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Max Gronke

Publications and source records attributed to Max Gronke.

At least 55 records · Page 3Linked to original sources
arXiv2023

CLASSY VII Lyα Profiles: The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-Era Analogs

Lyman-alpha line profiles are a powerful probe of ISM structure, outflow speed, and Lyman continuum escape fraction. In this paper, we present the Ly$α$ line profiles of the COS Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Ly$α$ emission profile in the bottom of a damped, Ly$α$ absorption trough. Such profiles reveal an inhomogeneous interstellar medium (ISM). We successfully fit the damped Ly$α$ absorption (DLA) and the Ly$α$ emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Ly$α$ exchange between high-$N_\mathrm{HI}$ and low-$N_\mathrm{HI}$ paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Ly$α$ peak separation and the [O III]/[O II] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Ly$α$ peak separation decreases. We combine measurements of Ly$α$ peak separation and Ly$α$ red peak asymmetry in a diagnostic diagram which identifies six Lyman continuum leakers in the CLASSY sample. We find a strong correlation between the Ly$α$ trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Ly$α$ peak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Ly$α$ photons outside the spectroscopic aperture reshapes Ly$α$ profiles as the distances to these compact starbursts span a large range.

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arXiv2023

Cooling driven coagulation

Astrophysical gases such as the interstellar-, circumgalactic- or intracluster-medium are commonly multiphase, which poses the question of the structure of these systems. While there are many known processes leading to fragmentation of cold gas embedded in a (turbulent) hot medium, in this work, we focus on the reverse process: coagulation. This is often seen in wind-tunnel and shearing layer simulations, where cold gas fragments spontaneously coalesce. Using 2D and 3D hydrodynamical simulations, we find that sufficiently large ($\gg c_{\rm s} t_{\rm cool}$), perturbed cold gas clouds develop pulsations which ensure cold gas mass growth over an extended period of time ($\gg r / c_{\rm s}$). This mass growth efficiently accelerates hot gas which in turn can entrain cold droplets, leading to coagulation. The attractive inverse square force between cold gas droplets has interesting parallels with gravity; the `monopole' is surface area rather than mass. We develop a simple analytic model which reproduces our numerical findings.

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arXiv2023

ALPACA: A New Semi-Analytic Model for Metal Absorption Lines Emerging from Clumpy Galactic Environments

We present a new semi-analytic formalism for modeling metal absorption lines that emerge from a clumpy galactic environment, ALPACA. We predict the ''down-the-barrel'' (DTB) metal absorption line profiles and the EW of absorption at different impact parameters as a function of the properties of the clumps, including the clump kinematics, the clump volume filling factor, the clump number density profile and the clump ion column densities. With ALPACA, we jointly model the stacked DTB CII$λ$1334 spectrum of a sample of $z \sim$ 3 Lyman break galaxies and the EW v.s. $b$ profile of a sample of $z \sim$ 2 star-forming galaxy-galaxy pairs. ALPACA successfully reproduced two datasets simultaneously, and the best-fit prefers a low clump volume filling factor ($\sim 3 \times 10^{-3}$). The radial velocities of the clumps are a superposition of a rapidly accelerated outflow with a maximum velocity of $\sim 400\,\rm km\,s^{-1}$ and a velocity dispersion of $σ_{\rm cl} \sim\,120 \rm km\,s^{-1}$. The joint modeling reveals a physical scenario where the absorption observed at a particular velocity is contributed by the clumps distributed over a fairly broad range of radii. We also find that the commonly adopted Sobolev approximation is at best only applicable within a narrow range of radii where the clumps are undergoing rapid acceleration in a non-volume-filling clumpy medium. Lastly, we find that the clump radial velocity profile may not be fully constrained by the joint modeling and spatially-resolved Ly$α$ emission modeling may help break the degeneracy.

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arXiv2023

On the evolution of the size of Lyman alpha halos across cosmic time: no change in the circumgalactic gas distribution when probed by line emission

Lyman $α$ (Ly$α$) is now routinely used as a tool for studying high-redshift galaxies and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Ly$α$ emitters indicate that significant extended Ly$α$ halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-$z$ samples while optimizing the observations for detection of low surface brightness Ly$α$ emission. The Ly$α$ escape fractions range between 0.7\% and 37\%, and we detect extended Ly$α$ emission around six out of seven targets. We find Ly$α$ halo to UV scale length ratios around 6:1 which is marginally lower than high-redshift observations, and halo flux fractions between 60\% and 85\% -- consistent with high-redshift observations -- when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Ly$α$ halo fractions. We find that deeper H$α$ data would be required to firmly determine the origin of Ly$α$ halo emission, however, there are indications that H$α$ is more extended than the central FUV profile, potentially indicating conditions favorable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation.

