Physical properties of a very diffuse HI structure at high Galactic latitude

The main goal of this analysis is to present a new method to estimate the physical properties of diffuse cloud of atomic hydrogen observed at high Galactic latitude. This method, based on a comparison of the observations with fractional Brownian motion simulations, uses the statistical properties of the integrated emission, centroid velocity and line width to constrain the physical properties of the 3D density and velocity fields, as well as the average temperature of HI. We applied this method to interpret 21 cm observations obtained with the Green Bank Telescope of a very diffuse HI cloud at high Galactic latitude located in Firback North 1. We first show that the observations cannot be reproduced solely by highly-turbulent CNM type gas and that there is a significant contribution of thermal broadening to the line width observed. To reproduce the profiles one needs to invoke two components with different average temperature and filling factor. We established that, in this very diffuse part of the ISM, 2/3 of the column density is made of WNM and 1/3 of thermally unstable gas (T ~2600 K). The WNM gas is mildly supersonic (~1) and the unstable phase is definitely sub-sonic (~0.3). The density contrast (i.e., the standard deviation relative to the mean of density distribution) of both components is close to 0.8. The filling factor of the WNM is 10 times higher that of the unstable gas, which has a density structure closer to what would be expected for CNM gas. This field contains a signature of CNM type gas at a very low level (N_H ~ 3 x 10^19) which could have been formed by a convergent flow of WNM gas.Comment: 13 pages, 12 figures, accepted for publication in A&

Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium

We compare the performance of several dust models in reproducing the dust spectral energy distribution (SED) per unit extinction in the diffuse interstellar medium (ISM). We use our results to constrain the variability of the optical properties of big grains in the diffuse ISM, as published by the Planck collaboration. We use two different techniques to compare the predictions of dust models to data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared emission spectrum to recover the dust extinction and the intensity of the interstellar radiation field (ISRF). Second, we infer the ISRF intensity from the total power emitted by dust per unit extinction, and then predict the emission spectrum. In both cases, we test the ability of the models to reproduce dust emission and extinction at the same time. We identify two issues. Not all models can reproduce the average dust emission per unit extinction: there are differences of up to a factor $\sim2$ between models, and the best accord between model and observation is obtained with the more emissive grains derived from recent laboratory data on silicates and amorphous carbons. All models fail to reproduce the variations in the emission per unit extinction if the only variable parameter is the ISRF intensity: this confirms that the optical properties of dust are indeed variable in the diffuse ISM. Diffuse ISM observations are consistent with a scenario where both ISRF intensity and dust optical properties vary. The ratio of the far-infrared opacity to the $V$ band extinction cross-section presents variations of the order of $\sim20\%$ ($40-50\%$ in extreme cases), while ISRF intensity varies by $\sim30\%$ ($\sim60\%$ in extreme cases). This must be accounted for in future modelling.Comment: A&A, in pres

FIRBACK: II. Data Reduction and Calibration of the 170 micron ISO Deep Cosmological Survey

We present the final reduction and calibration of the FIRBACK ISOPHOT data. FIRBACK is a deep cosmological survey performed at 170 microns. This paper deals with the ISOPHOT C200 camera with the C160 filter. We review the whole data reduction process and compare our final calibration with DIRBE (for the extended emission) and IRAS (for point sources). The FIRBACK source extraction and galaxy counts is discussed in a companion paper (Dole et al., 2001).Comment: Accepted for publication in A&A. 9 pages, includes new aa.cls. Also available (with better quality figures) at http://wwwfirback.ias.u-psud.fr and http://mips.as.arizona.edu/~hdole/firback (new aa.cls is here

Power Spectrum Analysis of Far-IR Background Fluctuations in 160 Micron Maps From the Multiband Imaging Photometer for Spitzer

We describe data reduction and analysis of fluctuations in the cosmic far-IR background (CFIB) in observations with the Multiband Imaging Photometer for Spitzer (MIPS) instrument 160 micron detectors. We analyzed observations of an 8.5 square degree region in the Lockman Hole, part of the largest low-cirrus mapping observation with this instrument. We measured the power spectrum of the CFIB in these observations by fitting a power law to the IR cirrus component, the dominant foreground contaminant, and subtracting this cirrus signal. The CFIB power spectrum in the range 0.2 arc min^{-1} <k< 0.5 arc min^{-1} is consistent with previous measurements of a relatively flat component. However, we find a large power excess at low k, which falls steeply to the flat component in the range 0.03 arc min^{-1} <k< 0.1 arc min^{-1}. This low-k power spectrum excess is consistent with predictions of a source clustering "signature". This is the first report of such a detection in the far-IR.Comment: This is the version of the paper accepted by A&A, which includes various changes and new material. The superior-quality PDF with integrated figures may be downloaded at http://www-astro.lbl.gov/~bruce/spitzerpaper1/cfibaa_pub.pdf 15 pages, figures integrated with text. This paper supersedes astro-ph/050416

