2MASS wide-field extinction maps

Probability distribution functions of the total hydrogen column density (N-PDFs) are a valuable tool for distinguishing between the various processes (turbulence, gravity, radiative feedback, magnetic fields) governing the morphological and dynamical structure of the interstellar medium. We present N-PDFs of 29 Galactic regions obtained from Herschel imaging at high angular resolution (18″), covering diffuse and quiescent clouds, and those showing low-, intermediate-, and high-mass star formation (SF), and characterize the cloud structure using the ∆-variance tool. The N-PDFs show a large variety of morphologies. They are all double-log-normal at low column densities, and display one or two power law tails (PLTs) at higher column densities. For diffuse, quiescent, and low-mass SF clouds, we propose that the two log-normals arise from the atomic and molecular phase, respectively. For massive clouds, we suggest that the first log-normal is built up by turbulently mixed H2 and the second one by compressed (via stellar feedback) molecular gas. Nearly all clouds have two PLTs with slopes consistent with self-gravity, where the second one can be flatter or steeper than the first one. A flatter PLT could be caused by stellar feedback or other physical processes that slow down collapse and reduce the flow of mass toward higher densities. The steeper slope could arise if the magnetic field is oriented perpendicular to the LOS column density distribution. The first deviation point (DP), where the N-PDF turns from log-normal into a PLT, shows a clustering around values of a visual extinction of AV (DP1) ~ 2–5. The second DP, which defines the break between the two PLTs, varies strongly. In contrast, the width of the N-PDFs is the most stable parameter, with values of σ between ~0.5 and 0.6. Using the ∆-variance tool, we observe that the AV value, where the slope changes between the first and second PLT, increases with the characteristic size scale in the ∆-variance spectrum. We conclude that at low column densities, atomic and molecular gas is turbulently mixed, while at high column densities, the gas is fully molecular and dominated by self-gravity. The best fitting model N-PDFs of molecular clouds is thus one with log-normal low column density distributions, followed by one or two PLTs.

Aim: The aim of the paper is to understand the emission from the photon dominated regions in Cepheus B, estimate the column densities of neutral carbon in bulk of the gas in Cepheus B and to derive constraints on the factors which determine the abundance of neutral carbon relative to CO. Methods: This paper presents 15'x15' fully sampled maps of CI at 492 GHz and 12CO 4-3 observed with KOSMA at 1' resolution. The new observations have been combined with the FCRAO 12CO 1-0, IRAM-30m 13CO 2-1 and C18O 1-0 data, and far-infrared continuum data from HIRES/IRAS. The KOSMA-tau spherical PDR model has been used to understand the CI and CO emission from the PDRs in Cepheus B and to explain the observed variation of the relative abundances of both C^0 and CO. Results: The emission from the PDR associated with Cepheus B is primarily at V_LSR between -14 and -11 km s^-1. We estimate about 23% of the observed CII emission from the molecular hotspot is due to the ionized gas in the HII region. Over bulk of the material the C^0 column density does not change significantly, (2.0+-1.4)x10^17 cm^-2, although the CO column density changes by an order of magnitude. The observed \cbyco abundance ratio varies between 0.06 and 4 in Cepheus B. We find an anti-correlation of the observed C/CO abundance ratio with the observed hydrogen column density, which holds even when all previous observations providing C/CO ratios are included. Here we show that this observed variation of C/CO abundance with total column density can be explained only by clumpy PDRs consisting of an ensemble of clumps. At high H2 column densities high mass clumps, which exhibit low C/CO abundance, dominate, while at low column densities, low mass clumps with high C/CO abundance dominate.

Abstract. The use of ro-vibronic spectra of I2 in the region of 543 nm to 578 nm as reference spectra for atmospheric Differential Optical Absorption Spectroscopy is studied. It is shown that the retrieval of atmospheric column densities with Differential Optical Absorption Spectroscopy set-ups at FWHM at and above 1 nm depends critically on the column density, under which the used reference spectrum was recorded. Systematic overestimation of the comparatively low atmospheric column density of I2 of the order of 13% is possible. Under low pressure conditions relevant in laboratory studies, the systematic deviations may grow up to 45%. To avoid such effects with respect to field measurements, new reference spectra of I2 were determined under column density of the order of 1016 cm-2 close to that expected for an atmospheric measurement. Two typical configurations of Differential Optical Absorption Spectroscopy, which use grating spectrometers, were chosen for the spectroscopic set-up. One spectrum was recorded at similar resolution (0.25 nm FWHM) but finer binning (0.035 nm/pixel) than previously published data. For the other (0.59 nm FWHM, 0.154 nm/pixel) no previously published spectra exist. Wavelength calibration is accurate to ±0.04 nm and ±0.11 nm respectively. The absorption cross section for the recordings was determined under low column density with an accuracy of ±4% and ±3% respectively. The absolute absorption cross section of I2 at 500 nm (wavelength: in standard air) in the continuum absorption region was determined using a method independent of iodine vapour pressure. Obtained was σI2 (500 nm)=(2.186±0.021·10-18 cm2 in very good agreement with previously published results, but at 50% smaller uncertainty. From this and previously published results a weighted average of σI2(500 nm)=(2.191±0.02)·10-18 cm2 is determined.

