H2 Column Density Research Articles

CO-to-H2 conversion and spectral column density in molecular clouds: the variability of the XCO factor

ABSTRACT Analysing the Galactic plane CO survey with the Nobeyama 45-m telescope, we compared the spectral column density (SCD) of $N_{\rm H_2}$ calculated for the 12CO (J = 1–0) line using the current conversion factor $X_{\rm ^{12}CO}$ to that for the 13CO (J = 1–0) line under the LTE (local thermal equilibrium) assumption in the M16 and W43 regions. Here, SCD is defined by $\mathrm{d}N_{\rm H_2}/\mathrm{d}v$ with $N_{\rm H_2}$ and v being the column density and radial velocity, respectively. It is found that the $X_{\rm ^{12}CO}$ method significantly underestimates the H2 density in a cloud or region, where SCD exceeds a critical value (∼3 × 1021 [H2 cm−2 (km s−1)−1]), but overestimates in lower SCD regions. We point out that the actual CO-to-H2 conversion factor varies with the H2 column density or with the CO line intensity: it increases in the inner and opaque parts of molecular clouds, whereas it decreases in the low-density envelopes. However, in so far as the current $X_{^{12}{\rm CO}}$ is used combined with the integrated 12CO intensity averaged over an entire cloud, it yields a consistent value with that calculated using the 13CO intensity by LTE. Based on the analysis, we propose a new CO-to-H2 conversion relation, $N_{\rm H_2}^* = \int X^*_{\rm CO} (T_{\rm B}) T_{\rm B}\ \mathrm{d}v$, where $X^*_{\rm CO} (T_{\rm B})=(T_{\rm B}/T_{\rm B}^*)^\beta X_{\rm ^{12}CO}$ is the modified spectral conversion factor as a function of the brightness temperature, TB, of the 12CO (J = 1–0) line, and β ∼ 1–2 and $T_{\rm B}^*=12\!-\!16$ K are empirical constants obtained by fitting to the observed data. The formula corrects for the over/underestimation of the column density at low/high CO line intensities, and is applicable to molecular clouds with TB ≥ 1 K (12CO (J = 1–0) line rms noise in the data) from envelope to cores at sub-parsec scales (spatial resolution).

Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies

We investigate the relation between the detection of the 11.3 μm polycyclic aromatic hydrocarbon (PAH) feature in the nuclear (∼24−230 pc) regions of 22 nearby Seyfert galaxies and the properties of the cold molecular gas. For the former we use ground-based (0.3−0.6″ resolution) mid-infrared (mid-IR) spectroscopy. The cold molecular gas is traced by ALMA and NOEMA high (0.2−1.1″) angular resolution observations of the CO(2–1) transition. Galaxies with a nuclear detection of the 11.3 μm PAH feature contain more cold molecular gas (median 1.6 × 107 M⊙) and have higher column densities (N(H2) = 2 × 1023 cm−2) over the regions sampled by the mid-IR slits than those without a detection. This suggests that molecular gas plays a role in shielding the PAH molecules in the harsh environments of Seyfert nuclei. Choosing the PAH molecule naphthalene as an illustration, we compute its half-life in the nuclear regions of our sample when exposed to 2.5 keV hard X-ray photons. We estimate shorter half-lives for naphthalene in nuclei without a 11.3 μm PAH detection than in those with a detection. TheSpitzer/IRS PAH ratios on circumnuclear scales (∼4″ ∼ 0.25−1.3 kpc) are in between model predictions for neutral and partly ionized PAHs. However, Seyfert galaxies in our sample with the highest nuclear H2column densities are not generally closer to the neutral PAH tracks. This is because in the majority of our sample galaxies, the CO(2–1) emission in the inner ∼4″ is not centrally peaked and in some galaxies traces circumnuclear sites of strong star formation activity. Spatially resolved observations with the MIRI medium-resolution spectrograph on theJames WebbSpace Telescope will be able to distinguish the effects of an active galactic nucleus (AGN) and star formation on the PAH emission in nearby AGN.

