Excitation Of CO Research Articles

VLA Legacy Survey of Molecular Gas in Massive Star-forming Galaxies at High Redshift

We present the initial results of an ongoing survey with the Karl G. Jansky Very Large Array targeting the CO(J = 1–0) transition in a sample of 30 submillimeter-selected, dusty star-forming galaxies (SFGs) at z = 2–5 with existing mid-J CO detections from the Atacama Large Millimeter/submillimeter Array and NOrthern Extended Millimeter Array, of which 17 have been fully observed. We detect CO(1–0) emission in 11 targets, along with three tentative (∼1.5σ–2σ) detections; three galaxies are undetected. Our results yield total molecular gas masses of 6–23 × 1010 (α CO/1) M ⊙, with gas mass fractions, f gas = M mol/(M *+M mol), of 0.1–0.8 and a median depletion time of (140 ± 70) Myr. We find median CO excitation ratios of r 31 = 0.75 ± 0.39 and r 41 = 0.63 ± 0.44, with significant scatter. We find no significant correlation between the excitation ratio and a number of key parameters such as redshift, CO(1–0) line width, or ΣSFR. We only find a tentative positive correlation between r 41 and the star-forming efficiency, but we are limited by our small sample size. Finally, we compare our results to predictions from the SHARK semi-analytical model, finding a good agreement between the molecular gas masses, depletion times, and gas fractions of our sources and their SHARK counterparts. Our results highlight the heterogeneous nature of the most massive SFGs at high redshift, and the importance of CO(1–0) observations to robustly constrain their total molecular gas content and interstellar medium properties.

Photodissociation study of CO2 on the formation of state-correlated CO(X1Σ+, v) with O(3P2) photoproducts in the low energy band centered at 148 nm.

The spin-forbidden O(3P2) + CO(X1Σ+, v) channel formed from the photodissociation of CO2 in the low energy band centered at 148nm is investigated by using the time-sliced velocity-mapped ion imaging technique. The vibrational-resolved images of the O(3P2) photoproducts measured in the photolysis wavelength range of 144.62-150.45nm are analyzed to give the total kinetic energy releases (TKER) spectra, CO(X1Σ+) vibrational state distributions, and anisotropy parameters (β). The TKER spectra reveal the formation of correlated CO(X1Σ+) with well resolved v = 0-10 (or 11) vibrational bands. Several high vibrational bands that were observed in the low TKER region for each studied photolysis wavelength exhibit a bimodal structure. The CO(X1Σ+, v) vibrational distributions all present inverted characteristics, and the most populated vibrational state changes from a low vibrational state to a relatively higher vibrational state with a change in the photolysis wavelength from 150.45 to 144.62nm. However, the vibrational-state specific β-values for different photolysis wavelengths present a similar variation trend. The measured β-values show a significant bulge at the higher vibrational levels, in addition to the overall slow decreasing trend. The observed bimodal structures with mutational β-values for the high vibrational excited state CO(1Σ+) photoproducts suggest the existence of more than one nonadiabatic pathway with different anisotropies in the formation of O(3P2) + CO(X1Σ+, v) photoproducts across the low energy band.

PHANGS–JWST First Results: Stellar-feedback-driven Excitation and Dissociation of Molecular Gas in the Starburst Ring of NGC 1365?

We compare embedded young massive star clusters (YMCs) to (sub-)millimeter line observations tracing the excitation and dissociation of molecular gas in the starburst ring of NGC 1365. This galaxy hosts one of the strongest nuclear starbursts and richest populations of YMCs within 20 Mpc. Here we combine near-/mid-IR PHANGS–JWST imaging with new Atacama Large Millimeter/submillimeter Array multi-J CO (1–0, 2–1 and 4–3) and [C i] (1–0) mapping, which we use to trace CO excitation via R 42 = I CO(4−3)/I CO(2−1) and R 21 = I CO(2−1)/I CO(1−0) and dissociation via R CICO = I [CI](1−0)/I CO(2−1) at 330 pc resolution. We find that the gas flowing into the starburst ring from northeast to southwest appears strongly affected by stellar feedback, showing decreased excitation (lower R 42) and increased signatures of dissociation (higher R CICO) in the downstream regions. There, radiative-transfer modeling suggests that the molecular gas density decreases and temperature and [CI/CO] abundance ratio increase. We compare R 42 and R CICO with local conditions across the regions and find that both correlate with near-IR 2 μm emission tracing the YMCs and with both polycyclic aromatic hydrocarbon (11.3 μm) and dust continuum (21 μm) emission. In general, R CICO exhibits ∼0.1 dex tighter correlations than R 42, suggesting C i to be a more sensitive tracer of changing physical conditions in the NGC 1365 starburst than CO (4–3). Our results are consistent with a scenario where gas flows into the two arm regions along the bar, becomes condensed/shocked, forms YMCs, and then these YMCs heat and dissociate the gas.

