Multi-component Decompositions Research Articles

Appreciating mergers for understanding the non-linear Mbh–M*,spheroid and Mbh–M*, galaxy relations, updated herein, and the implications for the (reduced) role of AGN feedback

ABSTRACT We present revised (black hole mass)–(spheroid stellar mass) and (black hole mass)–(galaxy stellar mass) scaling relations based on colour-dependent (stellar mass)-to-light ratios. Our 3.6 $\mu$m luminosities were obtained from multicomponent decompositions, which accounted for bulges, discs, bars, ansae, rings, nuclear components, etc. The lenticular galaxy bulges (not associated with recent mergers) follow a steep $M_{\rm {bh}}\propto ~M_{\rm {*,bulge}}^{1.53\pm 0.15}$ relation, offset by roughly an order of magnitude in black hole mass from the $M_{\rm {bh}}\propto ~M_{\rm {*,ellip}}^{1.64\pm 0.17}$ relation defined by the elliptical (E) galaxies which, in Darwinian terms, are shown to have evolved by punctuated equilibrium rather than gradualism. We use the spheroid (i.e. bulge and elliptical) size–mass relation to reveal how disc-galaxy mergers explain this offset and the dramatically lower Mbh/M*,sph ratios in the elliptical galaxies. The deceptive near-linear Mbh–M*,sph ‘red sequence’, followed by neither the bulge population nor the elliptical galaxies, is shown to be an artefact of sample selection, combining bulges and elliptical galaxies from disparate Mbh–M*,sph sequences. Moreover, both small bulges with ‘undermassive’ black holes and big lenticular galaxies (including relic ‘red nuggets’) with ‘overmassive’ black holes – relative to the near-linear Mbh–M*,sph sequence – are no longer viewed as outliers. We confirm a steep $M_{\rm {bh}}\propto ~M_{\rm {*,bulge}}^{2.25\pm 0.39}$ relation for spiral galaxies and discuss numerous implications of this work, including how mergers, rather than (only) feedback from active galactic nuclei, have shaped the high-mass end of the galaxy mass function. We also explain why there may be no useful Mbh–M*,sph–Re,sph plane due to M*,sph scaling nearly linearly with Re,sph.

Disc cloaking: Establishing a lower limit to the number density of local compact massive spheroids/bulges and the potential fate of some high-z red nuggets

ABSTRACT The near-absence of compact massive quiescent galaxies in the local Universe implies a size evolution since z ∼ 2.5. It is often theorized that such ‘red nuggets’ have evolved into today’s elliptical (E) galaxies via an E-to-E transformation. We examine an alternative scenario in which a red nugget develops a rotational disc through mergers and accretion, say, at 1 ≲ z ≲ 2, thereby cloaking the nugget as the extant bulge/spheroid component of a larger, now old, galaxy. We have performed detailed, physically motivated, multicomponent decompositions of a volume-limited sample of 103 massive ($M_*/\rm M_{\odot } \gtrsim 1\times 10^{11}$) galaxies within 110 Mpc. Many less massive nearby galaxies are known to be ‘fast-rotators’ with discs. Among our 28 galaxies with existing elliptical classifications, we found that 18 have large-scale discs, and two have intermediate-scale discs, and are reclassified here as lenticulars (S0) and elliculars (ES). The local spheroid stellar mass function, size–mass diagram and bulge-to-total (B/T) flux ratio are presented. We report lower limits for the volume number density of compact massive spheroids, nc, Sph ∼ (0.17–$1.2) \times 10^{-4}\, \rm Mpc^{-3}$, based on different definitions of ‘red nuggets’ in the literature. Similar number densities of local compact massive bulges were reported by de la Rosa et al. using automated two-component decompositions and their existence is now abundantly clear with our multicomponent decompositions. We find disc-cloaking to be a salient alternative for galaxy evolution. In particular, instead of an E-to-E process, disc growth is the dominant evolutionary pathway for at least low-mass ($1\times 10^{10}\lt M_*/\rm M_{\odot } \lessapprox 4 \times 10^{10}$) red nuggets, while our current lower limits are within an alluring factor of a few of the peak abundance of high-mass red nuggets at 1 ≲ z ≲ 2.

