Cloud Mass Function Research Articles

From the molecular-cloud- to the embedded-cluster-mass function with a density threshold for star formation

The mass function of molecular clouds and clumps is shallower than the mass function of young star clusters, gas-embedded and gas-free alike, as their respective mass function indices are $\beta_0 \simeq 1.7$ and $\beta_\star \simeq 2$. We demonstrate that such a difference can arise from different mass-radius relations for the embedded-clusters and the molecular clouds (clumps) hosting them. In particular, the formation of star clusters with a constant mean {\it volume} density in the central regions of molecular clouds of constant mean {\it surface} density steepens the mass function from clouds to embedded-clusters. This model is observationally supported since the mean surface density of molecular clouds is approximately constant, while there is a growing body of evidence, in both Galactic and extragalactic environments, that efficient star-formation requires a hydrogen molecule number density threshold of $n_{th} \simeq 10^{4-5}\,cm^{-3}$. In the framework of the same model, the radius distribution steepens from clouds (clumps) to embedded-clusters, which contributes to explaining observed cluster radius distributions. [Abridged]

UNUSUALLY LUMINOUS GIANT MOLECULAR CLOUDS IN THE OUTER DISK OF M33

We use high spatial resolution (~7pc) CARMA observations to derive detailed properties for 8 giant molecular clouds (GMCs) at a galactocentric radius corresponding to approximately two CO scale lengths, or ~0.5 optical radii (r25), in the Local Group spiral galaxy M33. At this radius, molecular gas fraction, dust-to-gas ratio and metallicity are much lower than in the inner part of M33 or in a typical spiral galaxy. This allows us to probe the impact of environment on GMC properties by comparing our measurements to previous data from the inner disk of M33, the Milky Way and other nearby galaxies. The outer disk clouds roughly fall on the size-linewidth relation defined by extragalactic GMCs, but are slightly displaced from the luminosity-virial mass relation in the sense of having high CO luminosity compared to the inferred virial mass. This implies a different CO-to-H2 conversion factor, which is on average a factor of two lower than the inner disk and the extragalactic average. We attribute this to significantly higher measured brightness temperatures of the outer disk clouds compared to the ancillary sample of GMCs, which is likely an effect of enhanced radiation levels due to massive star formation in the vicinity of our target field. Apart from brightness temperature, the properties we determine for the outer disk GMCs in M33 do not differ significantly from those of our comparison sample. In particular, the combined sample of inner and outer disk M33 clouds covers roughly the same range in size, linewidth, virial mass and CO luminosity than the sample of Milky Way GMCs. When compared to the inner disk clouds in M33, however, we find even the brightest outer disk clouds to be smaller than most of their inner disk counterparts. This may be due to incomplete sampling or a potentially steeper cloud mass function at larger radii.

NEW TESTS FOR DISRUPTION MECHANISMS OF STAR CLUSTERS: METHODS AND APPLICATION TO THE ANTENNAE GALAXIES

We present new tests for disruption mechanisms of star clusters based on the bivariate mass–age distribution g(M, τ). In particular, we derive formulae for g(M, τ) for two idealized models in which the rate of disruption depends on the masses of the clusters and one in which it does not. We then compare these models with our Hubble Space Telescope observations of star clusters in the Antennae galaxies over the mass–age domain in which we can readily distinguish clusters from individual stars: τ ≲ 107(M/104 M☉)1.3 yr. We find that the models with mass-dependent disruption are poor fits to the data, even with complete freedom to adjust several parameters, while the model with mass-independent disruption is a good fit. The successful model has the simple form g(M, τ) ∝ M−2τ−1, with power-law mass and age distributions, dN/dM ∝ M−2 and dN/dτ ∝ τ−1. The predicted luminosity function is also a power law, dN/dL ∝ L−2, in good agreement with our observations of the Antennae clusters. The similarity of the mass functions of star clusters and molecular clouds indicates that the efficiency of star formation in the clouds is roughly independent of their masses. The age distribution of the massive young clusters is plausibly explained by the following combination of disruption mechanisms: (1) removal of interstellar material by stellar feedback, τ ≲ 107 yr; (2) continued stellar mass loss, 107 yr ≲ τ ≲ 108 yr; (3) tidal disturbances by passing molecular clouds, τ ≳ 108 yr. None of these processes is expected to have a strong dependence on mass, consistent with our observations of the Antennae clusters. We speculate that this simple picture also applies—at least approximately—to the clusters in many other galaxies.