The excitation of CO in CO-dominated cometary comae

ABSTRACTAn abundance of CO significantly surpassing the abundance of H2O is observed in the comae of comets at large heliocentric distances. In these environments, CO molecules can be the most abundant species and they may be therefore the dominant projectiles inducing collisional excitation of the cometary molecules. It is thus of high interest to investigate the excitation of CO by CO. This article provides a new set of CO–CO collisional rate coefficients for temperatures up to 150 K and for CO rotational levels j1 up to 10. These data are obtained from quantum scattering calculations using the coupled states approximation. They are used in a simple radiative transfer model in order to test their impact on the excitation of cometary CO. Because mutual (de-)excitations of the target and projectile are important, the CO projectile was assumed to be thermalized at the kinetic temperature. We found that the non-local thermodynamical equilibrium regime extends for CO densities in the range 103–107 cm−3. We also observed that as soon as the CO/H2O ratio is larger than 70 per cent/30 per cent, the contribution of H2O collisions can be neglected. Similarly, the excitation of CO by CO may be ignored for relatively low CO/H2O density ratios (≤30 per cent/70 per cent). Finally, when the coma is a ∼50 per cent/50 per cent mixture of CO and H2O, the contribution of both colliders is similar and has to be considered.

In situ infrared absorption probing of plasma catalysis: vibrationally-excited species induced Mars–van Krevelen type mechanism

Nonthermal plasma (NTP) assisted catalysis has recently received attention as an attractive technology that can overcome the energy barriers involved in conventional thermal catalysis. It is important to obtain fundamental insight into the synergistic effects arising from interactions between NTP-activated species and catalysis. However, it remains a challenge due to difficulties associated with the in situ characterization of NTP-activated species. In this study, CO was explored in dielectric barrier discharge by applying in situ transmission infrared (TIR) absorption spectroscopy; TIR enables the measurement of the vibrational temperature of CO while observing the surface reaction of excited CO to form intermediates species over wurtzite ZnO. The reaction behavior of activated CO correlates with the vibrational temperature of CO, showing vibrationally-excited CO reacts with lattice oxygen via the Mars–van Krevelen type mechanism; thus leading to higher activity compared with conventional thermal catalysis. This work helps to understand the fundamental chemistry and provides new insight into NTP-activated CO and its catalysis.

Comprehensive Studies of Magnetic Transitions and Spin-Phonon Couplings in the Tetrahedral Cobalt Complex Co(AsPh3)2I2.

A combination of inelastic neutron scattering (INS), far-IR magneto-spectroscopy (FIRMS), and Raman magneto-spectroscopy (RaMS) has been used to comprehensively probe magnetic excitations in Co(AsPh3)2I2 (1), a reported single-molecule magnet (SMM). With applied field, the magnetic zero-field splitting (ZFS) peak (2D') shifts to higher energies in each spectroscopy. INS placed the ZFS peak at 54 cm-1, as revealed by both variable-temperature (VT) and variable-magnetic-field data, giving results that agree well with those from both far-IR and Raman studies. Both FIRMS and RaMS also reveal the presence of multiple spin-phonon couplings as avoided crossings with neighboring phonons. Here, phonons refer to both intramolecular and lattice vibrations. The results constitute a rare case in which the spin-phonon couplings are observed with both Raman-active (g modes) and far-IR-active phonons (u modes; space group P21/c, no. 14, Z = 4 for 1). These couplings are fit using a simple avoided crossing model with coupling constants of ca. 1-2 cm-1. The combined spectroscopies accurately determine the magnetic excited level and the interaction of the magnetic excitation with phonon modes. Density functional theory (DFT) phonon calculations compare well with INS, allowing for the assignment of the modes and their symmetries. Electronic calculations elucidate the nature of ZFS in the complex. Features of different techniques to determine ZFS and other spin-Hamiltonian parameters in transition-metal complexes are summarized.