Stellar masses, sizes, and radial profiles for 465 nearby early-type galaxies: An extension to the Spitzer survey of stellar structure in Galaxies (S4G)

Context. The Spitzer Survey of Stellar Structure in Galaxies (S4G) is a detailed study of over 2300 nearby galaxies in the near-infrared (NIR), which has been critical to our understanding of the detailed structures of nearby galaxies. Because the sample galaxies were selected only using radio-derived velocities, however, the survey favored late-type disk galaxies over lenticulars and ellipticals. Aims. A follow-up Spitzer survey was conducted to rectify this bias, adding 465 early-type galaxies (ETGs) to the original sample, to be analyzed in a manner consistent with the initial survey. We present the data release of this ETG extension, up to the third data processing pipeline (P3): surface photometry. Methods. We produce curves of growth and radial surface brightness profiles (with and without inclination corrections) using reduced and masked Spitzer IRAC 3.6 μm and 4.5 μm images produced through Pipelines 1 and 2, respectively. From these profiles, we derive the following integrated quantities: total magnitudes, stellar masses, concentration parameters, and galaxy size metrics. We showcase NIR scaling relations for ETGs among these quantities. Results. We examine general trends across the whole S4G and ETG extension among our derived parameters, highlighting differences between ETGs and late-type galaxies (LTGs). The latter are, on average, more massive and more concentrated than LTGs, and subtle distinctions are seen among ETG morphological subtypes. We also derive the following scaling relations and compare them with previous results in visible light: mass-size (both half-light and isophotal), mass-concentration, mass-surface brightness (central, effective, and within 1 kpc), and mass-color. Conclusions. We find good agreement with previous works, though some relations (e.g., mass-central surface brightness) will require more careful multicomponent decompositions to be fully understood. The relations between mass and isophotal radius and between mass and surface brightness within 1 kpc, in particular, show notably small scatter. The former provides important constraints on the limits of size growth in galaxies, possibly related to star formation thresholds, while the latter–particularly when paired with the similarly tight relation for LTGs–showcases the striking self-similarity of galaxy cores, suggesting they evolve little over cosmic time. All of the profiles and parameters described in this paper will be provided to the community via the NASA/IPAC database on a dedicated website.

The Morphology-dependent Black Hole–Host Galaxy Correlations: A Consequence of Physical Formation Processes

For decades, astronomers have been investigating how the central supermassive black hole (BH) may govern the host galaxy’s properties and vice versa. Our work adds another step to this study. We have performed state-of-theart 2D modeling and multi-component photometric decompositions of the largest-to-date sample of galaxies with dynamically-measured black hole masses (MBH). The multi-component decomposition allows us to accurately extract the bulge (spheroid) stellar luminosity/mass and structural parameters (also for other galaxy components) and provides detailed galaxy morphologies. We investigated the correlations between MBH and various host galaxy properties, including the bulge (M*,sph) and total galaxy (M*,gal) stellar masses discussed here. Importantly, we analyzed the role of galaxy morphology in these correlations. Our work reveals that the BH scaling relations depend on galaxy morphology and thus depend on the galaxy’s formation and evolution physics. Here we discuss that in the MBH–M*,sph diagram, early-type galaxies (ETGs) with a disk, ETGs without a disk, and late-type galaxies (LTG-spirals) define distinct relations, with quadratic slopes but different zero-points. We also review the MBH–M*,gal relation, where ETGs and LTGs define different relations. Notably, the existence of the MBH–M*,gal relations enables one to quickly estimate MBH in other galaxies without going through the multi-component decomposition process to obtain M*,sph. The final morphology-dependent black hole scaling relations provide tests for morphology-aware simulations of galaxies with a central BH and hold insights for BH-galaxy co-evolution theories based on BH accretion and feedback.

