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Laser Interferometer Space Antenna double black holes: dynamics in gaseous nuclear discs
We study the inspiral of double black holes, with masses in the LaserInterferometer Space Antenna (LISA) window of detectability, orbitinginside a massive circumnuclear, rotationally supported gaseous disc.Using high-resolution smoothed particle hydrodynamics simulations, wefollow the black hole dynamics in the early phase when gas-dynamicalfriction acts on the black holes individually, and continue oursimulation until they form a close binary. We find that in the earlysinking the black holes lose memory of their initial orbitaleccentricity if they corotate with the gaseous disc. As a consequence,the massive black holes bind forming a binary with a low eccentricity,consistent with zero within our numerical resolution limit. The cause ofcircularization resides in the rotation present in the gaseousbackground where dynamical friction operates. Circularization may hindergravitational waves from taking over and leading the binary tocoalescence. In the case of counter-rotating orbits, the initialeccentricity (if present) does not decrease, and the black holes maybind forming an eccentric binary. When dynamical friction has subsided,for equal mass black holes and regardless their initial eccentricity,angular momentum loss, driven by the gravitational torque exerted on thebinary by surrounding gas, is nevertheless observable down to thesmallest scale probed (~=1 pc). In the case of unequal masses, dynamicalfriction remains efficient down to our resolution limit, and there is nosign of formation of any ellipsoidal gas distribution that may furtherharden the binary. During inspiral, gravitational capture of gas by theblack holes occurs mainly along circular orbits; eccentric orbits implyhigh relative velocities and weak gravitational focusing. Thus, theactive galactic nucleus activity may be excited during the black holepairing process and double active nuclei may form when circularizationis completed, on distance scales of tens of parsecs.
| Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| The possible detection of high-redshift Type II QSOs in deep fields
The colours of high-redshift Type II quasi-stellar objects (QSOs) aresynthesized from observations of moderate-redshift systems. It is shownthat Type II QSOs are comparable to starbursts at matching the coloursof z850-dropouts and i775-drops in the HubbleUltraDeep Field, and more naturally account for the bluest objectsdetected. Type II QSOs may also account for some of thei775-drops detected in the Great Observatories Origins DeepSurvey (GOODS) fields. It is shown that by combining imaging data fromthe Hubble Space Telescope and the James Webb Space Telescope, it willbe possible to clearly separate Type II QSOs from Type I QSOs andstarbursts based on their colours. Similarly, it is shown that theUnited Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey(UKIDSS) ZYJ filters may be used to discriminate high-redshift Type IIQSOs from other objects. If Type II QSOs are prevalent at highredshifts, then active galactic nuclei (AGNs) may be major contributorsto the re-ionization of the intergalactic medium.
| Unveiling the nature of Ultraluminous Infrared Galaxies with 3-4μm spectroscopy*
We present the results of L-band spectroscopical observations of localbright Ultraluminous Infrared Galaxies (ULIRGs), performed with theInfrared Spectrometer And Array Camera (ISAAC) at the Very LargeTelescope. The excellent sensitivity of the telescope and of theinstrument provided spectra of unprecedented quality for this class ofobjects, which allowed a detailed study of the active galactic nucleus(AGN)/starburst contribution to the energy output, and of thecomposition of the circumnuclear absorber. We discuss the L-bandspectral features of seven single sources, and the statisticalproperties of a complete sample of 15 sources obtained combining ourobservations with other published 3-4 μm spectra. Our main resultsare as follows. (i) When a spectral indicator suggesting the presenceof an AGN (low equivalent width of the 3.3-μm emission line, steepλ-fλ spectrum, presence of an absorptionfeature at 3.4 μm) is found, the AGN is always confirmed byindependent analysis at other wavelengths. Conversely, in all known AGNsat least one of the above indicators is present. (ii) Two newdiagnostic diagrams are proposed combining the above indicators, inwhich starbursts and AGNs are clearly and completely separated. (iii)The above diagnostic techniques are possible with spectra of relativelylow quality, which can be obtained for several tens of ULIRGs withcurrently available telescopes. This makes L-band spectroscopy thecurrent best tool to disentangle AGN and starburst contributions inULIRGs. (iv) The L-band properties of ULIRGs are heterogeneous.However, we show that all the spectral differences among ULIRGs can bereproduced starting from pure intrinsic AGN and starburst spectra andtwo varying parameters: the amount of dust extinction of the AGNcomponent, and the relative AGN/starburst contribution to the bolometricluminosity. (v) Using the above decomposition model, we show that AGNsin ULIRGs have a low dust-to-gas ratio and a dust extinction curvedifferent from Galactic. (vi) The estimate of the presence andcontribution of AGNs in a complete sample shows that AGNs are hosted byapproximately two-thirds of ULIRGs, but their energetic contribution isrelevant (>30 per cent of the bolometric luminosity) only in ~20 percent of the sample.
