arXiv subjects

P. E. J. Nulsen

Publications and source records attributed to P. E. J. Nulsen.

At least 127 records · Page 7Linked to original sources

arXiv2004

A Chandra X-ray Observation of Abell 1991: The Late Stages of Infall?

We present results from a 38 ks Chandra X-ray observation of the z=0.059 galaxy cluster A1991. The cluster has a bright X-ray core and a central temperature gradient that declines inward from 2.7 keV at 130 kpc to approximately 1.6 keV at the cluster center. The radiative cooling time of the gas in the inner 10 kpc is about 0.5 Gyr, and rises to 1 Gyr at a radius of 20 kpc. The cooling rate of the gas within the latter radius is less than about 25 solar mass/yr. The Chandra ACIS-S3 image shows that the intracluster medium has an asymmetric surface brightness distribution with respect to the central galaxy. Bright knots of soft X-ray emission embedded in a cometary structure are located approximately 10 arcsec north of the optical center of the cD galaxy. Unlike the structures seen in other cooling flow clusters, the knots have no obvious association with the radio source. The structure's temperature of 0.83 keV makes it nearly 1 keV cooler than its surroundings, and its mass is 3.4E+09 solar masses. Based on its bow-shaped appearance and modest overpressure with respect to its surroundings, we interpret the structure as a cool mass concentration that is breaking apart as it travels northward through the center of the cluster.

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arXiv2004

A Systematic Study of Radio-Induced X-ray Cavities in Clusters, Groups, and Galaxies

We present an analysis of sixteen galaxy clusters, one group and one galaxy drawn from the Chandra X-ray Observatory's data archive. These systems possess prominent X-ray surface brightness depressions associated with cavities or bubbles that were created by interactions between powerful radio sources and the surrounding hot gas. The minimum energy associated with the cavities ranges between pV~10^55 erg in galaxies, groups, and poor clusters to pV~10^60 erg in rich clusters. We evaluate the hypothesis that cooling in the hot gas can be quenched by energy injected into the surrounding gas by the rising bubbles. Nearly half of the systems in this study may have instantaneous mechanical luminosities large enough to balance cooling, at least for a short period of time, if the cavities are filled with a relativistic gas. We find a trend or upper envelope in the distribution of central X-ray luminosity versus instantaneous mechanical luminosity with the sense that the most powerful cavities are found in the most X-ray luminous systems. Such a trend would be expected if many of these systems produce bubbles at a rate that scales in proportion to the cooling rate of the surrounding gas. Finally, we use the X-ray cavities to measure the mechanical power of radio sources over six decades of radio luminosity, independently of the radio properties themselves. We find that the ratio of the instantaneous mechanical (kinetic) luminosity to the 1.4 GHz synchrotron luminosity ranges from a few to roughly a thousand. This wide range implies that the 1.4 GHz synchrotron luminosity is an unreliable gauge of the mechanical power of radio sources.

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arXiv2004

In Search of the Dark Ages -- An Experimental Challenge

Most direct source detections beyond z~7 are likely to arise from wide-field narrowband surveys of Ly-a emission in the J band. For this to be true, the Ly-a emission must somehow escape from compact star-forming regions (CSR) presumably associated with massive haloes. Since the Ly-a escape fraction is <10% from an emitting region of size roughly 1 kpc, these objects will be difficult to find and hard to detect. For CSR sources, existing large-format IR arrays are close to ideal in terms of their noise characteristics for conducting "wide-field" narrowband surveys where pixel sizes are 0.1" or larger. However, we stress that Ly-a can also arise from external large-scale shocks (ELS) due to starburst winds, powered by CSRs, ploughing into gas actively accreting onto the dark halo. The winds effectively carry energy from the dense, dusty environment of a starburst into lower density regions, where the escape probability for Ly-a photons is greater. ELS emission is expected to be considerably more clumpy (<100 pc; ~0.01") than CSR emission. For a "targetted" observation of an ELS source, IR arrays will need two orders of magnitude improvement in dark current if we wish to detect dispersed clumpy emission within the environment of a massive halo. For either targetted or wide-field surveys, the deepest observations will be those where the pixel sampling is well matched to the size of the emitting regions. We show that there are only 3 -- 4 J-band windows [z = 7.72, (8.22), 8.79, 9.37] suitable for observing the Dark Ages in Ly-a; we summarize their cosmological properties in Table 1.

