Diagnostics of the molecular component of PDRs with mechanical heating. II: line intensities and ratios

CO observations in active galactic nuclei and star-bursts reveal high kinetic temperatures. Those environments are thought to be very turbulent due to dynamic phenomena such as outflows and high supernova rates. We investigate the effect of mechanical heating (MH) on atomic fine-structure and molecular lines, and their ratios. We use those ratios as a diagnostic to constrain the amount of MH in an object and also study its significance on estimating the H2 mass. Equilibrium PDRs models were used to compute the thermal and chemical balance for the clouds. The equilibria were solved for numerically using the optimized version of the Leiden PDR-XDR code. Large velocity gradient calculations were done as post-processing on the output of the PDR models using RADEX. High-J CO line ratios are very sensitive to MH. Emission becomes at least one order of magnitude brighter in clouds with n~10^5~cm^-3 and a star formation rate of 1 Solar Mass per year (corresponding to a MH rate of 2 * 10^-19 erg cm^-3 s^-1). Emission of low-J CO lines is not as sensitive to MH, but they do become brighter in response to MH. Generally, for all of the lines we considered, MH increases excitation temperatures and decreases the optical depth at the line centre. Hence line ratios are also affected, strongly in some cases. Ratios involving HCN are a good diagnostic for MH, such as HCN(1-0)/CO(1-0) and HCN(1-0)/HCO^+(1-0). Both ratios increase by a factor 3 or more for a MH equivalent to > 5 percent of the surface heating, as opposed to pure PDRs. The first major conclusion is that low-J to high-J intensity ratios will yield a good estimate of the MH rate (as opposed to only low-J ratios). The second one is that the MH rate should be taken into account when determining A_V or equivalently N_H, and consequently the cloud mass. Ignoring MH will also lead to large errors in density and radiation field estimates.Comment: 38 pages, to appear in A&

Radiative and mechanical feedback into the molecular gas of NGC 253

Starburst galaxies are undergoing intense periods of star formation. Understanding the heating and cooling mechanisms in these galaxies can give us insight to the driving mechanisms that fuel the starburst. Molecular emission lines play a crucial role in the cooling of the excited gas. With SPIRE on the Herschel Space Observatory we have observed the rich molecular spectrum towards the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed and together with low-J line fluxes from ground based observations, these lines trace the excitation of CO. By studying the CO excitation ladder and comparing the intensities to models, we investigate whether the gas is excited by UV radiation, X-rays, cosmic rays, or turbulent heating. Comparing the $^{12}$CO and $^{13}$CO observations to large velocity gradient models and PDR models we find three main ISM phases. We estimate the density, temperature,and masses of these ISM phases. By adding $^{13}$CO, HCN, and HNC line intensities, we are able to constrain these degeneracies and determine the heating sources. The first ISM phase responsible for the low-J CO lines is excited by PDRs, but the second and third phases, responsible for the mid to high-J CO transitions, require an additional heating source. We find three possible combinations of models that can reproduce our observed molecular emission. Although we cannot determine which of these are preferable, we can conclude that mechanical heating is necessary to reproduce the observed molecular emission and cosmic ray heating is a negligible heating source. We then estimate the mass of each ISM phase; $6\times 10^7$ M$_\odot$ for phase 1 (low-J CO lines), $3\times 10^7$ M$_\odot$ for phase 2 (mid-J CO lines), and $9\times 10^6$ M$_\odot$ for phase 3 (high-J CO lines) for a total system mass of $1\times10^{8}$ M$_\odot$

Modelling mechanical heating in star-forming galaxies: CO and 13CO Line ratios as sensitive probes

