Secular Evolution Of Disk Galaxies Research Articles

Galactic Bar Resonances with Diffusion: An Analytic Model with Implications for Bar–Dark Matter Halo Dynamical Friction

The secular evolution of disk galaxies is largely driven by resonances between the orbits of “particles” (stars or dark matter) and the rotation of non-axisymmetric features (spiral arms or a bar). Such resonances may also explain kinematic and photometric features observed in the Milky Way and external galaxies. In simplified cases, these resonant interactions are well understood: for instance, the dynamics of a test particle trapped near a resonance of a steadily rotating bar is easily analyzed using the angle-action tools pioneered by Binney, Monari, and others. However, such treatments do not address the stochasticity and messiness inherent to real galaxies—effects that have, with few exceptions, been previously explored only with complex N-body simulations. In this paper, we propose a simple kinetic equation describing the distribution function of particles near an orbital resonance with a rigidly rotating bar, allowing for diffusion of the particles’ slow actions. We solve this equation for various values of the dimensionless diffusion strength Δ, and then apply our theory to the calculation of bar–halo dynamical friction. For Δ = 0, we recover the classic result of Tremaine and Weinberg that friction ultimately vanishes, owing to the phase mixing of resonant orbits. However, for Δ > 0, we find that diffusion suppresses phase mixing, leading to a finite torque. Our results suggest that stochasticity—be it physical or numerical—tends to increase bar–halo friction, and that bars in cosmological simulations might experience significant artificial slowdown, even if the numerical two-body relaxation time is much longer than a Hubble time.

The EDGE-CALIFA survey: The role of spiral arms and bars in driving central molecular gas concentrations

Shocks and torques produced by non-axisymmetric structures such as spiral arms and bars may transport gas to galaxy central regions. We test this hypothesis by studying the dependence of the concentration of CO luminosity (CCO) and molecular gas (Cmol) and the star formation rate (CSFR) in the central ∼2 kpc on the strength of non-axisymmetric disk structure using a sample of 57 disk galaxies selected from the EDGE-CALIFA survey. The Cmol is calculated using a CO-to-H2 conversion factor that decreases with higher metallicity and higher stellar surface density. We find that Cmol is systematically 0.22 dex lower than CCO. We confirm that high Cmol and strong non-axisymmetric disk structure are more common in barred galaxies than in unbarred galaxies. However, we find that spiral arms also increase Cmol. We show that there is a good correlation between Cmol and the strength of non-axisymmetric structure (which can be due to a bar, spiral arms, or both). This suggests that the stronger the bars and spirals, the more efficient the galaxy is at transporting cold gas to its center. Despite the small subsample size, the Cmol of the four Seyferts are not significantly reduced compared to inactive galaxies of similar disk structure, implying that the active galactic nucleus feedback in Seyferts may not notably affect the molecular gas distribution in the central ∼2 kpc. We find that CSFR tightly correlates with Cmol in both unbarred and barred galaxies. Likewise, elevated CSFR is found in galaxies with strong disk structure. Our results suggest that the disk structure, either spirals or bars, can transport gas to the central regions, with higher inflow rates corresponding to stronger structure, and consequently boost central star formation. Both spirals and bars play, therefore, an essential role in the secular evolution of disk galaxies.

Strong spiral arms drive secular growth of pseudo bulges in disk galaxies

Spiral-driven instabilities may drive gas inflow to enhance central star formation in disk galaxies. We investigate this hypothesis using the Sloan Digital Sky Survey (SDSS) in a sample of 2779 nearby unbarred star-forming main sequence spiral galaxies. The strength of spiral arms is quantified by their average Fourier amplitude relative to the axisymmetric disk. The star formation properties in the central 1–3 kpc region were derived from the SDSS spectra. We show that galaxies with stronger spiral arms not only tend to have more intense central specific star formation rates (sSFRs), larger Balmer absorption line indices, and lower 4000 Å break strengths, but also have enhanced central sSFRs relative to the sSFR measured for the whole galaxy. This link is independent of redshift, stellar mass, surface density, and concentration. There is a lack of evidence for strong spiral arms being associated with a significant fraction of starburst or post-starburst galaxies, implying that the spiral-induced central star formation is likely continuous rather than bursty. We also show that stronger spiral arms tend to have an increasing fraction of pseudo bulges, a relatively unchanged fraction of star-forming classical bulges, and a decreasing fraction of quenched classical bulges. Moreover, the concentration of galaxies hosting pseudo bulges mildly increases with stronger spiral arms, implying that spirals help pseudo bulges grow. The connection between spirals and bulge type is partly attributed to the suppression of spirals by classical bulges and partly to the enhanced central star formation driven by spirals. We explain our results in the context of a scenario where spiral arms transport cold gas inward to trigger continuous central star formation, which facilitates the buildup of pseudo bulges. Spiral arms thus play a role in the secular evolution of disk galaxies.