A Massive Molecular Torus inside a Gas-poor Circumnuclear Disk in the Radio Galaxy NGC 1052 Discovered with ALMA

Abstract We report ALMA observations of NGC 1052 to search for mass accretion in a gas-poor active galactic nucleus. We detected CO emission representing a rotating ring-like circumnuclear disk (CND) seen edge-on with a gas mass of 5.3 × 105 M ⊙. The CND has smaller gas mass than that in typical Seyfert galaxies with circumnuclear star formation and is too gas-poor to drive mass accretion onto the central engine. The continuum emission casts molecular absorption features of CO, HCN, HCO+, SO, SO2, CS, CN, and H2O, with H13CN and HC15N and vibrationally excited (v 2 = 1) HCN and HCO+. Broader absorption line widths than CND emission-line widths imply the presence of a geometrically thick molecular torus with a radius of 2.4 ± 1.3 pc and a thickness ratio of 0.7 ± 0.3. We obtain an H2 column density of (3.3 ± 0.7) × 1025 cm−2 using H12CN, H13CN, and HCO+ absorption features and adopting abundance ratios of 12C to 13C and HCO+ to H2, and we derived a torus gas mass of (1.3 ± 0.3) × 107 M ⊙, which is ∼9% of the central black hole mass. The molecular gas in the torus is clumpy, with an estimated covering factor of . The gas density of the clumps inside the torus is inferred to be (6.4 ± 1.3) × 107 cm−3, which meets the excitation conditions for an H2O maser. The specific angular momentum in the torus exceeds the flat rotation curve extrapolated from that of the CND, indicating a Keplerian rotation inside a 14.4 pc sphere of influence.

The column density structure of Orion A depicted by N-PDF

We have conducted a large-field simultaneous survey of 12CO, 13CO and C18O J = 1 – 0 emission toward the Orion A giant molecular cloud (GMC) with a sky coverage of ∼4.4 deg2 using the Purple Mountain Observatory (PMO)-13.7 m millimeter-wavelength telescope. We use the probability distribution function of the column density (N-PDF) to investigate the distribution of molecular hydrogen in the Orion A GMC. The H2 column density, derived from the 13CO emission, of the GMC is dominated by a log-normal distribution in the range from ∼ 4 × 1021 to ∼ 1.5 × 1023 cm−2 with excesses both at the low-density and high-density ends. The excess of the low-density end is possibly caused by an extended and low-temperature (∼10 K) component with velocities in the range of 5 – 8 km s−1. Compared with the northern sub-regions, the southern sub-regions of the Orion A GMC contain less gas with column density in NH2 > 1.25 × 1022 cm−2. The dispersions of the N-PDFs of the sub-regions are found to correlate with the evolutionary stages of the clouds across the Orion A GMC. The structure hierarchy of Orion A GMC is explored with the DENDROGRAM algorithm, and it is found that the GMC is composed of two branches. All structures except one in the tree have virial parameters less than 2, indicating self-gravity is important on the spatial scales from ∼0.3 to ∼4 pc. Although power-laws and departures from lognormal distributions are found at the high-density end of N-PDFs for active star-forming regions, the N-PDFs of structures in the Orion A GMC are predominantly lognormal on scales from R∼0.4 to 4 pc.

Effect of optical depth on study of chemical properties of massive star forming clumps

Here we present the study on chemical properties of massive star forming clumps using N2H+(1-0), H13CO+(1-0), HCN(1-0) and HN13C(1-0) data from the literature [Astron. Astrophys. 563, A97 (2014)]. We found that abundances of H13CO+ and HN13C are affected by H2 column densities. As the median values of these two abundances increase by nearly 10 times from stages A to B, H13CO+ and HN13C are suitable for tracing the evolution of massive star forming clumps. The order of rapidity in growth of abundances of all the four studied molecules from stages A to B, is H13CO+, HCN, HN13C, and N2H+, from the highest to the lowest. Our results suggest that the observing optically thin molecular lines with high angular resolution are necessary to study the chemical evolution of massive star forming clumps.

Direct Measurement of the [C i] Luminosity to Molecular Gas Mass Conversion Factor in High-redshift Star-forming Galaxies

