Radial Transport in High-Redshift Disk Galaxies Dominated by Inflowing Streams

TL;DR

This work tests whether radial transport of cold gas in high-redshift disks is primarily driven by internal violent disk instabilities or by accretion via cold streams from the cosmic web. Using the VELA zoom-in simulations, the authors map radial velocity fields, compute radius-dependent and disk-wide average inflows, and separately quantify inflows associated with recently accreted streams through metallicity- and streamline-based selections. They find that disk-averaged inflows are modest (a few percent of $V_{ m rot}$) and that the outer disk transport is dominated by stream inflows, a result that aligns with observed large radial motions at cosmic noon when the signal traces streams rather than disk instabilities. Comparisons with analytic instability-based transport models show systematic overpredictions of inflow unless extreme assumptions hold, suggesting the cosmic-web streams play a central role in shaping gas transport and disk evolution at these epochs. The study highlights the need for tracer-enabled simulations to robustly identify inflowing streams and motivates observational modeling with stream-aware mocks to interpret high-redshift disk kinematics.

Abstract

We study the radial transport of cold gas within simulated disk galaxies at cosmic noon, aiming at distinguishing between disk instability and accretion along cold streams from the cosmic web as its driving mechanism. Disks are selected based on kinematics and flattening from the VELA zoom-in hydro-cosmological simulations. The radial velocity fields in the disks are mapped, their averages are computed as a function of radius and over the whole disk, and the radial mass flux in each disk as a function of radius is obtained. The transport directly associated with fresh incoming streams is identified by selecting cold gas cells that are either on incoming streamlines or have low metallicity. The radial velocity fields in VELA disks are found to be highly non-axisymmetric, showing both inflows and outflows. However, in most cases, the average radial velocities, both as a function of radius and over the whole disk, are directed inwards, with the disk-averaged radial velocities typically amounting to a few percent of the disk-averaged rotational velocities. This is significantly lower than the expectations from various models that analytically predict the inward mass transport as driven by torques associated with disk instability. Under certain simplifying assumptions, the latter typically predict average inflows of more than $10\%$ of the rotational velocities. Analyzing the radial motions of streams and off-stream material, we find that the radial inflow in VELA disks is dominated by the stream inflows themselves, especially in the outer disks. The high inward radial velocities inferred in observed disks at cosmic noon, at the level of $\sim \! 20\%$ of the rotational velocities, may reflect inflowing streams from the cosmic web rather than being generated by disk instability.

Figures (23)

