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ABSTRACT: Recent hydrostatic X-ray studies of the hot interstellar medium (ISM) in
early-type galaxies underestimate the gravitating mass as compared to stellar
dynamics, implying modest, but significant deviations from exact hydrostatic
equilibrium. We present a method for combining X-ray measurements and stellar
dynamical constraints in the context of Bayesian statistics that allows the
radial distribution of the implied nonthermal pressure or bulk motions in the
hot ISM to be constrained. We demonstrate the accuracy of the method with
hydrodynamical simulations tailored to produce a realistic galaxy model.
Applying the method to the nearby elliptical galaxy NGC4649, we find a
significant but subdominant nonthermal pressure fraction (0.27+/-0.06) in the
central (<5 kpc) part of the galaxy, similar to the level of deviations from
hydrostatic equilibrium expected in galaxy clusters. Plausible sources of
systematic error, if important, may reduce this fraction. This would imply >360
km/s random turbulence or a magnetic field B=(39+/-6)(n_e/0.1
cm^{-3})^{0.59+/-0.09} muG, whereas gas rotation alone is unlikely to explain
the detailed nonthermal profile. Future observations with Astro-H will allow
turbulence or gas rotation at this level to be detected.
05/2012;
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ABSTRACT: The Elliptical Isolated X-ray (ElIXr) Galaxy Survey is a volume-limited
(<110Mpc) study of optically selected, isolated, Lstar elliptical galaxies, to
provide an X-ray census of galaxy-scale (virial mass, Mvir < 1e13 Msun)
objects, and identify candidates for detailed hydrostatic mass modelling. In
this paper, we present a Chandra and XMM study of one such candidate, NGC1521,
and constrain its distribution of dark and baryonic matter. We find a
morphologically relaxed hot gas halo, extending almost to R500, that is well
described by hydrostatic models similar to the benchmark, baryonically closed,
Milky Way-mass elliptical galaxy NGC720. We obtain good constraints on the
enclosed gravitating mass (M500=3.8e12+/-1e12 Msun, slightly higher than
NGC\thin 720), and baryon fraction (fb500=0.13+/-0.03). We confirm at 8.2-sigma
the presence of a dark matter (DM) halo consistent with LCDM. Assuming a
Navarro-Frenk-White DM profile, our self-consistent, physical model enables
meaningful constraints beyond R500, revealing that most of the baryons are in
the hot gas. Within the virial radius, fb is consistent with the Cosmic mean,
suggesting that the predicted massive, quasi-hydrostatic gas halos may be more
common than previously thought. We confirm that the DM and stars conspire to
produce an approximately powerlaw total mass profile (rho \propto r^-alpha)
that follows the recently discovered scaling relation between alpha and optical
effective radius. Our conclusions are insensitive to modest, observationally
motivated, deviations from hydrostatic equilibrium. Finally, after correcting
for the enclosed gas fraction, the entropy profile is close to the self-similar
prediction of gravitational structure formation simulations, as observed in
massive galaxy clusters.
04/2012;
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ABSTRACT: We present a Chandra, Suzaku, and ROSAT study of the hot intragroup medium (IGrM) of the relaxed fossil group/poor cluster RX J1159+5531. This group exhibits an advantageous combination of flat surface brightness profile, high luminosity, and optimal distance, allowing the gas to be detected out to the virial radius (R vir≡ R 108 = 1100 kpc) in a single Suzaku pointing, while the complementary Chandra data reveal a round morphology and relaxed IGrM image down to kpc scales. We measure the IGrM entropy profile over ~3 orders of magnitude in radius, including three data bins beyond ~0.5R 200 that have good azimuthal coverage (>30%). We find no evidence that the profile flattens at large scales (>R 500), and when corrected for the enclosed gas fraction, the entropy profile is very close to the predictions from self-similar structure formation simulations, as seen in massive clusters. Within R vir, we measure a baryon fraction of 0.17 ± 0.02, consistent with the cosmological value. These results are in sharp contrast to the gas behavior at large scales recently reported in the Virgo and Perseus clusters and indicate that substantial gas clumping cannot be ubiquitous near R vir, at least in highly evolved (fossil) groups.