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arXiv2003

X-ray Cavities and Cooling Flows

Recent data have radically altered the X-ray perspective on cooling flow clusters. X-ray spectra show that very little of the hot intracluster medium is cooler than about 1 keV, despite having short cooling times. In an increasing number of cooling flow clusters, the lobes of a central radio source are found to have created cavities in the hot gas. Generally, the cavities are not overpressured relative to the intracluster gas, but act as buoyant bubbles of radio emitting plasma that drive circulation as they rise, mixing and heating the intracluster gas. All this points to the radio source, i.e. an active galactic nucleus, as the heat source that prevents gas from cooling to low temperatures. However, heating due to bubbles alone seems to be insufficient, so the energetics of cooling flows remain obscure. We briefly review the data and theory supporting this view and discuss the energetics of cooling flows.

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arXiv2001

Discovery of Ghost Cavities in Abell 2597's X-ray Atmosphere

A Chandra image of the central 100 kpc of the Abell 2597 cluster of galaxies shows bright, irregular, X-ray emission within the central dominant cluster galaxy (CDG), and two low surface brightness cavities located 30 kpc from the CDG's nucleus. Unlike the cavities commonly seen in other clusters, Abell 2597's ``ghost'' cavities are not coincident with the bright central radio source. Instead, they appear to be associated with faint, extended radio emission seen in a deep VLA radio map. We interpret the ghost cavities as buoyantly-rising relics of a radio outburst that occurred between 50--100 Myr ago. The demography of cavities in the few clusters studied thus far shows that galactic radio sources experience recurrent outbursts on a $\sim 100$ Myr timescale. Over the lifetime of a cluster, ghost cavities emerging from CDGs deposit $\gae 10^{59-61}$ erg of energy into the intracluster medium. If a significant fraction of this energy is deposited as magnetic field, it would account for the high field strengths in the cooling flow regions of clusters. The similarity between the central cooling time of the keV gas and the radio cycling timescale suggests that feedback between cooling gas and the radio source may be retarding or quenching the cooling flow.

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arXiv2001

Interaction of Radio Lobes with the Hot Intracluster Medium: Driving Convective Outflow in Hydra A

The radio lobes of Hydra A lie within cavities surrounded by a rim of enhanced X-ray emission in the intracluster gas. Although the bright rim appears cooler than the surrounding gas, existing Chandra data do not exclude the possibility that the rim is produced by a weak shock. A temperature map shows that cool gas extends out along the radio axis of Hydra A. The age of the radio source and equipartition pressure of the radio lobe argue against a shock, and comparison with similar structure in the Perseus Cluster also suggests that the rim is cool. We show that the cool bright rim cannot be the result of shock induced cooling, or due to the effect of magnetic fields in shocks. The most likely source of low entropy (cool) gas is entrainment by the rising cavity. This requires some means of communicating the bouyant force on the cavity to the surrounding gas. The magnetic field required to produce the Faraday rotation in Hydra A has the appropriate properties for this, if the Faraday screen is mainly in this bright rim. In Hydra A, the mass outflow due to the rising cavities could be sufficient to balance cooling driven inflow, so preventing the build up of low entropy gas in the cluster core.

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arXiv2001

Chandra Observations of NGC 4636 - An Elliptical Galaxy in Turmoil

Chandra images show symmetric, 8 kpc long, arm-like features in the X-ray halo surrounding NGC 4636. The leading edges of these features are sharp and are accompanied by temperature increases of ~30%. These properties, along with their scale and symmetry, suggest that the arm-like structures are produced by nuclear outburst driven shocks. We interpret these observations as part of a cycle in which cooling gas originally fueled a nuclear outburst about 3 X 10^{6} years ago leading to shocks that reheat the cooling gas, thus preventing the accumulation of significant amounts of cooled gas in the galaxy center and temporarily starving the central AGN.

