Clouds In NGC Research Articles

Extreme giant molecular clouds in the luminous infrared galaxy NGC 3256

ABSTRACT We present a cloud decomposition of 12CO (2–1) observations of the merger and nearest luminous infrared galaxy, NGC 3256. 185 spatially and spectrally resolved clouds are identified across the central ≈130 kpc2 at 90 pc resolution and completeness is estimated. We compare our cloud catalogue from NGC 3256 to ten galaxies observed in the PHANGS-ALMA survey. Distributions in NGC 3256 of cloud velocity dispersions (median 23 km s−1), luminosities (1.5 × 107 K km s−1 pc2), CO-estimated masses (2.1 × 107 M⊙), mass surface densities (470 M⊙ pc−2), virial masses (5.4 × 107 M⊙), virial parameters (4.3), size-linewidth coefficients (6.3 km2 s−2 pc−1), and internal turbulent pressures ( 1.0 × 10 7K cm−3$\, k_{\mathrm{B}}^{-1}$) are significantly higher than in the PHANGS-ALMA galaxies. Cloud radii (median 88 pc) are slightly larger in NGC 3256 and free-fall times (4.1 Myr) are shorter. The distribution of cloud eccentricities in NGC 3256 (median of 0.8) is indistinguishable from many PHANGS-ALMA galaxies, possibly because the dynamical state of clouds in NGC 3256 is similar to that of nearby spiral galaxies. However, the narrower distribution of virial parameters in NGC 3256 may reflect a narrower range of dynamical states than in PHANGS-ALMA galaxies. No clear picture of cloud alignment is detected, despite the large eccentricities. Correlations between cloud properties point to high external pressures in NGC 3256 keeping clouds bound and collapsing given such high velocity dispersions and star-formation rates. A fit to the cloud mass function gives a high-mass power-law slope of $-2.75^{+0.07}_{-0.01}$, near the average from PHANGS-ALMA galaxies. We also compare our results to a pixel-based analysis of these observations and find molecular-gas properties agree qualitatively, though peak brightness temperatures are somewhat higher and virial parameters and free-fall times are somewhat lower in this cloud-based analysis.

Broad-line region clouds orbiting an AGN sample

ABSTRACT We present a spectral and temporal analysis of XMM–Newton data from a sample of six galaxies (NGC 3783, Mrk 279, Mrk 766, NGC 3227, NGC 7314, and NGC 3516). Using the hardness-ratio curves, we identify time intervals in which clouds are eclipsing the central X-ray source in five of the six sources. We detect three occultations in NGC 3227 and one occultation in NGC 3783, NGC 7314, and NGC 3516, together with the well-known occultations in Mrk 766. We estimate the physical properties of the eclipsing clouds. The derived physical size of the X-ray sources (∼(3–28) × 1013 cm) is less than that of the eclipsing clouds with column densities of ∼1022–1023 cm−2, thus a single cloud may block the X-ray source, leading to notorious temporal variability of the X-ray flux. The eclipsing clouds in Mrk 766, NGC 3227, NGC 7314, and NGC 3516 with distances from the X-ray source of ∼(0.3–3.6) $\times 10^4\, R_\mathrm{ g}$ are moving at Keplerian velocities >1122 km s−1, typical parameters of broad-line region clouds, while the eclipsing cloud in NGC 3783 is likely located in the dusty torus. We also find a good anticorrelation with a slope of −187 ± 62 between the known masses of the supermassive black hole in the centre of the galaxies with the equivalent width (EW) of the 6.4 keV Fe line for the five type 1 Seyfert galaxies of our sample, while the type 2 Seyfert galaxy NGC 7314 shows an average EW value of 100 ± 11 eV inconsistent with the above anticorrelation.