Statistical properties of dust far-infrared emission

The description of the statistical properties of dust emission gives important constraints on the physics of the interstellar medium but it is also a useful way to estimate the contamination of diffuse interstellar emission in the cases where it is considered a nuisance. The main goals of this analysis of the power spectrum and non-Gaussian properties of 100 micron dust emission are 1) to estimate the power spectrum of interstellar matter density in three dimensions, 2) to review and extend previous estimates of the cirrus noise due to dust emission and 3) to produce simulated dust emission maps that reproduce the observed statistical properties. The main results are the following. 1) The cirrus noise level as a function of brightness has been previously overestimated. It is found to be proportional to instead of ^1.5, where is the local average brightness at 100 micron. This scaling is in accordance with the fact that the brightness fluctuation level observed at a given angular scale on the sky is the sum of fluctuations of increasing amplitude with distance on the line of sight. 2) The spectral index of dust emission at scales between 5 arcmin and 12.5 degrees is =-2.9 on average but shows significant variations over the sky. Bright regions have systematically steeper power spectra than diffuse regions. 3) The skewness and kurtosis of brightness fluctuations is high, indicative of strong non-Gaussianity. 4) Based on our characterization of the 100 micron power spectrum we provide a prescription of the cirrus confusion noise as a function of wavelength and scale. 5) Finally we present a method based on a modification of Gaussian random fields to produce simulations of dust maps which reproduce the power spectrum and non-Gaussian properties of interstellar dust emission.Comment: 13 pages, 13 figures. Accepted for publication in A&

Direct Estimate of Cirrus Noise in Herschel Hi-GAL Images

In Herschel images of the Galactic plane and many star forming regions, a major factor limiting our ability to extract faint compact sources is cirrus confusion noise, operationally defined as the "statistical error to be expected in photometric measurements due to confusion in a background of fluctuating surface brightness". The histogram of the flux densities of extracted sources shows a distinctive faint-end cutoff below which the catalog suffers from incompleteness and the flux densities become unreliable. This empirical cutoff should be closely related to the estimated cirrus noise and we show that this is the case. We compute the cirrus noise directly, both on Herschel images from which the bright sources have been removed and on simulated images of cirrus with statistically similar fluctuations. We connect these direct estimates with those from power spectrum analysis, which has been used extensively to predict the cirrus noise and provides insight into how it depends on various statistical properties and photometric operational parameters. We report multi-wavelength power spectra of diffuse Galactic dust emission from Hi-GAL observations at 70 to 500 microns within Galactic plane fields at l= 30 degrees and l= 59 degrees. We find that the exponent of the power spectrum is about -3. At 250 microns, the amplitude of the power spectrum increases roughly as the square of the median brightness of the map and so the expected cirrus noise scales linearly with the median brightness. Generally, the confusion noise will be a worse problem at longer wavelengths, because of the combination of lower angular resolution and the rising power spectrum of cirrus toward lower spatial frequencies, but the photometric signal to noise will also depend on the relative spectral energy distribution of the source compared to the cirrus.Comment: 4 pages (in journal), 3 figures, Astronomy and Astrophysics, accepted for publication 13 May 201

An Imprint of Molecular Cloud Magnetization in the Morphology of the Dust Polarized Emission

We describe a morphological imprint of magnetization found when considering the relative orientation of the magnetic field direction with respect to the density structures in simulated turbulent molecular clouds. This imprint was found using the Histogram of Relative Orientations (HRO): a new technique that utilizes the gradient to characterize the directionality of density and column density structures on multiple scales. We present results of the HRO analysis in three models of molecular clouds in which the initial magnetic field strength is varied, but an identical initial turbulent velocity field is introduced, which subsequently decays. The HRO analysis was applied to the simulated data cubes and mock-observations of the simulations produced by integrating the data cube along particular lines of sight. In the 3D analysis we describe the relative orientation of the magnetic field $\mathbf{B}$ with respect to the density structures, showing that: 1.The magnetic field shows a preferential orientation parallel to most of the density structures in the three simulated cubes. 2.The relative orientation changes from parallel to perpendicular in regions with density over a critical density $n_{T}$ in the highest magnetization case. 3.The change of relative orientation is largest for the highest magnetization and decreases in lower magnetization cases. This change in the relative orientation is also present in the projected maps. In conjunction with simulations HROs can be used to establish a link between the observed morphology in polarization maps and the physics included in simulations of molecular clouds.Comment: (16 pages, 11 figures, submitted to ApJ 05MAR2013, accepted 07JUL2013

Adaptive dual-comb spectroscopy in the green region

Dual-comb spectroscopy is extended to the visible spectral range with a set-up based on two frequency-doubled femtosecond ytterbium-doped fiber lasers. The dense rovibronic spectrum of iodine around 19240 cm-1 is recorded within 12 ms at Doppler-limited resolution with a simple scheme that only uses free-running femtosecond lasers

Correlated Anisotropies in the Cosmic Far-Infrared Background Detected by MIPS/Spitzer: Constraint on the Bias

We report the detection of correlated anisotropies in the Cosmic Far-Infrared Background at 160 microns. We measure the power spectrum in the Spitzer/SWIRE Lockman Hole field. It reveals unambiguously a strong excess above cirrus and Poisson contributions, at spatial scales between 5 and 30 arcminutes, interpreted as the signature of infrared galaxy clustering. Using our model of infrared galaxy evolution we derive a linear bias b=1.74 \pm 0.16. It is a factor 2 higher than the bias measured for the local IRAS galaxies. Our model indicates that galaxies dominating the 160 microns correlated anisotropies are at z~1. This implies that infrared galaxies at high redshifts are biased tracers of mass, unlike in the local Universe.Comment: ApJ Letters, in pres