High-density Tracers Research Articles

Deuterium fractionation of the starless core L 1498

Molecular deuteration is commonly seen in starless cores and is expected to occur on a timescale comparable to that of the core contraction. Thus, the deuteration serves as a chemical clock, allowing us to investigate dynamical theories of core formation. We aim to provide a 3D cloud description for the starless core L\,1498 located in the nearby low-mass star-forming region Taurus and explore its possible core formation mechanism. We carried out nonlocal thermal equilibrium radiative transfer with multi-transition observations of the high-density tracer to derive the density and temperature profiles of the L\,1498 core. By combining these observations with the spectral observations of the deuterated species, ortho- and we derived the abundance profiles for the observed species and performed chemical modeling of the deuteration profiles across L\,1498 to constrain the contraction timescale. We present the first ortho- detection toward L\,1498. We find a peak molecular hydrogen density of $ cm$^ $, a temperature of 7.5$_ $ K, and a deuteration of $ in the center. We derived a lower limit of the core age for L\,1498 of 0.16 Ma, which is compatible with the typical free-fall time, indicating that L\,1498 likely formed rapidly.

Relative alignment between gas structures and magnetic field in Orion A at different scales using different molecular gas tracers

Context. Magnetic fields can play a crucial role in high-mass star formation. Nonetheless, the significance of magnetic fields at various scales and their relationship with gas structures have been largely overlooked. Aims. Our goal is to examine the relationship between the magnetic field and molecular gas structures within the Orion A giant molecular cloud at different scales and density regimes. Methods. We assessed the gas intensity structures and column densities in Orion A using 12CO, 13CO, and C18O from Nobeyama observations. By comparing Nobeyama observations with Planck polarization observations on large scales (~0.6 pc) and JCMT polarization observations on small scales (~0.04 pc), we investigate how the role of magnetic fields changes with scale and density. Results. We find a similar trend from parallel to perpendicular alignment with increasing column density in Orion A at both large and small spatial scales. In addition, when changing from low-density to high-density tracers, the relative orientation preference changes from random to perpendicular. The self-similar results at different scales indicate that magnetic fields are dynamically important in both cloud formation and filament formation. However, magnetic field properties at small scales are relative complicated, and the interplay between magnetic field and star-forming activity needs to be discussed case by case.

HCN as a probe of the inner disc in a candidate proto-brown dwarf

ABSTRACT The detection of Keplerian rotation is rare among Class 0 protostellar systems. We have investigated the high-density tracer HCN as a probe of the inner disc in a Class 0 proto-brown dwarf candidate. Our ALMA high angular resolution observations show the peak in the HCN (3–2) line emission arises from a compact component near the proto-brown dwarf with a small bar-like structure and a deconvolved size of ∼50 au. Radiative transfer modelling indicates that this HCN feature is tracing the innermost, dense regions in the proto-brown dwarf where a small Keplerian disc is expected to be present. The limited velocity resolution of the observations, however, makes it difficult to confirm the rotational kinematics of this feature. A brightening in the HCN emission towards the core centre suggests that HCN can survive in the gas phase in the inner, dense regions of the proto-brown dwarf. In contrast, modelling of the HCO+ (3–2) line emission indicates that it originates from the outer pseudo-disc/envelope region and is centrally depleted. HCN line emission can reveal the small-scale structures and can be an efficient observational tool to study the inner disc properties in such faint compact objects where spatially resolving the disc is nearly impossible.

