High-velocity Clouds Research Articles

Spatial metallicity variations of mono-temperature stellar populations revealed by early-type stars in LAMOST

We investigate the radial metallicity gradients and azimuthal metallicity distributions on the Galactocentric X–Y plane using mono-temperature stellar populations selected from the LAMOST-MRS young stellar sample. The estimated radial metallicity gradient ranges from −0.015 dex/kpc to −0.07 dex/kpc, which decreases as the effective temperature decreases (or when the stellar age increases) at 7500 < Teff < 12 500 K (τ < 1.5 Gyr). The azimuthal metallicity excess (the metallicity after subtracting the radial metallicity gradient, Δ [M/H]) distributions exhibit inhomogeneities with dispersions of 0.04 dex to 0.07 dex, which decrease as the effective temperature decreases. We also identify five potential metal-poor substructures with large metallicity excess dispersions. The metallicity excess distributions of these five metal-poor substructures suggest that they contain a larger fraction of metal-poor stars compared to other control samples. These metal-poor substructures may be associated with high-velocity clouds that infall into the Galactic disk from the Galactic halo, which are not quickly well mixed with the pre-existing interstellar medium (ISM) of the Galactic disk. As a result, these high-velocity clouds produce some metal-poor stars and the observed metal-poor substructures. The variations of metallicity inhomogeneities with different stellar populations indicate that high-velocity clouds are not well mixed with the pre-existing Galactic disk ISM within 0.3 Gyr.

Magnetic field draping around clumpy high-velocity clouds in galactic halo

ABSTRACT Throughout the passage within the Galactic halo, high-velocity clouds (HVCs) sweep up ambient magnetic fields and form stretched and draped configurations of magnetic fields around them. Many earlier numerical studies adopt spherically symmetric uniform-density clouds as initial conditions for simplicity. However, observations demonstrate that HVCs are clumpy and turbulent. In this paper, we perform 3D magnetohydrodynamic simulations to study the evolution of clouds with initial density distributions described by power-law spatial power spectra. We systematically study the role of (i) the initial density structure, (ii) halo magnetic fields, and (iii) radiative cooling efficiency upon infalling HVCs. We find that (i) the clouds’ density structure regulates mixing and mass growth. Uniform clouds grow from the onset of the simulations, while clumpy clouds initially lose gas and then grow at later times. Along the same lines, the growth curve of clumpy clouds depends on the slope of the initial density power spectra. (ii) Magnetic fields suppress hydrodynamic instabilities and the growth of small-scale structures. As a result, magnetized clouds develop long filaments extended along the streaming direction, whereas non-magnetized clouds are fragmented into many small clumps. (iii) Efficient cooling keeps the main cloud body more compact and produces decelerated dense clumps condensed from the halo gas. This work potentially helps us understand and predict the observed properties of HVCs such as the detectability of magnetized clouds, the presence of decelerated HI structures associated with HVC complexes and small-scale features, and a possible link between the origin and the fate of HVCs.

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.

The circumgalactic medium of Milky Way-like galaxies in the TNG50 simulation – II. Cold, dense gas clouds and high-velocity cloud analogs

ABSTRACT We use the TNG50 simulation of the IllustrisTNG project to study cold, dense clouds of gas in the circumgalactic media (CGM) of Milky Way-like galaxies. We find that their CGM is typically filled with of order one hundred (thousand) reasonably (marginally) resolved clouds, possible analogs of high-velocity clouds (HVCs). There is a large variation in cloud abundance from galaxy to galaxy, and the physical properties of clouds that we explore – mass, size, metallicity, pressure, and kinematics – are also diverse. We quantify the distributions of cloud properties and cloud-background contrasts, providing cosmological inputs for idealized simulations. Clouds characteristically have subsolar metallicities, diverse shapes, small overdensities (χ = ncold/ nhot ≲ 10), are mostly inflowing, and have sub-virial rotation. At TNG50 resolution, resolved clouds have median masses of ∼ $10^6\, \rm {M_\odot }$ and sizes of ∼10 kpc. Larger clouds are well converged numerically, while the abundance of the smallest clouds increases with resolution, as expected. In TNG50 MW-like haloes, clouds are slightly (severely) underpressurized relative to their surroundings with respect to total (thermal) pressure, implying that magnetic fields may be important. Clouds are not distributed uniformly throughout the CGM but are clustered around other clouds, often near baryon-rich satellite galaxies. This suggests that at least some clouds originate from satellites, via direct ram-pressure stripping or otherwise. Finally, we compare with observations of intermediate and high velocity clouds from the real Milky Way halo. TNG50 shows a similar cloud velocity distribution as observations and predicts a significant population of currently difficult-to-detect low velocity clouds.

