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Full list of authors: Bao, Dong-Wei; Brotherton, Michael S.; Du, Pu; McLane, Jacob N.; Zastrocky, T. E.; Olson, Kianna A.; Fang, Feng-Na; Zhai, Shuo; Huang, Zheng-Peng; Wang, Kai; Zhao, Bi-Xuan; Li, Sha-Sha; Yang, Sen; Chen, Yong-Jie; Liu, Jun-Rong; Yao, Zhu-Heng; Peng, Yue-Chang; Guo, Wei-Jian; Songsheng, Yu-Yang; Li, Yan-Rong; Jiang, Bo-Wei; Kasper, David H.; Chick, William T.; Nguyen, My L.; Maithil, Jaya; Kobulnicky, H. A.; Dale, D. A.; Hand, Derek; Adelman, C.; Carter, Z.; Murphree, A. M.; Oeur, M.; Schonsberg, S.; Roth, T.; Winkler, Hartmut; Marziani, Paola; D'Onofrio, Mauro; Hu, Chen; Xiao, Ming; Xue, Suijian; Czerny, Bozena; Aceituno, Jesus; Ho, Luis C.; Bai, Jin-Ming; Wang, Jian-Min; MAHA Collaboration.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited., In this third paper of the series reporting on the reverberation mapping campaign of active galactic nuclei with asymmetric Hβ emission-line profiles, we present results for 15 Palomar–Green quasars using spectra obtained between the end of 2016–2021 May. This campaign combines long time spans with relatively high cadence. For eight objects, both the time lags obtained from the entire light curves and the measurements from individual observing seasons are provided. Reverberation mapping of nine of our targets has been attempted for the first time, while the results for six others can be compared with previous campaigns. We measure the Hβ time lags over periods of years and estimate their black hole masses. The long duration of the campaign enables us to investigate their broad-line region (BLR) geometry and kinematics for different years by using velocity-resolved lags, which demonstrate signatures of diverse BLR geometry and kinematics. The BLR geometry and kinematics of individual objects are discussed. In this sample, the BLR kinematics of Keplerian/virialized motion and inflow is more common than that of outflow. © 2022. The Author(s). Published by the American Astronomical Society., We acknowledge the support by the National Science Foundation of China through grants NSFC-12022301, -11991051, -11991054, -11873048, -11833008, by National Key R&D Program of China (grants 2016YFA0400701), by grant No. QYZDJ-SSW-SLH007 from the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (CAS), by the Strategic Priority Research Program of CAS grant No.XDB23010400, and by the International Partnership Program of CAS, grant No.113111KYSB20200014. L.C.H. was supported by the National Science Foundation of China (11721303, 11991052) and the National Key R&D Program of China (2016YFA0400702). M.B. enjoyed support from the Chinese Academy of Sciences Presidents International Fellowship Initiative, grant No. 2018VMA0005. YRL acknowledges financial support from the National Natural Science Foundation of China through grant NSFC-11922304 and from the Youth Innovation Promotion Association CAS. H.C. acknowledges financial support from grant NSFC-12122305. We also acknowledge support from a University of Wyoming Science Initiative Faculty Innovation Seed grant. This work is supported by the National Science Foundation under REU grant AST 1852289. T.E.Z. acknowledges support from NSF grant 1005444I., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

The last few years have seen the confirmation of several trends associated with the quasar main sequence. The idea of a main sequence for quasars is relatively recent, and its full potential for the observational classification and contextualization of quasars' properties has yet to be fully exploited. The main sequence drivers are discussed in terms of the properties of extreme objects. We briefly summarize developments that constrain the viewing angle of the accretion disk in a particular class of quasars (extreme Population B, radiating at low Eddington ratio), as well as inferences on the chemical composition of the broad line emitting gas, and on the nature of radio emission along the quasar main sequence. © 2021 Wiley-VCH GmbH., N. Bon and E. Bon acknowledge the support of Serbian Ministry of Education, Science and Technological Development, through the contract number 451-03-68/2020-14/200002., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

Nuclear Activity in Galaxies Across Cosmic Time, Proceedings of the conference held 7-11 October 2019 in Addis Ababa, Ethiopia. Edited by Mirjana Pović et al. Proceedings of the International Astronomical Union, Volume 356, pp. 66-71, The main sequence offers a method for the systematization of quasar spectral properties. Extreme FeII emitters (or extreme Population A, xA) are believed to be sources accreting matter at very high rates. They are easily identifiable along the quasar main sequence, in large spectroscopic surveys over a broad redshift range. The very high accretion rate makes it possible that massive black holes hosted in xA quasars radiate at a stable, extreme luminosity-to-mass ratio. After reviewing the basic interpretation of the main sequence, we report on the possibility of identifying virial broadening estimators from low-ionization line widths, and provide evidence of the conceptual validity of redshift-independent luminosities based on virial broadening for a known luminosity-to-mass ratio., AdO acknowledges financial support from Spanish grants AYA2016-76682-C3-1-P and SEV-2017-0709.