| A Unified, Merger-driven Model of the Origin of Starbursts, Quasars, the Cosmic X-Ray Background, Supermassive Black Holes, and Galaxy Spheroids
We present an evolutionary model for starbursts, quasars, and spheroidalgalaxies in which mergers between gas-rich galaxies drive nuclearinflows of gas, producing starbursts and feeding the buried growth ofsupermassive black holes (BHs) until feedback expels gas and renders abriefly visible optical quasar. The quasar lifetime and obscuring columndensity depend on both the instantaneous and peak quasar luminosity, andwe determine this dependence using a large set of galaxy mergersimulations varying galaxy properties, orbital geometry, and gasphysics. We use these fits to deconvolve observed quasar luminosityfunctions and obtain the evolution of the formation rate of quasars withpeak luminosity, n dot(Lpeak,z). Quasars spend extendedperiods at luminosities well below peak, so n dot(Lpeak) hasa maximum corresponding to the ``break'' in the observed luminosityfunction. From n dot(Lpeak) and our simulations, we obtainself-consistent hard and soft X-ray and optical luminosity functions andpredict many observables at multiple redshifts, including column densitydistributions of optical and X-ray samples, the luminosity function ofbroad-line quasars in X-ray samples and broad-line fraction versusluminosity, active BH mass functions, the distribution of Eddingtonratios, the mass function of relic BHs and total BH mass density, andthe cosmic X-ray background. In every case, our predictions agree wellwith observed estimates, without invoking ad hoc assumptions aboutsource properties or distributions. We provide a library of Monte Carlorealizations of our models for comparison with observations.
| The Star-forming Torus and Stellar Dynamical Black Hole Mass in the Seyfert 1 Nucleus of NGC 3227
We report R~4300 VLT SINFONI adaptive optics integral field K-bandspectroscopy of the nucleus of the Seyfert 1 galaxy NGC 3227 at aspatial resolution of 0.085" (7 pc). We present the morphologies andkinematics of emission lines and absorption features and give the firstderivation of a black hole mass in a Seyfert 1 nucleus from stellardynamics (marginally resolving the black hole's sphere of influence). Weshow that the gas in the nucleus has a mean column density of order1024 cm-2 and that it is geometrically thick, inagreement with the standard ``molecular torus'' scenario. We discusspossible heating processes responsible for maintaining the verticalheight of the torus. We also resolve the nuclear stellar distributionand find that within a few parsecs of the AGN there has been an intensestarburst, the most recent episode of which began ~40 Myr ago but hasnow ceased. The current luminosity of stars within 30 pc of the AGN,~3×109 Lsolar, is comparable to that of theAGN. We argue that the star formation has been occurring in theobscuring material. Finally, we apply Schwarzschild orbit superpositionmodels to our full two-dimensional data and derive the mass of the blackhole, paying careful attention to the input parameters, which are oftenuncertain. Our models yield a 1 σ range for the black hole mass ofMBH=7×106-2×107Msolar.Based on observations at the European Southern Observatory VLT(074.B-9012).
