Strange and Odd Morphology Extragalactic Radio Sources (STROMERSs): A Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) Look at the Strange and Odd Radio Sources

Abstract

We report the identification of an extremely rare and peculiar set of irregular radio sources, termed “STROMERSs” (STRange and Odd Morphology Extragalactic Radio Sources).ingThe irregular radio sources with very anomalous morphologies that make them exceptionally different from all the known classes and subclasses of irregular radio sources are detected as STROMERSs. A thorough search for this class of sources from the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) gave a total of nine such candidates. We checked the corresponding morphology of the identified sources in other frequency surveys. We found a detectable radio emission for all of the nine sources in the NRAO VLA Sky Survey (NVSS) at 1.4 GHz and in the TIFR GMRT Sky Survey (TGSS) at 150 MHz, while the same was found for only three sources in the Westerbork Northern Sky Survey (WENSS) at 625 MHz. However, the strange morphology was not found in all of those other survey images. We also characterized the sources with their corresponding physical parameters like optical counterpart, size, spectral index, and radio luminosity. ingThe estimated spectral values of the sources indicated that the STROMERSs were most likely radio galaxies. The presence of any nearby galaxy clusters for the STROMERSs was also checked.

1. Introduction

Radio sources are one of the strong and energetic astronomical objects with significant radio emission. The radio emission from these objects is primarily non-thermal and originates from the synchrotron radiation produced by relativistic electrons spiraling in the strong magnetic fields present in the jets and lobes of radio galaxies. A classical double-lobed radio galaxy is characterized by a pair of extended lobes in the opposite direction with a bright compact radio core at the center.

Apart from the typical radio galaxy, there are some irregular radio galaxies. The irregular radio galaxies are a type of active galactic nuclei (AGNs) that exhibit complex, non-standard radio morphologies that do not conform to the typical double-lobed structure. In the set of the irregular class of galaxies, there are some subsets; for example, winged radio galaxies (WRGs; these include both X-shaped radio galaxies (XRGs) and Z-shaped radio galaxies (ZRGs) [1,2,3,4,5]), double double radio galaxies (DDRGs [6,7]), bet-tail (BT) radio galaxies (which comprise the wide angle tail (WAT) and narrow angle tail (NAT) [8,9,10,11,12,13]), and recently identified odd radio circles (ORCs [14,15]). In addition, there are some individual mentions of peculiar radio sources in the literature, e.g., [16,17,18]. Very recently, Sasmal et al. [19] reported the findings of four miscellaneous radio sources from the LOFAR Two-meter Sky Survey First Data Release (LoTSS DR1) [20] at 144 MHz.

As extensively documented in the literature and clearly illustrated by the classic case of BL Lacs [21], valuable insights into the “central engine” of radio galaxies can be obtained by examining their rare specimens that exhibit highly unusual traits, such as a distinctive radio morphology, spectrum, polarization, or time variability. These “odd entities” can challenge and refine both theoretical models of double radio sources and their numerical simulations. HYMORSs (HYbrid MOrphology Radio Sources) are one such example, where the two lobes of this type of radio sources exhibit different FR morphology on the other sides of AGNs [22]. Though they are less populated (<1%) radio galaxies, they strongly constrain the notion that the jets in radio galaxies belonging to the two FR classes have different energy carriers [23]. In retrospect, we conclude that seemingly unusual radio sources exhibiting uncommon characteristics are far from regular types of radio sources and that casual annotations such as “freaks” [24], “curiosities” [25] or simply “miscellaneous” [19] do not sufficient capture their astrophysical potential. A modification is thus necessary, commensurate with the true potential of such peculiar types of radio galaxies. Motivated by this, we suggest a novel annotation for extended radio galaxies and quasars with radio morphologies that are unique from one another, or as “erroneous”; that is, in a way that differs significantly from the recognized standard morphological types mentioned in the most recent review paper by Baldi [26]. The present work focuses on searching for radio sources that fall within the broad category of “STROMERSs” (STRange and Odd Morphology Extragalactic Radio Sources), which is the term we propose for such radio morphological outliers.

