Total Hydrogen Column Density Research Articles

COSMIC ORIGINS SPECTROGRAPH OBSERVATIONS OF WARM INTERVENING GAS ATz∼ 0.325 TOWARD 3C 263

We present HST/COS high S/N observations of the z = 0.32566 multi-phase absorber towards 3C263. The COS data shows absorption from H I, O VI, C III, N III, Si III and C II. The Ne VIII in this absorber is detected in the FUSE spectrum. The low and intermediate ions are kinematically aligned with each other and H I and display narrow line widths of 6 km/s. The O VI lines are kinematically offset by 12 km/s from the low ions and are a factor of four broader. All metal ions except O VI and Ne VIII are consistent with an origin in gas photoionized by the extragalactic background radiation. The bulk of the observed H I is also traced by this photoionized medium. The carbon abundance in this gas phase is near-solar. The O VI and Ne VIII favor an origin in collisionally ionized gas at T = 5.2 x 10^5 K. The H I absorption associated with this warm absorber is a BLA marginally detected in the COS spectrum. This warm gas phase has total hydrogen column density of N(H) ~ 3 x 10^19 which is 2 dex higher than what is traced by the photoionized gas. Simultaneous detection of O VI, Ne VIII and BLAs in an absorber can be a strong diagnostic of gas with temperature in the range of 10^5 - 10^6 K corresponding to the warm phase of the WHIM or shock-heated gas in the extended halos of galaxies.

EVIDENCE FOR ENVIRONMENTAL CHANGES IN THE SUBMILLIMETER DUST OPACITY

The submillimeter opacity of dust in the diffuse Galactic interstellar medium (ISM) has been quantified using a pixel-by-pixel correlation of images of continuum emission with a proxy for column density. We used three BLAST bands at 250, 350, and 500 \mu m and one IRAS at 100 \mu m. The proxy is the near-infrared color excess, E(J-Ks), obtained from 2MASS. Based on observations of stars, we show how well this color excess is correlated with the total hydrogen column density for regions of moderate extinction. The ratio of emission to column density, the emissivity, is then known from the correlations, as a function of frequency. The spectral distribution of this emissivity can be fit by a modified blackbody, whence the characteristic dust temperature T and the desired opacity \sigma_e(1200) at 1200 GHz can be obtained. We have analyzed 14 regions near the Galactic plane toward the Vela molecular cloud, mostly selected to avoid regions of high column density (N_H > 10^{22} cm^-2) and small enough to ensure a uniform T. We find \sigma_e(1200) is typically 2 to 4 x 10^{-25} cm^2/H and thus about 2 to 4 times larger than the average value in the local high Galactic latitude diffuse atomic ISM. This is strong evidence for grain evolution. There is a range in total power per H nucleon absorbed (re-radiated) by the dust, reflecting changes in the interstellar radiation field and/or the dust absorption opacity. These changes affect the equilibrium T, which is typically 15 K, colder than at high latitudes. Our analysis extends, to higher opacity and lower T, the trend of increasing opacity with decreasing T that was found at high latitudes. The recognition of changes in the emission opacity raises a cautionary flag because all column densities deduced from dust emission maps, and the masses of compact structures within them, depend inversely on the value adopted.

INVESTIGATING THE COSMIC-RAY IONIZATION RATE IN THE GALACTIC DIFFUSE INTERSTELLAR MEDIUM THROUGH OBSERVATIONS OF H+3

Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (zeta_2) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H3+ observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7+-1.3)x10^-16 s^-1<zeta_2<(10.6+-8.2)x10^-16 s^-1 with a mean value of zeta_2=(3.5^+5.3_-3.0)x10^-16 s^-1. Upper limits (3 sigma) derived from non-detections of H3+ are as low as zeta_2<0.4x10^-16 s^-1. These low upper-limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in zeta_2 between different diffuse cloud sight lines. A study of zeta_2 versus N_H (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when N_H increases to values typical of dense molecular clouds. Both the difference in ionization rates between diffuse and dense clouds and the variation of zeta_2 among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.