| A Compact Supermassive Binary Black Hole System
We report on the discovery of a supermassive binary black hole system inthe radio galaxy 0402+379, with a projected separation between the twoblack holes of just 7.3 pc. This is the closest black hole pair yetfound by more than 2 orders of magnitude. These results are based onrecent multifrequency observations using the Very Long Baseline Array(VLBA), which reveal two compact, variable, flat-spectrum, active nucleiwithin the elliptical host galaxy of 0402+379. Multiepoch observationsfrom the VLBA also provide constraints on the total mass and dynamics ofthe system. Low spectral resolution spectroscopy using the Hobby-EberlyTelescope indicates two velocity systems with a combined mass of the twoblack holes of ~1.5×108 Msolar. The twonuclei appear stationary, while the jets emanating from the weaker ofthe two nuclei appear to move out and terminate in bright hot spots. Thediscovery of this system has implications for the number of close binaryblack holes that might be sources of gravitational radiation. Green BankTelescope observations at 22 GHz to search for water masers in thisinteresting system are also presented.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Imaging Molecular Gas in the Luminous Merger NGC 3256: Detection of High-Velocity Gas and Twin Gas Peaks in the Double Nucleus
Molecular gas in the merging starburst galaxy NGC 3256 has been imagedwith the Submillimeter Array at a resolution of1''×2'' (170×340 pc at 35 Mpc). Thisis the first interferometric imaging of molecular gas in the mostluminous galaxy within z=0.01. There is a large disk of molecular gas(r>3 kpc) in the center of the merger with a strong gas concentrationtoward the double nucleus. The gas disk having a mass of~3×109 Msolar in the central 3 kpc rotatesaround a point between the two nuclei that are 850 pc apart on the sky.The molecular gas is warm and turbulent and shows spatial variation ofthe intensity ratio between CO isotopomers. High-velocity molecular gasis discovered at the galactic center. Its velocity in our line of sightis up to 420 km s-1 offset from the systemic velocity of thegalaxy; the terminal velocity is twice as large as that due to therotation of the main gas disk. The high-velocity gas is most likely dueto a molecular outflow from the gas disk, entrained by thestarburst-driven superwind in the galaxy. The molecular outflow isestimated to have a rate of ~10 Msolar yr-1 and toplay a significant role in the dispersal or depletion of molecular gasfrom the galactic center. A compact gas concentration and steep velocitygradient are also found around each of the twin nuclei. They aresuggestive of a small gas disk rotating around each nucleus. If theseare indeed minidisks, their dynamical masses are ~109Msolar within a radius of 170 pc.
| Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations
We use very deep Spitzer MIPS 24 μm observations to examine thebolometric luminosities (Lbol) and UV extinction propertiesof more than 200 spectroscopically identified, optically selected(UnGR) z~2 galaxies, supplemented with near-IR-selected(``BzK'' and ``DRG'') and submillimeter galaxies at similar redshifts,in the GOODS-N field. Focusing on redshifts 1.51012 Lsolar, with a mean~=2×1011 Lsolar. Using24 μm observations as an independent probe of dust extinction, wefind that, as in the local universe, the obscurationLIR/L1600 is strongly dependent on Lboland ranges in value from <1 to ~1000 within the sample considered.However, the obscuration is generally ~10 times smaller at a givenLbol at z~2 than at z~0. We show that the values ofLIR and obscuration inferred from the UV spectral slopeβ generally agree well with the values inferred fromL5-8.5μm for Lbol<1012Lsolar. Using the specific SFRs of galaxies as a proxy forcold gas fraction, we find a wide range in the evolutionary state ofgalaxies at z~2, from galaxies that have just begun to form stars tothose that have already accumulated most of their stellar mass and areabout to become, or already are, passively evolving.Based, in part, on data obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation. Alsobased in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.
| Chandra and Spitzer Unveil Heavily Obscured Quasars in the Chandra/SWIRE Survey
Using the large multiwavelength data set in the Chandra/SWIRE Survey(0.6 deg2 in the Lockman Hole), we show evidence for theexistence of highly obscured (Compton-thick) AGNs, estimate a lowerlimit to their surface density, and characterize their multiwavelengthproperties. Two independent selection methods based on the X-ray andinfrared spectral properties are presented. The two selected samplescontain (1) five X-ray sources with hard X-ray spectra and columndensities >~1024 cm-2 and (2) 120 infraredsources with red and AGN-dominated infrared SEDs. We estimate a surfacedensity of at least 25 Compton-thick AGNs deg-2 detected inthe infrared in the Chandra/SWIRE field, of which ~40% show distinct AGNsignatures in their optical/near-infrared SEDs, the remaining beingdominated by the host galaxy emission. Only ~33% of all Compton-thickAGNs are detected in the X-rays at our depth [F(0.3-8keV)>10-15 ergs cm-2 s-1]. We reportthe discovery of two sources in our sample of Compton-thick AGNs, SWIREJ104409.95+585224.8 (z=2.54) and SWIRE J104406.30+583954.1 (z=2.43),which are the most luminous Compton-thick AGNs at high z currentlyknown. The properties of these two sources are discussed in detail withan analysis of their spectra, SEDs, luminosities, and black hole masses.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. Based on observations at the Kitt Peak National Observatory,National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation. The NationalRadio Astronomy Observatory is a facility of the National ScienceFoundation operated under a cooperative agreement by AssociatedUniversities, Inc.