Stellar mass distribution of S4G disk galaxies and signatures of bar-induced secular evolution

We use 3.6 $\mu$m photometry from the Spitzer Survey of Stellar Structure in Galaxies (S$^{4}$G) to trace the stellar distribution in nearby disk galaxies ($z\approx0$) with total stellar masses $10^{8.5}\lesssim M_{\ast}/M_{\odot}\lesssim10^{11}$ and mid-IR Hubble types $-3 \le T \le 10$, and to provide observational constraints for galaxy formation models to be checked against. For 1154 galaxies with disk inclinations lower than $65^{\circ}$, we Fourier decompose and rescale their images to a common frame determined (i) by the size in physical units, (ii) by their disk scalelength, and for 748 barred galaxies (iii) by both the length and orientation of their bars. We stack the resized density profiles and images to obtain statistically representative average stellar disks and bars in bins of $M_{\ast}$ and $T$. We also calculate the mean stellar contribution to the circular velocity. We infer the gravitational potentials from the synthetic bars to obtain the tangential-to-radial force ratio ($Q_{\rm T}$) and $A_2$ profiles in the different bins. We provide observational evidence for bar-induced secular evolution of disk galaxies. For $M_{\ast} \ge 10^{9}M_{\odot}$, we find a significant difference in the stellar density profiles of barred and non-barred systems: (i) disks in barred galaxies show larger scalelengths ($h_{\rm R}$) and fainter extrapolated central surface brightnesses ($\Sigma_{0}$), (ii) the mean surface brightness profiles ($\Sigma_{\ast}$) of barred and non-barred galaxies intersect each other slightly beyond the mean bar length, most likely at the bar corotation, and (iii) the central mass concentration of barred galaxies is larger (by almost a factor 2 when $T\le5$) than in their non-barred counterparts. We also show that bars hosted by early-type galaxies are more centrally concentrated and have larger density amplitudes than their late-type counterparts (Abridged).

The stellar mass distribution of S4G disk galaxies

AbstractWe use 3.6 μm imaging from the S4G survey to characterize the typical stellar density profiles (Σ*) and bars as a function of fundamental galaxy parameters (e.g. the total stellar massM*), providing observational constraints for galaxy simulation models to be compared with. We rescale galaxy images to a common frame determined by the size in physical units, by their disk scalelength, or by their bar size and orientation. We stack the resized images to obtain statistically representative average stellar disks and bars. For a givenM*bin (≥ 109M⊙), we find a significant difference in the stellar density profiles of barred and non-barred systems that gives evidence for bar-induced secular evolution of disk galaxies: (i) disks in barred galaxies show larger scalelengths and fainter extrapolated central surface brightnesses, (ii) the mean surface brightness profiles of barred and non-barred galaxies intersect each other slightly beyond the mean bar length, most likely at the bar corotation, and (iii) the central mass concentration of barred galaxies is larger (by almost a factor 2 whenT< 5) than in their non-barred counterparts. We also show that early- and intermediate-type spirals (0 ≤T< 5) host intrinsically narrower bars than the later types and S0s, whose bars are oval-shaped. We show a clear correlation between galaxy family and bar ellipticity.

STAR FORMATION PROPERTIES IN BARRED GALAXIES. III. STATISTICAL STUDY OF BAR-DRIVEN SECULAR EVOLUTION USING A SAMPLE OF NEARBY BARRED SPIRALS

Stellar bars are important internal drivers of secular evolution in disk galaxies. Using a sample of nearby spiral galaxies with weak and strong bars, we explore the relationships between the star formation feature and stellar bars in galaxies. We find that galaxies with weak bars tend to be coincide with low concentrical star formation activity, while those with strong bars show a large scatter in the distribution of star formation activity. We find enhanced star formation activity in bulges towards stronger bars, although not predominantly, consistent with previous studies. Our results suggest that different stages of the secular process and many other factors may contribute to the complexity of the secular evolution. In addition, barred galaxies with intense star formation in bars tend to have active star formation in their bulges and disks, and bulges have higher star formation densities than bars and disks, indicating the evolutionary effects of bars. We then derived a possible criterion to quantify the different stages of bar-driven physical process, while future work is needed because of the uncertainties.

Internal and environmental secular evolution of disk galaxies

Internal and environmental secular evolution of disk galaxies

Multi-wavelength study of star formation properties in barred galaxies

AbstractStellar bars are important internal drivers of the secular evolution of disk galaxies. Using a sample of nearby barred galaxies with weak and strong bars, we evaluate the correlations between star formation properties in different galactic structures and their associated bars, and try to interpret the complex process of bar-driven secular evolution. We find that weaker bars tend to associate with lower concentrical star formation activities, while stronger bars appear to have large scatter in the distribution of the global star formation activities. In general, the star formation activities in early- and late-type galaxies have different behavior, with similar star formation rate density distributions. In addition, there are only weak trends toward increased star formation activities in bulges and galaxies with stronger bars, which is consistent with previous works. Our results suggest that the different stages of the evolutionary sequence and many factors besides bars may contribute to the complexity of this process. Furthermore, significant correlations are found between the star formation activities in different galactic structures, in which barred galaxies with intense star formation in bulges tend to also have active star formation in their bars and disks. Most bulges have higher star formation densities than their associated bars and disks, indicating the presence of bar-driven evolution. Therefore, we derived a possible criterion (Figure 1) to quantify the different stages of a bar-driven evolutionary sequence. Future work is needed to improve on the uncertainties of this study.