REVISITING THE CHLORINE ABUNDANCE IN DIFFUSE INTERSTELLAR CLOUDS FROM MEASUREMENTS WITH THECOPERNICUSSATELLITE

We reanalyzed interstellar Cl I and Cl II spectra acquired with the Copernicus satellite. The directions for this study come from those of Crenny & Federman and sample the transition from atomic to molecular rich clouds where the unique chemistry leading to molecules containing chlorine is initiated. Our profile syntheses relied on up-to-date laboratory oscillator strengths and component structures derived from published high-resolution measurements of K I absorption that were supplemented with Ca II and Na I D results. We obtain self-consistent results for the Cl I lines at 1088, 1097, and 1347 A from which precise column densities are derived. The improved set of results reveals clearer correspondences with H2 and total hydrogen column densities. These linear relationships arise from rapid conversion of Cl^+ to Cl^0 in regions where H2 is present.

Outflow in Overlooked Luminous Quasar: Subaru Observations of AKARI J1757$+$5907

Abstract We present Subaru observations of the newly discovered luminous quasar AKARI J1757$+$5907, which shows an absorption outflow in its spectrum. The absorption consists of 9 distinct troughs, and our analysis focuses on the troughs at $\sim-$1000 km s$^{-1}$ for which we could measure accurate column densities of He I$^*$, Fe II, and Mg II. We used photoionization models to constrain the ionization parameter, total hydrogen column density, and number density of the outflowing gas. These constraints yielded lower limits for the distance, mass-flow rate, and kinetic luminosity for outflows of 3.7 kpc, 70 $M_{\odot}$ yr$^{-1}$, and 2.0 $\times$10$^{43}$ erg s$^{-1}$, respectively. Such a mass-flow rate value can contribute significantly to metal enrichment of the intra-cluster medium. We found that this moderate velocity outflow is similar to those recently discovered in massive post-starburst galaxies. Finally, we describe the scientific potential of future observations targeting this object.

MULTIPLE ABSORPTION-LINE SPECTROSCOPY OF THE INTERGALACTIC MEDIUM. I. MODEL

We present a physically-based absorption-line model for the spectroscopic study of the intergalactic medium (IGM). This model adopts results from Cloudy simulations and theoretical calculations by Gnat and Sternberg (2007) to examine the resulting observational signatures of the absorbing gas with the following ionization scenarios: collisional ionization equilibrium (CIE), photoionization equilibrium, hybrid (photo- plus collisional ionization), and non-equilibrium cooling. As a demonstration, we apply this model to new observations made with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope of the IGM absorbers at z~0.1877 along the 1ES 1553+113 sight line. We identify Ly alpha, C III, O VI, and N V absorption lines with two distinct velocity components (blue at z_b=0.18757; red at z_r=0.18772) separated by Delta(cz)/(1+z)~38 km/s. Joint analyses of these lines indicate that none of the examined ionization scenarios can be applied with confidence to the blue velocity component, although photoionization seems to play a dominant role. For the red component, CIE can be ruled out, but pure photoionization and hybrid scenarios (with T<1.3E5 K) are more acceptable. The constrained ranges of hydrogen density and metallicity of the absorbing gas are n_H=(1.9-2.3)E-5 cm^-3 and Z=(0.43-0.67)Z_solar. These constraints indicate OVI and HI ionization fractions, f_OVI=0.10-0.15 and f_HI=(3.2-5.1)E-5, with total hydrogen column density N_H=(0.7-1.2)E18 cm^-2. This demonstration shows that joint analysis of multiple absorption lines can constrain the ionization state of an absorber, and results used to estimate the baryonic matter contained in the absorber.

