High angular resolution near-infrared integral field observations of young star cluster complexes in NGC 1365^⋆

¹ Observatório Nacional, Rua General José Cristino, 77, 20921-400 São Cristovão, Rio de Janeiro, Brazil
e-mail: egallian@on.br; etelles@on.br
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: mkissler@eso.org
³ Observatoire de Haute-Provence, OAMP, 04870 Saint Michel l’Observatoire, France
e-mail: danielle.alloin@oamp.fr

Received: 11 January 2012
Accepted: 21 July 2012

Abstract

This paper presents and examines new near-infrared integral field observations of the three so-called “embedded star clusters” located in the nuclear region of NGC 1365. Adaptive-optics-corrected K-band data cubes were obtained with the ESO/VLT instrument SINFONI. The continuum in the K-band and emission lines such as He I, Brγ, and several H₂ lines were mapped at an achieved angular resolution of 0.2′′ over a field of 3  ×  3′′² around each source. We find that the continuum emission of the sources is spatially resolved. This means that they are indeed cluster complexes confined to regions of about 50 pc extension. We performed robust measurements of the equivalent width of the CO absorption band at 2.3   μm and of Brγ. For the main mid-infrared bright sources, the data only allow us to determine an upper limit to the equivalent width of the CO bands. Under the assumption of an instantaneously formed standard initial mass function Starburst99 model, the new measurements are found to be incompatible with previously published mid-infrared line ratios. We show that an upper mass limit of 25 to 30 M_⊙, lower than the typically assumed 100 M_⊙, allows one to simply remove this inconsistency. For such a model, the measurements are consistent with ages in the range of 5.5 Myr to 6.5 Myr, implying masses in the range from 3 to 10  ×  10⁶ M_⊙. We detect extended gas emission both in H ii and H₂. We argue that the central cluster complexes are the sources of excitation for the whole nebulae, through ionisation and shock heating. We detect a blue wing on the Brγ emission profile, suggesting the existence of gas outflows centred on the cluster complexes. We do not find any evidence for the presence of a lower mass cluster population, which would fill up a “traditional” power law cluster mass function.