Ram Pressure Research Articles

Young stars raining through the galactic halo: the nature and orbit of price-whelan 1

ABSTRACT We present radial velocities for five member stars of the recently discovered young (age ≃ 100−150 Myr) stellar system Price-Whelan 1 (PW 1), which is located far away in the Galactic Halo (D≃ 29 kpc, Z≃ 15 kpc), and that is probably associated with the leading arm (LA) of the Magellanic Stream. We measure the systemic radial velocity of PW 1, Vr = 275 ± 10 km s−1, significantly larger than the velocity of the LA gas in the same direction. We re-discuss the main properties and the origin of this system in the light of these new observations, computing the orbit of the system and comparing its velocity with that of the H i in its surroundings. We show that the bulk of the gas at the velocity of the stars is more than 10 deg (5 kpc) away from PW 1 and the velocity difference between the gas and the stars becomes larger as gas closer to the stars is considered. We discuss the possibilities that (1) the parent gas cloud was dissolved by the interaction with the Galactic gas, and (2) that the parent cloud is the high-velocity cloud (HVC) 287.5+22.5 + 240, lagging behind the stellar system by ≃ 25 km s−1 and ≃10 deg ≃ 5 kpc. This HVC, which is part of the LA, has metallicity similar to PW 1, displays a strong magnetic field that should help to stabilize the cloud against ram pressure, and shows traces of molecular hydrogen. We also show that the system is constituted of three distinct pieces that do not differ only by position in the sky but also by stellar content.

High-resolution three-dimensional simulations of gas removal from ultrafaint dwarf galaxies

Context. The faintest Local Group galaxies found lurking in and around the Milky Way halo provide a unique test bed for theories of structure formation and evolution on small scales. Deep Subaru and Hubble Space Telescope photometry demonstrates that the stellar populations of these galaxies are old and that the star formation activity did not last longer than 2 Gyr in these systems. A few mechanisms that may lead to such a rapid quenching have been investigated by means of hydrodynamic simulations, but these have not provided any final assessment so far. Aims. This is the first in a series of papers aimed at analyzing the roles of stellar feedback, ram pressure stripping, host-satellite tidal interactions, and reionization in cleaning the lowest mass Milky Way companions of their cold gas using high-resolution, three-dimensional hydrodynamic simulations. Methods. We simulated an isolated ultrafaint dwarf galaxy loosely modeled after Boötes I, and examined whether or not stellar feedback alone could drive a substantial fraction of the ambient gas out from the shallow potential well. Results. In contrast to simple analytical estimates, but in agreement with previous hydrodynamical studies, we find that most of the cold gas reservoir is retained. Conversely, a significant amount of the metal-enriched stellar ejecta crosses the boundaries of the computational box with velocities exceeding the local escape velocity and is, thus, likely lost from the system. Conclusions. Although the total energy output from multiple supernova explosions exceeds the binding energy of the gas, no galactic-scale outflow develops in our simulations and as such, most of the ambient medium remains trapped within the weak potential well of the model galaxy. It seems thus unavoidable that to explain the dearth of gas in ultrafaint dwarf galaxies, we will have to resort to environmental effects. This will be the subject of a forthcoming paper.

ALMA Unveils Widespread Molecular Gas Clumps in the Ram Pressure Stripped Tail of the Norma Jellyfish Galaxy

Abstract We present the first high-resolution map of the cold molecular gas distribution as traced by CO(2−1) emission with ALMA in a long ram pressure stripped tail. The Norma cluster galaxy ESO 137-001 is undergoing a strong interaction with the surrounding intracluster medium and is one of the nearest jellyfish galaxies with a long multiphase and multicomponent tail. We have mapped the full extent of the tail at 1″ (350 pc) angular resolution and found a rich distribution of mostly compact CO regions extending to nearly 60 kpc in length and 25 kpc in width. In total, about 109 M ⊙ of molecular gas was detected with ALMA. From comparison with previous APEX observations, we also infer the presence of a substantial extended molecular component in the tail. The ALMA CO features are found predominantly at the heads of numerous small-scale (∼1.5 kpc) fireballs (i.e., star-forming clouds with linear streams of young stars extending toward the galaxy) but also large-scale (∼8 kpc) superfireballs and double-sided fireballs that have additional diffuse ionized gas tails extending in the direction opposite the stellar tails. The new data help to shed light on the origin of the molecular tail; CO filaments oriented in the direction of the tail are likely young molecular features formed in situ, whereas large CO features tilted with respect to the tail may have originated from dense gas complexes that were gradually pushed away from the disk.

