Herschel Hi-GAL Research Articles

Supervised machine learning on Galactic filaments. II. Encoding the position to optimize the detection of filaments over a wide range of column density and contrast

Filaments host star formation and are fundamental structures of galaxies. Their diversity, as observed in the interstellar medium, from very low-density structures to very dense hubs, and their complex life cycles make their complete detection challenging over this large diversity range. Using 2D H$_2$ column density images obtained as part of the Herschel Hi-GAL survey of the Galactic plane (Gp), we want to detect, simultaneously and using a single model, filaments over a large range of column density and contrast over the whole Gp. In particular, we target low-contrast and low-density structures that are particularly difficult to detect with classical algorithms. The whole H$_2$ column density image of the Gp was subdivided into individual patches of 32times 32 pixels. Following our proof of concept study aimed at exploring the potential of supervised learning for the detection of filaments, we propose an innovative supervised learning method based on adding information by encoding the position of these patches in the Gp. To allow the segmentation of the whole Gp, we introduced a random procedure that preserves the balance within the model training and testing datasets over the Gp plane. Four architectures and six models were tested and compared using different metrics. For the first time, a segmentation of the whole Gp has been obtained using supervised deep learning. A comparison of the models based on metrics and astrophysical results shows that one of the architectures (PE-UNet-Latent), where the position encoding was done in the latent space gives the best performance to detect filaments over the whole range of density and contrast observed in the Gp. A normalized map of the whole Gp was also produced and reveals the highly filamentary structure of the Gp in all density regimes. We successfully tested the generalization of our best model by applying it to the 2D 12CO COHRS molecular data obtained on a 52 portion (in longitude) of the plane. We demonstrate the interest of position encoding to allow the detection of filaments over the wide range of density and contrast observed in the Gp. The produced maps (both normalized and segmented) offer a unique opportunity for follow-up studies of the life cycle of Galactic filaments. The promising generalization possibility tested on a molecular dataset of the Gp opens new opportunities for systematic detection of filamentary structures in the big data context available for the Gp.

CHIMPS2: 13CO J = 3→2 emission in the central molecular zone

ABSTRACT We present the initial data for the $(J = 3 \rightarrow 2)$ transition of $^{13}\text{CO}$ obtained from the central molecular zone (CMZ) of the Milky Way as part of the CO Heterodyne Inner Milky Way Plane Survey 2 (CHIMPS2). Covering $359^{\circ } \le l \le 1^{\circ }$ and $|b| \le 0.5^{\circ }$ with an angular resolution of 19 arcsec, velocity resolution of 1 km s$^{-1}$, and rms $\Delta {T_{\rm A}^{*}} = 0.59\, \mathrm{K}$ at these resolutions, our observations unveil the complex structure of the CMZ molecular gas in improved detail. Complemented by the $\rm {^{12}CO}$ CHIMPS2 data, we estimate a median optical depth of $\tau _{13} = 0.087$. The preliminary analysis yields a median $^{13}\text{CO}$ column-density range equal to $N(^{13}{\rm CO}) = 2{\!-\!}5 \times 10^{18}\, \mathrm{cm}^{-2}$, median H$_{2}$ column density equal to $N(\mathrm{H_{2}}) = 4 \times 10^{22}\, \mathrm{cm}^{-2}$ to $1 \times 10^{23}\, \mathrm{cm}^{-2}$. We derive $N({\rm H_{2}})$-based total mass estimates of $M({\rm H}_{2}) = 2{\!-\!}6 \times 10^{7}\, \mathrm{M}_{\odot }$, in agreement with previous studies. We analyse the relationship between the integrated intensity of $^{13}\text{CO}$ and the surface density of compact sources identified by Herschel Hi-GAL, and find that younger Hi-GAL sources detected at 500 $\rm{\mu m}$ but not at 70 $\rm{\mu m}$ follow the dense gas of the CMZ more closely than those that are bright at 70 $\rm{\mu m}$. The latter, actively star-forming sources appear to be more associated with material in the foreground spiral arms.

