Unveiling the environment and faint features of the isolated galaxy CIG 96 with deep optical and HI observations^⋆

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: prm@iaa.es, pramirezmoreta@gmail.com
² Joint ALMA Observatory – ESO, Av. Alonso de Córdova, 3104 Santiago, Chile
³ Kapteyn Instituut, Postbus 800, 9700 AV Groningen, The Netherlands
⁴ Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
⁵ Department of Astronomy, University of Massachusetts-Amherst, LGRT-B 522 710 North Pleasant Street, Amherst, MA, USA
⁶ Observatori Astronòmic de la Universitat de València, Catedrático José Beltrán, 2, 46980 Paterna, Spain
⁷ National Astronomical Observatory of Japan (NAOJ), 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁸ The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, 181-0015 Tokyo, Japan
⁹ Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France
¹⁰ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ UK
¹¹ Universidad de Antofagasta, Unidad de Astronomía, Facultad Cs. Básicas, Av. U. de Antofagasta 02800, Antofagasta, Chile
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ Estación Experimental de Zonas Áridas (CSIC), Ctra. de Sacramento s/n, La Cañada de San Urbano, 04120 Almería, Spain
¹⁴ Instituto de Astrofísica de Canarias, c/Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain

Received: 1 May 2018
Accepted: 30 July 2018

Abstract

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 = 27.5 mag arcsec⁻² (Cousins R) and μ_VST = 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 N_HI = 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 N_HI > 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 N_HI 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.