeppoSAX view of bright Compton-thin Seyfert 2 galaxies , 2002, astro-ph/0202392.

[94]  J. Kneib,et al.  Submillimeter Galaxies , 2002, astro-ph/0202228.

[95]  M. Mignoli,et al.  The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy , 2002, astro-ph/0202080.

[96]  J. Kneib,et al.  Faint Submillimeter Counts from Deep 850 Micron Observations of the Lensing Clusters A370, A851, and A2390 , 2002, astro-ph/0201426.

[97]  R. McMahon,et al.  The SCUBA Bright Quasar Survey (SBQS): 850-μm observations of the z>≳ 4 sample , 2001, astro-ph/0109438.

[98]  J. Dunlop,et al.  The SCUBA 8-mJy survey - I. Submillimetre maps, sources and number counts , 2001, astro-ph/0107446.

[99]  M. Page,et al.  Submillimeter Evidence for the Coeval Growth of Massive Black Holes and Galaxy Bulges , 2001, Science.

[100]  W. Brandt,et al.  X-Ray Spectroscopy of Quasi-Stellar Objects with Broad Ultraviolet Absorption Lines , 2001, astro-ph/0110579.

[101]  Shiro Ueno,et al.  Chandra observations of the luminous infrared galaxy NGC 3256 , 2001, astro-ph/0109198.

[102]  D. M. Alexander,et al.  The Chandra Deep Field North Survey. V. 1 Ms Source Catalogs , 2001, astro-ph/0108404.

[103]  J. Dunlop,et al.  Coupled spheroid and black hole formation, and the multifrequency detectability of active galactic nuclei and submillimetre sources , 2001, astro-ph/0108122.

[104]  Astrophysics,et al.  Exploratory Chandra Observations of the Highest-Redshift Quasars: X-Rays from the Dawn of the Modern Universe , 2001, astro-ph/0108001.

[105]  W. Brandt,et al.  The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-Ray Source Population , 2001, astro-ph/0107450.

[106]  Alice Shapley,et al.  A Multivariate Statistical Analysis of Spiral Galaxy Luminosities. I. Data and Results , 2001, astro-ph/0106361.

[107]  M. Yun,et al.  Radio Properties of Infrared-selected Galaxies in the IRAS 2 Jy Sample , 2001, astro-ph/0102154.

[108]  M. Gerin,et al.  ISOCAM-CVF 5-12 Micron Spectroscopy of Ultraluminous Infrared Galaxies , 2001, astro-ph/0101187.

[109]  A. Fabian,et al.  Chandra detection of reflected X‐ray emission from the type 2 QSO in IRAS 09104+4109 , 2000, astro-ph/0011518.

[110]  R. Mushotzky,et al.  The Nature of the Hard X-Ray Background Sources: Optical, Near-Infrared, Submillimeter, and Radio Properties , 2000, astro-ph/0007175.

[111]  W. Brandt,et al.  Detection of X-Ray Emission from Gravitationally Lensed Submillimeter Sources in the Field of Abell 370 , 2000, astro-ph/0008050.

[112]  A. Fabian,et al.  The X-ray spectra of Compton-thick Seyfert 2 galaxies as seen by BeppoSAX , 2000, astro-ph/0005219.

[113]  E. Feigelson,et al.  X-Ray Sources in the Hubble Deep Field Detected by Chandra , 2000, astro-ph/0004260.

[114]  J. Kneib,et al.  Testing the connection between the X-ray and submillimetre source populations using Chandra , 2000, astro-ph/0002322.

[115]  U. Berkeley,et al.  Extragalactic Results from the Infrared Space Observatory , 2000, astro-ph/0002184.

[116]  J. Kneib,et al.  The diversity of SCUBA-selected galaxies , 1999, astro-ph/9911069.

[117]  M. Birkinshaw.  Large-Scale Structure in the X-ray Universe , 2000 .

[118]  K. Nandra,et al.  X-Ray Observations of Optically Selected, Radio-quiet Quasars. I. The ASCA Results , 1999, astro-ph/9910218.

[119]  E. Richards.  The Nature of Radio Emission from Distant Galaxies: The 1.4 GHz Observations , 1999, astro-ph/9908313.

[120]  D. Rigopoulou,et al.  A Large Mid-Infrared Spectroscopic and Near-Infrared Imaging Survey of Ultraluminous Infrared Galaxies: Their Nature and Evolution , 1999, astro-ph/9908300.

[121]  A. Fabian.  THE OBSCURED GROWTH OF MASSIVE BLACK HOLES , 1999, astro-ph/9908064.

[122]  D. Helfand,et al.  X-Rays from NGC 3256: High-Energy Emission in Starburst Galaxies and Their Contribution to the Cosmic X-Ray Background , 1999, astro-ph/9907036.

