Neutral Hydrogen Column Density Research Articles

Reconciling X-Ray and λ21 cm H I Absorption Gas Column Densities toward Obscured AGN

Abstract Hydrogen column densities inferred from X-ray absorption are typically 5–30 times larger than the neutral atomic hydrogen column densities derived from λ21 cm H I absorption toward radio-loud active galactic nuclei (AGN). Some part of the difference is ascribed to uncertainty in the spin temperature T sp = 100 K that is often used to convert λ21 cm H I absorption to N(H I). Here we propose another way to infer the gas column from H I absorption. In our Galaxy there is a nearly linear correlation between the inteferometrically measured integrated λ21 cm absorption ϒH I and reddening, ϒH I ∝ E(B–V)1.10 for ϒH I ≳ 0.7 km s−1 or E(B–V) ≳ 0.04 mag. Scaling E(B–V) then provides the total gas column density N(H) from the same dust column that is responsible for optical obscuration and X-ray absorption, without calculating N(H I). Values of N(H) so derived typically exceed N(H I) by a factor 4 because the ubiquitous Galactic λ21 cm H I absorption samples only a portion of the interstellar gas. If the well-studied case of Hydra-A is a guide, even very large disparities in X-ray and λ21 cm gas column densities can be explained by resolving the core radio continuum and inferring N(H) from λ21 cm absorption. Milky Way conditions are often invoked in discussion of obscured AGN, so the empirical relationship seen in the Milky Way should be a relevant benchmark.

Significant H i and Metal Differences around the z = 0.83 Lens Galaxy toward the Doubly Lensed Quasar SBS 0909+532

Abstract We report a large difference in neutral hydrogen (H i) and metal column densities between the two sight lines probing opposite sides of the lensing galaxy at z lens = 0.83 toward the doubly lensed quasar SBS 0909+532. Using archival HST-STIS and Keck HIRES spectra of the lensed quasar images, we measure log N H I = 18.77 ± 0.12 cm−2 toward the brighter image (A) at an impact parameter of r A = 3.15 kpc and log N H I = 20.38 ± 0.20 cm−2 toward the fainter image (B) at an impact parameter of r B = 5.74 kpc. This difference by a factor of ∼41 is the highest difference between sight lines for a lens galaxy in which H i has been measured, suggesting patchiness and/or anisotropy on these scales. We estimate an average Fe abundance gradient between the sight lines to be ≥+0.35 dex kpc−1. The N Fe II/N Mg II ratios for the individual components detected in the Keck HIRES spectra have supersolar values for all components in sight line A and for 11 out of 18 components in sight line B, suggesting that Type Ia supernovae may have contributed to the chemical enrichment of the galaxy’s environment. Additionally, these observations provide complementary information to detections of cold gas in early-type galaxies and the tension between these and some models of cloud survival.

Magnetic fields in the Milky Way from pulsar observations: effect of the correlation between thermal electrons and magnetic fields

ABSTRACTPulsars can act as an excellent probe of the Milky Way magnetic field. The average strength of the Galactic magnetic field component parallel to the line of sight can be estimated as $\langle B_\parallel \rangle = 1.232 \, \text{RM}/\text{DM}$, where RM and DM are the rotation and dispersion measure of the pulsar. However, this assumes that the thermal electron density and magnetic field of the interstellar medium are uncorrelated. Using numerical simulations and observations, we test the validity of this assumption. Based on magnetohydrodynamical simulations of driven turbulence, we show that the correlation between the thermal electron density and the small-scale magnetic field increases with increasing Mach number of the turbulence. We find that the assumption of uncorrelated thermal electron density and magnetic fields is valid only for subsonic and trans-sonic flows, but for supersonic turbulence, the field strength can be severely overestimated by using $1.232 \, \text{RM}/\text{DM}$. We then correlate existing pulsar observations from the Australia Telescope National Facility with regions of enhanced thermal electron density and magnetic fields probed by 12CO data of molecular clouds, magnetic fields from the Zeeman splitting of the 21 cm line, neutral hydrogen column density, and H α observations. Using these observational data, we show that the thermal electron density and magnetic fields are largely uncorrelated over kpc scales. Thus, we conclude that the relation $\langle B_\parallel \rangle = 1.232 \, \text{RM}/\text{DM}$ provides a good estimate of the magnetic field on Galactic scales, but might break down on sub-kpc scales.

