Astrophys Space Sci (2008) 313: 293–296DOI 10.1007/s10509-007-9620-7

O R I G I NA L A RT I C L E

Observations of molecular clouds in nearby galaxies with ALMA

Nario Kuno · Akihiko Hirota · Tomoka Tosaki ·Rie Miura

Received: 31 January 2007 / Accepted: 26 July 2007 / Published online: 9 September 2007© Springer Science+Business Media B.V. 2007

Abstract We present recent results of the observationsof giant molecular clouds in nearby galaxies with theNobeyama 45 m telescope and Millimeter Array. We givesome brief comments about observations of GMCs in nearbygalaxies with ALMA.

Keywords Galaxies · Star formation · Molecular clouds

1 Nobeyama CO atlas of nearby spiral galaxies

We made a CO mapping survey of 40 nearby spiral galaxieswith the multi-beam receiver BEARS (25-Beam Array Re-ceiver System) mounted on the 45 m telescope (Kuno et al.2007). It is often said that spiral density waves trigger starformation, while bars suppress star formation by large shear.So, it is interesting to investigate the relation between SFEand strength of arm and bar. Using the CO data, we are in-vestigating the spatial variation of star formation efficiency(SFE) in some galaxies to study the influence of spiral andbar structures on star formation. We defined arm, interarm,bar, and central regions using 2MASS image (Fig. 1). SFRin each region was derived from Hα data and total gas masswas derived from HI and CO data. Figure 2 shows SFE inarm, interarm, bar, and central region in NGC 4321. In NGC4321, SFE in bar is lower than that in the arms, although the

N. Kuno (�) · T. TosakiNobeyama Radio Observatory, Minamimaki-mura,Minamisaku-gun, Nagano 384-1305, Japane-mail: kuno@nro.nao.ac.jp

A. Hirota · R. MiuraUniversity of Tokyo, Bunkyo-ku, Tokyo 113-033, Japan

Fig. 1 CO integrated intensity map of NGC 4321 (Kuno et al. 2007).The arm, interarm, bar, and central regions are divided by solid line.The crosses indicate the observing grid

surface density of total gas is comparable. SFE in the inter-arm regions is also lower than that in the arms. These arepreliminary results and we are making same comparison forother galaxies to see the relation between SFE and strengthof arm and bar.

2 Observations of GMCs in nearby galaxies

To understand the reason for the spatial variation of SFE, itis important to investigate the difference of the properties of

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Fig. 2 SFE in arm, interarm, bar, and central regions in NGC 4321derived from Hα, HI and CO data (Hirota et al. 2007)

Fig. 3 CO integrated intensity map of IC342 obtained with the 45 mtelescope and NMA (Hirota et al. 2007). The circles indicate identifiedmolecular clouds

molecular clouds, such as mass function and dense gas frac-tion. For such studies, we have to resolve individual molec-ular cloud in nearby galaxies. We present some observationsof giant molecular clouds in nearby galaxies performed withNMA.

Fig. 4 Comparison betweenvirial mass and CO luminositymass of the molecular cloudsidentified in the arm region(Hirota et al. 2007)

2.1 IC 342

We observed the spiral arm and bar regions with the 45m telescope and NMA to investigate how these structureschange the properties of molecular clouds. Here, we presentthe results of the arm region. Figure 3 is the CO map ob-tained by combining the data with the 45 m telescope andNMA. We identified 25 clouds in the arm region. We foundthe velocity change across the arm as often seen in many spi-ral galaxies. So, we separated these clouds into two groups,namely, clouds located upstream and downstream of the ve-locity change.

Figure 4 is the comparison of virial mass and CO lumi-nosity mass of the clouds. We can see the property of molec-ular clouds changes across the arm. That is, upstream diffuseclouds become gravitationally bound downstream. Or theupstream clouds may be simply assembly of small cloudswhich are not gravitationally bound. If we can get higherspatial resolution, we may be able to distinguish them andsee the formation process of GMCs directly.

2.2 M31

In many spiral galaxies, complexes of star forming regionswhose size is comparable with GMAs (Giant Molecular As-sociations) are often seen. In order to study how star forma-tion proceeds in GMAs, we made observations of a GMA inM31 to resolve its internal structure. Figures 5 and 6 are theCO map obtained with the 45 m telescope and NMA, respec-tively. Within the GMA, we can see many clumps whosesize is similar to GMC in Milky Way. The missing flux isestimated to be about 50%. These results suggest that theGMA consists of many GMCs and diffuse envelope. We aresearching where dense gas is formed in the GMA by observ-ing with dense gas tracers.

2.3 M33

We observed molecular clouds around the giant HII regionNGC 604. Figure 7 (right) is the CO map of NGC 604 ob-tained with the 45 m telescope. We found some complexesof molecular clouds that were not detected in the previous

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Fig. 5 CO map of M31 obtained with NRO 45 m telescope. The circleindicates the field of view of NMA (Tosaki et al. 2007)

Fig. 6 CO map of M31 obtained with NMA. The circle indicates thefield of view of NMA (Tosaki et al. 2007)

Fig. 7 CO map of M33 obtained with NRO 45 m telescope (right) and NMA (left). The circle indicates the field of view of NMA (Miura et al.2007)

observations. Figure 7 (left) is the CO map of the two com-plexes in Fig. 1 obtained with NMA. We identified 8 molec-ular clouds in the complexes whose size and mass are com-

parable with those of GMCs in our Galaxy. Furthermore, wemade observations of HCN and radio continuum and foundthat these clouds are separated into four types, although the

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mass of the molecular clouds are almost the same; molecularclouds with (1) CO, (2) CO + HCN, (3) CO + HCN + radiocontinuum, (4) CO + radio continuum. In this region, se-quential star formation seems to be triggered by NGC 604.The difference of the properties of molecular clouds maymean the difference of the stage of star formation, that is,from early stage clouds which have dense gas traced byHCN to late stage clouds which have star forming regionstraced by radio continuum.

3 Observations of GMCs in nearby galaxies withALMA

We can resolve only giant molecular clouds in nearby galax-ies with present instruments. For example, however, if wecan get 0.1′′ resolution with ALMA, it corresponds to 5 pcat a distance of 10 Mpc. So, we can expect to resolveeven much smaller molecular clouds in nearby galaxies with

ALMA. Furthermore, we can observe dense gas in individ-ual molecular cloud with high sensitivity of ALMA. So, wecan study the relation between star formation efficiency andthe properties of molecular clouds in detail. And it must giveus some clues for understanding of massive star formation.

However, even for ALMA, it takes long observing timeto map entire disk of nearby galaxies with such high angularresolution and high sensitivity. So, we have to make obser-vations with present instruments which can be used to selecttarget area for ALMA observations.

References

Hirota, et al.: (2007, in preparation)Kuno, et al.: Publ. Astron. Soc. Jpn. 59, 117 (2007)Miura, et al.: (2007, in preparation)Tosaki, et al.: (2007, in preparation)