Aim. H I 21-cm and 12CO 2.6-mm line emissions trace the atomic and molecular gas phases, respectively, but they miss most of the opaque H I and diffuse H2 present in the dark neutral medium (DNM) at ...the transition between the H I-bright and CO-bright regions. Jointly probing H I, CO, and DNM gas, we aim to constrain the threshold of the H I–H2 transition in visual extinction, AV, and in total hydrogen column densities, NHtot $N_{\textrm{H}}^{\textrm{tot}}$ N H tot . We also aim to measure gas mass fractions in the different phases and to test their relation to cloud properties. Methods. We have used dust optical depth measurements at 353 GHz, γ-ray maps at GeV energies, and H I and CO line data to trace the gas column densities and map the DNM in nearby clouds toward the Galactic anticentre and Chamaeleon regions. We have selected a subset of 15 individual clouds, from diffuse to star-forming structures, in order to study the different phases across each cloud and to probe changes from cloud to cloud. Results. The atomic fraction of the total hydrogen column density is observed to decrease in the (0.6–1) × 1021 cm−2 range in NHtot $N_{\textrm{H}}^{\textrm{tot}}$ N H tot (AV ≈ 0.4 mag) because of the formation of H2 molecules. The onset of detectable CO intensities varies by only a factor of 4 from cloud to cloud, between 0.6 × 1021 cm−2 and 2.5 × 1021 cm−2 in total gas column density. We observe larger H2 column densities than linearly inferred from the CO intensities at AV > 3 mag because of the large CO optical thickness; the additional H2 mass in this regime represents on average 20% of the CO-inferred molecular mass. In the DNM envelopes, we find that the fraction of diffuse CO-dark H2 in the molecular column densities decreases with increasing AV in a cloud. For a half molecular DNM, the fraction decreases from more than 80% at 0.4 mag to less than 20% beyond 2 mag. In mass, the DNM fraction varies with the cloud properties. Clouds with low peak CO intensities exhibit large CO-dark H2 fractions in molecular mass, in particular the diffuse clouds lying at high altitude above the Galactic plane. The mass present in the DNM envelopes appears to scale with the molecular mass seen in CO as MHDNM = 62 ± 7 MH2CO0.51 ± 0.02 $M_{\textrm{H}}^{\textrm{DNM}} = (62 \pm 7) {M_{\textrm{H_2}}^{\textrm{CO}}}^{\,0.51 \pm 0.02}$ M H DNM =(62±7) M H 2 CO 0.51±0.02 across two decades in mass. Conclusions. The phase transitions in these clouds show both common trends and environmental differences. These findings will help support the theoretical modelling of H2 formation and the precise tracing of H2 in the interstellar medium.
Aims. We aim to explore the capabilities of dust emission and γ rays for probing the properties of the interstellar medium in the nearby anti-centre region, using γ-ray observations with the Fermi ...Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. We also aim to study massive star-forming clouds including the well known Taurus, Auriga, Perseus, and California molecular clouds, as well as a more diffuse structure which we refer to as Cetus. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H2 conversion factor, XCO, in different environments. Methods. We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in H i 21-cm and 12CO 2.6-mm line emission. We have jointly modelled the γ-ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth τ353 at 353 GHz as a combination of H i-bright, CO-bright, and ionised gas components. The complementary information from dust emission and γ rays was used to reveal the gas not seen, or poorly traced, by H i, free-free, and 12CO emissions, namely (i) the opaque H iand diffuse H2 present in the Dark Neutral Medium at the atomic-molecular transition, and (ii) the dense H2 to be added where 12CO lines saturate. Results. The measured interstellar γ-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds, from the atomic envelopes to the 12CO-bright cores, and with a small ± 9% cloud-to-cloud dispersion in particle flux. We detect the ionised gas from the H iiregion NGC 1499 in the dust and γ-ray emissions and measure its mean electron density and temperature. We find a gradual increase in grain opacity as the gas (atomic or molecular) becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the XCO factor derived from dust is systematically larger by 30% to 130% than the γ-ray estimate. We also evaluate the average γ-ray XCO factor for each cloud, and find that XCO tends to decrease from diffuse to more compact molecular clouds, as expected from theory. We find XCO factors in the anti-centre clouds close to or below 1020 cm-2 K-1 km-1 s, in agreement with other estimates in the solar neighbourhood. Together, they confirm the long-standing unexplained discrepancy, by a factor of two, between the mean XCO values measured at parsec scales in nearby clouds and those obtained at kiloparsec scale in the Galaxy. Our results also highlight large quantitative discrepancies in 12CO intensities between simulations and observations at low molecular gas densities.