Enhanced luminosity of young stellar objects in cometary globules

In this study we investigated the effects of external trigger on the characteristics of young stellar objects (YSOs) associated with cometary globules (CGs). We made optical spectroscopy of stars associated with star-forming CGs. We find that the masses of the most massive stars associated with CGs are correlated with the masses of the parent cloud but they are systematically larger than expected for clouds of similar mass from the relation Mmax-star=0.33Mcl0.43 given by Larson (Mon. Not. R. Astron. Soc. 200:159, 1982). We have also estimated the luminosities of the IRAS sources found associated with CGs as a function of cloud mass and then compared them with those of the IRAS sources found associated with isolated opacity class 6 clouds (isolated and relatively away from large star forming regions). We find that the luminosities of IRAS sources associated with CGs are larger than those of the opacity class 6 clouds. These findings support results from recent simulations in which it was shown that the Radiation Driven Implosion (RDI) process, believed to be responsible for the cometary morphology and star formation, can increase the luminosity 1–2 orders of magnitudes higher than those of protostars formed without external triggering due to an increase in accretion rates. Thus implying that the massive stars can have profound influence on the star formation in clouds located in their vicinity.

The Origin of the Universal Globular Cluster Mass Function

AbstractEvidence favouring a Gaussian initial mass function for systems of old globular clusters has accumulated over recent years. We show that a bell-shaped mass function may be the imprint of expulsion from protoclusters of the leftover star forming gas due to supernova activity. Owing to the corresponding weakening of its gravitational potential, a protocluster retains a fraction only of its newly formed stars. The mass fraction of bound stars extends from zero to unity depending on the star formation efficiency achieved by the protoglobular cloud. We investigate how such wide variations affect the mapping of the protoglobular cloud mass function to the initial globular cluster mass function. We conclusively demonstrate that the universality of the globular cluster mass function originates from a common protoglobular cloud mass-scale of about 106 M⊙ among galaxies. Moreover, gas removal during star formation in massive gas clouds is highlighted as the likely prime cause of the predominance of field stars in the Galactic Halo.

Determination of the mass function of extra-galactic GMCs via NIR color maps

The giant molecular clouds (GMCs) of external galaxies can be mapped with sub-arcsecond resolution using multiband observations in the near-infrared. However, the interpretation of the observed reddening and attenuation of light, and their transformation into physical quantities, is greatly hampered by the effects arising from the unknown geometry and the scattering of light by dust particles. We examine the relation between the observed near-infrared reddening and the column density of the dust clouds. In this paper we particularly assess the feasibility of deriving the mass function of GMCs from near-infrared color excess data. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions. We include the scattered light in the models and calculate near-infrared color maps from the simulated data. The color maps are compared with the true line-of-sight density distributions of the models. We extract clumps from the color maps and compare the observed mass function to the true mass function. For the physical configuration chosen in this study, essentially a face-on geometry, the observed mass function is a non-trivial function of the true mass function with a large number of parameters affecting its exact form. The dynamical range of the observed mass function is confined to ~103.5 105.5 regardless of the dynamical range of the true mass function. The color maps are more sensitive in detecting the high-mass end of the mass function, and on average the masses of clouds are underestimated by a factor of ∼10 depending on the parameters describing the dust distribution. A significant fraction of clouds is expected to remain undetected at all masses. The simulations show that the cloud mass function derived from JHK color excess data using simple foreground screening geometry cannot be regarded as a one-to-one tracer of the underlying mass function.

