Orthogonal vertical velocity dispersion distributions produced by bars

In barred galaxies, the contours of stellar velocity dispersions ($\sigma$) are generally expected to be oval and aligned with the orientation of bars. However, many double-barred (S2B) galaxies exhibit distinct $\sigma$ peaks on the minor axis of inner bars, which we termed "$\sigma$-humps," while two local $\sigma$ minima are present close to the ends of inner bars, i.e., "$\sigma$-hollows." Analysis of numerical simulations shows that $\sigma_z$-humps or hollows should play an important role in generating the observed $\sigma$-humps+hollows in low-inclination galaxies. In order to systematically investigate the properties of $\sigma_z$ in barred galaxies, we apply the vertical Jeans equation to a group of well-designed three-dimensional bar+disk(+bulge) models. A vertically thin bar can lower $\sigma_z$ along the bar and enhance it perpendicular to the bar, thus generating $\sigma_z$-humps+hollows. Such a result suggests that $\sigma_z$-humps+hollows can be generated by the purely dynamical response of stars in the presence of a, sufficiently massive, vertically thin bar, even without an outer bar. Using self-consistent $N$-body simulations, we verify the existence of vertically thin bars in the nuclear-barred and S2B models which generate prominent $\sigma$-humps+hollows. Thus the ubiquitous presence of $\sigma$-humps+hollows in S2Bs implies that inner bars are vertically thin. The addition of a bulge makes the $\sigma_z$-humps more ambiguous and thus tends to somewhat hide the $\sigma_z$-humps+hollows. We show that $\sigma_z$ may be used as a kinematic diagnostic of stellar components that have different thickness, providing a direct perspective on the morphology and thickness of nearly face-on bars and bulges with integral field unit spectroscopy.Comment: 14 pages, 16 figures. Accepted for publication in Ap

Galaxies with multiple bars : constraints on their formation scenarios

Much of current astrophysical research is aimed at addressing one key issue: how galaxies form and evolve. We still do not fully understand the evolutionary processes driving the lifetimes of the zoo of galaxies which populate the Universe. Galaxies may be isolated, or in groups or clusters; they may appear as pure ellipsoids or discs or include a variety of structures; they may be forming stars violently or passing quietly through their lives; and the many other properties that we are progressively discovering. Each piece we add to the puzzle pile complicates the picture a bit more. This thesis is aimed at fixing one of these pieces, a specific one related to the very interesting, but so far not well characterised, double-barred galaxies. Barred galaxies are rather common structures in the Universe and, more importantly, they are key elements for secular evolution theories. In fact, bars can transport gas to the central regions of galaxies and trigger the formation of bulges and other new structures. Double bars go a step farther: they allow the material to reach the very central regions where the gas driven inwards by a single bar cannot get to. Therefore, nested bar systems are considered a very efficient way to bring about the internal secular evolution of galaxies and even to feed active galactic nuclei. This hypothesis has, however, some detractors, as it is very promising from a theoretical point of view but there is not much observational evidence that it is valid. In this thesis we have carefully observed and studied a sample of five doublebarred galaxies using the state-of-the-art techniques for the analysis of their kinematics and stellar populations. This has led to original work not previously performed, so each result is relevant and provides clues to the nature of these objects. We are pleased to present the discovery of the σ-hollows, the only known kinematical signature of the presence of stellar inner bars. Moreover, we disentangle the complex structural composition of the galaxies of our sample, dealing with different formation scenarios and rejecting those which do not account for the observational properties derived here. We find out that one out of the five galaxies is probably hosting a disc-like bulge, whereas the other four present a classical bulge in their centres. Finally, we ascertain that inner bars tend to be younger and more metal-rich than the outer structures. Nevertheless, the characteristic stellar populations for the bulge and the inner bar are indistinguishable and there is no evidence of star-forming structures in the central regions of these galaxies, so we have to conclude that these five inner bars are not playing a major role in the secular evolution of their host galaxiesGran parte de la investigación astrofísica actual gira en torno a una única e importante cuestión: cómo se forman y evolucionan las galaxias. Todavía hoy no comprendemos los procesos evolutivos que dirigen la vida de ese zoo de objetos que puebla el Universo. Las galaxias pueden estar aisladas o viviendo en grupos o cúmulos; pueden aparentar ser simple elipsoides o discos o estar constituidas por variedad de estructuras; pueden formar estrellas violentamente o dejar pasar tranquilamente sus vidas; y un largo etcétera de propiedades que vamos descubriendo poco a poco. Cada pieza que añadimos al puzle complica la imagen final un poco más. Esta tesis pretende colocar por fin una de esas piezas, aquella relacionada con el fascinante, aunque todavía poco estudiado, caso de las galaxias con dos barras. Las galaxias con barra son estructuras comunes en el Universo y elementos clave en las teorías de evolución secular, puesto que las barras son capaces de transportar gas a las regiones centrales y, por lo tanto, promover la formación de bulbos y otras estructuras. Las dobles barras van un paso más allá al permitir que el material alcance las partes más internas de las galaxias, donde el gas transportado por una barra simple no puede llegar. Es por tanto lógico que estos sistemas dobles sean considerados fundamentales y muy eficientes para la evolución secular interna de las galaxias, e incluso se les relaciona a menudo con los núcleos activos galácticos. Esta hipótesis encuentra, sin embargo, varios detractores, pues aunque es muy prometedora desde un punto de vista puramente teórico, no hay evidencias observacionales que la sustenten. En la presente tesis hemos observado y estudiado en detalle una muestra de cinco galaxias con dos barras y hemos hecho uso de las técnicas de análisis más novedosas con el objetivo de caracterizar su cinemática y poblaciones estelares. Se trata de un trabajo completamente original jamás antes realizado, por lo que cada resultado es relevante y proporciona nuevas pistas sobre la naturaleza de estos objetos. Así pues, presentamos el descubrimiento de los llamados σ-hollows, las únicas señales cinemáticas conocidas de la presencia de barras internas. Desentrañamos también la compleja composición estructural de las galaxias de nuestra muestra, considerando los diferentes escenarios de formación posibles y rechazando aquellos que no explican las propiedades observacionales encontradas. Obtenemos así que una de las cinco galaxias en estudio probablemente contiene un bulbo tipo disco, mientras que el resto de la muestra está compuesta por bulbos clásicos. Finalmente, averiguamos que las barras internas tienden a ser más jóvenes y metálicas que las estructuras externas; sin embargo, las poblaciones estelares características para el bulbo y la barra interna son indistinguibles y no hay evidencias de formación estelar relevante en las regiones centrales, por lo que concluimos que al menos estas cinco barras internas no están desempeñando un papel importante en la evolución secular de sus galaxias anfitrionas

