arXiv subjects

David A. Buote

Publications and source records attributed to David A. Buote.

At least 55 records · Page 3Linked to original sources

arXiv2008

X-ray Group and cluster mass profiles in MOND: Unexplained mass on the group scale

Although very successful in explaining the observed conspiracy between the baryonic distribution and the gravitational field in spiral galaxies without resorting to dark matter (DM), the modified Newtonian dynamics (MOND) paradigm still requires DM in X-ray bright systems. Here, to get a handle on the distribution and importance of this DM, and thus on its possible form, we deconstruct the mass profiles of 26 X-ray emitting systems in MOND, with temperatures ranging from 0.5 to 9 keV. Initially we compute the MOND dynamical mass as a function of radius, then subtract the known gas mass along with a component of galaxies which includes the cD galaxy with $M/L_K=1$. Next we test the compatibility of the required DM with ordinary massive neutrinos at the experimental limit of detection ($m_ν=2$ eV), with density given by the Tremaine-Gunn limit. Even by considering that the neutrino density stays constant and maximal within the central 100 or 150 kpc (which is the absolute upper limit of a possible neutrino contribution there), we show that these neutrinos can never account for the required DM within this region. The natural corollary of this finding is that, whereas clusters (T $\ga$ 3 keV) might have most of their mass accounted for if ordinary neutrinos have a 2 eV mass, groups (T $\lsim$ 2 keV) cannot be explained by a 2 eV neutrino contribution. This means that, for instance, cluster baryonic dark matter (CBDM, Milgrom 2007) or even sterile neutrinos would present a more satisfactory solution to the problem of missing mass in MOND X-ray emitting systems.

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arXiv2007

Trouble for AGN Feedback ? The puzzle of the core of the Galaxy Cluster AWM 4

The core of the relaxed cluster AWM 4 is characterized by a unique combination of properties which defy a popular scenario for ANG heating of cluster cores. A flat inner temperature profile is indicative of a past, major heating episode which completely erased the cool core, as testified by the high central cooling time (~ 3 Gyr) and by the high central entropy level (~ 60 keV cm^2). Yet the presence of a 1.4 GHz active central radio galaxy with extended radio lobes out to 100 kpc, reveals recent feeding of the central massive black hole. A system like AWM 4 should have no radio emission at all if only feedback from the cooling hot gas regulates the AGN activity.

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arXiv2007

Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X-ray observations from Chandra and XMM

We present radial mass profiles within 0.3 r_vir for 16 relaxed galaxy groups-poor clusters (kT range 1-3 keV) selected for optimal mass constraints from the Chandra and XMM data archives. After accounting for the mass of hot gas, the resulting mass profiles are described well by a two-component model consisting of dark matter (DM), represented by an NFW model, and stars from the central galaxy. The stellar component is required only for 8 systems, for which reasonable stellar mass-to-light ratios (M/L_K) are obtained, assuming a Kroupa IMF. Modifying the NFW dark matter halo by adiabatic contraction does not improve the fit and yields systematically lower M/L_K. In contrast to previous results for massive clusters, we find that the NFW concentration parameter (c_vir) for groups decreases with increasing M_vir and is inconsistent with no variation at the 3 sigma level. The normalization and slope of the c_vir-M_vir relation are consistent with the standard LambdaCDM cosmological model with sigma_8 = 0.9. The small intrinsic scatter measured about the c_vir-M_vir relation implies the groups represent preferentially relaxed, early forming systems. The mean gas fraction (f =0.05 +/- 0.01) of the groups measured within an overdensity Delta=2500 is lower than for hot, massive clusters, but the fractional scatter (sigma_f/f=0.2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.