The Milky Way atlas for linear filaments

Context. Filamentary structure is important for the ISM and star formation. Galactic distribution of filaments may regulate the star formation rate in the Milky Way. However, interstellar filaments are intrinsically complex, making them difficult to study quantitatively. Aims. Here we focus on linear filaments, the simplest morphology that can be treated as building blocks of any filamentary structure. Methods. We present the first catalog of 42 straight-line filaments across the full Galactic plane, identified by clustering of far-IR Herschel HiGAL clumps in position–position–velocity space. We investigated the dynamics along the filaments using molecular line cubes, compared the filaments with Galactic spiral arms, and compared ambient magnetic fields with the filaments’ orientation. Results. The selected filaments show extreme linearity (> 10), aspect ratio (7–48), and velocity coherence over a length of 3–40 pc (mostly > 10 pc). About one-third of them are associated with spiral arms, but only one is located in the arm center (known as the “skeleton” of the Milky Way). A few of them extend perpendicular to the Galactic plane, and none is located in the Central Molecular Zone (CMZ) near the Galactic center. Along the filaments, prevalent periodic oscillation (both in velocity and density) is consistent with gas flows channeled by the filaments and feeding the clumps that harbor diverse star formation activity. No correlation is found between the filament orientations with Planck measured global magnetic field lines. Conclusions. This work highlights some of the fundamental properties of molecular filaments and provides a golden sample for follow-up studies on star formation, ISM structure, and Milky Way structure.

Supervised machine learning on Galactic filaments

Context. Filaments are ubiquitous in the Galaxy, and they host star formation. Detecting them in a reliable way is therefore key towards our understanding of the star formation process. Aims. We explore whether supervised machine learning can identify filamentary structures on the whole Galactic plane. Methods. We used two versions of UNet-based networks for image segmentation. We used H2 column density images of the Galactic plane obtained with Herschel Hi-GAL data as input data. We trained the UNet-based networks with skeletons (spine plus branches) of filaments that were extracted from these images, together with background and missing data masks that we produced. We tested eight training scenarios to determine the best scenario for our astrophysical purpose of classifying pixels as filaments. Results. The training of the UNets allows us to create a new image of the Galactic plane by segmentation in which pixels belonging to filamentary structures are identified. With this new method, we classify more pixels (more by a factor of 2 to 7, depending on the classification threshold used) as belonging to filaments than the spine plus branches structures we used as input. New structures are revealed, which are mainly low-contrast filaments that were not detected before. We use standard metrics to evaluate the performances of the different training scenarios. This allows us to demonstrate the robustness of the method and to determine an optimal threshold value that maximizes the recovery of the input labelled pixel classification. Conclusions. This proof-of-concept study shows that supervised machine learning can reveal filamentary structures that are present throughout the Galactic plane. The detection of these structures, including low-density and low-contrast structures that have never been seen before, offers important perspectives for the study of these filaments.

FUGIN hot core survey. I. Survey method and initial results for l = 10°–20°

Abstract We have developed a method to make a spectral-line-based survey of hot cores, which represent an important stage of high-mass star formation, and applied the method to the data of the FUGIN (FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope) survey. First, we select hot core candidates by searching the FUGIN data for the weak hot core tracer lines (HNCO and CH3CN) by stacking, and then we conduct follow-up pointed observations on these candidates in C34S, SO, OCS, HC3N, HNCO, CH3CN, and CH3OH J = 2–1 and J = 8–7 lines to confirm and characterize them. We applied this method to the l = 10°–20° portion of the FUGIN data and identified 22 “HotCores” (compact sources with more than two significant detections of the hot core tracer lines, i.e., SO, OCS, HC3N, HNCO, CH3CN, or CH3OH J = 8–7 lines) and 14 “DenseClumps” (sources with more than two significant detection of C34S, CH3OH J = 2–1, or the hot core tracer lines). The identified HotCores are found to be associated with signposts of high-mass star formation such as ATLASGAL clumps, WISE H ii regions, and Class II methanol masers. Many of the FUGIN HotCores are identified with the Herschel Hi-GAL clumps with a median mass of 6.8 × 102 M⊙ and a median bolometric luminosity of 7.4 × 103 L⊙. Five of the seven HotCores with stronger CH3CN lines exhibit elevated gas temperatures of 50–100 K. These observations suggest that FUGIN HotCores are closely related to the formation of stars with medium to high mass. For those associated with ATLASGAL clumps, their bolometric luminosity to clump mass ratios are consistent with the star formation stages centered at the hot core phase. The catalog of FUGIN HotCores provides a useful starting point for further statistical studies and detailed observations of high-mass star forming regions.