| Mid-Infrared Spitzer Spectra of X-Ray-Selected Type 2 QSOs: QSO2s Are Not Ultraluminous Infrared Galaxies
We have performed a spectroscopic study of seven Type 2 QSOs using themid-infrared spectrometer IRS on board the Spitzer Space Telescope.These are (to our knowledge) the first mid-IR spectra of X-ray-selectedQSO2s taken. The objects have been selected according to their highintrinsic luminosities and column densities in X-rays. Their spectrastrongly differ from template spectra of Type 2 active galactic nuclei(AGNs) at lower luminosities. They do not exhibit strong PAH dustemission features from circumnuclear star-forming regions, typical forlower luminosity Type 2 Seyfert galaxies or other previously used QSO2templates, such as the (ultra)luminous infrared galaxy ([U]LIRG) NGC6240. They also do not show the ice and silicate absorption features ofhighly luminous but deeply embedded compact nuclei seen in some ULIRGs.Instead, they reveal a relatively featureless, rising continuum similarto luminous Type 1 AGNs. We also find evidence for a 10 μm silicatefeature in emission. Models of dusty tori in the AGN unificationscenario predict this only for Type 1 AGNs. The ratio of the AGNcontinuum luminosity at 6 μm to the absorption-corrected 2-10 keVX-ray AGN luminosity is very similar to that found in Seyfert galaxies.X-ray-selected QSO2s are thus characterized by powerful AGNs in hostswith a luminosity due to star formation <~1011Lsolar. The dominance of the AGN light in the mid-IR spectraof QSO2s together with their flatter spectral energy distributions(SEDs) places important constraints on models of the cosmic infraredbackground and of the star formation history of the universe.
| Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies?
We present the results of the first HCO+ survey probing thedense molecular gas content of a sample of 16 luminous and ultraluminousinfrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN (1-0)observations, had shown that LIRGs and ULIRGs possess a significantlyhigher fraction of dense molecular gas compared to normal galaxies.While the picture issued from HCO+ partly confirms thisresult, we have discovered an intriguing correlation between the HCN(1-0)/HCO+ (1-0) luminosity ratio and the IR luminosity ofthe galaxy (LIR). This trend casts doubts on the use of HCNas an unbiased quantitative tracer of the dense molecular gas content inLIRGs and ULIRGs. A plausible scenario explaining the observed trendimplies that X-rays coming from an embedded active galactic nucleus mayplay a dominant role in the chemistry of molecular gas atLIR>=1012 Lsolar. We discuss theimplications of this result for the understanding of LIRGs, ULIRGs, andhigh-redshift gas-rich galaxies.
