Photoionized Gas in Dark-Matter Minihalos in the Galactic Halo and Local Group

Aims. We aim to investigate the ionisation conditions and distances of Galactic high-velocity clouds (HVCs) in the Galactic halo and beyond in the direction of the Local Group (LG) barycentre and anti-barycentre, by studying spectral data of 29 extragalactic background sources obtained with Cosmic Origins Spectropgraph (COS) installed on the Hubble Space Telescope (HST). Methods. We model column-densities of low, intermediate, and high ions, such as Si II, C II, Si III, Si IV...

Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Local Group model and would have HI volume densities of order 0.0003/cm^3. Clouds with these volume densities and the observed neutral hydrogen column densities will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. Here we examine the implications of this process for models of CHVCs. We have modeled the ionization structure of spherical clouds (with and without ...

We analyse the distribution and origin of OVI in the Circumgalactic Medium (CGM) of dark-matter haloes of $\sim 10^{12}$ M$_\odot$ at $z\sim1$ in the VELA cosmological zoom-in simulations. We find that the OVI in the inflowing cold streams is primarily photoionized, while in the bulk volume it is primarily collisionally ionized. The photoionized component dominates the observed column density at large impact parameters ($\gtrsim 0.3 R_{\rm vir}$), while the collisionally ioni...

Observations reveal massive amounts of OVI around star-forming $L_*$ galaxies, with covering fractions of near unity extending to the host halo's virial radius. This OVI absorption is typically kinematically centered upon photoionized gas, with line widths that are suprathermal and kinematically offset from the galaxy. We discuss various scenarios and whether they could result in the observed phenomenology (cooling gas flows, boundary layers, shocks, virialized gas, photoioni...

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 dif...

We consider the physical conditions and origin of the z = 0.0777 absorption system observed in C III, C II, Si III, C IV, O VI, and H I absorption along the line of sight towards the quasar PHL 1811. We analysed the HST/STIS and FUSE spectra of this quasar and compared the results to Cloudy photoionization and collisional ionization models in order to derive densities, temperatures, and metallicities of the absorbing gas. The absorption can be explained by two C III clouds, o...

We report on the detection of a population of weak metal-line absorbers in the halo or nearby intergalactic environment of the Milky Way. Using high-resolution ultraviolet absorption-line spectra of bright QSOs obtained with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we have observed unsaturated, narrow absorption in OI and SiII together with mildly saturated CII absorption at high radial velocities (|v_LSR|=100-320 km/s). The measured OI column de...

A consensus is developing on the nature of various populations of absorbers at different redshift regimes and in particular on their relationships to galaxies at those epochs. As one example we discuss the population of z < 1 MgII absorbers. Kinematic models are presented that show this population to be consistent with some combination of halo and disk kinematics and not with a pure disk or halo model. In contrast, the 2 < z < 3 CIV absorbers are likely to represent a mix of ...

We address the spatial scale, ionization structure, mass and metal content of gas at the Milky Way disk-halo interface detected as absorption in the foreground of seven closely-spaced, high-latitude halo blue horizontal branch stars (BHBs) with heights z = 3 - 14 kpc. We detect transitions that trace multiple ionization states (e.g. CaII, FeII, SiIV, CIV) with column densities that remain constant with height from the disk, indicating that the gas most likely lies within z < ...

We present detailed photoionization models of well aligned optically thin C III absorption components at $2.1 < z < 3.4$. Using our models we estimate density ($n_{\rm \tiny H}$), metallicity ($[C/H]$), total hydrogen column density and line-of-sight thickness ($L$) in each C III components. We estimate the systematic errors in these quantities contributed by the allowed range of the quasar spectral index used in the ultraviolet background radiation calculations. Our inferred...