Ram pressure stripping: an analytical approach

ABSTRACT We take an analytical approach to study ram pressure stripping, using simple models for discs and the distribution of halo gas to look at this phenomenon in cluster, group and galaxy haloes. We also study variations in galaxy properties and redshift. In each case, we model the worst-case scenario (i.e. the maximum effect resulting from ram pressure). We show that the worst-case scenario is not affected greatly by changes in redshift. We find that gas discs in galaxies with a higher spin parameter are stripped sooner than galaxies with a smaller spin parameter. Galaxies in cluster haloes are stripped of gas more efficiently compared with group and galaxy haloes, because they have a higher infall speed and a higher density of gas in the intra-cluster medium (i.e. as a result of a greater retention of baryons). We comment on the limitations of our model, and we look at and illustrate a situation where a significant amount of gas may be retained in the galaxy disc. Finally, we discuss the implications for star formation in galaxies during infall into haloes.

The geometry of the magnetic field in the central molecular zone measured by PILOT

We present the first far infrared (FIR) dust emission polarization map covering the full extent of Milky Way’s central molecular zone (CMZ). The data, obtained with the PILOT balloon-borne experiment, covers the Galactic center region − 2° < ℓ < 2°, − 4° < b < 3° at a wavelength of 240 μm and an angular resolution of 2.2′. From our measured dust polarization angles, we infer a magnetic field orientation projected onto the plane of the sky (POS) that is remarkably ordered over the full extent of the CMZ, with an average tilt angle of ≃22° clockwise with respect to the Galactic plane. Our results confirm previous claims that the field traced by dust polarized emission is oriented nearly orthogonally to the field traced by GHz radio synchrotron emission in the Galactic center region. The observed field structure is globally compatible with the latest Planck polarization data at 353 and 217 GHz. Upon subtraction of the extended emission in our data, the mean field orientation that we obtain shows good agreement with the mean field orientation measured at higher angular resolution by the JCMT within the 20 and 50 km s−1 molecular clouds. We find no evidence that the magnetic field orientation is related to the 100 pc twisted ring structure within the CMZ. The low polarization fraction in the Galactic center region measured with Planck at 353 GHz combined with a highly ordered projected field orientation is unusual. This feature actually extends to the whole inner Galactic plane. We propose that it could be caused by the increased number of turbulent cells for the long lines of sight towards the inner Galactic plane or to dust properties specific to the inner regions of the Galaxy. Assuming equipartition between magnetic pressure and ram pressure, we obtain magnetic field strength estimates of the order of 1 mG for several CMZ molecular clouds.

Effect of Shape Complexity on Ram Pressure and Metal Flow in Aluminum Extrusion

Product output and quality are directly affected by metal flow through the extrusion die. The current paper investigates the effect of profile complexity on extrusion pressure, metal flow, and product defects. Cold extrusion experiments were performed on three solid profiles of different complexities. Simulations were carried out for these three shapes using the commercial finite element package DEFORM-3D. After verifying against experimental results, numerical work was extended to six more profiles of varying complexity. It was found that profiles of higher complexity usually result in more inhomogeneous metal flow, require larger extrusion forces, and are more susceptible to product defects. Current complexity definitions need to be improved for consistent ranking of die profiles. Factors such as extrusion ratio and die profile symmetry may also play a significant role in the distortion of metal flow through an extrusion die. These findings can be of direct utility in extrusion die design improvement and reduction of extrusion defects related to metal flow.

