Search

Aims. We aim to systematically study the properties of the different transitions of the dense molecular gas tracer HC3N in galaxies. Methods. We have conducted single-dish observations of HC3N emission lines towards a sample of nearby gas-rich galaxies. HC3N(J = 2-1) was observed in 20 galaxies with the Effelsberg 100-m telescope. HC3N(J = 24-23) was observed in nine galaxies with the 10-m Submillimeter Telescope (SMT). Results. HC3N 2-1 is detected in three galaxies: IC 342, M 66, and NGC 660 (>3σ). HC3N 24-23 is detected in three galaxies: IC 342, NGC 1068, and IC 694. These are the first measurements of HC3N 2-1 in a relatively large sample of external galaxies, although the detection rate is low. For the HC3N 2-1 non-detections, upper limits (2σ) are derived for each galaxy, and stacking the non-detections is attempted to recover the weak signal of HC3N. The stacked spectrum, however, does not show any significant signs of HC3N 2-1 emission. The results are also compared with other transitions of HC3N observed in galaxies. Conclusions. The low detection rate of both transitions suggests low abundance of HC3N in galaxies, which is consistent with other observational studies. The comparison between HC3N and HCN or HCO+ shows a large diversity in the ratios between HC3N and HCN or HCO+. More observations are needed to interpret the behavior of HC3N in different types of galaxies. [ABSTRACT FROM AUTHOR]

The peculiar QSO J0927+2943 shows multiple sets of emission lines in its optical spectrum. This signature has been interpreted as the relative motion between a black hole, either recoiling or bound in a binary system, and its host galaxy, or as a superposition of two galaxies along the line of sight. In order to test these scenarios, we have collected 2 mm CO(2-1) observations using the IRAM Plateau de Bure Interferometer, and optical images and spectroscopy at the Calar Alto observatory. Together with archival HST images, these data provide unique insights on the nature of this system. The recoiling/binary black hole scenarios are ruled out by the clear detection of a galactic-scale molecular gas reservoir at the same redshift of the QSO broad lines. The observations presented here also disfavour the superposition model, although with less constraints. Thus, the origin of the second, bright set of narrow emission lines in J0927+2943 is still unknown. [ABSTRACT FROM AUTHOR]

ABSTRACT The spatial distribution of 3.3 [ABSTRACT FROM AUTHOR]

ABSTRACT Observations of the dust emission using millimetre/submillimetre bolometer arrays can be contaminated by molecular line flux, such as flux from 12CO. As the brightest molecular line in the submillimetre, it is important to quantify the contribution of CO flux to the dust continuum bands. Conversion factors were used to convert molecular line integrated intensities to flux detected by bolometer arrays in mJy beam−1. These factors were calculated for 12CO line integrated intensities to the SCUBA-2 850 and 450 μm bands. The conversion factors were then applied to HARP 12CO 3-2 maps of NGC 1333 in the Perseus complex and NGC 2071 and NGC 2024 in the Orion B molecular cloud complex to quantify the respective 12CO flux contribution to the 850 μm dust continuum emission. Sources with high molecular line contamination were analysed in further detail for molecular outflows and heating by nearby stars to determine the cause of the 12CO contribution. The majority of sources had a 12CO 3-2 flux contribution under 20 per cent. However, in regions of molecular outflows, the 12CO can dominate the source dust continuum (up to 79 per cent contamination) with 12CO fluxes reaching ∼68 mJy beam−1. [ABSTRACT FROM AUTHOR]

ABSTRACT The form of depleted sulphur in dense clouds is still unknown. Until now, only two molecules, OCS and SO2, have been detected in interstellar ices but cannot account for the elemental abundance of sulphur observed in the diffuse medium. Chemical models suggest that solid H2S is the main form of sulphur in denser sources but observational constraints exist that greatly weaken this hypothesis. We have used the Nautilus gas-grain code in which new chemical reactions have been added, based on recent experiments of H2S ice irradiation with UV photons and high energy protons. In particular, we included the new species S n, H2S n and C2S. We found that at the low temperature observed in dense clouds, i.e. 10 K, these new molecules are not efficiently produced and our modifications of the network do not change the previous predictions. At slightly higher temperature, 20 K in less dense clouds or in the proximity of protostars, H2S abundance on the surfaces is strongly decreased in favour of the polysulphanes H2S3. Such a result can also be obtained if the diffusion barriers on the grains are less important. In the context of the life cycle of interstellar clouds and the mixing between diffuse and denser parts of the clouds, the depletion of sulphur in the form of polysulphanes or other sulphur polymers may have occurred in regions where the temperature is slightly higher than the cold inner parts of the clouds. [ABSTRACT FROM AUTHOR]

