Deciphering the Large-scale Environment of Radio Galaxies in the Local Universe: Where Are They Born? Where Do They Grow? Where Do They Die?

Abstract

The role played by the large-scale environment on the nuclear activity of radio galaxies (RGs), is still not completely understood. Accretion mode, jet power and galaxy evolution are connected with their large-scale environment from tens to hundreds of kpc. Here we present a detailed, statistical, analysis of the large-scale environment for two samples of RGs up to redshifts z src =0.15. The main advantages of our study, with respect to those already present in the literature, are due to the extremely homogeneous selection criteria of catalogs adopted to perform our investigation. This is also coupled with the use of several clustering algorithms. We performed a direct search of galaxy-rich environments around RGs using them as beacon. To perform this study we also developed a new method that does not appear to suffer by a strong z src dependence as other algorithms. We conclude that, despite their radio morphological (FR I vs FR II) and/or their optical (HERG vs LERG) classification, RGs in the local Universe tend to live in galaxy-rich large-scale environments having similar characteristics and richness. We highlight that the fraction of FR Is-LERG, inhabiting galaxy rich environments, appears larger than that of FR IIs-LERG. We also found that 5 out of 7 FR II-HERGs, with z src ≤0.11, lie in groups/clusters of galaxies. However, we recognize that, despite the high level of completeness of our catalogs, when restricting to the local Universe, the low number of HERGs (∼10% of the total FR IIs investigated) prevent us to make a strong statistical conclusion about this source class.

Hard X-ray selected giant radio galaxies – III. The LOFAR view

“…To confirm this result, we performed an environment analysis adopting the same methods and procedures outlined in Massaro et al (2019Massaro et al ( , 2020a. We searched for cosmological neighbours in the SDSS lying within the 2 Mpc radius computed at the same redshift of the central radio galaxy, with all SDSS magnitude flags indicating a galaxy-type object, and having a maximum spectroscopic redshift difference z ≤ 0.005 with respect to that of the central target, corresponding to the maximum velocity dispersion in groups and clusters of galaxies (see e.g.…”

“…9). The probability to have this number of cosmological neighbours at z ∼ 0.06 is less than a few per cent, as shown in Massaro et al (2019), thus strongly suggesting the presence of a galaxy cluster. Finally, we highlight that B2 1144+35B is most probably the BGC of this galaxy cluster being located at ∼70 kpc from the centroid of all positions of all other cosmological neighbours.…”

“…z = 0.21) and centred on its position. Finally, red circles mark the location of three galaxies, lying within 1 Mpc, and having a redshift difference, with respect to that of the central radio galaxy, less than 0.005 (known as cosmological neighbours; see Massaro et al 2019Massaro et al , 2020b. Their presence clearly indicate that 4C +34.47 belongs to a galaxy group/cluster.…”

“…Red circles mark the location of cosmological neighbours, i.e. optical galaxies having a redshift difference, with respect to that of the central radio galaxy, less than 0.005 (see Massaro et al 2019Massaro et al , 2020b for more details), as well as that of SDSS J114720.45+350438.5 at z = 0.05777 lying at ∼250 kpc and SDSS J114724.51+350246.5 with a redshift estimate of 0.05686 at ∼120 kpc distance, which could belong to the same cosmological structure. B2 1144+35B appears to be the BCG of a galaxy group/cluster.…”

Turin-SyCAT: A multifrequency catalog of Seyfert galaxies

“…Having strong dust absorption, unknown Seyfert galaxies could not emit significantly in the optical energy range, but could show mid-IR to hard X-ray flux ratios similar to those listed in our Turin-SyCAT. We also expect to investigate the large-scale environments of Seyfert galaxies (Dultzin-Hacyan et al 1999;Koulouridis et al 2006;Villarroel & Korn 2014;Jiang et al 2016) using sources listed in Turin-SyCAT adopting the same statistical procedures described in Massaro et al (2019Massaro et al ( , 2020 as an additional test for the unification scenario.…”