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.
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.
We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope (LAT) in the Milky Way halo region, searching for a signal from dark matter annihilation or decay. ...In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e super(+)/e super(-) produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum, and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as the annihilation of dark matter.
We present an analysis of the gamma-ray measurements by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope in the region of the supernova remnant (SNR) Cygnus Loop (G74.0--8.5). We ...detect significant gamma-ray emission associated with the SNR in the energy band 0.2-100 GeV. The gamma-ray spectrum shows a break in the range 2-3 GeV. The gamma-ray luminosity is ~1 X 1033 erg s--1 between 1 and 100 GeV, much lower than those of other GeV-emitting SNRs. The morphology is best represented by a ring shape, with inner/outer radii 07 ? 01 and 16 ? 01. Given the association among X-ray rims, H Delta *a filaments, and gamma-ray emission, we argue that gamma rays originate in interactions between particles accelerated in the SNR and interstellar gas or radiation fields adjacent to the shock regions. The decay of neutral pions produced in nucleon-nucleon interactions between accelerated hadrons and interstellar gas provides a reasonable explanation for the gamma-ray spectrum.
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.
The dynamical evolution of the Gould Belt has been modelled in 3D and compared to the spatial and velocity distributions of all HI and H2 clouds found within a few hundred parsecs from the Sun and to ...the Hipparcos distances of the nearby OB associations. The model describes the expansion of a shock wave that sweeps momentum from the ambient medium. It includes the effects of the Galactic differential rotation and its gravitational torque, as well as interstellar density gradients within and away from the Galactic plane, possible fragmentation and drag forces in the late stages, and an initial rotation of the system. The evolved Belt geometry and velocity field have been fitted to the directions and velocities of the nearby clouds using a maximum-likelihood test. In order to do so, local clouds have been systematically searched for in the available HI and CO surveys. The likelihood function also included a distance estimate for a subset of well-known clouds. The best fit to the data yields values for the current Belt semi-axes of $(373 \pm 5)$ pc and $(233 \pm 5)$ pc, and an inclination of $17.2\degr \pm 0.5\degr$. These characteristics are consistent with earlier results, but a different Belt orientation has been found because of the presence of new molecular clouds and the revised distance information: the Belt centre currently lies $(104 \pm 4)$ pc away from the Sun, towards the Galactic longitude $l_{\rm centre} = 180.4\degr \pm 2.0\degr$, and the ascending node longitude is $l_{\Omega} = 296.1\degr \pm 2.0\degr$. The Belt characteristics are independent of initial rotation. The present Belt rim is found to coincide with most of the nearby OB associations and H2 clouds, but the Belt expansion bears little relation to the average association velocities and the younger ones are surprisingly found farther out from the Belt center. An initial kinetic energy of $(1.0 \pm 0.1)$ 1045 J and an expansion age of $(26.4 \pm 0.4)$ Myr are required, in good agreement with earlier 2D estimates. The factor of 2 discrepancy that exists between the dynamical Belt age and that derived from photometric stellar ages could not be solved by adding a vertical dimension in the expansion, nor by adding drag forces and fragmentation, nor by introducing an initial rotation. Allowing the Belt to cross the Galactic disc before reaching its present position would require a longer age of 52 Myr, but the very poor fit to the data does not support this possibility.
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.
Numerical simulations based on the LambdaCDM model of cosmology predict a large number of as yet unobserved Galactic dark matter satellites. We report the results of a Large Area Telescope (LAT) ...search for these satellites via the gamma -ray emission expected from the annihilation of weakly interacting massive particle (WIMP) dark matter. Some dark matter satellites are expected to have hard gamma -ray spectra, finite angular extents, and a lack of counterparts at other wavelengths. We sought to identify LAT sources with these characteristics, focusing on gamma -ray spectra consistent with WIMP annihilation through the bb channel. We found no viable dark matter satellite candidates using one year of data, and we present a framework for interpreting this result in the context of numerical simulations to constrain the velocity-averaged annihilation cross section for a conventional 100 GeV WIMP annihilating through the bb channel.
The dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are some of the most dark matter (DM) dominated objects known. We report on γ-ray observations of Milky Way dSphs based on six years ...of Fermi Large Area Telescope data processed with the new Pass8 event-level analysis. None of the dSphs are significantly detected in γ rays, and we present upper limits on the DM annihilation cross section from a combined analysis of 15 dSphs. These constraints are among the strongest and most robust to date and lie below the canonical thermal relic cross section for DM of mass ≲100 GeV annihilating via quark and τ-lepton channels.
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