Hot Halo Gas Research Articles

The unexpectedly large dust and gas content of quiescent galaxies at z > 1.4

Early type galaxies (ETG) contain most of the stars present in the local Universe and, above a stellar mass of ~5e10 Msun, vastly outnumber spiral galaxies like the Milky Way. These massive spheroidal galaxies have, in the present day, very little gas or dust, and their stellar populations have been evolving passively for over 10 billion years. The physical mechanisms that led to the termination of star formation in these galaxies and depletion of their interstellar medium remain largely conjectural. In particular, there are currently no direct measurements of the amount of residual gas that might be still present in newly quiescent spheroids at high redshift. Here we show that quiescent ETGs at z~1.8, close to their epoch of quenching, contained 2-3 orders of magnitude more dust at fixed stellar mass than local ETGs. This implies the presence of substantial amounts of gas (5-10%), which was however consumed less efficiently than in more active galaxies, probably due to their spheroidal morphology, and consistently with our simulations. This lower star formation efficiency, and an extended hot gas halo possibly maintained by persistent feedback from an active galactic nucleus (AGN), combine to keep ETGs mostly passive throughout cosmic time.

The Circum-Galactic Medium of Massive Spirals. II. Probing the Nature of Hot Gaseous Halo around the Most Massive Isolated Spiral Galaxies

We present the analysis of the XMM-Newton data of the Circum-Galactic Medium of MASsive Spirals (CGM-MASS) sample of six extremely massive spiral galaxies in the local Universe. All the CGM-MASS galaxies have diffuse X-ray emission from hot gas detected above the background extending $\sim(30-100)\rm~kpc$ from the galactic center. This doubles the existing detection of such extended hot CGM around massive spiral galaxies. The radial soft X-ray intensity profile of hot gas can be fitted with a $\beta$-function with the slope typically in the range of $\beta=0.35-0.55$. This range, as well as those $\beta$ values measured for other massive spiral galaxies, including the Milky Way (MW), are in general consistent with X-ray luminous elliptical galaxies of similar hot gas luminosity and temperature, and with those predicted from a hydrostatic isothermal gaseous halo. Hot gas in such massive spiral galaxy tends to have temperature comparable to its virial value, indicating the importance of gravitational heating. This is in contrast to lower mass galaxies where hot gas temperature tends to be systematically higher than the virial one. The ratio of the radiative cooling to free fall timescales of hot gas is much larger than the critical value of $\sim10$ throughout the entire halos of all the CGM-MASS galaxies, indicating the inefficiency of gas cooling and precipitation in the CGM. The hot CGM in these massive spiral galaxies is thus most likely in a hydrostatic state, with the feedback material mixed with the CGM, instead of escaping out of the halo or falling back to the disk. We also homogenize and compare the halo X-ray luminosity measured for the CGM-MASS galaxies and other galaxy samples and discuss the "missing" galactic feedback detected in these massive spiral galaxies.

The Properties of the Galactic Hot Gaseous Halo from X-Ray Emission

Abstract The extended hot X-ray emitting gaseous halo of the Milky Way has an optical depth ∼1 for the dominant emission lines of O vii and O viii, which are used to infer the halo properties. To improve on halo gas properties, we treat optical depth effects with a Monte Carlo radiative transfer model, which leads to slightly steeper density profiles ( β ≈ 0.5 ) than if optical depths effects were ignored. For the preferred model where the halo is rotating on cylinders at 180 km s−1, independent fits to both lines lead to identical results, where the core radius is 2.5 kpc and the turbulent component of the Doppler-b parameter is 100–120 km s−1; the turbulent pressure is 20% of the thermal pressure. The fit is improved when emission from a disk is included, with a radial scale length of 3 kpc (assumed) and a fitted vertical scale height of approximately 1.3 kpc. The disk component is a minor mass constituent and has low optical depth, except at low latitudes. The gaseous mass is 3 – 4 × 10 10 M ⊙ within 250 kpc , similar to our previous determinations and significantly lower than the mass of the missing baryons, which is 1.7 × 10 11 M ⊙ .

