Galactic Fountain Research Articles

Apostle–Auriga: Effects of stellar feedback subgrid models on the evolution of angular momentum in disc galaxies

Abstract Utilizing the Apostle–Auriga simulations, which start from the same zoom-in initial conditions of Local Group-like systems but run with different galaxy formation subgrid models and hydrodynamic solvers, we study the impact of stellar feedback models on the evolution of angular momentum in disc galaxies. At z = 0, auriga disc galaxies tend to exhibit higher specific angular momenta compared to their cross-matched apostle counterparts. By tracing the evolution history of the Lagrangian mass tracers of the in-situ star particles in the z = 0 galaxies, we find that the specific angular momentum distributions of the gas tracers from the two simulations at the halo accretion time are relatively similar. The present-day angular momentum difference is mainly driven by the physical processes occurring inside dark matter haloes, especially galactic fountains. Due to the different subgrid implementations of stellar feedback processes, auriga galaxies contain a high fraction of gas that has gone through recycled fountain (∼65 per cent) which could acquire angular momentum through mixing with the high angular momentum circumgalactic medium (CGM). In apostle, however, the fraction of gas that has undergone the recycled fountain process is significantly lower (down to ∼20 per cent for Milky Way-sized galaxies) and the angular momentum acquisition from the CGM is marginal. As a result, the present-day auriga galaxies overall have higher specific angular momenta.

Fermi and eROSITA Bubbles as Persistent Structures of the Milky Way

The Fermi and eROSITA bubbles (FBs and eRBs), large diffuse structures in our Galaxy, can be the by-products of steady star formation activity. To simultaneously explain the star formation history of the Milky Way (MW) and the metallicity of ∼Z ⊙ at the Galactic disk, a steady Galactic wind driven by cosmic rays (CRs) is required. For tenuous gases with a density of ≲10−3 cm−3, CR heating dominates over radiative cooling, and the gas can maintain the virial temperature of ∼0.3 keV, ideal for escape from the Galactic system as the wind. A part of the wind falls back onto the disk like a Galactic fountain flow. We model the wind dynamics according to the Galactic evolution scenario and find that the scale height and surface brightness of the X-ray and the hadronic gamma-ray emissions from such fountain flow region can be consistent with the observed properties of the FBs and eRBs. This implies that the bubbles are persistent structures of the MW existing over (at least) the last ∼1 Gyr rather than evanescent structures formed by nontrivial, ∼10 Myr past Galactic center transient activities.

First detection of the [CII] 158 µm line in the intermediate-velocity cloud Draco

High-latitude intermediate-velocity clouds (IVCs) are part of the Milky Way’s H I halo and originate from either a galactic fountain process or extragalactic gas infall. They are partly molecular and can most of the time be identified in CO. Some of these regions also exhibit high-velocity cloud gas, which is mostly atomic, and gas at local velocities (LVCs), which is partly atomic and partly molecular. We conducted a study on the IVCs Draco and Spider, both were exposed to a very weak UV field, using the spectroscopic receiver upGREAT on the Stratospheric Observatory for Infrared Astronomy (SOFIA). The 158 µm fine-structure line of ionized carbon ([C II]) was observed, and the results are as follows: In Draco, the [C II] line was detected at intermediate velocities (but not at local or high velocities) in four out of five positions. No [C II] emission was found at any velocity in the two observed positions in Spider. To understand the excitation conditions of the gas in Draco, we analyzed complementary CO and H I data as well as dust column density and temperature maps from Herschel. The observed [C II] intensities suggest the presence of shocks in Draco that heat the gas and subsequently emit in the [C II] cooling line. These shocks are likely caused by the fast cloud’s motion toward the Galactic plane that is accompanied by collisions between H I clouds. The nondetection of [C II] in the Spider IVC and LVC as well as in other low-density clouds at local velocities that we present in this paper (Polaris and Musca) supports the idea that highly dynamic processes are necessary for [C II] excitation in UV-faint low-density regions.