The Star Formation–Gas Density Relation in Four Galactic GMCs: Effects of Stellar Feedback

We present maps of four Galactic giant molecular clouds (GMCs) in the J = 2-1 emission of both CO and 13CO. We use an LTE analysis to derive maps of the CO excitation temperature and column density and the distribution of total molecular gas column density, Σgas. The depletion of CO by freeze-out onto cold dust grains is accounted for by an approximation to the results of Lewis et al., which were derived from far-IR observations with Herschel. The surface density of young stellar objects (YSOs) is obtained from published catalogs. The mean YSO surface density exhibits a power-law dependence on Σgas, with exponents in the range 0.9–1.9. Gas column density probability distribution functions show power-law tails extending to high column densities. The distributions of sonic Mach number, M S , are sharply peaked at MS ∼ 5–8 for 3 GMCs; a fourth has a broad distribution up to M S = 30, possibly a result of feedback effects from multiple OB stars. An analysis following the methodology of Pokhrel et al. finds that our sample of GMCs shows power-law relations that are somewhat shallower than those found by Pokhrel et al. for the star formation rate versus 〈Σgas〉 and versus 〈Σgas〉/t ff in a different sample of clouds. We discuss possible differences in the two samples of star-forming clouds and the effects of stellar feedback on the relation between gas density and star formation rate.

CO Excitation and its Connection to Star Formation at 200 pc in NGC 1365

We report high-resolution (2″ ∼ 200 pc) mappings of the central region of the nearby barred spiral galaxy NGC 1365 in the CO(1–0) and CO(2–1) emission lines. The 2–1/1–0 ratio of integrated intensities shows a large scatter (0.15) with a median value of 0.67. We also calculate the ratio of velocity dispersions and peak temperatures and find that in most cases the velocity dispersion ratio is close to unity and thus the peak temperature ratio is comparable to the integrated intensity ratio. This result indicates that both CO(1–0) and CO(2–1) lines trace similar components of molecular gas, with their integrated intensity (or peak temperature) ratios reflecting the gas density and/or temperature. Similar to recent kiloparsec-scale studies, these ratios show a positive correlation with a star formation rate indicator (here we use an extinction-corrected Hα map), suggesting that molecular gas associated with recent star formation is denser and/or warmer. We also find that some CO spectra show two peaks owing to complicated kinematics, and such two components likely trace molecular gas at different conditions. This result demonstrates the importance of spectral fitting to measure integrated intensities and their ratios more accurately.

Cooperative Catalysis of Vibrationally Excited CO2 and Alloy Catalyst Breaks the Thermodynamic Equilibrium Limitation

Using nonthermal plasma (NTP) to promote CO2 hydrogenation is one of the most promising approaches that overcome the limitations of conventional thermal catalysis. However, the catalytic surface reaction dynamics of NTP-activated species are still under debate. The NTP-activated CO2 hydrogenation was investigated in Pd2Ga/SiO2 alloy catalysts and compared to thermal conditions. Although both thermal and NTP conditions showed close to 100% CO selectivity, it is worth emphasizing that when activated by NTP, CO2 conversion not only improves more than 2-fold under thermal conditions but also breaks the thermodynamic equilibrium limitation. Mechanistic insights into NTP-activated species and alloy catalyst surface were investigated by using in situ transmission infrared spectroscopy, where catalyst surface species were identified during NTP irradiation. Moreover, in in situ X-ray absorption fine-structure analysis under reaction conditions, the catalyst under NTP conditions not only did not undergo restructuring affecting CO2 hydrogenation but also could clearly rule out catalyst activation by heating. In situ characterizations of the catalysts during CO2 hydrogenation depict that vibrationally excited CO2 significantly enhances the catalytic reaction. The agreement of approaches combining experimental studies and density functional theory (DFT) calculations substantiates that vibrationally excited CO2 reacts directly with hydrogen adsorbed on Pd sites while accelerating formate formation due to neighboring Ga sites. Moreover, DFT analysis deduces the key reaction pathway that the decomposition of monodentate formate is promoted by plasma-activated hydrogen species. This work enables the high designability of CO2 hydrogenation catalysts toward value-added chemicals based on the electrification of chemical processes via NTP.

