Galactic Winds Research Articles

Any way the wind blows: quantifying superbubbles and their outflows in simulated galaxies across z ≈ 0 − 3

Abstract We present an investigation of clustered stellar feedback in the form of superbubbles identified within eleven galaxies from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, at both cosmic noon (1 < z < 3) and in the local Universe. We study the spatially-resolved multiphase outflows that these supernovae drive, comparing our findings with recent theory and observations. These simulations consist of five LMC-mass galaxies and six Milky Way-mass progenitors (with a minimum baryonic particle mass of mb.min = 7, 100M⊙). For all galaxies, we calculate the local and galaxy-averaged mass and energy loading factors from the identified outflows. We also characterize the multiphase morphology and properties of the identified superbubbles, including the ‘shell’ of cool (T < 105 K) gas and break out of energetic hot (T > 105 K) gas when the shell bursts. We find that these simulations, regardless of redshift, have mass-loading factors and momentum fluxes in the cool gas that largely agree with recent observations. Lastly, we also investigate how methodological choices in measuring outflows can affect loading factors for galactic winds.

Gas-phase metallicity gradients in galaxies at z ∼ 6–8

The study of gas-phase metallicity and its spatial distribution at high redshift is crucial to understand the processes that shaped the growth and evolution of galaxies in the early Universe. Here we study the spatially resolved metallicity in three systems at z ∼ 6 − 8, namely A2744-YD4, BDF-3299, and COSMOS24108, with JWST NIRSpec IFU low-resolution (R ∼ 100) spectroscopic observations. These are among the highest-z sources in which metallicity gradients have been probed so far. Each of these systems hosts several spatial components in the process of merging within a few kiloparsecs, identified from the rest-frame UV and optical stellar continuum and ionised gas emission line maps. The sources have heterogeneous properties, with stellar masses log(M*/M⊙) ∼7.6–9.3, star formation rates (SFRs) ∼1–15 M⊙ yr−1, and gas-phase metallicities 12+log(O/H) ∼7.7–8.3, which exhibit a large scatter within each system. Their properties are generally consistent with those of the highest-redshift samples to date (z ∼ 3 − 10), though the sources in A2744-YD4 and COSMOS24108 are at the high end of the mass-metallicity relation (MZR) defined by the z ∼ 3 − 10 sources. Moreover, the targets in this work follow the predicted slope of the MZR at z ∼ 6 − 8 from most cosmological simulations. The gas-phase metallicity gradients are consistent with being flat in the main sources of each system. Flat metallicity gradients are thought to arise from gas mixing processes on galaxy scales, such as mergers or galactic outflows and supernova winds driven by intense stellar feedback, which wash out any gradient formed in the galaxy. The existence of flat gradients at z ∼ 6 − 8 sets also important constraints on future cosmological simulations and chemical evolution models, whose predictions on the cosmic evolution of metallicity gradients often differ significantly, especially at high redshift, but are mostly limited to z ≲ 3 so far.

CHANG-ES. XXXIII. A 20 kpc radio bubble in the halo of the star-forming galaxy NGC 4217

Cosmic rays may be dynamically very important in driving large-scale galactic winds. Edge-on galaxies give us an outsider's view of radio haloes, and of their extra-planar cosmic-ray electrons and magnetic fields. We present a new radio continuum imaging study of the nearby edge-on galaxy NGC\,4217. We examine the distribution of extra-planar cosmic rays and magnetic fields. We observed it with both the Jansky Very Large Array (JVLA) in the $S$ band (2--4\,GHz) and the LOw Frequency ARray (LOFAR) at 144\,MHz. We measured vertical intensity profiles and exponential scale heights. We re-imaged both the JVLA and LOFAR data at matched angular resolution in order to measure radio spectral indices between 144\,MHz and 3\,GHz. Confusing point-like sources were subtracted prior to imaging. We then fitted intensity profiles with cosmic-ray electron advection models, using an isothermal wind model that is driven by a combination of pressure from the hot gas and cosmic rays. We discover a large-scale radio halo on the north-western side of the galactic disc. The morphology is reminiscent of a bubble extending up to 20\,kpc from the disc. We find spectral ageing in the bubble, which allowed us to measure the advection speeds of the cosmic-ray electrons, which accelerate from 300 to $600\ $. Assuming energy equipartition between the cosmic rays and the magnetic field, we estimate the bubble may have been inflated by a modest 10\,<!PCT!> of the kinetic energy injected by supernovae over its dynamical timescale of 35\,Myr. While no active galactic nucleus (AGN) has been detected, such activity in the recent past cannot be ruled out. Non-thermal bubbles with sizes of tens of kiloparsecs may be a ubiquitous feature of star-forming galaxies, and if so this would demonstrate the influence of feedback. Determining possible contributions by AGN feedback will require deeper observations.