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arXiv2001

A High Resolution Study of the Hydra A Cluster with Chandra: Comparison of the Core Mass Distribution with Theoretical Predictions and Evidence for Feedback in the Cooling Flow

The cooling flow cluster Hydra A was observed during the orbital activation and calibration phase of the Chandra Observatory. While the X-ray image of the cluster exhibits complex structure in the central region as reported in McNamara $etal$, the large scale X-ray morphology of the cluster is fairly smooth. A spectroscopic analysis of the ACIS data shows that the gas temperature in Hydra A increases outward, reaches a maximum temperature of 4 keV at 200 kpc, and then decreases slightly at larger radii. The distribution of heavy elements is nonuniform, with a factor of two increase in the Fe and Si abundances within the central 100 kpc. Beyond the central 100 kpc the Si-to-Fe abundance ratio is twice solar, while the Si-to-Fe ratio of the central excess is consistent with the solar value. One of the more surprising results is the lack of spectroscopic evidence for multiphase gas within the bulk of the cooling flow. Beyond the central 30 kpc, the ACIS spectra are adequately fit with a single temperature model. The addition of a cooling flow component does not significantly improve the fit. Only within the central 30 kpc (where the cooling time is less than 1~Gyr), is there spectroscopic evidence for multiphase gas. However, the spectroscopic mass deposition rate is more than a factor of 10 less than the morphologically derived mass accretion rate at 30 kpc. We propose that the cooling flow region is convectively unstable due to heating by the central radio source which significantly reduces the net accretion rate.

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arXiv2001

Chandra X-ray observations of the 3C295 cluster core

We examine the properties of the X-ray gas in the central regions of the distant (z=0.46), X-ray luminous cluster of galaxies surrounding the powerful radio source 3C 295, using observations made with the Chandra Observatory. Between radii of 50-500 kpc, the cluster gas is approximately isothermal with an emission-weighted temperature, kT ~5 keV. Within the central 50 kpc radius this value drops to kT ~3.7 keV. The spectral and imaging Chandra data indicate the presence of a cooling flow within the central 50 kpc radius of the cluster, with a mass deposition rate of approximately 280 solar masses per year. We estimate an age for the cooling flow of 1-2 Gyr, which is approximately one thousand times older than the central radio source. We find no evidence in the X-ray spectra or images for significant heating of the X-ray gas by the radio source. We report the detection of an edge-like absorption feature in the spectrum for the central 50 kpc region, which may be due to oxygen-enriched dust grains. The implied mass in metals seen in absorption could have been accumulated by the cooling flow over its lifetime. Combining the results on the X-ray gas density profile with radio measurements of the Faraday rotation measure in 3C295, we estimate the magnetic field strength in the region of the cluster core to be B ~12 \muG.

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arXiv2000

New Perspectives on Cooling Flows and Cluster Radio Sources

We discuss new results from the Chandra X-ray Observatory and XMM-Newton on cluster cooling flows, emphasizing early results from our Chandra programs. We find cooling rates reduced by factors of 5-10 compared to those from earlier missions. Nevertheless, substantial amounts of keV gas appear to be cooling and fueling star formation in central dominant cluster galaxies (CDGs). The structure of the keV thermal gas is remarkably complex, particularly in regions surrounding the the radio source and sites of star formation in CDGs. The radio sources are displacing the thermal gas leaving cavities filled with radio emission. The cavities are apparently supported against the local gas pressure by magnetic fields and cosmic rays. In addition, radio-faint, ``ghost'' cavities are seen in some clusters. They may be relics of earlier radio outbursts rising buoyantly in the intracluster medium. The radio sources may reduce the mass deposition rates by mechanical heating, and by inducing convective currents that move cool material outward.