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arXiv2007

The X-Ray Concentration-Virial Mass Relation

We present the concentration (c)-virial mass (M) relation of 39 galaxy systems ranging in mass from individual early-type galaxies up to the most massive galaxy clusters, (0.06-20) x 10^{14} M_sun. We selected for analysis the most relaxed systems possessing the highest quality data currently available in the Chandra and XMM public data archives. A power-law model fitted to the X-ray c-M relation requires at high significance (6.6 sigma) that c decreases with increasing M, which is a general feature of CDM models. The median and scatter of the c-M relation produced by the flat, concordance LCDM model (Omega_m=0.3, sigma_8=0.9) agrees with the X-ray data provided the sample is comprised of the most relaxed, early forming systems, which is consistent with our selection criteria. Holding the rest of the cosmological parameters fixed to those in the concordance model the c-M relation requires 0.76< sigma_8 <1.07 (99% conf.), assuming a 10% upward bias in the concentrations for early forming systems. The tilted, low-sigma_8 model suggested by a new WMAP analysis is rejected at 99.99% confidence, but a model with the same tilt and normalization can be reconciled with the X-ray data by increasing the dark energy equation of state parameter to w ~ -0.8. When imposing the additional constraint of the tight relation between sigma_8 and Omega_m from studies of cluster abundances, the X-ray c-M relation excludes (>99% conf.) both open CDM models and flat CDM models with Omega_m ~1. This result provides novel evidence for a flat, low-Omega_m universe with dark energy using observations only in the local (z << 1) universe. Possible systematic errors in the X-ray mass measurements of a magnitude ~10% suggested by CDM simulations do not change our conclusions.

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arXiv2007

Serendipitous XMM-Newton discovery of a cluster of galaxies at z=0.28

We report the discovery of a galaxy cluster serendipitously detected as an extended X-ray source in an offset observation of the group NGC 5044. The cluster redshift, z=0.281, determined from the optical spectrum of the brightest cluster galaxy, agrees with that inferred from the X-ray spectrum using the Fe K alpha complex of the hot ICM (z=0.27 +/- 0.01). Based on the 50 ks XMM observation, we find that within a radius of 383 kpc the cluster has an unabsorbed X-ray flux, f_X (0.5-2 keV) = 3.34 (+0.08, -0.13) x 10^{-13} erg/cm^2/s, a bolometric X-ray luminosity, L_X = 2.21 (+0.34, -0.19) x 10^{44} erg/s, kT = 3.57 +/- 0.12 keV, and metallicity, 0.60 +/- 0.09 solar. The cluster obeys the scaling relations for L_X and T observed at intermediate redshift. The mass derived from an isothermal NFW model fit is, M_vir = 3.89 +/- 0.35 x 10^{14} solar masses, with a concentration parameter, c = 6.7 +/- 0.4, consistent with the range of values expected in the concordance cosmological model for relaxed clusters. The optical properties suggest this could be a ``fossil cluster''.

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arXiv2006

The Absence of Adiabatic Contraction of the Radial Dark Matter Profile in the Galaxy Cluster A2589

We present an X-ray analysis of the radial mass profile of the radio-quiet galaxy cluster A2589 between 0.015-0.25 r_vir using an XMM-Newton observation. Except for a ~16 kpc shift of the X-ray center of the R=45-60 kpc annulus, A2589 possesses a remarkably symmetrical X-ray image and is therefore an exceptional candidate for precision studies of its mass profile by applying hydrostatic equilibrium. The total gravitating matter profile is well described by the NFW model (fractional residuals <~10%) with c_vir=6.1 +/- 0.3 and M_vir = 3.3 +/- 0.3 x 10^{14} M_sun (r_vir = 1.74 +/- 0.05 Mpc) in excellent agreement with LCDM. When the mass of the hot ICM is subtracted from the gravitating matter profile, the NFW model fitted to the resulting dark matter (DM) profile produces essentially the same result. However, if a component accounting for the stellar mass (M_*) of the cD galaxy is included, then the NFW fit to the DM profile is substantially degraded in the central r ~50 kpc for reasonable M_*/L_V. Modifying the NFW DM halo by adiabatic contraction arising from the early condensation of stellar baryons in the cD galaxy further degrades the fit. The fit is improved substantially with a Sersic-like model recently suggested by high resolution N-body simulations but with an inverse Sersic index, alpha ~0.5, a factor of ~3 higher than predicted. We argue that neither random turbulent motions nor magnetic fields can provide sufficient non-thermal pressure support to reconcile the XMM mass profile with adiabatic contraction of a CDM halo assuming reasonable M_*/L_V. Our results support the scenario where, at least for galaxy clusters, processes during halo formation counteract adiabatic contraction so that the total gravitating mass in the core approximately follows the NFW profile.

