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A&A
Volume 699, July 2025
Article Number A357
Number of page(s) 27
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202554200
Published online 23 July 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1. Introduction

Studies of galaxies in clusters over the past almost five decades have accumulated overwhelming evidence that galaxies infalling into clusters are subject to the effects of the ram pressure exerted by the hot intracluster medium on the galaxy interstellar medium (Gunn & Gott 1972). Both observations at different wavelengths and hydrodynamical simulations have established that gas is stripped by ram pressure when it exceeds the gravitational force of the galaxy, thereby depriving the galaxy of its fuel for star formation.

Whether ram pressure can be the driving force behind the differences between cluster and field galaxy populations is a long-standing question, concerning both the morphologies of galaxies (Dressler 1980) and their star formation histories (Poggianti et al. 1999). The answer to this question requires knowing how many cluster galaxies are subject to ram pressure during their lifetime (see, e.g., Vulcani et al. 2022), at each cosmological epoch. It also requires an understanding of the consequences of ram pressure on those “transient” galaxy properties that strongly depend on environment, such as star formation and morphological appearance (Bösch et al. 2013), as opposed to more fundamental galaxy properties, such as stellar mass, whose distribution at least above ∼1010 M⊙ seems invariant with halo mass (Vulcani et al. 2013).

Although a conclusive answer to this question has not been reached, several efforts have improved our understanding of this phenomenon. On the simulation side, wind-tunnel simulations of individual galaxies have clarified several aspects of the stripping process (Schulz & Struck 2001; Tonnesen & Bryan 2009, 2012, 2021; Roediger et al. 2006, 2014) and recently addressed a number of questions posed by observations (Akerman et al. 2023, 2024; Zhu et al. 2024). Cosmological (magneto)hydrodynamical simulations can now provide large samples of stripped galaxies as a function of halo mass and epoch (Bahé & McCarthy 2015; Pillepich et al. 2018; Göller et al. 2023; Kulier et al. 2023), thus offering sufficient statistics to investigate timescales and global populations (Rohr et al. 2023; Zinger et al. 2024).

Directly observing large samples of ram-pressure stripped galaxies is clearly key. At low redshift, unilaterally stripped (and thus extraplanar) material with morphologies suggestive of ram pressure has been directly observed in HI, radio continuum, Hα, CO, X-rays, optical, and ultraviolet imaging, as well as in integral-field spectroscopy (IFS). These studies include detailed analyses of individual galaxies as well as searches for ram pressure candidates in entire clusters or across several clusters (see also Sect. 5). An incomplete list of recent surveys at wavelengths other than optical includes VESTIGE (Hα in Virgo, Boselli et al. 2018, 2020, 2021, 2022, 2023), VERTICO (CO in Virgo, Brown et al. 2021, 2023), the MeerKAT Fornax survey (HI, Serra et al. 2023, 2024), the LOFAR surveys of Roberts et al. (2021a,b, 2022b, 2024), and Ignesti et al. (2023b), and the UV surveys of Smith et al. (2010) and George et al. (2024).

In this paper, we discuss a sample that was originally selected from optical B-band imaging. Optical imaging searches have been carried out in Coma (Roberts & Parker 2020), in the WINGS/OmegaWINGS imaging of 71 galaxy clusters at z = 0.04–0.07 (Poggianti et al. 2016, hereafter P16 and Vulcani et al. 2022) and the UNIONS imaging of some SDSS fields (Roberts et al. 2022a). A recent search for ram pressure stripped galaxies in Fornax, Antlia, and Hydra using S-PLUS imaging (Gondhalekar et al. 2024) adopts a semiautomated pipeline based on self-supervised learning to find stripping candidates, exploring the feasibility of more automated searches in large imaging surveys. At higher redshifts (z = 0.2–0.9), HST imaging has provided increasingly larger samples of stripping candidates (Cortese et al. 2007; Owers et al. 2012; McPartland et al. 2016; Ebeling et al. 2014; Ebeling & Kalita 2019; Roman-Oliveira et al. 2019, 2021; Durret et al. 2021, 2022), and the first IFS studies of cluster galaxies at z = 0.3–0.5 have confirmed the importance of ram pressure at intermediate redshifts (Kalita & Ebeling 2019; Moretti et al. 2022; Werle et al. 2022; Lee et al. 2022a, b; Bellhouse et al. 2022; Vulcani et al. 2024), with the currently holding IFS record of two galaxies at z = 0.7 (Boselli et al. 2019).

Optical/UV studies of the stellar light can provide candidates. However, only by adding observations that probe the gas in one of its phases (ionized, neutral, or molecular) can the ram pressure stripping (RPS) nature of these objects be confirmed, by finding and characterizing extraplanar gas. Generally speaking, though, the mere presence of extraplanar gas (whether in HI, Hα, or other phases) is not sufficient per se to prove ram pressure. However, the morphology of the gas with respect to the stars can often come a long way in identifying the physical process at work and distinguishing RPS from other processes, such as tidal interactions or outflows resulting from internal feedback. The most compelling evidence for ram pressure stripping, however, arises from a comparison between the gas and stellar kinematics, which can be effectively carried out using with IFS, or at least by analyzing the gas kinematics, as in HI studies. The IFS observations of individual galaxies (Merluzzi et al. 2013, 2016; Fumagalli et al. 2014; Fossati et al. 2016; Consolandi et al. 2017) have shown the power of spatially resolving all the main gaseous and stellar properties, providing a detailed picture of where and how stripping proceeds in the galaxy, and what its consequences on the star formation activity are in both the disk and stripped tails.

The GAs Stripping Phenomena in galaxies (GASP; Poggianti et al. 2017b, hereafter P17) is an ESO Large Program with the MUSE spectrograph designed to study the physical processes that can remove gas from galaxies and their consequences for galaxy star formation activity and evolution. GASP has observed 114 galaxies at z = 0.04–0.07, of which 76 are in galaxy cluster fields and the remaining 38 are in groups, filaments, and in isolation. Several GASP papers have presented a detailed analysis of individual galaxies, or of a specific aspect of a subset of the sample. The non-cluster subsample has been presented in Vulcani et al. (2021). In this paper, we present an overview of all 76 galaxies selected in 39 cluster fields, showing the MUSE data and the variety of stripping stages in our sample (Sect. 3.1). The success rate in identifying ram pressure at work and the characteristics of the galaxies undergoing ram pressure are discussed in Sect. 3.2, while Sect. 3.3 is dedicated to those galaxies with a truncated ionized gas disk. We present the properties of the star-forming galaxies of the control sample in Sect. 4 and discuss the pros and cons of selecting ram pressure candidates based on optical imaging and other wavelengths in Sect. 5. In the following, we use a Chabrier (2003) initial mass function and a standard Λ cold dark matter cosmology with ΩM = 0.3, ΩΛ = 0.7, and H0 = 70  km s−1 Mpc−1.

