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Register a new userMulti-temperature mapping of dust structures throughout the Galactic Plane using the PPMAP tool with Herschel Hi-GAL data
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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.
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Past and recent observations have revealed unexpected variations in the FIR-mm dust emissivity. In the Herschel spectral range, those are often referred to as a 500{mu}m emission excess. Several dust emission models have been developed to interpret astrophysical data in the FIR-mm domain. However, these are commonly unable to fully reconcile theoretical predictions with observations. In contrast, the recently revised two level system (TLS) model seems to provide a promising way of interpreting the existing data. The newly available Herschel Hi-GAL data which covers most of the inner Milky-Way offers a unique opportunity to investigate possible variations in the dust emission properties both with wavelength and environment. By combining the IRIS 100 {mu}m with the Hi-GAL 160, 250, 350 and 500 {mu}m data, we model the dust emission spectra in each pixel of the Hi-GAL maps, using both the TLS model and, for comparison, a single modified black-body fit. The effect of temperature mixing along the line of sight is investigated. We find a slight decrease in the dust temperature with distance from the Galactic center. We also report the detection of a significant 500 {mu}m emissivity excess in the peripheral regions of the plane (35circ<|l|<70circ) of about 13-15% of the emissivity, that can reach up to 20% in some HII regions. We present the spatial distribution of the best-fit values for the two main parameters of the TLS model, i.e. the charge correlation length, lc, used to characterize the disordered charge distribution (DCD) part of the model, and the amplitude A of the TLS processes, with respect to the DCD effect. They highlight the plausible existence of an overall gradient with distance to the Galactic center. A comparison with previous findings in the solar neighborhood shows that the local value of the excess is less than expected from the Galactic gradient observed here.
The recent data collected by {it Herschel} have confirmed that interstellar structures with filamentary shape are ubiquitously present in the Milky Way. Filaments are thought to be formed by several physical mechanisms acting from the large Galactic scales down to the sub-pc fractions of molecular clouds, and they might represent a possible link between star formation and the large-scale structure of the Galaxy. In order to study this potential link, a statistically significant sample of filaments spread throughout the Galaxy is required. In this work we present the first catalogue of $32,059$ candidate filaments automatically identified in the Hi-GAL survey of the entire Galactic Plane. For these objects we determined morphological (length, $l^{a}$, and geometrical shape) and physical (average column density, $N_{rm H_{2}}$, and average temperature, $T$) properties. We identified filaments with a wide range of properties: 2$$,$leq l^{a}leq$, 100$$, $10^{20} leq N_{rm H_{2}} leq 10^{23}$,cm$^{-2}$ and $10 leq Tleq$ 35,K. We discuss their association with the Hi-GAL compact sources, finding that the most tenuous (and stable) structures do not host any major condensation and we also assign a distance to $sim 18,400$ filaments for which we determine mass, physical size, stability conditions and Galactic distribution. When compared to the spiral arms structure, we find no significant difference between the physical properties of on-arm and inter-arm filaments. We compared our sample with previous studies, finding that our Hi-GAL filament catalogue represents a significant extension in terms of Galactic coverage and sensitivity. This catalogue represents an unique and important tool for future studies devoted to understanding the filament life-cycle.
Variations in the dust emissivity are critical for gas mass determinations derived from far-infrared observations, but also for separating dust foreground emission from the Cosmic Microwave Background (CMB). Hi-GAL observations allow us for the first time to study the dust emissivity variations in the inner regions of the Galactic plane at resolution below 1 degree. We present maps of the emissivity spectral index derived from the combined Herschel PACS 160 mu m, SPIRE 250 mu m, 350 mu m, and 500 mu m data, and the IRIS 100 mu m data, and we analyze the spatial variations of the spectral index as a function of dust temperature and wavelength in the two Science Demonstration Phase Hi-GAL fields, centered at l=30{deg} and l=59{deg}. Applying two different methods, we determine both dust temperature and emissivity spectral index between 100 and 500 mu m, at an angular resolution of 4. Combining both fields, the results show variations of the emissivity spectral index in the range 1.8-2.6 for temperatures between 14 and 23 K. The median values of the spectral index are similar in both fields, i.e. 2.3 in the range 100-500 mu m, while the median dust temperatures are equal to 19.1 K and 16.0 K in the l=30{deg} and l=59{deg} field, respectively. Statistically, we do not see any significant deviations in the spectra from a power law emissivity between 100 and 500 mu m. We confirm the existence of an inverse correlation between the emissivity spectral index and dust temperature, found in previous analyses.
We present the physical and evolutionary properties of prestellar and protostellar clumps in the Herschel Infrared GALactic plane survey (Hi-GAL) in two large areas centered in the Galactic plane and covering the tips of the long Galactic bar at the intersection with the spiral arms. The areas fall in the longitude ranges 19 < l < 33 and 340 < l < 350, while latitude is -1 < b < 1. Newly formed high mass stars and prestellar objects are identified and their properties derived and compared. A study is also presented on five giant molecular complexes at the further edge of the bar. The star-formation rate was estimated from the quantity of proto-stars expected to form during the collapse of massive turbulent clumps into star clusters. This new method was developed by applying a Monte Carlo procedure to an evolutionary model of turbulent cores and takes into account the wide multiplicity of sources produced during the collapse. The star-formation rate density values at the tips are 1.2 +- 0.3 10-3 Msol/yr/kpc2 and 1.5+-0.3 10-3 Msol/yr/kpc2 in the first and fourth quadrant, respectively. The same values estimated on the entire field of view, that is including the tips of the bar and background and foreground regions, are 0.9+-0.2 10-3 Msol/yr/kpc2 and 0.8+-0.2 10-3 Msol/yr/kpc2. The conversion efficiency is approximately 0.8% in the first quadrant and 0.5% in the fourth quadrant, and does not show a significant difference in proximity of the bar. The star forming regions identified through CO contours at the further edge of the bar show star-formation rate densities larger than the surrounding regions but their conversion efficiencies are comparable. Our results suggest that the star-formation activity at the bar is due to a large amount of dust and molecular material rather than being due to a triggering process.
We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key-project that will map the inner Galactic Plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2{deg} x 2{deg} tiles approximately centered at l=30{deg} and l=59{deg}. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around A_V of about 1 is exceeded for the regions in the l=59{deg} field; a A_V value between 5 and 10 is found for the l=30{deg} field, likely due to the relatively larger distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm-2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.
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