[123]  D. B. Sanders,et al.  Resolving the Submillimeter Background: The 850 Micron Galaxy Counts , 1999, astro-ph/9904126.

[124]  R. Genzel,et al.  Accepted for publication in the Astrophysical Journal Letters Mid-infrared and optical spectroscopy of ultraluminous infrared galaxies: A comparison 1 , 1999 .

[125]  J. Kneib,et al.  Redshift Distribution of the Faint Submillimeter Galaxy Population , 1999, astro-ph/9903142.

[126]  R. Maiolino,et al.  The Distribution of Absorbing Column Densities among Seyfert 2 Galaxies , 1999, astro-ph/9902377.

[127]  J. Bond,et al.  The Canada-United Kingdom Deep Submillimeter Survey. II. First Identifications, Redshifts, and Implications for Galaxy Evolution , 1999, astro-ph/9901047.

[128]  G. Zamorani,et al.  A Three-dimensional Diagnostic Diagram for Seyfert 2 Galaxies: Probing X-Ray Absorption and Compton Thickness , 1998, astro-ph/9811074.

[129]  R. Mushotzky,et al.  X-Ray Constraints on Accretion and Starburst Processes in Galactic Nuclei. I. Spectral Results , 1998, astro-ph/9808159.

[130]  O. Fèvre,et al.  The Canada-UK Deep Submillimeter Survey: First Submillimeter Images, the Source Counts, and Resolution of the Background , 1998, astro-ph/9808040.

[131]  Jean-Paul Kneib,et al.  Deep Counts of Submillimeter Galaxies , 1998, astro-ph/9812412.

[132]  J. Dunlop,et al.  High-redshift star formation in the Hubble Deep Field revealed by a submillimetre-wavelength survey , 1998, Nature.

[133]  X. Barcons,et al.  Do nuclear starbursts obscure the X-ray background? , 1998, astro-ph/9803289.

[134]  Rob Ivison,et al.  A hyperluminous galaxy at z = 2.8 found in a deep submillimetre survey , 1997, astro-ph/9712161.

[135]  D. Kunze,et al.  What Powers Ultraluminous IRAS Galaxies? , 1997, astro-ph/9711255.

[136]  Kirpal Nandra,et al.  ASCA Observations of Type 2 Seyfert Galaxies. I. Data Analysis Results , 1997 .

[137]  I. Smail,et al.  A Deep Submillimeter Survey of Lensing Clusters: A New Window on Galaxy Formation and Evolution , 1997, astro-ph/9708135.

[138]  Steve Rawlings,et al.  High-redshift radio galaxies and quasars at submillimetre wavelengths: assessing their evolutionary status , 1997, astro-ph/9705094.

[139]  T. J. Turner,et al.  ASCA Observations of Type 2 Seyfert Galaxies. II. The Importance of X-Ray Scattering and Reflection , 1997, astro-ph/9705087.

[140]  D. Sanders,et al.  LUMINOUS INFRARED GALAXIES , 1996 .

[141]  ROBERT E. Williams,et al.  The Hubble Deep Field: Observations, Data Reduction, and , 1996, astro-ph/9607174.

[142]  Giorgio Matt,et al.  The iron Kα line complex in Compton-thick Seyfert 2 galaxies , 1996 .

[143]  David A. Smith,et al.  Unified theories of active galactic nuclei: a hard X-ray sample of Seyfert 2 galaxies , 1996 .

[144]  A. Kinney,et al.  Multiwavelength tests of the dusty torus model for Seyfert galaxies , 1994 .

[145]  M. S. Oey,et al.  Atlas of quasar energy distributions , 1994 .

[146]  K. Nandra,et al.  GINGA observations of the X-ray spectra of Seyfert galaxies. , 1994 .

[147]  Robert Antonucci,et al.  Unified models for active galactic nuclei and quasars , 1993 .

[148]  Giuseppina Fabbiano,et al.  The X-ray spectra of galaxies. II - Average spectral properties and emission mechanisms , 1992 .

[149]  G. Trinchieri,et al.  The X-ray spectra of galaxies. I - Spectral fits of individual galaxies and X-ray colors , 1992 .

[150]  John A. Nousek,et al.  Chi-squared and C statistic minimization for low count per bin data. [Sampling in X ray astronomy] , 1989 .

[151]  N. Gehrels.  Confidence limits for small numbers of events in astrophysical data , 1986 .

[152]  George Helou,et al.  Thermal infrared and nonthermal radio: remarkable correlation in disks of galaxies , 1985 .

[153]  Richard F. Mushotzky,et al.  X-ray spectra and time variability of active galactic nuclei , 1993 .

[154]  W. Cash,et al.  Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques , 1979 .

[155]  Y. Avni,et al.  Energy spectra of X-ray clusters of galaxies , 1976 .