arXiv subjects

A. Giannetti

Publications and source records attributed to A. Giannetti.

At least 37 records · Page 2Linked to original sources

arXiv2020

The SEDIGISM survey: first data release and overview of the Galactic structure

The SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) survey used the APEX telescope to map 84 deg^2 of the Galactic plane between l = -60 deg and l = +31 deg in several molecular transitions, including 13CO(2-1) and C18O(2-1), thus probing the moderately dense (~10^3 cm^-3) component of the interstellar medium. With an angular resolution of 30'' and a typical 1-sigma sensitivity of 0.8-1.0 K at 0.25 km/s velocity resolution, it gives access to a wide range of structures, from individual star-forming clumps to giant molecular clouds and complexes. The coverage includes a good fraction of the first and fourth Galactic quadrants, allowing us to constrain the large scale distribution of cold molecular gas in the inner Galaxy. In this paper we provide an updated overview of the full survey and the data reduction procedures used. We also assess the quality of these data and describe the data products that are being made publicly available as part of this first data release (DR1). We present integrated maps and position-velocity maps of the molecular gas and use these to investigate the correlation between the molecular gas and the large scale structural features of the Milky Way such as the spiral arms, Galactic bar and Galactic centre. We find that approximately 60 per cent of the molecular gas is associated with the spiral arms and these appear as strong intensity peaks in the derived Galactocentric distribution. We also find strong peaks in intensity at specific longitudes that correspond to the Galactic centre and well known star forming complexes, revealing that the 13CO emission is concentrated in a small number of complexes rather than evenly distributed along spiral arms.

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arXiv2020

The SEDIGISM survey: Molecular clouds in the inner Galaxy

We use the 13CO(2-1) emission from the SEDIGISM high-resolution spectral-line survey of the inner Galaxy, to extract the molecular cloud population with a large dynamic range in spatial scales, using the SCIMES algorithm. This work compiles a cloud catalogue with a total of 10663 molecular clouds, 10300 of which we were able to assign distances and compute physical properties. We study some of the global properties of clouds using a science sample, consisting of 6664 well resolved sources and for which the distance estimates are reliable. In particular, we compare the scaling relations retrieved from SEDIGISM to those of other surveys, and we explore the properties of clouds with and without high-mass star formation. Our results suggest that there is no single global property of a cloud that determines its ability to form massive stars, although we find combined trends of increasing mass, size, surface density and velocity dispersion for the sub-sample of clouds with ongoing high-mass star formation. We then isolate the most extreme clouds in the SEDIGISM sample (i.e. clouds in the tails of the distributions) to look at their overall Galactic distribution, in search for hints of environmental effects. We find that, for most properties, the Galactic distribution of the most extreme clouds is only marginally different to that of the global cloud population. The Galactic distribution of the largest clouds, the turbulent clouds and the high-mass star-forming clouds are those that deviate most significantly from the global cloud population. We also find that the least dynamically active clouds (with low velocity dispersion or low virial parameter) are situated further afield, mostly in the least populated areas. However, we suspect that part of these trends may be affected by some observational biases, and thus require further follow up work in order to be confirmed.

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arXiv2020

SEDIGISM-ATLASGAL: Dense Gas Fraction and Star Formation Efficiency Across the Galactic Disk

By combining two surveys covering a large fraction of the molecular material in the Galactic disk we investigate the role the spiral arms play in the star formation process. We have matched clumps identified by ATLASGAL with their parental GMCs as identified by SEDIGISM, and use these giant molecular cloud (GMC) masses, the bolometric luminosities, and integrated clump masses obtained in a concurrent paper to estimate the dense gas fractions (DGF$_{\rm gmc}=\sum M_{\rm clump}/M_{\rm gmc}$) and the instantaneous star forming efficiencies (i.e., SFE$_{\rm gmc} = \sum L_{\rm clump}/M_{\rm gmc}$). We find that the molecular material associated with ATLASGAL clumps is concentrated in the spiral arms ($\sim$60% found within $\pm$10 km s$^{-1}$ of an arm). We have searched for variations in the values of these physical parameters with respect to their proximity to the spiral arms, but find no evidence for any enhancement that might be attributable to the spiral arms. The combined results from a number of similar studies based on different surveys indicate that, while spiral-arm location plays a role in cloud formation and HI to H$_2$ conversion, the subsequent star formation processes appear to depend more on local environment effects. This leads us to conclude that the enhanced star formation activity seen towards the spiral arms is the result of source crowding rather than the consequence of a any physical process.

