2018A&A...619A.163R

Context. Asymmetries in atomic hydrogen (HI) in galaxies are often caused by the interaction with close companions, making isolated galaxies an ideal framework to study secular evolution. The AMIGA project has demonstrated that isolated galaxies show the lowest level of asymmetry in their HI integrated profiles compared to even field galaxies, yet some present significant asymmetries. CIG 96 (NGC 864) is a representative case reaching a 16% level.
Aims. Our aim is to investigate the HI asymmetries of the spiral galaxy CIG 96 and what processes have triggered the star-forming regions observed in the XUV pseudo-ring.
Methods. We performed deep optical observations at CAHA1.23m, CAHA2.2m and VST (OmegaCAM wide-field camera) telescopes. We reach surface brightness (SB) limits of µ_CAHA2.2m=27.5mag/arcsec² (Cousins R) and µ_VST=28.7mag/arcsec² (SDSS r) that show the XUV pseudo-ring of the galaxy in detail. Additionally, a wavelet filtering of the HI data cube from our deep observations with VLA/EVLA telescope allowed us to reach a column density of N_HI=8.9x10¹⁸cm^–2 (5σ) (28''x28'' beam), lower than in any isolated galaxy.
Results. We confirm that the HI of CIG 96 extends farther than 4xr₂₅ in all directions. Furthermore, we detect for the first time two gaseous structures (∼10⁶M_☉) in the outskirts. The SDSS g-r colour index image from CAHA1.23m shows extremely blue colours in certain regions of the pseudo-ring where N_HI>8.5x10²⁰cm^–2, whereas the rest show red colours. Galactic cirrus contaminate the field, setting an unavoidable detection limit at 28.5mag/arcsec² (SDSS r).
Conclusions. At the current SB and N_HI levels, we detect no stellar link within 1°x1° or gaseous link within 40'x40' between CIG 96 and any companion. The isolation criteria rule out interactions with other similar-sized galaxies for at least ∼2.7Gyr. Using existing stellar evolution models, the age of the pseudo-ring is estimated at 1Gyr or older. Undetected previously accreted companions and cold gas accretion remain as the main hypothesis to explain the optical pseudo-ring and HI features of CIG 96.