We construct an analytic phenomenological model for extended warm/hot gaseous coronae of L * galaxies. We consider UV OVI COS-Halos absorption line data in combination with Milky Way X-ray OVII and OVIII absorption and emission. We fit these data with a single model representing the COS-Halos galaxies and a Galactic corona. Our model is multi-phased, with hot and warm gas components, each with a (turbulent) lognormal distribution of temperatures and densities. The hot gas, traced by the X-ray absorption and emission, is in hydrostatic equilibrium in a Milky Way gravitational potential. The median temperature of the hot gas is 1.5 × 10 6 K and the mean hydrogen density is ∼ 5 × 10 −5 cm −3 . The warm component as traced by the OVI, is gas that has cooled out of the high density tail of the hot component. The total warm/hot gas mass is high and is 1.2 × 10 11 M . The gas metallicity we require to reproduce the oxygen ion column densities is 0.5 solar. The warm OVI component has a short cooling time (∼ 2 × 10 8 years), as hinted by observations. The hot component, however, is ∼ 80% of the total gas mass and is relatively long-lived, with t cool ∼ 7 × 10 9 years. Our model supports suggestions that hot galactic coronae can contain significant amounts of gas. These reservoirs may enable galaxies to continue forming stars steadily for long periods of time and account for "missing baryons" in galaxies in the local universe.

We construct a new analytic phenomenological model for the extended circumgalactic material (CGM) of L * galaxies. Our model reproduces the OVII/OVIII absorption observations of the Milky Way (MW) and the OVI measurements reported by the COS-Halos and eCGM surveys. The warm/hot gas is in hydrostatic equilibrium in a MW gravitational potential, and we adopt a barotropic equation of state, resulting in a temperature variation as a function of radius. A pressure component with an adiabatic index of γ = 4/3 is included to approximate the effects of a magnetic field and cosmic rays. We introduce a metallicity gradient motivated by the enrichment of the inner CGM by the Galaxy. We then present our fiducial model for the corona, tuned to reproduce the observed OVI-OVIII column densities, and with a total mass of M CGM ≈ 5.5 × 10 10 M ⊙ inside r CGM ≈ 280 kpc. The gas densities in the CGM are low (n H = 10 −5 − 3 × 10 −4 cm −3 ) and its collisional ionization state is modified by the metagalactic radiation field (MGRF). We show that for OVI-bearing warm/hot gas with typical observed column densities N OVI ∼ 3 × 10 14 cm −2 at large ( 100 kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, t cool /t dyn , has a model-independent upper limit of ∼ 5. In our model, t cool /t dyn at large radii is ∼ 2 − 3. We present predictions for a wide range of future observations of the warm/hot CGM, from UV/X-ray absorption and emission spectroscopy, to dispersion measure (DM) and Sunyaev-Zeldovich CMB measurements.

We present dark matter minihalo models for the Ultra-Compact, High-Velocity H i Clouds (UCHVCs) recently discovered in the 21 cm ALFALFA survey. We assume gravitational confinement of 10 4 K H i gas by flat-cored darkmatter subhalos within the Local Group. We show that for flat cores, typical (median) tidally stripped cosmological subhalos at redshift z = 0 have dark-matter masses of ∼10 7 M within the central 300 pc (independent of total halo mass), consistent with the "Strigari mass scale" observed in low-luminosity dwarf galaxies. Flat-cored subhalos also resolve the mass discrepancy between simulated and observed satellites around the Milky Way. For the UCHVCs, we calculate the photoionization-limited hydrostatic gas profiles for any distance-dependent total observed H i mass and predict the associated (projected) H i half-mass radii, assuming the clouds are embedded in distant (d 300 kpc) and unstripped subhalos. For a typical UCHVC (0.9 Jy km s −1 ), we predict physical H i half-mass radii of 0.18 to 0.35 kpc (or angular sizes of 0. 6 to 2. 1) for distances ranging from 300 kpc to 2 Mpc. As a consistency check, we model the gas-rich dwarf galaxy Leo T, for which there is a well-resolved H i column density profile and a known distance (420 kpc). For Leo T, we find that a subhalo with M 300 = 8 (±0.2) × 10 6 M best fits the observed H i profile. We derive an upper limit of P HIM 150 cm −3 K for the pressure of any enveloping hot intergalactic medium gas at the distance of Leo T. Our analysis suggests that some of the UCHVCs may in fact constitute a population of 21 cm-selected but optically faint dwarf galaxies in the Local Group.

We present neutral hydrogen (H i) imaging observations with the Westerbork Synthesis Radio Telescope of AGC198606, an H i cloud discovered in the ALFALFA 21cm survey. This object is of particular note as it is located 16 km s −1 and 1.• 2 from the gas-bearing ultrafaint dwarf galaxy Leo T while having a similar H i linewidth and approximately twice the flux density. The H i imaging observations reveal a smooth, undisturbed H i morphology with a full extent of 23 × 16 at the 5 × 10 18 atoms cm −2 level. The velocity field of AGC198606 shows ordered motion with a gradient of ∼25 km s −1 across ∼20 . The global velocity dispersion is 9.3 km s −1 with no evidence for a narrow spectral component. No optical counterpart to AGC198606 is detected. The distance to AGC198606 is unknown, and we consider several different scenarios: physical association with Leo T, a minihalo at a distance of ∼150 kpc based on the models of Faerman et al. (2013, ApJ, 777, 119), and a cloud in the Galactic halo. At a distance of 420 kpc, AGC198606 would have an H i mass of 6.2 × 10 5 M , an H i radius of 1.4 kpc, and a dynamical mass within the H i extent of 1.5 × 10 8 M .

