On the formation of bulges from their observed properties in nearby galaxies

The question whether the observed properties of galaxies are imprinted by the initial conditions of their formation or determined by different evolutionary processes is still open. In the local Universe, more than 60% of stars reside in elliptical galaxies and bulges of lenticular and spiral galaxies. Thus, unveiling the paths of bulge formation and evolution ensures a better understanding of galaxies themselves. The current paradigm separates galactic bulges into two broad categories, namely classical and disk-like bulges. In this scenario, we focus on the description of the bulge component in nearby galaxies from an observational point of view. We aim to properly interpret how observed properties of bulges provide evidences of different mechanisms responsible for their formation and evolution. Indeed, the separation of bulge types according to their observed properties has become a common task in extragalactic astronomy, even if there are no unambiguous ways of doing it yet. We present the technical procedures followed to characterize the structural properties of the bulge component as well as to determine its three-dimensional shape. We elucidate how the photometric algorithm allows to describe the surface brightness distribution of galaxies. We also revise and fine-tune the procedures and methods adopted to constrain the three-dimensional shape of bulges, detailing both the geometrical and statistical analysis. We describe the two-dimensional multi-component photometric decomposition of 404 galaxies from the Calar Alto Legacy Integral Field Area (CALIFA) data release 3. We provide the community with an accurate photometric characterization of the multiple stellar structures shaping the CALIFA galaxies, describing them with the suitable combination of a nuclear point source, a bulge, a single or double bar, and a pure exponential or double-exponential disk component. Moreover, we use a human-supervised approach to evaluate the optimal number of structures to be accounted for fitting the surface brightness distribution. We release the photometric parameters of the CALIFA galaxies, together with statistical errors and a visual analysis of the quality of each fit. The analysis of the photometric components reveals a clear segregation of the structural composition of galaxies with stellar mass. At high masses (log(M_star/M_sun) > 11), the galaxy population is dominated by galaxies modeled with a single Sérsic or a bulge+disk with a bulge-to-total luminosity ratio B/T > 0.2. At intermediate masses (9.5 < log(M_star/M_sun) < 11), galaxies described with bulge+disk and B/T < 0.2 are preponderant, whereas, at the low mass end (log(M_star/M_sun) < 9.5), the prevailing population is constituted by galaxies modeled with either pure disks or nuclear point sources+disks (i.e., with no discernible bulge). We set the basis for new studies combining photometric information with the wealth of two-dimensional spatially resolved spectroscopic information provided by the CALIFA survey. In an effort to push the limits of the scaling relations studied so far to the very low-sigma regime we describe the small bulges at the end of the Hubble sequence. To this aim, we derive the photometric and kinematic properties of 9 nearby late-type spiral galaxies. We analyze the i-band images of the Sloan Digital Sky Survey (SDSS) of these galaxies to characterize the structural parameters of their bulges by means of a two-dimensional photometric decomposition. Moreover, we measure the line-of-sight stellar velocity distribution within the bulge effective radius from the long-slit spectra taken with high spectral resolution at the Telescopio Nazionale Galileo. Finally, we combine the photometric and kinematic information of the sample bulges to study their location in the fundamental plane, Kormendy, and Faber-Jackson relations defined for elliptical galaxies and large bulges. We find that each of our bulge follows the same scaling relations of elliptical galaxies, massive bulges, and compact early-type galaxies so they cannot be classified as disk-like systems. This analysis suggests that a single population of galaxy spheroids follows the same scaling relations, where the mass seems to lead to a smooth transition in the photometric and kinematic properties from less to more massive bulges and elliptical galaxies. The thorough description of the bulge structural features in the CALIFA sample results in the characterization of the bulge three-dimensional shape. Constraining the intrinsic shape of bulges allows to provide new clues on the bulge formation mechanisms and set new limitations for future simulations. Firstly, we take advantage of GalMer numerical simulations to estimate the reliability of the procedure. Thus, we create a set of mock SDSS i-band images at different galaxy inclinations for a set of simulated galaxies, that closely resemble lenticular galaxies. We perform a two-dimensional photometric decomposition of all the mock images applying the same procedure as for real galaxies, in order to characterize the geometrical parameters of bulge and disk which we use to recover the bulge intrinsic shape. We conclude that for galaxies in the inclination range 25° < theta < 65° we can safely derive the intrinsic shape of their bulges. Moreover, we also realize that a very accurate photometric decomposition is mandatory to retrieve the bulge intrinsic shape. Secondly, we obtain the intrinsic shape of 83 bulges from the CALIFA survey. We introduce the (B/A, C/A) diagram to analyze possible correlations between the intrinsic shape and properties of bulges. We find that our CALIFA bulges tend to be nearly oblate systems (66%), with a smaller fraction of prolate spheroids (19%) and triaxial ellipsoids (15%). The majority of triaxial bulges are in barred galaxies (75%). Moreover, we find that bulges with low Sérsic indices or in galaxies with low bulge-to-total luminosity ratios form a heterogeneous class of objects; additionally, bulges in late-type galaxies or in less massive galaxies have no preference for being oblate, prolate, or triaxial. On the contrary, bulges with high Sérsic index, in early-type galaxies, or in more massive galaxies are mostly oblate systems. We conclude that various evolutionary pathways may coexist in galaxies, with merging events and dissipative collapse being the main mechanisms driving the formation of the most massive oblate bulges and bar evolution reshaping the less massive triaxial bulges. Finally, we discuss the observational criteria usually applied to discriminate bulge types in classical and disk-like spheroids. We derive the photometric, kinematic, stellar population, and intrinsic shape properties of bulges in nine lenticular galaxies from the ATLAS3D survey. Our analysis is based on all the observed diagnostics commonly adopted in recent works and tests their efficiency on our sample of meticulously selected systems. Indeed, the morphology of our sample galaxies is chosen to deal with the most simple examples of disk galaxies. We argue that the bulge Sérsic index is a poor tool to discriminate the different bulge types. Moreover, we find that the kinematic properties and line-strength indices of the sample bulges provide no clear identification of the bulge type; this remains true also when comparing the results obtained from the line-strength indices with those obtained from the photometric analysis. We conclude that the common practice of applying the observational criteria for distinguishing bulge types, based on a priori classification according to their morphology or Sérsic index, has to be carefully reconsidered. We remark that, even if the different observational characteristics look well motivated in terms of distinct formation paths, their interplay might result in contradictory outcomes. We propose to characterize the disk-like bulges in terms of their intrinsic shape and dynamical status as the most reliable way to separate them from the classical bulges.