Aside from very rare kinds of sources, the study of STROMERSs may give some prominent or crucial information on those specific sources and radio galaxies in general. The complex radio morphologies might be an indication of the interactions between the AGN jets or outflows and the host galaxy’s interstellar or intergalactic medium. The irregular radio galaxies are frequently associated with merging or interacting galaxies, and it could also be a similar scenario for the STROMERSs. Thus, the study of such intriguing sources may ensure the understanding of the connection between irregular radio morphologies and galaxy mergers or how those interactions are crucial for models of AGN triggering and co-evolution with the host galaxy. The sources may also represent an important transitional phase in the life cycle of radio-loud AGNs. Mapping the detailed radio structures of these sources can reveal signatures of shocks, backflows, and turbulence induced by the AGN activity. Hence, studying such exceptional radio structures of these galaxies hopefully provide important insights into the physics of AGN feedback and its impact on galaxy evolution.

The recently improved telescopes and their cutting-edge technology allow us to observe more and in detail the fine structures of distant radio sources. Here, we present the search result for this kind of sources from the NRAO Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) at 1.4 GHz [27].

In this paper, Section 2 describes our identification procedure, which includes the data and source selection, cross-reference of the sources in other frequency surveys, and corresponding optical counterpart identification. Section 3 and Section 4 present the results and the summary and discussion portions, respectively. In the results section, we include a short note on each of the individual sources and the presence of nearby galaxy clusters for the STROMERS candidates. We used the following cosmology parameters in this paper: $H_0=67.4$ km s⁻¹ Mpc⁻¹, ${\Omega }_m=0.315$, and ${\Omega }_{vac}=0.685$ [28]. In the required parameter calculations, we used the cosmological calculator for the flat universe by Nick Gnedin1. The spectral index ($\alpha$) was defined as $S_{\nu }\propto {\nu }^{\alpha }$, where $S_{\nu }$ is the flux density at the particular frequency $\nu$. The contour map drawing, overlaying the radio map of the sources on their respective optical image, and drawing of the presence of nearby galaxy clusters were performed using the Common Astronomy Software Applications (CASA)2. We also used the Astronomical Image Processing System (AIPS)3 [29] for measuring the flux density, source size, etc. Every position presentation used the J2000 coordinate system.

2. Identification of the STROMERS Candidates

2.1. Data Selection: The VLA FIRST Survey Data

Sasmal et al. [19] presented the search result for miscellaneous sources from the low-frequency LoTSS DR1. The LoTSS DR1 has a small sky coverage of 424 square degrees [20]. In this work, we shifted our focus to the high-frequency survey data, preferentially those that had a comparatively larger sky-coverage area. The available high-frequency surveys that have a relatively large sky coverage are mainly the NRAO Very Large Array Sky Survey (NVSS) at 1.4 GHz [30] and the VLA FIRST at 1.4 GHz. Now, the VLA FIRST outperforms the other with its better-resolving power, which is needed to observe fine morphological details.

The FIRST survey uses the NRAO VLA B configuration, and it covers 10,575 square degrees of sky area, which is 25% of the total sky. Among the total sky coverage, it has 80% sky coverage in the north, and 20% in the south galactic cap [27]. Here, we used the latest data release of VLA FIRST, i.e., the FIRST data of 14 December 2017 (14Dec17 version). The 14Dec17 version covers the sky from R.A. = 07.0 hr to 17.5 hr, Decl. = −08.0 deg to +57.6 deg in the northern sky. In the southern sky, the survey covers from R.A. = 20.4 hr to 4.0 hr and Decl. = −11.5 deg to +15.4 deg. The typical rms of the survey is 0.15 mJy [27].

2.2. Selection of the STROMERS Candidates

The 14Dec17 data release of FIRST has a total of 946,432 sources. We filtered the total sources and created our investigating sample as the sources with an angular size higher than or equal to 10″. We set the angular size selection criterion as twice the convolution beam size, in order to spot the sufficiently large sources. Additionally, to find bright sources, we considered only the sources whose total flux density given in the catalog was greater than 10 mJy at 1400 MHz. After this filtering, a sample size of 95,243 targets was obtained. Then, for each of those 95,243 sources, we conducted a manual visual search (MVS). The MVS included checking for their grayscale images and contour images. Radio sources with strange morphologies that do not resemble any known classes and subclasses of irregular radio galaxies are identified as STROMERSs. Hence, in selecting such candidates, we first spotted the odd morphology sources from their grayscale radio images, then we drew the respective contour plots for each of those sources. After scrutinizing the peculiar morphology from the contour image, we finally selected the particular source as a STROMERS candidate. To ensure a reliable morphology, the minimum contour levels were drawn at least from three times the level up to the rms level. We aimed to find the irregular radio galaxies from the FIRST survey. From the above selection criterion, first, we reported 296 “winged” radio galaxies Bera et al. [1], 717 bent-tail radio galaxies Sasmal et al. [13], and in this paper, we reported nine peculiar radio morphologies named “STROMERSs”.