Vibrational Relaxation of Highly Vibrationally Excited Molecules Scattered from Au(111): Role of the Dissociation Barrier

Energy transfer during molecular collisions at a metal surface represents a sensitive probe of the molecule–surface interaction potential. Here, via molecular dynamics calculations on several first-principles neural network potentials, we find that the vibrational energy transfer dynamics of highly vibrationally excited NO and CO molecules scattered from Au(111) are strongly correlated with their respective potential energy landscapes in the vicinity of the dissociation barrier. Our results not only reproduce the observed significantly less vibrational relaxation of CO (vi = 17) than NO (vi = 16) scattered from Au(111) and attribute it to the different dissociation barriers in the two systems, but also show a dramatic change of dynamics due to a minor adjustment of the energy landscape near the barrier in the CO case. We find that the BEEF-vdW density functional based potential largely overestimates the vibrational relaxation of CO (vi = 17) even in the absence of any nonadiabatic energy loss, despite its good description for CO adsorption on Au(111). We also discuss the possibility of integrating the validated adiabatic potential with first-principles determined diabatic states towards a more complete description of the non-adiabatic energy transfer in these benchmark systems.

Comprehensive study on plasma chemistry and products in [formula omitted] pulsed discharges under Martian pressure

With the development of Mars exploration, the In-Situ resource utilization on Mars has been drawn significant attention in recent years. The abundant CO2 in the Martian atmosphere makes it possible to decompose CO2 to produce high-value platform chemicals by plasma technology. In this comprehensive study, the discharge characteristics, products distribution, and reaction pathways in CO2 pulsed discharges under Martian pressure are investigated, and the effects of pulsed waveforms on discharge characteristics and produced species are also discussed. The simulation results show that electrons and CO2+ ions are the main components of charged particles in the discharge region; CO and O are mainly produced by the electron impact dissociation and electron impact attachment reactions with ground state and vibrationally excited state of CO2 during the power-on phase and consumed by the ion–neutral reactions. The recombination reaction of electrons and CO2+ ions is very important for the formation of O2. The simulation data also give the main generation and consumption pathways of vibrationally excited CO2 molecules, and indicate that the vibrational energy of excited CO2 molecules can lower the energy barrier of the reaction and facilitate the decomposition of CO2. In addition, the experimental and computational data also suggest that the pulsed waveforms could affect the discharge characteristics and plasma chemistry in CO2 pulsed discharges under Martian pressure, the densities of product species in CO2 plasma increase with the pulse rise rate and pulse rise duration of pulsed voltage. This study provides an enhanced understanding of discharge characteristics and plasma chemistry in the CO2 pulsed discharges under Martian pressure, and suggests optimized ways to utilize the CO2 on Mars.

Correlated rotational excitations in NO-CO inelastic collisions.

We present a joint experimental and theoretical study of rotationally inelastic collisions between NO (X2Π1/2, ν = 0, j = 1/2, f) radicals and CO (X1Σ+, ν = 0, j = 0) molecules at a collision energy of 220 cm-1. State-to-state scattering images for excitation of NO radicals into various final states were measured with high resolution by combining the Stark deceleration and velocity map imaging techniques. The high image resolution afforded the observation of correlated rotational excitations of NO-CO pairs, which revealed a number of striking scattering phenomena. The so-called "parity-pair" transitions in NO are found to have similar differential cross sections, independent of the concurrent excitation of CO, extending this well-known effect for collisions between NO and rare gas atoms into the realm of bimolecular collisions. Forward scattering is found for collisions that induce a large amount of rotational energy transfer (in either NO, CO, or both), which require low impact parameters to induce sufficient energy transfer. This observation is interpreted in terms of the recently discovered hard collision glory scattering mechanism, which predicts the forward bending of initially backward receding trajectories if the energy uptake in the collision is substantial in relation to the collision energy. The experimental results are in good agreement with the predictions from coupled-channels quantum scattering calculations based on an ab initio NO-CO potential energy surface.

