arXiv subjects

R. M. Johnstone

Publications and source records attributed to R. M. Johnstone.

At least 37 records · Page 2Linked to original sources

arXiv2008

Cold gas in the Perseus cluster core: Excitation of molecular gas in filaments

We have recently detected CO lines in the well-known filaments around NGC 1275, the galaxy at the centre of the Perseus cluster of galaxies. These previous observations, with the HERA multi-beam array at the IRAM 30m telescope enabled us to make a large map of the CO(2-1) line and to see hints of molecular gas far away from the cluster centre. To confirm the presence of CO emission lines in the outer filaments and to study the CO(2-1)/CO(1-0) line ratio, we observed seven regions of interest again with the 30m telescope in both CO(1-0) and CO(2-1). The regions we observed were: the eastern filament, the horseshoe, the northern filament and a southern extension, all selected from Halpha emission line mapping. Molecular gas is detected in all the observed regions. This result confirms the large extent of the cold molecular gas filaments. We discuss the CO(2-1)/CO(1-0) ratios in the filaments. The eastern filament has optically thick gas, whereas further away, the line ratio increases close to values expected for a warmer optically thin medium. We also show CO(1-0) and CO(2-1) lines in 9 regions closer to the centre. The kinematics of the CO is studied here in more detail and confirms that it follows the motions of the warm H_2 gas found in the near-infrared. Finally, we searched for dense gas tracers around 3C84 and claim here the first detection of HCN(3-2).

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arXiv2008

The origin of molecular hydrogen emission in cooling-flow filaments

The optical filaments found in many cooling flows in galaxy clusters consist of low density ($\sim 10^3 \pcc$) cool ($\sim 10^3$ K) gas surrounded by significant amounts of cosmic-ray and magnetic-field energy. Their spectra show anomalously strong low-ionization and molecular emission lines when compared with galactic molecular clouds exposed to ionizing radiation such as the Orion complex. Previous studies have shown that the spectra cannot be produced by O-star photoionization. Here we calculate the physical conditions in dusty gas that is well shielded from external sources of ionizing photons and is energized either by cosmic rays or dissipative MHD waves. Strong molecular hydrogen lines, with relative intensities similar to those observed, are produced. Selection effects introduced by the microphysics produce a correlation between the \htwo line upper level energy and the population temperature. These selection effects allow a purely collisional gas to produce \htwo emission that masquerades as starlight-pumped \htwo but with intensities that are far stronger. This physics may find application to any environment where a broad range of gas densities or heating rates occur.

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arXiv2008

Cool X-ray emitting gas in the core of the Centaurus cluster of galaxies

We use a deep XMM-Newton Reflection Grating Spectrometer observation to examine the X-ray emission from the core of the Centaurus cluster of galaxies. We clearly detect Fe-XVII emission at four separate wavelengths, indicating the presence of cool X-ray emitting gas in the core of the cluster. Fe ions from Fe-XVII to XXIV are observed. The ratio of the Fe-XVII 17.1A lines to 15.0A line and limits on O-VII emission indicate a lowest detected temperature in the emitting region of 0.3 to 0.45 keV (3.5 to 5.2x10^6 K). The cluster also exhibits strong N-VII emission, making it apparent that the N abundance is supersolar in its very central regions. Comparison of the strength of the Fe-XVII lines with a Solar metallicity cooling flow model in the inner 17 kpc radius gives mass deposition rates in the absence of heating of 1.6-3 Msun/yr. Spectral fitting implies an upper limit of 0.8 Msun/yr below 0.4 keV, 4 Msun/yr below 0.8 keV and 8 Msun/yr below 1.6 keV. The cluster contains X-ray emitting gas over at least the range of 0.35 to 3.7 keV, a factor of more than 10 in temperature. We find that the best fitting metallicity of the cooler components is smaller than the hotter ones, confirming that the apparent metallicity does decline within the inner 1 arcmin radius.

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arXiv2006

Cold molecular gas in the Perseus cluster core - Association with X-ray cavity, Halpha filaments and cooling flow -

Cold molecular gas has been recently detected in several cooling flow clusters of galaxies where huge optical nebulosities often stand. These optical filaments are tightly linked to the cooling flow and to the related phenomena, like the rising bubbles of relativistic plasma, fed by the radio jets. We present here a map in the CO(2-1) rotational line of the cold molecular gas associated with some Halpha filaments surrounding the central galaxy of the Perseus cluster: NGC 1275. The map, extending to about 50 kpc (135 arcsec) from the center of the galaxy, has been made with the 18-receiver array HERA, at the focus of the IRAM 30m telescope. Although most of the cold gas is concentrated to the center of the galaxy, the CO emission is also clearly associated to the extended filaments conspicuous in ionised gas and could trace a possible reservoir fueling the star formation there. Some of the CO emission is also found where the X-ray gas could cool down more efficiently: at the rims of the central X-ray cavity (where the hot gas is thought to have been pushed out and compressed by the central AGN expanding radio lobes). The CO global kinematics does not show any rotation in NGC 1275. The cold gas is probably a mixture of gas falling down on the central galaxy and of uplifted gas dragged out by a rising bubble in the intracluster medium. As recenlty suggested in other cluster cores, the cold gas peculiar morphology and kinematics argue for the picture of an intermittent cooling flow scenario where the central AGN plays an important role.