The Fornax Deep Survey (FDS) with the VST

Context. Galaxies either live in a cluster, a group, or in a field environment. In the hierarchical framework, the group environment bridges the field to the cluster environment, as field galaxies form groups before aggregating into clusters. In principle, environmental mechanisms, such as galaxy–galaxy interactions, can be more efficient in groups than in clusters due to lower velocity dispersion, which lead to changes in the properties of galaxies. This change in properties for group galaxies before entering the cluster environment is known as preprocessing. Whilst cluster and field galaxies are well studied, the extent to which galaxies become preprocessed in the group environment is unclear. Aims. We investigate the structural properties of cluster and group galaxies by studying the Fornax main cluster and the infalling Fornax A group, exploring the effects of galaxy preprocessing in this showcase example. Additionally, we compare the structural complexity of Fornax galaxies to those in the Virgo cluster and in the field. Methods. Our sample consists of 582 galaxies from the Fornax main cluster and Fornax A group. We quantified the light distributions of each galaxy based on a combination of aperture photometry, Sérsic+PSF (point spread function) and multi-component decompositions, and non-parametric measures of morphology. From these analyses, we derived the galaxy colours, structural parameters, non-parametric morphological indices (Concentration C; Asymmetry A, Clumpiness S; Gini G; second order moment of light M20), and structural complexity based on multi-component decompositions. These quantities were then compared between the Fornax main cluster and Fornax A group. The structural complexity of Fornax galaxies were also compared to those in Virgo and in the field. Results. We find significant (Kolmogorov-Smirnov test p-value < α = 0.05) differences in the distributions of quantities derived from Sérsic profiles (g′−r′, r′−i′, Re, and μ̄e,r′), and non-parametric indices (A and S) between the Fornax main cluster and Fornax A group. Fornax A group galaxies are typically bluer, smaller, brighter, and more asymmetric and clumpy. Moreover, we find significant cluster-centric trends with r′−i′, Re, and μ̄e,r′, as well as A, S, G, and M20 for galaxies in the Fornax main cluster. This implies that galaxies falling towards the centre of the Fornax main cluster become fainter, more extended, and generally smoother in their light distribution. Conversely, we do not find significant group-centric trends for Fornax A group galaxies. We find the structural complexity of galaxies (in terms of the number of components required to fit a galaxy) to increase as a function of the absolute r′-band magnitude (and stellar mass), with the largest change occurring between −14 mag ≲Mr′ ≲ −19 mag (7.5 ≲ log10(M*/M⊙) ≲ 9.7). This same trend was found in galaxy samples from the Virgo cluster and in the field, which suggests that the formation or maintenance of morphological structures (e.g., bulges, bar) are largely due to the stellar mass of the galaxies, rather than the environment they reside in.

Defining the (Black Hole)–Spheroid Connection with the Discovery of Morphology-dependent Substructure in the MBH–nsph and MBH–Re,sph Diagrams: New Tests for Advanced Theories and Realistic Simulations

Abstract For 123 local galaxies with directly measured black hole masses (M BH), we provide the host spheroid’s Sérsic index (n sph), effective half-light radius (R e,sph), and effective surface brightness ( ), obtained from careful multicomponent decompositions, and we use these to derive the morphology-dependent M BH–n sph and M BH–R e,sph relations. We additionally present the morphology-dependent M *,sph–n sph and M *,sph–R e,sph relations. We explored differences due to early-type galaxies (ETGs) versus late-type galaxies (LTGs), Sérsic versus core-Sérsic galaxies, barred versus non-barred galaxies, and galaxies with and without a stellar disk. We detect two different M BH–n sph relations due to ETGs and LTGs with power-law slopes 3.95 ± 0.34 and 2.85 ± 0.31. We additionally quantified the correlation between M BH and the spheroid’s central concentration index, which varies monotonically with the Sérsic index. Furthermore, we observe a single, near-linear M *,sph– relation for ETGs and LTGs, which encompasses both classical and alleged pseudobulges. In contrast, ETGs and LTGs define two distinct M BH–R e,sph relations with (cf. ∼0.51 dex for the M BH–σ relation and ∼0.58 dex for the M BH–M *,sph relation), and the ETGs alone define two steeper M BH–R e,sph relations, offset by ∼1 dex in the direction, depending on whether they have a disk or not and explaining their similar offset in the M BH–M *,sph diagram. This trend holds using 10%, 50%, or 90% radii. These relations offer pivotal checks for simulations trying to reproduce realistic galaxies, and for theoretical studies investigating the dependence of black hole mass on basic spheroid properties.

