William G. Mathews

Hot Gas in and Around Elliptical Galaxies

▪ Abstract We review the origin, evolution, and physical nature of hot gas in elliptical galaxies and associated galaxy groups. Unanticipated recent X-ray observations with Chandra and XMM indicate much less cooling than previously expected. Consequently, many long-held assumptions must be reexamined or discarded and new approaches must be explored. Chief among these are the role of heating by active galactic nuclei, the influence of radio lobes on the hot gas, details of the cooling process, possible relation between the hot and colder gas in elliptical galaxies, and the complexities of stellar enrichment of the hot gas.

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☉.

Multidirectional views of the active nucleus of NGC 1068

Previous spectropolarimetric work has shown that the type 2 Seyfert galaxy NGC 1068 contains a broad-line region hidden from our direct view and visible only in scattered, polarized light. New observations, including imaging polarimetry as well as spectropolarimetry, allow us to confirm and extend the earlier work. Off-nucleus regions of high polarization are identified as places in the body of the galaxy where dust reflects light from the hidden broad-line region. This allows for the first time the ability to obtain spectra of an active nucleus as it would appear from several different viewing directions.

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.

THE FERMI BUBBLES. I. POSSIBLE EVIDENCE FOR RECENT AGN JET ACTIVITY IN THE GALAXY

The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in the Galaxy, which extend about 50° (~10 kpc) above and below the Galactic center (GC) and are symmetric about the Galactic plane. Using axisymmetric hydrodynamic simulations with a self-consistent treatment of the dynamical cosmic ray (CR)-gas interaction, we show that the bubbles can be created with a recent active galactic nucleus (AGN) jet activity about 1-3 Myr ago, which was active for a duration of ~0.1-0.5 Myr. The bipolar jets were ejected into the Galactic halo along the rotation axis of the Galaxy. Near the GC, the jets must be moderately light with a typical density contrast 0.001 η 0.1 relative to the ambient hot gas. The jets are energetically dominated by kinetic energy, and overpressured with either CR or thermal pressure which induces lateral jet expansion, creating fat CR bubbles as observed. The sharp edges of the bubbles imply that CR diffusion across the bubble surface is strongly suppressed. The jet activity induces a strong shock, which heats and compresses the ambient gas in the Galactic halo, potentially explaining the ROSAT X-ray shell features surrounding the bubbles. The Fermi bubbles provide plausible evidence for a recent powerful AGN jet activity in our Galaxy, providing new insights into the origin of the halo CR population and the channel through which massive black holes in disk galaxies release feedback energy during their growth.

Far-Infrared Spitzer Observations of Elliptical Galaxies: Evidence for Extended Diffuse Dust

Far-infrared Spitzer observations of elliptical galaxies are inconsistent with simple steady state models of dust creation in red giant stars and destruction by grain sputtering in the hot interstellar gas at T ~ 107 K. The flux at 24 μm correlates with optical fluxes, suggesting that this relatively hot dust is largely circumstellar. But fluxes at 70 and 160 μm do not correlate with optical fluxes. Elliptical galaxies with similar LB have luminosities at 70 and 160 μm (L70 and L160) that vary over a factor of ~100, implying an additional source of dust unrelated to that produced by ongoing local stellar mass loss. Neither L70/LB nor L160/LB correlate with the stellar age or metallicity. Optical line fluxes from warm gas at T ~ 104 K correlate weakly with L70 and L160, suggesting that the dust may be responsible for cooling this gas. Many normal elliptical galaxies have emission at 70 μm that is extended to 5-10 kpc. Extended far-infrared emission with sputtering lifetimes of ~108 yr is difficult to maintain by mergers with gas-rich galaxies. Instead, we propose that this cold dust is buoyantly transported from reservoirs of dust in the galactic cores, which are supplied by mass loss from stars in the core. Intermittent energy outbursts from AGNs can drive the buoyant outflow.

XMM-NEWTON AND CHANDRA OBSERVATIONS OF THE GALAXY GROUP NGC 5044. I. EVIDENCE FOR LIMITED MULTIPHASE HOT GAS

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 ( = 4.5 ? 0.2 M? yr-1) models within the central ~30 kpc. Alternatively, the data can be fitted equally well if the temperature within each spherical shell varies continuously from ~Th to Tc ~ Th/2, but no lower. The high spatial resolution of the Chandra data allows us to determine that the temperature excursion Th ? Tc 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 multiphase gas having a limited temperature range. We do not find any evidence that azimuthal temperature variations within each annulus on the sky can account for the range in temperatures within each shell. We provide a detailed investigation of the systematic errors on the derived spectral models considering the effects of calibration, plasma codes, bandwidth, variable NH, and background rate. We find that the RGS gratings and the EPIC and ACIS CCDs give fully consistent results when the same models are fitted over the same energy ranges for each instrument. The cooler component of the 2T model has a temperature (Tc ~ 0.7 keV) similar to the kinetic temperature of the stars. The hot phase has a temperature (Th ~ 1.4 keV) characteristic of the virial temperature of the ~1013 M? 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 ~Th 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.

Cold Dust in Early-Type Galaxies. I. Observations*

We describe far-infrared observations of early-type galaxies selected from the Infrared Space Observatory (ISO) archive. This rather inhomogeneous sample includes 39 giant elliptical galaxies and 14 S0 (or later) galaxies. These galaxies were observed with the array photometer PHOT on-board the ISO satellite using a variety of different observing modes—sparse maps, mini-maps, oversampled maps, and single pointings—each of which requires different and often rather elaborate photometric reduction procedures. The ISO background data agree well with the COBE-DIRBE results to which we have renormalized our calibrations. As a further check, the ISO fluxes from galaxies at 60 and 100 μm agree very well with those previously observed with IRAS at these wavelengths. The spatial resolution of ISO is several times greater than that of IRAS, and the ISO observations extend out to 200 μm, which views a significantly greater mass of colder dust not assessable to IRAS. Most of the galaxies are essentially point sources at ISO resolution, but a few are clearly extended at FIR wavelengths with image sizes that increase with FIR wavelength. The integrated far-infrared luminosities do not correlate with optical luminosities, suggesting that the dust may have an external, merger-related origin. In general, the far-infrared spectral energy distributions can be modeled with dust at two temperatures, ~43 and ~20 K, which probably represent limits of a continuous range of temperatures. The colder dust component dominates the total mass of dust, 106-107 M⊙, which is typically more than 10 times larger than the dust masses previously estimated for the same galaxies using IRAS observations. For S0 galaxies we find that the optically normalized far-infrared luminosity LFIR/LB correlates strongly with the mid-infrared luminosity L15 μm/LB, but that correlation is weaker for elliptical galaxies.

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 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 Sersic-like model recently suggested by high-resolution N-body simulations but with an inverse Sersic 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.