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arXiv2020

Studying the effects and cause of the massive star formation in Messier 8 East

Messier 8 (M8), one of the brightest HII regions in our Galaxy, is associated with two prominent massive star-forming regions: M8-Main, the particularly bright part of the large scale HII region (mainly) ionised by the stellar system Herschel 36 (Her 36) and M8 East (M8 E), which is mainly powered by a deeply embedded young stellar object (YSO), a bright infrared (IR) source, M8E-IR. We aim to study the interaction of the massive star-forming region M8 E with its surroundings and to compare the star-forming environments of M8-Main and M8 E. We used the IRAM 30 m telescope to perform an imaging spectroscopy survey of the molecular environment of M8E-IR. We imaged and analysed data for the $J$ = 1 $\to$ 0 rotational transitions of $^{12}$CO, $^{13}$CO, N$_2$H$^+$, HCN, H$^{13}$CN, HCO$^+$, H$^{13}$CO$^+$, HNC and HN$^{13}$C observed for the first time toward M8~E. We used LTE and non-LTE techniques to determine column densities of the observed species and to constrain the physical conditions of the gas responsible for their emission. Examining the YSO population in M8~E allows us to explore the observed ionization front (IF) as seen in GLIMPSE 8~$μ$m emission image. We find that $^{12}$CO probes the warm diffuse gas also traced by the GLIMPSE 8~$μ$m emission, while N$_2$H$^+$ and HN$^{13}$C trace the cool and dense gas. We find that the star-formation in M8~E appears to be triggered by the earlier formed stellar cluster NGC~6530, which powers an HII region giving rise to an IF that is moving at a speed $\geq$ 0.26~km~s$^{-1}$ across M8~E. We derive temperatures of 80 K and 30 K for the warm and cool gas components, respectively, and constrain H$_2$ volume densities to be in the range of 10$^4$--10$^6$~cm$^{-3}$. Comparison of the observed abundances of various species reflects the fact that M8~E is at an earlier stage of massive star formation than M8-Main.

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arXiv2020

Survey of ortho-H$_2$D$^+$ in high-mass star-forming regions

(Abridged) We present a large sample of o-H$_2$D$^+$ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. APEX observations of the ground-state transition of o-H$_2$D$^+$ were analysed in a sample of massive clumps selected from ATLASGAL at different evolutionary stages. Column densities and beam-averaged abundances of o-H$_2$D$^+$ with respect to H$_2$, $X$(o-H$_2$D$^+$), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. We detect 16 sources in o-H$_2$D$^+$ and find clear correlations between $X$(o-H$_2$D$^+$) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-to-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H$_3^+$ are more abundant in the early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the $X$(o-H$_2$D$^+$) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H$^{13}$CO$^+$, DCO$^+$, and C$^{17}$O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Our study presents the largest sample of o-H$_2$D$^+$ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H$_2$D$^+$ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence.

