arXiv subjects
Publications and source records attributed to David A. Buote.
I review constraints on the radial density profiles and ellipticities of the dark matter obtained from recent X-ray observations with Chandra and XMM of elliptical galaxies and galaxy clusters and discuss their implications, especially for the self-interacting dark matter model.
Using Chandra archival data, we quantify the evolution of cluster morphology with redshift. To quantify cluster morphology, we use the power ratio method developed by Buote and Tsai (1995). Power ratios are constructed from moments of the two-dimensional gravitational potential and are, therefore, related to a cluster's dynamical state. Our sample will include 40 clusters from the Chandra archive with redshifts between 0.11 and 0.89. These clusters were selected from two fairly complete flux-limited X-ray surveys (the ROSAT Bright Cluster Sample and the Einstein Medium Sensitivity Survey), and additional high-redshift clusters were selected from recent ROSAT flux-limited surveys. Here we present preliminary results from the first 28 clusters in this sample. Of these, 16 have redshifts below 0.5, and 12 have redshifts above 0.5.
The failure of the XMM-Newton and Chandra X-ray telescopes to detect cooling gas in elliptical galaxies and clusters of galaxies has led many to adopt the position that the gas is not cooling at all and that heating by an active nucleus in the central E or cD galaxy is sufficient to offset radiative cooling. In this paper we explore an idealized limiting example of this point of view in which hot, buoyant bubbles formed near the center return the inflowing, radiatively cooling gas to distant regions in the flow. We show that idealized steady state, centrally heated non-cooling flows can indeed be constructed. In addition, the emission-weighted temperature profiles in these circulating flows resemble those of normal cooling flows. However, these solutions are valid only (1) for a range of bubble parameters for which there is no independent justification, (2) for a limited spatial region in the cooling flow and (3) for a limited period of time after which cooling seems inevitable. Our exploration of non-cooling flows is set in the context of galaxy/group flows.
Using new XMM and Chandra observations we present an analysis of the metal abundances of the hot gas within a radius of 100 kpc of the bright nearby galaxy group NGC 5044. Motivated by the inconsistent abundance and temperature determinations obtained by different observers for X-ray groups, we provide a detailed investigation of the systematic errors on the derived abundances considering the effects of the temperature distribution, calibration, plasma codes, bandwidth, Galactic Nh, and background rate. The iron abundance (Fe) drops from Fe ~1 solar within R ~50 kpc to Fe ~0.4 solar near R=100 kpc. This radial decline in Fe is highly significant: Fe=1.09 +/- 0.04 solar (stat) +/- 0.05 solar + 0.18 solar (sys) within R=48 kpc (5') compared to Fe=0.44 +/- 0.02 solar (stat) +/- 0.10 solar + 0.13 solar (sys) over R=48-96 kpc (5'-10'). The data rule out with high confidence a very sub-solar value for Fe within R=48 kpc confirming that previous claims of very sub-solar central Fe values in NGC 5044 were primarily the result of the Fe Bias: i.e., the incorrect assumption of spatially isothermal and single-phase gas when in fact temperature variations exist. Within R=48 kpc we obtain Si/Fe = 0.83 +/- 0.02 (stat) +/- 0.02 + 0.07 (sys) and S/Fe = 0.54 +/- 0.02 (stat) +/- 0.01 + 0.01 (sys) in solar units. These ratios are consistent with their values at larger radii and imply that SNIa have contributed ~80% of the iron mass within a 100 kpc radius of NGC 5044. This SNIa fraction is similar to the Sun and suggests an IMF similar to that of the Milky Way. At the very center (R ~2 kpc) the XMM and Chandra CCDs and the XMM RGS show that the Fe drops to ~50% of its value at immediately larger radius analogously to that seen in some galaxy clusters. (Abridged)
Using new XMM and Chandra observations we present an analysis of the temperature structure of the hot gas within a radius of 100 kpc of the bright nearby galaxy group NGC 5044. A spectral deprojection analysis of data extracted from circular annuli reveals that a two-temperature model (2T) of the hot gas is favored over single-phase or cooling flow (M_dot = 4.5 +/- 0.2 M_{sun}/yr) models within the central ~30 kpc. Alternatively, the data can be fit equally well if the temperature within each spherical shell varies continuously from ~T_h to T_c ~ T_h/2, but no lower. The high spatial resolution of the Chandra data allows us to determine that the temperature excursion T_h --> T_c required in each shell exceeds the temperature range between the boundaries of the same shell in the best-fitting single-phase model. This is strong evidence for a multi-phase gas having a limited temperature range. The cooler component of the 2T model has a temperature (T_c \~ 0.7 keV) similar to the kinetic temperature of the stars. The hot phase has a temperature (T_h ~ 1.4 keV) characteristic of the virial temperature of the \~10^{13} M_{sun} halo expected in the NGC 5044 group. However, in view of the morphological disturbances and X-ray holes visible in the Chandra image within R ~10 kpc, bubbles of gas heated to ~T_h in this region may be formed by intermittent AGN feedback. Some additional heating at larger radii may be associated with the evolution of the cold front near R ~50 kpc, as suggested by the sharp edge in the EPIC images.
