Dynamical Evolution of Galaxies

Xiaolei Zhang

• Publisher: De Gruyter
• Year: 2017
• ISBN:
  9783110525441
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

This research monograph presents a new dynamical framework for the study of secular morphological evolution of galaxies along the Hubble sequence. Classical approaches based on Boltzmann's kinetic equation, as well as on its moment-equation descendants the Euler and Navier-Stokes fluid equations, are inadequate for treating the maintenance and long-term evolution of systems containing self-organized structures such as galactic density-wave modes. A global and synthetic approach, incorporating correlated fluctuations of the constituent particles during a nonequilibrium phase transition, is adopted to supplement the continuum treatment. The cutting-edge research combining analytical, N-body simulational, and observational aspects, as well as the fundamental-physics connections it provides, make this work a valuable reference for researchers and graduate students in astronomy, astrophysics, cosmology, many-body physics, complexity theory, and other related fields.

Contents
Dynamical Drivers of Galaxy Evolution
N-Body Simulations of Galaxy Evolution
Astrophysical Implications of the Dynamical Theory
Putting It All Together
Concluding Remarks
Appendix: Relation to Kinetics and Fluid Mechanics

1Introduction

During the past few decades, the dominant view was that the structural properties of galaxies remain largely unchanged over time, unless galaxies were perturbed by violent events such as major or minor mergers (Toomre & Toomre 1972). This view is particularly favored in the currently popular hierarchical clustering/Lambda cold dark matter (LCDM) paradigm of structure formation and evolution (Ostriker & Steinhardt 1995; Liddle 2003 and the references therein; Mo, van den Bosch, & White 2010 and the references therein). Even though the role of mergers during the early phase of galaxy assembly at high redshifts could indeed be important, growing evidence has shown that at least since redshift z ∼ 1 the rate of merger appears to have been significantly reduced (Cohen 2002; Conselice et al. 2003; López-Sanjuan et al. 2009. See also Conselice et al. 2016 which indicates that the role of merger was already insignificant starting from z = 2, when the universe was only a quarter of its current age), and subsequent galaxy morphological transformation is likely to be dominated by the slower internal secular evolution process.

In the late 1970s, photometric and kinematic evidence in the bulges of late-type galaxies, as well as hints from N-body simulations of barred galaxies which incorporated a dissipative gas component, prompted several investigators to speculate that the morphology of late-Hubble-type disk galaxies may be transformed into that of earlier Hubble-types by gas accretion under barred potential (Kormendy 1979, 1982; Combes & Sanders 1981). These initial speculations have since been developed into one version of the secular evolution scenario, which emphasizes the role of dissipative gas accretion in the formation of the so-called pseudo bulges (or disky-bulges) in late-type galaxies (Kormendy & Kennicutt 2004 and the references therein). More careful examination, on the other hand, has since demonstrated that the relevant gas-dynamical process is not powerful enough to transform the morphology of intermediate- to early-Hubble-type disk galaxies, considering the observed fraction of gas and the rate of star formation in these galaxies (Kormendy & Kennicutt 2004).

Beginning in a series of papers published in the late 1990s (Zhang 1996, 1998, 1999), the present author showed that the stellar component, which constitutes the major portion of the galaxy mass in all but the very late-type galaxies, in fact is the main contributor of the secular morphological evolution of galaxies, through a collective interaction process between the spontaneously formed density wave modes and the basic state of the galactic disk2. This process had previously been overlooked in galactic dynamical analyses employing the stellar dynamical equation (which is the same as the collisionless Boltzmann equation, or the Vlasov equation), as well as the moment equation descendant of the collisional Boltzmann equation, the Eulerian equation set, because the Boltzmann equation itself was derived in a context that ignored collective interactions and interparticle correlations (see the detailed derivation of Boltzmann equation in the Appendix of this monograph), thus is not suitable for applications that focus on the self-organization and collective dissipation processes such as the study of the secular morphological evolution of galaxies mediated by spontaneously formed density wave modes.