Abstract The amount of cold, molecular gas in high-redshift galaxies is typically inferred from proxies of molecular hydrogen (H2), such as carbon monoxide (CO) or neutral atomic carbon ([C i]) and molecular gas mass conversion factors. The use of these proxies, however, relies on modeling and observations that have not been directly measured outside the local universe. Here, we use recent samples of high-redshift gamma-ray burst (GRB) and quasar molecular gas absorbers to determine this conversion factor from the column density of H2, which gives us the mass per unit column, and the [C i](J = 1) column density, which provides the luminosity per unit column. This technique allows us to make direct measurements of the relative abundances in high-redshift absorption-selected galaxies. Our sample spans redshifts of z = 1.9−3.4 and covers two orders of magnitude in gas-phase metallicity. We find that the [C i]-to-M mol conversion factor is metallicity dependent, with α [C i] scaling linearly with the metallicity: , with a scatter of σ α [CI] = 0.2 dex. Using a sample of emission-selected galaxies at z ∼ 0–5, with both [C i] and CO line detections, we apply the α [C i] conversion to derive independent estimates of the molecular gas mass and the CO-to-M mol, α CO, conversion factor. We find a remarkable agreement between the molecular gas masses inferred from the absorption-derived α [C i] compared to typical α CO-based estimates, which we confirm here to be metallicity-dependent as well, with an inferred slope that is consistent with α CI and previous estimates from the literature. These results thus support the use of the absorption-derived α [C i] conversion factor for emission-selected star-forming galaxies and demonstrate that both methods probe the same universal properties of molecular gas in the local and high-redshift universe.

The Forgotten Quadrant Survey

Aims. We present the Forgotten Quadrant Survey (FQS), an ESO large project that used the 12 m antenna of the Arizona Radio Observatory to map the Galactic plane in the range 220° < l < 240° and −2.°5 < b < 0°, both in 12CO (1–0) and 13CO (1–0), at a spectral resolution of 0.65 km s−1 and 0.26 km s−1. Methods. We used the (1–0) transition of carbon monoxide to trace the molecular component of the interstellar medium. Our data set allows us to easily identify how the molecular dense gas is organised at different spatial scales: from the giant clouds with their denser filamentary networks, down to the clumps and cores that host the new-born stars and to obtain reliable estimates of their key physical parameters such as size and mass. Results. We present the first release of the data of the FQS survey and discuss their quality. Spectra with 0.65 km s−1 velocity channels have noise ranging from 0.8 K to 1.3 K for 12CO (1–0) and from 0.3 K to 0.6 K for 13CO (1–0). In this first paper, we used the 12CO (1–0) spectral cubes to produce a catalogue of 263 molecular clouds. The clouds are grouped in three main structures corresponding to the Local, Perseus, and Outer arms up to a distance of ∼8.6 kpc from the Sun. This is the first self-consistent statistical catalogue of molecular clouds of the outer Galaxy obtained with a subarcminute spatial resolution and therefore able to detect not only the classical giant molecular clouds, but also the small clouds and to resolve the cloud structure at the sub-parsec scale up to a distance of a few kiloparsec. We found two classes of objects: structures with sizes above a few parsecs that are typical molecular clouds and may be self-gravitating, and subparsec structures that cannot be in gravitational equilibrium and are likely transient or confined by external pressure. We used the ratio between the Herschel H2 column density and the integrated intensity of the CO lines to calculate the CO conversion factor and we found mean values of (3.3 ± 1.4) × 1020 cm−2 (K km s−1)−1 and (1.2 ± 0.4) × 1021 cm−2 (K km s−1)−1, for 12CO (1–0) and 13CO (1–0), respectively. Conclusions. The FQS contributes to the general effort in producing a new generation of high-quality spectroscopic data for the Galactic plane in the less-studied third Galactic quadrant toward the outer Galaxy. The FQS has produced a data set of great legacy value, largely improving the data quality both in terms of sensitivity and spatial resolution over previous data sets.