The Astrophysical Journal 02/2012; 748(1):11. · 6.02 Impact Factor
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ABSTRACT: This is the first of two papers investigating the deprojection and spherical
averaging of ellipsoidal galaxy clusters. We specifically consider applications
to hydrostatic X-ray and Sunyaev-Zel'dovich (SZ) studies, though many of the
results also apply to isotropic dispersion-supported stellar dynamical systems.
Here we present analytical formulas for galaxy clusters described by a
gravitational potential that is a triaxial ellipsoid of constant shape and
orientation. For this model type we show that the mass bias due to spherically
averaging X-ray observations is independent of the temperature profile, and for
the special case of a scale-free logarithmic potential, there is exactly zero
mass bias for any shape, orientation, and temperature profile. The ratio of
spherically averaged intracluster medium (ICM) pressures obtained from SZ and
X-ray measurements depends only on the ICM intrinsic shape, projection
orientation, and H_0, which provides another illustration of how cluster
geometry can be recovered through a combination of X-ray and SZ measurements.
We also demonstrate that Y_SZ and Y_X have different biases owing to spherical
averaging, which leads to an offset in the spherically averaged Y_SZ - Y_X
relation. A potentially useful application of the analytical formulas presented
is to assess the error range of an observable (e.g., mass, Y_SZ) accounting for
deviations from assumed spherical symmetry, without having to perform the
ellipsoidal deprojection explicitly. Finally, for dedicated ellipsoidal
studies, we also generalize the spherical onion peeling method to the triaxial
case for a given shape and orientation.
09/2011;
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ABSTRACT: This is the second of two papers investigating the spherical averaging of
ellipsoidal galaxy clusters in the context of X-ray and Sunyaev-Zel'dovich (SZ)
observations. In the present study we quantify the orientation-average bias and
scatter in observables that result from spherically averaging clusters
described by ellipsoidal generalizations of the NFW profile or a nearly
scale-free logarithmic potential. Although the mean biases are small and mostly
<1%, the flattest cluster models generally have a significant mean bias; i.e.,
averaging over all orientations does not always eliminate projection biases.
Substantial biases can result from different viewing orientations, where the
integrated Compton-y parameter (Y_SZ) and the concentration have the largest
scatter (as large as sigma ~10% for Y_SZ), and the emission-weighted
temperature (T_X) has the smallest (sigma < ~0.5%). The very small scatter for
T_X leads to Y_X and M_gas having virtually the same orientation biases.
Substantial scatter is expected for individual clusters (up to sigma ~8%) in
the correlation between Y_SZ and Y_X in comparison to the small mean bias
(sigma < ~1%) applicable to a random sample of clusters of sufficient size. For
ellipsoidal NFW models we show that the orientation bias for the total cluster
mass attains a minimum near the radius r_2500 so that the spherically averaged
mass computed at this radius is always within ~0.5% of the true value for any
orientation. Finally, to facilitate the accounting for orientation bias in
X-ray and SZ cluster studies, we provide cubic polynomial approximations to the
mean orientation bias and 1-sigma scatter for each cluster observable as a
function of axial ratio for the ellipsoidal NFW models.
09/2011;
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ABSTRACT: We present a Chandra, Suzaku and Rosat study of the hot Intra Group Medium
(IGrM) of the relaxed fossil group/ poor cluster RXJ1159+5531. This group
exhibits an advantageous combination of flat surface brightness profile, high
luminosity and optimal distance, allowing the gas to be detected out to the
virial radius (Rvir=R108=1100 kpc) in a single Suzaku pointing, while the
complementary Chandra data reveal a round morphology and relaxed IGrM image
down to kpc scales. We measure the IGrM entropy profile over 3 orders of
magnitude in radius, including 3 data bins beyond 0.5R200 with azimuthal
coverage (>30%). We find no evidence that the profile flattens at large scales
(>R500), and when corrected for the enclosed gas fraction, the entropy profile
is very close to the predictions from self-similar structure formation
simulations, as seen in massive clusters. Within Rvir, we measure a baryon
fraction of 0.17+/-0.02, consistent with the Cosmological value. These results
are in sharp contrast to the gas behaviour at large scales recently reported in
the Virgo and Perseus clusters, and indicate that substantial gas clumping
cannot be ubiquitous near Rvir, at least in highly evolved (fossil) groups.