Context. The quasar class of extreme Population A (xA) (also known as super-Eddington accreting massive black holes, SEAMBHs) has been hailed as potential distance indicators for cosmology. Aims. The aim of this paper is to define tight criteria for their proper identification, starting from the main selection criterion RFeII > 1, and to identify potential intruders that do not meet the selection criteria, which nonetheless have been selected as xA because of the coarseness of automatic searches. The inclusion of the spurious xA sources may dramatically increase the dispersion in the Hubble diagram of quasars, which were obtained from virial luminosity estimates. Methods. We studied a sample of 32 low-z quasars that were originally selected from the seventh data release of the Sloan Digital Sky Survey as xA or SEAMBHs, which have been proved to be almost certainly misclassified sources. All of them show moderate to strong Fe II emission and the large majority show strong absorption features in their spectra which are typical of fairly evolved stellar populations. We performed a simultaneous fit of a host galaxy spectrum, active galactic nucleus (AGN) continuum, FeII template, and emission lines to spectra, using the fitting technique based on ULySS, the full spectrum fitting package. We derived the main accretion parameters (i.e., luminosity, black hole mass, and Eddington ratio) and investigate the relation between host galaxy properties and AGN. Results. For sources in our sample with spectral types that correspond to a relatively low Eddington ratio, we find an overall consistency between HβNC, [O III]λλ4959,5007 line shifts, and the mean stellar velocity obtained from the host galaxy fit (within|60| km s-1). Only one source in our sample qualifies as a xA source. Conclusions. The correct classification of spectra that were contaminated by heavy absorption requires careful determination of the host galaxy spectrum. Contamination and misclassification are not usual in the identification of the xAs, nor at low z or at high z. We find a high fraction of host galaxy spectrum; in half of the sample this is even higher than 40%. When absorption lines are prominent, and the fraction of the host galaxy is high, host galaxy spectrum mimics FeII, which may result in a mistaken identification of FeII spectral features. We have identified several stellar absorption lines that, along with the continuum shape, may lead to an overestimate of RFeII, and therefore to the misclassification of sources as xA sources. © ESO 2020., This research is part of the projects 176001 "Astrophysical spectroscopy of extragalactic objects" and 176003 "Gravitation and the large scale structure of the Universe", funded by Ministry of Education, Science and Technological Development of the Republic of Serbia. PM and MDO acknowledge funding from the INAF PRIN-SKA 2017 program 1.05.01.88.04. PM also acknowledges the Programa de Estancias de Investigacion (PREI) No. DGAP/DFA/2192/2018 of UNAM. A.d.O. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness through grant AYA2016-76682-C3-1-P and from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). ML. M. A. acknowledges financial support of National Science Centre, Poland, grant No. 2017/26/A/ST9/00756 (Maestro 9). DD acknowledges support from grants PAPIIT, UNAM 113719, Mexico D.F. 04510, Mexico. AN acknowledges support from grant CONACyT research fellow -Instituto de Astronomia, UNAM, Mexico D.F. 04510, Mexico. We thank to Pu Du for his help and constructive comments. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/.The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.

Quasars accreting matter at very high rates (known as extreme Population A [xA] quasars, possibly associated with super-Eddington accreting massive black holes) may provide a new class of distance indicators covering cosmic epochs from present day up to less than 1 Gyr from the Big Bang. At a more fundamental level, xA quasars are of special interest in studies of the physics of AGNs and host galaxy evolution. However, their observational properties are largely unknown. xA quasars can be identified in relatively large numbers from major optical surveys over a broad range of redshifts, and efficiently separated from other type-1 quasars thanks to selection criteria defined from the systematically-changing properties along the quasars main sequence. It has been possible to build a sample of ~250 quasars at low and intermediate redshift, and larger samples can be easily selected from the latest data releases of the Sloan Digital Sky Survey. A large sample can clarify the main properties of xA quasars which are expected—unlike the general population of quasars—to radiate at an extreme, well-defined Eddington ratio with small scatter. As a result of the small scatter in Eddington ratio shown by xA quasars, we propose a method to derive the main cosmological parameters based on redshift-independent “virial luminosity” estimates from measurements of emission line widths, roughly equivalent to the luminosity estimates based from line width in early and late type galaxies. A major issue related to the cosmological application of the xA quasar luminosity estimates from line widths is the identification of proper emission lines whose broadening is predominantly virial over a wide range of redshift and luminosity. We report on preliminary developments using the AlIIIλ1860 intermediate ionization line and the Hydrogen Balmer line Hβ as virial broadening estimators, and we briefly discuss the perspective of the method based on xA quasars. © Copyright © 2020 Dultzin, Marziani, de Diego, Negrete, Del Olmo, Martínez-Aldama, D'Onofrio, Bon, Bon and Stirpe., This research is part of the projects 176001-Astrophysical spectroscopy of extragalactic objects and 176003-Gravitation and the large scale structure of the Universe, funded by Ministry of Education, Science and Technological Development of the Republic of Serbia.

Context. This is the second u-band extension of the WIde-field Nearby Galaxy-cluster Survey (WINGS) whereby 39 clusters were imaged with the ESO-VLT survey telescope. This follows the first part of the survey which was performed with several telescopes of the northern hemisphere in the U Cousin–Bessel filter band covering 17 clusters. Aims. The u-band data, in combination with those already collected by the WINGS survey, permit a detailed multi-wavelength investigation of the properties of the member galaxies from the cluster center out to the periphery. Methods. We used SExtractor to derive the main properties of the galaxies in the observed fields and measure the u − V colors on circular apertures of increasing radius. The photometric accuracy of the magnitudes was calibrated with the standard stars and was tested by means of comparisons with the u-band data of the Sloan Digital Sky Survey (SDSS). Results. We present the catalogs of the photometric analysis performed by SExtractor. We also provide a brief analysis of the u − V versus V color–magnitude diagram of our clusters, the plots of the color as a function of the cluster-centric distance (for cluster members only), the mass–color relation and the correlation of the current star formation rate (SFR) with the absolute V and u magnitudes for the galaxies in the observed fields.

Context. This is the second work dedicated to the observed parallelism between galaxy clusters and early-type galaxies. The focus is on the distribution of these systems in the scaling relations (SRs) observed when effective radii, effective surface brightness, total luminosities and velocity dispersions are mutually correlated. Aims. Using the data of the Illustris simulation we try to speculate on the origin of the observed SRs. Methods. We compare the observational SRs extracted from the database of the WIde-field Nearby Galaxy-cluster Survey (WINGS) with the relevant parameters coming from the Illustris simulations. Then we use the simulated data at different redshift to infer the evolution of the SRs. Results. The comparison demonstrate that galaxy clusters (GCs) at z~0 follow the same log(L)-log(sigma) relation of early-type galaxies (ETGs) and that both in the log(Ie)-log(Re) and log(Re)-log(M*) planes the distribution of GCs is along the sequence defined by the brightest and massive early-type galaxies (BCGs). The Illustris simulation reproduces the tails of the massive galaxies visible both in the log(Ie)-log(Re) and log(Re)-log(M*) planes, but fail to give the correct estimate of the effective radii of the dwarf galaxies that appear too large and those of GCs that are too small. The evolution of the SRs up to z=4 permits to reveal the complex evolutionary paths of galaxies in the SRs and indicate that the line marking the Zone of Exclusion (ZoE), visible both in the log(Ie)-log(Re) and log(Re)-log(M*) planes, is the trend followed by virialized and passively evolving systems. Conclusions. We speculate that the observed SRs originate from the intersection of the virial theorem and a relation L=L_0 x sigma^beta where the luminosities depend on the star formation history., 22 pages, 14 figures, 4 tables