Physical conditions in the gas phases of the giant H II region LMC-N 11

Context. The ambiguous origin of the [C II] 158μm line in the interstellar medium complicates its use for diagnostics concerning the star-formation rate and physical conditions in photodissociation regions. Aims. We investigate the origin of [C II] in order to measure the total molecular gas content, the fraction of CO-dark H2 gas, and how these parameters are impacted by environmental effects such as stellar feedback. Methods. We observed the giant H II region N 11 in the Large Magellanic Cloud with SOFIA/GREAT. The [C II] line is resolved in velocity and compared to H I and CO, using a Bayesian approach to decompose the line profiles. A simple model accounting for collisions in the neutral atomic and molecular gas was used in order to derive the H2 column density traced by C+. Results. The profile of [C II] most closely resembles that of CO, but the integrated [C II] line width lies between that of CO and that of H I. Using various methods, we find that [C II] mostly originates from the neutral gas. We show that [C II] mostly traces the CO-dark H2 gas but there is evidence of a weak contribution from neutral atomic gas preferentially in the faintest components (as opposed to components with low [C II]/CO or low CO column density). Most of the molecular gas is CO-dark. The CO-dark H2 gas, whose density is typically a few 100s cm−3 and thermal pressure in the range 103.5−5 K cm−3, is not always in pressure equilibrium with the neutral atomic gas. The fraction of CO-dark H2 gas decreases with increasing CO column density, with a slope that seems to depend on the impinging radiation field from nearby massive stars. Finally we extend previous measurements of the photoelectric-effect heating efficiency, which we find is constant across regions probed with Herschel, with [C II] and [O I] being the main coolants in faint and diffuse, and bright and compact regions, respectively, and with polycyclic aromatic hydrocarbon emission tracing the CO-dark H2 gas heating where [C II] and [O I] emit. Conclusions. We present an innovative spectral decomposition method that allows statistical trends to be derived for the molecular gas content using CO, [C II], and H I profiles. Our study highlights the importance of velocity-resolved photodissociation region (PDR) diagnostics and higher spatial resolution for H I observations as future steps.

KFPA Examinations of Young STellar Object Natal Environments (KEYSTONE): Hierarchical Ammonia Structures in Galactic Giant Molecular Clouds

Abstract We present initial results from the K-band Focal Plane Array Examinations of Young STellar Object Natal Environments survey, a large project on the 100 m Green Bank Telescope mapping ammonia emission across 11 giant molecular clouds at distances of 0.9–3.0 kpc (Cygnus X North, Cygnus X South, M16, M17, Mon R1, Mon R2, NGC 2264, NGC 7538, Rosette, W3, and W48). This data release includes the NH3 (1,1) and (2,2) maps for each cloud, which are modeled to produce maps of kinetic temperature, centroid velocity, velocity dispersion, and ammonia column density. Median cloud kinetic temperatures range from 11.4 ± 2.2 K in the coldest cloud (Mon R1) to 23.0 ± 6.5 K in the warmest cloud (M17). Using dendrograms on the NH3 (1,1) integrated intensity maps, we identify 856 dense gas clumps across the 11 clouds. Depending on the cloud observed, 40%–100% of the clumps are aligned spatially with filaments identified in H2 column density maps derived from spectral energy distribution fitting of dust continuum emission. A virial analysis reveals that 523 of the 835 clumps (∼63%) with mass estimates are bound by gravity alone. We find no significant difference between the virial parameter distributions for clumps aligned with the dust-continuum filaments and those unaligned with filaments. In some clouds, however, hubs or ridges of dense gas with unusually high mass and low virial parameters are located within a single filament or at the intersection of multiple filaments. These hubs and ridges tend to host water maser emission, multiple 70 μm detected protostars, and have masses and radii above an empirical threshold for forming massive stars.

Nobeyama 45 m mapping observations toward the nearby molecular clouds Orion A, Aquila Rift, and M17: Project overview

Abstract We carried out mapping observations toward three nearby molecular clouds, Orion A, Aquila Rift, and M 17, using a new 100 GHz receiver, FOREST, on the Nobeyama 45 m telescope. We describe the details of the data obtained such as intensity calibration, data sensitivity, angular resolution, and velocity resolution. Each target contains at least one high-mass star-forming region. The target molecular lines were 12CO (J = 1–0), 13CO (J = 1–0), C18O (J = 1–0), N2H+ (J = 1–0), and CCS (JN = 87–76), with which we covered the density range of 102 cm−3 to 106 cm−3 with an angular resolution of ∼20″ and a velocity resolution of ∼0.1 km s−1. Assuming the representative distances of 414 pc, 436 pc, and 2.1 kpc, the maps of Orion A, Aquila Rift, and M17 cover most of the densest parts with areas of about 7 pc × 15 pc, 7 pc × 7 pc, and 36 pc × 18 pc, respectively. On the basis of the 13CO column density distribution, the total molecular masses are derived to be $3.86 \times 10^{4}\, M_\odot$, $2.67 \times 10^{4}\, M_{\odot }$, and $8.1\times 10^{5}\, M_{\odot }$ for Orion A, Aquila Rift, and M17, respectively. For all the clouds, the H2 column density exceeds the theoretical threshold for high-mass star formation of ≳ 1 g cm−2 only toward the regions which contain current high-mass star-forming sites. For other areas, further mass accretion or dynamical compression would be necessary for future high-mass star formation. This is consistent with the current star formation activity. Using the 12CO data, we demonstrate that our data have enough capability to identify molecular outflows, and for the Aquila Rift we identify four new outflow candidates. The scientific results will be discussed in detail in separate papers.

