arXiv subjects

David S. Davis

Publications and source records attributed to David S. Davis.

14 recordsLinked to original sources

arXiv2011

Fermi/LAT Observations of Swift/BAT Seyferts: on the Contribution of Radio-quiet AGN to the Extragalactic Gamma-ray Background

We present the analysis of 2.1 years of Fermi/LAT data on 491 Seyfert galaxies detected by the Swift/BAT survey. Only the two nearest objects, NGC 1068 and NGC 4945, which were identified in the Fermi First-year Catalog, are detected. Using the Swift/BAT and radio 20 cm fluxes, we define a new radio-loudness parameter $R_{X, BAT}$ where radio loud objects have $\log R_{X, BAT} > - 4.7$. Based on this parameter, only radio loud sources are detected by Fermi/LAT. An upper limit to the flux of the undetected sources is derived to be $\sim 2 \times 10^{-11}$ photons cm$^{-2}$ s$^{-1}$, approximately seven times lower than the observed flux of NGC 1068. Assuming a median redshift of 0.031, this implies an upper limit to the $γ$-ray (1--100 GeV) luminosity of $\lesssim 3 \times 10^{41}$ erg s$^{-1}$. In addition, we identified 120 new Fermi/LAT sources near the Swift/BAT Seyferts with significant Fermi/LAT detections. A majority of these objects do not have \bat counterparts, but their possible optical counterparts include blazars, FSRQs, and quasars.

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arXiv2009

Testing the Reliability of Cluster Mass Indicators with a Systematics Limited Dataset

We present the mass-X-ray observable scaling relationships for clusters of galaxies using the XMM-Newton cluster catalog of Snowden et al. Our results are roughly consistent with previous observational and theoretical work, with one major exception. We find 2-3 times the scatter around the best fit mass scaling relationships as expected from cluster simulations or seen in other observational studies. We suggest that this is a consequence of using hydrostatic mass, as opposed to virial mass, and is due to the explicit dependence of the hydrostatic mass on the gradients of the temperature and gas density profiles. We find a larger range of slope in the cluster temperature profiles at r_{500} than previous observational studies. Additionally, we find only a weak dependence of the gas mass fraction on cluster mass, consistent with a constant. Our average gas mass fraction results argue for a closer study of the systematic errors due to instrumental calibration and analysis method variations. We suggest that a more careful study of the differences between various observational results and with cluster simulations is needed to understand sources of bias and scatter in cosmological studies of galaxy clusters.

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arXiv2008

A novel method for measuring the extragalactic background light: Fermi application to the lobes of Fornax A

We describe a new method for measuring the extragalactic background light (EBL) through the detection of $γ$-ray inverse Compton (IC) emission due to scattering of the EBL photons off relativistic electrons in the lobes of radio galaxies. Our method has no free physical parameters and is a powerful tool when the lobes are characterized by a high energy sharp break or cutoff in their electron energy distribution (EED). We show that such a feature will produce a high energy IC `imprint' of the EBL spectrum in which the radio lobes are embedded, and show how this imprint can be used to derive the EBL. We apply our method to the bright nearby radio galaxy Fornax A, for which we demonstrate, using WMAP and EGRET observations, that the EED of its lobes is characterized by a conveniently located cutoff, bringing the IC EBL emission into the {\sl Fermi} energy range. We show that {\sl Fermi} will set upper limits to the optical EBL and measure the more elusive infrared EBL.