Physical conditions and redshift evolution of optically thin C iii absorbers: low-z sample

ABSTRACT We present a detailed analysis of 99 optically thin C iii absorption systems at redshift 0.2 ≤ z ≤ 0.9 associated with neutral hydrogen column densities in the range 15 ≤ log $N({\rm{H\,{\small I}}})$ (cm−2) ≤ 16.2. Using photoionization models, we infer the number density (nH), C-abundance ([C/H]), and line-of-sight thickness (L) of these systems in the ranges −3.4 ≤ log nH (cm−3) ≤ −1.6, −1.6 ≤ [C/H] ≤ 0.4, and 1.3 pc ≤ L ≤ 10 kpc, respectively, with most of the systems having sub-kpc scale thickness. We combine the low-z and previously reported high-z (2.1 ≤ z ≤ 3.3) optically thin C iii systems to study the redshift evolution and various correlations between the derived physical parameters. We see a significant redshift evolution in nH, [C/H], and L. We compare the redshift evolution of metallicity in C iii systems with those of various types of absorption systems. We find that the slope of [C/H] versus z for C iii absorbers is steeper compared to the redshift evolution of cosmic metallicity of the damped Ly α sample (DLA) but consistent with that of sub-DLAs. We find the existence of strong anticorrelation between L and [C/H] for the combined sample with a significance level of 8.39σ. We see evidence of two distinct [C/H] branch C iii populations (low-[C/H] branch, $[\mathrm{ C/H}]\, \le -1.2$, and high-[C/H] branch, $[\mathrm{ C/H}]\, \gt -1.2$) in the combined C iii sample when divided appropriately in the L versus $N({\rm{C\,{\small III}}})$ plane. Further studies of C iii absorbers in the redshift range 1.0 ≤ z ≤ 2.0 are important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium.

The ALPINE-ALMA [CII] survey

Context. The Lyman-α line in the ultraviolet (UV) and the [CII] line in the far-infrared (FIR) are widely used tools to identify galaxies in the early Universe and to obtain insights into interstellar medium (ISM) properties in high-redshift galaxies. By combining data obtained with ALMA in band 7 at ∼320 GHz as part of the ALMA Large Program to INvestigate [CII] at Early Times (ALPINE) with spectroscopic data from DEIMOS at the Keck Observatory, VIMOS and FORS2 at the Very Large Telescope, we assembled a unique sample of 53 main-sequence star-forming galaxies at 4.4 < z < 6 in which we detect both the Lyman-α line in the UV and the [CII] line in the FIR. Aims. The goal of this paper is to constrain the properties of the Lyα emission in these galaxies in relation to other properties of the ISM. Methods. We used [CII], observed with ALMA, as a tracer of the systemic velocity of the galaxies, and we exploited the available optical spectroscopy to obtain the Lyα-[CII] and ISM-[CII] velocity offsets. Results. We find that 90% of the selected objects have Lyα-[CII] velocity offsets in the range 0 < ΔvLyα − [CII] < 400 km s−1, in line with the few measurements available so far in the early Universe, and significantly smaller than those observed at lower redshifts. At the same time, we observe ISM-[CII] offsets in the range −500 < ΔvISM−[CII] < 0 km s−1, in line with values at all redshifts, which we interpret as evidence for outflows in these galaxies. We find significant anticorrelations between ΔvLyα−[CII] and the Lyα rest-frame equivalent width EW0(Lyα) (or equivalently, the Lyα escape fraction fesc(Lyα)): galaxies that show smaller ΔvLyα−[CII] have larger EW0(Lyα) and fesc(Lyα). Conclusions. We interpret these results in the framework of available models for the radiative transfer of Lyα photons. According to the models, the escape of Lyα photons would be favored in galaxies with high outflow velocities, producing large EW0(Lyα) and small ΔvLyα-[CII], in agreement with our observations. The uniform shell model would also predict that the Lyα escape in galaxies with slow outflows (0 < vout < 300 km s−1) is mainly determined by the neutral hydrogen column density (NHI) along the line of sight, while the alternative model by Steidel et al. (2010, ApJ, 717, 289) would more highly favor a combination of NHI at the systemic velocity and covering fraction as driver of the Lyα escape. We suggest that the increase in Lyα escape that is observed in the literature between z ∼ 2 and z ∼ 6 is not due to a higher incidence of fast outflows at high redshift, but rather to a decrease in average NHI along the line of sight, or alternatively, a decrease in HI covering fraction.

