Giant star-forming clumps? Ivison, R J; Richard, J; Biggs, A D ...
Monthly notices of the Royal Astronomical Society. Letters,
06/2020, Volume:
495, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT
With the spatial resolution of the Atacama Large Millimetre Array (ALMA), dusty galaxies in the distant Universe typically appear as single, compact blobs of dust emission, with a median ...half-light radius, ≈1 kpc. Occasionally, strong gravitational lensing by foreground galaxies or galaxy clusters has probed spatial scales 1–2 orders of magnitude smaller, often revealing late-stage mergers, sometimes with tantalizing hints of sub-structure. One lensed galaxy in particular, the Cosmic Eyelash at z = 2.3, has been cited extensively as an example of where the interstellar medium exhibits obvious, pronounced clumps, on a spatial scale of ≈100 pc. Seven orders of magnitude more luminous than giant molecular clouds in the local Universe, these features are presented as circumstantial evidence that the blue clumps observed in many z ∼ 2–3 galaxies are important sites of ongoing star formation, with significant masses of gas and stars. Here, we present data from ALMA which reveal that the dust continuum of the Cosmic Eyelash is in fact smooth and can be reproduced using two Sérsic profiles with effective radii, 1.2 and 4.4 kpc, with no evidence of significant star-forming clumps down to a spatial scale of ≈80 pc and a star formation rate of <3 M⊙ yr−1.
Context. Large-scale motions in galaxies (supernovae explosions, galaxy collisions, galactic shear etc.) generate turbulence, which allows a fraction of the available kinetic energy to cascade down ...to small scales before it is dissipated. Aims. We establish and quantify the diagnostics of turbulent dissipation in mildly irradiated diffuse gas in the specific context of shock structures. Methods. We incorporated the basic physics of photon-dominated regions into a state-of-the-art steady-state shock code. We examined the chemical and emission properties of mildly irradiated (G0 = 1) magnetised shocks in diffuse media (nH = 102 to 104 cm-3) at low- to moderate velocities (from 3 to 40 km s-1). Results. The formation of some molecules relies on endoergic reactions. Their abundances in J-type shocks are enhanced by several orders of magnitude for shock velocities as low as 7 km s-1. Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30 km s-1, where H2 dissociation sets in. C-type shocks display a more gradual molecular enhancement with increasing shock velocity. We quantified the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities. We fitted various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephan’s Quintet and of a Galactic line of sight which samples diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5 km s-1), dissipation is due to ion-neutral friction and it powers H2 low-energy transitions and atomic lines. In moderate velocity shocks (20 km s-1 and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas in the line of sight is shocked (from 4% to 66%). For example, C+ emission may trace shocks in UV irradiated gas where C+ is the dominant carbon species. Conclusions. Low- and moderate velocity shocks are important in shaping the chemical composition and excitation state of the interstellar gas. This allows one to probe the statistical distribution of shock velocities in interstellar turbulence.
Aims. Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium (ISM) and its 158 μm fine structure transition C ii is the most important cooling line of the ...diffuse ISM. We combine C ii absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. Methods. We present high-resolution C ii spectra obtained with the Herschel/HIFI instrument towards bright dust continuum regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by single pointings in the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 μm with Herschel/PACS. Results. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution PACS data and that high spectral resolution spectra are needed to interpret the C ii and O i emission and the C ii/FIR ratio. This phenomenon may explain part of the C ii/FIR deficit seen in external luminous infrared galaxies where the bright emission from the nuclear regions may be partially canceled by absorption from diffuse gas in the foreground. The C+ and C excitation in the diffuse gas is consistent with a median pressure of ~5900 K cm-3 for a mean kinetic temperature of ~100 K. A few higher pressure regions are detected along the lines of sight, as emission features in both fine structure lines of atomic carbon. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight. The derived median value of the filling factor is 2.4%, in good agreement with the properties of the Galactic cold neutral medium. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 × 10-26 erg-1H-1. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 ± 0.4 × 10-4.