2.3. Cross-Referencing the Sources with Other Survey Data

We cross-checked the identified STROMERSs and their respective morphology in other available prominent surveys at different frequencies like NVSS at 1.4 GHz, the Westerbork Northern Sky Survey (WENSS) at 625 MHz [31], and the first alternative data release of the TIFR GMRT Sky Survey (TGSS ADR1) at 150 MHz [32]. The angular resolution and rms of these three surveys (NVSS, WENSS, and TGSS ADR1) are inferior to that of the FIRST. Thus, we may not have any strange morphology as seen in FIRST; however, this cross-referencing is mainly important for the source characterization (e.g., galaxy, nebulae, or H-I cloud).

2.4. Optical Counterpart Identification

Often, we found an optical host galaxy, typically located near the central core of a radio source. That specific optical host galaxy was taken as the optical counterpart of that corresponding radio source. A visual assessment allowed us to select the optical counterparts for the STROMERSs, based on their relative position in the radio map. We used the Sloan Digital Sky Survey (SDSS) [33] data catalog to look for the optical counterparts. Here, the SDSS data release 13 (SDSS DR13) [34] and the SDSS-i band images were used. The optical sources near to the radio core were found using the NASA/IPAC Extragalactic Database (NED)4. We used the associated infrared source (IR counterpart), such as the 2MASS (Two Micron All Sky Survey) [35] or WISE (Wide-field Infrared Survey Explorer) [36,37], when the optical counterpart was not available.

3. Results

Our exhaustive search gave a total of nine STROMERS candidates. In

Table 1. List of candidates for the identified STROMERSs.

Sl.	Name	R.A.	Decl.	Ref.	z	$\mathit{\theta }$	l	${\mathit{F}}_{1400}$	${\mathit{F}}_{150}$	${\mathit{\alpha }}_{150}^{1400}$	${\mathit{L}}_{1400\mathit{MHz}}$ (erg/s)
No.		(J2000.0)	(J2000.0)			(arcsec)	(Mpc)	(mJy)	(mJy)		($\times {\mathbf{10}}^{\mathbf{41}}$)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)
1	J0707+4327	07 07 11.57	+43 27 10.8	WISEA	—	35	—	22.5	85.5	−0.60	—
2	J0708+4114	07 08 06.78	+41 14 06.2	2MASS	0.124	61	0.178	55.6	298.7	−0.75	1.70
3	J0739+3947	07 39 31.43	+39 47 20.0	SDSS	0.098	63	0.144	43.7	181.4	−0.64	0.86
4	J0803+1050	08 03 37.68	+10 50 42.4	SDSS	0.142	112	0.379	85.2	663.1	−0.92	3.44
5	J0858−0107	08 58 28.92	−01 07 17.3	2MASS	0.197	75	0.363	101.9	448.0	−0.66	8.68
6	J1440+1125	14 40 20.78	+11 25 07.1	SDSS	0.311	68	0.553	119.9	578.7	−0.70	27.60
7	J1452+0212	14 52 24.61	+02 12 00.9	—	—	63	—	201.8	970.9	−0.70	—
8	J1453+2210	14 53 17.40	+22 10 55.8	SDSS	0.128	85	0.258	96.2	495.1	−0.73	3.19
9	J2155+0846	21 55 26.16	+08 46 48.4	SDSS	0.150	58	0.208	125.5	776.7	−0.82	5.71

The basic parameters of the STROMERSs are mentioned in this table. The sources are cataloged in the table in the ascending order of Right Ascension (RA). The table contains the following columns: column 1: catalog number, column 2: host name, column 3: RA (J2000.0), column 4: declination (J2000.0), column 5: position reference, column 6: redshift (z), column 7: angular size ($\theta$) in arc-second (major axis), column 8: linear size (l) in Mpc, column 9: flux density at 1400 MHz in mJy ($F_{1400}$), column 10: flux density in TGSS at 150 MHz in mJy ($F_{150}$), column 11: spectral index (${\alpha }_{150}^{1400}$), column 12: luminosity in erg/s (L). References—WISEA: Wide-field Infrared Survey Explorer—ALL-SKY [36,37]; 2MASS: 2 Micron All Sky Survey Point Source Catalog—Final Release [35]; SDSS: Sloan Digital Sky Survey [34,39,40].

