arXiv subjects

W. R. Forman

Publications and source records attributed to W. R. Forman.

At least 109 records · Page 6Linked to original sources

arXiv2009

The Jet-Heated X-ray Filament in the Centaurus A Northern Middle Radio Lobe

We present results from a 40 ks {\em XMM-Newton} observation of the X-ray filament coincident with the southeast edge of the Centaurus A Northern Middle Radio Lobe (NML). We find that the X-ray filament consists of five spatially resolved X-ray knots embedded in a continuous diffuse bridge. The spectrum of each knot is well fitted by a thermal model with temperatures ranging from 0.3-0.7 keV and subsolar elemental abundances. In four of the five knots, non-thermal models are a poor fit to the spectra, conclusively ruling out synchrotron or IC/CMB mechanisms for their emission. The internal pressures of the knots exceed that of the ambient ISM or the equipartition pressure of the NML by more than an order of magnitude, demonstrating that they must be short lived ($\sim3\times10^6$ yrs). Based on energetic arguments, it is implausible that these knots have been ionized by the beamed flux from the active galactic nucleus of Cen A or that they have been shock-heated by supersonic inflation of the NML. In our view, the most viable scenario for the origin of the X-ray knots is that they are the result of cold gas shock heated by a direct interaction with the jet. The lifetimes of the X-ray knots in the NML are roughly the same as the age of the strong shock around the SW inner lobe, suggesting they were created in the same AGN outburst. The most plausible model of the NML is that it is a bubble from a previous outburst that is being re-energized by the current outburst. We also report the discovery of a large scale (at least 35 kpc radius) gas halo around Cen A.

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arXiv2009

The Growth of Supermassive Black Holes Across Cosmic Time

One of the main themes in extragalactic astronomy for the next decade will be the evolution of galaxies over cosmic time. Many future observatories, including JWST, ALMA, GMT, TMT and E-ELT will intensively observe starlight over a broad redshift range, out to the dawn of the modern Universe when the first galaxies formed. It has, however, become clear that the properties and evolution of galaxies are intimately linked to the growth of their central black holes. Understanding the formation of galaxies, and their subsequent evolution, will therefore be incomplete without similarly intensive observations of the accretion light from supermassive black holes (SMBH) in galactic nuclei. To make further progress, we need to chart the formation of typical SMBH at z>6, and their subsequent growth over cosmic time, which is most effectively achieved with X-ray observations. Recent technological developments in X-ray optics and instrumentation now bring this within our grasp, enabling capabilities fully matched to those expected from flagship observatories at longer wavelengths.

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arXiv2009

Merging Cold Fronts in the Galaxy Pair NGC 7619 and NGC 7626

We present results from {\it Chandra} observations of the galaxy pair NGC 7619 and NGC 7626, the two dominant members of the Pegasus group. The X-ray images show a brightness edge associated with each galaxy, which we identify as merger cold fronts. The edges are sharp, and the axes of symmetry of the edges are roughly anti-parallel, suggesting that these galaxies are falling towards one another in the plane of the sky. The detection of merger cold fronts implies a merging subgroup scenario, since the alternative is that the galaxies are falling into a pre-existing $\sim1$ keV halo without a dominant galaxy of its own, and such objects are not observed. We estimate the 3D velocities from the cold fronts and show that the velocity vectors are indeed most likely close to the plane of the sky, with a relative velocity of $\sim1190\kms$. The relative velocity is consistent with what is expected from the infall of two roughly equal mass subgroups whose total viral mass equals that of the Pegasus group. We conclude that the Pegasus cluster is currently forming from a major merger of two subgroups, dominated by NGC 7619 and NGC 7626. NGC 7626 contains a strong radio source, a core with two symmetric jets and radio lobes. Although we find no associated structure in the X-ray surface brightness map, the temperature map reveals a clump of cool gas just outside the southern lobe, presumably entrained by the lobe, and an extension of cooler gas into the lobe itself. The jet axis is parallel with the projected direction of motion of NGC 7626 (inferred from the symmetry axis of the merger cold front), and the southern leading jet is foreshortened as compared to the northern trailing one, possibly due to the additional ram pressure the forward jet encounters.

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arXiv2009

High-energy particle acceleration at the radio-lobe shock of Centaurus A

We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations. The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption -- thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We estimate that the lobe is expanding to the southwest with a velocity of ~2600 km/s, roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain gamma_max for the accelerated particles to be 10^8 at the strongest part of the shock, consistent with expectations from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strengths.