Jellyfish: Ram Pressure Stripping As a Diagnostic Tool in Studies of Cluster Collisions∗ †

Abstract Prompted by the discovery of A1758N_JFG1, a spectacular case of ram pressure stripping (RPS) in the galaxy cluster A1758N, we investigate the properties of other galaxies suspected to undergo RPS in this equal-mass, post-collision merger. Exploiting constraints derived from Hubble Space Telescope images and Keck longslit spectroscopy, our finding of apparent debris trails and dramatically enhanced star formation rates in an additional seven RPS candidates support the hypothesis that RPS, and hence rapid galaxy evolution in high-density environments, is intricately linked to cluster collisions. Unexpectedly, we find the vast majority of RPS candidates in A1758N to be moving toward us, and in a shared direction as projected on the plane of the sky. We hypothesize that this directional bias is the result of two successive events: (1) the quenching, during and after the first core passage, of star formation in galaxies with an approximately isotropic velocity distribution within the central region of the merger, and (2) RPS events triggered in late-type galaxies falling into the merging system along a filament, possibly enhanced by a shock front expanding into the outskirts of the southeastern subcluster. Since this explanation implies that the merger axis of A1758N must be significantly inclined with respect to the plane of the sky, our findings open the possibility of RPS events becoming important diagnostic tools to constrain the geometry of cluster collisions that, due to the orientation of the merger axis, lack the classic observational signatures of face-on mergers.

Origin of the galaxy H i size–mass relation

ABSTRACT We analytically derive the observed size–mass relation of galaxies’ atomic hydrogen (H i), including limits on its scatter, based on simple assumptions about the structure of H i discs. We trial three generic profiles for H i surface density as a function of radius. First, we assert that H i surface densities saturate at a variable threshold, and otherwise fall off exponentially with radius or, secondly, radius squared. Our third model assumes the total gas surface density is exponential, with the H i fraction at each radius depending on local pressure. These are tested against a compilation of 110 galaxies from the THINGS, LITTLE THINGS, LVHIS, and Bluedisk surveys, whose H i surface density profiles are well resolved. All models fit the observations well and predict consistent size–mass relations. Using an analytical argument, we explain why processes that cause gas disc truncation – such as ram-pressure stripping – scarcely affect the H i size–mass relation. This is tested with the IllustrisTNG(100) cosmological, hydrodynamic simulation and the Dark Sage semi-analytic model of galaxy formation, both of which capture radially resolved disc structure. For galaxies with $m_* \ge 10^9\, {\rm M}_{\odot }$ and $m_{\rm H\, {\small {I}}} \ge 10^8\, {\rm M}_{\odot }$, both simulations predict H i size–mass relations that align with observations, show no difference between central and satellite galaxies, and show only a minor, second-order dependence on host halo mass for satellites. Ultimately, the universally tight H i size–mass relation is mathematically inevitable and robust. Only by completely disrupting the structure of H i discs, e.g. through overly powerful feedback, could a simulation predict the relation poorly.

Group pre-processing versus cluster ram-pressure stripping: the case of ESO156−G029

ABSTRACT We report on observations of ESO156−G029, member of a galaxy group which is positioned at the virial radius of cluster Abell 3193. ESO156−G029 is located ∼1.4 Mpc in projected distance from the brightest cluster galaxy NGC1500. We show that ESO156−G029 has disturbed gas kinematics and a highly asymmetric neutral hydrogen (H i) distribution, which are consequences of group pre-processing, and possibly of ram pressure. Based on the current data we propose a scenario in which ESO156−G029 had a minor gas-rich merger in the past and now starts to experience ram pressure. We infer that the galaxy will undergo rapid evolution once it gets closer to the cluster centre (less than 0.5 Mpc) where ram pressure is strong enough to begin stripping the H i from the galaxy.

Exploring the multiphase medium in MKW 08: from the central active galaxy up to cluster scales

