arXiv subjects

Alberto D. Bolatto

Publications and source records attributed to Alberto D. Bolatto.

At least 145 records · Page 8Linked to original sources

arXiv2015

Spatially extended and high-velocity dispersion molecular component in spiral galaxies: single-dish vs. interferometric observations

Recent studies of the molecular medium in nearby galaxies have provided mounting evidence that the molecular gas can exist in two phases: one that is clumpy and organized as molecular clouds and another one that is more diffuse. This last component has a higher velocity dispersion than the clumpy one. In order to investigate these two molecular components further, we compare the fluxes and line widths of CO in NGC 4736 and NGC 5055, two nearby spiral galaxies for which high-quality interferometric as well as single-dish data sets are available. Our analysis leads to two main results: 1) Employing three different methods, we determine the flux recovery of the interferometer as compared to the single-dish to be within a range of 35-74% for NGC4736 and 81-92% for NGC5055, and 2) when focusing on high (SNR>5) lines of sight, the single-dish line widths are larger by ~(40+-20)% than the ones derived from interferometric data; which is in agreement with stacking all lines of sight. These results point to a molecular gas component that is distributed over spatial scales larger than 30"(~1kpc), and is therefore filtered out by the interferometer. The available observations do not allow us to distinguish between a truly diffuse gas morphology and a uniform distribution of small clouds that are separated by less than the synthesized beam size (~3" or ~100pc), as they would both be invisible for the interferometer. This high velocity dispersion component has a dispersion similar to what is found in the atomic medium, as traced through observations of the HI line.

astro-ph.GA↗

arXiv2014

Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. II. Gas-to-Dust Ratio Variations across ISM Phases

The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and lifecycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and Halpha observations. In the diffuse atomic ISM, we derive the gas-to-dust ratio as the slope of the dust-gas relation and find gas-to-dust ratios of 380+250-130 in the LMC, and 1200+1600-420 in the SMC, not including helium. The atomic-to-molecular transition is located at dust surface densities of 0.05 Mo pc-2 in the LMC and 0.03 Mo pc-2 in the SMC, corresponding to AV ~ 0.4 and 0.2, respectively. We investigate the range of CO-to-H2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on XCO to be 6x1020 cm-2 K-1 km-1 s in the LMC (Z=0.5Zo) at 15 pc resolution, and 4x 1021 cm-2 K-1 km-1 s in the SMC (Z=0.2Zo) at 45 pc resolution. In the LMC, the slope of the dust-gas relation in the dense ISM is lower than in the diffuse ISM by a factor ~2, even after accounting for the effects of CO-dark H2 in the translucent envelopes of molecular clouds. Coagulation of dust grains and the subsequent dust emissivity increase in molecular clouds, and/or accretion of gas-phase metals onto dust grains, and the subsequent dust abundance (dust-to-gas ratio) increase in molecular clouds could explain the observations. In the SMC, variations in the dust-gas slope caused by coagulation or accretion are degenerate with the effects of CO-dark H2. Within the expected 5--20 times Galactic XCO range, the dust-gas slope can be either constant or decrease by a factor of several across ISM phases. Further modeling and observations are required to break the degeneracy between dust grain coagulation, accretion, and CO-dark H2.

astro-ph.GA↗

arXiv2014

ALMA Reveals the Molecular Medium Fueling the Nearest Nuclear Starburst

We use ALMA to derive the mass, length, and time scales associated with the nuclear starburst in NGC 253. This region forms ~2 M_sun/yr of stars and resembles other starbursts in scaling relations, with star formation consuming the gas reservoir 10 times faster than in galaxy disks. We present observations of CO, the high effective density transitions HCN(1-0), HCO+(1-0), CS(2-1), and their isotopologues. We identify ten clouds that appear as peaks in line emission and enhancements in the HCN-to-CO ratio. These clouds are massive (~10^7 M_sun) structures with sizes (~30 pc) similar to GMCs in other systems. Compared to disk galaxy GMCs, they show high line widths (~20-40 km/s) given their size, with implied Mach numbers ~90. The clouds also show high surface (~6,000 M_sun/pc^2) and volume densities (n_H2~2,000 cm^-3). Given these, self-gravity can explain the line widths. This short free fall time (~0.7 Myr) helps explain the more efficient star formation in NGC 253. We also consider the starburst region as a whole. The geometry is confused by the high inclination, but simple models support a non-axisymmetric, bar-like geometry with a compact, clumpy region of high gas density embedded in an extended CO distribution. Even for the whole region, the surface density still exceeds that of a disk galaxy GMC. The orbital time (~10 Myr), disk free fall time (<~ 3 Myr), and disk crossing time (<~ 3 Myr) are each much shorter than in a normal spiral galaxy disk. Some but not all aspects of the structure correspond to predictions from assuming vertical dynamical equilibrium or a marginally stable rotating disk. Finally, the CO-to-H2 conversion factor implied by our cloud calculations is approximately Galactic, contrasting with results showing a low value for the whole starburst region. The contrast provides resolved support for the idea of mixed molecular ISM phases in starburst galaxies.

