Gas Filaments Research Articles

Analytical models of rotating magnetized gas filaments

ABSTRACT Several rotating molecular gas filaments with striking helical morphology have been observed in the Central Molecular Zone, including the Double Helix Nebula (DHN), the Galactic Centre Tornado (GCT), and the Pigtail Nebula (PTN). We present a model for isothermal, rotating, magnetized, pressure-truncated, non-self-gravitating filaments, as a model for these objects. We find a novel analytical stationary solution for these molecular tornadoes by assuming a propagating torsional Alfvén wave resulting in balance between toroidal magnetic stress and centrifugal force, as well as constant flux to mass ratio for the poloidal magnetic field. Our model allows for differential rotation and the density profile for the stationary solution goes asymptotically as r−1 at large radius for a special case where the flux to mass ratios for the poloidal and toroidal fields are constant inside the filament. We subject our stationary solutions to a virial analysis and show that they satisfy the virial equation for cylindrical symmetry $2\mathcal {K}+\mathcal {M}=0$. Finally, we estimate the ratio of gravitational potential energy per unit length, $\mathcal {W}$, to the bulk kinetic energy per unit length, $\mathcal {K}_0$, and calculate $|\mathcal {W}|/\mathcal {K}_0$ for the DHN, GCT, and PTN. Our estimates suggest that self-gravity is negligible in the DHN and the PTN and GCT are at most weakly self-gravitating. We also briefly address an application of our model to filamentary plasma lenses in the interstellar medium.

Streams of cold cosmic fuel for galaxies.

An expansive filament of cold gas may connect galaxies and spur star formation.

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.

Stellar cluster formation in a Milky Way-sized galaxy at z > 4 – I. The proto-globular cluster population and the imposter amongst us

ABSTRACT The formation history of globular clusters (GCs) at redshift z > 4 remains an unsolved problem. In this work, we use the cosmological, N-body hydrodynamical ‘zoom-in’ simulation GigaEris to study the properties and formation environment of proto-GC candidates in the region surrounding the progenitor of a Milky Way-sized galaxy. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion. We define proto-GC candidate systems as gravitationally bound stellar systems with baryonic mass fraction Fb ≥ 0.75 and stellar velocity dispersion σ⋆ < 20 km s−1. At z = 4.4, we identify nine systems that satisfy our criteria, all of which form between 10 and 30 kpc from the centre of the main host. Their baryonic masses are in the range 105–107 M⊙. By the end of the simulation, they still have a relatively low stellar mass (M⋆ ∼ 104–105 M⊙) and a metallicity (−1.8 ≲ [Fe/H] ≲ −0.8) similar to the blue Galactic GCs. All of the identified systems except one appear to be associated with gas filaments accreting onto the main galaxy in the circum-galactic region and formed at z = 5–4. The exception is the oldest object, which appears to be a stripped compact dwarf galaxy that has interacted with the main halo between z = 5.8 and z = 5.2 and has lost its entire dark matter content due to tidal mass loss.

The complex interplay of AGN jet-inflated bubbles and the intracluster medium

ABSTRACTWe use SWIFT, a smoothed particle hydrodynamics code, to simulate the evolution of bubbles inflated by active galactic nuclei (AGNs) jets, as well as their interactions with the ambient intracluster medium (ICM). These jets inflate lobes that turn into bubbles after the jets are turned off (at t = 50 Myr). Almost all of the energy injected into the jets is transferred to the ICM very quickly after they are turned off, with roughly 70 per cent of it in thermal form and the rest in kinetic. At late times (t > 500 Myr) we find the following: (1) the bubbles draw out trailing filaments of low-entropy gas, similar to those recently observed, (2) the action of buoyancy and the uplift of the filaments dominates the energetics of both the bubbles and the ICM, and (3) almost all of the originally injected energy is in the form of gravitational potential energy, with the bubbles containing 15 per cent of it, and the rest contained in the ICM. These findings indicate that feedback proceeds mainly through the displacement of gas to larger radii. We find that the uplift of these filaments permanently changes the thermodynamic properties of the ICM by reducing the central density and increasing the central temperature (within 30 kpc). We propose that jet feedback proceeds not only through the heating of the ICM (which can delay cooling), but also through the uplift-related reduction of the central gas density. The latter also delays cooling, on top of reducing the amount of gas available to cool.