Aims. Supersonic turbulence is a large reservoir of suprathermal energy in the interstellar medium. Its dissipation, because it is intermittent in space and time, can deeply modify the chemistry of ...the gas. This is clearly seen in the framework of shock chemistry. Intense turbulent dissipation also occurs in regions of large velocity shears, sharing with shocks the property of intermittency. Whether these bursts of dissipation, short-lived and localized, have a measurable impact on molecular abundances in the diffuse medium, and how the chemical enrichment they drive compares to observations, are the questions we address here. Methods. We further explore a hybrid method to compute the chemical and thermal evolution of a magnetized dissipative structure, under the energetic constraints provided by the observed properties of turbulence in the cold neutral medium. For the first time, we model a random line of sight by taking into account the relative duration of the bursts with respect to the thermal and chemical relaxation timescales of the gas. The key parameter is the turbulent rate of strain a due to the ambient turbulence. With the gas density, it controls the size of the dissipative structures, therefore the strength of the burst. It also sets the relative importance of viscous dissipation and ion-neutral friction in the gas heating and chemical enrichment. Results. For a large range of rates of strain and densities, the models of turbulent dissipation regions (TDR) reproduce the CH+ column densities observed in the diffuse medium and their correlation with highly excited H2. They do so without producing an excess of CH. As a natural consequence, they reproduce the abundance ratios of HCO+/OH and HCO+/H2O, and their dynamic range of about one order of magnitude observed in diffuse gas. Large C2H and CO abundances, also related to those of HCO+, are another outcome of the TDR models that compare well with observed values. Neutral carbon exceeds the abundance expected at ionization equilibrium, in agreement with fine-structure line observations. The abundances and column densities computed for CN, HCN and HNC are one order of magnitude above PDR model predictions, although still significantly smaller than observed values. The dependence of our results on the rate of strain and density reveals that the chemical enhancements are in better agreement with observations if the dissipation is dominated by ion-neutral friction, involving shear structures of thickness ~100 AU.
Within four nearby (d < 160 pc) molecular clouds, we statistically evaluated the structure of the interstellar magnetic field, projected on the plane of the sky and integrated along the line of ...sight, as inferred from the polarized thermal emission of Galactic dust observed by Planck at 353 GHz and from the optical and near-infrared polarization of background starlight. We compared the dispersion of the field orientation directly in vicinities with an area equivalent to that subtended by the Planck effective beam at 353 GHz (10′) and using the second-order structure functions of the field orientation angles. We found that the average dispersion of the starlight-inferred field orientations within 10′-diameter vicinities is less than 20°, and that at these scales the mean field orientation is on average within 5° of that inferred from the submillimetre polarization observations in the considered regions. We also found that the dispersion of starlight polarization orientations and the polarization fractions within these vicinities are well reproduced by a Gaussian model of the turbulent structure of the magnetic field, in agreement with the findings reported by the Planck Collaboration at scales ℓ > 10′ and for comparable column densities. At scales ℓ > 10′, we found differences of up to 14.̊7 between the second-order structure functions obtained from starlight and submillimetre polarization observations in the same positions in the plane of the sky, but comparison with a Gaussian model of the turbulent structure of the magnetic field indicates that these differences are small and are consistent with the difference in angular resolution between both techniques. The differences between the second-order structure functions calculated with each technique suggests that the increase in the angular resolution obtained with the starlight polarization observations does not introduce significant corrections to the dispersion of polarization orientations used in the calculation of the molecular-cloud-scale magnetic field strengths reported in previous studies by the Planck Collaboration.
Models of irradiated molecular shocks Godard, B.; Pineau des Forêts, G.; Lesaffre, P. ...