CMZoom III: Spectral line data release

ABSTRACT We present an overview and data release of the spectral line component of the SMA Large Program, CMZoom. CMZoom observed 12CO (2–1), 13CO (2–1), and C18O (2–1), three transitions of H2CO, several transitions of CH3OH, two transitions of OCS, and single transitions of SiO and SO within gas above a column density of N(H2) ≥ 1023 cm−2 in the Central Molecular Zone (CMZ; inner few hundred pc of the Galaxy). We extract spectra from all compact 1.3 mm CMZoom continuum sources and fit line profiles to the spectra. We use the fit results from the H2CO 3(0, 3)–2(0, 2) transition to determine the source kinematic properties. We find ∼90 per cent of the total mass of CMZoom sources have reliable kinematics. Only four compact continuum sources are formally self-gravitating. The remainder are consistent with being in hydrostatic equilibrium assuming that they are confined by the high external pressure in the CMZ. We find only two convincing proto-stellar outflows, ruling out a previously undetected population of very massive, actively accreting YSOs with strong outflows. Finally, despite having sufficient sensitivity and resolution to detect high-velocity compact clouds (HVCCs), which have been claimed as evidence for intermediate mass black holes interacting with molecular gas clouds, we find no such objects across the large survey area.

Caught in the Act: A Metal-rich High-velocity Cloud in the Inner Galaxy

We characterize the chemical and physical conditions in an outflowing high-velocity cloud (HVC) in the inner Galaxy. We report a supersolar metallicity of [O/H] = +0.36 ± 0.12 for the HVC at v LSR = 125.6 km s−1 toward the star HD 156359 (l = 328.°7, b = −14.°5, d = 9 kpc, z = −2.3 kpc). Using archival observations from the Far-Ultraviolet Spectroscopic Explorer (FUSE), the Hubble Space Telescope Imaging Spectrograph, and the European Southern Observatory Fiber-fed Extended Range Optical Spectrograph we measure high-velocity absorption in H i, O i, C ii, N ii, Si ii, Ca ii, Si iii, Fe iii, C iv, Si iv, N v, and O vi. We measure a low H i column density of log N(H i) = 15.54 ± 0.05 in the HVC from multiple unsaturated H i Lyman series lines in the FUSE data. We determine a low dust depletion level in the HVC from the relative strength of silicon, iron, and calcium absorption relative to oxygen, with [Si/O] = −0.33 ± 0.14, [Fe/O] = −0.30 ± 0.20, and [Ca/O] = −0.56 ± 0.16. Analysis of the high-ion absorption using collisional ionization models indicates that the hot plasma is multiphase, with the C iv and Si iv tracing 104.9 K gas and N v and O vi tracing 105.4 K gas. The cloud’s metallicity, dust content, kinematics, and close proximity to the disk are all consistent with a Galactic wind origin. As the HD 156359 line of sight probes the inner Galaxy, the HVC appears to be a young cloud caught in the act of being entrained in a multiphase Galactic outflow and driven out into the halo.

The Distance to High-velocity Cloud Complex M

λ-21 cm HI4PI survey data are used to study the anomalous-velocity hydrogen gas associated with high-velocity cloud Complex M. These high-sensitivity, high-resolution, high-dynamic-range data show that many of the individual features, including MI, MIIa, and MIIb, are components of a long, arched filament that extends from about (l, b) = (105°, 53°) to (l, b) = (196°, 55°). Maps at different velocities, results from Gaussian analysis, and observations of associated high-energy emission make a compelling case that the MI cloud and the arched filament are physically interacting. If this is the case, we can use the distance to MI, 150 pc as reported by Schmelz & Verschuur, to set the distance to Complex M. The estimated mass of Complex M is then about 120 M ⊙, and the energy implied using the observed line-of-sight velocity, −85 km s−1, is 8.4 × 1048 erg. Integrating over 4π sr, the total energy for a spherically symmetrical explosion is estimated to be 1.9 × 1050 erg, well within the energy budget of a typical supernova.