ABSTRACT We have examined the ratio between the integrated luminosity of massive young stellar objects detected by the Red MSX Source (RMS) survey and the mass of molecular clouds in the Galactic Ring Survey (GRS) region, as a function of Galactocentric radius. The results indicate that 60-80 per cent of the observed increases in the star formation rate density associated with spiral-arm features are due to source crowding within the arms. Of the remainder, most of the increase in the inner Sagittarius arm is due to an enhancement in the simple star formation efficiency, i.e. in the number of RMS sources per unit molecular gas mass. In the inner Perseus arm, the residual increase is due to a higher than average mean source luminosity, which implies a top-heavy initial mass function, and this is entirely due to the presence, in the GRS region, of the W49 star-forming complex, which appears to be exceptional in its nature. The results also suggest that there is little or no increase in the star formation efficiency on kiloparsec scales in the Scutum tangent region which includes W43. We discuss the possible role played by the spiral arms in influencing the star formation efficiency and conclude that the most likely mechanisms are related to orbit crowding within the arms. [ABSTRACT FROM AUTHOR]

ABSTRACT We report results from a deep Jansky Very Large Array (JVLA) search for 12CO J = 1-0 line emission from galaxies in a candidate galaxy cluster at z ∼ 1.55 in the COSMOS field. We target four galaxies with optical spectroscopic redshifts in the range z = 1.47-1.59, consistent with the likely redshift for the candidate galaxy cluster. Two of these four galaxies, ID 51613 and ID 51813, are nominally detected in CO 1-0 line emission at the 3-4σ level. We find CO luminosities of (2.42 ± 0.58) × 1010 K km s−1 pc2 and (1.26 ± 0.38) × 1010 K km s−1 pc2, respectively. Taking advantage from the clustering and expanded 2-GHz bandwidth of the JVLA, we perform a search for emission lines in the proximity of optical sources within the field of view of our observations (60 arcsec). We limit our search to galaxies with KS < 23.5 (AB) and zphot = 1.2-1.8. We find two bright optical galaxies, ID 51207 and ID 51380, to be associated with significant emission line peaks (>4σ) in the data cube, which we identify with the CO 1-0 line emission. To test the reliability of the line peaks found, we performed a parallel search for line peaks using a Bayesian inference method. Both CO line emitting candidates are identified with probabilities of 13 per cent and 72 per cent that there are line emitting sources in each case, respectively. Monte Carlo simulations show that such associations are statistically significant, with probabilities of chance association of 3.5 per cent and 10.7 per cent for ID 51207 and ID 51380, respectively. Modelling of their optical/IR spectral energy distributions indicates that the CO detected galaxies and candidates have stellar masses and star formation rates in the range (0.3-1.1) × 1011 M⊙ and 60-160 M⊙ yr−1, with star formation efficiencies comparable to that found in other star-forming galaxies at similar redshifts. By comparing the space density of CO emitters derived from our observations with the space density derived from previous CO detections at z ∼ 1.5, and with semi-analytic predictions for the CO luminosity function, we suggest that the latter tend to underestimate the number of CO galaxies detected at high redshift. Finally, we argue about the benefits of future searches for molecular gas line emission in clustered fields with upcoming submillimetre/radio facilities. [ABSTRACT FROM AUTHOR]

ABSTRACT Observations of molecular clouds in metal-poor environments typically find that they have much higher star formation rates than one would expect based on their observed CO luminosities and the molecular gas masses that are inferred from them. This finding can be understood if one assumes that the conversion factor between CO luminosity and H2 mass is much larger in these low-metallicity systems than in nearby molecular clouds. However, it is unclear whether this is the only factor at work, or whether the star formation rate of the clouds is directly sensitive to the metallicity of the gas. To investigate this, we have performed numerical simulations of the coupled dynamical, chemical and thermal evolution of model clouds with metallicities ranging from 0.01 to 1 Z⊙. We find that the star formation rate in our model clouds has little sensitivity to the metallicity. Reducing the metallicity of the gas by two orders of magnitude delays the onset of star formation in the clouds by no more than a cloud free-fall time and reduces the time-averaged star formation rate by at most a factor of 2. On the other hand, the chemical state of the clouds is highly sensitive to the metallicity, and at the lowest metallicities, the clouds are completely dominated by atomic gas. Our results not only confirm that the CO-to-H2 conversion factor in these systems depends strongly on the metallicity, but also show that the precise value is highly time-dependent, as the integrated CO luminosity of the most metal poor clouds is dominated by emission from short-lived gravitationally collapsing regions. Finally, we find evidence that the star formation rate per unit H2 mass increases with decreasing metallicity, owing to the much smaller H2 fractions present in our low-metallicity clouds. [ABSTRACT FROM AUTHOR]