Computational astrophysic

Constraining cloud parameters using high density gas tracers in galaxies

Far-infrared molecular emission is an important tool used to understand the excitation mechanisms of the gas in the inter-stellar medium of star-forming galaxies. In the present work, we model the emission from rotational transitions with critical densities n >~ 10^4 cm-3. We include 4-3 < J <= 15-14 transitions of CO and 13CO, in addition to J <= 7-6 transitions of HCN, HNC, and HCO+ on galactic scales. We do this by re-sampling high density gas in a hydrodynamic model of a gas-rich disk galaxy, assuming that the density field of the interstellar medium of the model galaxy follows the probability density function (PDF) inferred from the resolved low density scales. We find that in a narrow gas density PDF, with a mean density of ~10 cm-3 and a dispersion \sigma = 2.1 in the log of the density, most of the emission of molecular lines, emanates from the 10-1000 cm-3 part of the PDF. We construct synthetic emission maps for the central 2 kpc of the galaxy and fit the line ratios of CO and 13CO up to J = 15-14, as well as HCN, HNC, and HCO+ up to J = 7-6, using one photo-dissociation region (PDR) model. We attribute the goodness of the one component fits for our model galaxy to the fact that the distribution of the luminosity, as a function of density, is peaked at gas densities between 10 and 1000 cm-3. We explore the impact of different log-normal density PDFs on the distribution of the line-luminosity as a function of density, and we show that it is necessary to have a broad dispersion, corresponding to Mach numbers >~ 30 in order to obtain significant emission from n > 10^4 cm-3 gas. Such Mach numbers are expected in star-forming galaxies, LIRGS, and ULIRGS. By fitting line ratios of HCN(1-0), HNC(1-0), and HCO+(1-0) for a sample of LIRGS and ULIRGS using mechanically heated PDRs, we constrain the Mach number of these galaxies to 29 < M < 77.Comment: 13 pages, 11 figure

Diagnostics of the molecular component of photon-dominated regions with mechanical heating. II. Line intensities and ratios

CO observations in active galactic nuclei and starbursts reveal high kinetic temperatures. Those environments are thought to be very turbulent due to dynamic phenomena, such as outflows and high supernova rates. We investigate the effect of mechanical heating on atomic fine-structure and molecular lines and on their ratios. We try to use those ratios as a diagnostic to constrain the amount of mechanical heating in an object and also study its significance on estimating the H2 mass. Equilibrium photodissociation models (PDRs hereafter) were used to compute the thermal and chemical balance for the clouds. The equilibria were solved for numerically using the optimized version of the Leiden PDR-XDR code. Large velocity-gradient calculations were done as post-processing on the output of the PDR models using RADEX. High-J CO line ratios are very sensitive to mechanical heating (γmech hereafter). These emission becomes at least one order of magnitude brighter in clouds with n ~ 105 cm-3 and a star formation rate of 1M⊙ yr-1 (corresponding to γmech = 2 × 10-19 erg cm-3 s-1). The emission of low-J CO lines is not as sensitive to γmech, but they do become brighter in response to γmech. Generally, for all of the lines we considered, γmech increases excitation temperatures and decreases the optical depth at the line centre. Hence line ratios are also affected, strongly in some cases. Ratios involving HCN are a good diagnostic for γmech, where the HCN(1–0)/CO(1–0) increases from 0.06 to 0.25, and the HCN(1–0)/HCO+(1–0) increase from 0.15 to 0.5 for amounts of γmech that are equivalent to 5% of the surface heating rate. Both ratios increase to more than 1 for higher γmech, as opposed to being much less than unity in pure PDRs. The first major conclusion is that low-J to high-J intensity ratios will yield a good estimate of the mechanical heating rate (as opposed to only low-J ratios). The second one is that the mechanical heating rate should be taken into account when determing AV or, equivalently, NH, and consequently the cloud mass. Ignoring γmech will also lead to large errors in density and radiation field estimates. Key words: galaxies: ISM / photon-dominated region / turbulence / ISM: molecules / ISM: clouds    Interstellar matter and star formationGalaxie