A CO isotopologue Line Atlas within the Whirlpool galaxy Survey (CLAWS)

We present the CO isotopologue Line Atlas within the Whirpool galaxy Survey (CLAWS), which is based on an IRAM 30 m large programme that provides a benchmark study of numerous faint CO isotopologues in the millimetre-wavelength regime across the full disc of the nearby grand-design spiral galaxy M 51 (NGC 5194). The survey’s core goal is to use the low-J CO isotopologue lines to constrain CO excitation and chemistry, and therefrom the local physical conditions of the gas. In this survey paper, we describe the CLAWS observing and data reduction strategies. We map the J = 1 → 0 and 2 → 1 transitions of the CO isotopologues 12CO, 13CO, C18O, and C17O, as well as several supplementary lines within the 1 mm and 3 mm window (CN(1-0), CS (2-1), CH3OH(2-1), N2H+ (1–0), and HC3N (10–9)) at ~1 kpc resolution. A total observation time of 149 h offers unprecedented sensitivity. We use these data to explore several CO isotopologue line ratios in detail, study their radial (and azimuthal) trends, and investigate whether changes in line ratios stem from changes in interstellar medium properties such as gas temperatures, densities, or chemical abundances. For example, we find negative radial trends for the 13CO/12CO, C18O/12CO, and C18O/13CO line ratios in their J = 1 → 0 transitions. We also find variations with the local environment, such as higher 12CO (2–1)/(1–0) or 13CO/12CO (1-0) line ratios in interarm regions compared to spiral arm regions. We propose that these aforementioned variations in CO line ratios are most likely due to a variation in the optical depth, though abundance variations due to selective nucleosynthesis on a galaxy-wide scale could also play a role. We also study the CO spectral line energy distribution (SLED) using archival JCMT 12CO (3–2) data and find a variation in the SLED shape with local environmental parameters, suggesting changes in the optical depth, gas temperatures, or densities.

CO excitation and line energy distributions in gas-selected galaxies

ABSTRACT While emission-selected galaxy surveys are biased towards the most luminous part of the galaxy population, absorption selection is a potentially unbiased galaxy selection technique with respect to luminosity. However, the physical properties of absorption-selected galaxies are not well characterized. Here, we study the excitation conditions in the interstellar medium (ISM) in damped Ly α (DLA) absorption-selected galaxies. We present a study of the CO spectral-line energy distribution (SLED) in four high-metallicity absorption-selected galaxies with previously reported CO detections at intermediate (z ∼ 0.7) and high (z ∼ 2) redshifts. We find further evidence for a wide variety of ISM conditions in these galaxies. Two out of the four galaxies show CO SLEDs consistent with that of the Milky Way inner disc. Interestingly, one of these galaxies is at z ∼ 2 and has a CO SLED below that of main-sequence galaxies at similar redshifts. The other two galaxies at z > 2 show more excited ISM conditions, with one of them showing thermal excitation of the mid-J (J = 3, 4) levels, similar to that seen in two massive main-sequence galaxies at these redshifts. Overall, we find that absorption selection traces a diverse population of galaxies.

New physical insights: Formamide discharge decomposition and the role of fragments in the formation of large biomolecules

In this work we present a time-resolved FTIR spectroscopic study on kinetics of atomic and molecular species, specifically CO, CN radical, N2, HCN and CO2 generated in a glow discharge of formamide-nitrogen-water mixture in a helium buffer gas. Radicals such as NH, CH and OH have been proven to be fundamental stones of subsequent chemical reactions having a crucial role in a prebiotic synthesis of large organic molecules.This work contains three main goals. Firstly, we present our time-resolved spectra of formamide decomposition products and discuss the mechanism of collisional excitations between specific species. Secondly, according to our time resolution, we demonstrate and explain the band shape of CO’s first overtone and the energy transfer between excited nitrogen and CO, present in our spectra. Lastly, we present theoretical results for the non-LTE modelling of the spectra using bi-temperature approach and a 1D harmonic Franck-Condon approach for the multi-molecule spectra of the formamide decomposition process in the 1800–5600 cm−1 spectral range.