Aims. We have explored the capabilities of dust extinction and γ rays to probe the properties of the interstellar medium in the nearby anti-centre region. In particular, we aim at quantifying the ...variations of the dust properties per gas nucleon across the different gas phases and different clouds. The comparison of dust extinction and emission properties with other physical quantities of large grains (emission spectral index β, dust colour temperature Tdust, total-to-selective extinction factor RV) helps the theoretical modelling of grains as they evolve from diffuse to dense cloud environments. Methods. We have jointly modelled the γ-ray intensity, recorded between 0.4 and 100 GeV with the Fermi Large Area Telescope (LAT), and the stellar reddening, E(B − V), inferred from Pan-STARRS and 2MASS photometry, as a combination of HI-bright, CO-bright, and ionised gas components. The complementary information from dust reddening and γ rays is used to reveal the gas not seen, or poorly traced, by HI, free-free, and 12CO emissions, namely (i) the opaque HI and diffuse H2 present in the dark neutral medium (DNM) at the atomic-molecular transition, and (ii) the dense H2 to be added where 12CO lines saturate (COsat). We compare the total gas column densities, NH, derived from the γ rays and stellar reddening with those inferred from a similar, previously published analysis of γ rays and of the optical depth of the thermal dust emission, τ353, at 353 GHz. We can therefore compare environmental variations in specific dust reddening, E(B − V)∕NH, and in dust emission opacity (dust optical depth per gas nucleon), τ353∕NH. Results. The gas column densities obtained when combining γ rays with either dust reddening or dust emission compare reasonably well in the atomic and DNM gas phases and over most of the CO-bright phase, but we find localised differences in the dense media (COsat component) due to differences in the two dust tracers. Over the whole anti-centre region, we find an average E(B − V)∕NH ratio of (2.02 ± 0.48) ×10−22 mag cm2, with maximum local variations of about ± 30% at variance with the two to six fold coincident increase seen in emission opacity as the gas column density increases. We show how the specific reddening and opacity vary with the colour temperature and spectral index of the thermal emission of the large grains. Additionally, we find a better agreement between the XCO = N(H2)/WCO conversion factors derived with dust reddening or with γ rays than with those inferred from dust emission, especially towards clouds with large τ353 optical depths. The comparison confirms that the high XCO values found with dust emission are biased by the significant rise in emission opacity inside molecular clouds. Conclusions. In the diffuse medium, we find only small variations in specific reddening, E(B − V)∕NH, compatible with the dispersion in the RV factor reported by other studies. This implies a relatively uniform dust-to-gas mass ratio in the diffuse parts of the anti-centre clouds. The small amplitude of the E(B − V)∕NH variations with increasing NH column density confirms that the large opacity τ353∕NH rise seen towards dense CO clouds is primarily due to changes in dust emissivity. The environmental changes are qualitatively compatible with model predictions based on mantle accretion on the grains and the formation of grain aggregates.
ABSTRACT High resolution stellar spectroscopic surveys provide massive amounts of diffuse interstellar bands (DIBs) measurements. Data can be used to study the distribution of the DIB carriers and ...those environmental conditions that favor their formation. In parallel, recent studies have also proved that DIBs extracted from stellar spectra constitute new tools for building the 3D structure of the Galactic interstellar medium (ISM). The amount of details on the structure depends directly on the quantity of available lines of sight. Therefore there is a need to construct databases of high-quality DIB measurements as large as possible. We aim at providing the community with a catalog of high-quality measurements of the 1.5273 m DIB toward a large fraction of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) hot stars observed to correct for the telluric absorption and not used for ISM studies so far. This catalog would complement the extensive database recently extracted from the APOGEE observations and used for 3D ISM mapping. We devised a method to fit the stellar continuum of the hot calibration stars and extracted the DIB from the normalized spectrum. Severe selection criteria based on the absorption characteristics are applied to the results. In particular limiting constraints on the DIB widths and Doppler shifts are deduced from the H i 21 cm measurements, following a new technique of decomposition of the emission spectra. From ∼16,000 available hot telluric spectra we have extracted ∼6700 DIB measurements and their associated uncertainties. The statistical properties of the extracted absorptions are examined and our selection criteria are shown to provide a robust dataset. The resulting catalog contains the DIB total equivalent widths, central wavelengths and widths. We briefly illustrate its potential use for the stellar and interstellar communities.