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arXiv2006

A Chandra View of Dark Matter in Early-Type Galaxies

We present a Chandra study of mass profiles in 7 elliptical galaxies, of which 3 have galaxy-scale and 4 group-scale halos demarcated at 1E13Msun. These represent the best available data for nearby objects with comparable X-ray luminosities. We measure ~flat mass-to-light (M/L) profiles within an optical half-light radius (Reff), rising by an order of magnitude at ~10Reff, which confirms the presence of dark matter (DM). The data indicate hydrostatic equilibrium, which is also supported by agreement with studies of stellar kinematics in elliptical galaxies. The data are well-fitted by a model comprising an NFW DM profile and a baryonic component following the optical light. The distribution of DM halo concentration parameters (c) versus Mvir agrees with LCDM predictions and our observations of bright groups. Concentrations are slightly higher than expected, which is most likely a selection effect. Omitting the stellar mass drastically increases c, possibly explaining large concentrations found by some past observers. The stellar M/LK agree with population synthesis models, assuming a Kroupa IMF. Allowing adiabatic compression (AC) of the DM halo by baryons made M/L more discrepant, casting some doubt on AC. Our best-fitting models imply total baryon fractions \~0.04--0.09, consistent with models of galaxy formation incorporating strong feedback. The groups exhibit positive temperature gradients, consistent with the "Universal" profiles found in other groups and clusters, whereas the galaxies have negative gradients, suggesting a change in the evolutionary history of the systems around Mvir=1E13 Msun.

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arXiv2005

A Chandra Survey of Early-Type Galaxies, I: Metal Enrichment in the ISM

We present the first in a series of papers studying the X-ray properties of 28 early type galaxies spanning ~3 orders of magnitude in L_x. We report emission-weighted constraints for Fe, O, Ne, Mg, Si, S and Ni. We find no evidence of the very sub-solar Z_Fe historically reported, confirming a trend in recent X-ray observations of bright galaxies and groups, nor do we find Z_Fe correlates with L_x. In general we find no evidence for multi-phase ISM, indicating multi-temperature fits found with ASCA were due to temperature gradients Chandra can resolve. We compare the stellar Z_Fe, estimated from simple stellar population model fits, to Z_Fe(ISM). Excepting one possible outlier we find no evidence that the gas is much more metal-poor than the stars and, in a few systems Z_Fe(ISM) is higher. In general the two exhibit similar Z_Fe, which is inconsistent with both galactic wind models and recent hierarchical chemical enrichment simulations. Adopting standard SNIa and SNII metal yields our abundance ratio constraints imply 66+/-11% of the Fe within the ISM was produced in SNIa, similar to the Solar neighbourhood and implying similar enrichment histories for the ISM in a spiral and in elliptical galaxies. Although these values are sensitive to systematic uncertainties in the SN yields, they are in good agreement with observations of more massive systems, indicating remarkable homology in the enrichment process operating from cluster scales to low-to-intermediate L_x galaxies. The data also uniformly exhibit the low Z_O/Z_Mg abundance ratios which have been reported in some other systems. This is inconsistent with metal-production calculations in SNII and may indicate an additional source of enrichment, such as Population III hypernovae.(abridged)

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arXiv2005

Dark-Matter Halos of Early-Type Galaxies

Cosmological simulations of galaxy formation predict a universal form for the mass profile of dark matter (DM) halos from cluster to galaxy scales. Remarkably few interesting constraints exist, however, on DM halos in early-type galaxies. Using Chandra we present the temperature, density and mass profiles of a small sample of early-type galaxies, revealing significant DM in each case. When a component is included to account for stellar mass and the DM halo is allowed to respond adiabatically to the baryonic condensation into stars, the mass profiles are well-fitted by the universal profile, with Virial masses and concentrations in agreement with simulations. However, only ~half, or less, of the mass within R_e seems attributable to the stars, implying stellar M_*/L_B ~1-5.