The Baryonic Collapse Efficiency of Galaxy Groups in the RESOLVE and ECO Surveys

Abstract We examine the z = 0 group-integrated stellar and cold baryonic (stars + cold atomic gas) mass functions (group SMF and CBMF) and the baryonic collapse efficiency (group cold baryonic to dark matter halo mass ratio) using the RESOLVE and ECO survey galaxy group catalogs and a galform semi-analytic model (SAM) mock catalog. The group SMF and CBMF fall off more steeply at high masses and rise with a shallower low-mass slope than the theoretical halo mass function (HMF). The transition occurs at the group-integrated cold baryonic mass 1011 . The SAM, however, has significantly fewer groups at the transition mass ∼1011 and a steeper low-mass slope than the data, suggesting that feedback is too weak in low-mass halos and conversely too strong near the transition mass. Using literature prescriptions to include hot halo gas and potential unobservable galaxy gas produces a group BMF with a slope similar to the HMF even below the transition mass. Its normalization is lower by a factor of ∼2, in agreement with estimates of warm-hot gas making up the remaining difference. We compute baryonic collapse efficiency with the halo mass calculated two ways, via halo abundance matching (HAM) and via dynamics (extended all the way to three-galaxy groups using stacking). Using HAM, we find that baryonic collapse efficiencies reach a flat maximum for groups across the halo mass range of , which we label “nascent groups.” Using dynamics, however, we find greater scatter in baryonic collapse efficiencies, likely indicating variation in group hot-to-cold baryon ratios. Similarly, we see higher scatter in baryonic collapse efficiencies in the SAM when using its true groups and their group halo masses as opposed to friends-of-friends groups and HAM masses.

Hot Gaseous Halos in Early Type Galaxies

The hot gas in early type galaxies (ETGs) plays a crucial role in their formation and evolution. As the hot gas is often extended to the outskirts beyond the optical size, the large scale structural features identified by Chandra (including cavities, cold fronts, filaments, and tails) point to key evolutionary mechanisms, e.g., AGN feedback, merging history, accretion/stripping, as well as star formation and quenching. We systematically analyze the archival Chandra data of ETGs to study the hot ISM. Using uniformly derived data products with spatially resolved spectral information, we revisit the X-ray scaling relations of ETGs and address their implications by comparing them with those of groups/clusters and simulations.

The formation of hot gaseous haloes around galaxies

We use a suite of hydrodynamical cosmological simulations from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) project to investigate the formation of hot hydrostatic haloes and their dependence on feedback mechanisms. We find that the appearance of a strong bimodality in the probability density function (PDF) of the ratio of the radiative cooling and dynamical times for halo gas provides a clear signature of the formation of a hot corona. Haloes of total mass $10^{11.5}-10^{12}{\rm{M}}_{\odot}$ develop a hot corona independent of redshift, at least in the interval $z=0-4$ where the simulation has sufficiently good statistics. We analyse the build up of the hot gas mass in the halo, $M_{\rm{hot}}$, as a function of halo mass and redshift and find that while more energetic galactic winds powered by SNe increases $M_{\rm{hot}}$, AGN feedback reduces it by ejecting gas from the halo. We also study the thermal properties of gas accreting onto haloes and measure the fraction of shock-heated gas as a function of redshift and halo mass. We develop analytic and semianalytic approaches to estimate a `critical halo mass', $M_{\rm{crit}}$, for hot halo formation. We find that the mass for which the heating rate produced by accretion shocks equals the radiative cooling rate, reproduces the mass above which haloes develop a significant hot atmosphere. This yields a mass estimate of $M_{\rm{crit}} \approx 10^{11.7}{\rm{M}}_{\odot}$ at $z=0$, which agrees with the simulation results. The value of $M_{\rm{crit}}$ depends more strongly on the cooling rate than on any of the feedback parameters.