Limits on the OH Molecule in the Smith High-velocity Cloud

We have used the Green Bank Telescope to search for the OH molecule at several locations in the Smith Cloud, one of the most prominent of the high-velocity clouds surrounding the Milky Way. Five positions with high H i column density were selected as targets for individual pointings, along with a square degree around a molecular cloud detected with the Planck telescope near the tip of the Smith Cloud. Gas in the Galactic disk with similar values of N HI has detectable OH emission. Although we found OH at velocities consistent with the foreground Aquila molecular cloud, nothing was found at the velocity of the Smith Cloud to an rms level of 0.7 mK (T b ) in a 1 km s−1 channel. The three positions that give the strictest limits on OH are analyzed in detail. Their combined data imply a 5σ limit on N(H2)/N HI ≤ 0.03, scaled by a factor dependent on the OH excitation temperature and background continuum T ex/(T ex − T bg). There is no evidence for far-infrared emission from dust within the Smith Cloud. These results are consistent with expectations for a low-metallicity diffuse cloud exposed to the radiation field of the Galactic halo, rather than a product of a galactic fountain.

The soft X-ray background with Suzaku. II. Supervirial temperature bubbles?

Abstract Observations of the hot X-ray emitting interstellar medium in the Milky Way are important for studying the stellar feedback and for understanding the formation and evolution of galaxies. We present measurements of the soft X-ray background emission for 130 Suzaku observations at 75° < l < 285° and |b| > 15°. With the standard soft X-ray background model consisting of the local hot bubble and of the Milky Way halo, residual structures remain at 0.7–1 keV in the spectra of some regions. Adding a collisional-ionization-equilibrium component with a temperature of ∼0.8 keV, much higher than the virial temperature of the Milky Way, significantly reduces the derived C-statistic for 56 out of 130 observations. The emission measure of the 0.8 keV component varies by more than an order of magnitude: assuming the solar abundance, the median value is $3 \times 10^{-4}\, \rm {cm^{-6}\ pc}$ and the 16th–84th percentile range is $(1\!-\!8) \times 10^{-4}\, \rm {cm^{-6}\ pc}$. Regions toward the Orion–Eridanus superbubble, having a large cavity extending from the Ori OB1 association, have the highest emission measures of the 0.8 keV component. While the scatter is large, the emission measures tend to be higher toward lower galactic latitudes. We discuss possible biases caused by the solar wind charge exchange, stars, and background groups. The 0.8 keV component is probably heated by supernovae in the Milky Way disk, possibly related to Galactic fountains.

Phosphorus-bearing molecules PO and PN at the edge of the Galaxy

Despite its importance in planet formation and biology1, phosphorus has been identified only in the inner 12 kpc of the Galaxy2–19. The study of this element has been hindered in part by unfavourable atomic transitions2,4,20. Phosphorus is thought to be created by neutron capture on 29Si and 30Si in massive stars20,21, and released into the interstellar medium by Type II supernova explosions2,22. However, models of galactic chemical evolution must arbitrarily increase the supernovae production23 to match observed abundances. Here we present the detection of gas-phase phosphorus in the Outer Galaxy through millimetre spectra of PO and PN. Rotational lines of these molecules were observed in the dense cloud WB89-621, located 22.6 kpc from the Galactic Centre24. The abundances of PO and PN in WB89-621 are comparable to values near the Solar System25. Supernovae are not present in the Outer Galaxy26, suggesting another source of phosphorus, such as ‘Galactic Fountains’, where supernova material is redistributed through the halo and circumgalactic medium27. However, fountain-enriched clouds are not found at such large distances. Any extragalactic source, such as the Magellanic Clouds, is unlikely to be metal rich28. Phosphorus instead may be produced by neutron-capture processes in lower mass asymptotic giant branch stars29 which are present in the Outer Galaxy. Asymptotic giant branch stars also produce carbon21, flattening the extrapolated metallicity gradient and accounting for the high abundances of C-containing molecules in WB89-621.

