Molecular gas in NUclei of GAlaxies (NUGA) XIII. The interacting Seyfert 2/LINER galaxy NGC 5953^*

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: casasola@arcetri.astro.it
² INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
³ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
⁴ Observatorio Astronómico Nacional (OAN) – Observatorio de Madrid, C/ Alfonso XII, 3, 28014 Madrid, Spain
⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicherstrasse 77, 50937 Köln, Germany
⁶ IRAM – Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d'Hères, France
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 3 August 2009
Accepted: 7 November 2009

Abstract

We present ¹²CO(1–0) and ¹²CO(2–1) maps of the interacting Seyfert 2/LINER galaxy NGC 5953 obtained with the IRAM interferometer at resolutions of 21 14 and 11 07, respectively. We also present single-dish IRAM 30 m observations of the central region of NGC 5953 for the ¹²CO(1–0), ¹²CO(2–1), and HCN(1–0) transitions at resolutions of 22´´, 12´´, and , respectively. The CO emission is distributed over a disk of diameter ~16´´(~2.2 kpc), within which are several, randomly distributed peaks. The strongest peak does not coincide with the nucleus, but is instead offset from the center, ~2–3 (~340 pc) toward the west/southwest. The kinematics of the molecular component are quite regular, as is typical of a rotating disk. We also compared the ¹²CO distribution of NGC 5953 with observations at other wavelengths in order to study correlations between different tracers of the interstellar medium. The HST/F606W WFPC2 images show flocculent spiral structures and an “S-shape” feature ≳60 pc in radius, possibly associated with a nuclear bar or with the radio jet. A two-dimensional bulge/disk decomposition of the H-band (HST/F160W) and 3.6 μm (Spitzer/IRAC) images reveals a circumnuclear “ring” ~10–14´´ in diameter, roughly coincident in size with the CO disk and with a star-forming ring previously identified in ionized gas. This ring is not present in the near-infrared (NIR) color image, nor is it present in the “dust-only” image constructed from the 8  μm IRAC map. The implication is that the excess residual ring is stellar, with colors similar to the surrounding disk. We interpret this ring, visible in ionized gas, which appears as stars in the NIR, and with no sign of hot dust, as due to a red super giant population at least 10–15 Myr old. However, star formation is still ongoing in the disk and in the ring itself. Using NIR images, we computed the gravity torques exerted by the stellar potential on the gas. The torques are predominantly positive in both ¹²CO(1–0) and ¹²CO(2–1), suggesting that gas is not flowing into the center, and less than 5% of the gas angular momentum is exchanged in each rotation. This comes from the regular and almost axisymmetric total mass and gas distributions in the center of the galaxy. In NGC 5953, the AGN is apparently not being actively fueled in the current epoch.