Galactic bars are already mature at Cosmic Noon: bar strength and flatness at z ~ 1.5

TL;DR

The paper investigates whether bars at Cosmic Noon are transient or long-lived by analyzing a sample of nine massive barred galaxies at $z\sim1.5$ using rest-frame near-IR JWST/NIRCam F444W imaging. It combines ellipse fitting, spiral-arm masking, and 1D/2D bulge–disk–bar modeling to measure bar profiles, lengths, flux contributions, and gravitational torques via $Q_b$, finding that seven bars display flat profiles with $n<0.4$ and torques comparable to local bars, while two flat bars exhibit very low $Q_b$ potentially due to gas inflows or minor mergers. Shorter absolute bar lengths relative to local samples, and the presence of mature bars at this epoch, imply dynamically settled disks by $z\sim2.5-3.5$ and sustained secular evolution over $\gtrsim1$–$2$ Gyr. These results support a picture of long-lived bars forming early and persisting to at least $z\sim1.5$, shaping disk dynamics and enabling larger high-redshift bar studies in the future.

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 ($>10^{10.5}\,\rm M_{\odot}$) star-forming barred-spiral galaxies at $z_{\rm spec} \sim 1.5$. 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 $z\sim1.5$. To assess the gravitational impact of the bars, we calculate the maximum transverse-to-radial force ratio ($Q_{b}$). We find that $Q_{b}$ 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 $z\sim 1.5$ 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.

Figures (14)

• Figure 1: Ellipse fitting: (for each galaxy; left) The F444W image with ellipses fit to constant surface brightness isophotes. The ellipse with SMA $=$ bar length is highlighted. (top right) The ellipticity vs SMA plot. The SMA value at the peak ellipticity is used as the bar length. The exception being ID: 89 (and ID: 1147, not shown), which rather shows a plateau. In this case we select the outer edge of this plateau as the peak since it also corresponds to a change in the position angle (PA) change of $>10^{\circ}$ right after the fall of the peak in ellipticity. This can be seen in the PA vs SMA plot (bottom right).
• Figure 2: The 1D surface brightness profiles along the bar major axis of the galaxies in our sample (continued in Fig. \ref{['fig:profile_examples_2']}). Also shown are the major-axis profiles of the 2D best fit bar models (Sec. \ref{['subsec:2d_fit']}) for comparison. The x-axis gives the radial distance in kpc from the galaxy center. The red dashed lines denote the $R_\mathrm{bar}$, which is considered the approximate extent of the bar. Hence, the region within these lines contain the majority of the bulge and bar flux. In each case, we also provide the bar $n$-value from the 2D morphological fitting (Sec. \ref{['subsec:2d_fit']}), as well as the resulting classification into flat, intermediate and exponential bar profiles. In the five galaxies with flat profiles, the characteristic shoulders beyond the central bulge can be seen, giving the appearance of a "flattening" of the profile within $R_\mathrm{bar}$. The gaps in the profile are regions which have been flagged as they likely are regions influenced by spiral arms (Sec. \ref{['subsec:spiral_mask']}).
• Figure 3: Continuation of Fig. \ref{['fig:profile_examples']} for the remaining galaxies.
• Figure 4: The 2D morphological fitting of the rest-frame near-IR F444W images for six of the nine galaxies in our sample. The bulge, disk and bar models are shown separately, along with the residual $=$ Image $-$ (bulge+disk+bar) model. The final image is scaled based on the signal-to-noise (S/N), which is the ratio of the residual flux and the noise image.
• Figure 5: 2D morphological fitting of the rest-frame near-IR F444W images for the remaining three of the nine galaxies in our sample.