Galactic coronae in Milky Way-like galaxies: the role of stellar feedback in gas accretion

ABSTRACT Star-forming galaxies like the Milky Way are surrounded by a hot gaseous halo at the virial temperature – the so-called galactic corona – that plays a fundamental role in their evolution. The interaction between the disc and the corona has been shown to have a direct impact on accretion of coronal gas onto the disc with major implications for galaxy evolution. In this work, we study the gas circulation between the disc and the corona of star-forming galaxies like the Milky Way. We use high-resolution hydrodynamical N-body simulations of a Milky Way-like galaxy with the inclusion of an observationally motivated galactic corona. In doing so, we use SMUGGLE, an explicit interstellar medium (ISM), and stellar feedback model coupled with the moving-mesh code arepo. We find that the reservoir of gas in the galactic corona is sustaining star formation: the gas accreted from the corona is the primary fuel for the formation of new stars, helping in maintaining a nearly constant level of cold gas mass in the galactic disc. Stellar feedback generates a gas circulation between the disc and the corona (the so-called galactic fountain) by ejecting different gas phases that are eventually re-accreted onto the disc. The accretion of coronal gas is promoted by its mixing with the galactic fountains at the disc–corona interface, causing the formation of intermediate temperature gas that enhances the cooling of the hot corona. We find that this process acts as a positive feedback mechanism, increasing the accretion rate of coronal gas onto the galaxy.

The SATIN project – I. Turbulent multiphase ISM in Milky Way simulations with SNe feedback from stellar clusters

ABSTRACT We introduce the star formation and supernova (SN) feedback model of the satin (Simulating AGNs Through ISM with Non-Equilibrium Effects) project to simulate the evolution of the star forming multiphase interstellar medium (ISM) of entire disc galaxies. This galaxy-wide implementation of a successful ISM feedback model tested in small box simulations naturally covers an order of magnitude in gas surface density, shear and radial motions. It is implemented in the adaptive mesh refinement code ramses at a peak resolution of 9 pc. New stars are represented by star cluster (sink) particles with individual SN delay times for massive stars. With SN feedback, cooling, and gravity, the galactic ISM develops a three-phase structure. The star formation rates naturally follow observed scaling relations for the local Milky Way gas surface density. SNe drive additional turbulence in the warm (300 < T < 104 K) gas and increase the kinetic energy of the cold gas, cooling out of the warm phase. The majority of the gas leaving the galactic ISM is warm and hot with mass loading factors of 3 ≤ η ≤ 10 up to h = 5 kpc away from the galaxy. While the hot gas is leaving the system, the warm and cold gas falls back onto the disc in a galactic fountain flow. The inclusion of other stellar feedback processes from massive stars seems to be needed to reduce the rate at which stars form at higher surface densities and to increase/decrease the amount of warm/cold gas.

Detection of Dust in High-velocity Cloud Complex C–Enriched Gas Accreting onto the Milky Way * * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with program

We present the detection of dust depletion in Complex C, a massive, infalling, low-metallicity high-velocity cloud in the northern Galactic hemisphere that traces the ongoing accretion of gas onto the Milky Way. We analyze a very high signal-to-noise Hubble Space Telescope Cosmic Origins Spectrograph spectrum of active galactic nucleus (AGN) Mrk 817 formed by coadding 165 individual exposures taken under the AGN STORM 2 program, allowing us to determine dust-depletion patterns in Complex C at unprecedented precision. By fitting Voigt components to the O i, S ii, N i, Si ii, Fe ii, and Al ii absorption and applying ionization corrections from customized Cloudy photoionization models, we find subsolar elemental abundance ratios of [Fe/S] = −0.42 ± 0.08, [Si/S] = −0.29 ± 0.05, and [Al/S] = −0.53 ± 0.08. These ratios indicate the depletion of Fe, Si, and Al into dust grains, since S is mostly undepleted. The detection of dust provides an important constraint on the origin of Complex C, as dust grains indicate the gas has been processed through galaxies, rather than being purely extragalactic. We also derive a low metallicity of Complex C of [S/H] = −0.51 ± 0.16 (≈31% solar), confirming earlier results from this sight line. We discuss origin models that could explain the presence of dust in Complex C, including Galactic fountain models, tidal stripping from the Magellanic Clouds or other satellite galaxies, and precipitation of coronal gas onto dust-bearing “seed” clouds.