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arXiv2020

Square Kilometre Array Science Data Challenge 1: analysis and results

As the largest radio telescope in the world, the Square Kilometre Array (SKA) will lead the next generation of radio astronomy. The feats of engineering required to construct the telescope array will be matched only by the techniques developed to exploit the rich scientific value of the data. To drive forward the development of efficient and accurate analysis methods, we are designing a series of data challenges that will provide the scientific community with high-quality datasets for testing and evaluating new techniques. In this paper we present a description and results from the first such Science Data Challenge (SDC1). Based on SKA MID continuum simulated observations and covering three frequencies (560 MHz, 1400MHz and 9200 MHz) at three depths (8 h, 100 h and 1000 h), SDC1 asked participants to apply source detection, characterization and classification methods to simulated data. The challenge opened in November 2018, with nine teams submitting results by the deadline of April 2019. In this work we analyse the results for 8 of those teams, showcasing the variety of approaches that can be successfully used to find, characterise and classify sources in a deep, crowded field. The results also demonstrate the importance of building domain knowledge and expertise on this kind of analysis to obtain the best performance. As high-resolution observations begin revealing the true complexity of the sky, one of the outstanding challenges emerging from this analysis is the ability to deal with highly resolved and complex sources as effectively as the unresolved source population.

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arXiv2019

The 3D structure of CO depletion in high-mass prestellar regions

Disentangling the different stages of the star-formation process, in particular in the high-mass regime, is a challenge in astrophysics. Chemical clocks could help alleviating this problem, but their evolution strongly depends on many parameters, leading to degeneracy in the interpretation of the observational data. One of these uncertainties is the degree of CO depletion. We present here the first self-consistent magneto-hydrodynamic simulations of high-mass star-forming regions at different scales, fully coupled with a non-equilibrium chemical network, which includes C-N-O bearing molecules. Depletion and desorption processes are treated time-dependently. The results show that full CO-depletion (i.e. all gas-phase CO frozen-out on the surface of dust grains), can be reached very quickly, in one third or even smaller fractions of the free-fall time, whether the collapse proceeds on slow or fast timescales. This leads to a high level of deuteration in a short time both for typical tracers like N$_2$H$^+$, as well as for the main ion H$_3^+$, the latter being in general larger and more extended. N$_2$ depletion is slightly less efficient, and no direct effects on N-bearing molecules and deuterium fractionation are observed. We show that CO depletion is not the only driver of deuteration, and that there is a strong impact on $D_{frac}$ when changing the grain-size. We finally apply a two-dimensional gaussian Point Spread Function to our results to mimic observations with single-dish and interferometers. Our findings suggest that the low-values observed in high-mass star-forming clumps are in reality masking a full-depletion stage in the inner 0.1 pc region.

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arXiv2019

On the size of the CO-depletion radius in the IRDC G351.77-0.51

An estimate of the degree of CO-depletion ($f_D$) provides information on the physical conditions occurring in the innermost and densest regions of molecular clouds. A key parameter in these studies is the size of the depletion radius, i.e. the radius within which the C-bearing species, and in particular CO, are largely frozen onto dust grains. A strong depletion state (i.e. $f_D>10$, as assumed in our models) is highly favoured in the innermost regions of dark clouds, where the temperature is $<20$ K and the number density of molecular hydrogen exceeds a few $\times$10$^{4}$ cm$^{-3}$. In this work, we estimate the size of the depleted region by studying the Infrared Dark Cloud (IRDC) G351.77-0.51. Continuum observations performed with the $Herschel$ $Space$ $Observatory$ and the $LArge$ $APEX$ $BOlometer$ $CAmera$, together with APEX C$^{18}$O and C$^{17}$O J=2$\rightarrow$1 line observations, allowed us to recover the large-scale beam- and line-of-sight-averaged depletion map of the cloud. We built a simple model to investigate the depletion in the inner regions of the clumps in the filament and the filament itself. The model suggests that the depletion radius ranges from 0.02 to 0.15 pc, comparable with the typical filament width (i.e.$\sim$0.1 pc). At these radii, the number density of H$_2$ reaches values between 0.2 and 5.5$\times$10$^{5}$ cm$^{-3}$. These results provide information on the approximate spatial scales on which different chemical processes operate in high-mass star-forming regions and also suggest caution when using CO for kinematical studies in IRDCs.