A Carbon-enhanced Lyman Limit System: Signature of the First Generation of Stars?

Abstract We present the study of a Lyman limit system (LLS) at z abs = 1.5441 toward quasar J134122.50+185213.9 observed with VLT X-shooter. This is a very peculiar system with strong C i absorption seen associated with a neutral hydrogen column density of log N(H i) (cm−2) = 18.10, too small to shield the gas from any external UV flux. The low-ionization absorption lines exhibit a simple kinematic structure consistent with a single component. Using CLOUDY models to correct for ionization, we find that the ionization parameter of the gas is in the range −4.5 < log U < –4.2 and the gas density –1.5 < log n(H) (cm−3) < –1.2. The models suggest that carbon is overabundant relative to iron, [C/Fe] > +2.2 at [Fe/H] ∼–1.6. Such a metal abundance pattern is reminiscent of carbon-enhanced metal-poor stars detected in the Galaxy halo. Metal enrichment by the first generation of supernovae provide a plausible explanation for the inferred abundance pattern in this system.

New results concerning the environment of the heliosphere, nearby interstellar clouds, and physical processes in the inter–cloud medium

We present our new results concerning the interface between the outer heliosphere and the local interstellar medium (LISM). The three dimensional shape of the Local Interstellar Cloud (LIC) based on 62 sightlines to nearby stars shows a region of very low neutral hydrogen density in the direction of the star ε CMa, the brightest source of extreme-UV (EUV) radiation. This “hydrogen hole” with very weak neutral hydrogen absorption by the LIC and Blue clouds results from photoionization by the EUV radiation from ε CMa. The LIC likely surrounds the heliosphere, but in the direction of the hydrogen hole its neutral hydrogen column density is too low to be measured. Upper limits to this column density and the direction of the Sun’s motion through space indicate that the Sun will leave the outer edge of the LIC in less than 1, 900 years. The measured difference between the speed and direction of incoming neutral hydrogen atoms (measured by IBEX and Ulysses) and the flow vector of the LIC indicate that the plasma at the edge of the LIC has a different flow vector than the LIC core. The inter-cloud plasma and much of the Local Cavity are inside the Str¨omgren sphere (also called an H II region) surrounding ε CMa. The outer edges of the LIC and other clouds are Str¨omgren shells that are partially ionized by the EUV radiation from ε CMa and white dwarfs. The Local Cavity could be a Str¨omgren sphere plasma photoionized by ε CMa and hot white dwarfs that contains low density ionized gas that is not hot. An interstellar probe should measure magnetic field and plasma properties in the VLISM between the heliopause and 600–700 au from the Sun and then enter what is likely the Str¨omgren shell outer edge of the LIC. When the Sun leaves the LIC, it will either enter the G cloud, a transition region between the LIC and the G cloud, or ionized Str¨omgren sphere plasma.

Lyman continuum leakage in faint star-forming galaxies at redshiftz= 3−3.5 probed by gamma-ray bursts

Context.The identification of the sources that reionized the Universe and their specific contribution to this process are key missing pieces of our knowledge of the early Universe. Faint star-forming galaxies may be the main contributors to the ionizing photon budget during the epoch of reionization, but their escaping photons cannot be detected directly due to inter-galactic medium opacity. Hence, it is essential to characterize the properties of faint galaxies with significant Lyman continuum (LyC) photon leakage up toz∼ 4 to define indirect indicators allowing analogs to be found at the highest redshift.Aims.Long gamma-ray bursts (LGRBs) typically explode in star-forming regions of faint, star-forming galaxies. Through LGRB afterglow spectroscopy it is possible to detect directly LyC photons. Our aim is to use LGRBs as tools to study LyC leakage from faint, star-forming galaxies at high redshift.Methods.Here we present the observations of LyC emission in the afterglow spectra ofGRB 191004Batz = 3.5055, together with those of the other two previously known LyC-leaking LGRB host galaxies (GRB 050908atz = 3.3467, andGRB 060607Aatz = 3.0749), to determine their LyC escape fraction and compare their properties.Results.From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of log(NH I/cm−2) = 17.2 ± 0.15, and negligible extinction (AV = 0.03 ± 0.02 mag). The only metal absorption lines detected are C IVand Si IV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Lyman-alpha (Lyα) emission. From its Lyαemission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to 1 ≤ SFR ≤ 4.7 M⊙yr−1. We fit the Lyαemission with a shell model and find parameters values consistent with the observed ones. The absolute (relative) LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of 0.35−0.11+0.10(0.43−0.13+0.12), 0.08−0.04+0.05(0.08−0.04+0.05) and 0.20−0.05+0.05(0.45−0.15+0.15), respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRBs presented here have allM1600 > −19.5 mag, with the GRB 060607A host atM1600 > −16 mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore, the time involved is minimal compared to galaxy studies.