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arXiv2009

The large-scale shock in the cluster of galaxies Hydra A

We analyzed a deep XMM-Newton observation of the cluster of galaxies Hydra A, focusing on the large-scale shock discovered as a surface brightness discontinuity in Chandra images. The shock front can be seen both in the pressure map and in temperature profiles in several sectors. The Mach numbers determined from the temperature jumps are in good agreement with the Mach numbers derived from EPIC/pn surface brightness profiles and previously from Chandra data and are consistent with M~1.3. The estimated shock age in the different sectors using a spherically symmetric point explosion model ranges between 130 and 230 Myr and the outburst energy between 1.5 and 3e61 ergs. The shape of the shock seen in the pressure map can be approximated with an ellipse centered 70 kpc towards the NE from the cluster center. We aimed to develop a better model that can explain the offset between the shock center and the AGN and give a consistent result on the shock age and energy. To this end, we performed 3D hydrodynamical simulations in which the shock is produced by a symmetrical pair of AGN jets launched in a spherical galaxy cluster. As an explanation of the observed offset of the shock center, we consider large-scale bulk flows in the intracluster medium. The simulation successfully reproduces the size, ellipticity, and average Mach number of the observed shock front. The predicted age of the shock is 160 Myr and the total input energy 3e61 erg. Both values are within the range determined by the spherically symmetric model. Matching the observed 70 kpc offset of the shock ellipse from the cluster center requires large-scale coherent motions with a high velocity of 670 km/s. We discuss the feasibility of this scenario and offer alternative ways to produce the offset and to further improve the simulation.

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arXiv2008

Chandra Cluster Cosmology Project II: Samples and X-ray Data Reduction

We discuss the measurements of the galaxy cluster mass functions at z=~0.05 and z=~0.5 using high-quality Chandra observations of samples derived from the ROSAT PSPC All-Sky and 400deg^2 surveys. We provide a full reference for the data analysis procedures, present updated calibration of relations between the total cluster mass and its X-ray indicators (T_X, Mgas, and Y_X) based on a subsample of low-z relaxed clusters, and present a first measurement of the evolving L_X-Mtot relation (with Mtot estimated from Y_X) obtained from a well-defined statistically complete cluster sample and with appropriate corrections for the Malmquist bias applied. Finally, we present the derived cluster mass functions, estimate the systematic uncertainties in this measurement, and discuss the calculation of the likelihood function. We confidently measure the evolution in the cluster comoving number density at a fixed mass threshold, e.g., by a factor of 5.0 +- 1.2 at M_500=2.5e14 h^-1 Msun between z=0 and 0.5. This evolution reflects the growth of density perturbations and can be used for the cosmological constraints complementing those from the distance-redshift relation.

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arXiv2008

Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints

Chandra observations of large samples of galaxy clusters detected in X-rays by ROSAT provide a new, robust determination of the cluster mass functions at low and high redshifts. Statistical and systematic errors are now sufficiently small, and the redshift leverage sufficiently large for the mass function evolution to be used as a useful growth of structure based dark energy probe. In this paper, we present cosmological parameter constraints obtained from Chandra observations of 36 clusters with =0.55 derived from 400deg^2 ROSAT serendipitous survey and 49 brightest z=~0.05 clusters detected in the All-Sky Survey. Evolution of the mass function between these redshifts requires Omega_Lambda>0 with a ~5sigma significance, and constrains the dark energy equation of state parameter to w0=-1.14+-0.21, assuming constant w and flat universe. Cluster information also significantly improves constraints when combined with other methods. Fitting our cluster data jointly with the latest supernovae, WMAP, and baryonic acoustic oscillations measurements, we obtain w0=-0.991+-0.045 (stat) +-0.039 (sys), a factor of 1.5 reduction in statistical uncertainties, and nearly a factor of 2 improvement in systematics compared to constraints that can be obtained without clusters. The joint analysis of these four datasets puts a conservative upper limit on the masses of light neutrinos, Sum m_nu<0.33 eV at 95% CL. We also present updated measurements of Omega_M*h and sigma_8 from the low-redshift cluster mass function.