Hydrodynamic shielding in radiative multicloud outflows within multiphase galactic winds

Abstract Stellar-driven galactic winds regulate the mass and energy content of star-forming galaxies. Emission- and absorption-line spectroscopy shows that these outflows are multiphase and comprised of dense gas clouds embedded in much hotter winds. Explaining the presence of cold gas in such environments is a challenging endeavour that requires numerical modelling. In this paper we report a set of 3D hydrodynamical simulations of supersonic winds interacting with radiative and adiabatic multicloud systems, in which clouds are placed along a stream and separated by different distances. As a complement to previous adiabatic, subsonic studies, we demonstrate that hydrodynamic shielding is also triggered in supersonic winds and operates differently in adiabatic and radiative regimes. We find that the condensation of warm, mixed gas in between clouds facilitates hydrodynamic shielding by replenishing dense gas along the stream, provided that its cooling length is shorter than the cloud radius. Small separation distances between clouds also favour hydrodynamic shielding by reducing drag forces and the extent of the mixing region around the clouds. In contrast, large separation distances promote mixing and dense gas destruction via dynamical instabilities. The transition between shielding and no-shielding scenarios across different cloud separation distances is smooth in radiative supersonic models, as opposed to their adiabatic counterparts for which clouds need to be in close proximity. Overall, hydrodynamic shielding and re-condensation are effective mechanisms for preserving cold gas in multiphase flows for several cloud-crushing times, and thus can help understand cold gas survival in galactic winds.

Dust Survival in Galactic Winds

We present a suite of high-resolution numerical simulations to study the evolution and survival of dust in hot galactic winds. We implement a novel dust framework in the Cholla hydrodynamics code and use wind tunnel simulations of cool, dusty clouds to understand how thermal sputtering affects the dust content of galactic winds. Our simulations illustrate how various regimes of cloud evolution impact dust survival, dependent on cloud size, wind properties, and dust grain size. We find that significant amounts of dust can survive in winds in all scenarios, even without shielding from the cool phase of outflows. We present an analytic framework that explains this result, along with an analysis of the impact of cloud evolution on the total fraction of dust survival. Using these results, we estimate that 60% of 0.1 μm dust that enters a starburst-driven wind could survive to populate both the hot and cool phases of the halo, based on a simulated distribution of cloud properties. We also investigate how these conclusions depend on grain size, exploring grains from 0.1 μm to 10 Å. Under most circumstances, grains smaller than 0.01 μm cannot withstand hot-phase exposure, suggesting that the small grains observed in the circumgalactic medium (CGM) are either formed in situ due to the shattering of larger grains, or must be carried there in the cool phase of outflows. Finally, we show that the dust-to-gas ratio of clouds declines as a function of distance from the galaxy due to cloud–wind mixing and condensation. These results provide an explanation for the vast amounts of dust observed in the CGMs of galaxies and beyond.