, we present the sources with their basic parameters. The contour images of the sources are presented in Figure 1. We found a redshift (z) for seven sources. Here, we measured the angular ($\theta$) and linear size (l), when the redshift information of the radio sources was available. The two-point spectral index (${\alpha }_{1400}^{150}$) was calculated at frequencies between 1400 MHz and 150 MHz. We used the NVSS flux for the 1400 MHz flux, and for the 150 MHz flux, we used the TGSS flux. Here, we used the NVSS data for the 1400 MHz flux instead of FIRST because of the configuration of these two survey systems. Due to the high resolution and the absence of closely spaced antennas, the FIRST data could miss some flux. On the other hand, the NVSS could compromise the accuracy of a source position, as it measures more exact flux values. The spectral index was calculated for all nine sources. The optical counterpart was found for five sources and the IR counterpart was identified for three sources, when no optical or IR counterpart was found for the STROMERS J1452+0212. Using the redshift and flux information ($F_{1400}$), we calculated the luminosity value ($L_{1400MHz}$) for the possible seven sources. We calculated the radio luminosity ($L_{rad}$) using the formula (adapted from O’Dea Owen [38]):

$$L_{rad}=1.2\times {10}^{27}{D}_{\mathrm{Mpc}}^2{S}_0{\nu }_0^{-\alpha }{(1+z)}^{-(1+\alpha )}\times ({\nu }_u^{(1+\alpha )}-{\nu }_l^{(1+\alpha )}){(1+\alpha )}^{-1}\mathrm{erg}{\mathrm{s}}^{-1}$$

(1)

In this equation, the following notations are used: $D_{\mathrm{Mpc}}$ is the luminosity distance to the source (Mpc), $S_0$ is the flux density (Jy) at a given frequency ${\nu }_0$ (Hz), z is the redshift of the radio galaxy, $\alpha$ is the spectral index, ${\nu }_u$ (Hz) is the cutoff frequency, and ${\nu }_l$ (Hz) is the lower cutoff frequency. In this luminosity calculation, we used values of 15 GHz and 100 MHz for the upper and lower cutoff frequencies, respectively.

3.1. Notes on Individual Sources

In this section, we present a few details of the individual sources and their respective morphological structure, overall appearance, and nature.

3.1.1. ingJ0707+4327

The FIRST radio map of the source at 1.4 GHz showed that the source had three lobes, which were quite symmetrically ejected from the central core. The overall radio morphology gave the radio source a triangular “spinnerlike” shape. The lobes were directed towards the north-east (NE), west (W), and south (S) directions. Concerning our viewing plane, the angles in between the lobes were measured as 110 degrees for the W–S lobes and 125 degrees for both the S–NE and NE–W lobes, respectively. At 1.4 GHz, we found that the west lobe had a slightly greater flux density of 6.9 mJy, whereas the NE and south lobes had almost similar flux densities of 5.7 and 5.8 mJy, respectively. The core region had a radio flux density of 4.0 mJy. No optical or infrared (IR) counterpart was found for the source. The radio source was identified as NVSS J070711+432710 [30] and 7C 0703+4331 [41] in the NVSS and the seventh Cambridge Catalogue of Radio Sources (7C) [41]. We cross-checked this peculiar morphology in other frequencies, but due to inadequate resolution, the source appeared to be a point source in both TGSS and WENSS.

3.1.2. J0708+4114

The radio source had a circular morphology. The angular size of the source was ∼1 arcmin. The source is known as NVSS J070805+411342 [30] in the NVSS catalog. However, the poor resolution does not show this fascinating circular shape. This ring morphology is also absent in the TGSS and WENSS maps. The source 2MASS J07080679+4114063 [35] may be the possible IR counterpart of this source. The source had a redshift of 0.12410 [42]. The radio emission along the circumference was not smoothly distributed. Some region showed a compact and collimated emission while in some regions, it had a diffuse emission. The radio map showed a more compact emission on both sides of the possible host and a diffuse emission on the opposite side of the host.

3.1.3. J0739+3947

Similar to the previous source, this source also had a circular morphology. The angular size was also quite similar at ∼1 arcmin. However, this source showed a more compact ring-shaped structure with respect to the earlier one. The only small diffuse emission was observed on the opposite side of the optical core. The optical host of this radio source was identified as SDSS J073931.42+394719.9 [34] with a redshift value of 0.09766 [34]. The circular morphology was unavailable in the NVSS, TGSS, and WENSS data.

