A neighbor Search Code for Galaxies - Preliminary Results

TL;DR

This paper introduces a neighbor search code applied to SDSS galaxy data to analyze galaxy environments, morphology, activity, and star formation history, using a novel 3D-distance counting method.

Contribution

The paper presents a new neighbor search algorithm that combines spectroscopic and photometric data to estimate galaxy densities and their relation to galaxy properties.

Findings

N correlates with galaxy morphology and activity type

N relates to star formation history of galaxies

Preliminary results show the effectiveness of the neighbor search method

Abstract

We present preliminary results of applying a neighbour search code to a large sample of galaxies drawn from the Sloan Digitial Sky Survey (SDSS). We draw our sample of target galaxies from the spectroscopic catalogue of the SDSS, which has redshift measurements for all its $\sim8\times10^5$ galaxies. We count the neighbours from the photometric catalogue of the SDSS, which is a flux-limited catalogue that has a photometric redshift estimation for all its $\sim2\times10^8$ galaxies. The code counts the number (N) of galaxies within a given 3D-distance from the target galaxy, using a database manager and a simple decision tree. The 3D-distance is composed of a 2D projected distance on the sky and a radial component, which is the difference between the spectroscopic target redshift and the photometric redshift of the "neighbour" galaxies. As we keep the search volume constant throughout the sample, N is a proxy for the number density of galaxies around the target galaxies. We also present an analysis of the preliminary results, showing the relation between N, the inferred morphology and the dominant activity type of the target galaxies, as well as the relation between N and the star formation history of the target galaxies.