New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-α absorbers

We report the detections of molecular hydrogen (H2), vibrationally-excited H2 (H2∗), and neutral atomic carbon (C I), an efficient tracer of molecular gas, in two new afterglow spectra of GRBs 181020A (z = 2.938) and 190114A (z = 3.376), observed with X-shooter at the Very Large Telescope (VLT). Both host-galaxy absorption systems are characterized by strong damped Lyman-α absorbers (DLAs) and substantial amounts of molecular hydrogen with logN(H I, H2) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and logN(H I, H2) = 22.15 ± 0.05, 19.44 ± 0.04 (GRB 190114A). The DLA metallicites, depletion levels, and dust extinctions are within the typical regimes probed by GRBs with [Zn/H] = −1.57 ± 0.06, [Zn/Fe] = 0.67 ± 0.03, and AV = 0.27 ± 0.02 mag (GRB 181020A) and [Zn/H] = −1.23 ± 0.07, [Zn/Fe] = 1.06 ± 0.08, and AV = 0.36 ± 0.02 mag (GRB 190114A). In addition, we examine the molecular gas content of all known H2-bearing GRB-DLAs and explore the physical conditions and characteristics required to simultaneously probe C I and H2∗. We confirm that H2 is detected in all C I- and H2∗-bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H2 absorbers. We find that a large molecular fraction of fH2 ≳ 10−3 is required for C I to be detected. The defining characteristic for H2∗ to be present is less clear, though a large H2 column density is an essential factor. We also find that the observed line profiles of the molecular-gas tracers are kinematically “cold”, with small velocity offsets of δv < 20 km s−1 from the bulk of the neutral absorbing gas. We then derive the H2 excitation temperatures of the molecular gas and find that they are relatively low with Tex ≈ 100−300 K, however, there could be evidence of warmer components populating the high-J H2 levels in GRBs 181020A and 190114A. Finally, we demonstrate that even though the X-shooter GRB afterglow campaign has been successful in recovering several H2-bearing GRB-host absorbers, this sample is still hampered by a significant dust bias excluding the most dust-obscured H2 absorbers from identification. C I and H2∗ could open a potential route to identify molecular gas even in low-metallicity or highly dust-obscured bursts, though they are only efficient tracers for the most H2-rich GRB-host absorption systems.

Dynamics of cluster-forming hub-filament systems: The case of the high-mass star-forming complex Monoceros R2.

High-mass stars and star clusters commonly form within hub-filament systems. Monoceros R2 (hereafter Mon R2), at a distance of 830 pc, harbors one of the closest such systems, making it an excellent target for case studies. We investigate the morphology, stability and dynamical properties of the Mon R2 hub-filament system. We employ observations of the 13CO and C18O 1→0 and 2→1 lines obtained with the IRAM-30m telescope. We also use H2 column density maps derived from Herschel dust emission observations. We identified the filamentary network in Mon R2 with the DisPerSE algorithm and characterized the individual filaments as either main (converging into the hub) or secondary (converging to a main filament) filaments. The main filaments have line masses of 30-100 M ⊙ pc-1 and show signs of fragmentation, while the secondary filaments have line masses of 12-60 M ⊙ pc-1 and show fragmentation only sporadically. In the context of Ostriker's hydrostatic filament model, the main filaments are thermally supercritical. If non-thermal motions are included, most of them are trans-critical. Most of the secondary filaments are roughly transcritical regardless of whether non-thermal motions are included or not. From the morphology and kinematics of the main filaments, we estimate a mass accretion rate of 10-4-10-3 M ⊙ yr-1 into the central hub. The secondary filaments accrete into the main filaments with a rate of 0.1-0.4×10-4 M ⊙ yr-1. The main filaments extend into the central hub. Their velocity gradients increase towards the hub, suggesting acceleration of the gas.We estimate that with the observed infall velocity, the mass-doubling time of the hub is ~ 2:5 Myr, ten times larger than the free-fall time, suggesting a dynamically old region. These timescales are comparable with the chemical age of the Hii region. Inside the hub, the main filaments show a ring- or a spiral-like morphology that exhibits rotation and infall motions. One possible explanation for the morphology is that gas is falling into the central cluster following a spiral-like pattern.