Surveys of clumps, cores, and condensations in Cygnus-X

Context.Theories and models have suggested that circumstellar disks could channel material to the central protostar, and resist star formation feedback. Our current knowledge of the picture and role of disks around massive protostars is unclear because the observational evidence of these circumstellar disks is limited.Aims.To investigate whether disk-mediated accretion is the primary mechanism in high-mass star formation, we have established a survey of a large sample of massive dense cores within a giant molecular cloud.Methods.We used high angular resolution (~1.8″) observations with SMA to study the dust emission and molecular line emission of about 50 massive dense cores in Cygnus-X. At a typical distance of 1.4 kpc for Cygnus-X, these massive dense cores are resolved into ~2000 au condensations. We combined the CO outflow emission and gas kinematics traced by several high-density tracers to search for disk candidates.Results.We extracted hundreds of dust condensations from the SMA 1.3 mm dust continuum emission. The CO data show bipolar or unipolar outflow signatures toward 49 dust condensations. Among them, only 27 sources are detected in dense gas tracers, which reveals the gas kinematics, and nine sources show evidence of rotating envelopes, suggesting the existence of embedded accretion disks. The position-velocity diagrams along the velocity gradient of all rotating condensations suggest that four condensations are possible to host Keplerian-like disks.Conclusions.A detailed investigation of the 27 sources detected in dense gas tracers suggests that the nine disk candidates are at earlier evolutionary stages compared to the remaining 18 sources. Non-detection of rotating disks in our sample may be due to several factors, including an unknown inclination angle of the rotation axis and an early evolutionary stage of the central source, and the latter could be important, considering that young and powerful outflows could confuse the observational evidence for rotation. The detection rate of disk candidates in our sample is 1/3, which confirms that disk accretion is a viable mechanism for high-mass star formation, although it may not be the only one.

Deuterium fractionation in cold dense cores in the low-mass star-forming region L1688

ABSTRACT In this work, we study deuterium fractionation in four starless cores in the low-mass star-forming region L1688 in the Ophiuchus molecular cloud. We study how the deuterium fraction (RD) changes with environment, compare deuteration of ions and neutrals, core centre and its envelope, and attempt to reproduce the observed results with a gas–grain chemical model. We chose high and low gas density tracers to study both core centre and the envelope. With the IRAM 30 m antenna, we mapped N2H+(1–0), N2D+(1–0), H13CO+ (1–0) and (2–1), DCO+(2–1), and p-NH2D(111–101) towards the chosen cores. The missing p-NH3 and N2H+(1–0) data were taken from the literature. To measure the molecular hydrogen column density, dust and gas temperature within the cores, we used the Herschel/SPIRE dust continuum emission data, the Green Bank Ammonia Survey data (NH3), and the COMPLETE survey data to estimate the upper limit on CO depletion. We present the deuterium fraction maps for three species towards four starless cores. Deuterium fraction of the core envelopes traced by DCO+/H13CO+ is one order of magnitude lower (∼0.08) than that of the core central parts traced by the nitrogen-bearing species (∼0.5). Deuterium fraction increases with the gas density as indicated by high deuterium fraction of high gas density tracers and low deuterium fraction of lower gas density tracers and by the decrease of RD with core radii, consistent with the predictions of the chemical model. Our model results show a good agreement with observations for RD(N2D+/N2H+) and RD(DCO+/HCO+) and underestimate the RD(NH2D/NH3).

ALMA imaging of the cold molecular and dusty disk in the type 2 active nucleus of the Circinus galaxy

Context. The central engines of many active galactic nuclei (AGNs) are thought to be surrounded by warm molecular and dusty material in an axisymmetric geometry, thus explaining part of the observational diversity of active nuclei. Aims. We aim to shed light on the physical properties and kinematics of the molecular material in the nucleus of one of the closest type 2 active galaxies. Methods. To this end, we obtained high angular resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of the nucleus of the Circinus galaxy. The observations map the emission at 350 GHz and 690 GHz with spatial resolutions of ∼3.8 pc and ∼2.2 pc, respectively. Results. The continuum emission traces cold (T ≲ 100 K) dust in a circumnuclear disk with spiral arms on scales of 25 pc, plus a marginally resolved nuclear emission peak. The latter is not extended in polar direction as claimed based on earlier ALMA observations. A significant amount (of the order of 40%) of the 350 GHz emission is not related to dust, but most likely free-free emission instead. We detect CO(3−2) and CO(6−5) as well as HCO+(4−3), HCN(4−3), and CS(4−3). The CO emission is extended, showing a spiral pattern, similar to the extended dust emission. Towards the nucleus, CO is excited to higher transitions and its emission is self-absorbed, leading to an apparent hole in the CO(3−2) but not the CO(6−5) emission. On the other hand, the high gas density tracers HCO+, HCN, and CS show a strong, yet unresolved (≲4 pc) concentration of the emission at the nucleus, pointing at a very small ‘torus’. The kinematics are dominated by rotation and point at a geometrically thin disk down to the resolution limit of our observations. In contrast to several other AGNs, no HCN enhancement is found towards the nucleus. Conclusions. The Circinus nucleus is therefore composed of at least two distinct components: (1) an optically thin, warm outflow of ionised gas containing clouds of dust which are responsible for the polar mid-infrared emission, but not seen at submillimetre wavelengths; and (2) a cold molecular and dusty disk, traced by submillimetre continuum and line emission. The latter is responsible for the bulk of the obscuration of the nucleus. These findings support the most recent radiative transfer calculations of the obscuring structures in AGNs, which find a similar two-component structure.

