S. Molinari

MIPSGAL: A Survey of the Inner Galactic Plane at 24 and 70 μm

MIPSGAL is a 278 deg^2 survey of the inner Galactic plane using the Multiband Infrared Photometer for Spitzer aboard the Spitzer Space Telescope. The survey field was imaged in two passbands, 24 and 70 μm with resolutions of 6″ and 18″, respectively. The survey was designed to provide a uniform, well-calibrated and well-characterized data set for general inquiry of the inner Galactic plane and as a longer-wavelength complement to the shorter-wavelength Spitzer survey of the Galactic plane: Galactic Plane Infrared Mapping Survey Extraordinaire. The primary science drivers of the current survey are to identify all high-mass (M > 5 M⊙) protostars in the inner Galactic disk and to probe the distribution, energetics, and properties of interstellar dust in the Galactic disk. The observations were planned to minimize data artifacts due to image latents at 24 μm and to provide full coverage at 70 μm. Observations at ecliptic latitudes within 15° of the ecliptic plane were taken at multiple epochs to help reject asteroids. The data for the survey were collected in three epochs, 2005 September–October, 2006 April, and 2006 October with all of the data available to the public. The estimated point-source sensitivities of the survey are 2 and 75 mJy (3 σ) at 24 and 70 μm, respectively. Additional data processing was needed to mitigate image artifacts due to bright sources at 24 μm and detector responsivity variations at 70 μm due to the large dynamic range of the Galactic plane. Enhanced data products including artifact-mitigated mosaics and point-source catalogs are being produced with the 24 μm mosaics already publicly available from the NASA/IPAC Infrared Science Archive. Some preliminary results using the enhanced data products are described.

The data reduction pipeline for the Hi-GAL survey

We present the data reduction pipeline for the Herschel Infrared Galactic Plane survey (Hi-GAL). Hi-GAL is a key project of the Herschel satellite, which is mapping the inner part of the Galactic plane (|l| ≤ 70° and |b| ≤ 1°), using two Photodetector Array Camera and Spectrometer (PACS) and three Spectral and Photometric Imaging Receiver (SPIRE) frequency bands, from 70 to 500 μm. Our pipeline relies only partially on the Herschel Interactive Processing Environment (hipe). It features several newly developed routines to perform data reduction, including accurate data culling, noise estimation and minimum variance map-making, the latter performed with the romagal algorithm, a deep modification of the roma code already tested on cosmological surveys. We discuss in depth the properties of the Hi-GAL science demonstration phase data.

Cores in infrared dark clouds (IRDCs) seen in the Hi-GAL survey between l=300 degrees and 330 degrees

We have used data taken as part of the Herschel infrared Galactic Plane survey (Hi-GAL) to study 3171 infrared dark cloud (IRDC) candidates that were identified in the mid-IR (8 μm) by Spitzer (we refer to these as ‘Spitzer-dark’ regions). They all lie in the range l= 300–330° and |b|≤ 1°. Of these, only 1205 were seen in emission in the far-IR (250–500 μm) by Herschel (we call these ‘Herschel-bright’ clouds). It is predicted that a dense cloud will not only be seen in absorption in the mid-IR, but will also be seen in emission in the far-IR at the longest Herschel wavebands (250–500 μm). If a region is dark at all wavelengths throughout the mid-IR and far-IR, then it is most likely to be simply a region of lower background IR emission (a ‘hole in the sky’). Hence, it appears that previous surveys, based on Spitzer and other mid-IR data alone, may have overestimated the total IRDC population by a factor of ∼2. This has implications for estimates of the star formation rate in IRDCs in the Galaxy. We studied the 1205 Herschel-bright IRDCs at 250 μm and found that 972 of them had at least one clearly defined 250-μm peak, indicating that they contained one or more dense cores. Of these, 653 (67 per cent) contained an 8-μm point source somewhere within the cloud, 149 (15 per cent) contained a 24-μm point source but no 8-μm source and 170 (18 per cent) contained no 24- or 8-μm point sources. We use these statistics to make inferences about the lifetimes of the various evolutionary stages of IRDCs.