We present a study of the physical properties of JO201, a unique disk galaxy with extended tails undergoing extreme ram-pressure stripping as it moves through the massive cluster Abell 85 at supersonic speeds mostly along the line of sight. JO201 was observed with MUSE as part of the GASP programme. In a previous paper (GASP II) we studied the stellar and gas kinematics. In this paper we present emission-line ratios, gas-phase metallicities and ages of the stellar populations across the galaxy disk and tails. We find that while the emission at the core of the galaxy is dominated by an active galactic nucleus (AGN), the disk is composed of star-forming knots surrounded by excited diffuse gas. The collection of star-forming knots presents a metallicity gradient steadily decreasing from the centre of the galaxy outwards, and the ages of the stars across the galaxy show that the tails formed, 17 pages, 12 figures

Context. Galaxy evolution and the effect of environment are most often studied using scaling relations or some regression analyses around some given property. These approaches however do not take into account the complexity of the physics of the galaxies and their diversification. Aims. We here investigate the effect of cluster environment on the evolution of galaxies through multivariate unsupervised classification and phylogenetic analyses applied to two relatively large samples from the WINGS survey, one of cluster members and one of field galaxies (2624 and 1476 objects respectively). Methods. These samples are the largest ones ever analysed with a phylogenetic approach in astrophysics. To be able to use the Maximum Parsimony (cladistics) method, we first performed a pre-clustering in 300 clusters with a hierarchical clustering technique, before applying it to these pre-clusters. All these computations used seven parameters: B-V, log(Re), nV , $\mu$e , H$\beta$ , D4000 , log(M *). Results. We have obtained a tree for the combined samples and do not find different evolutionary paths for cluster and field galaxies. However, the cluster galaxies seem to have accelerated evolution in the sense they are statistically more diversified from a primitive common ancestor. The separate analyses show a hint for a slightly more regular evolution of the variables for the cluster galaxies, which may indicate they are more homogeneous as compared to field galaxies in the sense that the groups of the latter appear to have more specific properties. On the tree for the cluster galaxies, there is a separate branch which gathers rejunevated or stripped-off groups of galaxies. This branch is clearly visible on the colour-magnitude diagram, going back from the red sequence towards the blue one. On this diagram, the distribution and the evolutionary paths of galaxies are strikingly different for the two samples. Globally, we do not find any dominant variable able to explain either the groups or the tree structures. Rather, co-evolution appears everywhere, and could depend itself on environment or mass. Conclusions. This study is another demonstration that unsupervised machine learning is able to go beyond the simple scaling relations by taking into account several properties together. The phylogenetic approach is invaluable to trace the evolutionary scenarii and project them onto any biavariate diagram without any a priori modelling. Our WINGS galaxies are all at low redshift, and we now need to go to higher redshfits to find more primitve galaxies and complete the map of the evolutionary paths of present day galaxies., Comment: Astronomy and Astrophysics - A&A, EDP Sciences, In press

Tables A.1–A.3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/630/A110, Context. When can an active galactic nucleus (AGN) be considered radio loud (RL)? Following the established view of the AGNs inner workings, an AGN is RL if associated with relativistic ejections emitting a radio synchrotron spectrum (i.e., it is a “jetted” AGN). In this paper we exploit the AGN main sequence that offers a powerful tool to contextualize radio properties. Aims. If large samples of optically-selected quasars are considered, AGNs are identified as RL if their Kellermann’s radio loudness ratio RK > 10. Our aims are to characterize the optical properties of different classes based on radio loudness within the main sequence and to test whether the condition RK > 10 is sufficient for the identification of RL AGNs, since the origin of relatively strong radio emission may not be necessarily due to relativistic ejection. Methods. A sample of 355 quasars was selected by cross-correlating the Very Large Array Faint Images of the Radio Sky at Twenty-Centimeters survey (FIRST) with the twelfth release of the Sloan Digital Sky Survey Quasar Catalog published in 2017. We classified the optical spectra according to their spectral types along the main sequence of quasars. For each spectral type, we distinguished compact and extended morphology (providing a FIRST-based atlas of radio maps in the latter case), and three classes of radio loudness: detected ( specific flux ratio in the g band and at 1.4 GHz, R′K < 10), intermediate (10 ≤ R′K < 70), and RL (R′K ≥ 70). Results. The analysis revealed systematic differences between radio-detected (i.e., radio-quiet), radio-intermediate, and RL classes in each spectral type along the main sequence. We show that spectral bins that contain the extreme Population A sources have radio power compatible with emission by mechanisms ultimately due to star formation processes. RL sources of Population B are characteristically jetted. Their broad Hβ profiles can be interpreted as due to a binary broad-line region. We suggest that RL Population B sources should be preferential targets for the search of black hole binaries, and present a sample of binary black hole AGN candidates. Conclusions. The validity of the Kellermann’s criterion may be dependent on the source location along the quasar main sequence. The consideration of the main sequence trends allowed us to distinguish between sources whose radio emission mechanisms is jetted from the ones where the mechanism is likely to be fundamentally different.© ESO 2019, PM and MDO acknowledge funding from the INAF PRIN-SKA 2017 program 1.05.01.88.04. PM also acknowledges the Programa de Estancias de Investigación (PREI) No. DGAP/DFA/2192/2018 of UNAM. AdO acknowledges financial support from the Spanish Ministry of Economy and Competitiveness through grant AYA2016-76682-C3-1-P and from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). The relevant research is part of the project 176001 “Astrophysical spectroscopy of extragalactic objects” supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia

We investigate the color-magnitude diagram (CMD) of cluster galaxies in the hierarchical $\Lambda$-CDM cosmological scenario using both single stellar populations and simple galaxy models. First, we analyze the effect of bursts and mergers and companion chemical pollution and rejuvenation of the stellar content on the integrated light emitted by galaxies. The dispersion of the galaxy magnitudes and colors on the $M_V-(B-V)$ plane is mainly due to mixing of ages and metallicities of the stellar populations, with mergers weighting more than bursts of similar mass fractions. The analysis is made using the Monte-Carlo technique applied to ideal model galaxies reduced to single stellar populations with galaxy-size mass to evaluate mass, age and metallicity of each object. We show that separately determining the contributions by bursts and mergers leads to a better understanding of observed properties of CMD of cluster galaxies. Then we repeat the analysis using suitable chemo-photometric models of galaxies whose mass is derived from the cosmological predictions of the galaxy content of typical clusters. Using the halo mass function and the Monte-Carlo technique, we derive the formation redshift of each galaxy and its photometric history. These are used to simulate the CMD of the cluster galaxies. The main conclusion is that most massive galaxies have acquired the red color they show today in very early epochs and remained the same ever since. The simulations nicely reproduce the Red Sequence, the Green Valley and the Blue Cloud, the three main regions of the CMD in which galaxies crowd., Comment: Accepted for publication in ApJ