Down but Not Out: Properties of the Molecular Gas in the Stripped Virgo Cluster Early-type Galaxy NGC 4526

We present Atacama Large Millimeter/submillimeter Array data on the 3 mm continuum emission, CO isotopologues (12CO, 13CO, and C18O), and high-density molecular tracers (HCN, HCO+, HNC, HNCO, CS, CN, and CH3OH) in NGC 4526. These data enable a detailed study of the physical properties of the molecular gas in a longtime resident of the Virgo Cluster; comparisons to more commonly studied spiral galaxies offer intriguing hints into the processing of molecular gas in the cluster environment. Many molecular line ratios in NGC 4526, along with our inferred abundances and CO/H2 conversion factors, are similar to those found in nearby spirals. One striking exception is the very low observed 12CO/13CO(1−0) line ratio, 3.4 ± 0.3, which is unusually low for spirals though not for Virgo Cluster early-type galaxies. We carry out radiative transfer modeling of the CO isotopologues with some archival (2−1) data, and we use Bayesian analysis with Markov Chain Monte Carlo techniques to infer the physical properties of the CO-emitting gas. We find surprisingly low [12CO/13CO] abundance ratios of and at radii of 0.4 kpc and 1 kpc. The emission from the high-density tracers HCN, HCO+, HNC, CS, and CN is also relatively bright, and CN is unusually optically thick in the inner parts of NGC 4526. These features hint that processing in the cluster environment may have removed much of the galaxy’s relatively diffuse, optically thinner molecular gas along with its atomic gas. Angular momentum transfer to the surrounding intracluster medium may also have caused contraction of the disk, magnifying radial gradients such as we find in [13CO/C18O]. More detailed chemical evolution modeling would be interesting in order to explore whether the unusual [12CO/13CO] abundance ratio is entirely an environmental effect or whether it also reflects the relatively old stellar population in this early-type galaxy.

The sharp ALMA view of infall and outflow in the massive protocluster G31.41+0.31

Context.To better understand the formation of high-mass stars, it is fundamental to investigate how matter accretes onto young massive stars, how it is ejected, and how all this differs from the low-mass case. The massive protocluster G31.41+0.31 is the ideal target to study all these processes because observations at millimeter and centimeter wavelengths have resolved the emission of the Main core into at least four massive dust continuum sources, named A, B, C, and D, within 1″ or 0.018 pc, and have identified signatures of infall and several outflows associated with the core.Aims.We study the interplay between infall and outflow in G31.41+0.31, at a spatial resolution of a few 100 au, by investigating their properties and their possible impact on the core.Methods.We carried out molecular line observations of typical high-density tracers, such as CH3CN or H2CO, and shock and outflow tracers, such as SiO, with ALMA at 1.4 mm that achieved an angular resolution of ~0.′′09 (~340 au).Results.The observations reveal inverse P Cygni profiles in CH3CN and H2CO toward the four sources embedded in the Main core, suggesting that all of them are undergoing collapse. The infall rates estimated from the redshifted absorption are on the order of ~10−2M⊙yr−1. The individual infall rates imply that the accretion timescale of the Main core is an order of magnitude smaller than its rotation timescale. This confirms that rotating toroids such as the G31 Main core are non-equilibrium, transient collapsing structures that need to be constantly replenished with fresh material from a large-scale reservoir. For sources B, C, and D the infall could be accelerating inside the sources, while for source A the presence of a second emission component complicates the interpretation. The SiO observations have revealed the presence of at least six outflows in the G31.41+0.31 star-forming region, and suggest that each of the four sources embedded in the Main core drives a molecular outflow. The outflow rates are on the order of ~10−5–10−4M⊙yr−1, depending on the SiO abundance. The feedback of the outflows appears sufficient to sustain the turbulence in the core and to eventually disrupt the core or prevent its further collapse. The mass accretion rates onto the individual sources, estimated from the highest values of the outflow mass loss rates, are on the order of 10−4M⊙yr−1. The difference of about two orders of magnitude between the accretion rates and the infall rates suggests that the central stars and the jets and outflows might not be efficient in removing disk material, which could lead to gravitational instabilities, fragmentation of the disk, and episodic accretion events.Conclusions.Infall and outflows are simultaneously present in all four sources embedded in the high-mass star-forming core G31.41+0.31. This indicates that these sources are still actively accreting and have not reached their final mass yet, which confirms the youth of this massive core.