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arXiv2008

A Multiwavelength View of Star Formation in Interacting Galaxies in the Pavo Group

We combine Spitzer IRAC mid-infrared (MIR) and Chandra X-ray observations of the dominant galaxies NGC6872 and NGC6876 in the Pavo group with archival optical and HI data to study interaction-induced star formation. In spiral galaxy NGC6872, 8.0 and 5.8 micron nonstellar emission having colors consistent with polycyclic aromatic hydrocarbons (PAHs) is concentrated in clumps in three regions: in a 5 kpc radius outer ring about the center of the spiral galaxy, in a bridge of emission connecting NGC6872's northern spiral arm to IC4970, and along the full extent of NGC6872's tidal arms. PAH emission is correlated with young star clusters and dense HI regions. We find no strong differences in the MIR colors of star-forming regions in the spiral galaxy NGC6872 as a function of position relative to the tidally interacting companion galaxy IC4970. We find 11 very luminous X-ray sources (>~ (0.5 - 5) x 10^{39} ergs/s) clustered to the southwest in NGC6872, near bright star-forming regions. In NGC6872's tidal features, young star clusters form at the boundaries of diffuse X-ray gas, suggesting that stars form as gas stripped by the interactions cools. The nucleus of NGC6872 is a weak X-ray point source (0.5-8 keV luminosity of 8.5 x 10^{39} ergs/s), but there is little evidence in the inner 1 kpc of NGC6872 for PAH emission from recent star formation or nuclear activity. However, a 4 kpc `stream', leading from the outer ring of NGC6872 to the nucleus, may signal transport of interstellar matter into NGC6872's nuclear region. Nonstellar emission, consistent with PAH emission, is also found in the central region of elliptical galaxy NGC6877, companion to dominant Pavo group elliptical NGC6876. However, in the central region of NGC6876, the dust emission is more likely due to silicate emission from old AGB stars.

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arXiv2008

Discovery of heavily-obscured AGN among 7 INTEGRAL hard X-ray sources observed by Chandra

We observed 7 INTEGRAL sources with the Chandra X-ray Observatory to refine their localization to ~2 arcsec and to study their X-ray spectra. Two sources are inferred to have a Galactic origin: IGR J08390-4833 is most likely a magnetic cataclysmic variable with a white dwarf spin period ~1,450 s; and IGR J21343+4738 is a high-mass X-ray binary. Five sources (IGR J02466-4222, IGR J09522-6231, IGR J14493-5534, IGR J14561-3738, and IGR J23523+5844) prove to be AGN with significant intrinsic X-ray absorption along the line of sight. Their redshifts and hard X-ray (17-60 keV) luminosities range from 0.025 to 0.25 and from ~2x10^43 to ~2x10^45 erg/s, respectively, with the distance to IGR J14493-5534 remaining unknown. The sources IGR J02466-4222 and IGR J14561-3738 are likely Compton-thick AGN with absorption column densities NH>10^24 cm^-2, and the former further appears to be one of the nearest X-ray bright, optically-normal galaxies. With the newly-identified sources, the number of heavily-obscured (NH>10^24 cm^-2) AGN detected by INTEGRAL has increased to ~10. Therefore, such objects constitute 10-15% of hard X-ray bright, non-blazar AGN in the local Universe. The small ratio (<<1%) of soft (0.5-8.0 keV) to hard (17-60 keV) band fluxes (Chandra to INTEGRAL) and the non-detection of optical narrow-line emission in some of the Compton-thick AGN in our sample suggests that there is a new class of objects in which the central massive black hole may be surrounded by a geometrically-thick dusty torus with a narrow ionization cone.

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arXiv2008

Chandra's View of the Ram Pressure Stripped Galaxy M86

We present results from a mosaic of nine Chandra observations of M86 and the surrounding field. We detect three main diffuse components: the Virgo ICM at ~2.4 keV, the extended halo of M86 at ~1.2 keV, and the cooler central and stripped gas of M86 at ~0.8 keV. The most striking feature is a long tail of emission, which consists of a plume ~ 4' north of M86 and two main extensions emanating from the plume. Based on the morphology and temperature structure of the tail, we conclude that it is formed by ram pressure stripping of M86 as it falls into the Virgo cluster and interacts with the Virgo ICM, in agreement with earlier work. The tail is 150 kpc in projection, and a simple estimate gives a lower limit on the true length of the tail of 380 kpc, making this the longest ram pressure stripped tail presently known. The total gas mass in the plume (7x10^8 M_sun) and tail (1x10^9 M_sun) is about three times that in the core of M86, which supports the scenario where most of the gas was stripped rapidly and recently. The projected position of the plume can be understood if M86 has an aspherical potential, as suggested by optical isophotes. Ram pressure stripping from an aspherical potential can also explain the split "double tails" seen in M86 and in other Virgo cluster galaxies in the field. The large line-of-sight velocity of M86 (1550 km/s with respect to M87), its position relative to the Virgo cluster, and the orientation of the tail tightly constrain its orbital parameters. The data are inconsistent with a radial orbit, and imply inner and outer turning radii of r_i ~ 300 kpc and r_o > 8.8 Mpc, indicating that M86 is, at best, only weakly bound to the Virgo cluster.