ATOMS: ALMA three-millimeter observations of massive star-forming regions – XIII. Ongoing triggered star formation within clump-fed scenario found in the massive (∼1500 M⨀) clump

ABSTRACT Whether ionization feedback triggers the formation of massive stars is highly debated. Using ALMA 3-mm observations with a spatial resolution of ∼0.05 pc and a mass sensitivity of 1.1 $\rm M_\odot$ per beam at 20 K, we investigate the star formation and gas flow structures within the ionizing feedback-driven structure, a clump-scale massive (≳ 1500 $\rm M_\odot$) bright-rimmed cloud (BRC) associated with IRAS 18290–0924. This BRC is bound only if external compression from ionized gas is considered. A small-scale (≲ 1 pc) age sequence along the direction of ionizing radiation is revealed for the embedded cores and protostars, which suggests triggered star formation via radiation-driven implosion (RDI). Furthermore, filamentary gas structures converge towards the cores located in the BRC’s centre, indicating that these filaments are fueling mass towards cores. The local core-scale mass infall rate derived from H13CO+ J = 1 − 0 blue profile is of the same order of magnitude as the filamentary mass inflow rate, approximately 1 $\rm M_\odot$ kyr−1. A photodissociation region (PDR) covering the irradiated clump surface is detected in several molecules, such as CCH, HCO+, and CS whereas the spatial distribution stratification of these molecules is indistinct. CCH spectra of the PDR possibly indicate a photoevaporation flow leaving the clump surface with a projected velocity of ∼2 km s−1. Our new observations show that RDI accompanied by a clump-fed process is operating in this massive BRC. Whether this combined process works in other massive BRCs is worth exploring with dedicated surveys.

Dissecting the active galactic nucleus in Circinus – IV. MUSE-NFM observations unveil a tuning-fork ionized outflow morphology

ABSTRACT We present the ionized gas outflow morphology in the Circinus galaxy using the narrow-field mode (NFM) of the Multi Unit Spectroscopic Explorer (MUSE) instrument onboard the Very Large Telescope (VLT). The NFM observations provide a spatial resolution of ∼0.1 arcsec, corresponding to a physical scale of ∼2 pc, one of the highest spatial resolution achievable using ground-based adaptive optics-assisted observations in the optical wavelengths. The MUSE observations reveal a collimated clumpy outflow profile originating near the active galactic nucleus (AGN) location and extending up to 1.5 arcsec (∼30 pc) in the north-west direction. The collimated structure then fragments into two filaments, giving the entire outflowing gas a ‘tuning-fork’ morphology. These structures remain undetected in the lower spatial resolution MUSE wide-field mode data. We explain the origin of this tuning-fork structure to the interaction of the outflow with a dense clump in the interstellar medium (ISM) as the outflow propagates outwards. The origin of the collimated structure itself could be from jet–ISM interactions on small scales. These observations also provide evidence to the origin of the ionized gas filaments previously observed in the Circinus galaxy out to kiloparsec scales. We find instantaneous and time-averaged mass outflow rates of 10−2 and 10−4 M⊙ yr−1, respectively. Based on the star formation rate in the Circinus galaxy reported in the literature, the observed ionized outflows are not expected to regulate star formation within the ∼100 pc scales probed by the NFM data.

