Deconstructing double-barred galaxies in 2D and 3D. II. Two distinct groups of inner bars

The intrinsic photometric properties of inner and outer stellar bars within 17 double-barred galaxies are thoroughly studied through a photometric analysis consisting of: i) two-dimensional multi-component photometric decompositions, and ii) three-dimensional statistical deprojections for measuring the thickening of bars, thus retrieving their 3D shape. The results are compared with previous measurements obtained with the widely used analysis of integrated light. Large-scale bars in single- and double-barred systems show similar sizes, and inner bars may be longer than outer bars in different galaxies. We find two distinct groups of inner bars attending to their in-plane length and ellipticity, resulting in a bimodal behaviour for the inner/outer bar length ratio. Such bimodality is related neither to the properties of the host galaxy nor the dominant bulge, and it does not show a counterpart in the dimension off the disc plane. The group of long inner bars lays at the lower end of the outer bar length vs. ellipticity correlation, whereas the short inner bars are out of that relation. We suggest that this behaviour could be due to either a different nature of the inner discs from which the inner bars are dynamically formed, or a different assembly stage for the inner bars. This last possibility would imply that the dynamical assembly of inner bars is a slow process taking several Gyr to happen. We have also explored whether all large-scale bars are prone to develop an inner bar at some stage of their lives, possibility we cannot fully confirm or discard.Comment: 14 pages, 8 figures, 1 table. Accepted for publication in MNRA

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

The intrinsic three-dimensional shape of galactic bars

We present the first statistical study on the intrinsic three-dimensional (3D) shape of a sample of 83 galactic bars extracted from the CALIFA survey. We use the galaXYZ code to derive the bar intrinsic shape with a statistical approach. The method uses only the geometric information (ellipticities and position angles) of bars and discs obtained from a multi-component photometric decomposition of the galaxy surface-brightness distributions. We find that bars are predominantly prolate-triaxial ellipsoids (68%), with a small fraction of oblate-triaxial ellipsoids (32%). The typical flattening (intrinsic C/A semiaxis ratio) of the bars in our sample is 0.34, which matches well the typical intrinsic flattening of stellar discs at these galaxy masses. We demonstrate that, for prolate-triaxial bars, the intrinsic shape of bars depends on the galaxy Hubble type and stellar mass (bars in massive S0 galaxies are thicker and more circular than those in less massive spirals). The bar intrinsic shape correlates with bulge, disc, and bar parameters. In particular with the bulge-to-total (B/T) luminosity ratio, disc g-r color, and central surface brightness of the bar, confirming the tight link between bars and their host galaxies. Combining the probability distributions of the intrinsic shape of bulges and bars in our sample we show that 52% (16%) of bulges are thicker (flatter) than the surrounding bar at 1$\sigma$ level. We suggest that these percentages might be representative of the fraction of classical and disc-like bulges in our sample, respectively.Comment: 18 pages, 11 figures, accepted for publication in MNRA

Stellar populations in the bulges of isolated galaxies

open7siWe present photometry and long-slit spectroscopy for 12 S0 and spiral galaxies selected from the Catalogue of Isolated Galaxies. The structural parameters of the sample galaxies are derived from the Sloan Digital Sky Survey i-band images by performing a two-dimensional photometric decomposition of the surface brightness distribution. This is assumed to be the sum of the contribution of a Sersic bulge, an exponential disc, and a Ferrers bar characterized by elliptical and concentric isophotes with constant ellipticity and position angles. The rotation curves and velocity dispersion profiles of the stellar component are measured from the spectra obtained along the major axis of galaxies. The radial profiles of the Hβ, Mg and Fe line- strength indices are derived too. Correlations between the central values of the Mg2 and Fe line-strength indices and the velocity dispersion are found. The mean age, total metallicity and total α/Fe enhancement of the stellar population in the centre and at the radius, where the bulge gives the same contribution to the total surface brightness as the remaining components, are obtained using stellar population models with variable element abundance ratios. We identify intermediate-age bulges with solar metallicity and old bulges with a large spread in metallicity. Most of the sample bulges display supersolar α/Fe enhancement, no gradient in age and negative gradients of metallicity and α/Fe enhancement. These findings support a formation scenario via dissipative collapse where environmental effects are remarkably less important than in the assembly of bulges of galaxies in groups and clusters.openMorelli, Lorenzo; Parmiggiani, Marco; Corsini, ENRICO MARIA; Costantin, Luca; DALLA BONTA', Elena; Méndez Abreu, J.; Pizzella, AlessandroMorelli, Lorenzo; Parmiggiani, Marco; Corsini, ENRICO MARIA; Costantin, Luca; DALLA BONTA', Elena; Méndez Abreu, J.; Pizzella, Alessandr

No evidence for small disk-like bulges in a sample of late-type spirals

About 20% of low-redshift galaxies are late-type spirals with a small or no bulge component. Although they are the simplest disk galaxies in terms of structure and dynamics, the role of the different physical processes driving their formation and evolution is not yet fully understood. We investigated whether small bulges of late-type spirals follow the same scaling relations traced by ellipticals and large bulges and if they are disk-like or classical bulges. We derived the photometric and kinematic properties of 9 nearby late-type spirals. To this aim, we analyzed the surface brightness distribution from the i-band images of the Sloan Digital Sky Survey and obtained the structural parameters of the galaxies from a two-dimensional photometric decomposition. We measured 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. We used the photometric and kinematic properties of the sample bulges to study their location in the Fundamental Plane, Kormendy, and Faber-Jackson relations defined for ellipticals and large bulges. We found that our sample bulges satisfy some of the photometric and kinematic prescriptions for being considered disk-like bulges such as small sizes and masses with nearly exponential light profiles, small bulge-to-total luminosity ratios, low stellar velocity dispersions, and ongoing star formation. However, each of them follows the same scaling relations of ellipticals, massive bulges, and compact early-type galaxies so they cannot be classified as disk-like systems. We find a single population of galaxy spheroids that follow the same scaling relations, where the mass seems to lead to a smooth transition in the photometric and kinematic properties from less massive bulges to more massive bulges and ellipticals.Comment: Accepted for publication in A&A, 20 pages, 10 figure