In the course of presentation of this monograph, we will make extensive use of the phrases “collective dissipation” or “collective effects.” These phrases describe a general tendency for nonequilibrium, complex systems to spontaneously form global patterns, which lead to emergent new dynamics through coherent interactions of their component parts. Other phrases used in this context in the literature, ranging in applications from condensed matter physics, fluid dynamics, economics, biological and social sciences, include “cooperative effect,” “coupling and interaction among the degrees-of-freedom of a complex system,” and “synergetics.”

In the case of a galaxy possessing a density wave pattern, the traditional approach employed in constructing galaxy models displays a top-down type of organization, meaning that the stellar orbits in such galaxy models respond passively to an applied potential. Though still adequate for the study of the density wave modal emergence phase, the top-down approach is entirely powerless to deal with the self-consistent evolution of the galaxy basic state together with the corresponding self-limiting density wave modal set. The density wave modes of galaxies are able to maintain their quasi-steady amplitude only at the expense of the dissipative secular evolution of the mass distribution of the basic state of the galactic disk. So the long-term survival of the mode and the secular evolution of the basic state of the disk are closely linked.

The secular evolution process, which slowly transforms the morphology of a galaxy over its lifetime, could naturally account for the observed properties of the great majority of physical galaxies, if both the stellar and the gaseous accretion processes are taken into account. As an emerging paradigm for galaxy evolution, its dynamical foundation has been gradually established in the past few decades, and its astrophysical consequences are just beginning to be explored. In the current monograph, we seek to establish that the secular evolution scenario provides a coherent framework for understanding the extraordinary regularity and the systematic variation of galaxy properties along the Hubble sequence.

1.1Observational Background

Observational data on the characteristics of galaxies have been accumulating for more than a century, but evidence for the importance of internal secular evolution processes has only become apparent in the recent decades, largely as a result of the increased sensitivity, spectral coverage, and angular resolution of the space and ground-based telescopes. The supporting evidence ranges from the characteristics of individual galaxies both in isolated and in group/cluster environments, as well as the statistical properties of galaxy populations as a whole.

The most detailed characteristics of an individual galaxy come from the observation of our own Galaxy, the Milky Way. It has long been known that the observed kinematics of the different age groups of stars in the Milky Way Disk differ systematically, manifesting as the well-known age–velocity dispersion relation of the solar neighborhood stars (Wielen 1977). It is obvious that there exists a dynamical mechanism that heats the Disk stars secularly as they age, which operates smoothly across the entirety of a Hubble time (Gilmore, King, & van der Kruit 1990). Furthermore, the stellar population in the Galactic Bulge region has the well-known stratified distribution, with younger populations closer to the Galactic central region. This stratification trend also extends to the Thick and Thin Disks away from the Bulge region (Gilmore et al. 1990). These distributions also hint at a secular evolution origin for their formation.

Recent deep surveys have found that galaxies in the general field environment similar to that occupied by the Milky Way have undergone significant morphological transformation over the cosmic time. It is found that more field galaxies are of earlier Hubble types in the nearby universe than at higher redshifts (Lilly et al. 1998). There exist also the so-called faint-blue galaxies, which are in fact L∗ galaxies having luminosities and sizes similar to the Milky Way, which are found at the intermediate redshifts, but which have all but disappeared in the nearby universe (Ellis 1997). Since the total number density of galaxies of all Hubble types have not evolved significantly between redshifts z = 1 and z = 0 (Cohen 2002; Conselice et al. 2016), and the merger fraction since z = 1 is low (Conselice et al. 2003; López-Sanjuan et al. 2009), the most likely explanation for these observed statistical differences in galaxies between the higher redshifts and the nearby universe is the internal morphological (as well as the accompanying stellar population and color) evolution of galaxies.

