arXiv subjects

Joel N. Bregman

Publications and source records attributed to Joel N. Bregman.

At least 145 records · Page 8Linked to original sources

arXiv2001

Detection of Bulk Motions in the ICM of the Centaurus Cluster

Several recent numerical simulations of off-center cluster mergers predict that significant angular momentum with associated velocities of a few x 10^{3} km/s can be imparted to the resulting cluster. Such gas bulk velocities can be detected by the Doppler shift of X-ray spectral lines with ASCA spectrometers. Using two ASCA observations of the Centaurus cluster, we produced a velocity map for the gas in the cluster's central regions. We also detected radial and azimuthal gradients in temperature and metal abundance distributions, which seem to be associated with the infalling sub-group centered at NGC 4709 (Cen 45). More importantly, we found a significant (>99.8% confidence level) velocity gradient along a line near-perpendicular to the direction of the incoming sub-group and with a maximum velocity difference of ~3.4+-1.1 x 10^{3} km/s. It is unlikely (P < 0.002) that the observed velocity gradient is generated by gain fluctuations across the detectors. While the observed azimuthal temperature and abundance variations can be attributed to the interaction with Cen 45, we argue that the intracluster gas velocity gradient is more likely due to a previous off-center merging event in the main body of the Centaurus cluster.

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arXiv2001

The Detection of a Cooling Flow Elliptical Galaxy from OVI Emission

Cooling flow models for the hot gas in elliptical galaxies predict that gas is cooling at a rate of ~ 1 solar mass/yr, yet there is little evidence for this phenomenon beyond the X-ray waveband. If hot gas is cooling, it will pass through the 3x10^5 K regime and radiate in the OVI 1032,1038 ultraviolet lines, which can be detected with the Far Ultraviolet Spectroscopic Explorer (FUSE) and here we report on FUSE observations of the X-ray bright early-type galaxies NGC 1404 and NGC 4636. In NGC 1404, the OVI doublet is not detected, implying a cooling rate < 0.3 solar masses/yr, which is below the predicted values from the cooling flow model of 0.4-0.9 solar masses/yr. In NGC 4636, both OVI lines are clearly detected, indicating a cooling rate of 0.43 +/- 0.06 solar masses/yr, which falls within the range of values from the cooling flow prediction, 0.36-2.3 solar masses/yr and is closest to the model where the production of the cooled gas is distributed through the galaxy. The emission line widths, 44 +/- 15 km/s, are close to the Doppler broadening value (30 km/s), indicating that the flow is quiescent rather than turbulent, and that the flow velocity is < 30 km/s.

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arXiv2001

Chandra X-ray Observations of the X-ray Faint Elliptical Galaxy NGC 4697

(abridged) A Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which ~80 are low mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. The X-ray emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with {\it ROSAT}, is due to interstellar gas. The cumulative LMXB spectrum is well-fit by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component. Three of the resolved sources in NGC 4697 are supersoft sources. In the outer regions of NGC 4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. The luminosity function of the LMXBs has a ``knee'' at 3.2 x 10^38 ergs/s, which is approximately the Eddington luminosity of a 1.4 Msun neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates black hole and NS binaries. This characteristic luminosity could be used as a distance estimator.

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arXiv2000

Are There ADAFs in Nearby Giant Elliptical Galaxies?

Recently it has been suggested that the hard x-ray power-law tails exhibited in the ASCA spectra of several nearby giant elliptical galaxies are caused by low-radiative efficiency accretion flows (ADAFs) onto a massive black hole in the centers of the galaxies. The estimated fluxes from these central ADAFs as derived from the ASCA x-ray spectral analyses indicate that they may be visible as x-ray point sources. We analyze archival ROSAT HRI images of three such galaxies, NGCs 1399, 4636, and 4696, to determine whether point x-ray sources consistent with ASCA-derived fluxes are present in the centers of the galaxies. We find that the upper limit for the flux of a central point x-ray source in each of these galaxies as determined by the ROSAT HRI data is only marginally consistent with the predicted flux inferred from the ASCA spectra. We suggest that although a central point source, such as a ADAF associated with a massive black hole, cannot be completely ruled out, they can only account for a fraction of the flux of the observed hard x-ray power law components.