Superdense massive galaxies in the Nearby Universe

Superdense massive galaxies (r_e~1 kpc; M~10^{11} Msun) were common in the early universe (z>1.5). Within some hierarchical merging scenarios, a non-negligible fraction (1-10%) of these galaxies is expected to survive since that epoch retaining their compactness and presenting old stellar populations in the present universe. Using the NYU Value-Added Galaxy Catalog from the SDSS Data Release 6 we find only a tiny fraction of galaxies (~0.03%) with r_e<1.5 kpc and M_*>8x10^{10} Msun in the local Universe (z<0.2). Surprinsingly, they are relatively young (~2 Gyr) and metal-rich ([Z/H]~0.2). The consequences of these findings within the current two competing size evolution scenarios for the most massive galaxies ("dry" mergers vs "puffing up" due to quasar activity) are discussed.Comment: Accepted for publication in ApJ Letters; 3 figure

The universal variability of the stellar initial mass function probed by the TIMER survey

The debate about the universality of the stellar initial mass function (IMF) revolves around two competing lines of evidence. While measurements in the Milky Way, an archetypal spiral galaxy, seem to support an invariant IMF, the observed properties of massive early-type galaxies (ETGs) favor an IMF somehow sensitive to the local star-formation conditions. However, the fundamental methodological and physical differences between the two approaches have hampered a comprehensive understanding of IMF variations. Here, we describe an improved modeling scheme that, for the first time, allows consistent IMF measurements across stellar populations with different ages and complex star-formation histories (SFHs). Making use of the exquisite MUSE optical data from the TIMER survey and powered by the MILES stellar population models, we show the age, metallicity, [Mg/Fe], and IMF slope maps of the inner regions of NGC 3351, a spiral galaxy with a mass similar to that of the Milky Way. The measured IMF values in NGC 3351 follow the expectations from a Milky Way-like IMF, although they simultaneously show systematic and spatially coherent variations, particularly for low-mass stars. In addition, our stellar population analysis reveals the presence of metal-poor and Mg-enhanced star-forming regions that appear to be predominantly enriched by the stellar ejecta of core-collapse supernovae. Our findings therefore showcase the potential of detailed studies of young stellar populations to provide the means to better understand the early stages of galaxy evolution and, in particular, the origin of the observed IMF variations beyond and within the Milky Way

Towards a new classification of galaxies: principal component analysis of CALIFA circular velocity curves