The distribution and origin of C2H in NGC 253 from ALCHEMI

Context. Observations of chemical species can provide insights into the physical conditions of the emitting gas however it is important to understand how their abundances and excitation vary within different heating environments. C2H is a molecule typically found in PDR regions of our own Galaxy but there is evidence to suggest it also traces other regions undergoing energetic processing in extragalactic environments. Aims. As part of the ALCHEMI ALMA large program, we map the emission of C2H in the central molecular zone of the nearby starburst galaxy NGC 253 at 1.6″ (28 pc) resolution and characterize it to understand its chemical origins. Methods. We used spectral modeling of the N = 1−0 through N = 4−3 rotational transitions of C2H to derive the C2H column densities towards the dense clouds in NGC 253. We then use chemical modeling, including photodissociation region (PDR), dense cloud, and shock models to investigate the chemical processes and physical conditions that are producing the molecular emission. Results. We find high C2H column densities of ∼1015 cm−2 detected towards the dense regions of NGC 253. We further find that these column densities cannot be reproduced if it is assumed that the emission arises from the PDR regions at the edge of the clouds. Instead, we find that the C2H abundance remains high even in the high visual extinction interior of these clouds and that this is most likely caused by a high cosmic-ray ionization rate.

High-mass star formation in Orion B triggered by cloud–cloud collision: Merging molecular clouds in NGC 2024

AbstractWe performed new comprehensive 13CO(J = 2–1) observations toward NGC 2024, the most active star-forming region in Orion B, with an angular resolution of ∼100″ obtained with Nanten2. We found that the associated cloud consists of two independent velocity components. The components are physically connected to the H ii region as evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of ∼0.6 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that a cloud–cloud collision between the two components triggered the formation of the late O-type stars and early B stars localized within 0.3 pc of the cloud peak. The duration time of the collision is estimated to be 0.3 million years from a ratio of the displacement and the relative velocity ∼3 km s−1 corrected for probable projection. The high column density of the colliding cloud ∼1023 cm−2 is similar to those in the other high-mass star clusters in RCW 38, Westerlund 2, NGC 3603, and M 42, which are likely formed under trigger by cloud–cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud–cloud collision in Orion.

ALMA Observations of NGC 6334S. I. Forming Massive Stars and Clusters in Subsonic and Transonic Filamentary Clouds

Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) and Karl G. Jansky Very Large Array (JVLA) observations of the massive infrared dark cloud NGC 6334S (also known as IRDC G350.56+0.44), located at the southwestern end of the NGC 6334 molecular cloud complex. The H13CO+ and NH2D lines covered by the ALMA observations at a ∼3″ angular resolution (∼0.02 pc) reveal that the spatially unresolved nonthermal motions are predominantly subsonic and transonic, a condition analogous to that found in low-mass star-forming molecular clouds. The observed supersonic nonthermal velocity dispersions in massive star-forming regions, often reported in the literature, might be significantly biased by poor spatial resolutions that broaden the observed line widths owing to unresolved motions within the telescope beam. Our 3 mm continuum image resolves 49 dense cores, whose masses range from 0.17 to 14 M ⊙. The majority of them are resolved with multiple velocity components. Our analyses of these gas velocity components find an anticorrelation between the gas mass and the virial parameter. This implies that the more massive structures tend to be more gravitationally unstable. Finally, we find that the external pressure in the NGC 6334S cloud is important in confining these dense structures and may play a role in the formation of dense cores and, subsequently, the embedded young stars.

ALMA Observations of the Molecular Clouds in NGC 625

Abstract We present the highest-resolution (1″) 12CO observations of molecular gas in the dwarf starburst galaxy NGC 625 to date, obtained with Atacama Large Millimeter/submillimeter Array. The molecular gas, which is distributed in discrete clouds within an area of 0.4 kpc2, does not have well-ordered large-scale motions. We measure a molecular mass in NGC 625 of 5.3 × 106 , assuming a Milky Way CO-to- conversion factor. We use the CPROPS package to identify molecular clouds and measure their properties. The 19 resolved CO clouds have a median radius of 20 pc, a median linewidth 2.5 , and a median surface density of 169 . Larson scaling relations suggest that molecular clouds in NGC 625 are mostly in virial equilibrium. Comparison of our high-resolution CO observations with a star formation rate map, inferred from ancillary optical observations, suggests that about 40% of the molecular clouds coincide with the brightest H ii regions. These bright H ii regions have a range of molecular gas depletion timescales, all within a factor of ∼3 of the global depletion time in NGC 625 of 106–134 Myr. The highest surface density molecular clouds toward the southwest of the galaxy, in a region we call the Butterfly, do not show strong star formation activity and suggest a depletion timescale longer than 5 Gyr.