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arXiv2000

Detection of Intracluster Gas Bulk Velocities in the Perseus and Centaurus Clusters

We report the results of spatially resolved X-ray spectroscopy of 8 different ASCA pointings distributed symmetrically around the center of the Perseus cluster. The outer region of the intracluster gas is roughly isothermal, with temperature ~6-7 keV, and metal abundance ~ 0.3 Solar. Spectral analysis of the central pointing is consistent with the presence of a cooling flow and a central metal abundance gradient. A significant velocity gradient is found along an axis highly discrepant with the major axis of the X-ray elongation. The radial velocity difference is found to be greater than 1000 km/sMpc at the 90% confidence level. Simultaneous fittings of GIS 2 & 3 indicate that two symmetrically opposed regions have different radial velocities at the 95% confidence level and the F-test rules out constant velocities for these regions at the 99% level. Intrinsic short and long term variations of gain are unlikely (P < 0.03) to explain the velocity discrepancies. We also report the preliminary results of a similar analysis carried out for the Centaurus cluster, where long-exposure SIS data is available. We also find a significant velocity gradient near the central regions (3'-8') of Centaurus. If attributed to bulk rotation the correspondent circular velocity is ~1500+-150km/s (at 90% confidence). The line of maximum velocity gradient in Centaurus is near-perpendicular to the infalling galaxy group associated with NGC 4709.

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arXiv2000

Emission Mechanisms in X-Ray Faint Galaxies

Hot gas dominates the emission in X-ray luminous early-type galaxies, but in relatively X-ray faint systems, integrated X-ray emission from discrete stellar-like sources is thought to be considerable, although the amount of the contribution is controversial. To help resolve this issue, we examine the radial X-ray surface brightness distribution of 17 X-ray faint galaxies observed with the ROSAT HRI and PSPC. We assume that the stellar contribution follows a de Vaucouleurs law while the hot gas component follows a King beta model. For some galaxies, both models fit equally well, but for a number of systems, a dual component model yields the best fit, from which upper bounds are placed on the stellar contribution. Best-fit values for the stellar contribution are inconsistent with (lower than) that suggested by Fabbiano, Gioia, & Trinchieri (1989) and estimated from the bulge of M31, but are consistent with the Forman, Jones, & Tucker (1985) estimate of the stellar fraction in X-ray faint elliptical and S0 galaxies. Our results indicate an upper limit to discrete sources of L_X/L_B = 1.6 x 10^29 ergs/s/L(solar).

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arXiv2000

Resolving the Mystery of X-ray Faint Elliptical Galaxies: Chandra X-ray Observations of NGC 4697

Chandra observations of the X-ray faint elliptical galaxy NGC 4697 resolve much of the X-ray emission (61% within one effective radius) into ~80 point sources, of which most are low mass X-ray binaries (LMXBs). These LMXBs provide the bulk of the hard emission and much of the soft emission as well. Of the remaining unresolved emission, it is likely that about half is from fainter LMXBs, while the other half (~23% of the total emission) is from interstellar gas. Three of the resolved sources are supersoft sources. In the outer regions of NGC 4697, eight of the LMXBs (about 25%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from ~5e37 to ~2.5e39 ergs/s. The luminosity function of the LMXBs has a "knee" at 3.2e38 ergs/s, which is roughly the Eddington luminosity of a 1.4 M_sun neutron star (NS); this knee might be useful as a distance indicator. The highest luminosity source has the Eddington luminosity of a ~20 M_sun black hole (BH). The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main sequence stars.

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arXiv2000

Velocity Gradients in the Intracluster Gas of the Perseus Cluster

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arXiv2000

Iron Abundance Profiles of 12 Clusters of Galaxies Observed With BeppoSAX

We have derived azimuthally-averaged radial iron abundance profiles of the X-ray gas contained within 12 clusters of galaxies with redshift 0.03 < z < 0.2 observed with BeppoSAX. We find evidence for a negative metal abundance gradient in most of the clusters, particularly significant in clusters that possess cooling flows. The composite profile from the 12 clusters resembles that of cluster simulations of Metzler & Evrard (1997). This abundance gradient could be the result of the spatial distribution of gas-losing galaxies within the cluster being more centrally condensed than the primordial hot gas. Both inside and outside the core region, we find a higher abundance in cooling flow clusters than in non-cooling flow clusters. Outside of the cooling region this difference cannot be the result of more efficient sputtering of metals into the gaseous phase in cooling flow clusters, but might be the result of the mixing of low metallicity gas from the outer regions of the cluster during a merger.