Using Chandra archival data, we have begun a study to quantify the evolution of cluster morphology with redshift. To quantify cluster morphology, we use the power ratio method developed by Buote and Tsai (1995). Power ratios are constructed from moments of the two-dimensional gravitational potential and are, therefore, related to a cluster's dynamical state. Our sample will include around 50 clusters from the Chandra archive with redshifts between 0.11 and 1.1. These clusters were selected from two fairly complete flux-limited X-ray surveys (the ROSAT Bright Cluster Sample and the Einstein Medium Sensitivity Survey), and additional high-redshift clusters were selected from recent ROSAT flux-limited surveys. Here we present preliminary results from the first 15 clusters in this sample. Of these, eight have redshifts below 0.5, and seven have redshifts above 0.5.
We have used a high resolution Chandra observation to examine the core mass distribution of the unusually regular cD cluster Abell 2029. This system is especially well-suited for analysis of its mass distribution under the assumption of hydrostatic equilibrium: it exhibits an undisturbed X-ray morphology and a single-phase ICM. We estimate the total mass, and the contributions of the gas and dark matter components from <3 arcsec to ~3 arcmin (<4.4-260 kpc, 0.001-0.1 r_vir). The gas density is well fitted by either a double beta-model, or a ``cusped'' beta-model. The temperature data are increasing, and can be approximated by a power-law, with alpha_T = 0.27+/-0.01. Fitting smooth functions to density and T, we obtain a mass profile, which is remarkably well described down to 0.002 r_vir by the NFW model, for which we measure r_scale = 540+/-90 kpc (~0.2 r_vir) and c = 4.4 +/- 0.9. The mass profile is also approximated by a power-law fit with alpha_m = 1.81+/-0.04 (corresponding to a logarithmic density slope of -1.19+/-0.04). The density profile is too shallow to be fitted with a Moore et al. model. The consistency with NFW at <0.01 r_vir is incompatible with SIDM, and contrasts with the results from rotation curves of LSB and dwarf galaxies. This suggests that while CDM may adequately describe clusters, it does not currently account properly for small halos. Using the cD light, we observe a total M/L_V ~ 12 at r < 20 kpc, rising to > 100 beyond 200 kpc. The consistency with an NFW halo model and the large M/L suggest the cluster is DM-dominated down to <~ 0.005 r_vir. We observe a gas mass fraction = 0.139+/-0.004 at the limit of our observations, finding an upper limit to Omega_m <= 0.29+/-0.03 (abridged).