2. Galaxy sample and data

The GASP sample used in this paper comprises 64 stripping candidates from the atlas of P16 and 12 galaxies that were selected as a control sample. Here, we consider only the galaxies that make up the GASP cluster sample (see P17), while field galaxies have been characterized separately in Vulcani et al. (2017, 2018c,b, 2019b, 2021).

The 64 galaxies were selected from the P16 sample, which utilized images and spectra from the WINGS/OmegaWINGS survey (Fasano et al. 2006; Cava et al. 2009; Varela et al. 2009; Moretti et al. 2014, 2017; Gullieuszik et al. 2015) to identify galaxies with a disturbed morphology. A visual inspection of B-band images revealed the presence of tentacles of material that appear to be stripped from the galaxy, most likely due to gas-only removal mechanisms. In some cases, the tails were extremely long and spectacular, while in many others, the evidence of debris tails and stripped material was less pronounced. The candidates were assigned by P16 to five classes according to the visual evidence for stripping signatures, ranging from extreme cases (JClass = 5) to progressively weaker cases, down to the weakest (JClass = 1). The selection was based solely on the images; therefore, a subset of the candidates did not have a known spectroscopic redshift. Galaxies with morphologies clearly disturbed due to mergers or tidal interactions were removed from the sample, although the most doubtful cases where either gas stripping or tidal forces, or both, might be at work were retained and flagged. In fact, the eventual presence of tidal forces does not exclude the possibility that gas stripping mechanisms, such as ram pressure, are also at work, as it is sometimes observed (e.g., Fritz et al. 2017; Serra et al. 2024; Watson et al. 2025). Further details on the sample can be found in P16 and P17.

Galaxies in the control sample belonging to clusters were selected from the same WINGS/OmegaWINGS sample. This sample of 12 objects was assembled with the goal of contrasting the properties of stripping candidates with those of galaxies that show no optical evidence of ongoing gas removal. We selected 12 previously known spectroscopic cluster members, of which four were selected to be star-forming (having emission lines in their OmegaWINGS spectra) morphologically spiral galaxies that did not show any clear sign of extraplanar debris in the B-band WINGS+OmegaWINGS optical images. The remaining eight control galaxies were selected to be passive galaxies devoid of ongoing star formation, as testified by the lack of emission lines in their OmegaWINGS fiber spectra. Of these eight (five spirals and three S0s by choice), six were previously known post-starburst (k+a) galaxies (Paccagnella et al. 2017) and two had neither emission nor strong Balmer lines in absorption (spectral k-type, Dressler et al. 1999). GASP k+a galaxies have been the subject of a dedicated publication (Vulcani et al. 2020a) and will not be discussed in detail hereafter.

For all 76 galaxies, T-type morphologies (Hubble types) were assessed from V-band images using MORPHOT (see Vulcani et al. 2011, 2023; Fasano et al. 2012, for details), an automatic tool purposely devised in the framework of the WINGS project. MORPHOT was designed with the aim to reproduce as closely as possible visual morphological classifications.

The 76 galaxies were observed with MUSE/VLT as part of the GASP program to probe the physics of gas and stars and establish the main physical mechanisms, if any, acting on them. Details on observations, data reduction, and data analysis can be found in P17. Relevant for this paper are stellar masses, Hα fluxes, and the gas and stellar kinematics. To obtain emission-only data cubes on which we measured line fluxes, we subtracted the stellar-only component of each spectrum derived with our spectrophotometric code SINOPSIS (Fritz et al. 2014, 2017). In addition, SINOPSIS provided us with spatially resolved estimates of the following stellar population properties: stellar masses; luminosity-weighted age; average star formation rate and total mass formed in four age bins (=star formation histories, SFH): young (ongoing star formation) = t<2×107 yr, recent = 2×107<t<5.7×108 yr, intermediate-age = 5.7×108<t<5.7×109 yr, and old = >5.7×109 yr. The latter outputs of SINOPSIS are used only in Sect. 3.4.

To derive emission line fluxes, velocities, and velocity dispersions with associated errors, we used KUBEVIZ (Fossati et al. 2016). Before performing the fits, we averaged the data cube in the spatial direction with a 5 × 5 kernel, corresponding to our worst seeing conditions of 1″ = 0.7–1.3 kpc at the redshifts of our galaxies. To extract the stellar kinematics from the spectrum, we used the Penalized Pixel-Fitting (PPXF) code (Cappellari & Emsellem 2004), fitting the observed spectra with the stellar population templates by Vazdekis et al. (2010).

In Gullieuszik et al. (2020), we determined the galaxy disk boundaries computed from the map of the stellar continuum in the Hα region and from the isophote with a surface brightness 1σ above the average sky background level. Because of the (stellar and gaseous) emission from the stripped gas tails, this isophote does not have an elliptical symmetry. To obtain a symmetric isophote, we fit an ellipse to the undisturbed side of the isophote and replaced the isophote on the disturbed side with the ellipse. Everything inside of this isophote represents the galaxy disk; the rest constitutes the galaxy tail.

3. Results

3.1. Assessing the physical process at work and estimating the level of ram pressure

We consider a galaxy to be undergoing RPS if there is extraplanar ionized gas (i.e., Hα emission) preferentially on one side of the disk while the disk stellar kinematics is undisturbed. A chaotic stellar kinematics is considered a signature of strong tidal effects or mergers (e.g., Mihos et al. 1993; Struck 1999).

All galaxies were inspected and classified according to the degree of RPS (see Table 1), based on the visible extension of the Hα-emitting extraplanar (and unilateral) gas obtained from the MUSE data cubes. The classification ranges from weakest and mild stripping (JTypes = 0.3 and 0.5, respectively) with only weak signs of extraplanar gas, to strong stripping (JType = 1) that show a significant gaseous tail, to extreme stripping (JType = 2) that have a gas tail at least as long as the stellar disk diameter. The latter are the most striking cases, which we refer also to as jellyfish galaxies. Finally, a JType = 3 was assigned to galaxies that have a truncated Hα disk with gas present only in the central regions of the disk and little extraplanar gas. These are most probably in an advanced stage of stripping and might have gone through one of the other classes during their evolution (see Sect. 3.3). A JType = 4 was assigned to those galaxies that have no ionized gas left, and a JType = 0 to those galaxies with an undisturbed ionized gas distribution without any sign of stripping, while −9 represents unknown cases and −99 is assigned to mergers.

Table 1.

Stripping types scheme and number of galaxies in the stripping candidate sample.

We note that both JType = 0.3 and 0.5 galaxies are at a very early stage of stripping and are similar from the point of view of the strength of the tail. The only difference between them is that, for the purposes of studying the galaxy disks, JType = 0.3 galaxies have often been used in previous GASP papers as control sample galaxies (Vulcani et al. 2018c, 2019a, 2020b; Franchetto et al. 2020, 2021b; Bellhouse et al. 2021; Tomičić et al. 2021a, b; Peluso et al. 2023) because the properties of the gas within their stellar disk can be considered unaffected by stripping. For the purposes of this paper, given the similarities in the extraplanar gas amount between JType = 0.3 and JType = 0.5, in the following figures, we consider these two classes together.

