Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe link between E-B polarization modes and gas column density from interstellar dust emission
83
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The analysis of the Planck polarization E and B mode power spectra of interstellar dust emission at 353 GHz recently raised new questions on the impact of Galactic foregrounds to the detection of the polarization of the Cosmic Microwave Background (CMB) and on the physical properties of the interstellar medium (ISM). In the diffuse ISM a clear E-B asymmetry is observed, with twice as much power in E modes than in B modes; as well as a positive correlation between the total power, T, and both E and B modes, presently interpreted in terms of the link between the structure of interstellar matter and that of the Galactic magnetic field. In this paper we aim at extending the Planck analysis of the high-latitude sky to low Galactic latitude, investigating the correlation between the TEB power spectra with the gas column density from the diffuse ISM to molecular clouds. We divide the sky between Galactic latitude |b|>5 deg and |b|<60 deg in 552 circular patches and we study the cross-correlations between the TEB power spectra and the column density of each patch using the latest release of the Planck polarization data. We find that the B-to-E power ratio (BB/EE) and the TE correlation ratio (rTE) depend on column density. While the former increases going from the diffuse ISM to molecular clouds in the Gould Belt, the latter decreases. This systematic variation must be related to actual changes in ISM properties. The data show significant scatter about this mean trend. The variations of BB/EE and rTE are observed to be anti-correlated for all column densities. In the diffuse ISM, the variance of these two ratios is consistent with a stochastic non-Gaussian model in which the values of BB/EE and rTE are fixed. We finally discuss the dependencies of TB and EB with column density, which are however hampered by instrumental noise.
rate research
Read More
The quest for a B-mode imprint from primordial gravity waves on the polarization of the cosmic microwave background (CMB) requires the characterization of foreground polarization from Galactic dust. We present a statistical study of the filamentary structure of the 353 GHz Planck Stokes maps at high Galactic latitude, relevant to the study of dust emission as a polarized foreground to the CMB. We filter the intensity and polarization maps to isolate filaments in the range of angular scales where the power asymmetry between E-modes and B-modes is observed. Using the Smoothed Hessian Major Axis Filament Finder, we identify 259 filaments at high Galactic latitude, with lengths larger or equal to 2deg (corresponding to 3.5 pc in length for a typical distance of 100 pc). These filaments show a preferred orientation parallel to the magnetic field projected onto the plane of the sky, derived from their polarization angles. We present mean maps of the filaments in Stokes I, Q, U, E, and B, computed by stacking individual images rotated to align the orientations of the filaments. Combining the stacked images and the histogram of relative orientations, we estimate the mean polarization fraction of the filaments to be 11 %. Furthermore, we show that the correlation between the filaments and the magnetic field orientations may account for the E and B asymmetry and the $C_{ell}^{TE}/C_{ell}^{EE}$ ratio, reported in the power spectra analysis of the Planck 353 GHz polarization maps. Future models of the dust foreground for CMB polarization studies will need to take into account the observed correlation between the dust polarization and the structure of interstellar matter.
We present a detailed study of an estimator of the HI column density, based on a combination of HI 21cm absorption and HI 21cm emission spectroscopy. This isothermal estimate is given by $N_{rm HI,ISO} = 1.823 times 10^{18} int left[ tau_{rm tot} times {rm T_B} right] / left[ 1 - e^{-tau_{rm tot}} right] {rm dV}$, where $tau_{rm tot}$ is the total HI 21cm optical depth along the sightline and ${rm T_B}$ is the measured brightness temperature. We have used a Monte Carlo simulation to quantify the accuracy of the isothermal estimate by comparing the derived $N_{rm HI,ISO}$ with the true HI column density $N_{rm HI}$. The simulation was carried out for a wide range of sightlines, including gas in different temperature phases and random locations along the path. We find that the results are statistically insensitive to the assumed gas temperature distribution and the positions of different phases along the line of sight. The median value of the ratio of the true H{sc i} column density to the isothermal estimate, $N_{rm HI}/{N_{rm HI, ISO}}$, is within a factor of 2 of unity while the 68.2% confidence intervals are within a factor of $approx 3$ of unity, out to high HI column densities, $le 5 times 10^{23}$,cm$^{-2}$ per 1 km s$^{-1}$ channel, and high total optical depths, $le 1000$. The isothermal estimator thus provides a significantly better measure of the HI column density than other methods, within a factor of a few of the true value even at the highest columns, and should allow us to directly probe the existence of high HI column density gas in the Milky Way.
We study infrared emission of 17 isolated, diffuse clouds with masses of order solar masses, to test the hypothesis that grain property variations cause the apparently low gas-to-dust ratios that have been measured in those clouds. Maps of the clouds were constructed from WISE data and directly compared to the maps of dust optical depth from Planck. The mid-infrared emission per unit dust optical depth has a significant trend toward lower values at higher optical depths. The trend can be quantitatively explained by extinction of starlight within the clouds. The relative amounts of PAH and very small grains traced by WISE, compared to large grains tracked by Planck, are consistent with being constant. The temperature of the large grains significantly decreases for clouds with larger dust optical depth; this trend is partially due to dust property variations but is primarily due to extinction of starlight. We updated the prediction for molecular hydrogen column density, taking into account variations in dust properties, and find it can explain the observed dust optical depth per unit gas column density. Thus the low gas-to-dust ratios in the clouds are most likely due to `dark gas that is molecular hydrogen.
We present $^{12}$CO(1-0) and $^{12}$CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 $mu$m emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses <~ 10$^9$ M$_{odot}$, contrary to the atomic hydrogen fraction, M$_{HI}$/M$_*$, which increases inversely with M$_*$. The flattening of the M$_{H_2}$/M$_*$ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, $tau_{dep}$, are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between HI deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity.
Planck has mapped the intensity and polarization of the sky at microwave frequencies with unprecedented sensitivity. We make use of the Planck 353 GHz I, Q, and U Stokes maps as dust templates, and cross-correlate them with the Planck and WMAP data at 12 frequencies from 23 to 353 GHz, over circular patches with 10 degree radius. The cross-correlation analysis is performed for both intensity and polarization data in a consistent manner. We use a mask that focuses our analysis on the diffuse interstellar medium at intermediate Galactic latitudes. We determine the spectral indices of dust emission in intensity and polarization between 100 and 353 GHz, for each sky-patch. The mean values, $1.59pm0.02$ for polarization and $1.51pm0.01$ for intensity, for a mean dust temperature of 19.6 K, are close, but significantly different ($3.6,sigma$). We determine the mean spectral energy distribution (SED) of the microwave emission, correlated with the 353 GHz dust templates, by averaging the results of the correlation over all sky-patches. We find that the mean SED increases for decreasing frequencies at $ u < 60$ GHz, for both intensity and polarization. The rise of the polarization SED towards low frequencies may be accounted for by a synchrotron component correlated with dust, with no need for any polarization of the anomalous microwave emission. We use a spectral model to separate the synchrotron and dust polarization and to characterize the spectral dependence of the dust polarization fraction. The polarization fraction ($p$) of the dust emission decreases by $(21pm6)$ % from 353 to 70 GHz. The decrease of $p$ could indicate differences in polarization efficiency among components of interstellar dust (e.g., carbon versus silicate grains). Our observational results provide inputs to quantify and optimize the separation between Galactic and cosmological polarization.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
