Symposium Proceedings

Energy Cycles and Outflows

The talks are in the same order as the Program Schedule.

Key Results in X-ray Spectroscopy of AGN

Jane Turner (UMBC/GSFC)

I review some of the important results gained from grating spectroscopy of AGN with Chandra and XMM. I focus on results that have provided new, and sometimes unexpected insight into the environs of supermassive black holes.

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The First Chandra Field: The Discovery of "Leon X-1"

Martin Weisskopf (NASA/MSFC), Thomas L. Aldcroft (SAO), Robert Cameron (SLAC), Poshak Gandhi, Cedric Foellmi (ESO), Ronald Elsner, Sandeep Patel (NASA/MSFC), Kinwah Wu (MSSLK, University College London), Stephen O'Dell (NASA/MSFC)

Before the official first-light images, the Chandra X-Ray Observatory obtained an X-ray image of the field to which its focal plane was first exposed. We describe this historic observation and report our study of the first Chandra field. Chandra's Advanced CCD Imaging Spectrometer (ACIS) detected 15 X-ray sources, the brightest being dubbed Leon X-1 to honor the Chandra Telescope Scientist, Leon Van Speybroeck. Based upon our analysis of the X-ray data and spectroscopy at the European Southern Observatory (ESO; La Silla, Chile), we find that Leon X-1 is a Type-1 (unobscured) active galactic nucleus (AGN) at a redshift z=0.3207. Leon X-1 exhibits strong Fe II emission and a broad-line Balmer decrement that is unusually flat for an AGN. Within the context of the Eigenvector-1 correlation space, these properties suggest that Leon X-1 may be a massive ($\ge 10^{9}\,M_{\odot}$) black hole, accreting at a rate approaching its Eddington limit.

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X-ray Properties of Intermediate-mass Black Holes in Active Galaxies

Jenny E. Greene (Harvard-Smithsonian Center for Astrophysics), Luis C. Ho (Carnegie Observatories)

We present Chandra observations of the only uniformly-selected sample of intermediate-mass ( $< 10^{6}\,M_{\odot}$) black holes (BHs) in active galactic nuclei (AGNs). This sample probes a new regime in BH mass, host galaxy morphology, and AGN demographics. The sample also currently provides the best observational constraints on the mass spectrum and spectral properties of primordial seed BHs. Remarkably, the objects appear to obey the M-sigma relation established locally for more massive systems, suggesting physical continuity over many orders of magnitude in BH mass. By selection, these systems radiate close to their Eddington luminosities, and thus we explicitly compare their properties to those of narrow-line Seyfert 1s. In conjunction with radio constraints from the VLA, the Chandra observations allow us to explore the broad spectral energy distributions for this unique sample.

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A Bridge from Sgr A* to LLAGN: Results from a 300ks Simultaneous Multiwavelength Campaign on M81* with the Chandra HETGS

Sera Markoff, Claude Canizares (MIT Kavli Institute), Geoff Bower (UC Berkeley), Poonam Chandra (TIFR), C. C. Cheung (MIT Kavli Institute), Sarah Gallagher (UCLA), Sebastian Heinz, Mario Jimenez-Garate, Herman Marshall (MIT Kavli Institute), Michael Muno (UCLA), Michael Nowak (MIT Kavli Institute), Alison Peck (Harvard-Smithsonian Center for Astrophysics), Alak Ray (TIFR), Rainer Schoedel (Uni Koeln), Andy Young (MIT Kavli Institute)

Sgr A* has been the focus of many fruitful multiwavelength campaigns using *Chandra* as well as several other instruments. It is the lowest luminosity (in Eddington units) black hole that we can currently study with high statistics, and is thus a valuable testbed for theories of black hole accretion. There are several unanswered questions regarding Sgr A* beyond its weak emission, such as what is the exact nature of its accretion flow, why does it show regular X-ray flares, and how does it relate to "normal" LLAGN? M81* offers itself as a unique bridge source: it is also the weak nucleus of a spiral galaxy, with a 20 times greater mass, and it shares very similar radio spectra and polarization properties with Sgr A*. Yet M81* is four orders of magnitude brighter in the radio, has significant nonthermal X-ray emission, and does have resolved jets. In this talk I will present some of the initial results from our 2005 campaign on M81*, using $\sim$300ks GTO time on the *Chandra HETGS*. We have obtained the first gratings-resolution X-ray spectrum of an isolated LLAGN nucleus to date, the details of which will be presented in another talk at this meeting. Here I will focus on the broadband continuum. The first 50ks were successfully coordinated with radio (GMRT, VLA), mm-submm (SMA, IRAM PdBI) and NIR (Lick w/laser guide star AO) and the rest had partial coverage by the GMRT, VLA/VLBA, SMA and PdBI. I will discuss both the long term variability and theoretical modeling, and compare it to what we see in Sgr A* and other LLAGN.