Figure 1 shows an example for each JType 0.5, 1, 2, and 3, presenting the MUSE Hα flux and velocity maps, the stellar velocity map, and an RGB image obtained by combining the g−r−i MUSE images. Figures for all the other galaxies, as well as their classification and properties, are presented in the Appendix A.

thumbnail Fig. 1.

Illustrative examples of stripped galaxies of different JTypes. Top left: JType = 0.5, mild stripping. Top right: JType = 1, strong stripping. Bottom left: JType = 2, extreme stripping. Bottom right: JType = 3, truncated disks. For each galaxy, the Hα flux (top left), the gas kinematics (top right), the stellar kinematics (bottom left), and the color composite image (bottom right) are shown. The pink line delimits the stellar disk as described in Sect. 2.

The classification described above is based on visual inspection of the Hα extension with respect to the stellar disk. For galaxies with 0.3 ≤ JType ≤ 3, we also measured two quantities that should be linked with the degree of stripping: the fraction f H α out $ f_{\mathrm {H\alpha }}^{\mathrm {out}} $, defined as the percentage of Hα emission1 that is outside of the stellar disk, and the total Hα luminosity LHα outside of the stellar disk. These quantities are shown in Fig. 2 for the different JTypes. A discussion of the stellar mass distribution of the different JTypes is deferred to Sect. 3.3. At some level, f H α out $ f_{\mathrm {H\alpha }}^{\mathrm {out}} $ and f H α out $ f_{\mathrm {H\alpha }}^{\mathrm {out}} $ are linked, but they are not equivalent: for example, the two galaxies with the highest fraction of Hα in the tail (JO149 and JW56) are low-mass galaxies that have over 30% of their total Hα emission in their tails, but have only a moderate tail Hα luminosity because their overall Hα luminosity is quite low.

thumbnail Fig. 2.

Tail Hα luminosity versus fraction of Hα emission in the tail for different JTypes: 3 = blue, 2 = red, 1 = orange, and 0.5 = gray. Values are computed for S/N Hα = 4, and the error bars denote the range of values with cuts at S/N = 3 and 5. The size of the points is proportional to the stellar mass.

As shown in Fig. 2, above f H α out ∼ 0.1 $ f_{\mathrm {H\alpha }}^{\mathrm {out}} \sim 0.1 $, the great majority of stripped galaxies have a JType = 2 and a LHα≥1040 erg/s. On the other extreme, all truncated disks have f H α out < 0.05 $ f_{\mathrm {H\alpha }}^{\mathrm {out}} < 0.05 $ and LHα≤4×1039 erg/s. The JType = 1 and 0.5 classes span a range of values that is intermediate between the two classes described above, with typical values of f H α out ≥ 0.05 $ f_{\mathrm {H\alpha }}^{\mathrm {out}} \geq 0.05 $ and LHα≥5×1039 erg/s for JType = 1 and f H α out $ f_{\mathrm {H\alpha }}^{\mathrm {out}} $ between 0.01 and 0.05 and LHα between 0.5 and 5×1039 erg/s for JType = 0.5. The visual discrimination between JTypes 1 and 2 is clearly driven more by the Hα tail length than by the amount of extraplanar ionized gas, although galaxies with the longest ionized gas tails (JType = 2) tend to have the most luminous tails and the largest fractions of Hα emission residing in the tail.

3.2. Comparison with stellar light classifications

The JType classification shown above is independent of the “JClass” assigned in P16. While the latter was based on B-band stellar emission, which may originate from stars formed in the stripped gas, here we directly assess the ionized gas that is detected outside the galaxy disk. This provides direct evidence of stripping and, in principle, is not related to the ionization mechanism of such gas, which could be star formation or other processes. It is therefore interesting to check a posteriori the correspondence between the MUSE (gas)-based GASP classification (JType) and the imaging-based P16 classification (JClass), as shown in Fig. 3. In this plot, we consider together JType = 3 and 4 galaxies, given their similar appearance in the B-band images.

thumbnail Fig. 3.

Comparison between the original JClass by Poggianti et al. (2016) and the JType presented in Table 1, for the GASP galaxies constituting the stripping sample. Galaxies with JType ≤ 0 are considered as failures; galaxies with JType = 0.3 and 0.5 are the weakest and mild cases of stripping; galaxies with JType = 1 are cases of strong stripping; galaxies with JType = 2 are cases of extreme stripping; and galaxies with JType = 3 and 4 are galaxies at the final stages of stripping, i.e., truncated disks and fully stripped, respectively. The number of galaxies is given inside the boxes, and the color darkness increases with the number of galaxies.

Overall, there is a good correspondence between JType and JClass for all types, from the weakest to the most extreme stripping (0.3–2), as well as for truncated disks (JType = 3). In fact, the latter are expected to have only weak signatures still visible in the images and are in fact dominated by JClass = 1 objects. The JClass-JType correspondence is particularly striking for the JClass = 5: GASP has confirmed the most secure P16 stripping candidates as jellyfish JType = 2 galaxies in 8 spectroscopically confirmed cluster members, the only mismatch being JO190, a foreground merger discussed in Sect. 3.2.

We note two other interesting trends. First, there are a number of cases with very weak imaging signatures (JClass = 1) that turn out to be spectacular cases (JType = 2 and 1) when the MUSE observations of the gas are considered. Being able to detect directly the stripped gas is clearly a great advantage compared to trying to detect the stars that form within this gas. Second, we note that, vice versa, a significant fraction of the weak MUSE cases (JTypes 0.3 and 0.5) were considered much more promising cases from the images (JClass 3 and 4). This is not due, as it might be expected, to contamination in the images from superimposed background or foreground B-band sources2. Inspecting again these cases, it turns out that the reason for the mismatch is that while the JClass was meant to give an assessment of how secure we were that the galaxy was subject to ram pressure, the JType focuses on the direct observation of extraplanar gas. The two aspects are correlated in the majority of cases, but not always, as even the B-band morphology in the disk provide strong evidence for ram pressure at work without a long tail. For example, JO13 (a JClass = 4 and a JType = 0.5; see Appendix) has a clear half-ring with very bright HII regions on one side of the disk, strongly suggesting the impact of ram pressure, though it has only a small amount of extraplanar gas.

