How does star formation proceed in the circumnuclear starburst ring of NGC 6951?^⋆,⋆⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: vanderlaan@iram.fr
² European Southern Observatory, Karl-Schwarzschild-Str 2, 85748 Garching, Germany
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Gemini Observatory, Northern Operations center, 670 N. A’ohoku Place, Hilo, HI 96720, USA
⁵ Center for Astrophysical Sciences, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA

Received: 24 August 2012
Accepted: 8 January 2013

Abstract

Gas inflowing along stellar bars is often stalled at the location of circumnuclear rings, which form an effective reservoir for massive star formation and thus shape the central regions of galaxies. However, how exactly star formation proceeds within these circumnuclear starburst rings is the subject of debate. Two main scenarios for this process have been put forward. In the first, the onset of star formation is regulated by the total amount of gas present in the ring with star forming starting, once a mass threshold has been reached, in “random” positions within the ring like “popcorn”. In the second, star formation primarily takes place near the locations where the gas enters the ring. This scenario has been dubbed “pearls-on-a-string”. Here we combine new optical IFU data covering the full stellar bar with existing multiwavelength data to study the 580 pc radius circumnuclear starburst ring in detail in the nearby spiral galaxy NGC 6951. Using Hubble Space Telescope (HST) archival data together with SAURON  and OASIS  IFU data, we derive the ages and stellar masses of star clusters, as well as the total stellar content of the central region. Adding information on the molecular gas distribution, stellar and gaseous dynamics, and extinction, we find that the circumnuclear ring in NGC 6951 is ~1–1.5 Gyr old and has been forming stars for most of that time. We see evidence for preferred sites of star formation within the ring, consistent with the “pearls-on-a-string” scenario, when focusing on the youngest stellar populations. The ring’s longevity means that this signature is washed out when older stellar populations are included in the analysis.