No evidence for large-scale outflows in the extended ionised halo of ULIRG Mrk273

TL;DR

Deep imaging and spectroscopy of Mrk273 reveal an extended ionised halo with no signs of large-scale outflows, suggesting the halo is tidal debris from a merger rather than outflowing gas.

Contribution

This study provides the first detailed kinematic analysis of the extended ionised halo in Mrk273, challenging previous assumptions of large-scale outflows.

Findings

Extended halo up to 45 kpc with narrow emission lines

No evidence of kinematic disturbance beyond 6 kpc

Halo likely consists of tidal debris from a merger

Abstract

We present deep new GTC/OSIRIS narrow-band images and optical WHT/ISIS long-slit spectroscopy of the merging system Mrk273 that show a spectacular extended halo of warm ionised gas out to a radius of $\sim45$ kpc from the system nucleus. Outside of the immediate nuclear regions (r > 6 kpc), there is no evidence for kinematic disturbance in the ionised gas: in the extended regions covered by our spectroscopic slits the emission lines are relatively narrow (FWHM $\lesssim$ 350 km$\rm s^{-1}$) and velocity shifts small (|$\Delta$V| $\lesssim{} $250 km$\rm s^{-1}$). This is despite the presence of powerful near-nuclear outflows (FWHM > 1000 km$\rm s^{-1}$; |$\Delta$V| > 400 km$\rm s^{-1}$; r < 6 kpc). Diagnostic ratio plots are fully consistent with Seyfert 2 photo-ionisation to the NE of the nuclear region, however to the SW the plots are more consistent with low-velocity radiative shock models. The kinematics of the ionised gas, combined with the fact that the main structures are aligned with low-surface-brightness tidal continuum features, are consistent with the idea that the ionised halo represents tidal debris left over from a possible triple-merger event, rather than a reservoir of outflowing gas.