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Radio Galaxies, Jets, and their Environments

Diana Worrall (Univ. of Bristol)

Jet physics is again flourishing as a result of Chandra's ability to resolve high-energy emission from the radio-emitting structures of active galaxies and separate it from the X-ray-emitting thermal environments of the jets. I will review the progress that has been made in answering several important questions, including: Are the radio structures in a state of minimum energy? How fast are jets? What keeps them collimated? What type of plasma produces the jet structures we observe -- electron-positron or electron proton? Where and how does particle acceleration occur? Which structures are dynamical and which have reached equilibrium? How is jet energy transferred to the surrounding medium?

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Deceleration in AGN Jets

Rita Sambruna (GFSC), M. Gliozzi, D. Donato (GMU), L. Maraschi (Oss Brera), F. Tavecchio (Oss Merate), C. C. Cheung (MIT), C. M. Urry (Yale), J. F. Wardle (Brandeis)

We present deep (70-80 ks) Chandra and multicolor HST ACS images of two jets hosted by the powerful quasars 1136-135 and 1150+497, together with new radio observations. The sources have an FRII morphology and were selected from our previous X-ray and optical jet survey for detailed follow up aimed at obtaining better constraints on the jet multiwavelength morphology and X-ray and optical spectra of individual knots, and to test emission models deriving physical parameters more accurately. All the X-ray and optical knots detected in our previous short exposures are confirmed, together with a few new faint features. The jet radial profiles show good correspondence between the knots at the various wavelengths; a few show offsets between the knots peaks of $<$1 arcsec. In 1150+497 the X-ray, optical, and radio profiles decrease in similar ways with distance from the core up to $\sim$ 7 arcsec, after which the radio emission increases more than the X-ray one. No X-ray spectral variations are observed in 1150+497. In 1136-135 an interesting behavior is observed, whereby, downstream of the most prominent knot at $\sim$ 6.5 arcsec from the core, the X-ray emission fades while the radio emission brightens. The X-ray spectrum also varies, with the X-ray photon index flattening from $\Gamma_x \sim 2$ in the inner part to $\Gamma_x \sim 1.7$ to the end of the jet. We intepret the jet behavior in 1136-135 in a scenario where the relativistic flow suffers systematic deceleration along the jet, and briefly discuss the major consequences of this scenario. Moreover, we find evidence for faint, soft intra-knot X-ray emission, and quantify its contribution to the X-ray flux of individual knots. The origin of this emission is addressed.

Chandra Deep Imaging of a Large Scale Jet in the z=1 Quasar PKS 1127-145

Aneta Siemiginowska, Thomas L. Aldcroft (SAO/CXC), Jill Bechtold (Steward Observatory), Chi C. Cheung (MIT), Daniel E. Harris (SAO/CXC), Marek Sikora (CAMK, Warsaw), Lukasz Stawarz (Max-Planck Institut, Heidelberg)

The discovery of many X-ray jets associated with Active Galactic Nuclei (AGN) during the six years of the Chandra mission has changed the way we think about extragalactic jets. although jets span distances of hundreds of kpc to Mpc and constitute the largest physical manifestation of the AGN phenomenon, before Chandra only a few were known to emit X-rays. The fact that X-ray emission originates so far away from AGN suggests low energy losses during the jet propagation, re-acceleration processes, and interaction with the environment at large distances. In May 2005 we obtained a deep Chandra ACIS-S image of the projected 300 kpc jet in the redshift 1.18 quasar PKS 1127-145, with the main goal of studying details of the jet X-ray morphology in the context of competing theoretical models. The X-ray jet extends $\sim$30 arcsec away from the quasar core, has distinct knots and diffuse jet emission. Here we present the new X-ray data together with new radio observations and discuss details of the jet broad-band morphology. The high quality of the Chandra data enable us to study spectral properties along the jet in the high redshift large scale X-ray jet for the first time. We also consider the implications of our observation on theoretical models of the large scale jet emission.

Penetrating the Deep Cover of Compton Thick AGN

Nancy A. Levenson (Univ. Kentucky), T. M. Heckman (JHU), J. H. Krolik (JHU), K. A. Weaver (NASA/JHU), P. T. Zycki (Copernicus Astron. Ctr.)