A possible far-ultraviolet flux-dependent core mass function in NGC 6357

Context. NGC 6357 is a galactic star-forming complex (d ~ 1.7 kpc) composed of several HII regions, a few young stellar clusters, and giant molecular clouds. In particular, the HII regions G353.2+0.9, G353.1+0.6, and G353.2+0.7 are associated with three young clusters; the most prominent of these, Pismis 24, contains some of the most massive stars known. Aims. We aim to derive the properties of the densest compact gas structures (cores) in the region as well as the effects of an intense far-ultraviolet (FUV) radiation field on their global properties. Methods. We mapped the NGC 6357 region at 450 and 850 μm with SCUBA-2 and in the CO(3–2) line with HARP at the JCMT. We also made use of the Herschel Hi-GAL data at 70 and 160 μm. We used the algorithm Gaussclumps to retrieve the compact cores embedded in the diffuse sub-millimetre emission and constructed their spectral energy distribution from 70 to 850 μm, from which we derived mass and temperature. We divided the observed area into an ‘active’ region (i.e. the eastern half, which is exposed to the FUV radiation from the more massive members of the three clusters) and a ‘quiescent’ region (i.e. the western half, which is less affected by FUV radiation). We compared the core mass functions and the temperature distributions in the two areas to look for any differences that could be due to the different levels of FUV radiation. Results. We retrieved 686 dense cores, 411 in the active region and 275 in the quiescent region, with an estimated mass completeness limit of ~5 M⊙. We also attempted to select a sample of pre-stellar cores based on cross-correlation with 70 μm emission and red WISE point sources, which unfortunately is biased due to distance, emission at 70 μm from the dust on the surface of the cores that is heated by the FUV radiation, and saturation in the WISE bands. Most of the cores above the mass completeness limit are likely to be gravitationally bound. The fraction of gas in dense cores is very low, 1.4%. We found a mass-size relation log(M∕M⊙) ~ a × log(D∕arcsec), with a in the range 2.0–2.4, depending on the precise selection of the sample. The temperature distributions in the two sub-regions are clearly different, peaking at ~25 K in the quiescent region and at ~35 K in the active region. The core mass functions are different as well, at a 2σ level, consistent with a Salpeter initial mass function in the quiescent region and flatter than that in the active region. The dense cores lying close to the HII regions are consistent with pre-existing cores being gradually engulfed by a photon dominated region and photoevaporating. A comparison of the obtained distribution of core masses with those derived from simulations of cloud-cloud collisions yields no conclusive evidence of ongoing cloud-cloud collisions. Conclusions. We attribute the different global properties of dense cores in the two sub-regions to the influence of the FUV radiation field.

The High-mass Protostellar Population of a Massive Infrared Dark Cloud

Abstract We conduct a census of the high-mass protostellar population of the ∼70,000 M ⊙ infrared dark cloud (IRDC) G028.37+00.07, identifying 35 sources based on their 70 μm emission, as reported in the Herschel Hi-GAL catalog of Molinari et al. We perform aperture photometry to construct spectral energy distributions, which are then fit with the massive protostar models of Zhang & Tan. We find that the sources span a range of isotropic luminosities from ∼20 to 4500 L ⊙. The most luminous sources are predicted to have current protostellar masses of m * ∼ 10 M ⊙ forming from cores of mass M c ∼ 40 to 400 M ⊙. The least luminous sources in our sample are predicted to be protostars with masses as low as ∼0.5 M ⊙ forming from cores with M c ∼ 10 M ⊙, which are the minimum values explored in the protostellar model grid. The detected protostellar population has a total estimated protostellar mass of M * ∼ 100 M ⊙. Allowing for completeness corrections, which are constrained by comparison with an ALMA study in part of the cloud, we estimate a star formation efficiency per freefall time of ∼3% in the IRDC. Finally, analyzing the spatial distribution of the sources, we find relatively low degrees of central concentration of the protostars. The protostars, including the most massive ones, do not appear to be especially centrally concentrated in the protocluster as defined by the IRDC boundary.