Cores in infrared dark clouds (IRDCs) seen in the Hi-GAL survey between l= 300° and 330°

We have used data taken as part of the Herschel infrared Galactic Plane survey (Hi-GAL) to study 3171 infrared dark cloud (IRDC) candidates that were identified in the mid-IR (8 μm) by Spitzer (we refer to these as ‘Spitzer-dark’ regions). They all lie in the range l= 300–330° and |b|≤ 1°. Of these, only 1205 were seen in emission in the far-IR (250–500 μm) by Herschel (we call these ‘Herschel-bright’ clouds). It is predicted that a dense cloud will not only be seen in absorption in the mid-IR, but will also be seen in emission in the far-IR at the longest Herschel wavebands (250–500 μm). If a region is dark at all wavelengths throughout the mid-IR and far-IR, then it is most likely to be simply a region of lower background IR emission (a ‘hole in the sky’). Hence, it appears that previous surveys, based on Spitzer and other mid-IR data alone, may have overestimated the total IRDC population by a factor of ∼2. This has implications for estimates of the star formation rate in IRDCs in the Galaxy. We studied the 1205 Herschel-bright IRDCs at 250 μm and found that 972 of them had at least one clearly defined 250-μm peak, indicating that they contained one or more dense cores. Of these, 653 (67 per cent) contained an 8-μm point source somewhere within the cloud, 149 (15 per cent) contained a 24-μm point source but no 8-μm source and 170 (18 per cent) contained no 24- or 8-μm point sources. We use these statistics to make inferences about the lifetimes of the various evolutionary stages of IRDCs.

The G305 star-forming complex: embedded massive star formation discovered by Herschel Hi-GAL

We present a Herschel far-infrared study towards the rich massive starforming complex G305, utilising PACS 70, 160µm and SPIRE 250, 350, and 500µm 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)> 1 and log(F160/F350)> 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 � 2 × 10 4 young stellar objects (YSOs) present, corresponding to a star formation rate (SFR) of 0.01-0.02 M⊙ 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 >2 in comparison to the SFR derived from the YSO population.

The initial conditions of high-mass star formation : radiative transfer models of IRDCs seen in the Herschel Hi-GAL survey

The densest infrared dark clouds (IRDCs) may represent the earliest observable stage of high-mass star formation. These clouds are very cold, hence they emit mainly at far-infrared and sub-mm wavelengths. For the first time, Herschel has provided multi-wavelength, spatially resolved observations of cores within IRDCs, which, when combined with radiative transfer modelling, can constrain their properties, such as mass, density profile and dust temperature. We use a 3D, multi-wavelength Monte Carlo radiative transfer code to model in detail the emission from six cores in three typical IRDCs seen in the Hi-GAL survey (G030.50+00.95, G031.03+00.26 and G031.03+00.76), and thereby to determine the properties of these cores and compare them with their low-mass equivalents. We found masses ranging from 90 to 290 M⊙ with temperatures from 8 to 11K at the centre of each core and 18 to 28 K at the surface. The maximum luminosity of an embedded star within each core was calculated, and we rule out the possibility of significant high mass star formation having yet occurred in three of our cores.

Isolated starless cores in infrared dark clouds in the Hi-GAL survey

In a previous paper, we identified cores within infrared dark clouds. We regarded those without embedded sources as the least evolved and labelled them starless. Here we identify the most isolated starless cores and model them using a three-dimensional multiwavelength Monte Carlo radiative transfer code. We derive the cores’ physical parameters and discuss the relation between the mass, temperature, density, size and the surrounding interstellar radiation field (ISRF) for the cores. The masses of the cores were found not to correlate with their radial size or central density. The temperature at the surface of a core was seen to depend almost entirely on the level of the ISRF surrounding the core. No correlation was found between the temperature at the centre of a core and its local ISRF. This was seen to depend, instead, on the density and mass of the core.

Calibration and performance of the LWS

The status of calibration and performance of the ISO Long-Wavelength Spectrometer eleven months after launch is described. The strategy followed for the calibration observations and first results are summarized. The overall performance of the instrument essentially fulfills the expectations; certain changes in sensitivity of the detectors are reported. Some improvements in the way observations are executed, which resulted from the in-flight experience, are explained.