Measuring cosmic filament spin with the kinetic Sunyaev–Zel’dovich effect

ABSTRACT The spin of intergalactic filaments has been predicted from simulations, and supported by tentative evidence from redshift-space filament shapes in a galaxy redshift survey: generally, a filament is redshifted on one side of its axis, and blueshifted on the other. Here, we investigate whether filament spins could have a measurable kinetic Sunyaev–Zel’dovich (kSZ) signal, from cosmic microwave background (CMB) photons being scattered by moving ionized gas; this pure velocity information is complementary to filament redshift-space shapes. We propose to measure the kSZ dipole by combining galaxy redshift surveys with CMB experiments. We base our signal-to-noise ratio analyses first on an existing filament catalogue, and its combination with Planck data. We then investigate the detectability of the kSZ dipole using the combination of DESI or SKA-2 with next-stage CMB experiments. We find that the gas haloes of filament galaxies co-rotating with filaments induce a stronger kSZ dipole signal than that from the diffuse filamentary gas, but both signals seem too small to be detected in near-term surveys such as DESI+future CMB experiments. But the combination of SKA-2 with future CMB experiments could give a more than 10σ detection. The gain comes mainly from an increased area overlap and an increased number of filaments, but also the low noise and high resolution in future CMB experiments are important to capture signals from filaments small on the sky. Successful detection of the signals may help to find the gravitomagnetic effect in large-scale structure and advance our understanding of baryons in the cosmic web.

Directly Tracing Cool Filamentary Accretion over >100 kpc into the Interstellar Medium of a Quasar Host at z = 1

We report the discovery of giant (50−100 kpc) [O ii] emitting nebulae with MUSE in the field of TXS 0206−048, a luminous quasar at z = 1.13. “Down-the-barrel” UV spectra of the quasar show absorption at velocities coincident with those of the extended nebulae, enabling new insights into inflows and outflows around the quasar host. One nebula exhibits a filamentary morphology extending over 120 kpc from the halo toward the quasar and intersecting with another nebula surrounding the quasar host with a radius of 50 kpc. This is the longest cool filament observed to date and arises at higher redshift and in a less massive system than those in cool-core clusters. The filamentary nebula has line-of-sight velocities >300 km s−1 from nearby galaxies but matches that of the nebula surrounding the quasar host where they intersect, consistent with accretion of cool intergalactic or circumgalactic medium or cooling hot halo gas. The kinematics of the nebulae surrounding the quasar host are unusual and complex, with redshifted and blueshifted spiral-like structures. The emission velocities at 5−10 kpc from the quasar match those of inflowing absorbing gas observed in UV spectra of the quasar. Together, the extended nebulae and associated redshifted absorption represent a compelling case of cool, filamentary gas accretion from halo scales into the extended interstellar medium and toward the nucleus of a massive quasar host. The inflow rate implied by the combined emission and absorption constraints is well below levels required to sustain the quasar’s radiative luminosity, suggesting anisotropic or variable accretion.

Testing the Limits of AGN Feedback and the Onset of Thermal Instability in the Most Rapidly Star-forming Brightest Cluster Galaxies

We present new, deep, narrow- and broadband Hubble Space Telescope observations of seven of the most star-forming brightest cluster galaxies (BCGs). Continuum-subtracted [OII] maps reveal the detailed, complex structure of warm (T ∼ 104 K) ionized gas filaments in these BCGs, allowing us to measure spatially resolved star formation rates (SFRs) of ∼60–600 M ⊙yr−1. We compare the SFRs in these systems and others from the literature to their intracluster medium cooling rates (), measured from archival Chandra X-ray data, finding a best-fit relation of + (−3.22 ± 0.38) with an intrinsic scatter of 0.39 ± 0.09 dex. This steeper-than-unity slope implies an increasingly efficient conversion of hot (T ∼ 107 K) gas into young stars with increasing , or conversely a gradual decrease in the effectiveness of AGN feedback in the strongest cool cores. We also seek to understand the physical extent of these multiphase filaments that we observe in cluster cores. We show, for the first time, that the average extent of the multiphase gas is always smaller than the radii at which the cooling time reaches 1 Gyr, the t cool/t ff profile flattens, and that X-ray cavities are observed. This implies a close connection between the multiphase filaments, the thermodynamics of the cooling core, and the dynamics of X-ray bubbles. Interestingly, we find a one-to-one correlation between the average extent of cool multiphase filaments and the radius at which the cooling time reaches 0.5 Gyr, which may be indicative of a universal condensation timescale in cluster cores.

