Total Hydrogen Column Density Research Articles

Lyman Limit System with O vi in the Circumgalactic Environment of a Pair of Galaxies

We report on the analysis of a multiphase Lyman limit system (LLS) at z = 0.39047 identified toward the background quasar FBQS J0209–0438. The O VI doublet lines associated with this absorber have a different profile from the low-ionization metals and H i. Ly α has a very broad H i (b ≈ 150 km s−1) component well-aligned with one of the O VI components. The Doppler b-parameters for the broad H i and O VI indicate gas with T = (0.8 − 2.0) × 106 K and a total hydrogen column density that is an order of magnitude larger than the cooler phase of gas responsible for the LLS. Observations by the Very Large Telescope MUSE show two moderately star-forming galaxies within ρ ≲ 105 kpc and ∣Δv∣ ≲ 130 km s−1 of the absorber, one of them a dwarf galaxy (M * ≈ 106 M ⊙) overlapping with the quasar point-spread function, and the other a larger galaxy (R 1/2 ≈ 4 kpc) with M * ≈ 3 × 1010 M ⊙ and M h ≈ 7 × 1011 M ⊙, and the dwarf galaxy within its virial radius. Although the absorption is aligned with the extended major axis of the larger galaxy, the line-of-sight velocity of the absorbing gas is inconsistent with corotating accretion. The metallicity inferred for the LLS is lower than the gas phase [O/H] of the two galaxies. The mixture of cool and warm/hot gas phases for the absorbing gas and its proximity and orientation to the galaxy pair points to the LLS being a high-velocity gas in the combined halo environment of both galaxies.

Molecular cloud matching in CO and dust in M33

This study is aimed to contribute to a more comprehensive understanding of the molecular hydrogen distribution in the galaxy M33 by introducing novel methods for generating high angular resolution (18.2″, equivalent to 75 pc for a distance of 847 kpc) column density maps of molecular hydrogen (NH2). M33 is a local group galaxy that has been observed with Herschel in the far-infrared (FIR) wavelength range from 70 to 500 μm. Previous studies have presented total hydrogen column density maps (NH), using these FIR data (partly combined with mid-IR maps), employing various methods. We first performed a spectral energy distribution (SED) fit to the 160, 250, 350, and 500 μm continuum data obtain NH, using a technique similar to one previously reported in the literature. We also use a second method which involves translating only the 250 μm map into a NH map at the same angular resolution of 18.2″. An NH2 map via each method is then obtained by subtracting the H I component. Distinguishing our study from previous ones, we adopt a more versatile approach by considering a variable emissivity index, β, and dust absorption coefficient, κ0. This choice enables us to construct a κ0 map, thereby enhancing the depth and accuracy of our investigation of the hydrogen column density. We address the inherent biases and challenges within both methods (which give similar results) and compare them with existing maps available in the literature. Moreover, we calculate a map of the carbon monoxide CO(1 − 0)-to-molecular hydrogen (H2) conversion factor (XCO factor), which shows a strong dispersion around an average value of 1.8 × 1020 cm−2/(K km s−1) throughout the disk. We obtain column density probability distribution functions (N-PDFs) from the NH, NH2, and NH I maps and discuss their shape, consisting of several log-normal and power-law tail components.

The Observed O vi Is Just the Tip of the Iceberg: Estimating the Hidden Material in Circumgalactic and Intergalactic Clouds

This paper proposes a new method for estimating the total quantity of material in moving circumgalactic and intergalactic clouds from O vi measurements. We simulate high-velocity clouds (HVCs) with the FLASH hydrodynamic code and track the ionization and recombination of all ionization levels of oxygen as a function of time. We calculate the O vi/oxygen ratio (f O VI ) in our dynamic nonequilibrium ionization clouds, finding that it differs significantly from that in static gas. We find that O vi exists in cool, medium, and hot gas in the clouds. As such, it traces all of the hydrogen rather than merely the ionized hydrogen. The total quantity of hydrogen along a typical observed line of sight through a cloud can be estimated from the observed O vi column density, metallicity, and our f O VI . We provide the simulations’ f O VI values, a prescription for finding f O VI for observed dynamic clouds, and a methodology for calculating the total hydrogen column density from f O VI and an observed O vi column density. As examples, we use our f O VI values to estimate the total hydrogen column densities along various observed sight lines through two HVCs, Complex C and the Magellanic Stream, finding that these clouds contain more material than previous lower limits. We also extend this analysis to low-redshift intergalactic O vi clouds, finding that they contain several times more baryonic material than previously thought and therefore may account for a significant fraction of the Universe’s baryons.