Cloudy with a chance of rain: accretion braking of cold clouds

ABSTRACT Understanding the survival, growth, and dynamics of cold gas is fundamental to galaxy formation. While there has been a plethora of work on ‘wind tunnel’ simulations that study such cold gas in winds, the infall of this gas under gravity is at least equally important, and fundamentally different since cold gas can never entrain. Instead, velocity shear increases and remains unrelenting. If these clouds are growing, they can experience a drag force due to the accretion of low-momentum gas, which dominates over ram pressure drag. This leads to subvirial terminal velocities, in line with observations. We develop simple analytic theory and predictions based on turbulent radiative mixing layers. We test these scalings in 3D hydrodynamic simulations, both for an artificial constant background and a more realistic stratified background. We find that the survival criterion for infalling gas is more stringent than in a wind, requiring that clouds grow faster than they are destroyed ($t_{\rm grow} \lt 4\, t_{\rm cc}$). This can be translated to a critical pressure, which for Milky Way-like conditions is $P \sim 3000 \, {k}_\mathrm{ B} \, {\rm K}\, {\rm cm}^{-3}$. Cold gas that forms via linear thermal instability (tcool/tff < 1) in planar geometry meets the survival threshold. In stratified environments, larger clouds need only survive infall until cooling becomes effective. We discuss applications to high-velocity clouds and filaments in galaxy clusters.

A Component of the Smith High-velocity Cloud Now Crossing the Galactic Plane

We have identified a new structure in the Milky Way: a leading component of the Smith high-velocity cloud that is now crossing the Galactic plane near longitude 25°. Using new 21 cm H i data from the Green Bank Telescope (GBT), we measured the properties of several dozen clouds that are part of this structure. Their kinematics is consistent with that of the Smith Cloud with a V LSR exceeding that permitted by circular rotation in their direction. Most of the clouds in the leading component show evidence that they are interacting with disk gas allowing the location of the interaction to be estimated. The leading component crosses the Galactic plane at a distance from the Sun of 9.5 kpc, about 4.5 kpc from the Galactic Center. Its H i mass may be as high as 106 M ⊙, comparable to the mass of the neutral component of the Smith Cloud, but only a fraction of this is contained in clouds that are resolved in the GBT data. Like the Smith Cloud, the leading component appears to be adding mass and angular momentum to the interstellar medium in the inner Galaxy. We suggest that the Smith Cloud is not an isolated object, but rather part of a structure that stretches more than 40° (∼7 kpc) across the sky, in two pieces separated by a gap of ∼1 kpc.

Radio and γ-ray Evidence for the Supernova Origin of High-velocity Cloud Complex M

Using λ21 cm galactic neutral hydrogen data from the HI4PI survey and 0.75–30 MeV γ-ray emission from the Imaging Compton Telescope, we have searched for the origin event that accelerated high-velocity cloud Complex M. Radio plots of l − b, l − v, and b − v show a cavity centered at (l, b) ∼ (150°, 50.°) and extending about ±33°. The best view of the cavity is at a velocity of −25 km s−1, which shows a circular cross section on the back (receding) face. Complex M, at −85 km s−1, is on the front (approaching) face. The γ-ray emission reveals several minima, the largest centered at (l, b) ∼(150°, 50.°) and coincident with the position and extent of the cavity seen in the radio data. Using the known distance to Complex M and assuming that the cavity is spherical, we can bootstrap the distance to the original, explosive source of the cavity D = 307 pc, calculate the radius of the cavity R = 166 pc, and approximate the expansion velocity V E ≈ 40 km s−1 of the cavity. The total energy of the expanding cavity is 3.0 ± 1.0 × 1050 erg, well within the range of a single supernova. These results indicate that this explosion took place about four million years ago. As the blast wave from this supernova propagated outwards, it began to sweep up interstellar gas and carved out the Local Chimney, a low-density extension of the Local Bubble that reaches into the galactic halo.