06/2011;
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ABSTRACT: We review X-ray constraints on dark matter in giant elliptical galaxies
(10^{12} M_sun <~ M_vir <~ 10^{13} M_sun) obtained using the current generation
of X-ray satellites, beginning with an overview of the physics of the hot
interstellar medium and mass modeling methodology. Dark matter is now firmly
established in many galaxies, with inferred NFW concentration parameters
somewhat larger than the mean theoretical relation. X-ray observations confirm
that the total mass profile (baryons+DM) is close to isothermal (M ~ r), and
new evidence suggests a more general power-law relation for the slope of the
total mass profile that varies with the stellar half-light radius. We also
discuss constraints on the baryon fraction, super-massive black holes, and
axial ratio of the dark matter halo. Finally, we review constraints on
non-thermal gas motions and discuss the accuracy of the hydrostatic equilibrium
approximation in elliptical galaxies.
03/2011;
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ABSTRACT: Since the launch of the Einstein X-ray Observatory in the 1970s, a number of
broad absorption features have been reported in the X-ray spectra of BL Lac
objects. These features are often interpreted as arising from high velocity
outflows intrinsic to the BL Lac object, therefore providing important
information about the inner environment around the central engine. However,
such absorption features have not been observed more recently with
high-resolution X-ray telescopes such as Chandra and XMM-Newton. In this paper,
we report the detection of a transient X-ray absorption feature intrinsic to
the BL Lac object H 2356-309 with the Chandra X-ray Telescope. This BL Lac
object was observed during XMM cycle 7, Chandra cycle 8 and 10, as part of our
campaign to investigate X-ray absorption produced by the warm-hot intergalactic
medium (WHIM) residing in the foreground large scale superstructure. During one
of the 80 ksec, Chandra cycle 10 observations, a transient absorption feature
was detected at 3.3-sigma (or 99.9% confidence level, accounting for the number
of trials), which we identify as the OVIII K-alpha line produced by an absorber
intrinsic to the BL Lac object. None of the other 11 observations showed this
line. We constrain the ionization parameter (25 <~ Xi <~ 40) and temperature
(10^5 < T < 2.5 10^7 K) of the absorber. This absorber is likely produced by an
outflow with a velocity up to 1,500 km/s. There is a suggestion of possible
excess emission on the long-wavelength side of the absorption line; however,
the derived properties of the emission material are very different from those
of the absorption material, implying it is unlikely a typical P Cygni-type
profile.
02/2011;
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ABSTRACT: We present a study of the dark and luminous matter in the isolated elliptical
galaxy NGC720, based on deep X-ray observations made with Chandra and Suzaku.
The gas is reliably measured to ~R2500, allowing us to place good constraints
on the enclosed mass and baryon fraction (fb) within this radius
(M2500=1.6e12+/-0.2e12 Msun, fb(2500)=0.10+/-0.01; systematic errors are
<~20%). The data indicate that the hot gas is close to hydrostatic, which is
supported by good agreement with a kinematical analysis of the dwarf satellite
galaxies. We confirm a dark matter (DM) halo at ~20-sigma. Assuming an NFW DM
profile, our physical model for the gas distribution enables us to obtain
meaningful constraints at scales larger than R2500, revealing that most of the
baryons are in the hot gas. We find that fb within Rvir is consistent with the
Cosmological value, confirming theoretical predictions that a ~Milky Way-mass
(Mvir=3.1e12+/-0.4e12 Msun) galaxy can sustain a massive, quasi-hydrostatic gas
halo. While fb is higher than the cold baryon fraction typically measured in
similar-mass spiral galaxies, both the gas fraction (fg) and fb in NGC720 are
consistent with an extrapolation of the trends with mass seen in massive galaxy
groups and clusters. After correcting for fg, the entropy profile is close to
the self-similar prediction of gravitational structure formation simulations,
as observed in galaxy clusters. Finally, we find a strong heavy metal abundance
gradient in the ISM similar to those observed in massive galaxy groups.