CO (J = 1–0) Observations toward Filamentary Molecular Clouds in the Galactic Region with l = [169.°75, 174.°75], b = [−0.°75, 0.°5

Abstract We present observations of the CO isotopologues (12CO, 13CO, and C18O) toward the Galactic region with 169.°75 ≤ l ≤ 174.°75 and −0.°75 ≤ b ≤ 0.°5 using the Purple Mountain Observatory 13.7 m millimeter-wavelength telescope. Based on the 13CO (J = 1 − 0) data, we find five molecular clouds within the velocity range between −25 and 8 km s−1 that are all characterized by conspicuous filamentary structures. We have identified eight filaments with a length of 6.38–28.45 pc, a mean H2 column density of 0.70 × 1021–6.53 × 1021 cm−2, and a line mass of 20.24–161.91 M ☉ pc−1, assuming a distance of ∼1.7 kpc. Gaussian fittings to the inner parts of the radial density profiles lead to a mean FWHM width of 1.13 ± 0.01 pc. The velocity structures of most filaments present continuous distributions with slight velocity gradients. We find that turbulence is the dominant internal pressure to support the fragmentation of filaments instead of thermal pressure. Most filaments have virial parameters smaller than 2; thus, they are gravitationally bound. Four filaments have an LTE line mass close to the virial line mass. We further extract dense clumps using the 13CO data and find that 64% of the clumps are associated with the filaments. According to the complementary IR data, most filaments have associated Class II young stellar objects. Class I objects are mainly found to be located in the filaments with a virial parameter close to 1. Within two virialized filaments, 12CO outflows have been detected, indicating ongoing star-forming activity therein.

Spatial Variation of the Chemical Properties of Massive Star-forming Clumps

Abstract We selected 90 massive star-forming clumps with strong N2H+(1−0), HCO+(1−0), HCN(1−0), and HNC(1−0) emission from the Millimetre Astronomy Legacy Team 90 GHz survey. We obtained Herschel data for all 90 sources and NRAO VLA Sky Survey data for 51 of them. We convolved and regridded all images to the same resolution and pixel size and derived the temperature, H2 column density, molecules’ abundances and abundance, and ratios of each pixel. Our analysis yields three main conclusions. First, the abundances of N2H+, HCO+, HCN, and HNC increase when the column density decreases and the temperature increases, with spatial variations in their abundances dominated by changes in the H2 column density. Second, the abundance ratios between N2H+, HCO+, HCN, and HNC also display systemic variations as a function of the column density due to the chemical properties of these molecules. Third, the sources associated with the 20 cm continuum emission can be classified into four types based on the behavior of the abundances of the four molecules considered here as a function of this emission. The variations of the first three types could also be attributed to the variation of the H2 column density.

ALMA Observations of Molecular Gas in the Host Galaxy of AT2018cow

Abstract We investigate the molecular gas in and star formation properties of the host galaxy (CGCG 137–068) of a mysterious transient, AT2018cow, at kpc and larger scales, using archival band-3 data from the Atacama Large Millimeter/submillimeter Array (ALMA). AT2018cow is the nearest fast-evolving luminous transient (FELT); this is the first study unveiling molecular-gas properties of FELT hosts. The achieved rms and beam size are 0.21 mJy beam−1 at a velocity resolution of 40 km s−1 and 3.″66 × 2.″71 (1.1 kpc × 0.8 kpc), respectively. CO(J = 1–0) emission is successfully detected. The total molecular gas mass inferred from the CO data is (1.85 ± 0.04) × 108 M ⊙ with the Milky Way CO-to-H2 conversion factor. The H2 column density at the AT2018cow site is estimated to be 8.6 × 1020 cm−2. The ALMA data reveal that (1) CGCG 137–068 is a normal star-forming (SF) dwarf galaxy in terms of its molecular gas and star formation properties, and (2) that AT2018cow is located between a CO peak and a blue star cluster. These properties suggest ongoing star formation and favor the explosion of a massive star as the progenitor of AT2018cow. We also find that CGCG 137–068 has a solar or super-solar metallicity. If the metallicity of the other FELT hosts is not higher than average, then some properties of SF dwarf galaxies other than metallicity may be related to FELTs.

