Galaxy and Mass Assembly (GAMA): the GAMA galaxy group catalogue (G3Cv1)

Abstract

Using the complete Galaxy and Mass Assembly I (GAMA‐I) survey covering ∼142 deg2 to rAB= 19.4, of which ∼47 deg2 is to rAB= 19.8, we create the GAMA‐I galaxy group catalogue (G3Cv1), generated using a friends‐of‐friends (FoF) based grouping algorithm. Our algorithm has been tested extensively on one family of mock GAMA lightcones, constructed from Λ cold dark matter N‐body simulations populated with semi‐analytic galaxies. Recovered group properties are robust to the effects of interlopers and are median unbiased in the most important respects. G3Cv1 contains 14 388 galaxy groups (with multiplicity ≥2), including 44 186 galaxies out of a possible 110 192 galaxies, implying ∼40 per cent of all galaxies are assigned to a group. The similarities of the mock group catalogues and G3Cv1 are multiple: global characteristics are in general well recovered. However, we do find a noticeable deficit in the number of high multiplicity groups in GAMA compared to the mocks. Additionally, despite exceptionally good local spatial completeness, G3Cv1 contains significantly fewer compact groups with five or more members, this effect becoming most evident for high multiplicity systems. These two differences are most likely due to limitations in the physics included of the current GAMA lightcone mock. Further studies using a variety of galaxy formation models are required to confirm their exact origin. The G3Cv1 catalogue will be made publicly available as and when the relevant GAMA redshifts are made available at http://www.gama-survey.org.

The masses of satellites in GAMA galaxy groups from 100 square degrees of KiDS weak lensing data

“…The spectroscopic group-satellite catalogue used in this work has a high, but not 100 per cent, purity. For groups with N FoF ≥ 5 the purity approaches 90 per cent; groups with fewer members have significantly lower purity (Robotham et al 2011). Li et al (2013) have shown that a contamination fraction of 10 per cent in the satellite sample would lead to a +15 per cent bias in the inferred satellite masses, well within the reported uncertainties (which amount to up to a factor 2).…”

“…1 the stacked ESD of all 9683 satellites residing in groups with N FoF ≥ 5. We also show the ESD around all galaxies in the GAMA catalogue, which is dominated by (central) field galaxies (Robotham et al 2011). The lensing signal around the two samples is qualitatively different.…”

“…In most cases (∼90 per cent), the iterative central galaxy coincides with the BCG, while the centre of light is more discrepant. Viola et al (2015) performed a detailed analysis of the lensing signal of GAMA groups comparing the different centre definitions and confirm the results of Robotham et al (2011): the BCG and the iterative centre both represent the group centre of mass to a good degree, while the centre of light is a very poor indicator of the group centre. In this work, we use the central-satellite classification that uses the BCG as the central, and therefore measure the lensing signal around all group members except the BCGs.…”

“…3 These are the same groups used by Viola et al (2015). Robotham et al (2011) identified the central galaxy in each group using three definitions of group centre: the weighted centre of light, an iterative method rejecting the galaxy farthest away from the centre of light until one galaxy remained (the 'iterative' centre), and the brightest cluster galaxy (hereafter BCG). All galaxies that are not centrals are classified as satellites.…”

“…Here, we use data over three different regions on the sky, centred at right ascensions 9 h , 12 h and 15 h (the G09, G12 and G15 fields), which overlap with SDSS data. Below we briefly describe the GAMA galaxy group sample constructed by Robotham et al (2011), who discuss the properties, possible systematics and limitations of the catalogue in greater detail. Galaxy photometric properties such as luminosity and stellar mass are measured from the five-band optical SDSS imaging.…”