Internal 1000 au Scale Structures of the R CrA Cluster-forming Cloud. I. Filamentary Structures

We have recently detected CO lines in the well-known filaments around NGC 1275, the galaxy at the centre of the Perseus cluster of galaxies. These previous observations, with the HERA multi-beam array at the IRAM 30m telescope enabled us to make a large map of the CO(2-1) line and to see hints of molecular gas far away from the cluster centre. To confirm the presence of CO emission lines in the outer filaments and to study the CO(2-1)/CO(1-0) line ratio, we observed seven regions of interest again with the 30m telescope in both CO(1-0) and CO(2-1). The regions we observed were: the eastern filament, the horseshoe, the northern filament and a southern extension, all selected from Halpha emission line mapping. Molecular gas is detected in all the observed regions. This result confirms the large extent of the cold molecular gas filaments. We discuss the CO(2-1)/CO(1-0) ratios in the filaments. The eastern filament has optically thick gas, whereas further away, the line ratio increases close to values expected for a warmer optically thin medium. We also show CO(1-0) and CO(2-1) lines in 9 regions closer to the centre. The kinematics of the CO is studied here in more detail and confirms that it follows the motions of the warm H_2 gas found in the near-infrared. Finally, we searched for dense gas tracers around 3C84 and claim here the first detection of HCN(3-2).

Aims. This paper belongs to a series of four, dedicated to the analysis of the dynamical, thermal and chemical properties of translucent molecular gas, with the perspective of characterizing the processes driving the dissipation of supersonic turbulence, an anticipated prerequisite of dense core formation. Methods. We analyze the small scale morphology and velocity structure of the parsec-scale environment of a low mass dense core (1 M� ). Our work is based on large maps made with the IRAM-30 m telescope in the two lowest rotational transitions of 12 CO and 13 CO with high angular (20 �� or 0.015 pc at 115 GHz) and spectral (0.055 km s −1 ) resolutions. The field is translucent, hence providing strong constraints on the column density and physical conditions in the gas. Results. More than one third of the field mass (6.5 M� ) lies in an elongated tail of dense and cold gas, possibly extending beyond the edge of the map and connected to the core in space and velocity. This core tail is highly turbulent and sub-structured into narrow filaments of aspect ratio up to 20. These are pure velocity structures with velocity shears in the range 2–10 km s −1 pc −1 . Another third of the mass, according to the weak extinction of the field, lies in more dilute molecular and atomic gas. Its molecular fraction, largely traced by optically thick 12 CO lines, is even more turbulent than the dense core tail. The gas emitting in the broad wings of the 12 CO lines is organized into a conspicuous network of narrow criss-crossed filaments, whose pattern at the parsec scale is seen for the first time. The gas there is optically thin in the 12 CO(1–0) line (τ12 < 0.2), warmer than 25 K and more dilute than 1000 cm −3 . These optically thin 12 CO-filaments, though contributing to about 10% of the mass of the environment, have a CO cooling rate a few times larger than that of the whole field on average. Whether dense or dilute, all the filamentary structures in the field (with transverse sizes 0.015–0.03 pc), are preferentially oriented along the direction of the magnetic fields, as measured a few parsecs away. Using the Chandrasekhar-Fermi method, we estimate the intensity of the magnetic fields intensity in the dilute molecular gas to be Bpos = 15 µG. We infer that the turbulent motions in the dilute gas are in the trans-Alfvenic range. –––

Using the NSF’s Karl G. Jansky Very Large Array (VLA), we report six detections of CO(J = 1 → 0) emission and one upper limit in z = 2–3 galaxies originally detected in higher-J CO emission in the Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). From the CO(J = 1 → 0) line strengths, we measure total cold molecular gas masses of M gas = (2.4–11.6) × 1010 (α CO/3.6)M ⊙. We also measure a median CO(J = 3 → 2) to CO(J = 1 → 0) line brightness temperature ratio of r 31 = 0.84 ± 0.26, and a CO(J = 7 → 6) to CO(J = 1 → 0) ratio range of r 71 &lt; 0.05 to r 71 = 0.17. These results suggest that CO(J = 3 → 2) selected galaxies may have a higher CO line excitation on average than CO(J = 1 → 0) selected galaxies, based on the limited, currently available samples from the ASPECS and VLA CO Luminosity Density at High Redshift (COLDz) surveys. This implies that previous estimates of the cosmic density of cold gas in galaxies based on CO(J = 3 → 2) measurements should be revised down by a factor of ≃2 on average based on assumptions regarding CO excitation alone. This correction further improves the agreement between the best currently existing constraints on the cold gas density evolution across cosmic history from line scan surveys, and the implied characteristic gas depletion times.