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arXiv2000

The X-ray Faint Early-Type Galaxy NGC4697

We analyze archival ROSAT HRI, ROSAT PSPC, and ASCA data of the X-ray faint early-type galaxy NGC4697. The joint ROSAT PSPC + ASCA spectrum is fit by a two-component thermal model, a MEKAL model with kT_{MEKAL}=0.26^{+0.04}_{-0.03} keV with low metallicity and a bremsstrahlung model with kT_{BREM}=5.2^{+3.0}_{-1.6} keV. A similar model was found to fit the spectra of another faint early-type galaxy (NGC4382) and the bulge of M31. We interpret this soft emission as a combination of emission from a soft component of low mass X-ray binaries (LMXBs) and from a low temperature interstellar medium, although the relative contributions of the two components could not be determined. Twelve point sources were identified within 4' of NGC4697, of which 11 are most likely LMXBs associated with the galaxy. The soft X-ray colors of four of the LMXBs in NGC4697 support the claim that LMXBs possess a soft spectral component. Finally, we present a simulation of what we believe the Chandra data of NGC4697 will look like.

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arXiv2000

Maximum Entropy Reconstruction of the Interstellar Medium: I. Theory

We have developed a technique to map the three-dimensional structure of the local interstellar medium using a maximum entropy reconstruction technique. A set of column densities N to stars of known distance can in principle be used to recover a three-dimensional density field n, since the two quantities are related by simple geometry through the equation N = C n, where C is a matrix characterizing the stellar spatial distribution. In practice, however, there is an infinte number of solutions to this equation. We use a maximum entropy reconstruction algorithm to find the density field containing the least information which is consistent with the observations. The solution obtained with this technique is, in some sense, the model containing the minimum structure. We apply the algorithm to several simulated data sets to demonstrate its feasibility and success at recovering ``real'' density contrasts. This technique can be applied to any set of column densities whose end points are specified. In a subsequent paper we shall describe the application of this method to a set of stellar color excesses to derive a map of the dust distribution, and to soft X-ray absorption columns to hot stars to derive a map of the total density of the interstellar medium.

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arXiv2000

Radial Temperature Profiles of 11 Clusters of Galaxies Observed With BeppoSAX

We have derived azimuthally-averaged radial temperature profiles of the X-ray gas contained within 11 clusters of galaxies with redshift z=0.03-0.2 with BeppoSAX. Each of the 11 clusters have had their radial temperature profiles previously determined with ASCA. We find that the temperature profiles of these clusters are generally flat or increase slightly out to ~30% of the virial radius, and that a decline in temperature of 14% out to 30% of the virial radius is ruled out at the 99% confidence level. This is in accordance with a previous ROSAT PSPC study and an ASCA study by White (1999), but in disagreement with an ASCA study by Markevitch et al. (1998) that found on average that cluster temperature profiles decreased significantly with radius.

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arXiv2000

On the Internal Absorption of Galaxy Clusters

A study of the cores of galaxy clusters with the Einstein SSS indicated the presence of absorbing material corresponding to 1E+12 Msun of cold cluster gas, possibly resulting from cooling flows. Since this amount of cold gas is not confirmed by observations at other wavelengths, we examined whether this excess absorption is present in the ROSAT PSPC observations of 20 bright galaxy clusters. For 3/4 of the clusters, successful spectral fits were obtained with absorption due only to the Galaxy, and therefore no extra absorption is needed within the clusters, in disagreement with the results from the Einstein SSS data for some of the same clusters. For 1/4 of the clusters, none of our spectral fits was acceptable, suggesting a more complicated cluster medium than the two-temperature and cooling flow models considered here. However, even for these clusters, substantial excess absorption is not indicated.

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arXiv1999

Constraints on Cool Gas in Rich Clusters of Galaxies

Cool gas should be present in galaxy clusters due to stripping of galactic gas, infall onto the cluster, and from cooling flows. We searched for this gas through resonance absorption line observations from low-ionization metal line gas toward six background quasars. Both cooling flow and non-cooling flow clusters were observed, with lines of sight from the inner to outer parts of the cluster (0.2 <= r_proj/R_cluster <= 0.7). The HST/FOS spectra failed to detect Fe II or Mg II absorption at the cluster redshift, with 1-sigma upper limits on the ion column densities of N <= 10^12 - 10^13 cm^-2. The failure to detect absorption lines implies that the cool cluster gas either is more highly ionized or it has a low covering factor.