3.1.4. J0803+1050

For this radio source, the lobes on the opposite side of the central host showed different kinds of morphologies with each other. The lobe in the north side traveled straight and then arced towards the west direction. This north-side lobe seemed to have a double-lobe structure. The outer lobe was diffuse whereas the inner one was more compact, which is the characteristic of a DDRG. However, the south lobe did not show a similar trend. The lobe on the south side showed a compact radio lobe emission. The source was quite peculiar as a possible recurrent activity may have happened on the north side but the same was not seen on the south side. The source had an optical host SDSS J080337.68+105042.4 [39] with a redshift of 0.142356 [34]. The galaxy cluster WHL J080337.7+105042 [43] was found to be located near the optical host with an angular separation of less than $0.1^{\prime \prime }$. The closeness implied that the galaxy cluster may be associated with the source. The approximate angular dimension of the source was 112″, which corresponds to a linear size as 0.38 Mpc. We found no radio counterpart in WENSS for the source, whereas the TGSS data showed just two compact lobes directed in the opposite direction. No such peculiar kind of morphology was seen in that radio map. In the NVSS data, a pointlike structure was found for the source.

3.1.5. J0858–0107

This peculiar source had a “C”-type morphology [13] on the north side. On the south side, the source had a diffused tail. A weak diffused-tailed emission was also observed in the southwest direction. The source had a total of three prominent optical sources inside the radio extension. However, 2MASS J08582891-0107169 [35] is supposed to be the host of the galaxy. The source had a redshift of 0.197057 [42]. We found the galaxy cluster WHL J085829.2–010704 [43] was located at an angular distance of 0.24′ from the proposed host. The radio source was cataloged as NVSS J085829-010720 [30] in the NVSS data. Due to the inferior resolution, the source looked like a point source both in the NVSS and TGSS data. However, no radio data were found in WENSS.

3.1.6. J1440+1125

The radio image of the source showed a “C”-shaped structure, with its primary lobes directed towards the same southwest direction. However, in addition to this, the primary lobe in the lower side (with respect to our image plane) was further enhanced towards the southeast direction. On the other hand, the upper-side lobe showed an extra diffuse emission in the west direction. The combination of these further extensions of the lobes resulted in an overall strange morphology. The optical galaxy SDSS J144020.78+112507.1 [39] was identified as the optical counterpart of this source with a redshift value of 0.310929 [34]. The galaxy cluster WHL J144020.8+112507 [43] was located at an angular distance of 0.24″ from the optical host. The source was unresolved in both the NVSS and TGSS data, while there was no radio counterpart found in the WENSS data.

3.1.7. J1452+0212

This peculiar radio source had an overall triangular shape, a bit similar to the first STROMERS in this list (STROMERS J0707+4327). The three lobes were directed towards the northeast (NE), southeast (SE), and west directions. However, in contradiction to the STROMERS J0707+4327, the lobes of this source were not linearly directed and equispaced in the respective directions. The radio map showed that the west lobe was further bent towards the north direction and was more compact with respect to the other two lobes. In an anticlockwise direction, the angles between the west, NE, and SE lobes were 110, 105, and 145 degrees, respectively. The measured flux densities were 61.6 mJy, 64.1 mJy, and 76.3 mJy for the SE, west, and NE lobes, respectively. There was no optical host found at the central core of the radio map. However, the optical galaxy SDSS J145222.56+021203.4 [34] with a redshift value of 0.30932 [34] was located in the west lobe and it might be the possible optical counterpart of that source. The source is also known as PMN J1452+0212 [44] and 87 GB 144952.0+022404 [45] in other respective radio survey catalogs. Neither NVSS nor TGSS showed such an integral structure in their respective radio data, whereas the WENSS had no radio data for this source.

3.1.8. J1453+2210

The jets of this radio source showed some unusual bending. When seen from the central core, the lobes were initially directed towards the northeast (NE) and northwest (NW) directions. The edge of the NE lobe was further diffused and bent towards the west direction. On the other hand, the NW lobe first bent in the west direction and then pointed to the north direction. SDSS J145317.40+221055.8 [39] was identified as the optical host of this radio source. The source had a redshift of 0.12816 [34]. Though an extended structure was found in the TGSS data, the poor resolution was unable to give a detailed morphological structure. No radio counterpart was found in the WENSS data.