Constraining cosmic ray transport with observations of the circumgalactic medium

ABSTRACT Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, ∼L* galaxies could have substantial non-thermal pressure support in the form of cosmic rays. However, these predictions are sensitive to the specific model of cosmic ray transport employed, which is theoretically and observationally underconstrained. In this work, we propose a novel observational constraint for calculating the lower limit of the radially averaged, effective cosmic ray transport rate, ${\kappa _{\rm eff}^{\rm min}}$. Under a wide range of assumptions (so long as cosmic rays do not lose a significant fraction of their energy in the galactic disc, regardless of whether the cosmic ray pressure is important or not in the CGM), we demonstrate a well-defined relationship between ${\kappa _{\rm eff}^{\rm min}}$ and three observable galaxy properties: the total hydrogen column density, the average star formation rate, and the gas circular velocity. We use a suite of Feedback in Realistic Environments 2 galaxy simulations with a variety of cosmic ray transport physics to demonstrate that our analytical model of ${\kappa _{\rm eff}^{\rm min}}$ is a robust lower limit of the true cosmic ray transport rate. We then apply our new model to calculate ${\kappa _{\rm eff}^{\rm min}}$ for galaxies in the COS-Halos sample, and confirm this already reveals strong evidence for an effective transport rate that rises rapidly away from the interstellar medium to values ${\kappa _{\rm eff}^{\rm min}}\gtrsim 10^{30\!-\!31}\, {\rm cm}^2\, {\rm s}^{-1}$ (corresponding to anisotropic streaming velocities of $v^{\rm stream}_{\rm eff} \gtrsim 1000\, {\rm km}\, {\rm s}^{-1}$) in the diffuse CGM, at impact parameters larger than 50–100 kpc. We discuss how future observations can provide qualitatively new constraints in our understanding of cosmic rays in the CGM and intergalactic medium.

Atomic oxygen abundance toward Sagittarius B2

A substantial fraction of oxygen in diffuse clouds is unaccounted for by observations and is postulated to be in an unknown refractory form, referred to as unidentified depleted oxygen (UDO), which, depending on the local gas density, may contribute up to 50% of the total oxygen content. Previous Infrared Space Observatory (ISO) observations suggest that a significant fraction of oxygen in even denser, translucent clouds may be in atomic form. We have analyzed velocity-resolved archival SOFIA observations of the 63 μm fine-structure [O I] transition toward the high-mass star-forming region Sgr B2(M) in the Central Molecular Zone. The foreground spiral-arm clouds as well as the extended Sgr B2 envelope between the Sun and the background dust continuum source produce multiple [O I] absorption components, spectrally separated in velocity space. The gas-phase atomic oxygen column density in foreground clouds toward Sgr B2 is well correlated with the total hydrogen column density, with an average atomic oxygen abundance of (2.51 ± 0.69) × 10−4 with respect to hydrogen nuclei. This value is in good agreement with the earlier ISO measurements on the same line of sight, and is about 35% lower than the total interstellar medium oxygen abundance in the low-density warm gas, as measured in the UV. We find no evidence that a significant fraction of the oxygen on the line of sight toward Sagittarius B2 is in the form of UDO.

The mixing of dust and gas in the high latitude translucent cloud MBM 40

Context. High latitude molecular clouds (hereafter HLMCs) permit the study of interstellar gas dynamics and astrochemistry with good accuracy due to their proximity, generally clear lines of sight, and lack of internal star-forming activity which can heavily modify the physical context. MBM 40, one of the nearest HLMCs, has been extensively studied, making it a superb target to infer and study the dust-to-gas mixing ratio (DGMR). Aims. The mixing of dust and gas in the interstellar medium remains a fundamental issue to keep track of astrochemistry evolution and molecular abundances. Accounting for both molecular and atomic gas is difficult because H2 is not directly observable and H I spectra always show different dynamical profiles blended together which are not directly correlated with the cloud. We used two independent strategies to infer the molecular and atomic gas column densities and compute the dust-to-gas mixing ratio. Methods. We combined H I 21 cm and 12CO line observations with the IRAS 100 μm image to infer the dust-to-gas mixing ratio within the cloud. The cloud 21 cm profile was extracted using a hybrid Gaussian decomposition where 12CO was used to deduce the total molecular hydrogen column density. Infrared images were used to calculate the dust emission. Results. The dust-to-gas mixing ratio is nearly uniform within the cloud as outlined by the hairpin structure. The total hydrogen column density and 100 μm emissivity are linearly correlated over a range in N(Htot) of one order of magnitude.