Dense cluster was the environment where morphological transformation of galaxies was first hinted at through the so-called Butcher–Oemler effect (Butcher & Oemler 1978a, 1978b). When it was discovered, the Butcher–Oemler effect referred to the bluer colors of galaxies in dense clusters at the intermediate redshifts, compared to similar-density clusters in the local universe which contain mostly red, early-type galaxies. Subsequent Hubble Space Telescope (HST) observations (Couch et al. 1994; Dressler et al. 1994) have been able to resolve the morphology of these intermediate-redshift Butcher–Oemler galaxies, and show that they are mostly late-type disks; therefore the Butcher–Oemler effect is now considered not only a color evolution effect but also a morphological transformation effect. Because of the high-speed nature of encounters of galaxies in dense clusters, mergers are known to be infrequent in the virialized regions of dense clusters, so once again internal dynamical mechanisms are likely to have played a prominent role in the transformation of Butcher–Oemler galaxy morphologies between the intermediate redshifts and the nearby universe.

In the late 1970s, from observational studies of late-type galaxy bulges, J. Kormendy concluded that many of these bulges appear to have disk-like morphological and kinematic characteristics. He subsequently proposed (Kormendy 1979, 1982) that these late-type bulges are evolutionarily linked to disks, and named these bulges “pseudo bulges” to distinguish them from “true” (or “classical”) bulges in early-type disk galaxies, which were believed to be formed either primordially, or else through galaxy mergers.

Since it was at the time universally believed that stars “do not dissipate”, or that their orbital behavior is adiabatic under a steady potential (see, e. g. Binney & Tremaine 2008), Kormendy proposed that the formation of pseudo-bulges and the associated mild Hubble-type evolution among late-type galaxies were made possible through gas accretion in barred potentials, and these inwardly accreted gas subsequently formed new bulge stars. This proposed pathway for secular evolution, though very influential since then, has a number of problems from comparison with the observed properties of galaxies, as well as from detailed analysis of the physics of the accretion process. First of all, as emphasized by Andredakis, Peletier, & Balcells (1995), the continuity of galaxy properties across the entire Hubble sequence as highlighted for example by the smooth variation of the Sersic index n in fitting the bulge surface density profile, indicates that there is not an apparent break in the formation mechanism between late-type and early-type bulges. However, because of the paucity of gas compared to stars in most galaxies of earlier Hubble types, there simply is not enough of a reservoir of gas to build up the bulges of galaxies such as our own (of type Sb), which has its Bulge mass comparable to the Disk mass, not to say for galaxies of even earlier Hubble types. Secondly, bulges of intermediate- to early-type galaxies, including our own, consist mostly of stars of very old ages (Jablonka, Gorgas, & Goudfrooij 2002), despite also possessing a stratified color distribution (Wirth & Shaw 1983), and could not have been built up by an extended process of secular gas accretion which subsequently formed stars. A good fraction of these apparently old bulges have stellar kinematics which are also rotation-dominated and are related to the kinematics of their disks, hinting at a secular evolution origin for their formation (Kormendy & Kennicutt 2004 and the references therein). Therefore, a dynamical mechanism which could channel the pre-existing disk stars from the outer to the inner region of a galaxy is needed to explain these rotation-dominated older bulges. Thirdly, recent results of the ATLAS3D team (Cappellari et al. 2013) have shown that the morphologies and other internal properties of spirals, S0s, as well as disky ellipticals, form a continuous trend of evolution, which also coincides with the trend of aging of stellar population of their galactic disks. This provides further support for a unified formation history of the majority of the Hubble sequence galaxies through internal processes, which necessarily involves the participation of the stellar component. Fourth, recent results of the COSMOS team (Cisternas et al. 2011a, 2011b) showed that the establishment and evolution of the well-known black-hole-mass/bulge-mass correlation since z = 1, on the bulge-building side, was mainly due to the radial mass accretion process on pre-existing stellar disks, which were already in place by z = 1. The COSMOS team had also excluded merger as a significant contributor to the build-up of the black-hole-mass and bulge-mass correlation since z = 1. This further motivates a serious consideration of the internal secular mass redistribution process that emphasizes the role of stellar mass accretion in bulge building3. Finally, as we will show later in this monograph, even the gas contribution to the secular mass accretion process is through the same collective dissipation process involving large-scale, coherent density wave modes, rather than through the particle-level viscosity commonly attributed to gas dissipation. The mean free path of the collision/scattering of the galactic molecular clouds in a density wave crest region is similar to that of the stars, and both components should be considered together in supporting a star-gas combined two-fluid instability. This common mean free path of the two-fluid instability is many orders of magnitude larger than the microscopic collisional mean free path of the gas. So the roles of stars and gas are now parallel, rather than distinct, in the collective process that leads to the secular morphological transformation of galaxies.

