Molecular Clouds In M33 Research Articles

Clusters, clouds, and correlations: relating young clusters to giant molecular clouds in M33 and M31

ABSTRACT We use young clusters and giant molecular clouds (GMCs) in the galaxies M33 and M31 to constrain temporal and spatial scales in the star formation process. In M33, we compare the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) catalogue of 1214 clusters with ages measured via colour–magnitude diagram (CMD) fitting to 444 GMCs identified from a new 35 pc resolution Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(2–1) survey. In M31, we compare the Panchromatic Hubble Andromeda Treasury (PHAT) catalogue of 1249 clusters to 251 GMCs measured from a Combined Array for Research in Millimeter-wave Astronomy (CARMA) 12CO(1–0) survey with 20 pc resolution. Through two-point correlation analysis, we find that young clusters have a high probability of being near other young clusters, but correlation between GMCs is suppressed by the cloud identification algorithm. By comparing the positions, we find that younger clusters are closer to GMCs than older clusters. Through cross-correlation analysis of the M33 cluster data, we find that clusters are statistically associated when they are ≤10 Myr old. Utilizing the high precision ages of the clusters, we find that clusters older than ≈18 Myr are uncorrelated with the molecular interstellar medium (ISM). Using the spatial coincidence of the youngest clusters and GMCs in M33, we estimate that clusters spend ≈4–6 Myr inside their parent GMC. Through similar analysis, we find that the GMCs in M33 have a total lifetime of ≈11–15 Myr. We also develop a drift model and show that the above correlations can be explained if the clusters in M33 have a 5–10 km s−1 velocity dispersion relative to the molecular ISM.

ALMA Observations of Giant Molecular Clouds in M33. III. Spatially Resolved Features of the Star formation Inactive Million-solar-mass Cloud

Abstract We present 12CO (J = 2–1), 13CO (J = 2–1), and C18O (J = 2–1) observations toward GMC-8, one of the most massive giant molecular clouds (GMCs) in M33 using ALMA with an angular resolution of 0.″44 × 0.″27 (∼2 pc × 1 pc). The earlier studies revealed that its high-mass star formation is inactive in spite of a sufficient molecular reservoir with a total mass of ∼106 M ⊙. The high-angular resolution data enable us to resolve this peculiar source down to a molecular clump scale. One of the GMC’s remarkable features is that a round-shaped gas structure (the “Main cloud”) extends over the ∼50 pc scale, which is quite different from the other two active star-forming GMCs dominated by remarkable filaments/shells obtained by our series of studies in M33. The fraction of the relatively dense gas traced by the 13CO data with respect to the total molecular mass is only ∼2%, suggesting that their spatial structure and the density are not well developed to reach an active star formation. The CO velocity analysis shows that the GMC is composed of a single component as a whole, but we found some local velocity fluctuations in the Main cloud and extra blueshifted components at the outer regions. Comparing the CO with previously published large-scale H i data, we suggest that an external atomic gas flow supplied a sufficient amount of material to grow the GMC up to ∼106 M ⊙.

ALMA Observations of Giant Molecular Clouds in M33. II. Triggered High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604 Complex

Abstract We present the results of ALMA observations in 12CO(J = 2 − 1), 13CO(J = 2 − 1), and C18O(J = 2 − 1) lines and 1.3 mm continuum emission toward a massive (∼106 M ⊙) giant molecular cloud associated with the giant H ii region NGC 604 in one of the nearest spiral galaxies, M33, at an angular resolution of 0.″44 × 0.″27 (1.8 pc × 1.1 pc). The 12CO and 13CO images show highly complicated molecular structures composed of a lot of filaments and shells whose lengths are 5–20 pc. We found three 1.3 mm continuum sources to be dense clumps at the edges of two shells and also at the intersection of several filaments. We examined the velocity structures of the 12CO(J = 2 − 1) emission in the shells and filaments containing dense clumps, and concluded that expansion of the H ii regions cannot explain the formation of such dense cores. Alternatively, we suggest that cloud–cloud collisions induced by an external H i gas flow and the galaxy’s rotation compressed the molecular material into dense filaments/shells which are ongoing high-mass star formation sites. We propose that multiple gas converging/colliding events with a velocity of a few tens of kilometers per second are necessary to build up NGC 604, the most significant cluster-forming complex in the Local Group of galaxies.