3.3. Success rate and properties of stripped galaxies

Of the 64 stripping candidates, 56 have been confirmed to be subject to RPS, which disturbs their gas in a detectable way according to the criteria described above. From now on, we refer to these as “stripped galaxies”. Three of them (JO24, JO73, and JO1343) turned out to be members of a background cluster or group that exerts ram pressure on them. In addition, JW36 is most likely the end product of stripping, being a blue, comet-like galaxy devoid of ionized gas, with strong Hβ in absorption (plot not shown), and thus has characteristics quite similar to IC3418, one of the prototypical jellyfish galaxies in the Coma cluster (Hester et al. 2010). Therefore, overall, 89% (57/64) of the stripping candidates have been confirmed by the GASP data. Of the remaining seven galaxies, one (JW105) is a chance superposition between a passive cluster member early-type galaxy and some background sources at z∼0.38 and z∼0.15; three galaxies show signs of a merger (the cluster member JO153 and the nonmembers JO20 and JO190; see Vulcani et al. 2021), two are likely interacting with a close neighbor (JO119 and JO157), and one (JW10) has an uncertain classification given the morphology of the Hα-emitting extraplanar gas.

With a large sample of IFU-confirmed ram pressure stripped galaxies in a wide range of stripping stages in 39 different clusters, it is possible to address questions regarding the intrinsic properties of the stripped galaxies and the occurrence of different levels of stripping in the various regions of clusters and in different types of clusters. In what follows, we also include three galaxies from the control sample, which are discussed in detail in Sec. 4. Briefly, only one control sample galaxy is truly undisturbed, while the other three star-forming galaxies of the control sample show signs of stripping. These latter are classified in the same way as the stripping sample, following the classification scheme presented in Table 1.

The stellar mass distribution of galaxies of different JTypes is shown in Fig. 4. Extreme and strong stripping cases (JTypes 2 and 1) are found in a very wide range of masses, from 6×108 to 3×1011 M⊙. The most massive stripped galaxies (≥1011 M⊙) are all spectacular jellyfish cases (JType = 2) (see also Luber et al. 2022), but the majority of jellyfish galaxies in the GASP sample (9 out of 16) have masses ≤4×1010 M⊙. In Fig. 2, we also see that the JType = 2 galaxies with the highest Hα tail luminosities (∼1041 erg s−1 and above) are have large stellar masses.

thumbnail Fig. 4.

Stellar mass distribution for galaxies of different stripping types, as indicated in the legend. A3376_B_0261 is the only galaxy from the control sample with JType = 0, reported as a black square in the JType ≤ 0 panel. The gray distribution in the background represents the entire sample of 76 galaxies.

Moreover, the masses of truncated disks range from 4.6×109 to 6.5×1010 M⊙, showing that this phase of stripping can also be observed in galaxies over a wide mass range. Finally, it is noticeable that all the weakest and mild cases (JTypes 0.3 and 0.5) have masses below 2×1010 M⊙. The median mass is ∼8×109 M⊙ for JTypes = 0.3+0.5 and JType = 1, while it is five times higher (4×1010 M⊙) for JType = 2. Note that in this and the following figure, we likewise include a histogram for JType = 4 galaxies for completeness.

Given that the JType classification is influenced by visibility effects and observational biases, it is hard to draw conclusions regarding the occurrence of different levels of ram pressure as a function of galaxy mass without risking over-interpretation. In fact, as discussed in detail in Gullieuszik et al. (2020), the mass of the galaxy is only one of many factors that determine the level of stripping. However, the fact that strong stripping effects are observed over more than two orders of magnitude in mass is a solid result.

Moving to galaxy Hubble types, the great majority of GASP stripped galaxies are spirals of types between Sab and Sc, as shown in Fig. 5, and no T-type segregation is observed going from weakest to extreme stripping. The fully stripped galaxy sample, instead, by selection, included S0s and early spirals.

thumbnail Fig. 5.

Morphological type distribution for galaxies of different stripping types, as indicated in the legend. Colors and symbols are the same as in Fig. 4. Ttypes and Hubble types are 6 = Scd, 5 = Sc, 4 = Sbc, 3 = Sb, 2 = Sab, 1 = Sa, 0 = S0/a, −1 = S0+, −2 = S0, −4 = E/S0, and −5 = E.

Next, we present the location of the various JTypes in the projected position versus projected velocity phase-space diagram in Fig. 6. Distances of the galaxies are measured from the Brightest Cluster Galaxy, velocity dispersions are from Biviano et al. (2017) and Gullieuszik et al. (2020). Several simulation studies have characterized the typical time since infall of galaxies located in different regions of the phase-space diagram. We use here the representation from Rhee et al. (2017), who identified the regions where the majority of galaxies lie at a given epoch after they enter the cluster halo (see their Fig. 5). Rhee et al. (2017) separated galaxies into first (not fallen yet; turquoise), recent (0<tinfall<3.63 Gyr; purple), intermediate (3.63<tinfall<6.45 Gyr; yellow), and ancient (6.45<tinfall<13.7 Gyr; red) infallers. Obviously, these numbers should be taken with caution, as each galaxy can be located beyond the corresponding region, and the observed radii and velocities, being projected quantities, are lower limits to the real values. In Jaffé et al. (2018), we studied the phase-space diagram of a subset of the sample of this paper (those available at the time) and concluded that jellyfish galaxies are moving at very high speeds close to the cluster center and are a recently infallen population moving mostly on radial orbits. In Gullieuszik et al. (2020), we studied the location in the phase-space diagram of a subset of the galaxies of this paper as a function of the star formation rate in the tails, finding a good correlation between star formation and the phase-space region.

thumbnail Fig. 6.

Stripping types distributed across the projected phase space diagram, as indicated in the legend. Overplotted are colored lines that delimit the regions defined by Rhee et al. (2017): first (not fallen yet; turquoise), recent (0<tinfall<3.63 Gyr; purple), intermediate (3.63<tinfall<6.45 Gyr; yellow), and ancient (6.45<tinfall<13.7 Gyr; red) infallers. The dashed line indicates the limit of subhalos, in order to define galaxies bounded to the clusters. The size of the circles is proportional to the galaxy stellar mass, and masses are given in Table A.1. The gray contours in the background represent the density of points from all spectroscopic cluster members in the WINGS/OmegaWINGS sample. The control sample galaxies are not shown here.

In Fig. 6, we now show the full sample and confirm the results from both Jaffé et al. (2018) and Gullieuszik et al. (2020). The most striking segregation in the plot is the location of the extreme stripping (cyan circles) galaxies, which are also those with the highest Hα tail luminosities and, thus, the highest SFR in the tails (cf. Fig. 3). These galaxies are all located in the region at high velocities and low clustercentric radii, where recently infallen galaxies are mostly located, above the purple line. The strong stripping galaxies (green circles) occupy, on average, larger radii and lower velocities than JType = 2 but are characterized by a large range of both quantities. They are spread across most regions of the plot, except the recently infallen region, and some approach at high velocities close or beyond the cluster virial radius. JType = 0.3+0.5 galaxies (blue circles) are found at projected radii between 0.5 and 1.7 r/r200 and thus avoid the cluster central region, on average at larger radii than the previous types, and in most cases at velocities below 1 v/σ. Finally, JTypes = 3 (truncated disks) are consistent with having entered the cluster earlier than the other JTypes, as they are located in or close to the ancient infallers region.