The present-day gas content of simulated field dwarf galaxies

ABSTRACT We examine the gas content of field dwarf galaxies in a high-resolution cosmological simulation. In agreement with previous work, we find that galaxies inhabiting dark matter haloes with mass below a critical value, M200 ≲ Mcrit ≈ 5 × 109 M⊙, are quiescent at the present day. The gas content of these galaxies is thus insensitive to feedback from evolving stars. Almost half of these quiescent systems today have gas masses much smaller than that expected for their mass. We find that gas-deficient galaxies originate from (1) past interactions with massive hosts, in which a dwarf loses gas and dark matter via tidal and ram-pressure forces; and (2) from hydrodynamic interactions with the gaseous filaments and sheets of the cosmic web, in which a dwarf loses gas via ram pressure. We refer to these systems as ‘flybys’ and ‘COSWEBs’. Flybys locate in high-density regions, tracing the location of the most massive galaxies in the simulation. In contrast, COSWEBs are dispersed throughout the volume and trace the cosmic web. For sub-critical systems, M200 < Mcrit, the fraction of COSWEB galaxies can be as high as $35 {{\ \rm per\ cent}}$, and much higher for flybys, which make up 100 per cent of the galaxies with $M_{200}\lt 3\times 10^8 \ \rm M_{\odot }$. The deficit of gas caused by these mechanisms may preclude the detection of a large fraction of field dwarfs in future H i surveys. For galaxies inhabiting haloes with mass M200 > Mcrit, we find that cosmic web stripping, on average, shuts down star formation in more than $70{{\ \rm per\ cent}}$ of the affected systems.

Magnetic field evolution in cosmic filaments with LOFAR data

ABSTRACT Measuring the magnetic field in cosmic filaments reveals how the Universe is magnetized and the process that magnetized it. Using the Rotation Measures (RM) at 144 MHz from the LoTSS DR2 data, we analyse the rms of the RM extragalactic component as a function of redshift to investigate the evolution with redshift of the magnetic field in filaments. From previous results, we find that the extragalactic term of the RM rms at 144 MHz is dominated by the contribution from filaments (more than 90 per cent). Including an error term to account for the minor contribution local to the sources, we fit the data with a model of the physical filament magnetic field, evolving as $B_f = B_{f,0}\, (1+z)^\alpha$ and with a density drawn from cosmological simulations of five magnetogenesis scenarios. We find that the best-fitting slope is in the range α = [ − 0.2, 0.1] with uncertainty of σα = 0.4–0.5, which is consistent with no evolution. The comoving field decreases with redshift with a slope of γ = α − 2 = [ − 2.2, −1.9]. The mean field strength at z = 0 is in the range Bf, 0 = 39–84 nG. For a typical filament gas overdensity of δg = 10 the filament field strength at z = 0 is in the range $B_{f,0}^{10}=8$–26 nG. A primordial stochastic magnetic field model with initial comoving field of BMpc = 0.04–0.11 nG is favoured. The primordial uniform field model is rejected.

Self-regulated AGN feedback of light jets in cool-core galaxy clusters

ABSTRACT Heating from active galactic nuclei (AGNs) is thought to stabilize cool-core clusters, limiting star formation and cooling flows. We employ radiative magnetohydrodynamic (MHD) simulations to model light AGN jet feedback with different accretion modes (Bondi–Hoyle–Lyttleton and cold accretion) in an idealized Perseus-like cluster. Independent of the probed accretion model, accretion efficiency, jet density and resolution, the cluster self-regulates with central entropies and cooling times consistent with observed cool-core clusters in this non-cosmological setting. We find that increased jet efficiencies lead to more intermittent jet powers and enhanced star formation rates. Our fiducial low-density jets can easily be deflected by orbiting cold gaseous filaments, which redistributes angular momentum and leads to more extended cold gas distributions and isotropic bubble distributions. In comparison to our fiducial low momentum-density jets, high momentum-density jets heat less efficiently and enable the formation of a persistent cold gas disc perpendicular to the jets that is centrally confined. Cavity luminosities measured from our simulations generally reflect the cooling luminosities of the intracluster medium and correspond to averaged jet powers that are relatively insensitive to short periods of low-luminosity jet injection. Cold gas structures in our MHD simulations with low momentum-density jets generally show a variety of morphologies ranging from discy to very extended filamentary structures. In particular, magnetic fields are crucial to inhibit the formation of unrealistically massive cold gas discs by redistributing angular momentum between the hot and cold phases and by fostering the formation of elongated cold filaments that are supported by magnetic pressure.