10/2010;
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ABSTRACT: In a previous paper, we reported a 3σ detection of an absorption line from the warm-hot intergalactic medium (WHIM) using the Chandra and XMM X-ray grating spectra of the blazar H2356-309, the sight line of which intercepts the Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify our initial detection, we obtained a deep (500 ks), follow-up exposure of H2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect the redshifted O VII WHIM line at a significance level of 3.4σ, a substantial improvement over the 1.7σ level reported previously when using only the Cycle-8 data. The significance increases to 4.0σ when the existing XMM grating data are included in the analysis, thus confirming at higher significance the existence of the line at the redshift of the Sculptor Wall with an equivalent width of 28.5 ± 10.5 mÅ (90% confidence). We obtain a 90% lower limit on the O VII column density of 0.8 × 1016 cm–2 and a 90% upper limit on the Doppler b parameter of 460 km s–1. Assuming the absorber is uniformly distributed throughout the ~15 Mpc portion of the blazar's sight line that intercepts the Sculptor Wall, that the O VII column density is 2 × 1016 cm–2 (corresponding to b 150 km –1 where the inferred column density is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we estimate a baryon over-density of ~30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations. The clear detection of O VII absorption in the Sculptor Wall demonstrates the viability of using current observatories to study WHIM in the X-ray absorption spectra of blazars behind known large-scale structures.
The Astrophysical Journal 04/2010; 714(2):1715. · 6.02 Impact Factor
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ABSTRACT: In a previous paper we reported a 3-sigma detection of an absorption line from the Warm-Hot Intergalactic Medium (WHIM) using the Chandra and XMM X-ray grating spectra of the blazar H2356-309, the sight-line of which intercepts the Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify our initial detection, we obtained a deep (500 ks), follow-up exposure of H2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect the redshifted OVII WHIM line at a significance level of 3.4-sigma, a substantial improvement over the 1.7-sigma level reported previously when using only the Cycle-8 data. The significance increases to 4.0-sigma when the existing XMM grating data are included in the analysis, thus confirming at higher significance the existence of the line at the redshift of the Sculptor Wall with an equivalent width of 28.5+/-10.5 mA (90% confidence). We obtain a 90% lower limit on the OVII column density of 0.8 10^16 cm^-2 and a 90% upper limit on the Doppler-b parameter of 460 km/s. Assuming the absorber is uniformly distributed throughout the ~ 15 Mpc portion of the blazar's sight-line that intercepts the Sculptor Wall, that the OVII column density is ~ 2 10^16 cm^-2 (corresponding to b > 150 km/s where the inferred column density is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we estimate a baryon over-density of ~ 30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations. The clear detection of OVII absorption in the Sculptor Wall demonstrates the viability of using current observatories to study WHIM in the X-ray absorption spectra of blazars behind known large-scale structures. Comment: 11 pages, 6 figures; accepted for publication in the Astrophysical Journal; minor revisions to match the accepted version; conclusions unchanged.
01/2010;
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ABSTRACT: We present a survey, using the Chandra X-ray observatory, of the central gravitating mass profiles in a sample of 10 galaxies, groups and clusters, spanning ~2 orders of magnitude in virial mass. We find the total mass distributions from ~0.2--10Re, where Re is the optical effective radius of the central galaxy, are remarkably similar to powerlaw density profiles. The negative logarithmic slope of the mass density profiles, alpha, systematically varies with Re, from alpha=2, for systems with Re~4kpc to alpha=1.2 for systems with Re>30kpc. Departures from hydrostatic equilibrium are likely to be small and cannot easily explain this trend. We show that the conspiracy between the baryonic (Sersic) and dark matter (NFW/ Einasto) components required to maintain a powerlaw total mass distribution naturally predicts an anti-correlation between alpha and Re that is very close to what is observed. The systematic variation of alpha with Re implies a dark matter fraction within Re that varies systematically with the properties of the galaxy in such a manner as to reproduce, without fine tuning, the observed tilt of the fundamental plane. We speculate that establishing a nearly powerlaw total mass distribution is therefore a fundamental feature of galaxy formation and the primary factor which determines the tilt of the fundamental plane. Comment: 10 pages, 5 figures, 2 tables. Accepted for publication in MNRAS. Minor revisions to match accepted version
11/2009;