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arXiv2000

On the soft X-ray spectrum of cooling flows

Strong evidence for cooling flows has been found in low resolution X-ray imaging and spectra of many clusters of galaxies. However high resolution X-ray spectra of several clusters from the Reflection Grating Spectrometer (RGS) on XMM-Newton now show a soft X-ray spectrum inconsistent with a simple cooling flow. The main problem is a lack of the emission lines expected from gas cooling below 1--2 keV. Lines from gas at about 2--3 keV are observed, even in a high temperature cluster such as A 1835, indicating that gas is cooling down to about 2--3 keV, but is not found at lower temperatures. Here we discuss several solutions to the problem; heating, mixing, differential absorption and inhomogeneous metallicity. Continuous or sporadic heating creates further problems, including the targetting of the heat at the cooler gas and also the high total energy required. So far there is no clear observational evidence for widespread heating, or shocks, in cluster cores, except in radio lobes which occupy only part of the volume. The implied ages of cooling flows are short, at about 1 Gyr. Mixing. or absorption, of the cooling gas are other possibilities. Alternatively, if the metals in the intracluster medium are not uniformly spread but are clumped, then little line emission is expected from the gas cooling below 1 keV. The low metallicity part cools without line emission whereas the strengths of the soft X-ray lines from the metal-rich gas depend on the mass fraction of that gas and not on the abundance, since soft X-ray line emission dominates the cooling function below 2 keV.

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arXiv2000

ASCA and ROSAT observations of nearby cluster cooling flows

We present a detailed analysis of the X-ray properties of the cooling flows in a sample of nearby, X-ray bright clusters of galaxies using high-quality ASCA spectra and ROSAT X-ray images. We demonstrate the need for multiphase models to consistently explain the spectral and imaging X-ray data for the clusters. The mass deposition rates of the cooling flows, independently determined from the ASCA spectra and ROSAT images, exhibit reasonable agreement. We confirm the presence of intrinsic X-ray absorption in the clusters using a variety of spectral models. We also report detections of extended $100μ$m infrared emission, spatially coincident with the cooling flows, in several of the systems studied. The observed infrared fluxes and flux limits are in good agreement with the predicted values due to reprocessed X-ray emission from the cooling flows. We present precise measurements of the abundances of iron, magnesium, silicon and sulphur in the central regions of the Virgo and Centaurus clusters. Our results firmly favour models in which a high mass fraction (70-80 per cent) of the iron in the X-ray gas in these regions is due to Type Ia supernovae. Finally, we present a series of methods which may be used to measure the ages of cooling flows from the X-ray data. The results for the present sample of clusters indicate ages of between 2.5 and 7 Gyr. If the ages of cooling flows are primarily set by subcluster merger events, then our results suggest that in the largest clusters, mergers with subclusters with masses of approximately 30 per cent of the final cluster mass are likely to disrupt cooling flows.

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arXiv2000

A physical model for the hard X-ray background

We combine a semi-analytic galaxy formation model (Nulsen and Fabian 1997) with a prescription for the obscured growth of massive black holes (Nulsen and Fabian 2000; Fabian 1999), to reproduce the hard X-ray background (XRB), the local 2-10keV AGN luminosity function and the source counts, including recent Chandra results. The model also complies with constraints on the AGN contribution to the far-infrared and sub-mm backgrounds. The co-moving density of luminous AGN [L(2-10keV)>1E44 erg/s, in the unabsorbed rest- frame] in the model declines sharply since z=2, mimicking the observed evolution of the quasar population. The abundance of lower luminosity AGN simultaneously increases, and we identify such objects with the Seyfert galaxies. These features reflect an epoch-dependent rate of Bondi accretion from the hot, cooling atmosphere: at z<2.5 most AGN are fed at much less than 10 per cent of the Eddington rate, whilst at z>2.5 most accrete at 10-50 per cent Eddington. Unlike other models for quasar and galaxy formation, we do not consider major mergers between normal galaxies. The model produces an excess soft XRB below 4keV which can be removed by modifying the wind expulsion model of Fabian (1999) to incorporate anisotropic ejection.