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arXiv2023

Tracing the kinematics of the whole ram pressure stripped tails in ESO 137-001

Ram pressure stripping (RPS) is an important process to affect the evolution of cluster galaxies and their surrounding environment. We present a large MUSE mosaic for ESO 137-001 and its stripped tails, and study the detailed distributions and kinematics of the ionized gas and stars. The warm, ionized gas is detected to at least 87 kpc from the galaxy and splits into three tails. There is a clear velocity gradient roughly perpendicular to the stripping direction, which decreases along the tails and disappears beyond $\sim45$ kpc downstream. The velocity dispersion of the ionized gas increases to $\sim80$ km s$^{-1}$ at $\sim20$ kpc downstream and stays flat beyond. The stars in the galaxy disc present a regular rotation motion, while the ionized gas is already disturbed by the ram pressure. Based on the observed velocity gradient, we construct the velocity model for the residual galactic rotation in the tails and discuss the origin and implication of its fading with distance. By comparing with theoretical studies, we interpreted the increased velocity dispersion as the result of the oscillations induced by the gas flows in the galaxy wake, which may imply an enhanced degree of turbulence there. We also compare the kinematic properties of the ionized gas and molecular gas from ALMA, which shows they are co-moving and kinematically mixed through the tails. Our study demonstrates the great potential of spatially resolved spectroscopy in probing the detailed kinematic properties of the stripped gas, which can provide important information for future simulations of RPS.

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arXiv2023

Spectral Shapes of the Lya Emission from Galaxies. II. the influence of stellar properties and nebular conditions on the emergent Lya profiles

We demonstrate how the stellar and nebular conditions in star-forming galaxies modulate the emission and spectral profile of HI Lya emission line. We examine the net Lya output, kinematics, and in particular emission of blue-shifted Lya radiation, using spectroscopy from with the Cosmic Origins Spectrograph on HST, giving a sample of 87 galaxies at redshift z=0.05-0.44. We contrast the Lya spectral measurements with properties of the ionized gas (from optical spectra) and stars (from stellar modeling). We demonstrate correlations of unprecedented strength between the Lya escape fraction (and equivalent width) and the ionization parameter (p~10^-15). The relative contribution of blue-shifted emission to the total Lya also increases from ~0 to ~40% over the range of O_32 ratios (p~10^-6). We also find particularly strong correlations with estimators of stellar age and nebular abundance, and weaker correlations regarding thermodynamic variables. Low ionization stage absorption lines suggest the Lya emission and line profile are predominantly governed by the column of absorbing gas near zero velocity. Simultaneous multi-parametric analysis over many variables shows we can predict 80% of the variance on Lya luminosity, and ~50% on the EW. We determine the most crucial predictive variables, finding that for tracers of the ionization state and Hb luminosity dominate the luminosity prediction whereas the Lya EW is best predicted by Hb EW and the Ha/Hb ratio. We discuss our results with reference to high redshift observations, focussing upon the use of Lya to probe the nebular conditions in high-z galaxies and cosmic reionization.

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arXiv2023

Cloudy with A Chance of Rain: Accretion Braking of Cold Clouds

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

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arXiv2023

Lyman-alpha Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems

T Tauri stars produce broad Lyman-alpha emission lines that contribute $\sim$88% of the total UV flux incident on the inner circumstellar disks. Lyman-alpha photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyman-alpha emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high velocity emission line wings, providing estimates of the average velocities within the bulk intervening H I. The model velocities are significantly correlated with the K band veiling, indicating a turnover from Lyman-alpha profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with red-shifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyman-alpha photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyman-alpha irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.

