The effects of bar strength and kinematics on galaxy evolution II: The global and local impacts of slow-strong bars

Technical Deep Dive: The effects of bar strength and kinematics on galaxy evolution II

Overview

This technical deep dive examines how galactic bars of different strengths and rotation speeds (kinematics) impact the star formation histories of their host galaxies. The study uses a sample of 210 nearby barred galaxies with detailed measurements of bar properties from the MaNGA survey.

Problem & Context

Galactic bars are known to significantly influence the evolution of their host galaxies through processes like radial gas inflow and redistribution. However, the net impact of bars on global galaxy properties like star formation rate (SFR) and quenching remains debated. This is in part because bars have a range of strengths and rotation speeds, which appear to have different effects.

Methodology

The authors:

• Classified bars as "strong" or "weak" based on Galaxy Zoo crowdsourced morphological assessments.
• Measured bar rotation speeds (pattern speeds) using the Tremaine-Weinberg method applied to MaNGA kinematic data to identify "fast" and "slow" bars.
• Constructed radial profiles of various star formation tracers (Hα, Hβ, Hδ, D4000) to probe the spatial distribution of ongoing and recent star formation in different bar subsets.
• Examined the co-evolution of star formation indicators and stellar population ages across the barred regions.
• Compared on-bar and off-bar regions to isolate local vs. global bar impacts.

Data & Experimental Setup

• Sample of 210 barred galaxies from the MaNGA survey with both morphological (Galaxy Zoo) and spectroscopic (MaNGA) data.
• Classified into strong/weak and fast/slow bar subsets.
• Divided into star-forming and quenching host galaxies based on position relative to the star-forming sequence.

Results

Key findings:

• Slow-strong bars drive the most enhancement of star formation in the central regions of their star-forming host galaxies, while fast-strong bars suppress star formation in the bar region.
• Slow-strong bars also show the most quenching in their host galaxies, with very low star formation across the entire bar region.
• The ends of slow-strong bars in star-forming hosts show the most localized enhancement of recent and ongoing star formation compared to off-bar regions.
• Quenching galaxies with fast bars maintain more ongoing star formation at their bar ends compared to those with slow bars.

Interpretation

The authors propose that the bar's impact on host galaxy evolution depends on a combination of bar strength and bar speed:

• Strong bars drive the most efficient radial gas inflow, enhancing central star formation when hosts are star-forming but quenching them more completely when they start to quench.
• Fast bars generate the highest shear in the bar region, suppressing star formation along the bar arms.
• The length of the bar relative to the corotation radius (bar speed) determines the extent of star formation enhancement or suppression at the bar ends.

Limitations & Uncertainties

• Small sample size, especially for some bar type/host SFR subsets.
• Challenges in constraining the properties of the Milky Way's own bar due to our internal vantage point.
• Limitations in using integrated galaxy properties to infer bar-driven gas flows and cloud dynamics.

What Comes Next

Further expanding this analysis to larger, more diverse samples of barred galaxies with reliable bar property measurements will be important to fully characterize the role of bars in galaxy evolution. Connecting these observations to hydrodynamic simulations can also provide more detailed physical insights.