Improved measurements of turbulence in the hot gaseous atmospheres of nearby giant elliptical galaxies

We present significantly improved measurements of turbulent velocities in the hot gaseous haloes of nearby giant elliptical galaxies. Using deep XMM–NewtonReflection Grating Spectrometer (RGS) observations and a combination of resonance scattering and direct line broadening methods, we obtain well bounded constraints for 13 galaxies. Assuming that the turbulence is isotropic, we obtain a best-fitting mean 1D turbulent velocity of ∼110 km s−1. This implies a typical 3D Mach number ∼0.45 and a typical non-thermal pressure contribution of ∼6 per cent in the cores of nearby massive galaxies. The intrinsic scatter around these values is modest – consistent with zero, albeit with large statistical uncertainty – hinting at a common and quasi-continuous mechanism sourcing the velocity structure in these objects. Using conservative estimates of the spatial scales associated with the observed turbulent motions, we find that turbulent heating can be sufficient to offset radiative cooling in the inner regions of these galaxies (<10 kpc, typically 2–3 kpc). The full potential of our analysis methods will be enabled by future X-ray micro-calorimeter observations.

Constraining the Physical State of the Hot Gas Halos in NGC 4649 and NGC 5846

Abstract We present results of a joint Chandra/XMM-Newton analysis of the early-type galaxies NGC 4649 and NGC 5846 aimed at investigating differences between mass profiles derived from X-ray data and those from optical data, to probe the state of the hot interstellar medium (ISM) in these galaxies. If the hot ISM is at a given radius in hydrostatic equilibrium (HE), the X-ray data can be used to measure the total enclosed mass of the galaxy. Differences from optically derived mass distributions therefore yield information about departures from HE in the hot halos. The X-ray mass profiles in different angular sectors of NGC 4649 are generally smooth with no significant azimuthal asymmetries within 12 kpc. Extrapolation of these profiles beyond this scale yields results consistent with the optical estimate. However, in the central region ( kpc) the X-ray data underpredict the enclosed mass, when compared with the optical mass profiles. Consistent with previous results, we estimate a nonthermal pressure component accounting for 30% of the gas pressure, likely linked to nuclear activity. In NGC 5846 the X-ray mass profiles show significant azimuthal asymmetries, especially in the NE direction. Comparison with optical mass profiles in this direction suggests significant departures from HE, consistent with bulk gas compression and decompression due to sloshing on ∼15 kpc scales; this effect disappears in the NW direction, where the emission is smooth and extended. In this sector we find consistent X-ray and optical mass profiles, suggesting that the hot halo is not responding to strong nongravitational forces.

The unorthodox evolution of major merger remnants into star-forming spiral galaxies

Galaxy mergers are believed to play a key role in transforming star-forming disk galaxies into quenched ellipticals. Most of our theoretical knowledge about such morphological transformations does, however, rely on idealised simulations where processes such as cooling of hot halo gas into the disk and gas accretion in the post-merger phase are not treated in a self-consistent cosmological fashion. In this paper we study the morphological evolution of the stellar components of four major mergers occurring at z=0.5 in cosmological hydrodynamical zoom-simulations. In all simulations the merger reduces the disk mass-fraction, but all galaxies simulated at our highest resolution regrow a significant disk by z=0 (with a disk fraction larger than 24%). For runs with our default physics model, which includes galactic winds from star formation and black hole feedback, none of the merger remnants are quenched, but in a set of simulations with stronger black hole feedback we find that major mergers can indeed quench galaxies. We conclude that major merger remnants commonly evolve into star-forming disk galaxies, unless sufficiently strong AGN feedback assists in the quenching of the remnant.