The headlight cloud in NGC 628: An extreme giant molecular cloud in a typical galaxy disk

Context.Cloud-scale surveys of molecular gas reveal the link between giant molecular cloud properties and star formation across a range of galactic environments. Cloud populations in galaxy disks are considered to be representative of the normal star formation process, while galaxy centers tend to harbor denser gas that exhibits more extreme star formation. At high resolution, however, molecular clouds with exceptional gas properties and star formation activity may also be observed in normal disk environments. In this paper we study the brightest cloud traced in CO(2–1) emission in the disk of nearby spiral galaxy NGC 628.Aims.We characterize the properties of the molecular and ionized gas that is spatially coincident with an extremely bright H IIregion in the context of the NGC 628 galactic environment. We investigate how feedback and large-scale processes influence the properties of the molecular gas in this region.Methods.High-resolution ALMA observations of CO(2–1) and CO(1−0) emission were used to characterize the mass and dynamical state of the “headlight” molecular cloud. The characteristics of this cloud are compared to the typical properties of molecular clouds in NGC 628. A simple large velocity gradient (LVG) analysis incorporating additional ALMA observations of13CO(1−0), HCO+(1−0), and HCN(1−0) emission was used to constrain the beam-diluted density and temperature of the molecular gas. We analyzed the MUSE spectrum using Starburst99 to characterize the young stellar population associated with the H IIregion.Results.The unusually bright headlight cloud is massive (1 − 2 × 107 M⊙), with a beam-diluted density ofnH2 = 5 × 104cm−3based on LVG modeling. It has a low virial parameter, suggesting that the CO emission associated with this cloud may be overluminous due to heating by the H IIregion. A young (2 − 4 Myr) stellar population with mass 3 × 105 M⊙is associated.Conclusions.We argue that the headlight cloud is currently being destroyed by feedback from young massive stars. Due to the large mass of the cloud, this phase of the its evolution is long enough for the impact of feedback on the excitation of the gas to be observed. The high mass of the headlight cloud may be related to its location at a spiral co-rotation radius, where gas experiences reduced galactic shear compared to other regions of the disk and receives a sustained inflow of gas that can promote the mass growth of the cloud.

The spatial relation between young star clusters and molecular clouds in M51 with LEGUS

We present a study correlating the spatial locations of young star clusters with those of molecular clouds in NGC~5194, in order to investigate the timescale over which clusters separate from their birth clouds. The star cluster catalogues are from the Legacy ExtraGalactic UV Survey (LEGUS) and the molecular clouds from the Plateau de Bure Interefrometer Arcsecond Whirpool Survey (PAWS). We find that younger star clusters are spatially closer to molecular clouds than older star clusters. The median ages for clusters associated with clouds is 4~Myr whereas it is 50~Myr for clusters that are sufficiently separated from a molecular cloud to be considered unassociated. After $\sim$6~Myr, the majority of the star clusters lose association with their molecular gas. Younger star clusters are also preferentially located in stellar spiral arms where they are hierarchically distributed in kpc-size regions for 50-100~Myr before dispersing. The youngest star clusters are more strongly clustered, yielding a two-point correlation function with $\alpha=-0.28\pm0.04$, than the GMCs ($\alpha=-0.09\pm0.03$) within the same PAWS field. However, the clustering strength of the most massive GMCs, supposedly the progenitors of the young clusters for a star formation efficiency of a few percent, is comparable ($\alpha=-0.35\pm0.05$) to that of the clusters. We find a galactocentric-dependence for the coherence of star formation, in which clusters located in the inner region of the galaxy reside in smaller star-forming complexes and display more homogeneous distributions than clusters further from the centre. This result suggests a correlation between the survival of a cluster complex and its environment.