The initial conditions for stellar protocluster formation - III. The Herschel counterparts of the Spitzer Dark Cloud catalogue

Context. Galactic plane surveys of pristine molecular clouds are key for establishing a Galactic-scale view of star formation. For this reason, an unbiased sample of infrared dark clouds in the 10° < | l | < 65°, | b | < 1° region of the Galactic plane was built using Spitzer 8 μm extinction. However, intrinsic fluctuations in the mid-infrared background can be misinterpreted as foreground clouds. Aims. The main goal of this study is to disentangle real clouds in the Spitzer Dark Cloud (SDC) catalogue from artefacts due to fluctuations in the mid-infrared background. Methods. We constructed H2 column density maps at ~18″ resolution using the 160 μm and 250 μm data from the Herschel Galactic plane survey Hi-GAL. We also developed an automated detection scheme that confirms the existence of a SDC through its association with a peak on these Herschel column density maps. Detection simulations, along with visual inspection of a small sub-sample of SDCs, have been performed to get more insight into the limitations of our automated identification scheme. Results. Our analysis shows that 76( ± 19)% of the catalogued SDCs are real. This fraction drops to 55( ± 12)% for clouds with angular diameters larger than ~1 arcmin. The contamination of the PF09 catalogue by large spurious sources reflects the large uncertainties associated to the construction of the 8 μm background emission, a key stage in identiying SDCs. A comparison of the Herschel confirmed SDC sample with the BGPS and ATLASGAL samples shows that SDCs probe a unique range of cloud properties, reaching down to more compact and lower column density clouds than any of these two (sub-)millimetre Galactic plane surveys. Conclusions. Even though about half of the large SDCs are spurious sources, the vast majority of the catalogued SDCs do have a Herschel counterpart. The Herschel-confirmed sample of SDCs offers a unique opportunity to study the earliest stages of both low- and high-mass star formation across the Galaxy.

The molecular environment of the Galactic star forming region G19.61–0.23

Context. Although current facilities allow the study of Galactic star formation at high angular resolution, our current understanding of the high-mass star-formation process is still very poor. In particular, we still need to characterize the properties of clouds giving birth to high-mass stars in our own Galaxy and use them as templates for our understanding of extragalactic star formation. Aims. We present single-dish (sub)millimeter observations of gas and dust in the Galactic high-mass star-forming region G19.61–0.23, with the aim of studying the large-scale properties and physical conditions of the molecular gas across the region. The final aim is to compare the large-scale (about 100 pc) properties with the small-scale (about 3 pc) properties and to consider possible implications for extragalactic studies. Methods. We have mapped CO isotopologues in the J = 1–0 transition using the FCRAO-14m telescope and the J = 2–1 transition using the IRAM-30m telescope. We have also used APEX 870 μm continuum data from the ATLASGAL survey and FCRAO supplementary observations of the ^(13)CO J = 1–0 line from the BU-FCRAO Galactic Ring Survey, as well as the Spitzer infrared Galactic plane surveys GLIMPSE and MIPSGAL to characterize the star-formation activity within the molecular clouds. Results. We reveal a population of molecular clumps in the ^(13)CO(1–0) emission, for which we derived the physical parameters, including sizes and masses. Our analysis of the ^(13)CO suggests that the virial parameter (ratio of kinetic to gravitational energy) varies over an order of magnitude between clumps that are unbound and some that are apparently “unstable”. This conclusion is independent of whether they show evidence of ongoing star formation. We find that the majority of ATLASGAL sources have MIPSGAL counterparts with luminosities in the range 10^(4)–5×10^4 L_☉ and are presumably forming relatively massive stars. We compare our results with previous extragalactic studies of the nearby spiral galaxies M 31 and M 33; and the blue compact dwarf galaxy Henize 2–10. We find that the main giant molecular cloud surrounding G19.61–0.23 has physical properties typical for Galactic GMCs and which are comparable to the GMCs in M 31 and M 33. However, the GMC studied here shows smaller surface densities and masses than the clouds identified in Henize 2–10 and associated with super star cluster formation.