Star cluster formation in the nuclear regions of nearby spiral galaxies.

摘要

Super star clusters (SSCs) are young and massive star clusters that contain hundreds to thousands of OB stars. Their masses are in excess of 10 5--106 M⊙ , suggesting that they could be the progenitors of globular clusters. The youngest SSCs are often embedded in their natal cocoons, thus their stellar continuum is obscured at optical and near-infrared wavelengths. However, they can be identified by the thermal free-free emission from their surrounding H II nebulae at radio wavelengths or by their recombination line emission in the near infrared.;In this thesis, I study the compact H II nebulae, or the "supernebulae", surrounding the embedded SSCs in nearby active star forming galaxies. Using the subarcsecond resolution radio mapping of Very Large Array (VLA) at 2 cm and 6 cm to trace free-free emission, a total of 21 supernebulae are found in four spiral galaxies---IC 342, Maffei II, NGC 2903, and NGC 6946. They are compact, a few pc in extent, and are excited by young star clusters with over a hundred O-type stars. Similar conditions in supernebulae in M 82 are investigated at 7 mm. Over 50% of these nebulae have high turnover frequencies of free-free emission, implying high electron densities of 103--104 cm -3. They lie in regions of high visual extinction, suggesting their locations at deep in the natal molecular clouds. They are young, ≲ 1.5 Myr in age. In addition to the sub-arcsecond resolution studies on compact supernebulae, the free-free emission in the arcsecond VLA 7mm map of M82 reveals a high correlation with the distribution of CO(J=1--0) line, HCN(J=1--0) line, and 8 micron PAH emission. The implications of these correlations are discussed.;Finally, utilizing the high resolution near-infrared spectroscopy of OSIRIS integral field spectrograph with adaptive optics on the Keck II Telescope, I present the a near-infrared followup study on Br gamma recombination line emission in the central 10″(300 pc) of NGC 6946. The Keck integral field spectra in K-band resolve the stellar continuum, H 2 vibrational-rotational lines, and Br gamma line with 0''. 3 resolution. The extinction map of stellar continuum agrees well with CO distribution. The visual extinction is up to AV ∼ 10, which affects the near-infrared morphology of stellar continuum significantly. In spite of the high ionization photons produced in this region, the ratio between H2 v =1-0 S(1) line and v =1-0 S(0) line suggests that collisions, rather than UV fluorescence, dominates the excitation of H2. This implies that molecular gas in this region is dense, as suggested by previous molecular gas observations. The Br gamma line emission suggests one-third of the current star formation in the NGC 6946 nuclear region consist of star clusters with masses of Mcl > 2 x 10 4 M⊙ . These clusters are forming in the principal shocks arms of nuclear molecular spirals and the nuclear molecular ring. The overall star formation efficiency reaches 20% in the nuclear CO ring. For cluster-hosting molecular clouds, the star formation efficiency could be as high as 40%. The clusters which excite the H II nebulae were formed close to their current locations. They are on their journey of inward migration, but they will be dynamically dissolved into nuclear stellar disk or bulge before reaching the center of NGC 6946.