Overall, the location within the phase-space diagram appears to be more relevant than the cluster velocity dispersion, which can be considered a proxy for the cluster mass. The distribution of velocity dispersions σ of clusters with GASP stripped galaxies is shown in the top panel of Fig. 7. Velocity dispersions range from 400 to above 1000 km s−1, thus encompassing clusters such as Fornax, Virgo, and Coma. Most of the GASP clusters have intermediate σ values between 500 and 900 km s−1. Interestingly, weakest+mild (JType = 0.3+0.5), strong (JType = 1), and extreme (JType = 2) stripping cases are observed across the entire range of σ from ∼550 to 950 km s−1.

thumbnail Fig. 7.

Top: Velocity dispersion distribution of the clusters hosting at least one ram pressure stripped galaxy (JType ≥ 0.3). Cluster masses, computed from the velocity dispersions assuming virialization, are shown in the top axis. The vertical lines show the velocity dispersion of well-studied clusters in the local Universe (from left to right: Fornax, Virgo, and Coma). Bottom: Frequency of galaxies of different JTypes as a function of the velocity dispersion of the hosting cluster. The symbol size is proportional to the frequency.

The GASP sample does not include mild cases in the most massive clusters (σ>1000 km s−1) and extreme cases in very low-mass clusters σ<550 km s−1, but these two trends could be due to low number statistics. What the GASP data clearly show is that strong and extreme ram pressure can be effective even in clusters as small as Virgo or even smaller: being hosted in a very high-mass cluster is not a necessary condition for jellyfish galaxies and strong ram pressure.

3.4. The truncated disks

Four of our stripped galaxies have a truncated Hα disk (JO10, JO23, JO36, JW108). All of them have ionized gas only left in the central region of the disk and less than 5% of their Hα luminosity in small clouds or tendrils outside of the disk. For these reasons, they have been assigned a JType = 3. One of them has been discussed in detail in Fritz et al. (2017), and the other three are shown in Figures 1, 8, and A.2.

As shown by numerous hydrodynamical simulations (e.g., Roediger & Hensler 2005; Roediger et al. 2014; Kapferer et al. 2008, 2009; Tonnesen et al. 2011; Tonnesen & Bryan 2012; Zhu et al. 2024; Akerman et al. 2024), ram pressure strips the gas first from the outer regions of the disk where it is less gravitationally bound and then proceeds inward. When a galaxy is hit by ram pressure face-on, the stripping occurs in progressively inner cylindrical annuli in an axisymmetric manner, but in the majority of cases, there will be an angle between the wind direction and the disk axis. Depending on the viewing angle of the observer, in projection, we observe only one side of the galaxy devoid of gas as long as there is a significant tail. This is the case for some of the best-studied galaxies in the GASP sample, such as JO206, JO204, or JW100 (see the appendix), which all have a “gas disk that is truncated” but still have a long tail.

What we define as “truncated disks” in this paper, instead, are galaxies that have very little extraplanar gas and are in an advanced stage of stripping, occurring before the eventual stripping of all gas and after a phase in which the extraplanar gas tail was probably more prominent. This recent past can be recovered from the MUSE data, as shown in Fig. 8 for JO23 as an example. The RGB image of JO23 (middle right) clearly shows an elongation of the blue stellar light (stellar tail) toward the southwest, while the gas is left only in an approximately circular (when deprojected) region around the galaxy center. Analyzing the maps of star formation history (bottom panels), as derived with the code SINOPSIS (see Sect. 2), we see where stars formed during four intervals in time. Currently (based on the ongoing star formation rate, 2×107 yr), stars form only in the central region, where gas remains. Quite recently (between 2×107 and 5.7×108 yr ago, and at a lower level in the previous age bin), stars formed both in a larger central area and along a tail to the southwest, where gas must still have been present. This allows us to understand the projected direction of stripping. At older times (>5.7×109 yr ago), stars were forming in a disk regularly centered on the galaxy center, with no star-forming tails. Unfortunately, given that the age resolution of the spectrophotometric reconstruction deteriorates at progressively older ages, we cannot age-date the beginning of the stripping more precisely than in these four age bins, but this analysis is already sufficient to observe the progressive evolution of the star formation and, therefore, of the gas stripping.

thumbnail Fig. 8.

Example of a JType = 3 (truncated galaxy): JO23. Top: Hα flux (top left), the gas kinematics (top right), the stellar kinematics (bottom left), and the color composite image (bottom right). Panels and colors are the same as in Fig. 1. Bottom: Stellar maps of different ages, illustrating the average star formation rate per kpc2 during the last 2×107 yr (left), between 2×107 and 5.7×108 yr (central left), between 5.7×108 and 5.7×109 yr (central right), and >5.7×109 yr ago (right). The green ellipses show the contour that defines the stellar disk (see text for details).

By similarly studying the star formation history from MUSE data for galaxies in the subsequent stage of stripping, i.e., with no gas left, we previously showed that the great majority of galaxies with post-starburst spectra in clusters are the end product of ram pressure. It has also been possible to reconstruct the stripping (i.e., quenching) histories both in the GASP sample (Vulcani et al. 2020a) and in a sample of galaxies in clusters at z = 0.3–0.5 (Werle et al. 2022).

4. The control sample

We remind the reader that, in addition to the stripping candidate galaxies discussed in the previous sections, GASP observed 12 “control cluster galaxies” that did not show any sign of stripping from optical B-band imaging. The main properties of control cluster galaxies are listed in Table A.2. As already mentioned, eight of these galaxies were selected to be devoid of gas (and therefore are JType = 4) and a dedicated study has been presented in Vulcani et al. (2020a). Out of the remaining four star-forming control cluster galaxies that were not expected to be undergoing stripping based on B-band data, three turn out to be stripped at some level when observed by MUSE, with some Hα emission outside and preferentially on one side of the stellar disk, as visible in Figures 9 and A.1. Two of them are very weak cases of stripping and are classified as JType = 0.5: A970_B_0338 has short Hα filaments extending to the west of the disk, and A3128_B_0148 has two small tendrils of gas coming out from the southwest side of the disk. The other galaxy, A3266_B_0257, is a JType = 1 with detached gas clouds at the northeast side of the disk. The Hα fractions and luminosity outside of the disk are consistent with those of stripped galaxies of similar JType: that is fHαout∼0.01−0+.02 in A970_B_0338 and A3128_B_0148 and 0.18 in A3266_B_0.257. This confirms the well-known fact that stripping signatures are much more visible when observing the ionized gas rather than the total optical light, and that cases such as these go unnoticed when selecting from optical imaging.

thumbnail Fig. 9.

Illustrative example of the control sample galaxy A3128_B_0148. Panels and colors are the same as in Fig. 1.