ABSTRACT The linewidth-size (σ- R) relationship of the interstellar medium (ISM) has been extensively measured and analysed, in both the local ISM and in nearby normal galaxies. Generally, a power law describes the relationship well with an index ranging from 0.2 to 0.6, and is now referred to as one of the 'Larson's Relationships'. The nature of turbulence and star formation is considered to be intimately related to these relationships, so evaluating the σ- R correlations in various environments is important for developing a comprehensive understanding of the ISM. We measure the linewidth-size relationship in the Central Molecular Zone (CMZ) of the Galactic Centre using spectral line observations of the high-density tracers N2H+, HCN, H13CN and HCO+. We construct dendrograms, which map the hierarchical nature of the position-position-velocity (PPV) data, and compute the linewidths and sizes of the dendrogram-defined structures. The dispersions range from ∼2 to 30 km s−1 in structures spanning sizes 2-40 pc. By performing Bayesian inference, we show that a power law with exponent 0.3-1.1 can reasonably describe the σ- R trend. We demonstrate that the derived σ- R relationship is independent of the locations in the PPV data set where σ and R are measured. The uniformity in the σ- R relationship indicates that turbulence in the CMZ is driven on the large scales beyond ≳30 pc. We compare the CMZ σ- R relationship to that measured in the Galactic molecular cloud Perseus. The exponents between the two systems are similar, suggestive of a connection between the turbulent properties within a cloud to its ambient medium. Yet, the velocity dispersion in the CMZ is systematically higher, resulting in a scaling coefficient that is approximately five times larger. The systematic enhancement of turbulent velocities may be due to the combined effects of increased star formation activity, larger densities and higher pressures relative to the local ISM. [ABSTRACT FROM AUTHOR]

ABSTRACT We make use of interferometric CO and H i observations, and optical integral-field spectroscopy from the ATLAS3D survey, to probe the origin of the molecular and ionized interstellar medium (ISM) in local early-type galaxies. We find that 36 ± 5 per cent of our sample of fast-rotating early-type galaxies have their ionized gas kinematically misaligned with respect to the stars, setting a strong lower limit on the importance of externally acquired gas (e.g. from mergers and cold accretion). Slow rotators have a flat distribution of misalignments, indicating that the dominant source of gas is external. The molecular, ionized and atomic gas in all the detected galaxies are always kinematically aligned, even when they are misaligned from the stars, suggesting that all these three phases of the ISM share a common origin. In addition, we find that the origin of the cold and warm gas in fast-rotating early-type galaxies is strongly affected by environment, despite the molecular gas detection rate and mass fractions being fairly independent of group/cluster membership. Galaxies in dense groups and the Virgo cluster nearly always have their molecular gas kinematically aligned with the stellar kinematics, consistent with a purely internal origin (presumably stellar mass loss). In the field, however, kinematic misalignments between the stellar and gaseous components indicate that at least 42 ± 5 per cent of local fast-rotating early-type galaxies have their gas supplied from external sources. When one also considers evidence of accretion present in the galaxies' atomic gas distributions, ≳46 per cent of fast-rotating field ETGs are likely to have acquired a detectable amount of ISM from accretion and mergers. We discuss several scenarios which could explain the environmental dichotomy, including preprocessing in galaxy groups/cluster outskirts and the morphological transformation of spiral galaxies, but we find it difficult to simultaneously explain the kinematic misalignment difference and the constant detection rate. Furthermore, our results suggest that galaxy mass may be an important independent factor associated with the origin of the gas, with the most massive fast-rotating galaxies in our sample (M K≲−24 mag; stellar mass of ≈8 × 1010 M⊙) always having kinematically aligned gas. This mass dependence appears to be independent of environment, suggesting it is caused by a separate physical mechanism. [ABSTRACT FROM AUTHOR]

Based on [Ci] (1-0) observations and complementary Hi, 12/13CO (1-0), 12CO (2-1) and [Cii] data we estimated the column density and fraction of molecular hydrogen in GMCs along the major axis of the galaxy M33. We found that on average 75%±14% of the hydrogen is in molecular form. Roughly 40-45% of the H2 is both traced by [CII] and CO, while ~15% by the [Ci] lines. The found CO-dark H2 gas fractions are within the predictions by Wolfire et al. (2010) for a half solar metallicity, Z = 0.5Zʘ, as in M33. [ABSTRACT FROM AUTHOR]