Richard E. Griffiths

Publications (4)12.05 Total impact

• Source

  Available from: adsabs.harvard.edu

  Article: ASCA Observations of Starbursting Dwarf Galaxies: The Case of NGC 1569

  Roberto della Ceca, Richard E. Griffiths, Timothy M. Heckman, John W. MacKenty
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  ABSTRACT: We present ASCA observations of the nearby star-forming dwarf galaxy NGC 1569. Combining the ASCA data with archival ROSAT PSPC and HRI data, optical broad and narrow Hα images, and new infrared K-band images, we have investigated, in greater detail than previously possible, the X- ray properties of this prototype dwarf starburst. The principal results of this investigation are as follows: 1. The ASCA SIS broad-band (0.5-6 keV) X-ray spectrum reveals the presence of at least two spectral components. The soft component is best described by a thermal model with a temperature of ~0.64-0.8 keV, while the hard component can be described by a thermal model with a temperature of ~3.7 keV or by a power law with photon index of ~2.1. The total unabsorbed X-ray luminosity is ~3.1 x 10^38^ ergs s^-1^(0.5-2.0 keV) and ~1.3 x 10^38^ ergs s^-1^ (2-10 keV). The soft thermal component provides ~60% of the total luminosity in the (0.5-2.0) keV energy band but less than ~10% of the total luminosity in the 2-10 keV energy band. 2. The ROSAT PSPC image of NGC 1569 shows that the soft (E < 2 keV) X- ray emission is clearly extended, is morphologically associated with the system of Hα filaments (in which NGC 1569 is immersed), and seems to extend preferentially along the optical minor axis of the galaxy. In the azimuthally averaged and background-corrected radial profile, the soft X-ray emission (from the ROSAT PSPC) can be detected out to a radius of ~1.9' (~1.2 kpc); the half-light radius is ~28" (~0.29 kpc). The soft X-ray emission profile (from the ASCA SIS) can be described by a simple model consisting of a pointlike X-ray source along with diffuse X-ray emission described by a bidimensional Gaussian distribution with σ ~30" (~0.31 kpc). About 40% of the soft X-ray emission derives from the pointlike source and the remainder from the diffuse emission. The hard (E > 2 keV) X-ray emission is consistent with a pointlike source (the central starburst) at the spatial resolution of the ASCA XRT + SIS system. 3. The ROSAT HRI image resolves the central starburst into a group of at least four bright knots of X-ray emission. Almost all the flux in the hard spectral component could be associated with the two brightest knots of X-ray emission. If this is the case, the spectral shape that we observe for the hard spectral component suggests an origin of the X-ray emission in young supernova remnants or in low-mass X-ray binaries. The new ASCA data provide crucial support for the idea that starbursts can drive gas out of dwarf galaxies. First, the data establish that the diffuse X-ray emission in NGC 1569 is produced by hot gas. Second, they show that the temperature of this gas far exceeds the depth of the galaxy's shallow potential well. The data also suggest that a population of galaxies like NGC 1569 would have X-ray spectra that are too soft to contribute significantly to the hard (>2 keV) cosmic X-ray background. These objects contribute in a major way to the soft X-ray background only.
  The Astrophysical Journal 09/1996; 469:662. · 6.02 Impact Factor
• Source