Herschel 158 μm [C ii] Observations of “CO-dark” Gas in the Perseus Giant Molecular Cloud

Abstract We present observations of velocity-resolved [C ii] 158 μm emission from both a dense and a more diffuse photodissociation region (PDR) in the Perseus giant molecular cloud using the Heterodyne Instrument for the Far-Infrared on board the Herschel Space Telescope. We detect [C ii] emission from 80% of the total positions, with a 95% detection rate from the dense boundary region. The integrated intensity of the [C ii] emission remains relatively constant across each boundary, despite the observed range in optical extinction between 1 and 10 mag. This flat profile indicates a constant heating and cooling rate within both regions observed. The integrated intensity of [C ii] emission is reasonably well correlated with the neutral hydrogen (H i) column density, as well as the total gas column density. This, in addition to the 80′ (7 pc) extent of the [C ii] emission from the cloud center, suggests that the H i envelope plays a dominant role in explaining the [C ii] emission emanating from Perseus. We compare the [C ii] and 12CO integrated intensities with predictions from a 1D, two-sided slab PDR model and show that a simple core + envelope, equilibrium model without an additional “CO-dark” H2 component can reproduce observations well. Additional observations are needed to disentangle how much of the [C ii] emission is associated with the “CO-dark” H2 gas, as well as constrain spatial variations of the dust-to-gas ratio across Perseus.

Properties of five z ∼ 0.3–0.4 confirmed LyC leakers: VLT/XShooter observations

ABSTRACT Using new Very Large Telescope (VLT)/XShooter spectral observations we analyse the physical properties of five z ∼ 0.3–0.4 confirmed Lyman continuum (LyC) leakers. Strong resonant Mg ii λλ2796, 2803 Å emission lines (I(λλ2796, 2803)/I(Hβ) ≃ 10–38 per cent) and non-resonant Fe ii* λλ2612, 2626 Å emission lines are observed in spectra of five and three galaxies, respectively. We find high electron densities Ne ∼ 400 cm−3, significantly higher than in typical low-z, but comparable to those measured in z ∼ 2–3 star-forming galaxies (SFGs). The galaxies have a mean value of log N/O = –1.16, close to the maximum values found for SFGs in the metallicity range of 12 + log O/H ≃ 7.7–8.1. All 11 low-z LyC emitting galaxies found by Izotov et al., including the ones considered in this study, are characterized by high equivalent width (EW) (Hβ) ∼ 200–400 Å, high ionization parameter (log(U) = –2.5 to –1.7), high average ionizing photon production efficiency ξ = 1025.54 Hz erg−1, and hard ionizing radiation. On the Baldwin–Phillips–Terlevich (BPT) diagram we find the same offset of our leakers from low-z main-sequence SFGs as that for local analogues of Lyman-break galaxies (LBGs) and extreme SFGs at z ∼ 2–3. We confirm the effectiveness of the He i emission lines diagnostics proposed by Izotov et al. in searching for LyC leaker candidates and find that their intensity ratios correspond to those in a median with low neutral hydrogen column density N(H i) = 1017–5 × 1017 cm−2 that permit leakage of LyC radiation, likely due to their density-bounded H ii regions.

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.

Physical conditions in two high-redshift H2-bearing GRB-DLAs, 120815A and 121024A

