arXiv subjects

David A. Buote

Publications and source records attributed to David A. Buote.

90 records · Page 5Linked to original sources

arXiv2000

Iron Gradients in Cooling Flow Galaxies and Groups

Previous studies of the Fe abundances in the hot gas of galaxies and groups have reported conflicting results with most studies finding very sub-solar Fe abundances that disagree with standard theory. To investigate the possible role of Fe abundance gradients on these measurements we present deprojection analysis of the ROSAT PSPC data of 10 of the brightest cooling flow galaxies and groups. The PSPC allows for spatially resolved spectral analysis on a half-arcminute scale, and interesting constraints on both the temperatures and Fe abundances are possible because the ~1 keV temperatures of these systems are well matched to the bandpass of the PSPC. In 9 out of 10 systems we find clear evidence that the Fe abundance decreases with increasing radius: Z_Fe ~1 solar - (several) solar within the central radial bin (r < ~10 kpc) which decreases to Z_Fe ~0.5 solar at the largest radii examined (r ~50-100 kpc). The Fe abundances (and temperatures) are consistent with the average values for these systems that we obtained in our previous analyses of the ASCA data using multitemperature models which confirms that previous inferences of very sub-solar Fe abundances from ASCA arise from the incorrect assumption of isothermal gas and not the presence of Fe abundance gradients. We discuss why this ``Fe Bias'' affects much more seriously the measurements of Z_Fe from ASCA data than from ROSAT data. We show that the Fe abundance profiles for these galaxies and groups are consistent with a gas-dynamical model where the gas is enriched by stellar ejecta and supernovae in the ``solar supernova proportion'', the stars formed with a Galactic IMF, and the gas is diluted by mixing with primordial gas at large radii.

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arXiv2000

Oxygen Absorption in Cooling Flows

The inhomogeneous cooling flow scenario predicts the existence of large quantities of gas in massive elliptical galaxies, groups, and clusters that have cooled and dropped out of the flow. Using spatially resolved, deprojected X-ray spectra from the ROSAT PSPC we have detected strong absorption over energies ~0.4-0.8 keV intrinsic to the central ~1 arcmin of the galaxy, NGC 1399, the group, NGC 5044, and the cluster, A1795. These systems have amongst the largest nearby cooling flows in their respective classes and low Galactic columns. Since no excess absorption is indicated for energies below ~0.4 keV the most reasonable model for the absorber is warm, collisionally ionized gas with T=10^{5-6} K where ionized states of oxygen provide most of the absorption. Attributing the absorption only to ionized gas reconciles the large columns of cold H and He inferred from Einstein and ASCA with the lack of such columns inferred from ROSAT, and also is consistent with the negligible atomic and molecular H inferred from HI, and CO observations of cooling flows. The prediction of warm ionized gas as the product of mass drop-out in these and other cooling flows can be verified by Chandra, XMM, and ASTRO-E.

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arXiv1999

X-Ray Evidence for Multiphase Hot Gas with Nearly Solar Fe Abundances in the Brightest Groups of Galaxies

We analyze the ASCA spectra accumulated within ~100 kpc radii of 12 of the brightest groups of galaxies. Upon fitting isothermal models (1T) jointly to the ASCA SIS and GIS spectra we obtain fits for most groups that are of poor or at best marginal quality and give very sub-solar metallicities similar to previous studies, = 0.29 +/- 0.12 Z_sun. Two-temperature models (2T) provide significantly better fits for 11 out of the 12 groups and in every case have metallicities that are substantially larger than obtained for the 1T models, = 0.75 +/- 0.24 Z_sun. Although not very well constrained, for most of the groups absorption in excess of the Galactic value is indicated for the cooler temperature component of the 2T models. A simple multiphase cooling flow model gives results analogous to the 2T models including large metallicities, = 0.65 +/- 0.17 Z_sun. The nearly solar Fe abundances and also solar alpha/Fe ratios indicated by the 2T and cooling flow models are consistent with models of the chemical enrichment of ellipticals, groups, and clusters which assume ratios of Type Ia to Type II supernova and an IMF similar to those of the Milky Way. Thus, we have shown that the very sub-solar Fe abundances and Si/Fe enhancements obtained from most previous studies within r ~100 kpc of galaxy groups are an artifact of their fitting isothermal models to the X-ray spectra which also has been recently demonstrated for the brightest elliptical galaxies. Owing to the importance of these results for interpreting X-ray spectra, in an appendix we use simulated ASCA observations to examine in detail the ``Fe bias'' and ``Si bias'' associated with the spectral fitting of ellipticals, groups, and clusters of galaxies.