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arXiv2006

The Two-dimensional XMM-Newton Group Survey: z<0.012 groups

We present the results of the 2-dimensional XMM-Newton Group Survey (2dXGS), an archival study of nearby galaxy groups. In this paper we consider eleven nearby systems (z<0.012) in Mulchaey et al. (2003), which span a broad range in X-ray luminosity from 10^40 to 10^43 ergs/s. We measure the iron abundance and temperature distribution in these systems and derive pressure and entropy maps. We find statistically significant evidence for structure in the entropy and pressure of the gas component of seven groups on the 10-20% level. The XMM-Newton data for the three groups with best statistics also suggest patchy metalicity distributions within the central 20--50 kpc of the brightest group galaxy, probed with 2-10 kpc resolution. This provides insights into the processes associated with thermalization of the stellar mass loss. Analysis of the global properties of the groups reveals a subclass of X-ray faint groups, which are characterized by both higher entropy and lower pressure. We suggest that the merger history of the central elliptical is responsible for both the source and the observed thermodynamical properties of the hot gas of the X-ray faint groups.

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arXiv2004

Substructure Analysis of Selected Low Richness 2dFGRS Clusters of Galaxies

Complementary one-, two-, and three-dimensional tests for detecting the presence of substructure in clusters of galaxies are applied to recently obtained data from the 2dF Galaxy Redshift Survey. The sample of 25 clusters used in this study includes 16 clusters not previously investigated for substructure. Substructure is detected at or greater than the 99% CL level in at least one test for 21 of the 25 clusters studied here. From the results, it appears that low richness clusters commonly contain subclusters participating in mergers. About half of the clusters have two or more components within 0.5 h^{-1} Mpc of the cluster centroid, and at least three clusters (Abell 1139, Abell 1663, and Abell S333) exhibit velocity-position characteristics consistent with the presence of possible cluster rotation, shear, or infall dynamics. The geometry of certain features is consistent with influence by the host supercluster environments. In general, our results support the hypothesis that low richness clusters relax to structureless equilibrium states on very long dynamical time scales (if at all).

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arXiv2003

XMM Observations of an IXO in NGC 2276

We present the results from a ~53 ksec XMM observation of NGC 2276. This galaxy has an unusual optical morphology with the disk of this spiral appearing to be truncated along the western edge. This XMM observation shows that the X-ray source at the western edge is a bright Intermediate X-ray Object (IXO). Its spectrum is well fit by a multi-color disk blackbody model used to fit optically thick standard accretion disks around black holes. The luminosity derived for this IXO is 1.1x10^41 erg/s in the 0.5 - 10 keV band making it one of the most luminous discovered to date. The large source luminosity implies a large mass black hole if the source is radiating at the Eddington rate. On the other hand, the inner disk temperature determined here is too high for such a massive object given the standard accretion disk model. In addition to the IXO we find that the nuclear source in this galaxy has dimmed by at least a factor of several thousand in the eight years since the ROSAT HRI observations.

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arXiv2003

An X-ray Atlas of Groups of Galaxies

A search was conducted for a hot intragroup medium in 109 low-redshift galaxy groups observed with the ROSAT PSPC. Evidence for diffuse, extended X-ray emission is found in at least 61 groups. Approximately one-third of these detections have not been previously reported in the literature. Most of the groups are detected out to less than half of the virial radius with ROSAT. Although some spiral-rich groups do contain an intragroup medium, diffuse emission is restricted to groups that contain at least one early-type galaxy.

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arXiv1999

Gas, Iron and Gravitational Mass in Galaxy Clusters: The General Lack of Cluster Evolution at z < 1.0

We have analyzed the ASCA data of 29 nearby clusters of galaxies systematically, and obtained temperatures, iron abundances, and X-ray luminosities of their intracluster medium (ICM). We also estimate ICM mass using the beta model, and then evaluate iron mass contained in the ICM and derive the total gravitating mass. This gives the largest and most homogeneous information about the ICM derived only by the ASCA data. We compare these values with those of distant clusters whose temperatures, abundances, and luminosities were also measured with ASCA, and find no clear evidence of evolution for the clusters at z<1.0. Only the most distant cluster at z=1.0, AXJ2019.3+1127, has anomalously high iron abundance, but its iron mass in the ICM may be among normal values for the other clusters, because the ICM mass may be smaller than the other clusters. This may suggest a hint of evolution of clusters at z ~ 1.0.

