We summarize and reanalyze observations bearing upon missing galactic baryons, where we propose a consistent picture for halo gas in L L* galaxies. The hot X-ray emitting halos are detected to 50-70 kpc, where typically, M hot (< 50 kpc) ∼ 5 × 10 9 M , and with density n ∝ r −3/2 . When extrapolated to R 200 , the gas mass is comparable to the stellar mass, but about half of the baryons are still missing from the hot phase. If extrapolated to 1.9-3R 200 , the baryon to dark matter ratio approaches the cosmic value. Significantly flatter density profiles are unlikely for R < 50 kpc and they are disfavored but not ruled out for R > 50 kpc. For the Milky Way, the hot halo metallicity lies in the range 0.3-1 solar for R < 50 kpc. Planck measurements of the thermal Sunyaev-Zeldovich effect toward stacked luminous galaxies (primarily early-type) indicate that most of their baryons are hot, near the virial temperature, and extend beyond R 200 . This stacked SZ signal is nearly an order of magnitude larger than that inferred from the X-ray observations of individual (mostly spiral) galaxies with M * > 10 11.3 M . This difference suggests that the hot halo properties are distinct for early and late type galaxies, possibly due to different evolutionary histories. For the cooler gas detected in UV absorption line studies, we argue that there are two absorption populations: extended halos; and disks extending to ∼ 50 kpc, containing most of this gas, and with masses a few times lower than the stellar masses. Such extended disks are also seen in 21 cm HI observations and in simulations.

We present simultaneous X-ray and radio observations of the black hole X-ray binary V404 Cygni at the end of its 2015 outburst. From 2015 July 11-August 5, we monitored V404 Cygni with Chandra, Swift, and NuSTAR in the X-ray, and with the Karl G. Jansky Very Large Array and the Very Long Baseline Array in the radio, spanning a range of luminosities that were poorly covered during its previous outburst in 1989 (our 2015 campaign covers10 erg s 33 X 34 1 ). During our 2015 campaign, the X-ray spectrum evolved rapidly from a hard photon index of). We argue that V404 Cygni reaching G » 2 marks the beginning of the quiescent spectral state, which occurs at a factor of ≈3-4 higher X-ray luminosity than the average pre-outburst luminosity of »´-8 10 erg s 32 1. V404 Cygni falls along the same radio/X-ray luminosity correlation that it followed during its previous outburst in 1989, implying a robust disk-jet coupling. We exclude the possibility that a synchrotron-cooled jet dominates the X-ray emission in quiescence, leaving synchrotron self-Compton from either a hot accretion flow or from a radiatively cooled jet as the most likely sources of X-ray radiation, and/or particle acceleration along the jet becoming less efficient in quiescence. Finally, we present the first indications of correlated radio and X-ray variability on minute timescales in quiescence, tentatively measuring the radio emission to lag the X-ray by  15 4 minute, suggestive of X-ray variations propagating down a jet with alength of<3.0 au.

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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 a2 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 s 3 and a co-rotating halo at s 2 , and leading to a best-fit rotational velocity of =  f v 183 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.

We report the discovery of diffuse ultraviolet light around late-type galaxies out to 5-20 kpc from the midplane using Swift and GALEX images. The emission is consistent with the stellar outskirts in the early-type galaxies but not in the late-type galaxies, where the emission is quite blue and consistent with a reflection nebula powered by light escaping from the galaxy and scattering off dust in the halo. Our results agree with expectations from halo dust discovered in extinction by Menard et al. (2010) to within a few kpc of the disk and imply a comparable amount of hot and cold gas in galaxy halos (a few×10 8 M ⊙ within 20 kpc) if the dust resides primarily in Mg II absorbers. The results also highlight the potential of UV photometry to study individual galaxy halos.