Context. The study of the brightest cluster galaxy (BCG) coronae embedded in noncool core (NCC) galaxy clusters is crucial to understand the BCG’s role in galaxy cluster evolution as well as the activation of the self-regulated cooling and heating mechanism in the central regions of galaxy clusters. Aims. We explore the X-ray properties of the intracluster medium (ICM) of the NCC galaxy cluster MKW 08 and the BCG corona, along with their interface region. With recent and deep archival Chandra observations, we study the BCG corona in detail, and with archival XMM-Newton observations, we investigate the implications of the central active galactic nuclei (AGN) on the BCG. Methods. We carry out imaging and spectral analyses of MKW 08 with archival XMM-Newton and Chandra X-ray observations. Results. Our spectral analysis suggests the presence of a central AGN by a power-law with a photon index of Γ ≃ 1.8 at the core of its BCG. Although the ICM does not exhibit a cluster scale cool core, the BCG manifests itself as a mini cool core characterized by a cooling time as short as 64 Myr at r = 3 kpc centered at the galaxy. The isothermality of the BCG corona seems to favor mechanical feedback from the central AGN as the major source of gas heating. The gas pressure profile of this mini cool core suggests that the BCG coronal gas reaches pressure equilibrium with the hotter and less dense ICM inside an interface of nearly constant pressure, delimited by radii 4 ≤ r ≤ 10 kpc at the galactic center. As revealed by the presence of a metal enriched tail (Z ≃ 0.5–0.9 Z⊙) extending up to 40 kpc, the BCG corona seems to be experiencing ram-pressure stripping by the surrounding ICM and/or interacting with a nearby galaxy, IC 1042.

Numerical Analysis of Bonded Composite T Joint Subjected to Hydrodynamic Ram Pressures

When a high-velocity projectile impacts a fluid-filled structure, hydrodynamic ram occurs from the overpressure caused by the transfer of energy and momentum from the projectile to the fluid, often with catastrophic effects. The critical damage is usually located at the seams/joints of said enclosure. A RamGun can be used as a less expensive alternative for assessing joint resistance to hydrodynamic ram. Numerical techniques are regularly used to supplement testing and to predict hydrodynamic ram damage in structures. This study was used to evaluate the viability of using a coarse mesh cohesive zone modeling technique to model a bonded joint in concert with LS-DYNA arbitrary Lagrangian–Euler fluid structure interaction algorithm to ascertain the joint resistance to hydrodynamic ram pressures generated in a RamGun. The ballistic testing approach was used to determine the failure pressure range for a joint that was used to calibrate the numerical model. The calibrated coarse mesh was able to predict joint failure in line with the test pressure values and also captured failure modes for various coupons of varying skin thicknesses. This will serve as a baseline for modeling and assessing joint failure in larger structures like fuel tanks and storage containers subjected to hydrodynamic ram.

Investigation of a Ducted Rocket with a Paraffin/Oxygen Hybrid Gas Generator

Analysis and testing of a ducted rocket employing a hybrid gas generator is presented. Over 150 successful static firing tests have been performed using a connected pipe facility supplying vitiated air to the ram combustor at stagnation temperature and pressure encountered in actual flights. The hybrid gas generator consisting of paraffin wax and gaseous oxygen operated unchoked, supplying hot, fuel-rich gases to the ram combustor for further burning with air. Empirical correlation of the fuel regression rate versus oxidizer mass flux in the gas generator has been displayed, and good overall combustion efficiency has been revealed. Thrust and fuel/air ratio control via the cold oxidizer flow rate into the gas generator is a major advantage of employing a hybrid gas generator. It was shown that such control could be done over broad ranges of flight altitude, flight Mach number, and thrust, while maintaining the ram pressure throughout the engine. This is in contrast to the thrust control of a ducted rocket employing a fuel-rich solid propellant. In that case, the process involves variation of the gas generator nozzle throat area by a mechanical device within the hot flow, implying a large increase of pressure in the gas generator combustor. Different scenarios of engine control and thrust modulation for variable flight conditions and requirements have been demonstrated.

Living with Neighbors. I. Observational Clues to Hydrodynamic Impact of Neighboring Galaxies on Star Formation

Abstract Galaxies in pairs show enhanced star formation (SF) compared to their counterparts in isolation, which is often explained by the tidal effect of neighboring galaxies. Recent observations, however, reported that galaxies paired with early-type neighbors do not undergo the SF enhancement. Here we revisit the influence of neighbors using a large sample of paired galaxies from the Sloan Digital Sky Survey and a carefully constructed control sample of isolated counterparts. We find that star-forming neighbors enhance SF, and even more so for more star-forming (and closer) neighbors, which can be attributed to collisions of interstellar medium (ISM) leading to SF. We further find that, contrary to the anticipated tidal effect, quiescent neighbors quench SF, and even more so for more quiescent (and closer) neighbors. This seems to be due to removal of gas reservoirs via ram pressure stripping and gas accretion cut off by hot gas halos of quiescent neighbors, on top of their paucity of ISM to collide to form stars. Our findings, especially the intimate connection of SF to the status and strength of neighbors’ SF, imply that the hydrodynamic mechanisms, along with the tidal effect, play a crucial role during the early phase of galactic interactions.