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arXiv2008

Evidence for Non-Hydrostatic Gas Motions in the Hot ISM of Centaurus A

We present preliminary results from a deep (600 ks) {\em Chandra} observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A (Cen A). We find a surface brightness discontinuity in the gas $\sim$3.5 kpc from the nucleus spanning a 120$^\circ$ arc. The temperature of the gas is 0.60$\pm$0.05 and 0.68$\pm$0.10 keV, interior and exterior to the discontinuity, respectively. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of 2.3$\pm$0.4 pressure jump across the discontinuity. This would imply that the gas is moving at 470$\pm$100 km s$^{-1}$, or Mach 1.0$\pm$0.2 (1.2$\pm$0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of $\sim$7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of non-hydrostatic motion of the gas core (i.e. sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kpc-scale jet. The jet could have decollimated crossing the discontinuity and thereby forming the northeast radio lobe.

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arXiv2007

Where Centaurus A gets its X-ray knottiness

We report an X-ray spectral study of the transverse structure of the Centaurus A jet using new data from the Chandra Cen A Very Large Project. We find that the spectrum steepens with increasing distance from the jet axis, and that this steepening can be attributed to a change in the average spectrum of the knotty emission. Such a trend is unexpected if the knots are predominantly a surface feature residing in a shear layer between faster and slower flows. We suggest that the spectral steepening of the knot emission as a function of distance from the jet axis is due to knot migration, implying a component of transverse motion of knots within the flow.

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arXiv2007

A large population of mid-infrared selected, obscured active galaxies in the Bootes field

We identify a population of 640 obscured and 839 unobscured AGNs at redshifts 0.7 ~80%. The observed numbers of IRAGNs are comparable to predictions from previous X-ray, optical, and IR luminosity functions, for the given redshifts and IRAC flux limits. We observe a bimodal distribution in R-[4.5] color, suggesting that luminous IR-selected AGNs have either low or significant dust extinction, which may have implications for models of AGN obscuration.

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arXiv2007

The Active Nucleus of IC4970: A Nearby Example of Merger-Induced Cold-Gas Accretion

We present results from Chandra X-ray and Spitzer mid-infrared observations of the interacting galaxy pair NGC6872/IC4970 in the Pavo galaxy group and show that the smaller companion galaxy IC4970 hosts a highly obscured active galactic nucleus (AGN). The 0.5-10 keV X-ray luminosity of the nucleus is variable, increasing by a factor 2.9 to 1.7 x 10^{42} erg/s (bright state) on ~100 ks timescales. The X-ray spectrum of the is heavily absorbed (N_H = 3 x 10^{23} cm^{-2}) for power law models with Gamma = 1.5-2.0 and shows a clear 6.4 keV Fe Kalpha line with equivalent width of 144-195 eV. Limits on the diffuse emission in IC4970 from Chandra X-ray data suggest that the available power from Bondi accretion of hot interstellar gas may be an order of magnitude too small to power the AGN. Spitzer images show that 8 micron nonstellar emission is concentrated in the central 1 kpc of IC4970, consistent with high obscuration in this region. The mid-infrared colors of the nucleus are consistent with those expected for a highly obscured AGN. Taken together these data suggest that the nucleus of IC4970 is a Seyfert 2, triggered and fueled by cold material supplied to the central supermassive black hole as a result of the off-axis collision of IC4970 with the cold-gas rich spiral galaxy NGC6872.

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arXiv2007

New results on particle acceleration in the Centaurus A jet and counterjet from a deep Chandra observation

We present new deep Chandra observations of the Centaurus A jet, with a combined on-source exposure time of 719 ks. These data allow detailed X-ray spectral measurements to be made along the jet out to its disappearance at 4.5 kpc from the nucleus. We distinguish several regimes of high-energy particle acceleration: while the inner part of the jet is dominated by knots and has properties consistent with local particle acceleration at shocks, the particle acceleration in the outer 3.4 kpc of the jet is likely to be dominated by an unknown distributed acceleration mechanism. In addition to several compact counterjet features we detect probable extended emission from a counterjet out to 2.0 kpc from the nucleus, and argue that this implies that the diffuse acceleration process operates in the counterjet as well. A preliminary search for X-ray variability finds no jet knots with dramatic flux density variations, unlike the situation seen in M87.

