We study the fragmentation of the nearest high line-mass filament, the integral shaped filament (ISF, line-mass ~400 M⊙ pc-1) in the Orion A molecular cloud. We have observed a 1.6 pc long section of ...the ISF with the Atacama Large Millimetre/submillimeter Array (ALMA) at 3 mm continuum emission, at a resolution of ~3″ (1200 AU). We identify from the region 43 dense cores with masses about a solar mass. 60% of the ALMA cores are protostellar and 40% are starless. The nearest neighbour separations of the cores do not show a preferred fragmentation scale; the frequency of short separations increases down to 1200 AU. We apply a two-point correlation analysis on the dense core separations and show that the ALMA cores are significantly grouped at separations below ~17 000 AU and strongly grouped below ~6000 AU. The protostellar and starless cores are grouped differently: only the starless cores group strongly below ~6000 AU. In addition, the spatial distribution of the cores indicates periodic grouping of the cores into groups of ~30 000 AU in size, separated by ~50 000 AU. The groups coincide with dust column density peaks detected by Herschel. These results show hierarchical, two-mode fragmentation in which the maternal filament periodically fragments into groups of dense cores. Critically, our results indicate that the fragmentation models for lower line-mass filaments (~16 M⊙ pc-1) fail to capture the observed properties of the ISF. We also find that the protostars identified with Spitzer and Herschel in the ISF are grouped at separations below ~17 000 AU. In contrast, young stars with disks do not show significant grouping. This suggests that the grouping of dense cores is partially retained over the protostar lifetime, but not over the lifetime of stars with disks. This is in agreement with a scenario where protostars are ejected from the maternal filament by the slingshot mechanism, a model recently proposed for the ISF. The separation distributions of the dense cores and protostars may also provide an evolutionary tracer of filament fragmentation.
Context. Filamentary structures are common morphological features of the cold, molecular interstellar medium (ISM). Recent studies have discovered massive, hundred-parsec-scale filaments that may be ...connected to the large-scale, Galactic spiral arm structure. Addressing the nature of these giant molecular filaments (GMFs) requires a census of their occurrence and properties. Aims. We perform a systematic search of GMFs in the fourth Galactic quadrant and determine their basic physical properties. Methods. We identify GMFs based on their dust extinction signatures in the near- and mid-infrared and the velocity structure probed by super(13) CO line emission. We use the super(13) CO line emission and ATLASGAL dust emission data to estimate the total and dense gas masses of the GMFs. We combine our sample with an earlier sample from literature and study the Galactic environment of the GMFs. Results. We identify nine GMFs in the fourth Galactic quadrant: six in the Centaurus spiral arm and three in inter-arm regions. Combining this sample with an earlier study using the same identification criteria in the first Galactic quadrant results in 16 GMFs, nine of which are located within spiral arms. The GMFs have sizes of 80-160 pc and super(13) CO-derived masses between 5?90 x 10 super(4)M sub(?). Their dense gas mass fractions are between 1.5-37%, which is higher in the GMFs connected to spiral arms. We also compare the different GMF-identification methods and find that emission and extinction-based techniques overlap only partially, thereby highlighting the need to use both to achieve a complete census.
We present the first study of the relationship between the column density distribution of molecular clouds within nearby Galactic spiral arms and their evolutionary status as measured from their ...stellar content. We analyze a sample of 195 molecular clouds located at distances below 5.5 kpc, identified from the ATLASGAL 870 μm data. We define three evolutionary classes within this sample: starless clumps, star-forming clouds with associated young stellar objects, and clouds associated with H ii regions. We find that the N(H2) probability density functions (N-PDFs) of these three classes of objects are clearly different: the N-PDFs of starless clumps are narrowest and close to log-normal in shape, while star-forming clouds and H ii regions exhibit a power-law shape over a wide range of column densities and log-normal-like components only at low column densities. We use the N-PDFs to estimate the evolutionary time-scales of the three classes of objects based on a simple analytic model from literature. Finally, we show that the integral of the N-PDFs, the dense gas mass fraction, depends on the total mass of the regions as measured by ATLASGAL: more massive clouds contain greater relative amounts of dense gas across all evolutionary classes.