Interaction of Active Janus Colloids with Tracers.

Understanding the motion of artificial active swimmers in complex surroundings, such as a dense bath of passive particulate matter, is essential for their successful utilization as cargo (drug) carriers and sensors or for medical imaging, under microscopic domains. In this study, we experimentally investigated the motion of active SiO2-Pt Janus particles (JPs) in a two-dimensional bath of smaller silica tracers dispersed with varying areal densities. Our observations indicate that when an active JP undergoes a collision with an isolated tracer, their interaction can have a significant impact on the swimmer's motion. However, the overall impact of tracers on the active JPs' motion (translation and rotation) depends on the frequency of collisions and also on the nature of the collision, which is marked by the time-duration for which the particles maintain contact during the collisions. Further, in the high-density tracer bath, our experiments reveal that the motion of the active JP results in a novel organizational behavior of the tracers on the trailing Pt (depletion of tracers) and the leading SiO2 (accumulation of tracers) side. In laboratory frame the emergence and the subsequent vanishing of the depletion zone are discussed in detail.

Gas properties in the Early Universe deciphered from spectral line surveys of high-z objects: The Cloverleaf Quasar

Molecular lines observed in remote quasars arise from exceptionnally strong starburst regions in the vicinity of the QSO. Their study offers a unique opportunity to probe the gas physical and chemical properties at the epoch of peak star formation (z=2-3). We report the preliminary results of an on-going mm/sub-mm spectral line survey of the Cloverleaf Quasar (z=2.6), the archetype of gravitationally lensed quasars. We identify 14 molecular species, including rare isotopologues of CO and HCO+. The high gas density tracers HCO+, HCN, HNC and CN are detected through several rotational transitions, up to J=8-7 (Eup/k ≃ 155 K). Their emission appears to arise from dense (> 106 cm−3) gas clumps embedded in a circumnuclear disk with a flat rotation curve.

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): Detection of a Dense SiO Jet in the Evolved Protostellar Phase

Abstract Jets and outflows trace the accretion history of protostars. High-velocity molecular jets have been observed from several protostars in the early Class 0 phase of star formation, detected with the high-density tracer SiO. Until now, no clear jet has been detected with SiO emission from isolated evolved Class I protostellar systems. We report a prominent dense SiO jet from a Class I source G205S3 (HOPS-315: T bol ∼ 180 K, spectral index ∼0.417), with a moderately high mass-loss rate (∼0.59 × 10−6 M ⊙ yr−1) estimated from CO emission. Together, these features suggest that G205S3 is still in a high-accretion phase, similar to that expected of Class 0 objects. We compare G205S3 to a representative Class 0 system G206W2 (HOPS-399) and literature Class 0/I sources to explore the possible explanations behind the SiO emission seen at the later phase. We estimate a high inclination angle (∼40°) for G205S3 from CO emission, which may expose the infrared emission from the central core and mislead the spectral classification. However, the compact 1.3 mm continuum, C18O emission, location in the bolometric luminosity to submillimeter fluxes diagram, outflow force (∼3.26 × 10−5 M ⊙ km s−1 yr−1) are also analogous to that of Class I systems. We thus consider G205S3 to be at the very early phase of Class I, and in the late phase of high accretion. The episodic ejection could be due to the presence of an unknown binary, a planetary companion, or dense clumps, where the required mass for such high accretion could be supplied by a massive circumbinary disk.

