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Register a new userAn analysis of star formation with Herschel in the Hi-GAL Survey. II. The tips of the Galactic bar
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Marcella Veneziani Ms.
Publication date
2016
fields
Physics
and research's language is
English
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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.
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The Herschel survey of the Galactic Plane (Hi-GAL) provides a unique opportunity to study star formation over large areas of the sky and different environments in the Milky Way. We use the best studied Hi-GAL fields to date, two 2x2 tiles centered on (l, b) = (30, 0) deg and (l, b) = (59, 0) deg, to study the star formation activity using a large sample of well selected young stellar objects (YSOs). We estimate the star formation rate (SFR) for these fields using the number of candidate YSOs and their average time scale to reach the Zero Age Main Sequence, and compare it with the rate estimated using their integrated luminosity at 70 micron combined with an extragalactic star formation indicator. We measure a SFR of (9.5 +- 4.3)*10^{-4} Msol/yr and (1.6 +- 0.7)*10^{-4} Msol/yr with the source counting method, in l=30 deg and l=59 deg, respectively. Results with the 70 micron estimator are (2.4 +- 0.4)*10^{-4} Msol/yr and (2.6 +- 1.1)*10^{-6} Msol/yr. Since the 70 micron indicator is derived from averaging extragalactic star forming complexes, we perform an extrapolation of these values to the whole Milky Way and obtain SFR_{MW} = (0.71 +- 0.13) Msol/yr from l = 30 deg and SFR_{MW} = (0.10 +- 0.04) Msol/yr from l=59 deg. The estimates in l=30 deg are in agreement with the most recent results on the Galactic star formation activity, indicating that the characteristics of this field are likely close to those of the star-formation dominated galaxies used for its derivation. Since the sky coverage is limited, this analysis will improve when the full Hi-GAL survey will be available.
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.
We present a Herschel far-infrared study towards the rich massive star- forming complex G305, utilising PACS 70, 160 {mu}m and SPIRE 250, 350, and 500 {mu}m observations from the Hi-GAL survey of the Galactic plane. The focus of this study is to identify the embedded massive star-forming population within G305, by combining far-infrared data with radio continuum, H2O maser, methanol maser, MIPS, and Red MSX Source survey data available from previous studies. By applying a frequentist technique we are able to identify a sample of the most likely associations within our multi-wavelength dataset, that can then be identified from the derived properties obtained from fitted spectral energy distributions (SEDs). By SED modelling using both a simple modified blackbody and fitting to a comprehensive grid of model SEDs, some 16 candidate associations are identified as embedded massive star-forming regions. We derive a two-selection colour criterion from this sample of log(F70/F500)geq 1 and log(F160/F350)geq 1.6 to identify an additional 31 embedded massive star candidates with no associated star-formation tracers. Using this result we can build a picture of the present day star-formation of the complex, and by extrapolating an initial mass function, suggest a current population of approx 2 times 10^4 young stellar objects (YSOs) present, corresponding to a star formation rate (SFR) of 0.01-0.02 Modot yr^-1. Comparing this resolved star formation rate, to extragalactic star formation rate tracers (based on the Kennicutt-Schmidt relation), we find the star formation activity is underestimated by a factor of geq 2 in comparison to the SFR derived from the YSO population.
We demonstrate the use of the 3D Monte Carlo radiative transfer code PHAETHON to model infrared-dark clouds (IRDCs) that are externally illuminated by the interstellar radiation field (ISRF). These clouds are believed to be the earliest observed phase of high-mass star formation, and may be the high-mass equivalent of lower-mass prestellar cores. We model three different cases as examples of the use of the code, in which we vary the mass, density, radius, morphology and internal velocity field of the IRDC. We show the predicted output of the models at different wavelengths chosen to match the observing wavebands of Herschel and Spitzer. For the wavebands of the long- wavelength SPIRE photometer on Herschel, we also pass the model output through the SPIRE simulator to generate output images that are as close as possible to the ones that would be seen using SPIRE. We then analyse the images as if they were real observations, and compare the results of this analysis with the results of the radiative transfer models. We find that detailed radiative transfer modelling is necessary to accurately determine the physical parameters of IRDCs (e.g. dust temperature, density profile). This method is applied to study G29.55+00.18, an IRDC observed by the Herschel Infrared Galactic Plane survey (Hi-GAL), and in the future it will be used to model a larger sample of IRDCs from the same survey.
We present a first study of the star-forming compact dust condensations revealed by Herschel in the two 2 times 2 degr Galactic Plane fields centered at [l;b] = [30degr; 0 degr] and [l;b] = [59degr; 0 degr], respectively, and observed during the Science Demonstration Phase for the Herschel infrared Galactic Plane survey (Hi-GAL) Key-Project. Compact source catalogs extracted for the two fields in the five Hi-GAL bands (70, 160, 250, 350 and 500 $mu$m) were merged based on simple criteria of positional association and spectral energy distribution (SED) consistency into a final catalog which contains only coherent SEDs with counterparts in at least three adjacent Herschel bands. These final source lists contain 528 entries for the l = 30degr field, and 444 entries for the l = 59degr field. The SED coverage has been augmented with ancillary data at 24 $mu$m and 1.1 mm. SED modeling for the subset of 318 and 101 sources (in the two fields, respectively) for which the distance is known was carried out using both a structured star/disk/envelope radiative transfer model and a simple isothermal grey-body. Global parameters like mass, luminosity, temperature and dust properties have been estimated. The Lbol/Menv ratio spans four orders of magnitudes from values compatible with the pre-protostellar phase to embedded massive zero-age main sequence stars. Sources in the l = 59degr field have on average lower L/M, possibly outlining an overall earlier evolutionary stage with respect to the sources in the l = 30degr field. Many of these cores are actively forming high-mass stars, although the estimated core surface densities appear to be an order of magnitude below the 1 g cm$^{-2}$ critical threshold for high-mass star formation.
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