Jorge L. Pineda

A Herschel [C ii] Galactic plane survey - I. The global distribution of ISM gas components

Context. The [C ii] 158 μm line is an important tool for understanding the life cycle of interstellar matter. Ionized carbon is present in a variety of phases of the interstellar medium (ISM), including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions. Aims. Velocity-resolved observations of [C ii] are the most powerful technique available to disentangle the emission produced by these components. These observations can also be used to trace CO-dark H2 gas and determine the total mass of the ISM. Methods. The Galactic Observations of Terahertz C+ (GOTC+) project surveys the [C ii] 158 μm line over the entire Galactic disk with velocity-resolved observations using the Herschel/HIFI instrument. We present the first longitude-velocity maps of the [C ii] emission for Galactic latitudes b = 0 ◦ , ±0.5 ◦ ,a nd±1.0 ◦ . We combine these maps with those of H i, 12 CO, and 13 CO to separate the different phases of the ISM and study their properties and distribution in the Galactic plane. Results. [C ii] emission is mostly associated with spiral arms, mainly emerging from Galactocentric distances between 4 and 10kpc. It traces the envelopes of evolved clouds as well as clouds that are in the transition between atomic and molecular. We estimate that most of the observed [C ii] emission is produced by dense photon dominated regions (∼47%), with smaller contributions from COdark H2 gas (∼28%), cold atomic gas (∼21%), and ionized gas (∼4%). Atomic gas inside the Solar radius is mostly in the form of cold neutral medium (CNM), while the warm neutral medium gas dominates the outer galaxy. The average fraction of CNM relative to total atomic gas is ∼43%. We find that the warm and diffuse CO-dark H2 is distributed over a larger range of Galactocentric distances (4−11kpc) than the cold and dense H2 gas traced by 12 CO and 13 CO (4−8kpc). The fraction of CO-dark H2 to total H2 increases with Galactocentric distance, ranging from ∼20% at 4kpc to ∼80% at 10kpc. On average, CO-dark H2 accounts for ∼30% of the molecular mass of the Milky Way. When the CO-dark H2 component is included, the radial distribution of the CO-to-H2 conversion factor is steeper than that when only molecular gas traced by CO is considered. Most of the observed [C ii] emission emerging from dense photon dominated regions is associated with modest far-ultraviolet fields in the range χ0 � 1−30.

THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD

We report a study of the relation between dust and gas over a 100 deg 2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12 CO and 13 CO J = 1 → 0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200 �� corresponding to 0.14 pc at a distance of 140 pc. We find that the relation between visual extinction AV and N (CO) is linear between AV � 3 and 10 mag in the region associated with the B213-L1495 filament. In other regions, the linear relation is flattened for AV 4 mag. We find that the presence of temperature gradients in the molecular gas affects the determination of N (CO) by ∼30%–70% with the largest difference occurring at large column densities. Adding a correction for this effect and accounting for the observed relation between the column density of CO and CO2 ices and AV, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data � 23 mag. We have used these data to study a sample of dense cores in Taurus. Fitting an analytical column density profile to these cores we derive an average volume density of about 1.4 × 10 4 cm −3 and a CO depletion age of about 4.2 × 10 5 yr. At visual extinctions smaller than ∼3 mag, we find that the CO fractional abundance is reduced by up to two orders of magnitude. The data show a large scatter suggesting a range of physical conditions of the gas. We estimate the H2 mass of Taurus to be about 1.5 × 10 4 M� , independently derived from the AV and N (CO) maps. We derive a CO integrated intensity to H2 conversion factor of about 2.1 × 10 20 cm −2 (K km s −1 ) −1 , which applies even in the region where the [CO]/[H2] ratio is reduced by up to two orders of magnitude. The distribution of column densities in our Taurus maps resembles a log-normal function but shows tails at large and low column densities. The length scale at which the high column density tail starts to be noticeable is about 0.4 pc.