Black Hole Mass Scaling Relations for Early-type Galaxies. I. MBH–M*, sph and MBH–M*,gal

Abstract Analyzing a sample of 84 early-type galaxies (ETGs) with directly measured supermassive black hole masses—nearly doubling the sample size of such galaxies with multicomponent decompositions—a symmetric linear regression on the reduced (merger-free) sample of 76 galaxies reveals with a total scatter of Δrms = 0.52 dex in the direction. Importantly, however, we discover that the ES/S0-type galaxies with disks are offset from the E-type galaxies by more than a factor of ten in their ratio, with ramifications for formation theories, simulations, and some virial factor measurements used to convert AGN virial masses into M BH. Separately, each population follows a steeper relation with slopes of 1.86 ± 0.20 and 1.90 ± 0.20, respectively. The offset mass ratio is mainly due to the exclusion of the disk mass, with the two populations offset by only a factor of two in their ratio in the M BH– diagram where and dex depending on the sample. For , we detect no significant bend nor offset in either the M BH– or M BH– relations due to barred versus non-barred, or core-Sérsic versus Sérsic, ETGs. For reference, the ensemble of late-type galaxies (which invariably are Sérsic galaxies) follow M BH– and M BH– relations with slopes equal to 2.16 ± 0.32 and 3.05 ± 0.70, respectively. Finally, we provide some useful conversion coefficients, υ, accounting for the different stellar mass-to-light ratios used in the literature, and we report the discovery of a local, compact massive spheroid in NGC 5252.

Spectro-photometric decomposition of galaxy structural components

Galaxies are complex systems made up of different structural components such as bulges, discs, and bars. Understanding galaxy evolution requires unveiling, independently, their history of stellar mass and metallicity assembly. We introduce C2D, a new algorithm to perform spectro-photometric multi-component decompositions of integral field spectroscopy (IFS) datacubes. The galaxy surface-brightness distribution at each wavelength (quasi-monochromatic image) is fitted using GASP2D, a 2D photometric decomposition code. As a result, C2D provides both a characteristic one-dimensional spectra and a full datacube with all the spatial and spectral information for every component included in the fit. We show the basic steps of the C2D spectro-photometric fitting procedure, tests on mock datacubes demonstrating its reliability, and a first application of C2D to a sample of three early-type galaxies (ETGs) observed within the CALIFA survey. The resulting datacubes from C2D are processed through the PIPE3D pipeline obtaining both the stellar populations and ionised gas properties of bulges and discs. This paper presents an overview of the potential of C2D+PIPE3D to unveil the formation and evolution of galaxies.