We analyze observations of three active galaxies obtained with Chandra. All of these active galactic nuclei (AGN) suffer from Compton thick obscuration, with column densities in excess of $1.5\times 10^{24}$ cm$^2$ along the lines of sight. We therefore view the powerful central engines only indirectly, even at X-ray energies. Approximately 1% of the continuum's intrinsic flux is detected in reflection in each case. The only hard X-ray feature is the prominent Fe K$\alpha$ fluorescence line, with equivalent width greater than 1 keV in all sources. In detail, the morphologies of the extended soft X-ray emission and optical line emission are similar, and line emission dominates the soft X-ray spectra. Thus, we interpret the soft X-ray emission as a consequence of photoionization by the central engines. Because the resulting spectra are complex and do not reveal the AGN directly, crude analysis techniques such as hardness ratios would mis-classify these galaxies as hosts of intrinsically weak, unabsorbed AGN and would fail to identify the luminous, absorbed nuclei that are present. The active nuclei produce most of the galaxies' total observed emission over a broad spectral range, and much of their light emerges at far-infrared wavelengths.

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Iron K-band Features in the 522ks Chandra/HETGS Spectrum of MCG-6-30-15: a Narrow View of the Broad Iron Line

Christopher Reynolds (Univ. of Maryland, College Park), Andrew Young (MIT), Andrew Fabian (Univ. of Cambridge), Julia Lee (Harvard), Claude Canizares (MIT), Robert Gibson (MIT)

We present results from a deep (522ks) Chandra HETG observation of the Seyfert 1.2 galaxy MCG-6-30-15, focusing on spectral features in the iron K-band and the robustness of the presence of a relativistically broadened iron emission line from the inner accretion disk. The combination of high signal-to-noise and high spectral resolution allows us to address in detail models in which a photoionized absorber mimics the highly broadened iron emission line by curving the spectrum in the 3-6keV. We can rule out these alternatives to the broad iron line; the very iron ions required to produce the observed curvature in a pure absorption model (via L-shell bound-free transitions) would also produce prominent K-shell absorption lines that are not observed. We also note FeXXV and FeXXVI absorption lines, revealing the presence of a highly ionized outflow from the nucleus of this AGN.

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Why are they not AGN?

Roberto Soria (Harvard-Smithsonian Center for Astrophysics/Univ. College London), Giuseppina Fabbiano (Harvard-Smithsonian Center for Astrophysics), Alister Graham (Australian National Univ.), Alessandro Baldi, Martin Elvis (Harvard-Smithsonian Center for Astrophysics), Helmut Jerjen (Australian National Univ.), Silvia Pellegrini (Univ. of Bologna), Aneta Siemiginowska (Harvard-Smithsonian Center for Astrophysics)

Why are most supermassive black holes (BHs) in the local universe so faint? Is it because of a lack of fuel, low radiative efficiency, or both? At what rate are they accreting today, and what fraction of the gas inflow gets re-ejected by outflows? Is accretion in a steady state, or is gas building up until the next outburst? To answer these questions, we have studied the nuclear activity in a sample of very faint elliptical/S0 galaxies with supermassive BHs. Their nuclei have typical X-ray luminosities $\sim 10^{38}$-$10^{39}$ erg/s ($\sim
10^{-8}$- $10^{-7} L_{\rm Edd}$), and are surrounded by X-ray-emitting hot gas. If this were the only source of fuel for the BH, the observed X-ray luminosities would be too faint for standard disk accretion, but brighter than predicted by radiatively-inefficient solutions (eg, ADAF). A second source of fuel, stellar mass losses from inside the sphere of influence, can be estimated from optical images. By adding the two components, we constrain the true accretion rate and efficiency. We show that a self-regulating, slow outflow, powered by the accretion luminosity (mechanical plus radiative), can remove all the gas that does not sink into the BH, using only a small fraction of the available power. The rest of the energy may be carried out in a jet or advected. Typical values consistent with the observations and with the condition of mass balance are that 90-99% of the injected gas is lost in outflows, 1-10% accretes onto the BH, and 0.01-0.1% may go in a relativistic jet.

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AGN Outflows: Agents of Galaxy Feedback

Sarah Gallagher (UCLA)

Theoretical modeling of structure formation in the early Universe overpredicts the masses of large galaxies unless some form of feedback is included. Recently, outflows from actively accreting black holes have emerged as the favored agents of this energy injection. The significant correlation of black-hole and host galaxy bulge masses in the local Universe is another indication of a connection between black holes and their hosts, and mass ejection accompanying mass accretion onto the black hole during its active accretion phase is probably important in mediating this relationship. While these theoretical considerations motivate the study of AGN outflows in the context of understanding massive galaxy formation, winds are also a fundamental product of accretion physics, particularly in luminous quasars. I will review the evidence on AGN outflows in the local Universe and at cosmologically interesting redshifts, focusing on the primary role that X-ray studies have played in detecting and constraining the most massive winds. These constraints are essential for estimating the kinetic power of AGNs and thus their impact on galaxy formation.