Understanding star formation in molecular clouds

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.

Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition

Context. The atomic-to-molecular hydrogen (H/H2) transition has been extensively studied as it controls the fraction of gas in a molecular state in an interstellar cloud. This fraction is linked to star-formation by the Schmidt–Kennicutt law. While theoretical estimates of the column density of the H I layer have been proposed for static photodissociation regions (PDRs), Herschel and well-resolved Atacama Large Millimeter Array observations have revealed dynamical effects in star forming regions, caused by the process of photoevaporation. Aims. We extend the analytic study of the H/H2 transition to include the effects of the propagation of the ionization front, in particular in the presence of photoevaporation at the walls of blister H II regions, and we find its consequences on the total atomic hydrogen column density at the surface of clouds in the presence of an ultraviolet field, and on the properties of the H/H2 transition. Methods. We solved semi-analytically the differential equation giving the H2 column density profile by taking into account H2 formation on grains, H2 photodissociation, and the ionization front propagation dynamics modeled as advection of the gas through the ionization front. Results. Taking this advection into account reduces the width of the atomic region compared to static models. The atomic region may disappear if the ionization front velocity exceeds a certain value, leading the H/H2 transition and the ionization front to merge. For both dissociated and merged configurations, we provide analytical expressions to determine the total H I column density. Our results take the metallicity into account. Finally, we compared our results to observations of PDRs illuminated by O-stars, for which we conclude that the dynamical effects are strong, especially for low-excitation PDRs.

Discovery of a multiphase O VI and O VII absorber in the circumgalactic/intergalactic transition region

Aims. The observational constraints on the baryon content of the warm-hot intergalactic medium (WHIM) rely almost entirely on far ultraviolet (FUV) measurements. However, cosmological, hydrodynamical simulations predict strong correlations between the spatial distributions of FUV and X-ray absorbing WHIM. We investigate this prediction by analyzing XMM-Newton X-ray counterparts of FUV-detected intergalactic O VI absorbers known from FUSE and HST/STIS data, thereby aiming to gain understanding on the properties of the hot component of FUV absorbers and to compare this information to the predictions of simulations. Methods. We study the X-ray absorption at the redshift of the only significantly detected O VI absorber in the Ton S 180 sightline’s FUV spectrum, found at zOVI = 0.04579 ± 0.00001. We characterize the spectral properties of the O VI-O VIII absorbers and explore the ionization processes behind the measured absorption. The observational results are compared to the predicted warm-hot gas properties in the EAGLE simulation to infer the physical conditions of the absorber. Results. We detect both O VI and O VII absorption at a 5σ confidence level, whereas O VIII absorption is not significantly detected. Collisional ionization equilibrium (CIE) modeling constrains the X-ray absorbing gas temperature to log TCIE (K) = 6.22 ± 0.05 with a total hydrogen column density NH = 5.8−2.2+3.0 × Z⊙/Zabs × 1019 cm−2. This model predicts an O VI column density consistent with that measured in the FUV, but our limits on the O VI line width indicate > 90% likelihood that the FUV-detected O VI arises from a different, cooler phase. We find that the observed absorber lies about a factor of two further away from the detected galaxies than is the case for similar systems in EAGLE Conclusions. The analysis suggests that the detected O VI and O VII trace two different – warm and hot – gas phases of the absorbing structure at z ≈ 0.046, of which the hot component is likely in collisional ionization equilibrium. As the baryon content information of the studied absorber is primarily imprinted in the X-ray band, understanding the abundance of similar systems helps to define the landscape for WHIM searches with future X-ray telescopes. Our results highlight the crucial role of line widths for the interpretation and detectability of WHIM absorbers.

