The Host Galaxies of Hybrid Morphology Radio Sources Andra Stroe1and Victoria Catlett2 1 Jeremy J. Harwood3Tessa Vernstrom4and Beatriz Mingo5

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The Host Galaxies of Hybrid Morphology Radio Sources

Andra Stroe1, ∗and Victoria Catlett2, 1

Jeremy J. Harwood,3Tessa Vernstrom,4and Beatriz Mingo5

1Center for Astrophysics |Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA

2The University of Texas at Dallas, 800 W Campbell Rd, Richardson, TX 75080, USA

3Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatﬁeld,

Hertfordshire AL10 9AB, UK

4ICRAR, The University of Western Australia, 35 Stirling Hw, 6009 Crawley, Australia

5School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

(Received August 12, 2022; Revised October 13, 2022; Accepted October 14, 2022)

ABSTRACT

Based on their diﬀering radio morphologies, powerful radio galaxies can be separated into the

Fanaroﬀ-Riley I (FR-I) and II (FR-II) classes. Hybrid morphology radio sources (HyMoRS) contain

morphologies consistent with each type of jet on either side: a powerful, highly relativistic FR-II-

like jet terminating in a hotspot on one side and an FRI-like plume on the other. HyMoRS present

a unique opportunity to study the conditions which give rise to the dichotomy. Using host galaxy

properties, we conduct the ﬁrst multiwavelength investigation into whether orientation can explain

HyMoRS morphology. Through optical spectroscopy and mid-infrared photometry, we analyze the

emission characteristics, and evaluate the broad characteristics of ﬁve HyMoRS host galaxies at inter-

mediate redshifts (0.4<z<1.5). The HyMoRS host galaxies in our sample have properties consistent

with typical host galaxies of FR-II sources, suggesting that the observed hybrid morphologies may be

caused by a dense, cluster-like environment bending FR-II jets combined with a favorable orientation

which can make one side appear similar to an FR-I jet. Our results thus support the hypothesis that

HyMoRS are mainly caused by environment and orientation.

Keywords: Active galaxies (17), Early-type galaxies (429), Emission line galaxies (459), Fanaroﬀ-Riley

radio galaxies (526), Galaxy clusters (584), Galaxy environments (2029), Galaxy evolution

(594), Ionization (2068), Radio galaxies (1343), Shocks (2086), Spectroscopy (1558)

1. INTRODUCTION

The prominent radio emission in powerful active

galactic nuclei (AGN) typically manifests through twin,

symmetrical relativistic jets, spanning several hundreds

of kpc to Mpc scales outward from their host galaxy.

Fanaroﬀ & Riley (1974) divided powerful radio sources

into two morphological classiﬁcations, Fanaroﬀ-Riley

class I (FR-I) and class II (FR-II). In FR-I sources,

the emission is brightest near the host galaxy, followed

by extended diﬀuse emission outwards. On the other

hand, FR-IIs feature prominent brightness peaks called

“hotspots” farther from the host galaxy with lobes of

Corresponding author: Andra Stroe

andra.stroe@cfa.harvard.edu

∗Clay Fellow

diﬀuse emission. The diﬀerent morphology was later at-

tributed to diﬀering jet physics: unlike FR-I jets, which

get disrupted quickly within the host and decelerate on

tens of kpcs scales, FR-II jets stay collimated over large

distances (Bicknell 1994;Laing & Bridle 2002).

The underlying cause of the FR-I/II dichotomy has

been a topic of debate (e.g.; Kaiser & Alexander 1997),

and theories include both intrinsic and extrinsic causes.

For example, the properties of the black hole engine

could aﬀect the magnetic ﬁelds which collimate and

power the jets (e.g.; Celotti et al. 1997), which could

determine the initial jet velocities. The material in the

host galaxy could aﬀect the deceleration of the jet mate-

rial (e.g.; Kaiser & Best 2007;Mingo et al. 2019). This

scenario is supported by the observed diﬀerences be-

tween the host galaxies of FR-I and FR-II sources (e.g.;

Miraghaei & Best 2017). FR-I sources are hosted by

arXiv:2210.09315v1 [astro-ph.GA] 17 Oct 2022

2Stroe et al.

Table 1. Positions, spectroscopic redshifts (zspec), spectrum origin instrument, SDSS magnitudes in the g, r, and i bands, and

radio ﬂuxes (at 1.4 GHz and 4.9 GHz) for each of the ﬁve HyMoRS in our sample.

Source R.A. Decl. zspec Instrument g r i S1.4 GHz S4.9GHz

hh mm ss ◦ 0 00 mag mag mag mJy mJy

J1315+516 13 14 38.12 51 34 13.4 0.47799 SDSS 20.52 ±0.03 19.75 ±0.02 18.98 ±0.02 93 51

J1348+286 13 47 51.58 28 36 29.6 0.74058 BOSS 17.39 ±0.01 17.20 ±0.01 17.43 ±0.01 241 117

J1313+507 13 13 25.78 50 42 06.2 0.88000 GMOS 21.45 ±0.05 20.96 ±0.05 20.36 ±0.04 277 84

J1154+513 11 53 46.43 51 17 04.1 1.37250 GMOS 22.10 ±0.12 21.44 ±0.11 21.37 ±0.16 495 137

J1206+503 12 06 22.39 50 17 44.3 1.45423 BOSS 21.38 ±0.04 20.89 ±0.04 20.69 ±0.05 241 75

massive red elliptical galaxies with spectra dominated

by absorption features, no broad lines, and little-to-no

narrow emission lines (e.g.; Matthews et al. 1964;Hickox

et al. 2009;Best & Heckman 2012;Janssen et al. 2012;