We use the EAGLE cosmological, hydrodynamical simulations to predict the column density and equivalent width distributions of intergalactic O vii ($E=574 \, \rm {eV}$) and O viii ($E=654 \, \rm {eV}$) absorbers at low redshift. These two ions are predicted to account for $40 \, \hbox{ per cent}$ of the gas-phase oxygen, which implies that they are key tracers of cosmic metals. We find that their column density distributions evolve little at observable column densities from redshift 1 to 0, and that they are sensitive to active galactic nucleus feedback, which strongly reduces the number of strong (column density $N \gtrsim 10^{16} \, \rm {cm}^{-2}$) absorbers. The distributions have a break at $N \sim 10^{16}\, \rm {cm}^{-2}$, corresponding to overdensities of ∼102, likely caused by the transition from sheet/filament to halo gas. Absorption systems with $N \gtrsim 10^{16} \, \rm {cm}^{-2}$ are dominated by collisionally ionized O vii and O viii, while the ionization state of oxygen at lower column densities is also influenced by photoionization. At these high column densities, O vii and O viii arising in the same structures probe systematically different gas temperatures, meaning their line ratio does not translate into a simple estimate of temperature. While O vii and O viii column densities and covering fractions correlate poorly with the H i column density at ${N}_{\rm {H}\, \rm {I}} \gtrsim 10^{15} \, \rm {cm}^{-2}$, O vii and O viii column densities are higher in this regime than at the more common, lower H i column densities. The column densities of O vi and especially Ne viii, which have strong absorption lines in the UV, are good predictors of the strengths of O vii and O viii absorption and can hence aid in the detection of the X-ray lines.

We use observations of the C I, C II, H I, and -->H2 column densities along lines of sight in the Galactic plane to determine the formation rate of -->H2 on grains and to determine chemical reaction rates with polycyclic aromatic hydrocarbons (PAHs). Photodissociation region models are used to find the best-fit parameters to the observed columns. We find the -->H2 formation rate on grains has a low rate ( -->R ~ 1 × 10−17 cm3 s−1) along lines of sight with low column density ( -->AV 0.25) and low molecular fraction ( -->fH2 10−4). At higher column densities ( -->0.25 ≤ AV ≤ 2.13), we find a rate of -->R ~ 3.5 × 10−17 cm3 s−1. The lower rate at low column densities could be the result of grain processing by interstellar shocks, which may deplete the grain surface area or process the sites of -->H + H formation, thereby inhibiting -->H2 production. Alternatively, the formation rate may be normal, and the low molecular fraction may be the result of lines of sight that graze larger clouds. Such lines of sight would have a reduced -->H2 self-shielding compared to the line-of-sight column. We find the reaction -->C+ + PAH−→ C + PAH0 is best fit with a rate -->2.4 × 10−7ΦPAHT−0.52 cm3 s−1 with -->T2 = T/100 K, and the reaction -->C+ + PAH0→ C + PAH+ is best fit with a rate -->8.8 × 10−9ΦPAH cm3 s−1. In high-column-density gas, we find -->ΦPAH ~ 0.4. In low-column-density gas, -->ΦPAH is less well constrained, with -->ΦPAH ~ 0.2–0.4.