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arXiv1999

X-Ray Emission Line Ratios and Multiphase Gas in Elliptical Galaxies and Galaxy Clusters

We examine the K shell emission lines produced by isothermal and simple multiphase models of the hot gas in elliptical galaxies and galaxy clusters to determine the most effective means for constraining the width of the differential emission measure (xi(T)) in these systems which we characterize by a dimensionless parameter, sigma_xi. Comparison of line ratios of two-temperature (sigma_xi << 1) and cooling flow (sigma_xi ~1) models is presented in detail. We find that a two-temperature model can approximate very accurately a cooling flow spectrum over 0.5-10 keV. We have re-analyzed the ASCA spectra of three of the brightest galaxy clusters to assess the evidence for multiphase gas in their cores: M87 (Virgo), the Centaurus cluster, and the Perseus cluster. K-alpha emission line blends of Si, S, Ar, Ca, and Fe are detected in each system as is significant Fe K-beta emission. The Fe K-beta/K-alpha ratios are consistent with optically thin plasma models and do not suggest resonance scattering in these systems. Consideration of both the ratios of H-like to He-like K-alpha lines and the local continuum temperatures clearly rules out isothermal gas in each case. To obtain more detailed constraints we fitted plasma models over 1.6-9 keV where the emission is dominated by these K shell lines and by continuum. In each case the ASCA spectra cannot determine whether the gas emits at only two temperatures or over a continuous range of temperatures as expected for a cooling flow. The metal abundances are near solar for all of the multiphase models. We discuss the implications of these results and examine the prospects for determining the temperature structure in these systems with upcoming X-ray missions.

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arXiv1999

X-Ray Evidence for Multiphase Hot Gas with Solar Abundances in the Brightest Elliptical Galaxies

We examine whether isothermal models of the hot gas can successfully describe the ASCA and ROSAT spectra of NGC 1399, NGC 4472, NGC 4636, and NGC 5044. Broad-band spectral fitting of the ASCA SIS and GIS data accumulated within a radius of ~5 arcmin for each galaxy shows that isothermal models are unable to fit the SIS data near 1 keV. In addition, these isothermal models typically fail to produce the large equivalent widths of the K-alpha line blends of the H-like and He-like ions of Si and S which are measured independently of the Fe L emission lines. Two-temperature models provide excellent broad-band fits to both the SIS and GIS data of each galaxy with the relative abundances (except for NGC 4636) fixed at their solar values. A simple multiphase cooling flow model fits nearly as well as the two-temperature model for NGC 1399, NGC 4472, and NGC 5044. The multiphase models also predict more accurately the Si and S equivalent widths and the ratios of Si XIV/XIII and S XVI/XV than the isothermal models. Using various approaches we find that the temperature gradients inferred from the ROSAT PSPC data of these galaxies, especially for NGC 1399 and NGC 5044, are inconsistent with the isothermal models derived from fitting the ASCA data within a single-aperture but are very consistent with the multiphase models. Therefore, models which assume isothermal gas within r ~5 arcmin are inconsistent with both the ASCA and ROSAT PSPC data of these elliptical galaxies. Simple two-temperature models and multiphase cooling flows provide good descriptions of these data sets with Fe abundances of ~1-2 solar and (except for NGC 4636) relative abundances fixed at their solar values. We discuss the implications of these nearly solar abundances. (Abridged)