Butler et al. 2018). At low jet power, both FR-Is and

FR-IIs are predominantly low-excitation radio galaxies

(LERGs) (Laing et al. 1994;Mingo et al. 2022). High-

excitation radio galaxies (HERG) appear at high jet

powers, in which FR-IIs dominate. HERGs, hosted by

bluer galaxies with lower masses, higher star formation

(SF) rates (SFR), and diskier morphologies, have strong

narrow (an order of magnitude stronger than LERGs)

and, in some cases broad, emission lines (e.g.; Baum &

Heckman 1989;Zirbel & Baum 1995;Best & Heckman

2012;Miraghaei & Best 2017;Butler et al. 2018). Fi-

nally, FR-Is tend to live in denser cluster environments

than FR-II galaxies, suggesting that the density of the

intergalactic/intracluster medium could decelerate the

jets outside of the galaxy (e.g.; Prestage & Peacock 1988;

Mingo et al. 2019).

Hybrid morphology radio sources (HyMoRS) possess

both FR-I and FR-II structures, presenting a unique

opportunity to probe the conditions which cause the

dichotomy. First identiﬁed in Gopal-Krishna & Wiita

(2000), their existence suggests that the black hole en-

gine cannot alone cause the morphological diﬀerences

between FR-I’s and FR-II’s. The number of HyMoRS

candidates has rapidly grown over the past few years,

amounting to hundreds of sources and ∼5% of the re-

solved radio AGN population in modern radio surveys

(Mingo et al. 2019;Kapi´nska et al. 2017).

In this paper, using the intrinsic properties of the

host galaxy, we investigate whether orientation can give

rise to these morphological diﬀerences. As a relativistic

jet from the black hole engine travels through the host

galaxy, it may be slowed down by intervening material

in a host galaxy or travel unimpeded and slow down on

larger scales, eventually terminating in a bright hotspot

(e.g.; Kaiser & Best 2007;Mingo et al. 2019). The dis-

tribution and type of material in the galaxy could aﬀect

how quickly the jet slows, aﬀecting the location of the

ﬁnal emission peak (Gopal-Krishna & Wiita 1996). We

aim to unveil the cause of the dichotomy by focusing our

analysis on the best-studied ﬁve sources that Gawro´nski

et al. (2006) securely identiﬁed as HyMoRS given their

spatially-resolved radio observations (see Figure 1). L-

band (1.0−2.0 GHz) images from the Very Large Array

(VLA) show the presence of one FR-I-like and one FR-II

set of structures in each system, but, using spectral ag-

ing techniques, Harwood et al. (2020) largely attribute

the FR-I-like morphology to a favorable projection of

an FR-II jet, hotspot, and lobe. We measure detailed

galaxy properties by investigating their spectral prop-

erties with newly-obtained 1D spectroscopy from the

Gemini Multi-Object Spectrograph North (GMOS-N) in

combination with MIR color diagnostics.

In Section 2, we present our new data taken with Gem-

ini, as well as archival images and spectra, and we walk

through the data reduction methods. Section 3discusses

how we analyze the spectral and ancillary data. In Sec-

tion 4, we discuss the insights into HyMoRS host galaxy

properties and the implications for the broader forma-

tion context of powerful radio galaxies. Conclusions can

be found in Section 5.

We use the ΛCDM cosmological model of a ﬂat uni-

verse with H0= 71 km s−1Mpc−1, Ωm= 0.27, and

Ω∆= 0.73. A Salpeter (1955) initial mass function is

used throughout.

2. SAMPLE, OBSERVATIONS AND DATA

REDUCTION

2.1. HyMoRS sample

The ﬁve sources in this study have been identiﬁed as

bona-ﬁde HyMoRS through a uniform radio selection

by Gawro´nski et al. (2006). After selecting all sources

with 1.4 GHz ﬂuxes >20 mJy and angular size θ > 800

in ﬁve random high galactic latitude ≈16◦×16◦ar-

eas within the VLA Faint Images of the Radio Sky at

Twenty-centimeters (FIRST) survey (White et al. 1997),

Gawro´nski et al. (2006) visually selected HyMoRS can-

didates and followed them with pointed, narrow-band

VLA observations, resulting in a secure sample of ﬁve

Host Galaxies of HyMoRS 3

Figure 1. The ﬁve HyMoRS in our sample. The background images are Pan-STARRS RGB composites (using g,r,iﬁlters),

with overlaid L-band VLA contours at σrms ×2n. Each image shows the host galaxy (zoomed-in section) with an FR-I-like

plume on one side and an FR-II jet, hotspot, and lobe on the other.

HyMoRS sources. The resolved spatial and spectral ra-

dio properties of these ﬁve sources were then studied in

detail by Harwood et al. (2020) through high-resolution,

wide-band (C, L), JVLA observations. In the present

paper, we follow up these ﬁve best-studies sources, with-

out imposing any additional selection criteria.

2.2. GMOS-N Observations and Data Reduction

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TheHostGalaxiesofHybridMorphologyRadioSourcesAndraStroe1,andVictoriaCatlett2,1JeremyJ.Harwood,3TessaVernstrom,4andBeatrizMingo51CenterforAstrophysicsjHarvard&Smithsonian,60GardenSt.,Cambridge,MA02138,USA2TheUniversityofTexasatDallas,800WCampbellRd,Richardson,TX75080,USA3CentreforAstrophysicsResearc...

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The Host Galaxies of Hybrid Morphology Radio Sources Andra Stroe1and Victoria Catlett2 1 Jeremy J. Harwood3Tessa Vernstrom4and Beatriz Mingo5

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