We present a study of the evolution of the column density distribution, f(N, z), and total neutral hydrogen mass in high column density quasar absorbers using candidates from a recent high-redshift survey for damped Lyman α (DLA) and Lyman-limit system (LLS) absorbers. The observed number of LLS [N(H_i) >1.6 × 10^(17) atom cm^(−2)] is used to constrain f(N, z) below the classical DLA definition of 2 × 10^(20) atom cm^(−2). The evolution of the number density of LLS is consistent with our previous work but steeper than previously published work of other authors. At z= 5, the number density of Lyman-limit systems per unit redshift is ∼5, implying that these systems are a major source of ultraviolet (UV) opacity in the high-redshift Universe. The joint LLS–DLA analysis shows unambiguously that f(N, z) deviates significantly from a single power law and that a Γ-law distribution of the form f(N,z) = (f_*/N_*)(N/N_*)^(−β)exp(−N/N_*) provides a better description of the observations. These results are used to determine the amount of neutral gas contained in DLAs and in systems with lower column density. Whilst in the redshift range 2–3.5, ∼90 per cent of the neutral H i mass is in DLAs, we find that at z > 3.5 this fraction drops to only 55 per cent and that the remaining ‘missing’ mass fraction of the neutral gas lies in sub-DLAs with N(H i) 10^(19)–2 × 10^(20) atom cm^(−2). The characteristic column density, N_*, changes from 1.6 × 10^(21) atom cm^(−2) at z 3.5, supporting a picture where at z > 3.5, we are directly observing the formation of high column density neutral hydrogen DLA systems from lower column density units. Moreover, since current metallicity studies of DLA systems focus on the higher column density systems they may be giving a biased or incomplete view of global galactic chemical evolution at z > 3. After correcting the observed mass in H i for the ‘missing’ neutral gas the comoving mass density now shows no evidence for a decrease above z= 2.

We present measurements of the C18O/C17O abundance ratio based on observations of J = 1 → 0 lines of these isotopomers toward star-forming cores in the Taurus molecular cloud. Our data set includes measurements along 648 lines of sight through these clouds, covering both low and high column density regions. We compare the integrated intensity ratio for each line of sight with a simple model of emission from a dense cloud to determine this abundance ratio. Using this model, we find that a C18O/C17O abundance ratio of 4.0 ± 0.5 is consistent with the data. However, at low column densities, it appears that a higher abundance ratio may be more appropriate. We examine ways in which the abundance ratio might be changed in the outer parts of molecular clouds and conclude that selective photodissociation of C17O by external ultraviolet light can increase the abundance ratio. A two-phase model, incorporating a C17O-free "sheath" of cloud material surrounding a self-shielded inner cloud region, is fitted to the data. Using this model and an assumed sheath H2 column density of 4 × 1021 cm-2, we find a lower abundance ratio of 2.8 ± 0.4 for the material in the shielded inner cloud. This new result is consistent with recent results from ultraviolet absorption spectroscopy through translucent clouds and measurements of the 13C18O/13C17O ratio in the Ophiuchus molecular cloud.

This paper examines whether a fractal cloud geometry can reproduce the emission-line spectra of active galactic nuclei (AGNs). The nature of the emitting clouds is unknown, but many current models invoke various types of magnetohydrodynamic confinement. Recent studies have argued that a fractal distribution of clouds, in which subsets of clouds occur in self-similar hierarchies, is a consequence of such confinement. Whatever the confinement mechanism, fractal cloud geometries are found in nature and may be present in AGNs too. We first outline how a fractal geometry can apply at the center of a luminous quasar. Scaling laws are derived that establish the number of hierarchies, typical sizes, column densities, and densities. Photoionization simulations are used to predict the integrated spectrum from the ensemble. Direct comparison with observations establishes all model parameters so that the final predictions are fully constrained. Theory suggests that denser clouds might form in regions of higher turbulence and that larger turbulence results in a wider dispersion of physical gas densities. An increase in turbulence is expected deeper within the gravitational potential of the black hole, resulting in a density gradient. We mimic this density gradient by employing two sets of clouds with identical fractal structuring but different densities. The low-density clouds have a lower column density and large covering factor similar to the warm absorber. The high-density clouds have high column density and smaller covering factor similar to the broad-line region (BLR). A fractal geometry can simultaneously reproduce the covering factor, density, column density, BLR emission-line strengths, and BLR line ratios as inferred from observation. Absorption properties of the model are consistent with the integrated line-of-sight column density as determined from observations of X-ray absorption, and when scaled to a Seyfert galaxy, the model is consistent with the number of multiple UV absorption components observed in them. Rough estimates show that about one in 100 of the galaxies that harbor a supermassive black hole will show activity, assuming that material needs to be within its EUV continuum emitting radius for activity to occur. This is close to the observationally determined duty cycle. Stochastic feeding of the central engine of fractal cloud distribution of material may therefore account for continuum variations and long-term activity. The total cloud mass is much larger than that measured in ionized gas alone since the clouds are mutually self-shielding.