| Detection of the Buried Active Galactic Nucleus in NGC 6240 with the Infrared Spectrograph on the Spitzer Space Telescope
We present mid-infrared spectra of the nearby ultraluminous infraredgalaxy NGC 6240 taken with the Infrared Spectrograph (IRS) on theSpitzer Space Telescope. The spectrum of NGC 6240 is dominated by strongfine-structure lines, rotational H2 lines, and polycyclicaromatic hydrocarbon (PAH) emission features. The H2 linefluxes suggest molecular gas at a variety of temperatures. A simpletwo-temperature fit to the S(0) through S(7) lines implies a mass of~6.7×106 Msolar at T~957 K and~1.6×109 Msolar at T~164 K, or about 15% ofthe total molecular gas mass in this system. Notably, we have detectedthe [Ne V] 14.3 μm emission line, with a flux of5×10-14 ergs cm-2 s-1, providingthe first direct detection of the buried active galactic nucleus (AGN)in the mid-infrared. Modeling of the total spectral energy distribution(SED) from near- to far-infrared wavelengths requires the presence of ahot dust (T~700 K) component, which we also associate with the buriedAGN. The small [Ne V]/[Ne II] and [Ne V]/IR flux ratios, the relativefraction of hot dust emission, and the large 6.2 μm PAH equivalentwidth (EQW), are all consistent with an apparent AGN contribution ofonly 3%-5% to the bolometric luminosity. However, correcting themeasured [Ne V] flux by the extinction implied by the silicate opticaldepth and our SED fitting suggests an intrinsic fractional AGNcontribution to the bolometric luminosity of ~20%-24% in NGC 6240, whichlies within the range implied by fits to the hard X-ray spectrum.Based on observations obtained with the Spitzer Space Telescope, whichis operated by the Jet Propulsion Laboratory, California Institute ofTechnology, under NASA contract 1407.
| Powerful High-Velocity Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's Quintet
We present the discovery of strong mid-infrared emission lines ofmolecular hydrogen of apparently high-velocity dispersion (~870 kms-1) originating from a group-wide shock wave in Stephan'sQuintet. These Spitzer Space Telescope observations reveal emissionlines of molecular hydrogen and little else. This is the first time analmost pure H2 line spectrum has been seen in anextragalactic object. Along with the absence of PAH-dust features andvery low excitation ionized gas tracers, the spectra resemble shockedgas seen in Galactic supernova remnants, but on a vast scale. Themolecular emission extends over 24 kpc along the X-ray-emitting shockfront, but it has 10 times the surface luminosity as the soft X-rays andabout one-third the surface luminosity of the IR continuum. We suggestthat the powerful H2 emission is generated by the shock wavecaused when a high-velocity intruder galaxy collides with filaments ofgas in the galaxy group. Our observations suggest a close connectionbetween galaxy-scale shock waves and strong broad H2 emissionlines, like those seen in the spectra of ultraluminous infrared galaxieswhere high-speed collisions between galaxy disks are common.
| Evolution of the Circumnuclear Radio Supernova SN 2000ft in NGC 7469
SN 2000ft is the first radio supernova detected in the circumnuclearstarburst of a luminous infrared Seyfert 1 galaxy. It is located at adistance of 600 pc from the QSO-like nucleus of NGC 7469. We report thetemporal evolution of SN 2000ft during the 3 years after its discovery.Although SN 2000ft has exploded in the dusty and very dense environmentthat exists in the nuclear regions of luminous infrared galaxies, itshows the radio evolution properties characteristic of radio supernovaeidentified as Type II supernovae, aside from some foreground free-freeabsorption. The peak luminosity and circumstellar matter opacity of SN2000ft are similar to other compact radio sources detected in luminousinfrared galaxies such as NGC 6240, Arp 299, and Arp 220 and identifiedas Type II supernovae.
| Dynamical Properties of Ultraluminous Infrared Galaxies. I. Mass Ratio Conditions for ULIRG Activity in Interacting Pairs
We present first results from our Very Large Telescope large program tostudy the dynamical evolution of ultraluminous infrared galaxies(ULIRGs), which are the products of mergers of gas-rich galaxies. Thefull data set consists of high-resolution long-slit H- and K-bandspectra of 38 ULIRGs and 12 QSOs (in the range 0.0423:1 typically do not force enough gas into the center togenerate ULIRG luminosities.
| The Double Active Galactic Nucleus in NGC 6240 Revealed through 3-5 μm Spectroscopy
We present 3-5 μm spectroscopy of the interacting system NGC 6240,revealing the presence of two active galactic nuclei (AGNs). Thebrightest (southern) nucleus shows up with a starburst-like emission,with a prominent 3.3 μm emission feature. However, the presence of anAGN is revealed by the detection of a broad Brα emission line,with a width of ~1800 km s-1. The spectrum of the faintest(northern) nucleus shows typical AGN features, such as a steep continuumand broad absorption features in the M band. We discuss the physicalproperties of the dusty absorbers/emitters, and we show that in bothnuclei, the AGN is dominant in the 3-5 μm band but that itscontribution to the total luminosity is small (a few percent of thestarburst emission).Based on observations collected at the European Southern Observatory,Chile (proposal 73.B-0574).
