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The Milky Way's million degree gaseous halo contains a considerable amount of mass that, depending on its structural properties, can be a significant mass component. In order to analyze the structure of the Galactic halo, we use XMM-Newton Reflection Grating Spectrometer archival data and measure O VII Kα absorption-line strengths toward 26 active galactic nuclei, LMC X-3, and two Galactic sources (4U 1820-30 and X1735-444). We assume a β-model as the underlying gas density profile and find best-fit parameters of n • = 0.46 +0.74 −0.35 cm −3 , r c = 0.35 +0.29 −0.27 kpc, and β = 0.71 +0.13 −0.14 . These parameters result in halo masses ranging between M (18 kpc) = 7.5 +22.0 −4.6 × 10 8 M ⊙ and M (200 kpc) = 3.8 +6.0 −0.5 × 10 10 M ⊙ assuming a gas metallicity of Z = 0.3 Z ⊙ , which are consistent with current theoretical and observational work. The maximum baryon fraction from our halo model of f b = 0.07 +0.03−0.01 is significantly smaller than the universal value of f b = 0.171, implying the mass contained in the Galactic halo accounts for 10% -50% of the missing baryons in the Milky Way. We also discuss our model in the context of several Milky Way observables, including ram pressure stripping in dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5 -2 keV band, the Milky Way's star formation rate, spatial and thermal properties of cooler gas (∼10 5 K) and the observed Fermi bubbles toward the Galactic center. Although the metallicity of the halo gas is a large uncertainty in our analysis, we place a lower limit on the halo gas between the Sun and the Large Magellanic Cloud (LMC). We find that Z 0.2 Z ⊙ based on the pulsar dispersion measure toward the LMC.

We present a detailed strong‐lensing, weak‐lensing and X‐ray analysis of Abell 2744 (z= 0.308), one of the most actively merging galaxy clusters known. It appears to have unleashed ‘dark’, ‘ghost’, ‘bullet’ and ‘stripped’ substructures, each ∼1014 M⊙. The phenomenology is complex and will present a challenge for numerical simulations to reproduce. With new, multiband Hubble Space Telescope (HST) imaging, we identify 34 strongly lensed images of 11 galaxies around the massive Southern ‘core’. Combining this with weak‐lensing data from HST, VLT and Subaru, we produce the most detailed mass map of this cluster to date. We also perform an independent analysis of archival Chandra X‐ray imaging. Our analyses support a recent claim that the Southern core and Northwestern substructure are post‐merger and exhibit morphology similar to the Bullet Cluster viewed from an angle. From the separation between X‐ray emitting gas and lensing mass in the Southern core, we derive a new and independent constraint on the self‐interaction cross‐section of dark matter particles σ/m < 3 ± 1 cm2 g−1. In the Northwestern substructure, the gas, dark matter and galaxy components have become separated by much larger distances. Most curiously, the ‘ghost’ clump (primarily gas) leads the ‘dark’ clump (primarily dark matter) by more than 150 kpc. We propose an enhanced ‘ram‐pressure slingshot’ scenario which may have yielded this reversal of components with such a large separation, but needs further confirmation by follow‐up observations and numerical simulations. A secondary merger involves a second ‘bullet’ clump in the North and an extremely ‘stripped’ clump to the West. The latter appears to exhibit the largest separation between dark matter and X‐ray emitting baryons detected to date in our sky.

The Milky Way hosts a hot (≈ 2 × 10 6 K), diffuse, gaseous halo based on detections of z = 0 O VII and O VIII absorption lines in quasar spectra and emission lines in blank-sky spectra. Here we improve constraints on the structure of the hot gas halo by fitting a radial model to a much larger sample of O VII and O VIII emission line measurements from XMM-Newton/EPIC-MOS spectra compared to previous studies (≈650 sightlines). We assume a modified β-model for the halo density distribution and a constant-density Local Bubble from which we calculate emission to compare with the observations. We find an acceptable fit to the O VIII emission line observations with χ 2 red (dof) = 1.08 (644) for best-fit parameters of n o r 3β c = 1.35 ± 0.24 cm −3 kpc 3β and β = 0.50 ± 0.03 for the hot gas halo and negligible Local Bubble contribution. The O VII observations yield an unacceptable χ 2 red (dof) = 4.69 (645) for similar best-fit parameters, which is likely due to temperature or density variations in the Local Bubble. The O VIII fitting results imply hot gas masses of M (<50 kpc) = 3.8 +0.3 −0.3 × 10 9 M ⊙ and M (<250 kpc) = 4.3 +0.9 −0.8 × 10 10 M ⊙ , accounting for 50% of the Milky Way's missing baryons. We also explore our results in the context of optical depth effects in the halo gas, the halo gas cooling properties, temperature and entropy gradients in the halo gas, and the gas metallicity distribution. The combination of absorption and emission line analyses implies a sub-solar gas metallicity that decreases with radius, but that also must be ≥ 0.3Z ⊙ to be consistent with the pulsar dispersion measure toward the Large Magellanic Cloud.