ABSTRACT The gamma-ray burst (GRB) afterglows provide a unique opportunity to study the interstellar medium (ISM) of star-forming galaxies at high-redshift. The GRB-DLAs (damped Lyman-α absorbers) contain a large neutral hydrogen column density, N(H i), and are observed against the GRB afterglow. A large fraction of GRB-DLAs show presence of molecular hydrogen (H2) which is an indicator of star-formation. Hence it is important to study those GRB-DLAs which have H2 lines to decipher and understand their physical conditions. The GRB-DLAs 121024A and 120815A, situated at redshift 2.30 and 2.36, respectively, are two such important H2-bearing GRB-DLAs. Besides H2, these two GRB-DLAs also show many metal lines. In this work we have carried out a detail numerical study on the H2 lines, as well as on those metal lines, in GRB-DLAs 121024A and 120815A self-consistently. We use the spectral synthesis code cloudy for this study. This modelling helps us to determine the underlying physical conditions which give rise to such lines and hence to understand these two GRB-DLAs in much more detail than any other previous investigation. We find that the hydrogen densities for these two H2-bearing DLAs are ≥60 cm−3. Moreover our study infers that the linear sizes are ≤17.7 pc for these two GRB-DLAs, and the mean gas temperatures averaged over the cloud thickness, are ≤140 K. Overall, we find that these two H2-bearing GRB-DLAs are denser, cooler, and smaller compared to those without H2.

High spin temperatures at large impact parameters: Ionisation in the outskirts of galaxies

By including the most recent observations of H I 21-cm absorption through nearby galactic discs, we confirm our previous assertion that there is an anti-correlation between the abundance of cool neutral atomic gas and impact parameter. In comparing the measured neutral hydrogen column densities of the sample with the absorption strength, we find a peak in the mean spin temperature of ⟨Tspin/f ⟩ ≈ 2310 K at an impact parameter of ρ ≈ 14 kpc, with ⟨Tspin/f ⟩≳1000 K in the remainder of the disc. This is significantly different to the spin temperature distribution in the Milky Way, which exhibits a constant ≈250 − 400 K over ρ = 8 − 25 kpc. The measured column densities may, however, suffer from beam dilution, which we show appears to be the case for the observations of H I 21-cm emission in which the beam subtends radii of ≳10 kpc. We therefore applied the column density profile of the Milky Way, in addition to the mean of the sample, observed at sufficiently high resolution, and the mean profile for the nearby ∼1012 M⊙ galaxies in the IllustrisTNG simulations. All of the models yield a peak in the mean spin temperature at similar impact parameters (r ≈ 10 − 15 kpc) as the measured column densities. These radii are similar to those of the spiral arms where H II regions are often concentrated. We therefore suggest that the elevated spin temperatures trace the H II regions observed in the outer disc of many spiral galaxies.

The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60Fe Accretion*

Abstract We describe the interface between the outer heliosphere and the local interstellar medium (LISM) surrounding the Sun. The components of the inner LISM are the four partially ionized clouds (the Local Interstellar Cloud (LIC), G cloud, Blue cloud, and Aql cloud) that are in contact with the outer heliosphere, and ionized gas produced by EUV radiation primarily from ϵ CMa. We construct the three-dimensional shape of the LIC based on interstellar line absorption along 62 sightlines and show that in the directions of ϵ CMa, β CMa, and Sirius B the neutral hydrogen column density from the center of the LIC is a minimum. We call this region the “hydrogen hole.” In this direction, the presence of Blue cloud absorption and the absence of LIC absorption can be simply explained by the Blue cloud lying just outside the heliosphere. We propose that the outer edge of the Blue cloud is a Strömgren shell driven toward the heliosphere by high pressures in the H ii region. We find that the vectors of neutral and ionized helium flowing through the heliosphere are inconsistent with the LIC flow vector, and that the nearby intercloud gas is consistent with ionization by ϵ CMa and other stellar sources without requiring additional sources of ionization or million-degree plasma. In the upwind direction, the heliosphere is passing through an environment of several LISM clouds, which may explain the recent influx of interstellar grains containing 60Fe from supernova ejecta measured in Antarctic snow.

Diverse properties of Ly α emission in low-redshift compact star-forming galaxies with extremely high [O iii]/[O ii] ratios