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arXiv2019

ATLASGAL --- Molecular fingerprints of a sample of massive star forming clumps

We have conducted a 3-mm molecular-line survey towards 570 high-mass star-forming clumps, using the Mopra telescope. The sample is selected from the 10,000 clumps identified by the ATLASGAL survey and includes all of the most important embedded evolutionary stages associated with massive star formation, classified into five distinct categories (quiescent, protostellar, young stellar objects, \hii\ regions and photo-dominated regions). The observations were performed in broadband mode with frequency coverage of 85.2 to 93.4\,GHz and a velocity resolution of $\sim$0.9\,\kms, detecting emission from 26 different transitions. We find significant evolutionary trends in the detection rates, integrated line intensities, and abundances of many of the transitions and also identify a couple of molecules that appear to be invariant to changes in the dust temperature and evolutionary stage (N$_2$H$^+$\,(1-0) and HN$^{13}$C\,(1-0)). We use the K-ladders for CH$_3$C$_2$H\,(5-4) and CH$_3$CH\,(5-4) to calculate the rotation temperatures and find $\sim$1/3 of the quiescent clumps have rotation temperatures that suggest the presence of an internal heating source. These sources may constitute a population of very young protostellar objects that are still dark at 70\,\mum\ and suggest that the fraction of truly quiescent clumps may only be a few per cent. We also identify a number of line ratios that show a strong correlation with the evolutionary stage of the embedded objects and discuss their utility as diagnostic probes of evolution.

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arXiv2019

Small-scale physical and chemical structure of diffuse and translucent molecular clouds along the line of sight to Sgr B2

The diffuse and translucent molecular clouds traced in absorption along the line of sight to strong background sources have so far been investigated mainly in the spectral domain because of limited angular resolution or small sizes of the background sources. We aim to resolve and investigate the spatial structure of molecular clouds traced by several molecules detected in absorption along the line of sight to SgrB2(N). We have used spectral line data from the EMoCA survey performed with ALMA, taking advantage of the high sensitivity and angular resolution. We identify, on the basis of the spectral analysis of c-C3H2 across the field of view, 15 main velocity components along the line of sight to SgrB2(N) and several components in the envelope of SgrB2. The c-C3H2 column densities reveal two categories of clouds. Clouds in Category I (3 kpc arm, 4 kpc arm, and some GC clouds) have smaller c-C3H2 column densities, smaller linewidths, and smaller widths of their column density PDFs than clouds in Category II (Scutum arm, Sgr arm, and other GC clouds). To investigate the spatial structure we derive opacity maps for the following molecules: c-C3H2, H13CO+, 13CO, HNC, HN13C, HC15N, CS, C34S, 13CS, SiO, SO, and CH3OH. These maps reveal that most molecules trace relatively homogeneous structures that are more extended than the field of view defined by the background continuum emission (about 15", that is 0.08-0.6pc depending on the distance). SO and SiO show more complex structures with smaller clumps of size ~5-8". Our analysis suggests that the driving of the turbulence is mainly solenoidal in the investigated clouds. On the basis of HCO+, we conclude that most line-of-sight clouds towards SgrB2 are translucent, including all clouds where complex organic molecules were recently detected. We also conclude that CCH and CH are good probes of H2 in both diffuse and translucent clouds.

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arXiv2019

A timeline for massive star-forming regions via combined observation of o-H$_2$D$^+$ and N$_2$D$^+$