Measurements of molecular hydrogen (H2) column densities are presented for the first six rotational levels (J = 0-5) for 73 extragalactic targets observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). All of these have a final signal-to-noise ratio larger than 10 and are located at Galactic latitude |b| > 20°. The individual observations were calibrated with the FUSE calibration pipeline CalFUSE version 2.1 or higher and then carefully aligned in velocity. The final velocity shifts for all the FUSE segments are listed. H2 column densities or limits are determined for the six lowest rotational (J) levels for each H I component in the line of sight, using a curve-of-growth approach at low column densities (<16.5) and Voigt-profile fitting at higher column densities. Detections include 65 measurements of low-velocity H2 in the Galactic disk and lower halo. Eight sight lines yield nondetections for Galactic H2. The measured column densities range from log N(H2) = 14 to 20. Strong correlations are found between log N(H2) and T01, the excitation temperature of the H2, as well as between log N(H2) and the level population ratios (log[N(J')/N(J)]). The average fraction of nuclei in molecular hydrogen [f(H2)] in each sight line is calculated; however, because there are many H I clouds in each sight line, the physics of the transition from H I to H2 cannot be studied. Detections also include H2 in 16 intermediate-velocity clouds in the Galactic halo (out of 35 IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm of the Magellanic Stream), although 19 high-velocity clouds are intersected; this strongly suggests that dust is rare or absent in these objects. Finally, there are five detections of H2 in external galaxies.

We study the properties of low-column density gas clumps in the halo of the Milky Way based on high-resolution 21-cm observations. Using interferometric data from the WSRT and the VLA we study HI emission at low-, intermediate- and high radial velocities along four lines of sight towards quasars. Along these sightlines we previously detected weak CaII and NaI absorbers in their optical spectra. The analysis of the high-resolution HI data reveals the presence of several compact and cold clumps of neutral gas at velocities similar to the optical absorption. The clumps have narrow HI line widths in the range of 1.8 to 13 km/s, yielding upper limits for the kinetic temperature of the gas of 70 to 3700 K. The neutral gas has low HI column densities in the range of 5E18 to 3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes. Our high-resolution 21-cm observations indicate that many of the CaII and NaI absorbers seen in our optical quasar spectra are associated with low-column density HI clumps at small angular scales. This suggests that next to the massive, high-column density neutral gas clouds in the halo (the common 21-cm LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas structures in the halo that remain mostly unseen in the existing 21-cm all-sky surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected HI clumps are consistent with what is expected from theoretical models of gas in the inner and outer Milky Way halo.

The Lyα emission line is one of the main observables of galaxies at high redshift, but its output depends strongly on the neutral gas distribution and kinematics around the star-forming regions where UV photons are produced. We present observations of Lyα and 21 cm H i emission at comparable scales with the goal to qualitatively investigate how the neutral interstellar medium (ISM) properties impact Lyα transfer in galaxies. We have observed 21 cm H i at the highest possible angular resolution (≈3″ beam) with the Very Large Array in two local galaxies from the Lyman Alpha Reference Sample. We compare these data with Hubble Space Telescope Lyα imaging and spectroscopy, and Multi Unit Spectroscopic Explorer and Potsdam MultiAperture Spectrophotometer ionized gas observations. In LARS08, high-intensity Lyα emission is cospatial with high column density H i where the dust content is the lowest. The Lyα line is strongly redshifted, consistent with a velocity redistribution that allows Lyα escape from a high column density neutral medium with a low dust content. In eLARS01, high-intensity Lyα emission is located in regions of low column density H i, below the H i data sensitivity limit ( < 2 × 1020 cm−2). The perturbed ISM distribution with low column density gas in front of the Lyα emission region plays an important role in the escape. In both galaxies, the faint Lyα emission (∼1×10−16 erg s−1cm−2 arcsec−2) traces intermediate Hα emission regions where H i is found, regardless of the dust content. Dust seems to modulate, but not prevent, the formation of a faint Lyα halo. This study suggests the existence of scaling relations between dust, Hα, H i, and Lyα emission in galaxies.

New intermediate-resolution spectroscopy for six members of a sample of 68 moderate- to high-redshift QSOs is presented. Evidence is reported which indicates that seven strong absorption features in the QSO spectra are due to damped Ly-alpha absorption. A standard curve-of-growth analysis on five of the damped systems is performed, and relevant properties are tabulated and discussed. Six of the seven damped Ly-alpha systems have H I column densities of 2 x 10 to the 20th/sq cm or larger, while the remaining system has an H I column density of about 10 to the 20th/sq cm. It is suggested that damped Ly-alpha systems arise when a sight line intercepts a high-redshift protogalaxy disk containing a quiescent cloud component characterized by high column density and low effective velocity dispersion. At the same time, the sight line usually intercepts a broader turbulent component, which is identified as the halo, characterized by much lower column density and higher effective velocity dispersion.

