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-Celestial distribution of galaxies with known recessional velocities in the cluster of galaxies A1367, with two contours of the X-ray emission sketched around the X-ray center (X). Small symbols mark galaxies within the gaussian velocity distribution of the cluster (see inset). Larger dots refer to galaxies with redshift exceeding 7700 km s −1 , i.e. in the high velocity tail of the distribution (shaded). Notice that these galaxies are clustered in a small region, revealing the existence of a group falling onto the cluster. Galaxies inside the small square box (BIG), corresponding to Fig.1, have been all detected in Hα.
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![Fig. 3.— Net (H α + N[II]) band frame of the BIG group.](profile/Luis-Carrasco-7/publication/231072684/figure/fig2/AS:300456938098696@1448645924743/Net-H-a-NII-band-frame-of-the-BIG-group_Q320.jpg)
The discovery of a striking astrophysical laboratory in the cluster of galaxies A1367 by Sakai and coworkers is confirmed with independent imaging and spectroscopic observations and further investigated in the present analysis. Two giant and 10 dwarf/H II galaxies, members of a group, are simultaneously undergoing a burst of star formation. Redshif...
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Context 1
... exceeds significantly the mean cluster velocity of < V >= 6420 (σ V = 822) km s −1 (Struble & Rood 1991). In addition to 12 galaxies in Fig.1, redshifts in excess of 7700 km s −1 are found for another 8 galaxies in A1367. They are all spatially segregated near the cluster center, and form a secondary peak in the cluster velocity distribution of Fig. 4. We consider unlikely that the group sits in the background of Abell 1367, in free Hubble flow for two reasons. Firstly, at 30 arcmin projected angular separation from the cluster center (see Fig. 4) the group velocity of 8200 km s −1 falls within the caustics associated with the density enhancement of A1367 (see Fig. 7 of Gavazzi, ...
Context 2
... 8 galaxies in A1367. They are all spatially segregated near the cluster center, and form a secondary peak in the cluster velocity distribution of Fig. 4. We consider unlikely that the group sits in the background of Abell 1367, in free Hubble flow for two reasons. Firstly, at 30 arcmin projected angular separation from the cluster center (see Fig. 4) the group velocity of 8200 km s −1 falls within the caustics associated with the density enhancement of A1367 (see Fig. 7 of Gavazzi, Randone & Branchini 1995). In other words the group is well within the turn-around radius of the cluster. This points out the existence of a separate dynamical unit falling into the cluster from the ...
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... In the more complex scenario of ram-pressure stripping and "preprocessing", i.e. the environmental transformation occurring within galaxy groups before they are accreated onto galaxy clusters, we find the Blue Infalling Group, also known as BIG. Studied by Sakai et al. (2002); Gavazzi et al. (2003); Cortese et al. (2006); Ge et al. (2021b), the more recent observations with MUSE (Fossati et al. 2019) confirm that BIG is an exceptional system in order to understand the role of different processes in the group environment. The BIG is mainly composed of two galaxies CGCG097-125 and CGCG097-114 with mass of log( * /M ) ≈ 10.32, log( * /M ) ≈ 9.22 respectively. ...
We present an analysis of the kinematics and ionization conditions in a sample composed of seven star-forming galaxies undergoing ram-pressure stripping in the A1367 cluster, and the galaxy ESO137-001 in the Norma cluster. MUSE observations of two new galaxies in this sample, CGCG097-073 and CGCG097-079, are also presented. This sample is characterized by homogeneous integral field spectroscopy with MUSE and by a consistent selection based on the presence of ionised gas tails. The ratio [O i]/Hα is consistently elevated in the tails of these objects compared to what observed in unperturbed galaxy disks, an ubiquitous feature which we attribute to shocks or turbulent phenomena in the stripped gas. Compact star-forming regions are observed in only $\approx 50 {{\ \rm per\ cent}}$ of the tails, implying that specific (currently unknown) conditions are needed to trigger star formation inside the stripped gas. Focusing on the interface regions between the interstellar and intracluster medium, we observe different line ratios that we associate to different stages of the stripping process, with galaxies at an early stage of perturbation showing more prominent signatures of elevated star formation. Our analysis thus demonstrates the power of a well selected and homogeneous sample to infer general properties arising from ram-pressure stripping inside local clusters.