Extended neutral hydrogen filamentary network in NGC 2403

We present new neutral hydrogen (H I) observations of the nearby galaxy NGC 2403 to determine the nature of a low-column-density cloud that was detected earlier by the Green Bank Telescope. We find that this cloud is the tip of a complex of filaments of extraplanar gas that is coincident with the thin disk. The total H I mass of the complex is 2 × 107 M⊙ or 0.6% of the total H I mass of the galaxy. The main structure, previously referred to as the 8 kpc filament, is now seen to be even more extended, along a 20 kpc stream. The kinematics and morphological properties of the filaments are unlikely to be the result of outflows related to galactic fountains. It is more likely that the 20 kpc filament is related to a recent galaxy interaction. In this context, a ∼50 kpc long stellar stream has recently been detected connecting NGC 2403 with the nearby dwarf satellite DDO 44. Intriguingly, the southern edge of this stream overlaps with the tip of the 20 kpc H I filament. We conclude that the H I anomalies in NGC 2403 are the result of a recent (∼2Gyr) interaction with DDO 44 leading to the observed filamentary complex.

Fountain-driven gas accretion feeding star formation over the disc of NGC 2403

ABSTRACT We use a dynamical model of galactic fountain to study the neutral extraplanar gas (EPG) in the nearby spiral galaxy NGC 2403. We have modelled the EPG as a combination of material ejected from the disc by stellar feedback (i.e. galactic fountain) and gas accreting from the inner circumgalactic medium (CGM). This accretion is expected to occur because of cooling/condensation of the hot CGM (corona) triggered by the fountain. Our dynamical model reproduces the distribution and kinematics of the EPG H i emission in NGC 2403 remarkably well and suggests a total EPG mass of $4.7^{+1.2}_{-0.9}\times 10^8\, \mathrm{M_\odot }$, with a typical scale height of around 1 kpc and a vertical gradient of the rotation velocity of $-10.0\pm 2.7\, \mathrm{km\, s^{-1}\, kpc^{-1}}$. The best-fitting model requires a characteristic outflow velocity of $50\pm 10\, \mathrm{km\, s^{-1}}$. The outflowing gas starts out mostly ionized and only becomes neutral later in the trajectory. The accretion rate from the condensation of the inner hot CGM inferred by the model is 0.8 $\mathrm{M}_\odot \, \mathrm{yr}^{-1}$, approximately equal to the star-formation rate in this galaxy (0.6 $\mathrm{M}_\odot \, \mathrm{yr}^{-1}$). We show that the accretion profile, which peaks at a radius of about 4.5 kpc, predicts a disc growth rate compatible with the observed value. Our results indicate that fountain-driven corona condensation is a likely mechanism to sustain star formation, as well as the disc inside-out growth in local disc galaxies.

HALOGAS: Strong constraints on the neutral gas reservoir and accretion rate in nearby spiral galaxies

