Documentation

1. Fifth-nearest neighbour surface density estimates

With the newly released data from the cluster regions, the SAMI DR3 data span the full range of environmental densities, from the dense cores of rich clusters to the lower density field regions. To quantify the environmental density for SAMI targets, we make available the fifth-nearest neighbour surface density estimates (DensityCatDR3), which are determined in a similar manner to that outlined in Brough et al. (2013) and Brough et al. (2017), and described in Croom et al. (2020). The estimates are determined using a density-defining population drawn from the parent redshift surveys used to select the SAMI targets (for the GAMA regions, redshifts come from Driver et al. 2011 and Liske et al. 2015, while for the clusters redshifts come from Owers et al. 2017). The surface density is estimated as

$\Sigma_5 = 5/(\pi d_n^2$)

where $d_n$ is the projected comoving distance to the 5th nearest galaxy in the density-defining population that is within 1000 km/s of the SAMI target redshift. The estimates are corrected for the spectroscopic completeness of the parent survey, and the multiplicative value used to correct the raw $\Sigma_5$ values is given by the "CompletenessCorrection" parameter. An area correction ("AreaCorrection") is also determined for SAMI targets where $d_n$ is larger than the distance to the nearest survey edge.

The "SurfaceDensity_err" parameter gives the uncertainty estimate for the $\Sigma_5$ values, and is determined as

$\Sigma_{5, err} = {\rm max}(|\Sigma_5 - \Sigma_4|, |\Sigma_5 -􀀀\Sigma_6|)$, where $\Sigma_4$ and $\Sigma_6$ are the surface densities determined to the 4th and 6th nearest neighbours, respectively.

1.1 Usage and flags

For both the cluster and GAMA regions, two values for the fifth nearest neighbour surface densities are estimated around each galaxy in the input catalogue: the primary measurement, labelled "SurfaceDensity", is determined using a density-defining population with absolute $r$-band magnitudes $M_r < -18.6$, while a secondary measurement ("SurfaceDensity_m19"; corresponding parameters have the "_m19" tag) is determined using a density-defining population with a brighter absolute magnitude limit $M_r < -19$ . The SurfaceDensity estimates are most useful for the primary SAMI targets (see Target Selection) since, at the depth of the GAMA redshift survey ($r=19.8$), the density-defining population is volume-limited to $z=0.103$, which is just beyond the redshift limit of the primary targets. The SurfaceDensity_m19 are useful for the secondary SAMI targets, which have a maximum redshift of $z=0.115$, and so require the brighter absolute magnitude limit in order that the density-defining population is volume-limited to this redshift. The SurfaceDensity_m19 therefore allow a comparable surface density estimate for the secondary and primary targets, albeit with slightly lower precision due to the smaller number density of the brighter targets.

Each estimate is accompanied by a flag ("SurfaceDensityFlag" and "SurfaceDensityFlag_m19") in the range 0-9, which gives an indication of problems that may have arisen in determining the surface density measurement. The flag meanings are listed in the table below. It is recommended that only SurfaceDensity estimates where SurfaceDensityFlag <=1 are used.