After almost one century since their definition, the origin of lenticular galaxies is still a matter of debate. Several formation processes have been proposed in order to explain the wide variety of their observational properties. These properties could indicate that the class of lenticulars is a family formed by galaxies with different formations and evolutions. Here I review the main observational properties of the photometric components of lenticular galaxies reported in recent decades. I revise the main processes proposed in order to explain their origin and evolution. I also explain the different properties of the lenticular galaxies that have evolved through each of these formation processes. A unique opportunity for understanding the origin of S0 galaxies will shortly be forthcoming. This is due to the morphological classifications of large galaxy samples that have recently been published. These classifications have given us our first ever opportunity to study large samples of lenticulars within a wide range of masses and located in a great variety of environments. These large samples will provide us with a real census of nearby lenticular galaxies and could be crucial in finally helping us to understand the origin and evolution of these galaxies.
Lenticular galaxies were introduced by Hubble in 1936 in order to fill the gap between elliptical and spiral galaxies. Their observational properties have been analyzed for decades. Several studies have shown that lenticular galaxies exhibit a great diversity of properties, being similar to both elliptical and spiral galaxies. This makes their origin and evolution still a matter of debate, and several questions still need to be answered. Are lenticular galaxies a well-defined and homogeneous class of galaxies, or are they on the contrary a morphological class of galaxies containing galaxies with different origin and evolution?
Concerning their formation, are lenticulars formed by major mergers of galaxies, similarly to bright elliptical galaxies, or did they form through slow galaxy processes including minor satellite accretion or other secular evolution processes? Are they the final steps in the evolution of late-type galaxies due to environmental mechanisms? Some light can be shed on these questions by analyzing the different observational properties of the structural components present in lenticulars. This is the main aim of this paper. I have compiled information from the literature about the main observational properties of the photometric components of lenticulars and I review the different formation mechanisms proposed. There is a wide diversity of formation mechanisms of S0 galaxies that act on different time scales and galaxy environments. For each formation mechanism, I review the expected properties of the lenticular galaxies formed.
This paper is organized as follows. In Section
Galaxy classification is a visual task. Several visual classifications have been proposed since the discovery of galaxies (extragalactic nebulae) at the beginning of the 20th century (see [
The Hubble tuning fork diagram. A schematic view of the Hubble galaxy classification. Credit: NASA and ESA.
Several galaxy catalogues show visual classifications of galaxies with a high level of detail. These collections contain examples of the different morphological types of galaxies, including lenticulars, and give the classification criteria for each galaxy type. Among the earlier generations of catalogues, we may mention the
HST images of two S0 galaxies, NGC5866 (a) and NGC2787 (b), and two ellipticals, NGC1132 (c) and ESO306-17 (d). Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).
These studies indicate how difficult the classification of E and S0 galaxies is and how difficult it is to select galaxy samples containing only E or S0 galaxies using only photometric information (see [
Galaxy classification scheme proposed by the ATLAS3D project. Galaxies are classified as fast and slow rotators. Fast rotators form a parallel sequence to spiral galaxies. Credit: Cappellari et al. [
The main problem with visual galaxy classifications is that they are not reproducible. Although the classifications are done by specialists, visual galaxy classifications are not universal due to the lack of objectivity of the human eye. Automatic algorithms can provide reproducible galaxy classifications and give important clues to the origin and evolution of galaxies. Automatic classifications also have the advantage that biases can be fully understood and characterized based on extensive simulations of galaxies treated as real ones. Nevertheless, automated visual galaxy classifications can only provide broad galaxy classes. They are not as accurate as visual classifications made by trained observers.