ALMA Observations of Giant Molecular Clouds in M33. I. Resolving Star Formation Activities in the Giant Molecular Filaments Possibly Formed by a Spiral Shock

Abstract We report molecular line and continuum observations toward one of the most massive giant molecular clouds (GMCs), GMC-16, in M33 using ALMA with an angular resolution of 0.″44 × 0.″27 (∼2 pc × 1 pc). We have found that the GMC is composed of several filamentary structures in 12CO and 13CO(J = 2–1). The typical length, width, and total mass are ∼50–70 pc, ∼5–6 pc, and ∼105 M ⊙, respectively, which are consistent with those of giant molecular filaments (GMFs) as seen in the Galactic GMCs. The elongations of the GMFs are roughly perpendicular to the direction of the galaxy’s rotation, and several H ii regions are located at the downstream side relative to the filaments with an offset of ∼10–20 pc. These observational results indicate that the GMFs are considered to be produced by a galactic spiral shock. The 1.3 mm continuum and C18O(J = 2–1) observations detected a dense clump with the size of ∼2 pc at the intersection of several filamentary clouds, which is referred to as the “hub filament,” possibly formed by a cloud–cloud collision. A strong candidate for protostellar outflow in M33 has also been identified at the center of the clump. We have successfully resolved the parsec-scale local star formation activity in which the galactic scale kinematics may induce the formation of the parental filamentary clouds.

The Origin of Interstellar Turbulence in M33

Abstract We utilize the multi-wavelength data of M33 to study the origin of turbulence in its interstellar medium. We find that the H i turbulent energy surface density inside 8 kpc is ∼1–3 × 1046 erg pc−2, and has no strong dependence on galactocentric radius because of the lack of variation in H i surface density and H i velocity dispersion. Then, we consider the energies injected by supernovae (SNe), the magneto-rotational instability (MRI), and the gravity-driven turbulence from accreted materials as the sources of turbulent energy. For a constant dissipation time of turbulence, the SNe energy can maintain turbulence inside ∼4 kpc radius (equivalent to ∼0.5 R 25), while the MRI energy is always smaller than the turbulent energy within 8 kpc radius. However, when we let the dissipation time to be equal to the crossing time of turbulence across the H i scale height, the SNe energy is enough to maintain turbulence out to 7 kpc radius, and the sum of SNe and MRI energies is able to maintain turbulence out to 8 kpc radius. Due to lack of constraint in the mass accretion rate through the disk of M33, we cannot rule out the accretion driven turbulence as a possible source of energy. Furthermore, by resolving individual giant molecular clouds in M33, we also show that the SNe energy can maintain turbulence within individual molecular clouds with ∼1% of coupling efficiency. This result strengthens the proposition that stellar feedback is an important source of energy to maintain turbulence in nearby galaxies.

Revisiting the Extended Schmidt Law: The Important Role of Existing Stars in Regulating Star Formation

Abstract We revisit the proposed extended Schmidt law, which posits that the star formation efficiency in galaxies depends on the stellar mass surface density, by investigating spatially resolved star formation rates (SFRs), gas masses, and stellar masses of star formation regions in a vast range of galactic environments, from the outer disks of dwarf galaxies, to spiral disks and to merging galaxies, as well as individual molecular clouds in M33. We find that these regions are distributed in a tight power law as ∝ , which is also valid for the integrated measurements of disk and merging galaxies at high-z. Interestingly, we show that star formation regions in the outer disks of dwarf galaxies with down to 10−5 yr−1 kpc−2, which are outliers of both the Kennicutt–Schmidt and Silk–Elmegreen laws, also follow the extended Schmidt law. Other outliers in the Kennicutt–Schmidt law, such as extremely metal-poor star formation regions, also show significantly reduced deviation from the extended Schmidt law. These results suggest an important role for existing stars in helping to regulate star formation through the effect of their gravity on the midplane pressure in a wide range of galactic environments.