Context. Herschel has revolutionized our ability to measure column densities (NH) and temperatures (T) of molecular clouds thanks to its far infrared multiwavelength coverage. However, the lack of a ...well defined background intensity level in the Herschel data limits the accuracy of the NH and T maps. Aims. We aim to provide a method that corrects the missing Herschel background intensity levels using the Planck model for foreground Galactic thermal dust emission. For the Herschel/PACS data, both the constant-offset as well as the spatial dependence of the missing background must be addressed. For the Herschel/SPIRE data, the constant-offset correction has already been applied to the archival data so we are primarily concerned with the spatial dependence, which is most important at 250 μm. Methods. We present a Fourier method that combines the publicly available Planck model on large angular scales with the Herschel images on smaller angular scales. Results. We have applied our method to two regions spanning a range of Galactic environments: Perseus and the Galactic plane region around l = 11deg (HiGal–11). We post-processed the combined dust continuum emission images to generate column density and temperature maps. We compared these to previously adopted constant-offset corrections. We find significant differences (≳20%) over significant (~15%) areas of the maps, at low column densities (NH ≲ 1022 cm-2) and relatively high temperatures (T ≳ 20 K). We have also applied our method to synthetic observations of a simulated molecular cloud to validate our method. Conclusions. Our method successfully corrects the Herschel images, including both the constant-offset intensity level and the scale-dependent background variations measured by Planck. Our method improves the previous constant-offset corrections, which did not account for variations in the background emission levels.
Density profiles of isolated cores derived from thermal dust continuum emission rely on models of dust properties, such as mass opacity, that are poorly constrained. With complementary measures from ...near-infrared extinction maps, we can assess the reliability of commonly used dust models. In this work, we compare Herschel-derived maps of the optical depth with equivalent maps derived from CFHT WIRCAM near-infrared observations for three isolated cores: CB 68, L 429, and L 1552. We assess the dust opacities provided from four models: OH1a, OH5a, Orm1, and Orm4. Although the consistency of the models differs between the three sources, the results suggest that the optical properties of dust in the envelopes of the cores are best described by either silicate and bare graphite grains (e.g., Orm1) or carbonaceous grains with some coagulation and either thin or no ice mantles (e.g., OH5a). None of the models, however, individually produced the most consistent optical depth maps for every source. The results suggest that either the dust in the cores is not well-described by any one dust property model, the application of the dust models cannot be extended beyond the very center of the cores, or more complex SED fitting functions are necessary.
Aims. We aim to better understand the heating of gas by observing the prominent gas cooling line CII at 158 mu m in the low-metallicity environment of the Local Group spiral galaxy M33 on scales of ...280 pc. In particular, we describe the variation of the photoelectric heating efficiency with the galactic environment. Methods. In this study, we present CII observations along the major axis of M33 using the Infrared Space Observatory in combination with Herschel continuum maps, IRAM 30m CO 2-1, and VLA H I data to study the variation in velocity integrated intensities. The ratio of CII emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, far-ultraviolet radiation fields, and average column densities of the molecular clouds. Results. The heating efficiency stays constant at 0.8% in the inner 4.5 kpc radius of the galaxy, where it increases to reach values of ~3% in the outskirts at about a 6 kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields for optical extinctions of only a few magnitudes at constant metallicity. For the significant fraction of H I emission stemming from PDRs and for typical pressures found in the Galactic cold neutral medium (CNM) traced by H I emission, the CNM contributes ~15% to the observed CII emission in the inner 2 kpc radius of M33. The CNM contribution remains largely undetermined in the south, while positions between radial distances of 2 and 7.3 kpc in the north of M33 show a contribution of ~40% + or - 20%.