astro-ph.SR↗
arXiv2023

Extended Lyman-$α$ emission towards the SPT2349-56 protocluster at $z=4.3$

Context. Deep spectroscopic surveys with the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed that some of the brightest infrared sources in the sky correspond to concentrations of dusty star-forming galaxies (DSFG) at high redshift. Among these, the SPT2349-56 protocluster system at z = 4.304 is amongst the most extreme examples due to its high source density and integrated star formation rate. Aims. We conducted a deep Lyman-$α$ line emission survey around SPT2349-56 using the Multi-Unit Spectroscopic Explorer (MUSE) at Very Large Telescope (VLT) in order to characterize this uniquely dense environment. Methods. Taking advantage of the deep three-dimensional nature of this survey, we performed a sensitive search for Lyman-$α$ emitters (LAEs) toward the core and northern extension of the protocluster, which correspond to the brightest infrared regions in this field. Using a smoothed narrowband image extracted from the MUSE datacube around the protocluster redshift, we searched for possible extended structures. Results. We identify only three LAEs at z = 4.3 in this field, in concordance with expectations for blank-fields, and an extended Lyman-$α$ structure spatially associated with core of the protocluster. All the previously-identified DSFGs in this field are undetected in Lyman-$α$ emission, consistent with the conspicuous dust obscuration in these systems. We find an extended Lyman-$α$ structure, about $60 \times 60$ kpc$^{2}$ in size, and located 56 kpc west of the protocluster core. Three DSFGs coincide spatially with the location of this structure. We conclude that either the three co-spatial DSFGs or the protocluster core itself are feeding ionizing photons to the Lyman-$α$ structure.

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arXiv2022

Molecular Shattering

Recent observations suggest galaxies may ubiquitously host a molecular component to their multiphase circumgalactic medium (CGM). However, the structure and kinematics of the molecular CGM remains understudied theoretically and largely unconstrained observationally. Recent work suggests molecular gas clouds with efficient cooling survive acceleration in hot winds similar to atomic clouds. Yet the pressure-driven fragmentation of molecular clouds when subjected to external shocks or undergoing cooling remains unstudied. We perform radiative, inviscid hydrodynamics simulations of clouds perturbed out of pressure equilibrium to explore the process of hydrodynamic fragmentation to molecular temperatures. We find molecular clouds larger than a critical size can shatter into a mist of tiny droplets, with the critical size deviating significantly from the atomic case. We find that cold clouds shatter only if the sound crossing time exceeds the local maximum of the cooling time ~8000 K. Moreover, we find evidence for a universal mechanism to 'shatter' cold clouds into a 'mist' of tiny droplets as a result of rotational fragmentation -- a process we dub 'splintering.' Our results have implications for resolving the molecular phase of the CGM in observations and cosmological simulations.

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arXiv2022

Lyα Halos around [O III]-Selected Galaxies in HETDEX

We present extended Lyman-α (Lyα) emission out to 800 kpc of 1034 [O III]-selected galaxies at redshifts 1.9 40 kpc from the galaxy centers. The surface brightness in the inner parts (r<10 kpc) around the [O III]-selected galaxies, however, is ten times fainter than that of the LAEs. Our results are consistent with the notion that photons dominating the outer regions of the Lyα halos are not produced in the central galaxies but originate outside of them.

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arXiv2022

Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris

The Magellanic Stream is sculpted by its infall through the Milky Way's circumgalactic medium, but the rates and directions of mass, momentum, and energy exchange through the Stream-halo interface are relative unknowns critical for determining the origin and fate of the Stream. Complementary to large-scale simulations of LMC-SMC interactions, we apply new insights derived from idealized, high-resolution "cloud-crushing" and radiative turbulent mixing layer simulations to the Leading Arm and Trailing Stream. Contrary to classical expectations of fast cloud breakup, we predict that the Leading Arm and much of the Trailing Stream should be surviving infall and even gaining mass due to strong radiative cooling. Provided a sufficiently supersonic tidal swing-out from the Clouds, the present-day Leading Arm could be a series of high-density clumps in the cooling tail behind the progenitor cloud. We back up our analytic framework with a suite of converged wind-tunnel simulations, finding that previous results on cloud survival and mass growth can be extended to high Mach number ($\mathcal{M}$) flows with a modified drag time $t_{drag} \propto 1 + \mathcal{M}$ and longer growth time. We also simulate the Trailing Stream; we find that the growth time is long ($\sim$ Gyrs) compared to the infall time, and approximate H$α$ emission is low on average ($\sim$few mR) but can be up to tens of mR in bright spots. Our findings also have broader extragalactic implications for e.g. galactic winds, which we discuss.

