We present new constraints on lagging atomic halos in the edge-ons NGC 4244 and NGC 4565 from recent WSRT data. In NGC 4244, any possible lag is limited to <10km s−1kpc−1 up to z = 2kpc, while NGC 4565 may have a lag of ∼10km s−1kpc−1, up to z = 2kpc, but only at about R ≤ 3′. These small lags in galaxies with little star formation may disagree with recently reported trends for extraplanar ionized gas in three edge-ons.

Extra-planar gas in spiral galaxies can originate from disk-halo cycling driven by star formation in the disk or from infalling gas clouds. Studying the kinematics of halo gas can provide evidence for which scenario is more plausible. Edge-on galaxies provide a direct view of the extra-planar gas. We measure gas velocities from Hα emission as a function of height above the galactic disk for NGC 4244, which has a low star formation rate, and NGC 891, which has a high star formation rate. Using a multi-slit setup on the ARC 3.5 m telescope, we are able to measure velocities along 16 slits simultaneously. We compare our preliminary results to studies of HI velocities in these galaxies.

This contribution summarizes some recent results concerning gas in Galaxy halos. In the past few years, distances have been determined for six high-velocity clouds (HVCs), including the second largest, complex C. The derived distances range from about 2 to 10 kpc, placing these HVCs in the near Galactic halo, but far above the Galactic plane. Distances of ∼10 kpc were determined to three different concentrations in Complex C, implying a mass of ∼5 × 107M⊙; this cloud is known to have a metallicity of ∼0.15 solar, and represents an inflow of 0.15–0.25 M⊙ yr−1 of low-metallicity material. A statistical analysis of the sky and velocity distribution of the ∼200 small HVCs shows that they probably form a population of clouds orbiting the Milky Way, with distances up to 80 kpc, and a radial inflow component of 50 km s−1. A study of 115 Lyα and 14 O VI absorption lines at redshifts z < 0.017 (v < 5000 km s−1) in the spectra of 76 AGNs shows that half of the intergalactic Lyα absorption lines originate within 450 kpc of galaxies. The amount of gas inside this distance is a factor 2–4 more than the total amount of mass inside the galaxies.

We discuss the support for the cooling of gas directly at the disk-halo interface as a mechanism to continually fuel a galaxy. This may be an important reservoir as there is not enough cold gas observed in galaxy halos and simulations indicate the existing cold clouds will be rapidly destroyed as they move through the surrounding halo medium. We show possible evidence for a net infall of the WIM layer in the Milky Way, simulation results showing the recooling of warm clouds at the disk-halo interface, and GALFA HI data of small, cold HI clouds that could represent this recooling.

We present GBT neutral hydrogen observations of two nearby galaxy groups. The M 81/M 82 and NGC 2403 groups represent opposite ends of the range in intensity of galaxy interactions, and have remarkably different properties in Hi. We have discovered 5 new Hi clouds in the highly interacting M 81/M 82 group of galaxies. Based on the velocity and angular distributions of these clouds, we conclude that they are relics of ongoing interactions between galaxies in the group. In contrast, the non-interacting NGC 2403 galaxy group shows no signs of Hi clouds or faint, extended Hi. We also detect a compact Hi cloud at v = −205 km s−1, which we believe to be part of HVC Complex A. Our observations suggest that the number density of Hi clouds in galaxy groups is strongly correlated with the level and age of ongoing galaxy interactions in the group.

We report on new optical emission line observations toward the northern part of the Magellanic Stream. Using the Wisconsin H-Alpha Mapper (WHAM), we have detected Hα emission toward 17 directions spread across 50∘ of the Stream. We find strong variation in the Hα surface brightness with no significant correlation with Magellanic longitude. We compare our data with HI observations and briefly discuss the implications for the physical conditions of the Stream.

We have detected hundreds of HI clouds in the lower Galactic halo within the Galactic All-Sky Survey pilot region, a region spanning 325∘≤ l ≤ 343∘ and |b|≤ 20∘. The motions of these clouds are governed by Galactic rotation, they are seen within the disk and up to heights of 2.5 kpc, and they span a wide range of Galactocentric radii. However, these clouds are not uniformly distributed and appear to be associated with Galactic structures such as spiral arms or superbubbles.