  Available from: Brian Boyle

  Article: A deep ROSAT Survey - XII. The X-ray spectra of faint ROSAT sources

  Omar Almaini, Tom Shanks, Brian J. Boyle, Richard E. Griffiths, Nathan Roche, Gordon C. Stewart, Ioannis Georgantopoulos
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  ABSTRACT: Optical spectroscopy has enabled us to identify the optical counterparts to over 200 faint X-ray sources to a flux limit of S_(0.5-2keV)=4x10^-15 erg s^-1 cm^-2 on 5 deep ROSAT fields. Here we present a spectral analysis of all the X-ray sources to investigate claims that the average source spectra harden at faint X-ray flux. From a hardness ratio analysis we confirm that the average spectra from 0.5-2 keV harden from an equivalent photon index of $\Gamma=2.2$ at S_(0.5-2keV)=1x10^-13 erg s^-1 cm^-2 to $\Gamma=1.7$ below 1x10^-14 erg s^-1 cm^-2. These spectral changes are due to the emergence of an unidentified source population rather than the class of X-ray QSOs already identified. The 128 QSOs detected so far show no evidence for spectral hardening over this energy range and retain a mean photon index of $\Gamma=2.2$. Recent work suggests that many of the remaining unidentified sources are X-ray luminous galaxies. Taking a subset identified as the most likely galaxy candidates we find that these show significantly harder spectra than QSOs. The emission line galaxies in particular show spectra more consistent with the residual X-ray background, with $\Gamma=1.51 \pm 0.1$ from 0.1-2 keV. Individually the galaxies appear to be a mixture of absorbed and unabsorbed X-ray sources. Combined with recent cross-correlation results and work on the source number count distribution, these results suggest that we may be uncovering the missing hard component of the cosmic X-ray background.
  06/1996;
• Source

  Available from: Brian Boyle

  Article: A deep ROSAT Survey X: X-ray Luminous Narrow Emission Line Galaxies

  Richard E. Griffiths, Roberto Della Ceca, Ioannis Georgantopoulos, Brian J. Boyle, Gordon C. Stewart, Tom Shanks, Antonella Fruscione
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  ABSTRACT: X-ray luminous narrow emision-line galaxies (NELG) have been previously identified and proposed as an important class of extragalactic X-ray sources, with a potentially significant contribution to the total extragalactic X-ray flux at energies below $\sim$ 10 keV. In order to investigate and clarify this possibility, we have used a sample of NELG found in 5 deep ROSAT fields and similar samples belonging to the Cambridge-Cambridge ROSAT Serendipity Survey and to the {\it Einstein} Observatory Extended Medium Sensitivity Survey sample. The principal results of this investigation are as follows: a) for a given optical luminosity, the typical X-ray luminosity of NELGs is about one or two orders of magnitude higher than that of normal galaxies; b) the ratio of the surface density of NELGs compared with BLAGN increases from about 0.04 at fx >= 6 x 10^{-13} erg cm^{-2} s^{-1} to about 0.1 at fx >= 10^{-14} erg cm^{-2} s^{-1}, suggesting that the surface density of NL galaxies might be very close to that of BLAGN at fx ~ 10^{-15} erg cm^{-2} s^{-1}; c) we find that these objects are described by a cosmological evolution rate similar to that of soft X-ray selected BLAGN; d) the de-evolved (z=0) XLF of NELGs in the luminosity range 10^{41} - 5 x 10^{43} erg s^{-1} is steeper than the BLAGN (z=0) XLF in the same luminosity interval. Their spatial density is significantly lower than the spatial density of X-ray selected BLAGN at Lx(z=0) ~ 5 x 10^{43} erg s^{-1}, but this difference decreases at lower luminosities such that at Lx(z=0) < 10^{42} erg s^{-1} the spatial density of NELGs is very close to that of BLAGN. The implications of these results for the contribution of this class of objects to the cosmic X-ray background are discussed.
  04/1996;
• Source

  Available from: adsabs.harvard.edu

  Article: Search for starbursts among X-ray-selected galaxies - Optical spectroscopy

  Antonella Fruscione, Richard E. Griffiths
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  ABSTRACT: To establish the nature of X-ray-selected galaxies and to probe the potential contribution of starburst galaxies to the X-ray background, low-resolution 350-730-nm spectra have been obtained for the optical counterparts of 15 X-ray sources selected from the Einstein Observatory Extended Medium-Sensitivity Survey and Imaging Proportional Counter Source Catalog. At least three X-ray sources can be classified as starburst galaxies on the basis of line-ratio diagnostics alone. This is consistent with the number expected from recent models which predict that starburst galaxies could account for at least 15-20 percent of the extragalactic X-ray background at 2 keV.
  The Astrophysical Journal 09/1991; 380:L13-L16. · 6.02 Impact Factor