We describe the CO Luminosity Density at High-z (COLDz) survey, the first spectral line deep field targeting CO(1–0) emission from galaxies at z = 1.95–2.85 and CO(2–1) at z = 4.91–6.70. The main goal of COLDz is to constrain the cosmic density of molecular gas at the peak epoch of cosmic star formation. By targeting both a wide (∼51 arcmin2) and a deep (∼9 arcmin2) area, the survey is designed to robustly constrain the bright end and the characteristic luminosity of the CO(1–0) luminosity function. An extensive analysis of the reliability of our line candidates and new techniques provide detailed completeness and statistical corrections as necessary to determine the best constraints to date on the CO luminosity function. Our blind search for CO(1–0) uniformly selects starbursts and massive main-sequence galaxies based on their cold molecular gas masses. Our search also detects CO(2–1) line emission from optically dark, dusty star-forming galaxies at z &gt; 5. We find a range of spatial sizes for the CO-traced gas reservoirs up to ∼40 kpc, suggesting that spatially extended cold molecular gas reservoirs may be common in massive, gas-rich galaxies at z ∼ 2. Through CO line stacking, we constrain the gas mass fraction in previously known typical star-forming galaxies at z = 2–3. The stacked CO detection suggests lower molecular gas mass fractions than expected for massive main-sequence galaxies by a factor of ∼3–6. We find total CO line brightness at ∼34 GHz of 0.45 ± 0.2 μK, which constrains future line intensity mapping and CMB experiments.

We study the CO(1–0)-to-H2 conversion factor (X CO) and the line ratio of CO(2–1)-to-CO(1–0) (R 21) across a wide range of metallicity (0.1 ≤ Z/Z ⊙ ≤ 3) in high-resolution (∼0.2 pc) hydrodynamical simulations of a self-regulated multiphase interstellar medium. We construct synthetic CO emission maps via radiative transfer and systematically vary the observational beam size to quantify the scale dependence. We find that the kpc-scale X CO can be overestimated at low Z if assuming steady-state chemistry or assuming that the star-forming gas is H2 dominated. On parsec scales, X CO varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO. The parsec-scale X CO drops to the Milky Way value of once dust shielding becomes effective, independent of Z. The CO lines become increasingly optically thin at lower Z, leading to a higher R 21. Most cloud area is filled by diffuse gas with high X CO and low R 21, while most CO emission originates from dense gas with low X CO and high R 21. Adopting a constant X CO strongly over- (under-)estimates H2 in dense (diffuse) gas. The line intensity negatively (positively) correlates with X CO (R 21) as it is a proxy of column density (volume density). On large scales, X CO and R 21 are dictated by beam averaging, and they are naturally biased toward values in dense gas. Our predicted X CO is a multivariate function of Z, line intensity, and beam size, which can be used to more accurately infer the H2 mass.

Observations with theHerschelSpace Telescope have established that most star forming gas is organised in filaments, a finding that is supported by numerical simulations of the supersonic interstellar medium (ISM) where dense filamentary structures are ubiquitous. We aim to understand the formation of these dense structures by performing observations covering the12CO(4→3),12CO(3→2), and various CO(2–1) isotopologue lines of the Musca filament, using the APEX telescope. The observed CO intensities and line ratios cannot be explained by PDR (photodissociation region) emission because of the low ambient far-UV field that is strongly constrained by the non-detections of the [C II] line at 158μm and the [O I] line at 63μm, observed with the upGREAT receiver on SOFIA, as well as a weak [C I] 609μm line detected with APEX. We propose that the observations are consistent with a scenario in which shock excitation gives rise to warm and dense gas close to the highest column density regions in the Musca filament. Using shock models, we find that the CO observations can be consistent with excitation by J-type low-velocity shocks. A qualitative comparison of the observed CO spectra with synthetic observations of dynamic filament formation simulations shows a good agreement with the signature of a filament accretion shock that forms a cold and dense filament from a converging flow. The Musca filament is thus found to be dense molecular post-shock gas. Filament accretion shocks that dissipate the supersonic kinetic energy of converging flows in the ISM may thus play a prominent role in the evolution of cold and dense filamentary structures.

