Studying the slow processes of galaxy evolution through bars
Note: this is a post by Galaxy Zoo science team member Edmond Cheung. He is a graduate student in astronomy at UC Santa Cruz, and his first Galaxy Zoo paper was accepted to the Astrophysical Journal last week. Below, Edmond discusses in more depth the new discoveries we’ve made using the Galaxy Zoo 2 data.
Observations show that bars – linear structures of stars in the centers of disk galaxies – have been present in galaxies since z ~ 1, about 8 billion years ago. In addition, more and more galaxies are becoming barred over time. In the present-day Universe, roughly two-thirds of all disk galaxies appear to have bars. Observations have also shown that there is a connection between the presence of a bar and the properties of its galaxy, including morphology, star formation, chemical abundance gradients, and nuclear activity. Both observations and simulations argue that bars are important influences on galaxy evolution. In particular, this is what we call secular evolution: changes in galaxies taking place over very long periods of time. This is opposed to processes like galaxy mergers, which effect changes in the galaxy extremely quickly.
Examples of galaxies with strong bars (linear features going through the center) as identified in Galaxy Zoo 2.
To date, there hasn’t been much evidence of secular evolution driven by bars. In part, this is due to a lack of data – samples of disk galaxies have been relatively small and are confined to the local Universe at z ~ 0. This is mainly due to the difficulty of identifying bars in an automated manner. With Galaxy Zoo, however, the identification of bars is done with ~ 84,000 pairs of human eyes. Citizen scientists have created the largest-ever sample of galaxies with bar identifications in the history of astronomy. The Galaxy Zoo 2 project represents a revolution to the bar community in that it allows, for the first time, statistical studies of barred galaxies over multiple disciplines of galaxy evolution research, and over long periods of cosmic time.
In this paper, we took the first steps toward establishing that bars are important drivers of galaxy evolution. We studied the relationship of bar properties to the inner galactic structure in the nearby Universe. We used the bar identifications and bar length measurements from Galaxy Zoo 2, with images from the Sloan Digital Sky Survey (SDSS). The central finding was a strong correlation between these bar properties and the masses of the stars in the innermost regions of these galaxies (see plot).
This plot shows the central surface stellar mass density plotted against the specific star formation rate for disks identified in Galaxy Zoo 2. The colors show the average value of the bar fraction for all galaxies in that bin. This plot shows that the presence of a bar is clearly correlated with the global properties of its galaxy (Σ and SSFR).
We compared these results to state-of-the-art simulations and found that these trends are consistent with bar-driven secular evolution. According to the simulations, bars grow with time, becoming stronger (they exert more torque) and longer. During this growth, bars drive an increasing amount of material in towards the centers of galaxies, resulting in the creation and growth of dense central components, known as “disky pseudobulges”. Thus our findings match the predictions of bar-driven secular evolution. We argue that our work represents the best evidence of bar-driven secular evolution yet, implying that bars are not stagnant structures within disk galaxies, but are instead a critical evolutionary driver of their host galaxies.
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