A notable property of these four star-forming galaxies is their combination of emission line ratios at the edges of their disks. Figure 10 shows the Baldwin et al. (1981) (BPT) diagrams and maps based on the [NII] and [OI] lines, which are commonly used to discriminate between regions powered by star formation, by a central AGN, or by other mechanisms (LINER, shocks, etc.). While, according to the [NII] diagram, the ionizing source of the gas is star formation throughout the disk, there is a clear excess of [OI] emission at the disk edges, in a ring shape. Such an excess has been observed to be quite common in significant portions of the tails of stripped cluster galaxies (Fossati et al. 2016; Poggianti et al. 2019a; Tomičić et al. 2021b), and it may arise from a layer of mixed intracluster medium–interstellar medium gas at temperatures ∼1 keV, as reproduced by photoionization models published in Campitiello et al. (2021).

thumbnail Fig. 10.

Diagnostic diagrams for [O III]5007/Hβ vs. [N II]6583/Hα and [OI]6300/Hα for the control sample galaxies. For each galaxy, the top panel shows the BPT line-ratio map, while the bottom panel shows the corresponding spatially resolved BPT diagram. Lines are included from Kauffmann et al. (2003, K03), Kewley et al. (2001, K01), and Sharp & Bland-Hawthorn (2010, SB10) to separate star-forming (red), composite (orange), AGN (green), and LINER (cyan) galaxies. Only spaxels with a S/N > 3 in all of the emission lines involved are shown. Adopting a S/N > 4 does not modify these plots.

The GASP sample includes 14 control field galaxies (outside of clusters) lacking signs of stripping from B-band imaging. These are galaxies mostly in small groups and filaments, which are not expected to be embedded in a very dense and hot intergalactic medium and, therefore, are not expected to be ram pressure stripped. Interestingly, the [OI] excess seen in Fig. 10 is also observed in our field control sample galaxies (plots not shown; sample presented in Vulcani et al. 2018a) in a similar way. This indicates that the unusual emission line ratio at the periphery of disks is not a cluster-specific effect. We note that the regions with the [OI] excess are regions of diffuse ionized gas (DIG) (see the plots in Tomičić et al. 2021a)4. We also stress that the [OI] excess observed in the tails of stripping galaxies, as well as at the edges of control cluster and field galaxies in the GASP sample, cannot originate from contamination by sky lines, nor can it be an artifact due to low S/N. Even when retaining only spaxels with a much higher S/N cut (even above 4), the effect remains visible.

The presence of DIG in the extraplanar regions up to several kpc from the disk, with enhanced low-ionization line ratios ([NII]/Hα, [SII]/Hα, [OI]/Hα) in disagreement with photoionization by OB stars, was already revealed a few decades ago by long-slit spectroscopy of a few edge-on galaxies (see, e.g., Sokolowski et al. 1991). Studies based on MaNGA (Bundy et al. 2015) and SAMI (Croom et al. 2012) data observed this excess in a few cases of edge-on galaxies (Belfiore et al. 2016; Fogarty et al. 2012) but these previous integral field surveys did not have the necessary areal coverage to observe the whole LINER-like [OI] ring.

Different ionization mechanisms were proposed to reproduce such ratios: shocks originated, for example, in expanding structures such as superbubbles (Rand 1998; Collins & Rand 2001), or starburst-driven shocks (Croom et al. 2012); the so-called Turbulent Mixing Layers (Slavin et al. 1993), where the line-emitting regions are at the interface between cold (T≤104 K) and hot (T>105 K) gas; hot evolved (pAGB) stars (Belfiore et al. 2022), or younger (age < 25 Myr) stars in low-density gas (McClymont et al. 2024), producing a ionizing radiation harder than OB stars. However, while in the cases cited above the excess occurs for all the BPT low-ionization line ratios, in the GASP data, we often observe an excess only in [OI]/Hα, whereas [NII]/Hα and [SII]/Hα are more in agreement with ionization by OB stars: this introduces a further challenge to the interpretation. Whether this can arise from the interaction with a surrounding medium hotter than the interstellar medium, which can be the circumgalactic and/or the intergalactic medium, with a mechanism similar to the one proposed for the stripped tails at the interface of the stripped gas and the intracluster medium, is a possibility that is worth exploring in the future. We defer the discussion of the [OI] excess issue to a separate paper (Radovich et al., in prep.), where a more detailed comparison of different photoionization models will be given.

5. Discussion: Identifying ram pressure using observations at different wavelengths

In the previous sections, we showed that an optically selected sample of stripping candidates obtained from blue-light imaging of galaxy clusters (P16) has a high success rate in identifying galaxies subject to RPS. As discussed in Vulcani et al. (2021), this is not the case when selecting galaxies in the same way from general field samples: based on MUSE data as those used in this paper, among 27 GASP non-cluster galaxies in Vulcani et al. (2021), we identified a plethora of physical mechanisms as the cause for the galaxy disturbance (galaxy-galaxy interactions, mergers, cosmic web enhancement, starvation, gas accretion, and ram pressure in groups and filaments). Together, these results show that optical imaging selection is only effective when applied in cluster fields and requires deep and high-quality imaging.

Blue or UV imaging (Smith et al. 2010; Vulcani et al. 2021; Durret et al. 2021, 2022; McPartland et al. 2016; Ebeling et al. 2014; Cortese et al. 2007; Roman-Oliveira et al. 2019; Roberts et al. 2022a; Owers et al. 2012; George et al. 2024) is just one of the several methods used to identify RPS candidates. Observations of gas in different phases (molecular (typically CO, Brown et al. 2021, 2023), neutral (HI, Serra et al. 2023, 2024; Ramatsoku et al. 2019, 2020; Deb et al. 2022, 2023), ionized gas (through Hα imaging or integral field spectroscopy such as GASP, Boselli et al. 2018; Moretti et al. 2022; Gondhalekar et al. 2024), as well as radio continuum and X-ray studies (Sun et al. 2010, 2021; Roberts et al. 2021b, 2022b; Ignesti et al. 2023a) all provide samples of ram pressure candidates, which are all affected by intrinsic incompleteness and observational biases. In the coming years, LOFAR and SKA-Low will be efficient at finding radio tails over large areas of sky, also because the length of the radio tails increases at low frequencies (Ignesti et al. 2022b; Roberts et al. 2024). However, no observational method for identifying ram pressure ensures a complete, holistic view of the stripping phenomenon. An optical/ultraviolet imaging selection method assumes that at least some extraplanar star formation must be present in the stripped gas. This is not always the case; however, as shown in this paper, such an optical selection results in a very diverse range of observed stripping strengths, and even cases with very little star formation in the tail can be identified (see Gullieuszik et al. 2020).