The accretion history of high-mass stars: an ArTéMiS pilot study of infrared dark clouds

ABSTRACT The mass growth of protostars is a central element to the determination of fundamental stellar population properties such as the initial mass function. Constraining the accretion history of individual protostars is therefore an important aspect of star formation research. The goal of the study presented here is to determine whether high-mass (proto)stars gain their mass from a compact (<0.1 pc) fixed-mass reservoir of gas, often referred to as dense cores, in which they are embedded, or whether the mass growth of high-mass stars is governed by the dynamical evolution of the parsec-scale clump that typically surrounds them. To achieve this goal, we performed a 350-μm continuum mapping of 11 infrared dark clouds, along side some of their neighbouring clumps, with the ArTéMiS camera on APEX. By identifying about 200 compact ArTéMiS sources, and matching them with Herschel Hi-GAL 70 -μm sources, we have been able to produce mass versus temperature diagrams. We compare the nature (i.e. starless or protostellar) and location of the ArTéMiS sources in these diagrams with modelled evolutionary tracks of both core-fed and clump-fed accretion scenarios. We argue that the latter provide a better agreement with the observed distribution of high-mass star-forming cores. However, a robust and definitive conclusion on the question of the accretion history of high-mass stars requires larger number statistics.

2 mm GISMO Observations of the Galactic Center. I. Dust Emission*

Abstract The central molecular zone, covering the inner ∼1° of the Galactic plane has been mapped at 2 mm using the Goddard-IRAM Superconducting 2-Millimeter Observer (GISMO) bolometric camera on the 30 m IRAM telescope. The 21″ resolution maps show abundant emission from cold molecular clouds, from star-forming regions, and from one of the Galactic center nonthermal filaments. In this work we use the Herschel Hi-GAL data to model the dust emission across the Galactic center. We find that a single-temperature fit can describe the 160–500 μm emission for most lines of sight, if the long-wavelength dust emissivity scales as λ −β with β ≈ 2.25. This dust model is extrapolated to predict the 2 mm dust emission. Subtraction of the model from the GISMO data provides a clearer look at the 2 mm emission of star-forming regions and the brightest nonthermal filament.

The role of spiral arms in Milky Way star formation

What role does Galactic structure play in star formation? We have used the Herschel Hi-GAL compact-clump catalogue to examine trends in evolutionary stage over large spatial scales in the inner Galaxy. We examine the relationship between the fraction of clumps with embedded star formation (the star-forming fraction, or SFF) and other measures of star-formation activity. Based on a positive correlation between SFF and evolutionary indicators such as the luminosity-to-mass ratio, we assert that the SFF principally traces the average evolutionary state of a sample and must depend on the local fraction of rapidly-evolving, high-mass young stellar objects. The spiral-arm tangent point longitudes show small excesses in the SFF, though these can be accounted for by a small number of the most massive clusters, just 7.6% of the total number of clumps in the catalogue. This suggests that while the arms tend to be home to the Galaxy's massive clusters, the remaining 92.4% of Hi-GAL clumps in our catalogue do not show an enhancement of star formation within arms. Globally, the SFF is highest at the Galactic midplane and inner longitudes. We find no significant trend in evolutionary stage as a function of position across spiral arms at the tangent-point longitudes. This indicates that the angular offset observed between gas and stars, if coordinated by a density wave, is not evident at the clump phase; alternatively, the onset of star formation is not triggered by the spiral density wave.

Testing the Larson relations in massive clumps

We tested the validity of the three Larson relations in a sample of 213 massive clumps selected from the Herschel Hi-GAL survey and combined with data from the MALT90 survey of 3mm emission lines. The clumps have been divided in 5 evolutionary stages to discuss the Larson relations also as function of evolution. We show that this ensemble does not follow the three Larson relations, regardless of clump evolutionary phase. A consequence of this breakdown is that the virial parameter $\alpha_{vir}$ dependence with mass (and radius) is only a function of the gravitational energy, independent of the kinetic energy of the system, and $\alpha_{vir}$ is not a good descriptor of clump dynamics. Our results suggest that clumps with clear signatures of infall motions are statistically indistinguishable from clumps with no such signatures. The observed non-thermal motions are not necessarily ascribed to turbulence acting to sustain the gravity, but they may be due to the gravitational collapse at the clump scales. This seems particularly true for the most massive (M$\geq$1000 M$_{\odot}$) clumps in the sample, where also exceptionally high magnetic fields may not be enough to stabilize the collapse.