3.1.9. J2155+0846

The radio lobe of the source was primarily bent towards the northwest (NW) and southwest (SW) directions. Both lobes were then further extended towards the south and gave an overall unique morphology (looking like a mirror image of the question mark “?”) and made the source a STROMERS candidate. The optical counterpart of the source was SDSS J215526.16+084648.4 [34], which had a redshift of 0.150143 [34]. The galaxy cluster RM J215526.2+084648.4 [46] was associated with this source. The TGSS data showed the source as a pointlike source, whereas no radio data were found in WENSS.

3.2. The Presence of Nearby Galaxy Clusters

A STROMERS is a very new kind of identification; hence, as a first step toward understanding this kind of source, we tried to see if there was any association with any nearby cluster for this class of sources. In this context, we tried to check for the presence of any galaxy cluster in the vicinity of such sources. The main purpose of finding the relationship between STROMERSs and associated galaxy clusters was to obtain a better understanding of their strange and odd morphologies. However, there are several other purposes for the study of the environment of the sources: (a) the determination of the physical condition of the radio jets, (b) the triggering of AGN activity, (c) the determination of the particular modes of AGN feedback, (d) testing models of source dynamics and environmental impact, and (e) finding the galaxy cluster at high redshift using the source. Here, we considered a spherical region of radius 1 Mpc from the source as the nearby environment of the respective source. We used the search radius of 1 Mpc at the redshift of each source. We assumed that the STROMERSs and clusters of galaxies were associated if $z=|z-z_{cl}|\le 0.05$, where $z_{cl}$ is the redshift of the galaxy cluster. By keeping the optical host at the center, we drew a circle of radius 1 Mpc as shown in Figure 2. In Figure 2, the position of the identified nearby galaxy clusters is presented and marked as square boxes.

From

Table 2. List of nearby galaxy clusters for the identified STROMERSs.

STROMERS	Nearby Galaxy Cluster	${\mathit{S}}_{\mathit{\theta }}$	z	${\mathit{S}}_{\mathit{l}}$	${\mathit{m}}_{\mathit{r}}$	${\mathit{r}}_{500}$	${\mathit{R}}_{\mathit{L}\ast 500}$	${\mathit{M}}_{500}$	${\mathit{N}}_{500}$
Candidate		In arcmin		In kpc				$\times {\mathbf{10}}^{\mathbf{14}}{\mathit{M}}_{\odot }$
STROMERS J0708+4114	RXGCC 258	6.35	0.104	879.19
STROMERS J0739+3947	MSPM 06508	2.60	0.098	353.36
STROMERS J0803+1050	WHL J080337.7+105042	0.01	0.142 *	1.55	16.98	0.70	20.43	1.11	14
	MSPM 09531	1.78	0.142 *	276.47
STROMERS J0858−0107	WHL J085829.2−010704	0.24	0.245 *	48.65	17.07	0.85	41.99	2.41	15
STROMERS J1440+1125	WHL J144020.8+112507	0.005	0.311 *	1.42	17.58	0.79	26.14	1.45	15
STROMERS J2155+0846	RM J215526.2+084648.4	0.003	0.153	0.48
	WHL J215529.8+084736	1.21	0.154 *	196.76	15.82	1.06	65.28	3.89	29
	NSC J215521+084647	1.27	0.114	206.52
	ABELL 2395	2.86	0.151	465.07

In this table, we have listed the galaxy clusters in the vicinity of the identified STROMERSs. The nearby galaxy clusters are listed in column 2. The measured angular separation ($S_{\theta }$ in arcmin) and linear separation ($S_l$ in kpc) between the source and their respective clusters are presented in columns 3 and 5, respectively. The redshift (z) of the galaxy clusters is cataloged in column 4. The “*”-marked redshifts indicate that these z values are spectroscopic redshift (SPEC), otherwise, the redshifts are photometric (PHOT). The value $m_r$ is the r-filter magnitude of the galaxy clusters. In column 7, we cataloged $r_{500}$, which is the cluster radius for optical luminosity. Column 8 presents the cluster richness, $R_{L\ast 500}$. The cluster mass ($M_{500}$ in the unit of $\times {10}^{14}{M}_{\odot }$) and the number of all member galaxies ($N_{500}$) within the cluster radius ($r_{500}$) are listed in columns 9 and 10, respectively. References—RXGCC: ROSAT All-Sky Survey X-ray Galaxy Cluster Catalog [47]; MSPM: MultiScale Probability Mapping [48]; WHL: Wen+Han+Liu [43]; RM: RedMapper Cluster [46]; NSC: Northern Sky optical Cluster [49,50,51]; ABELL: Abell Clusters of Galaxies [52,53].