NICER, NuSTAR, and Swift follow-up observations of the γ-ray flaring blazar BL Lacertae in 2020 August–October

ABSTRACT During a period of strong γ-ray flaring activity from BL Lacertae, we organized Swift, Neutron star Interior Composition Explorer (NICER), and Nuclear Spectroscopic Telescope Array (NuSTAR) follow-up observations. The source has been monitored by Swift-XRT (X-ray Telescope) between 2020 August 11 and October 16, showing a variability amplitude of 65, with a flux varying between 1.0 × 10−11 and 65.3 × 10−11 erg cm−2 s−1. On 2020 October 6, Swift-XRT has observed the source during its historical maximum X-ray flux. A softer-when-brighter behaviour has been observed by XRT, suggesting an increasing importance of the synchrotron emission in the X-ray part of the spectrum covered by XRT during this bright state. Rapid variability in soft X-rays has been observed with both the Swift-XRT and NICER observations with a minimum variability time-scale of 60 and 240 s, and a doubling time-scale of 274 and 1008 s, respectively, suggesting very compact emitting regions (1.1 × 1014 and 4.0 × 1014 cm). At hard X-rays, a minimum variability time-scale of ∼5.5 ks has been observed by NuSTAR. We report the first simultaneous NICER and NuSTAR observations of BL Lacertae during 2020 October 11–12. The joint NICER and NuSTAR spectra are well fitted by a broken power law with a significant difference of the photon index below (2.10) and above (1.60) an energy break at ∼2.7 keV, indicating the presence of two different emission components (i.e. synchrotron and inverse Compton) in the broad-band X-ray spectrum. Leaving the total hydrogen column density towards BL Lacertae free to vary, a value of NH,tot = (2.58 ± 0.09) × 1021 cm−2 has been estimated.

Three-dimensional dust geometry of the LMC H i ridge region as revealed by the IRSF/SIRIUS survey

Abstract We present a new method to evaluate the dust extinction (AV) along the line of sight using the InfraRed Survey Facility (IRSF) near-infrared (NIR) data of the Large Magellanic Cloud (LMC) H i ridge region. In our method, we estimate an AV value for each star from the NIR color excess and sort them from bluer to redder in each line of sight. Using the percentile values of the sorted AV, we construct a new three-dimensional AV map. We compare the resultant AV map with the total hydrogen column density N(H) traced by velocity-resolved H i and CO observations. In the LMC H i ridge region, Fukui et al. (2017, PASJ, 69, L5) found two velocity components and an intermediate velocity one bridging them. Comparing our three-dimensional AV maps with N(H) maps at the different velocities, we find that the dust geometry is consistent with the scenario of ongoing gas collision between the two velocities as suggested in the previous study. In addition, we find a difference by a factor of 2 in AV/N(H) between the two velocity components, which suggests that inflow gas from the Small Magellanic Clouds (SMC) is mixed in this region. Overall, our results support triggered star formation in 30 Doradus due to the large-scale gas collision caused by tidal interaction between the LMC and the SMC.

The Abell 3391/95 galaxy cluster system

Context.Inferences about dark matter, dark energy, and the missing baryons all depend on the accuracy of our model of large-scale structure evolution. In particular, with cosmological simulations in our model of the Universe, we trace the growth of structure, and visualize the build-up of bigger structures from smaller ones and of gaseous filaments connecting galaxy clusters.Aims.Here we aim to reveal the complexity of the large-scale structure assembly process in great detail and on scales from tens of kiloparsecs up to more than 10 Mpc with new sensitive large-scale observations from the latest generation of instruments. We also aim to compare our findings with expectations from our cosmological model.Methods.We used dedicated SRG/eROSITA performance verification (PV) X-ray, ASKAP/EMU Early Science radio, and DECam optical observations of a ~15 deg2region around the nearby interacting galaxy cluster system A3391/95 to study the warm-hot gas in cluster outskirts and filaments, the surrounding large-scale structure and its formation process, the morphological complexity in the inner parts of the clusters, and the (re-)acceleration of plasma. We also used complementary Sunyaev-Zeldovich (SZ) effect data from thePlancksurvey and custom-made Galactic total (neutral plus molecular) hydrogen column density maps based on the HI4PI and IRAS surveys. We relate the observations to expectations from cosmological hydrodynamic simulations from the Magneticum suite.Results.We trace the irregular morphology of warm and hot gas of the main clusters from their centers out to well beyond their characteristic radii,r200. Between the two main cluster systems, we observe an emission bridge on large scale and with good spatial resolution. This bridge includes a known galaxy group but this can only partially explain the emission. Most gas in the bridge appears hot, but thanks to eROSITA’s unique soft response and large field of view, we discover some tantalizing hints for warm, truly primordial filamentary gas connecting the clusters. Several matter clumps physically surrounding the system are detected. For the “Northern Clump,” we provide evidence that it is falling towards A3391 from the X-ray hot gas morphology and radio lobe structure of its central AGN. Moreover, the shapes of these X-ray and radio structures appear to be formed by gas well beyond the virial radius,r100, of A3391, thereby providing an indirect way of probing the gas in this elusive environment. Many of the extended sources in the field detected by eROSITA are also known clusters or new clusters in the background, including a known SZ cluster at redshiftz= 1. We find roughly an order of magnitude more cluster candidates than the SPT and ACT surveys together in the same area. We discover an emission filament north of the virial radius of A3391 connecting to the Northern Clump. Furthermore, the absorption-corrected eROSITA surface brightness map shows that this emission filament extends south of A3395 and beyond an extended X-ray-emitting object (the “Little Southern Clump”) towards another galaxy cluster, all at the same redshift. The total projected length of this continuous warm-hot emission filament is 15 Mpc, running almost 4 degrees across the entire eROSITA PV observation field. The Northern and Southern Filament are each detected at >4σ. ThePlanckSZ map additionally appears to support the presence of both new filaments. Furthermore, the DECam galaxy density map shows galaxy overdensities in the same regions. Overall, the new datasets provide impressive confirmation of the theoretically expected structure formation processes on the individual system level, including the surrounding warm-hot intergalactic medium distribution; the similarities of features found in a similar system in the Magneticum simulation are striking. Our spatially resolved findings show that baryons indeed reside in large-scale warm-hot gas filaments with a clumpy structure.