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arXiv1999

X-Ray Emission from Rotating Elliptical Galaxies

The slow inward flow of the hot gas in elliptical galaxy cooling flows is nearly impossible to detect directly due to instrumental limitations. However, in rotating galaxies, if the inflowing gas conserves angular momentum, it will eventually form a disk. The X-ray signature of this phenomenon is a flattening of the X-ray isophotes in the inner 1-10 kpc region. This effect is observable, so we have searched for it in X-ray observations of six rotating and non-rotating early-type galaxies, obtained mainly with the ROSAT PSPC and HRI imagers. The ellipticities of the X-ray emission never increase toward the central region, nor are the X-ray ellipticities significantly greater than the ellipticities for the optical stellar emission. Central ellipticities in excess of 0.5 were expected in rotating ellipticals whereas values of 0-0.2 are measured. The failure to detect the expected signature requires a modification to the standard cooling flow picture, possibly including partial galactic winds, rapid mass drop-out, or turbulent redistribution of angular momentum.

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arXiv1999

The Effect of Environment on the X-Ray Emission from Early-Type Galaxies

In order to help understand the phenomena of X-ray emission from early-type galaxies, we obtained an optically flux-limited sample of 34 early-type galaxies, observed with ROSAT. A previous analysis of this sample suggested that the most X-ray luminous galaxies were in rich environments. Here we investigate environmental influences quantitatively, and find a positive correlation between L_B/L_X and the local galaxy density. We suggest that this correlation occurs because the X-ray luminosity is enhanced either through accretion of the intergalactic gas or because the ambient medium stifles galactic winds. When the ambient medium is unimportant, partial or global galactic winds can occur, reducing L_B/L_X. These effects lead to the large observed dispersion in L_X at fixed L_B. We argue that the transition from global winds to partial winds is one of the principle reasons for the steep relationship between L_X and L_B. We discuss details of the data reduction not previously presented, and examine the dependence of L_X on the choice of outer source radius and background location. Effects of Malmquist bias are shown not to be important for the issues addressed. Finally, we compare the temperature deduced for these galaxies from different analyses of ROSAT and ASCA data.

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arXiv1999

Using the Bulge of M31 as a Template for the Integrated X-ray Emission from LMXBs

We have performed joint ASCA + ROSAT PSPC spectral fitting on the inner 5' of the bulge of M31. We find that single component spectral models provide an inadequate fit to the spectrum, in contrast to previous studies by Einstein and Ginga. Although the 2-10 keV spectrum can be fit adequately with a bremsstrahlung model with kT = 7.4 +/- 0.3 keV, an additional soft component with kT = 0.38 +/- 0.03 keV is required to fit the spectrum below 2 keV. This soft component comprises 38% +/- 6% of the total emission in the 0.1-2.0 keV band, and possibly more depending on the absorption value used in the fit. Since previous spatial studies of the bulge of M31 indicate that < 25% of the X-ray emission from the bulge in this band is from a diffuse gaseous component, this implies that stellar sources, namely low-mass X-ray binaries (LMXBs), are responsible for some if not all of the soft component. The spectral properties of M31 are very similar to those of the X-ray faint early-type galaxy NGC4382. This supports the claim that the unexplained soft X-ray emission seen previously in these galaxies also emanates from LMXBs.

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arXiv1999

Radial Temperature Profiles of X-Ray--Emitting Gas Within Clusters of Galaxies

Previous analyses of ASCA data of clusters of galaxies have found conflicting results regarding the slope of the temperature profile of the hot X-ray gas within clusters, mainly because of the large, energy-dependent point spread function (PSF) of the ASCA mirrors. We present a summary of all ASCA-determined cluster temperature profiles found in the literature, and find a discrepancy in the radial temperature trend of clusters based on which PSF-correction routine is used. This uncertainty in the cluster temperature profile in turn can lead to large uncertainties in the amount of dark matter in clusters. In this study, we have used ROSAT PSPC data to obtain independent relative temperature profiles for 26 clusters, most of which have had their temperature profiles determined by ASCA. Our aim is not to measure the actual temperature values of the clusters, but to use X-ray color profiles to search for a hardening or softening of the spectra with radius for comparison to ASCA-derived profiles. The radial color profiles indicate that outside of the cooling flow region, the temperature profiles of clusters are in general constant. Within 35% of the virial radius, we find a temperature drop of 20% at 10 keV and 12% at 5 keV can be ruled out at the 99% confidence level. A subsample of non-cooling flow clusters shows that the condition of isothermality applies at very small radii too, although cooling gas complicates this determination in the cooling flow subsample. The colors predicted from the temperature profiles of a series of hydrodynamical cluster simulations match the data very well, although they cannot be used to discriminate among different cosmologies. An additional result is that the color profiles show evidence for a central peak in metallicity in low temperature clusters.

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