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arXiv2007

A Chandra Study of the Lobe/ISM Interactions Around the Inner Radio Lobes of Centaurus A: Constraints on the Temperature Structure and Transport Processes

We present results from deeper {\em Chandra} observations of the southwest radio lobe of Centaurus A, first described by Kraft et al. (2003). We find that the sharp X-ray surface brightness discontinuity extends around $\sim$75% of the periphery of the radio lobe, and detect significant temperature jumps in the brightest regions of this discontinuity nearest to the nucleus. This demonstrates that this discontinuity is indeed a strong shock which is the result of an overpressure which has built up in the entire lobe over time. Additionally, we demonstrate that if the mean free path for ions to transfer energy and momentum to the electrons behind the shock is as large as the Spitzer value, the electron and proton temperatures will not have equilibrated along the SW boundary of the radio lobe where the shock is strongest. Thus the proton temperature of the shocked gas could be considerably larger than the observed electron temperature, and the total energy of the outburst correspondingly larger as well. We investigate this using a simple one-dimensional shock model for a two-fluid (proton/electron) plasma. We find that for the thermodynamic parameters of the Cen A shock the electron temperature rises rapidly from $\sim$0.29 keV (the temperature of the ambient ISM) to $\sim$3.5 keV at which point heating from the protons is balanced by adiabatic losses. The proton and electron temperatures do not equilibrate in a timescale less than the age of the lobe. We note that the measured electron temperature of similar features in other nearby powerful radio galaxies in poor environments may considerably underestimate the strength and velocity of the shock.

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arXiv2007

The Disturbed 17 keV Cluster Associated with the Radio Galaxy 3C 438

We present results from a {\em Chandra} observation of the cluster gas associated with the FR II radio galaxy 3C 438. This radio galaxy is embedded within a massive cluster with gas temperature $\sim$17 keV and bolometric luminosity of 6$\times10^{45}$ ergs s$^{-1}$. It is unclear if this high temperature represents the gravitational mass of the cluster, or if this is an already high ($\sim$ 11 keV) temperature cluster that has been heated transiently. We detect a surface brightness discontinuity in the gas that extends $\sim$600 kpc through the cluster. The radio galaxy 3C 438 is too small ($\sim$110 kpc across) and too weak to have created this large disturbance in the gas. The discontinuity must be the result of either an extremely powerful nuclear outburst or the major merger of two massive clusters. If the observed features are the result of a nuclear outburst, it must be from an earlier epoch of unusually energetic nuclear activity. However, the energy required ($\sim10^{63}$ ergs) to move the gas on the observed spatial scales strongly supports the merger hypothesis. In either scenario, this is one of the most extreme events in the local Universe.

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arXiv2007

X-ray Constraints on Galaxy-Gas-Jet Interactions in the Dumbbell Galaxies NGC4782 and NGC4783 in the LGG316 Group

We present results from a 49.3 ks Chandra X-ray observation of the strongly interacting dumbbell galaxies NGC4782(3C278) and NGC4783 that constrain the kinematics of the interaction and models for bending the radio jets associated with NGC4782. The galaxies are embedded in an approximately spherical distribution of group gas, centered on NGC4782,that is nearly isothermal with mean kT ~ 1.4+/- 0.4 keV. The X-ray morphology suggests that NGC4783 is infalling into a single, massive galaxy group (LGG316) with NGC4782 nearly at rest at the center of the group potential.NGC4783 shows a sharp X-ray surface brightness edge (cold front) to the east and a ~15 kpc ram-pressure-stripped tail to the west. Analysis of this cold front indicates NGC4783 is moving east with a total velocity ~870+270-400 km/s (Mach ~1.4+0.5-0.7) at an inclination angle ~46 deg.(>33 deg.)towards us with respect to theplane of the sky. A ~45Myr old X-ray cavity, with enthalpy of 4.4 x 10^{57}ergs, coincides with the eastern radio lobe of 3C278. X-ray knots are found on both the radio jet and counter-jet, coincident with peaks in the radio emission. Assuming a light, mildly relativistic jet in 3C278, ram pressure velocities of 100-200 km/s impacting the eastern jet and ~170 km/s acting on the western radio lobe are sufficient to produce their observed bending. These velocities may be caused by bulk motions established in the IGM by the high velocity interaction between the galaxies, by the motion of NGC4782 relative to the IGM, or both.

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