Discussing the distance bias in the estimation of Hi-GAL compact source physical properties – II. Evolutionary status and star formation rate

The derivation of the evolutionary stage and rate of star formation of star-forming sites from Galactic single-dish far-infrared and submillimetre surveys suffers from the relatively limited spatial resolution that prevents access to ‘core’ scales (r ≤ 0.1 pc) for heliocentric distances d ≳ 1 kpc. In a previous article, we studied the implications of this ‘distance bias’ for the mass–radius relationship and its ability to diagnose potential sites for high-mass star formation, using a method that simulates the appearance at large distances of nearby and well-resolved star-forming regions mapped with Herschel. In the present article, we use the same method to quantify the bias introduced in the estimate of the evolutionary stage of dense ‘clumps’ (r ≥ 0.1 pc) revealed from the Herschel Hi-GAL survey, focusing in particular on the Lbol/Menv ratio, which is widely used as an evolutionary indicator. Furthermore, we discuss how the star formation rate (SFR) and efficiency (SFE) change with distance. The location of sources extracted from the virtual distance-displaced maps in the Lbol versus Menv diagram provides evolutionary indications that are consistent with those derived from the underlying population of cores and do not fluctuate substantially with distance. We also show that estimates of the SFR from integrated clump properties are consistent with estimates from the underlying resolved source population and show only minor variations with virtual distance. We conclude that methodologies commonly used to infer evolutionary indicators from clump-integrated quantities from large-scale single-dish Galactic far-infrared and submillimetre surveys are robust against distance and angular-resolution bias.

The Infrared and Radio Flux Densities of Galactic H ii regions

Abstract We derive infrared and radio flux densities of all ∼1000 known Galactic H ii regions in the Galactic longitude range . Our sample comes from the Wide-Field Infrared Survey Explorer (WISE) catalog of Galactic H ii regions. We compute flux densities at six wavelengths in the infrared (Spitzer GLIMPSE 8 μm, WISE 12 μm and 22 μm, Spitzer MIPSGAL 24 μm, and Herschel Hi-GAL 70 μm and 160 μm) and two in the radio (MAGPIS 20 cm and VGPS 21 cm). All H ii region infrared flux densities are strongly correlated with their ∼20 cm flux densities. All H ii regions used here, regardless of physical size or Galactocentric radius, have similar infrared to radio flux density ratios and similar infrared colors, although the smallest regions (r < 1 pc), have slightly elevated IR to radio ratios. The colors and , and and reliably select H ii regions, independent of size. The infrared colors of ∼22% of H ii regions, spanning a large range of physical sizes, satisfy the IRAS color criteria of Wood & Churchwell for H ii regions, after adjusting the criteria to the wavelengths used here. Because these color criteria are commonly thought to select only ultra-compact H ii regions, this result indicates that the true ultra-compact H ii region population is uncertain. Compared to a sample of IR color indices from star-forming galaxies, H ii regions show higher ratios. We find a weak trend of decreasing infrared to ∼20 cm flux density ratios with increasing R gal, in agreement with previous extragalactic results, possibly indicating a decreased dust abundance in the outer Galaxy.

Multitemperature mapping of dust structures throughout the Galactic Plane using the PPMAP tool with Herschel Hi-GAL data

We describe new Hi-GAL based maps of the entire Galactic Plane, obtained using continuum data in the wavelength range 70-500 $\mu$m. These maps are derived with the PPMAP procedure, and therefore represent a significant improvement over those obtained with standard analysis techniques. Specifically they have greatly improved resolution (12 arcsec) and, in addition to more accurate integrated column densities and mean dust temperatures, they give temperature-differential column densities, i.e., separate column density maps in twelve distinct dust temperature intervals, along with the corresponding uncertainty maps. The complete set of maps is available on-line. We briefly describe PPMAP and present some illustrative examples of the results. These include (a) multi-temperature maps of the Galactic HII region W5-E, (b) the temperature decomposition of molecular cloud column-density probability distribution functions, and (c) the global variation of mean dust temperature as a function of Galactocentric distance. Amongst our findings are: (i) a strong localised temperature gradient in W5-E in a direction orthogonal to that towards the ionising star, suggesting an alternative heating source and providing possible guidance for models of the formation of the bubble complex, and (ii) the overall radial profile of dust temperature in the Galaxy shows a monotonic decrease, broadly consistent both with models of the interstellar radiation field and with previous estimates at lower resolution. However, we also find a central temperature plateau within ~ 6 kpc of the Galactic centre, outside of which is a pronounced steepening of the radial profile. This behaviour may reflect the greater proportion of molecular (as opposed to atomic) gas in the central region of the Galaxy.