Deep near-infrared survey towards the W40 and Serpens South region in the Aquila Rift: A comprehensive catalogue of young stellar objects

ABSTRACT Active star-forming regions are excellent laboratories for studying the origins and evolution of young stellar object (YSO) clustering. The W40–Serpens South region is such a region, and we compile a large near- and mid-infrared catalogue of point sources in it, based on deep near-infrared observations of Canada-France-Hawaii Telescope (CFHT) in combination with Two Micron All Sky Survey (2MASS), UKIRT Infrared Deep Sky Survey (UKIDSS), and Spitzer catalogues. From this catalogue, we identify 832 YSOs, and classify 15, 135, 647, and 35 of them to be deeply embedded sources, Class I YSOs, Class II YSOs, and transition disc sources, respectively. In general, these YSOs are well correlated with the filamentary structures of molecular clouds, especially the deeply embedded sources and the Class I YSOs. The W40 central region is dominated by Class II YSOs, but in the Serpens South region, half of the YSOs are Class I. We further generate a minimum spanning tree (MST) for all the YSOs. Around the W40 cluster, there are eight prominent MST branches that may trace the vestigial molecular gas filaments that once fed gas to the central natal gas clump. Of the eight, only two now include detectable filamentary gas in Herschel data and corresponding Class I YSOs, while the other six are populated exclusively with Class II YSOs. Four MST branches overlap with the Serpens South main filament, and where they intersect, molecular gas ‘hubs’ and more Class I YSOs are found. Our results imply a mixture of YSO distributions composed of both primordial and somewhat evolved YSOs in this star-forming region.

Study on Townsend first ionization coefficient in a streamer filament of the pulsed electric discharge in water

Streamers in water usually consist of several weakly ionized gaseous filaments. It is important to understand the ionization and excitation processes occurring in a streamer filament. In this paper, we studied the Townsend first ionization coefficient α in a streamer filament. The emission images of streamers in water were acquired by using an ultra-high speed camera system. Based on the emission images, it was found that the luminance of a streamer filament decreased exponentially as the distance increased from the anode tip. The Townsend first ionization coefficient in the streamer filament in water was then estimated by the emission profile method. It was found that the Townsend first ionization coefficient in a streamer filament was 5.3 cm−1 on average and did not depend on the water conductivity or the applied voltage of interest in the present paper. The Townsend first ionization coefficient estimated by the emission profile method is in agreement with that deduced from the electron density profile of the streamer filament in water, indicating that the emission profile method for measuring the Townsend first ionization coefficient can be applicable to the streamers in water. We also estimated the reduced Townsend first ionization coefficient α/N in a streamer filament in water and found that the obtained value for α/N was four orders of magnitude lower than those obtained from the glow discharge in low pressure water vapor, but at least ten orders of magnitude greater than those predicted by the empirical formulas.

Filamentary structures of ionized gas in Cygnus X

Context.Ionized gas probes the influence of massive stars on their environment. The Cygnus X region (d~ 1.5 kpc) is one of the most massive star-forming complexes in our Galaxy, within which the Cyg OB2 association (age of 3–5 Myr and stellar mass 2 × 104M⊙) has a dominant influence.Aims.We observe the Cygnus X region at 148 MHz using the Low Frequency Array (LOFAR) and take short-spacing information into account during image deconvolution into account. Together with data from the Canadian Galactic Plane Survey, we investigate the morphology, distribution, and physical conditions of low-density ionized gas in a 4° × 4° (~100 pc × 100 pc) region at a resolution of 2′ (0.9 pc).Methods.The Galactic radio emission in the region analyzed is almost entirely thermal (free-free) at 148 MHz, with emission measures (EM) of 103< EM [pc cm−6] < 106. As filamentary structure is a prominent feature of the emission, we use DisPerSE and Fil ChaP to identify filamentary ridges and characterize their radial (EM) profiles.Results.The distribution of radial profiles has a characteristic width of 4.3 pc and a power-law distribution (β= −1.8 ± 0.1) in peak EM down to our completeness limit of 4200 pc cm−6. The electron densities of the filamentary structure range between 10 ≲ne[cm−3] ≲ 400 with a median value of 35 cm−3, remarkably similar to [N II] surveys of ionized gas.Conclusions.Cyg OB2 may ionize at most two-thirds of the total ionized gas and the ionized gas in filaments. More than half of the filamentary structures are likely photoevaporating surfaces flowing into a surrounding diffuse (~5 cm−3) medium. However, this is likely not the case for all ionized gas ridges. A characteristic width in the distribution of ionized gas indicates that the stellar winds of Cyg OB2 create a fraction of the ionized filaments through swept-up ionized gas or dissipated turbulence.