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arXiv2000

Non-Gravitational Heating in the Hierarchical Formation of X-ray Clusters

The strong deviation in the properties of X-ray clusters from simple scaling laws highlights the importance of non-gravitational heating and cooling processes in the evolution of proto-cluster gas. We investigate this from two directions: by finding the amount of `excess energy' required in intra-cluster gas in order to reproduce the observed X-ray cluster properties, and by studying the excess energies obtained from supernovae in a semi-analytic model of galaxy formation. Using the insights obtained from the model, we then critically discuss possible ways of achieving the high excess specific energies required in clusters. These include heating by supernovae and active galactic nuclei, the role of entropy, and the effect of removing gas through radiative cooling. Our model self-consistently follows the production of excess energy as well as its effect on star formation in galaxies. Excess energy is retained in gas as gravitational, kinetic and/or thermal energy. The gas distribution of virialized halos are modelled with a 2-parameter family of gas density profiles; gas profiles are then chosen according to the total energy of the gas. We investigate four contrasting ways of modifying gas density profiles in the presence of excess energy, in order to study the sensitivity of results to the model. In addition, we estimate the minimum excess specific energy required when all available gas profiles are considered, using a fiducial cluster with a temperature of around 2 keV. We find that the excess specific energies required in clusters lie roughly in the range 1--3 keV/particle. (abridged)

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arXiv2000

Chandra X-ray Observations of the Hydra A Cluster: An Interaction Between the Radio Source and the X-Ray-Emitting Gas

We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the cluster's X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, $\sim 25-35$ kpc diameter, that are coincident with Hydra A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is $\sim 600$ Myr in the inner 10 kpc. These properties are consistent with the presence of a $\sim 34 \msunyr$ cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently-accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material cooling at larger radii remains elusive.

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arXiv2000

Chandra Observation of Abell 2142: Survival of Dense Subcluster Cores in a Merger

We use Chandra data to map the gas temperature in the central region of the merging cluster A2142. The cluster is markedly nonisothermal; it appears that the central cooling flow has been disturbed but not destroyed by a merger. The X-ray image exhibits two sharp, bow-shaped, shock-like surface brightness edges or gas density discontinuities. However, temperature and pressure profiles across these edges indicate that these are not shock fronts. The pressure is reasonably continuous across these edges, while the entropy jumps in the opposite sense to that in a shock (i.e. the denser side of the edge has lower temperature, and hence lower entropy). Most plausibly, these edges delineate the dense subcluster cores that have survived a merger and ram pressure stripping by the surrounding shock-heated gas.

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arXiv1999

Chandra X-Ray Detection of the Radio Hotspots of 3C295

An observation of the radio galaxy 3C295 during the calibration phase of the Chandra X-ray Observatory reveals X-ray emission from the core of the galaxy, from each of the two prominent radio hotspots, and from the previously known cluster gas. We discuss the possible emission processes for the hotspots and argue that a synchrotron self-Compton model is preferred for most or all of the observed X-ray emission. SSC models with near equipartition fields thus explain the X-ray emission from the hotspots in the two highest surface brightness FRII radio galaxies, Cygnus A and 3C295. This lends weight to the assumption of equipartition and suggests that relativistic protons do not dominate the particle energy density.

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arXiv1999

The soft X-ray background: evidence for widespread disruption of the gas halos of galaxy groups

Almost all of the extragalactic X-ray background (XRB) at 0.25 keV can be accounted for by radio-quiet quasars, allowing us to derive an upper limit of 4 \bgunit\ for the remaining background at 0.25 keV. However, the XRB from the gas halos of groups of galaxies, with gas removal due to cooling accounted for, exceeds this upper limit by an order of magnitude if non-gravitational heating is not included. We calculate this using simulations of halo merger trees and realistic gas density profiles, which we require to reproduce the observed gas fractions and abundances of X-ray clusters. In addition, we find that the entire mass range of groups, from $\sim 5\times 10^{12}$ to $\sim 10^{14}$\Ms, contributes to the 0.25 keV background in this case. In a further study, we reduce the luminosities of groups by maximally heating their gas halos while maintaining the same gas fractions. This only reduces the XRB by a factor of 2 or less. We thus argue that most of the gas associated with groups must be outside their virial radii. This conclusion is supported by X-ray studies of individual groups. The properties of both groups and X-ray clusters can be naturally explained by a model in which the gas is given excess specific energies of $\sim 1$ keV/particle by non-gravitational heating. With this excess energy, the gas is gravitationally unbound from groups, but recollapses with the formation of a cluster of temperature $\ga 1$ keV. This is similar to a model proposed by Pen, but is contrary to the evolution of baryons described by Cen \& Ostriker. (abridged)

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