The Scale of Stellar Yields: Implications of the Measured Mean Iron Yield of Core Collapse Supernovae

The scale of α-element yields is difficult to predict from theory because of uncertainties in massive star evolution, supernova physics, and black hole formation, and it is difficult to constrain empirically because the impact of higher yields can be compensated by greater metal loss in galactic winds. We use a recent measurement of the mean iron yield of core collapse supernovae (CCSN) by Rodriguez et al., y¯Fecc=0.058±0.007M⊙ , to infer the scale of α-element yields by assuming that the plateau of [α/Fe] abundance ratios observed in low-metallicity stars represents the yield ratio of CCSN. For a plateau at [α/Fe]cc = 0.45, we find that the population-averaged yields of O and Mg are about equal to the solar abundance of these elements, logyOcc/ZO,⊙=logyMgcc/ZMg,⊙=−0.01±0.1 , where yXcc is the mass of element X produced by massive stars per unit mass of star formation. The inferred O and Fe yields agree with predictions of the Sukhbold et al. CCSN models assuming their Z9.6+N20 neutrino-driven engine, a scenario in which many progenitors with M < 40M ⊙ implode to black holes rather than exploding. The yields are lower than assumed in many models of the galaxy mass–metallicity relation, reducing the level of outflows needed to match observed abundances. Our one-zone chemical evolution models with η=Ṁout/Ṁ*≈0.6 evolve to solar metallicity at late times. By further requiring that models reach [α/Fe] ≈ 0 at late times, we infer a Hubble-time integrated Type Ia supernova rate of 1.1×10−3M⊙−1 , compatible with estimates from supernova surveys.

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.

Theory and Observation of Winds from Star-Forming Galaxies

Galactic winds shape the stellar, gas, and metal content of galaxies. To quantify their impact, we must understand their physics. We review potential wind-driving mechanisms and observed wind properties, with a focus on the warm ionized and hot X-ray-emitting gas. Energy and momentum injection by supernovae (SNe), cosmic rays, radiation pressure, and magnetic fields are considered in the light of observations: ▪Emission and absorption line measurements of cool/warm gas provide our best physical diagnostics of galactic outflows.▪The critical unsolved problem is how to accelerate cool gas to the high velocities observed. Although conclusive evidence for no one mechanism exists, the momentum, energy, and mass-loading budgets observed compare well with theory.▪A model in which star formation provides a force ∼L/c, where L is the bolometric luminosity, and cool gas is pushed out of the galaxy's gravitational potential, compares well with available data. The wind power is ∼0.1 of that provided by SNe.▪The very hot X-ray-emitting phase may be a (or the) prime mover. Momentum and energy exchange between the hot and cooler phases is critical to the gas dynamics.▪Gaps in our observational knowledge include the hot gas kinematics and the size and structure of the outflows probed with UV absorption lines. Simulations are needed to more fully understand mixing, cloud–radiation, cloud–cosmic ray, andcloud–hot wind interactions, the collective effects of star clusters, and both distributed andclustered SNe. Observational works should seek secondary correlations in the wind data thatprovide evidence for specific mechanisms and compare spectroscopy with the column density–velocity results from theory.

Retardation theory of eleven galaxies

Abstract The missing mass problem has been with us since the 1970s, as Newtonian gravity using baryonic mass cannot account for various observations. We investigate the viability of retardation theory, an alternative to the Dark Matter paradigm (DM) which does not seek to modify the General Principal of Relativity but to improve solutions within it by exploring its weak field approximation to solve the said problem in a galactic context. This approach have yielded satisfactory results, with respect to galactic rotation curves, the Tully-Fisher relation and missing mass derived from gravitational lensing. Recently it was able to introduce a necessary correction to the virial theorem explaining mass excess in clusters of galaxies. The current work presents eleven rotation curves calculated using Retardation Theory. The calculated rotation curves are compared with observed rotation curves. Values for the change in mass flux to mass ratio are extracted from the fitting process as a free fitting parameter. Those quantities are interpreted here and in previous works using galactic processes. Retardation Theory was able to successfully reproduce rotation curves and a preliminary correlation with star birthrate index is seen, suggesting a possible link between galactic winds and observed rotation curves. Retardation Theory shows promising results within current observations. More research is needed to elucidate the suggested mechanism and the processes which contribute to it. Galactic mass outflows carried by galactic winds may affect rotation curves.