THE DISCOVERY OF THE FIRST He Iλ10830 BROAD ABSORPTION LINE QUASAR

We report the discovery of the first He i*λ10830 broad absorption line quasar FBQS J1151+3822. Using new infrared and optical spectra, as well as the SDSS spectrum, we extracted the apparent optical depth profiles as a function of velocity of the 3889 Å and 10830 Å He i* absorption lines. Since these lines have the same lower levels, inhomogeneous absorption models could be used to extract the average true He i* column density; the log of that number was 14.9. The total hydrogen column density was obtained using Cloudy models. A range of ionization parameters and densities were allowed, with the lower limit on the ionization parameter of log U = −1.4 determined by the requirement that there be sufficient He i*, and the upper limit on the density of log n = 8 determined by the lack of Balmer absorption. Simulated UV spectra showed that the ionization parameter could be further constrained in principle using a combination of low- and high-ionization lines (such as Mg ii and P v), but the only density-sensitive line predicted to be observable and not significantly blended was C iiiλ1176. We estimated the outflow rate and kinetic energy, finding them to be consistent but on the high side compared with analyses of other objects. Assuming that radiative line driving is the responsible acceleration mechanism, a force multiplier model was constructed. A dynamical argument using the model results strongly constrained the density to be log n ⩾ ∼7. Consequently, the log hydrogen column density is constrained to be between 21.7 and 22.9, the mass outflow rate to be between 11 and 56 solar masses per year, the ratio of the mass outflow rate to the accretion rate to be between 1.2 and 5.8, and the kinetic energy to be between 1 and 5 × 1044 erg s−1. We discuss the advantages of using He i* to detect high column density BALQSOs and measure their properties. We find that the large λfik ratio of 23.3 between the 10830 Å and 3889 Å components makes He i* analysis sensitive to a large range of high column densities. We discuss the prospects for finding other He i*λ10830 BALQSOs and examine the advantages of studying the properties of a sample identified using He i*.

THE MID-INFRARED HIGH-IONIZATION LINES FROM ACTIVE GALACTIC NUCLEI AND STAR-FORMING GALAXIES

We used Spitzer/IRS spectroscopic data on 426 galaxies including quasars, Seyferts, LINER and HII galaxies to investigate the relationship among the mid-IR emission lines. There is a tight linear correlation between the [Ne V]14.3 um and 24.3 um (97.1 eV) and the [O IV]25.9 um (54.9 eV) high-ionization emission lines. The correlation also holds for these high-ionization emission lines and the [Ne III]15.56 um (41 eV) emission line, although only for active galaxies. We used these correlations to calculate the [Ne III] excess due to star formation in Seyfert galaxies. We also estimated the [O IV] luminosity due to star formation in active galaxies and determined that it dominates the [O IV] emission only if the contribution of the active nucleus to the total luminosity is below 5%. We find that the AGN dominates the [O IV] emission in most Seyfert galaxies, whereas star-formation adequately explains the observed [O IV] emission in optically classified HII galaxies. Finally we computed photoionization models to determine the physical conditions of the narrow line region where these high-ionization lines originate. The estimated ionization parameter range is -2.8 < log U < -2.5 and the total hydrogen column density range is 20 < log nH (cm-2) < 21.

A Detailed Observational Study of Molecular Loops 1 and 2 in the Galactic Center

Fukui et al. (2006, Science, 314, 106) discovered two huge molecular loops in the galactic center located at ($l, b$) $\simeq$ (355$^\circ$–359$^\circ$, 0$^\circ$–2$^\circ$) in a large velocity range of $-$180—40 km s$^{-1}$. Following the discovery, we present detailed observational properties of the two loops based on NANTEN $^{12}$CO ($J =$ 1–0) and $^{13}$CO ($J =$ 1–0) datasets at 10 pc resolution, including a complete set of velocity channel distributions and comparisons with HI and dust emissions as well as with the other broad molecular features. We have found new features on smaller scales in the loops, including helical distributions in the loop tops and vertical spurs. The loops have counterparts of the HI gas, indicating that the loops include atomic gas. The IRAS far-infrared emission is also associated with the loops, and was used to derive an X-factor of 0.7 ($\pm$ 0.1) $\times$ 10$^{20}\ $cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ to convert the $^{12}$CO intensity into the total molecular hydrogen column density. From the $^{12}$CO, $^{13}$CO, HI, and dust datasets we estimated the total mass of loops 1 and 2 to be $\sim\ $1.4 $\times$ 10$^6$$M_\odot$ and $\sim\ $1.9 $\times$ 10$^6$$M_\odot$, respectively, where the HI mass corresponds to $\sim\ $10%–20% of the total mass and the total kinetic energy of the two loops is $\sim\ $10$^{52}\ $erg. An analysis of the kinematics of the loops yields that the loops are rotating at $\sim\ $47 km s$^{-1}$ and expanding at $\sim\ $141 km s$^{-1}$ at a radius of $\sim\ $670 pc from the center. Fukui et al. (2006) presented a model that the loops are created by magnetic flotation due to the Parker instability with an estimated magnetic field strength of $\sim\ $150$\ \mu$G. We present comparisons with the recent numerical simulations of the magnetized nuclear disk by Machida et al. (2009, PASJ, 61, 411) and Takahashi et al. (2009, PASJ, 61, 957), and show that the theoretical results are in good agreement with the observations. The helical distributions also suggest that some magnetic instability plays a role similarly to the solar helical features.