Open Access.-Attribution 4.0 International (CC BY 4.0), The last 25 years saw a major step forward in the analysis of optical and UV spectroscopic data of large quasar samples. Multivariate statistical approaches have led to the definition of systematic trends in observational properties that are the basis of physical and dynamical modeling of quasar structure. We discuss the empirical correlates of the so-called >main sequence> associated with the quasar Eigenvector 1, its governing physical parameters and several implications on our view of the quasar structure, as well as some luminosity effects associated with the virialized component of the line emitting regions. We also briefly discuss quasars in a segment of the main sequence that includes the strongest FeII emitters. These sources show a small dispersion around a well-defined Eddington ratio value, a property which makes them potential Eddington standard candles.© Copyright © 2018 Marziani, Dultzin, Sulentic, Del Olmo, Negrete, Martínez-Aldama, D'Onofrio, Bon, Bon and Stirpe., AD and MM-A acknowledge financial support from the Spanish Ministry for Economy and Competitiveness through grants AYA2013-42227-P and AYA2016-76682-C3-1-P. DD and CN acknowledge support from grants PAPIIT108716, UNAM, and CONACyT221398. EB and NB acknowledge grants 176003 Gravitation and the large scale structure of the Universe and 176001 Astrophysical spectroscopy of extragalactic objects supported by the Ministry of Education and Science of the Republic of Serbia.

Jellyfish galaxies in clusters are key tools to understand environmental processes at work in dense environments. The advent of Integral Field Spectroscopy has recently allowed to study a significant sample of stripped galaxies in the cluster environment at z$\sim 0.05$, through the GAs Stripping Phenomena in galaxies with MUSE (GASP) survey. However, optical spectroscopy can only trace the ionized gas component through the H$_{\alpha}$ emission that can be spatially resolved on kpc scale at this redshift. The complex interplay between the various gas phases (ionized, neutral, molecular) is however yet to be understood. We report here the detection of large amounts of molecular gas both in the tails and in the disks of 4 jellyfish galaxies from the GASP sample with stellar masses $\sim 3.5\times 10^{10}-3\times 10^{11} M_{\odot}$, showing strong stripping. The mass of molecular gas that we measure in the tails amounts to several $10^9 M_{\odot}$ and the total mass of molecular gas ranges between 15 and 100 \% of the galaxy stellar mass. The molecular gas content within the galaxies is compatible with the one of normal spiral galaxies, suggesting that the molecular gas in the tails has been formed in-situ. We find a clear correlation between the ionized gas emission $\rm H\alpha$ and the amount of molecular gas. The CO velocities measured from APEX data are not always coincident with the underlying $\rm H\alpha$ emitting knots, and the derived Star Formation Efficiencies appear to be very low.

Open Access.--Attribution 4.0 International (CC BY 4.0), We present preliminary results of a spectroscopic analysis for a sample of type 1 highly accreting quasars (L/L-Edd similar to 1.0) at high redshift, z similar to 2-3. The quasars were observed with the OSIRIS spectrograph on the GTC 10.4 m telescope located at the Observatorio del Roque de los Muchachos in La Palma. The highly accreting quasars were identified using the 4D Eigenvector 1 formalism, which is able to organize type 1 quasars over a broad range of redshift and luminosity. The kinematic and physical properties of the broad line region have been derived by fitting the profiles of strong UV emission lines such as AlIII lambda 1860, SiIII]lambda 1892 and CIII]lambda 1909. The majority of our sources show strong blueshifts in the high-ionization lines and high Eddington ratios which are related with the productions of outflows. The importance of highly accreting quasars goes beyond a detailed understanding of their physics: their extreme Eddington ratio makes them candidates standard candles for cosmological studies.© Copyright © 2018 Martínez-Aldama, Del Olmo, Marziani, Sulentic, Negrete, Dultzin, Perea and D'Onofrio., MM-A acknowledge the postdoctoral grant from the CONACyT. MM-A and AD acknowledge financial support from Spanish Ministry for Economy and Competitiveness through grants AYA2013-42227-P and AYA2016-76682-C3-3-1-P.

The conference was sponsored jointly by the National Institute for Astrophysics, the Padova Astronomical Observatory, the Department of Physics and Astronomy of the University of Padova, and the Instituto de Astrofisica de Andalucia (IAA) of the Consejo Superior de Investigacion Cientifica (CSIC) whose support is gratefully acknowldeged. We are very grateful for the facilitation and sponsorship (including the best poster contest) that was provided by the Journal Frontiers in Astronomy and Space Sciences. Sponsors Officina stellare and Springer Verlag provided conference material free-of-charge. The meeting has been part of the events meant to celebrate the 250th anniversary of the foundation of the Padova Observatory.