JWST MIRI and NIRCam observations of NGC 891 and its circumgalactic medium

We present new JWST observations of the nearby, prototypical edge-on, spiral galaxy NGC 891. The northern half of the disk was observed with NIRCam in its F150W and F277W filters. Absorption is clearly visible in the mid-plane of the F150W image, along with vertical dusty plumes that closely resemble the ones seen in the optical. A $ kpc ^2$ area of the lower circumgalactic medium (CGM) was mapped with MIRI F770W at 12 pc scales. Thanks to the sensitivity and resolution of JWST, we detect dust emission out to $ 4$ kpc from the disk, in the form of filaments, arcs, and super-bubbles. Some of these filaments can be traced back to regions with recent star formation activity, suggesting that feedback-driven galactic winds play an important role in regulating baryonic cycling. The presence of dust at these altitudes raises questions about the transport mechanisms at play and suggests that small dust grains are able to survive for several tens of million years after having been ejected by galactic winds in the disk-halo interface. We lay out several scenarios that could explain this emission: dust grains may be shielded in the outer layers of cool dense clouds expelled from the galaxy disk, and/or the emission comes from the mixing layers around these cool clumps where material from the hot gas is able to cool down and mix with these cool cloudlets. This first set of data and upcoming spectroscopy will be very helpful to understand the survival of dust grains in energetic environments, and their contribution to recycling baryonic material in the mid-plane of galaxies.

GA-NIFS: NIRSpec reveals evidence for non-circular motions and AGN feedback in GN20

ABSTRACT We present rest-frame optical data of the $z\sim 4$ submillimetre galaxy GN20 obtained with the JWST Near Infrared Spectrograph (NIRSpec) in integral field spectroscopy mode. The H$\alpha$ emission is asymmetric and clumpy and extends over a projected distance of &amp;gt;15 kpc. To first order, the large-scale ionized gas kinematics are consistent with a turbulent ($\sigma \sim 90$ km s$^{-1}$), rotating disc ($v_{\rm rot}\sim 500$ km s$^{-1}$), congruent with previous studies of its molecular and ionized gas kinematics. However, we also find clear evidence for non-circular motions in the H$\alpha$ kinematics. We discuss their possible connection with various scenarios, such as external perturbations, accretion, or radial flows. In the centre of GN20, we find broad-line emission (full width at half-maximum $\sim 1000{-}2000$ km s$^{-1}$) in the H$\alpha$ + [N ii] complex, suggestive of fast, active galactic nucleus-driven winds or, alternatively, of the broad-line region of an active black hole. Elevated values of [N ii] $\lambda 6583$/H$\alpha \ \gt\ 0.4$ and of the Hα equivalent width EW(H$\alpha)\ \gt\ 6$ Å throughout large parts of GN20 suggest that feedback from the active black hole is able to photoionize the interstellar medium. Our data corroborate that GN20 offers a unique opportunity to observe key processes in the evolution of the most massive present-day galaxies acting in concert, over 12 billion years ago.

Revealing Gas Inflows Toward the Galactic Central Molecular Zone

We study the gas inflows toward the Galactic Central Molecular Zone (CMZ) based on the gas morphological and kinematic features from the Milky Way Imaging Scroll Painting in the region of l = 1.°2–19.°0 and ∣b∣ ≲ 3.°0. We find that the near dust lane appears to extend to l ∼ 15°, in which the end of the large-scale gas structure intersects with the 3 kpc ring at a distance of ∼5 kpc. Intriguingly, many filamentary molecular clouds (MCs), together with the bow-like/ballistic-like clouds and continuous CO features with notable velocity gradient, are finely outlined along the long structure. These MCs also have relatively large velocity dispersions, indicating the shocked gas generated by local continuous accretion and thus the enhanced turbulence along the entire gas structure. We suggest that the ∼3.1–3.6 kpc-long CO structure originates from the accretion molecular gas driven by the Galactic bar. The gas near the bar end at the 3 kpc ring region becomes an important reservoir for the large-scale accreting flows inward to the CMZ through the bar channel. The inclination angle of the bar is estimated to be ϕ bar = 23° ± 3°, while the pattern speed of the bar is Ωbar ≲ 32.5 ± 2.5 km s−1 kpc−1. The total mass of the whole near gas lane is about 1.3 ± 0.4 × 107 M ⊙ according to the calculated X CO ∼ 1.0 ± 0.4 × 1020 cm−2(K km s−1)−1 from the large-scale 12CO and 13CO data and the complementary H i data. We revisit the gas inflow rate as a mean value of 1.1 ± 0.3 M ⊙ yr−1, which seems to be comparable to the outflow's rate of the Galactic nuclear winds after applying the updated lower X-factor above.