COSMIC ORIGINS SPECTROGRAPH ANDFUSEOBSERVATIONS OFT∼ 105K GAS IN A NEARBY GALAXY FILAMENT

We present a detection of a broad Ly-alpha absorber (BLA) with a matching O VI line in the nearby universe. The BLA is detected at z = 0.01028 in the high S/N spectrum of Mrk 290 obtained using the Cosmic Origins Spectrograph. The Ly-alpha absorption has two components, with b(HI) = 55 +/- 1 km/s and b(HI) = 33 +/- 1 km/s, separated in velocity by v ~ 115 km/s. The O VI, detected by FUSE at z = 0.01027, has a b(OVI) = 29 +/- 3 km/s and is kinematically well aligned with the broader HI component. The different line widths of the BLA and OVI suggest a temperature of T = 1.4 x 10^5 K in the absorber. The observed line strength ratios and line widths favor an ionization scenario in which both ion-electron collisions and UV photons contribute to the ionization in the gas. Such a model requires a low-metallicity of -1.7 dex, ionization parameter of log U ~ -1.4, a large total hydrogen column density of N(H) ~ 4 x 10^19 cm^-2, and a path length of 400 kpc. The line of sight to Mrk 290 intercepts at the redshift of the absorber, a megaparsec scale filamentary structure extending over 20 deg in the sky, with several luminous galaxies distributed within 1.5 Mpc projected distance from the absorber. The collisionally ionized gas in this absorber is likely tracing a shock-heated gaseous structure, consistent with a few different scenarios for the origin, including an over-dense region of the WHIM in the galaxy filament or highly ionized gas in the extended halo of one of the galaxies in the filament. In general, BLAs with metals provide an efficient means to study T ~ 10^5 - 10^6 K gas in galaxy halos and in the intergalactic medium. A substantial fraction of the baryons "missing" from the present universe is predicted to be in such environments in the form of highly ionized plasma.

The origins of a rich absorption line complex in a quasar at redshift 3.45

We discuss the nature and origin of a rich complex of narrow absorption lines in the quasar J102325.31+514251.0 at redshift 3.447. We measure nine C iv(λλ1548, 1551) absorption line systems with velocities from −1400 to −6200 km s−1, and full widths at half-minimum ranging from 16 to 350 km s−1. We also detect other absorption lines in these systems, including H i, C iii, N v, O vi and Si iv. Lower ionization lines are not present, indicating a generally high degree of ionization in all nine systems. The total hydrogen column densities range from ≲ 1017.2 to 1019.1 cm−2. The tight grouping of these lines in the quasar spectrum suggests that most or all of the absorbing regions are physically related. We examine several diagnostics to estimate more directly the location and origin of each absorber. Four of the systems can be attributed to a quasar-driven outflow based on line profiles that are smooth and broad compared to thermal linewidths and to the typical absorption lines formed in intergalactic gas or galaxy haloes. Several systems also have other indicators of a quasar outflow origin, including partial covering of the quasar emission source (e.g. in systems with speeds too high for a starburst-driven flow), O vi column densities above 1015 cm−2 and an apparent line-lock in C iv (in two of the narrow profile systems). A search for line variability yielded null results, although with very poor constraints because the comparison spectra have much lower resolution. Altogether (but not including the tentative line-lock) there is direct evidence for six of the nine systems forming in a quasar outflow. Consistent with a near-quasar origin, eight of the systems have metallicity values or lower limits in the range Z≥ 1–8 Z⊙. The lowest velocity system, which has an ambiguous location based on the diagnostics mentioned above, also has the lowest metallicity, Z≤ 0.3 Z⊙, and might form in a non-outflow environment farther from the quasar. Overall, however, this complex of narrow absorption lines can be identified with a highly structured, multicomponent outflow from the quasar. The high metallicities are similar to those derived for other quasars at similar redshifts and luminosities, and are consistent with evolution scenarios wherein quasars appear after the main episodes of star formation and metal enrichment in the host galaxies.