astro-ph.GA↗

arXiv2014

Extended HCN and HCO$^{+}$ emission in the starburst galaxy M82

We mapped 3 mm continuum and line emission from the starburst galaxy M82 using the Combined Array for Research in Millimeter-wave Astronomy. We targeted the HCN, HCO$^{+}$, HNC, CS and HC$_{3}$N lines, but here we focus on the HCN and HCO$^{+}$ emission. The map covers a field of 1.2' with a ~5" resolution. The HCN and HCO$^{+}$ observations are combined with single dish images. The molecular gas in M82 had been previously found to be distributed in a molecular disk, coincident with the central starburst, and a galactic scale outflow which originates in the central starburst. With the new short spacings-corrected maps we derive some of the properties of the dense molecular gas in the base of the outflow. From the HCN and HCO$^{+}$ J=(1-0) line emission, and under the assumptions of the gas being optically thin and in local thermodynamic equilibrium, we place lower limits to the amount of dense molecular gas in the base of the outflow. The lower limits are $7\times10^{6}$ $M_{\odot}$ and $21\times10^{6}$ $M_{\odot}$, or $\gtrsim2\%$ of the total molecular mass in the outflow. The kinematics and spatial distribution of the dense gas outside the central starburst suggests that it is being expelled through chimneys. Assuming a constant outflow velocity, the derived outflow rate of dense molecular gas is $\geq0.3$ $M_{\odot}$ yr$^{-1}$, which would lower the starburst lifetime by $\geq5\%$. The energy required to expel this mass of dense gas is $(1-10)\times10^{52}$ erg.

astro-ph.GA↗

arXiv2014

The CARMA Paired Antenna Calibration System: Atmospheric Phase Correction for Millimeter Wave Interferometry and its Application to Mapping the Ultraluminous Galaxy Arp 193

Phase fluctuations introduced by the atmosphere are the main limiting factor in attaining diffraction limited performance in extended interferometric arrays at millimeter and submillimeter wavelengths. We report the results of C-PACS, the Combined Array for Research in Millimeter-Wave Astronomy Paired Antenna Calibration System. We present a systematic study of several hundred test observations taken during the 2009-2010 winter observing season where we utilize CARMA's eight 3.5-m antennas to monitor an atmospheric calibrator while simultaneously acquiring science observations with 6.1-m and 10.4-m antennas on baselines ranging from a few hundred meters to ~2 km. We find that C-PACS is systematically successful at improving coherence on long baselines under a variety of atmospheric conditions. We find that the angular separation between the atmospheric calibrator and target source is the most important consideration, with consistently successful phase correction at CARMA requiring a suitable calibrator located $\lesssim$6$^\circ$ away from the science target. We show that cloud cover does not affect the success of C-PACS. We demonstrate C-PACS in typical use by applying it to the observations of the nearby very luminous infrared galaxy Arp 193 in $^{12}$CO(2-1) at a linear resolution of ~70 pc (0.12" x 0.18"), 3 times better than previously published molecular maps of this galaxy. We resolve the molecular disk rotation kinematics and the molecular gas distribution and measure the gas surface densities and masses on 90 pc scales. We find that molecular gas constitutes $\sim30\%$ of the dynamical mass in the inner 700 pc of this object with a surface density $\sim10^4 M_\odot$ pc$^{-2}$; we compare these properties to those of the starburst region of NGC 253.