Context: In cold and dense gas prior to the formation of young stellar objects, heavy molecular species (including CO) are accreted onto dust grains. Under these conditions H$_3^+$ and its deuterated isotopologues become more abundant, enhancing the deuterium fraction of molecules such as N$_2$H$^+$ that are formed via ion-neutral reactions. Because this process is extremely temperature sensitive, the abundance of these species is likely linked to the evolutionary stage of the source. Aims: We investigate how the abundances of o-H$_2$D$^+$ and N$_2$D$^+$ vary with evolution in high-mass clumps. Methods: We observed with APEX the ground-state transitions of o-H$_2$D$^+$ near 372 GHz, and N$_2$D$^+$(3-2) near 231 GHz for three massive clumps in different evolutionary stages. The sources were selected within the G351.77-0.51 complex to minimise the variation of initial chemical conditions, and to remove distance effects. We modelled their dust continuum emission to estimate their physical properties, and also modelled their spectra under the assumption of local thermodynamic equilibrium to calculate beam-averaged abundances. Results: We find an anticorrelation between the abundance of o-H$_2$D$^+$ and that of N$_2$D$^+$, with the former decreasing and the latter increasing with evolution. With the new observations we are also able to provide a qualitative upper limit to the age of the youngest clump of about 10$^5$ yr, comparable to its current free-fall time. Conclusions: We can explain the evolution of the two tracers with simple considerations on the chemical formation paths, depletion of heavy elements, and evaporation from the grains. We therefore propose that the joint observation and the relative abundance of o-H$_2$D$^+$ and N$_2$D$^+$ can act as an efficient tracer of the evolutionary stages of the star-formation process.

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arXiv2018

Characterising the high-mass star forming filament G351.776--0.527 with Herschel and APEX dust continuum and gas observations

G351.776-0.527 is among the most massive, closest, and youngest filaments in the inner Galactic plane and therefore it is an ideal laboratory to study the kinematics of dense gas and mass replenishment on a large scale. In this paper, we present far-infrared (FIR) and submillimetre wavelength continuum observations combined with spectroscopic C$^{18}$O (2-1) data of the entire region to study its temperature, mass distribution, and kinematics. The structure is composed of a main elongated region with an aspect ratio of $\sim 23$, which is associated with a network of filamentary structures. The main filament has a remarkably constant width of 0.2 pc. The total mass of the network (including the main filament) is $\geq 2600$ M$_\odot$, while we estimate a mass of $\sim 2000$ M$_\odot$ for the main structure. Therefore, the network harbours a large reservoir of gas and dust that could still be accreted onto the main structure. From the analysis of the gas kinematics, we detect two velocity components in the northern part of the main filament. The data also reveal velocity oscillations in C$^{18}$O along the spine in the main filament and in at least one of the branches. Considering the region as a single structure, we find that it is globally close to virial equilibrium indicating that the entire structure is approximately in a stable state.

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arXiv2018

SEDIGISM: The kinematics of ATLASGAL filaments

Analysing the kinematics of filamentary molecular clouds is a crucial step towards understanding their role in the star formation process. Therefore, we study the kinematics of 283 filament candidates in the inner Galaxy, that were previously identified in the ATLASGAL dust continuum data. The $^{13}$CO(2 - 1) and C$^{18}$O(2 - 1) data of the SEDIGISM survey (Structure, Excitation, and Dynamics of the Inner Galactic Inter Stellar Medium) allows us to analyse the kinematics of these targets and to determine their physical properties at a resolution of 30 arcsec and 0.25 km/s. To do so, we developed an automated algorithm to identify all velocity components along the line-of-sight correlated with the ATLASGAL dust emission, and derive size, mass, and kinematic properties for all velocity components. We find two-third of the filament candidates are coherent structures in position-position-velocity space. The remaining candidates appear to be the result of a superposition of two or three filamentary structures along the line-of-sight. At the resolution of the data, on average the filaments are in agreement with Plummer-like radial density profiles with a power-law exponent of p = 1.5 +- 0.5, indicating that they are typically embedded in a molecular cloud and do not have a well-defined outer radius. Also, we find a correlation between the observed mass per unit length and the velocity dispersion of the filament of $m \sim σ_v^2$. We show that this relation can be explained by a virial balance between self-gravity and pressure. Another possible explanation could be radial collapse of the filament, where we can exclude infall motions close to the free-fall velocity.

