MAGNUS II: Rotational support of massive early-type galaxies decreased over the past 7 billion years

TL;DR

The study tests whether massive early-type galaxies lose rotational support from intermediate redshift to the present by comparing PSF-corrected $\lambda_R$ in 200 MAGNUS ETGs at $0.25<z<0.75$ with 787 local MaNGA ETGs ($z\le0.05$). Using a JAM-based PSF correction and a 2$R_e^{\rm maj}$ aperture, the authors find a higher median $\lambda_R^{\rm corr}$ at intermediate redshift ($0.48\pm0.05$) than locally ($0.34\pm0.03$), with a combined evolution slope of $d\lambda_R^{\rm corr}/dz = 0.30\pm0.04$ and a pronounced decline for $\log M_* /M_\odot > 11.3$. The results robustly support a spin-down of massive ETGs over the last ~7 Gyr, consistent with dry-merger–driven angular momentum loss and hierarchical growth in simulations. The findings underscore the role of mass in shaping kinematic evolution and emphasize the importance of PSF-corrected, spatially resolved spectroscopy for tracing dynamical histories across cosmic time.

Abstract

Understanding how the internal kinematics of massive galaxies evolve is key to constraining the physical processes that drive their assembly. We investigate the evolution of rotational support in massive ($\log M_{\ast}/M_{\odot} \geq 10.6$) early-type galaxies (ETGs) over the past $\sim$7 Gyr. We use MUSE integral-field spectroscopic (IFS) data for 212 ETGs at intermediate redshift ($0.25 < z < 0.75$) from the MAGNUS sample. We compare their kinematics to a carefully matched local sample of 787 ETGs ($z \leq 0.05$) from the MaNGA survey. Using the specific stellar angular momentum proxy, $λ_R$, we quantify the balance between ordered rotation and random motions. We derive intrinsic $λ_R$ values by applying a uniform correction for seeing and point-spread function (PSF) effects to both samples. We find a significant evolutionary trend: the intermediate-redshift ETGs are systematically more rotationally supported than their local counterparts. The median PSF-corrected $λ_R$ for the MAGNUS sample is $0.48 \pm 0.05$, substantially higher than the median of $0.34 \pm 0.03$ for the matched MaNGA sample. This corresponds to a positive slope in the $λ_R-z$ relation of $\mathrm{d} λ_R / \mathrm{d} z = 0.3 \pm 0.04$ for the combined sample. The decline in rotational support is most pronounced for the most massive galaxies ($\log M_{\ast}/M_{\odot} > 11.3$). Our results provide robust evidence that massive ETGs have undergone significant kinematic evolution, losing angular momentum as they evolve towards the present day, consistent with theoretical models where processes such as dry mergers play a crucial role in shaping the dynamical state of galaxies.

Figures (7)

• Figure 1: Distribution in stellar mass (left) and stellar velocity dispersion (right) of the MaNGA ETG sample (dashed line) and MAGNUS sample (solid line). A mass cut of log $M_{\ast} (M_{\odot}) \geq 10.6$ and a redshift cut of $z \leq 0.05$ were applied to select the MaNGA ETG sample. For both samples, the stellar mass was measured from SPS fitting using pPXF with templates from the FSPS model. The p-values of the corresponding K-S tests are shown in the title of the respective plots.
• Figure 2: Sérsic model fitting of a MAGNUS galaxy (MACS1149_60, z=0.533) based on HST F814W image, using GALIGHT. The panels from left to right are as follows: (1) observed data, (2) best-fit model, (3) residuals divided by the variance, and (4) top-1D surface brightness profiles of data (open circles) and best-fit model (blue line); bottom - the corresponding residual. Note that the 1D surface brightness profiles are only for illustration purposes. The actual fitting is based on the 2D images.
• Figure 3: Correction of the observed angular momentum parameter $\lambda_R^{\rm obs}$ for both samples. The ratio of observed to corrected $\lambda_{R}$ is plotted as a function of the galaxy Sérsic index, n, and colored by the 'resolution parameter' r $=\frac{\sigma_{\rm PSF}}{R^{\rm maj}}$, which quantifies how well resolved a galaxy is. For the MaNGA sample (right pointing triangle) $R^{\rm maj} =$$R_{\rm e}^{\rm maj}$ and for the MAGNUS galaxies (square) this is $R^{\rm maj} \approx$ 2$R_{\rm e}^{\rm maj}$. Solid lines show the effect of the Sérsic index on the corrections for a fixed value of r.
• Figure 4: Comparison of observed and PSF-corrected $\lambda_{R}$ values for the MAGNUS (top row) and MaNGA sample (bottom row). From left to right, the values are colored with ellipticity, normalized semi-major axis, $R_{\rm e}^{\rm maj}$, stellar mass, $M_{\ast}$, and integrated velocity dispersion, $\sigma_e$. The black dashed line is the one-to-one line.
• Figure 5: PSF-corrected $\lambda_{R}$ of both samples with redshift. The MAGNUS galaxies are marked using black circles, while the MaNGA sample is marked with black right-facing triangles. The solid lines show the best-fit line -- the blue one is from a linear fit with the MAGNUS sample alone, while the red one is from the combined sample. The dashed lines show the corresponding 1$\sigma$ uncertainty of the best-fit line. The parameters from the best-fit lines and associated intrinsic scatters are also reported using the respective color of the lines.