Context. Galaxies in the local Universe are thought to require ongoing replenishment of their gas reservoir in order to maintain the observed star formation rates. Cosmological simulations predict that this accretion can occur in both a dynamically hot and cold mode, depending on the redshift, halo mass, and the details of the included feedback processes. However, until now, observational evidence of the accretion required to match the observed star formation histories is lacking. Aims. Within the framework of the Hydrogen Accretion in LOcal GalaxieS (HALOGAS) survey, this paper attempts to determine whether galaxies in the local Universe possess a significant reservoir of cold neutral gas and the possible accretion rates these reservoirs could provide. Additionally, with this moderately sized sample, we can start to investigate whether the observed accretion is connected to intrinsic properties of the galaxies such as Hubble type, star formation rate, or environment. Methods. We searched the vicinity of 22 nearby galaxies in a systematic and automated manner for isolated H I clouds or distinct streams that are not yet connected to the galaxy disk. The HALOGAS observations were carried out with the Westerbork Synthesis Radio Telescope and represent one of the most sensitive and detailed H I surveys to date. These observations typically reach column density sensitivities of ∼1019 cm−2 over a 20 km s−1 line width. Results. We find 14 secure H I cloud candidates without an observed optical counterpart in the entire HALOGAS sample. These cloud candidates appear to be analogs to the most massive clouds detected in the extensive cloud distributions around the Milky Way and M 31. However, their numbers seem significantly reduced on average compared to the numbers in these galaxies. Within the framework of cold accretion, we constrain upper limits for H I accretion in the local Universe. The average H I mass currently observed in a state suggestive of accretion onto the galaxies amounts to a rate of 0.05 M⊙ yr−1 with a stringent upper limit of 0.22 M⊙ yr−1, confirming previous estimates. This is much lower than the average star formation rate in this sample. Our best estimate, based on the Green Bank Telescope detection limits of several galaxies in the sample, suggests that another 0.04 M⊙ yr−1 of neutral hydrogen at most could be accreted from clouds and streams that remain undetected. Conclusions. These results show that in nearby galaxies, neutral hydrogen is not being accreted at the same rate as stars are currently being formed. Our study cannot exclude that other forms of gas accretion are at work, such as those provided by direct infall of ionized intergalactic gas or the condensation of coronal gas, triggered by galactic fountain activities. However, these observations do not reveal extended neutral gas reservoirs around most nearby spiral galaxies either.

A Semianalytical Line Transfer Model. III. Galactic Inflows

We present calculations of ultraviolet spectra resulting from the scattering of photons by gas infalling onto an isotropically emitting source of radiation. The model is based on an adaptation of the semianalytical line transfer (SALT) code of Scarlata & Panagia, and designed to interpret the inverse P Cygni profiles observed in the spectra of partially ionized galactic inflows. In addition to presenting the model, we explore the parameter space of the inflowing SALT model and recreate various physically motivated scenarios including spherical inflows, inflows with covering fractions less than unity, and galactic fountains (i.e., galactic systems with both an inflowing and outflowing component). The resulting spectra from inflowing gas show spectral features that could be misinterpreted as interstellar medium features in low resolution spectroscopy (σ ≈ 120 km s−1), suggesting that the total number of galactic systems with inflows is undercounted. Our models suggest that observations at medium resolution (R = 6000 or σ ≈ 50 km s−1) that can be obtained with 8 m class telescopes will be able to resolve the characteristic inverse P Cygni profiles necessary to identify inflows.

Metal content of the circumgalactic medium around star-forming galaxies at z ∼ 2.6 as revealed by the VIMOS Ultra-Deep Survey