The origin of the Gaussian initial mass function of old globular cluster systems

Evidence favouring a Gaussian initial mass function for systems of old globular clusters has accumulated over recent years. We show that an approximately Gaussian mass function is naturally generated from a power-law mass distribution of protoglobular clouds by expulsion from the protocluster of star-forming gas due to supernova activity, provided that the power-law mass distribution shows a lower mass limit. As a result of gas loss, the gravitational potential of the protocluster gets weaker and only a fraction of the newly formed stars is retained. The mass fraction of bound stars ranges from zero to unity, depending on the local star formation efficiency ε. Assuming that ε is independent of the protoglobular cloud mass, we investigate how such variations affect the mapping of a protoglobular cloud mass function to the resulting globular cluster initial mass function. A truncated power-law cloud mass spectrum generates bell-shaped cluster initial mass functions, with a turnover location mostly sensitive to the lower limit of the cloud mass range. Assuming instantaneous gas removal and a slope α≃−1.7 for the cloud mass spectrum, we evolve the derived cluster initial mass functions up to an age of 13 Gyr in a potential like that of the Milky Way. We obtain a good match to the Old Halo cluster mass function, with a present-day mass fraction of clusters in the halo of 2 per cent, as is observed, with mlow≃ 6 × 105 M⊙, mup≥ 5 × 106 M⊙, δ≃−2.9 and rc≃ 0.025, respectively, the lower and upper limits of the cloud mass range, the slope and the core of the power-law spectrum for the star formation efficiency. The steep slope δ means that most protoglobular clouds achieve too low a star formation efficiency to give rise to bound star clusters following gas removal. As a result, most newly formed stars are scattered into the field soon after their formation. Gas removal during star formation in massive clouds is thus likely the prime cause of the predominance of field stars in the Galactic halo. The shape of the present-day cluster mass function depends weakly on the underlying distribution of the star formation efficiency. Finally, we show that a Gaussian mass function for the protoglobular clouds with a mean log mG≃ 6.1–6.2 and an s.d. σ≲ 0.4 provides results very similar to those resulting from a truncated power-law cloud mass spectrum, that is, the distribution function of masses of protoglobular clouds influences only weakly the shape of the resulting globular star cluster initial mass function. The gas removal process and the protoglobular cloud mass scale dominate the relevant physics.

Cloud mass function in a gas rich dwarf galaxy

We study cloud mass function (here after CMF) obtained in a Grape-SPH simulation of model galaxy. The CMF can be fitted by a simple power law, and its power law index is ∼ −2.0, which closes to the observational results.

Interstellar Gas Mass Functions: Fractality and Scale Dependence

Abstract We present a theoretical model for the observed mass functions (MF’s) of interstellar clouds, and test it by comparison with interferometrically observed MF’s. We consider the time evolution of velocities using the stochastic differential Langevin equation for the objects (in fact gas clumps of sizes below 1 pc) under Brownian motion, with friction and magnetic field. We also study geometrical binding, and hence the implications of sizes of the regions in which the gas clouds move, and also of the departures from sphericity of the clouds themselves. The size of the global region in which the objects move under the influence of the general dynamics of the galaxy leads to a box-effect modelled as a Chandrasekhar barrier, which naturally leads to the superposition of Gaussian distributions over different ranges of displacement, and with progressively decreasing amplitude. This accounts for the general power law trend observed for the MF’s, as well as its accompanying peaked structure. These results fit well the observed distributions even in very low mass ranges, which show global power law characteristics (scale independence, fractality, departures from sphericity for individual clouds), accompanied by turnovers, for a wide range of masses, characteristics of Gaussians (scale dependence, a tendency for clouds to be spherical).