The Magellanic Mopra Assessment (MAGMA). I. the molecular cloud population of the large magellanic cloud

We present the properties of an extensive sample of molecular clouds in the Large Magellanic Cloud (LMC) mapped at 11?pc resolution in the CO(1-0) line. Targets were chosen based on a limiting CO flux and peak brightness as measured by the NANTEN survey. The observations were conducted with the ATNF Mopra Telescope as part of the Magellanic Mopra Assessment. We identify clouds as regions of connected CO emission and find that the distributions of cloud sizes, fluxes, and masses are sensitive to the choice of decomposition parameters. In all cases, however, the luminosity function of CO clouds is steeper than dN/dLL ?2, suggesting that a substantial fraction of mass is in low-mass clouds. A correlation between size and linewidth, while apparent for the largest emission structures, breaks down when those structures are decomposed into smaller structures. We argue that the correlation between virial mass and CO luminosity is the result of comparing two covariant quantities, with the correlation appearing tighter on larger scales where a size-linewidth relation holds. The virial parameter (the ratio of a clouds kinetic to self-gravitational energy) shows a wide range of values and exhibits no clear trends with the CO luminosity or the likelihood of hosting young stellar object (YSO) candidates, casting further doubt on the assumption of virialization for molecular clouds in the LMC. Higher CO luminosity increases the likelihood of a cloud harboring a YSO candidate, and more luminous YSOs are more likely to be coincident with detectable CO emission, confirming the close link between giant molecular clouds and massive star formation.

C + detection of warm dark gas in diffuse clouds

We present the first results of the Herschel open time key program, Galactic Observations of Terahertz C + (GOT C+) survey of the [CII] 2 P3/2‐ 2 P1/2 fine-structure line at 1.9 THz (158 μm) using the HIFI instrument on Herschel. We detected 146 interstellar clouds along sixteen lines-of-sight towards the inner Galaxy. We also acquired HI and CO isotopologue data along each line-of-sight for analysis of the physical conditions in these clouds. Here we analyze 29 diffuse clouds (AV < 1.3 mag) in this sample characterized by having [CII] and HI emission, but no detectable CO. We find that [CII] emission is generally stronger than expected for diffuse atomic clouds, and in a number of sources is much stronger than anticipated based on their HI column density. We show that excess [CII] emission in these clouds is best explained by the presence of a significant diffuse warm H2, dark gas, component. This first [CII] 158 μm detection of warm dark gas demonstrates the value of this tracer for mapping this gas throughout the Milky Way and in galaxies.

A Herschel [C ii] Galactic plane survey - II. CO-dark H2 in clouds

Context. H i and CO large scale surveys of the Milky Way trace the diffuse atomic clouds and the dense shielded regions of molecular hydrogen clouds, respectively. However, until recently, we have not had spectrally resolved C + surveys in sufficient lines of sight to characterize the ionized and photon dominated components of the interstellar medium, in particular, the H2 gas without CO, referred to as CO-dark H2, in a large sample of interstellar clouds. Aims. We use a sparse Galactic plane survey of the 1.9 THz (158 μm) [C ii] spectral line from the Herschel open time key programme, Galactic Observations of Terahertz C+ (GOT C+), to characterize the H2 gas without CO in a statistically significant sample of interstellar clouds. Methods. We identify individual clouds in the inner Galaxy by fitting the [C ii] and CO isotopologue spectra along each line of sight. We then combine these spectra with those of H i and use them along with excitation models and cloud models of C + to determine the column densities and fractional mass of CO-dark H2 clouds. Results. We identify1804 narrow velocity [C ii] components corresponding to interstellar clouds in different categories and evolutionary states. About 840 are diffuse molecular clouds with no CO, ∼510 are transition clouds containing [C ii ]a nd 12 CO, but no 13 CO, and the remainder are dense molecular clouds containing 13 CO emission. The CO-dark H2 clouds are concentrated between Galactic radii of ∼3.5 to 7.5 kpc and the column density of the CO-dark H2 layer varies significantly from cloud to cloud with a global average of 9 × 10 20 cm −2 . These clouds contain a significant fraction by mass of CO-dark H2, that varies from ∼75% for diffuse molecular clouds to ∼20% for dense molecular clouds. Conclusions. We find a significant fraction of the warm molecular ISM gas is invisible in H i and CO, but is detected in [C ii]. The fraction of CO-dark H2 is greatest in the diffuse clouds and decreases with increasing total column density, and is lowest in the massive clouds. The column densities and mass fraction of CO-dark H2 are less than predicted by models of diffuse molecular clouds using solar metallicity, which is not surprising as most of our detections are in Galactic regions where the metallicity is larger and shielding more effective. There is an overall trend towards a higher fraction of CO-dark H2 in clouds with increasing Galactic radius, consistent with lower metallicity there.