When are galactic winds molecular?

ABSTRACT The molecular phase of supernova-driven outflows originates from the cold, molecular gas in the disc of a star-forming galaxy, and may carry a substantial fraction of the wind mass flux in some galaxies, but it remains poorly understood. Observations of this phase come mostly from very nearby galaxies due its low-surface brightness and covering fraction, and simulations often lack the spatial resolution necessary to resolve it. Here, we analytically estimate the survivability of this phase in order to understand under what conditions a galactic wind can contain a significant molecular phase. We show that the molecular content of outflows is primarily determined by two dimensionless numbers: a generalized Eddington ratio describing the strength of the outflow and the dissociation parameter, an ionization parameter-like quantity describing the strength of the radiation field per baryon. We apply this model to a sample of galaxies and show that, while any molecules entrained in the winds of normal star-forming galaxies should be destroyed close to the galactic disc, the outflows of strong starburst should become increasingly dominated by molecules.

Probing cold gas with Mg ii and Ly α radiative transfer

ABSTRACT The Mg ii resonance doublet at 2796 Å and 2803 Å is an increasingly important tool to study cold, $T \sim 10^{4}\,$ K, gas – an observational-driven development requiring theoretical support. We develop a new Monte Carlo radiative transfer code to systematically study the joined Mg ii and Ly α escape through homogeneous and ‘clumpy’ multiphase gas with dust in arbitrary three-dimensional geometries. Our main findings are (i) the Mg ii spectrum differs from Ly α due to the large difference in column densities, even though the atomic physics of the two lines are similar. (ii) The Mg ii escape fraction is generally higher than that of Ly α because of lower dust optical depths and path lengths – but large variations due to differences in dust models and the clumpiness of the cold medium exist. (iii) Clumpy media possess a ‘critical covering factor’ above which Mg ii radiative transfer matches a homogeneous medium. The critical covering factors for Mg ii and Ly α differ, allowing constraints on the cold gas structure. (iv) The Mg ii doublet ratio $R_{\rm MgII}$ varies for strong outflows/inflows ($\gtrsim 700 \,\mathrm{km\, s}^{-1}$), in particular, $R_{\rm MgII}\lt 1$ being an unambiguous tracer for powerful galactic winds. (v) Scattering of stellar continuum photons can decrease $R_{\rm MgII}$ from two to one, allowing constraints on the scattering medium. Notably, we introduce a novel probe of the cold gas column density – the halo doublet ratio – which we show to be a powerful indicator of ionizing photon escape. We discuss our results in the context of interpreting and modelling observations as well as their implications for other resonant doublets.