A Catalog of High-velocity C iv Mini-broad Absorption Lines in the VLT-UVES and Keck-HIRES Archives

Abstract We present a catalog of high-velocity C iv λ1548,1551 mini-broad absorption lines (mini-BALs) in the archives of the Very Large Telescope-UV Visual Echelle Spectrograph and Keck-High Resolution Echelle Spectrometer. We identify C iv mini-BALs based on smooth rounded BAL-like profiles with velocity blueshifts <−4000 km s−1 and widths in the range 70 ≲ FWHM(1548) ≲ 2000 km s−1. We find 105 mini-BALs in 44 quasars from a total sample of 638 quasars. The fraction of quasars with at least one mini-BAL meeting our criteria is roughly ∼9% after correcting for incomplete velocity coverage. All of the systems are highly ionized based on the strong presence of N v and O vi and/or the absence of Si ii and C ii when within the wavelength coverage. Two of the mini-BAL systems in our catalog, plus three others at smaller velocity shifts, have P v λ1118,1128 absorption indicating highly saturated C iv absorption and total hydrogen column densities ≳1022 cm−2. Most of the mini-BALs are confirmed to have optical depths ≳1 with partial covering of the quasar continuum source. The covering fractions are as small as 0.06 in C iv and 0.03 in Si iv, corresponding to outflow absorbing structures <0.002 pc across. When multiple lines are measured, the lines of less abundant ions tend to have narrower profiles and smaller covering fractions indicative of inhomogeneous absorbers where higher column densities occur in smaller clumps. This picture might extend to BAL outflows if the broader and generally deeper BALs form in either the largest clumps or collections of many mini-BAL-like clumps that blend together in observed quasar spectra.

Using realistic host galaxy metallicities to improve the GRB X-ray equivalent total hydrogen column density and constrain the intergalactic medium density

ABSTRACT It is known that the GRB equivalent hydrogen column density (NHX) changes with redshift and that, typically, NHX is greater than the GRB host neutral hydrogen column density. We have compiled a large sample of data for GRB NHX and metallicity [X/H]. The main aims of this paper are to generate improved NHX for our sample by using actual metallicities, dust corrected where available for detections, and for the remaining GRB, a more realistic average intrinsic metallicity using a standard adjustment from solar. Then, by approximating the GRB host intrinsic hydrogen column density using the measured neutral column (NHI, IC) adjusted for the ionization fraction, we isolate a more accurate estimate for the intergalactic medium (IGM) contribution. The GRB sample mean metallicity is = −1.17 ± 0.09 rms (or 0.07 ± 0.05 Z/Zsol) from a sample of 36 GRB with a redshift 1.76 ≤ z ≤ 5.91, substantially lower than the assumption of solar metallicity used as standard for many fitted NHX. Lower GRB host mean metallicity results in increased estimated NHX with the correction scaling with redshift as Δlog (NHX cm−2) = (0.59 ± 0.04)log(1 + z) + 0.18 ± 0.02. Of the 128 GRB with data for both NHX and NHI, IC in our sample, only six have NHI, IC > NHX when revised for realistic metallicity, compared to 32 when solar metallicity is assumed. The lower envelope of the revised NHX – NHI, IC, plotted against redshift can be fit by log(NHX – NHI, IC cm−2) = 20.3 + 2.4 log(1 + z). This is taken to be an estimate for the maximum IGM hydrogen column density as a function of redshift. Using this approach, we estimate an upper limit to the hydrogen density at redshift zero (n0) to be consistent with n0 = 0.17 × 10−7cm−3.