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arXiv1998

X-ray Isophote Shapes and the Mass of NGC 3923

We present analysis of the shape and radial mass distribution of the E4 galaxy NGC 3923 using archival X-ray data from the ROSAT PSPC and HRI. The X-ray isophotes are significantly elongated with ellipticity e_x=0.15 (0.09-0.21) (90% confidence) for semi-major axis a\sim 10h^{-1}_70 kpc and have position angles aligned with the optical isophotes within the estimated uncertainties. Applying the Geometric Test for dark matter, which is independent of the gas temperature profile, we find that the ellipticities of the PSPC isophotes exceed those predicted if M propto L at a marginal significance level of 85% (80%) for oblate (prolate) symmetry. Detailed hydrostatic models of an isothermal gas yield ellipticities for the gravitating matter, e_mass=0.35-0.66 (90% confidence), which exceed the intensity weighted ellipticity of the R-band optical light, = 0.30 (e_R^max=0.39). We conclude that mass density profiles with rho\sim r^{-2} are favored over steeper profiles if the gas is essentially isothermal (which is suggested by the PSPC spectrum) and the surface brightness in the central regions (r<~15") is not modified substantially by a multi-phase cooling flow, magnetic fields, or discrete sources. We argue that these effects are unlikely to be important for NGC 3923. (The derived e_{mass} range is very insensitive to these issues.) Our spatial analysis also indicates that the allowed contribution to the ROSAT emission from a population of discrete sources with Sigma_x propto Sigma_R is significantly less than that indicated by the hard spectral component measured by ASCA.

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arXiv1997

X-Ray Spectral Analysis of Elliptical Galaxies from ASCA: the Fe Abundance in a Multi-Phase Medium

We present spectral analysis of ASCA data of 17 elliptical and 3 lenticular galaxies most of which have high L_x/L_B. Single-temperature models (MEKAL and Raymond-Smith) give unacceptable fits (chi^2_red > 1.5) in most cases and, in agreement with previous studies, give very sub-solar abundances, = 0.19 +/- 0.12 Z_sun (MEKAL). The spectra for approximately half the sample are better fit with a cooling-flow model which in three cases gives a substantially better fit. The abundances derived from the cooling-flow model are also significantly larger, = 0.6 +/- 0.5 Z_sun. We empirically tested the reliability of the plasma codes in the Fe-L region and found no evidence for serious problems with the determined temperatures and abundances. Two-temperature models give substantially better fits that are formally acceptable (chi^2_red ~ 1.0) in all but a few cases. The highest S/N galaxies (which also have highest L_x/L_B) have fitted temperatures < 2 keV for both components consistent with each being distinct phases of hot gas. The lowest S/N galaxies (which also have lowest L_x/L_B) generally have a hot component with temperature, T_H > ~5 keV, which is consistent with emission from discrete sources. (We discuss the origin of these two components from analysis of L_x-L_B and L_x/L_B.) The abundances of these two-temperature models are approximately solar, = 0.9 +/- 0.7 Z_sun (MEKAL), consistent with a recent multi-phase model for the evolution of hot gas in ellipticals. Finally, for several galaxies we find evidence for absorption in excess of the Galactic value and discuss its implications.

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arXiv1997

The Shapes and Sizes of Elliptical Galaxy Halos from X-Ray Observations

We review the theory of how the shapes of the X-ray isophotes probe both the shape and radial distribution of gravitating matter in elliptical galaxies in a way that is more robust than the traditional spherical approach. We summarize and update the previous analyses of X-ray observations of NGC 720 and NGC 1332 and describe preliminary results for NGC 3923.

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arXiv1997

Omega_0 and Substructure in Galaxy Clusters

(Abridged) We examine the theoretical relationship between Omega_0 and substructure in galaxy clusters which are formed by the collapse of high density peaks in a gaussian random field. The radial mass distributions of the clusters are computed from the spherical accretion model using the adiabatic approximation following Ryden & Gunn. For a cluster of mass, M(r,t), we compute the quantity dM/M_bar at a cosmic time t and within a radius r, where dM is the accreted mass and M_bar is the average mass of the cluster during the previous relaxation time, which is computed individually for each cluster. For a real cluster in three dimensions we argue that dM/M_bar should be strongly correlated with the low order multipole ratios, Phi^{int}_l/Phi^{int}_0, of the potential due to matter interior to r. It is shown that the expected correlation between dM/M_bar and Phi^{int}_l/Phi^{int}_0 extends to the two-dimensional multipole ratios, Psi^{int}_m/Psi^{int}_0, which are well defined observables of the cluster density distribution. The strongest dependence of dM/M_bar on Omega_0 (lambda_0=0) occurs at z=0 where dM/M_bar propto Omega_0^{1/2} for relaxation times ~1-2 crossing times and only very weakly depends on mass and radius. The fractional accreted mass in CDM models with Omega_0+lambda_0=1 depends very weakly on Omega_0 and has a magnitude similar to the Omega_0=1 value. dM/M_bar evolves more rapidly with redshift in low-density universes and decreases significantly with radius for Omega_0=1 models for z > ~0.5. We discuss how to optimize constraints on Omega_0 and lambda_0 using cluster morphologies.