Galactic Superbubbles in 3D: Wind Formation and Cloud Shielding

Abstract Galactic superbubbles are triggered by stellar feedback in the discs of star-forming galaxies. They are important in launching galactic winds, which play a key role in regulating the mass and energy exchange in galaxies. Observations can only reveal projected information and the 3D structure of such winds is quite complex. Therefore, numerical simulations are required to further our understanding of such structures. Here, we describe hydrodynamical simulations targeting two spatial scales. Large-scale superbubble models reveal supernova-driven outflows, and their subsequent merging, which leads to galactic wind formation. Additionally, the turbulence parameter σt not only affects disc formation, but also influences mass and energy characteristics, controlling gas distribution and the injection rate in the simulated star formation zone. Small-scale wind-multicloud models indicate that isolated clouds are susceptible to instabilities, leading to fragmentation and dense gas destruction. In contrast, in closer cloud configurations, the condensation mechanism becomes important owing to hydrodynamic shielding, which helps to maintain the cold material throughout the evolution of the system. These simulations provide a comprehensive picture of galactic winds, showing how large-scale superbubble dynamics create the environment where small-scale wind-multicloud interactions shape the interstellar and circumgalactic media, ultimately regulating galaxy evolution.

Observational Constraints on Sunyaev–Zeldovich Effect Halos around High-z Quasars

We present continuum observations from the Atacama Large Millimeter/submillimeter Array of 10 high-redshift (2.2 ≤ z ≤ 2.7) ultraluminous quasars (QSOs) and constrain the presence of hot, ionized, circumgalactic gas in a stacking analysis. We measure a Compton-y parameter profile with a peak value of (1.7 ± 1.1) × 10−6 at a radius of ∼50 kpc. We compare our stacked observations to active galactic nucleus feedback wind models and generalized Navarro–Frenk–White pressure profile models to constrain the wind luminosity and halo mass of the stacked QSOs. Our observations constrain the observed average halo mass to M 500 < 1 × 1013 M ⊙ and the average feedback wind power <1 × 1012 L ⊙, which is <1% of the bolometric luminosity of the quasar.

The Role of Active Galactic Nucleus Winds in Galaxy Formation: Connecting AGN Outflows at Low Redshifts to the Formation/Evolution of Their Host Galaxies

Using Sloan Digital Sky Survey (SDSS) spectra, we applied an automatic method to search for outflows (OFs) in three large samples of narrow-line active galactic nuclei (AGN) at low redshifts (z < 0.4), separated into three spectral activity classes: radio-loud galaxies (RGs), 15,793; radio-quiet Seyfert 2 AGN (Sy2), 18,585; and LINERs, 25,656. In general, the probability of detecting an OF decreases along the sequence Sy1→Sy2→LINER/RG and independently of the AGN class, the wind velocity, traced by W80, increases with the AGN luminosity. Moreover W80 is systematically higher in RGs or any of the other AGN classes when detected in radio. These results support the idea that there are two main modes of production of OF, the radiative mode dominant in radio-quiet AGN and the jet mode dominant in RGs, although both modes could also happen simultaneously at different levels. From the spectra and SDSS photometry, the characteristics of the AGN host galaxies and their supermassive black holes (SMBHs) were also retrieved using the stellar population synthesis code STARLIGHT. This revealed that, independently of the AGN spectral class, (1) galaxy hosts with OFs have systematically later morphological types and higher star formation rates (SFRs) than their counterparts without OF, (2) the AGN occupy different positions in the specific diagnostic diagram (specific black hole accretion rate (sBHAR) versus specific SFR), which suggests they follow different evolutionary paths congruent with the morphology of their galaxy hosts, and (3) they show no evidence of AGN quenching or triggering of star formation. These results are consistent with a scenario explaining the different AGN classes as consequences of different formation processes of galaxies: early-type galaxies (LINERs and RGs) formed bigger bulges and more massive SMBHs, exhausting their reservoir of gas more rapidly than late-type galaxies (Sy2 and Sy1), and thereby quenching their star formation and starving their SMBHs.

