Molecular tendrils feeding star formation in the Eye of the Medusa

The Medusa merger in high resolution ¹²CO 2–1 maps

¹ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d’Hères, France
e-mail: koenig@iram.fr
² Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, 43992 Onsala, Sweden
³ Department of Astronomy, University of Wisconsin, 475 N. Charter Street, Madison, WI 53706, USA
⁴ University of Manchester, Jodrell Bank Centre for Astrophysics, Oxford Road, Manchester, M13 9PL, UK
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Astronomisches Institut Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany

Received: 31 January 2014
Accepted: 23 June 2014

Abstract

Studying molecular gas properties in merging galaxies gives us important clues to the onset and evolution of interaction-triggered starbursts. NGC 4194 (the Medusa merger) is particularly interesting to study, since its FIR-to-CO luminosity ratio rivals that of ultraluminous galaxies (ULIRGs), despite its lower luminosity compared to ULIRGs, which indicates a high star formation efficiency (SFE) that is relative to even most spirals and ULIRGs. We study the molecular medium at an angular resolution of 0.65′′ × 0.52′′ (~120 × 98 pc) through our observations of ¹²CO 2−1 emission using the Submillimeter Array (SMA). We compare our ¹²CO 2−1 maps with the optical Hubble Space Telescope and high angular resolution radio continuum images to study the relationship between molecular gas and the other components of the starburst region. The molecular gas is tracing the complicated dust lane structure of NGC 4194 with the brightest emission being located in an off-nuclear ring-like structure with ~320 pc radius, the Eye of the Medusa. The bulk CO emission of the ring is found south of the kinematical center of NGC 4194. The northern tip of the ring is associated with the galaxy nucleus, where the radio continuum has its peak. Large velocity widths associated with the radio nucleus support the notion of NGC 4194 hosting an active galactic nucleus. A prominent, secondary emission maximum in the radio continuum is located inside the molecular ring. This suggests that the morphology of the ring is partially influenced by massive supernova explosions. From the combined evidence, we propose that the Eye of the Medusa contains a shell of swept up material where we identify a number of giant molecular associations. We propose that the Eye may be the site of an efficient starburst of 5−7 M_⊙ yr^-1, but it would still constitute only a fraction of the 30−50 M_⊙ yr^-1 star formation rate of the Medusa. Furthermore, we find that ~50% of the molecular mass of NGC 4194 is found in extended filamentary-like structures tracing the minor and major axis dust lanes. We suggest that molecular gas is transported along these lanes, providing the central starburst region with fuel. Interestingly, a comparison with locations of super star clusters (SSCs) reveal that the molecular gas and the SSCs are not co-spatial.