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High Velocity Outflows in Near Eddington AGN

James Reeves (NASA/GSFC/JHU), Ken Pounds (Univ. of Leicester)

We present evidence for ultra fast (0.1c) outflows in several AGN which are likely to be accreting at a high fraction of the Eddington limit. In particular we discuss the Chandra grating and XMM-Newton spectra of PG 1211+143 and PDS 456, both of which are high luminosity AGN likely to be accreting near the Eddington rate. The X-ray spectra of both sources exhibit deep absorption from high ionization material, in the form of ionized absorption lines and edges from Fe, O, Ne, Mg and S. The high column absorber appears to be outflowing with velocities near 0.1c in both sources, implying that the kinetic power of the fast outflows is a significant fraction of the quasar energy budget. We also present new Chandra observations of PG 1211+143, which shows evidence for variable, redshifted iron K absorption lines. The relativistic velocities measured (0.2-0.3c) imply that we are seeing the long sought-after evidence for infall of matter in the last few gravitational radii around a supermassive black hole.

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The Nature of Simultaneous near-IR and X-ray Flares from Sgr A* at the Galactic center

Farhad Yusef-Zadeh (Northwestern Univ.), Howard Bushouse (STScI), Craig Heinke (Northwestern Univ.), Mark Wardle (Macquarie Univ.), Darren Dowell (Cal Tech), Douglas Roberts (Northwestern/Adler), Stu Shapiro (UIUC)

Sgr A* is considered to be a massive black hole at the Galactic center and is known to be variable in radio, millimeter, near-IR and X-rays. However, the correlation of the variability across its spectrum has not been fully studied. Here I review highlights of the results of two observing campaigns in 2004, designed to investigate the correlation of flare activity from Sgr A* at high energies. We report the detection of a simultaneous X-ray and near-IR flare, as well as a sub-millimeter and near-IR flare using the NICMOS of HST, XMM-Newton and CSO. We explain the X-ray emission as arising from the population of near-IR-synchrotron-emitting relativistic particles, scattering sub-millimeter seed photons within the inner 10 horizon radii of Sgr A* up to X-rays and soft $\gamma$-rays, detected by INTEGRAL. Using the inverse Compton scattering picture, we explain the lack of one-to-one X-ray counterparts to near-IR flares by the variation of the magnetic field and the spectral index distributions of the nonthermal particles. We also describe a power spectrum analysis of the NICMOS data during which Sgr A* was active in near-IR wavelengths. Lastly, I will present Chandra results of X-ray emission from nonthermal radio filaments and diffuse features in the Galactic center and discuss their emission mechanism in X-rays.


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The Chandra View of X-ray Binaries

Saeqa Vrtilek (SAO)

This talk will present some highlights from the many remarkable studies of X-ray binary systems that have been conducted using Chandra. Chandra's unprecedented spatial resolution and sensitivity have enabled us to determine luminosity functions for entire classes of X-ray binaries as observed in other galaxies and in globular clusters within our own and nearby galaxies, and to measure and analyze scattering halos around X-ray binaries at an accuracy high enough to provide a new method for measuring cosmic distances. It has been used for identifications of sources through accurate x-ray positions, and to place constraints on the chemical state of interstellar matter by measuring absorption lines in X-ray binaries, to measure the speed of powerful X-ray winds with the first detections of X-ray P-Cygni features, and to determine the size and separation of the jet material in galactic microquasars by measuring X-ray line velocities to an accuracy comparable to that of optical spectroscopy.

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Accretion Disk Winds in Black Hole X-ray Binaries

Jon Miller (Univ. of Michigan)

The exact nature of warm absorbers in Seyfert AGN is unclear, but accretion disk winds provide a viable explanation. Chandra HETGS observations of black hole X-ray binaries in outburst have revealed variable, blue-shifted absorption lines which imply ionized outflows. These outflows also likely represent disk-driven winds, providing evidence that winds may be a generic property of accretion disks around compact objects. In this presentation, I will briefly review both recent and new Chandra results which establish ionized outflows in black hole binaries and discuss parallels with the warm absorbers inferred in Seyfert AGN.

