Unveiling the environment and faint features of the isolated galaxy CIG
  96 with deep optical and HI observations
  Asymmetries in 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, yet some present
significant asymmetries. CIG 96 (NGC 864) is a representative case reaching a
16% level. Our aim is to investigate the HI asymmetries of this spiral galaxy
and what processes have triggered the star-forming regions observed in the XUV
pseudoring. We performed deep optical observations at CAHA 1.23m, 2.2m and VST
telescopes. We reach surface brightness (SB) limits of mu_2.2m = 27.5 mag
arcsec-2 (Cous R) and mu_VST = 28.7mag arcsec-2 (r) that show the XUV
pseudoring of the galaxy in detail. Additionally, a wavelet filtering of the HI
data cube from our deep observations with E/VLA telescope allowed us to reach a
column density of N_HI = 8.9x10^18 cm -2 (5sigma) (28"x28" beam), lower than in
any isolated galaxy. We confirm that the HI extends farther than 4xr_25 in all
directions. Furthermore, we detect for the first time two gaseous structures
(10^6 Msol) in the outskirts. The g-r colour index image from 1.23m shows
extremely blue colours in certain regions of the pseudoring where
N_HI>8.5x10^20 cm-2 , whereas the rest show red colours. Galactic cirrus
contaminate the field, setting an unavoidable detection limit at 28.5mag
arcsec-2 (r). We detect no stellar link within 1degx1deg 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 pseudoring is estimated at
1Gyr or older. Undetected previously accreted companions and cold gas accretion
remain as the main hypothesis to explain the optical pseudoring and HI features
of CIG 96.
