"In a landmark study of more than 2,000 spiral galaxies, Hubble has found clear evidence that majestic barred spirals—galaxies that show a distinctive bar-shaped structure of stars and gas that slice across their nuclei—were far less common 7 billion years ago than they are today. The findings come from Hubble’s largest galaxy census, which surveyed 10 times more spiral galaxies than previous observations.
.. A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20% of the spiral galaxies in the distant past possessed bars
[demonstrating lack of mergers?], compared with nearly 70% of their modern counterparts [demonstrating effect of mergers?]. Bars have been forming steadily over the last 7 billion years, more than tripling in number.
Galactic bars develop when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. The tiny elongations in the stars’ orbits grow and get locked into place, forming a bar. The bar becomes even more pronounced as it collects more and more stars in elliptical orbits. Eventually, a high fraction of the stars in the galaxy’s inner region join the bar. This process has been demonstrated repeatedly with computer-based simulations.
As they develop, bars apparently become one of the most important catalysts for galaxy evolution. They force a large amount of gas towards the galactic center, fueling new star formation, building central bulges of stars, and feeding massive black holes. Galaxies are thought to initially assemble from, and grow in size through, mergers with other smaller galaxies. After this phase is complete, however, the only other dramatic way for galaxies to evolve is through the action of bars.
.. A second key finding from the COSMOS study is that recently forming bars are not uniformly distributed across galaxy masses. They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars today is the same as it was in the past. This finding has important ramifications for understanding galaxy evolution. Low-mass galaxies are known to form stars at a slower pace, and Hubble now shows that their bars form more slowly as well.
.. Our own Milky Way galaxy has a central bar that probably formed somewhat early, like the bars in other large galaxies in the Hubble survey."http://hubblesite.org/hubble_discoverie ... lution.pdf
"The spectacular barred spiral galaxy NGC 6872 ..
Measuring tip-to-tip across its two outsized spiral arms, NGC 6872 spans more than 522,000 light-years, making it more than five times the size of our Milky Way galaxy.
.. By analyzing the distribution of energy by wavelength, the team uncovered a distinct pattern of stellar age along the galaxy's two prominent spiral arms. The youngest stars appear in the far end of the northwestern arm, within the tidal dwarf candidate, and stellar ages skew progressively older toward the galaxy's center.
The southwestern arm displays the same pattern, which is likely connected to waves of star formation triggered by the galactic encounter.
As in all barred spirals, NGC 6872 contains a stellar bar component that transitions between the spiral arms and the galaxy's central regions. Measuring about 26,000 light-years in radius, or about twice the average length
found in nearby barred spirals, it is a bar that befits a giant galaxy. "https://www.jpl.nasa.gov/news/news.php?release=2013-016
" .. The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out [a.k.a. "growth"], which creates a self-perpetuating bar structure."
" .. Bars are thought to be temporary phenomena in the lives of spiral galaxies [
]; the bar structures decay over time, transforming galaxies from barred spirals to more "regular" spiral patterns. [Really?] Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, stubby bars. Since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. [Really?] The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years."https://en.wikipedia.org/wiki/Barred_spiral_galaxy
Note the change in perspective from 2002 to 2008 (from above), both utilizing Hubble:
"Michael Merrifield (University of Nottingham, England) says that analysis of Hubble Deep Field galaxies indicates that barred spirals were virtually absent [?] until the universe was about half its present age. "At these early epochs disks were either too dynamically hot or too low in mass to undergo the conventional bar instability," [directly clashes with the hubblesite's link stating low-mass galaxies are where bars are forming]30 maart 2002https://web.archive.org/web/20020512044 ... 401_st.htm