Dust and gas environment of the young embedded cluster IRAS 18511+0146

IRAS 18511+0146 is a young embedded (proto)cluster located at 3.5 kpc surrounding what appears to be an intermediate mass protostar. In this paper, we investigate the nature of cluster members (two of which are believed to be the most massive and luminous) using imaging and spectroscopy in the near and mid-infrared. The brightest point-like object associated with IRAS 18511+0146 is referred to as S7 in the present work (designated UGPS J185337.88+015030.5 in the UKIRT Galactic Plane survey). Seven of the nine objects show rising spectral energy distributions (SED) in the near-infrared, with four objects showing Br-gamma emission. Three members: S7, S10 (also UGPS J185338.37+015015.3) and S11 (also UGPS J185338.72+015013.5) are bright in mid-infrared with diffuse emission being detected in the vicinity of S11 in PAH bands. Silicate absorption is detected towards these three objects, with an absorption maximum between 9.6 and 9.7 um, large optical depths (1.8-3.2), and profile widths of 1.6-2.1 um. The silicate profiles of S7 and S10 are similar, in contrast to S11 (which has the largest width and optical depth). The cold dust emission investigated using Herschel HiGal peaks at S7, with temperature at 26 K and column density N(H2) ~ 7 x 10^(22) cm^(-2). The bolometric luminosity of IRAS 18511 region is L ~ 1.8 x 10^4 L_sun. S7 is the main contributor to the bolometric luminosity, with L (S7) > 10^4 L_sun. S7 is a high mass protostellar object with ionised stellar winds, evident from the correlation between radio and bolometric luminosity as well as the asymmetric Br-gamma profile. The differences in silicate profiles of S7 and S11 could be due to different radiation environment as we believe the former to be more massive and in an earlier phase than the latter.

Physical properties of GalacticPlanckcold cores revealed by the Hi-GAL survey

Previous studies of the initial conditions of massive star formation have mainly targeted Infrared-Dark Clouds (IRDCs) toward the inner Galaxy. This is due to the fact that IRDCs were first detected in absorption against the bright mid-IR background, requiring a favourable location to be observed. By selection, IRDCs represent only a fraction of the Galactic clouds capable of forming massive stars and star clusters. Due to their low dust temperatures, IRDCs are bright in the far-IR and millimeter and thus, observations at these wavelengths have the potential to provide a complete sample of star-forming massive clouds across the Galaxy. Our aim is to identify the clouds at the initial conditions of massive star formation across the Galaxy and compare their physical properties as a function of their Galactic location. We have examined the physical properties of a homogeneous galactic cold core sample obtained with the Planck satellite across the Galactic Plane. With the use of Herschel Hi-GAL observations, we have characterized the internal structure of them. By using background-subtracted Herschel images, we have derived the H2 column density and dust temperature maps for 48 Planck clumps. Their basic physical parameters have been calculated and analyzed as a function of location within the Galaxy. These properties have also been compared with the empirical relation for massive star formation derived by Kauffmann & Pillai (2010). Most of the Planck clumps contain signs of star formation. About 25% of them are massive enough to form high mass stars. Planck clumps toward the Galactic center region show higher peak column densities and higher average dust temperatures than those of the clumps in the outer Galaxy. Although we only have seven clumps without associated YSOs, the Hi-GAL data show no apparent differences in the properties of Planck cold clumps with and without star formation.

Large-scale latitude distortions of the inner Milky Way disk from theHerschel/Hi-GAL Survey

We use the Herschel Hi-GAL survey data to study the spatial distribution in Galactic longitude and latitude of the interstellar medium and of dense, star-forming clumps in the inner Galaxy. The peak position and width of the latitude distribution of the dust column density as well as of number density of compact sources from the band-merged Hi-GAL photometric catalogues are analysed as a function of longitude. The width of the diffuse dust column density traced by the Hi-GAL 500 micron emission varies across the inner Galaxy, with a mean value of 1{\deg}.2-1{\deg}.3, similar to that of the 250um Hi-GAL sources. 70um Hi-GAL sources define a much thinner disk, with a mean FWHM of 0{\deg}.75, and an average latitude of b=0{\deg}.06, coincident with the results from ATLASGAL. The GLAT distribution as a function of GLON shows modulations, both for the diffuse emission and for the compact sources, with ~0{\deg}.2 displacements mostly toward negative latitudes at l~ +40{\deg}, +12{\deg}, -25{\deg} and -40{\deg}. No such modulations can be found in the MIPSGAL 24 or WISE 22 um data when the entire source samples are considered. The distortions revealed by Herschel are interpreted as large-scale bending modes of the Plane. The lack of similar distortions in tracers of more evolved YSOs or stars rules out gravitational instabilities or satellite-induced perturbations, as they should act on both the diffuse and stellar disk components. We propose that the observed bends are caused by incoming flows of extra-planar gas interacting with the gaseous disk. Stars decouple from the gaseous ISM and relax into the stellar disk potential. The time required for the disappearance of the distortions from the diffuse ISM to the relatively evolved YSO stages are compatible with star-formation timescales.