There is now ample evidence that the interstellar thick disks of spiral galaxies are dusty. Although the majority of extraplanar gas in the first few kiloparsecs above the plane of a spiral galaxy is matter that has been expelled from the thin disk, the feedback-driven expulsion does not destroy dust grains altogether (and there is not yet any good measure suggesting it changes the dust-to-gas mass ratio). Direct optical imaging of a majority of edge-on spiral galaxies shows large numbers of dusty clouds populating the thick disk to heights z ∼ 2 kpc. These observations are likely revealing a cold, dense phase of the thick disk interstellar medium. New observations in the mid-infrared show emission from traditional grains and polycyclic aromatic hydrocarbons (PAHs) in the thick disks of spiral galaxies. PAHs are found to have large scale heights and to arise both in the dense dusty clouds traced through direct optical imaging and in the diffuse ionized gas. In this contribution, we briefly summarize these probes of dust in the thick disks of spiral galaxies. We also argue that not only can dust can be used to trace extraplanar material that has come from within the thick disk, but that its absence can be a marker for newly accreted matter from the circumgalactic or intergalactic medium. Thus, observations of dust can perhaps provide a quantitative measure of the importance of “outflow versus infall” in spiral galaxies.

A review of the observational constraints and theoretical models of extra-planar rotation (lagging halos) is presented. It is becoming clear that lagging halos, with vertical gradients in rotation speeds ranging from dvΘ/dz = −8to − 60km s−1kpc−1, are a ubiquitous phenomenon. The observational data so far does not allow one to say definitively whether this number changes with radius or with vertical height in a galaxy, but it is pointed out that such measurements, along with constraints on the presence of radial flows, will provide a crucial test that may allow one to distinguish between the different physical models that produce lagging halos. Some challenges in kinematically distinguishing between Galactic circulation and extragalactic accretion are also discussed.

Globular clusters (GC) are powerful tools to analyse the Halo environment: spread throughout the Halo at distances of 2– 122 kpc, moving with space velocities of ∼200 km s−1, they provide unique tracers of local Halo conditions. Each cluster enriches the Halo with several M⊙ Myr−1 through stellar winds. The interaction of these winds with the Halo can be used as a probe of the local Halo environment.

Using spectral energy distributions of 6875 stars in the GC ω Cen, we quantify the cluster’s total mass loss. Comparing this to observed limits on intra-cluster material (ICM), we find the ICM is being cleared from these clusters on timescales of <105 yr. We also present new 21 cm observations of the high velocity H i cloud near ω Cen in the context of recent Spitzer observations, discussing its structure and likely associations.

Low-ionization transitions such as the Mg iiλ2796/2803 doublet trace cold gas in the vicinity of galaxies. The exact physical origin of this cold gas traced by QSO absorption lines is debated. Does it trace gas in disks, halos, merger tails, and even in outflows? Recent studies of strong starbursts and post-starbursts at intermediate redshifts have shown that Mg ii are seen in outflows, with blue-shifted velocities ∼500 − 1500km s−1. Strong intervening Mg ii absorbers (with equivalent width Wrλ2796 > 1Å) may be tracing the denser and colder gas of starburst-driven outflows. Indeed, based on Mg ii statistics, our clustering analysis has shown that the host-galaxy mass is anti-correlated with the line-of-sight velocity Δv (as measured from Wrλ2796). If Mg ii absorbers were virialized in galaxy halos, a positive Mh–Wr correlation would have been observed. Our result thus shows that the Mg ii clouds are not virialized in the gaseous halos of the host-galaxies.

We analyse the distribution and physical properties of low-column density gas in the halo of the Milky Way with optical and UV absorption spectroscopy and with 21-cm observations.

Hα emission from neutral halo clouds probes the radiation and hydrodynamic conditions in the halo. Armed with such measurements, we can explore how radiation escapes from the Galactic plane and how infalling gas can survive a trip through the halo. The Wisconsin H-Alpha Mapper (WHAM) is one of the most sensitive instruments for detecting and mapping optical emission from the ISM. Here, we present recent results exploring the ionization of two infallling high-velocity complexes. First, we report on our progress mapping Hα emission covering the full extent of Complex A. Intensities are faint (100 mR; EM 0.2pccm−6) but correlate on the sky and in velocity with 21-cm emission. Second, we explore the ionized component of some Anti-Center Complex clouds studied by Peek et al. (2007) that show dynamic shaping from interaction with the Galactic halo.

Indirect evidence from detailed chemical evolution studies shows that infall of low metallicity gas appears to have proceeded at a rather constant rate during the lifetime of the Galactic disk. We explore two possible implications: (a) the effect of an infalling high velocity cloud (HVC) on interstellar cloud stability leading to enhanced star formation, and (b) the presence of these HVC’s around neighbouring galaxies, which would generalize this aspect of the evolution of galaxies in groups. (a) Using a simple analytical model we show how the overpressure produced in the galactic plane by HVC infall reduces the effective Jeans mass of the existing clouds, thus tending to enhance the SFR. (b) Using HI data from the THINGS survey we detect HI “fluff” around 4 galaxies of a total sample of 32 observed. Projecting some of the largest Galactic HVC’s to the distances of the THINGS galaxies we produce a useful estimate of the fraction of the HVC masses that we can detect at these distances, finding a value of order 10%, and go on to make tentative estimates about infall rates for nearby galaxies.