One might think that relying directly on a gas tracer (such as, for example, Hα tracing the ionized gas, or an HI survey) could provide a “unbiased” complete sample. However, this is not the case, as demonstrated by the multiwavelength observations of a sample such as GASP. For example, there can be galaxies with prominent Hα tails and little neutral extraplanar gas (see, e.g., Ramatsoku et al. 2020; Deb et al. 2022). In Ramatsoku et al. (in preparation), we begin with a large sample of HI-selected galaxies that have neutral gas [HI] tails and show that they do not always have significant star formation in the tail and thus do not display visible extraplanar debris in optical and ultraviolet imaging. Radio continuum tails, which are numerous in clusters and groups of galaxies (Roberts et al. 2021a,b, 2022b; Serra et al. 2024), show the same behavior.

It is thus important to realize that depending on the wavelength of selection, we obtain different samples, only partially overlapping. Much of the mismatch of the samples selected with different techniques can be attributed to different stages of the stripping process. In GASP, we observe that the HI content of Hα-selected jellyfish galaxies is generally already low (Deb et al. 2022; Moretti et al. 2020a, 2023), whereas HI-rich galaxies with spectacular HI tails have not (yet?) developed a star-forming tail. While a detailed discussion of this is beyond the scope of this paper and is deferred to Ramatsoku et al. (in prep.), here it is important to consider whether the different JTypes we discuss throughout this paper correspond to different stages of stripping.

Truncated disks are, for the reasons discussed in Sect. 3.3, in an advanced stage that probably follows one of the other JTypes, and are the precursors of post-starburst cluster galaxies. For JTypes = 0.3, 0.5, 1, and 2, it is harder to demonstrate an evolutionary sequence. Logically, such a sequence would go from an initial phase when the stripping has just begun (JType = 0.3–0.5) to a subsequent stage with more prominent tails (JTypes = 1 and 2). However, as we discussed when commenting Fig. 7, the location in the phase space indicates that the most spectacular tails in jellyfish galaxies form on first infall on radial orbits of gas-rich galaxies approaching with large velocities relative to the cluster (see also Biviano et al. 2024)5. As not all gas-rich galaxies infalling into clusters will have these orbital characteristics, in principle it might well be that some galaxies will never go through a JType = 2 phase and will be stripped more gently. For example, most JTypes = 0.3–0.5 galaxies have relatively low velocities with respect to the cluster systemic velocity, although as they approach the cluster center, they may gain higher velocities, moving upward and leftward in Fig. 7. To conclude, most of the JTypes cannot be necessarily interpreted as different stages of an evolutionary sequence, and the reasons for the different tail morphologies and luminosities lie in the many factors determining the physics of the ram pressure phenomenon (the most important ones being galaxy mass, position in the phase-space diagram, and cluster mass; see Gullieuszik et al. 2020) as well as the amount of gas in the galaxy when they fall into the cluster.

6. Summary

We presented the complete sample of cluster galaxies from the GASP survey6, including 64 RPS candidates and 12 control (undisturbed) galaxies in 39 galaxy cluster fields at z = 0.04−0.07.

Using MUSE IFS of these galaxies, we assessed whether the gas disturbed morphology originates from RPS based on ionized gas and stellar velocity maps. The visual classification of gas disturbance (stripping strength, JType), ranging from very weak or weak to strong to extreme cases (jellyfishes) of ram pressure, is well correlated with the fraction and the amount of Hα luminosity found in a stripped tail, outside the stellar disk. GASP targets were selected from an optically selected sample of stripping candidates based on B-band images. Comparing the imaging-based and the MUSE-based assessment of stripping strength, we find a very good correlation, as well as some interesting cases where the MUSE data demonstrate the superiority of directly observing ionized gas to investigate not only the existence but also the extent of the tail.

Of the 64 RPS imaging candidates, the MUSE data confirm 56+17. Thus, 89% of the candidates have been confirmed, showing that blue-light imaging selection in clusters, and only in clusters, is likely to yield confirmed cases in the vast majority of instances. Failures are mostly due to interactions or mergers, and only one is a chance superposition of sources at different redshifts. We discussed the properties of the whole sample of ram pressure stripped galaxies, including stellar mass and morphological distributions, and we presented the location of the various JTypes in the phase-space diagram. The velocity dispersions of clusters that host ram pressure stripped galaxies in the GASP sample range from less than 500 to over 1000 km s−1. We show that all levels of stripping strength, from weak to extreme, are present in clusters across the entire range of velocity dispersion, and thus cluster masses.

By analyzing in detail one of the JType classes, the class of truncated Hα disks, we conclude that these galaxies are at an advanced stage of stripping and show that their past stripping history can be studied from star formation history reconstruction.

Analyzing the star-forming galaxies in the control sample, we find that three out of the four already show weak but detectable signs of stripping that were not identified in the imaging. A notable property of these galaxies, as well as all GASP control sample galaxies – even those in the field – is the fact that they display a ring of peculiar emission-line ratios surrounding the disk. According to BPT diagnostic diagrams, such rings appear simply star-forming when using the [NII] line, but show an excess of LINER-like [OI] emission. We mention possible causes of this phenomenon, which has not been identified before as a ring and is common in the outskirts of all spirals, both in clusters and in the field. Lastly, we discussed the biases and incompleteness of this and other methods for selecting ram pressure stripped galaxies, and reasoned that different JTypes do not always necessarily correspond to an evolutionary sequence in representing different stages of RPS.

Acknowledgments

We thank the referee whose comments helped us to improve the manuscript. Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere under ESO program 196.B-0578. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 833824). We thank the INAF GO grant 2023 “Identifying ram pressure induced unwinding arms in cluster spirals” (PI Vulcani). RS acknowledges financial support from FONDECYT Regular projects 1230441 and 1241426, and also gratefully acknowledges financial support from ANID – MILENIO – NCN2024_112. JF acknowledges financial support from the UNAM- DGAPA-PAPIIT IN110723 grant, México.

1

The Hα emission is computed irrespective of the ionization mechanisms of such emissions, i.e., including emissions powered by star formation, LINER, and AGNs, according to BPT (Baldwin et al. 1981) diagrams.

2

The only case of such a superposition is a clear failure, JW105, described in the next section.

3

JO134 is subject to both ram pressure and a minor merger, as discussed in detail in Vulcani et al. (2021).

4

We note that the inverse is not true, i.e., not all the DIG spaxels lie in the LINER region of the [OI] diagram.

5

As discussed in Jaffé et al. (2018) and Luber et al. (2022), more massive galaxies plunge deeper into the cluster before being stripped, and at that point they are in such a hostile environment that their tails become spectacular. However, in the GASP sample, there are also some lower mass galaxies with extreme tails that can be observed at quite low radial velocities; see Fig. 7.

6

A constantly updated list of all the papers from the GASP survey can be found at https://web.oapd.inaf.it/gasp/

7

We note that 56 still have gas; and one has no gas left, but rather a visible tail of recently formed stars.

