Philip J. Humphrey

A Chandra View of Dark Matter in Early-Type Galaxies

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 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) × 1014 M☉. We selected for analysis the most relaxed systems possessing the highest quality data currently available in the Chandra and XMM-Newton public data archives. A power-law model fitted to the X-ray c-M relation requires at high significance (6.6 σ) 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 ΛCDM model (Ωm = 0.3, σ8 = 0.9) agrees with the X-ray data, provided that the sample is comprised of the most relaxed, early-forming systems, which is consistent with our selection criteria. When allowing only σ8 to vary in the concordance model, the c-M relation requires 0.76 99% confidence) both open CDM models and flat CDM models with Ωm ≈ 1. This result provides novel evidence for a flat, low-Ωm 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.

Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X-Ray Observations from Chandra and XMM-Newton

We present radial mass profiles within ~0.3rvir for 16 relaxed galaxy groups—poor clusters (kT range 1-3 keV) selected for optimal mass constraints from the Chandra and XMM-Newton 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, represented by an NFW model, and stars from the central galaxy. The stellar component is required only for eight systems, for which reasonable stellar mass-to-light ratios (M/LK) 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/LK. In contrast to previous results for massive clusters, we find that the NFW concentration parameter (cvir) for groups decreases with increasing Mvir and is inconsistent with no variation at the 3 σ level. The normalization and slope of the cvir-Mvir relation are consistent with the standard ΛCDM cosmological model with σ8 = 0.9 (considering a 10% bias for early forming systems). The small intrinsic scatter measured about the cvir-Mvir relation implies that the groups represent preferentially relaxed, early forming systems. The mean gas fraction (f = 0.05 ± 0.01) of the groups measured within an overdensity Δ = 2500 is lower than for hot, massive clusters, but the fractional scatter (σf/f = 0.2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.

A Chandra Survey of Early-Type Galaxies. I. Metal Enrichment in the Interstellar Medium

We present a Chandra study of the emission-weighted metal abundances in 28 early-type galaxies, spanning ~3 orders of magnitude in X-ray luminosity (LX). We report constraints for Fe, O, Ne, Mg, Si, S, and Ni. We find no evidence of the very subsolar Fe abundance (ZFe) historically reported, confirming a trend in recent observations of bright galaxies and groups, nor do we find any correlation between ZFe and luminosity. Excepting one case, the ISM is single-phase, indicating that multitemperature fits found with ASCA reflected temperature gradients that we resolve with Chandra. We find no evidence that ZFe (ISM) is substantially lower than the stellar metallicity estimated from simple stellar population models. In general, these quantities are similar, which is inconsistent with galactic wind models and recent hierarchical chemical enrichment simulations. Our abundance ratio constraints imply that 66% ± 11% of the ISM Fe was produced in SNe Ia, similar to the solar neighborhood, indicating similar enrichment histories for elliptical galaxies and the Milky Way. Although these values are sensitive to the considerable systematic uncertainty in the supernova yields, they are in agreement with observations of more massive systems. This indicates considerable homology in the enrichment process operating from cluster scales to low-to-intermediate-LX galaxies. The data uniformly exhibit low ZO/ZMg ratios, which have been reported in some clusters, groups, and galaxies. This is inconsistent with standard SN II metal yield calculations and may indicate an additional source of enrichment, such as Population III hypernovae.

Sterile neutrino dark matter bounds from galaxies of the Local Group

We show that the canonical oscillation-based (nonresonant) production of sterile neutrino dark matter is inconsistent at >99% confidence with observations of galaxies in the Local Group. We set lower limits on the nonresonant sterile neutrino mass of 2.5 keV (equivalent to 0.7 keV thermal mass) using phase-space densities derived for dwarf satellite galaxies of the Milky Way as well as limits of 8.8 keV (equivalent to 1.8 keV thermal mass) based on subhalo counts of N-body simulations of M 31 analogs. Combined with improved upper mass limits derived from significantly deeper x-ray data of M 31 with full consideration for background variations, we show that there remains little room for nonresonant production if sterile neutrinos are to explain 100% of the dark matter abundance. Resonant and nonoscillation sterile neutrino production remain viable mechanisms for generating sufficient dark matter sterile neutrinos.

CONFIRMATION OF X-RAY ABSORPTION BY WARM-HOT INTERGALACTIC MEDIUM IN THE SCULPTOR WALL

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-10Chandra 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 3.4σ , 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 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 −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 Ovii column density is ≈2 × 10 16 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 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.