The IRAM-30m EMPIRE Nearby Galaxy Dense Gas Survey

This work presents an overview of the main results from the EMIR Multi-Line Probe of the ISM Regulating Galaxy Evolution (EMPIRE) survey, an IRAM-30m Large Program (∼ 500 h) which observed tracers of high density molecular gas across the disks of nine nearby, star-forming galaxies. EMPIRE is the first comprehensive and systematic study mapping high-density tracers including HCN, HCO+ and HNC, as well as optically thin J = 1−0 transitions of 13CO and C18O. Such a combination of spectroscopic tracers offers the best way to study cold, immediately star-forming gas to address how dense gas fractions and star formation efficiencies vary across and among galaxies. The extensive and sensitive data collected from EMPIRE has allowed us to relate the fraction of star-forming gas and its ability to form stars to local interstellar medium (ISM) and dynamical conditions, such as stellar surface densities, ISM dynamical pressure or molecular gas surface densities. The main results from EMPIRE show that the star formation efficiency in the dense gas varies systematically in all galactic disks. Therefore, this provides support for a context-dependent role of gas density, where dense gas fractions follow interstellar pressure, but star formation only takes place in local over-densities. The EMPIRE survey has successfully turned into a stepping stone for on-going and future projects, aiming to link the large-scale EMPIRE extragalactic results to high-resolution measurements, accessible from our Milky Way.

Determination of Suitable Imaging Techniques for the Investigation of the Bonding Zones of Asphalt Layers.

The material behavior of asphalt depends on its composition of aggregate, bitumen, and air voids. Asphalt pavements consist of multiple layers, making the interaction of the materials at the layer boundary important so that any stresses that occur can be relieved. The material behavior at the layer boundary is not yet understood in detail, as further methods of analysis are lacking in addition to mechanical methods. For this reason, the layer boundary of asphalt structures was analyzed using imaging methods. The aim of this research was to find an imaging method that allows a detailed analysis of the bonding zone of asphalt layers. Two different imaging techniques were used for this purpose. One is a 2-D imaging technique (asphalt petrology) and the other is a 3-D imaging technique (high-resolution computed tomography). Image analysis is a widely used technique in materials science that allows to analyze the material behavior and their composition. In this research, attention was paid to the analysis of the position of the bitumen emulsion, because the contained bitumen is supposed to bond the layers together. It was found that the application of 2-D imaging (asphalt petrology) lacked the precision for a detailed analysis of the individual materials at the layer boundary. With high-resolution computed tomography, a detailed view is possible to visualize the individual materials at the layer boundary in 3D. However, it is difficult to differentiate the materials because there are no gradations in the gray values due to the identical densities. However, it is possible to differentiate between the bitumen from the asphalt and from the emulsion if a high-density tracer is added to the bitumen emulsion for the CT studies. The results of the investigations are presented in this article.

Molecular line ratio diagnostics and gas kinematics in the AGN host Seyfert galaxy NGC 5033

ABSTRACT Multiple molecular lines are useful for studying the physical properties of multiphase star-forming gas in different types of galaxies. We probe the molecular gas throughout the disc of the spiral galaxy NGC 5033, hosting an active galactic nucleus (AGN), using multiple low-J CO lines [12CO(1–0, 2–1, 3–2 and 13CO(1–0, 2–1)] and dense gas tracers [HCN(1–0) and HCO+(1–0)]. First, we determine the ratios of the integrated intensity maps and the ratio of intensities in position velocity diagrams. Secondly, we obtain the ratios of CO lines and high-density tracers at the centre; and thirdly, we model these line ratios using a radiative transfer code. Line ratio diagnostics reveal that the south of the gaseous disc contains cooler gas than the northern part, and the centre hosts warmer and less tenuous gas with a similar dense gas fraction compared to most galaxies of similar type. Our model results mostly agree with the empirical ones in the sense that the central region of NGC 5033 harbours warmer gas than that in the centres of normal spirals and lenticulars without showing AGN activity. Finally, the beam-averaged total molecular gas mass and gas surface density along the galaxy’s major axis show a radial gradient, i.e. increasing from the outskirts up to the central region of size 1 kpc where there is a depression in both gas mass and surface density.