Barlenses in the CALIFA survey: Combining photometric and stellar population analyses

Aims: It is theoretically predicted that, at low galaxy inclinations, boxy/peanut bar components have a barlens appearance of a round central component embedded in the narrow bar. We investigate barlenses in the Calar Alto Legacy Integral Field Area (CALIFA) survey galaxies, studying their morphologies, stellar populations, and metallicities. We show that, when present, barlenses account for a significant portion of light of photometric bulges, i.e., the excess light on top of the disks, which highlights the importance of bars in accumulating central galaxy mass concentrations in the cosmic timescale. Methods: We made multi-component decompositions for a sample of 46 barlens galaxies drawn from the CALIFA survey, where M⋆/M⊙ = 109.7 − 1011.4 and z = 0.005 − 0.03. Unsharp masks of the Sloan Digital Sky Survey (SDSS) r′-band mosaics were used to identify the boxy/peanut or X-shaped features. Barlenses are identified in the images using our simulation snapshots as an additional guide. Our decompositions with GALFIT include bulges, disks, and bars as well as barlenses as a separate component. For 26 of the decomposed galaxies the CALIFA DR2 V500 grating data cubes were used to explore stellar ages and metallicities at the regions of various structure components. Results: We find that 25 ± 2% of the 1064 galaxies in the whole CALIFA sample show either X-shaped or barlens features. In the decomposed galaxies with barlenses, on average 13% ± 2% of the total galaxy light belongs to this component, leaving less than 10% for possible separate bulge components. Most importantly, bars and barlenses are found to have similar cumulative stellar age and metallicity distributions. The metallicities in barlenses are on average near solar, but exhibit a large range. In some of the galaxies barlenses and X-shaped features appear simultaneously, in which case the bar origin of the barlens is unambiguous. Conclusion: This is the first time that a combined morphological and stellar population analysis is used to study barlenses. We show that their stars are accumulated in a prolonged time period concurrently with the evolution of the narrow bar.

The Carnegie-Irvine Galaxy Survey. VII. Constraints on the Origin of S0 Galaxies from Their Photometric Structure

Abstract Using high-quality optical images from the Carnegie-Irvine Galaxy Survey, we perform multicomponent decompositions of S0 galaxies (S0s) to derive accurate structural parameters to constrain their physical origin. Many S0s do not host prominent bulges. S0s have a broad distribution of bulge-to-total ratios (B/T) and Sérsic indices (n), with average values of B/T = 0.34 ± 0.15 and n = 2.62 ± 1.02, qualitatively consistent with the notion that S0s define a parallel sequence with and may have evolved from spiral galaxies. This is further reinforced by the incidence of bars and lenses in S0s, which, when compared with the statistics in spirals, supports the idea that lenses are demised bars. However, despite their wide range of prominence, the bulges of S0s form a surprisingly uniform sequence on both the Kormendy and fundamental plane relations. There is no evidence for population dichotomy or other indications of differentiation into classical and pseudo bulges. Most of the S0s reside in the field and in groups; cluster environment is not a necessary condition for S0 production. The properties of S0 bulges show little correlation with environmental indicators, after the dependence of galaxy stellar mass on environment is taken into account. As the bulges of late-type spirals and S0s are intrinsically different, and environmental effects that may account for such evolution appear to be minimal, we conclude that late-type spirals are not plausible progenitors of S0s. The bulges of S0s likely formed at an early epoch, after which secular processes contributed little to their subsequent evolution.

Star Formation in the Local Universe from the CALIFA Sample. II. Activation and Quenching Mechanisms in Bulges, Bars, and Disks

Abstract We estimate the current extinction-corrected Hα star formation rate (SFR) of the different morphological components that shape galaxies (bulges, bars, and disks). We use a multicomponent photometric decomposition based on Sloan Digital Sky Survey imaging to Calar Alto Legacy Integral Field Area Integral Field Spectroscopy (IFS) datacubes for a sample of 219 galaxies. This analysis reveals an enhancement of the central SFR and specific SFR (sSFR = SFR/M ⋆) in barred galaxies. Along the main sequence, we find that more massive galaxies in total have undergone efficient suppression (quenching) of their star formation, in agreement with many studies. We discover that more massive disks have had their star formation quenched as well. We evaluate which mechanisms might be responsible for this quenching process. The presence of type 2 AGNs plays a role at damping the sSFR in bulges and less efficiently in disks. Also, the decrease in the sSFR of the disk component becomes more noticeable for stellar masses around for bulges, it is already present at . The analysis of the line-of-sight stellar velocity dispersions (σ) for the bulge component and of the corresponding Faber–Jackson relation shows that AGNs tend to have slightly higher σ values than star-forming galaxies for the same mass. Finally, the impact of environment is evaluated by means of the projected galaxy density, Σ5. We find that the SFR of both bulges and disks decreases in intermediate- to high-density environments. This work reflects the potential of combining IFS data with 2D multicomponent decompositions to shed light on the processes that regulate the SFR.

