Galaxy and mass assembly (gama): Galaxy radial alignments in gama groups

Michael D. Schneider*, Shaun Cole, Carlos S. Frenk, Lee Kelvin, Rachel Mandelbaum, Peder Norberg, Joss Bland-Hawthorn, Sarah Brough, Simon Driver, Andrew Hopkins, Jochen Liske, Jon Loveday, Aaron Robotham

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)


We constrain the distributions of projected radial alignment angles of satellite galaxy shapes within the Galaxy And Mass Assembly survey group catalogue. We identify the galaxy groups using spectroscopic redshifts and measure galaxy projected ellipticities from Sloan Digital Sky Survey imaging. With a sample of 3850 groups with 13 655 satellite galaxies with high quality shape measurements, we find a less than 2σ signal of radial alignments in the mean projected ellipticity components and the projected position angle when using galaxy shape estimates optimized for weak lensing measurements. Our radial alignment measurement increases to greater than 3σ significance relative to the expectation for no alignments if we use 2D Śersic model fits to define galaxy orientations. Our weak measurement of radial alignments is in conflict with predictions from dark-matter N-body simulations, which we interpret as evidence for large misalignments of baryons and dark matter in group and cluster satellites. Within our uncertainties, that are dominated by our small sample size, we find only weak and marginally significant trends of the radial alignment angle distributions on projected distance from the group centre, host halo mass, and redshift that could be consistent with a tidal torquing mechanism for radial alignments. Using our lensing optimized shape estimators, we estimate that intrinsic alignments of galaxy group members may contribute a systematic error to the mean differential projected surface mass density of groups inferred from weak lensing observations by -1 ± 20 per cent at scales around 300 h-1 kpc from the group centre assuming a photometric redshift rms error of 10 per cent, and given our group sample with median redshift of 0.17 and median virial masses ~1013 h-1

Original languageEnglish
Article numberstt855
Pages (from-to)2727-2738
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - Aug 2013
Externally publishedYes


  • Galaxies: Clusters: General
  • Galaxies: Formation
  • Galaxies: Statistics

Fingerprint Dive into the research topics of 'Galaxy and mass assembly (gama): Galaxy radial alignments in gama groups'. Together they form a unique fingerprint.

Cite this