Astronomy and astrophysics (Berlin),
02/2019, Volume:
622
Journal Article
Peer reviewed
Open access
Context. The recent discovery of excited molecules in starburst galaxies observed with ALMA and the Herschel space telescope has highlighted the necessity to understand the relative contributions of ...radiative and mechanical energies in the formation of molecular lines and explore the conundrum of turbulent gas bred in the wake of galactic outflows. Aims. The goal of the paper is to present a detailed study of the propagation of low velocity (5–25 km s−1) stationary molecular shocks in environments illuminated by an external ultraviolet (UV) radiation field. In particular, we intend to show how the structure, dynamics, energetics, and chemical properties of shocks are modified by UV photons and to estimate how efficiently shocks can produce line emission. Methods. We implemented several key physico-chemical processes in the Paris-Durham shock code to improve the treatment of the radiative transfer and its impact on dust and gas particles. We propose a new integration algorithm to find the steady-state solutions of magnetohydrodynamics equations in a range of parameters in which the fluid evolves from a supersonic to a subsonic regime. We explored the resulting code over a wide range of physical conditions, which encompass diffuse interstellar clouds and hot and dense photon-dominated regions. Results. We find that C-type shock conditions cease to exist as soon as G0 > 0.2 (nH/cm−3)1/2 $G_0\,{>}\,0.2\,\, ({n_{\textrm{H}}}/{\textrm{cm}^{-3}})^{1/2}$ G0 > 0.2 (nH/cm−3)1/2 . Such conditions trigger the emergence of another category of stationary solutions, called C*-type and CJ-type shocks, in which the shocked gas is momentarily subsonic along its trajectory. These solutions are shown to be unique for a given set of physical conditions and correspond to dissipative structures in which the gas is heated up to temperatures comprised between those found in C-type and adiabatic J-type shocks. High temperatures combined with the ambient UV field favour the production or excitation of a few molecular species to the detriment of others, hence leading to specific spectroscopic tracers such as rovibrational lines of H2 and rotational lines of CH+. Unexpectedly, the rotational lines of CH+ may carry as much as several percent of the shock kinetic energy. Conclusions. Ultraviolet photons are found to strongly modify the way the mechanical energy of interstellar shocks is processed and radiated away. In spite of what intuition dictates, a strong external UV radiation field boosts the efficiency of low velocity interstellar shocks in the production of several molecular lines which become evident tracers of turbulent dissipation.
Aims. The condensation of diffuse gas into molecular clouds and dense cores occurs at a rate driven largely by turbulent dissipation. This process still has to be caught in action and characterized. ...Methods. We observed a mosaic of 13 fields with the IRAM-PdB interferometer (PdBI) to search for small-scale structure in the 12CO(1-0) line emission of the turbulent and translucent environment of a low-mass dense core in the Polaris Flare. The large size of the mosaic (1' $\times$ 2') compared to the resolution (4'') is unprecedented in the study of the small-scale structure of diffuse molecular gas. Results. The interferometer data uncover eight weak and elongated structures with thicknesses as small as ≈3 mpc (600 AU) and lengths up to 70 mpc, close to the size of the mosaic. These are not filaments because once merged with short-spacings data, the PdBI-structures appear to be the sharp edges, in space and velocity-space, of larger-scale structures. Six out of eight form quasi-parallel pairs at different velocities and different position angles. This cannot be the result of chance alignment. The velocity-shears estimated for the three pairs include the highest values ever measured in regions that do not form stars (up to 780 km s-1 pc-1). The CO column density of the PdBI-structures is in the range $N({\rm CO)}$ = 1014 to 1015 cm-2 and their H2 density, estimated in several ways, does not exceed a few 103 cm-3. Because the larger scale structures have sharp edges (with little or no overlap for those that are pairs), they have to be thin layers of CO emission. We call them SEE(D)S for sharp-edged extended (double) structures. These edges mark a transition, on the milliparsec scale, between a CO-rich component and a gas undetected in the 12CO(1-0) line because of its low CO abundance, presumably the cold neutral medium. Conclusions. We propose that these SEE(D)S are the first directly-detected manifestations of the intermittency of interstellar turbulence. The large velocity-shears reveal an intense straining field, responsible for a local dissipation rate several orders of magnitude above average, possibly at the origin of the thin CO layers.