Aims. We aim to better understand the heating of gas by observing the prominent gas cooling line C ii at 158 μm in the low-metallicity environment of the Local Group spiral galaxy M 33 on scales of ...280 pc. In particular, we describe the variation of the photoelectric heating efficiency with the galactic environment. Methods. In this study, we present C ii observations along the major axis of M 33 using the Infrared Space Observatory in combination with Herschel continuum maps, IRAM 30 m CO 2−1, and VLA H i data to study the variation in velocity integrated intensities. The ratio of C ii emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, far-ultraviolet radiation fields, and average column densities of the molecular clouds. Results. The heating efficiency stays constant at 0.8% in the inner 4.5 kpc radius of the galaxy, where it increases to reach values of ~3% in the outskirts at about a 6 kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields for optical extinctions of only a few magnitudes at constant metallicity. For the significant fraction of H i emission stemming from PDRs and for typical pressures found in the Galactic cold neutral medium (CNM) traced by H i emission, the CNM contributes ~15% to the observed C ii emission in the inner 2 kpc radius of M 33. The CNM contribution remains largely undetermined in the south, while positions between radial distances of 2 and 7.3 kpc in the north of M 33 show a contribution of ~40% ± 20%.
Context. Filamentary structures are common morphological features of the cold, molecular interstellar medium (ISM). Recent studies have discovered massive, hundred-parsec-scale filaments that may be ...connected to the large-scale, Galactic spiral arm structure. Addressing the nature of these giant molecular filaments (GMFs) requires a census of their occurrence and properties. Aims. We perform a systematic search of GMFs in the fourth Galactic quadrant and determine their basic physical properties. Methods. We identify GMFs based on their dust extinction signatures in the near- and mid-infrared and the velocity structure probed by 13CO line emission. We use the 13CO line emission and ATLASGAL dust emission data to estimate the total and dense gas masses of the GMFs. We combine our sample with an earlier sample from literature and study the Galactic environment of the GMFs. Results. We identify nine GMFs in the fourth Galactic quadrant: six in the Centaurus spiral arm and three in inter-arm regions. Combining this sample with an earlier study using the same identification criteria in the first Galactic quadrant results in 16 GMFs, nine of which are located within spiral arms. The GMFs have sizes of 80–160 pc and 13CO-derived masses between 5−90 × 104M⊙. Their dense gas mass fractions are between 1.5–37%, which is higher in the GMFs connected to spiral arms. We also compare the different GMF-identification methods and find that emission and extinction-based techniques overlap only partially, thereby highlighting the need to use both to achieve a complete census.
Aims. Within the framework of the Herschel M 33 extended survey HerM33es and in combination with multi-wavelength data we study the spectral energy distribution (SED) of a set of H ii regions in the ...Local Group galaxy M 33 as a function of the morphology. We analyse the emission distribution in regions with different morphologies and present models to infer the Hα emission measure observed for H ii regions with well defined morphology. Methods. We present a catalogue of 119 H ii regions morphologically classified: 9 filled, 47 mixed, 36 shell, and 27 clear shell H ii regions. For each object we extracted the photometry at twelve available wavelength bands, covering a wide wavelength range from FUV-1516 Å (GALEX) to IR-250 μm (Herschel), and we obtained the SED for each object. We also obtained emission line profiles in vertical and horizontal directions across the regions to study the location of the stellar, ionised gas, and dust components. We constructed a simple geometrical model for the clear shell regions, whose properties allowed us to infer the electron density of these regions. Results. We find trends for the SEDs related to the morphology of the regions, showing that the star and gas-dust configuration affects the ratios of the emission in different bands. The mixed and filled regions show higher emission at 24 μm, corresponding to warm dust, than the shells and clear shells. This could be due to the proximity of the dust to the stellar clusters in the case of filled and mixed regions. The far-IR peak for shells and clear shells seems to be located towards longer wavelengths, indicating that the dust is colder for this type of object. The logarithmic 100 μm/70 μm ratio for filled and mixed regions remains constant over one order of magnitude in Hα and FUV surface brightness, while the shells and clear shells exhibit a wider range of values of almost two orders of magnitude. We derive dust masses and dust temperatures for each H ii region by fitting the individual SEDs with dust models proposed in the literature. The derived dust mass range is between 102−104 M⊙ and the cold dust temperature spans Tcold ~ 12−27 K. The spherical geometrical model proposed for the Hα clear shells is confirmed by the emission profile obtained from the observations and is used to infer the electron density within the envelope: the typical electron density is 0.7 ± 0.3 cm-3, while filled regions can reach values that are two to five times higher.
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