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arXiv2022

Tracing Lyman-alpha and Lyman Continuum Escape in Galaxies with Mg II Emission

Star-forming galaxies are considered the likeliest source of the H I ionizing Lyman Continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z >~ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the Epoch of Reionization. Emission from Mg II \ly\ly 2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for 8 LyC emitter candidates, selected to have strong Mg II emission lines. We securely detect LyC emission in 50% (4/8) galaxies with 2$σ$ significance. This high detection rate suggests that strong Mg II emitters might be more likely to leak LyC than similar galaxies without strong Mg II. Using photoionization models, we constrain the escape fraction of Mg II as ~ 15 -- 60%. We confirm that the escape fraction of Mg II correlates tightly with that of Lyman-alpha (LyA), which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg II emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg II emission can be adopted to estimate the escape fraction of LyA and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the Epoch of Reionization.

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arXiv2022

Where Outflows Meet Inflows: Gas Kinematics in SSA22 Lyman-$α$ Blob 2 Decoded by Advanced Radiative Transfer Modelling

We present new spectroscopic observations of Lyman-$α$ (Ly$α$) Blob 2 ($z \sim$ 3.1). We observed extended Ly$α$ emission in three distinct regions, where the highest Ly$α$ surface brightness (SB) center is far away from the known continuum sources. We searched through the MOSFIRE slits that cover the high Ly$α$ SB regions, but were unable to detect any significant nebular emission near the highest SB center. We further mapped the flux ratio of the blue peak to the red peak and found it is anti-correlated with Ly$α$ SB with a power-law index of $\sim$ -0.4. We used radiative transfer models with both multiphase, clumpy and shell geometries and successfully reproduced the diverse Ly$α$ morphologies. We found that most spectra suggest outflow-dominated kinematics, while 4/15 spectra imply inflows. A significant correlation exists between parameter pairs, and the multiphase, clumpy model may alleviate previously reported discrepancies. We also modeled Ly$α$ spectra at different positions simultaneously and found that the variation of the inferred clump outflow velocities can be approximately explained by line-of-sight projection effects. Our results support the `central powering + scattering' scenario, i.e. the Ly$α$ photons are generated by a central powering source and then scatter with outflowing, multiphase HI gas while propagating outwards. The infalling of cool gas near the blob outskirts shapes the observed blue-dominated Ly$α$ profiles, but its energy contribution to the total Ly$α$ luminosity is less than 10%, i.e. minor compared to the photo-ionization by star-forming galaxies and/or AGNs.

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arXiv2022

(Re)Solving Reionization with Lyα: How Bright Lyα Emitters account for the $z\approx2-8$ Cosmic Ionizing Background

The cosmic ionizing emissivity from star-forming galaxies has long been anchored to UV luminosity functions. Here we introduce an emissivity framework based on Ly$α$ emitters (LAEs), which naturally hones in on the subset of galaxies responsible for the ionizing background due to the intimate connection between the production and escape of Ly$α$ and LyC photons. Using constraints on the escape fractions of bright LAEs ($L_{\rm{Lyα}}>0.2 L^{*}$) at $z\approx2$ obtained from resolved Ly$α$ profiles, and arguing for their redshift-invariance, we show that: (i) quasars and LAEs together reproduce the relatively flat emissivity at $z\approx2-6$, which is non-trivial given the strong evolution in both the star-formation density and quasar number density at these epochs and (ii) LAEs produce late and rapid reionization between $z\approx6-9$ under plausible assumptions. Within this framework, the $>10\times$ rise in the UV population-averaged $f_{\rm{esc}}$ between $z\approx3-7$ naturally arises due to the same phenomena that drive the growing Ly$α$ emitter fraction with redshift. Generally, a LAE dominated emissivity yields a peak in the distribution of the ionizing budget with UV luminosity as reported in latest simulations. Using our adopted parameters ($f_{\rm{esc}}=50\%$, $ξ_{\rm{ion}}=10^{25.9}$ Hz erg$^{-1}$ for half the bright LAEs), a highly ionizing minority of galaxies with $M_{\rm UV}<-17$ accounts for the entire ionizing budget from star-forming galaxies. Rapid flashes of LyC from such rare galaxies produce a "disco" ionizing background. We conclude proposing tests to further develop our suggested Ly$α$-anchored formalism.