A detailed CO2(1B2) chemiluminescence chemical kinetics model for carbon monoxide and hydrocarbon oxidation

The CO2(1B2) - CO2(X1Σ+g) transition is a source of chemiluminescence from CO and hydrocarbon premixed flames and can be used as a diagnostic; however, its chemistry is not well known due to its broadband nature. Although several attempts have been made to model CO2(1B2) chemiluminescence, none performs well in hydrocarbon flames. We propose a new detailed kinetic model for CO2(1B2) chemiluminescence, based on shock tube experiments in the literature and on opposed flame data presented here. The mechanism consists of 26 reactions which describe the formation of the lower excited state molecule CO2(3B2) (R1), the inter-system crossing reaction between CO2(3B2) and CO2(1B2) (R2), CO2(1B2), the formatting reaction path in hydrocarbon flames (R3), CO2(1B2) radiative quenching (R4) and collisional quenching of CO2(3B2) and CO2(1B2) (R5-R26). The reaction rates constants of R1 and R3 within ± 60% and ± 32% uncertainty, respectively, were determined as follows:CO + O + M = CO2(3B2) + M.k1 = 1 × 1013exp(-10/RT) cm6 mol2 s−1, KJ mol−1.CH + O2 = CO2(1B2) + H.k3 = 8 × 1010 cm3 mol−1 s−1.The mechanism was evaluated against several shock tube experiments at low and elevated pressures and also the CO2(1B2)/OH* chemiluminescent intensity ratio for premixed CH4-air and C3H8-air opposed flames measured in the current study. The comparison showed good agreement for CO2(1B2) temporal profiles for CO-based, CH4-based and C2H4-based mixtures. The prediction of temperature dependence of the CO2(1B2) peak intensity for the CH4-based mixture at both low and elevated pressures was much improved relative to previous models. The CO2(1B2)/OH* chemiluminescent intensity ratio for premixed CH4-air flames predicted by the new model agrees quite well with experiment data, while a small discrepancy remains for C3H8-air flames. Overall, the developed CO2(1B2) chemiluminescence model reproduces, a wide range of experimental data and extends knowledge of CO2(1B2) chemistry.

Spin-Forbidden Carbon-Carbon Bond Formation in Vibrationally Excited α-CO.

Fourier transform infrared spectroscopy of laser-irradiated cryogenic crystals shows that vibrational excitation of CO leads to the production of equal amounts of CO2 and C3O2. The reaction mechanism is explored using electronic structure calculations, demonstrating that the lowest-energy pathway involves a spin-forbidden reaction of (CO)2 yielding C(3P) + CO2. C(3P) then undergoes barrierless recombination with two other CO molecules forming C3O2. Calculated intersystem crossing rates support the spin-forbidden mechanism, showing subpicosecond spin-flipping time scales for a (CO)2 geometry that is energetically consistent with states accessed through vibrational energy pooling. This spin-flip occurs with an estimated ∼4% efficiency; on the singlet surface, (CO)2 reconverts back to CO monomers, releasing heat which induces CO desorption. The discovery that vibrational excitation of condensed-phase CO leads to spin-forbidden C–C bond formation may be important to the development of accurate models of interstellar chemistry.

Scanning Disk Rings and Winds in CO at 0.01–10 au: A High-resolution M-band Spectroscopy Survey with IRTF-iSHELL