Giant molecular clouds in M33: are they susceptible to dynamical friction?

Most of giant molecular clouds (GMCs) in M 33 are connected with spiral-like gaseous arms (filaments) with the exception of the inner 2 kpc region where the link between the arms and GMCs disappears (see Tosaki et al. 2011). We check whether it may be caused by the dynamic friction retarding the clouds. Using semi-analytical model for this galaxy we calculate the dynamics of GMCs of different masses situated at different initial galactocentric distances in the disk plane. We demonstrate that the dynamical friction may really change the orbits of GMCs in the central 2 kpc-size region. However in this case the typical lifetimes of GMCs should be close to or greater than $10^8$~yr, which is larger than the usually accepted values.

The population of young stellar clusters throughout the disk of M 33

The properties of young stellar clusters (YSCs) in M33, identified from the center out to about twice the size of the bright star-forming disk,are investigated. We find 915 discrete MIR sources as far as the extent of the warped HI disk, i.e. 16 kpc from the galaxy center. Their surface density has a steep radial decline beyond 4.5 kpc, and flattens out beyond the optical radius at 8.5 kpc. We are able to identify YSCs out to 12 kpc. At large galactocentric radii, the paucity of very luminous clusters and the relevance of hot dust emission become evident from the analysis of the bolometric and MIR luminosity functions. The YSC mass and size are correlated with a log-log slope of 2.09, similar to that measured for giant molecular clouds in M33 and the Milky Way, which represent the protocluster environment. Most of the YSCs in our sample have low extinction and ages between 3 and 10 Myr. In the inner regions of M33 the clusters span a wide range of mass (10^2<M<3 10^5 msun) and luminosity 10^38<L{bol}<3 10^{41}erg/s, while at galactocentric radii larger than 4 kpc we find a deficiency of massive clusters. Beyond 7 kpc, where the Halpha surface brightness drops significantly, the dominant YSC population has M<10^3 msun and a slightly older age (10 Myrs). This implies the occurrence of star formation events about 10 Myr ago as far as 10-12 kpc from the center of M33. The cluster L{FUV}--L{Halpha} relation is non-linear for L{FUV}<10^{39}erg/s, in agreement with randomly sampled models of the IMF which, furthermore, shows no appreciable variation throughout the M33 disk.

Minimal HCN emission from molecular clouds in M33

Since HCN emission has been shown to be a linear tracer of ongoing star formation activity, we have searched for HCN (J = 1->0) emission from known GMCs in the nearby galaxy M33. No significant HCN emission has been found along any of the lines of sight. We find two lines of sight where CO-to-HCN integrated intensity ratios up to 280, nearly a factor of 6 above what is found in comparable regions of the Milky Way. Star formation tracers suggest that the HCN-to-star formation rate ratio (L_HCN/M_SFR) is a factor of six lower than what is observed in the Milky Way (on average) and local extragalactic systems. Simple chemical models accounting for the sub-solar N/O ratio suggest that depletion cannot account for the high CO-to-HCN ratios. Given HCN formation requires high extinction (A_V > 4), low metallicity may yield reduced dust shielding and thus a high CO/HCN ratio. The turbulence and structure of GMCs in M33 are comparable to those found in other systems, so the differences are unlikely to result from different GMC properties. Since lower CO-to-HCN ratios are associated with the highest rates of star formation, we attribute the deficits in part to evolutionary effects within GMCs.