The hidden heart of the luminous infrared galaxy IC 860

High-resolution (0.​​″03–0.​​″09 (9–26 pc)) ALMA (100–350 GHz (λ3 to 0.8 mm)) and (0.​​″04 (11 pc)) VLA 45 GHz measurements have been used to image continuum and spectral line emission from the inner (100 pc) region of the nearby infrared luminous galaxy IC 860. We detect compact (r ∼ 10 pc), luminous, 3 to 0.8 mm continuum emission in the core of IC 860, with brightness temperatures TB > 160 K. The 45 GHz continuum is equally compact but significantly fainter in flux. We suggest that the 3 to 0.8 mm continuum emerges from hot dust with radius r ∼ 8 pc and temperature Td ∼ 280 K, and that it is opaque at millimetre wavelengths, implying a very large H2 column density N(H2)≳1026 cm−2. Vibrationally excited lines of HCN ν2 = 1f J = 4 − 3 and 3–2 (HCN-VIB) are seen in emission and spatially resolved on scales of 40–50 pc. The line-to-continuum ratio drops towards the inner r = 4 pc, resulting in a ring-like morphology. This may be due to high opacities and matching HCN-VIB excitation- and continuum temperatures. The HCN-VIB emission reveals a north–south nuclear velocity gradient with projected rotation velocities of ν = 100 km s−1 at r = 10 pc. The brightest emission is oriented perpendicular to the velocity gradient, with a peak HCN-VIB 3–2 TB of 115 K (above the continuum). Vibrational ground-state lines of HCN 3–2 and 4–3, HC15N 4–3, HCO+ 3–2 and 4–3, and CS 7–6 show complex line absorption and emission features towards the dusty nucleus. Redshifted, reversed P-Cygni profiles are seen for HCN and HCO+ consistent with gas inflow with νin ≲ 50 km s−1. Foreground absorption structures outline the flow, and can be traced from the north-east into the nucleus. In contrast, CS 7–6 has blueshifted line profiles with line wings extending out to −180 km s−1. We suggest that a dense and slow outflow is hidden behind a foreground layer of obscuring, inflowing gas. The centre of IC 860 is in a phase of rapid evolution where an inflow is building up a massive nuclear column density of gas and dust that feeds star formation and/or AGN activity. The slow, dense outflow may be signaling the onset of feedback. The inner, r = 10 pc, IR luminosity may be powered by an AGN or a compact starburst, which then would likely require a top-heavy initial mass function.

A Large-field J = 1 – 0 Survey of CO and Its Isotopologues toward the Cassiopeia A Supernova Remnant

Abstract We have conducted a large-field simultaneous survey of 12CO, 13CO, and C18O J = 1 − 0 emission toward the Cassiopeia A (Cas A) supernova remnant (SNR), which covers a sky area of 3.°5 × 3.°1. The Cas giant molecular cloud (GMC) mainly consists of three individual clouds with masses on the order of 104−105 M ☉. The total mass derived from the 13CO emission of the GMC is 2.1 × 105 M ☉ and is 9.5 × 105 M ☉ from the 12CO emission. Two regions with broadened (6–7 km s−1) or asymmetric 12CO line profiles are found in the vicinity (within a 10′ × 10′ region) of the Cas A SNR, indicating possible interactions between the SNR and the GMC. Using the GAUSSCLUMPS algorithm, 547 13CO clumps are identified in the GMC, 54% of which are supercritical (i.e., α vir < 2). The mass spectrum of the molecular clumps follows a power-law distribution with an exponent of −2.20. The pixel-by-pixel column density of the GMC can be fitted with a log-normal probability distribution function (N-PDF). The median column density of molecular hydrogen in the GMC is 1.6 × 1021 cm−2 and half the mass of the GMC is contained in regions with H2 column density lower than 3 × 1021 cm−2, which is well below the threshold of star formation. The distribution of the YSO candidates in the region shows no agglomeration.