• Figure 1: Disk Selection: The ratio of the disk-averaged rotational velocity to the disk-averaged radial velocity dispersion, $\langle V_{\rm rot} \rangle/\langle \sigma_{r} \rangle$, is plotted against the ratio of the disk radius to the disk height, $R_{\rm d}/H_{\rm d}$, for all 1098 snapshots belonging to 34 VELA galaxies with the blue, round points. Of these, 425 snapshots satisfy the conditions $R_{\rm d}/H_{\rm d}>3$ and $V_{\rm rot}/\sigma_{r}>1$ at all $r$, the distance from the galactic center in cylindrical coordinates, and are selected as rotation-supported disks, highlighted by the red stars. Here, $V_{\rm rot}$ and $\sigma_{r}$ are the average rotational velocity and the radial velocity dispersion of cold gas as a function of $r$, respectively. The gray, dashed vertical and horizontal lines indicate $R_{{\rm d}}/H_{{\rm d}}=3$ and $\langle V_{\rm rot} \rangle/\langle \sigma_{r} \rangle=1$, respectively. Some of the blue points with $R_{\rm d}/H_{\rm d}>3$ and $\langle V_{\rm rot} \rangle/\langle \sigma_{r}\rangle>1$ do not satisfy the condition $V_{\rm rot}/\sigma_{r}>1$ at all $r$. As such, they are not in the disk galaxy sample.
• Figure 2: Radial Transport in a VELA Disk: For the simulated galaxy VELA 7 at a redshift of $z=1.5$, the top left- and right-hand panels show face-on maps of the cold gas surface density ($\Sigma$) and the two-dimensional radial velocity ($v_{r,\rm 2D}$), respectively, within a region spanning $3\ R_{\rm d} \times 3\ R_{\rm d}$, centered on the galactic center. The disk is large ($R_{\rm d}=18.43\ \rm kpc$) and thin ($R_{\rm d}/H_{\rm d} \sim 9$). It is mainly supported by rotation, with $\langle V_{\rm rot} \rangle/\langle \sigma_{r} \rangle \sim 3$, and has a cold gas fraction of $f=0.18$. Regions with no cold gas are shown as white in the surface density map and black in the radial velocity map. Dashed red circles mark the radial extent of the disk. Both maps exhibit strong non-axisymmetry, largely due to incoming cold gas streams. The middle left- and right-hand panels display the average radial velocity ($V_r$, in units of $V_{\rm rot}$) and the radial mass flux ($F_r$, in units of $\mathscr{M}/t_{\rm dyn}$), respectively, as functions of $r$, normalized by $R_{\rm d}$, out to the disk radius. Here, $V_{\rm rot}$ and $\mathscr{M}$ are the average rotational velocity and mass of all cold gas at $r$, and $t_{\rm dyn}=r/V_{\rm rot}$ is the local dynamical time. The non-normalized $V_r$ and $F_r$ are shown in the bottom left- and right-hand panels, respectively. Beyond roughly $0.2\ R_{\rm d}$, both $V_r$ and $V_{\rm rot}$ are negative and tend to grow in magnitude with increasing $r$, albeit with some fluctuations, indicating a predominantly inflow-driven cold gas radial transport within the disk. The disk-averaged radial velocity is also negative and has a magnitude about $8 \%$ of the disk-averaged rotational velocity.
• Figure 3: Trends with Distance from the Galactic Center: The left- and right-hand panels show, respectively, the medians (solid curves) and the $16^{\rm th}$–$84^{\rm th}$ percentile ranges (shaded regions) for the average radial velocity ( $V_r$, in units of $V_{\rm rot}$), and the radial mass flux ($F_r$, in units of $\mathscr{M}/t_{\rm dyn}$), as functions of $r$ (in units of $R_{\rm d}$), derived from our sample of disk galaxies. The median values of both $V_r$ and $F_r$ are consistently negative and become more negative with increasing $r$, except within approximately $\sim \! 0.2\ R_{\rm d}$, where the trend reverses. Although the $84^{\rm th}$ percentile values of both $F_r$ and $V_r$ are positive at all radii, the magnitudes at the $16^{\rm th}$ percentile are substantially larger. Therefore, statistically speaking, the radial transport of cold gas across our sample is dominated by inflows at all radii.
• Figure 4: Trends with Cold Gas Fraction and Redshift: The left- and right-hand panels show the ratio of the disk-averaged radial velocity to the disk-averaged rotational velocity, $\langle V_r \rangle/\langle V_{\rm rot} \rangle$, plotted as a function of the cold gas fraction ($f$) and redshift ($z$), respectively, using blue circles. Red squares represent the median values within bins of $f$ and $z$ (chosen to ensure adequate snapshot counts), while the vertical error bars correspond to the $16^{\rm th}$–$84^{\rm th}$ percentile ranges. The horizontal error bars indicate the bin widths, and the squares are placed at the mean values of $f$ and $z$ for each bin. For the majority of VELA disks ($82 \%$), $\langle V_r \rangle$ is negative (note that $\langle V_{\rm rot} \rangle$ is strictly positive), underscoring the dominance of cold gas radial inflows over outflows in our disk galaxy sample. On average, $\langle V_r \rangle/\langle V_{\rm rot} \rangle$ becomes more negative with increasing $f$ and $z$, suggesting that galaxies with higher cold gas content or located at higher redshifts typically exhibit stronger radial inflows. Although both correlations are relatively weak, the trend with cold gas fraction is stronger than that with redshift, as reflected in the Spearman rank correlation coefficients of $-0.26$ and $-0.13$, respectively.
• Figure 5: Comparison of Disk Instability Based Radial Transport Models with Simulations: $\langle V_r \rangle/\langle V_{\rm rot} \rangle$ versus $\langle V_{\rm rot} \rangle/\langle \sigma_r \rangle$ is plotted for each snapshot in our disk galaxy sample, using blue circles. Red squares represent the medians within bins of $\langle V_{\rm rot} \rangle/\langle \sigma_r \rangle$ of varying widths, selected to ensure a reasonable number of snapshots per bin. Vertical error bars indicate the $16^{\rm th}$–$84^{\rm th}$ percentile ranges, and horizontal error bars show the bin widths. The red squares are positioned at the average $\langle V_{\rm rot} \rangle/\langle \sigma_r \rangle$ for each bin. Overlaid curves correspond to predictions from disk-instability-based models of radial inflow, as labeled. The left-hand panel adopts $Q = 0.68$goldreich65, and the right-hand panel uses $Q = 1$ (marginal instability for thin disks). The dekel20 model describes inflow via ring migration, while the dekel09 and dekel13 models focus on clump migration. The krumholz10 and krumholz18 models consider global gas transport in viscous disks and are likely the most applicable for comparison with the analysis of the VELA disks presented in this paper. It is worth noting that for most VELA disks, these two models tend to overestimate the disk-averaged radial inflow, regardless of whether $Q = 0.68$ or $Q = 1$ is assumed. Furthermore, all five models consistently predict radial inflows, whereas about $18 \%$ of the VELA disks show disk-averaged outflows.