Abstract We present the detection of the average H2 absorption signal in the overall population of neutral gas absorption systems at z∼ 3 using composite absorption spectra built from the Sloan ...Digital Sky Survey-III damped Lyman α catalogue. We present a new technique to directly measure the H2 column density distribution function $f_{\rm H_2}(N)$ from the average H2 absorption signal. Assuming a power-law column density distribution, we obtain a slope $\beta = -1.29 \pm 0.06(\rm stat) \pm 0.10 (\rm sys)$ and an incidence rate of strong H2 absorptions with N(H2) ≳ 1018 cm−2 to be $4.0 \pm 0.5(\rm stat) \pm 1.0 (\rm sys)\, \hbox{ per cent}$ in H i absorption systems with N(H i) ≥1020 cm−2. Assuming the same inflexion point where $f_{\rm H_2}(N)$ steepens as at z = 0, we estimate that the cosmological density of H2 in the column density range $\log N(\rm H_2) ({\rm cm}^{-2})= 18{\text{--}}22$ is ${\sim } 15\hbox{ per cent}$ of the total. We find one order of magnitude higher H2 incident rate in a sub-sample of extremely strong damped Lyman α absorption systems (DLAs) $\log N(\rm{H\,\small {I}}) ({\rm cm}^{-2}) \ge 21.7$, which, together with the derived shape of $f_{\rm H_2}(N)$, suggests that the typical H i–H2 transition column density in DLAs is log N(H)(cm−2) ≳ 22.3 in agreement with theoretical expectations for the average (low) metallicity of DLAs at high-z.
Abstract We have used the Green Bank Telescope to search for the OH molecule at several locations in the Smith Cloud, one of the most prominent of the high-velocity clouds surrounding the Milky Way. ...Five positions with high H i column density were selected as targets for individual pointings, along with a square degree around a molecular cloud detected with the Planck telescope near the tip of the Smith Cloud. Gas in the Galactic disk with similar values of N HI has detectable OH emission. Although we found OH at velocities consistent with the foreground Aquila molecular cloud, nothing was found at the velocity of the Smith Cloud to an rms level of 0.7 mK ( T b ) in a 1 km s −1 channel. The three positions that give the strictest limits on OH are analyzed in detail. Their combined data imply a 5 σ limit on N (H 2 )/ N HI ≤ 0.03, scaled by a factor dependent on the OH excitation temperature and background continuum T ex /( T ex − T bg ). There is no evidence for far-infrared emission from dust within the Smith Cloud. These results are consistent with expectations for a low-metallicity diffuse cloud exposed to the radiation field of the Galactic halo, rather than a product of a galactic fountain.
ABSTRACT
We present a study of C i/H2 relative abundance in the diffuse cold neutral medium (CNM). Using the chemical and thermal balance model, we calculate the dependence of C i/H2 on the main ...parameters of the medium: hydrogen number density, metallicity, strength of the UV field, and cosmic ray ionization rate (CRIR). We show that the observed relative C i and H2 column densities in damped Lyman alpha systems (DLAs) at high redshifts can be reproduced within our model assuming the typically expected conditions in the diffuse CNM. Using additional observed information on metallicity, H i column density, and excitation of C i fine-structure levels, as well as temperature, we estimated for a wide range metallicities in the CNM at high redshifts that CRIRs are in the range from ∼10−16 to a $\rm few \times 10^{-15}\, \rm s^{-1}$, hydrogen number densities are in the range ∼10−103 cm−3, and the UV field is in the range from 10−2 to a $\rm few \times 10^2$ of the Mathis field. We argue that because the observed quantities used in this work are quite homogeneous and much less affected by radiative transfer effects (in comparison with, for example, the dissociation of HD and UV pumping of H2 rotational levels), our estimates are quite robust against the assumption of the exact geometrical model of the cloud and local sources of the UV field.
