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Register a new userSecular evolution and the assembly of bulges
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Authors
F. Combes
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Bulges are of different types, morphologies and kinematics, from pseudo-bulges, close to disk properties (Sersic index, rotation fraction, flatenning), to classical de Vaucouleurs bulges, close to elliptical galaxies. Secular evolution and bar development can give rise to pseudo-bulges. To ensure prolonged secular evolution, gas flows are required along the galaxy life-time. There is growing evidence for cold gas accretion around spiral galaxies. This can explain the bar cycle of destruction and reformation, together with pseudo-bulge formation. However, bulges can also be formed through major mergers, minor mergers, and massive clumps early in the galaxy evolution. Bulge formation is so efficient that it is difficult to explain the presence of bulgeless galaxies today.
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We use high resolution collisionless $N$-body simulations to study the secular evolution of disk galaxies and in particular the final properties of disks that suffer a bar and perhaps a bar-buckling instability. Although we find that bars are not destroyed by the buckling instability, when we decompose the radial density profiles of the secularly-evolved disks into inner Sersic and outer exponential components, for favorable viewing angles, the resulting structural parameters, scaling relations and global kinematics of the bar components are in good agreement with those obtained for bulges of late-type galaxies. Round bulges may require a different formation channel or dissipational processes.
(Abridged) We study the incidence, as well as the nature, of composite bulges in a sample of 10 face-on barred galaxies to constrain the formation and evolutionary processes of the central regions of disk galaxies. We analyze the morphological, photometric, and kinematic properties of each bulge. Then, by using a case-by-case analysis we identify composite bulges and classify every component into a classical or pseudobulge. In addition, bar-related boxy/peanut (B/P) structures were also identified and characterised. We find only three galaxies hosting a single-component bulge (two pseudobulges and one classical bulge). We find evidence of composite bulges coming in two main types based on their formation: secular-built and merger- and secular-built. We call secular-built to composite bulges made of entirely by structures associated with secular processes such as pseudo bulges, central disks, or B/P bulges. We find four composite bulges of this kind in our sample. On the other hand, merger- and secular-built bulges are those where structures with different formation paths coexist within the same galaxy, i.e., a classical bulge coexisting with a secular-built structure (pseudobulge, central disk, or B/P). Three bulges of this kind were found in the sample. We remark on the importance of detecting kinematic structures such as sigma-drops to identify composite bulges. A large fraction (80%) of galaxies were found to host sigma-drops or sigma-plateaus in our sample revealing their high incidence in barred galaxies. The high frequency of composite bulges in barred galaxies points towards a complex formation and evolutionary scenario. Moreover, the evidence for coexisting merger- and secular-built bulges reinforce this idea. We discuss how the presence of different bulge types, with different formation histories and timescales, can constrain current models of bulge formation.
We determine central values and radial trends in the stellar populations of the bulges of a sample of 28 edge-on S0-Sb disk galaxies, 22 of which are boxy/peanut-shaped (and therefore barred). Our principal findings are the following. (1) At a given velocity dispersion, the central stellar populations of galaxies with boxy/peanut-shaped bulges are indistinguishable from those of early-type (elliptical and S0) galaxies. Either secular evolution affects stellar populations no differently to monolithic collapse or mergers, or secular evolution is not important in the central regions of these galaxies, despite the fact that they are barred. (2) The radial metallicity gradients of boxy/peanut-shaped bulges are uncorrelated with velocity dispersion and are, on average, shallower than those of unbarred early-type galaxies. This is qualitatively consistent with chemodynamical models of bar formation, in which radial inflow and outflow smears out pre-existing gradients.
We examine the evolution of assembly bias using a semi-analytical model of galaxy formation implemented in the Millennium-WMAP7 N-body simulation. We consider fixed number density galaxy samples ranked by stellar mass or star formation rate. We investigate how the clustering of haloes and their galaxy content depend on halo formation time and concentration, and how these relationships evolve with redshift. At $z=0$ the dependences of halo clustering on halo concentration and formation time are similar. However, at higher redshift, halo assembly bias weakens for haloes selected by age, and reverses and increases for haloes selected by concentration. The variation of the halo occupation with concentration and formation time is also similar at $z=0$ and changes at higher redshifts. In this case, the occupancy variation with halo age stays mostly constant with redshift but decreases for concentration. Finally, we look at the evolution of assembly bias reflected in the galaxy distribution by examining the galaxy correlation functions relative to those of shuffled galaxy samples which remove the occupancy variation. This correlation functions ratio monotonically decreases with larger redshift and for lower number density samples, going below unity in some cases, leading to reduced galaxy clustering. While the halo occupation functions themselves vary, the assembly bias trends are similar whether selecting galaxies by stellar mass or star formation rate. Our results provide further insight into the origin and evolution of assembly bias. Our extensive occupation function measurements and fits are publicly available and can be used to create realistic mock catalogues.
We use data from the Multi-Unit Spectroscopic Explorer (MUSE), recently commissioned at the Very Large Telescope (VLT), to study the kinematics and stellar population content of NGC 4371, an early-type massive barred galaxy in the core of the Virgo cluster. We integrate this study with a detailed structural analysis using imaging data from the Hubble and Spitzer space telescopes, which allows us to perform a thorough investigation of the physical properties of the galaxy. We show that the rotationally supported inner components in NGC 4371, an inner disc and a nuclear ring - which, according to the predominant scenario, are built with stars formed from gas brought to the inner region by the bar - are vastly dominated by stars older than 10 Gyr. Our results thus indicate that the formation of the bar occurred at a redshift of about $z=1.8^{+0.5}_{-0.4}$ (error bars are derived from 100 Monte Carlo realisations). NGC 4371 thus testifies to the robustness of bars. In addition, the mean stellar age of the fraction of the major disc of the galaxy covered by our MUSE data is above 7 Gyr, with a small contribution from younger stars. This suggests that the quenching of star formation in NGC 4371, likely due to environmental effects, was already effective at a redshift of about $z=0.8^{+0.2}_{-0.1}$. Our results point out that bar-driven secular evolution processes may have an extended impact in the evolution of galaxies, and thus on the properties of galaxies as observed today, not necessarily restricted to more recent cosmic epochs.
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