The VLA-COSMOS 3 GHz Large Project: Cosmic evolution of radio AGN and implications for radio-mode feedback sincez ~ 5

Based on a sample of over 1,800 radio AGN at redshifts out to z~5, which have typical stellar masses within ~3x(10^{10}-10^{11}) Msol, and 3 GHz radio data in the COSMOS field, we derived the 1.4 GHz radio luminosity functions for radio AGN (L_1.4GHz ~ 10^{22}-10^{27} W/Hz) out to z~5. We constrained the evolution of this population via continuous models of pure density and pure luminosity evolutions, and we found best-fit parametrizations of Phi*~(1+z)^{(2.00+/-0.18)-(0.60+/-0.14)z}, and L*~(1+z)^{(2.88+/-0.82)-(0.84+/-0.34)z}, respectively, with a turnover in number and luminosity densities of the population at z~1.5. We converted 1.4 GHz luminosity to kinetic luminosity taking uncertainties of the scaling relation used into account. We thereby derived the cosmic evolution of the kinetic luminosity density provided by the AGN and compared this luminosity density to the radio-mode AGN feedback assumed in the Semi-Analytic Galaxy Evolution (SAGE) model, i.e., to the redshift evolution of the central supermassive black hole accretion luminosity taken in the model as the source of heating that offsets the energy losses of the cooling, hot halo gas, and thereby limits further stellar mass growth of massive galaxies. We find that the kinetic luminosity exerted by our radio AGN may be high enough to balance the radiative cooling of the hot gas at each cosmic epoch since z~5. However, although our findings support the idea of radio-mode AGN feedback as a cosmologically relevant process in massive galaxy formation, many simplifications in both the observational and semi-analytic approaches still remain and need to be resolved before robust conclusions can be reached.

A massive, dead disk galaxy in the early Universe.

At z=2, when the Universe was just 3 Gyr old, half of the most massive galaxies were extremely compact and had already exhausted their fuel for star formation1–4. It is believed that they were formed in intense nuclear starbursts and that they ultimately grew into the most massive local elliptical galaxies seen today, through mergers with minor companions5,6, but validating this scenario requires higher resolution observations of their centers than currently possible, even from space. Magnification due to gravitational lensing offers a unique opportunity to resolve their inner regions, as demonstrated in a recent study of a z=2.6 compact spheroidal galaxy which revealed a bulge, rotating at velocities comparable to the fastest rotating local ellipticals7. Following the same approach, here we map the stellar populations and kinematics of a lensed z=2.1478 compact galaxy, which surprisingly turn out to be a fast spinning, rotationally supported disk galaxy. Rather than in a merger-driven nuclear starburst8, its stars must thus have formed in a disk, likely fed by streams of cold gas, which were able to penetrate the hot halo gas until they were cut off by shock heating from the dark matter halo9. This result unambiguously confirm indications from a growing body of indirect evidence10–13 that the first galaxies to cease star formation must go through major changes not just in their structure, but also in their kinematics to evolve into present day ellipticals.

SDSS IV MaNGA—Rotation Velocity Lags in the Extraplanar Ionized Gas from MaNGA Observations of Edge-on Galaxies

Abstract We present a study of the kinematics of the extraplanar ionized gas around several dozen galaxies observed by the Mapping of Nearby Galaxies at the Apache Point Observatory (MaNGA) survey. We considered a sample of 67 edge-on galaxies out of more than 1400 extragalactic targets observed by MaNGA, in which we found 25 galaxies (or 37%) with regular lagging of the rotation curve at large distances from the galactic midplane. We model the observed Hα emission velocity fields in the galaxies, taking projection effects and a simple model for the dust extinction into account. We show that the vertical lag of the rotation curve is necessary in the modeling, and estimate the lag amplitude in the galaxies. We find no correlation between the lag and the star formation rate in the galaxies. At the same time, we report a correlation between the lag and the galactic stellar mass, central stellar velocity dispersion, and axial ratio of the light distribution. These correlations suggest a possible higher ratio of infalling-to-local gas in early-type disk galaxies or a connection between lags and the possible presence of hot gaseous halos, which may be more prevalent in more massive galaxies. These results again demonstrate that observations of extraplanar gas can serve as a potential probe for accretion of gas.