Aims. The comparative study of several molecular species at the origin of the gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in unraveling the coupled chemical and ...dynamical evolution of the ISM. Methods. The lowest rotational lines of HCO+, HCN, HNC, and CN were observed at the IRAM-30m telescope in absorption against the λ3 mm and λ1.3 mm continuum emission of massive star-forming regions in the Galactic plane. The absorption lines probe the gas over kiloparsecs along these lines of sight. The excitation temperatures of HCO+ are inferred from the comparison of the absorptions in the two lowest transitions. The spectra of all molecular species on the same line of sight are decomposed into Gaussian velocity components. Most appear in all the spectra of a given line of sight. For each component, we derived the central opacity, the velocity dispersion, and computed the molecular column density. We compared our results to the predictions of UV-dominated chemical models of photodissociation regions (PDR models) and to those of non-equilibrium models in which the chemistry is driven by the dissipation of turbulent energy (TDR models). Results. The molecular column densities of all the velocity components span up to two orders of magnitude. Those of CN, HCN, and HNC are linearly correlated with each other with mean ratios N(HCN)/N(HNC) = 4.8 ± 1.3 and N(CN)/N(HNC) = 34 ± 12, and more loosely correlated with those of HCO+, N(HNC)/N(HCO+) = 0.5 ± 0.3, N(HCN)/N(HCO+) = 1.9 ± 0.9, and N(CN)/N(HCO+) = 18 ± 9. These ratios are similar to those inferred from observations of high Galactic latitude lines of sight, suggesting that the gas sampled by absorption lines in the Galactic plane has the same chemical properties as that in the Solar neighbourhood. The FWHM of the Gaussian velocity components span the range 0.3 to 3 km s-1 and those of the HCO+ lines are found to be 30% broader than those of CN-bearing molecules. The PDR models fail to reproduce simultaneously the observed abundances of the CN-bearing species and HCO+, even for high-density material ( 100 cm-3 < nH < 104 cm-3). The TDR models, in turn, are able to reproduce the observed abundances and abundance ratios of all the analysed molecules for the moderate gas densities (30 cm-3 < nH < 200 cm-3) and the turbulent energy observed in the diffuse interstellar medium. Conclusions. Intermittent turbulent dissipation appears to be a promising driver of the gas phase chemistry of the diffuse and translucent gas throughout the Galaxy. The details of the dissipation mechanisms still need to be investigated.
Cosmic rays are predominantly accelerated in shocks associated with star formation such as supernova remnants and stellar wind bubbles, so the cosmic-ray flux and thus cosmic-ray ionization rate, ζH, ...should correlate with the star formation rate in a galaxy. Submillimeter bright galaxies (SMGs) are some of the most prolific star-forming galaxies in the universe, and gravitationally lensed SMGs provide bright continuum sources suitable for absorption line studies. Abundances of OH+ and H2O+ are useful for inferring ζH when combined with chemical models, and have been used for this purpose within the Milky Way. At redshifts z 2 transitions out of the ground rotational states of OH+ and H2O+ are observable with ALMA, and we present observations of both molecules in absorption toward the lensed SMGs SMM J2135−0102 and SDP 17b. These detections enable an exploration of ζH in galaxies with extreme star formation and high supernova rates, both of which should significantly enhance cosmic-ray production. The observed OH+ and H2O+ absorption is thought to arise in massive, extended halos of cool, diffuse gas that surround these galaxies. Using a chemical model designed to focus on the reaction network important to both species, we infer cosmic-ray ionization rates of ζH ∼ 10−16-10−14 s−1 in these extended gaseous halos. Because our estimates come from gas that is far away from the sites of cosmic-ray acceleration, they imply that cosmic-ray ionization rates in the compact regions where star formation occurs in these galaxies are orders of magnitude higher.
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