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arXiv2005

On the origin and excitation of the extended nebula surrounding NGC 1275

We use line-of-sight velocity information on the filamentary emission-line nebula of NGC 1275 to infer a dynamical model of the nebula's flow through the surrounding intracluster gas. We detect outflowing gas and flow patterns that match simulations of buoyantly rising bubbles from which we deduce that some of the nebula filaments have been drawn out of NGC 1275. We find a radial gradient of the ratio [NII]6584/H-alpha which may be due to a variation in metallicity, interactions with the surrounding intracluster medium or a hardening of the excitation mechanism. We find no preferred spatial correlation of stellar clusters within the filaments and there is a notable lack of [OIII]5007 emission, therefore it is unlikely that the filaments are ionized by stellar UV.

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arXiv2005

A very deep Chandra observation of the Perseus cluster: shocks, ripples and conduction

We present the first results from a very deep Chandra X-ray observation of the core of the Perseus cluster of galaxies. A pressure map reveals a clear thick band of high pressure around the inner radio bubbles. The gas in the band must be expanding outward and the sharp front to it is identified as a shock front, yet we see no temperature jump across it; indeed there is more soft emission behind the shock than in front of it. We conclude that in this inner region either thermal conduction operates efficiently or the co-existing relativistic plasma seen as the radio mini-halo is mediating the shock. If common, isothermal shocks in cluster cores mean that we cannot diagnose the expansion speed of radio bubbles from temperature measurements alone. They can at times expand more rapidly than currently assumed without producing significant regions of hot gas. Bubbles may also be significantly more energetic. The pressure ripples found in earlier images are identified as isothermal sound waves. A simple estimate based on their amplitude confirms that they can be an effective distributed heat source able to balance radiative cooling. We see multiphase gas with about 10^9 Msun at a temperature of about 0.5 keV. Much, but not all, of this cooler gas is spatially associated with the optical filamentary nebula around the central galaxy, NGC 1275. A residual cooling flow of about 50 Msun/yr may be taking place. A channel is found in the pressure map along the path of the bubbles, with indications found of outer bubbles. The channel connects in the S with a curious cold front.

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arXiv2005

The Prevalence of Cooling Cores in Clusters of Galaxies at z~0.15-0.4

We present a Chandra study of 38 X-ray luminous clusters of galaxies in the ROSAT Brightest Cluster Sample (BCS) that lie at z~0.15-0.4. We find that the majority of clusters at moderate redshift generally have smooth, relaxed morphologies with some evidence for mild substructure perhaps indicative of recent minor merger activity. Using spatially-resolved spectral analyses, cool cores appear to still be common at these redshifts. At a radius of 50 kpc, we find that at least 55 per cent of the clusters in our sample exhibit signs of mild cooling [t(cool)<10 Gyr], while in the central bin at least 34 per cent demonstrate signs of strong cooling [t(cool)<2 Gyr]. These percentages are nearly identical to those found for luminous, low-redshift clusters of galaxies, suggesting little evolution in cluster cores since z~0.4 and that heating and cooling mechanisms may already have stabilised by this epoch. Comparing the central cooling times to central Halpha emission in BCS clusters, we find a strong correspondence between the detection of Halpha and central cooling time. (Abridged)

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arXiv2005

The nature of the molecular gas system in the core of NGC 1275

We present near-infrared integral field spectroscopy of the central kiloparsec of NGC 1275 at the heart of the Perseus cluster of galaxies, obtained with the UIST IFU on UKIRT. The nuclear ro-vibrational H_2 emission is spatially resolved and is likely to originate approximately 50 pc from the active nucleus. The Pa-alpha emission is, by contrast, spatially unresolved. The requirements for thermal excitation of the H_2 by nuclear X-radiation, its kinematics on sub-arcsec scales, and its stability against self-gravity, together suggest that the observed H_2 is part of a clumpy disk rotating about the radio-jet axis. The sharp jump in H_2 velocity across the nucleus implies a black hole mass of 3.4E8 M(Solar), with a systematic error of 0.18 dex due to the uncertainty in the radio jet inclination. This agrees well with the value implied by the empirical correlation between black hole mass and stellar velocity dispersion for nearby elliptical galaxies, and is ~100 times the stellar mass in this region.