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arXiv2018

ALMACAL IV: A catalogue of ALMA calibrator continuum observations

We present a catalogue of ALMA flux density measurements of 754 calibrators observed between August 2012 and September 2017, for a total of 16,263 observations in different bands and epochs. The flux densities were measured reprocessing the ALMA images generated in the framework of the ALMACAL project, with a new code developed by the Italian node of the European ALMA Regional Centre. A search in the online databases yielded redshift measurements for 589 sources ($\sim$78 per cent of the total). Almost all sources are flat-spectrum, based on their low-frequency spectral index, and have properties consistent with being blazars of different types. To illustrate the properties of the sample we show the redshift and flux density distributions as well as the distributions of the number of observations of individual sources and of time spans in the source frame for sources observed in bands 3 (84$-$116 GHz) and 6 (211$-$275 GHz). As examples of the scientific investigations allowed by the catalogue we briefly discuss the variability properties of our sources in ALMA bands 3 and 6 and the frequency spectra between the effective frequencies of these bands. We find that the median variability index steadily increases with the source-frame time lag increasing from 100 to 800 days, and that the frequency spectra of BL Lacs are significantly flatter than those of flat-spectrum radio quasars. We also show the global spectral energy distributions of our sources over 17 orders of magnitude in frequency.

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arXiv2018

The fragmentation properties of massive protocluster gas clumps: an ALMA study

Fragmentation of massive dense molecular clouds is the starting point in the formation of rich clusters and massive stars. Theory and numerical simulations indicate that the population of the fragments (number, mass, diameter, separation) resulting from the gravitational collapse of such clumps is probably regulated by the balance between the magnetic field and the other competitors of self-gravity, in particular turbulence and protostellar feedback. We have observed 11 massive, dense and young star-forming clumps with the Atacama Large Millimeter Array (ALMA) in the thermal dust continuum emission at $\sim 1$~mm with an angular resolution of 0.25 arcseconds with the aim of determining their population of fragments. We find fragments on sub-arcsecond scales in 8 out of the 11 sources. The ALMA images indicate two different fragmentation modes: a dominant fragment surrounded by companions with much smaller mass and size, and many ($\geq 8$) fragments with a gradual change in masses and sizes. On average, the largest number of fragments is found towards the warmer and more massive clumps. Also, the warmer clumps tend to form fragments with larger mass and size. To understand the role of the different physical parameters to regulate the final population of the fragments, we have simulated the collapse of a massive clump of $100$ and $300$ M$_{\odot}$ having different magnetic support. The simulations indicate that: (1) fragmentation is inhibited when the initial turbulence is low, independent of the other physical parameters. (2) a filamentary distribution of the fragments is favoured in a highly magnetised clump. We conclude that the clumps that show many fragments distributed in a filamentary-like structure are likely characterised by a strong magnetic field, while the others are possible also in a weaker magnetic field.

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arXiv2017

Unveiling the inner morphology and gas kinematics of NGC 5135 with ALMA

The local Seyfert 2 galaxy NGC5135, thanks to its almost face-on appearance, a bulge overdensity of stars, the presence of a large-scale bar, an AGN and a Supernova Remnant, is an excellent target to investigate the dynamics of inflows, outflows, star formation and AGN feedback. Here we present a reconstruction of the gas morphology and kinematics in the inner regions of this galaxy, based on the analysis of Atacama Large Millimeter Array (ALMA) archival data. To our purpose, we combine the available $\sim$100 pc resolution ALMA 1.3 and 0.45 mm observations of dust continuum emission, the spectroscopic maps of two transitions of the CO molecule (tracer of molecular mass in star forming and nuclear regions), and of the CS molecule (tracer of the dense star forming regions) with the outcome of the SED decomposition. By applying the $^{\rm 3D}$BAROLO software (3D-Based Analysis of Rotating Object via Line Observations), we have been able to fit the galaxy rotation curves reconstructing a 3D tilted-ring model of the disk. Most of the observed emitting features are described by our kinematic model. We also attempt an interpretation for the emission in few regions that the axisymmetric model fails to reproduce. The most relevant of these is a region at the northern edge of the inner bar, where multiple velocity components overlap, as a possible consequence of the expansion of a super-bubble.