The MASSIVE Survey. VI. The Spatial Distribution and Kinematics of Warm Ionized Gas in the Most Massive Local Early-type Galaxies

Abstract We present the first systematic investigation of the existence, spatial distribution, and kinematics of warm ionized gas as traced by the [O ii] 3727 Å emission line in 74 of the most massive galaxies in the local universe. All of our galaxies have deep integral-field spectroscopy from the volume- and magnitude-limited MASSIVE survey of early-type galaxies with stellar mass (M K &lt; −25.3 mag) and distance D &lt; 108 Mpc. Of the 74 galaxies in our sample, we detect warm ionized gas in 28, which yields a global detection fraction of 38 ± 6% down to a typical [O ii] equivalent width limit of 2 Å. MASSIVE fast rotators are more likely to have gas than MASSIVE slow rotators with detection fractions of 80 ± 10% and 28 ± 6%, respectively. The spatial extents span a wide range of radii (0.6–18.2 kpc; 0.1–4R e ), and the gas morphologies are diverse, with 17/28 ≈ 61 ± 9% being centrally concentrated, 8/28 ≈ 29 ± 9% exhibiting clear rotation out to several kiloparsecs, and 3/28 ≈ 11 ± 6% being extended but patchy. Three out of four fast rotators show kinematic alignment between the stars and gas, whereas the two slow rotators with robust kinematic measurements available exhibit kinematic misalignment. Our inferred warm ionized gas masses are roughly ∼105 M ⊙. The emission line ratios and radial equivalent width profiles are generally consistent with excitation of the gas by the old underlying stellar population. We explore different gas origin scenarios for MASSIVE galaxies and find that a variety of physical processes are likely at play, including internal gas recycling, cooling out of the hot gaseous halo, and gas acquired via mergers.

Forming disc galaxies in major mergers – III. The effect of angular momentum on the radial density profiles of disc galaxies

We study the effect of angular momentum on the surface density profiles of disc galaxies, using high resolution simulations of major mergers whose remnants have downbending radial density profiles (type II). As described in the previous papers of this series, in this scenario, most of the disc mass is acquired after the collision via accretion from a hot gaseous halo. We find that the inner and outer disc scalelengths, as well as the break radius, correlate with the total angular momentum of the initial merging system, and are larger for high angular momentum systems. We follow the angular momentum redistribution in our simulated galaxies, and find that, like the mass, the disc angular momentum is acquired via accretion, i.e. to the detriment of the gaseous halo. Furthermore, high angular momentum systems give more angular momentum to their discs, which affects directly their radial density profile. Adding simulations of isolated galaxies to our sample, we find that the correlations are valid also for disc galaxies evolved in isolation. We show that the outer part of the disc at the end of the simulation is populated mainly by inside-out stellar migration, and that in galaxies with higher angular momentum, stars travel radially further out. This, however, does not mean that outer disc stars (in type II discs) were mostly born in the inner disc. Indeed, generally the break radius increases over time, and not taking this into account leads to overestimating the number of stars born in the inner disc.

A HOT GASEOUS GALAXY HALO CANDIDATE WITH Mg X ABSORPTION

ABSTRACT The hot gas in galaxy halos may account for a significant fraction of missing baryons in galaxies, and some of these gases can be traced by high ionization absorption systems in QSO UV spectra. Using high S/N Hubble Space Telescope/Cosmic Origins Spectrograph spectra, we discovered a high ionization state system at z = 1.1912 in the sightline toward LBQS 1435-0134, and two-component absorption lines are matched for Mg x, Ne viii, Ne vi, O vi, Ne v, O v, Ne iv, O iv, N iv, O iii, and H i. Mg x, detected for the first time (5.8σ), is a particularly direct tracer of hot galactic halos, as its peak ion fraction occurs near 106.1 K, about the temperature of a virialized hot galaxy halo of mass . With Mg x and Ne viii, a photoionization model cannot reproduce the observed column densities with path lengths of galaxy halos. For collisional ionization models, one or two-temperature models do not produce acceptable fits, but a three-temperature model or a power-law model can produce the observed results. In the power-law model, with temperatures in the range of , the total hydrogen column density is and the positive power-law index indicates most of the mass is at the high temperature end. We suggest that this absorption system is a hot volume-filled galaxy halo rather than interaction layers between the hot halo and cool clouds. The temperature dependence of the column density is likely due to the local mixture of multiple phase gases.