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Appendix A: Properties of all galaxies

In this appendix, we present the properties of the full sample discussed in the main paper. Table A.1 presents the JType classification of all stripping candidates together with other relevant quantities. The main properties of control cluster galaxies are listed in Table A.2. Figures A.1 and A.2 show the Hα flux, the gas kinematics, the stellar kinematics and the color composite image for the control sample and the, the stripping sample, respectively, in alphabetical order.

thumbnail Fig. A.1.

Control sample galaxies. Panels and colors are the same as in Fig.1.

thumbnail Fig. A.2.

Stripping candidates. Panels and colors are the same as in Fig.1.

Table A.1.

Properties of galaxies in the stripping sample.

Table A.1.

Continued.

Table A.2.

Properties of galaxies in the control sample.

thumbnail Fig. A.2.

Continued.

thumbnail Fig. A.2.

Continued.

thumbnail Fig. A.2.

Continued. Stripping candidates.

thumbnail Fig. A.2.

Continued.

thumbnail Fig. A.2.

Continued.

thumbnail Fig. A.2.

Continued.

thumbnail Fig. A.2.

Continued.

thumbnail Fig. A.2.

Continued.

thumbnail Fig. A.2.

Continued.

All Tables

Table 1.

Stripping types scheme and number of galaxies in the stripping candidate sample.

In the text
Table A.1.

Properties of galaxies in the stripping sample.

In the text
Table A.1.

Continued.

In the text
Table A.2.

Properties of galaxies in the control sample.

In the text

All Figures

thumbnail Fig. 1.

Illustrative examples of stripped galaxies of different JTypes. Top left: JType = 0.5, mild stripping. Top right: JType = 1, strong stripping. Bottom left: JType = 2, extreme stripping. Bottom right: JType = 3, truncated disks. For each galaxy, the Hα flux (top left), the gas kinematics (top right), the stellar kinematics (bottom left), and the color composite image (bottom right) are shown. The pink line delimits the stellar disk as described in Sect. 2.
In the text
thumbnail Fig. 2.

Tail Hα luminosity versus fraction of Hα emission in the tail for different JTypes: 3 = blue, 2 = red, 1 = orange, and 0.5 = gray. Values are computed for S/N Hα = 4, and the error bars denote the range of values with cuts at S/N = 3 and 5. The size of the points is proportional to the stellar mass.
In the text
thumbnail Fig. 3.

Comparison between the original JClass by Poggianti et al. (2016) and the JType presented in Table 1, for the GASP galaxies constituting the stripping sample. Galaxies with JType ≤ 0 are considered as failures; galaxies with JType = 0.3 and 0.5 are the weakest and mild cases of stripping; galaxies with JType = 1 are cases of strong stripping; galaxies with JType = 2 are cases of extreme stripping; and galaxies with JType = 3 and 4 are galaxies at the final stages of stripping, i.e., truncated disks and fully stripped, respectively. The number of galaxies is given inside the boxes, and the color darkness increases with the number of galaxies.
In the text
thumbnail Fig. 4.

Stellar mass distribution for galaxies of different stripping types, as indicated in the legend. A3376_B_0261 is the only galaxy from the control sample with JType = 0, reported as a black square in the JType ≤ 0 panel. The gray distribution in the background represents the entire sample of 76 galaxies.
In the text
thumbnail Fig. 5.

Morphological type distribution for galaxies of different stripping types, as indicated in the legend. Colors and symbols are the same as in Fig. 4. Ttypes and Hubble types are 6 = Scd, 5 = Sc, 4 = Sbc, 3 = Sb, 2 = Sab, 1 = Sa, 0 = S0/a, −1 = S0+, −2 = S0, −4 = E/S0, and −5 = E.
In the text
thumbnail Fig. 6.

Stripping types distributed across the projected phase space diagram, as indicated in the legend. Overplotted are colored lines that delimit the regions defined by Rhee et al. (2017): first (not fallen yet; turquoise), recent (0<tinfall<3.63 Gyr; purple), intermediate (3.63<tinfall<6.45 Gyr; yellow), and ancient (6.45<tinfall<13.7 Gyr; red) infallers. The dashed line indicates the limit of subhalos, in order to define galaxies bounded to the clusters. The size of the circles is proportional to the galaxy stellar mass, and masses are given in Table A.1. The gray contours in the background represent the density of points from all spectroscopic cluster members in the WINGS/OmegaWINGS sample. The control sample galaxies are not shown here.
In the text
thumbnail Fig. 7.

Top: Velocity dispersion distribution of the clusters hosting at least one ram pressure stripped galaxy (JType ≥ 0.3). Cluster masses, computed from the velocity dispersions assuming virialization, are shown in the top axis. The vertical lines show the velocity dispersion of well-studied clusters in the local Universe (from left to right: Fornax, Virgo, and Coma). Bottom: Frequency of galaxies of different JTypes as a function of the velocity dispersion of the hosting cluster. The symbol size is proportional to the frequency.
In the text
thumbnail Fig. 8.

Example of a JType = 3 (truncated galaxy): JO23. Top: Hα flux (top left), the gas kinematics (top right), the stellar kinematics (bottom left), and the color composite image (bottom right). Panels and colors are the same as in Fig. 1. Bottom: Stellar maps of different ages, illustrating the average star formation rate per kpc2 during the last 2×107 yr (left), between 2×107 and 5.7×108 yr (central left), between 5.7×108 and 5.7×109 yr (central right), and >5.7×109 yr ago (right). The green ellipses show the contour that defines the stellar disk (see text for details).
In the text
thumbnail Fig. 9.

Illustrative example of the control sample galaxy A3128_B_0148. Panels and colors are the same as in Fig. 1.
In the text
thumbnail Fig. 10.

Diagnostic diagrams for [O III]5007/Hβ vs. [N II]6583/Hα and [OI]6300/Hα for the control sample galaxies. For each galaxy, the top panel shows the BPT line-ratio map, while the bottom panel shows the corresponding spatially resolved BPT diagram. Lines are included from Kauffmann et al. (2003, K03), Kewley et al. (2001, K01), and Sharp & Bland-Hawthorn (2010, SB10) to separate star-forming (red), composite (orange), AGN (green), and LINER (cyan) galaxies. Only spaxels with a S/N > 3 in all of the emission lines involved are shown. Adopting a S/N > 4 does not modify these plots.
In the text
thumbnail Fig. A.1.

Control sample galaxies. Panels and colors are the same as in Fig.1.
In the text
thumbnail Fig. A.2.

Stripping candidates. Panels and colors are the same as in Fig.1.
In the text
thumbnail Fig. A.2.

Continued.
In the text
thumbnail Fig. A.2.

Continued.
In the text
thumbnail Fig. A.2.

Continued. Stripping candidates.
In the text
thumbnail Fig. A.2.

Continued.
In the text
thumbnail Fig. A.2.

Continued.
In the text
thumbnail Fig. A.2.

Continued.
In the text
thumbnail Fig. A.2.

Continued.
In the text
thumbnail Fig. A.2.

Continued.
In the text
thumbnail Fig. A.2.

Continued.
In the text

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