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arXiv2005

The Shape of Galaxy Cluster Dark Matter Haloes: Systematics of Its Imprint on Cluster Gas, and Comparison to Observations

(Abridged) We study predictions for galaxy cluster observables that can test the statistics of dark matter halo shapes expected in a flat LCDM universe. We present a simple analytical model for the prediction of cluster-scale X-ray observations, approximating clusters as isothermal systems in hydrostatic equilibrium, and dark matter haloes as ellipsoids with uniform axial ratios. We test the model against high-resolution, hydrodynamic cluster simulations to gauge its reliability. We find that this simple prescription does a good job of predicting the distribution of cluster X-ray ellipticities compared to the simulations as long as one focuses on cluster regions that are less sensitive to recent mergers. Based on this simple model, the distribution of cluster-size halo shapes expected in the concordance LCDM cosmology implies an X-ray ellipticity distribution with a mean of 0.32 +- 0.01 and a scatter of 0.14 +- 0.01 for the mass range (1-4)x10^{14} Msun/h. We find it important to include the mass dependence of halo shape to make comparisons to observational samples that contain many, very massive clusters. We analyse the systematics of four observational samples of cluster ellipticities and find that our results are statistically compatible with observations. In particular, we find remarkably good agreement between two recent ROSAT samples and LCDM predictions that DO NOT include gas cooling. We also test how well our analytical model can predict Sunyaev-Zel'dovich decrement maps and find that it is less successful although still useful; the model does not perform as well as a function of flux level in this case because of the changing triaxiality of dark matter haloes as a function of radial distance. Both this effect and the changing alignment of isodensity shells of dark matter haloes leave an imprint on cluster gas...

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arXiv2005

The Heated Core of the Radio-Quiet Galaxy Cluster A644

We present an analysis of a Chandra ACIS-I observation of the massive galaxy cluster A644. This cluster was previously classified as a cooling flow, but no radio emission has been detected from its cD galaxy. Outside the core (R ~75 kpc ~0.03R_vir) the hot ICM has properties consistent with a (relaxed) cool-core cluster out to the largest radii investigated (R ~415 kpc ~0.14 R_vir). Over this region the gravitating mass profile is described well by a Navarro-Frenk-White profile with concentration parameter, c = 6.1 +/- 1.2, and virial radius, R_vir = 2.9 +/- 0.4 Mpc. However, inside the core the temperature and entropy profiles reverse their inward radial decline and rise at the center; the inner temperature profile is inconsistent with a constant at the 2.3 sigma level. Although the core region does not display X-ray cavities or filamentary structures characteristic of radio-loud, cool-core clusters, the peak of the X-ray emission is offset from that of the centroid of the global X-ray halo by ~60 kpc. The position of the cD galaxy lies approximately between the X-ray peak and centroid, further testifying to a merger origin for the properties of the X-ray emission in the core. We discuss the implications of A644 and the small number of radio-quiet, cool-core clusters for the AGN feedback paradigm to suppress cooling flows in clusters.

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arXiv2005

The Evolution of Structure in X-ray Clusters of Galaxies

Using Chandra archival data, we quantify the evolution of cluster morphology with redshift. Clusters form and grow through mergers with other clusters and groups, and the amount of substructure in clusters in the present epoch and how quickly it evolves with redshift depend on the underlying cosmology. Our sample includes 40 X-ray selected, luminous clusters from the Chandra archive, and we quantify cluster morphology using the power ratio method (Buote & Tsai 1995). The power ratios are constructed from the moments of the X-ray surface brightness and are related to a cluster's dynamical state. We find that, as expected qualitatively from hierarchical models of structure formation, high-redshift clusters have more substructure and are dynamically more active than low-redshift clusters. Specifically, the clusters with z>0.5 have significantly higher average third and fourth order power ratios than the lower redshift clusters. Of the power ratios, $P_3/P_0$ is the most unambiguous indicator of an asymmetric cluster structure, and the difference in $P_3/P_0$ between the two samples remains significant even when the effects of noise and other systematics are considered. After correcting for noise, we apply a linear fit to $P_3/P_0$ versus redshift and find that the slope is greater than zero at better than 99% confidence. This observation of structure evolution indicates that dynamical state may be an important systematic effect in cluster studies seeking to constrain cosmology, and when calibrated against numerical simulations, structure evolution will itself provide interesting bounds on cosmological models.