astro-ph.IM↗

arXiv2014

The Role of Stellar Feedback in the Dynamics of HII Regions

Stellar feedback is often cited as the biggest uncertainty in galaxy formation models today. This uncertainty stems from a dearth of observational constraints as well as the great dynamic range between the small scales (<1 pc) where the feedback occurs and the large scales of galaxies (>1 kpc) that are shaped by this feedback. To bridge this divide, in this paper we aim to assess observationally the role of stellar feedback at the intermediate scales of HII regions. In particular, we employ multiwavelength data to examine several stellar feedback mechanisms in a sample of 32 HII regions in the Large and Small Magellanic Clouds (LMC and SMC, respectively). Using optical, infrared, radio, and X-ray images, we measure the pressures exerted on the shells from the direct stellar radiation, the dust-processed radiation, the warm ionized gas, and the hot X-ray emitting gas. We find that the warm ionized gas dominates over the other terms in all of the sources, although two have comparable dust-processed radiation pressures to their warm gas pressures. The hot gas pressures are comparatively weak, while the direct radiation pressures are 1-2 orders of magnitude below the other terms. We discuss the implications of these results, particularly highlighting evidence for hot gas leakage from the HII shells and regarding the momentum deposition from the dust-processed radiation to the warm gas. Furthermore, we emphasize that similar observational work should be done on very young HII regions to test whether direct radiation pressure and hot gas can drive the dynamics at early times.

astro-ph.SR↗

arXiv2014

ALFALFA Discovery of the Nearby Gas-rich Dwarf Galaxy Leo P. V. Neutral Gas Dynamics and Kinematics

We present new HI spectral line imaging of the extremely metal-poor, star-forming dwarf irregular galaxy Leo P. Our HI images probe the global neutral gas properties and the local conditions of the interstellar medium (ISM). The HI morphology is slightly elongated along the optical major-axis. We do not find obvious signatures of interaction or infalling gas at large spatial scales. The neutral gas disk shows obvious rotation, although the velocity dispersion is comparable to the rotation velocity. The rotation amplitude is estimated to be V_c = 15 +/- 5 km/s. Within the HI radius probed by these observations, the mass ratio of gas to stars is roughly 2:1, while the ratio of the total mass to the baryonic mass is ~15:1. We use this information to place Leo P on the baryonic Tully-Fisher relation, testing the baryonic content of cosmic structures in a sparsely populated portion of parameter space that has hitherto been occupied primarily by dwarf spheroidal galaxies. We detect the signature of two temperature components in the neutral ISM of Leo P; the cold and warm components have characteristic velocity widths of 4.2 +/- 0.9 km/s and 10.1 +/- 1.2 km/s, corresponding to kinetic temperatures of ~1100 K and ~6200 K, respectively. The cold HI component is unresolved at a physical resolution of 200 pc. The highest HI surface densities are observed in close physical proximity to the single HII region. A comparison of the neutral gas properties of Leo P with other extremely metal-deficient (XMD) galaxies reveals that Leo P has the lowest neutral gas mass of any known XMD, and that the dynamical mass of Leo P is more than two orders of magnitude smaller than any known XMD with comparable metallicity.

astro-ph.GA↗

arXiv2014

Search for [CII] emission in z=6.5-11 star-forming galaxies

We present the search for the [CII] emission line in three $z>6.5$ Lyman-alpha emitters (LAEs) and one J-Dropout galaxy using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the Plateau de Bure Interferometer (PdBI). We observed three bright $z\sim6.5-7$ LAEs discovered in the SUBARU deep field (SDF) and the Multiple Imaged lensed $z\sim 11$ galaxy candidate found behind the galaxy cluster MACSJ0647.7+7015. For the LAEs IOK-1 ($z=6.965$), SDF J132415.7+273058 ($z=6.541$) and SDF J132408.3+271543 ($z=6.554$) we find upper limits for the [CII] line luminosity of $<2.05$, $<4.52$ and $<10.56\times10^{8}{\rm L}_{\odot}$ respectively. We find upper limits to the FIR luminosity of the galaxies using a spectral energy distribution template of the local galaxy NGC 6946 and taking into account the effects of the Cosmic Microwave Background on the mm observations. For IOK-1, SDF J132415.7+273058 and SDF J132408.3+271543 we find upper limits for the FIR luminosity of $<2.33$, $3.79$ and $7.72\times10^{11}{\rm L}_{\odot}$ respectively. For the lensed galaxy MACS0647-JD, one of the highest redshift galaxy candidate to date with $z_{\rm ph}=10.7^{+0.6}_{-0.4}$ we put an upper limit in the [CII] emission of $<1.36\times10^{8}\times(μ/15)^{-1}{\rm L}_{\odot}$ and an upper limit in the FIR luminosity of $<6.1\times10^{10}\times(μ/15)^{-1}{\rm L}_{\odot}$ (where $μ$ is the magnification factor). We explore the different conditions relevant for the search for [CII] emission in high redshift galaxies as well as the difficulties for future observations with ALMA and CCAT.

