We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71 GMCs in total, based on wide field and high sensitivity CO(J=3-2) observations with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing archival optical da
ta, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and HII regions enable us to classify GMCs into four categories: Type A showing no sign of massive star formation (SF), Type B being associated only with HII regions, Type C with both HII regions and <10 Myr-old YSGs and Type-D with both HII regions and 10--30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10^5 Mo is estimated to be 20--40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J=3-2)/CO(J=1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.
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.
We have mapped the northern area (30 times 20) of a local group spiral galaxy M33 in 12CO(J=1-0) line with the 45-m telescope at the Nobeyama Radio Observatory. Along with Halpha and Spitzer 24-micron data, we have investigated the relationship betwe
en the surface density of molecular gas mass and that of star formation rate (SFR) in an external galaxy (Kennicutt-Schmidt law) with the highest spatial resolution (~80pc) to date, which is comparable to scales of giant molecular clouds (GMCs). At positions where CO is significantly detected, the SFR surface density exhibits a wide range of over four orders of magnitude, from Sigma(SFR)<10^{-10} to ~10^{-6}M_solar yr^{-1} pc^{-2}, whereas the Sigma(H2) values are mostly within 10 to 40 M_solar pc^{-2}. The surface density of gas and that of SFR correlate well at a 1-kpc resolution, but the correlation becomes looser with higher resolution and breaks down at GMC scales. The scatter of the Sigma(SFR)-Sigma(H2) relationship in the 80-pc resolution results from the variety of star forming activity among GMCs, which is attributed to the various evolutionary stages of GMCs and to the drift of young clusters from their parent GMCs. This result shows that the Kennicutt-Schmidt law is valid only in scales larger than that of GMCs, when we average the spatial offset between GMCs and star forming regions, and their various evolutionary stages.
We present a new on-the-fly (OTF) mapping of CO(J=3-2) line emission with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 8 x 8 (or 10.5 x 10.5 kpc at the distance of 4.5 Mpc) region of the nearby barred spiral galaxy M 83 at an effe
ctive resolution of 25. Due to its very high sensitivity, our CO(J=3-2) map can depict not only spiral arm structures but also spur-like substructures extended in inter-arm regions. This spur-like substructures in CO(J=3-2) emission are well coincident with the distribution of massive star forming regions traced by Halpha luminosity and Spitzer/IRAC 8 um emission. We have identified 54 CO(J=3-2) clumps as Giant Molecular-cloud Associations (GMAs) employing the CLUMPFIND algorithm, and have obtained their sizes, velocity dispersions, virial masses, and CO luminosity masses. We found that the virial parameter alpha, which is defined as the ratio of the virial mass to the CO luminosity mass, is almost unity for GMAs in spiral arms, whereas there exist some GMAs whose alpha are 3 -- 10 in the inter-arm region. We found that GMAs with higher $alpha$ tend not to be associated with massive star forming regions, while other virialized GMAs are. Since alpha mainly depends on velocity dispersion of the GMA, we suppose the onset of star formation in these unvirialized GMAs with higher alpha are suppressed by an increase in internal velocity dispersions of Giant Molecular Clouds within these GMAs due to shear motion.
We present aperture synthesis high-resolution (~ 7 x 3) observations in CO(J=1-0) line, HCN(J=1-0) line, and 95 GHz continuum emission toward the central (~ 1.5 kpc) region of the nearby barred spiral galaxy M 83 with the Nobeyama Millimeter Array. O
ur high-resolution CO(J=1-0) mosaic map depicts the presence of molecular ridges along the leading sides of the stellar bar and nuclear twin peak structure. On the other hand, we found the distribution of the HCN(J=1-0) line emission which traces dense molecular gas (nH2 > a few x 10^4 cm^-3) shows nuclear single peak structure and coincides well with that of the 95 GHz continuum emission which traces massive starburst. The peaks of the HCN(J=1-0) line and the 95 GHz continuum emission are not spatially coincident with the optical starburst regions traced by the HST V-band image. This suggests the existence of deeply buried ongoing starburst due to strong extinction (A_v ~ 5 mag) near the peaks of the HCN(J=1-0) line and the 95 GHz continuum emission. We found that the HCN(J=1-0)/CO(J=1-0) intensity ratio R_HCN/CO correlates well with extinction-corrected SFE in the central region of M 83 at a resolution of 7.5 (~ 160 pc). This suggests that SFE is controlled by dense gas fraction traced by R_HCN/CO even on a Giant Molecular cloud Association (GMA) scale. Moreover, the correlation between R_HCN/CO and the SFE in the central region of M 83 seems to be almost coincident with that of the Gao & Solomon (2004a) sample. This suggests that the correlation between R_HCN/CO and the SFE on a GMA (~ 160 pc) scale found in M 83 is the origin of the global correlation on a few kpc scale shown by Gao & Solomon (2004a).