Context. The circumgalactic medium (CGM) is the location where the interplay between large-scale outflows and accretion onto galaxies occurs. Metals in different ionization states flowing between the circumgalactic and intergalactic mediums are affected by large galactic outflows and low-ionization state inflowing gas. Observational studies on their spatial distribution and their relation with galaxy properties may provide important constraints on models of galaxy formation and evolution. Aims. The main goal of this paper is to provide new insights into the spatial distribution of the circumgalactic of star-forming galaxies at 1.5 < z < 4.5 (⟨z⟩∼2.6) in the peak epoch of cosmic star formation activity in the Universe. We also look for possible correlations between the strength of the low- and high-ionization absorption features (LIS and HIS) and stellar mass, star formation rate, effective radius, and azimuthal angle ϕ that defines the location of the absorbing gas relative to the galaxy disc plane. Methods. The CGM has been primarily detected via the absorption features that it produces on the continuum spectrum of bright background sources. We selected a sample of 238 close pairs from the VIMOS Ultra Deep Survey to examine the spatial distribution of the gas located around star-forming galaxies and generate composite spectra by co-adding spectra of background galaxies that provide different sight-lines across the CGM of star-forming galaxies. Results. We detect LIS (C II and Si II) and HIS (Si IV, C IV) up to separations ⟨b⟩ = 172 kpc and 146 kpc. Beyond this separation, we do not detect any significant signal of CGM absorption in the background composite spectra. Our Lyα, LIS, and HIS rest-frame equivalent width (W0) radial profiles are at the upper envelope of the W0 measurements at lower redshifts, suggesting a potential redshift evolution for the CGM gas content producing these absorptions. We find a correlation between C II and C IV with star formation rate and stellar mass, as well as trends with galaxy size estimated by the effective radius and azimuthal angle. Galaxies with high star formation rate (log[SFR/(M⊙ yr−1)] > 1.5) and stellar mass (log[M⋆/M⊙] > 10.2) show stronger C IV absorptions compared with those low SFR (log[SFR/(M⊙ yr−1)] < 0.9) and low stellar mass (log[M⋆/M⊙] < 9.26). The latter population instead shows stronger C II absorption than their more massive or more star-forming counterparts. We compute the C II/C IVW0 line ratio that confirms the C II and C IV correlations with impact parameter, stellar mass, and star formation rate. We do not find any correlation with ϕ in agreement with other high-redshift studies and in contradiction to what is observed at low redshift where large-scale outflows along the minor axis forming bipolar outflows are detected. Conclusions. We find that the stronger C IV line absorptions in the outer regions of these star-forming galaxies could be explained by stronger outflows in galaxies with higher star formation rates and stellar masses that are capable of projecting the ionized gas up to large distances and/or by stronger UV ionizing radiation in these galaxies that is able to ionize the gas even at large distances. On the other hand, low-mass galaxies show stronger C II absorptions, suggesting larger reservoirs of cold gas that could be explained by a softer radiation field unable to ionize high-ionization state lines or by the galactic fountain scenario where metal-rich gas ejected from previous star formation episodes falls back to the galaxy. These large reservoirs of cold neutral gas around low-mass galaxies could be funnelled into the galaxies and eventually provide the necessary fuel to sustain star formation activity.

On the Kinematics of Cold, Metal-enriched Galactic Fountain Flows in Nearby Star-forming Galaxies

We use medium-resolution Keck/Echellette Spectrograph and Imager spectroscopy of bright quasars to study cool gas traced by Ca ii λλ3934, 3969 and Na i λλ5891, 5897 absorption in the interstellar/circumgalactic media of 21 foreground star-forming galaxies at redshifts 0.03 < z < 0.20 with stellar masses 7.4 ≤ log M */M ⊙ ≤ 10.6. The quasar–galaxy pairs were drawn from a unique sample of Sloan Digital Sky Survey quasar spectra with intervening nebular emission, and thus have exceptionally close impact parameters (R ⊥ < 13 kpc). The strength of this line emission implies that the galaxies’ star formation rates (SFRs) span a broad range, with several lying well above the star-forming sequence. We use Voigt profile modeling to derive column densities and component velocities for each absorber, finding that column densities N(Ca ii) > 1012.5 cm−2 (N(Na i) > 1012.0 cm−2) occur with an incidence f C(Ca ii) = 0.63+0.10 −0.11 (f C(Na i) = 0.57+0.10 −0.11). We find no evidence for a dependence of f C or the rest-frame equivalent widths W r (Ca ii K) or W r (Na i 5891) on R ⊥ or M *. Instead, W r (Ca ii K) is correlated with local SFR at >3σ significance, suggesting that Ca ii traces star formation-driven outflows. While most of the absorbers have velocities within ±50 km s−1 of the host redshift, their velocity widths (characterized by Δv 90) are universally 30–177 km s−1 larger than that implied by tilted-ring modeling of the velocities of interstellar material. These kinematics must trace galactic fountain flows and demonstrate that they persist at R ⊥ > 5 kpc. Finally, we assess the relationship between dust reddening and W r (Ca ii K) (W r (Na i 5891)), finding that 33% (24%) of the absorbers are inconsistent with the best-fit Milky Way E(B−V)-W r relations at >3σ significance.