astro-ph.GA↗

arXiv2013

Formation of Molecular Clouds and Global Conditions for Star Formation

Giant molecular clouds (GMCs) are the primary reservoirs of cold, star-forming molecular gas in the Milky Way and similar galaxies, and thus any understanding of star formation must encompass a model for GMC formation, evolution, and destruction. These models are necessarily constrained by measurements of interstellar molecular and atomic gas, and the emergent, newborn stars. Both observations and theory have undergone great advances in recent years, the latter driven largely by improved numerical simulations, and the former by the advent of large-scale surveys with new telescopes and instruments. This chapter offers a thorough review of the current state of the field.

astro-ph.GA↗

arXiv2013

Dust May Be More Rare Than Expected in Metal Poor Galaxies

'Normal' galaxies observed at z>6, when the Universe was <1 billion years old, thus far show no evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is 1%. A prototypical example is 'Himiko' (z=6.6), which a mere 840 Myr after the Big Bang is forming stars at a rate of 30-100 Msun/yr, yielding a mass assembly time M^{star}/SFR 150x10^6 yr. Himiko is estimated to have a low fraction (2-3% of the Solar value) of elements heavier than helium (metallicity), and although its gas mass cannot be asserted at this time its dust-to-stellar mass ratio is constrained to be <0.05%. The local galaxy I Zw 18, with a metallicity 4% solar and forming stars less rapidly than Himiko but still vigorously for its mass (M^{star}/SFR 1.6x10^9 yr), is also very dust deficient and perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18 from which we determine its dust mass to be 450-1800 Msun, yielding a dust-to-stellar mass ratio \approx 10^{-6}-10^{-5} and a dust-to-gas mass ratio 3.2-13x10^{-6}. If I Zw 18 is a reasonable analog of Himiko, then Himiko's dust mass is \approx 50,000 Msun, a factor of 100 below the current upper limit. These numbers are considerably uncertain, but if most high-z galaxies are more like Himiko than like the quasar host SDSS J114816.64+525150.3, then the prospects for detecting the gas and dust in them are much poorer than hitherto anticipated.

astro-ph.CO↗

arXiv2013

Indications of a sub-linear and non-universal Kennicutt-Schmidt relationship

We estimate the parameters of the Kennicutt-Schmidt (KS) relationship, linking the star formation rate (Sigma_SFR) to the molecular gas surface density (Sigma_mol), in the STING sample of nearby disk galaxies using a hierarchical Bayesian method. This method rigorously treats measurement uncertainties, and provides accurate parameter estimates for both individual galaxies and the entire population. Assuming standard conversion factors to estimate Sigma_SFR and Sigma_mol from the observations, we find that the KS parameters vary between galaxies, indicating that no universal relationship holds for all galaxies. The KS slope of the whole population is 0.76, with the 2sigma range extending from 0.58 to 0.94. These results imply that the molecular gas depletion time is not constant, but varies from galaxy to galaxy, and increases with the molecular gas surface density. Therefore, other galactic properties besides just Sigma_mol affect Sigma_SFR, such as the gas fraction or stellar mass. The non-universality of the KS relationship indicates that a comprehensive theory of star formation must take into account additional physical processes that may vary from galaxy to galaxy.

astro-ph.CO↗

arXiv2013

CARMA Survey Toward Infrared-bright Nearby Galaxies (STING). III. The Dependence of Atomic and Molecular Gas Surface Densities on Galaxy Properties