The Evolution of NGC 7465 as Revealed by Its Molecular Gas Properties

Abstract We present ALMA observations of CO isotopologues and high-density molecular tracers (HCN, HCO+, CN, etc.) in NGC 7465, an unusually gas-rich early-type galaxy that acquired its cold gas recently. In the inner 300 pc, the molecular gas kinematics are misaligned with respect to all other galaxy components; as the gas works its way inward, it is torqued into polar orbits about the stellar kinematically decoupled core (KDC), indicating that the stellar KDC is not related to the current gas accretion event. The galaxy also exhibits unusually high 12CO/13CO line ratios in its nucleus but typical 13CO/C18O ratios. Our calculations show that this result does not necessarily indicate an unusual [12CO/13CO] abundance ratio but rather that 12CO (1–0) is optically thin due to high temperatures and/or large line widths associated with the inner decoupled, misaligned molecular structure. Line ratios of the higher-density tracers suggest that the densest phase of molecular gas in NGC 7465 has a lower density than is typical for nearby galaxies, possibly as a result of the recent gas accretion. All of the observed molecular properties of NGC 7465 are consistent with it having acquired its molecular (and atomic) gas from a spiral galaxy. Further detailed studies of the CO isotopologues in other early-type galaxies would be valuable for investigating the histories of those that may have acquired their gas from dwarfs. Finally, these ALMA data also show an unidentified line source that is probably a background galaxy similar to those found at z = 1–3 in blind CO surveys.

Physical characterization of S169: a prototypical IR bubble associated with the massive star-forming region IRAS 12326-6245

Aims. With the aim of studying the physical properties of Galactic IR bubbles and to explore their impact in massive star formation, we present a study of the IR bubble S169, associated with the massive star forming region IRAS 12326-6245. Methods. We used CO (2–1),13CO (2–1), C18O (2–1), HCN (3–2), and HCO+ (3–2) line data obtained with the APEX telescope using the on-the-fly full sampling technique to study the properties of the molecular gas in the nebula and the IRAS source. To analyze the properties and distribution of the dust, we made use of images obtained from the IRAC-GLIMPSE, Herschel, and ATLASGAL archives. The properties of the ionized gas in the nebula were studied using radio continuum and Hα images obtained from the SUMSS survey and SuperCOSMOS database, respectively. In our search for stellar and protostellar objects in the region, we used point source calalogs obtained from the MSX, WISE, GLIMPSE, 2MASS, AAVSO, ASCC-2.5V3, and GAIA databases. Results. The new APEX observations allowed us to identify three molecular components, each one associated with different regions of the nebula, namely: at −39 km s−1 (component A), −25 km s−1 (component B), and −17 km s−1 (component C). Component A is shown to be the most dense and clumpy. Six molecular condensations (MC1 to MC6) were identified in this component, with MC3 (the densest and more massive one) being the molecular counterpart of IRAS 12326-6245. For this source, we estimated an H2 column density up to 8 × 1023 cm−2. An LTE analysis of the high density tracer lines HCO+ (3–2) and HCN (3–2) on this source, assuming 50 and 150 K, respectively, indicates column densities of N(HCO+) = (5.2 ± 0.1) × 1013 cm−2 and N(HCN) = (1.9 ± 0.5) × 1014 cm−2. To explain the morphology and velocity of components A, B, and C, we propose a simple model consisting of a partially complete semisphere-like structure expanding at ~12 km s−1. The introduction of this model has led to a discussion about the distance to both S169 and IRAS 12326-6245, which was estimated to be ~2 kpc. Several candidate YSOs were identified, projected mostly onto the molecular condensations MC3, MC4, and MC5, which indicates that the star-formation process is very active at the borders of the nebula. A comparison between observable and modeled parameters was not enough to discern whether the collect-and-collapse mechanism is acting at the edge of S169. However, other processes such as radiative-driven implosion or even a combination of both mechanisms, namely, collect-and-collapse and radiative-driven implosion, could be acting simultaneously in the region.