We present an investigation of candidate Infrared Dark Cloud cores as identified by Simon et al. (2006) located within the SCUBA Legacy Catalogue. After applying a uniform noise cut to the Catalogue data we identify 154 Infrared Dark Cloud cores that were detected at 850um and 51 cores that were not. We derive column densities for each core from their 8um extinction and find that the IRDCs detected at 850um have higher column densities (a mean of 1.7x10^22 cm-2) compared to those cores not detected at 850um (a mean of 1.0x10^22 cm-2). Combined with sensitivity estimates, we suggest that the cores not detected at 850um are low mass, low column density and low temperature cores that are below the sensitivity limit of SCUBA at 850um. For a subsample of the cores detected at 850um those contained within the MIPSGAL area) we find that two thirds are associated with 24um sources. Cores not associated with 24um emission are either ``starless'' IRDC cores that perhaps have yet to form stars, or contain low mass YSOs below the MIPSGAL detection limit. We see that those ``starless'' IRDC cores and the IRDC cores associated with 24um emission are drawn from the same column density population and are of similar mass. If we then assume the cores without 24um embedded sources are at an earlier evolutionary stage to cores with embedded objects we derive a statistical lifetime for the quiescent phase of a few 10^3-10^4 years. Finally, we make conservative predictions for the number of observed IRDCs that will be observed by the Apex Telescope Galactic Plane Survey (ATLASGAL), the Herschel Infrared Galactic Plane Survey (Hi-GAL), the JCMT Galactic Plane Survey (JPS) and the SCUBA-2 ``All Sky'' Survey (SASSy).

The physical state of cold cloud clumps has a great impact on the process and efficiency of star formation and the masses of the forming stars inside these objects. The sub-millimetre survey of the Planck space observatory and the far-infrared follow-up mapping of the Herschel space telescope provide an unbiased, large sample of these cold objects. We have observed $^{12}$CO(1$-$0) and $^{13}$CO(1$-$0) emission in 35 clumps in 26 Herschel fields sampling different environments in the Galaxy. Densities and temperatures were calculated from both the dust continuum and the molecular line data, kinematic distances were derived using $^{13}$CO line velocities and clump sizes and masses were calculated by fitting 2D Gaussian functions to the optical depth distribution maps. Clump masses and virial masses were estimated assuming an upper and lower limit on the kinetic temperatures and considering uncertainties due to distance limitations. The excitation temperatures are between 8.5$-$19.5 K, while the Herschel-derived dust colour temperatures are 12$-$16 K. The sizes (0.1$-$3 pc), $^{13}$CO column densities (0.5$-$44$\times$10$^{15}$ cm$^{-2}$) and masses (from less than 0.1 $M_{\odot}$ to more than 1500 $M_{\odot}$) of the objects span broad ranges. Eleven gravitationally unbound clumps were found, many of them smaller than 0.3 pc, but large, parsec-scale clouds with a few hundred solar masses appear as well. Colder clumps have generally high column densities but warmer objects appear at both low and higher column densities. The clump column densities derived from the line and dust observations correlate well, but are heavily affected by uncertainties of the dust properties, varying molecular abundances and optical depth effects.

The relationship between B-field orientation and density structure in molecular clouds is often assessed using the histogram of relative orientations (HRO). We perform a plane-of-the-sky geometrical analysis of projected B-fields, by interpreting HROs in dense, spheroidal, pre-stellar, and protostellar cores. We use James Clerk Maxwell Telescope POL-2 850 $\mu$m polarization maps and Herschel column density maps to study dense cores in the Ophiuchus molecular cloud complex. We construct two-dimensional core models, assuming Plummer column density profiles and modelling both linear and hourglass B-fields. We find that high-aspect ratio ellipsoidal cores produce strong HRO signals, as measured using the shape parameter $\xi$. Cores with linear fields oriented $&amp;lt;\!\! 45 ^{\circ }$ from their minor axis produce constant HROs with $-1 \lt \xi \lt 0$, indicating that fields are preferentially parallel to column density gradients. Fields parallel to the core minor axis produce the most negative value of $\xi$. For low-aspect ratio cores, $\xi \approx 0$ for linear fields. Hourglass fields produce a minimum in $\xi$ at intermediate densities in all cases, converging to the minor-axis-parallel linear field value at high and low column densities. We create HROs for six dense cores in Ophiuchus. $\rho$ Oph A and IRAS 16293 have high aspect ratios and preferentially negative HROs, consistent with moderately strong field behaviour. $\rho$ Oph C, L1689A, and L1689B have low aspect ratios, and $\xi \approx 0$. $\rho$ Oph B is too complex to be modelled using a simple spheroidal field geometry. We see no signature of hourglass fields, agreeing with previous findings that dense cores generally exhibit linear fields on these size scales.