The impact of cosmic rays on dynamical balance and disc–halo interaction in L⋆ disc galaxies

ABSTRACT Cosmic rays (CRs) are an important component in the interstellar medium, but their effect on the dynamics of the disc–halo interface (&amp;lt;10 kpc from the disc) is still unclear. We study the influence of CRs on the gas above the disc with high-resolution FIRE-2 cosmological simulations of late-type L⋆ galaxies at redshift z ∼ 0. We compare runs with and without CR feedback (with constant anisotropic diffusion κ∥ ∼ 3 × 1029 cm2 s−1 and streaming). Our simulations capture the relevant disc–halo interactions, including outflows, inflows, and galactic fountains. Extra-planar gas in all of the runs satisfies dynamical balance, where total pressure balances the weight of the overlying gas. While the kinetic pressure from non-uniform motion (≳1 kpc scale) dominates in the mid-plane, thermal and bulk pressures (or CR pressure if included) take over at large heights. We find that with CR feedback, (1) the warm (∼104 K) gas is slowly accelerated by CRs; (2) the hot (&amp;gt;5 × 105 K) gas scale height is suppressed; (3) the warm-hot (2 × 104–5 × 105 K) medium becomes the most volume-filling phase in the disc–halo interface. We develop a novel conceptual model of the near-disc gas dynamics in low-redshift L⋆ galaxies: with CRs, the disc–halo interface is filled with CR-driven warm winds and hot superbubbles that are propagating into the circumgalactic medium with a small fraction falling back to the disc. Without CRs, most outflows from hot superbubbles are trapped by the existing hot halo and gravity, so typically they form galactic fountains.

The Galactic dynamics revealed by the filamentary structure in atomic hydrogen emission

We present a study of the filamentary structure in the neutral atomic hydrogen (H I) emission at the 21 cm wavelength toward the Galactic plane using the 16′.2-resolution observations in the H I 4π (HI4PI) survey. Using the Hessian matrix method across radial velocity channels, we identified the filamentary structures and quantified their orientations using circular statistics. We found that the regions of the Milky Way’s disk beyond 10 kpc and up to roughly 18 kpc from the Galactic center display H I filamentary structures predominantly parallel to the Galactic plane. For regions at lower Galactocentric radii, we found that the H I filaments are mostly perpendicular or do not have a preferred orientation with respect to the Galactic plane. We interpret these results as the imprint of supernova feedback in the inner Galaxy and Galactic rotation and shear in the outer Milky Way. We found that the H I filamentary structures follow the Galactic warp and flaring and that they highlight some of the variations interpreted as the effect of the gravitational interaction with satellite galaxies. In addition, the mean scale height of the filamentary structures is lower than that sampled by the bulk of the H I emission, thus indicating that the cold and warm atomic hydrogen phases have different scale heights in the outer galaxy. Finally, we found that the fraction of the column density in H I filaments is almost constant up to approximately 18 kpc from the Galactic center. This is possibly a result of the roughly constant ratio between the cold and warm atomic hydrogen phases inferred from the H I absorption studies. Our results indicate that the H I filamentary structures provide insight into the dynamical processes shaping the Galactic disk. Their orientations record how and where the stellar energy input, the Galactic fountain process, the cosmic ray diffusion, and the gas accretion have molded the diffuse interstellar medium in the Galactic plane.

