Intrinsic alignments of SDSS-III BOSS LOWZ sample galaxies

TL;DR

This study measures intrinsic alignments (IA) of SDSS-III BOSS LOWZ galaxies to 200 $h^{-1}$Mpc and demonstrates that IA amplitudes scale with luminosity and halo mass via power laws, with no strong redshift or color evolution in the examined red sequence sample. The authors employ a linear alignment framework augmented by a halo-model 1-halo term, calibrated against galaxy-galaxy lensing to obtain halo masses, and use a Landy-Szalay-like estimator to derive projected correlations. They find that large-scale IA is best predicted by luminosity or mass, while small-scale IA correlates with the shape bias; environmental effects reveal central galaxies align with halos and tidal fields, whereas satellites show radial alignments within groups. These results yield flexible IA templates and mass- or luminosity-based priors that can improve weak lensing analyses in current and future surveys. Overall, the work provides a detailed, multi-faceted view of IA dependencies across scale, mass, luminosity, and environment, with implications for mitigating IA foregrounds in cosmic shear studies.

Abstract

Intrinsic alignments (IA) of galaxies, i.e. correlations of galaxy shapes with each other or with the density field, are a major astrophysical source of contamination for weak lensing surveys. We present the results of IA measurements of galaxies on 0.1- 200 Mpc/h scales using the SDSS-III BOSS LOWZ sample, in the redshift range 0.16<z<0.36. We extend the existing IA measurements for spectroscopic LRGs to lower luminosities, and show that the luminosity dependence of large-scale IA can be well-described by a power law. Within the limited redshift and color range of our sample, we observe no significant redshift or color dependence of IA. We measure the halo mass of LOWZ galaxies using galaxy-galaxy lensing, and show that the mass dependence of large-scale IA is also well described by a power law. We detect variation in the scale dependence of IA with mass and luminosity, which underscores the need to use flexible templates in order to remove the IA signal. We also study the environment dependence of IA by splitting the sample into field and group galaxies, which are further split into satellite and central galaxies. We show that group central galaxies are aligned with their halos at small scales and also are aligned with the tidal fields out to large scales. We also detect the radial alignments of satellite galaxies within groups, which results in a null detection of large-scale intrinsic alignments for satellites. These results can be used to construct better intrinsic alignment models for removal of this contaminant to the weak lensing signal.

Figures (22)

• Figure 1: Redshift distribution of LOWZ galaxies and our group sub-sample. Vertical lines mark the boundary of the redshift range that we use in this paper, $z=[0.16,0.36]$.
• Figure 2: Average $k+e$ corrected $r$-band absolute magnitude, $\langle M_r \rangle$, for the LOWZ sample as function of redshift.
• Figure 3: The stacked distribution of satellite galaxies in our group sample, with respect to BGGs in the groups. The apparent sharp boundary at $r_p=0.8h^{-1}\text{Mpc}$ is due to the size of our CiC cylinders and the fact that our sample is dominated by two-galaxy (single satellite) groups. The strong peak at $\Pi=0$ and $r_p<0.3h^{-1}$Mpc arises primarily from the fiber collision corrected pairs.
• Figure 4:
• Figure 5: $w_{g+}$ for the full LOWZ sample (same data as Fig. \ref{['fig:lowz_w']}). Here we show the fits combining the halo model fitting function and the NLA model. The blue line is the joint fit ($0.3<r_p<65h^{-1}\text{Mpc}$) with an additional free parameter ($q_{21}$) in the halo model fitting function. The green line shows the combination (not a joint fit) of a smoothed NLA model with $k_{\text{smooth}}=1h/$Mpc and our usual halo model fitting function with best-fit parameters from Table \ref{['tab:haloparams']} and Table \ref{['tab:samples']}.