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arXiv1999

Reconstructing the Merger History of the A3266 Galaxy Cluster

A temperature map of the A3266 galaxy cluster has been derived from the ASCA GIS observations. It shows an asymmetric pattern of heating indicative of an ongoing merger between a group sized sub-cluster and the main cluster. The galaxy distribution shows two peaks connected in a barlike structure running NE to SW through the central region of the main cluster, defining the merger axis. The temperature of the intergalactic medium generally decreases from SW to NE along the merger axis with a peak of 13.2 +3.4/-2.0 keV in a region which is perpendicular to the merger axis and extends through the main cluster density peak. The central bar has a velocity dispersion of ~1300 km/s, compared to \~1000 km/s for the surrounding cluster. The central bar also has two distinct density peaks in the distribution of galaxies, yet it has a velocity distribution which is consistent with a single Gaussian. This implies a merger in the plane of the sky. The optical and X-ray data taken together show that a loose group of about ~30 galaxies has penetrated the main cluster from the SW, decoupling from their original intergroup medium and passing through a strong shock front. Two radio galaxies, one a wide angle tail morphology (WAT) and the other a narrow angle tail (NAT), are located to the SW of the main cluster in the post-shock gas. Since the merger is in the plane of the sky, a dynamical analysis cannot be applied to derive the velocity of the merger. Alternatively, using the pre- and post-shock gastemperature and assuming an adiabatic shock, we calculate a relative gas velocity of ~1400 km/s. The alignment of the NAT and WAT relative to the shock front combined with the high gas velocity from the merger implies that the observed radio morphologies formed via ram-pressure as a result of the merger.

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arXiv1998

The Enrichment History of Hot Gas in Poor Galaxy Groups

We have analyzed the ASCA SIS and GIS data for seventeen groups and determined the average temperature and abundance of the hot x-ray emitting gas. For groups with gas temperatures less than 1.5 keV we find that the abundance is correlated with the gas temperature and luminosity. We have also determined the abundance of the alpha-elements and iron independently for those groups with sufficient counts. We find that for the cool groups (i.e. kT <1.5 keV) the ratio of alpha-elements to iron is ~1, about half that seen in clusters. Spectral fits with the S, Si and Fe abundances allowed to vary separately suggest the S/Fe ratio is similar to that seen in clusters while the Si/Fe ratio in groups is half the value determined for richer systems. The mass of metals per unit blue luminosity drops rapidly in groups as the temperature drops. There are two possible explanations for this decrease. One is that the star formation in groups is very different from that in rich clusters. The other explanation is that groups lose much of their enriched material via winds during the early evolution of ellipticals. If the latter is true, we find that poor groups will have contributed significantly (roughly 1/3 of the metals) to the enrichment of the intergalactic medium.

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arXiv1997

An X-Ray Temperature Map of the Merging Cluster Abell 2255

We present a spatially resolved map of intracluster gas temperatures in the rich cluster Abell 2255, using ROSAT PSPC data. The intracluster gas is strongly non-isothermal and the temperature distribution supports the conclusions of Burns et al. (1995) that Abell~2255 is currently undergoing a merger. The hottest regions are near the core, northwest of the center. A map of the ratio of softer to harder X-ray photons reveals an anomalously soft region northeast of the center, coincident with a radio ridge discovered by Burns et al (1995). We suggest that this cooler region is associated with an infalling group of galaxies which is giving rise to the radio ridge. Meanwhile, a much more substantial recent merger between comparably sized subcluster components can account for the more global merger signatures, which include the extensive central radio halo, the large scale gas temperature variations, the pair of central dominant ellipticals, elliptical X-ray isophotes offset from these dominant galaxies, and the very high velocity dispersion of galaxies in the cluster.

