Galaxy Zoo: CANDELS Barred Disks and Bar Fractions

Abstract: The formation of bars in disk galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in disks decreases from the local Universe to z ~ 1, and by z > 1 simulations predict that bar features in dynamically mature disks should be extremely rare. Here we report the discovery of strong barred structures in massive disk galaxies at z ~ 1.5 in deep rest-frame optical images from CANDELS. From within a sample of 876 disk galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a sub-sample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5< z < 2 (f_bar = 10.7 +6.3 -3.5% after correcting for incompleteness) does not significantly evolve. We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disk galaxies have evolved over the last 11 billion years.

• The paper shows that barred structures exist at z ≈ 1.5, contradicting theories that expected their rarity in dynamically unsettled disks.
• The study uses CANDELS imaging and over 200,000 Galaxy Zoo classifications to analyze 875 disk galaxies, deriving a bar fraction of ~10.7% from z 0.5 to 2.
• The presence of bars in high-redshift disks implies early dynamical maturity, prompting a reevaluation of bar formation mechanisms in massive galaxies.

Bar Formation and Fractions in High-Redshift Disk Galaxies

The paper "Galaxy Zoo: CANDELS Barred Disks and Bar Fractions" by Simmons et al. presents a comprehensive study on the presence and evolution of barred structures in disk galaxies up to a redshift of $z \sim 2$. Utilizing visual classifications from the Galaxy Zoo project, this investigation shines a light on the dynamics of disk galaxies over 11 billion years of cosmic time.

Methodology and Data

The study capitalizes on data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), covering infrared and optical observations from the Hubble Space Telescope (HST). The Galaxy Zoo initiative has amassed classifications from over 200,000 volunteers, allowing for an extensive morphological analysis of galaxies. In this study, more than 875 disk galaxies were selected using visual classification schemes that assess features such as clumpiness and edge-on orientation, crucial for identifying potential bars.

Key Findings

1. Bar Incidence at High Redshifts: A significant discovery is the existence of strong barred structures in disk galaxies at $z \sim 1.5$. This contradicts previous theories suggesting bars should be rare at such early times, given that disks would be too dynamically unsettled to support them.
2. Bar Fractions: The study reports a bar fraction of approximately $10.7^{+6.3}_{-3.5}\%$ in disk galaxies between $0.5 \leq z \leq 2$. This result is notable for its lack of significant evolution over this broad redshift range, suggesting a relatively stable environment for bar formation despite the changing cosmic landscape.
3. Implications for Disk Maturity: The presence of bars at $z > 1$ implies that some massive disk galaxies reached dynamical maturity earlier than previously thought. This requires refinement in our understanding of disk evolution and bar formation mechanisms in the context of galaxy assembly.

Theoretical Implications

The findings imply that while many high-redshift disks are dynamically warmer (as known from increased rotational velocities and velocity dispersions), some manage to host bars. This reveals a potential subset of massive, early-maturing galaxies that have stable enough environments to sustain such structures. The work challenges bar formation theories, urging the need for simulations that accurately incorporate both early disk formations and the evolutionary processes preserving or disrupting bar structures.

Future Directions

The study suggests that the exploration of bar fractions across a larger, more diverse sample could lead to a better understanding of the conditions required for bar formation in early galaxies. Moreover, comparison with high-resolution simulations will be invaluable. These simulations should encompass a wider range of stellar masses and incorporate interactions like tidal disturbances, which might explain the initiation or destruction of bars.

Overall, this research contributes critical insights into the cosmological evolution of disk galaxies and highlights the complexity of large-scale galactic structures across cosmic time. The potential for galaxy interactions and mergers to influence bar presence makes it a compelling area for further study, with implications for models of galaxy evolution and the broader understanding of cosmic structure formation.