Down-bending Breaks in Galactic Disks Are an Intrinsic Byproduct of Inside-out Growth

TL;DR

This paper shows that down-bending Type II breaks in galactic disks arise intrinsically from inside-out growth driven by late-time cold-gas accretion, rather than solely from external perturbations. Using high-resolution TNG50 simulations, the authors classify disk surface-density profiles into Type I, II, and III, finding Type II disks dominate at $M_star < 10^{10.6} M_odot$ and exhibit larger scale-lengths, strong rotation, and minimal merger histories. They reveal a synchronized evolution where delayed angular-momentum growth and a localized sSFR peak near $R_{ m break}$ induce a break in the stellar density profile and a U-shaped age profile, with in-situ star formation contributing most to outer disks and radial migration playing a secondary role. The results argue for viewing Type II breaks as an intrinsic disk state, with Type I and III reflecting increasing external influence, and provide a coherent framework consistent with observed mass–size trends and age distributions.

Abstract

The exponential profile has long been hypothesized as the fundamental morphology of galactic disks. The IllustrisTNG simulations reproduce diverse surface-density profiles: Type I (single exponential), Type II (down-bending), and Type III (up-bending), consistent with observed mass-size relations and kinematics. Type II disks dominate the stellar-mass regime $M_\star < 10^{10.6} M_\odot$ with a prevalence of about 40%, exhibiting systematically extended morphologies. Conversely, Type III and Type I galaxies are more compact while following the same mass-size scaling relation. Evolutionary histories show that Type II galaxies experience minimal external perturbations, suggesting that Type II disks represent an intrinsic disk form and challenging conventional single-exponential paradigms. We demonstrate that Type II breaks arise naturally via inside-out growth since $z=1$, governed by synchronized cold-gas accretion and localized peaks in specific star formation rate. This mechanism also produces the characteristic U-shaped age profiles of Type II disks. Stellar dynamical redistribution plays a minor role in their formation.

Figures (5)

• Figure 1: Left panels: Examples of surface density profiles (black dots) of galaxies with Type I, II, and III disks, from top to bottom. The exponential (blue-dashed lines) and Sérsic (red-dashed) functions are use to decompose the disk and bulge component in morphology, their combined fitting result correspond to the yellow solid profiles. Vertical black dashed lines mark the break radius ($R_{\rm break}$) , where the profile first deviates from the inner exponential part. Right panels: The fraction of galaxies with different disk types as a function of their stellar masses. Solid and dashed lines denote results from two galaxy samples: our kinematically-selected primary sample ($f_{\rm disk} > 0.4$; du2019du2020) and a morphologically-defined sample incorporating visually classified disks to enable direct comparison with observational surveys tang2020. The upper panel gives their number counts in each stellar mass bin.
• Figure 2: Kinematic and structural properties of Type I (circles), Type II (triangles), and Type III (crosses) galaxies in the $h_R$--$M_\star$ plane. From left to right, the data points are color-coded by rotational support $\kappa_{\rm rot}$, stellar halo mass fraction $f_{\rm halo}$, and ex-situ stellar mass fraction $f_{\rm ex\text{-}situ}$. The gray dashed, gray dotted, and black solid lines represent the linear fits to the $h_R$--$M_\star$ relation for Type I, Type III, and the total disk sample, respectively; shaded regions denote the 1$\sigma$ fitting uncertainties. The error bars indicate the standard deviation of the residuals from the global linear
• Figure 3: Accumulated fraction of mergers for Type I (green), Type II (blue), and Type III (red) disk galaxies across three stellar mass bins: $\log(M_\star/M_\odot) = 10.0$--10.4 (left), 10.4--10.8 (middle), and 10.8--11.2 (right). The fraction of galaxies experiencing at least one major merger (solid profiles, mass ratio $> 1:4$) or minor mergers (dashed profiles, mass ratio $1:4$--$1:10$) as a function of the lookback time.
• Figure 4: Evolution of radial profiles for Type II disk galaxies from $z=1.5$ to $z=0$, plotted against the normalized radius $R/R_{\rm break}$. The panels represent, from top to bottom: normalized stellar surface density ($\Sigma_\star / \Sigma_{\star, 1\rm{kpc}}$), mass-weighted mean stellar age, cold-gas surface density ($\Sigma_{\rm gas}$), specific star formation rate (sSFR), and star formation efficiency (SFE). Colored curves correspond to different redshifts: $z=0$ (orange), $z=0.5$ (pink), $z=1.0$ (blue), and $z=1.5$ (gray). Solid lines denote the median values, while shaded regions indicate the interquartile range (25th–75th percentiles). The vertical dashed line marks the break radius identified at $z=0$.
• Figure 5: Decomposition of the $z=0$ stellar population by origin, age, and kinematic components. Upper panels: radial surface-density profiles for the kinematically defined disk (blue), bulge (orange), and stellar halo (red), following the decomposition method of du2019du2020. Lower panels: radial mass fractions of ex-situ stars (brown) and in-situ stars. The in-situ population is subdivided into three age bins: young, intermediate-age, and old (dark to light blue shading). In both panels, solid lines denote median values, shaded regions indicate the interquartile range (25th--75th percentiles), and the vertical dashed line marks the break radius $R_{\rm break}$.