Planck’s Dusty GEMS

We present ALMA, NOEMA, and IRAM-30 m/EMIR observations of the high-density tracer molecules HCN, HCO+, and HNC in three of the brightest lensed dusty star-forming galaxies at z ≃ 3–3.5, part of the Planck’s Dusty Gravitationally Enhanced subMillimetre Sources (GEMS), with the aim of probing the gas reservoirs closely associated with their exceptional levels of star formation. We obtained robust detections of ten emission lines between Jup = 4 and 6, as well as several additional upper flux limits. In PLCK_G244.8+54.9, the brightest source at z = 3.0, the HNC(5–4) line emission at 0.1″ resolution, together with other spatially-integrated line profiles, suggests comparable distributions of dense and more diffuse gas reservoirs, at least over the most strongly magnified regions. This rules out any major effect from differential lensing. This line is blended with CN(4–3) and in this source, we measure a HNC(5–4)/CN(4–3) flux ratio of 1.76 ±0. 86. Dense-gas line profiles generally match those of mid-J CO lines, except in PLCK_G145.2+50.8, which also has dense-gas line fluxes that are relatively lower, perhaps due to fewer dense cores and more segregated dense and diffuse gas phases in this source. The HCO+/HCN ≳ 1 and HNC/HCN ∼ 1 line ratios in our sample are similar to those of nearby ultraluminous infrared galaxies (ULIRGs) and consistent with photon-dominated regions without any indication of important mechanical heating or active galactic nuclei feedback. We characterize the dense-gas excitation in PLCK_G244.8+54.9 using radiative transfer models assuming pure collisional excitation and find that mid-J HCN, HCO+, and HNC lines arise from a high-density phase with an H2 density of n ∼ 105–106 cm−3, although important degeneracies hinder a determination of the exact conditions. The three GEMS are consistent with extrapolations of dense-gas star-formation laws derived in the nearby Universe, adding further evidence that the extreme star-formation rates observed in the most active galaxies at z ∼ 3 are a consequence of their important dense-gas contents. The dense-gas-mass fractions traced by HCN/[CI] and HCO+/[CI] line ratios are elevated, but not exceptional as compared to other lensed dusty star-forming galaxies at z > 2, and they fall near the upper envelope of local ULIRGs. Despite the higher overall gas fractions and local gas-mass surface densities observed at high redshift, the dense-gas budget of rapidly star-forming galaxies seems to have evolved little between z ∼ 3 and z ∼ 0. Our results favor constant dense-gas depletion times in these populations, which is in agreement with theoretical models of star formation.

ATLASGAL-selected massive clumps in the inner Galaxy

Aims. We study ten molecular transitions obtained from an unbiased 3 mm molecular line survey using the IRAM 30 m telescope toward 409 compact dust clumps identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) to better understand the photodissociation regions (PDRs) associated with these clumps. The main goal of this study is to investigate whether the abundances of the selected molecules show any variations resulting from the PDR chemistry in different clump environments. Methods. We selected HCO, HOC+, C2H, c-C3H2, CN, H13CN, HC15N, and HN13C as PDR tracers, and H13CO+ and C18O as dense gas tracers. By using estimated optical depths of C2H and H13CN and assuming optically thin emission for other molecular transitions, we derived the column densities of those molecules and their abundances. To assess the influence of the presence and strength of ultra-violet radiation, we compare abundances of three groups of the clumps: HII regions, infrared bright non-HII regions, and infrared dark non-HII regions. Results. We detected C18O, H13CO+, C2H, c-C3H2, CN, and HN13C toward most of the observed dust clumps (detection rate >94%), and H13CN is also detected with a detection rate of 75%. On the other hand, HCO and HC15N show detection rates of 32 and 39%, respectively, toward the clumps, which are mostly associated with HII region sources: detection rates of HCO and HC15N toward the HII regions are 66 and 79%. We find that the abundances of HCO, CN, C2H, and c-C3H2 decrease as the H2 column density increases, indicating high visual extinction, while those of high-density tracers (i.e., H13CO+ and HC15N) are constant. In addition, N(HCO)/N(H13CO+) ratios significantly decrease as H2 column density increases, and, in particular, 82 clumps have X(HCO) ≳ 10−10 and N(HCO)/N(H13CO+) ≳ 1, which are indications of far-ultraviolet (FUV) chemistry. This suggests the observed HCO abundances are likely associated with FUV radiation illuminating the PDRs. We also find that high N(c-C3H2)/N(C2H) ratios found for HII regions that have high HCO abundances (≳10−10) are associated with more evolved clumps with high Lbol/Mclump. This trend might be associated with grain-surface processes, which determine the initial abundances of these molecules, and time-dependent effects in the clumps corresponding to the envelopes around dense PDRs and HII regions. In addition, some fraction of the measured abundances of the small hydrocarbons of the HII sources may be the result of the photodissociation of PAH molecules.