Abstract We present an overview and first results from a M-band spectroscopic survey of planet-forming disks performed with iSHELL on the Infrared Telescope Facility, using two slits that provide resolving power R ≈ 60,000–92,000 (5–3.3 km s−1). iSHELL provides a nearly complete coverage at 4.52–5.24 μm in one shot, covering >50 lines from the R and P branches of 12CO and 13CO for each of multiple vibrational levels, and providing unprecedented information on the excitation of multiple emission and absorption components. Some of the most notable new findings of this survey are: (1) the detection of two CO Keplerian rings at <2 au (in HD 259431), (2) the detection of H2O rovibrational lines at 5 μm (in AS 205 N), and (3) the common kinematic variability of CO lines over timescales of 1–14 yr. By homogeneously analyzing this survey together with a previous survey of cooler stars, we discuss a unified view of CO spectra where emission and absorption components scan the disk surface across radii from a dust-free region within dust sublimation out to ≈10 au. We classify two fundamental types of CO line shapes interpreted as emission from Keplerian rings (double-peak lines) and a disk surface plus a low-velocity part of a wind (triangular lines), where CO excitation reflects different emitting regions (and their gas-to-dust ratio) rather than just the irradiation spectrum. A disk+wind interpretation for the triangular lines naturally explains several properties observed in CO spectra, including the line blueshifts, line shapes that turn into narrow absorption at high inclinations, and the frequency of disk winds as a function of the stellar type.

Molecular Gas Excitation of the Massive Dusty Starburst CRLE and the Main-sequence Galaxy HZ10 at z = 5.7 in the COSMOS Field

Abstract We report CO(5 → 4) and CO(6 → 5) line observations in the dusty starbursting galaxy CRLE (z = 5.667) and the main-sequence (MS) galaxy HZ10 (z = 5.654) with the Northern Extended Millimeter Array. CRLE is the most luminous z > 5 starburst in the COSMOS field and HZ10 is the most gas-rich “normal” galaxy currently known at z > 5. We find line luminosities for CO(5 → 4) and CO(6 → 5) of (4.9 ± 0.5) and (3.8 ± 0.4) × 1010 K km s−1 pc2 for CRLE and upper limits of < 0.76 and < 0.60 × 1010 K km s−1 pc2 for HZ10, respectively. The CO excitation of CRLE appears comparable to other z > 5 dusty star-forming galaxies. For HZ10, these line luminosity limits provide the first significant constraints of this kind for an MS galaxy at z > 5. We find the upper limit of L 5 → 4 ′ / L 2 → 1 ′ in HZ10 could be similar to the average value for MS galaxies around z ≈ 1.5, suggesting that MS galaxies with comparable gas excitation may already have existed one billion years after the Big Bang. For CRLE we determine the most likely values for the H2 density, kinetic temperature, and dust temperature based on excitation modeling of the CO line ladder. We also derive a total gas mass of (7.1 ± 1.3) × 1010 M ⊙. Our findings provide some of the currently most detailed constraints on the gas excitation that sets the conditions for star formation in a galaxy protocluster environment at z > 5.

Photodissociation dynamics of CO2 + hv → CO(X1Σ+) + O(1D2) via the 3P1Πu state.

The vacuum ultraviolet (VUV) photodissociation of CO2 is important to understand the primary photochemical processes of CO2 induced by solar VUV excitation in the Earth's atmosphere. Here, we report a detailed study of vibrational-state-specific photodissociation dynamics of the CO(X1Σ+) + O(1D2) channel via the 3P1Πu state by using the time-sliced velocity-mapped ion imaging apparatus combined with the single VUV photoionization detection scheme. By recording the sliced images of the O(1D2) photoproducts formed by VUV photoexcitation of CO2 to the individual vibrational structure of the 3P1Πu state, both the vibrational state distributions of the counterpart CO(X1Σ+) photoproducts and the vibrational-state-specific product anisotropy parameters (β) are determined. The experimental results show that photodissociation of CO2 at 108.22, 107.50, 106.10, and 104.76nm yields less anisotropic (β > 0) and inverted distributed CO(X1Σ+, v) photoproducts. The possible dissociation mechanism for the CO(X1Σ+) + O(1D2) channel may involve the non-adiabatic transition of excited CO2* from the initially prepared state to the 31A' state with potential energy barriers. While at 108.82 and 107.35nm, the vibrational distributions are found to have the population peaked at a low vibrational state, and the anisotropy parameters turn out to be negative. Such variation indicates the possibility of another non-adiabatic dissociation pathway that may involve Coriolis-type coupling to the low-lying dissociative state. These observations show sclear evidence of the influence of the initially vibrational excitations on the photodissociation dynamics of CO2 via the 3P1Πu state.