From the above arguments, plus more that will be discussed in the main text, we see that internal secular evolution processes involving the participation of both the stellar and gaseous components appear to be crucial to the transformation of galaxy morphologies along the Hubble sequence at least since z = 1, and possibly since z = 2. The admittance of the stellar component into the secular mass redistribution process in galaxies, however, cannot be accomplished without a major breakthrough in the dynamical foundation of the theories of galactic structure and evolution.

1.2Theoretical Background

The ultimate engine for the morphological evolution of galaxies is in a self-gravitating system’s tendency to increase its entropy with time. It has long since been known (e. g., Antonov 1962; Lynden-Bell & Wood 1968) that the direction of this entropy evolution for self-gravitating systems is toward configurations with ever more centrally concentrated cores, together with increasingly extended outer envelopes.

Even with the above understanding, it was long held that the intrinsic speed of this evolution process is extremely slow. This slowness is due partly to the well-known pressure and angular momentum barriers in the galactic systems. Through an order of magnitude calculation, one can show that the natural speed of galactic evolution through microscopic transport processes results in a time scale for energy and angular momentum redistribution which is several orders of magnitude longer than the age of the universe (Zhang 1992).

It is clear that in order for secular morphological transformation of galaxies to be a relevant process in the observed history of the universe, so as to explain statistical evolution of galaxy properties with redshifts, dynamical mechanisms other than diffusion or dynamical friction have to be identified. An analogy here is the atmospheric heat flow (or Rayleigh–Bénard convection) problem (Kreuzer 1981). It is well known that when the natural speed of heat conduction is too slow in an atmospheric layer possessing a temperature gradient, an organized macroscopic flow pattern, facilitated by hexagonal convection cells, will spontaneously develop when the temperature gradient exceeds a certain threshold. These convection flow patterns greatly accelerate the speed of reducing the original temperature nonequilibrium in the atmospheric distribution, compared to the speed due to the conduction/diffusion process alone. As we will demonstrate in subsequent text, the new dynamical mechanism in galaxies operates in a similar fashion to the convection process in atmospheric flow. This mechanism will be shown to be closely related to the emergence of density wave patterns (or more specifically, unstable density wave modes) in galaxies, and to the maintenance of these modes as a dynamical equilibrium state, balanced by the opposing tendencies of the spontaneous growth of the unstable mode and the irreversible local dissipation process, with the latter process both damps the growth of the wave mode and simultaneously leads to the secular evolution of the basic state of the galactic-disk mass distribution.

Bertil Lindblad was the first astronomer to come up with the idea that spiral patterns in galaxies may be waves of density enhancement. In the 1950s, he made a series of numerical studies using an assembly of point-particles to simulate the appearance of spiral galaxies, but failed to obtain self-sustained patterns. C. C. Lin and F. Shu (1964, 1966) were the first to succeed in obtaining linear perturbative solutions of self-consistent spiral patterns in the tightly wrapped, or the so-called WKBJ (which stands for Wentz...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• 1 Introduction
• 2 Dynamical Drivers of Galaxy Evolution
• 3 N-Body Simulations of Galaxy Evolution
• 4 Astrophysical Implications of the Dynamical Theory
• 5 Putting It All Together
• 6 Concluding Remarks
• 7 Appendix: Relation to Kinetics and Fluid Mechanics
• References
• Index