Diagnosing cosmic ray modified shocks with H α polarimetry

ABSTRACT A novel diagnostic of cosmic ray modified shocks by polarimetry of H α emissions is suggested. In a cosmic ray modified shock, the pressure of cosmic rays is sufficiently high compared to the upstream ram pressure to force the background plasma to decelerate (measured in the shock rest frame). Simultaneously, a fraction of the hydrogen atoms co-existing in the upstream plasma collide with the decelerated protons and undergo charge-exchange reactions. As a result, hydrogen atoms with the same bulk velocity of the decelerated protons are generated. We show that when the shock is observed from edge-on, the H α radiated by these upstream hydrogen atoms is linearly polarized with a sizable degree of a few per cent as a result of resonant scattering of Ly β. The polarization direction depends strongly on the velocity modification; the direction is parallel to the shock surface for the case of no modification, while the direction is parallel to the shock velocity for the case of a modified shock.

Smaller stellar disc scale lengths in rich environments

Abstract We investigate the dependence of stellar disc scale lengths on environment for a sample of Sloan Digital Sky Survey Data Release 7 galaxies with published photometric bulge-disc decompositions. We compare disc scale lengths at fixed bulge mass for galaxies in an isolated field environment to galaxies in X-ray rich and X-ray poor groups. At low bulge mass, stellar disc scale lengths in X-ray rich groups are smaller compared to discs in both X-ray poor groups and in isolated field environments. This decrease in disc scale length is largely independent of halo mass, though shows some dependence on group-centric distance. We also find that stellar disc scale lengths are smaller in X-ray rich environments for a subset of star-forming galaxies and for galaxies of different morphological types. We note that disc scale lengths of low mass galaxies are known to have large systematic uncertainties, however we focus on differences between samples with the same measurement biases. Our results show that stellar disc scale lengths depend on X-ray brightness, a tracer of IGM density, suggesting a role for hydrodynamic processes such as ram-pressure stripping and/or starvation.

Cosmological Simulations of Satellites around Isolated Dwarf Galaxies

Abstract We trace the cosmological origin of satellites around a dwarf galaxy using a very high-resolution (12 pc/h) cosmological hydrodynamic zoom simulation. To realistically describe the formation and evolution of small-mass stellar satellites, our model includes a full baryonic physics treatment including a recipe for UV self-shielding. We find that the majority of their star-forming gas is accreted after reionization, thus the survival of a mini-halo gas to reionization is not an important factor. Instead, the key factor seems to be the ability for a mini-halo to cool its recently accreted gas, which is more efficient in more massive halos. We find that halos in denser environments suffer more mergers, enabling them to grow their mass such that cooling of accreted gas can occur efficiently. Although the host galaxy is only a dwarf galaxy itself, we find that ram pressure is an efficient means by which accreted mini-halos lose their gas content, both by interacting with hot halo gas but also in direct collisions with the gas disk of the host. The satellites are also disrupted by the tidal forces near the center of the host galaxy. In summary, our results suggest that the characteristics of satellites are mainly determined by their ability to efficiently cool gas that is accreted in the redshift range of z = 3–5, prior to their infall into the host galaxy.

On the nature of radio filaments near the Galactic Centre

ABSTRACT We suggest that narrow, long radio filaments near the Galactic Centre arise as kinetic jets – streams of high-energy particles escaping from ram pressure confined pulsar wind nebulae (PWNe). The reconnection between the PWN and interstellar magnetic field allows pulsar wind particles to escape, creating long narrow features. They are the low-frequency analogues of kinetic jets seen around some fast-moving pulsars, such as The Guitar and The Lighthouse PWNe. The radio filaments trace a population of pulsars also responsible for the Fermi GeV excess produced by the Inverse Compton scattering by the pulsar wind particles. The magnetic flux tubes are stretched radially by the large-scale Galactic winds. In addition to PWNe accelerated particles can be injected at supernovae remnants. The model predicts variations of the structure of the largest filaments on scales of ∼dozens of years – smaller variations can occur on shorter time-scales. We also encourage targeted observations of the brightest sections of the filaments and of the related unresolved point sources in search of the powering PWNe and pulsars.