Another problem with visual classifications is the time needed for detailed classifications of large galaxy samples. Present galaxy surveys such as Sloan Digital Sky Survey (SDSS; [
The amount of time needed for detailed visual classifications of galaxies makes the use of automated algorithms a must. Automatic classifications of galaxies can be divided into two broad groups: parametric and nonparametric. Parametric classifications reproduce galaxy measurements using parametric laws. One of the most extended parametric classifications classifies galaxies according to some properties of their structural parameters obtained through the fitting of their surface brightness distribution. These algorithms assume that the luminosity distribution of a galaxy is the sum of the distributions of its individual components. The surface brightness of each component is modelled by a parametric law, which has to be strictly considered as an empirical fitting function (see the review of these algorithms in Section
Nonparametric galaxy classifications are based on the measurements of a set of galaxy parameters that are correlated with the Hubble sequence. The advantage of this method is that it assumes no analytic models and can classify regular and irregular galaxies. Several galaxy parameters have been used for decades for galaxy classification. The colour of galaxies was one of the first parameters used in such classifications since colours are easy to obtain and correlate with galaxy morphological types. Thus, E and S0 galaxies show redder colours than spirals. This correlation is due to colours of galaxies reflecting the dominant constituent stellar population and this correlates with morphology. de Vaucouleurs [
Spectra of galaxies also reflect their stellar populations and have been used for galaxy classification purposes. This property has been used in several spectroscopic galaxy classifications performed using certain features or full galaxy spectra in different wavelength and redshift ranges (e.g., [
Scatter plot of Hubble versus spectral types. The sample of galaxies correspond to those galaxies classified visually by Nair and Abraham [
Galaxies can also be classified on the basis of their light distribution. Morgan [
It is very difficult to classify galaxies with classical approaches using more than three of the previous parameters. Several classification methods take into account the information contained in more than three parameters, but the final calibrations are done in three dimensions (e.g., [
In the near future, all these new large galaxy samples will provide new insights in understanding the nature and evolution of lenticular and other morphological types of galaxies.
From a structural point of view, galaxies can be simple systems formed by one main structural component, such as elliptical galaxies, or they can be more complex systems, such as spiral galaxies, showing structures such as spheroids, discs, bars, spiral arms, rings, and lenses. The luminosity distribution of a galaxy is the sum of the distributions of its individual components. The surface brightness distribution of the galaxies then provides information about their different structural components. It is assumed that each galaxy component can be modelled by a parametric law. Thus, the fit of the surface brightness distribution of galaxies by one or more components provides their structural parameters (size, scale, shape, and luminosity). Comparison of these structural parameters with different types of galaxies or galaxies of the same type but at different redshifts provides useful information for classifying galaxies and understanding their formation and evolution.
One of the basic problems of this quantitative morphology is prior knowledge, gained from solely photometrical information, of the number of structural components of the galaxies. This is crucial because the values of the resulting structural parameters of the galaxies depend on the number of structures used in the fit of their surface brightness distributions (e.g., [
Several multicomponent fitting algorithms have been developed for fitting the surface brightness distribution of galaxies (e.g., [
(left panels) r-band images the NGC4643. Sérsic (middle upper panel), Sérsic plus disc (middle middle panel), and Sérsic plus disc plus bar (middle-lower panel) models of the galaxy. (right panels) the corresponding residuals. Credit: Weinzirl et al. [
Several mathematical functions have been used in order to fit the distribution of the stellar surface brightness of the different structural components of galaxies. These fitting functions include those for bulges, discs, lenses, and bars.
