Active Galaxies in the Sloan Digital Sky Survey II: galaxy and activity evolution

TL;DR

This study examines the evolution of galaxy morphology and nuclear activity over the past 2 billion years using SDSS data, revealing that less luminous galaxies show more significant evolutionary changes in activity and structure.

Contribution

It provides new insights into how galaxy properties and activity levels evolve with time, emphasizing the role of intrinsic potential wells and secular processes in shaping galaxy evolution.

Findings

Radio power and emission-line strength evolve with time, especially in less luminous galaxies.

Star-formation becomes more centrally concentrated in more evolved galaxies.

Environment has a lesser impact compared to intrinsic properties on galaxy activity.

Abstract

In this second paper of a series of papers based on the FIRST and SDSS surveys we investigate the evolution of galaxy morphology and nuclear activity in the look-back time of the SDSS (~2 Gyrs) for a sample of ~150000 galaxies in the local universe. We demonstrate an evolution in the strength of the radio power and the spectroscopic emission-lines typical of AGN, as well as in the morphology of their hosts. Such evolution appears more substantial for less luminous systems, and is possibly the low-redshift tail of the downsizing in star-formation, AGN activity and supermassive black hole build-up observed in higher redshift surveys.This suggests that the differences in intrinsic properties of galaxies along the Hubble Sequence may arise from the difference in the depth of their potential wells which leads to different evolutionary paths because of different timescales for gas infall. This primordial infall and the following secular evolution mediated by bar and density wave instabilities may bring galaxies of different mass to have the different activity levels and morphological features in the local universe shown in this study. In agreement with such a hypothesis, we find that star-formation as traced by radio emission is progressively more centrally concentrated in more evolved star-forming galaxies and we show that the environment in which a galaxy resides plays a lesser role in shaping the features and activity for the majority of systems.