| MAMBO 1.2 mm Observations of BzK-selected Star-forming Galaxies at z ~ 2
We present MAMBO 1.2 mm observations of five BzK-preselected vigorousstarburst galaxies at z~2. Two of these were detected at more than 99.5%confidence levels, with 1.2 mm fluxes around 1.5 mJy. These millimeterfluxes imply vigorous activity with star formation rates (SFRs) of~500-1500 Msolar yr-1, which were also confirmedby detections at 24 μm with the Multiband Imaging Photometer onSpitzer (MIPS). The two detected galaxies are the ones in the samplewith the highest SFRs estimated from the rest-frame UV, and theirfar-IR-derived and UV-derived SFRs agree reasonably well. This isdifferent from local ultraluminous infrared galaxies (ULIRGs) and high-zsubmillimeter/millimeter-selected galaxies, for which the UV is reportedto underestimate SFRs by factors of 10-100, but similar to the averageBzK ULIRG at z~2. The two galaxies detected at 1.2 mm are brighter in Kthan the typical NIR counterparts of MAMBO and SCUBA sources, implying asignificantly different K-band-to-submillimeter/millimeter flux ratio aswell. This suggests a scenario in which z~2 galaxies, after their rapid(sub)millimeter-bright phase that is opaque to optical/UV light, evolveinto a longer lasting phase of K-band-bright and massive objects.Targeting the most UV-active BzK galaxies could yield substantialdetection rates at submillimeter/millimeter wavelengths.Based on observations carried out with the IRAM 30 m Telescope at PicoVeleta. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN(Spain). Also based on ESO observations (program IDs 072.A-0506 and075.A-0439) and on Subaru observations (programs S02B-101 and S04A-081),and on observations made with the Spitzer Space Telescope, which isoperated by the JPL, Caltech, under a contract with NASA.
| AGN or starburst powered? Untangling the pan-spectral SEDs of the high-redshift radio galaxies and sub-mm galaxies
Here we show how theoretical pan-spectral modelling is beginning todecode the physical parameters of the interstellar medium of starburstand high-redshift galaxies, is revealing the star formation rate,identifying the bolometric contribution due to an embedded AGN and isdetermining the attenuation of the galaxian light due to foregrounddust.
| Millimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Markarian 273, and IRAS 17208-0014, Based on HCN-to-HCO+ Ratios
We present interferometric observations of three ultraluminous infraredgalaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3 mmwavelength range, using the Nobeyama Millimeter Array. Both the HCN(J=1-0) and HCO+ (J=1-0) molecular lines were observedsimultaneously. HCN emission was clearly detected at the nuclearpositions of these ULIRGs, and HCO+ emission was detected atthe nuclear positions of UGC 5101 and IRAS 17208-0014. TheHCN-to-HCO+ brightness-temperature ratios toward the nucleiof the three ULIRGs were derived and compared with those of lowerluminosity galaxies known to be dominated by active galactic nuclei(AGNs) or starbursts. In UGC 5101 and Mrk 273, where there is evidencefor obscured AGNs from previous observations at other wavelengths, wefound high HCN/HCO+ ratios (>1.8) that are in the rangefound for AGN-dominated galaxies. In IRAS 17208-0014, where the presenceof a powerful obscured AGN has been unclear, the ratio (~1.7) is betweenthe observed values for starburst- and AGN-dominated galaxies. The highHCN/HCO+ brightness-temperature ratios in UGC 5101 and Mrk273 could be the consequence of an HCN abundance enhancement, which isexpected from chemical effects of the central X-ray-emitting AGN on thesurrounding dense molecular gas. Our proposed millimeter interferometricmethod based on HCN/HCO+ ratios may be an effective tool forunveiling elusive buried AGNs at the cores of ULIRGs, especially becauseof the negligible dust extinction at these wavelengths.
