Context.
The Mid-Infrared Instrument (MIRI) onboard the
James Webb
Space Telescope (JWST) will provide imaging, coronagraphy, low-resolution spectroscopy, and medium-resolution spectroscopy at ...unprecedented sensitivity levels in the mid-infrared wavelength range. The Medium Resolution Spectrometer (MRS) of MIRI is an integral field spectrograph that provides diffraction-limited spectroscopy between 4.9 and 28.3 μm, within a field of view (FOV) varying from ∼13 to ∼56 arcsec square. The design for MIRI MRS conforms with the goals of the JWST mission to observe high redshift galaxies and to study cosmology as well as observations of galactic objects, and stellar and planetary systems.
Aims.
From ground testing, we calculate the physical parameters essential for general observers and calibrating the wavelength solution and resolving power of the MRS which is critical for maximizing the scientific performance of the instrument.
Methods.
We have used ground-based observations of discrete spectral features in combination with Fabry-Perot etalon spectra to characterize the wavelength solution and spectral resolving power of the MRS. We present the methodology used to derive the MRS spectral characterization, which includes the precise wavelength coverage of each MRS sub-band, computation of the resolving power as a function of wavelength, and measuring slice-dependent spectral distortions.
Results.
The ground calibration of the MRS shows that it will cover the wavelength ranges from 4.9 to 28.3 μm, divided in 12 overlapping spectral sub-bands. The resolving power is
R
≳ 3500 in channel 1,
R
≳ 3000 in channel 2,
R
≳ 2500 in channel 3, and
R
≳ 1500 in channel 4. The MRS spectral resolution optimizes the sensitivity for detection of spectral features with a velocity width of ∼100 km s
−1
which is characteristic of most astronomical phenomena JWST aims to study in the mid-infrared. Based on the ground test data, the wavelength calibration accuracy is estimated to be below one-tenth of a pixel (0.1 nm at 5 μm and 0.4 at 28 μm), with small systematic shifts due to the target position within a slice for unresolved sources that have a maximum amplitude of about 0.25 spectral resolution elements. The absolute wavelength calibration is presently uncertain at the level of 0.35 nm at 5 μm and 46 nm at 28 μm, and it will be refined using in-flight commissioning observations.
Conclusions.
Based on ground test data, the MRS complies with the spectral requirements for both the
R
and wavelength accuracy for which it was designed. We also present the commissioning strategies and targets that will be followed to update the spectral characterization of the MRS.
We present a comprehensive statistical analysis of star-forming objects located in the vicinities of 1360 bubble structures throughout the Galactic plane and their local environments. The compilation ...of ~70 000 star-forming sources, found in the proximity of the ionized (Hii) regions and detected in both Hi-GAL and GLIMPSE surveys, provided a broad overview of the different evolutionary stages of star-formation in bubbles, from prestellar objects to more evolved young stellar objects (YSOs). Surface density maps of star-forming objects clearly reveal an evolutionary trend where more evolved star-forming objects (Class II YSO candidates) are found spatially located near the center, while younger star-forming objects are found at the edge of the bubbles. We derived dynamic ages for a subsample of 182 H ii regions for which kinematic distances and radio continuum flux measurements were available. We detect approximately 80% more star-forming sources per unit area in the direction of bubbles than in the surrounding fields. We estimate the clump formation efficiency (CFE) of Hi-GAL clumps in the direction of the shell of the bubbles to be ~15%, around twice the value of the CFE in fields that are not affected by feedback effects. We find that the higher values of CFE are mostly due to the higher CFE of protostellar clumps, in particular in younger bubbles, whose density of the bubble shells is higher. We argue that the formation rate from prestellar to protostellar phase is probably higher during the early stages of the (H ii ) bubble expansion. Furthermore, we also find a higher fraction of massive YSOs (MYSOs) in bubbles at the early stages of expansion (<2 Myr) than older bubbles. Evaluation of the fragmentation time inside the shell of bubbles advocates the preexistence of clumps in the medium before the bubble expansion in order to explain the formation of MYSOs in the youngest H ii regions (<1 Myr), as supported by numerical simulations. Approximately 23% of the Hi-GAL clumps are found located in the direction of a bubble, with 15% for prestellar clumps and 41% for protostellar clumps. We argue that the high fraction of protostellar clumps may be due to the acceleration of the star-formation process cause by the feedback of the (Hii) bubbles.