ROTATION AND TURBULENCE OF THE HOT INTRACLUSTER MEDIUM IN GALAXY CLUSTERS

Cosmological simulations of galaxy clusters typically find that the weight of a cluster at a given radius is not balanced entirely by the thermal gas pressure of the hot intracluster medium (ICM), with theoretical studies emphasizing the role of random turbulent motions to provide the necessary additional pressure support. Using a set of high-resolution, hydrodynamical simulations of galaxy clusters that include radiative cooling and star formation and are formed in a cold dark matter (CDM) universe, we find instead that in the most relaxed clusters rotational support exceeds that from random turbulent motions for radii 0.1-0.5 r 500, while at larger radii, out to 0.8 r 500, they remain comparable. We also find that the X-ray images of the ICM flatten prominently over a wide radial range, 0.1-0.4 r 500. When compared with the average ellipticity profile of the observed X-ray images computed for nine relaxed nearby clusters, we find that the observed clusters are much rounder than the relaxed CDM clusters within ≈0.4 r 500. Moreover, while the observed clusters display an average ellipticity profile that does not vary significantly with radius, the ellipticity of the relaxed CDM clusters declines markedly with increasing radius, suggesting that the ICM of the observed clusters rotates less rapidly than that of the relaxed CDM clusters out to ≈0.6 r 500. When these results are compared with those obtained from a simulation without radiative cooling, we find a cluster ellipticity profile in much better agreement with the observations, implying that overcooling has a substantial impact on the gasdynamics and morphology out to larger radii than previously recognized. It also suggests that the 10-20% systematic errors from nonthermal gas pressure support reported for simulated cluster masses, obtained from fitting simulated X-ray data over large radial ranges within r 500, may need to be revised downward. These results demonstrate the utility of X-ray ellipticity profiles as a probe of ICM rotation and overcooling which should be used to constrain future cosmological cluster simulations.

The slope of the mass profile and the tilt of the Fundamental Plane in early‐type galaxies

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 to 10R e , where R e is the optical effective radius of the central galaxy, are remarkably similar to power-law density profiles. The negative logarithmic slope of the mass density profiles, α, systematically varies with R e , from α ≃ 2, for systems with R e ∼ 4 kpc to a ≃ 1.2 for systems with R e ≳ 30 kpc. 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 (Navarro-Frenk-White/Einasto) components required to maintain a power-law total mass distribution naturally predicts an anti-correlation between α and R e that is very close to what is observed. The systematic variation of α with R e implies a dark matter fraction within R e 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 power-law total mass distribution is therefore a fundamental feature of galaxy formation and the primary factor which determines the tilt of the Fundamental Plane.

A Census of Baryons and Dark Matter in an Isolated, Milky Way Sized Elliptical Galaxy

We present a study of the dark and luminous matter in the isolated elliptical galaxy NGC 720, based on deep X-ray observations made with the Chandra and Suzaku observatories. The gas properties are reliably measured almost to R 2500 , allowing us to place good constraints on the enclosed mass and baryon fraction (f b ) within this radius (M 2500 = (1.6±0.2) x 10 12 M ⊙ , f b,2500 = 0.10±0.01; systematic errors are typically ≲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 at high significance (~20σ) the presence of a dark matter (DM) halo. Assuming a Navarro-Frenk-White DM profile, our physical model for the gas distribution enables us to obtain meaningful constraints at scales larger than R 2500, revealing that most of the baryons are in the hot gas. We find that f b within the virial radius is consistent with the Cosmological value, confirming theoretical predictions that a ~ Milky Way mass (M vir = 3.1 +0.4 -0.3 × 10 12 M ⊙ ) galaxy can sustain a massive, quasi-hydrostatic gas halo. While f b is higher than the cold (cool gas plus stars) baryon fraction typically measured in similar-mass spiral galaxies, both the gas fraction (f g ) and f b in NGC 720 are consistent with an extrapolation of the trends with mass seen in massive galaxy groups and clusters. After correcting for f g , the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in massive galaxy clusters. Finally, we find a strong heavy metal abundance gradient in the interstellar medium, qualitatively similar to those observed in massive galaxy groups.

Low-Mass X-Ray Binaries and Globular Clusters in Early-Type Galaxies. I. Chandra Observations

We present a Chandra survey of LMXBs in 24 early-type galaxies. Correcting for detection incompleteness, the X-ray luminosity function (XLF) of each galaxy is consistent with a power law with negative logarithmic differential slope, β ~ 2.0. However, β strongly correlates with incompleteness, indicating the XLF flattens at low-LX. The composite XLF is well fitted by a power law with a break at (2.21+ 0.65−0.56) × 1038 ergs s−1 and β = 1.40+ 0.10−0.13 and 2.84+ 0.39−0.30 below and above it, respectively. The break is close to the Eddington limit for a 1.4 M☉ neutron star, but the XLF shape rules out its representing the division between neutron star and black hole systems. Although the XLFs are similar, we find evidence of some variation between galaxies. The high-LX XLF slope does not correlate with age, but may correlate with [α/Fe]. Considering only LMXBs with LX > 1037 ergs s−1, matching the LMXBs with globular clusters (GCs) identified in HST observations of 19 of the galaxies, we find the probability a GC hosts an LMXB is proportional to LαGCZγFe where α = 1.01 ± 0.19 and γ = 0.33 ± 0.11. Correcting for GC luminosity and color effects, and detection incompleteness, we find no evidence that the fraction of LMXBs with LX > 1037 ergs s−1 in GCs (40%), or the fraction of GCs hosting LMXBs (~6.5%) varies between galaxies. The spatial distribution of LMXBs resembles that of GCs, and the specific frequency of LMXBs is proportional to the GC specific luminosity, consistent with the hypothesis that all LMXBs form in GCs. If the LMXB lifetime is τL and the duty cycle is Fd, our results imply ~1.5(τL/108 yr )−1F−1d LMXBs are formed per gigayear per GC, and we place an upper limit of one active LMXB in the field per 3.4 × 109 L☉ of V-band luminosity.