We report the first detailed measurement of the shape of the CO luminosity function at high redshift, based on &gt;320 hr of the NSF’s Karl G. Jansky Very Large Array (VLA) observations over an area of ∼60 arcmin2 taken as part of the CO Luminosity Density at High Redshift (COLDz) survey. COLDz “blindly” selects galaxies based on their cold gas content through CO(J = 1 → 0) emission at z ∼ 2–3 and CO(J = 2 → 1) at z ∼ 5–7 down to a CO luminosity limit of log( /K km s−1 pc2) ≃ 9.5. We find that the characteristic luminosity and bright end of the CO luminosity function are substantially higher than predicted by semi-analytical models, but consistent with empirical estimates based on the infrared luminosity function at z ∼ 2. We also present the currently most reliable measurement of the cosmic density of cold gas in galaxies at early epochs, i.e., the cold gas history of the universe, as determined over a large cosmic volume of ∼375,000 Mpc3. Our measurements are in agreement with an increase of the cold gas density from z ∼ 0 to z ∼ 2–3, followed by a possible decline toward z ∼ 5–7. These findings are consistent with recent surveys based on higher-J CO line measurements, upon which COLDz improves in terms of statistical uncertainties by probing ∼50–100 times larger areas and in the reliability of total gas mass estimates by probing the low-J CO lines accessible to the VLA. Our results thus appear to suggest that the cosmic star formation rate density follows an increased cold molecular gas content in galaxies toward its peak about 10 billion years ago, and that its decline toward the earliest epochs is likely related to a lower overall amount of cold molecular gas (as traced by CO) bound in galaxies toward the first billion years after the Big Bang.

We present our high-resolution (0.″15 × 0.″13, ∼34 pc) observations of the CO (6−5) line emission, which probes the warm and dense molecular gas, and the 434 μm dust continuum emission in the nuclear region of the starburst galaxy IC 5179, conducted with the Atacama Large Millimeter Array (ALMA). The CO (6−5) emission is spatially distributed in filamentary structures with many dense cores and shows a velocity field that is characteristic of a circumnuclear rotating gas disk, with 90% of the rotation speed arising within a radius of ≲150 pc. At the scale of our spatial resolution, the CO (6−5) and dust emission peaks do not always coincide, with their surface brightness ratio varying by a factor of ∼10. This result suggests that their excitation mechanisms are likely different, as further evidenced by the southwest to northeast spatial gradient of both CO-to-dust continuum ratio and Pa-α equivalent width. Within the nuclear region (radius ∼ 300 pc) and with a resolution of ∼34 pc, the CO line flux (dust flux density) detected in our ALMA observations is 180 ± 18 Jy km s−1 (71 ± 7 mJy), which accounts for 22% (2.4%) of the total value measured by Herschel.

When a streamer discharge occurs in water, several luminous plasma filaments will be created in the water during the discharge. After the discharge, these plasma filaments turn into neutral gas phase and remain in water. The gas filament remained in water is a good object for studying the basic processes involved in the streamer propagation. We investigated the evolution of the gas filaments remained in water after a streamer discharge at different experimental conditions. We recorded eight successive images during one discharge pulse. The density of gas in the gas filament and the radius of the gas filament were measured from the obtained images. We found that the radius of the gas filament and the density of gas in the gas filament are almost not influenced by the impulse voltage within the range studied. While the conductivity of water has strong effect on the radius of the gas filament and the density of gas in the gas filament. The radius of the gas filament becomes thicker and expands faster as the conductivity of water becomes larger. The density of gas in the gas filament remained in water oscillates between 400 to 800 kg/m3 with an duration of ~10 μs during the expansion period of 4–39 μs after the HV pulse starts. Both the impulse voltage and the conductivity of water do not affect the oscillation duration of the density of gas in the gas filament.