THE FRACTAL DIMENSION OF STAR-FORMING REGIONS AT DIFFERENT SPATIAL SCALES IN M33

We study the distribution of stars, HII regions, molecular gas, and individual giant molecular clouds in M33 over a wide range of spatial scales. The clustering strength of these components is systematically estimated through the fractal dimension. We find scale-free behavior at small spatial scales and a transition to a larger correlation dimension (consistent with a nearly uniform distribution) at larger scales. The transition region lies in the range 500-1000 pc. This transition defines a characteristic size that separates the regime of small-scale turbulent motion from that of large-scale galactic dynamics. At small spatial scales, bright young stars and molecular gas are distributed with nearly the same three-dimensional fractal dimension (Df <= 1.9), whereas fainter stars and HII regions exhibit higher values (Df = 2.2-2.5). Our results indicate that the interstellar medium in M33 is on average more fragmented and irregular than in the Milky Way.

NRO Legacy Project: Survey of Giant Molecular Clouds in M33

As the Nobeyama Radio Observatory Legacy Project: Survey of Giant Molecular Clouds in M33, we have been mapping M33 in CO(1-0) with the multi-beam receiver BEARS equipped on the 45-m telescope using the OTF mapping technique since 2007. The purpose of this project is to investigate the physical properties of GMCs and understand the evolutionary process from GMC formation to star formation in GMCs by comparing with various data such as CO(3-2), 1.1 mm continuum obtained with ASTE10m telescope at Atacama and the optical data obtained with SUBARU. We identified 87 GMCs using the first year data of CO(1-0) and observed 28 GMCs among them in CO(3-2) with ASTE (Onodera 2009, PhD thesis, University of Tokyo). From the comparison of these lines, it was shown that the CO(3-2)/CO(1-0) ratio increases with star forming activity in the GMCs. Furthermore, we found that more massive GMCs tend to have higher CO(3-2)/CO(1-0) ratio. Since the ratio is thought to be an indicator of the fraction of warm and dense molecular gas, our results imply that the fraction of warm and dense gas increases with GMC mass. Especially, since the ratio in the GMCs with low star forming activity is in the range where the ratio depends mainly on the density, we speculate that dense gas fraction increases with GMC mass.

Fitting power-law distributions to data with measurement errors

If X, which follows a power-law distribution, is observed subject to Gaussian measurement errore, then X + e is distributed as the convolution of the power-law and Gaussian distributions. Maximum-likelihood estimation of the parameters of the two distributions is considered. Large-sample formulae are given for the covariance matrix of the estimated parameters, and implementation of a small-sample method (the jackknife) is also described. Other topics dealt with are tests for goodness of fit of the posited distribution, and tests whether special cases (no measurement errors or an infinite upper limit to the power-law distribution) may be preferred. The application of the methodology is illustrated by fitting convolved distributions to masses of giant molecular clouds in M33 and the Large Magellanic Cloud (LMC), and to H I cloud masses in the LMC.

Atomic Carbon Emission from Individual Molecular Clouds in M33

We present observations of the 492 GHz [CI] emission for four individual giant molecular clouds in the Local Group galaxy M33 obtained with the James Clerk Maxwell Telescope. The average [CI] to CO J=1-0 integrated intensity ratio of 0.10+/-0.03 is similar to what is observed in Galactic molecular clouds but smaller than what is seen in starburst galaxies. Similarly, the column density ratio N(C)/N(CO) is similar to that observed in the Orion Bar, but smaller than values obtained for starburst galaxies. The [CI] line is found to be a more important coolant than the lowest three rotational transitions of CO for all the clouds in the sample. The [CI] luminosity does not appear to be enhanced significantly in two low-metallicity clouds, which may be due to the unusual ionization environment of the clouds.