Modeling the Relativistic Jets in SS 433 Using Chandra X-ray Spectroscopy

Herman Marshall, Claude Canizares, Sebastian Heinz, Norbert Schulz, Michael Nowak (MIT Kavli Institute)

The unusual X-ray binary SS 433 has been observed with the Chandra X-ray observatory on four occasions at different orbital and precessional phases. These data have provided excellent views of the hottest parts of the oppositely directed jets. Emission line widths directly provide the opening angle of the jet, which varies by at least a factor of two between observations. We can also determine the composition, density, temperature, and ionization state of the jet gas from the X-ray spectra of the twin jets so that better models of the physical conditions in the jets can be constructed. In some observations, the soft X-ray emission lines are nearly absent, leading to a possible interpretation that jet cooling is predominantly radiative during these times. Observations during eclipse can be interpreted by blocking the cooler portions with the companion star, providing a measure of the size of the companion and the mass of the black hole. We will report on new, very long observations both in and out of eclipse using Chandra that were obtained in August, 2005 with simultaneous radio and optical coverage.

Reflections of AGN Outbursts in the Hot Gas in Galaxies and Clusters

Christine Jones (SAO)

Chandra images show the presense of shocks, jets, cavities, and buoyant bubbles in the hot gas in galaxies, groups and clusters. These features all owe their origin to outbursts from the SMBH at the nucleus of the system. In this talk I will review recent results on AGN outbursts in the rich clusters MS0735.6+7421, Perseus, Hercules A, Hydra A and Virgo as well as the effects of outbursts and the X-ray luminosities of LLAGN in a sample of 160 early-type galaxies.

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Dark Energy Studies with the Largest, Relaxed Galaxy Clusters

Steven Allen (KIPAC (Stanford/SLAC)), Robert Schmidt (Heidelberg), David Rapetti (KIPAC (Stanford/SLAC)), Harald Ebeling (IfA, Hawaii), Andrew Fabian (IoA, Cambridge), Leon van Speybroeck (Harvard-Smithsonian Center for Astrophysics)

I will present the latest results on the mean matter density, dark energy density and dark energy equation of state from Chandra measurements of the X-ray gas mass fraction in the largest, dynamically relaxed clusters. This method, like supernovae studies, allows us to measure the acceleration of the Universe directly and leads to comparable, though entirely independent, constraints. I will highlight the remarkably complementary nature of X-ray, cosmic microwave background and supernovae studies and show how the combination of these data already provides interesting constraints on the evolution of dark energy, requiring only minimal priors.

Cosmology with the Chandra Cluster Data

Alexey Vikhlinin (SAO) for the 400d Survey Team

We have serendipitously detected a large number of high-redshift galaxy clusters in ROSAT pointings covering over 400 square degrees of extragalactic sky. Each X-ray detection is optically confirmed and spectroscopic redshifts were obtained for all objects. The most massive distant clusters (43 objects at $z>0.35$) were followed up with Chandra. This program is nearly completed. These observations provide the most accurate to-date measurement of the cluster number density evolution. We will present the cosmological constraints provided by the 400d distant cluster sample.

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Measurement of the Cosmic Distance Scale from Chandra X-ray Imaging and Sunyaev-Zeldovich Effect Data of High Redshift Clusters of Galaxies

Max Bonamente (UAH - NASA/NSSTC), Marshall Joy (NASA/MSFC), Samuel LaRoque, John Carlstrom (Univ. of Chicago), Erik Reese (Univ. of California, Berkeley)

Galaxy clusters are unique probes of the expansion of the universe. Their strong X-ray emission, along with radio observations measuring the scattering of the cosmic microwave background, affords an independent method to obtain distances based on the physics of ionized plasmas. We determine the distance to 37 clusters of galaxies in the redshift range 0.14$<$0.89 using Chandra X-ray data and interferometric radio observations using the Owens Valley Radio Observatory and the Berkeley-Illinois-Maryland Association interferometric arrays. We model the plasma and the dark matter distribution in clusters using a hydrostatic equilibrium model that accounts for radial variation in density, temperature and abundances, and use the cluster distances to measure the Hubble constant.

