Galaxy-halo internal alignments across cosmic time

TL;DR

This paper investigates how the internal alignment between the stellar and dark matter components of central galaxies evolves across cosmic time and why blue disk-dominated centrals show weak galaxy–halo alignment signals. Using the IllustrisTNG300-1 simulation, the authors measure principal axes and angular momenta within $2\times R_{\rm h}$ and track major mergers via the SUBLINK trees along the main progenitor branch from $z=20$ to $z=0$, with a present-day sample defined by $\mathrm{M_r}< -21.5$ (and tests with $\mathrm{M_r}< -19.5$). They find that centrals with $\mathrm{M_{Tot}} > 10^{13}\,M_\odot$ align with the inner halo shape largely independent of color or merger history, while lower-mass red centrals and merger-rich systems show stronger evolution toward alignment; blue centrals are more influenced by the angular-momentum coupling between stars and dark matter, leading to a range of alignment outcomes. These results imply a multiscale origin of intrinsic alignments and help explain why blue centrals contribute weakly to large-scale anisotropy signals, with implications for interpreting weak-lensing systematics and guiding future observational campaigns such as Euclid and DESI.

Abstract

Investigations of intrinsic alignments suggest a link between the alignment of a central galaxy's major axis with the galaxy distribution and its internal galaxy-halo shape alignment. In contrast, blue central galaxies typically exhibit almost no alignment signal, owing to a stronger internal misalignment with their halo. We investigated how the internal alignment between the principal axes of the stellar and dark matter components evolves over time as a function of the total mass of central galaxies at z=0. In particular, we aim to understand why disk-dominated, blue central galaxies often show weak or absent alignment signals with the galaxy distribution in their group and in the larger-scale cosmic structure. We used data from the IllustrisTNG300-1 run and selected a sample of bright central galaxies at z=0. We computed the principal axes of the stellar and dark matter components, along with their angular momenta, to obtain the various alignment angles analyzed in this study. Also, we used the merger trees to determine the number of major mergers between z=20 and z=0, and to track their shapes along their main branch. We examined secondary dependencies of the galaxy-halo alignment on properties such as color and merger history. We analyzed how shape alignments relate to the dynamical coupling between the angular momentum directions of the stellar and dark matter components. The results show that massive centrals tend to align with the shape of their inner halo, and they are typically red and have undergone numerous mergers. Lower-mass red centrals, and those that have experienced many mergers exhibit the strongest evolution toward alignment. Blue centrals, in contrast, are more strongly influenced by the link between the stellar and dark matter angular momenta, such that they evolve toward either alignment or misalignment with both the shape and angular momentum of the inner halo.

Figures (10)

• Figure 1: Average misalignment angles $\theta_a$ (upper panel) and $\theta_c$ (lower panel) of the stellar and dark matter major and minor axes respectively calculated at redshifts $z=0,0.8,0.17,0.3,0.42,0.6,1.07,1.5,2$, as a function of the total mass (i.e., the sum of the stellar, gas and halo masses) at $z=0$. Lighter colors correspond to higher redshifts. The grey dotted line indicates an average angle of $60^{\circ}$, corresponding to random orientation, smaller angles indicate a tendency toward alignment. Error bars are calculated using the standard deviation of the mean.
• Figure 2: Evolution of the average misalignment angles $\theta_a$ (upper panels) and $\theta_c$ (lower panels) of the stellar and dark matter major and minor axes respectively, as a function of total mass. Left panels show the evolution of the alignments of BGGs that are classified as red at $z=0$, i.e $(g-r)_{z=0}>0.6$, while the right panels show the evolution of blue BGGs i.e $(g-r)_{z=0}<0.6$.
• Figure 3: Left, middle, and right panels illustrate the histogram of the number of major mergers from $z=15$ to $z=0$, total mass and color, respectively, for the entire sample of BGGs shown in grey. In light blue we select the galaxies that had 1 or 0 major mergers and in pink those that had 5 or more major mergers.
• Figure 4: Evolution of the average misalignment angles $\theta_a$ (upper panels) and $\theta_c$ (lower panels) between the stellar and dark matter major and minor axes respectively, as a function of total mass. Left panels show the evolution of the alignments for BGGs that have experienced many mergers ($\mathrm{N}_\mathrm{Merg} \ge 5$) from $z=15$ to $z=0$, while the right panels show the evolution of BGGs with few mergers ($\mathrm{N}_\mathrm{Merg} \le 1$).
• Figure 5: Evolution of the average misalignment angles $\theta_a$ (upper panels) and $\theta_c$ (lower panels) between the stellar and dark matter major and minor axes respectively, as a function of total mass. Left panels show the evolution of the alignments for red BGGs that have experienced many mergers ($\mathrm{N}_\mathrm{Merg} \ge 5$) from $z=15$ to $z=0$, while the right panels show the evolution of red BGGs with few mergers ($\mathrm{N}_\mathrm{Merg} \le 1$).