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arXiv2017

ATLASGAL-selected massive clumps in the inner Galaxy: VI. Kinetic temperature and spatial density measured with formaldehyde

We aim to directly determine the kinetic temperature and spatial density with formaldehyde for the $\sim$100 brightest ATLASGAL-selected clumps at 870 $μ$m representing various evolutionary stages of high-mass star formation. Ten transitions ($J$ = 3-2 and 4-3) of ortho- and para-H$_2$CO near 211, 218, 225, and 291 GHz were observed with the APEX 12 m telescope. Using non-LTE models with RADEX, we derive the gas kinetic temperature and spatial density using the measured p-H$_2$CO 3$_{21}$-2$_{20}$/3$_{03}$-2$_{02}$, 4$_{22}$-3$_{21}$/4$_{04}$-3$_{03}$, and 4$_{04}$-3$_{03}$/3$_{03}$-2$_{02}$ ratios. The gas kinetic temperatures derived from the p-H$_2$CO 3$_{21}$-2$_{20}$/3$_{03}$-2$_{02}$ and 4$_{22}$-3$_{21}$/4$_{04}$-3$_{03}$ line ratios are high, ranging from 43 to $>$300 K with an unweighted average of 91 $\pm$ 4 K. Deduced $T_{\rm kin}$ values from the $J$ = 3-2 and 4-3 transitions are similar. Spatial densities of the gas derived from the p-H$_2$CO 4$_{04}$-3$_{03}$/3$_{03}$-2$_{02}$ line ratios yield 0.6-8.3 $\times$ 10$^6$ cm$^{-3}$ with an unweighted average of 1.5 ($\pm$0.1) $\times$ 10$^6$ cm$^{-3}$. A comparison of kinetic temperatures derived from p-H$_2$CO, NH$_3$, and the dust emission indicates that p-H$_2$CO traces a distinctly higher temperature than the NH$_3$ (2,2)/(1,1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H$_2$CO linewidths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H$_2$CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H$_2$CO increase with time through the evolution of the clumps.

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arXiv2017

Galactocentric variation of the gas-to-dust ratio and its relation with metallicity

(Abridged) Context: The assumption of a gas-to-dust mass ratio (γ) is a common approach to estimate the basic properties of molecular clouds, such as total mass and column density of molecular hydrogen, from (sub)mm continuum observations of the dust. In the Milky Way a single value is used at all galactocentric radii, independently of the observed metallicity gradients. Both models and extragalactic observations suggest that this quantity increases for decreasing metallicity Z, typical of the outer regions in disks, where fewer heavy elements are available to form dust grains. Aims: We aim to investigate the variation of the gas-to-dust ratio as a function of galactocentric radius and metallicity, to allow a more accurate characterisation of the quantity of molecular gas across the galactic disk, as derived from observations of the dust. Methods: Observations of the optically thin C\$^{18}\$O (2-1) transition were obtained with the APEX telescope for a sample of 23 massive and dense star-forming regions in the far outer Galaxy (galactocentric distance greater than 14 kpc). From the modelling of this line and of the spectral energy distribution of the selected clumps we computed the gas-to-dust ratio and compared it to that of well-studied sources from the ATLASGAL TOP100 sample in the inner galactic disk. Results: The gradient in gas-to-dust ratio is found to be 0.087 dex/kpc (or equivalently γ proportional to Z\$^{-1.4}\$). The dust-to-metal ratio, decreases with galactocentric radius, which is the most common situation also for external late-type galaxies. This suggests that grain growth dominates over destruction. The predicted gas-to-dust ratio is in excellent agreement with the estimates in Magellanic clouds, for the appropriate value of Z.

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