Probing the Turbulence Dissipation Range and Magnetic Field Strengths in Molecular Clouds. II. Directly Probing the Ion–neutral Decoupling Scale

Abstract The line width of ions has been observed to be systematically narrower than that of the coexisting neutrals in molecular clouds and been interpreted as the signature of the decoupling of the neutral turbulence from magnetic fields in partially ionized medium. As a sequel of Li & Houde, here we present further observational evidence that supports these earlier proposals with the velocity coordinate spectrum analysis. We recover the turbulent energy spectra of HCN and HCO+ (4 − 3) in a starless molecular cloud in NGC 6334 where magnetic fields play a dynamically important role. Our analysis showed that the neutral spectrum is consistent with Kolmogorov type ( k − 5 / 3 , where k is the wavenumber), while that of the ions is the same on the large scale, but steeper (∼k −2), for scales smaller than 0.404 pc. We carefully ruled out the possibilities that the spectrum difference can stem from the differences of ion and neutral optical depth and hyperfine structure.

NGC 6334 and NGC 6357

Aims. The formation of high-mass stars is still debated. For this reason, several projects such as Herschel-HOBYS are focussed on the study of the earliest phases of massive star formation. As a result, massive star-forming complexes such as NGC 6334 and NGC 6357 have been observed in the far-infrared to study their massive dense cores where massive stars are expected to form. However, to better characterise the environments of these cores we need to understand the previous massive star formation history. To better characterise the environment of these massive dense cores we study the previous high-mass star formation and how these stars act on their environments. Methods. This study is based on the spectral classification of the OB stars identified towards NGC 6334 and NGC 6357 with spectra taken with the AAOmega spectrograph on the Anglo-Australian Telescope (AAT). From the subsequent spectral classification of 109 stars across these regions we were able to evaluate the following: distance, age, mass, global star-forming efficiency (SFE), and star formation rate (SFR) of the regions. The physical conditions of the ionised gas for both complexes was also derived. Results. We confirm that NGC 6334 and NGC 6357 belong to the Saggitarius-Carina arm which, in this direction, extends from 1 kpc to 2.2 kpc. From the location of the stars in Hertzprung-Russell diagram we show that stars older than ∼10 Myr are broadly spread across these complexes, while younger stars are mainly located in the H ii regions and stellar clusters. Our data also suggests that some of the young stars can be considered runaway stars. We evaluate a SFE of 0.019 +0.008 −0.007 and 0.021 +0.004 −0.003 and a SFR of 1.1 × 10 3 ± 300 M Myr −1 and 1.7 × 10 3 ± 400 M Myr −1 for NGC 6334 and NGC 6357, respectively. We note that 29 OB stars have X-ray counterparts, most of them belonging to NGC 6357. This suggests that molecular clouds in NGC 6357 are more impacted by X-ray flux and stellar winds than in NGC 6334. Finally, from the analysis of nebular lines (Hα, [NII], and [SII]) from spectra from several regions of ionised gas, we confirm that the filaments in NGC 6357 are shock heated.

The massive Class I eruptive variable V723 Carinae and its neighbour Car I-125★

Near-IR observations from 1993 to 2014 of V723 Carinae confirm this to be a young eruptive variable embedded in the Car I PDR/dust cloud in NGC 3372. In 1993, it was fainter than 16.6 in JHK and went into outburst before 2003, reaching K ≃ 12.9 in 2004. Since then, V723 Car has suffered erratic flux variations of up to ΔK = 2 in time-scales of years. The variations are also evident into the mid-infrared. The H − K index shows correlation with K, though the ratio ΔK/Δ(H − K) differs from that expected from dust extinction. The 1.91 to 2.48 μm spectrum of V723 Car shows emission lines of H2 and also Brγ, as well as 2.3–2.4 μm CO overtone bandheads. The system is extremely red (H − K ≥ 4), with mid- and far-IR colours of a Class I object. The fitted parameters of Robitaille et al.'s model to the 1.6–850 μm spectral energy distribution of V723 Car indicate a system composed of a 10 M⊙ central star with a 5.6 × 10−3 M⊙ disc and a 1.6 × 103 M⊙ envelope with AV ∼ 55 of extinction. The total luminosity of the system is about 4 × 103 L⊙. V723 Car is the most luminous, most massive, most deeply embedded, and possibly the youngest of all the young eruptive variables known. Evidence is provided of a 5000 au-long jet of shocked gas extending to opposite sides of the star. The infrared properties of a nearby 10 L⊙ Class I young stellar object, Car I-125, are also described, with variations in K typical of Herbig Ae/Be stars. Its near-IR spectrum is featureless except for bright Brγ line emission.

