Tobias Géron
Galaxy bars are very ubiquitous structures in the Universe. Observational studies report that between 30%-70% of galaxies host bars. In addition, it has been known that they can influence their galaxy by moving mass inwards and angular momentum outwards, which could possibly help to quench the galaxy. However, bars can have a variety of sizes and can thus have a varying effect on their host galaxy. Previous studies that look at bars often only have a 'barred' and a 'non-barred' sample, which could hide any potential effect that is only mediated by the strongest of bars. Other studies only include the most prominent bars in order to create a clean sample, which tells only half the story.
We have used Galaxy Zoo, a citizen science project, to investigate the differences between strong and weak bars in the context of galaxy evolution. Our sample includes a total of 1Astrophysics galaxies, of which 15% are strongly barred and 23% are weakly barred. We found that most of the strongly barred galaxies are massive quiescent galaxies, in agreement with previous studies, while weak bars seem to appear in any type of galaxy. However, when looking only at the star forming galaxies, we found that strongly barred galaxies have higher fibre star formation rates (SFRs), lower HI gas masses and lower depletion timescales. These differences were not found in weakly barred galaxies
This suggests that strong bars in star forming galaxies increase the rate of gas consumption and thereby mediate the quenching process.
If you plot all those galaxies on a stellar mass - star formation rate (SFR) plane, as in figure 2, and split up the sample in a star forming and quiescent group, an interesting result is obtained. Strong bars are more likely to be found in the quiescent group, which is in agreement with previous studies. About 10.7% of all galaxies in the star forming group have strong bars, whereas this increases to 17% of all quiescent galaxies. This is a first hint towards that strong bars might mediate quenching in some way. However, we also notice that this trend does not extend to weak bars. They appear in roughly equal fractions in the quiescent and star forming group (22.7% and 20% in the star forming and quiescent group, respectively).
We focused on only the star forming galaxies for the third figure. We wanted to see whether there are any differences in terms of SFR, HI gas mass and depletion timescale, which is the inverse of star formation efficiency (SFE). However, as strongly barred galaxies appear more frequently in more massive galaxies, we plotted these three quantities against stellar mass to remove any unwanted differences due to differences in stellar mass. In addition, as HI gas mass comes from ALFALFA (Haynes et al. (2018)), we need to incorporate non-detections to correctly compare both groups. This was done by calculating the upper limits for the non-detections and using the Python package linmix. This package is based on the hierarchical Bayesian model of Kelly B. C. (2007) and can generate first order fits while accounting for measurement errors in both dimensions and correctly implements non-detections. The stellar mass and SFR measurements come from MPA-JHU (Kauffmann et al. (2003), Brinchmann et al. (2004) and Tremonti et al. (2004)). Using this, we have shown that weakly barred galaxies don't differ significantly from galaxies without bars in any of the three quantities. However, strongly barred star forming galaxies have higher fibre SFRs, lower gas masses and lower depletion timescales compared to galaxies without bars across the stellar mass range.
Our results shows that strong bars are longer and more prominent structures in a galaxy than weakly barred galaxies, as we expected. However, we have also shown that strong bars appear more frequently in quiescent galaxies, while weak bars appear equally in star forming and quiescent galaxies. Finally, we have found that only strong bars in star forming galaxies, not weak bars, increase fibre SFR, lower gas masses and decrease depletion timescales compared with galaxies without bars. This will increase the rate of gas consumption in these galaxies and thereby facilitate the quenching process.