Discovery of a Premerger Shock in an Intercluster Filament in Abell 98

We report the first unambiguous detection of an axial merger shock in the early-stage merging cluster Abell 98 using deep (227 ks) Chandra observations. The shock is about 420 kpc south from the northern subcluster of Abell 98, in between the northern and central subclusters, with a Mach number of ≈ 2.3 ± 0.3. Our discovery of the axial merger shock front unveils a critical epoch in the formation of a massive galaxy cluster, when two subclusters are caught in the early phase of the merging process. We find that the electron temperature in the postshock region favors the instant collisionless model, where electrons are strongly heated at the shock front, by interactions with the magnetic field. We also report on the detection of an intercluster gas filament, with a temperature of kT = 1.07 ± 0.29 keV, along the merger axis of Abell 98. The measured properties of the gas in the filament are consistent with previous observations and numerical simulations of the hottest, densest parts of the warm–hot intergalactic medium (WHIM), where WHIM filaments interface with the virialization regions of galaxy clusters.

The eROSITA view of the Abell 3391/95 field: Case study from the Magneticum cosmological simulation

Context.Clusters of galaxies reside at the nodes of the cosmic web, interconnected by filamentary structures that contain tenuous diffuse gas, especially in the warm-hot phase. Galaxy clusters grow by mergers of smaller objects and gas that are mainly accreted through these large-scale filaments. For the first time, the large-scale cosmic structure and a long gas-emission filament have been captured by eROSITA on board the Spectrum-Roentgen-Gamma mission in a direct X-ray observation of the A3391/95 field.Aims.We investigate the assembly history of an A3391/95-like system of clusters and the thermo-chemical properties of the diffuse gas in it by connecting simulation predictions to the eROSITA observations with the aim to constrain the origin and nature of the gas in the pair-interconnecting bridge.Methods.We analysed the properties of a system resembling A3391/95, extracted from the (352h−1cMpc)3volume of the Magneticum Pathfinder cosmological simulations atz= 0.07. We tracked the main progenitors of the pair clusters and of surrounding groups back in time to study the assembly history of the system and its evolution.Results.Similarly to the observed A3391/95 system, the simulated cluster pair is embedded in a complex network of gas filaments, with structures aligned over more than 20 projected Mpc, and the whole region collapses towards the central overdense node. The spheres of influence (3 ×R200) of the two main clusters already overlap atz= 0.07, but their virial boundaries are still physically separated. The diffuse gas located in the interconnecting bridge closely reflects the warm-hot intergalactic medium, with a typical temperature of ~1 keV and an overdensityδ ~100 with respect to the mean baryon density of the Universe, and a lower enrichment level compared to the intra-cluster medium in clusters. We find that most of the bridge gas collapsed from directions roughly orthogonal to the intra-cluster gas accretion directions, and its origin is mostly unrelated to the two cluster progenitors. We find clear signatures in the surrounding groups of infall motion towards the pair, such as significant radial velocities and a slowdown of gas compared to dark matter. These findings further support the hypothesis that the Northern Clump (MCXC J0621.7-5242) cluster infalls along a cosmic gas filament towards Abell 3391 and might be merging with it.Conclusions.We conclude that in this configuration, the pair clusters of the A3391/95-like system are in a pre-merger phase and have not yet interacted. The diffuse gas in the interconnecting bridge is mostly warm filament gas and not tidally stripped cluster gas.