THE CIRCUM-GALACTIC MEDIUM OF MASSIVE SPIRALS. I. AN OVERVIEW AND A CASE STUDY OF NGC 5908

The Circum-Galactic Medium of MASsive Spirals (CGM-MASS) is a project studying the overall content, physical and chemical properties, and spatial distributions of the multi-phase circum-galactic medium (CGM) around a small sample of the most massive isolated spiral galaxies in the local Universe. We introduce the sample and present a detailed case study of the XMM-Newton observation of the hot gas halo of NGC5908. After data calibration, point source removal, and background analysis, we find that the diffuse soft X-ray emission of NGC5908 is significantly more extended than the stellar light in the vertical direction. The 0.5-1.25keV radial intensity profile tracing hot gas emission can be detected above the background out to about $2^\prime$, or $30\rm~kpc$ from the nucleus. The unresolved soft X-ray emission can be characterized with a $\beta$-model with a slope of $\beta\approx0.68$. The unresolved 0.5-2keV luminosity within $1^\prime$ is $6.8\times10^{39}\rm~ergs~s^{-1}$, but young stellar sources may contribute significantly to it. Assuming a metallicity of 0.2solar, an upper limit (without subtracting the very uncertain young stellar contribution) to the mass of hot gas within this radius is $2.3\times10^9\rm~M_\odot$. The cooling radius is $r_{\rm cool}\approx25\rm~kpc$ or $\approx0.06r_{\rm 200}$, within which the hot gas could cool radiatively in less than 10Gyr, and the cooling of hot gas could significantly contribute in replenishing the gas consumed in star formation. The hot gas accounts for $\approx1.9\%$ of the baryon detected within the cooling radius. By comparing NGC5908 to other galaxies, we find that its X-ray luminosity per stellar mass is consistent with lower-mass non-starburst field spiral galaxies. However, a large scatter in hot gas soft X-ray emissivity is indicated for spiral galaxies with $M_*\gtrsim2\times10^{11}\rm~M_\odot$.

THE INTERACTION OF THE FERMI BUBBLES WITH THE MILKY WAY’S HOT GAS HALO

ABSTRACT The Fermi bubbles are two lobes filled with non-thermal particles that emit gamma rays, extend vertically from the Galactic center, and formed from either nuclear star formation or accretion activity on Sgr A*. Simulations predict a range of shock strengths as the bubbles expand into the surrounding hot gas halo ( K), but with significant uncertainties in the energetics, age, and thermal gas structure. The bubbles should contain thermal gas with temperatures between 106 and 108 K, with potential X-ray signatures. In this work, we constrain the bubbles’ thermal gas structure by modeling O vii and O viii emission line strengths from archival XMM-Newton and Suzaku data. Our emission model includes a hot thermal volume-filled bubble component cospatial with the gamma-ray region, and a shell of compressed material. We find that a bubble/shell model with cm−3 and with log(T) ≈ 6.60–6.70 is consistent with the observed line intensities. In the framework of a continuous Galactic outflow, we infer a bubble expansion rate, age, and energy injection rate of km s−1, Myr, and erg s−1. These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation.

