Aperture Synthesis Observations of CO, HCN, and 89GHz Continuum Emission toward NGC 604 in M 33: Sequential Star Formation Induced by Supergiant Hii region

Abstract

We present the results from new Nobeyama Millimeter Array observations of CO(1-0), HCN(1-0), and 89-GHz continuum emissions toward NGC 604, known as the supergiant H ii region in a nearby galaxy M 33. Our high spatial resolution images of CO emission allowed us to uncover ten individual molecular clouds that have masses of (0.8 -7.4) 10${}^5$M$_{\sun }$ and sizes of 5 -- 29 pc, comparable to those of typical Galactic giant molecular clouds (GMCs). Moreover, we detected for the first time HCN emission in the two most massive clouds and 89 GHz continuum emission at the rims of the "H$\alpha$ shells". Three out of ten CO clouds are well correlated with the H$\alpha$ shells both in spatial and velocity domains, implying an interaction between molecular gas and the expanding H ii region. Furthermore, we estimated star formation efficiencies (SFEs) for each cloud from the 89-GHz and combination of H$\alpha$ and 24-$\mu$m data, and found that the SFEs decrease with increasing projected distance measured from the heart of the central OB star cluster in NGC 604, suggesting the radial changes in evolutionary stages of the molecular clouds in course of stellar cluster formation. Our results provide further support to the picture of sequential star formation in NGC604 initially proposed by Tosaki et al. (2007) with the higher spatially resolved molecular clouds, in which an isotropic expansion of the H ii region pushes gases outward and accumulates them to consecutively form dense molecular clouds, and then induces massive star formations.

23 pages, 8 figures, accepted for publication in ApJ

References in corpus (3)

• Triggered massive-star formation on the borders of Galactic HII regions. IV- Star formation at the periphery of Sh2-212
• The Chandra ACIS Survey of M33 (ChASeM33): Investigating the Hot Ionized Medium in NGC604
• A photodissociation model for the morphology of the HI near OB associations in M33

Cited by in corpus (15)

• NRO M33 All Disk Survey of Giant Molecular Clouds (NRO MAGiC): I. HI to H_2 Transition
• Dust Properties in HII Regions in M33
• Triggering the formation of the supergiant H II region NGC 604 in M33
• Enhancement of CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies in Supergiant HII Regions
• Photoevaporation of Molecular Gas Clumps Illuminated by External Massive Stars: Clump Lifetimes and Metallicity Dependence
• ALMA observations of the dense and shocked gas in the nuclear region of NGC 4038 (Antennae galaxies)
• ALMA Observations of Giant Molecular Clouds in M33. II. Triggered High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604 Complex
• Ongoing Massive Star Formation in NGC 604
• Investigation of Molecular Cloud Structure around Infrared Bubbles: CARMA Observations of N14, N22, and N74
• NRO M33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC): II. Dense Gas Formation within Giant Molecular Clouds in M33
• 13CO(J=1-0) On-the-fly Mapping of the Giant HII Region NGC 604: Variation in Molecular Gas Density and Temperature due to Sequential Star Formation
• Unveiling the new generation of stars in NGC 604 with Gemini-NIRI
• ALMA Observations of the Molecular Clouds in NGC 625
• ALMA ${}^{13}\mathrm{CO}$(J=1-0) Observations of NGC 604 in M33: Physical Properties of Molecular Clouds
• SIGNALS of Giant HII Regions: A Spatially Resolved Analysis of NGC 604