ABSTRACT We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of eight compact star-forming galaxies at redshifts z = 0.02811–0.06540, with low oxygen abundances 12 + log(O/H) = 7.43–7.82 and extremely high emission-line flux ratios O32 = [O iii] λ5007/[O ii] λ3727 ∼ 22–39, aiming to study the properties of Ly α emission in such conditions. We find a diversity in Ly α properties. In five galaxies Ly α emission line is strong, with equivalent width (EW) in the range 45–190 Å. In the remaining galaxies, weak Ly α emission with EW(Ly α) ∼ 2–7 Å is superposed on a broad Ly α absorption line, indicating a high neutral hydrogen column density N(H i) ∼ (1 – 3)× 1021 cm−2. We examine the relation between the Ly α escape fraction fesc(Ly α) and the Lyman continuum escape fraction fesc(LyC), using direct measures of the latter in eleven low-redshift LyC leakers, to verify whether fesc(Ly α) can be an indirect measure of escaping LyC radiation. The usefulness of O32, of the Ly α equivalent width EW(Ly α), and of the Ly α peak separation Vsep as indirect indicators of Ly α leakage is also discussed. It is shown that there is no correlation between O32 and fesc(Ly α). We find an increase of fesc(Ly α) with increasing EW(Ly α) for EW(Ly α) <100 Å, but for higher EW(Ly$\alpha)\gtrsim$150 Å the fesc(Ly α) is nearly constant attaining the value of ∼0.25. We find an anticorrelation between fesc(Ly α) and Vsep, though not as tight as the one found earlier between fesc(LyC) and Vsep. This finding makes Vsep a promising indirect indicator of both the Ly α and ionizing radiation leakage.

Cosmological Evolution of the Absorption of γ-Ray Burst X-Ray Afterglows

Abstract X-ray absorption of γ-ray burst (GRB) afterglows is prevalent yet poorly understood. X-ray-derived neutral hydrogen column densities (N H) of GRB X-ray afterglows show an increase with redshift, which might give a clue for the origin of this absorption. We use more than 350 X-ray afterglows with spectroscopic redshift (z) from the Swift X-ray Telescope repository as well as over 100 Lyα absorption measurements in z > 1.6 sources. The observed trend of the average optical depth τ at 0.5 keV is consistent with both a sharp increase of host N H(z), and an absorbing diffuse intergalactic medium (IGM), along with decreasing host contribution to τ. We analyze a subsample of high-z GRBs with N H derived both from the X-ray afterglow and the Lyα line. The increase of X-ray-derived N H(z) is contrasted by no such increase in the Lyα-derived column density. We argue that this discrepancy implies a lack of association between the X-ray and Lyα absorbers at high z. This points toward the X-ray absorption at high z being dominated by an intervening absorber, which lends credibility to an absorbing IGM contribution.

The VANDELS survey: the role of ISM and galaxy physical properties in the escape of Lyα emission in z ∼ 3.5 star-forming galaxies

Aims. We wish to investigate the physical properties of a sample of Lyα emitting galaxies in the VANDELS survey, with particular focus on the role of kinematics and neutral hydrogen column density in the escape and spatial distribution of Lyα photons. Methods. From all the Lyα emitting galaxies in the VANDELS Data Release 2 at 3.5 ≲ z ≲ 4.5, we selected a sample of 52 galaxies that also have a precise systemic redshift determination from at least one nebular emission line (HeII or CIII]). For these galaxies, we derived different physical properties (stellar mass, age, dust extinction, and star formation rate) from spectral energy distribution (SED) fitting of the exquisite multiwavelength photometry available in the VANDELS fields, using the dedicated spectral modeling tool BEAGLE and the UV β slope from the observed photometry. We characterized the Lyα emission in terms of kinematics, equivalent width (EW), full width at half-maximum, and spatial extension and then estimated the velocity of the neutral outflowing gas. The ultra-deep VANDELS spectra (up to 80 h on-source integration) enable this for individual galaxies without the need to rely on stacks. We then investigated the correlations between the Lyα properties and the other measured properties to study how they affect the shape and intensity of Lyα emission. Results. We reproduce some of the well-known correlations between Lyα EW and stellar mass, dust extinction, and UV β slope, in the sense that the emission line appears brighter in galaxies with lower mass that are less dusty and bluer. We do not find any correlation with the SED-derived star formation rate, while we find that galaxies with brighter Lyα tend to be more compact in both UV and in Lyα. Our data reveal an interesting correlation between the Lyα velocity offset and the shift of the interstellar absorption lines with respect to the systemic redshift, observed for the first time at high redshifts: galaxies with higher interstellar medium (ISM) outflow velocities show smaller Lyα velocity shifts. We interpret this relation in the context of the shell-model scenario, where the velocity of the ISM and the HI column density contribute together in determining the Lyα kinematics. In support to our interpretation, we observe that galaxies with high HI column densities have much more extended Lyα spatial profiles; this is a sign of increased scattering. However, we do not find any evidence that the HI column density is related to any other physical properties of the galaxies, although this might be due in part to the limited range of parameters that our sample spans.