... In the more complex scenario of ram-pressure stripping and "preprocessing", i.e. the environmental transformation occurring within galaxy groups before they are accreated onto galaxy clusters, we find the Blue Infalling Group, also known as BIG. Studied by Sakai et al. (2002); Gavazzi et al. (2003); Cortese et al. (2006); Ge et al. (2021b), the more recent observations with MUSE (Fossati et al. 2019) confirm that BIG is an exceptional system in order to understand the role of different processes in the group environment. The BIG is mainly composed of two galaxies CGCG097-125 and CGCG097-114 with mass of log( * /M ) ≈ 10.32, log( * /M ) ≈ 9.22 respectively. ...
We present an analysis of the kinematics and ionization conditions in a sample composed of seven star-forming galaxies undergoing ram-pressure stripping in the A1367 cluster, and the galaxy ESO137-001 in the Norma cluster. MUSE observations of two new galaxies in this sample, CGCG097-073 and CGCG097-079, are also presented. This sample is characterized by homogeneous integral field spectroscopy with MUSE and by a consistent selection based on the presence of ionised gas tails. The ratio [OI]/H${\alpha}$ is consistently elevated in the tails of these objects compared to what observed in unperturbed galaxy disks, an ubiquitous feature which we attribute to shocks or turbulent phenomena in the stripped gas. Compact star-forming regions are observed in only $\approx$ 50% of the tails, implying that specific (currently unknown) conditions are needed to trigger star formation inside the stripped gas. Focusing on the interface regions between the interstellar and intracluster medium, we observe different line ratios that we associate to different stages of the stripping process, with galaxies at an early stage of perturbation showing more prominent signatures of elevated star formation. Our analysis thus demonstrates the power of a well selected and homogeneous sample to infer general properties arising from ram-pressure stripping inside local clusters.
... Gravitational interactions within group members can strip material (stars and gas) into the intragroup space, which would be a source of redder stars for the ICL once the group is accreted by the main cluster, as observed, for instance, in A3888 (Krick et al. 2006) and Virgo (Mihos et al. 2017). Simultaneously, these tidal interactions can weaken the potential wells of the group members, favoring ram pressure stripping by the intracluster medium, which can produce a diffuse Hα emission and even an associated H I component (see, for instance, the case of A1367; Gavazzi et al. 2003;Cortese et al. 2006). In this cluster was also observed an unusual density of metal-rich, star-forming dwarf galaxies, in the region of the infalling group, which could end up disrupted into the ICL eventually. ...
We present a pilot study of the intracluster light (ICL) in massive clusters using imaging of the z = 0.566 cluster of galaxies WHL J013719.8–08284 observed by the RELICS project with the Hubble Space Telescope. We measure the ICL fraction in four optical ACS/WFC filters (F435W, F475W, F606W, and F814W) and five infrared WFC3/IR bands (F105W, F110W, F125W, F140W, and F160W). The ICL maps are calculated using the free-of-a-priori-assumptions algorithm CICLE, and the cluster membership is estimated from photometric properties. We find optical ICL fractions that range between ∼6% and 19%, in nice agreement with the values found in previous works for merging clusters. We also observe an ICL fraction excess between 3800 Å and 4800 Å, previously identified as a signature of merging clusters at 0.18 < z < 0.55. This excess suggests the presence of an enhanced population of young/low-metallicity stars in the ICL. All indicators thus point to WHL J013719.8–08284 as a disturbed cluster with a significant amount of recently injected stars, bluer than the average stars hosted by the cluster members and likely stripped out from infalling galaxies during the current merging event. Infrared ICL fractions are ∼50% higher than optical ones, which could be signatures of an older and/or higher-metallicity ICL population that can be associated with the buildup of the brightest cluster galaxy, passive evolution of previously injected young stars, or preprocessing in infalling groups. Finally, investigating the photometry of the cluster members, we tentatively conclude that WHL J013719.8–08284 fulfills the expected conditions for a fossil system progenitor.
... The group was discovered in the Hα survey of nearby clusters by Sakai et al. (2002). Its presence was also revealed in the Hα data by Iglesias-Páramo et al. (2002) and Gavazzi et al. (2003) who named it the Blue Infalling Group (BIG). Cortese et al. (2006) presented deep Hα imaging and multislit spectroscopy of the BIG, and suggested that the BIG experiences preprocessing. ...