First X-ray-based statistical tests for clumpy-torus models: eclipse events from 230 years of monitoring of Seyfert AGN

We present an analysis of multi-timescale variability in line-of-sight X-ray absorbing gas as a function of optical classification in a large sample of Seyfert AGN to derive the first X-ray statistical constraints for clumpy-torus models. We systematically search for discrete absorption events in the vast archive of RXTE monitoring of dozens of nearby type I and Compton-thin type II AGN. We are sensitive to discrete absorption events due to clouds of full-covering, neutral or mildly ionized gas with columns >~ 10^(22-25) cm^-2 transiting the line of sight. We detect 12 eclipse events in 8 objects, roughly tripling the number previously published from this archive. Peak column densities span ~ 4-26 x 10^22 cm^-2. Event durations span hours to months. The column density profile for an eclipsing cloud in NGC 3783 is doubly spiked, possibly indicating a cloud that is being tidally sheared. We infer the clouds' distances from the black hole to span ~0.3 -140 x 10^4 R_g. In seven objects, the clouds' distances are commensurate with the outer portions of Broad Line Regions (BLR), or the inner regions of infrared-emitting dusty tori. We discuss implications for cloud distributions in the context of clumpy-torus models. The probability of observing a source undergoing an absorption event, independent of constant absorption due to non-clumpy material, is 0.006(+0.160,-0.003) for type Is and 0.110(+0.461,-0.071) for type IIs.

PHOTODISSOCIATION CHEMISTRY FOOTPRINTS IN THE STARBURST GALAXY NGC 253

We report the first detection of PDR molecular tracers, namely HOC+, and CO+, and confirm the detection of the also PDR tracer HCO towards the starburst galaxy NGC 253, claimed to be mainly dominated by shock heating and in an earlier stage of evolution than M 82, the prototypical extragalactic PDR. Our CO+ detection suffers from significant blending to a group of transitions of 13CH3OH, tentatively detected for the first time in the extragalactic interstellar medium. These species are efficiently formed in the highly UV irradiated outer layers of molecular clouds, as observed in the late stage nuclear starburst in M 82. The molecular abundance ratios we derive for these molecules are very similar to those found in M 82. This strongly supports the idea that these molecules are tracing the PDR component associated with the starburst in the nuclear region of NGC 253. A comparison with the predictions of chemical models for PDRs shows that the observed molecular ratios are tracing the outer layers of UV illuminated clouds up to two magnitudes of visual extinction. Chemical models, which include grain formation and photodissociation of HNCO, support the scenario of a photo-dominated chemistry as an explanation to the abundances of the observed species. From this comparison we conclude that the molecular clouds in NGC 253 are more massive and with larger column densities than those in M 82, as expected from the evolutionary stage of the starbursts in both galaxies.

