Astronomers Find First Evidence Of Intrinsic Alignment Of Red Galaxies (Cosmology)

By cross-correlating the shape sample selected from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) data with the galaxy density sample from the Subaru FastSound survey, Korean astronomers reported on the first evidence of intrinsic alignment (IA) of red galaxies at redshift, z > 1. Their study recently appeared in Arxiv.

Intrinsic alignment (IA) is a coherent alignment of galaxy orientations with the surrounding large-scale structure caused by the local gravitational interaction. It has been considered as one of the main contaminants of weak lensing surveys, where the source galaxies are assumed to be randomly oriented. IA also contains useful information about galaxy formation and evolution. Galaxies at higher redshifts could be either more strongly or weakly aligned because of shorter time elapsed for internal and external interactions, e.g., structure formations and mergers, to boost or suppress IAs. Therefore, measuring the IA at various redshifts will help constrain dynamical aspects of galaxy evolution models. Furthermore, due to the fact that galaxy shapes are linearly related the gravitational potential on cosmological scales, there is a growing interest of using IA as a new tool to constrain cosmological models.

Figure 1. Projected correlation functions, wg+ (top) and wg× (bottom) as a function of transverse separation ¯rp. They used the FastSound sample for the galaxy density field for all the measurements presented here. They used the different shape samples as the galaxy shape field for different lines: red galaxies (red lines) and blue galaxies (blue solid lines). The blue dashed line is the GI correlation of the FastSound blue galaxies measured by Tonegawa et al. The dark and light red shaded regions in the top panel indicate the 68% and 95% confidence intervals of the best-fitting NLA model obtained for red galaxies at ¯rp > 3 h¯1 Mpc denoted by the vertical lines. © Tonegawa and Okumura

Now, inspired by these, Tonegawa and Okumura reported on the first evidence of IA of red galaxies at z > 1.

They first measured the gravitational shear-intrinsic ellipticity (GI) cross-correlation function at z ∼ 1.3 using galaxy positions from the FastSound spectroscopic survey and galaxy shapes from Canada-Hawaii-France telescope lensing survey data.

Finally, by adopting the non-linear alignment model, they obtained a 2.4σ level detection of the IA amplitude, ALA = 27.48, larger than the value extrapolated from the constraints obtained at lower redshifts.

“Our measured IA is translated into a ∼ 20% contamination to the weak lensing power spectrum for the red galaxies. This marginal detection of IA for red galaxies at z > 1 motivates the continuing investigation of the nature of IA for weak lensing studies. Furthermore, our result provides the first step to utilize IA measurements in future high-z surveys as a cosmological probe, complementary to galaxy clustering and lensing.”, they conclude.

References: (1) Motonari Tonegawa, Teppei Okumura, “First Evidence of Intrinsic Alignments of Red Galaxies at z>1: Cross-correlation between CFHTLenS and FastSound Samples”, Arxiv, pp. 1-7, 2021. (2)

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