Galaxy clusters grow primarily through the continuous accretion of group-scale haloes. Group galaxies experience preprocessing during their journey into clusters. A star-bursting compact group, the Blue Infalling Group (BIG), is plunging into the nearby cluster A1367. Previous optical observations reveal rich tidal features in the BIG members, and a long H$\alpha$ trail behind. Here we report the discovery of a projected $\sim 250$ kpc X-ray tail behind the BIG using Chandra and XMM-Newton observations. The total hot gas mass in the tail is $\sim 7\times 10^{10}\ {\rm M}_\odot$ with an X-ray bolometric luminosity of $\sim 3.8\times 10^{41}$ erg s$^{-1}$. The temperature along the tail is $\sim 1$ keV, but the apparent metallicity is very low, an indication of the multi-$T$ nature of the gas. The X-ray and H$\alpha$ surface brightnesses in the front part of the BIG tail follow the tight correlation established from a sample of stripped tails in nearby clusters, which suggests the multiphase gas originates from the mixing of the stripped interstellar medium (ISM) with the hot intracluster medium (ICM). Because thermal conduction and hydrodynamic instabilities are significantly suppressed, the stripped ISM can be long lived and produce ICM clumps. The BIG provides us a rare laboratory to study galaxy transformation and preprocessing.
... Simultaneously, these tidal interactions can weaken the potential wells of the group members, favouring rampressure stripping by the intracluster medium which can produce a diffuse Hα emission and even an associated HI component (see, for instance, the case of Abell 1367, Gavazzi et al. 2003;Cortese et al. 2006). In this cluster was also observed an inusual density of metal-rich, star-forming dwarf galaxies, in the region of the infalling group, which could end up disrupted into the ICL eventually. ...
We present a pilot study of the intracluster light (ICL) in massive clusters using imaging of the $z=0.566$ cluster of galaxies WHL J013719.8-08284 observed by the RELICS project with the HST. We measure the ICL fraction in four optical ACS/WFC filters (F435W, F475W, F606W, and F814W) and five infrared WFC3/IR bands (F105W, F110W, F125W, F140W, and F160W). The ICL maps are calculated using the free of a priori assumptions algorithm CICLE, and the cluster membership is estimated from photometric properties. We find optical ICL fractions that range between $\sim$6\% and 19\% in nice agreement with the values found in previous works for merging clusters. We also observe an ICL fraction excess between 3800 \AA and 4800 \AA, previously identified as a signature of merging clusters at $0.18<z<0.55$. This excess suggests the presence of an enhanced population of young/low-metallicity stars in the ICL. All indicators thus point to WHL J013719.8-08284 as a disturbed cluster with a significant amount of recently injected stars, bluer than the average stars hosted by the cluster members and likely stripped out from infalling galaxies during the current merging event. Infrared ICL fractions are $\sim$50\% higher than the optical ones, which could be signature of an older and/or higher-metallicity ICL population that can be associated with the build-up of the BCG, the passive evolution of young stars, previously injected, or preprocessing in infalling groups. Finally, investigating the photometry of the cluster members, we tentatively conclude that WHL J013719.8-08284 fulfills the expected conditions for a fossil system progenitor.
... Bekki 1999; Cortese et al. 2004;Poggianti et al. 2004;Roberts & Parker 2017). A third group, also with enhanced star formation, previously noted by Cortese et al. (2006) to be falling into the cluster, famously called BIG [blue infalling group, Gavazzi et al. (2003)], also lies in this region, though it is not included in our analysis due to our magnitude cut. It has been suggested in many studies that Abell 1367 is a dynamically unrelaxed cluster (e.g. ...
We take a close look at the galaxies in the Coma Supercluster and assess the role of the environment (in the form of cluster, group, and supercluster filament) in their evolution, in particular, examining the role of groups. We characterize the groups according to intrinsic properties such as richness and halo mass, as well as their position in the supercluster and proximity to the two rich clusters, Abell 1656 (Coma) and Abell 1367. We devise a new way of characterizing the local environment using a kernel density estimator. We find that apart from the dominant effects of the galaxy mass, the effect of the environment on galaxies is a complex combination of the overdensities on various scales, which is characterized in terms of membership of groups, and also of the position of the galaxy on filaments and their proximity to the infall regions of clusters. Whether the gas can be turned into stars depends upon the level of pre-processing, which plays a role in how star formation is enhanced in a given environment. Our results are consistent with gas accreted in the cold mode from the filaments, being made available to enhance star formation. Finally, we show that the Abell 1367 end of the supercluster is in the process of assembly at present, leading to heightened star formation activity, in contrast with the Coma-end of the filament system.
