Bars In GalaxiesEdit
Bars in galaxies are elongated, non-axisymmetric structures made primarily of stars that extend from the galactic center into the disk. They are common in disk galaxies and act as internal engines that reorganize material, angular momentum, and star formation over long timescales. In the local universe, surveys indicate that a substantial fraction of disk galaxies host a bar at some wavelength, with near-infrared observations often revealing bars that optical images miss due to dust. galaxys with bars are sometimes called barred spiral galaxy or discussed within the broader study of galaxy morphology and stellar dynamics. Through their rotation and gravitational influence, bars shape the evolution of their hosts in ways that rival more dramatic external encounters.
The existence of bars is tied to the fundamental physics of rotating, self-gravitating disks. They form through internal instabilities in dynamically cool stellar disks and can be triggered or enhanced by interactions with other galaxies or by changes in the surrounding dark matter halo. Once formed, bars promote secular (slow, internal) evolution by redistributing angular momentum: stars and gas exchange angular momentum with the bar itself, the outer disk, and the surrounding halo. This process can influence the overall structure of the galaxy for billions of years. The interaction between a bar and the surrounding halo, and the way that angular momentum is transferred, remains a central topic in galactic dynamics galactic dynamics and dark matter halos. The presence of a bar often correlates with a variety of resonant structures and orbital families that support the bar’s shape and continuing rotation x1 orbits, x2 orbits, and related resonances such as the corotation radius.
Structure and dynamics
Morphology and orbital structure
A bar is not just a long, straight feature; it represents a coherent set of stellar orbits that align to form an elongated, rotating pattern. The dominant family of stellar orbits that supports the bar in many models is the x1 family, which aligns with the bar’s major axis. Within the inner regions, other orbital families can support features like lenses or inner bars. In some galaxies, a secondary, inner bar—forming a nested bar system—appears within the primary bar and can drive gas inward even further. This nested-bar configuration has implications for fueling central activity and shaping the very inner structure of the galaxy. For readers exploring the orbital underpinnings, see x1 orbits and x2 orbits for the two commonly discussed families.
Pattern speed, resonances, and gas flows
Bars rotate as a pattern with a characteristic angular speed, known as the pattern speed. The locations where the bar’s pattern speed has resonances with the orbital motion—such as the corotation radius—play a crucial role in determining how gas responds to the bar. Gas tends to stream along the bar, lose angular momentum, and pile up at resonances, creating rings and enhanced central concentrations of gas and star formation. This inflow can feed central structures, including nuclear star clusters and, in some cases, supermassive black holes, linking bar dynamics to broader galactic evolution active galactic nucleus phenomena.
Evolution, buckling, and the stellar population
Bars interact with their host disks over time. They can transfer angular momentum to the outer disk and halo, potentially slowing their pattern speed through dynamical friction with a surrounding dark matter halo. Bars can also undergo vertical instabilities, sometimes buckling out of the disk plane and producing boxy or peanut-shaped bulges in edge-on galaxies. These vertical rearrangements illustrate how a single, elongated feature can reorganize a galaxy’s vertical and radial structure without requiring a major merger. The resulting central structures, often referred to as pseudo-bulges, reflect a history of secular evolution driven in part by the bar’s activity pseudo-bulge.
Formation, prevalence, and observational methods
How bars form and persist
The prevailing view is that bars emerge from internal instabilities in sufficiently massive, dynamically cold disks. Once formed, bars can persist for long periods, though their strength and pattern speed can evolve due to gas inflows, star formation, and interactions with the halo. Some simulations suggest bars may weaken or dissolve when central mass concentrations grow substantially from gas inflows, only to reform later as the disk cools and reorganizes. The balance between bar formation, dissolution, and possible reformation is a lively area of research, with different numerical experiments yielding somewhat different outcomes depending on the physics included (gas, star formation, feedback) and the properties of the host halo.
Observational programs and bar demographics
Detecting bars benefits from near-infrared observations, which trace the older stellar population that most faithfully marks the bar’s gravitational influence and are less affected by dust. Large surveys and targeted programs—such as infrared and multi-wavelength studies of nearby galaxies—have quantified the fraction of barred disks and examined how bar properties correlate with galaxy mass, color, and environment. For example, flows of gas and stars under the bar’s torque can be inferred from high-resolution imaging and spectroscopy, with results interpreted in light of dynamical models that include resonant orbits and gas dynamics. Readers may encounter discussions of the Spitzer Space Telescope–based survey programs and related datasets when exploring comprehensive catalogs of bar properties Spitzer Space Telescope and S4G.
Bars and galactic evolution
Secular transformation and central activity
Bars contribute to secular evolution by driving gas inward, fueling central star formation and contributing to the growth of central stellar densities. Over time, this secular buildup can alter a galaxy’s morphology, moving it toward an earlier-type appearance without the need for major mergers. The existence of pseudo-bulges is often cited as evidence for this slow, internal growth rather than catastrophic assembly. The interplay between bar-driven gas inflow and the activation of a central engine (if a supermassive black hole is present) is a topic of ongoing investigation, with some studies finding a statistical association between bars and central activity, while others emphasize a more nuanced or weak connection due to competing processes and selection effects AGN.
Environmental and internal drivers
While bars arise from internal dynamics, their frequency and properties can be influenced by a galaxy’s environment and mass distribution. Tidal interactions, minor mergers, and the angular momentum content of the surrounding dark matter halo can affect bar formation and longevity. In this sense, bars serve as a bridge between the quiet, ongoing evolution of isolated disks and the more dramatic influences of environmental perturbations. See galaxy morphology and dark matter for related discussions of how a galaxy’s broader context shapes its internal structure.
Controversies and debates
Longevity versus episodic bar activity
A central debate concerns whether bars are long-lived structures that persist for many billions of years or whether they form, dissolve, and reform multiple times over a galaxy’s lifetime. Proponents of long-lived bars emphasize cumulative angular momentum transfer and resonant locking that stabilizes the bar, while others point to gas inflows and central mass growth as possible disruptors. The answer likely depends on detailed balance among stellar dynamics, gas physics, and halo properties, and simulations continue to refine the conditions under which bars endure.
Bar fraction across cosmic time
Another area of active discussion is how bar prevalence evolves with redshift. Observational biases—such as limited resolution, dust, and surface-brightness effects—complicate measurements at earlier epochs. Some studies report a rising bar fraction toward the present day, consistent with gradual secular development, while others attribute apparent trends to selection effects. Careful treatment of biases and multi-wavelength data are essential to interpret any evolution in the barred fraction of galaxies over time galaxy evolution.
Bars and central activity
The degree to which bars promote or trigger central activity such as AGN remains contested. Some analyses find a higher incidence of bars in galaxies with active nuclei, suggesting bar-driven gas inflows as a fueling mechanism. Others find only weak or environment-dependent correlations, implying that multiple pathways (including stochastic accretion events and interactions) can feed central engines. This ongoing debate reflects the complexity of disentangling causal connections in galaxy centers.
Dark matter halos and bar dynamics
The role of the dark matter halo in bar dynamics—whether halos stabilize bars or facilitate their slowdown and evolution—remains an active area of inquiry. Differences in halo concentration, shape, and dynamical friction can lead to varying outcomes in simulations, and observational constraints are becoming more precise with high-resolution kinematic data. These questions connect to broader inquiries about the distribution of dark matter in galaxies and the nature of gravity on galactic scales dark matter.