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ABSTRACT: Understanding the process of metal enrichment is one of the key problems for our picture of structure formation and evolution, in which early-type galaxies are a crucial ingredient. X-ray observations provide a powerful tool for measuring the metal distributions in their hot ISM, which is shaped by their entire history of star-formation, evolution and feedback. In Fig 1 (left panel), we summarize the results of a Chandra survey of metals in early-type galaxies, supplemented with Suzaku data (Humphrey & Buote 2006, P. Humphrey et al., in prep.). Chandra is particularly suited to this study, as it enables temperature gradients and X-ray point sources to be resolved, mitigating two important sources of bias (e.g., Buote & Fabian 1998; Fabbiano et al. 1994). We found on average that the ISM is at least as metal-rich as the stars, and we did not find the problematical, highly sub-solar, abundances historically reported. The abundance ratios of O, Ne, Mg, Si and S with respect to Fe are similar to the centres of massive groups and clusters, suggesting homology in the enrichment process over a wide mass range. Finally, using high-quality Suzaku data, we were able to resolve, for the first time in a galaxy-scale (≲1013M⊙) object, a radial abundance gradient similar to those seen in some bright galaxy groups (Fig. 1, right panel).
Proceedings of the International Astronomical Union 10/2009; 5:283 - 283.
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ABSTRACT: We describe two-dimensional gasdynamical computations of the X-ray emitting gas in the rotating elliptical galaxy NGC 4649 that indicate an inflow of about one solar mass per year at every radius. Such a large instantaneous inflow cannot have persisted over a Hubble time. The central constant-entropy temperature peak recently observed in the innermost 150 parsecs is explained by compressive heating as gas flows toward the central massive black hole. Since the cooling time of this gas is only a few million years, NGC 4649 provides the most acutely concentrated known example of the cooling flow problem in which the time-integrated apparent mass that has flowed into the galactic core exceeds the total mass observed there. This paradox can be resolved by intermittent outflows of energy or mass driven by accretion energy released near the black hole. Inflowing gas is also required at intermediate kpc radii to explain the ellipticity of X-ray isophotes due to spin-up by mass ejected by stars that rotate with the galaxy and to explain local density and temperature profiles. We provide evidence that many luminous elliptical galaxies undergo similar inflow spin-up. A small turbulent viscosity is required in NGC 4649 to avoid forming large X-ray luminous disks that are not observed, but the turbulent pressure is small and does not interfere with mass determinations that assume hydrostatic equilibrium. Comment: 21 pages, 9 figures, accepted for publication by ApJ
10/2009;
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ABSTRACT: We present new mass measurements for the supermassive black holes (SMBHs) in the centres of three early-type galaxies. The gas pressure in the surrounding, hot interstellar medium (ISM) is measured through spatially resolved spectroscopy with the Chandra X-ray observatory, allowing the SMBH mass (Mbh) to be inferred directly under the hydrostatic approximation. This technique does not require calibration against other SMBH measurement methods and its accuracy depends only on the ISM being close to hydrostatic, which is supported by the smooth X-ray isophotes of the galaxies. Combined with results from our recent study of the elliptical galaxy NGC4649, this brings to four the number of galaxies with SMBHs measured in this way. Of these, three already have mass determinations from the kinematics of either the stars or a central gas disc, and hence join only a handful of galaxies with Mbh measured by more than one technique. We find good agreement between the different methods, providing support for the assumptions implicit in both the hydrostatic and the dynamical models. The stellar mass-to-light ratios for each galaxy inferred by our technique are in agreement with the predictions of stellar population synthesis models assuming a Kroupa initial mass function (IMF). This concurrence implies that no more than ~10-20% of the ISM pressure is nonthermal, unless there is a conspiracy between the shape of the IMF and nonthermal pressure. Finally, we compute Bondi accretion rates, finding that the two galaxies with the highest rates exhibit little evidence of X-ray cavities, suggesting that the correlation with the AGN jet power takes time to be established. Comment: Accepted for publication in the Astrophysical Journal, 23 pages, 10 figures. Minor corrections to text/ figures/ tables to match accepted version
06/2009;
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ABSTRACT: We present XMM RGS and Chandra LETG observations of the blazar, H 2356-309,
located behind the Sculptor Wall, a large-scale galaxy structure expected to
harbor high-density Warm-Hot Intergalactic Medium (WHIM). Our simultaneous
analysis of the RGS and LETG spectra yields a 3-sigma detection of the crucial
redshifted O vii K-alpha line with a column density (>~ 10^{16} cm^{-2})
consistent with similar large-scale structures produced in cosmological
simulations. This represents the first detection of non-local WHIM from X-ray
absorption studies where XMM and Chandra data are analyzed simultaneously and
the absorber redshift is already known, thus providing robust evidence for the
expected repository of the "missing baryons".