The Density and Temperature of Molecular Clouds in M33

We have observed the 12CO J = 2-1,13CO J = 2-1, and 12CO J = 3-2 lines in a sample of seven giant molecular clouds in the Local Group spiral galaxy M33 using the James Clerk Maxwell Telescope. The 12CO/13CO J = 2-1 line ratio is constant across the entire sample, while the observed 12CO J = 3-2/J = 2-1 line ratio has a weak dependence on the star formation environment of the cloud, with large changes in the line ratio seen only for clouds in the immediate vicinity of an extremely luminous H II region. A large velocity gradient analysis indicates that clouds without H II regions have temperatures of 10-20 K, clouds with H II regions have temperatures of 15-100 K, and the cloud in the giant H II region has a temperature of at least 100 K. Interestingly, the giant H II region appears capable of raising the kinetic temperature of the molecular gas only for clouds that are quite nearby (<100 pc). The continuous change of physical conditions across the observed range of star formation environments suggests that the unusual physical conditions in the cloud in the giant H II region are due to post-star formation changes in the molecular gas, rather than intrinsic properties of the gas related to the formation of the giant H II region. The results from this study of M33 suggest that similar observations of ensembles of giant molecular clouds in more distant normal spiral galaxies are likely to give meaningful measurements of the average physical conditions inside the molecular clouds. These results also imply that clouds with a factor of 3 difference in metallicity have similar density and temperature, which in turn implies that the differences in the CO-to-H2 conversion factor seen in these clouds can be attributed to metallicity effects entirely.

Interferometric observations of three giant molecular clouds in M31

One field in the northern spiral arm of M31 has been mapped at 11 x 6 arcsec resolution with the Berkeley-Illinois-Maryland Array. Three giant molecular clouds are detected in this field. The properties of the clouds, such as diameter, line width, and mass, are very similar to those of giant molecular clouds in the Galaxy and in M33, and are consistent with the size-line-width relation obtained for molecular clouds in those galaxies. The largest cloud has a mass of 7 x 10 exp 5 solar masses, 40 percent larger than the most massive cloud seen in a much larger survey of molecular clouds in M33. Thus, despite a low molecular gas surface density, the molecular interstellar medium of M31 appears capable of organizing itself into quite massive molecular clouds.

OB associations in the inner two kiloparsecs of M33

view Abstract Citations (60) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS OB Associations in the Inner Two Kiloparsecs of M33 Wilson, Christine D. Abstract UBV CCD photometry for the inner 7X10^arcmin^ of the Local Group spiral galaxy M33 is used to identify the OB associations and to study their properties. The association boundaries are identified using a "friends of friends" grouping algorithm, in which the major remaining source of subjectivity is the search radius chosen. We find 41 associations in this region of M33, each containing at least ten blue stars. These associations have mean radii of 40 pc, masses of 600M_sun_, and ages of 8X10^6^ yr, where the age is estimated from the V magnitudes of the three brightest main-sequence stars. One-third of the Wolf-Rayet stars in this region of the galaxy lie more than 75 pc away from the nearest OB association. Thus, in this galaxy, the most massive stars do not form exclusively in large associations. A comparison of the masses of the associations with the masses of molecular clouds in M33 implies a star formation efficiency of 10%. The OB associations in the southern spiral arm show a weak age gradient perpendicular to the arm, consistent with the presence of a spiral density wave. The associations in the northern arm lack any stars more massive than ~20M_sun_ while the southern arm contains several stars with masses of 60M_sun_. Either a recent cessation of star formation in the inner part of the northern arm or a difference in the upper mass cutoff to the initial mass function between the arms could explain this difference. Publication: The Astronomical Journal Pub Date: May 1991 DOI: 10.1086/115796 Bibcode: 1991AJ....101.1663W Keywords: Astronomical Photometry; B Stars; O Stars; Spiral Galaxies; Charge Coupled Devices; Molecular Clouds; Stellar Evolution; Ubv Spectra; Astrophysics; STARS: EARLY-TYPE; GALAXIES: INDIVIDUAL; GALAXIES: LOCAL GROUP; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (1) NED (1)