Historically, the first surface brightness models were proposed for fitting spheroidal galaxy components. The reason for this was that the first galaxies analysed were ellipticals. We can mention the surface-density models proposed by Reynolds [
One of the most popular fitting functions for describing the surface brightness of Es and bulges of S0s and spirals has been the de Vaucouleurs
More recent studies have found deviations from the
(a) Sérsic radial profiles with
Freeman [
Recently, detailed analyses of the external regions of disc galaxies have shown that not all galaxy discs show pure exponential surface brightness radial profiles. Most galaxies show truncations (downbending) or antitruncations (upbending) in their outermost regions. Thus, about 60% of galaxies have a break followed by downbending. Moreover, another 30% of galaxies have breaks followed by upbending. The shape of the surface brightness profiles of discs correlates with the Hubble type. Thus, downbending breaks are more frequent in late-type galaxies. In contrast, upbending breaks are more frequent in early-type galaxies (see [
Bars are elliptical structures located in discs of galaxies. It is well known since pioneering morphological classifications of galaxies that strong bars are common in discs of S0 and spiral galaxies (see e.g., [
There are not many studies of the surface-brightness decomposition of barred galaxies. The large number of free parameters needed in these kind of fits makes them difficult. Fortunately, the number of studies including bars in the photometrical decomposition is increasing in the last years. Thus, some studies have used the Sérsic functions for fitting the observed surface brightness profile of bars (see [
The radial surface intensity profile of a Ferrers ellipsoid (
In the case of
The radial surface intensity profile (
The radial surface intensity profile of a flat bar (
The surface radial profiles of Ferrers, Freeman and flat bars with the same bar radius can be seen in Figure
Lenses are photometric components commonly observed in lenticular and some early-type galaxies. In contrast, few late-type galaxies show lenses. Kormendy [
The relations between the structural parameters have been extensively studied in the literature in order to understand the formation and evolution processes of different galaxy components (for a recent review see [
Andredakis et al. [
Recent surface brightness decomposition of large galaxy samples, including S0s, show that the correlation between
Correlations between the bulge parameters for a sample of lenticulars and early-type spirals. Correlation between the Hubble type and effective radius (a), effective surface brightness (b), and Sérsic parameter (c). Correlations between the effective radius of the bulge and the Sérsic parameter (d), effective surface brightness (e), and absolute magnitude (f). The full line represents the best linear fit. The Pearson correlation coefficient (
Dynamically hot systems are not scattered randomly in the parameter space defined by their effective surface brightness (
Kormendy [
The Kormendy relation for bulges of S0, early-type and late-type spirals. In all panels the full line represents the KR of elliptical galaxies in the Coma cluster. The dotted black line corresponds to the extension of the KR of ellipticals towards galaxies with smaller effective radius. The red dashed line of the upper panel represents the KR for the S0 bulges. The red and green dashed lines in the lower panel show the KR for S0 bulges brighter and fainter than
Another projection of the FP is the so-called Faber-Jackson (FJ) relation ([see 144]), which relates the luminosity (
The Sérsic shape index
The effective radius (
Effective radius (a) and effective surface brightness (b) of bulges of S0 (red points), early-type (crosses), and late-type (squares) spirals as a function of the bulge absolute magnitude. The dashed line in the left panel corresponds to the relation followed by ellipticals galaxies from the Coma cluster. The continuous lines in the left panel correspond to the mean values of the data points in one- magnitude bins: S0s (red line) and Sa-Sbc (blue line). Credit: Laurikainen et al. [
Classical bulges can be distinguished from pseudobulges (Many bulges show disk-like features that do not resemble hot stellar systems. These kind of bulges are usually called pseudobulges (see [
In comparison with the large number of works investigating the correlations between the bulge parameters of S0 and spiral galaxies, little attention has been given to the three-dimensional shape of bulges (e.g., [
In a series of papers, Méndez-Abreu et al. [
Méndez-Abreu et al. [
Distribution of the triaxiality parameter
Several works found no correlation between the disc scale length and the Hubble type over a wide range of morphological types, including S0 discs (see [
Correlations between the disc parameters for a sample of lenticulars and early-type spirals. Correlation between the Hubble type and disc scale length (a) and the central surface brightness (b). Correlations between the disc scale length and the absolute magnitude of the disc (c) and the central surface brightness (d). The full line represents the best linear fit (
The scale length of discs of spiral and lenticular galaxies correlates with the disc luminosity. Thus, larger discs are located in brighter ones (see [
The Tully-Fisher relation (TFR; [
The TFR of S0 galaxies has been less investigated in the literature owing to the difficulty in obtaining the asymptotic circular velocity of these gas-poor galaxies. Nevertheless, several studies indicate that the TFR of S0 galaxies shows a similar slope to that of late-type ones but with a different zero point (see Figure
Tully-Fisher diagram for S0 Coma galaxies in I-band. The dotted line indicates the TF for late-type spirals and the full line shows the TF of S0 galaxies from the Coma cluster. Credit: Hinz et al. [
The distribution of disc galaxies in the rotational velocity-size (
Traditionally, the bulge-to-disc-luminosity ratio (
Recent surface brightness decomposition of samples of galaxies including S0s challenge the previous results. In particular, Balcells et al. [
The large spread found in the
The correlation between the bulge effective radius and disc scale length has been extensively studied in the literature (see [
The relation between the ratio of the bulge effective radius to disk scale length (
The presence of bars in disc galaxies has been extensively studied in the literature. Bars have been identified through visual inspection (e.g., [
The fraction of barred galaxies is not constant with the Hubble type. Neither is there agreement on whether the bar fraction is larger in S0 galaxies or in late-type ones. Thus, Aguerri et al. [
There are three main observational parameters of bars: length, strength, and pattern speed. I review below the most important results concerning these three parameters for S0 galaxies.