Aims. We aim to explore the capabilities of dust emission and γ rays for probing the properties of the interstellar medium in the nearby anti-centre region, using γ-ray observations with the Fermi ...Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. We also aim to study massive star-forming clouds including the well known Taurus, Auriga, Perseus, and California molecular clouds, as well as a more diffuse structure which we refer to as Cetus. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H2 conversion factor, XCO, in different environments. Methods. We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in H i 21-cm and 12CO 2.6-mm line emission. We have jointly modelled the γ-ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth τ353 at 353 GHz as a combination of H i-bright, CO-bright, and ionised gas components. The complementary information from dust emission and γ rays was used to reveal the gas not seen, or poorly traced, by H i, free-free, and 12CO emissions, namely (i) the opaque H iand diffuse H2 present in the Dark Neutral Medium at the atomic-molecular transition, and (ii) the dense H2 to be added where 12CO lines saturate. Results. The measured interstellar γ-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds, from the atomic envelopes to the 12CO-bright cores, and with a small ± 9% cloud-to-cloud dispersion in particle flux. We detect the ionised gas from the H iiregion NGC 1499 in the dust and γ-ray emissions and measure its mean electron density and temperature. We find a gradual increase in grain opacity as the gas (atomic or molecular) becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the XCO factor derived from dust is systematically larger by 30% to 130% than the γ-ray estimate. We also evaluate the average γ-ray XCO factor for each cloud, and find that XCO tends to decrease from diffuse to more compact molecular clouds, as expected from theory. We find XCO factors in the anti-centre clouds close to or below 1020 cm-2 K-1 km-1 s, in agreement with other estimates in the solar neighbourhood. Together, they confirm the long-standing unexplained discrepancy, by a factor of two, between the mean XCO values measured at parsec scales in nearby clouds and those obtained at kiloparsec scale in the Galaxy. Our results also highlight large quantitative discrepancies in 12CO intensities between simulations and observations at low molecular gas densities. Key words: gamma rays: ISM / solar neighborhood / ISM: clouds / cosmic rays / dust, extinction
Context
. Star-forming regions may play an important role in the life cycle of Galactic cosmic rays (CRs), notably as home to specific acceleration mechanisms and transport conditions. Gamma-ray ...observations of Cygnus X have revealed the presence of an excess of hard-spectrum gamma-ray emission, possibly related to a cocoon of freshly accelerated particles.
Aims
. We seek an improved description of the gamma-ray emission from the cocoon using ~13 yr of observations with the
Fermi
-Large Area Telescope (LAT) and use it to further constrain the processes and objects responsible for the young CR population.
Methods
. We developed an emission model for a large region of interest, including a description of interstellar emission from the background population of CRs and recent models for other gamma-ray sources in the field. Thus, we performed an improved spectro-morphological characterisation of the residual emission including the cocoon.
Results
. The best-fit model for the cocoon includes two main emission components: an extended component FCES G78.74+1.56, described by a 2D Gaussian of extension
r
68
= 4.4° ± 0.1°
−0.1°
+0.1°
and a smooth broken power law spectrum with spectral indices 1.67 ± 0.05
−0.01
+0.02
and 2.12 ± 0.02
−0.01
+0.00
below and above 3.0 ± 0.6
−0.2
+0.0
GeV, respectively; and a central component FCES G80.00+0.50, traced by the distribution of ionised gas within the borders of the photo-dissociation regions and with a power law spectrum of index 2.19 ± 0.03
−0.01
+0.00
that is significantly different from the spectrum of FCES G78.74+1.56. An additional extended emission component FCES G78.83+3.57, located on the edge of the central cavities in Cygnus X and with a spectrum compatible with that of FCES G80.00+0.50, is likely related to the cocoon. For the two brightest components FCES G80.00+0.50 and FCES G78.74+1.56, spectra and radial-azimuthal profiles of the emission can be accounted for in a diffusion-loss framework involving one single population of non-thermal particles with a flat injection spectrum. Particles span the full extent of FCES G78.74+1.56 as a result of diffusion from a central source, and give rise to source FCES G80.00+0.50 by interacting with ionised gas in the innermost region.
Conclusions
. For this simple diffusion-loss model, viable setups can be very different in terms of energetics, transport conditions, and timescales involved, and both hadronic and leptonic scenarios are possible. The solutions range from long-lasting particle acceleration, possibly in prominent star clusters such as Cyg OB2 and NGC 6910, to a more recent and short-lived release of particles within the last 10–100 kyr, likely from a supernova remnant. The observables extracted from our analysis can be used to perform detailed comparisons with advanced models of particle acceleration and transport in star-forming regions.