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arXiv2022

The Lyman Alpha Spectral Database (LASD)

Lyman $α$ (Ly$α$) emission from star-forming galaxies is an important tool to study a large range of astrophysical questions: it has the potential to carry information about the source galaxy, its nearby circumgalactic medium, and also the surrounding intergalactic medium. Much observational and theoretical work has therefore focused on understanding the details of this emission line. These efforts have been hampered, however, by an absence of spectroscopic reference samples that can be used both as comparisons for observational studies and as critical tests for theoretical work. For this reason we have compiled a large sample of Ly$α$ spectra, at both low and high redshift, and created a publicly available online database, at http://lasd.lyman-alpha.com. The Lyman alpha Spectral Database (LASD) hosts these spectra, as well as large set of spectral and kinematic quantities that have been homogeneously measured for the entire sample. These can easily be viewed online and downloaded in tabular form. The LASD has the capacity for users to easily upload their own Ly$α$ spectra, and all the same spectral measurements will be made, reported, and ingested into the database. We actively invite the community to do so, and the LASD is intended to be a long-term community resource. In this paper we present the design of the database as well as descriptions of the underlying algorithms and the initial Ly$α$ emitter samples that are in the database.

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arXiv2022

Survival and mass growth of cold gas in a turbulent, multiphase medium

Astrophysical gases are commonly multiphase and highly turbulent. In this work, we investigate the survival and growth of cold gas in such a turbulent, multi-phase medium using three-dimensional hydrodynamical simulations. Similar to previous work simulating coherent flow (winds), we find that cold gas survives if the cooling time of the mixed gas is shorter than the Kelvin-Helmholtz time of the cold gas clump (with some weak additional Mach number dependence). However, there are important differences. Near the survival threshold, the long-term evolution is highly stochastic, and subject to the existence of sufficiently large clumps. In a turbulent flow, the cold gas continuously fragments, enhancing its surface area. This leads to exponential mass growth, with a growth time given by the geometric mean of the cooling and the mixing time. The fragmentation process leads to a large number of small droplets which follow a scale-free $\mathrm{d} N/\mathrm{d} m \propto m^{-2}$ mass distribution, and dominate the area covering fraction. Thus, whilst survival depends on the presence of large `clouds', these in turn produce a `fog' of smaller droplets tightly coupled to the hot phase which are probed by absorption line spectroscopy. We show with the aid of Monte-Carlo simulations that the simulated mass distribution emerges naturally due to the proportional mass growth and the coagulation of droplets. We discuss the implications of our results for convergence criteria of larger scale simulations and observations of the circumgalactic medium.

astro-ph.GA↗
arXiv2021

The Survival of Multiphase Dusty Clouds in Hot Winds

Much progress has been made recently in the acceleration of $\sim10^{4}$\,K clouds to explain absorption-line measurements of the circumgalactic medium and the warm, atomic phase of galactic winds. However, the origin of the cold, molecular phase in galactic winds has received relatively little theoretical attention. Studies of the survival of $\sim10^{4}$\,K clouds suggest efficient radiative cooling may enable the survival of expelled material from galactic disks. Alternatively, gas colder than 10$^4$\,K may form within the outflow, including molecules if dust survives the acceleration process. We explore the survival of dusty clouds in a hot wind with three-dimensional hydrodynamic simulations including radiative cooling and dust modeled as tracer particles. We find that cold $\sim10^{3}$\,K gas can be destroyed, survive, or transformed entirely to $\sim 10^4\,$K gas. We establish analytic criteria distinguishing these three outcomes which compare characteristic cooling times to the system's `cloud crushing' time. In contrast to typically studied $\sim10^{4}$\,K clouds, colder clouds are entrained faster than the drag time as a result of efficient mixing. We find that while dust can in principle survive embedded in the accelerated clouds, the survival fraction depends critically on the time dust spends in the hot phase and on the effective threshold temperature for destruction. We discuss our results in the context of polluting the circumgalactic medium with dust and metals, as well as understanding observations suggesting rapid acceleration of molecular galactic winds and ram pressure stripped tails of jellyfish galaxies.

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