Impacts of bar-driven shear and shocks on star formation (2405.00107v1)

Abstract: Bars drive gas inflow. As the gas flows inwards, shocks and shear occur along the bar dust lanes. Such shocks and shear can affect the star formation and change the gas properties. For four barred galaxies, we present H{\alpha} velocity gradient maps that highlight bar-driven shocks and shear using data from the PHANGS-MUSE and PHANGS-ALMA surveys which allow us to study bar kinematics in unprecedented detail. Velocity gradients are enhanced along the bar dust lanes, where shocks and shear are shown to occur in numerical simulations. Velocity gradient maps also efficiently pick up expanding shells around HII regions. We put pseudo slits on the regions where velocity gradients are enhanced and find that H{\alpha} and CO velocities jump up to ~170 km/s, even after removing the effects of circular motions due to the galaxy rotation. Enhanced velocity gradients either coincide with the peak of CO intensity along the bar dust lanes or are slightly offset from CO intensity peaks, depending on the objects. Using the BPT diagnostic, we identify the source of ionization on each spaxel and find that star formation is inhibited in the high velocity gradient regions of the bar, and the majority of those regions are classified as LINER or composite. This implies that star formation is inhibited where bar-driven shear and shocks are strong. Our results are consistent with the results from the numerical simulations that show star formation is inhibited in the bar where shear force is strong.

• The paper reveals that bar-induced shear and shocks produce significant velocity gradients, exceeding 0.35 km/s/pc, along dust lanes in barred spiral galaxies.
• The study employs high-resolution Hα maps from PHANGS-MUSE and PHANGS-ALMA to demonstrate an anti-correlation between strong velocity gradients and star formation rates.
• The results support models where gravitational torques channel gas inward while intense shear inhibits molecular cloud collapse and local star formation.

Impacts of Bar-Driven Shear and Shocks on Star Formation

The paper "Impacts of Bar-Driven Shear and Shocks on Star Formation," authored by Taehyun Kim and colleagues, investigates the influence of bars in spiral galaxies on the distribution and dynamics of interstellar gas, and how these effects mediate star formation processes. This paper focuses on barred spiral galaxies, which host significant non-axisymmetric structures that induce complex kinematic phenomena, thus playing a crucial role in the evolution of such galaxies.

Study Overview

The research utilizes high-resolution kinematic data from the PHANGS-MUSE and PHANGS-ALMA datasets to analyze the velocity gradients within four nearby barred spiral galaxies. By employing H$\alpha$ velocity gradient maps, the paper provides detailed insights into the bar-driven shocks and shear along the bar dust lanes. The authors locate enhanced velocity gradients—indicative of shear and shocks—along the bar's dust lanes, a prominent site for non-circular gas motions due to gravitational torques.

Velocity Gradients and Shear

The paper presents intriguing findings regarding the kinematic behavior of gas in the bars. Significant velocity gradients, which often exceed 0.35 $\text{km s}^{-1} \text{pc}^{-1}$, are detected along the bar's dust lanes. These gradients are attributed to gravitational torques exerted by the bar structures, which funnel gas toward the galaxy's center, sometimes creating conditions conducive to star formation in nuclear regions. However, the associated shear and shocks can also suppress star formation by preventing the collapse of molecular clouds. The authors observe velocity jumps up to 170 $\text{km s}^{-1}$ across bar regions, confirming the presence of bar-induced shocks predicted by numerical simulations.

Star Formation Suppression

A critical result from this paper is the apparent suppression of star formation in regions experiencing the most intense shear and shock conditions. The authors quantify the star formation rate (SFR) surface density as a function of velocity gradient, revealing an anti-correlation where regions characterized by strong velocity gradients tend to exhibit diminished SFR surface densities. This finding corroborates theoretical models suggesting that high shear rates can inhibit the growth of instabilities necessary for star formation.

Ionization and Emission-Line Ratios

The research further explores the impact of the bar on the ionization state of gas, employing BPT diagrams to distinguish between ionizing mechanisms. High-velocity gradient regions often exhibit elevated emission-line ratios, classifying them as LINER or composite, rather than regions dominated by star formation. This suggests that shocks might alter the local ionization balance, modifying the emission line characteristics of the interstellar medium.

Theoretical and Practical Implications

The paper delivers important implications for both theoretical frameworks and observational strategies in galaxy evolution research. It substantiates the notion that while bars can ignite central starbursts by channeling gas inward, they simultaneously inhibit star formation along the bar—highlighting a complex interplay of dynamical processes. For future studies, this suggests a dual approach: considering both large-scale morphological features and localized kinematic phenomena to better understand the nuanced role of bars in galactic evolution.

Conclusions

In summary, the paper offers comprehensive observational evidence and analysis that enhance our understanding of the role of bar-driven dynamics in shaping star formation across galaxies. By leveraging advanced datasets and sophisticated modeling techniques, it sheds light on how shear and shocks contribute to the overall star formation landscape in barred spiral galaxies. This work underscores the importance of high-resolution, spatially-resolved kinematic studies in unraveling the complexities of galactic dynamics and evolution.