At the time of the Great Debate nebulæ where recognized to have different morphologies and first classifications, sometimes only descriptive, have been attempted. A review of these early classification systems are well documented by the Allan Sandage’s review in 2005 [107]. This review emphasized the debt, in term of continuity of forms of spiral galaxies, due by the Hubble’s classification scheme to the Reynold’s systems proposed in 1920 [97]. In The Realm of the Nebulæ [64] Edwin Hubble was first of all convinced about the need of a classification scheme to properly understand the nature of galaxies. ”The first step is obviously a study of the apparent features of the systems under investigations. The nebulæ might be members of a single family or they might represent a mixture of utterly different kinds of objects. The questions is very important for all investigations of a general nature. The nebulæ are so common that cannot all be studied individually. Therefore, it is necessary to know whether a fair sample can be assembled from the more conspicuous objects and, if so, the size of the sample required. The answer to this question, and to many others, is sought in the classification of nebulæ.” Hubble described his classification procedure as follows “sort out the nebulæ, by inspection of photographs, into groups of objects showing similar features. The more conspicuous members of each group can then be studied in detail and the results used for comparison of the groups themselves. The degree of success attained by the method depends largely upon the significance of the features selected as the basis of the classification. ... The features must be significant– they must indicate physical properties of the nebulæ themselves and not a chance effect of orientation– and also they must conspicuously enough to be seen in a large numbers of nebulæ.” In the 1936 Hubble’s classification scheme, nebulæ are divided into “two very unequal groups. The great majority are called regular nebulae, since they exhibit a common pattern, conspicuous evidence of rotational symmetry about dominating, central nuclei. The remaining objects, about 2 or 3 per cent of the total number, are called irregular, because they lack both rotational symmetry and, in general, dominating nuclei.” The pillars of the classification of the regular nebulæ are ”either elliptical or spirals. Objects in each group fall naturally into ordered sequences of structural forms ... The progression throughout the complete sequence thus runs from the most compact of the elliptical nebulæ to the most open of the spirals. ... The terms early and late are used to denote relative position in the empirical sequence without regard to their temporal implications.” Hubble concluded that his classification scheme, known today as the Hubble sequence (HS) and/or the tuning fork, is purely empirical in nature and emphasized that the above consideration “is important because the sequence closely resembles the line of development indicated by the current theory of nebular evolution as developed by Sir James Jeans.” So an empirical approach is needed according to Hubble to build a robust classification system, although the morphologist has to keep an eye on the theory! Since the HS definition, the galaxy morphological classification systems had an uninterrupted evolution whose main objective is to account for the plethora of substructures emerging from galaxy photographic observations. Up to the end of the 1980’s astronomers concentrated on organizing such variety of morphologies (in the optical band) underlying the smooth continuity between classification bins (e.g. [35]). At odds, classification systems taking into account the galaxy absolute magnitude, break the HS at high, where only cluster Dominators (cDs) are found, and at low luminosities (see e.g. the 3D classification system proposed by van den Bergh [127]), where dwarf galaxies display their variety . For decades the morphology has been considered the hollmark of a galaxy and the classification process should be made by a “specialist”. All galaxy catalogues report a classification, even if purely descriptive, we mention the Morphologicheskji Katalog Galaktik by Boris Vorontsov-Veliaminov (1962-1974), but very few have been considered a “must-see” catalog to refer in term of classification. We mention The Hubble Atlas of Galaxies by Allan Sandage (1961), A Revised Shapley-Ames Catalog of Bright Galaxies, by Sandage and Gustav Tammann (1981–1989) and multiple versions of The Reference Catalogue of Bright Galaxies by Gerard de Vaucouleurs and collaborators (1981-1989). Today the process of morphological classification of galaxies is going through the stress-tests of high-resolution, wide-field, deep imaging and multi wavelength observations through digital devices. Galaxies are today scrutinized with an unprecedented detail. The digital imaging provides the possibility to elaborate images, modeling galaxies, evidence asymmetries as well as identify sub-structures from the very center out to the extreme periphery of the galaxy. Different morphological structures can be investigated at several wavelengths and compared. Galaxies change, often dramatically, their morphology when observed at different wavelengths, still the basic HS is used to select galaxy samples over which to infer their global properties. We may synthesize the question about galaxy classification using two sentences which express two opposite views. From one side, the sentence of Halton Arp in the Introduction to the Atlas of Peculiar Galaxies comes to mind ”... But far from all galaxies fit the Hubble sequence of nebular forms. In fact, when looked at closely enough, every galaxy is peculiar.” Peculiar features tell us a story, maybe of that “unique” galaxy. On the other side, Ron Buta (1992), collaborator of Gearard and Antoinette de Vaucouleurs, resumed the need of a morphological classification in the following sentence ”As long as only a few criteria define a system, and if image material of a similar quality to that which formed the basis of the system is used, then there will be a greater ease of applicability and reproducibility of that system by independent observers. If one later finds correlations between fundamental observables and classifications, then the system could lead to physical insight ...” This Chapter focuses on the evolution of the galaxy classification schemes throughout this century and wishes to introduce the debate about the power entrusted, since the introduction of the HS, to the morphology in disentangling the galaxy history, i.e. in identifying the formation/evolutionary mechanisms that are believed to be at the origin of the observed morphology. In this context, the debate today enumerates very different positions from those suggesting that the galaxy morphology is the basic parameter, sufficient to identify an evolutionary path, to those who believe necessary to “isolate few ” additional galaxy physical parameters to that purpose, up to researchers that uses sophisticated statistical approaches in which the morphology is simply one of the parameters that come into play. In Section 3.2 Debra Elmegreen starts discussing the manifold of spiral galaxies that, since the daybreak of the extragalactic astronomy, charmed astronomers. In Section 3.3 the class of S0s is considered. In the original Hubble classification S0s are viewed as transition objects between Spiral and ellipticals at odds with van den Berg, more recent classification, which considers them a sequence parallel to that of normal and barred spirals. Gary Welch in 1999 [134] used colourful words to describe the S0 class “To span the abyss between the two main classes is the job of S0s or lenticular”. Eija Laurikainen provides here an historical and modern interpretation of this class of galaxies. S0s are often considered part of the vast class of the so-called early-type galaxies (ETGs) which also includes ellipticals. In Section 3.4 the classification of ETGs is considered by Pierre-Alain Duc, member of the ATLAS3D team. This research group is trying to organize ETGs into more physical classes, according to their bimodal, fast or slow, kinematic figures of rotation. Dwarfs, including irregular galaxies, were neglected by Hubble in The Realm of the Nebulae and have been considered only in later more complex classification schemes. Carme Gallart and Debra Elmegreen dealt with dwarfs in Section 3.5. “A good classification can drive the physics, but the physics must not be used to drive the classification. Otherwise the process becomes circular.” (A. Sandage, [107]). In Section 3.6 Didier Fraix-Bunet discusses the problem of galaxy classification and the use of multi-parametric approaches and genetic algorithms to provide a classification scheme independent from the HS. These techniques might potentially identify the evolutionary paths followed by galaxies in their evolution.