We investigate the correlation between CO and HI emission in 18 nearby galaxies from the CARMA Survey Toward IR-Bright Nearby Galaxies (STING) at sub-kpc and kpc scales. Our sample, spanning a wide range in stellar mass and metallicity, reveals evidence for a metallicity dependence of the HI column density measured in regions exhibiting CO emission. Such a dependence is predicted by the equilibrium model of McKee & Krumholz, which balances H_2 formation and dissociation. The observed HI column density is often smaller than predicted by the model, an effect we attribute to unresolved clumping, although values close to the model prediction are also seen. We do not observe HI column densities much larger than predicted, as might be expected were there a diffuse HI component that did not contribute to H_2 shielding. We also find that the H_2 column density inferred from CO correlates strongly with the stellar surface density, suggesting that the local supply of molecular gas is tightly regulated by the stellar disk.

astro-ph.GA↗

arXiv2013

The Starburst-Driven Molecular Wind in NGC 253 and the Suppression of Star Formation

The under-abundance of very massive galaxies in the universe is frequently attributed to the effect of galactic winds. Although ionized galactic winds are readily observable most of the expelled mass is likely in cooler atomic and molecular phases. Expanding molecular shells observed in starburst systems such as NGC 253 and M 82 may facilitate the entrainment of molecular gas in the wind. While shell properties are well constrained, determining the amount of outflowing gas emerging from such shells and the connection between this gas and the ionized wind requires spatial resolution <100 pc coupled with sensitivity to a wide range of spatial scales, hitherto not available. Here we report observations of NGC 253, a nearby starburst galaxy (D~3.4 Mpc) known to possess a wind, which trace the cool molecular wind at 50 pc resolution. At this resolution the extraplanar molecular gas closely tracks the Hα filaments, and it appears connected to molecular expanding shells located in the starburst region. These observations allow us to directly measure the molecular outflow rate to be > 3 Msun/yr and likely ~9 Msun/yr. This implies a ratio of mass-outflow rate to star formation rate of at least η~1-3, establishing the importance of the starburst-driven wind in limiting the star formation activity and the final stellar content.

astro-ph.CO↗

arXiv2013

The CO-to-H2 Conversion Factor

CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 "conversion factor," Xco, in different environments. In the Milky Way disk, we recommend a conversion factor Xco = 2x10^{20} cm^-2/(K km/s)^-1 with +/-30% uncertainty. Studies of other "normal galaxies" return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that Xco increases with decreasing metallicity, turning up sharply below metallicity ~1/3-1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, Xco appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower Xco is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments.

astro-ph.GA↗

arXiv2013

Dusty OB Stars in the Small Magellanic Cloud - I: Optical Spectroscopy Reveals Predominantly Main-Sequence OB Stars

We present the results of optical spectroscopic follow-up of 125 candidate main sequence OB stars in the Small Magellanic Cloud (SMC) that were originally identified in the S3MC infrared imaging survey as showing an excess of emission at 24 um indicative of warm dust, such as that associated with a transitional or debris disks. We use these long-slit spectra to investigate the origin of the 24 um emission and the nature of these stars. A possible explanation for the observed 24 um excess, that these are emission line stars with dusty excretion disks, is disproven for the majority of our sources. We find that 88 of these objects are normal stars without line emission, with spectral types mostly ranging from late-O to early-B; luminosity classes a sub-set of our sample indicate that most are main-sequence stars. We further identify 17 emission-line stars, 7 possible emission-line stars, and 5 other stars with forbidden-line emission. We discover a new O6 Iaf star; it exhibits strong HeII 4686 A emission but relatively weak NIII 4640 A emission that we attribute to the lower nitrogen abundance in the SMC. Two other objects are identified with planetary nebulae, one with a young stellar object, and two with X-ray binaries. To shed additional light on the nature of the observed 24 um excess we use optical and infrared photometry to estimate the dust properties of the objects with normal O and B star spectra and compare these properties to those of a sample of hot spots in the Galactic interstellar medium (ISM). We find that the dust properties of the dusty OB star sample resemble the properties of the Galactic sample of hot spots. Some may be runaway systems with bow-shocks resulting from a large velocity difference between star and ISM. We further investigate the nature of these dusty OB stars in a companion paper presenting mid-infrared spectroscopy and additional imaging.

astro-ph.SR↗

arXiv2013

Dusty OB stars in the Small Magellanic Cloud - II: Extragalactic Disks or Examples of the Pleiades Phenomenon?