Unstable Plasma Modes Amplified by Cosmic Rays in the Upstream of Collisionless Shock

Abstract—Nonequilibrium distributions of cosmic rays accelerated by the Fermi mechanism are formed by multiple interactions with magnetic fluctuations in the upstream region of a collisionless shock. The shock ram pressure is transferred to accelerated cosmic rays, which may in turn amplify the magnetic turbulence. A short-wave plasma instability induced in the upstream region of a collisionless shock caused by the interaction between cosmic rays and the background plasma is discussed.

Gone after one orbit: How cluster environments quench galaxies

ABSTRACT The effect of galactic orbits on a galaxy’s internal evolution within a galaxy cluster environment has been the focus of heated debate in recent years. To understand this connection, we use both the $(0.5 \,$Gpc)3 and the Gpc3 boxes from the cosmological hydrodynamical simulation set Magneticum Pathfinder. We investigate the velocity anisotropy, phase space, and the orbital evolution of up to ∼5 × 105 resolved satellite galaxies within our sample of 6776 clusters with $M_{\mathrm{vir}}\,\, \gt\,\, 10^{14} \, \mathrm{M_{\odot }}$ at low redshift, which we also trace back in time. In agreement with observations, we find that star-forming satellite galaxies inside galaxy clusters are characterized by more radially dominated orbits, independent of cluster mass. Furthermore, the vast majority of star-forming satellite galaxies stop forming stars during their first passage. We find a strong dichotomy both in line-of-sight and radial phase space between star-forming and quiescent galaxies, in line with observations. The tracking of individual orbits shows that the star formation of almost all satellite galaxies drops to zero within $1 \, \mathrm{Gyr}$ after infall. Satellite galaxies that are able to remain star forming longer are characterized by tangential orbits and high stellar mass. All this indicates that in galaxy clusters the dominant quenching mechanism is ram-pressure stripping.

The Fornax Deep Survey with the VST

Context. We present the study of a magnitude limited sample (mB ≤ 16.6 mag) of 13 late type galaxies (LTGs), observed inside the virial radius, Rvir ∼ 0.7 Mpc, of the Fornax cluster within the Fornax Deep Survey (FDS). Aims. The main objective is to use surface brightness profiles and g − i colour maps to obtain information on the internal structure of these galaxies and find signatures of the mechanisms that drive their evolution in high-density environments inside the virial radius of the cluster. Methods. By modelling galaxy isophotes, we extract the azimuthally averaged surface brightness profiles in four optical bands. We also derive g − i colour profiles, and relevant structural parameters like total magnitude and effective radius. For ten of the galaxies in this sample, we observe a clear discontinuity in their typical exponential surface brightness profiles, derive their “break radius”, and classify their disc-breaks into Type II (down-bending) or Type III (up-bending). Results. We find that Type II galaxies have bluer average (g − i) colour in their outer discs while Type III galaxies are redder. The break radius increases with stellar mass and molecular gas mass while it decreases with molecular gas-fractions. The inner and outer scale-lengths increase monotonically with absolute magnitude, as found in other works. For galaxies with CO(1-0) measurements, there is no detected cold gas beyond the break radius (within the uncertainties). In the context of morphological segregation of LTGs in clusters, we also find that, in Fornax, galaxies with morphological type 5 < T ≤ 9 (∼60% of the sample) are located beyond the high-density, ETG-dominated regions, however there is no correlation between T and the disc-break type. We do not find any correlation between the average (g − i) colours and cluster-centric distance, but the colour-magnitude relation holds true. Conclusions. The main results of this work suggest that the disc-breaks of LTGs inside the virial radius of the Fornax cluster seem to have arisen through a variety of mechanisms (e.g. ram-pressure stripping, tidal disruption), which is evident in their outer-disc colours and the absence of molecular gas beyond their break radius in some cases. This can result in a variety of stellar populations inside and outside the break radii.