On the Redshifts of the BL Lac 3FGL J0909.0+2310 and its Close Companion

TL;DR

This study determines the redshift of the BL Lac object 3FGL J0909.0+2310 at 0.432, identifies a close companion galaxy at a similar redshift, and explores its environment using multi-object spectroscopy.

Contribution

First spectroscopic redshift measurement of 3FGL J0909.0+2310 using OSIRIS MOS, revealing a nearby galaxy potentially influencing its activity.

Findings

Redshift of 0.432±0.002 for the BL Lac host galaxy.

Identification of a close companion galaxy at similar redshift.

Discovery of unrelated galaxy groups at different redshifts.

Abstract

We report on the redshift of the BL Lac object 3FGL J0909.0+2310 based on observations obtained with the OSIRIS Multi Object Spectrograph (MOS) mounted on the 10.4-m Gran Telescopio Canarias. A redshift of 0.432+/-0.002 was obtained by the identification of three absorption features (CaII K&H and G-band) detected in the spectrum of the BL Lac host galaxy. The closest object to the BL Lac at an angular separation of 3.8" (~21 kpc at this distance) has a similar redshift of 0.431+/-0.002. This companion galaxy could be the most likely cause of the nuclear activity as postulated by studies based on more extended data sets and cosmological models. MOS allows us to study the object's neighbourhood within a field of view of approximately 7'x2' and we find two small groups of galaxies at redshifts 0.28 and 0.39 which are probably not related to the activity of 3FGL~J0909.0+2310.