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arXiv1997

Gravitational Interactions in Poor Galaxy Groups

We report the results of the spatial analysis of deep ROSAT HRI observations, optical imaging and spectroscopy, and high-resolution VLA H I and continuum imaging of NGC 1961 and NGC 2276. These spirals were selected as showing some previous evidence for interaction with a surrounding (hot) diffuse medium. Our results favor most aspects of these galaxies as being shaped by gravitational interactions with companions, rather than the asymmetric pressure from motion through an external medium. The old stars follow the asymmetric structures of young stars and ionized gas, which suggests a tidal origin for the lopsided appearance of these galaxies. In NGC 2276, the H I and star-forming regions are strongly concentrated along the western edge of the disk. In this case, the ROSAT HRI detects the brightest star-forming regions as well as the diffuse disk emission, the most distant galaxy with such a detection. An asymmetric ionization gradient in the H II regions suggests radial movement of gas, which might have occurred in either tidal or wind scenarios. The X-ray emission from NGC 1961 is dominated by a point source near the nucleus of the galaxy but extended emission is seen out to a radius of ~0.8'. Previous studies of the enrichment of the intragroup medium in the NGC 2300 group indicates that stripping may be important in this system, but the density of the IGM is much too tenuous to effectively strip the gas from the galaxy. However, we propose that gravitational interactions in the group environment may enhance stripping. During a gravitational encounter the disk of the spiral galaxy may be warped, making ram pressure stripping more efficient than in a quiescent disk.

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arXiv1996

ROSAT Temperatures and Abundances for a Complete Sample of Elliptical Galaxies

We determine X-ray temperatures and abundances for elliptical galaxies drawn from a complete, optically selected sample. The optical magnitude-limited sample consists of 43 galaxies, complete to a (corrected) B magnitude of m_B^o=11.36. Of these, 30 have enough X-ray spectral counts to allow temperature determinations. We find that the temperatures of the X-ray emitting gas in these ellipticals are correlated with the central stellar velocity dispersions: T ~ sigma^1.45; this is a shallower trend than the simple thermal relation T ~ sigma^2. The diffuse gas is substantially hotter than the kinetic temperature of the luminous stars, since kT > mu m_p sigma^2 for all galaxies with measurable temperatures. This strongly indicates that dark matter halos are characterized by velocity dispersions which exceed those of the luminous stars. We see no evidence of emission from X-ray binaries becoming progressively more dominant in lower luminosity ellipticals. We find that so-called "supersoft" sources adhere to the observed kT-sigma relation, so they are no softer than expected for their velocity dispersions. We also find that ISM temperatures and abundances are correlated, with the gas in hotter systems being more enriched than in cooler galaxies. However, no correlation is found between gaseous abundances and stellar abundances, as inferred from Mg_2 indices.

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arXiv1996

X-Ray Emission from M32: X-Ray Binaries or a micro-AGN?

We have analysed archival {\it ROSAT} PSPC data for M32 in order to study the x-ray emission from this nearest elliptical galaxy. We fit spectra from three long exposures with Raymond-Smith, thermal bremsstrahlung, and power-law models. All models give excellent fits. The thermal fits have kT$\approx$4 keV, the Raymond-Smith iron abundance is $0.4^{+0.7}_{-0.3}$ Solar, the power-law fit has $α$=1.6$\pm$0.1, and all fits have $N_H$ consistent with the Galactic column. The source is centered on M32 to an accuracy of 9$''$, and unresolved at 27$''$ FWHM ($\sim$90 pc). M32 is x-ray variable by a factor of 3--5 on timescales of a decade down to minutes, with evidence for a possible period of $\sim$1.3 days. There are two plausible interpretations for these results: 1) Emission due to low-mass x-ray binaries; 2) Emission due to accretion onto a massive central black hole. Both of these possibilities are supported by arguments based on previous studies of M32 and other old stellar systems; the {\it ROSAT} PSPC data do not allow us to unambiguously choose between them. Observations with the {\it ROSAT} HRI and with {\it ASCA} are required to determine which of these two very different physical models is correct.

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