Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Single and double headed ORCs in the LOFAR surveys

F. De Gasperin, M. Brüggen, T. Pasini, H. Andernach, A. Ivleva, C. Spingola, L. M. Böss, J. Callingham, M. Hardcastle, G. Di Gennaro

Abstract

Deep radio surveys are now producing catalogues with millions of radio sources. Radio sources can have complex morphologies that depend both on the production mechanisms and the local environment. Recently, an unusual class of circular radio sources have been identified that were named "odd radio circles" (ORC). They have sizes of about 1', a circular/elliptical shape and appear edge brightened. Subsequent observations have suggested that this class may comprise a variety of sources. Despite various attempts to explain these sources, their origin remains unclear. The main goal of this work is to increase the number of known ORCs and derive common characteristics that can help identify the origin of these sources. We searched the LOFAR Two Metre Sky Survey (LoTSS) data release 3 (DR3) at 144 MHz for ORCs using a combination of parameter filtering on catalogue entries and visual inspection. We then identified possible optical counterparts and derived information such as redshift, physical size, and spectral index using further radio data at 54 and 1400 MHz. We isolated 18 sources with ORC structures. Four of these are double headed ORCs (ORCs with two rings), and two are new discoveries. We also found 5 new single headed ORCs and 9 candidate ORCs. With this work we significantly expanded the population of known ORCs. Our findings confirms that ORCs are a rare and heterogenous population of radio sources. We confirm the association with large ellipticals in most cases and we note a relation between the ORCs physical size and their integrated spectral index with small ORCs avoiding steep spectra.

Single and double headed ORCs in the LOFAR surveys

Abstract

Deep radio surveys are now producing catalogues with millions of radio sources. Radio sources can have complex morphologies that depend both on the production mechanisms and the local environment. Recently, an unusual class of circular radio sources have been identified that were named "odd radio circles" (ORC). They have sizes of about 1', a circular/elliptical shape and appear edge brightened. Subsequent observations have suggested that this class may comprise a variety of sources. Despite various attempts to explain these sources, their origin remains unclear. The main goal of this work is to increase the number of known ORCs and derive common characteristics that can help identify the origin of these sources. We searched the LOFAR Two Metre Sky Survey (LoTSS) data release 3 (DR3) at 144 MHz for ORCs using a combination of parameter filtering on catalogue entries and visual inspection. We then identified possible optical counterparts and derived information such as redshift, physical size, and spectral index using further radio data at 54 and 1400 MHz. We isolated 18 sources with ORC structures. Four of these are double headed ORCs (ORCs with two rings), and two are new discoveries. We also found 5 new single headed ORCs and 9 candidate ORCs. With this work we significantly expanded the population of known ORCs. Our findings confirms that ORCs are a rare and heterogenous population of radio sources. We confirm the association with large ellipticals in most cases and we note a relation between the ORCs physical size and their integrated spectral index with small ORCs avoiding steep spectra.
Paper Structure (29 sections, 13 figures, 3 tables)

This paper contains 29 sections, 13 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Single source analysis
  4. ORCs and DHORCs
  5. ORC J0823+6216 (Fig. \ref{['fig:J0823+6216']})
  6. DHORC J0825+6132 (Fig. \ref{['fig:J0825+6132']})
  7. DHORC J0847+7022 (Fig. \ref{['fig:J0847+7022']})
  8. ORC J1134+6642 (Fig. \ref{['fig:J1134+6642']})
  9. ORC J1150+2449 (Fig. \ref{['fig:J1150+2449']})
  10. ORC J1222+6436 (Fig. \ref{['fig:J1222+6436']})
  11. DHORC J1313+5003 (Fig. \ref{['fig:J1313+5003']})
  12. DHORC J1555+2726 "ORC 4" (Fig. \ref{['fig:J1555+2726']})
  13. ORC J1633+1441 "Uruk-hai" (Fig. \ref{['fig:J1633+1441']})
  14. Candidate ORCs
  15. J0016+2426
  16. ...and 14 more sections

Figures (13)

  • Figure 1: Top panel: image at 144 MHz ORC J0823+6216. The resolution is 5$^{\prime\prime}$, with the beam shape visible on the bottom left of the image. The insert shows the optical field from PanSTARR (DESI for all other images) with the identified optical counterpart. The red "x" in the radio image marks the position of the optical counterpart. Bottom panels: Images at 144 MHz and brightness profiles. Contours starts from $3\sigma$ and ends at the maximum value of the colour bars with a square root scaling. White contours trace the low-resolution (20$^{\prime\prime}$) 144 MHz brightness. Pixels are blanked (white) where a compact point source is identified within the ORC's confines. The green/blue dots identify the centre of the green/blue ellipses that trace the ORCs' shape. The right part of the panel shows the profile along elliptical annuli with the vertical dashed lines drawn at the length of the ellipses semi-major axis. The correspondence with a peak proves the enhancement of brightness along the ellipses line.
  • Figure 2: Same as Fig. \ref{['fig:J0823+6216']} but for ORC J0825+6132.
  • Figure 3: Same as Fig. \ref{['fig:J0823+6216']} but for DHORC J0847+7022. Contours starts at $2\sigma$.
  • Figure 4: Same as Fig. \ref{['fig:J0823+6216']} but for ORC J1134+6642. The resolution is 20$^{\prime\prime}$.
  • Figure 5: Same as Fig. \ref{['fig:J0823+6216']} but for ORC J1150+2449.
  • ...and 8 more figures