Finding Candidate TeV Halos among Very-high-energy Sources

We search for possible pulsar TeV halos among the very-high-energy (VHE) sources reported in different VHE surveys, among which in particular we use the results from the first Large High Altitude Air Shower Observatory catalog of γ-ray sources. Six candidates are found. They share similar properties of containing a middle-aged, γ-ray–bright pulsar in their positional error circles (the respective pulsars are J0248+6021, J0359+5414, J0622+3749, J0633+0632, J2006+3102, and J2238+5903), being in a rather clean field without any common Galactic VHE-emitting supernova remnants or (bright) pulsar wind nebulae (PWNe), and showing an absence of any γ-ray emissions in 0.1–500 GeV after removing the pulsars’ emissions. Combining these candidates with several reported (candidate) TeV halos, we obtain relationships between their luminosity at 50 TeV (L 50TeV) and the corresponding pulsars’ spin-down energy ( Ė ), which are L50TeV∼Ė0.9 and L50TeV/Ė∼6.4×10−4 , respectively. The relationships are nearly identical to previously reported ones. We probe possible connections between the extent/sizes of the VHE sources and the pulsars’ ages, and find a weak older-and-smaller trend. By comparing to the VHE detection results for PWNe, it is clear that the (candidate) TeV halos have hard emissions by either having power-law indices smaller than 2 in 1–25 TeV or by only being detected in 25–100 TeV. In addition, we also consider seven other VHE sources as possible TeV halos based on the results from different studies of them, but they do not fit cleanly with the properties listed above, indicating their potentially complex nature.

Highly-mass-loaded hot galactic winds are unstable to cool filament formation

Abstract As cool clouds are entrained by a hot supersonic galactic wind, they may be shredded by hydrodynamical instabilities and incorporated into the hot flow. One-dimensional steady-state calculations show how cool cloud entrainment affects the bulk thermodynamics and kinematics of the hot gas: mass-loading decelerates the hot flow and changes its entropy. Here, we investigate the stability of mass-loaded hot winds using both perturbation analysis and 3D time-dependent radiative hydrodynamical simulations. We show that mass-loading is stable over a broad range of parameters and that the 1D time-steady analytic solutions exactly reproduce the 3D time-dependent calculations, provided that the flow does not decelerate sufficiently to become subsonic. For higher values of the mass-loading, the flow develops a second sonic point, with the first being at the edge of the wind-driving region. Strong deceleration increases the wind density and the flow becomes radiative, undergoing a thermal instability to form elongated dense cometary filaments. We explore the mass-loading parameters required to trigger this behavior. For certain approximations, we can derive analytic criteria. In general a mass-loading rate similar to the initial hot mass outflow rate is required. In this sense, the destruction of small cool clouds by a hot flow may ultimately spontaneously generate fast cool filaments, as observed in starburst winds. Lastly, we find that the kinematics of filaments is sensitive to the slope of the mass-loading function. Filaments move faster than the surrounding wind if mass-loading is over long distances whereas filaments move slower than their surroundings if mass-loading is abrupt.

SAGAbg. I. A Near-unity Mass-loading Factor in Low-mass Galaxies via Their Low-redshift Evolution in Stellar Mass, Oxygen Abundance, and Star Formation Rate

Measuring the relation between star formation and galactic winds is observationally difficult. In this work we make an indirect measurement of the mass-loading factor (the ratio between the mass outflow rate and star formation rate) in low-mass galaxies using a differential approach to modeling the low-redshift evolution of the star-forming main sequence and mass–metallicity relation. We use Satellites Around Galactic Analogs (SAGA) background galaxies, i.e., spectra observed by the SAGA Survey that are not associated with the main SAGA host galaxies, to construct a sample of 11,925 spectroscopically confirmed low-mass galaxies from 0.01 ≲ z ≤ 0.21 and measure auroral line metallicities for 120 galaxies. The crux of the method is to use the lowest-redshift galaxies as the boundary condition of our model, and to infer a mass-loading factor for the sample by comparing the expected evolution of the low-redshift reference sample in stellar mass, gas-phase metallicity, and star formation rate against the observed properties of the sample at higher redshift. We infer a mass-loading factor of ηm=0.92−0.74+1.76 , which is in line with direct measurements of the mass-loading factor from the literature despite the drastically different sets of assumptions needed for each approach. While our estimate of the mass-loading factor is in good agreement with recent galaxy simulations that focus on resolving the dynamics of the interstellar medium, it is smaller by over an order of magnitude than the mass-loading factor produced by many contemporary cosmological simulations.