Recent millimeter interferometer observations of Arp 220 have yielded ~05 resolution images showing two strong concentrations of millimeter radio continuum and CO line emission embedded in a larger molecular gas disk (r ~ 1 kpc). The interferometer observations also revealed a complex velocity field with steep velocity gradients across each of the flux concentrations of Δv ~ 500 km s-1 within r = 03. The directions of these gradients are not aligned with each other or with that of the outer gas disk. This led to the conclusion that the two emission peaks represent either double nuclei with their own gas disks (r ~ 100 pc), which are counterrotating with respect to each other and rotate around the dynamical center of the system, or that they are two hot spots within a nuclear molecular gas disk. The overall structure of the molecular gas distribution and the corresponding complex velocity field, however, are highly symmetric, except for the unequal brightnesses of the two flux concentrations. This fact and similarities to the distribution and kinematics of the molecular gas in the central 2'' of NGC 1068 motivated us to try describing the Arp 220 nucleus as a warped molecular gas disk in which the peaks in the flux and line width distributions arise as a result of crowding of inclined circular orbits. Hubble Space Telescope (HST) images of the dust lanes flaring toward larger radii provide further support for a warped gas disk. The final model is surprisingly successful in explaining the CO line flux distribution, the velocity field, and position-velocity diagrams across the central flux concentrations. The model represents a strong 90° warp of the gas orbits out of the principal plane forming a circumnuclear polar ring and reproduces well the structures in HST near-IR color and continuum maps. The large number of young stars recently formed out of the warped, polar-ring gas implies that those stars will have kinematic properties similar to that of the molecular gas. We suggest that the red near-IR source at the center of the polar ring is the true nucleus of Arp 220. The success of the model implies that the two flux concentrations are not necessarily counterrotating nuclei, but rather the result of a combination of hot spots and orbit crowding due to the warp. The warp could be a direct consequence of the recent merger event indicated by tidal tails in optical images.

High-resolution ($ \sim$ 0$ .\!\!\!''$ 4) Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 observations of HCO$ ^+$ (4–3) and HCN (4–3) toward a midstage infrared bright merger, VV 114, have revealed a compact nuclear ($ &amp;lt;$ 200 pc) and extended ($ \sim$ 3–4 kpc) dense gas distribution across the eastern part of the galaxy pair. We have found a significant enhancement of HCN (4–3) emission in an unresolved compact and broad (290 km s$ ^{-1}$ ) component found in the eastern nucleus of VV 114, and suggest dense gas associated with the surrounding material around an Active Galactic Nucleus (AGN), with a mass upper limit of $ \lesssim$ 4 $ \times$ 10$ ^{8}$$ M_{\odot}$ . The extended dense gas is distributed along a filamentary structure with resolved dense gas concentrations ($ \sim$ 230 pc; $ \sim$ 10$ ^{6}$$ M_{\odot}$ ) separated by a mean projected distance of $ \sim$ 600 pc, many of which are generally consistent with the location of star formation traced in Pa$ \alpha$ emission. Radiative-transfer calculations suggest moderately dense ($ n_{\rm H_2}$$ =$ 10$ ^{5}$ –10$ ^{6}$ cm$ ^{-3}$ ) gas averaged over the entire emission region. These new ALMA observations demonstrate the strength of the dense gas tracers for identifying both the AGN and the star-formation activity in a galaxy merger, even in the most dust-enshrouded environment in the local universe.

[Abridged] We present a detailed study of the physical properties of the molecular gas in a sample of 18 molecular gas-rich early-type galaxies (ETGs) from the ATLAS$ 3D sample. Our goal is to better understand the star formation processes occurring in those galaxies, starting here with the dense star-forming gas. We use existing integrated $^{12}$CO(1-0, 2-1), $^{13}$CO(1-0, 2-1), HCN(1-0) and HCO$^{+}$(1-0) observations and present new $^{12}$CO(3-2) single-dish data. From these, we derive for the first time the average kinetic temperature, H$_{2}$ volume density and column density of the emitting gas, this using a non-LTE theoretical model. Since the CO lines trace different physical conditions than of those the HCN and HCO$^{+}$ lines, the two sets of lines are treated separately. We also compare for the first time the predicted CO spectral line energy distributions (SLEDs) and gas properties of our molecular gas-rich ETGs with those of a sample of nearby well-studied disc galaxies. The gas excitation conditions in 13 of our 18 ETGs appear analogous to those in the centre of the Milky Way. Such results have never been obtained before for ETGs and open a new window to explore further star-formation processes in the Universe. The conclusions drawn should nevertheless be considered carefully, as they are based on a limited number of observations and on a simple model. In the near future, with higher CO transition observations, it should be possible to better identify the various gas components present in ETGs, as well as more precisely determine their associated physical conditions. To achieve these goals, we show here from our theoretical study, that mid-J CO lines (such as the $^{12}$CO(6-5) line) are particularly useful.