New searches for H i 21 cm in damped Lyman α absorption systems

We present the results of three separate searches for HI 21-cm absorption in a total of twelve damped Lyman-alpha absorption systems (DLAs) and sub-DLAs over the redshift range z = 0.86-3.37. We find no absorption in the five systems for which we obtain reasonable sensitivities and add the results to those of other recent surveys in order to investigate factors which could have an effect on the detection rate: We provide evidence that the mix of spin temperature/covering factor ratios seen at low redshift may also exist at high redshift, with a correlation between the 21-cm line strength and the total neutral hydrogen column density, indicating a roughly constant spin temperature/covering factor ratio for all of the DLAs searched. Also, by considering the geometry of a flat expanding Universe together with the projected sizes of the background radio emission regions, we find, for the detections, that the 21-cm line strength is correlated with the size of the absorber. For the non-detections it is apparent that larger absorbers (covering factors) are required in order to exhibit 21-cm absorption, particularly if these DLAs do not arise in spiral galaxies. We also suggest that the recent z = 2.3 detection towards TXS 0311+430 arises in a spiral galaxy, but on the basis of a large absorption cross-section and high metallicity, rather than a low spin temperature

HUBBLE SPACE TELESCOPESURVEY OF INTERSTELLAR HIGH-VELOCITY Si III

We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong 1206.500 Angstrom line of Si III using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Because the Si III line is 4-5 times stronger than O VI 1031.926, it provides a sensitive probe of ionized gas down to column densities N(Si III) = 5x10^11 cm^-2 at Si III equivalent width 10 mA. We detect high-velocity Si III over (91+/-4)% of the sky (53 of 58 sight lines), and 59% of the HVCs show negative LSR velocities. Per sight line, the mean HVC column density is <log N(SiIII)> = 13.19 +/- 0.45, while the mean for all 90 velocity components is 12.92 +/- 0.46. Lower limits due to Si III line saturation are included in this average, so the actual mean/median values are even higher. The Si III appears to trace an extensive ionized component of Galactic halo gas at temperatures 10^4.0 K to 10^4.5 K indicative of a cooling flow. Photoionization models suggest that typical Si III absorbers with 12.5 < log N(Si III) < 13.5 have total hydrogen column densities N(H) = 10^18 to 10^19 cm^-2 for gas of hydrogen density n(H) = 0.1 cm^(-3) and 10% solar metallicity. With typical neutral fractions N(HI)/N(H) = 0.01, these HVCs may elude even long duration 21-cm observations at Arecibo, the EVLA, and other radio facilities. However, if Si III is associated with higher density gas, n(H) > 1 cm^(-3), the corresponding neutral hydrogen could be visible in deep observations. This reservoir of ionized gas may contain 10^8 M_sun and produce a mass infall rate of 1 M_sun/yr to the Galactic disk.

TRIGGERED STAR FORMATION IN A DOUBLE SHELL NEAR W51A

We present Heinrich Hertz Telescope CO observations of the shell structure near the active star-forming complex W51A to investigate the process of star formation triggered by the expansion of an HII region. The CO observations confirm that dense molecular material has been collected along the shell detected in Spitzer IRAC images. The CO distribution shows that the shell is blown out toward a lower density region to the northwest. Total hydrogen column density around the shell is high enough to form new stars. We find two CO condensations with the same central velocity of 59 km/s to the east and north along the edge of the IRAC shell. We identify two YSOs in early evolutionary stages (Stage 0/I) within the densest molecular condensation. From the CO kinematics, we find that the HII region is currently expanding with a velocity of 3.4 km/s, implying that the shell's expansion age is ~1 Myr. This timescale is in good agreement with numerical simulations of the expansion of the HII region (Hosokawa et al. 2006). We conclude that the star formation on the border of the shell is triggered by the expansion of the HII region.

Spatial and temporal variations in interstellar absorption towards HD 72127AB

New optical spectra of Ca II and Na I towards HD 72127AB provide additional evidence for both spatial and temporal variations in the complex interstellar absorption along the two sightlines; archival ultraviolet spectra yield information on the abundances, depletions and physical conditions in the gas towards HD 72127A. Similarities in the strengths of various tracers of interstellar material in the two lines of sight suggest that the total hydrogen column densities (N ∼ 2.5 × 10 20 cm −2 ) and the depletions and ionization in the main components at low local standard of rest (LSR) velocities also are similar. Towards HD 72127A, the main components are relatively cool (T 900 K), but with depletions resembling those found in warm, diffuse disc clouds; the generally weaker components at higher velocities have much milder depletions, more like those found in halo clouds. Several trace neutral species – Ca I, Cr I and Fe I – are much stronger towards HD 72127B, however. The column density of Cr I ,f or example, is about 30 times the value determined towards ζ Oph (the only previous detection of that species in the ISM). Dielectronic recombination in warmer gas (T 5000 K) may be largely responsible for the enhanced abundances of those trace neutral species towards HD 72127B. If the main components towards HD 72127AB are associated with material in the Vela supernova remnant, the differences in abundances and physical conditions occur on scales of about 1100 au.