| Subarcsecond Mid-Infrared Observations of NGC 6240: Limitations of Active Galactic Nucleus-Starburst Power Diagnostics
In order to examine the relative importance of powerful starbursts andCompton-thick active galactic nuclei (AGNs) in NGC 6240, we haveobtained mid-infrared images and low-resolution spectra of the galaxywith subarcsecond spatial resolution using the Keck telescopes. Despitethe high spatial resolution (~200 pc) of our data, no signature of thehidden AGNs has been detected in the mid-infrared. The southern nucleus,which we show provides 80%-90% of the total 8-25 μm luminosity of thesystem, has a mid-infrared spectrum and a mid-/far-infrared spectralenergy distribution consistent with starbursts. At the same time,however, it is also possible to attribute up to 60% of the bolometricluminosity to an AGN, consistent with X-ray observations, if the AGN isheavily obscured and emits mostly in the far-infrared. This ambiguityarises because the intrinsic variation of properties among a givengalaxy population (e.g., starbursts) introduces at least a factor of afew uncertainty even into the most robust AGN-starburst diagnostics. Weconclude that with present observations it is not possible to determinethe dominant power source in galaxies when AGN and starburstluminosities are within a factor of a few of each other.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| PKS 2149-306 and CXOCDFS J033225.3-274219: Two AGNs with Unusual Spectra Possibly Blueshifted?
PKS 2149-306 and CXOCDFS J033225.3-274219 each exhibit several emissionlines in their optical spectra from which their redshifts have beendetermined. However, an ASCA spectrum of the first object has detectedan emission line at 5 keV observed wavelength, and the Chandra ACIS-Ispectrum of the second object shows a very strong emission feature atthe observed wavelength of 6.2 keV. The two lines cannot be identifiedwith any known search line on the basis of their redshift values withoutinvoking extremely large outflow bulk velocities of 0.6c-0.75c. Suchhigh bulk velocities are practically unheard of in any otherobservations of extragalactic objects. We show here that all the linesin both the optical and X-ray spectra of the two objects can beidentified with lines at longer wavelengths that are blueshifted. Wepropose a scenario in which the spectrum is blueshifted due to netapparent motion toward the observer, resulting from an ejection.
| Examining the Seyfert-Starburst Connection with Arcsecond-Resolution Radio Continuum Observations
We compare the arcsecond-scale circumnuclear radio continuum propertiesof five Seyfert and five starburst galaxies, concentrating on the searchfor any structures that could imply a spatial or causal connectionbetween the nuclear activity and a circumnuclear starburst ring. Noevidence is found in the radio emission for a link between thetriggering or feeding of nuclear activity and the properties ofcircumnuclear star formation. Conversely, there is no clear evidence ofnuclear outflows or jets triggering activity in the circumnuclear ringsof star formation. Interestingly, the difference in the angle betweenthe apparent orientation of the most elongated radio emission and theorientation of the major axis of the galaxy is on average larger inSeyfert galaxies than in starburst galaxies, and Seyfert galaxies appearto have a larger physical size scale of the circumnuclear radiocontinuum emission. The concentration, asymmetry, and clumpinessparameters of radio continuum emission in Seyfert galaxies andstarbursts are comparable, as are the radial profiles of radio continuumand near-infrared line emission. The circumnuclear star formation andsupernova rates do not depend on the level of nuclear activity. Theradio emission usually traces the near-infrared Brγ andH2 1-0 S(1) line emission on large spatial scales, butlocally their distributions are different, most likely because of theeffects of varying local magnetic fields and dust absorption andscattering.