Dust Growth Timescale and Mass Function of Molecular Clouds in the Galaxy

We examine the characteristic growth time of dust grains in the Galaxy, where the grain growth in molecular clouds is the dominant factor for dust-mass increase. Since the timescale of grain growth depends on the gas density of molecular clouds, we reconsider the typical molecular-cloud density by using the “density function” of clouds [denoted as N(n) dn]. Here, the density function expresses the probability that a cloud has a volume-averaged gas density between n and n+dn, and it can be calculated by combining the observed mass function and the scaling law of molecular clouds. Through the formulation that combines N(n) with the characteristic dust growth timescale, we find that the typical dust growth timescale is in the range of 107–108 yr. This range is equivalent to the timescale of the dust growth in molecular clouds whose molecular number density is 3 × 102−3 × 103 cm−3. Thus, the dense molecular clouds (n > 104 cm−3) contribute little to dust growth. The timescale supports the value constrained in our previous model and provides an upper limit for the lifetimes of molecular clouds.

The Stellar Initial Mass Function from Random Sampling in Hierarchical Clouds. II. Statistical Fluctuations and a Mass Dependence for Starbirth Positions and Times

Observed variations in the slope of the stellar initial mass function (IMF) are shown to be consistent with a previously introduced model in which the protostellar gas is randomly sampled from clouds with a self-similar hierarchical structure. Root mean square variations in the IMF slope around the Salpeter value are ±0.4 when only 100 stars are observed, and ±0.1 when 1000 stars are observed. Similar variations should be present in other stochastic models as well. The hierarchical sampling model reproduces the tendency for massive stars to form closer to the center of a cloud at a time somewhat later than the formation time of the lower mass stars. The systematic variation in birth position results from the tendency for the trunk and larger branches of the hierarchical tree of cloud structure to lie closer to the cloud center, while the variations in birth order result from the relative infrequency of stars with larger masses. The hierarchical cloud sampling model has now reproduced most of the reliably observed features of the cluster IMF. The power-law part of the IMF comes from cloud hierarchical structure that is sampled during various star formation processes with a relative rate proportional to the square root of the local density. These processes include turbulence compression, magnetic diffusion, gravitational collapse, and clump or wavepacket coalescence, all of which have about this rate dependence. The low-mass flattening comes from the inability of gas to form stars below the thermal Jeans mass at typical temperatures and pressures. The thermal Jeans mass is the only relevant scale in the problem. Considerations of heating and cooling processes indicate why the thermal Jeans mass should be nearly constant in normal environments and why this mass might increase in starburst regions. In particular, the relative abundance of high-mass stars should increase where the average density of the interstellar medium is very large; accompanying this increase should be an increase in the average total efficiency of star formation. Alternative models in which the rate of star formation is independent of density and the local efficiency decreases systematically with increasing stellar mass can also reproduce the IMF, but this is an adjustable result and not a fundamental property of hierarchical cloud structure, as is the preferred model. The steep IMF in the extreme field is not explained by the model, but other origins are suggested, including one in which massive stars in low-pressure environments halt star formation in their clouds. In this case, the slope of the extreme field IMF is independent of the slope of each component cluster IMF and is given by (γ-1)/α for a cloud mass function slope, -γ~-2, and a power-law relation, ML∝Mαc, between the largest star in a low-pressure cloud, ML, and the cloud mass, Mc. A value of α~1/4 is required to explain the extreme field IMF as a superposition of individual cluster IMFs; cloud destruction by ionizing has this property. We note that the similarity between cluster IMFs and the average IMF from global studies of galaxies implies that most stars form in clusters and that massive stars do not generally halt star formation in the same cloud.