We present a galaxy classification system for 238 (E1-Sdm) CALIFA (Calar Alto Legacy Integral Field Area) galaxies based on the shapes and amplitudes of their circular velocity curves (CVCs). We infer the CVCs from the de-projected surface brightness of the galaxies, after scaling by a constant mass-to-light ratio based on stellar dynamics - solving axisymmetric Jeans equations via fitting the second velocity moment $V_{\mathrm{rms}}=\sqrt{V^2+\sigma^2}$ of the stellar kinematics. We use principal component analysis (PCA) applied to the CVC shapes to find characteristic features and use a $k$-means classifier to separate circular curves into classes. This objective classification method identifies four different classes, which we name slow-rising (SR), flat (FL), round-peaked (RP) and sharp-peaked (SP) circular curves. SR are typical for low-mass, late-type (Sb-Sdm), young, faint, metal-poor and disc-dominated galaxies. SP are typical for high-mass, early-type (E1-E7), old, bright, metal-rich and bulge-dominated galaxies. FL and RP appear presented by galaxies with intermediate mass, age, luminosity, metallicity, bulge-to-disk ratio and morphologies (E4-S0a, Sa-Sbc). The discrepancy mass factor, $f_d=1-M_{*}/M_{dyn}$, have the largest value for SR and SP classes ($\sim$ 74 per cent and $\sim$ 71 per cent, respectively) in contrast to the FL and RP classes (with $\sim$ 59 per cent and $\sim$ 61 per cent, respectively). Circular curve classification presents an alternative to typical morphological classification and appears more tightly linked to galaxy evolution.Comment: Accepted for publication in MNRAS (Minor changes), 123 pages, 19 figures, 87 Tables (containing the basic properties of the 238 E1-Sdm galaxies; the five main Principal Component Eigenvectors; the five main Principal Components - PC_i; the Multi-Gaussian Expansion models - MGEs; the circular velocity curve models and their uncertainties

Disc galaxies are still settling: The discovery of the smallest nuclear discs and their young stellar bars

When galactic discs settle and become massive enough, they are able to form stellar bars. These non-axisymmetric structures induce shocks in the gas, causing it to flow to the centre where nuclear structures, such as nuclear discs and rings, are formed. Previous theoretical and observational studies have hinted at the co-evolution of bars and nuclear discs, suggesting that nuclear discs grow "inside-out", thereby proposing that smaller discs live in younger bars. Nevertheless, it remains unclear how the bar and the nuclear structures form and evolve with time. The smallest nuclear discs discovered to date tend to be larger than $\sim200~\rm{pc}$, even though some theoretical studies find that when nuclear discs form they can be much smaller. Using MUSE archival data, we report for the first time two extragalactic nuclear discs with radius sizes below $100~\rm{pc}$. Additionally, our estimations reveal the youngest bars found to date. We estimate that the bars in these galaxies formed $4.50^{+1.60}_{-1.10}\rm{(sys)}^{+1.00}_{-0.75}\rm{(stat)}$ and $0.7^{+2.60}\rm{(sys)}^{+0.05}_{-0.05}\rm{(stat)}~\rm{Gyr}$ ago, for NGC\,289 and NGC\,1566, respectively. This suggests that at least some disc galaxies in the Local Universe may still be dynamically settling. By adding these results to previous findings in the literature, we retrieve a stronger correlation between nuclear disc size and bar length and we derive a tentative exponential growth scenario for nuclear discs.Comment: Accepted in A&A (in press), 13 pages, 5 figures, 2 tabel

Clocking the assembly of double-barred galaxies with the MUSE TIMER project

The formation of two stellar bars within a galaxy has proved challenging for numerical studies. It is not yet clear whether the inner bar is born via a star formation process promoted by gas inflow along the outer bar or whether it is dynamically assembled from instabilities in a small-scale stellar disc. Observational constraints to these scenarios are scarce. We present a thorough study of the stellar content of two double-barred galaxies observed by the MUSE TIMER project, NGC 1291 and NGC 5850, combined with a two-dimensional multicomponent photometric decomposition performed on the 3.6 μμm images from S^4G. Our analysis confirms the presence of σ-hollows appearing in the stellar velocity dispersion distribution at the ends of the inner bars. Both galaxies host inner discs matching in size with the inner bars, suggestive of a dynamical formation for the inner bars from small-scale discs. The analysis of the star formation histories for the structural components shaping the galaxies provides constraints on the epoch of dynamical assembly of the inner bars, which took place >6.5 Gyr ago for NGC 1291 and >4.5 Gyr ago for NGC 5850. This implies that inner bars are long-lived structures