ABSTRACT
High resolution stellar spectroscopic surveys provide massive amounts of diffuse interstellar bands (DIBs) measurements. Data can be used to study the distribution of the DIB carriers and ...those environmental conditions that favor their formation. In parallel, recent studies have also proved that DIBs extracted from stellar spectra constitute new tools for building the 3D structure of the Galactic interstellar medium (ISM). The amount of details on the structure depends directly on the quantity of available lines of sight. Therefore there is a need to construct databases of high-quality DIB measurements as large as possible. We aim at providing the community with a catalog of high-quality measurements of the 1.5273
μ
m DIB toward a large fraction of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) hot stars observed to correct for the telluric absorption and not used for ISM studies so far. This catalog would complement the extensive database recently extracted from the APOGEE observations and used for 3D ISM mapping. We devised a method to fit the stellar continuum of the hot calibration stars and extracted the DIB from the normalized spectrum. Severe selection criteria based on the absorption characteristics are applied to the results. In particular limiting constraints on the DIB widths and Doppler shifts are deduced from the H
i
21 cm measurements, following a new technique of decomposition of the emission spectra. From ∼16,000 available
hot telluric
spectra we have extracted ∼6700 DIB measurements and their associated uncertainties. The statistical properties of the extracted absorptions are examined and our selection criteria are shown to provide a robust dataset. The resulting catalog contains the DIB total equivalent widths, central wavelengths and widths. We briefly illustrate its potential use for the stellar and interstellar communities.
Aim.
H
I
21-cm and
12
CO 2.6-mm line emissions trace the atomic and molecular gas phases, respectively, but they miss most of the opaque H
I
and diffuse H
2
present in the dark neutral medium (DNM) ...at the transition between the H
I
-bright and CO-bright regions. Jointly probing H
I
, CO, and DNM gas, we aim to constrain the threshold of the H
I
–H
2
transition in visual extinction,
A
V
, and in total hydrogen column densities, N
H
tot
. We also aim to measure gas mass fractions in the different phases and to test their relation to cloud properties.
Methods.
We have used dust optical depth measurements at 353 GHz,
γ
-ray maps at GeV energies, and H
I
and CO line data to trace the gas column densities and map the DNM in nearby clouds toward the Galactic anticentre and Chamaeleon regions. We have selected a subset of 15 individual clouds, from diffuse to star-forming structures, in order to study the different phases across each cloud and to probe changes from cloud to cloud.
Results.
The atomic fraction of the total hydrogen column density is observed to decrease in the (0.6–1) × 10
21
cm
−2
range in N
H
tot
(
A
V
≈ 0.4 mag) because of the formation of H
2
molecules. The onset of detectable CO intensities varies by only a factor of 4 from cloud to cloud, between 0.6 × 10
21
cm
−2
and 2.5 × 10
21
cm
−2
in total gas column density. We observe larger H
2
column densities than linearly inferred from the CO intensities at
A
V
> 3 mag because of the large CO optical thickness; the additional H
2
mass in this regime represents on average 20% of the CO-inferred molecular mass. In the DNM envelopes, we find that the fraction of diffuse CO-dark H
2
in the molecular column densities decreases with increasing
A
V
in a cloud. For a half molecular DNM, the fraction decreases from more than 80% at 0.4 mag to less than 20% beyond 2 mag. In mass, the DNM fraction varies with the cloud properties. Clouds with low peak CO intensities exhibit large CO-dark H
2
fractions in molecular mass, in particular the diffuse clouds lying at high altitude above the Galactic plane. The mass present in the DNM envelopes appears to scale with the molecular mass seen in CO as
M
H
DNM
= 62 ± 7
M
H
2
CO
0.51 ± 0.02
across two decades in mass.
Conclusions.
The phase transitions in these clouds show both common trends and environmental differences. These findings will help support the theoretical modelling of H
2
formation and the precise tracing of H
2
in the interstellar medium.
The H.E.S.S. Galactic Plane Survey HGPS, 1 represents one of the most sensitive surveys of the Galactic Plane at very high energies (VHE, 0.1 < E < 100 TeV). However the source detection ...algorithm of the HGPS pipeline is not well-suited for complex regions, including sources with shell-like morphologies. As an alternative and complementary approach, we have investigated blind search methods for VHE γ-ray source detection based on well-known and widely used image processing and pattern recognition techniques.
