Hybrid Morphology Radio Sources from the MeerKAT Absorption Line Survey (MALS): Radio, Mid-infrared and Environmental Characteristics

TL;DR

HyMoRS are rare radio galaxies with FR I morphology on one side and FR II on the other, providing a testbed for jet physics and environmental effects. The study assembles the largest southern HyMoRS sample to date (36 sources) from the MeerKAT Absorption Line Survey (MALS) using a visual morphology–based search, with optical/IR cross-matches to confirm hosts and redshifts. The sample spans $L_{1.4\,\mathrm{GHz}}$ from $9.9\times 10^{23}$ to $5.7\times 10^{25}$ W Hz$^{-1}$ (median $4.4\times 10^{24}$ W Hz$^{-1}$) and $0.04<z<1.34$, and the two lobes show no statistically significant difference in spectral indices. Nine of the HyMoRS lie near cluster centers, including one with a giant jet extending $811$ kpc, and the majority host actively star-forming galaxies with elevated SFR, suggesting environmental deflection plus orientation effects may explain the morphologies; higher-resolution follow-up is required to confirm the morphologies.

Abstract

Hybrid morphology radio sources (HyMoRSs) are a rare subclass of radio galaxies that display a Fanaroff-Riley type I (FR I) morphology on one side of the central supermassive black hole (SMBH) and a type II (FR II) morphology on the other. In this study, we report the discovery of thirty-six new HyMoRSs, marking the largest collection of such sources in the southern sky to date, using data obtained from the MeerKAT absorption line survey (MALS). The identified HyMoRSs exhibit moderate radio luminosities in the range $9.9 \times 10^{23}$ to $5.7 \times 10^{25}$ W $Hz^{-1}$, with a median value of $4.4 \times 10^{24}$ W $Hz^{-1}$ at 1.4 GHz, and are located within the redshift range $0.04<z<1.34$. In this work, we show for the first time that the two lobes of HyMoRSs exhibit no statistically significant difference in their spectral indices. We also investigate the mid-infrared properties and environments of their host galaxies. Notably, nine out of the thirty-six sources are situated near the centers of galaxy clusters, including one with giant radio jets that extend over 811 kpc. Our analysis reveals that the majority of HyMoRSs are hosted by actively star-forming galaxies that exhibit elevated star formation rates. Furthermore, our findings suggest that HyMoRSs may arise from FR II jets being deflected by a dense, cluster-like environment, along with orientation effects that make one jet appear FR I-like. As our candidates are selected through visual inspection of MALS radio maps, higher-resolution follow-up observations are still necessary to confirm the nature of their morphologies.

Figures (1)

• Figure 1: A sample of four HyMoRSs identified from MALS at 1000 MHz and 1400 MHz. Sources are superimposed with 1006 MHz and 1381 MHz of the MAL survey (in green and red contours), and 1400 MHz of the FIRST survey (blue contours).