Subkiloparsec Imaging of Lyα Emission in a Low-mass, Highly Ionized, Gravitationally Lensed Galaxy at z = 1.84∗

Abstract Low-mass, low-metallicity galaxies at low to moderate (z ≲ 3) redshifts offer the best opportunity for detailed examination of the interplay between massive stars, ionizing radiation and gas in sources similar to those that likely reionized the universe. We present new narrowband Hubble Space Telescope observations of emission and the adjacent ultraviolet (UV) continuum in the low-mass (M ⋆ = 2 × 108 ), low-metallicity (Z ∼ 1/20 Z⊙), and highly ionized gravitationally lensed galaxy SL2S J02176−0513 at z = 1.844. The galaxy has strong emission with photometric equivalent width , at odds with the escape fraction of 10%. However, the spectroscopic profile suggests the presence of broad absorption underlying the emission, and the total equivalent width is consistent with the escape fraction once this underlying absorption is included. The emission is more spatially extended than the UV continuum, and the 0.″14 spatial resolution of HST coupled with the magnification of gravitational lensing enables us to examine the distribution of and the UV continuum on subkiloparsec scales. We find that the peaks of the emission and the UV continuum are offset by 650 pc, and there is no emission arising from the region with the strongest UV light. Our combined spectroscopic and imaging data imply a significant range in neutral hydrogen column density across the object. These observations offer indirect support for a model in which ionizing radiation escapes from galaxies through channels with low column density of neutral gas.

Shattering of Cosmic Sheets due to Thermal Instabilities: A Formation Channel for Metal-free Lyman Limit Systems

Abstract We present a new cosmological zoom-in simulation, where the zoom region consists of two halos with virial mass M v ∼ 5 × 1012 M ⊙ and an approximately megaparsec long cosmic filament connecting them at z ∼ 2. Using this simulation, we study the evolution of the intergalactic medium in between these two halos at unprecedented resolution. At 5 ≳ z ≳ 3, the two halos are found to lie in a large intergalactic sheet, or “pancake,” consisting of multiple coplanar dense filaments along which nearly all halos with M v > 109 M ⊙ are located. This sheet collapses at z ∼ 5 from the merger of two smaller sheets. The strong shock generated by this merger leads to thermal instabilities in the postshock region, and to a shattering of the sheet resulting in ≲ kiloparsec-scale clouds with temperatures of T ≳ 2 × 104 K and densities of n ≳ 10−3 cm−3, which are pressure confined in a hot medium with T ∼ 106 K and n ≳ 10−5 cm−3. When the sheet is viewed face-on, these cold clouds have neutral hydrogen column densities of N H i > 1017.2 cm−2, making them detectable as Lyman limit systems, though they lie well outside the virial radius of any halo and even well outside the dense filaments. Their chemical composition is pristine, having zero metallicity, similar to several recently observed systems. Since these systems form far from any galaxies, these results are robust to galaxy formation physics, resulting purely from the collapse of large-scale structure and radiative cooling, provided sufficient spatial resolution is available.

Search for H i emission from superdisk candidates associated with radio galaxies

Giant gaseous layers (termed “superdisks”) have been hypothesized in the past to account for the strip-like radio emission gap (or straight-edged central brightness depression) observed between twin radio lobes, in over a dozen relatively nearby powerful Fanaroff-Riley Class II radio galaxies. They could also provide a plausible alternative explanation for a range of observations. Although a number of explanations have been proposed for the origin of the superdisks, little is known about their material content. Some X-ray observations of superdisk candidates indicate the presence of hot gas, but a cool dusty medium also seems to be common. If they are entirely or partly composed of neutral gas, then it may be directly detectable and we report here a first attempt to detect/image any neutral hydrogen gas present in the superdisks that are inferred to be present in four nearby radio galaxies. We have not found a positive H i signal in any of the four sources, resulting in tight upper limits on the H i number density in the postulated superdisks, estimated directly from the central rms noise values of the final radio continuum subtracted image. The estimated ranges of the upper limit on neutral hydrogen number density and column density are 10−4-10−3 atoms per cm3 and 1019−1020 atoms per cm2, respectively. No positive H i signal is detected even after combining all the four available H i images (with inverse variance weighting). This clearly rules out an H i dominated superdisk as a viable model to explain these structures, however, the possibility of a superdisk being composed of warm/hot gas still remains open.