THE HIGHEST RESOLUTIONCHANDRAVIEW OF PHOTOIONIZATION AND JET–CLOUD INTERACTION IN THE NUCLEAR REGION OF NGC 4151

We report high resolution imaging of the nucleus of the Seyfert 1 galaxy NGC 4151 obtained with a 50 ks Chandra HRC observation. The HRC image resolves the emission on spatial scales of 0.5", ~30 pc, showing an extended X-ray morphology overall consistent with the narrow line region (NLR) seen in optical line emission. Removal of the bright point-like nuclear source and image deconvolution techniques both reveal X-ray enhancements that closely match the substructures seen in the Hubble Space Telescope [OIII] image and prominent knots in the radio jet. We find that most of the NLR clouds in NGC 4151 have [OIII] to soft X-ray ratio ~10, despite the distance of the clouds from the nucleus. This ratio is consistent with the values observed in NLRs of some Seyfert 2 galaxies, which indicates a uniform ionization parameter even at large radii and a density decreasing as $r^{-2}$ as expected for a nuclear wind scenario. The [OIII]/X-ray ratios at the location of radio knots show an excess of X-ray emission, suggesting shock heating in addition to photoionization. We examine various mechanisms for the X-ray emission and find that, in contrast to jet-related X-ray emission in more powerful AGN, the observed jet parameters in NGC 4151 are inconsistent with synchrotron emission, synchrotron self-Compton, inverse Compton of CMB photons or galaxy optical light. Instead, our results favor thermal emission from the interaction between radio outflow and NLR gas clouds as the origin for the X-ray emission associated with the jet. This supports previous claims that frequent jet-ISM interaction may explain why jets in Seyfert galaxies appear small, slow, and thermally dominated, distinct from those kpc scale jets in the radio galaxies.

Magnetic interaction of jets and molecular clouds in NGC 4258

Context. NGC 4258 is a well known spiral galaxy with a peculiar large scale jet flow detected in the radio and in H α . Due to the special geometry of the galaxy, the jets emerge from the nuclear region through the galactic disk – at least in the inner region. Aims. Also the distribution of molecular gas looks different from that in other spiral galaxies: 12 CO(1–0) emission has only been detected in the center and along the jets and only up to distances of about $50\hbox{$^{\prime\prime}$}$ (1.8 kpc) from the nucleus. This concentration of CO along the jets is similar to what is expected as fuel for jet-induced star formation in more distant objects. The reason for the CO concentration along the inner jets in NGC 4258 was not understood and is the motivation for the observations presented here. Methods. Using the IRAM interferometer at Plateau de Bure, we mapped the 12 CO(1–0) emission of the central part of NGC 4258 along the nuclear jet direction in the inner 3 kpc. In order to get a properly positioned overlay with H α we observed NGC 4258 in H α at the Hoher List Observatory of the University of Bonn. Results. We detected two parallel CO ridges along a position angle of $-25^\circ$ with a total length of about $80\hbox{$^{\prime\prime}$}$ (2.8 kpc), separated by a CO depleted funnel with a width of about $5\hbox{$^{\prime\prime}$}$ (175 pc). The H α emission is more extended and broader than the CO emission with its maximum just in between the two CO ridges. It seems to be mixed in location and in velocity with the CO emission. In CO we see a peculiar velocity distribution in the iso-velocity map and p-v diagrams. We discuss different scenarios for an interpretation and present a model which can explain the observational results consistently. Conclusions. We propose here that the concentration of CO along the ridges is due to interaction of the rotating gas clouds with the jet's magnetic field by ambipolar diffusion (ion-neutral drift). This magnetic interaction is thought to increase the time the molecular clouds reside near the jet thus leading to the quasi-static CO ridge.

Strongly induced collapse in the Class 0 protostar NGC 1333 IRAS 4A

Context. The onset of gravitational collapse in cluster-forming clouds is still poorly known. Aims. Our goal is to use the Class 0 protostar IRAS 4A, which is undergoing collapse in the active molecular cloud NGC 1333, to set constraints on this process. In particular we want to measure the mass infall rate and investigate whether the collapse could have been triggered by a strong external perturbation. Methods. We analyze existing continuum observations to derive the density structure of the envelope, and use our new molecular line observations with the IRAM 30 m telescope to probe its velocity structure. We perform a detailed comparison of this set of data with a numerical model of collapse triggered by a fast external compression. Results. Both the density and velocity structures of the envelope can be well fitted by this model of collapse induced by a fast external compression for a time of 1-2 x 10 4 yr after the formation of the central protostar. We deduce a high mass infall rate of 0.7-2 x 10 -4 M ⊙ yr -1 . The momentum required for the perturbation to produce this high mass infall rate is on the same order as the momenta measured for the NGC 1333 numerous outflows. Our analysis also shows that the turbulence is highly non uniform in the envelope, dropping from supersonic to subsonic values toward the center. The inner subsonic turbulence is most likely a relic of the conditions prevailing in the dense core before the onset of collapse. Conclusions. The vigorous collapse undergone by IRAS 4A was triggered by a fast external compression, probably related to the expansion of a nearby cavity, which could have simultaneously triggered the collapse of the nearby Class 0 protostar IRAS 4B. This cavity could have been generated by an outflow but we have not found a good protostellar candidate yet.