The multi-phase ISM in the nearby composite AGN-SB galaxy NGC 4945: large-scale (parsecs) mechanical heating

Context. Understanding the dominant heating mechanism in the nuclei of galaxies is crucial to understanding star formation in starbursts (SBs), active galactic nuclei (AGN) phenomena, and the relationship between star formation and AGN activity in galaxies. Analysis of the carbon monoxide (12CO) rotational ladder versus the infrared continuum emission (hereafter, 12CO/IR) in galaxies with different types of activity reveals important differences between them. Aims. We aim to carry out a comprehensive study of the nearby composite AGN-SB galaxy, NGC 4945, using spectroscopic and photometric data from the Herschel satellite. In particular, we want to characterize the thermal structure in this galaxy using a multi-transition analysis of the spatial distribution of the 12CO emission at different spatial scales. We also want to establish the dominant heating mechanism at work in the inner region of this object at smaller spatial scales (≲200 pc). Methods. We present far-infrared (FIR) and sub-millimeter (sub-mm) 12CO line maps and single spectra (from Jup = 3 to 20) using the Heterodyne Instrument for the Far Infrared (HIFI), the Photoconductor Array Camera and Spectrometer (PACS), and the Spectral and Photometric Imaging REceiver (SPIRE) onboard Herschel, and the Atacama Pathfinder EXperiment (APEX). We combined the 12CO/IR flux ratios and the local thermodynamic equilibrium (LTE) analysis of the 12CO images to derive the thermal structure of the interstellar medium (ISM) for spatial scales raging from ≲200 pc to 2 kpc. In addition, we also present single spectra of low- (12CO, 13CO and [CI]) and high-density (HCN, HNC, HCO+, CS and CH) molecular gas tracers obtained with APEX and HIFI applying LTE and non-LTE (NLTE) analyses. Furthermore, the spectral energy distribution of the continuum emission from the FIR to sub-mm wavelengths is also presented. Results. From the NLTE analysis of the low- and high-density tracers, we derive gas volume densities (103–106 cm−3) for NGC 4945 that are similar to those found in other galaxies with different types of activity. From the 12CO analysis we find a clear trend in the distribution of the derived temperatures and the 12CO/IR ratios. It is remarkable that at intermediate scales (360 pc–1 kpc, or 19″–57″) we see large temperatures in the direction of the X-ray outflow while at smaller scales (≲200 pc–360 pc, or ∼9″–19″), the highest temperature, derived from the high-J lines, is not found toward the nucleus but toward the galaxy plane. The thermal structure derived from the 12CO multi-transition analysis suggests that mechanical heating, like shocks or turbulence, dominates the heating of the ISM in the nucleus of NGC4945 located beyond 100 pc (≳5″) from the center of the galaxy. This result is further supported by published models, which are able to reproduce the emission observed at high-J (PACS) 12CO transitions when mechanical heating mechanisms are included. Shocks and/or turbulence are likely produced by the barred potential and the outflow observed in X–rays.

Early science with the LMT: molecular torus in UGC 5101

ABSTRACT As part of the Early Science Large Millimeter Telescope projects, we report the detection of nine double-peaked molecular lines, produced by a rotating molecular torus, in the ultraluminous infrared galaxies (ULIRG) – Compton-thick active galactic nuclei (AGN) galaxy UGC 5101. The double-peaked lines we report correspond to molecular transitions of HCN, HCO+, HNC, N2H+, CS, C18O, 13CO, and two CN lines; plus the detection of C2H that is a blend of six lines. The redshift search receiver spectra covers the 73–113 GHz frequency window. Low- and high-density gas tracers of the torus have different implied rotational velocities, with a rotational velocity of 149 ± 3 km s−1 for the low-density ones (C18O, 13CO) and 174 ± 3 km s−1 for high-density tracers (HCN, HCO+, HNC, N2H+, CS, and CN). In UGC 5101, we find that the ratio of integrated intensities of HCN to 13CO to be unusually large, probably indicating that the gas in the torus is very dense. Both the column densities and abundances are consistent with values found in AGN, starburst, and ULIRG galaxies. The observed abundance ratios cannot discriminate between X-ray and UV-field-dominated regions.