Pilot Hi survey of Planck Galactic Cold Clumps with FAST

We present a pilot Hi survey of 17 Planck Galactic Cold Clumps (PGCCs) with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Hi Narrow Self-Absorption (HINSA) is an effective method to detect cold Hi being mixed with molecular hydrogen H2 and improves our understanding of the atomic to molecular transition in the interstellar medium. HINSA was found in 58% PGCCs that we observed. The column density of HINSA was found to have an intermediate correlation with that of 13CO, following (N(HINSA)) = (0.52 ± 0.26) (N13CO) + (10 ± 4.1). Hi abundance relative to total hydrogen [Hi]/[H] has an average value of 4.4 × 10–3, which is about 2.8 times of the average value of previous HINSA surveys toward molecular clouds. For clouds with total column density NH > 5 × 1020 cm–2, an inverse correlation between HINSA abundance and total hydrogen column density is found, confirming the depletion of cold Hi gas during molecular gas formation in more massive clouds. Non-thermal line width of 13CO is about 0-0.5 km s−1 larger than that of HINSA. One possible explanation of narrower non-thermal width of HINSA is that HINSA region is smaller than that of 13CO. Based on an analytic model of H2 formation and H2 dissociation by cosmic ray, we found the cloud ages to be within 106.7 – 107.0 yr for five sources.

Extinction and dust/gas ratio in the H i ridge region of the LMC based on the IRSF/SIRIUS near-infrared survey

ABSTRACT We present a dust extinction AV map of the Large Magellanic Cloud (LMC) in the H i ridge region using the InfraRed Survey Facility (IRSF) near-infrared (IR) data, and compare the AV map with the total hydrogen column density N(H) maps derived from the CO and H i observations. In the LMC H i ridge region, the two-velocity H i components (plus an intermediate velocity component) are identified, and the young massive star cluster is possibly formed by collision between them. In addition, one of the components is suggested to be an inflow gas from the Small Magellanic Cloud (SMC) which is expected to have even lower metallicity gas (Fukui et al. 2017, PASJ, 69, L5). To evaluate dust/gas ratios in the H i ridge region in detail, we derive the AV map from the near-IR color excess of the IRSF data updated with the latest calibration, and fit the resultant AV map with a combination of the N(H) maps of the different velocity components to decompose it successfully into the three components. As a result, we find difference by a factor of 2 in AV/N(H) between the components. In additon, the CO-to-H2 conversion factor also indicates difference between the components, implying a difference in the metallicity. Our results are likely to support the scenario that the gas in the LMC H i ridge region is contaminated with an inflow gas from the SMC with a geometry consistent with the on-going collision between the two-velocity components.

Optical Reddening, Integrated H i Optical Depth, and Total Hydrogen Column Density

Abstract Despite the vastly different angular scales on which they are measured, the integrated λ21 cm H i optical depth measured interferometrically, , is a good proxy for the optical reddening derived from IR dust emission, with ∝ E(B–V)1.10 for 0.04 mag ≲ E(B–V) ≲ 4 mag. For E(B–V) ≲ 0.04 mag or < 0.7 , less-absorbent warm and ionized gases assert themselves and τ(H i) is a less reliable tracer of E(B–V). The –E(B–V) relationship can be inverted to give a broken power-law relationship between the total hydrogen column density N(H) and such that knowledge of alone predicts N(H) with an accuracy of a factor of 1.5 (±0.18 dex) across two orders of magnitude variation of . The –N(H) relation is invariant under a linear rescaling of the reddening measure used in the analysis and does not depend on knowing properties of the H i such as the spin temperature.