Unsaturated Low‐Ionization Broad Absorption Lines in the Quasar SDSS J144842.45+042403.1

We present a detailed analysis of the exceptional low-ionization broad absorption line (LoBAL) quasar SDSS J144842.45+042403.1. The C IV absorption trough, which is likely saturated, has a box-shaped bottom at a flux level of 2%-5% of the unabsorbed flux. A large fraction of at least 80% of the C IV and Fe II emission-line fluxes are obliterated by the C IV broad absorption lines (BALs), spanning 3000 km s−1 in velocity. This indicates that the outflow intercepts most of the lines of sight to the continuum source and the broad emission line region (BELR). Therefore, the BAL region (BALR) should be located outside the BELR, and its size is no less than that of the BELR. The Al III and Mg II BAL troughs, which are well resolved, are found to be unsaturated and to show the same velocity structures. Meanwhile, their velocities largely overlap that of the C IV BAL, indicating that the LoBALs and the HiBALs (high-ionization BALs) are closely associated and most likely produced from the same outflow. The column densities of the Mg+ and Al2+ ions in the outflow are measured. Results from an X-ray observation performed with Chandra suggest the presence of intrinsic X-ray absorption with a hydrogen column density of 1022-1023 cm−2. The measured Mg+ and Al2+ column densities can be well reproduced in models with a dimensionless ionization parameter of log U > − 1.5 and a total hydrogen column density of NH = 2.8 × 1022U cm−2. A comparison of models with the observed properties also suggests that the outflow is either very highly ionized (with a high column density) or clumpy.

Molecular hydrogen in high-redshift damped Lyman-αsystems: the VLT/UVES database

Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 z abs ≤ 4.2) Damped Lyman- α systems (DLAs) capitalising on observations performed with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log N (H I) ≥ 20 and z abs >1.8, which have data that include redshifted H 2 Lyman and/or Werner-band absorption lines. This sample of H I, H 2 and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H 2 could be completed so far, including all non-detections. Results. H 2 is detected in thirteen of the systems, which have molecular fractions of values between f $\simeq$ 5$\times$10 -7 and f $\simeq$ 0.1, where f = 2 N (H 2 )/(2 N (H 2 )+N(H I)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N (H 2 )~14.3, corresponding to log f ≥ -1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log f > -4.5, while H 2 is detected – regardless of the molecular fraction – in ∼ 50% of them. In contrast, only about 4% of the [X/H] f > -4.5. We show that the presence of H 2 does not strongly depend on the total neutral hydrogen column density, although the probability of finding log f > -4.5 is higher for log N (H I) ≥ 20.8 than below this limit (19% and 7% respectively). The overall H 2 detection rate in log N (H I) ≥ 20 DLAs is found to be about 16% (10% considering only log f > -4.5 detections) after correction for a slight bias towards large N (H I). There is a strong preference for H 2 -bearing DLAs to have significant depletion factors, [X/Fe] > 0.4. In addition, all H 2 -bearing DLAs have column densities of iron into dust grains larger than log $N({\rm Fe})_{\rm dust}$~ 14.7, and about 40% of the DLAs above this limit have detected H 2 lines with log f > -4.5. This demonstrates the importance of dust in governing the detectability of H 2 in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H 2 -bearing DLAs nor that of the molecular fraction in systems with H 2 detections over the redshift range 1.8 z abs ≤ 3.