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arXiv1996

X-ray Measurements of the Shapes of Clusters and Galaxies

We summarize a series of papers reporting studies of the ellipticities of X-ray images of five Abell clusters and two early type galaxies to learn about the shape of the gravitational potential and the underlying matter distributions. For the clusters we find the dark matter is centrally concentrated with ellipticity $ε_{DM} \sim 0.5$. For the galaxies we find independent evidence for extended, flattened dark matter halos. One case of a clear isophotal twist in the X-rays may indicate a triaxial halo.

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arXiv1996

Sensitivity of Galaxy Cluster Morphologies to Omega_0 and P(k)

We examine the sensitivity of the spatial morphologies of galaxy clusters to Omega_0 and P(k) using high-resolution N-body simulations with large dynamic range. Variants of the standard CDM model are considered having different spatial curvatures, SCDM (Omega_0=1), OCDM (Omega_0=0.35), LCDM (Omega_0=0.35, lambda_0=0.65), and different normalizations, sigma_8. We also explore critical density models with different spectral indices, n, of the scale-free power spectrum, P(k) propto k^n. Cluster X-ray morphologies are quantified with power ratios (PRs), where we take for the X-ray emissivity j_gas propto rho^2_DM, which we argue is a suitable approximation for analysis of PRs. We find that Omega_0 primarily influences the means of the PR distributions whereas the power spectrum (n and sigma_8) primarily affects their variances: log_10(P_3/P_0) is the cleanest probe of Omega_0 since its mean is very sensitive to Omega_0 but very insensitive to P(k). The PR means easily distinguish the SCDM and OCDM models, while the SCDM and LCDM means show a more modest, but significant, difference (3 sigma). The OCDM and LCDM models are largely indistinguishable in terms of the PRs. Finally, we compared these models to a sample of ROSAT clusters and find that the cluster morphologies strongly disfavor Omega_0=1, CDM while favoring low density, CDM models with or without a cosmological constant.

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arXiv1996

The Nature of the X-Ray Emission and the Mass Distributions in Two Early-Type Galaxies

We present spectral analysis of $ASCA$ observations of the early-type galaxies NGC 720 (E4) and NGC 1332 (E7/S0) with emphasis on constraining the relative contribution to the X-ray emission from hot gas and the integrated emission from X-ray binaries. Single-temperature spectral models yield poor fits to the spectrum ($χ^2_{red}\sim 3$) over the $\sim 0.5 - 5$ keV energy range. Two-temperature models significantly improve the spectral fits ($χ^2_{red}\sim 1.5$) and have soft-component temperatures and sub-solar abundances consistent with previous $ROSAT$ single-temperature models ($T_{soft}\sim 0.6$ keV, abundances $\sim 0.1$) and hard-component temperatures ($T_{hard}\gtrsim 3$ keV) consistent with those expected from a discrete component. The soft component dominates the emission in both galaxies, especially in the 0.4 - 2.4 keV band used in previous $ROSAT$ studies. Combining these spectral results with $ROSAT$ data we updated constraints on the mass distributions for NGC 720 and NGC 1332. For NGC 720, which yields the more precise constraints, the ellipticity of the intrinsic shape of the mass is slightly reduced $(Δε_{mass}\approx 0.05)$ when the discrete component is added, $ε_{mass}\sim 0.4-0.6$ $(90%)$. The estimates for the total mass increase with increasing discrete flux, and we find that models with $F_{hard}/F_{soft}=0.45$, the $2σ$ upper limit, have masses that exceed by $\sim 30% - 50%$ those where $F_{hard}/F_{soft}=0$.