| SINFONI adaptive optics integral field spectroscopy of the Circinus Galaxy
Aims.We investigate the star formation activity and the gas and stellardynamics on scales of a few parsecs in the nucleus of the CircinusGalaxy. Methods: .Using the adaptive optics near infrared integralfield spectrometer SINFONI on the VLT, we have obtained observations ofthe Circinus galaxy on scales of a few parsecs and at a spectralresolution of 70 km s-1 FWHM. The physical properties of thenucleus are analyzed by means of line and velocity maps extracted fromthe SINFONI datacube. Starburst models are constrained using theBrγ flux, stellar continuum (as traced via the CO absorptionbandheads longward of 2.3 μm), and radio continuum. Results:.The similarity of the morphologies of the H2 1-0 S(1) 2.12 μm andBrγ 2.17 μm lines to the stellar continuum and also theirkinematics, suggest a common origin in star formation. Within 8 pc ofthe AGN we find there has been a recent starburst in the last 100 Myr,which currently accounts for 1.4% of the galaxy's bolometric luminosity.The similarity of the spatial scales over which the stars and gas existindicates that this star formation is occuring within the torus; andcomparison of the gas column density through the torus to the maximumpossible optical depth to the stars implies the torus is a clumpymedium. The coronal lines show asymmetric profiles with a spatiallycompact narrow component and a spatially extended blue wing. Thesecharacteristics are consistent with some of the emission arising inclouds gravitationally bound to the AGN, and some outflowing incloudlets which have been eroded away from the bound clouds.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| Large-scale molecular shocks in galaxies: the SiO interferometer map of IC 342
We present the first high-resolution (5.6 arcsec×5.1 arcsec )images of the emission of silicon monoxide (SiO) in the nucleus of thenearby spiral IC 342, obtained with the IRAM Plateaude Bure Interferometer (PdBI). Using a two-field mosaic, we havesimultaneously mapped the emission of the SiO(v=0, J=2-1) andH13CO+(J=1-0) lines in a region of 0.9 kpc× 1.3 kpc (RA × Dec) centered around the nucleus ofIC 342. The bulk of the emission in the two linescomes from a 290 pc spiral arm located to the North and a centralcomponent that forms the southern ridge of a {r80} pc nuclear ringthat was identified in other interferometer maps of the galaxy. Wedetect continuum emission at 86.8 GHz in a 80-180 pc centralsource. The continuum emission, dominated by thermal free-freebremsstrahlung, is mostly anticorrelated with the observed distributionof SiO clouds. The SiO-to-H13CO+ intensity ratiois seen to increase by an order of magnitude from the nuclear ring (0.3) to the spiral arm ( 3.3). Furthermore the gas kinematics showsignificant differences between SiO and H13CO+over the spiral arm, where the linewidths of SiO are a factor of 2larger than those of H13CO+. The average abundanceof SiO in the inner {r320} pc of IC 342 isX(SiO) ≳2×10-10. This shows that shock chemistryis at work in the inner molecular gas reservoir of IC342. To shed light on the nature of shocks in IC342, we have compared the emission of SiO with another tracerof molecular shocks: the emission of methanol (CH3OH). We find that thesignificant difference of the abundance of SiO measured between thespiral arm (X(SiO) a few 10-9) and the nuclear ring (X(SiO) 10-10) is not echoed by a comparable variation in theSiO-to-CH3OH intensity ratio. This implies that the typical shockvelocities should be similar in the two regions. In contrast, thefraction of shocked molecular gas should be 5-7 times larger in thespiral arm (up to 10% of the available molecular gas mass over the armregion) compared to the nuclear ring. In the light of these results, werevise the validity of the various scenarios that have been proposed toexplain the onset of shock chemistry in galaxies and study theirapplicability to the nucleus of IC 342. We concludethat the large-scale shocks revealed by the SiO map of IC342 are mostly unrelated to star formation and arise insteadin a pre-starburst phase. Shocks are driven by cloud-cloud collisionsalong the potential well of the IC 342 bar. Thegeneral implications for the current understanding of galaxy evolutionare discussed.
| NGC 7679: an anomalous, composite Seyfert 1 galaxy whose X-ray luminous AGN vanishes at optical wavelengths
Morphological disturbances and gas kinematics of the SB0 galaxy NGC 7679= Arp 216 are investigated to understand the history of this highlycomposite object, where AGN and starburst signatures dominate in theX-ray and optical/IR regime, respectively. Perturbations of the ionizedgas velocity field appear quite mild within 15'' (~5 kpc) of the center,so it can be straightforwardly modeled as a circularly rotating disk.Outside that radius, significant disturbances are seen. In particular,the eastern distorted arm as well as the huge neutral hydrogen bridgeconnecting NGC 7679 to the nearby Seyfert spiral NGC 7682 unambiguouslyrepresent the vestige of a close encounter of the two objects ~500 Myrago. The relationship of such a past event with the much more recent,centrally located starburst (not older than 20 Myr) cannot be easilyestablished. Together, the classification of NGC 7679 is less extremethan that proposed in the past, being simply a (disturbed) galaxy wherestarburst and AGN activity coexist with a starburst dominating thebolometric luminosity.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
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