Giant molecular clouds in the Local Group galaxy M 33

Abstract

We present an analysis of a systematic CO(2-1) survey at 12 resolution covering most of the Local Group spiral M 33, which, at a distance of 840 kpc, is close enough for individual giant molecular clouds (GMCs) to be identified. The goal of this work is to study the properties of the GMCs in this subsolar metallicity galaxy. The CPROPS (Cloud PROPertieS) algorithm was used to identify 337 GMCs in M 33, the largest sample to date for an external galaxy. The sample is used to study the GMC luminosity function, or mass spectrum under the assumption of a constant N(H 2 )/I CO ratio. We find that n(L)dL ∝ L −2.0±0.1 for the entire sample. However, when the sample is divided into inner and outer disk samples, the exponent changes from 1.6 ± 0.2 in the center 2 kpc to 2.3 ± 0.2 for galactocentric distances larger than 2 kpc. On the basis of the emission in the FUV, Hα, 8 µm, and 24 µm bands, each cloud was classified in terms of its star-forming activity -no star formation or either embedded or exposed star formation (visible in FUV and Hα). At least one sixth of the clouds had no (massive) star formation, suggesting that the average time required for star formation to start is about one sixth of the total time for which the object is identifiable as a GMC. The clouds without star formation have significantly lower CO luminosities than those with star formation, whether embedded or exposed, a result that is presumably related to the lack of heating sources. Taking the cloud sample as a whole, the main non-trivial correlation is the decrease in cloud CO brightness (or luminosity) with galactocentric radius. The complete cloud catalog, including the CO and HI spectra and the CO contours overlaid on the FUV, Hα, 8 µm, and 24 µm images is presented in the appendix.

Cool and warm dust emission from M 33 (HerM33es)

“…This picture is in excellent agreement with other tracers of the ISM such as the atomic and the molecular hydrogen Gratier et al 2010Gratier et al , 2012. Many localized IR sources are well-detected in the Herschel bands.…”

The formation and destruction of molecular clouds and galactic star formation

“…Therefore, we expect that T d can be smaller than T f in such an environment, which may produce α = 1 + T f /T d > 2. This tendency is actually observed in the Milky Way outer disk, LMC, M33, and M51 (Rosolowsky 2005;Wong et al 2011;Gratier et al 2012;Hughes et al 2010;Colombo et al 2014) .…”

Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks

“…Therefore, the arm regions presumably have a larger number of massive stars forming HII regions and supernova remnants thus experience the recurrent supersonic compression twice more frequently than the fiducial case, and the inter-arm regions typically have a smaller number of massive stars thus experience less frequent supersonic compression, which results in about factor two longer formation timescale than the fiducial case. This may also explain the observed steep slope at outskirts of galaxies (e.g., the observed steep slope in the CO luminosity function in Galaxy M33; see Gratier et al 2012) where star formation is less active compared with normal disk regions. Note that the above argument might be modified, if the effect of large-scale dynamics (e.g., interaction with shock waves or strong shear flows) may play an important role in the destruction of GMCs than the stellar feedback, especially in the outskirts of galaxies with prominent spiral structures.…”