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 μCAHA2.2m = 27.5 mag arcsec-2 (Cousins R) and μVST = 28.7 mag arcsec-2 (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 NHI = 8.9 × 1018 cm-2 (5σ) (28″ × 28″ beam), lower than in any isolated galaxy. Results. We confirm that the HI of CIG 96 extends farther than 4 × r25 in all directions. Furthermore, we detect for the first time two gaseous structures (∼106 M⊙) 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 NHI > 8.5 × 1020 cm-2, whereas the rest show red colours. Galactic cirrus contaminate the field, setting an unavoidable detection limit at 28.5 mag arcsec-2 (SDSS r). Conclusions. At the current SB and NHI levels, we detect no stellar link within 1° × 1° or gaseous link within 40′ × 40′ between CIG 96 and any companion. The isolation criteria rule out interactions with other similar-sized galaxies for at least ∼2.7 Gyr. Using existing stellar evolution models, the age of the pseudo-ring is estimated at 1 Gyr 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.
- Galaxies: Evolution
- Galaxies: Individual: NGC 864
- Galaxies: Kinematics and dynamics
- Galaxies: Spiral
- Galaxies: Structure
- Radio lines: Galaxies