Secular evolution in disk galaxies

Abstract

Self-gravitating systems evolve toward the most tightly bound configuration that is reachable via the evolution processes that are available to them. They do this by spreading -the inner parts shrink while the outer parts expand -provided that some physical process efficiently transports energy or angular momentum outward. The reason is that self-gravitating systems have negative specific heats. As a result, the evolution of stars, star clusters, protostellar and protoplanetary disks, black hole accretion disks and galaxy disks are fundamentally similar. How evolution proceeds then depends on the evolution processes that are available to each kind of self-gravitating system. These processes and their consequences for galaxy disks are the subjects of my lectures and of this Canary Islands Winter School.I begin with a review of the formation, growth and death of bars. Then I review the slow ("secular") rearrangement of energy, angular momentum, and mass that results from interactions between stars or gas clouds and collective phenomena such as bars, oval disks, spiral structure and triaxial dark halos. The "existence-proof" phase of this work is largely over: we have a good heuristic understanding of how nonaxisymmetric structures rearrange disk gas into outer rings, inner rings and stuff dumped onto the center. The results of simulations correspond closely to the morphology of barred and oval galaxies. Gas that is transported to small radii reaches high densities. Observations confirm that many barred and oval galaxies have dense central concentrations of gas and star formation. The result is to grow, on timescales of a few Gyr, dense central components that are frequently mistaken for classical (elliptical-galaxy-like) bulges but that were grown slowly out of the disk (not made rapidly by major mergers). The resulting picture of secular galaxy evolution accounts for the richness observed in galaxy structure.

MUSE tells the story of NGC 4371: The dawning of secular evolution

“…Those merger-built spheroids are sometimes named classical bulges, while the internal processes driven by non-axisymmetric components are sometimes labelled as part of secular evolution (see e.g. Binney & Merrifield 1998;Elmegreen 1998;Athanassoula 2013;Kormendy 2013;Sellwood 2014). Secular evolution is expected to be the dominant set of processes in galaxy evolution after the peak of star formation activity that occurs between redshifts 1 and 2 or between about 8 and 10 Gyr ago.…”

Star formation sustained by gas accretion

“…This paper gives an overview of this rapidly evolving field, emphasizing the role of metal-poor gas accretion to sustain star formation in the local universe. We limit ourselves to the global picture, leaving aside details about star-formation processes Gnedin et al 2014), stellar and active galactic nucleus (AGN) feedback (Silich et al 2010;Hopkins et al 2013a;Trujillo-Gomez et al 2013), secular evolution (Binney 2013;Kormendy 2013), dense cluster environments (Santini 2011;Kravtsov and Borgani 2012), and the growth of black holes (BH) through cosmic gas accretion (Husemann et al 2011;Chen et al 2013). Other recent reviews covering cosmic gas accretion from different perspectives are in Sancisi et al (2008), Fig.…”

“…In addition to gas accretion, galaxies also grow through mergers (e.g, Guo et al 2011;Kormendy 2013). Simulations suggest that direct accretion from the cosmic web dominates mergers by about an order of magnitude (e.g., Wang et al 2011;L'Huillier et al 2012;Combes 2013;van de Voort et al 2011).…”

Boxy/Peanut/X Bulges, Barlenses and the Thick Part of Galactic Bars: What Are They and How Did They Form?

“…5. Recent reviews on this or related subjects have been given by Kormendy & Kennicutt (2004), Athanassoula (2008Athanassoula ( , 2013a and Kormendy (2008Kormendy ( , 2013.…”