The Astrophysical Journal 01/2009; 695(2). · 6.02 Impact Factor
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ABSTRACT: 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 Kα complex of the hot ICM (z = 0.27 ± 0.01). Based on the 50 ks XMM-Newton observation, we find that within a radius of 383 kpc the cluster has an unabsorbed X-ray flux fX(0.5-2 keV) = (3.34) × 10-13 ergs cm-2 s-1, a bolometric X-ray luminosity LX = (2.21) × 1044 ergs s-1, kT = 3.57 ± 0.12 keV, and metallicity 0.60 ± 0.09 Z☉. The cluster obeys the scaling relations for LX and T observed at intermediate redshift. The mass derived from an isothermal NFW model fit is Mvir = 3.89 ± 0.35 × 1014 M☉, 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."
The Astrophysical Journal 12/2008; 662(2):923. · 6.02 Impact Factor
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ABSTRACT: Optically similar elliptical galaxies have an enormous range of X-ray luminosities. We show that this range can be attributed to large variations in the dark halo mass Mvir determined from X-ray observations. The K-band luminosity of ellipticals varies with virial mass, LK M, but with considerable scatter, probably due to the stochastic incidence of massive satellite galaxies that merge by dynamical friction to form group-centered ellipticals. Both the observed X-ray luminosity LX M and LX/LK M are sufficiently sensitive to the virial mass to explain the wide variation observed in LX among galaxies of similar LK. The central galaxy supernova energy per particle of diffuse gas increases dramatically with decreasing virial mass, and elliptical galaxies with the lowest X-ray luminosities (and Mvir) are easily explained by supernova-driven outflows.
The Astrophysical Journal 12/2008; 652(1):L17. · 6.02 Impact Factor
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ABSTRACT: We present an X-ray analysis of the radial mass profile of the radio-quiet galaxy cluster A2589 between 0.015 and 0.25rvir, 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 with cvir = 6.1 ± 0.3 and Mvir = 3.3 ± 0.3 × 1014 M☉ (rvir = 1.74 ± 0.05 Mpc), in excellent agreement with ΛCDM. When the mass of the hot intracluster medium is subtracted from the gravitating matter profile, the NFW model fitted to the resulting dark matter (DM) profile produces essentially the same result. However, when accounting for the stellar mass (M*) of the cD galaxy, the NFW fit to the DM profile substantially degrades in the central r ~ 50 kpc for reasonable values of M*/LV. 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 Sérsic-like model recently suggested by high-resolution N-body simulations but with an inverse Sérsic index, α ~ 0.5, that is a factor of ~3 higher than predicted. We argue that neither random turbulent motions nor magnetic fields can provide sufficient nonthermal pressure support to reconcile the XMM-Newton mass profile with adiabatic contraction of a CDM halo, assuming reasonable values of M*/LV. Our results support the scenario in which, 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.
The Astrophysical Journal 12/2008; 650(2):777. · 6.02 Impact Factor
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ABSTRACT: We present a Chandra study of mass profiles in seven elliptical galaxies, of which three have galaxy-scale and four have group-scale halos, demarcated at 1013 M☉. These represent the best available data for nearby objects with comparable X-ray luminosities. We measure approximately flat mass-to-light (M/L) profiles within an optical half-light radius (Reff), rising by an order of magnitude at ~10 Reff, 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 ΛCDM 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 1013 M☉.
The Astrophysical Journal 12/2008; 646(2):899. · 6.02 Impact Factor