A Detection of H2 in a High-velocity Cloud toward the Large Magellanic Cloud

This work presents a new detection of H2 absorption arising in a high-velocity cloud associated with either the Milky Way or the Large Magellanic Cloud (LMC). The absorber was found in an archival Far Ultraviolet Spectroscopic Explorer spectrum of the LMC star Sk-70°32. This is the fifth well-characterized H2 absorber to be found in the Milky Way’s halo and the second such absorber outside the Magellanic Stream and Bridge. The absorber has a local standard of rest central velocity of +140 km s−1 and a H2 column density of 1017.5 cm−2. It is most likely part of a cool and relatively dense inclusion (T ≈ 75 K, n H ∼ 100 cm−3) in a warmer and more diffuse halo cloud. This halo cloud may be part of a still-rising Milky Way Galactic fountain flow or an outflow from the Large Magellanic Cloud.

Intermediate- and high-velocity clouds in the Milky Way – II. Evidence for a Galactic fountain with collimated outflows and diffuse inflows

ABSTRACTWe model the kinematics of the high- and intermediate-velocity clouds (HVCs and IVCs) observed in absorption towards a sample of 55 Galactic halo stars with accurate distance measurements. We employ a simple model of a thick disc whose main free parameters are the gas azimuthal, radial, and vertical velocities (vϕ, vR, and vz), and apply it to the data by fully accounting for the distribution of the observed features in the distance–velocity space. We find that at least two separate components are required to reproduce the data. A scenario where the HVCs and the IVCs are treated as distinct populations provides only a partial description of the data, which suggests that a pure velocity-based separation may give a biased vision of the gas physics at the Milky Way’s disc–halo interface. Instead, the data are better described by a combination of an inflow component and an outflow component, both characterized by rotation with vϕ comparable to that of the disc and vz of $50\!-\!100\, {\rm km\, s}^{-1}$. Features associated with the inflow appear to be diffused across the sky, while those associated with the outflow are mostly confined within a bicone pointing towards (l = 220°, b = +40°) and (l = 40°, b = −40°). Our findings indicate that the lower ($|z| \lesssim 10\, {\rm kpc}$) Galactic halo is populated by a mixture of diffuse inflowing gas and collimated outflowing material, which are likely manifestations of a galaxy-wide gas cycle triggered by stellar feedback, that is, the galactic fountain.

Intermediate- and high-velocity clouds in the Milky Way – I. Covering factors and vertical heights

ABSTRACT Intermediate- and high-velocity clouds (IVCs, HVCs) are a potential source of fuel for star formation in the Milky Way (MW), but their origins and fates depend sensitively on their distances. We search for IVCs and HVCs in HST high-resolution ultraviolet spectra of 55 halo stars at vertical heights $|z|\gtrsim \,1$ kpc. We show that IVCs (40 ≤ |$v$LSR| &amp;lt; 90 ${\rm km\, s}^{-1}$) have a high detection rate – the covering factor, fc – that is about constant (fc = 0.90 ± 0.04) from $z$ = 1.5 to 14 kpc, implying IVCs are essentially confined to |$z$| ≲ 1.5 kpc. For the HVCs (90 ≤ |$v$LSR| ≲ 170 ${\rm km\, s}^{-1}$), we find fc increases from fc ≃ 0.14 ± 0.10 at |$z$| ≲ 2–3 kpc to fc = 0.60 ± 0.15 at 6 ≲ |$z$| ≲ 14 kpc, the latter being similar to that found towards QSOs. In contrast, the covering factor of very high-velocity clouds (VHVCs; |$v$LSR| ≳ 170 ${\rm km\, s}^{-1}$) is $f_c \lt 0.04$ in the stellar sample compared to 20 per cent towards QSOs, implying these clouds must be at d ≳ 10–15 kpc (|$z$| ≳ 10 kpc). Gas clouds with |$v$LSR| &amp;gt; 40 ${\rm km\, s}^{-1}$ at |b| ≳ 15° have therefore |$v$LSR| decreasing with decreasing |$z$|. Our findings are consistent with a Galactic rain and/or fountain origin for these clouds. In the latter scenario, VHVCs may mostly serve as fuel for the MW halo. In view of their high covering factors and since all the IVCs and some HVCs are found in the thick disc, they appear good candidates as gas reservoirs to help sustain star formation in the MW.