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arXiv2004

Detections of Molecular Hydrogen in the Outer Filaments of NGC 1275

We present clear spectroscopic detections of molecular hydrogen in the outer filaments of the H-alpha nebula surrounding the central galaxy of the Perseus cluster, NGC 1275. This implies the presence of warm molecular gas clouds at projected distances up to 24 kpc from the nucleus of the host galaxy, embedded in the hot intracluster medium. The emission-line intensity ratios reveal that the molecular hydrogen is predominately thermally excited with excitation temperatures of 1600-2200K, suggesting a lack of pressure balance between the molecular component and its surrounding medium. Excitation by stellar UV or by the central AGN is shown to be unlikely, whilst thermal excitation by X-rays or conduction from the ICM or shocks are all possibilities. Evidence for a non-thermal component is found in the spectra of some regions based on a low ortho-to-para ratio.

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arXiv2004

The Galaxy Cluster Abell 3581 as seen by Chandra

We present results from an analysis of a Chandra observation of the cluster of galaxies Abell 3581. We discover the presence of a point-source in the central dominant galaxy which is coincident with the core of the radio source PKS 1404-267. The emission from the intracluster medium is analysed, both as seen in projection on the sky, and after correcting for projection effects, to determine the spatial distribution of gas temperature, density and metallicity. We find that the cluster, despite hosting a moderately powerful radio source, shows a temperature decline to around 0.4Tmax within the central 5 kpc. The cluster is notable for the low entropy within its core. We test and validate the xspec projct model for determining the intrinsic cluster gas properties.

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arXiv2003

Powerful obscured AGN among X-ray hard, optically-dim serendipitous Chandra sources

We present a small sample of Chandra X-ray sources selected from the fields of ACIS observations which probe fluxes around the break in the hard band source counts. The targets of these fields include 9 nearby galaxy clusters, 1 distant cluster and 2 powerful radio galaxy fields. The follow-up of this serendipitous sample was biased towards X-ray hard and optically-dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray fluxes, optical and near-infrared photometry leading to 51 photometric redshifts in all and 18 independently measured spectroscopic redshifts. Few sources are associated with the target fields themselves. Fifty-six of 58 sources imaged in the K-band are detected at K<~20 with K_median=18, and of these, 38 have hard X-ray count ratios and 24 of these are significantly hard with most of the counts emerging about 2 keV. We find that almost all are AGN hosted in massive early-type host galaxies with a photometric redshift distribution peaking at z~1. Two type 2 quasars with intrinsic X-ray luminosity L>~10^{45} erg/s, Fe K_alpha emission lines and absorbing column density nH>10^{23} cm^{-2} -- and nH>~10^{24} cm^{-2} in one case -- are discussed in detail; the sample contains at least 12 potential type 2 quasars in all. We discuss various detection strategies for type 2 quasars and calculate their inferred space density. This combines and extends a number of results from subsamples already published by us.

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arXiv2003

A deep Chandra observation of the Perseus cluster: shocks and ripples

We present preliminary results from a deep observation lasting almost 200 ks, of the centre of the Perseus cluster of galaxies around NGC 1275. The X-ray surface brightness of the intracluster gas beyond the inner 20 kpc, which contains the inner radio bubbles, is very smooth apart from some low amplitude quasi-periodic ripples. A clear density jump at a radius of 24 kpc to the NE, about 10 kpc out from the bubble rim, appears to be due to a weak shock driven by the northern radio bubble. A similar front may exist round both inner bubbles but is masked elsewhere by rim emission from bright cooler gas. The continuous blowing of bubbles by the central radio source, leading to the propagation of weak shocks and viscously-dissipating sound waves seen as the observed fronts and ripples, gives a rate of working which balances the radiative cooling within the inner 50 kpc of the cluster core.

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arXiv2002

A survey of molecular hydrogen in the central galaxies of cooling flows

We present a large sample of H- and K-band spectra of 32 optically line luminous central cluster galaxies. We find significant rovibrational H_2 emission in 23 of these galaxies as well as H recombination and/or [FeII] emission in another 5. This represents a fourfold increase in the number of molecular line detections known. A number of the detections are of extended emission (5-20 kpc). In several objects we find significant [SiVI] emission that appears to correlate with the strength of high ionization lines in the optical (e.g. [OIII]). This comprehensive sample builds on previous work and confirms that warm (1000-2500K) molecular hydrogen is present wherever there is ionized material in the cores of cooling flows and in most cases it also coincides with CO molecular line emission.