High mass Star formation in the giant molecular cloud NGC 6334: an infrared view

Sub-arcsec images in the JHKs, H2 and Br of three areas (I(N), F and NGC 6334 IV (MM3)) of the giant molecular cloud NGC 6334 are presented. The preliminary results indicate the presence of a deeply embedded young stellar cluster in the northermost part of the cloud (I(N)). We have identied the exciting source of the cometary UCHII NGC 6334 F. This source has an infrared luminosity LIR= 3 10 3 L and a very steep infrared spectral index. Finally, a new center of massive star formation associated with the millimeter peak MM3 has been found east of the bipolar HII region NGC 6334 A.

Properties of a Molecular Cloud in NGC 185

The dwarf elliptical galaxy NGC 185- the closest early-type galaxy with detectable molecular gas- offers a unique opportunity for high angular resolution studies of the interstellar medium in early-type galaxies. I present interferometric images (17 pc $\times$ 14 pc resolution) of CO emission from NGC 185. The majority of the molecular gas in NGC 185 is in one resolved cloud, closely associated with dust and an HI structure which may be a photodissociated envelope. The high spatial resolution also reveals a velocity gradient across the cloud, which allows a dynamical mass estimate independent of the H2/CO conversion factor or the virial theorem. The linear sizes, velocity gradients, and dynamical masses of the molecular clouds in NGC 185 and NGC 205, a similar dwarf elliptical, are comparable to those of the largest Galactic giant molecular clouds (GMCs). If the dynamical masses of the clouds are assumed to be good estimates of their true masses, the inferred H2/CO conversion factor in NGC 185 is at least eight times larger than a standard Galactic conversion factor, and the reasons for this difference are not yet understood. In any case, it seems clear that structures similar to Galactic GMCs can form in small elliptical or early type galaxies, even in the absence of spiral density waves.

Properties of the Molecular Clouds in NGC 205

The nearby dwarf elliptical galaxy NGC 205 offers a unique opportunity for high-resolution studies of the interstellar medium in an elliptical galaxy. The present paper investigates the distribution of molecular gas, molecular line ratios, and the relationships between atomic gas, molecular gas, and dust in this galaxy. The line ratios 12CO (2–1)/(1–0) and 12CO (1–0)/13CO (1–0) in one of the molecular clouds in NGC 205 are consistent with the ratios found in other elliptical galaxies and in Galactic giant molecular clouds; they suggest that the CO in this cloud is probably subthermally excited. Atomic gas, molecular gas, and dust are very closely associated on scales of ∼100 pc; the atomic gas can be understood as a photodissociated envelope around the molecular clouds. The atomic column densities in this galaxy are quite low (≈1020 cm-2) because the interstellar UV field is relatively low. The total gas-to-dust column density ratios are consistent with Galactic gas-to-dust ratios. In short, the molecular gas in NGC 205 seems to have properties very similar to the familiar molecular clouds in our own Galaxy except for the low atomic column densities.

Compact cores of molecular clouds in NGC 1333/IRAS 6–9

13CO(1-0)and C 18O(1-0)emissions in the NGC 1333/IRAS6–9 star forming region were mapped with the 13.7 m millimeter wave telescope of Purple Mountain Observatory. The areas covered are 10.3′ × 10′ for 13CO(1-0)and 6.8′ × 8′ for C 18O(1-0), respectively. Several new compact molecular cores were discovered and a bipolar outflow of 13CO(1-0)was identified along the SSV12-IRAS8 direction. We present the observed properties and derived the physical parameters for all the 13CO(1-0)and C 18O(1-0)cores. The core distribution and velocity structure are analyzed for this region. The relationship between cores, bipolar outflow and crowded young stellar objects (including infrared sources and HH objects) is also discussed in detail.