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arXiv2004

A Chandra View of the Normal SO Galaxy NGC 1332: II: Solar Abundances in the Hot Gas and Implications for SN Enrichment

We present spectral analysis of the diffuse emission in the normal, isolated, moderate-Lx S0 NGC 1332, constraining both the temperature profile and the metal abundances in the ISM. The characteristics of the point source population and the gravitating matter are discussed in two companion papers. The diffuse emission comprises hot gas, with an ~isothermal temperature profile (~0.5 keV), and emission from unresolved point-sources. In contrast with the cool cores of many groups and clusters, we find a small central temperature peak. We obtain emission-weighted abundance contraints within 20 kpc for several key elements: Fe, O, Ne, Mg and Si. The measured iron abundance (Z_Fe=1.1 in solar units; >0.53 at 99% confidence) strongly excludes the very sub-solar values often historically reported for early-type galaxies but agrees with recent observations of brighter galaxies and groups. The abundance ratios, with respect to Fe, of the other elements were also found to be ~solar, although Z_o/Z_Fe was significantly lower (<0.4). Such a low O abundance is not predicted by simple models of ISM enrichment by Type Ia and Type II supernovae, and may indicate a significant contribution from primordial hypernovae. Revisiting Chandra observations of the moderate-Lx, isolated elliptical NGC 720, we obtain similar abundance constraints. Adopting standard SNIa and SNII metal yields, our abundance ratio constraints imply 73+/-5% and 85+/-6% of the Fe enrichment in NGC 1332 and NGC 720, respectively, arises from SNIa. Although these results are sensitive to the considerable systematic uncertainty in the SNe yields, they are in good agreement with observations of more massive systems. These two moderate-Lx early-type galaxies reveal a consistent pattern of metal enrichment from cluster scales to moderate Lx/Lb galaxies. (abridged)

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arXiv2004

Absence of Dwarf Galaxies at High Redshifts: Evidence from a Galaxy Group

The galaxy group NGC 5044 consists of a luminous giant elliptical galaxy surrounded by a cluster of approximately 160 low luminosity and dwarf galaxies, mostly of early type. The cumulative projected radial distribution of dwarf galaxies in the NGC 5044 group, unlike distributions of more luminous galaxies in rich clusters, does not follow a projected dark matter (NFW) profile. Instead, a deficiency or absence of low luminosity galaxies is apparent in NGC 5044 within about 350 kpc, about one third of the current group virial radius. Most of the dwarf galaxies identified in NGC 5044 entered the virial radius at redshifts less than about 2 or 3, and very few entered at higher redshifts. The peculiar, non-NFW shape of the projected cumulative dwarf galaxy distribution in NGC 5044 within 350 kpc resembles the characteristic shallow cumulative distribution of dark subhalos that are also known to be relatively young. Dynamical friction is unlikely to explain the apparent lack of group member galaxies at small radii in NGC 5044.

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arXiv2004

Time-dependent Circulation Flows: Iron Enrichment in Cooling Flows with Heated Return Flows

We describe a new type of dynamical model for hot gas in galaxy groups and clusters in which gas moves simultaneously in both radial directions. Circulation flows are consistent with (1) the failure to observe cooling gas in X-ray spectra, (2) multiphase gas observed near the centers of these flows and (3) the accumulation of iron in the hot gas from Type Ia supernovae in the central galaxy. Dense inflowing gas cools, producing a positive central temperature gradient, as in normal cooling flows. Bubbles of hot, buoyant gas flow outward. Circulation flows eventually cool catastrophically if the outward flowing gas transports mass but no heat; to maintain the circulation both mass and energy must be supplied to the inflowing gas over a large volume, extending to the cooling radius. The rapid radial recirculation of gas produces a flat central core in the gas iron abundance, similar to many observations. We believe the circulation flows described here are the first gasdynamic, long-term evolutionary models that are in good agreement with all essential features observed in the hot gas: little or no gas cools as required by XMM spectra, the gas temperature increases outward near the center, and the gaseous iron abundance is about solar near the center and decreases outward.