With a Chandra ACIS-S3 observation, we detect 42 X-ray point sources in the elliptical galaxy NGC 720, including a possible central source. Most of these sources will be low-mass X-ray binaries (LMXBs), and 12 are located within 2" of globular cluster candidates. We investigate both the hardness ratios and combined spectra of the sources. They exhibit a distribution of X-ray colors similar to those seen in other early-type galaxies. We find that there is a population of highly absorbed sources located at large distances from the center of the galaxy. The overall spatial distribution of sources is consistent with the ellipticity and position angle of the galaxy, but the sources appear to form several arcs. NGC 720 contains nine ultraluminous sources (L_x >= 10^39 ergs/s). This number is more than have previously been detected in an early-type galaxy but similar to the number seen in the Antennae merger system. The ratio L_ULX/L_B for NGC 720 is more than double the ratio for the S0 galaxy NGC 1553 and a factor of seven higher than for the elliptical galaxy NGC 4697, although uncertainties in the distance and the source spectral properties could bring these ratios into agreement. The X-ray source luminosity function is also nearly as flat as those seen in disk and merger systems. The large number of ULXs in NGC 720 and its relatively flat XLF may provide evidence against the association of all ULXs with young stars. We also see a possible high luminosity break in the luminosity function at 2x10^39 h_50^-2 ergs/s.
X-ray observations with Chandra and XMM are providing valuable new measurements of the baryonic and dark matter content of groups and clusters. Masses of cD clusters obtained from X-ray and gravitational lensing studies generally show good agreement, therefore providing important validation of both methods. Gas fractions have been obtained for several clusters that verify previous results for a low matter density (Omega_m ~0.3). Chandra has also provided measurements of the mass profiles deep down into several cluster cores and has generally found no significant deviations from CDM predictions in contrast to the flat core density profiles inferred from the rotation curves of low-surface brightness galaxies and dwarf galaxies; i.e., there is no evidence for self-interacting dark matter in cluster cores. Finally, initial studies of the iron and silicon abundances in centrally E-dominated groups show that they have pronounced gradients from 1-2 solar values within the central 30-50 kpc that fall to values of 0.3-0.5 solar at larger radii. The Si/Fe ratios are consistent with approximately 80% of the metals originating from Type Ia supernovae. Several cD clusters also display central Fe enhancements suggestive of Type Ia supernova enrichment, though some have central dips that may provide a vital clue for solving the cooling flow mystery.
We present an initial analysis of a new XMM observation of NGC 1399, the central elliptical galaxy of the Fornax group. Spectral fitting of the spatially resolved spectral data of the EPIC MOS and pn CCDs reveals that a two-temperature model (2T) of the hot gas is favored over single-phase and cooling flow models within the central ~20 kpc. The preference for the 2T model applies whether or not the data are deprojected. The cooler component has a temperature (~0.9 keV) similar to the kinetic temperature of the stars while the hotter component has a temperature (~1.5 keV) characteristic of the virial temperature of a ~10^{13} M_sun halo. The two-phase model (and other multitemperature models) removes the ``Fe Bias'' within r < ~20 kpc and gives Z_Fe/Z_sun 1.5-2. At larger radii the iron abundance decreases until Z_Fe/Z_sun \~0.5 for r ~50 kpc. The Si abundance is super-solar (1.2-1.7 solar) within the central regions while Z_Si/Z_Fe ~0.8 over the entire region studied. These Fe and Si abundances imply that ~80% of the Fe mass within r ~50 kpc originates from Type Ia supernovae (SNIa). This SNIa fraction is similar to that inferred for the Sun and therefore suggests a stellar initial mass function similar to the Milky Way.