SUPERMASSIVE BLACK HOLES AND THEIR HOST SPHEROIDS. I. DISASSEMBLING GALAXIES

ABSTRACT Several recent studies have performed galaxy decompositions to investigate correlations between the black hole mass and various properties of the host spheroid, but they have not converged on the same conclusions. This is because their models for the same galaxy were often significantly different and not consistent with each other in terms of fitted components. Using 3.6 μm Spitzer imagery, which is a superb tracer of the stellar mass (superior to the K band), we have performed state-of-the-art multicomponent decompositions for 66 galaxies with directly measured black hole masses. Our sample is the largest to date and, unlike previous studies, contains a large number (17) of spiral galaxies with low black hole masses. We paid careful attention to the image mosaicking, sky subtraction, and masking of contaminating sources. After a scrupulous inspection of the galaxy photometry (through isophotal analysis and unsharp masking) and—for the first time—2D kinematics, we were able to account for spheroids; large-scale, intermediate-scale, and nuclear disks; bars; rings; spiral arms; halos; extended or unresolved nuclear sources; and partially depleted cores. For each individual galaxy, we compared our best-fit model with previous studies, explained the discrepancies, and identified the optimal decomposition. Moreover, we have independently performed one-dimensional (1D) and two-dimensional (2D) decompositions and concluded that, at least when modeling large, nearby galaxies, 1D techniques have more advantages than 2D techniques. Finally, we developed a prescription to estimate the uncertainties on the 1D best-fit parameters for the 66 spheroids that takes into account systematic errors, unlike popular 2D codes that only consider statistical errors.

THE SPITZER SURVEY OF STELLAR STRUCTURE IN GALAXIES (S 4 G): MULTI-COMPONENT DECOMPOSITION STRATEGIES AND DATA RELEASE

The Spitzer Survey of Stellar Structure in Galaxies (S^4G) is a deep 3.6 and 4.5 μm imaging survey of 2352 nearby (<40 Mpc) galaxies. We describe the S(^4)G data analysis pipeline 4, which is dedicated to two-dimensional structural surface brightness decompositions of 3.6 μm images, using GALFIT3.0. Besides automatic 1-component Sersic fits, and 2-component Sersic bulge + exponential disk fits, we present human-supervised multi-component decompositions, which include, when judged appropriate, a central point source, bulge, disk, and bar components. Comparison of the fitted parameters indicates that multi-component models are needed to obtain reliable estimates for the bulge Sersic index and bulge-to-total light ratio (B/T), confirming earlier results. Here, we describe the preparations of input data done for decompositions, give examples of our decomposition strategy, and describe the data products released via IRSA and via our web page (www.oulu.fi/astronomy/S4G_PIPELINE4/MAIN). These products include all the input data and decomposition files in electronic form, making it easy to extend the decompositions to suit specific science purposes. We also provide our IDL-based visualization tools (GALFIDL) developed for displaying/running GALFIT-decompositions, as well as our mask editing procedure (MASK_EDIT) used in data preparation. A detailed analysis of the bulge, disk, and bar parameters derived from multi-component decompositions will be published separately.