Galaxy clusters are the largest virialized structures in the observable Universe. The knowledge of their properties provides many useful astrophysical and cosmological information. Our aim is to derive the luminosity and stellar mass profiles of the nearby galaxy clusters of the Omega-WINGS survey and to study the main scaling relations valid for such systems. We have merged the data of the WINGS and Omega-WINGS databases, sorted the sources according to the distance from the brightest cluster galaxy (BCG) and calculated the integrated luminosity profiles in the $B$ and $V$ bands, taking into account extinction, photometric and spatial completeness, K-correction and background contribution. Then, by exploiting the spectroscopic sample we derived the stellar mass profiles of the clusters. We got the luminosity profiles of 46 galaxy clusters, reaching $r_{200}$ in 30 cases, and the stellar mass profiles of 42 of our objects. We successfully fitted all the integrated luminosity growth profiles with one or two embedded S\'ersic components, deriving the main clusters parameters. Finally, we checked the main scaling relation among the clusters parameters in comparison with those obtained for a selected sample of early-type galaxies (ETGs) of the same clusters. We found that the nearby galaxy clusters are non-homologous structures like ETGs and exhibit a color-magnitude (CM) red-sequence relation very similar to that observed for galaxies in clusters. These properties are not expected in the current cluster formation scenarios. In particular the existence of a CM relation for clusters, shown here for the first time, suggests that the baryonic structures grow and evolve in a similar way at all scales., Comment: 45 pages, 41 figures

We provide an explanation of the properties of the fundamental plane (FP) relation and its observed projections for a sample of nearby early-type galaxies (ETGs) in terms of a fine-tuning between the time-averaged star formation rate < {{\Psi }}> and their structural and dynamical characteristics. Their total V luminosity is linked with < {{\Psi }}> and the central velocity dispersion σ through the relation {log}(L)=0.48(+/- 0.06){log}(< {{\Psi }}> )+1.00 (+/- 0.13){log}(σ )+7.81(+/- 0.26), with an rms = 0.215 (R = 0.64 and P< 1.2× {10}-16). This fine-tuning permits us to obtain the FP in terms of two distinct “virtual planes” in the {log}({R}e){--}{log}(< {I}e> ){--}{log}(σ ) space. The first one (the virial plane; VP) represents the total galaxy mass derived from the scalar virial theorem and the mass-to-light ratio M/L, while the second plane comes from the relation L={L}0\prime {σ }-2, where {L}0\prime is a parameter connected with < {{\Psi }}> . This is a mathematically convenient way for expressing the independence of the galaxy light from the virial equilibrium. Each galaxy in the {log}({R}e){--}{log}(< {I}e> ){--}{log}(σ ) space is identified by the intersection of these two planes. A posteriori, we show that the properties of the FP (tilt and scatter) and the zone of exclusion visible in the FP projections are consequences of this fine-tuning. The link between the FP properties and the SFR of galaxies provides a new view of the star formation phenomenon. The star formation history of an unperturbed galaxy seems to be driven by the initial conditions in the protogalaxies and is regulated across cosmic epochs by the variation of the main galaxy parameters (mass, luminosity, structural shape, and velocity dispersion).

In the context of the GAs Stripping Phenomena in galaxies with Muse (GASP) survey, we present the characterization of JO204, a jellyfish galaxy in A957, a relatively low-mass cluster with $M=4.4 \times10^{14}M_\odot$. This galaxy shows a tail of ionized gas that extends up to 30 kpc from the main body in the opposite direction of the cluster center. No gas emission is detected in the galaxy outer disk, suggesting that gas stripping is proceeding outside-in. The stellar component is distributed as a regular disk galaxy; the stellar kinematics shows a symmetric rotation curve with a maximum radial velocity of 200km/s out to 20 kpc from the galaxy center. The radial velocity of the gas component in the central part of the disk follows the distribution of the stellar component; the gas kinematics in the tail retains the rotation of the galaxy disk, indicating that JO204 is moving at high speed in the intracluster medium. Both the emission and radial velocity maps of the gas and stellar components indicate ram-pressure as the most likely primary mechanism for gas stripping, as expected given that JO204 is close to the cluster center and it is likely at the first infall in the cluster. The spatially resolved star formation history of JO204 provides evidence that the onset of ram-pressure stripping occurred in the last 500 Myr, quenching the star formation activity in the outer disk, where the gas has been already completely stripped. Our conclusions are supported by a set of hydrodynamic simulations., accepted for publication in ApJ

Galaxies that abruptly interrupt their star formation in < 1.5 Gyr present recognizable features in their spectra (no emission and Hd in absorption) and are called post starburst (PSB) galaxies. By studying their stellar population properties and their location within the clusters, we obtain valuable insights on the physical processes responsible for star formation quenching. We present the first complete characterization of PSB galaxies in clusters at 0.04 < z < 0.07, based on WINGS and OmegaWINGS data, and contrast their properties to those of passive (PAS) and emission line (EML) galaxies. For V < 20, PSBs represent 7.2 +/- 0.2% of cluster galaxies within 1.2 virial radii. Their incidence slightly increases from the outskirts toward the cluster center and from the least toward the most luminous and massive clusters, defined in terms of X-ray luminosity and velocity dispersion. The phase-space analysis and velocity dispersion profile suggest that PSBs represent a combination of galaxies with different accretion histories. Moreover, PSBs with the strongest Hd are consistent with being recently accreted. PSBs have stellar masses, magnitudes, colors and morphologies intermediate between PAS and EML galaxies, typical of a population in transition from being star forming to passive. Comparing the fraction of PSBs to the fraction of galaxies in transition on longer timescales, we estimate that the short timescale star-formation quenching channel contributes two times more than the long timescale one to the growth of the passive population. Processes like ram-pressure stripping and galaxy-galaxy interactions are more efficient than strangulation in affecting star formation., 14 pages, 8 figures, ApJ in press

We analyzed the light curves (LCs) of several radio-quiet and radio-loud quasars belonging to the same parameter space volume in the 4D Eigenvector 1 (4DE1) quasar sequence, using data from the Catalina Real-time Transient Survey (CRTS). We report preliminary results on detected variability pattern, and discuss possible cases of periodic variability.

Can high ionization lines such as CIV λ 1549 provide useful virial broadening estimators for computing the mass of the supermassive black holes that power the quasar phenomenon? The question has been dismissed by several workers as a rhetorical one because blue-shifted, non-virial emission associated with gas outflows is often prominent in CIV λ 1549 line profiles. In this contribution, we first summarize the evidence suggesting that the FWHM of low-ionization lines like H β and MgII λ 2800 provide reliable virial broadening estimators over a broad range of luminosity. We confirm that the line widths of CIV λ 1549 is not immediately offering a virial broadening estimator equivalent to the width of low-ionization lines. However, capitalizing on the results of Coatman et al. (2016) and Sulentic et al. (2017), we suggest a correction to FWHM CIV λ 1549 for Eddington ratio and luminosity effects that, however, remains cumbersome to apply in practice. Intermediate ionization lines (IP ∼ 20–30 eV; AlIII λ 1860 and SiIII] λ 1892) may provide a better virial broadening estimator for high redshift quasars, but larger samples are needed to assess their reliability. Ultimately, they may be associated with the broad-line region radius estimated from the photoionization method introduced by Negrete et al. (2013) to obtain black hole mass estimates independent from scaling laws.