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Supercavities in the Hydra A Cluster: ICM Heating and the AGN Duty Cycle

Michael Wise (MIT Kavli Institute), Brian McNamara (Ohio Univ.), Paul Nulsen (Harvard-Smithsonian Center for Astrophysics), John Houck (MIT Kavli Institute), Larry David (Harvard-Smithsonian Center for Astrophysics)

We report on a new set of giant cavities recently discovered in a deep 200 ksec observation of the Hydra A cluster. These new cavities extend to a radius of 300 kpc from the central AGN with diameters of 250 kpc and 150 kpc for the northern and southern cavities, respectively, and appear to be connected to the previous smaller cavities seen in Hydra A. This connection suggests the the possibility that such supercavities represent merging bubbles generated by an ongoing series of outbursts from the central AGN. The mechanical energy necessary to inflate these supercavities is $\sim 10^{61}$ ergs and the buoyant rise time implies an age for the cavities of 100$--$300 Myr. Taken together, these estimates imply a mechanical luminosity of 1 $--3 \times 10^{45}$ erg s$^{-1}$ which is comparable to the luminosity associated with the cluster-scale shock in Hydra A discussed previously by Nulsen et al. (2005). As we demonstrate, the complete cavity system comprises 15$--$30% of the total cluster volume inside 350 kpc, and this entire volume is filled with 330 MHz radio emission. In this work, we present images and temperature maps of these new cluster-scale cavities and an analysis of their physical properties. We will discuss the implications of such supercavities on the evolution of the ICM in Hydra A as well as their potential contribution to the excess entropy or preheating seen in cluster cores. Finally, studies of these cavity systems and large-scale shock fronts produced by radio jets advancing into the ICM are currently some of the most reliable and accurate diagnostics for determining the ages and energetics of AGN. Based on these observations, we discuss what limits we can place on the age, total energy output, and duty-cycle of activity for the central AGN in Hydra A.

Ghost Cavities in Cluster Cores Viewed with Chandra and the VLA

Tracy Clarke (NRL), Craig Sarazin (UVa), Elizabeth Blanton (BU), Namir Kassim (NRL)

X-ray observations of the central regions of cooling core clusters have revealed a wealth of detail in the thermal gas. One of the most spectacular results is the profound effect that the central radio sources have on the structure of the thermal gas. X-ray data reveal depressions and filaments in the thermal gas which are connected to the active cluster-center radio galaxy. In addition, there are "ghost cavities" in several clusters which are located well beyond the currently active radio galaxy. The presence of numerous structures in cluster cores suggests that the central AGN may play a significant role in the energy budget of this region.

We present a study of the radio and X-ray interactions in the cores of two dense clusters. Our recent low frequency radio observations of these systems reveal low energy relativistic plasma which appears to connect the central radio source to the outer ghost cavities seen in Chandra images. We present details of the radio and X-ray observations of Abell 2597 which reveal several outburst episodes of the central AGN and provide the first suggestion of an X-ray tunnel which may be maintained over multiple outburst episodes. New radio observations of Abell 4059 reveal complex source morphology with the first evidence of emission extending into the southern ghost bubble.

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Insights on Physics of Gas and Dark Matter from Cluster Mergers

Maxim Markevitch, Scott Randall (SAO), Douglas Clowe (Univ. of Arizona), Anthony H. Gonzalez (Univ. of Florida)

I will present constraints on the physical parameters of dark matter and intracluster plasma from the 500 ks Chandra observation of the bullet cluster 1E0657-56 as well as other merging cluster data. In particular, new constraints on the dark matter self-interaction cross-section derived from a combination of X-ray and weak lensing mapping of 1E0657-56 will be presented.

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Galactic Halo or Local Group Intergalactic Medium?

Smita Mathur (The Ohio State Univ.)

Cosmological hydrodynamic simulations predict that the low redshift universe comprises of a web of warm-hot intergalactic gas and galaxies, groups of galaxies and clusters form at dense knots in these filaments. Our own Galaxy being no exception is also expected to be surrounded by the warm-hot intergalactic medium, filling the Local Group. Some theoretical models also predict the existence of a hot halo of the Galaxy. With X-ray and FUV observations of extragalactic sources, we can probe the warm-hot gas through absorption lines of highly ionized elements. Indeed, Chandra, XMM and FUSE observations have detected z=0 absorption lines towards many sightlines. The debate that has emerged is over the interpretation of these observations: are the z=0 absorption systems from the halo of our Galaxy or from the extended Local Group environment? This has important implications for our understanding of the mass of the Local Group, the physical conditions in the intergalactic medium, the structure of the Galaxy and galaxy formation in general. I will present the current status of the debate and discuss our ongoing observing program aimed at understanding the z=0 absorption systems, with emphasis on an exceptionally high quality Chandra spectrum of the Mrk 421 sightline. I'll also outline a new theoretical model.