Our goal is to build in a short amount of computational time a list of potentially valuable objects without prior case-specific morphological assumptions. We aim to classify and rank the detected objects in order to identify only the most promising source candidates for further multi-wavelength-association searches, dedicated analyses, or deeper observations.
In the approach proposed, we extract sparse and pertinent structural information from the significance maps using a edge detection operator. We then apply a Hough circle transform and detect a collection of objects as local maxima in the Hough space. On the basis of morphological parameters we can characterize different object classes. Classification can be used to identify valuable source candidates sharing the characteristics of well-known sources.
We show that using these pattern recognition techniques we can detect objects with partial circular symmetry irrespective of a morphological template (e.g. point-like, Gaussian-like, or shell-like). All the shell-type supernova remnants (SNRs) catalogued in the HGPS (from dedicated analyses) are associated with at least one detected object. Catalogue cross-matches indicate that several detected objects not catalogued in the HGPS are spatially coincident with multi-wavelength counterparts.
This paper can be seen as a prospective study for the search of VHE γ-ray sources based on Hough transform and morphological classification. The algorithm have been tested on bootstrap simulations and applied to significance maps of the H.E.S.S. Galactic survey. Further investigation on the most promising candidates will be conducted in dedicated follow-up analyses.
Aims.
We have explored the capabilities of dust extinction and
γ
rays to probe the properties of the interstellar medium in the nearby anti-centre region. In particular, we aim at quantifying the ...variations of the dust properties per gas nucleon across the different gas phases and different clouds. The comparison of dust extinction and emission properties with other physical quantities of large grains (emission spectral index
β
, dust colour temperature
T
dust
, total-to-selective extinction factor
R
V
) helps the theoretical modelling of grains as they evolve from diffuse to dense cloud environments.
Methods.
We have jointly modelled the
γ
-ray intensity, recorded between 0.4 and 100 GeV with the
Fermi
Large Area Telescope (LAT), and the stellar reddening,
E
(
B
−
V
), inferred from Pan-STARRS and 2MASS photometry, as a combination of HI-bright, CO-bright, and ionised gas components. The complementary information from dust reddening and
γ
rays is used to reveal the gas not seen, or poorly traced, by HI, free-free, and
12
CO emissions, namely (i) the opaque
HI
and diffuse H
2
present in the dark neutral medium (DNM) at the atomic-molecular transition, and (ii) the dense H
2
to be added where
12
CO lines saturate (CO
sat
). We compare the total gas column densities,
N
H
, derived from the
γ
rays and stellar reddening with those inferred from a similar, previously published analysis of
γ
rays and of the optical depth of the thermal dust emission,
τ
353
, at 353 GHz. We can therefore compare environmental variations in specific dust reddening,
E
(
B
−
V
)∕
N
H
, and in dust emission opacity (dust optical depth per gas nucleon),
τ
353
∕
N
H
.
Results.
The gas column densities obtained when combining
γ
rays with either dust reddening or dust emission compare reasonably well in the atomic and DNM gas phases and over most of the CO-bright phase, but we find localised differences in the dense media (CO
sat
component) due to differences in the two dust tracers. Over the whole anti-centre region, we find an average
E
(
B
−
V
)∕
N
H
ratio of (2.02 ± 0.48) ×10
−22
mag cm
2
, with maximum local variations of about ± 30% at variance with the two to six fold coincident increase seen in emission opacity as the gas column density increases. We show how the specific reddening and opacity vary with the colour temperature and spectral index of the thermal emission of the large grains. Additionally, we find a better agreement between the
X
CO
=
N
(H
2
)/
W
CO
conversion factors derived with dust reddening or with
γ
rays than with those inferred from dust emission, especially towards clouds with large
τ
353
optical depths. The comparison confirms that the high
X
CO
values found with dust emission are biased by the significant rise in emission opacity inside molecular clouds.
Conclusions.
In the diffuse medium, we find only small variations in specific reddening,
E
(
B
−
V
)∕
N
H
, compatible with the dispersion in the
R
V
factor reported by other studies. This implies a relatively uniform dust-to-gas mass ratio in the diffuse parts of the anti-centre clouds. The small amplitude of the
E
(
B
−
V
)∕
N
H
variations with increasing
N
H
column density confirms that the large opacity
τ
353
∕
N
H
rise seen towards dense CO clouds is primarily due to changes in dust emissivity. The environmental changes are qualitatively compatible with model predictions based on mantle accretion on the grains and the formation of grain aggregates.
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