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arXiv2002

Exciting molecular hydrogen in the central galaxies of cooling flows

The origin of rovibrational H_2 emission in the central galaxies of cooling flow clusters is poorly understood. Here we address this issue using data from our near-infrared spectroscopic survey of 32 of the most line luminous such systems, presented in the companion paper by Edge et al. (2002). We consider excitation by X-rays from the surrounding intracluster medium, UV radiation from young stars, and shocks. The v=1-0 K-band lines with upper levels within 10^4 K of the ground state appear to be mostly thermalised (implying gas densities above ~10^5 cm^-3), with excitation temperatures typically exceeding 2000K, as found earlier by Jaffe, Bremer and van der Werf (2001). Together with the lack of strong v=2-0 lines in the H-band, this rules out UV fluorescence. Using the CLOUDY photoionisation code, we deduce that the H_2 lines can originate in a population of dense clouds, exposed to the same hot (T~50000K) stellar continuum as the lower density gas which produces the bulk of the forbidden optical line emission in the H-alpha luminous systems. This dense gas may be in the form of self-gravitating clouds deposited directly by the cooling flow, or may instead be produced in the high-pressure zones behind strong shocks. Furthermore, the shocked gas is likely to be gravitationally unstable, so collisions between the larger clouds may lead to the formation of globular clusters.

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arXiv2002

Conduction and cooling flows

Chandra and XMM-Newton observations have confirmed the presence of large temperature gradients within the cores of many relaxed clusters of galaxies. Here we investigate whether thermal conduction operating over those gradients can supply sufficient heat to offset radiative cooling. Narayan & Medvedev (2001) and Gruzinov (2002) have noted, using published results on cluster temperatures, that conduction within a factor of a few of the Spitzer rate is sufficient to balance bremsstrahlung cooling. From a detailed study of the temperature and emission measure profiles of Abell 2199 and Abell 1835, we find that the heat flux required by conduction is consistent with or below the rate predicted by Spitzer in the outer regions of the core. Conduction may therefore explain the lack of observational evidence for large mass cooling rates inferred from arguments based simply on radiative cooling, provided that conductivity is suppressed by no more than a factor of three below the full Spitzer rate. To stem cooling in the cluster centre, however, would necessitate conductivity values at least a factor of two larger than the Spitzer values, which we consider implausible. This may provide an explanation for the observed star formation and optical nebulosities in cluster cores. The solution is likely to be time dependent. We briefly discuss the possible origin of the cooler gas and the implications for massive galaxies.

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arXiv2002

Chandra observations of Abell 2199

We present results from an analysis of two Chandra observations of the rich, nearby galaxy cluster Abell 2199. We find evidence (having corrected for projection effects) for radial gradients in temperature and metallicity in the X-ray emitting gas: the temperature drops from kT~4.2 keV at R=200 kpc to 1.6 keV within R=5 kpc of the centre. The metallicity rises from ~0.3 solar at R=200 kpc to ~0.7 solar at R=30 kpc before dropping to 0.3 solar within the central 5 kpc. We find evidence for structure in the surface brightness distribution associated with the central radio source 3C338. No evidence is found for the gas having a large spread in temperature at any particular location despite the cooling time being short (<10**9yr) within the central ~15 kpc. Heating and mass cooling rates are calculated for various assumptions about the state of the gas.

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arXiv2002

The physical conditions within dense cold clouds in cooling flows II

This is a progress report on our numerical simulations of conditions in the cold cores of cooling flow condensations. The physical conditions in any non-equilibrium plasma are the result of a host of microphysical processes, many involving reactions that are research areas in themselves. We review the dominant physical processes in our previously published simulations, to clarify those issues that have caused confusion in the literature. We show that conditions in the core of an X-ray illuminated cloud are very different from those found in molecular clouds, largely because carbon remains substantially atomic and provides powerful cooling through its far infrared lines. We show how the results of the Opacity Project have had a major impact on our predictions, largely because photoionization cross sections of atoms and first ions are now calculated to be far larger than previously estimated. Finally we show that the predicted conditions are strongly affected by such complexities as microturbulence or the presence of small amounts of dust. Large masses of cold dense gas, in addition to the warmer molecular gas detected recently, could be present in cooling flows.

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arXiv2002

Chandra reveals a double-sided X-ray jet in the quasar 3C9 at z=2.012

A Chandra observation of the radio-loud quasar 3C9 at redshift z=2.012 has revealed extended X-ray emission coincident with the radio jet. Of particular interest is the appearance of both jet and counterjet, which argues against the X-ray emission being highly beamed. We present the properties of the jets and discuss possible scenarios for the X-ray emission, contrasting them with the single sided jet found in PKS 0637-752, which has a similar X-ray luminosity. The jet X-ray emission in 3C9 is likely to be due to either nonthermal emission from a sheath with bulk Lorentz factor less than 1.5, or thermal emission from shocked cold gas surrounding the quasar. The thermal possibility implies a high mass for the cold gas unless it is highly clumped.

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