(Abridged) We present an analysis of a Chandra ACIS-S observation of the elliptical galaxy NGC 720 to verify the existence of a dark matter (DM) halo and to measure its ellipticity. The ACIS-S3 image reveals over 60 point sources. For semi-major axes a<~150" (18.2h_{70}^{-1} kpc) the ellipticity of the diffuse emission is ex ~0.15, which is less than the values 0.2-0.3 obtained from ROSAT because the point sources contaminated the ROSAT values. The Chandra data confirm the ~20 deg position angle (PA) twist discovered by ROSAT, but the Chandra twist is more gradual also because of the point sources contaminating the ROSAT values. Overall the ex and PA values for a<~150" can be explained by the triaxial model of NGC 720 published by Romanowsky & Kochanek. Since the optical image displays no substantial isophote twisting, the X-ray PA twist requires a massive DM halo if the hot gas is in hydrostatic equilibrium. The mass-follows-light hypothesis is also inconsistent with the Chandra ellipticities at the 96% (98%) level for oblate (prolate) symmetry. Thus, both the PA twist and the ellipticities of the Chandra image imply a DM halo independent of the gas T profile -- evidence that cannot be obviated by alternative gravity theories such as MOND. The DM density model, rho ~a^{-2}, provides the best fit and gives ellipticities of 0.37 +/- 0.03 (0.36 +/- 0.02) for oblate (prolate) models. These moderate ellipticities for the DM halo are inconsistent with both the nearly spherical halos predicted if the DM is self-interacting and with the highly flattened halos predicted if the DM is cold molecular gas. These ellipticities may also be too large to be explained by warm DM, but are consistent with galaxy-sized halos formed in the Lambda-CDM paradigm.
We have obtained high spatial resolution temperature and abundance profiles for the galaxy cluster Abell 2029 with the Chandra ACIS-S instrument. Our observations reveal that the spectra are well-fit by a single-phase gas in each annulus. While the temperature of the intracluster medium drops from ~9 keV at 3 arcmin (260 kpc) to 3 keV in the central 5 arcsec (5 kpc) of the cluster, there is no evidence for gas emitting at temperatures below 3 keV. The addition of a cooling flow component does not improve the fits, despite previous claims for a massive cooling flow. There is also no evidence for excess absorption above the Galactic N_H value. We also observe a steep Fe abundance gradient, such that Z_Fe >= 2 Z_solar (assuming meteoritic solar Fe abundance) in the core, consistent with significant enrichment from Fe-rich Type Ia supernovae in the cD galaxy. The Fe abundance drops to ~0.5 Z_solar at 3 arcmin (260 kpc), in good agreement with previous BeppoSAX measurements. The high resolution image reveals neither a strong central point source, nor any filamentary structure related to a cooling flow or a merger. The absence of a strong merger signature argues against the creation of the wide-angle-tail radio source morphology in a merger event. (modified for astro-ph).
X-ray observations have played a key role in the study of substructure and merging in galaxy clusters. I review the evidence for cluster substructure and mergers obtained from X-ray observations with satellites that operated before Chandra and XMM. Different techniques to study cluster mergers via X-ray imaging and spectral data are discussed with an emphasis on the quantitative analysis of cluster morphologies. I discuss the implications of measurements of cluster morphologies for cosmology and the origin of radio halos.
Previously it has been recognized that radio halos in galaxy clusters are preferentially associated with merging systems as indicated by substructure in the X-ray images and temperature maps. Since, however, many clusters without radio halos also possess substructure, the role of mergers in the formation of radio halos has remained unclear. By using power ratios to relate gravitational potential fluctuations to substructure in X-ray images, we provide the first quantitative comparison of the dynamical states of clusters possessing radio halos. A correlation between the 1.4 GHz power (P_{1.4}) of the radio halo (or relic) and the magnitude of the dipole power ratio (P_1/P_0) is discovered such that approximately P_{1.4} ~ P_1/P_0; i.e., the strongest radio halos appear only in those clusters currently experiencing the largest departures from a virialized state. From additional consideration of a small number of highly disturbed clusters without radio halos detected at 1.4 GHz, and recalling that radio halos are more common in clusters with high X-ray luminosity (Giovannini, Tordi, & Feretti), we argue that radio halos form preferentially in massive (L_x >~ 0.5 x 10^{45} erg/s) clusters experiencing violent mergers (P_1/P_0 >~ 0.5 x 10^{-4}) that have seriously disrupted the cluster core. The association of radio halos with massive, large-P_1/P_0, core-disrupted clusters is able to account for both the vital role of mergers in accelerating the relativistic particles responsible for the radio emission as well as the rare occurrence of radio halos in cluster samples.