OB star formation and H I production in molecular clouds in M33

view Abstract Citations (15) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS OB Star Formation and H i Production in Molecular Clouds in M33 Wilson, Christine D. ; Scoville, Nick Abstract For a sample of 38 resolved molecular clouds in M33, new Hα data and previously published high-resolution H I maps are analyzed to study the star formation properties of the clouds and the cycle of the gas from one phase of the interstellar medium to another. Roughly 2/3 of the molecular clouds with M > 0.5 x 10^5^ M_sun_ show recent massive star formation as traced by the Hα emission. The high- mass star formation rates for the individual clouds range from 5 x 10^-6^ to 5 x 10^-4^ M_sun_ yr^-1^. The photodissociating flux produced by these stars is sufficient to produce the amount of atomic gas seen near the molecular clouds if the molecular clouds have a clumped structure. In addition, the relative offset of the H I and the CO peaks in the southern spiral arm is not consistent with the picture in which the atomic gas condenses in the arms to form the molecular clouds, since the H I emission appears most strongly on the downstream side of the molecular gas. The atomic hydrogen column density predicted to be associated with the mantles of molecular clouds can account for the mean H I column density seen in the inner disk of M33. Publication: The Astrophysical Journal Pub Date: March 1991 DOI: 10.1086/169803 Bibcode: 1991ApJ...370..184W Keywords: B Stars; Molecular Clouds; O Stars; Spiral Galaxies; Star Formation; Galactic Structure; H Alpha Line; Interstellar Gas; Interstellar Matter; Photodissociation; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M33; GALAXIES: INTERSTELLAR MATTER; INTERSTELLAR: MOLECULES; STARS: FORMATION full text sources ADS | data products SIMBAD (2) NED (1)

The properties of individual giant molecular clouds in M33

view Abstract Citations (127) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Properties of Individual Giant Molecular Clouds in M33 Wilson, Christine D. ; Scoville, Nick Abstract Nineteen fields have been observed in the nearby spiral galaxy M33 with the Owens Valley Millimeter-Wave Interferometer at 7"-8" (30 pc) resolution detecting a total of 38 individual molecular clouds. The velocity widths, diameters, peak brightness temperatures, and masses of the clouds are very similar to those of Galactic giant molecular clouds (GMCs). The velocity widths and diameters of the M33 clouds are related by V_FWHM_ = (1.2^+1.3^_-0.6_)D^0.5+/-0.2^_pc_, which agrees within the errors with the velocity-diameter relation obtained for Galactic GMCs. The masses derived from the virial theorem and those derived from the integrated CO line fluxes agree to within 10% in the mean for this sample, implying that in this region of M33 the value of the conversion factor from CO flux to H_2_ column density, α, is similar to that of our own Galaxy. This is the first direct confirmation that the Galactic value of α is appropriate in an external galaxy. The clouds within 200 pc of the nucleus have elevated CO brightness temperatures, possibly due to an increased ultraviolet radiation field. The mass spectrum of clouds is consistent with that derived in the Galaxy for M = (0.8-4) x 10^5^ M_sun_ but shows a total lack of clouds with masses greater than 4 x 10^5^ M_sun_. Comparison of the flux detected with the interferometer with single-dish data indicates that roughly 50% of the molecular gas resides in structures less massive than 0.8 x 10^5^ M_sun_, in contrast to the Galaxy, where only 15% of the H_2_ mass is in these smaller structures. A simple model is proposed to explain the high-mass cutoff as arising from the competing processes of cloud growth through accretion and cloud destruction due to star formation. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169356 Bibcode: 1990ApJ...363..435W Keywords: Molecular Clouds; Spiral Galaxies; Galactic Structure; Interstellar Matter; Mass Spectra; Molecular Gases; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M33; GALAXIES: INTERSTELLAR MATTER; INTERSTELLAR: MOLECULES full text sources ADS | data products SIMBAD (40) NED (1)

Aperture synthesis observations of individual molecular clouds in M33

Aperture synthesis maps of six fields in the nearby spiral galaxy M33 have revealed the presence of a population of molecular clouds whose masses, sizes, velocity widths, and brightness temperatures are similar to Galactic giant molecular clouds. The masses of the clouds obtained from the virial theorem and from the integrated CO flux are in relatively good agreement, suggesting that the conversion factor from CO flux to molecular mass in M33 is unlikely to be much different from the value measured in our own Galaxy. The star formation properties of these regions are under study with deep optical CCD imaging and near-infrared data.