... Bekki 1999; Poggianti et al. 2004;Cortese et al. 2004;Roberts & Parker 2017). A third group, also with enhanced star formation, previously noted by Cortese et al. (2006) to be falling into the cluster, famously called BIG (blue infalling group, Gavazzi et al. (2003)) also lies in this region, though it is not included in our analysis due to our magnitude cut. It has been suggested in many studies that Abell 1367 is a dynamically unrelaxed cluster (e.g. ...
We take a close look at the galaxies in the Coma Supercluster and assess the role of the environment (in the form of cluster, group and supercluster filament) in their evolution, in particular examining the role of groups. We characterise the groups according to intrinsic properties such as richness and halo mass, as well as their position in the supercluster and proximity to the two rich clusters, Abell 1656 (Coma) and Abell 1367. We devise a new way of characterising the local environment using a kernel density estimator. We find that apart from the dominant effects of the galaxy mass, the effect of the environment on galaxies is a complex combination of the overdensities on various scales, which is characterised in terms of membership of groups, and also of the position of the galaxy on filaments and their proximity to the infall regions of clusters. Whether the gas can be turned into stars depends upon the level of pre-processing, which plays a role in how star formation is enhanced in a given environment. Our results are consistent with gas accreted in the cold mode from the filaments, being made available to enhance star formation. Finally, we show that the Abell~1367 end of the supercluster is in the process of assembly at present, leading to heightened star formation activity, in contrast with the Coma-end of the filament system.
... However, the sharp increase just after the pericentre passage is dampened (the maximum stellar mass is 114 per cent of the initial mass, compared to 120 per cent for the full sample at the same moment). This could suggest that a small fraction of galaxies experience a violent star formation burst, close to their passage at the pericentre: observations of such star formation bursts in infalling galaxies have been reported for example in Gavazzi et al. (2003), who argue that it is caused by enhanced ram-pressure during a high-velocity infall. ...
Satellite galaxies in galaxy clusters represent a significant fraction of the global galaxy population. Because of the unusual dense environment of clusters, their evolution is driven by different mechanisms than the ones affecting field or central galaxies. Understanding the different interactions they are subject to, and how they are influenced by them, is therefore an important step towards explaining the global picture of galaxy evolution. In this paper, we use the publicly available high-resolution hydrodynamical simulation Illustris-1 to study satellite galaxies in the three most massive host haloes (with masses |$M_{200} \gt 10^{14}\, h^{-1}{\rm M}_{\odot }$|) at z = 0. We measure the stellar-to-halo mass relation (hereafter SHMR) of the galaxies, and find that for satellites it is shifted towards lower halo masses compared to the SHMR of central galaxies. We provide simple fitting functions for both the central and satellite SHMR. To explain the shift between the two, we follow the satellite galaxies since their time of accretion into the clusters, and quantify the impact of dark matter stripping and star formation. We find that subhaloes start losing their dark matter as soon as they get closer than ∼1.5 × Rvir to the centre of their host, and that up to 80 per cent of their dark matter content gets stripped during infall. On the other hand, star formation quenching appears to be delayed, and galaxies continue to form stars for a few Gyr after accretion. The combination of these two effects impacts the ratio of stellar to dark matter mass, which varies drastically during infall, from 0.03 to 0.3.
... The Blue Infalling Group (BIG; Sakai et al. 2002;Cortese et al. 2006) is an ideal candidate due to its proximity. Named BIG by Gavazzi et al. (2003), the group features three galaxies more massive than 10 9 M , and a number of lower mass star-forming systems connected by a complex of ionized gas filaments discovered in deep H α emission line images by Cortese et al. (2006). The group is located ≈150 kpc from one of the subclusters that forms the A1367 cluster (Cortese et al. 2004), and has a mean recessional velocity of v 8230 km s −1 (Gavazzi et al. 2003). ...