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arXiv2004

A Chandra view of the normal S0 galaxy NGC 1332: I. An unbroken, steep power law luminosity function for the LMXB population

Chandra ACIS-S3 observations of the S0 galaxy NGC 1332 resolve much of the emission into 73 point-sources, of which 37 lie within the D_25 isophote. The unresolved emission is discussed in two companion papers. The source luminosity function (XLF) shows the break seen in other early-type galaxies at \~2x10^{38}ergs/s. After correcting for detection incompleteness due to source confusion and contamination from diffuse emission, the break vanishes and the data are well-described as a single power law. This casts further doubt on there being a ``universal'' XLF break in such galaxies dividing neutron-star and black-hole systems. The slope of the differential XLF (dN/dL), beta=2.7+/-0.5, is marginally (~2.5 sigma) steeper than has been found for analogous fits of other early-type galaxies but resembles what is generally seen at high luminosities. Two of the sources within D_25 are ULX, although neither have L_X>2x 10^{39} ergs/s. The absence of very luminous ULX in early-type galaxies suggests a break in the XLF slope at ~1-2x 10^{39} ergs/s, but we cannot constrain it in NGC 1332. The sources have a spatial distribution consistent with the optical light and display a range of properties expected for a LMXB population. The spectra of the individual sources, as well as the composite source spectra, agree with observations of other early-type galaxies, although a small number of highly-absorbed sources are seen. Two sources have very soft spectra, two show strong variability and one source shows evidence of an extended radial profile. We do not detect a central source in NGC 1332, but we find a faint (L_X=2+/-1 x 10^{38} ergs/s) point-source coincident with the centre of the companion dwarf galaxy NGC 1331. (Abridged)

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arXiv2004

Ultra-Low Iron Abundances in Distant Hot Gas in Galaxy Groups

A new XMM observation of the outer regions of the galaxy group NGC 5044 indicates hot gas iron abundances of only ~0.15 solar between r=0.2 - 0.4 r_vir. While the total baryon mass within the virial radius may be close to the cosmic mean value observed in rich clusters, the ratio of total iron mass to optical light in NGC 5044 is about 3 times lower than that in rich clusters. The remarkably low iron abundance over a large volume of the intergroup gas in the outer regions of NGC 5044 cannot be easily understood in terms of the outflow of enriched gas in a group wind during its early history or by the long term enrichment by the group member galaxies that currently occupy this region. It is possible that the stars in NGC 5044 did not produce iron with the same efficiency as in clusters, or the iron resides in non-luminous clouds or stars, or the entropy of the iron-enriched gas created in early galactic starburst winds was too high to penetrate the group gas of lower entropy.

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arXiv2004

The Dark Matter Radial Profile in the Core of the Relaxed Cluster A2589

We present an analysis of a Chandra--ACIS observation of the galaxy cluster A2589 to constrain the radial distribution of the total gravitating matter and the dark matter in the core of the cluster. A2589 is especially well-suited for this analysis because the hot gas in its core region (r < ~0.1 Rvir) is undisturbed by interactions with a central radio source. From the largest radius probed (r=0.07 Rvir) down to r ~0.02 Rvir dark matter dominates the gravitating mass. Over this region the radial profiles of the gravitating and dark matter are fitted well by the NFW and Hernquist profiles predicted by CDM. The density profiles are also described well by power laws, rho ~r^{-alpha}, where alpha=1.37 +/- 0.14 for the gravitating matter and alpha=1.35 +/- 0.21 for the dark matter. These values are consistent with profiles of CDM halos but are significantly larger than alpha ~0.5 found in LSB galaxies and expected from self-interacting dark matter models.

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