Broad emission lines in quasars enable us to "resolve" structure and kinematics of the broad line emitting region (BLR) thought to in- volve an accretion disk feeding a supermassive black hole. Interpretation of broad line measures within the 4DE1 formalism simplifies the apparent confusion among such data by contrasting and unifying properties of so-called high and low accreting Population A and B sources. H{\beta} serves as an estimator of black hole mass, Eddington ratio and source rest frame, the latter a valuable input for Civ{\lambda}1549 studies which allow us to isolate the blueshifted wind component. Optical and HST-UV spectra yield H{\beta} and Civ{\lambda}1549 spectra for low-luminosity sources while VLT-ISAAC and FORS and TNG-LRS provide spectra for high Luminosity sources. New high S/N data for Civ in high-luminosity quasars are presented here for comparison with the other previously published data. Comparison of H{\beta} and Civ{\lambda}1549 profile widths/shifts indicates that much of the emission from the two lines arise in regions with different structure and kinematics. Covering a wide range of luminosity and redshift shows evidence for a correlation between Civ{\lambda}1549 blueshift and source Eddington ratio, with a weaker trend with source luminosity (similar amplitude outflows are seen over 4 of the 5 dex luminosity range in our combined samples). At low luminosity (z < 0.7) only Population A sources show evidence for a significant outflow while at high luminosity the outflow signature begins to appear in Population B quasars as well., Comment: 18 pages, 11 figures

The most luminous quasars (with bolometric luminosities are ≳ 1047 erg/s) show a high prevalence of CIV λ1549 and [OIII]λλ4959,5007 emission line profiles with strong blueshifts. Blueshifts are interpreted as due to Doppler effect and selective obscuration, and indicate outflows occurring over a wide range of spatial scales. We found evidence in favor of the nuclear origin of the outflows diagnosed by [OIII]λλ4959,5007. The ionized gas mass, kinetic power, and mechanical thrust are extremely high, and suggest widespread feedback effects on the host galaxies of very luminous quasars, at cosmic epochs between 2 and 6 Gyr from the Big Bang. In this mini-review we summarize results obtained by our group and reported in several major papers in the last few years with an eye on challenging aspects of quantifying feedback effects in large samples of quasars.

Highly accreting quasars are possible cosmological probes, as their Eddington ratio is expected to saturate toward values of order unity. We present preliminary estimates of redshift- independent source luminosities and the Hubble diagram for quasars in the redshift range 0.1 ≲ z ≲ 2.6.

We present the analysis of the emission line galaxies members of 46 low redshift (0.04 < z < 0.07) clusters observed by WINGS (WIde-field Nearby Galaxy cluster Survey, Fasano et al. 2006). Emission line galaxies were identified following criteria that are meant to minimize biases against non-star forming galaxies and classified employing diagnostic diagrams. We have examined the emission line properties and frequencies of star forming galaxies, transition objects and active galactic nuclei (AGNs: LINERs and Seyferts), unclassified galaxies with emission lines, and quiescent galaxies with no detectable line emission. A deficit of emission line galaxies in the cluster environment is indicated by both a lower frequency with respect to control samples, and by a systematically lower Balmer emission line equivalent width and luminosity (up to one order of magnitude in equivalent width with respect to control samples for transition objects) that implies a lower amount of ionised gas per unit mass and a lower star formation rate if the source is classified as Hii region. A sizable population of transition objects and of low-luminosity LINERs (approx. 10 - 20% of all emission line galaxies) is detected among WINGS cluster galaxies. With respect to Hii sources they are a factor of approx. 1.5 more frequent than (or at least as frequent as) in control samples. Transition objects and LINERs in cluster are most affected in terms of line equivalent width by the environment and appear predominantly consistent with "retired" galaxies. Shock heating can be a possible gas excitation mechanism able to account for observed line ratios. Specific to the cluster environment, we suggest interaction between atomic and molecular gas and the intracluster medium as a possible physical cause of line-emitting shocks., Astronomy and Astrophysics, accepted

In Chapter 6 we outlined the research strategies implemented for the studies of high redshift galaxies, briefly addressed their structures, morphologies and star formation, discussed the ancestor problem and touched the theme of the origin of the first galaxies. With this Chapter we certainly enter in the modern view of galaxies, as members of a big society. Up to now we have essentially described and characterized the properties of nearby isolated galaxies, that are sufficiently relaxed to show their almost unperturbed properties. However the history of this society, not differently from the man society, is full of conflicts and battles for surviving. The present knowledge of the cosmic web, of the hierarchical nature of the structures in the Universe, and of the galaxy components have clearly demonstrated that gravitational interactions, gas accretion and outflows, as well as merging events and gravitational interactions are the true past history of galaxies. Galaxies change their structure and morphology across the Hubble time and many of their observed properties today depend on their past history, as well as in many cases on the environment in which they evolved. The title of this Chapter reflects our tentative effort of looking at galaxies along this viewpoint. What are the current boundaries of the galaxy concept? In other words, to what extent the structures we see today were present in the past and what are their limits in mass, luminosity, size, stellar populations and chemical enrichment across the Hubble time? What is the role of the dark matter? In which way the energetic phenomena observed in the galaxy nuclei and in SNe have changed the properties of galaxies? The suspect is that we have built the concept of galaxies looking only at their mature appearance and not at their whole life. Can we speak of man describing only its adult phase and not considering his interaction with the whole society? The following interviews will clarify several things connected to this new point of view. We start in Sec. 7.2 with a nice review of the effects of gravitational interactions and merging that will highlight the progresses done by simulations since the early Toomre’ pioneering work. We then address in Sec. 7.3 the problem of the disk assembling and the remarkable extended UV disks found in many galaxies. The current limits in the mass function of galaxies, from dwarfs to giants, are discussed in Sec. 7.4. The various effects of the presence of an AGN at the center of galaxies are analyzed in Sec. 7.5, while the activity of SNe is addressed in Sec. 7.6. Finally, Sec. 7.7 provide the different point of views on the role and nature of the dark matter.