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Detections of the Warm Hot Intergalactic Medium

Fabrizio Nicastro (SAO, UNAM), Katrien Steenbrugge (SAO), Martin Elvis (Presenter, SAO), Smita Mathur, Rik Williams (OSU)

I will review the current observational evidence for the presence of a large amount of baryonic matter in a tenous and hot filamentary web of interagalactic medium in the local Universe. I will show detections of intervening highly ionized metal absorption lines in the Chandra and XMM spectra of a number of blazars spanning redshifts from z=0.03 (Mkn 421) to z=0.89 (3C 454.3), and will discuss the implications of this detections in terms of number density of WHIM filaments in the Universe, and of total cosmological mass density of WHIM. I will show that our measurements are consistent with predictions from hydrodynamical simulations for the formation of structure in the Universe, and with the total amount of "missing baryons.''

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Study of the Soft X-ray Emission by Warm/Hot Intergalactic Medium

Andrzej Soltan (Copernicus Astronomical Center)

A positive detection of the soft X-ray emission generated by the Warm/Hot Intergalactic Medium (WHIM) at small separations from galaxies is reported. The signal strongly declines with the photon energy indicating the WHIM temperatures below kT = 1 keV. Low metallicity (below ~0.2) is strongly preffered. Systematic enhancements of the X-ray background (XRB) flux surrounding field galaxies are measured at scales of ~30 - 250 kpc. A technique is based on the correlation analysis of the XRB and the galaxy distribution. A large sample of the XMM-Newton observations is used in the investigation.

The amplitude of the signal is in agreement with an extrapolation of our earlier measurements of the WHIM emission performed at larger scales.

Preliminary assessments of the WHIM physical parameters based on the observed emission indicate densities of the order of 10^{-5} protons per cm^3 and typical mass of the halo surrounding the galaxy in the investigated sample of few x 10^11 solar masses.

[Author submitted updated abstract for proceedings.]

Where are the Baryons in the Local Group?

Joel Bregman, Edward Lloyd-Davies (Univ. of Michigan)

The absorption by OVII and OVIII at zero redshift can be interpreted as a very extensive Local Group medium, whose baryonic mass is greater than that of the known galaxies. This result is model dependent, and for different assumptions, this gas could be a halo around the Milky Way, with a hot gas mass that is a fraction of the Milky Way mass. These two possibilities, the Local Group and the Milky Way halo models, make different predictions that can be tested by both absorption and emission studies. The Local Group picture would predict that the OVII absorption column be greatest along the Milky Way - M31 axis, but from XMM observations of OVII absorption, we see no such effect. Also, in the Local Group picture, the soft X-ray emission would extend to Mpc scale and could be shadowed by HI clouds in the Magellanic Stream (70 kpc), but from Chandra observations, no such shadows are detected. We conclude that the hot gas responsible for emission and absorption resides primarily in a Galactic halo of extent 50 kpc, rather than in a massive medium filling the Local Group. Support for this program was provided by NASA.

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X-raying the Multi-Phase ISM Along the Sightline to the Galactic Center

Yangsen Yao (MIT/UMASS), Q. Daniel Wang (UMASS)

We present an X-ray absorption line spectroscopy of the cold, warm, and hot phases of the ISM toward Galactic center region. The OI, OII, OIII, OVII, OVIII, and NeIX Kalpha absorption lines are clearly detected in the Chandra grating spectra of 4U 820-303 (Galactic coordinates l,b =2.79, 7.91 and distance = 7.6 kpc). The detection of these lines allows us for the first time to measure the column densities of these different ISM phases in the same line of sight through much of the Galaxy. A joint-analysis of OVII, OVIII, and NeIX lines also provides tight constraints on the velocity dispersion, temperature, and Ne/O abundance ratio of the hot phase. These measurements have strong implications for our understanding of the global ISM in the Galaxy. Complemented by the pulsar dispersion measure along the same sightline, we obtain the first direct observational constraint on the global filling factor of the hot phase. If these the warm and hot phases are in a rough thermal pressure balance, the hot gas filling factor would then be greater than 0.9. If a filling factor of the hot phase is significantly less than 0.8, the thermal pressure in the hot phase should then be at least 10 times higher than that in the warm phase (i.e., a condition similar to the Local ISM).

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Chandra View of the Hot Interstellar Medium

Q. Daniel Wang (Univ. of Massachusetts)

Diffuse hot gas is thought to play an essential role in shaping the interstellar medium, dispersing the energy and chemically-enriched materials produced by stars, and feedbacking to the intergalactic medium. I will review our recent work aimed to quantify these effects. Based on the Chandra detection of various interstellar X-ray and far-UV absorption lines, we characterize for the first time the global distribution and filling factor of the hot gas as well as its thermal, chemical, and kinematic properties. This Galactic study is complemented by a survey of diffuse hot gas in and around nearby normal galaxies, based on X-ray imaging and far-UV spectroscopic observations. The results are being compared with various theories and simulations, providing new insights into the heating, transportation, and cooling mechanisms of the hot gas as well as its role in the formation and evolution of galaxies.