Recent XMM-Newton observations of the cooling flow gas in M87 indicate sharply decreasing oxygen, iron and silicon abundances within $\sim 5$ kpc of the galactic center. This result is unexpected since stellar mass loss and Type Ia supernovae are expected to produce pronounced central abundance maxima for all three elements. However, it has been suggested that many of the strong X-ray lines are optically thick and diffuse to larger radii in the cooling flow before escaping, falsifying the central abundances. We verify with radiation transfer calculations that this effect does indeed occur in the M87 cooling flow, but that it is insufficient to account for the M87 observations. We suggest that some source of continuous opacity is required to reduce the central X-ray line emission, perhaps by warm gas at $T \sim 10^5 - 10^6$ K. The radial surface brightness profiles of X-ray resonance lines are also sensitive to turbulence in cooling flows which reduces the line center optical depths considerably. Turbulence may provide sufficient energy to continuously heat the warm absorbing gas.
We summarize the discovery of oxygen absorption and warm (10^5-10^6 K) gas in cooling flows. Special attention is given to new results for M87 for which we find the strongest evidence to date for ionized oxygen absorption in these systems. We briefly discuss implications for observations of cooling flows with Chandra and XMM.
We present a re-analysis of the ROSAT PSPC data within the central 100 kpc of M87 to search for intrinsic oxygen absorption similar to that recently measured in several galaxies and groups. Using a spatial-spectral deprojection analysis we find the strongest evidence to date for intrinsic oxygen absorption in the hot gas of a galaxy, group, or cluster. Single-phase plasma models modified by intervening Galactic absorption cannot fit the 0.2-2.2 keV PSPC data as they under-predict the 0.2-0.4 keV region and over-predict the 0.5-0.8 keV region where the emission and absorption residuals are obvious upon visual inspection of the spectral fits. Since the excess emission between 0.2-0.4 keV rules out intrinsic absorption from cold gas or dust, the most reasonable model for the excess emission and absorption features is warm, collisionally ionized gas with a temperature of ~10^6 K. Simple multiphase models (cooling flow, two phases) modified by both intervening Galactic absorption and by a single oxygen edge provide good fits and yield temperatures and Fe abundances of the hot gas that agree with previous determinations by ASCA and SAX. The multiphase models of M87 inferred from the PSPC can account for the excess EUV emission observed with EUVE and the excess X-ray absorption inferred from EINSTEIN and ASCA data above 0.5 keV. Although the total mass of the warm gas implied by the oxygen absorption is consistent with the matter deposited by a cooling flow, the suppression of the mass deposition rate and the distortion of the X-ray isophotes in the region where the radio emission is most pronounced suggest some feedback effect from the AGN on the cooling gas. (Abridged)
Using spatially resolved, deprojected ROSAT PSPC spectra of 10 of the brightest cooling flow galaxies and groups with low Galactic column densities we have detected intrinsic absorption over energies ~0.4-0.8 keV in half of the sample. Since no intrinsic absorption is indicated for energies below ~0.4 keV, the most reasonable model for the absorber is collisionally ionized gas at temperatures T=10^{5-6} K with most of the absorption arising from ionized states of oxygen but with a significant contribution from carbon and nitrogen. The soft X-ray emission of this warm gas can explain the sub-Galactic column densities of cold gas inferred within the central regions of most of the systems. Attributing the absorption 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. Within the central ~10-20 kpc, where the constraints are most secure, the estimated mass of the ionized absorber is consistent with most (perhaps all) of the matter deposited by a cooling flow over the lifetime of the flow. Since the warm absorber produces no significant H or He absorption the large absorber masses are consistent with the negligible atomic and molecular H inferred from HI and CO observations of cooling flows. It is also found that if T > ~2x10^5 K then the optical and UV emission implied by the warm gas does not violate published constraints. Finally, we discuss how the prediction of warm ionized gas as the product of mass drop-out in these and other cooling flows can be verified with new CHANDRA and XMM observations. (Abridged)