Living on the edge of the Milky Way's central molecular zone

Context.The 1°.3 (G1.3) and 1°.6 (G1.6) cloud complexes in the central molecular zone (CMZ) of our Galaxy have been proposed to possibly reside at the intersection region of the X1 and X2 orbits for several reasons. This includes the detection of co-spatial low- and high-velocity clouds, high velocity dispersion, high fractional molecular abundances of shock-tracing molecules, and kinetic temperatures that are higher than for usual CMZ clouds.Aims.By investigating the morphology and deriving physical properties as well as chemical composition, we want to find the origin of the turbulent gas and, in particular, whether evidence of an interaction between clouds can be identified.Methods.We mapped both cloud complexes in molecular lines in the frequency range from 85 to 117 GHz with the IRAM 30 m telescope. The APEX 12m telescope was used to observe higher frequency transitions between 210 and 475 GHz from selected molecules that are emitted from higher energy levels. We performed non-local thermodynamic equilibrium (non-LTE) modelling of the emission of an ensemble of CH3CN lines to derive kinetic temperatures and H2volume densities. These were used as starting points for non-LTE modelling of other molecules, for which column densities and abundances were determined and compared with values found for other sources in the CMZ.Results.The kinematic structure of G1.3 reveals an ‘emission bridge’ at intermediate velocities (~150 km s−1) connecting low-velocity (~100 km s−1) and high-velocity (~180 km s−1) gas and an overall fluffy shell-like structure. These may represent observational evidence of cloud-cloud interactions. Low- and high-velocity gas components in G1.6 do not show this type of evidence of an interaction, suggesting that they are spatially separated. We selected three positions in each cloud complex for further analysis. Each position reveals several gas components at various peak velocities and of various line widths. We derived kinetic temperatures of 60–100 K and H2volume densities of 104–105cm−3in both complexes. Molecular abundances relative to H2suggest a similar chemistry of the two clouds, which is moreover similar to that of other GC clouds and, especially, agrees well with that of G+0.693 and G−0.11.Conclusions.We conclude that G1.3 may indeed exhibit signs of cloud-cloud interactions. In particular, we propose an interaction of gas that is accreted from the near-side dust lane to the CMZ, with gas pre-existing at this location. Low- and high-velocity components in G1.6 are rather coincidentally observed along the same line of sight. They may be associated with either overshot decelerated gas from the far-side dust line or actual CMZ gas and high-velocity gas moving on a dust lane. These scenarios would be in agreement with numerical simulations.

The Origin and Distance of the High-velocity Cloud MI

The high-velocity, neutral hydrogen feature known as MI may be the result of a supernova that took place about 100,000 yr ago at a distance of 163 pc. Low-velocity H i data show a clear cavity, a structure indicative of regions evacuated by old exploding stars, centered on the spatial coordinates of MI, (l, b) = (165°, 65.°5). The cavity is also visible in 100 μm dust data from IRAS. The invisible companion of the yellow giant star, 56 Ursae Majoris, may be the remains of the supernova that evacuated the cavity and blasted MI itself outward at 120 km s−1. The mass and energy of MI are easily in line with what is expected from a supernova. The diffuse X-rays seen by ROSAT provide evidence of a hot cavity, and the enhanced X-rays may result from the subsequent bow shock.

Catalog of High-velocity Dispersion Compact Clouds in the Central Molecular Zone of Our Galaxy

This study developed an automated identification procedure for compact clouds with broad velocity widths in the spectral-line data cubes of highly crowded regions. The procedure was applied to the CO J = 3 − 2 line data, obtained using the James Clerk Maxwell Telescope, to identify 184 high-velocity dispersion compact clouds (HVDCCs), which are a category of peculiar molecular clouds found in the central molecular zone of our Galaxy. A list of HVDCCs in the area −1.°4 ≤ l ≤ +2.°0, −0.°25 ≤ b ≤ +0.°25 was presented with their physical parameters, CO J = 3 − 2/J = 1 − 0 intensity ratios, and morphological classifications. Consequently, the list provides several intriguing sources that may have been driven by encounters with pointlike massive objects, local energetic events, or cloud-to-cloud collisions.

About Metallicity Variations in the Local Galactic Interstellar Medium

In this paper we discuss and confront recent results on metallicity variations in the local interstellar medium, obtained from observations of H ii regions and neutral clouds of the Galactic thin disk, and compare them with recent high-quality metallicity determinations of other tracers of the chemical composition of the interstellar medium as B-type stars, classical Cepheids, and young clusters. We find that the metallicity variations obtained for these last kinds of objects are consistent with each other and with that obtained for H ii regions but significantly smaller than those obtained for neutral clouds. We also discuss the presence of a large population of low-metallicity clouds as the possible origin for large metallicity variations in the local Galactic thin disk. We find that such a hypothesis does not seem compatible with: (a) what is predicted by theoretical studies of gas mixing in galactic disks, and (b) the models and observations on the metallicity of high-velocity clouds and their evolution as they mix with the surrounding medium in their fall onto the Galactic plane. We conclude that most of the evidence favors that the chemical composition of the interstellar medium in the solar neighborhood is highly homogeneous.

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.