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arXiv1996

The Twisting X-ray Isophotes of the Elliptical Galaxy NGC 720

We present spatial analysis of the deep (57ks) ROSAT HRI X-ray image of the E4 galaxy NGC 720. The orientation of the HRI surface brightness is consistent with the optical position angle $(PA)$ interior to semi-major axis $a\sim 60\arcsec$ (optical $R_e\sim 50\arcsec$). For larger $a$ the isophotes twist and eventually $(a\gtrsim 100\arcsec)$ orient along a direction consistent with the $PA$ measured with the PSPC data (Buote & Canizares 1994) -- the $\sim 30\arcdeg$ twist is significant at an estimated 99% confidence level. We argue that this twist is not the result of projected foreground and background sources, ram pressure effects, or tidal distortions. If spheroidal symmetry and a nearly isothermal hot gas are assumed, then the azimuthally averaged radial profile displays features which, when combined with the observed $PA$ twist, are inconsistent with the simple assumptions that the X-ray emission is due either entirely to hot gas or to the combined emission from hot gas and discrete sources. We discuss possible origins of the $PA$ twist and radial profile features (e.g., triaxiality).

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arXiv1996

Cosmological Implications of Galaxy Cluster Evolution

We analyze with hydrodynamical simulations the evolution of galaxy clusters in a cosmological environment. Power ratios (Buote \& Tsai 1995) are used to quantitatively relate cluster morphologies to their dynamical states. The simulated clusters follow the same ``evolutionary track'' obeyed by a sample of low-redshift $(z<0.2)$ ROSAT PSPC clusters (Buote \& Tsai 1996) indicating that the detailed evolution of individual simulated clusters is consistent with observed clusters. However, the distribution of simulated clusters (for $Ω=1$ standard Cold Dark Matter) along the evolutionary track at the present epoch, which indicates a measure of the present balance of cluster formation and relaxation rates, suggests that there are too many simulated clusters with significant amounts of substructure to be consistent with the observations, thus favoring a lower value of $Ω$. Perpendicular to the evolutionary track the distributions of observed and simulated clusters are consistent which may indicate a success of the cosmological model (e.g., power spectrum). Analysis of high-redshift simulated clusters suggests that the distribution of clusters both along and perpendicular to the evolutionary track is effectively constant from $z\sim 0.6$ to the present but changes significantly for $z > 0.6$.

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arXiv1995

Quantifying the Morphologies and Dynamical Evolution of Galaxy Clusters. II. Application to a Sample of ROSAT Clusters

We quantify the morphologies and dynamical states of 59 galaxy clusters using the power-ratio technique of Buote & Tsai applied to ROSAT PSPC X-ray images. The clusters exhibit a particularly strong $P_2/P_0 - P_4/P_0$ correlation in the $1h^{-1}_{80}$ Mpc aperture which may be interpreted as an evolutionary track; the location of a cluster on the correlation line indicates the dynamical state of the cluster and the distribution of clusters along this track measures the rate of formation and evolution of clusters in our sample. The power ratios anti-correlate with the cooling-flow rate indicating a reasonable dependence of the flow rate on cluster morphology. The relationship of the power ratios to the optical Bautz-Morgan (BM) Type is more complex. This is because the power ratios are sensitive to unrelaxed regions of clusters within a specified scale, whereas BM types are sensitive to unrelaxed regions over many scales. We discuss further astrophysical applications exploiting the relationship between the power ratios and the evolutionary states of clusters.

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arXiv1995

X-ray Constraints on the Intrinsic Shapes and Baryon Fractions of Five Abell Clusters

