Widespread Shocks in an Odd Radio Circle Host Galaxy

TL;DR

Odd Radio Circles (ORCs) are a newly identified class of extragalactic radio rings. This study targets the ORC4 central galaxy with Keck KCWI+KCRM integral-field spectroscopy to map spatially resolved emission lines and gas kinematics, aiming to distinguish between AGN photoionization and shock-driven ionization as the ring's origin. The data reveal that [OII] is the strongest and most extended optical line, with ionized gas reaching about $\sim20$ kpc from the center and velocity structures with gradients exceeding $>100$ km s$^{-1}$ and dispersions around $\sim200$ km s$^{-1}$. Emission-line ratios favor shock ionization with velocities near $200$–$300$ km s$^{-1}$, rather than AGN photoionization, suggesting the ionized gas is a relic of the event that produced the large-scale radio ring.

Abstract

Odd Radio Circles (ORCs) are a new class of extragalactic object, with large rings of faint radio continuum emission typically spanning 100s of kpc; their origins are unknown. Previous optical spectroscopy of the central galaxy in ORC4, a classic isolated ORC, revealed spatially-extended ionized gas with strong [OII] emission and line ratios consistent with LINER emission. We present new Keck/KCWI+KCRM integral field spectroscopy covering multiple strong optical emission lines to measure the extent, morphology, and spatially-resolved kinematics and line ratios of the ionized and neutral gas in the ORC4 central galaxy. We find that [OII] is the strongest optical emission line in this massive, old galaxy, and the [OII] emission is more spatially extended than other optical lines, including H-alpha. The gas kinematics show strong spatial asymmetries, high velocity gradients (>100 km/s), and high velocity dispersion (~200 km/s). The emission line ratios are most consistent with shock models with shock velocities of ~200-300 km/s and are not fit well by AGN photoionization models. These findings strongly suggest that the gas in the ORC4 central galaxy is the result of shock ionization in and around the central galaxy, likely associated with the event that created the large-scale radio ring of emission that identified this source as an ORC.

Figures (2)

• Figure 1: Spatially-integrated optical spectrum of the ORC4 central galaxy, showing stellar and line emission integrated in a circular aperture of 2$\hbox{$^{\prime\prime}$}$ diameter centered on the galaxy. The upper panel shows the spectra from both the blue and red arms of KCWI+KCRM (in black), while the lower panel shows the KCRM spectrum in greater detail. The gap in wavelength coverage from $\sim$3900-4700 Å is intentional (see text for details); the blue line shows the scaled GMOS long-slit spectrum of this galaxy from Rupke24 in the wavelength range not covered by KCWI+KCRM. Emission lines are marked with red dotted vertical lines and labelled. [Oii] is the strongest line in the optical spectrum.
• Figure 2: Combined stellar continuum and emission line fits (in red, upper panels) showing [Oii], [Oiii], H$\beta$, [Ni], [Oi], [Nii] H$\alpha$, and [Sii] in the central spaxel of the ORC4 central galaxy. The data are shown in black. The lower panels show the stellar continuum fit after subtracting the line emission, shown with dotted pink lines, and cyan regions indicate the wavelengths used to identify and fit the emission lines.