A secularly evolved model for the Milky Way bar and bulge

AbstractBars are strong drivers of secular evolution in disk galaxies. Bars themselves can evolve secularly through angular momentum transport, producing different boxy/peanut and X-shaped bulges. Our Milky Way is an example of a barred galaxy with a boxy bulge. We present a self-consistent N-body simulation of a barred galaxy which matches remarkably well the structure of the inner Milky Way deduced from star counts. In particular, features taken as signatures of a second “long bar“ can be explained by the interaction between the bar and the spiral arms of the galaxy (Martinez-Valpuesta & Gerhard 2011). Furthermore the structural change in the bulge inside l = 4° measured recently from VVV data can be explained by the high-density near-axisymmetric part of the inner boxy bulge (Gerhard & Martinez-Valpuesta 2012). We also compare this model with kinematic data from recent spectroscopic surveys. We use a modified version of the NMAGIC code (de Lorenzi et al. 2007) to study the properties of the Milky Way bar, obtaining an upper limit for the pattern speed of ~ 42 km/sec/kpc. See Fig. 1 for a comparison of one of our best models with BRAVA data (Kunder et al. 2012).

Star formation properties in barred galaxies (SFB) II. NGC 2903 and NGC 7080

Stellar bars are important for the secular evolution of disk galaxies because they can drive gas into the galactic central regions. To investigate the star formation properties in barred galaxies, we presented a multi-wavelength study of two barred galaxies: NGC 2903 and NGC 7080. We performed the three-component bulge-disk-bar decomposition using the 3.6 μm images, and identified the bulges in the two galaxies as pseudobulges. Based on the narrowband Hα images, the star formation clumps were identified and analyzed. The clumps in the bulge regions have the highest surface densities of star formation rates in both galaxies, while the star formation activities in the bar of NGC 2903 are more intense than those in the bar of NGC 7080. Finally, we compared our results with the scenario of bar-driven secular evolution in previous studies, and discussed the possible evolutionary stages of the two galaxies.

Internal secular evolution in disk galaxies: the growth of pseudobulges

AbstractObservational and theoretical evidence that internal, slow (“secular”) evolution reshapes galaxy disks is reviewed in Kormendy & Kennicutt (2004). This update has three aims. First, I emphasize that this evolution is very general – it is as fundamental to the evolution of galaxy disks as (e. g.) core collapse is to globular clusters, as the production of hot Jupiters is to the evolution of protoplanetary disks, and as evolution to red giants containing proto-white-dwarfs is to stellar evolution. One consequence for disk galaxies is the buildup of dense central components that get mistaken for classical (i. e., merger-built) bulges but that were grown out of disk stars and gas. We call these pseudobulges. Second, I review new results on pseudobulge star formation and structure and on the distinction between boxy and disky pseudobulges. Finally, I highlight how these results make a galaxy formation problem more acute. How can hierarchical clustering produce so many pure disk galaxies with no evidence for merger-built bulges?

The Potential-Density Phase-Shift Method for Determining the Corotation Radii in Spiral and Barred Galaxies

We have developed a new method for determining the corotation radii of density waves in disk galaxies, which makes use of the calculated radial distribution of an azimuthal phase shift between the potential and density wave patterns. The approach originated from improved theoretical understanding of the relation between the morphology and kinematics of galaxies and of the dynamical interaction between density waves and the basic-state disk stars, which results in the secular evolution of disk galaxies. In this paper we present the rationales behind the method and the first application of it to several representative barred and grand-design spiral galaxies, using near-infrared images to trace the mass distributions, as well as to calculate the potential distributions used in the phase-shift calculations. We compare our results with those from other existing methods for locating the corotations and show that the new method both confirms the previously established trends of bar-length dependence on galaxy morphological types and provides new insights into the possible extent of bars in disk galaxies. The method also facilitates the estimation of mass accretion/excretion rates due to bar and spiral density waves, providing an alternative way of quantifying the importance of these features in disk galaxies. A preliminary analysis of a larger sample shows that the phase-shift method is likely to be a generally applicable, accurate, and essentially model-independent method for determining the pattern speeds and corotation radii of single or nested density wave patterns in galaxies. Other implications of the results of this work include that most of the nearby bright disk galaxies appear to possess quasi-stationary spiral modes; that these density wave modes, as well as the associated basic states of the galactic disks, slowly transform over the time span of a Hubble time due to a collective dissipation process directly related to the presence of the phase shift between the potential and density patterns; and that self-consistent N-particle systems contain physics not revealed by the passive orbit analysis approaches.

The Distribution of Bar Strengths in Disk Galaxies

AbstractBars are the most important features that redistribute angular momentum and drive secular evolution in disk galaxies. We have derived the distribution of bar strengths in spirals and the Fourier properties of early-type galaxy bars in order to evaluate recent models of bar-halo angular momentum transfer and external gas accretion. A few results are presented here.

Bulges or Bars from Secular Evolution?

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.