Aims. We present the first public release of high-quality data products (DR1) from Hi-GAL, the Herschel infrared Galactic Plane Survey. Hi-GAL is the keystone of a suite of continuum Galactic plane ...surveys from the near-IR to the radio and covers five wavebands at 70, 160, 250, 350 and 500 mu m, encompassing the peak of the spectral energy distribution of cold dust for 8 < or = T< or = 50 K. This first Hi-GAL data release covers the inner Milky Way in the longitude range 68degrees > or = scriptl> or = -70degrees in a b < or = 1degrees latitude strip. Methods. Photometric maps have been produced with the ROMAGAL pipeline, which optimally capitalizes on the excellent sensitivity and stability of the bolometer arrays of the Herschel PACS and SPIRE photometric cameras. It delivers images of exquisite quality and dynamical range, absolutely calibrated with Planck and IRAS, and recovers extended emission at all wavelengths and all spatial scales, from the point-spread function to the size of an entire 2degreesx 2degrees "tile" that is the unit observing block of the survey. The compact source catalogues were generated with the CuTEx algorithm, which was specifically developed to optimise source detection and extraction in the extreme conditions of intense and spatially varying background that are found in the Galactic plane in the thermal infrared. Results. Hi-GAL DR1 images are cirrus noise limited and reach the 1sigma-rms predicted by the Herschel Time Estimators for parallel-mode observations at 60''s super(-1) scanning speed in relatively low cirrus emission regions. Hi-GAL DR1 images will be accessible through a dedicated web-based image cutout service. The DR1 Compact Source Catalogues are delivered as single-band photometric lists containing, in addition to source position, peak, and integrated flux and source sizes, a variety of parameters useful to assess the quality and reliability of the extracted sources. Caveats and hints to help in this assessment are provided. Flux completeness limits in all bands are determined from extensive synthetic source experiments and greatly depend on the specific line of sight along the Galactic plane because the background strongly varies as a function of Galactic longitude. Hi-GAL DR1 catalogues contain 123210, 308509, 280685, 160972, and 85460 compact sources in the five bands.
ABSTRACT
We present the 360° catalogue of physical properties of Hi-GAL compact sources, detected between 70 and 500 $\mu$m. This release not only completes the analogous catalogue previously ...produced by the Hi-GAL collaboration for −71° ≲ ℓ ≲ 67°, but also meaningfully improves it because of a new set of heliocentric distances, 120 808 in total. About a third of the 150 223 entries are located in the newly added portion of the Galactic plane. A first classification based on detection at 70 $\mu$m as a signature of ongoing star-forming activity distinguishes between protostellar sources (23 per cent of the total) and starless sources, with the latter further classified as gravitationally bound (pre-stellar) or unbound. The integral of the spectral energy distribution, including ancillary photometry from λ = 21 to 1100 $\mu$m, gives the source luminosity and other bolometric quantities, while a modified blackbody fitted to data for $\lambda \ge 160~\mu$m yields mass and temperature. All tabulated clump properties are then derived using photometry and heliocentric distance, where possible. Statistics of these quantities are discussed with respect to both source Galactic location and evolutionary stage. No strong differences in the distributions of evolutionary indicators are found between the inner and outer Galaxy. However, masses and densities in the inner Galaxy are on average significantly larger, resulting in a higher number of clumps that are candidates to host massive star formation. Median behaviour of distance-independent parameters tracing source evolutionary status is examined as a function of the Galactocentric radius, showing no clear evidence of correlation with spiral arm positions.
Abstract
We present a new derivation of the Milky Way’s current star formation rate (SFR) based on the data of the Herschel InfraRed Galactic Plane Survey (Hi-GAL). We estimate the distribution of ...the SFR across the Galactic plane from the star-forming clumps identified in the Hi-GAL survey and calculate the total SFR from the sum of their contributions. The estimate of the global SFR amounts to 2.0 ± 0.7
M
⊙
yr
−1
, of which 1.7 ± 0.6
M
⊙
yr
−1
coming from clumps with reliable heliocentric distance assignment. This value is in general agreement with estimates found in the literature of last decades. The profile of SFR density averaged in Galactocentric rings is found to be qualitatively similar to others previously computed, with a peak corresponding to the Central Molecular Zone and another one around Galactocentric radius
R
gal
∼ 5 kpc, followed by an exponential decrease as
log
(
Σ
SFR
/
M
⊙
yr
−
1
kpc
−
2
)
=
a
R
gal
/
kpc
+
b
, with
a
= −0.28 ± 0.01. In this regard, the fraction of SFR produced within and outside the solar circle is 84% and 16%, respectively; the fraction corresponding to the far outer Galaxy (
R
gal
> 13.5 kpc) is only 1%. We also find that, for
R
gal
> 3 kpc, our data follow a power law as a function of density, similarly to the Kennicutt–Schmidt relation. Finally, we compare the distribution of the SFR density across the face-on Galactic plane and those of median parameters, such as temperature, luminosity/mass ratio, and bolometric temperature, describing the evolutionary stage of Hi-GAL clumps. We found no clear correlation between the SFR and the clump evolutionary stage.
