Environments of dwarf galaxies with optical AGN characteristics

TL;DR

This study investigates the relationship between dwarf galaxies with optical AGN features and their environments, finding no significant environmental differences between AGN-hosting and non-AGN dwarf galaxies, suggesting environment does not trigger AGN activity.

Contribution

It provides a comprehensive analysis of dwarf galaxy environments with optical AGN features, highlighting the lack of environmental influence and discussing biases in AGN detection and environment measurement.

Findings

No environmental differences between AGN and non-AGN dwarf galaxies.

Environment is not a key factor in triggering AGN activity in dwarfs.

Biases in emission line measurements and environment estimation methods are identified.

Abstract

This study aims to explore the relation between dwarf galaxies ($M_* \leq 5\times10^9 M_\odot$) with AGNs and their environment by comparing neighbourhood parameters of AGN and non-AGN samples. Using the NASA-Sloan Atlas, both the local environment and the immediate environment of dwarf galaxies with $z \leq 0.055$ are analysed. Of the 145,155 galaxies in the catalogue, 62,258 of them are classified as dwarf galaxies, and by employing two AGN selection methods based on emission line fluxes (BPT and WHAN), 4,476 are found to have AGN characteristics in their optical spectra. Regardless of selection method, this study finds no discernible differences in environment between AGN and non-AGN host dwarf galaxies and these results indicate that environment is not an important factor in triggering AGN activity in dwarf galaxies. This is in line with existing literature on environments of regular galaxies with AGNs and suggests universality in terms of reaction to environment across the mass regime. The biases of AGN selection in low-mass galaxies, and the biases of different measures of environment are also considered. It is found that there are several mass-trends in emission line ratios and that the SDSS fiber covers galaxies non-uniformly with redshift. These biases should be accounted for in future work by possibly including other wavelength regimes or mass-weighting of emission line ratios. Lastly, a discussion of the environment estimation methods is included since they may not gauge the desired properties due to factors such as time delay or using loosely constrained proxy parameters.