Head-Tail Galaxies: Beacons of High-Density Regions in Clusters

TL;DR

This study identifies five head-tail galaxies in the Horologium-Reticulum Supercluster, demonstrating their association with extremely high-density regions and suggesting high environmental density causes their characteristic morphology.

Contribution

It provides new evidence linking head-tail galaxy morphology to high-density cluster environments using radio, optical, and X-ray data.

Findings

All five HT galaxies are in regions with >100 galaxies/Mpc^3 density.

HT galaxies are in denser environments than non-HT galaxies.

High densities likely cause ram pressure bending of HT galaxies.

Abstract

Using radio data at 1.4 GHz from the ATCA we identify five head-tail (HT) galaxies in the central region of the Horologium-Reticulum Supercluster (HRS). Physical parameters of the HT galaxies were determined along with substructure in the HRS to probe the relationship between environment and radio properties. Using a density enhancement technique applied to 582 spectroscopic measurements in the 2 degree x 2 degree region about A3125/A3128, we find all five HT galaxies reside in regions of extremely high density (>100 galaxies/Mpc^3). In fact, the environments surrounding HT galaxies are statistically denser than those environments surrounding non-HT galaxies and among the densest environments in a cluster. Additionally, the HT galaxies are found in regions of enhanced X-ray emission and we show that the enhanced density continues out to substructure groups of 10 members. We propose that it is the high densities that allow ram pressure to bend the HT galaxies as opposed to previously proposed mechanisms relying on exceptionally high peculiar velocities.