Simulations of ram-pressure stripping in galaxy-cluster interactions

Observationally, the quenching of star-forming galaxies appears to depend both on their mass and environment. The exact cause of the environmental dependence is still poorly understood, yet semi-analytic models (SAMs) of galaxy formation need to parameterise it to reproduce observations of galaxy properties. In this work, we use hydrodynamical simulations to investigate the quenching of disk galaxies through ram-pressure stripping (RPS) as they fall into galaxy clusters with the goal of characterising the importance of this effect for the reddening of disk galaxies. Our set-up employs a live model of a galaxy cluster that interacts with infalling disk galaxies on different orbits. We use the moving-mesh code AREPO, augmented with a special refinement strategy to yield high resolution around the galaxy on its way through the cluster in a computationally efficient way. Our direct simulations differ substantially from stripping models employed in current SAMs, which in most cases overpredict the mass loss from RPS. Furthermore, after pericentre passage, as soon as ram pressure becomes weaker, gas that remains bound to the galaxy is redistributed to the outer parts, an effect that is not captured in simplified treatements of RPS. Star formation in our model galaxies is quenched mainly because the hot gas halo is stripped, depriving the galaxy of its gas supply. The cold gas disk is only stripped completely in extreme cases, leading to full quenching and significant reddening on timescale of ~200 Myr. On the other hand, galaxies experiencing only mild ram pressure actually show an enhanced star formation rate that is consistent with observations and are quenched on timescales > 1 Gyr. Stripped gas in the wake is mixed efficiently with intracluster gas already a few tens of kpc behind the disk, and this gas is free of residual star formation.

HIDDEN ACTIVE GALACTIC NUCLEI IN EARLY-TYPE GALAXIES

ABSTRACT We present a stacking analysis of the complete sample of early-type galaxies (ETGs) in the Chandra COSMOS (C-COSMOS) survey, to explore the nature of the X-ray luminosity in the redshift and stellar luminosity ranges and . Using established scaling relations, we subtract the contribution of X-ray binary populations to estimate the combined emission of hot ISM and active galactic nuclei (AGNs). To discriminate between the relative importance of these two components, we (1) compare our results with the relation observed in the local universe for hot gaseous halos emission in ETGs, and (2) evaluate the spectral signature of each stacked bin. We find two regimes where the non-stellar X-ray emission is hard, consistent with AGN emission. First, there is evidence of hard, absorbed X-ray emission in stacked bins including relatively high z (∼1.2) ETGs with average high X-ray luminosity ( ). These luminosities are consistent with the presence of highly absorbed “hidden” AGNs in these ETGs, which are not visible in their optical–IR spectra and spectral energy distributions. Second, confirming the early indication from our C-COSMOS study of X-ray detected ETGs, we find significantly enhanced X-ray luminosity in lower stellar mass ETGs ( ), relative to the local relation. The stacked spectra of these ETGs also suggest X-ray emission harder than expected from gaseous hot halos. This emission is consistent with inefficient accretion onto .

THE ROTATION OF THE HOT GAS AROUND THE MILKY WAY

ABSTRACT The hot gaseous halos of galaxies likely contain a large amount of mass and are an integral part of galaxy formation and evolution. The Milky Way has a 2 &times; 10 6 -->?> K halo that is detected in emission and by absorption in the O vii resonance line against bright background active galactic nuclei (AGNs), and for which the best current model is an extended spherical distribution. Using XMM-Newton Reflection Grating Spectrometer data, we measure the Doppler shifts of the O vii absorption-line centroids toward an ensemble of AGNs. These Doppler shifts constrain the dynamics of the hot halo, ruling out a stationary halo at about 3 &sgr; -->?> and a co-rotating halo at 2 &sgr; -->?> , and leading to a best-fit rotational velocity of v &phi; = 183 &PlusMinus; 41 -->?> km s−1 for an extended halo model. These results suggest that the hot gas rotates and that it contains an amount of angular momentum comparable to that in the stellar disk. We examined the possibility of a model with a kinematically distinct disk and spherical halo. To be consistent with the emission-line X-ray data, the disk must contribute less than 10% of the column density, implying that the Doppler shifts probe motion in the extended hot halo.