Fully Sampled Maps of Ices and Silicates in Front of Cepheus A East with theSpitzer Space Telescope

We report the first fully sampled maps of the distribution of interstellar CO2 ices, H2O ices, and total hydrogen nuclei, as inferred from the 9.7 μm silicate feature, toward the star-forming region Cepheus A East with the IRS instrument on board the Spitzer Space Telescope. We find that the column density distributions for these solid state features all peak at, and are distributed around, the location of HW2, the protostar believed to power one of the outflows observed in this star-forming region. A correlation between the column density distributions of CO2 and water ice with that of total hydrogen indicates that the solid state features we mapped mostly arise from the same molecular clumps along the probed sight lines. We therefore derive average CO2 ice and water ice abundances with respect to the total hydrogen column density of X(CO2)ice ~ 1.9 × 10−5 and X(H2O)ice ~ 7.5 × 10−5. Within errors, the abundances for both ices are relatively constant over the mapped region exhibiting both ice absorptions. The fraction of CO2 ice with respect to H2O ice is also relatively constant at a value of 22% over that mapped region. A clear triple-peaked structure is seen in the CO2 ice profiles. Fits to those profiles using current laboratory ice analogs suggest the presence of both a low-temperature polar ice mixture and a high-temperature methanol-rich ice mixture along the probed sight lines. Our results further indicate that thermal processing of these ices occurred throughout the sampled region.

Particularly efficient star formation in M 33

The Star Formation (SF) rate in galaxies is an important parameter at all redshifts and evolutionary stages of galaxies. In order to understand the increased SF rates in intermediate redshift galaxies one possibility is to study star formation in local galaxies with properties frequently found at this earlier epoch like low metallicity and small size. We present sensitive observations of the molecular gas in M 33, a small Local Group spiral at a distance of 840 kpc which shares many of the characteristics of the intermediate redshift galaxies. The observations were carried out in the CO(2–1) line with the HERA heterodyne array on the IRAM 30 m telescope. A region in the northern part of M 33 was observed, reaching a detection threshold of a few 103 . The correlation in this field between the CO emission and tracers of SF (8 μm, 24 μm, Hα, FUV) is excellent and CO is detected very far North, showing that molecular gas forms far out in the disk even in a small spiral with a subsolar metallicity. One major molecular cloud was discovered in an interarm region with no HI peak and little if any signs of SF – without a complete survey this cloud would never have been found. The radial dependence of the CO emission has a scale length similar to the dust emission, less extended than the Hα or FUV. If, however, the ratio varies inversely with metallicity, then the scale length of the H2 becomes similar to that of the Hα or FUV. Comparing the SF rate to the H2 mass shows that M 33, like the intermediate redshift galaxies it resembles, has a significantly higher SF efficiency than large local universe spirals. The data presented here also provide an ideal test for theories of molecular cloud formation and cover a new region in parameter space, where . We find that a simple pressure-based prescription for estimating the molecular to atomic gas fraction does not perform well for M 33, at least in the outer parts. On the other hand, we show that the molecular gas fraction is influenced by (i) the total Hydrogen column density, dominated in M 33 by the HI, and (ii) the galactocentric distance.

The coronal structure of Speedy Mic – II. Prominence masses and off-disc emission*

Observations of stellar prominences on young rapidly rotating stars provide unique probes of their magnetic fields out to many stellar radii. We compare two independently obtained data sets of the K3 dwarf Speedy Mic (BO Mic, HD 197890) using the Anglo-Australian Telescope (AAT) and the European Southern Observatory (ESO) Very Large Telescope (VLT). Taken more than a fortnight apart, they provide the first insight into the evolution of the prominence system on such a young rapidly rotating star. The largest prominences observed transiting the stellar disc are found at very similar rotational phases between the epochs. This suggests that the magnetic structures supporting the prominences retain their identity on a two to three week time-scale. By taking advantage of the high signal-to-noise ratio and large wavelength range of the VLT observations, we identify prominences as transient absorption features in all lines of the hydrogen Balmer series down to H10. We use the ratios of the prominence equivalent widths (EWs) in these lines to determine their column densities in the first excited state of hydrogen. We determine the optical depths, finding prominences to be rather optically thick (τ≈ 20) in the Hα line. The total hydrogen column density and thus the prominence masses are determined via observations of the Ca II H&K lines. We find typical masses for four of the largest prominences to be in the range 0.5–2.3 × 1014 kg, slightly larger than giant solar prominence masses. Rotationally modulated emission is seen outside of the Hα line. These loops of emission are shown to be caused by prominences seen off the stellar disc. We find that all of the large emission loops can be associated with prominences we see transiting the stellar disc. This, combined with the fact that many prominences appear to eclipse the off-disc emission of others, strongly suggests that the prominence system is highly flattened and likely confined to low stellar latitudes.