Physical conditions in high-z optically thin C iii absorbers: origin of cloud sizes and associated correlations

We present detailed photoionization models of well aligned optically thin C III absorption components at $2.1 < z < 3.4$. Using our models we estimate density ($n_{\rm \tiny H}$), metallicity ($[C/H]$), total hydrogen column density and line-of-sight thickness ($L$) in each C III components. We estimate the systematic errors in these quantities contributed by the allowed range of the quasar spectral index used in the ultraviolet background radiation calculations. Our inferred $n_{\rm \tiny H}$ and overdensity ($\Delta$) are much higher than the measurements available in the literature and favor the absorption originating from gas associated with circumgalactic medium and probably not in hydrostatic equilibrium. We also notice $n_{\rm \tiny H}$, $L$ and $[C/H]$ associated with C III components show statistically significant redshift evolution. To some extent, these redshift evolutions are driven by the appearance of compact, high $n_{\rm \tiny H}$ and high $[C/H]$ components only in the low$-z$ end. We find more than 5$\sigma$ level correlation between $[C/H]$ and $L$, $L$ and neutral hydrogen column density (N (HI)), N (HI) and $[C/H]$. We show $L$ versus $[C/H]$ correlation can be well reproduced if $L$ is governed by the product of gas cooling time and sound speed as expected in the case of cloud fragmentation under thermal instabilities. This allows us to explain other observed correlations by simple photoionization considerations. Studying the optically thin C III absorbers over a large $z$ range and probably correlating their $z$ evolution with global star formation rate density evolution can shed light into the physics of cold clump formation and their evolution in the circumgalactic medium.

Molecular gas and dark neutral medium in the outskirts of Chamaeleon

Context. More gas is inferred to be present in molecular cloud complexes than can be accounted for by H I and CO emission, a phenomenon known as dark neutral medium (DNM) or CO-dark gas for the molecular part. Aims. We aim to investigate whether molecular gas can be detected in Chamaeleon where gas column densities in the DNM were inferred and CO emission was not detected. Methods. We took λ3 mm absorption profiles of HCO+ and other molecules toward 13 background quasars across the Chamaeleon complex, only one of which had detectable CO emission. We derived the H2 column density assuming N(HCO+)/N(H2) = 3 × 10−9 as before. Results. With the possible exception of one weak continuum target, HCO+ absorption was detected in all directions, C2H in eight directions and HCN in four directions. The sightlines divide into two groups according to their DNM content, with one group of eight directions having N(DNM) ≳ 2 × 1020 cm−2 and another group of five directions having N(DNM) &lt; 0.5 × 1020 cm−2. The groups have comparable mean N(H I) associated with Chamaeleon 6−7 × 1020 cm−2 and total hydrogen column density per unit reddening 6−7 × 1021 cm−2 mag−1. They differ, however, in having quite different mean reddening 0.33 vs. 0.18 mag, mean N(DNM) 3.3 vs. 0.14 × 1020 cm−2 and mean molecular column density 2N(H2) = 5.6 vs. 0.8 × 1020 cm−2. The gas at more positive velocities is enriched in molecules and DNM. Conclusions. Overall the quantity of H2 inferred from HCO+ can fully account for the previously inferred DNM along the sightlines studied here. H2 is concentrated in the high-DNM group, where the molecular fraction is 46% vs. 13% otherwise and 38% overall. Thus, neutral gas in the outskirts of the complex is mostly atomic but the DNM is mostly molecular. Saturation of the H I emission line profile may occur along three of the four sightlines having the largest DNM column densities, but there is no substantial reservoir of “dark” atomic or molecular gas that remains undetected as part of the inventory of dark neutral medium.

Young radio sources at high-energies and the γ-ray CSO PKS 1718–649

AbstractObservations at high-energies are important to define the first stages of the evolution of extragalactic radio sources and to characterize the interstellar medium of their host galaxies. In some of the X-ray-observed Compact Symmetric Objects (CSOs, among the youngest and most compact radio galaxies), we measured values of the total hydrogen column densities large enough to slow or prevent the radio source growth. The γ-ray window has the potential to constrain the non-thermal contribution of jets and lobes to the total high-energy emission. However, so far, young radio sources remain elusive in γ-rays, with only a handful of detections (or candidates) reported by Fermi. I present our γ-ray study of the CSO PKS 1718–649, and draw comparison with the restarted, γ-ray detected, radio galaxy 3C 84.