We use mid-infrared Spitzer spectroscopy and far-infrared Herschel photometry for a sample of twenty main sequence O9--B2 stars in the Small Magellanic Cloud (SMC) with strong 24 micron excesses to investigate the origin of the mid-IR emission. Either debris disks around the stars or illuminated patches of dense interstellar medium (ISM) can cause such mid-IR emission. In a companion paper, Paper I, we use optical spectroscopy to show that it is unlikely for any of these sources to be classical Be stars or Herbig Ae/Be stars. We focus our analysis on debris disks and cirrus hot spots. We find three out of twenty stars to be significantly extended in the mid-IR, establishing them as cirrus hot spots. We then fit the IR spectral energy distributions to determine dust temperatures and masses. We find the dust masses in the SMC stars to be larger than for any known debris disks, although this evidence against the debris disk hypothesis is circumstantial. Finally, we created a local comparison sample of bright mid-IR OB stars in the Milky Way (MW) by cross-matching the WISE and Hipparcos catalogs. All such local stars in the appropriate luminosity range that can be unambiguously classified are young stars with optical emission lines or are spatially resolved by WISE with sizes too large to be plausible debris disk candidates. We conclude that the very strong mid-IR flux excesses are most likely explained as cirrus hot spots, although we cannot rigorously rule out that a small fraction of the sample is made up of debris disks or transition disks. We present suggestive evidence that bow-shock heating around runaway stars may be a contributing mechanism to the interstellar emission. These sources, interpreted as cirrus hot spots, offer a new localised probe of diffuse interstellar dust in a low metallicity environment. (Abridged)

astro-ph.SR↗

arXiv2013

Misalignment of Magnetic Fields and Outflows in Protostellar Cores

We present results of 1.3 mm dust polarization observations toward 16 nearby, low-mass protostars, mapped with ~2.5" resolution at CARMA. The results show that magnetic fields in protostellar cores on scales of ~1000 AU are not tightly aligned with outflows from the protostars. Rather, the data are consistent with scenarios where outflows and magnetic fields are preferentially misaligned (perpendicular), or where they are randomly aligned. If one assumes that outflows emerge along the rotation axes of circumstellar disks, and that the outflows have not disrupted the fields in the surrounding material, then our results imply that the disks are not aligned with the fields in the cores from which they formed.

astro-ph.SR↗

arXiv2013

Millimeter dust emission compared with other mass estimates in N11 molecular clouds in the LMC

CO and dust emission at millimeter wavelengths are independent tracers of cold interstellar matter, which have seldom been compared on the scale of GMCs in other galaxies. In this study, and for the first time in the Large Magellanic Cloud, we compute the molecular cloud masses from the mm emission of the dust and compare them with the masses derived from their CO luminosity and virial theorem. We present CO (J=1-0,2-1) and 1.2 mm continuum observations of the N11 star forming region in the LMC obtained with the SEST telescope and the SIMBA bolometer, respectively. We use the CO data to identify individual molecular clouds and measure their physical properties. The correlations between the properties of the N11 clouds are in agreement with those found in earlier studies in the LMC that sample a larger set of clouds and a larger range of cloud masses. For the N11 molecular clouds, we compare the masses estimated from the CO luminosity (Xco\Lco), the virial theorem (Mvir) and the millimeter dust luminosity (L_d). The measured ratios Lco/Mvir and L_d/Mvir constrain the Xco and K_d (dust emissivity at 1.2 mm per unit gas mass) parameters as a function of the virial parameter a_vir. The comparison between the different mass estimates yields a Xco-factor of 8.8x10^20 cm-2/(K km s-1) x a_vir and a K_d parameter of 1.5x10^-3 cm2/g x a_vir. We compare our N11 results with a similar analysis for molecular clouds in the Gould's Belt. We do not find in N11 a large discrepancy between the dust mm and virial masses as reported in earlier studies of molecular clouds in the SMC. The ratio between L_d and Mvir in N11 is half of that measured for Gould's Belt clouds, which can be accounted for by a factor of two lower gas-to-dust mass ratio, as the difference in gas metallicities. If the two samples have similar a_vir values, this result implies that their dust far-IR properties are also similar.

astro-ph.CO↗