Context.
Supernova remnants (SNRs) represent a major feedback source from stars in the interstellar medium of galaxies. During the latest stage of supernova explosions, shock waves produced by the ...initial blast modify the chemistry of gas and dust, inject kinetic energy into the surroundings, and may alter star formation characteristics. Simultaneously,
γ
-ray emission is generated by the interaction between the ambient medium and cosmic rays (CRs), including those accelerated in the early stages of the explosion.
Aims.
We study the stellar and interstellar contents of IC443, an evolved shell-type SNR at a distance of 1.9 kpc with an estimated age of 30 kyr. We aim to measure the mass of the gas and characterize the nature of infrared point sources within the extended G region, which corresponds to the peak of
γ
-ray emission detected by VERITAS and
Fermi
.
Methods.
We performed 10′ × 10′ mapped observations of
12
CO,
13
CO
J
= 1–0,
J
= 2–1, and
J
= 3–2 pure rotational lines, as well as C
18
O
J
= 1–0 and
J
= 2–1 obtained with the IRAM 30 m and APEX telescopes over the extent of the
γ
-ray peak to reveal the molecular structure of the region. We first compared our data with local thermodynamic equilibrium models. We estimated the optical depth of each line from the emission of the isotopologs
13
CO and C
18
O. We used the population diagram and large velocity gradient assumption to measure the column density, mass, and kinetic temperature of the gas using
12
CO and
13
CO lines. We used complementary data (stars, gas, and dust at multiple wavelengths) and infrared point source catalogs to search for protostar candidates.
Results.
Our observations reveal four molecular structures: a shocked molecular clump associated with emission lines extending between −31 and 16 km s
−1
, a quiescent, dark cloudlet associated with a line width of ~2 km s
−1
, a narrow ring-like structure associated with a line width of ~1.5 km s
−1
, and a shocked knot. We measured a total mass of ~230, ~90, ~210, and ~4
M
⊙
, respectively, for the cloudlet, ring-like structure, shocked clump, and shocked knot. We measured a mass of ~1100
M
⊙
throughout the rest of the field of observations where an ambient cloud is detected. We found 144 protostar candidates in the region.
Conclusions.
Our results emphasize how the mass associated with the ring-like structure and the cloudlet cannot be overlooked when quantifying the interaction of CRs with the dense local medium. Additionally, the presence of numerous possible protostars in the region might represent a fresh source of CRs, which must also be taken into account in the interpretation of
γ
-ray observationsin this region.
Observations of molecular clouds reveal a complex structure, with gas and dust often arranged in filamentary, rather than spherical geometries. The association of pre- and proto-stellar cores with ...the filaments suggests a direct link with the process of star formation. Any study of the properties of such filaments requires representative samples from different environments for an unbiased detection method. We developed such an approach using the Hessian matrix of a surface-brightness distribution to identify filaments and determine their physical and morphological properties. After testing the method on simulated, but realistic, filaments, we apply the algorithms to column-density maps computed from Herschel observations of the Galactic plane obtained by the Hi-GAL project. We identified ~500 filaments, in the longitude range of l = 216.degrees5 to l = 225.degrees5, with lengths from ~1 pc up to ~30 pc and widths between 0.1 pc and 2.5 pc. Average column densities are between 1020 cm super(-2) and 10 super(22) cm super(-2). Filaments include the majority of dense material with N sub(H2) > 6 x 10 super(21) cm super(-2). We find that the pre- and proto-stellar compact sources already identified in the same region are mostly associated with filaments. However, surface densities in excess of the expected critical values for high-mass star formation are only found on the filaments, indicating that these structures are necessary to channel material into the clumps. Furthermore, we analyze the gravitational stability of filaments and discuss their relationship with star formation.
ABSTRACT
The recent data collected by Herschel have confirmed that interstellar structures with a filamentary shape are ubiquitously present in the Milky Way. Filaments are thought to be formed by ...several physical mechanisms acting from large Galactic scales down to subparsec 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 Herschel Infrared Galactic plane Survey (Hi-GAL) of the entire Galactic plane. For these objects, we determined morphological (length la 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 ≤ la ≤ 100 arcmin, $10^{20} \le N_{\rm H_{2}} \le 10^{23}$ cm−2 and 10 ≤ T ≤ 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. We also assign a distance to ∼18 400 filaments, for which we determine mass, physical size, stability conditions and Galactic distribution. When compared with the spiral arms structure, we find no significant difference between the physical properties of on-arm and inter-arm filaments. We compare 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 a unique and important tool for future studies devoted to understanding the filament life-cycle.
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