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A Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which ∼80 are low mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. Of the unresolved emission, it is likely that about half is from fainter unresolved LMXBs, while the other half (∼23% of the total count rate) is from interstellar gas. The X-ray emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. The physical state of the hot gas is uncertain; the X-ray luminosity and extended surface brightness are inconsistent with a global supersonic wind, a partial wind, or a global cooling inflow. The gas may be undergoing subsonic inflation, rotationally induced outflow, or ram pressure stripping. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with ROSAT, is due to interstellar gas. The cumulative LMXB spectrum is well-fit by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component.NGC 4697 has a central X-ray source with a luminosity of L X = 8 × 10 38 ergs s −1 , which may be due to an AGN and/or one or more LMXBs. At most, the massive black hole at the center of this galaxy is radiating at a very small fraction (≤ 4 × 10 −8 ) of its Eddington luminosity.Three of the resolved sources in NGC 4697 are supersoft sources. In the outer regions of NGC 4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. We suggest that all of the LMXBs may have been formed in globulars. The X-ray-to-optical luminosity ratio for the LMXBs in NGC 4697 is L X (LMXB, 0.3-10 keV)/L B = 8.1 × 10 29 ergs s −1 L −1 B⊙ , which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of 2) in the LMXB X-ray-to-optical ratios of early-type galaxies and spiral bulges. The X-ray luminosity function of NGC 4697 is also flatter than that found for the bulge of M31. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from ∼5×10 37 to ∼2.5×10 39 ergs s −1 . The luminosity function of the LMXBs has a "knee" at 3.2 × 10 38 ergs s −1 , which is approximately the Eddington luminosity of a 1.4 M ⊙ neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates black hole and NS binaries. This characteristic luminosity could be used as a distance estimator....

Galaxies are missing most of their baryons, and many models predict these baryons lie in a hot halo around galaxies. We establish observationally motivated constraints on the mass and radii of these haloes using a variety of independent arguments. First, the observed dispersion measure of pulsars in the Large Magellanic Cloud allows us to constrain the hot halo around the Milky Way: if it obeys the standard NFW profile, it must contain less than 4-5% of the missing baryons from the Galaxy. This is similar to other upper limits on the Galactic hot halo, such as the soft X-ray background and the pressure around high velocity clouds. Second, we note that the X-ray surface brightness of hot haloes with NFW profiles around large isolated galaxies is high enough that such emission should be observed, unless their haloes contain less than 10-25% of their missing baryons. Third, we place constraints on the column density of hot haloes using nondetections of OVII absorption along AGN sightlines: in general they must contain less than 70% of the missing baryons or extend to no more than 40 kpc. Flattening the density profile of galactic hot haloes weakens the surface brightness constraint so that a typical L * galaxy may hold half its missing baryons in its halo, but the OVII constraint remains unchanged, and around the Milky Way a flattened profile may only hold 6 − 13% of the missing baryons from the Galaxy (2 − 4 × 10 10 M ⊙ ). We also show that AGN and supernovae at low to moderate redshift -the theoretical sources of winds responsible for driving out the missing baryons -do not produce the expected correlations with the baryonic Tully-Fisher relationship and so are insufficient to explain the missing baryons from galaxies. We conclude that most of missing baryons from galaxies do not lie in hot haloes around the galaxies, and that the missing baryons never fell into the potential wells of protogalaxies in the first place. They may have been expelled from the galaxies as part of the process of galaxy formation.

We have investigated the X-ray spectral properties of a collection of low-mass X-ray binaries (LMXBs) within a sample of 15 nearby early-type galaxies using proprietary and archival data from the Chandra X-ray Observatory. We find that the spectrum of the sum of the sources in a given galaxy is remarkably similar from galaxy to galaxy when only sources with X-ray luminosities less than 10 39 ergs s −1 (0.3-10 keV) are considered. Fitting these lower luminosity sources in all galaxies simultaneously with a power law model led to a best-fit power law exponent of Γ = 1.56 ± 0.02 (90% confidence), and using a thermal bremsstrahlung model yielded kT brem = 7.3 ± 0.3 keV. This is the tightest constraint to date on the spectral properties of LMXBs in external galaxies. The spectral properties of the LMXBs do not vary with galactic radius out to three effective radii. There is also no apparent difference in the spectral properties of LMXBs that reside within globular clusters and those that do not. We demonstrate how the uniformity of the spectral properties of LMXBs can lead to more accurate determinations of the temperature and metallicity of the hot gas in galaxies that have comparable amounts of X-ray emission from hot gas and LMXBs.Although few in number in any given galaxy, sources with luminosities of 1 − 2 × 10 39 ergs s −1 are present in 10 of the galaxies. The spectra of these luminous sources are softer than the spectra of the rest of the sources, and are consistent with the spectra of Galactic black hole X-ray binary candidates when they are in their very high state. The spatial distribution of these sources is much flatter than the optical light distribution, suggesting that a significant portion of them must reside within globular clusters. The simplest explanation of these sources is that they are ∼ 10 − 15 M ⊙ black holes accreting near their Eddington limit. The spectra of these sources are very different than those of ultraluminous X-ray sources (ULXs) that have been found within spiral galaxies, suggesting that the two populations of X-ray luminous objects have different formation mechanisms. The number of sources with apparent luminosities above 2 × 10 39 ergs s −1 when determined using the distance of the galaxy is equal to the number of expected background AGN and thus appear to not be associated with the galaxy, indicating that very luminous sources are absent or very rare in early-type galaxies. The lack of ULXs within elliptical galaxies strengthens the argument that ULXs are associated with recent star formation.