The bar length has been obtained by eye estimates from galaxy images [
The bar length normalized by galaxy size depends on the Hubble type. Thus, lenticular galaxies show larger bars than late-type ones [
The bar strength represents the contribution of the bar to the total potential of the galaxy. Several methods have been developed to measure it. The most commonly used is the
The two methods give similar strength measurements of the bars due to the fact that bar strength computed with bar ellipticities correlates with
Histograms of the distribution (a) and (c) and cumulative distribution (b) and (d) of bar strengths in S0 (dotted line) and spiral galaxies (full line). This figure shows that bars are weaker in S0 than in spiral galaxies. Credit: Buta et al. [
Bar pattern speed is the main kinematic observable in barred galaxies and measures the tumbling pattern of the bar. The pattern speed is related to the so-called corotation radius of the galaxy. This is the radius at which the angular velocity of the disc is equal to the pattern speed of the bar. Barred galaxies are classified according to the distance-independent ratio
Several methods have been developed for measuring the pattern speed of bars, including hydrodynamic simulations [
The TW method has already been applied to a large sample of S0 barred galaxies [
The corotation radius (
The dependence of the bar pattern speed on the Hubble type is not well understood yet. Few measurements of bar pattern speeds for late-type galaxies are available in the literature. Nevertheless, there are some hints that S0 and late-type galaxies show different pattern speed. Thus, Aguerri et al. [
The
It is important to notice that all these results concerning bars indicate that discs of late-type and lenticular galaxies are different. Thus, lenticulars show a smaller fraction of bars than late-type ones. In addition, bars in lenticular galaxies are larger, weaker, and faster than late-type bars.
Dwarf galaxies are the most numerous type of galaxies in the universe. They are defined as those with low central surface brightness and luminosities below
In the Coma cluster, 56% of the dwarf galaxies in the magnitude interval
The structural relations of dwarf galaxies in the Coma cluster were analysed by Aguerri et al. [
Effective radius (a), Sérsic shape parameter (b), and central surface brightness (c) as functions of the R-band absolute magnitude of dE (asterisks) and bulges of dS0 (diamonds), late-type spirals (circles), early-type spirals (crosses), and S0 galaxies (squares) in the Coma cluster. See for more details [
In the previous sections I have reviewed the properties of the structural parameters of lenticular and spiral galaxies. It should be noticed that the scaling relations shown before give a wide variety of results. Thus, some results indicate that S0 bulges are similar to E galaxies (e.g., same FP, FJ, and size-mass relations, large
According to the current paradigm of galaxy formation, spheroidal systems (ellipticals and bulges of galaxies) have been formed by major or minor galaxy mergers. In contrast, discs do not survive after major mergers. They were formed by cooling of gas in rotating dark matter haloes. In this paradigm the central bulge of disc galaxies forms prior to the disc as a result of early merging (see the reviews by [
Galaxy major mergers produce remnants with similar shape and scaling relations to those observed in bulges of S0 galaxies. In particular, the gas content of the progenitors determines the shape and structural parameters of the remnants. Thus, dissipationless numerical simulations of major mergers produce triaxial remnants with a tendency to become prolate systems. In contrast, dissipational remnants are weakly triaxial, but close to oblate [
The major merger scenario could be applicable to some of the S0 galaxies, especially the most luminous and with prominent bulges (
Recently, it was discovered that a small fraction of nearby E/S0 galaxies show blue colours. These galaxies, called blue ellipticals, constitute only about 10% of the total population of nearby E/S0 galaxies (e.g., [
Major mergers are rare in rich galaxy clusters. They are more frequent in field or galaxy groups. Thus, S0 galaxies formed by this mechanism should be located preferentially in less dense galaxy environments.