The theoretical studies about galaxy formation and evolution are the subject of this Chapter. They started with the recognition that the Hubble sequence is not a simple morphological description of galaxies, but a possible scheme separating and characterizing the physical processes that bring galaxies to their present form. When the Hubble tuning fork reached its actual shape—around 1936 with the discovery of the S0 galaxies—, the Hertzsprung-Russell diagram that revealed the existence of the main sequence of stellar structures was already in place, but its explanation in terms of nuclear reactions was still to come (the p-p chain of Bethe appeared in 1939). Just before the end of the WWII the time was mature for the concept of stellar populations formulated by Baade, but only 􏰃 20 years later appeared the first monolithic collapse model of galaxies formation by Eggen et al. (1962). In 1964 Arno Penzias and Robert Woodrow Wilson measured the CMB radiation opening the way toward the current cosmological model. With the 1970s the idea that merging events have produced many of the actual galaxy structures appeared in the literature with the Toomre’ works. Leonard Searle and Robert Zinn proposed that galaxies form by the coalescence of smaller progenitors. At the same time the discovery of the existence of dark matter (DM) rapidly changed our idea of galaxies and how structures form in the Universe. White & Rees and Fall & Efstathiou developed the actual view of galaxy formation, in which baryons fall into the potential wells of hierarchically growing dark matter structures. The two decades 1980s–1990s have seen the development of Semi-Analytic Models (SAM) of galaxy formation and evolution and of numerical hydrodynamical simulations of increasing resolution and complexity. With the new century, the Universe was discovered to accelerate its expansion and thanks to the big redshift galaxy surveys the idea of the cosmic web started to be accepted. Today the dominating paradigm is that provided by the 􏰋CDM cosmology, a framework in which the Universe is believed to be composed of 􏰃 70% of dark energy (DE), 􏰃 26% of cold and hot DM, and 􏰃 4% of baryons. Within this model the values of the cosmological parameters, such as the Hubble expansion rate H0, are known with great accuracy. This is the precision cosmology era, a very awkward situation in which the cosmological parameters are known very well, but we do not know what the Universe is composed of. The following interviews go over some theoretical investigations that, along this century of extra-galactic research, tried to shed some light on the galaxy formation and evolution. Section 8.2 deals with the first attempts to simulate the Hubble tuning fork from basic physical principles. Then, in Sect.8.3 we better define the differences between the monolithic scenario of galaxy formation and the hierarchical scheme, introducing some hybrid versions of the two frameworks. In Sects. 8.4 and 8.5 we provide a much clear view of the star formation history, i.e. of the convolution of the Initial Mass Function (IMF) and the Star Formation Rate (SFR). The role of feedback is analyzed in Sect. 8.6, that of chemical enrichment in Sect. 8.7, and that of Magnetic Fields in Sect. 8.8. We finally propose a panoramic view of the comparisons between observations and models in Sect. 8.9.

Galaxies that are being stripped of their gas can sometimes be recognized from their optical appearance. Extreme examples of stripped galaxies are the so-called ``jellyfish galaxies'', that exhibit tentacles of debris material with a characteristic jellyfish morphology. We have conducted the first systematic search for galaxies that are being stripped of their gas at low-z (z=0.04-0.07) in different environments, selecting galaxies with varying degrees of morphological evidence for stripping. We have visually inspected B and V-band images and identified 344 candidates in 71 galaxy clusters of the OMEGAWINGS+WINGS sample and 75 candidates in groups and lower mass structures in the PM2GC sample. We present the atlas of stripping candidates and a first analysis of their environment and their basic properties, such as morphologies, star formation rates and galaxy stellar masses. Candidates are found in all clusters and at all clustercentric radii, and their number does not correlate with the cluster velocity dispersion sigma or X-ray luminosity L_X. Interestingly, convincing cases of candidates are also found in groups and lower mass haloes (10^{11}-10^{14} M_{sun}), although the physical mechanism at work needs to be securely identified. All the candidates are disky, have stellar masses ranging from log M/M_{sun} < 9 to > 11.5 and the majority of them form stars at a rate that is on average a factor of 2 higher (2.5 sigma) compared to non-stripped galaxies of similar mass. The few post-starburst and passive candidates have weak stripping evidence. We conclude that the stripping phenomenon is ubiquitous in clusters and could be present even in groups and low mass haloes. Further studies will reveal the physics of the gas stripping and clarify the mechanisms at work., revised version including referee comments

Some contributions in Chap. 1 have highlighted the impact of the discovery in the 1960s of a handful of radio galaxies and Quasars in the redshift range z ∼ 0.2–0.4. About 40 years later, at the end of the twentieth Century, the systematic exploration of galaxies reached z ∼ 1–3. The combination of HST deep imaging and the coming into operation of the 8–10 m class telescopes with their spectroscopic capabilities, move ahead the limits. At the same time, astronomers greatly improved their strategies to hunt high-redshift galaxies. Today, it is not infrequent the spectroscopic confirmation of galaxies at z ∼ 7–8, pushing the detection limits more or less to the end of the re-ionization era. The gauntlet to observe the so called “first galaxies”, i.e. those assembling during the first billion years of the cosmic time, is throw down.

With this book we neither aimed at tracing a history, even concise, of extragalactic astronomy nor at summarizing the vast panorama of scientific successes in this field of astrophysics. Our wish was to stimulate some distinguished researchers, via interviews in specific fields of extragalactic studies, at expressing their own convictions and perplexities about the progresses achieved in our understanding of galaxies across one century of research, if necessary emphasizing the scientific problems that still remain open. After about fifty interviews, we try in this chapter to delineate a picture underlying the facts and the views emerged from the presentations. The US of the second decade of XXth century has been the cradle where a bunch of scientists triggered a debate about the nature of spiral nebulæ, and put into discussion the size of The Galaxy set by Jacobus Kapteyn. The new research field, extragalactic astronomy, very soon reached the rank of a mature scientific branch of astrophysics “on the shoulder of giants” of the caliber of Hubble, Zwicky and Baade. However, it was only after WWII that the growth of extragalactic astronomy was overwhelming thanks to big investments in US and later on, for the worse economic conditions, also in some European countries and in Japan. In the last decade of the XXth century extragalactic astronomy assumed the today physiognomy of highly developed science branch in most of the advanced and emerging economies. The national level institutions and observing facilities characterizing the 1960-1980s decades boosted huge international collaborations and consortia, with hundreds of researchers involved with ground based telescopes located in the best observing sites. Space facilities with a complete wavelength coverage bypassed the limitations imposed by the Earth atmosphere providing a panchromatic view of galaxies. We refer to this as the Big Science era of extragalactic astronomy.