Physical properties of giant molecular clouds in the Large Magellanic Cloud

The Magellanic Mopra Assessment (MAGMA) is a high angular resolution ^(12)CO (J = 1 → 0) mapping survey of giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud using the Mopra Telescope. Here we report on the basic physical properties of 125 GMCs in the LMC that have been surveyed to date. The observed clouds exhibit scaling relations that are similar to those determined for Galactic GMCs, although LMC clouds have narrower linewidths and lower CO luminosities than Galactic clouds of a similar size. The average mass surface density of the LMC clouds is 50 M_⊙ pc^(−2), approximately half that of GMCs in the inner Milky Way. We compare the properties of GMCs with and without signs of massive star formation, finding that non-star-forming GMCs have lower peak CO brightness than star-forming GMCs. We compare the properties of GMCs with estimates for local interstellar conditions: specifically, we investigate the H i column density, radiation field, stellar mass surface density and the external pressure. Very few cloud properties demonstrate a clear dependence on the environment; the exceptions are significant positive correlations between (i) the H i column density and the GMC velocity dispersion, (ii) the stellar mass surface density and the average peak CO brightness and (iii) the stellar mass surface density and the CO surface brightness. The molecular mass surface density of GMCs without signs of massive star formation shows no dependence on the local radiation field, which is inconsistent with the photoionization-regulated star formation theory proposed by McKee. We find some evidence that the mass surface density of the MAGMA clouds increases with the interstellar pressure, as proposed by Elmegreen, but the detailed predictions of this model are not fulfilled once estimates for the local radiation field, metallicity and GMC envelope mass are taken into account.

A Herschel [C II] Galactic plane survey - III. [C II] as a tracer of star formation

Context. The [C ii] 158 μm line is the brightest far-infrared cooling line in galaxies, representing 0.1 to 1% of their far-infrared continuum emission, and is therefore a potentially powerful tracer of star formation activity. The [C ii] line traces different phases of the interstellar medium (ISM), including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions (PDRs). Therefore without being able to separate the contributions to the [C ii] emission, the relationship of this fine structure line emission to star formation has been unclear. Aims. We study the relationship between the [C ii] emission and the star formation rate (SFR) in the Galactic plane and separate the relationship of different ISM phases to the SFR. We compare these relationships to those in external galaxies and local clouds, allowing examinations of these relationships over a wide range of physical scales. Methods. Wecompare the distribution of the [C ii]emission, with its different contributing ISM phases, as a function of Galactocentric distance with the SFR derived from radio continuum observations. We also compare the SFR with the surface density distribution of atomic and molecular gas, including the CO-dark H2 component. Results. The [C ii] and SFR are well correlated at Galactic scales with a relationship that is in general agreement with that found for external galaxies. By combining [C ii] and SFR data points in the Galactic plane with those in external galaxies and nearby star forming regions, we find that a single scaling relationship between the [C ii] luminosity and SFR applies over six orders of magnitude. The [C ii] emission from different ISM phases are each correlated with the SFR, but only the combined emission shows a slope that is consistent with extragalactic observations. These ISM components have roughly comparable contributions to the Galactic [C ii] luminosity: dense PDRs (30%), cold H i (25%), CO-dark H2 (25%), and ionized gas (20%). The SFR‐gas surface density relationship shows a steeper slope compared to that observed in galaxies, but one that it is consistent with those seen in nearby clouds. The different slope is a result of the use of a constant CO-to-H2 conversion factor in the extragalactic studies, which in turn is related to the assumption of constant metallicity in galaxies. We find a linear correlation between the SFR surface density and that of the dense molecular gas.