, we see that a total of six out of seven (86%) STROMERSs with known redshift were associated with at least one galaxy cluster. For each of the STROMERSs, we found at least one galaxy cluster within the size of the STROMERS, except for the STROMERSs J0708+4114 and J0739+3947. STROMERS J2155+0846 was associated with four galaxy clusters within a 1 Mpc radius, whereas the STROMERS J0803+1050 consisted of two galaxy clusters. Thus, the final outcome of our search was that at least one cluster was associated inside the 1 Mpc radius for each STROMERS. Since our number of samples was too small to draw any conclusions from any of the aforementioned activities, we need to study a large number of such STROMERS samples in order to draw any further viable and more conclusive statement on the association of nearby galaxy clusters with STROMERSs.

4. Summary and Discussion

In this paper, we presented nine newly identified radio galaxies with a strange and rare kind of radio morphology, which did not match with other known classes and subclasses of radio galaxies; we named them as STROMERSs. Considering our sample size of 95,243, the proportion of such sources is nearly 0.01%. However, this is not an appropriate estimation, and additional similar identifications would allow us to give an accurate proportion value of these kinds of sources concerning all radio sources. With reference to our previous identification of such interesting and typical sources from LoTSS DR1 in Sasmal et al. [19], we can consider this as an extended list of the same but from the VLA FIRST at 1.4 GHz. Besides the fascinating morphology, the study of these sources may be helpful in understanding the surrounding medium and the galaxy’s evolution itself.

Here, we checked the basic properties of these sources like the available redshift (z), angular size, linear size (for the available redshift sources), flux densities, spectral index, and luminosity. We noticed that the identified STROMERSs had a moderately low redshift value. The redshift values lay in the range between 0.098 to 0.311. This range is well under the value of our previous identification in Sasmal et al. [19]. In our previous identification, we found the highest redshift for the source J1428+4556. However, we should remember that the probability of finding higher redshifted sources in VLA FIRST was lower than the same for the LoTSS DR1, and that might be a reason for the relatively low redshift values. The angular sizes of the sources were found in the span of 35 arcseconds to 112 arcseconds. Now, if we compare the angular sizes with the four sources from Sasmal et al. [19], we can see that the sizes of the sources from Sasmal et al. [19] are way larger than the sources identified in this paper. In this paper, the source with the largest size (STROMERS J0803+1050 with a size of ∼2 arcmin) was nearly 2.5 times smaller than the smallest source in the Sasmal et al. [19] list (J1323+5059 with a size of ∼5 arcmin). However, this is expected as FIRST is designed to observe more fine structures and sources, and hence, it has a tendency to miss the large-scale structures and sources. The calculated linear size showed that STROMERS J1440+1125 was the largest source with a size of ∼550 kpc, while STROMERS J0739+3947 was the smallest with a projected size of ∼150 kpc. Due to the unavailability of redshifts, we could not estimate the linear size for the two sources (STROMERS J0707+4327 and STROMERS J1452+0212). The previously identified sources from Sasmal et al. [19] had quite large sizes which led to a possibility of misidentification. The misidentification could be due to the overlaps of more than one source in our line of sight [19]. However, in this list, the probability of such misidentification was much less. The calculated two-point spectral index (${\alpha }_{150}^{1400}$) had a range of −0.60 to −0.92. This spectral index value is typical for radio galaxies [54,55,56,57,58]. However, the steep spectrum (${\alpha }_{150}^{1400}\le -0.50$) for all the sources indicated that these were lobe-dominated sources. This may indicate that there is a possibility of the sources being relic candidates [59]. However, the spectral index was less steep than that of the previously identified sources. The radio luminosity ($L_{1400MHz}$) of the sources was found to be on the order of $\times {10}^{41}$ erg sec⁻¹, except for the sources STROMERS J0739+3947 and STROMERS J1440+1125. The STROMERS J0739+3947 has the lowest luminosity of $8.6\times {10}^{40}$ erg sec⁻¹ and the STROMERS J1440+1125 has the highest luminosity of $2.76\times {10}^{42}$ erg sec⁻¹. The luminosity value was quite similar to previous sources in Sasmal et al. [19] and also similar to the average from Nilsson et al. [60]. Here, we also cross-identified the sources with other existing radio catalogs. The other catalogs’ identification was important to identify the radio sources’ character. We identified the radio sources as radio galaxies, and hence we could also treat them as such. However, in other frequencies, the sources did not show any such strange morphologies. We also checked the environment by identifying the presence of known galaxy clusters, and we found that there was at least one galaxy cluster inside 1 Mpc of the source for most cases. We also checked the same fact for other classes of radio sources from FIRST on a random basis and found that the presence of a nearby cluster was less frequent than for the STROMERSs. For the typical double-lobed radio galaxies and winged radio galaxies, the cluster association was less or equal to 10%, whereas for the BT sources and the HYMORSs, the values were 70% and 60%. The high value of the cluster association for BT sources was expected for the generation of ram pressure; however, the quantification of that association is still unknown for the HYMORSs and our STROMERSs. Thus, we can assert that there might be a possibility of some kind of association between the STROMERSs and their respective nearby galaxy clusters. However, the small number of samples restricted us from concluding and establishing any of the above activities.