Radio properties of the OH megamaser galaxy IIZw 096

Based on the two-epoch European Very Long Baseline Interferometry (VLBI) Network (EVN) archive data from OH line observations of IIZw 096, we confirm that the high-resolution OH emission in this source mainly comes from two spots (OH1 and OH2) of component (comp) D1 of this merging system. We found no significant variations in the OH 1667 MHz line emission, including flux densities and peak positions. The OH 1665 MHz line emission is detected at about the 6σ level in the OH1 region by combining two epochs of EVN observations. By using archival data from the Very Long Baseline Array (VLBA), Very Large Array (VLA), and Atacama Large Millimeter Array (ALMA) observations, we investigated the properties of the environment of this component through H I, CO(3-2), and HCO+(4-3) lines and the multi-band radio continuum emission. We found that the comp D1 shows the brightest CO, HCO+ line emission, as well as multi-band radio continuum emission. The environment around D1 shows no clear velocity structure associated with circular motions, making it different from most other OH megamasers (OHMs) in the literature, which might have been caused by an effect during the merger stage. Meanwhile, we found that the CO emission shows three velocity structures around D1, including the central broad full width at half maximum (FWHM) region, the double-peak region where the CO line profile shows two separated peaks, and the region of the high-velocity clouds where the CO line peaks at a high velocity (∼11 000 km s−1). Similarly, H Iobservations in absorption also show high-velocity clouds around the D1 region, which might be due to inflows caused by the merging of two or more galaxy components. Based on the high-resolution K-band VLA and L-band VLBA observations of the radio continuum emission, we derived the brightness temperature in the range 105 K–106 K, which is consistent with other starburst dominant OHM sources in the literature. The multi-band VLA observations show that the radio continuum emission of comp D might also have contributions from free-free emission in addition to synchrotron emission. As a consequence, these results support a starburst origin for the OHMs, without the presence of an active galactic nucleus (AGN).

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| < 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| > 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.

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.

Spatially resolved gas flows around the Milky Way

ABSTRACT We present spatially resolved measurements of cool gas flowing into and out of the Milky Way (MW), using archival ultraviolet spectra of background quasars from the Hubble Space Telescope/Cosmic Origins Spectrograph. We co-add spectra of different background sources at close projected angular separation on the sky. This novel stacking technique dramatically increases the signal-to-noise ratio of the spectra, allowing the detection of low-column-density gas (down to EW > 2 mÅ). We identify absorption as inflowing or outflowing, by using blue/redshifted high-velocity cloud absorption components in the Galactocentric rest frame, respectively. The mass surface densities of both inflowing and outflowing gases vary by more than an order of magnitude across the sky, with mean values of 〈Σin〉 ≳ 104.6 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for inflowing gas and 〈Σout〉 ≳ 103.5 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for outflowing gas. The mass flow rate surface densities (mass flow rates per unit area) also show large variation across the sky, with $\langle \dot{\Sigma }(d)_{\rm in}\rangle \gtrsim (10^{-3.6\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for inflowing gas and $\langle \dot{\Sigma }(d)_{\rm out}\rangle \gtrsim (10^{-4.8\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for outflowing gas. The regions with highest surface mass density of inflowing gas are clustered at smaller angular scales (θ < 40°). This indicates that most of the mass in inflowing gas is confined to small, well-defined structures, whereas the distribution of outflowing gas is spread more uniformly throughout the sky. Our study confirms that the MW is predominantly accreting gas, but it is also losing a non-negligible mass of gas via outflow.

The Long Tails of the Pegasus–Pisces Arch Intermediate Velocity Cloud

Abstract We present hydrodynamic simulations of the Pegasus–Pisces Arch (PP Arch), an intermediate velocity cloud in our Galaxy. The PP Arch, also known as IVC 86-36, is unique among intermediate and high velocity clouds, because its twin tails are unusually long and narrow. Its −50 km s−1 line-of-sight velocity qualifies it as an intermediate velocity cloud, but the tails’ orientations indicate that the cloud’s total three-dimensional speed is at least ∼100 km s−1. This speed is supersonic in the Reynold’s Layer and thick disk. We simulated the cloud as it travels supersonically through the Galactic thick and thin disks at an oblique angle relative to the midplane. Our simulated clouds grow long double tails and reasonably reproduce the H I 21 cm intensity and velocity of the head of the PP Arch. A bow shock protects each simulated cloud from excessive shear and lowers its Reynolds number. These factors may similarly protect the PP Arch and enable the survival of its unusually long tails. The simulations predict the future hydrodynamic behavior of the cloud when it collides with denser gas nearer to the Galactic midplane. It appears that the PP Arch’s fate is to deform, dissipate, and merge with the Galactic disk.