We analyzed ROSAT PSPC images of the bright, nearby $(z<0.1)$ galaxy clusters A401, A1656 (Coma), A2029, A2199, and A2256 to constrain their intrinsic shapes and baryon fractions. Following Buote & Tsai we probed the aggregate structure of the clusters on scales $\sim 1.5h^{-1}_{80}$ Mpc to reduce effects of possible substructure on smaller scales ($\lesssim$ a few hundred kpc). The ellipticities of the X-ray isophotes are typically $ε\approx 0.15 - 0.25$ and display negative radial gradients significant (except Coma). By assuming hydrostatic equilibrium and a variety of mass models we obtain ellipticities $ε_{mass} \approx 0.40-0.55$ for isothermal models of the total gravitating matter of the clusters; the $ε_{mass}$ constraints change by $<10%$ upon consideration of the small temperature gradients shown by ASCA to be typical for rich clusters; the observed X-ray ellipticity gradients require that mass models with constant ellipticity have steep density profiles, $ρ\propto r^{-4}$. Estimates of the gas masses are highly insensitive to the ellipticities of the X-ray isophotes. The clusters in our sample have increasing fractions of gas mass to total mass with radius and have $M_{gas}/M_{tot} = (4% - 11%)h^{-3/2}_{80}$ within a radius $1.5h^{-1}_{80}$ Mpc, in excellent agreement with the results of White & Fabian and the Baryon Catastrophe proposed by White et al. (1993). Finally, the ellipticities of the dark matter distributions are essentially identical to $ε_{mass}$ and are consistent with the shapes of dark halos predicted by N-body simulations and the shapes of the galaxy isopleths in the clusters in contrast to our previous conclusions using {\it Einstein} data.

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arXiv1995

X-ray Constraints on the Intrinsic Shape of the Lenticular Galaxy NGC 1332

We have analyzed ROSAT PSPC X-ray data of the optically elongated S0 galaxy NGC 1332 with the purposes of constraining the intrinsic shape of its underlying mass and presenting a detailed investigation of the uncertainties resulting from the assumptions underlying this type of analysis. The X-ray isophotes are elongated with ellipticity $0.10 - 0.27$ (90% confidence) for semi-major axes $75\arcsec -90\arcsec$ and have orientations consistent with the optical isophotes (ellipticity $\sim 0.43$). The spectrum is poorly constrained by the PSPC data and cannot rule out sizeable radial temperature gradients or an emission component due to discrete sources equal in magnitude to the hot gas. Using (and clarifying) the "geometric test" for dark matter, we determined that the hypothesis that mass-traces-light is not consistent with the X-ray data at 68% confidence and marginally consistent at 90% confidence independent of the gas temperature profile. Detailed modeling gives constraints on the ellipticity of the underlying mass of ε_{mass} = 0.47 - 0.72 (0.31 - 0.83) at 68% (90%) confidence for isothermal and polytropic models. The total mass of the isothermal models within a=43.6$ kpc (D = 20h^{-1}_{80} Mpc) is M_{tot} = (0.38 - 1.7) \times 10^{12}M_{\sun} (90% confidence) corresponding to total blue mass-to-light ratio Υ_B = (31.9 - 143) Υ_{\sun}. Similar results are obtained when the dark matter is fit directly using the known distributions of the stars and gas. When possible rotation of the gas and emission from discrete sources are included flattened mass distributions are still required, although the constraints on $ε_{mass}$, but not the total mass, are substantially weakened.

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arXiv1995

Quantifying the Morphologies and Dynamical Evolution of Galaxy Clusters. I. The Method

We describe and test a method to quantitatively classify clusters of galaxies according to their projected morphologies. This method will be subsequently used to place constraints on cosmological parameters ($Ω$ and the power spectrum of primordial fluctuations on scales at or slightly smaller than that of clusters) and to test theories of cluster formation. We specifically address structure that is easily discernible in projection and dynamically important to the cluster. The method is derived from the two-dimensional multipole expansion of the projected gravitational potential and yields dimensionless {\it power ratios} as morphological statistics. If the projected mass profile is used to characterize the cluster morphology, the power ratios are directly related to the cluster potential. However, since detailed mass profiles currently exist for only a few clusters, we use the X-ray--emitting gas as an alternative tracer of cluster morphology. In this case, the relation of the power ratios to the potential is qualitatively preserved. We demonstrate the feasibility of the method by analyzing simulated observations of simple models of X-ray clusters using the instrument parameters of the ROSAT PSPC. For illustrative purposes, we apply the method to ROSAT PSPC images of A85, A514, A1750, and A2029. These clusters, which differ substantially in their X-ray morphologies, are easily distinguished by their respective power ratios. We discuss the suitability of this method to address the connection between cluster morphology and cosmology and to assess whether an individual cluster is sufficiently relaxed for analysis of its intrinsic shape using hydrostatic methods. Approximately 50 X-ray observations of Abell clusters with the PSPC will be amenable to morphological analysis using the method of this paper.

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