Clumpy photon-dominated regions in Carina - I. [C I] and mid-J CO lines in two 4'

Context. The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. Aims. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas. Methods. The NANTEN2-4 m submillimeter telescope was used to map the [C i] 3 P1− 3 P0, 3 P2− 3 P1 and CO 4–3, 7–6 lines in two 4 � × 4 � regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr 14, and the second region is in the molecular cloud south of η Car and Tr 16. These data were combined with 13 CO SEST spectra, HIRES/IRAS 60 µm and 100 µm maps of the FIR continuum, and maps of 8 µm IRAC/Spitzer and MSX emission. Results. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV fi eld of af ew 10 3 in Draine units while the field of the southern region is about a factor 10 weaker. While the IRAC 8 µm emission lights up at the edges of the molecular clouds, CO and also [C i] appear to trace the H2 gas column density. The northern region shows a complex velocity and spatial structure, while the southern region shows an edge-on PDR with a single Gaussian velocity component. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38 �� beam (0.43 pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2 × 10 5 cm −3 is needed to reproduce the observed line emission at two selected interface positions. Conclusions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.

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We have carried out submillimeter 12CO( -->J = 3?2) observations of six giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10 m submillimeter telescope at a spatial resolution of 5 pc and very high sensitivity. We have identified 32 molecular clumps in the GMCs and revealed significant details of the warm and dense molecular gas with -->n(H2) ~ 103?105 cm?3 and -->Tkin ~ 60 K. These data are combined with 12CO( -->J = 1?0) and 13CO( -->J = 1?0) results and compared with LVG calculations. The results indicate that clumps that we detected are distributed continuously from cool (~10-30 K) to warm (30-200 K), and warm clumps are distributed from less dense (~103 cm?3) to dense (~103.5-105 cm?3). We found that the ratio of 12CO( -->J = 3?2) to 12CO( -->J = 1?0) emission is sensitive to and is well correlated with the local H? flux. We infer that differences of clump properties represent an evolutionary sequence of GMCs in terms of density increase leading to star formation. Type I and II GMCs (starless GMCs and GMCs with H?II regions only, respectively) are at the young phase of star formation where density does not yet become high enough to show active star formation, and Type III GMCs (GMCs with H?II regions and young star clusters) represent the later phase where the average density is increased and the GMCs are forming massive stars. The high kinetic temperature correlated with H? flux suggests that FUV heating is dominant in the molecular gas of the LMC.

4' fields

We present the first results from a new carbon monoxide (CO) survey of the southern Galactic plane being conducted with the Mopra radio telescope in Australia. The 12 CO, 13 CO, and C 18 O J = 1‐0 lines are being mapped over the l = 305 ◦ ‐345 ◦ ,b =± 0.5 ◦ portion of the fourth quadrant of the Galaxy, at 35 arcsec spatial and 0.1 km s −1 spectral resolution. The survey is being undertaken with two principal science objectives: (i) to determine where and how molecular clouds are forming in the Galaxy and (ii) to probe the connection between molecular clouds and the ‘missing’ gas inferred from gamma-ray observations. We describe the motivation for the survey, the instrumentation and observing techniques being applied, and the data reduction and analysis methodology. In this paper, we present the data from thefirst degree surveyed, l = 323 ◦ ‐324 ◦ ,b =± 0.5 ◦ . We compare the data to the previous CO survey of this region and present metrics quantifying the performance being achieved; the rms sensitivity per 0.1 km s −1 velocity channel is 1.5 K for 12 CO and 0.7 K for the other lines. We also present some results from the region surveyed, including line fluxes, column densities, molecular masses, 12 CO/ 13 CO line ratios, and 12 CO optical depths. We also examine how these quantities vary as a function of distance from the Sun when averaged over the 1 square degree survey area. Approximately 2 × 10 6 M� of molecular gas is found along the G323 sightline, with an average H2 number density of nH 2 ∼ 1c m −3 within the Solar