Satellite accretion must have occurred several times in a disc galaxy over a Hubble time. Mergers of galaxies with similar mass fully destroy the disc and leave remnants similar to E galaxies. In contrast, minor mergers damage the disc less and can produce bulge growth. The growth of the central bulge of the main galaxy depends on the density and relative mass of the accreted satellite. The remnants of these kind of mergers retain significant amounts of rotation (see [
The time evolution of the luminous matter of a minor merger (see details in [
The most important galaxy transformations were produced by the accretion of dense satellites (see [
Growth vectors of the bulges in the
The accretion of less dense satellites was studied by Eliche-Moral et al. [
The dependence of bulge and disc parameters, and
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In both cases the accretion of high- and low-density satellites hits the thin disc of the primary galaxy. This vertical heating of the disc depends on the orbit and mass of the satellite. The thick disc created by these types of mergers is reminiscent of the thick discs seen in S0 and early-type galaxies. Bekki [
The minor accretion models proposed by Aguerri et al. [
Since the first observations of galaxies in clusters it has been well known that the galaxies located in high-density environments have different properties from those located in the field [
One of the differences between galaxies in clusters and the field is morphology. In dense environments the fraction of ellipticals and S0 galaxies is larger than in the field. In contrast, the fraction of spiral and irregular galaxies is higher in less dense environments. This is the so-called morphology-density relation observed in nearby [
Galaxy population gradients in galaxy clusters as a function of radial distance from the cluster center (a) and local galaxy density (b) Credit: Dressler [
Galaxies in high-density environments evolve through many different physical processes. We may mention harassment [
X-ray observations of galaxy clusters reveal the presence of a large amount of hot gas (
The efficiency of the stripping depends on several properties of clusters and galaxies, such as the location of the stripped gas in the galaxy, the masses of the host cluster and galaxies, and the density of the gas. The gas in galaxies can be found as cold or hot gas. The cold gas is located inside galaxy discs, while the hot gas is located in the galaxy halo. Both types of gas can be stripped from galaxies. The gas from galaxy haloes can be stripped more easily than gas from the discs. The required relative velocity between the galaxy and cluster and the gas density of the intracluster gas are thus significantly much lower than those from disc stripping (see [
Gas stripping has been observed in several bright dwarf galaxies located in nearby galaxy clusters (e.g.,[
Morphological properties for different models from [
Galaxies in high-density environments like galaxy clusters can suffer several gravitational interactions with a wide variety of impact parameters, relative velocity between galaxies and mass ratios. Interactions in rich galaxy clusters are expected to be more abundant and faster than in small galaxy groups. The harassment scenario proposes that the cumulative effects of fast tidal encounters between galaxies and with the gravitational potential can produce dramatic morphological transformations in galaxies. Thus, bright galaxies can lose a significant fraction of their stellar and gas content. The fraction of mass lost can be high enough in order to transform bright galaxies into dwarfs ([
Observational indications of this mass loss have been found for the bright spiral galaxies located in the Coma cluster. Thus, Aguerri et al. [
Mastropietro et al. [
Change of total (a) and luminous (b) mass of simulated galaxies after different fast tidal events (see details in [
The final position of the resulting galaxies in the fundamental plane depends on the initial conditions of the simulations. Thus, models with initially large
One of the most prominent features that tidal interactions induce in the discs of galaxies is the presence of stable and prominent bars (see [
Aguerri and González-Garcia [
It is well known that early-type galaxies follow the luminosity-metallicity and velocity dispersion-metallicity relations (e.g., [