Bipolar H II regions – Morphology and star formation in their vicinity

Our goal is to identify bipolar HII regions and to understand their morphology, their evolution, and the role they play in the formation of new generations of stars. We use the Spitzer and Herschel Hi-GAL surveys to identify bipolar HII regions. We search for their exciting star(s) and estimate their distances using near-IR data. Dense clumps are detected using Herschel-SPIRE data. MALT90 observations allow us to ascertain their association with the central HII region. We identify Class 0/I YSOs using their Spitzer and Herschel-PACS emissions. These methods will be applied to the entire sample of candidate bipolar HII regions. This paper focuses on two bipolar HII regions, one interesting in terms of its morphology, G319.88$+$00.79, and one in terms of its star formation, G010.32$-$00.15. Their exciting clusters are identified and their photometric distances estimated to be 2.6 kpc and 1.75 kpc, respectively. We suggest that these regions formed in dense and flat structures that contain filaments. They have a central ionized region and ionized lobes perpendicular to the parental cloud. The remains of the parental cloud appear as dense (more than 10^4 per cm^3) and cold (14-17 K) condensations. The dust in the PDR is warm (19-25 K). Dense massive clumps are present around the central ionized region. G010.32-00.14 is especially remarkable because five clumps of several hundred solar masses surround the central HII region; their peak column density is a few 10^23 per cm^2, and the mean density in their central regions reaches several 10^5 per cm^3. Four of them contain at least one massive YSO; these clumps also contain extended green objects and Class II methanol masers. This morphology suggests that the formation of a second generation of massive stars has been triggered by the central bipolar HII region. It occurs in the compressed material of the parental cloud.

Star Formation Thresholds: The View from Inside the Galaxy

AbstractWe explore the relationship between the total gas surface density and star formation rate surface density, a.k.a., the “Kennicutt-Schmidt relation,” in a Galactic context. Specifically, we probe the origins of thresholds in the behaviour of the K-S relation at 10 M⊙ pc−2 and 100-200 M⊙ pc−2 using images from the Herschel Hi-GAL and Gould Belt surveys. In both cases, pervasive filamentary structures are seen, possibly due to turbulent motions. The Hi-GAL image supports the view that at ~10 M⊙ pc−2 gas becomes molecular, leading to the formation of clouds that harbour star formation. The GBS images suggest the 100-200 M⊙ pc−2 threshold originates from the nature of filaments being stable until a critical column density of ~160 M⊙ pc−2 is reached. Therefore, the transition between non-star-forming and star-forming gas in clouds (and galaxies) may be set universally by the dynamical properties of filaments.

Herschel Hi-GAL imaging of massive young stellar objects

We used Herschel Hi-GAL survey data to determine whether massive young stellar objects (MYSOs) are resolved at 70$\mu$m and to study their envelope density distribution. Our analysis of three relatively isolated sources in the l=30{\deg} and l=59{\deg} Galactic fields show that the objects are partially resolved at 70$\mu$m. The Herschel Hi-GAL survey data have a high scan velocity which makes unresolved and partially resolved sources appear elongated in the 70$\mu$m images. We analysed the two scan directions separately and examine the intensity profile perpendicular to the scan direction. Spherically symmetric radiative transfer models with a power law density distribution were used to study the circumstellar matter distribution. Single dish sub-mm data were also included to study how different spatial information affects the fitted density distribution. The density distribution which best fits both the 70$\mu$m intensity profile and SED has an average index of ~0.5. This index is shallower than expected and is probably due to the dust emission from bipolar outflow cavity walls not accounted for in the spherical models. We conclude that 2D axisymmetric models and Herschel images at low scan speeds are needed to better constrain the matter distribution around MYSOs.