ABSTRACT
We present a systematic study of deuterated molecular hydrogen (HD) at high redshift, detected in absorption in the spectra of quasars. We present four new identifications of HD lines ...associated with known $\rm H_2$-bearing Damped Lyman-α systems. In addition, we measure upper limits on the $\rm HD$ column density in 12 recently identified $\rm H_2$-bearing DLAs. We find that the new $\rm HD$ detections have similar $N({\rm HD})/N(\rm H_2)$ ratios as previously found, further strengthening a marked difference with measurements through the Galaxy. This is likely due to differences in physical conditions and metallicity between the local and the high-redshift interstellar media. Using the measured N(HD)/N(H2) ratios together with priors on the UV flux (χ) and number densities (n), obtained from analysis of $\rm H_2$ and associated C i lines, we are able to constrain the cosmic ray ionization rate (CRIR, ζ) for the new $\rm HD$ detections and for eight known HD-bearing systems where priors on n and χ are available. We find significant dispersion in ζ, from a few × 10−18 s−1 to a few × 10−15 s−1. We also find that ζ strongly correlates with χ – showing almost quadratic dependence, slightly correlates with Z, and does not correlate with n, which probably reflects a physical connection between cosmic rays and star-forming regions.
ABSTRACT
HD molecule is one of the most abundant molecule in the Universe and due to its sensibility to the conditions in the medium, it can be used to constrain physical parameters in the medium ...where HD resides. Lately, we have shown that HD abundance can be enhanced in the low-metallicity medium. Large and Small Magellanic Clouds give us an opportunity to study low-metallicity galaxies in detail towards different sightlines due to their proximity to our Galaxy. We revisited Far Ultraviolet Spectroscopic Explorer (FUSE) space telescope archival spectra towards bright stars in Magellanic Clouds to search for HD molecules, associated with the medium of these galaxies. We reanalysed H2 absorption lines and constrained HD column density at the positions of H2 components. We detected HD towards 24 sightlines (including 19 new detections). We try to measure cosmic ray ionization rate for several systems using measured N(HD)/N(H2), and in most cases get loose constraints due to insufficient quality of the FUSE spectra.
ABSTRACT
We present the detection of excited fine-structure energy levels of singly ionized silicon and neutral carbon associated with the proximate damped Lyman-α system at zabs = 2.811 towards Q ...0528−250. This absorber has an apparent relative velocity that is inconsistent with the Hubble flow indicating motion along the line-of-sight towards the quasar, i.e. zabs > zem. We measure the metallicity of the system to be Zn/H = −0.68 ± 0.02. Using the relative populations of the fine-structure levels of Si ii and C i, as well as the populations of H2 rotational levels, we constrain the physical conditions of the gas. We derive hydrogen number densities of $n_{\rm H}=190^{+70}_{-50}$ cm−3 and $260^{+30}_{-20}$ cm−3 in two velocity components where both C i and H2 are detected. Taking into account the kinetic temperature in each component, ∼150 K, we infer high values of thermal pressure in the cold neutral medium probed by the observations. The strengths of the UV field in Draine’s unit are $I_{\rm UV} = 10^{+5}_{-3}$ and $14^{+3}_{-3}$ in each of these two components, respectively. Such enhanced UV fluxes and thermal pressure compared to intervening DLAs are likely due to the proximity of the quasar. The typical size of the absorber is ∼104 au. Assuming the UV flux is dominated by the quasar, we constrain the distance between the quasar and the absorber to be ∼150−200 kpc. This favours a scenario where the absorption occurs in a companion galaxy located in the group where the quasar-host galaxy resides. This is in line with studies in emission that revealed the presence of several galaxies around the quasar.
We present the detection and detailed analysis of a diffuse molecular cloud at zabs = 2.4636 towards the quasar SDSS J 1513+0352 (zem ≃ 2.68) observed with the X-shooter spectrograph at the Very ...Large Telescope. We measured very high column densities of atomic and molecular hydrogen with log N(H I, H2) ≃ 21.8, 21.3. This is the highest H2 column density ever measured in an intervening damped Lyman-α system but we did not detect CO, implying log N(CO)/N(H2) < −7.8, which could be due to a low metallicity of the cloud. From the metal absorption lines, we derived the metallicity to be Z ≃ 0.15 Z⊙ and determined the amount of dust by measuring the induced extinction of the background quasar light, AV ≃ 0.4. We simultaneously detected Lyman-α emission at the same redshift with a centroid located at a most probable impact parameter of only ρ ≃ 1.4 kpc. We argue that the line of sight is therefore likely passing through the interstellar medium (ISM), as opposed to the circumgalactic medium (CGM), of a galaxy. The relation between the surface density of gas and that of star formation seems to follow the global empirical relation derived in the nearby Universe although our constraints on the star formation rate (SFR) and the galaxy extent remain too loose to be conclusive. We study the transition from atomic to molecular hydrogen using a theoretical description based on the microphysics of molecular hydrogen. We use the derived chemical properties of the cloud and physical conditions (Tk ≃ 90 K and n ≃ 250 cm−3) derived through the excitation of H2 rotational levels and neutral carbon fine structure transitions to constrain the fundamental parameters that govern this transition. By comparing the theoretical and observed H I column densities, we are able to bring an independent constraint on the incident ultra-violet (UV) flux, which we find to be in agreement with that estimated from the observed SFR.