Galaxies in high-density environments do not only interact with one another but also interact with the cluster gravitational potential. Thus, cluster and/or group tides can also produce strong morphological transformations in spiral galaxies. The tidal field of the clusters as a whole can trigger cluster spiral, nuclear, and disc activities. This effect is more important near the centre of clusters or groups. A higher fraction of starburst galaxies might be expected in the cores of clusters or groups of galaxies. The long-term effects of this activity would be to clear the gas from disc spirals and changing them into S0 galaxies [
Evidence for the morphological transformation of galaxies by the cluster environment has been observed in the S0 population of galaxies in the Coma cluster. Poggianti et al. [
The picture of galaxies falling into static galaxy clusters is very simplistic and far removed from reality. In the hierarchical structure formation theory dark matter haloes grow through the accretion of smaller units. This formation is scale-free. Thus, clusters and groups of galaxies grow through the accretion of smaller groups. Optical and X-ray studies of galaxy clusters show that a large fraction of them have recently accreted other groups or clusters [
Mergers of galaxy clusters could enhance star formation, as suggested by numerical simulations (e.g., [
The distribution of galaxies in colour-magnitude and colour-mass diagrams shows that they are located in two main regions: the blue cloud and the red sequence. Study of the colour-magnitude diagram of galaxies at different redshifts indicates that galaxies stop their star formation and pass from the blue cloud to the red sequence. Moreover, this migration is strongly mass dependent. Thus, more massive galaxies reach the red sequence at higher redshift than less massive ones (e.g., [
The reddening of spiral galaxies could produce S0s by fading the spiral arms of the progenitors. This reddening could be due to processes related to the galaxy environment, or they could be processes related only to the internal galaxy evolution. One of the internal processes that could regulate the star formation in galaxies is feedback processes heating the cold gas in galaxies and stopping their star formation. Dekel and Silk [
The red sequence is not only formed by ellipticals and early-type galaxies. It has recently been discovered that a small fraction of late-type spirals (about 6%) show red colour and are located in the red sequence (see [
These two internal processes (AGN feedback and passive evolution) could explain the similarities of S0s and spiral galaxies. The S0 galaxies formed by these mechanisms should be in intermediate or less dense galaxy environments. However, these internal processes could not explain the different bar properties observed between spiral and lenticular galaxies: different bar fractions (e.g, [
In this paper I have reviewed the observational properties of the photometric components of lenticular galaxies. I have selected samples of galaxies containing S0s and with structural parameters obtained in a homogeneous way by multicomponent surface brightness decompositions. The properties of the different components show that some lenticular galaxies are similar to ellipticals whereas others show similar relations to spiral galaxies. In particular, the most luminous S0s with
I have reviewed the different formation mechanisms proposed in the literature for understanding the formation of lenticular galaxies. There is a large variety of formation mechanisms that produce S0s over very different time scales. Each of these mechanisms produces S0 galaxies with different properties. Thus, lenticular galaxies with high
The morphological classifications of large galaxy samples that have recently appeared in the literature will in the near future provide important clues to understanding the formation and evolution of galaxies, including lenticulars. These classifications provide a unique opportunity for selecting large samples of lenticular galaxies located in a large diversity of environments in a homogeneous way, from field to galaxy clusters. These large samples will permit us to solve the biases of present small galaxy samples. Detailed analysis of the structural parameters, together with the analysis of the environments where lenticulars live, will in the near future provide new and important constrains to the formation and evolution of lenticular galaxies.
The authors would like to thank the anonymous referees for helping him to improve both the content and the presentation of the paper. This work was supported by the projects AYA2010-21887-C04-04 and by the Consolider-Ingenio 2010 Program grant CSD2006-00070.