Galactic bars are already mature at cosmic noon: bar strength and flatness at z ~ 1.5

Abstract

In this work, we explore the nature of z > 1 galactic bars. Once thought to be highly transient, our results demonstrate otherwise. Our sample consists of nine massive (>10^10.5 M⊙) star-forming barred-spiral galaxies at z_spec ∼ 1.5. Using rest-frame near-IR (F444W) JWST/NIRCam imaging, we apply ellipse fitting along with 1D and 2D morphological modeling to directly measure bar properties. We find that five galaxies host flat surface brightness profiles (bar Sérsic index < 0.4), indicative of highly evolved, “mature” bars. By contrast, only two galaxies show exponential profiles, characteristic of young bars, and these are also shorter in absolute length than the flat bars. We therefore conclude that a large fraction of bars at this epoch have already matured, thereby indicating the presence of well-settled disks required to facilitate bar formation and sustained evolution well before z ∼ 1.5. To assess the gravitational impact of the bars, we calculate the maximum transverse-to-radial force ratio (Q_b). We find that the Q_b values are comparable to, or weaker than, those of bars in the local Universe; seven of the nine bars show only a marginal increase in strength with maturity (from exponential to flat bars). Contrarily however, the remaining two bars are flat, but have the lowest Q_b values in our sample. We hence propose that the mature bars at z ∼ 1.5 may experience phases of weakening due to rapid gas inflows and/or minor mergers. In conclusion, our work sheds light on the rapidly evolving nature of high-z bars and paves the way for larger statistical studies.

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