ALMA reveals a pseudo-disc in a proto-brown dwarf

ABSTRACT We present the observational evidence of a pseudo-disc around the proto-brown dwarf Mayrit 1701117, the driving source of the large-scale HH 1165 jet. Our analysis is based on Atacama Large Millimeter/submillimeter Array 12CO (2–1) line and 1.37 mm continuum observations at an angular resolution of ∼0.4 arcsec. The pseudo-disc is a bright feature in the CO position–velocity diagram, elongated in a direction perpendicular to the jet axis, with a total (gas+dust) mass of ∼0.02 M$\odot$, size of 165–192 au, and a velocity spread of ±2 km s−1. The large velocity gradient is a combination of infalling and rotational motions, indicating a contribution from a pseudo-disc and an unresolved inner Keplerian disc. There is weak emission detected in the H2CO (3–2) and N2D+ (3–2) lines. H2CO emission likely probes the inner Keplerian disc where CO is expected to be frozen, while N2D+ possibly originates from an enhanced clump at the outer edge of the pseudo-disc. We have considered various models (core collapse, disc fragmentation, circumbinary disc) that can fit both the observed CO spectrum and the position–velocity offsets. The observed morphology, velocity structure, and the physical dimensions of the pseudo-disc are consistent with the predictions from the core collapse simulations for brown dwarf formation. From the best model fit, we can constrain the age of the proto-brown dwarf system to be ∼30 000–40 000 yr. A comparison of the H2 column density derived from the CO line and 1.37 mm continuum emission indicates that only about 2 per cent of the CO is depleted from the gas phase.

A multiwavelength study of filamentary cloud G341.244-00.265

We present a multiwavelength study toward the filamentary molecular cloud G341.244-00.265, to investigate the physical and chemical properties, as well as star formation activities taking place therein. Our radio continuum and molecular line data were obtained from the Sydney University Molonglo Sky Survey (SUMSS), Atacama Pathfinder Experiment Telescope Large Area Survey of the Galaxy (ATLASGAL), Structure, excitation, and dynamics of the inner Galactic interstellar medium (SEDIGISM) and Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90). The infrared archival data come from Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE), Wide-field Infrared Survey Explorer (WISE), and Herschel InfraRed Galactic Plane Survey (Hi-GAL). G341.244-00.265 displays an elongated filamentary structure both in far-infrared and molecular line emissions; the “head” and “tail” of this molecular cloud are associated with known infrared bubbles S21, S22, and S24. We made H2 column density and dust temperature maps of this region by the spectral energy distribution (SED) method. G341.244-00.265 has a linear mass density of about 1654 M⊙ pc−1 and has a projected length of 11.1 pc. The cloud is prone to collapse based on the virial analysis. Even though the interactions between this filamentary cloud and its surrounding bubbles are evident, we found these bubbles are too young to trigger the next generation of star formation in G341.244-00.265. From the ATLASGAL catalog, we found eight dense massive clumps associated with this filamentary cloud. All of these clumps have sufficient mass to form massive stars. Using data from the GLIMPSE and WISE survey, we search the young stellar objects (YSOs) in G341.244-00.265. We found an age gradient of star formation in this filamentary cloud: most of the YSOs distributed in the center are Class I sources, while most Class II candidates are located in the head and tail of G341.244-00.265, indicating star formation at the two ends of this filament is prior to the center. The abundance ratio of N(N2H+)/N(C18O) is higher in the center than that in the two ends, also indicating that the gas in the center is less evolved. Taking into account the distributions of YSOs and the N(N2H+)/N(C18O) ratio map, our study is in agreement with the prediction of the so-called “end-dominated collapse” star formation scenario.

H2self-shielding with non-LTE rovibrational populations: implications for cooling in protogalaxies

The abundance of molecular hydrogen (H2), the primary coolant in primordial gas, is critical for the thermodynamic evolution and star-formation histories in early protogalaxies. Determining the photodissociation rate of H2 by an incident Lyman-Werner (LW) flux is thus crucial, but prohibitively expensive to calculate on the fly in simulations. The rate is sensitive to the H2 rovibrational distribution, which in turn depends on the gas density, temperature, and incident LW radiation field. We use the publicly available cloudy package to model primordial gas clouds and compare exact photodissociation rate calculations to commonly-used fitting formulae. We find the fit from Wolcott-Green et al. (2011) is most accurate for moderate densities n~10^3 cm^{-3} and temperatures, T~10^3K, and we provide a new fit, which captures the increase in the rate at higher densities and temperatures, owing to the increased excited rovibrational populations in this regime. Our new fit has typical errors of a few percent percent up to n =<10^7 cm^{-3}, T =< 8000K, and H2 column density NH2 =<10^{17} cm^{-2}, and can be easily utilized in simulations. We also show that pumping of the excited rovibrational states of H2 by a strong LW flux further modifies the level populations when the gas density is low, and noticeably decreases self-shielding for J_21 > 10^3 and n < 10^2 cm^{-3}. This may lower the "critical flux" at which primordial gas remains H2-poor in some protogalaxies, enabling massive black hole seed formation.