Diagnostics of the Molecular Component of PDRs with Mechanical Heating

Context. Multitransition CO observations of galaxy centers have revealed that significant fractions of the dense circumnuclear gas have high kinetic temperatures, which are hard to explain by pure photon excitation, but may be caused by dissipation of turbulent energy. Aims. We aim to determine to what extent mechanical heating should be taken into account while modelling PDRs. To this end, the effect of dissipated turbulence on the thermal and chemical properties of PDRs is explored. Methods. Clouds are modelled as 1D semi-infinite slabs whose thermal and chemical equilibrium is solved for using the Leiden PDR-XDR code. Results. In a steady-state treatment, mechanical heating seems to play an important role in determining the kinetic temperature of the gas in molecular clouds. Particularly in high-energy environments such as starburst galaxies and galaxy centers, model gas temperatures are underestimated by at least a factor of two if mechanical heating is ignored. The models also show that CO, HCN and H2 O column densities increase as a function of mechanical heating. The HNC/HCN integrated column density ratio shows a decrease by a factor of at least two in high density regions with n \sim 105 cm-3, whereas that of HCN/HCO+ shows a strong dependence on mechanical heating for this same density range, with boosts of up to three orders of magnitude. Conclusions. The effects of mechanical heating cannot be ignored in studies of the molecular gas excitation whenever the ratio of the star formation rate to the gas density is close to, or exceeds, 7 \times 10-6 M yr-1 cm4.5 . If mechanical heating is not included, predicted column densities are underestimated, sometimes even by a few orders of magnitude. As a lower bound to its importance, we determined that it has non-negligible effects already when mechanical heating is as little as 1% of the UV heating in a PDR.Comment: 26 pages, 14 figures in the text and 13 figures as supplementary material. Accepted for publication in A&

Constraining cloud parameters using high density gas tracers in galaxies

Galaxie

Physiological constraints dictate toxin spatial heterogeneity in snake venom glands

Background: Venoms are ecological innovations that have evolved numerous times, on each occasion accompanied by the co-evolution of specialised morphological and behavioural characters for venom production and delivery. The close evolutionary interdependence between these characters is exemplified by animals that control the composition of their secreted venom. This ability depends in part on the production of different toxins in different locations of the venom gland, which was recently documented in venomous snakes. Here, we test the hypothesis that the distinct spatial distributions of toxins in snake venom glands are an adaptation that enables the secretion of venoms with distinct ecological functions. Results: We show that the main defensive and predatory peptide toxins are produced in distinct regions of the venom glands of the black-necked spitting cobra (Naja nigricollis), but these distributions likely reflect developmental effects. Indeed, we detected no significant differences in venom collected via defensive ‘spitting’ or predatory ‘biting’ events from the same specimens representing multiple lineages of spitting cobra. We also found the same spatial distribution of toxins in a non-spitting cobra and show that heterogeneous toxin distribution is a feature shared with a viper with primarily predatory venom. Conclusions: Our findings suggest that heterogeneous distributions of toxins are not an adaptation to controlling venom composition in snakes. Instead, it likely reflects physiological constraints on toxin production by the venom glands, opening avenues for future research on the mechanisms of functional differentiation of populations of protein-secreting cells within adaptive contexts

Machine-learning guided Venom Induced Dermonecrosis Analysis tooL: VIDAL

Snakebite envenoming is a global public health issue that causes significant morbidity and mortality, particularly in low-income regions of the world. The clinical manifestations of envenomings vary depending on the snake's venom, with paralysis, haemorrhage, and necrosis being the most common and medically relevant effects. To assess the efficacy of antivenoms against dermonecrosis, a preclinical testing approach involves in vivo mouse models that mimic local tissue effects of cytotoxic snakebites in humans. However, current methods for assessing necrosis severity are time-consuming and susceptible to human error. To address this, we present the Venom Induced Dermonecrosis Analysis tooL (VIDAL), a machine-learning-guided image-based solution that can automatically identify dermonecrotic lesions in mice, adjust for lighting biases, scale the image, extract lesion area and discolouration, and calculate the severity of dermonecrosis. We also introduce a new unit, the dermonecrotic unit (DnU), to better capture the complexity of dermonecrosis severity. Our tool is comparable to the performance of state-of-the-art histopathological analysis, making it an accessible, accurate, and reproducible method for assessing dermonecrosis in mice. Given the urgent need to address the neglected tropical disease that is snakebite, high-throughput technologies such as VIDAL are crucial in developing and validating new and existing therapeutics for this debilitating disease