In the absence of more detailed data and study, if we try to scrutinize only the radio morphology, then we may consider different possibilities for different sources, regarding the mechanism behind such morphology. If we consider each of the sources individually, the three-lobe structure of the STROMERS1 (J0707+4327) indicates a possible change in one of the jet directions. Now, the change in the jet direction may occur due to the reorientation caused by a spin-flip or the precision of jets or may be due to a merger. A spectral age analysis may give information about the aging of the individual lobes. The overall morphology of the seventh source (J1452+0212) in our list has a similarity with STROMERS1; however, the lobes and their extensions of the source are not as symmetrical as for STROMERS1. The further bend of the west lobe indicates that there might be some other attribution or other factor/s besides the jet reorientation or precision. The compactness of the west lobe may also be due to the fact that this source is relatively young (or active), and the other ones are relic ones. This may be confirmed by a spectral analysis. The second and third STROMERSs (J0708+4114 and J0739+3947) both have an overall similar circular morphology, although the detailed nature of the two is different. The sources have a similar one arcmin diameter as the ORC in [14]. However, these might not be the ORC candidates, as the sources have a ring-shaped emission instead of the circular diffused emission as in the case of ORCs. A possible explanation may be that the source is an extreme case of a bent-tail source. Initially, the source may have been bent due to “ram pressure” [61,62]; however, due to some external, environmental effect, the diffused tails were further bent to give such a circular shape. These two sources may also be a possible case of imaging artefact, a supernova remnant (SNR), ring galaxy, Einstein ring, etc. However, the possibility of an SNR is lower, as there are some radio hotspots in the ring of the sources, which is not conventional for an SNR. The fourth source J0803+1050 may be a special case of a DDRG as it has a pair of lobes on one side. However, there is a single lobe on the other side. Moreover, the diffuse lobe on the pair side has a bent structure, while the single-lobe side shows an arced structure. A recurrent activity is a possible case for the pair lobe side; however, the same is not true for the single arced lobe side. The “f”-like structure of STROMERS J0858–0107 has a mix of both, a bent feature and an extended emission. The three sources (STROMERS J1440+1124, STROMERS J1453+2211, and STROMERS J2155+0846) have an arced structure at the host region, but the orientation and extension of the lobe towards the edges are different, and these features make each of the sources a STROMERS candidate. The arced portion may be caused due to some “ram pressure” or bouncy effect, but we have to consider some alternative mechanism or environmental effect for the edges.

If we look individually at all of the nine STROMERSs, we see that the STROMERSs exhibit a diverse range of morphologies. We think this diversity is likely driven by a combination of factors, such as the properties of the central black hole, the dynamics of the accretion flow, and the environment in which the radio galaxy resides. Moreover, the combination might be different for different cases. However, there is also a possibility of imaging artefacts. A more extensive study of these sources is needed to determine the particular origin. For now, we conclude that the most likely explanation for the STROMERSs is that there is no particular scenario, mechanism, or event which is responsible for these peculiar kinds of morphologies. We think there is a possibility that more than one reason or event is responsible for this extremely rare kind of morphology. We also believe that the study of such source morphologies offers a unique window into the complex interplay between supermassive black hole activity and galaxy evolution, making them an important area of ongoing research in extragalactic astrophysics. Hence, we plan to extend our search for these STROMERSs with other available high-resolution data surveys and obtain an adequate number of such sources. We also plan to conduct observations on the individual sources and conduct some multi-wavelength studies. A larger number of such sources and further detailed studies of the sources will be helpful in understanding this strange and odd kind of morphology.