Galaxy Spin Alignment with Tidal Fields in the SDSS-IV MaNGA Survey

TL;DR

This paper tests the tidal torque theory by measuring three-dimensional spin alignments of disk galaxies with a locally reconstructed tidal field. Using the complete MaNGA data set and a $2\,h^{-1}{\rm Mpc}$ smoothed tidal field from the GEMA-VAC/ELUCID framework, the authors quantify $|\hat{\boldsymbol{j}}\cdot\hat{\boldsymbol{e}}_k|$ relative to the tidal eigenaxes $\hat{\boldsymbol{e}}_1$, $\hat{\boldsymbol{e}}_2$, and $\hat{\boldsymbol{e}}_3$ for $k=1,2,3$ across $M_*$ bins and environments. They find a mass-dependent trend: massive galaxies prefer spins perpendicular to $\hat{\boldsymbol{e}}_1$ and mildly parallel to $\hat{\boldsymbol{e}}_2$ (with detections up to $4.6\sigma$ and $2.5\sigma$), while low-mass galaxies show the opposite orientation, with a transition around $M_* \sim 10^{10}-10^{10.5}M_\odot$; gas spins mirror the stellar patterns. The alignment strengthens in regions of high tidal anisotropy $q$ and overdensity $\delta$, reaching up to $5.4\sigma$ for $\hat{\boldsymbol{e}}_1$, and a mutual information analysis singles out $q$ and $\delta$ as the principal environmental drivers. These results provide empirical support for a connection between galaxy spins and the cosmic tidal field, while also highlighting the potential roles of baryonic physics and scale dependence in shaping the observed alignments.

Abstract

The tidal torque theory (TTT) predicts that galaxy spins are correlated with the surrounding tidal field, reflecting how angular momentum is acquired during structure formation. We present a new observational test of this prediction using the final data release of the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory integral field spectroscopy survey, which enables direct spin measurements from stellar and ionized gas kinematics for a sample of 6325 disk galaxies. We utilize the three-dimensional tidal field reconstructed from the galaxy distribution, providing a physically defined reference frame for the analysis. We find that massive galaxies tend to align their spins parallel to the intermediate axis of the tidal field, consistent with the prediction of the TTT, while also showing a tendency to align perpendicular to the major axis. In contrast, low-mass galaxies exhibit the opposite trend, with a transition mass of $M_* \sim 10^{10}-10^{10.5}M_\odot$. No significant alignment is detected with respect to the minor axis across all stellar masses. We further examine the dependence on morphology and environment, finding that S0 and early-type spiral galaxies exhibit stronger alignment signals than late-type spirals. The alignment trend becomes particularly pronounced in regions of high tidal anisotropy and high overdensity. A mutual information analysis identifies these environmental factors as the dominant drivers of the observed trends. Our results provide new empirical evidence for the connection between galaxy spins and the cosmic tidal field.

Figures (7)

• Figure 1: Top: mean absolute values of the cosines of the angles between stellar spin vectors and the three principal axes of the local tidal tensor as a function of stellar mass. From left to right, the panels correspond to the major (green), intermediate (red), and minor (blue) principal axes. In each panel, the sample is divided into bins of equal size. The shaded regions indicate the 1$\sigma$ intervals from 1000 randomly shuffled samples, and the dashed lines mark the expected value for a random orientation. The detection significance for each bin is indicated below each point. Bottom: same as the top row, but for galaxies in regions of strong tidal anisotropy ($q > 1.235$).
• Figure 2: Same as Figure \ref{['fig:alignall']}, but for lenticulars ($-2\leq\mathrm{TType}\leq0$; top), early-type spirals ($1\leq\mathrm{TType}\leq3$; middle), and late-type spirals ($4\leq\mathrm{TType}\leq8$; bottom) in high-$q$ regions.
• Figure 3: Same as Figure \ref{['fig:alignall']}, but for galaxies residing in the sheet (top), filament (middle), and cluster (bottom) environments. Galaxies in void environments are not shown because of their small sample size.
• Figure 4: Same as Figure \ref{['fig:alignall']}, but for galaxies residing in high-$\delta$ (top), intermediate-$\delta$ (middle), and low-$\delta$ (bottom) regions.
• Figure 5: Normalized conditional mutual information $I(|\hat{\boldsymbol{j}} \cdot \hat{\boldsymbol{e}}_k|;Y|Z)$ ($k = 1, 2, 3$) for various combinations of $Y$ and $Z$. Each value is normalized by the mean and standard deviation estimated from 1000 randomly shuffled samples. Darker colors indicate stronger importance of $Y$ in determining spin alignments after controlling for $Z$.