Filamentary structures are ubiquitous in molecular clouds, and have been recently argued to play an important role in regulating the size and mass of embedded clumps through fragmentation and mass accretion. Here, we reveal the dynamical state and fragmentation of filamentary molecular gas associated with the Serpens South protocluster through analysis of wide ( ) observations of NH3 (1, 1) and (2, 2) inversion transitions with the Green Bank Telescope. Detailed modeling of the NH3 lines reveals that the kinematics of the cluster and surrounding filaments are complex. We identify hierarchical structure using a dendrogram analysis of the NH3 emission. The distance between neighbor structures that are embedded within the same parent structure is generally greater than expected from a spherical Jeans analysis, and is in better agreement with cylindrical fragmentation models. The NH3 line width-size relation is flat, and average gas motions are sub- or trans-sonic over all physical scales observed. Subsonic regions extend far beyond the typical 0.1 pc scale previously identified in star-forming cores. As a result, we find a strong trend of decreasing virial parameter with increasing structure mass in Serpens South. Extremely low virial parameters on the largest scales probed by our data suggest that the previously observed, ordered magnetic field is insufficient to support the region against collapse, in agreement with large radial infall motions previously measured toward some of the filaments. A more complex magnetic field configuration in the dense gas, however, may be able to support the filaments.

This paper presents a detailed analysis of a random piece of molecular gas, chosen in a weakly CO– emitting part of a cloud edge in the Perseus-Auriga complex. The data set consists of high angular resolution observations in the 12CO J = 1 − 0, J = 2 − 1, J = 3 − 2, and J = 4 − 3 and CS J = 2 − 1 and J = 3 − 2 lines, combined with CO (J = 3 − 2) and 13CO (J = 2 − 1) observations obtained previously. The observational results can be summarized as follows : (i) At many locations the CO line profiles exhibit weak, broad line wings, superimposed on a narrow intense line core. (ii) The CO line emission is highly structured down to the resolution of the observations (~0.014 pc), but the spatial distribution of the line core emission is different from that of the wing emission. The former is concentrated mostly in two distinct structures of size ~0.06 pc, while the latter is concentrated in a long filamentary structure (l ~ 0.2 pc) that crosses the mapped field and is barely resolved in its transverse dimension. (iii) The J = 2–1 to J = 1–0 CO line ratio is approximately constant across the entire field and has the same value, R = 0.62±0.08, in the CO-bright areas as in the almost 10 times weaker areas. (iv) A weak CS (2–1) line has been detected at the core velocity, and only an upper limit has been obtained for the CS (3–2) line, and (v) the low column density gas that emits in the line wings has been detected clearly in CO (3–2) and detected tentatively in the CO (4–3) line. The present work strengthens the results of our previous study, that the edges of molecular clouds are weak CO emitters when observed at the parsec scale, say, because the CO emission there is beam-diluted emission of dense (nH2 > 104 cm−3), cold (Tk < 15 K) structures. The moderate CO beam-averaged optical depth and the smoothness of the CO profiles set an upper limit of ~35 AU for the size of the cells within which each CO photon interacts with the gas. This result, though, does not necessarily imply that the CO-free regions are structured similarly and, indeed, a moderate-opacity region of CO-free gas and dust is required to surround the CO-emitting structures in order to provide shielding from the interstellar radiation field. Our work also reveals, for the first time, that the line wing emission also originates in small-scale structure. The CO (4–3) wing measurement is critically important in that it constrains the wing gas temperature to be in the range from Tk ~ 25 K, for dense gas at nH2 ~ 103 cm−3, up to Tk ~ 250 K for nH2 ~ 200 cm−3. Its filamentary morphology is consistent with the idea that it is dilute and warm gas confined to the specific structures in which the energy of turbulence is being dissipated, in an intermittent way. The large body of observational results presently available on the CO emission properties of non-starforming molecular clouds, from the smallest to the largest scales, is not consistent with a picture of randomly moving dense clumps immersed in a lower density medium. The AU scale dense CO cells have to be distributed on a fractal set with correlated velocities. They are likely to be dynamically connected to the turbulent velocity field of the gas that fills the volume (atomic and molecular hydrogen) and probably trace the active regions of large vorticity either because they would tend to be trapped in these structures or because those may be regions of enhanced CO formation rate.

A numerical comparison between ideal and dense gas flow structures in the supersonic regime for a cascade of wedge-shaped straight plates