We present WHAM observations of Hα, [N II], and [S II] in the Smith Cloud. A map of Hα emission from the cloud shows ionized gas coincident with the brightest H I emission, but nearly-as-bright Hα in some regions with faint H I. The ionized mass of the cloud is at least as large as the neutral mass, >106M⊙. Ionized gas in the core of the Smith Cloud has an electron temperature 6000 K < T < 16 000 K. The observed ratio [N II]/Hα = 0.39 ± 0.09 shows that the cloud has a non-primordial nitrogen abundance, 0.1 − 1× solar.

We review the evolution of the interstellar medium in disk galaxies, and show, both analytically and by numerical 3D hydrodynamic simulations, that the disk-halo connection is an essential ingredient in understanding the evolution of star forming galaxies. Depending on the star formation rate of the underlying gaseous disk, a galactic fountain is established. If the star formation rate is sufficiently high and/or cosmic rays are well coupled to the thermal plasma, a galactic wind will be formed and lead to a secular mass loss of the galaxy. Such a wind leaves a unique imprint on the soft X-ray spectra in edge-on galaxies, with delayed recombination being one of its distinctive features. We argue that synthetic spectra, obtained from self-consistent dynamical and thermal modeling of a galactic outflow, should be treated on an equal footing as observed spectra. We show that it is thus possible to successfully fit the spectrum of the starburst galaxy NGC 3079

There exist many physical processes that may contribute to the driving of turbulence in galactic disks. Some of them could drive turbulence even in the absence of star formation. For example, hydrodynamic (HD) or magnetohydrodynamic (MHD) instabilities, frequent mergers of small satellite clumps, ram pressure, or infalling gas clouds. In this work we present numerical simulations to study the interaction of compact high velocity clouds (CHVC) with the outskirts of magnetized gaseous disks. With our numerical simulations we show that the rain of small HVCs onto the disk is a potential source of random motions in the outer parts of Hi disks.

We present new developments on the cosmic-ray driven, galactic dynamo, modeled by means of direct, resistive CR–MHD simulations, performed with ZEUS and PIERNIK codes. The dynamo action, leading to the amplification of large–scale galactic magnetic-fields on galactic rotation timescales, appears as a result of galactic differential rotation, buoyancy of the cosmic-ray component and resistive dissipation of small–scale turbulent magnetic-fields. Our new results include demonstration of the global–galactic dynamo action driven by cosmic-rays supplied in supernova remnants. An essential outcome of the new series of global galactic dynamo models is the equipartition of the gas turbulent energy with magnetic-field energy and cosmic-ray energy, in saturated states of the dynamo on large galactic scales.

Over the last few years, evidence that nearby spiral galaxies are surrounded by massive halos of cold gas has been accumulating. This extra-planar cold gas, rotating more slowly than the disk gas, is observed in galaxies with a range of different properties (such as mass and star formation rate, SFR) and it appears analogous to the Intermediate and High Velocity Clouds of the Milky Way. Models for the origin of extra-planar gas have been proposed taking into account the effects of supernova feedback (galactic fountain), cooling flow accretion and hydrostatic equilibrium. Several techniques have been used from analytical treatments and ballistic orbit integration to hydrodynamical simulations. I present a new model where a galactic fountain sweeps up ambient medium as it travels through the halo. This seems to give the best results in reproducing the kinematics of the extra-planar gas and it implies a gas accretion rate of the order of the SFR of the host galaxy.

We present a new multi–fluid, grid MHD code PIERNIK, which is based on the Relaxing TVD scheme (Jin & Xin 1995). The original scheme (see Trac & Pen 2003; Pen et al 2003) has been extended by an addition of dynamically independent, but interacting fluids: dust and a diffusive cosmic ray gas, described within the fluid approximation, with an option to add other fluids in an easy way. The code has been equipped with shearing–box boundary conditions, and a selfgravity module, Ohmic resistivity module, as well as other facilities which are useful in astrophysical fluid–dynamical simulations. The code is parallelized by means of the MPI library. In this paper we present Ohmic resistivity extension of the original Relaxing TVD MHD scheme, and show examples of magnetic reconnection in cases of uniform and current–dependent resistivity prescriptions.