... Named BIG by Gavazzi et al. (2003), the group features three galaxies more massive than 10 9 M , and a number of lower mass star-forming systems connected by a complex of ionized gas filaments discovered in deep H α emission line images by Cortese et al. (2006). The group is located ≈150 kpc from one of the subclusters that forms the A1367 cluster (Cortese et al. 2004), and has a mean recessional velocity of v 8230 km s −1 (Gavazzi et al. 2003). The cluster has instead a mean velocity of v 6484 km s −1 (Cortese et al. 2004), which suggested that BIG is currently falling into the cluster potential. ...
We report new wide-field (≈4 × 4 arcmin ² ) Multi Unit Spectroscopic Explorer (MUSE) observations of the Blue Infalling Group (BIG), a compact group of galaxies located at a projected distance of 150 kpc from the X-ray centre of the A1367 cluster at z = 0.021. Our MUSE observations map in detail the extended ionized gas, primarily traced by H α emission, in between the members of the group. The gas morphology and its kinematics appear consistent with a tidal origin due to galaxy encounters, as also supported by the disturbed kinematics visible in one of the group members and the presence of tidal dwarf systems. A diffuse tail extending in the direction opposite to the cluster centre is also detected, hinting at a global ram-pressure stripping of the intragroup material as BIG falls inside A1367. Based on the analysis of spatially resolved emission line maps, we identify multiple ionization mechanisms for the diffuse gas filaments, including in situ photoionization from embedded H II regions and shocks. Combining spatially resolved kinematics and line ratios, we rule out the association of the most massive galaxy, CGCG097-120, with the group as this system appears to be decoupled from the intragroup medium and subject to strong ram pressure as it falls into A1367. Through our new analysis, we conclude that BIG is shaped by pre-processing produced by gravitational interactions in the Local Group environment combined with ram-pressure stripping by the global cluster halo. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
... The Blue Infalling Group (BIG Sakai et al. 2002;Cortese et al. 2006) is an ideal candidate due to its proximity. Named BIG by Gavazzi et al. (2003), the group features 3 galaxies more massive than 10 9 M , and a number of lower-mass star-forming systems connected by a complex of ionized gas filaments discovered in deep Hα emission line images by Cortese et al. (2006). The group is located ≈ 150 kpc from one of the subclusters that forms the A1367 cluster (Cortese et al. 2004), and has a mean recessional velocity of v 8230 km s −1 (Gavazzi et al. 2003). ...
... Named BIG by Gavazzi et al. (2003), the group features 3 galaxies more massive than 10 9 M , and a number of lower-mass star-forming systems connected by a complex of ionized gas filaments discovered in deep Hα emission line images by Cortese et al. (2006). The group is located ≈ 150 kpc from one of the subclusters that forms the A1367 cluster (Cortese et al. 2004), and has a mean recessional velocity of v 8230 km s −1 (Gavazzi et al. 2003). The cluster has instead a mean velocity of v 6484 km s −1 (Cortese et al. 2004), which suggested that BIG is currently falling into the cluster potential. ...
We report new wide-field (~ 4 x 4 square arcmin) MUSE observations of the Blue Infalling Group (BIG), a compact group of galaxies located at a projected distance of ~ 150 kpc from the X-Ray centre of the A1367 cluster at z = 0.021. Our MUSE observations map in detail the extended ionized gas, primarily traced by H$\alpha$ emission, in between the members of the group. The gas morphology and its kinematics appear consistent with a tidal origin due to galaxy encounters, as also supported by the disturbed kinematics visible in one of the group members and the presence of tidal dwarf systems. A diffuse tail extending in the direction opposite to the cluster centre is also detected, hinting at a global ram-pressure stripping of the intra-group material as BIG falls inside A1367. Based on the analysis of spatially-resolved emission line maps, we identify multiple ionization mechanisms for the diffuse gas filaments, including in situ photoionization from embedded HII regions and shocks. Combining spatially resolved kinematics and line ratios, we rule out the association of the most massive galaxy, CGCG097-120, with the group as this system appears to be decoupled from the intragroup medium and subject to strong ram pressure it falls into A1367. Through our new analysis, we conclude that BIG is shaped by pre-processing produced by gravitational interactions in the local group environment combined with ram pressure stripping by the global cluster halo.