The eROSITA view of the Abell 3391/95 field: The Northern Clump

Context.Galaxy clusters grow through mergers and the accretion of substructures along large-scale filaments. Many of the missing baryons in the local Universe may reside in such filaments as the warm-hot intergalactic medium (WHIM).Aims.SRG/eROSITA performance verification observations revealed that the binary cluster Abell 3391/3395 and the Northern Clump (the MCXC J0621.7–5242 galaxy cluster) are aligning along a cosmic filament in soft X-rays, similarly to what has been seen in simulations before. We aim to understand the dynamical state of the Northern Clump as it enters the atmosphere (3 ×R200) of Abell 3391.Methods.We analyzed joint eROSITA,XMM-Newton, andChandraobservations to probe the morphological, thermal, and chemical properties of the Northern Clump from its center out to a radius of 988 kpc (R200). We utilized the ASKAP/EMU radio data, the DECam optical image, and thePlanck y-map to study the influence of the wide-angle tail (WAT) radio source on the Northern Clump’s central intracluster medium. Using eROSITA data, we also analyzed the gas properties of the Northern Filament, the region between the virial radii of the Northern Clump and the A3391 cluster. From the Magneticum simulation, we identified an analog of the A3391/95 system along with an infalling group resembling the Northern Clump.Results.The Northern Clump is a weak cool-core cluster centered on a WAT radio galaxy. The gas temperature over 0.2–0.5R500iskBT500=1.99 ± 0.04 keV. We employed the mass-temperature(M – T) scaling relation and obtained a mass estimate ofM500= (7.68 ± 0.43) × 1013M⊙andR500= (63 6 ± 12) kpc. Its X-ray atmosphere has a boxy shape and deviates from spherical symmetry. We identify a southern surface brightness edge, likely caused by subsonic motion relative to the filament gas in the southern direction. At ~R500, the southern atmosphere (infalling head) appears to be 42% hotter than its northern atmosphere. We detect a downstream tail pointing toward the north with a projected length of ~318 kpc, plausibly the result of ram pressure stripping. Through a two-temperature fit, we identify a cooler component in the Northern Filament withkBT= 0.68- 0.64+ 0.38keV andne= 1.99-1.24+0.88× 10-5cm-3, which are consistent within the expected ranges of WHIM properties. The analog group in the Magneticum simulation is experiencing changes in its gas properties and a shift between the position of the halo center and that of the bound gas, while approaching the main cluster pair.Conclusions.The Northern Clump is a dynamically active system and far from being relaxed. Its atmosphere is affected by an interaction with the WAT and by gas sloshing or its infall toward Abell 3391 along the filament, consistent with the analog group-size halo in the Magneticum simulation.

Signature of Supersonic Turbulence in Galaxy Clusters Revealed by AGN-driven Hα Filaments

The hot intracluster medium (ICM) is thought to be quiescent with low observed velocity dispersions. Surface brightness fluctuations of the ICM also suggest that its turbulence is subsonic with a Kolmogorov scaling relation, indicating that the viscosity is suppressed and the kinetic energy cascades to small scales unscathed. However, recent observations of the cold gas filaments in galaxy clusters find that the scaling relations are steeper than that of the hot plasma, signaling kinetic energy losses and the presence of supersonic flows. In this work we use high-resolution simulations to explore the turbulent velocity structure of the cold filaments at the cores of galaxy clusters. Our results indicate that supersonic turbulent structures can be “frozen” in the cold gas that cools and fragments out of a fast, ∼107 K outflow driven by the central active galactic nucleus (AGN), when the radiative cooling time is shorter than the dynamical sound-crossing time. After the cold gas formation, however, the slope of the velocity structure function (VSF) flattens significantly over short, ∼10 Myr timescales. The lack of flattened VSF in observations of Hα filaments indicates that the Hα-emitting phase is short-lived for the cold gas in galaxy clusters. On the other hand, the ubiquity of supersonic turbulence revealed by observed filaments strongly suggests that supersonic outflows are an integral part of AGN–ICM interaction, and that AGN activity plays a crucial role at driving turbulence in galaxy clusters.