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Register a new userDust in the diffuse emission of the galactic plane - The Herschel/Spitzer SED fitting
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The first Herschel Hi-Gal images of the galactic plane unveil the far-infrared diffuse emission of the interstellar medium with an unprecedented angular resolution and sensitivity. In this paper, we present the first analysis of these data in combination with that of Spitzer Glimpse & Mipsgal. We selected a relatively diffuse and low excitation region of the l~59,^{circ} Hi-Gal Science Demonstration Phase field to perform a pixel by pixel fitting of the 8 to 500 microns SED using the DustEM dust emission model. We derived maps of the Very Small Grains (VSG) and PAH abundances from the model. Our analysis allows us to illustrate that the Aromatic Infrared Bands (AIB) intensity does not trace necessarily the PAH abundance but rather the product of abundance x column density x intensity of the exciting radiation field. We show that the spatial structure of PACS70microns map resembles the shorter wavelengths (e.g. IRAC8microns) maps, because they trace both the intensity of exciting radiation field and column density. We also show that the modeled VSG contribution to PACS70microns (PACS160microns) band intensity can be up to 50% (7%). The interpretation of diffuse emission spectra at these wavelengths must take stochastically heated particles into account. Finally, this preliminary study emphasizes the potential of analyzing the full dust SED sampled by Herschel and Spitzer data, with a physical dust model (DustEM) to reach the properties of the dust at simultaneously large and small scales.
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We use the SPIRE Fourier-Transform Spectrometer (FTS) on-board the ESA Herschel Space Telescope to analyse the submillimetre spectrum of the Ultra-compact HII region G29.96-0.02. Spectral lines from species including 13CO, CO, [CI], and [NII] are detected. A sparse map of the [NII] emission shows at least one other HII region neighbouring the clump containing the UCHII. The FTS spectra are combined with ISO SWS and LWS spectra and fluxes from the literature to present a detailed spectrum of the source spanning three orders of magnitude in wavelength. The quality of the spectrum longwards of 100 {mu}m allows us to fit a single temperature greybody with temperature 80.3pm0.6K and dust emissivity index 1.73pm0.02, an accuracy rarely obtained with previous instruments. We estimate a mass of 1500 Msol for the clump containing the HII region. The clumps bolometeric luminosity of 4 x 10^6 Lsol is comparable to, or slightly greater than, the known O-star powering the UCHII region.
It is well known that aligned, aspherical dust grains emit polarized radiation and that the degree of polarization depends on the angle $psi$ between the interstellar magnetic field and the line of sight. However, anisotropy of the dust absorption cross sections also modulates the $total intensity$ of the radiation as the viewing geometry changes. We report a detection of this effect in the high Galactic latitude $Planck$ data, finding that the 353 GHz dust intensity per $N_{rm HI}$ is smaller when the Galactic magnetic field is mostly in the plane of the sky and larger when the field is mostly along the line of sight. These variations are of opposite sign and roughly equal magnitude as the changes in polarized intensity per $N_{rm HI}$ with $psi$, as predicted. In principle, the variation in intensity can be used in conjunction with the dust polarization angle to constrain the full 3D orientation of the Galactic magnetic field.
Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium and its 158 micron fine structure transition [CII] is the most important cooling line of the diffuse interstellar medium (ISM). We combine [CII] absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. We present high resolution [CII] spectra obtained with the Herschel/HIFI instrument towards bright dust continuum sources regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by observations of the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 microns with Herschel/PACS. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution data and that high spectral resolution spectra are needed to interpret the [CII] and [OI] emission and the [CII]/FIR ratio. This phenomenon may explain part of the [CII]/FIR deficit seen in external luminous infrared galaxies. The C+ and C excitation in the diffuse gas is consistent with a median pressure of 5900 Kcm-3 for a mean TK ~100 K. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight: the median value is 2.4 %, in good agreement with the CNM properties. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 x 10^{-26} erg/s/H. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 +/- 0.4 x 10^{-4}.
We present the first large scale high angular resolution survey of ionized nitrogen in the Galactic Plane through emission of its two fine structure transitions ([NII]) at 122 $mu$m and 205 $mu$m. The observations were largely obtained with the PACS instrument onboard the Herschel Space Observatory. The lines-of-sight were in the Galactic plane, following those of the Herschel OTKP project GOT C+. Both lines are reliably detected at the 10$^{-8}$ - 10$^{-7}$ $W$m$^{-2}$sr$^{-1}$ level over the range -60$^{o}$ $leq$ $l$ $leq$ 60$^{o}$. The $rms$ of the intensity among the 25 PACS spaxels of a given pointing is typically less than one third of the mean intensity, showing that the emission is extended. [NII] is produced in gas in which hydrogen is ionized, and collisional excitation is by electrons. The ratio of the two fine structure transitions provides a direct measurement of the electron density, yielding $n(e)$ largely in the range 10 to 50 cm$^{-3}$ with an average value of 29 cm$^{-3}$ and N$^+$ column densities 10$^{16}$ to 10$^{17}$ cm$^{-2}$. [NII] emission is highly correlated with that of [CII], and we calculate that between 1/3 and 1/2 of the [CII] emission is associated with the ionized gas. The relatively high electron densities indicate that the source of the [NII] emission is not the Warm Ionized Medium (WIM), which has electron densities more than 100 times smaller. Possible origins of the observed [NII] include the ionized surfaces of dense atomic and molecular clouds, the extended low density envelopes of HII regions, and low-filling factor high-density fluctuations of the WIM.
Radio recombination lines (RRLs) can be used to determine the emission measure unambiguously along the Galactic plane. We use the deep (2100s per beam) HI Parkes Zone of Avoidance survey which includes 3 RRLs (H$166alpha$, H$167alpha$ and H$168alpha$) within its bandwidth. The region $ell = 36degr$ to $44degr$, $b = -4degr$ to $+4degr$ is chosen to include emission from the Local, Sagittarius and Scutum arms. An $8degr times 8degr$ data cube centred at $(ell, b) = (40degr, 0degr)$ is constructed of RRL spectra with velocity and spatial resolution of 27$kms$ and 15.5 arcmin, respectively. Well-known hii regions are identified as well as the diffuse RRL emission on the Galactic plane. A Galactic latitude section of the integrated RRL emission across the Galactic plane delineates the brightness temperature ($T_{b}$) distribution which has a half-power width in latitude of $simeq 1fdg5$. A value of the electron temperature $T_{e} simeq 8000$ K is derived from a comparison with the WMAP free-free MEM model. The $T_{b}$ distribution from the present RRL data is combined with the WMAP 5-yr data to derive the anomalous dust on the Galactic ridge. In this paper we demonstrate that diffuse ionized emission on the Galactic ridge can be recovered using RRLs from the ZOA survey. This method is therefore able to complement the ha data at low Galactic latitudes, to enable an all-sky free-free template to be derived.
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