Recent star formation in nearby 3CR radio-galaxies from UV HST observations

TL;DR

This study uses HST UV and optical imaging to identify recent star formation in nearby 3CR radio galaxies, revealing a strong link between star formation and high-excitation galaxies, likely triggered by recent mergers.

Contribution

It provides the first detailed UV-based analysis linking star formation activity to radio galaxy types and their nuclear activity, emphasizing the role of galaxy mergers in triggering AGN.

Findings

High-excitation galaxies (HEG) show frequent recent star formation.

Low-excitation galaxies (LEG) rarely exhibit recent star formation.

Star formation in HEG is likely triggered by recent major mergers.

Abstract

We analyzed HST images of 31 nearby (z <~ 0.1) 3CR radio-galaxies. We compared their UV and optical images to detect evidence of recent star formation. Six objects were excluded because they are highly nucleated or had very low UV count rates. After subtracting the emission from their nuclei and/or jets, 12 of the remaining 25 objects, presenting an UV/optical colors NUV - r < 5.4, are potential star-forming candidates. Considering the contamination from other AGN-related processes (UV emission lines, nebular continuum, and scattered nuclear light), there are 6 remaining star-forming "blue" galaxies. We then divide the radio galaxies, on the basis of the radio morphology, radio power, and diagnostic optical line ratios, into low and high excitation galaxies, LEG and HEG. While there is no correlation between the FR type (or radio power) and color, the FR type is clearly related to the spectroscopic type. In fact, all HEG (with one possible exception) show morphological evidence of recent star formation in UV compact knots, extended over 5-20 kpc. Conversely, there is only 1 "blue" LEG out of 19, including in this class also FR I galaxies. The picture that emerges, considering color, UV, optical, and dust morphology, is that only in HEG recent star formation is associated with these relatively powerful AGN, which are most likely triggered by a recent, major, wet merger. Conversely, in LEG galaxies the fraction of actively star-forming objects is not enhanced with respect to quiescent galaxies. The AGN activity in these sources can be probably self-sustained by their hot interstellar medium.