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Connecting Galaxy Formation, Star Formation and the X-ray Background

David Ballantyne (CITA, Univ. of Arizona), John Everett, Norm Murray (CITA)

The AGN which contribute the majority of the cosmic X-ray background (CXRB) peak in redshift at $z \sim 0.7$. Since this redshift is similar to the peak in the cosmic star-formation rate, we propose that the obscuring material required for AGN unification is regulated by star-formation within the host galaxy. We test this idea by computing CXRB synthesis models with a ratio of Type 2/Type 1 AGN that is a function of both $z$ and 2-10 keV X-ray luminosity $L$. The model which simultaneously best accounts for the Type 1 AGN fractions of Barger et al. (2005), the CXRB spectrum and the X-ray number counts predicts a Type 2 AGN fraction which evolves as $(1+z)^{0.3}$. This evolution predicts that the deep X-ray surveys are missing about half the obscured AGN with $\log L > 44$. Overall, these calculations show that the current data strongly supports a change to the AGN unification scenario where the obscuration is initiated by star formation in the host galaxy rather than an unevolving molecular torus.

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Using AGN to Observe the Growth of the Cosmic Web

Stephen Murray, Christine Jones, Almus Kenter, William Forman, Maxim Markevitch, Alexey Vikhlinin (Harvard-Smithsonian Center for Astrophysics) Katherine Brand, Buell Jannuzi (NOAO), Christopher Kochanek (OSU), D. J. Eisenstein (Univ. of Arizona)

We present X-ray and optical observations of the contiguous 9.3 sq. deg. XBootes survey made with the ACIS instrument on Chandra. The X-ray survey consists of 126 5ksec pointings that achieve a sensitivity of about $4\times10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in the 0.5-7 keV band. At this sensitivity limit we detect 4642 X-ray sources. As part of the AGES galaxy survey in the Bootes region (Kochanek et al. 2005), we have obtained 1800 redshifts of the X-ray selected objects most of which are AGN, yielding a density of $\sim150$ AGN per square degree. The mean AGN redshift is 1.3 with the distribution extending to $z>4$.

We have analyzed the spatial distribution of the X-ray selected AGN and compared this to the distribution of the $\sim20000$ AGES galaxies. To $z \sim 0.7$ (the limit of galaxy sample), the galaxies and AGN both trace the same structures and show the same web of voids and filaments. At larger redshifts, the X-ray AGN continue to show the characteristic structure of voids and filaments. Quantitatively, we computed the spatial 2-point correlation function for the X-ray selected AGN and find that the correlation length, $r_0\simeq6.4 \,
h^{-1}$ Mpc, and the exponent, $\gamma\simeq-1.7$, of the correlation function are similar to the canonical values derived for galaxies. In addition, we have compared the correlation function in several redshift intervals and find that the correlation length is approximately constant to $z\sim1.5$.

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First Results from the Extended Chandra Deep Field-South Survey

Anton Koekemoer (STScI), E-CDF-S Team

The Extended Chandra Deep Field-South (E-CDF-S) survey has been designed to complement the Chandra Deep Fields by significantly increasing the solid angle of sky with coverage to very sensitive X-ray flux levels. It is composed of four contiguous 250 ks ACIS-I observations flanking the original Chandra Deep Field-South (CDF-S). These observations cover a region with superb and growing multiwavelength coverage (e.g., COMBO-17, GEMS, GOODS, UDF, Spitzer, VLT, VLA, ATCA, etc), and they have the sensitivity to detect the X-ray emission from moderate-luminosity active galactic nuclei (AGN) to z $\sim$ 4-6 as well as X-ray luminous starburst galaxies to z $\sim$ 1. All the Chandra observations have now been successfully obtained and analyzed, and nearly 1000 X-ray sources, mostly AGN, are detected in the E-CDF-S region, increasing the previously known number by an additional 600. Multiwavelength follow-up studies are underway to investigate topics including the cosmic evolution and luminosity dependence of AGN X-ray emission, the efficacy of X-ray versus optical AGN-selection techniques, AGN clustering, moderate-luminosity AGN in the high-redshift universe, and off-nuclear ultraluminous X-ray sources. I will review the first results from the E-CDF-S survey, with highlights on some recent results for off-nuclear X-ray sources, alpha_ox studies and high-redshift X-ray sources, and will also discuss other ongoing projects and future prospects.


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