Nell’Universo locale, più del 60% della materia stellare si trova in galassie ellittiche e nei rigonfiamenti centrali, denominati sferoidi, di galassie lenticolari e a spirale. Tuttavia, ancora non si conosce a cosa sia dovuta l’eterogeneità delle loro proprietà osservate. In questo lavoro ci proponiamo di interpretare nel modo più accurato possibile come diversi meccanismi di formazione ed evoluzione possano produrre questa varietà di proprietà. Infatti, determinare gli scenari di formazione ed evoluzione di queste strutture permette una conoscenza più profonda delle galassie stesse. Presento le tecniche di analisi utilizzate per caratterizzare le proprietà strutturali degli sferoidi sia fotometricamente sia geometricamente. Infatti, descrivo gli algoritmi che permettono di analizzare la distribuzione di brillanza superficiale delle galassie e la forma intrinseca tridimensionale dei loro sferoidi. Discuto l’analisi fotometrica delle varie componenti di un campione di 404 galassie nell’ambito della survey Calar Alto Legacy Field Area (CALIFA). Le galassie vengono descritte con una combinazione di modelli, comprendenti sorgenti nucleari, sferoidi, dischi e barre. Ho riscontrato che le galassie più massicce sono meglio modellate con solamente uno sferoide o con sferoide+disco ma con un rapporto di luminosità B/T > 0.2. A masse intermedie, prevalgono le galassie con miglior modello dato da sferoide+disco e B/T < 0.2, mentre nelle galassie meno massicce la componente di sferoide tende a diventare sempre meno importante. Quest’analisi pone le basi per una serie di nuovi studi all’interno della collaborazione di CALIFA. In seguito, caratterizzo gli sferoidi di un campione di nove galassie di tipo morfologico avanzato. Questi sferoidi hanno la peculiarità di essere molto piccoli e presentare una bassa dispersione di velocità. Essi sono stati analizzati da un punto di vista sia fotometrico sia cinematico, usando spettroscopia a fenditura lunga ad alta risoluzione spettrale ottenuta con il Telescopio Nazionale Galileo. Queste proprietà sono state utilizzate per studiare il comportamento del campione di sferoidi nelle relazioni di scala (piano fondamentale, Kormendy, Faber-Jackson). L’analisi evidenzia come un’unica popolazione di sferoidi segua le stesse relazioni di scala, con una transizione nelle proprietà fotometriche e cinematiche osservate dovuta alla loro massa. Successivamente, sempre nell’ambito della survey CALIFA, analizzo la forma intrinseca tridimensionale degli sferoidi in 83 galassie. Utilizzo simulazioni numeriche (Galmer) per testare l’affidabilità del metodo statistico utilizzato per svolgere l’analisi geometrica. Introduco il diagramma (B/A, C/A), che permette di caratterizzare la forma degli sferoidi del nostro campione. Essi sono principalmente oblati (66%), con una piccola frazione di prolati (19%) e triassiali (15%). La maggior parte degli sferoidi triassiali si trova nelle galassie barrate (75%). Attraverso l’analisi della forma intrinseca e delle proprietà osservate degli sferoidi si conclude che il fenomeno di interazione tra galassie è uno dei maggiori responsabili della formazione degli sferoidi oblati più massicci, mentre l’evoluzione della barra tende a formare gli sferoidi triassiali meno massicci. Infine, propongo un’analisi dei criteri solitamente utilizzati in letteratura per discriminare diverse tipologie di sferoidi. A questo proposito vengono analizzati gli sferoidi in nove galassie nell’ambito del progetto ATLAS3D. L’analisi si basa su proprietà sia fotometriche sia spettroscopiche e sulla loro forma intrinseca, I risultati evidenziano come l’indice di Sérsic non sia un buon indicatore per differenziare diversi sferoidi. Invece, la forma intrinseca e lo stato dinamico dello sferoide vengono proposti come miglior combinazioni di parametri per caratterizzare gli sferoidi galattici.