Molecular Clouds in Cepheus and Cassiopeia

A large-scale 13CO(J = 1-0) survey for nearby molecular clouds was performed toward the Cepheus and Cassiopeia regions (100° < l < 130° and -10° < b < 20°) with the velocity coverage of -40 < VLSR < +20 km s-1 by using the two 4 m millimeter-wave telescopes at Nagoya University. An area of 866 square degrees was covered at an 8' grid spacing with a 27 beam, and 48,750 positions were observed. Significant 13CO emission (≥1.8 K km s-1 = 3 σ) is detected at 1015 positions. On the basis of the 13CO data, 188 distinct 13CO clouds are identified whose total mass is 1.0 × 105 M☉. Physical properties of the 13CO clouds such as molecular column densities, sizes, and masses are derived for each cloud. Astronomical objects associated with the 13CO clouds were searched for in the literature: 10 H II regions, eight reflection nebulae, 23 T Tauri type stars, 28 molecular outflows, and 125 IRAS point sources selected as candidates for protostars are likely to be associated with the 13CO clouds. Statistical studies on the 13CO clouds detected in this survey have been made. The mass spectra of the 13CO clouds in Cepheus and Cassiopeia are well represented by a power law, dNcloud/dMcloud ∝ Mcloud−α, with α = 1.7 ± 0.3 in a cloud mass range 101-105 M☉, which is similar to previously observed other regions like Cygnus. We also investigated the relation between the line width and the size of the 13CO clouds and found that there is no significant correlation among them in the cloud mass range 101-104 M☉. A virial analysis made for the 13CO clouds indicates that the relation between the virial mass Mvir and the mass measured in 13CO Mcloud is approximated well by (Mvir/M☉) = 2.8 × 101(Mcloud/M☉)0.62, suggesting that smaller clouds tend to be more weakly bound than larger clouds gravitationally or are dispersing if the possible external pressure is disregarded. It is found that the 13CO clouds associated with IRAS point sources, which are regarded as ongoing star-forming clouds, tend to be more massive and larger in size, and to have higher column densities than those without any signs of star formation. There seems to be a threshold value in the peak H2 column density of N(H2) = 2.5 × 1021 cm-2 for stars to form in a13CO cloud. In order to study star formation activities in the 13CO clouds, we investigated the global luminosity distribution of the IRAS point sources and attempted to determine the distribution as a function of the parent molecular cloud mass. The IRAS luminosity function in a given cloud with mass Mcloud is found to be well approximated by a power law dnstar/dLstar = 6 × 10−3(Mcloud/M☉)0.8(Lstar/L☉)−1.5 L☉−1 for the IRAS luminosity range 1 < Lstar/L☉ < 105 and the cloud mass range 101 < Mcloud/M☉ < 105. The luminosity function does not depend significantly on the cloud mass, the regions sampled, and the existence of H II regions. The luminosity of the most luminous IRAS point source in a given molecular cloud Lstar,max increases systematically with the mass of the associated cloud, which is fitted by the power law (Lstar,max/L☉) = 0.098 × (Mcloud/M☉)1.43. These relations are found to be consistent with those derived in Cygnus by Dobashi, Bernard, & Fukui, suggesting that they may represent general relations between the maximum IRAS luminosity and the molecular clouds. We finally demonstrate that the slope of the initial mass function estimated from the IRAS luminosity function derived here is consistent with the stellar initial mass function for a stellar mass to luminosity relation of (Lstar/L☉) = (Mstar/M☉)3.45.

Cosmology using the Parkes HI Multibeam Southern Sky Survey

AbstractI discuss the implications of the Parkes Multibeam Southern Sky HI Survey for cosmology. It will determine the local mass function of HI clouds, detecting several hundred per decade of mass. Each of these will come with a redshift and, for the more massive clouds, an estimate of the velocity width. This will provide an ideal database for peculiar motion studies and for measurements of biasing of galaxies relative to the underlying matter distribution.

A search for 'failed clusters' of galaxies

We describe a search for a new type of object - large clouds of hot gas with no visible galaxies - which we call failed clusters of galaxies. We calculate the expected X-ray luminosity, temperature, and angular diameter of such objects as a function of total cloud mass and convert the results to expected X-ray fluxes from failed clusters at different redshifts. Using the Einstein Imaging Proportional Counter (IPC) database, we establish a strategy to search for candidate failed clusters. From this initial screening of 1435 IPC fields, 17 candidates are selected for more detailed analysis, which indicates that 10 of these are very probably extended X-ray sources. Optical follow-up on the 10 prime candidates finds eight clusters of galaxies (including six reproted for the first time in this paper), one stellar identification, and one without an obvious optical counterpart (the candidate with the weakest evidence for X-ray extent). Investigation of several candidates with less evidence for X-ray extent yields two additional new clusters of galaxies. A conservative comparison of our results with the Einstein Extended Medium Sensitivity Survey demonstrates that failed clusters are a relatively unimportant contributor to the mass density of the universe. Our inability to find failed clusters is consistent with the hierarchical clustering scenario for the formation of galaxies and clusters.