ABSTRACT
Hydroxyl ($\rm OH$) is known to form efficiently in cold gas (T ∼ 100 K) along with the molecule $\rm H_2$ and can be used as an efficient tracer of the diffuse molecular gas in the ...interstellar medium (ISM). Using a simple formalism describing the $\rm H\, I/H_2$ transition and a reduced network of major chemical reactions, we present a semi-analytical prescription to estimate the abundances of O-bearing molecules in the diffuse ISM. We show that predictions based on our prescription are in good agreement with the estimates obtained using the meudon pdr code which utilizes the full reaction network. We investigate the dependence of the relative abundances of $\rm OH/H\, I$ and $\rm OH/H_2$ on the variations of physical conditions i.e. the metallicity, number density (n), cosmic ray ionization rate (ζ), and strength of UV field (χ) in the medium. We find that the $\rm OH/H\, I$ abundances observed in the Galactic ISM can be reproduced by models with n ∼ 50 cm−3, χ ∼ 1 (Mathis field), and ζ ∼ 3 × 10−17 s−1, with a variation of about 1 dex allowed around these values. Using the constrained $\rm H_2$ column density distribution function at z ∼ 3, we estimate the $\rm OH$ column density distribution function and discuss future prospects with the upcoming large radio absorption line surveys.
ABSTRACT
We present a comprehensive study of the excitation of C i fine-structure levels along 57 sight lines in the Large and Small Magellanic Clouds (LMC and SMC). The sightlines were selected by ...the detection of H2 in FUSE spectra. Using archival HST/COS and HST/STIS spectra, we detected absorption of C i fine-structure levels and measured their populations for 29 and 28 sightlines in the LMC and SMC, respectively. The C i column density ranges from 1013 to $10^{14}\, {\rm cm}^{-2}$ for the LMC and 1013 to $10^{15.4}\, {\rm cm}^{-2}$ for the SMC. We found excitation of C i fine-structure levels in the LMC and SMC to be 2–3 times higher than typical values in local diffuse interstellar medium (ISM). Comparing excitation of both C i fine-structure levels and H2 rotational levels with a grid of PDR Meudon models, we find that neutral cold gas in the LMC and SMC is illuminated by a stronger UV field than in local ISM ($\chi =5^{+7}_{-3}$ units of Mathis field for the LMC and $2^{+4}_{-1}$ for the SMC) and has on average higher thermal pressure (log p/k = 4.2 ± 0.4 and 4.3 ± 0.5, respectively). Magellanic Clouds sight lines likely probe regions near star-formation sites, which also affects the thermal state and C i/H2 relative abundances. At the same time, obtained enhanced UV field is consistent with some measurements at high redshifts. Together with low metallicities, this make Magellanic Clouds an interesting test case to study the central parts of high redshift galaxies.
We present a new method to infer parameters of the temperature-density relation in the intergalactic medium in the post-reionization epoch at z ∼ 2-4. This method is based on the analysis of the ...distribution of Ly absorbers over column densities and Doppler parameters by the model joint probability density function. This approach allows us to measure the power-law index γ of the temperature-density relation and a certain combination of the temperature at the mean density T0 and hydrogen photoionization rate Γ. To estimate T0 and Γ separately, we employ measurements of the Ly forest effective opacity and the model gas probability density function. We show that γ tends to be lower than 1.6 and reaches 1.3 at redshift ∼3. The inferred temperatures at the mean density are ∼(2 0.5) × 104 K in the studied redshift range. Both these estimates favor He II reionization at z 3. We find that the hydrogen photoionization rate is ∼0.6 × 10−12 s−1, which is consistent with previous measurements.
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