We study the components of cool and warm/hot gas in the circumgalactic medium (CGM) of simulated galaxies and address the relative production of OVI by photoionization versus collisional ionization, as a function of halo mass, redshift, and distance from the galaxy halo center. This is done utilizing two different suites of zoom-in hydro-cosmological simulations, VELA (6 halos; z > 1) and NIHAO (18 halos; to z = 0), which provide a broad theoretical basis because they use different codes and physical recipes for star formation and feedback. In all halos studied in this work, we find that collisional ionization by thermal electrons dominates at high redshift, while photoionization of cool or warm gas by the metagalactic radiation takes over near z ∼ 2. In halos of ∼ 10 12 M and above, collisions become important again at z < 0.5, while photoionization remains significant down to z = 0 for less massive halos. In halos with M v > 3 × 10 11 M , at z ∼ 0 most of the photoionized OVI is in a warm, not cool, gas phase (T 3 × 10 5 K). We also find that collisions are dominant in the central regions of halos, while photoionization is more significant at the outskirts, around R v , even in massive halos. This too may be explained by the presence of warm gas or, in lower mass halos, by cool gas inflows.

We combine the Santa Cruz semianalytic model (SAM) for galaxy formation and evolution with the circumgalactic medium (CGM) model presented in Faerman et al. to explore the CGM properties of L * galaxies. We use the SAM to generate a sample of galaxies with halo masses similar to the Milky Way (MW) halo, M vir ≈ 1012 M ⊙, and find that the CGM mass and mean metallicity in the sample are correlated. We use the CGM masses and metallicities of the SAM galaxies as inputs for the FSM20 model and vary the amount of nonthermal support. The density profiles in our models can be approximated by power-law functions with slopes in the range of 0.75 < a n < 1.25, with higher nonthermal pressure resulting in flatter distributions. We explore how the gas pressure, dispersion measure, O VI–O VIII column densities, and cooling rates behave with the gas distribution and total mass. We show that for CGM masses below ∼3 × 1010 M ⊙ photoionization has a significant effect on the column densities of O VI and O VIII. The combination of different MW CGM observations favors models with similar fractions in thermal pressure, magnetic fields/cosmic rays, and turbulent support and with M CGM ∼ (3–10) × 1010 M ⊙. The MW O VI column requires t cool/t dyn ∼ 4, independent of the gas distribution. The AGN jet-driven heating rates in the SAM are enough to offset the CGM cooling, although exact balance is not required in star-forming galaxies. We provide predictions for the column densities of additional metal ions—N V, Ne VIII, and Mg X.

Motivated by integral field units (IFUs) on large ground telescopes and proposals for ultraviolet-sensitive space telescopes to probe circumgalactic medium (CGM) emission, we survey the most promising emission lines and how such observations can inform our understanding of the CGM and its relation to galaxy formation. We tie our emission estimates to both HST/COS absorption measurements of ions around z ≈ 0.2 Milky Way mass halos and models for the density and temperature of gas. We also provide formulas that simplify extending our estimates to other samples and physical scenarios. We find that O iii 5007 Å and N ii 6583 Å, which at fixed ionic column density are primarily sensitive to the thermal pressure of the gas they inhabit, may be detectable with KCWI and especially IFUs on 30 m telescopes out to half a virial radius. O v 630 Å and O vi 1032,1038 Å are perhaps the most promising ultraviolet lines, with models predicting intensities &gt;100 γ cm−2 s−1 sr−1 in the inner 100 kpc of Milky Way-like systems. A detection of O vi would confirm the collisionally ionized picture and constrain the density profile of the CGM. Other ultraviolet metal lines constrain the amount of gas that is actively cooling and mixing. We find that C iii 978 Å and C iv 1548 Å may be detectable if an appreciable fraction of the observed O vi column is associated with mixing or cooling gas. Hα emission within 100 kpc of Milky Way-like galaxies is within reach of current IFUs even for the minimum signal from ionizing background fluorescence, while Hydrogen n &gt; 2 Lyman-series lines are too weak to be detectable.

We present dark-matter minihalo models for the Ultra-Compact High Velocity HI Clouds (UCHVCs) recently discovered in the 21 cm ALFALFA survey. We assume gravitational confinement of ~104 K HI gas by flat-cored dark-matter subhalos within the Local Group. For the UCHVCs we calculate the photoionization-limited hydrostatic gas profiles for any distance-dependent total observed HI mass and predict the associated (projected) HI half-mass radii. The observed 21 cm fluxes and half-mass angular radii then constrain the source distances or DM halo parameters. As a consistency check we model the gas-rich dwarf galaxy Leo T, for which the distance is known (420 kpc) and there is a well-resolved HI column density profile. We derive an upper limit for the pressure of any enveloping hot IGM gas at the distance of Leo T. Our analysis supports the scenario that some of the UCHVCs may constitute a population of 21-cm-selected but optically-faint dwarf galaxies in the Local Volume.