Early thin-disc assembly revealed by JWST edge-on galaxies

TL;DR

We address how disc thickness informs the formation of thin and thick discs by measuring the vertical structure of JWST-observed edge-on galaxies at $1<z<3$ using a forward-modeling 3D approach that jointly fits inclination and PSF effects with a $B(R,Z)=I_0 e^{-R/h_r}\,\mathrm{sech}^2\left(\frac{Z}{2 z_0}\right)$ profile. The study finds a median scale height $z_0\approx 0.25$ kpc and a median axis ratio $h_r/z_0\approx 8$–$9$, consistent with local thin discs but smaller than earlier local single-disc fits, implying thin discs are already in place by $z\sim3$; a thick disc with $<10\%$ of the thin-disc luminosity would be detectable, supporting a scenario where thick discs assemble gradually via dynamical heating rather than forming thick from birth. Comparisons with simulations (e.g., TNG50) show simulated galaxies tend to be thicker than observed, indicating a potential tension between models and real galaxies. The method also yields indirect constraints on gas kinematics, suggesting somewhat lower turbulence in typical main-sequence galaxies than inferred from H$\alpha$ studies, thereby highlighting the value of JWST imaging for constraining disc formation history.

Abstract

The vertical structure of stellar discs provides key constraints on their formation and evolution. Nearby spirals, including the Milky Way, host thin and thick components that may arise either from an early turbulent phase or from the subsequent dynamical heating of an initially thin disc; measuring disc thickness across cosmic time therefore offers a direct test of these scenarios. We present a new methodology to measure the thickness of edge-on galaxies that explicitly accounts for small departures from perfectly edge-on orientations by fitting a full three-dimensional model with forward modelling. This improves on traditional approaches that assume an inclination of $90^\circ$ and can bias thicknesses high. Applying the method to \textit{JWST} imaging of galaxies at $1<z<3$ with stellar masses $\gtrsim 10^9~\mathrm{M_{\odot}}$ from four major surveys, we measure a median scale height of $z_0 = 0.25\pm0.14~\mathrm{kpc}$ and a median ratio $h_r/z_0 = 8.4\pm3.7$. These values are consistent with the Milky Way and local thin discs, but imply scale heights $\sim 1.6$ times smaller than those inferred for local galaxies from single-disc fits. This result implies that thin discs are already present at $z\sim3$. We further show that a thick disc contributing 10\% of the thin-disc luminosity would be detectable in the data considered in this work, implying that any thick disc present must be fainter and favouring a scenario in which thick discs build up progressively through dynamical heating.

Figures (21)

• Figure 1: Examples of galaxies excluded from the final sample based on visual inspection. From left to right: compact objects that are too small for reliable fitting; systems that are not fully edge-on; a companion/contaminated case where a nearby or foreground source affects the image and cannot be effectively masked; and irregular galaxies excluded due to strong asymmetry.
• Figure 2: Four representative galaxies included in our final sample.
• Figure 3: Normalised distribution of redshift of the 90 galaxies in our sample (orange) compared to the parent sample (blue). The right axis shows the absolute count for the selected sources per redshift bin. The target selection provide a final sample biased towards the lower redshift sources, without any selected sources above z = 3.
• Figure 4: Distribution of SFR vs $M_\star$ per redshift bin of the selected sample (blue crosses) compared to the parent DJA sample (gray circles).
• Figure 5: The different panels show mock observations of the same disc and its isophotes (white contours) viewed at different inclinations.