Milky Way mass galaxies with X-shaped bulges are not rare in the local Universe

Abstract Boxy/Peanut/X-shaped (B/P/X) bulges are studied using the 3.6 μm images from the Spitzer Survey of Stellar Structure in Galaxies, and the Ks-band images from the Near-IR S0 galaxy Survey. They are compared with the properties of barlenses, defined as lens-like structures embedded in bars, with sizes of ∼50 per cent of bars and axial ratios of ∼0.6–0.9. Based on observations (extending Laurikainen et al.) and recent simulation models, we show evidence that barlenses are the more face-on counterparts of B/P/X-shaped bulges. Using unsharp masks 18 new X-shaped structures are identified, covering a large range of galaxy inclinations. The similar masses and red B−3.6 μm colours of the host galaxies, and the fact that the combined axial ratio distribution of the host galaxy discs is flat, support the interpretation that barlenses and X-shapes are physically the same phenomenon. In Hubble types −3 ≤ T ≤ 2 even half of the bars contain either a barlens or an X-shaped structure. Our detailed 2D multicomponent decompositions for 29 galaxies, fitting the barlens/X-shape with a separate component, indicate very small or non-existent classical bulges. Taking into account that the structures we study have similar host galaxy masses as the Milky Way (MW), our results imply that MW mass galaxies with no significant classical bulges are common in the nearby Universe.

Photometric scaling relations of lenticular and spiral galaxies

Photometric scaling relations are studied for S0 galaxies and compared with those for spirals. New 2D K_s-band multi-component decompositions are presented for 122 early-type disk galaxies. Combining with our previous decompositions, the final sample consists of 175 galaxies. As a comparison sample we use the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS), for which similar decompositions have previously been made by us. Our main results are: (1) Important scaling relations are present, indicating that the formative processes of bulges and disks in S0s are coupled like has been previously found for spirals. (2) We obtain median r_{eff}/h_r = 0.20, 0.15 and 0.10 for S0, S0/a-Sa and Sab-Sc galaxies: these are smaller than predicted by simulation models in which bulges are formed by galaxy mergers. (3) The properties of bulges of S0s are different from the elliptical galaxies, which is manifested in the M_K(bulge) vs r_{eff} relation, in the photometric plane, and to some extent also in the Kormendy relation. The bulges of S0s are similar to bulges of spirals with M_K(bulge) < -20 mag. Some S0s have small bulges, but their properties are not compatible with the idea that they could evolve to dwarfs by galaxy harassment. (4) The relative bulge flux B/T for S0s covers the full range found in the Hubble sequence. (5) The values and relations of the parameters of the disks of the S0 galaxies in NIRS0S are similar to those obtained for spirals in the OSUBSGS. Overall, our results support the view that spiral galaxies with bulges brighter than -20 mag in the K-band can evolve directly into S0s, due to stripping of gas followed by truncated star formation.

Multicomponent decompositions for a sample of S0 galaxies

We have estimated the bulge-to-total (B/T) light ratios in the Ks band for a sample of 24 SO, S0/a and Sa galaxies by applying a two-dimensional multicomponent decomposition method. For the disc an exponential function is used, the bulges are fitted by a Sersic R 1/n function and the bars and ovals are described either by a Sersic or a Ferrers function. In order to avoid non-physical solutions, preliminary characterization of the structural components is made by inspecting the radial profiles of the orientation parameters and the low azimuthal wavenumber Fourier amplitudes and phases. In order to identify also the inner structures, unsharp masks were created: previously undetected inner spiral arms were found in NGC 1415 and marginally in NGC 3941. Most importantly, we found that S0s have a mean (B/T) K ratio of 0.24 ± 0.11, which is significantly smaller than the mean (B/T) R = 0.6 generally reported in the literature. Also, the surface brightness profiles of the bulges in S0s were found to be more exponential-like than generally assumed, the mean shape parameter of the bulge being = 2.1 ± 0.7. We did not find examples of barred S0s lacking the disc component, but we found some galaxies (NGC 718, 1452 and 4608) having a non-exponential disc in the bar region. To our knowledge, our study is the first attempt to apply a multicomponent decomposition method for a moderately sized sample of early-type disc galaxies.