On the origin of H I absorption systems of quasars

view Abstract Citations (18) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Origin of H i Absorption Systems of Quasars Murakami, Izumi ; Ikeuchi, Satoru Abstract We examine intergalactic clouds confined by the gravity of cold dark matter (CDM), the so-called minihalo, as a unified model for neutral hydrogen (H I) absorption systems of quasars. The reported H I column density distribution from 10^13^ cm^-2^ of the Lyα forest to 10^22^ cm^-2^ of damped Lyα systems is well reproduced by the evolution of the two-phase nature of minihalos: a neutral core surrounded by an ionized envelope. Shielding effects of diffuse UV flux due to neutral hydrogen in the envelope cause the appearance of a neutral core in a massive minihalo, and this core is detected as a high column density system. Highly ionized envelopes and/or less massive one-phase minihalos correspond to the Lyα forest. In order to reproduce the number density evolution of the Lyα forest and the redshift variation of the continuum depression of high-redshift quasars, minihalos must experience an expansion that is caused by the increase of UV flux with the decrease of redshift at z > 2. Assuming a simplified evolution law of UV flux and mass function of gas clouds, we can reproduce all these observational properties by the minihalo model. At z < 2, the diffuse UV flux decreases and minihalos are apt to contract. The collapse of neutral cores is delayed, especially for small mass clouds, as late as z < 1, and these are considered to become dwarf galaxies. In this picture we present some predictions about low z Lyα clouds for observations of the Hubble Space Telescope. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/172640 Bibcode: 1993ApJ...409...42M Keywords: Absorption Spectra; Dark Matter; H I Regions; Intergalactic Media; Lyman Alpha Radiation; Quasars; Dwarf Galaxies; Line Spectra; Red Shift; Astrophysics; COSMOLOGY: DARK MATTER; GALAXIES: QUASARS: ABSORPTION LINES; GALAXIES: INTERGALACTIC MEDIUM full text sources ADS | data products SIMBAD (3)

THE H II REGION LUMINOSITY FUNCTION OF THE MILKY WAY

ABSTRACT Published radio surveys of H-II regions in the Milky Way are used to derive the radio continuum and equivalent H-alpha luminosity function for the Galaxy. It is found that the Galactic luminosity function can be fitted by a power-law function NL proportional to L exp -2.3 + or - 0.5. The luminosity function and the physical properties of the H-II regions are consistent with a Hubble type intermediate between Sb and Sc, based on comparisons with external galaxies. The qualitative resemblance of the Galactic luminosity function with the mass function of Galactic giant molecular clouds is also discussed.

Molecular clouds and cloud cores in the inner Galaxy

A compilation of CO emission regions and their measured parameters is presented which represents a nearly complete accounting of the molecular clouds in the first quadrant of the Galaxy. Emission regions associated with radio H II regions have systematically brighter CO peaks that are a factor of two to three times larger and have twice the mean velocity dispersion as the general cloud population. Both the H II region clouds and the hot core regions have a Galactic distribution characteristic of a spiral arm population, whereas the colder clouds are much less confined in Galactic azimuthal angle. Virial masses are obtained for the large sample of clouds with assigned kinematic distances. The mean H2 density for a GMC of diameter 40 pc is 180/cm. For these clouds, a linear relationship is found between the H2 column density and the integrated CO emission. The variation in the Z-dispersion of clouds as a function of cloud mass suggests that more massive GMCs have smaller random velocities.