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Register a new userNature of the DLA towards Q0528-250: High pressure and strong UV field revealed by excitation of CI, H2 and SiII
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We present the detection of excited fine-structure energy levels of singly-ionized silicon and neutral carbon associated with the proximate damped Lyman-$alpha$ system at $z_{rm abs}=2.811$ towards qso. 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., $z_{rm abs}>z_{rm em}$. We measure the metallicity of the system to be ${rm [Zn/H]}=-0.68pm 0.02$. Using the relative populations of the fine-structure levels of SiII and CI, as well as the populations of H$_2$ 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 CI and H$_2$ are detected. Taking into account the kinetic temperature in each component, $sim 150$K, we infer high values of thermal pressure in the cold neutral medium probed by the observations. The strengths of the UV field in Draines 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 $sim 10^4$ a.u. Assuming the UV flux is dominated by the quasar, we constrain the distance between the quasar and the absorber to be $sim 150-200$ 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.
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UV absorption studies with FUSE have observed H2 molecular gas in translucent and diffuse clouds. Observations of the 158 micron [C II] fine structure line with Herschel also trace the same H2 molecular gas in emission. We present [C II] observations along 27 lines of sight (LOSs) towards target stars of which 25 have FUSE H2 UV absorption. We detect [C II] emission features in all but one target LOS. For three Target LOSs, which are close to the Galactic plane, we also present position-velocity maps of [C II] emission observed by HIFI in on-the-fly spectral line mapping. We use the velocity resolved [C II] spectra towards the target LOSs observed by FUSE to identify C II] velocity components associated with the H2 clouds. We analyze the observed velocity integrated [C II] spectral line intensities in terms of the densities and thermal pressures in the H2 gas using the H2 column densities and temperatures measured by the UV absorption data. We present the H2 gas densities and thermal pressures for 26 target LOSs and from the [C II] intensities derive a mean thermal pressure in the range 6100 to 7700 K cm^-3 in diffuse H2 clouds. We discuss the thermal pressures and densities towards 14 targets, comparing them to results obtained using the UV absorption data for two other tracers CI and CO.
C$^+$ is a critical constituent of many regions of the interstellar medium, as it can be a major reservoir of carbon and, under a wide range of conditions, the dominant gas coolant. Emission from its 158$mu$m fine structure line is used to trace the structure of photon dominated regions in the Milky Way and is often employed as a measure of the star formation rate in external galaxies. Under most conditions, the emission from the single [CII] line is proportional to the collisional excitation rate coefficient. We here used improved calculations of the deexcitation rate of [CII] by collisions with H$_2$ to calculate more accurate expressions for interstellar C$^+$ fine structure emission, its critical density, and its cooling rate. The collision rates in the new quantum calculation are $sim$ 25% larger than those previously available, and narrow the difference between rates for excitation by atomic and molecular hydrogen. This results in [CII] excitation being quasi-independent of the molecular fraction and thus dependent only on the total hydrogen particle density. A convenient expression for the cooling rate at temperatures between 20 K and 400 K, assuming an LTE H$_2$ ortho to para ration is $Lambda ({rm LTE~OPR}) = left(11.5 + 4.0,e^{-100,mathrm K/T^{rm kin}}right);e^{-91.25,mathrm K/T^{rm kin}},n ({rm C}^{+}),n({rm H}_2)times 10^{-24};{rm ergs}~{rm cm}^{-3}~{rm s}^{-1}$. The present work should allow more accurate and convenient analysis of the [CII] line emission and its cooling.
Collisional de-excitation rates of partially deuterated molecules are different from the fully hydrogenated species because of lowering of symmetry. We compute the collisional (de)excitation rates of ND2H by ground state para-H2, extending the previous results for He- lium. We describe the changes in the potential energy surface of NH3- H2 involved by the pres- ence of two deuterium nuclei. Cross sections are calculated within the full close-coupling ap- proach and augmented with coupled-state calculations. Collisional rate coefficients are given between 5 and 35 K, a range of temperatures which is relevant to cold interstellar conditions. We find that the collisional rates of ND2H by H2 are about one order of magnitude higher than those obtained with Helium as perturber. These results are essential to radiative transfer modelling and will allow to interpret the millimeter and submillimeter detections of ND2H with better constraints than previously.
We present first results of neutral carbon ([CI], 3P1 - 3P0 at 492 GHz) and carbon monoxide (13CO, J = 1 - 0) mapping in the Vela Molecular Ridge cloud C (VMR-C) and G333 giant molecular cloud complexes with the NANTEN2 and Mopra telescopes. For the four regions mapped in this work, we find that [CI] has very similar spectral emission profiles to 13CO, with comparable line widths. We find that [CI] has opacity of 0.1 - 1.3 across the mapped region while the [CI]/13CO peak brightness temperature ratio is between 0.2 to 0.8. The [CI] column density is an order of magnitude lower than that of 13CO. The H2 column density derived from [CI] is comparable to values obtained from 12CO. Our maps show CI is preferentially detected in gas with low temperatures (below 20 K), which possibly explains the comparable H2 column density calculated from both tracers (both CI and 12CO underestimate column density), as a significant amount of the CI in the warmer gas is likely in the higher energy state transition ([CI], 3P2 - 3P1 at 810 GHz), and thus it is likely that observations of both the above [CI] transitions are needed in order to recover the total H2 column density.
To investigate the presence of small scale structure in the spatial distribution of H2 molecules we have undertaken repeated FUSE UV observations of the runaway O9.5V star, HD34078. In this paper we present five spectra obtained between January 2000 and October 2002. These observations reveal an unexpectedly large amount of highly excited H2. Column densities for H2 levels from (v = 0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2 levels are determined. These results are interpreted in the frame of a model involving essentially two components: i) a foreground cloud (unaffected by HD34078) responsible for the H2 (J = 0, 1), CI, CH, CH+ and CO absorptions; ii) a dense layer of gas (n = 10E4 cm-3) close to the O star and strongly illuminated by its UV flux which accounts for the presence of highly excited H2. Our model successfully reproduces the H2 excitation, the CI fine-structure level populations as well as the CH, CH+ and CO column densities. We also examine the time variability of H2 absorption lines tracing each of these two components. From the stability of the J = 0, 1 and 2 damped H2 profiles we infer a 3 sigma upper limit on column density variations Delta(N(H2))/N(H2) of 5% over scales ranging from 5 to 50 AU. This result clearly rules out any pronounced ubiquitous small scale density structure of the kind apparently seen in HI. The lines from highly excited gas are also quite stable (equivalent to Delta(N)/N <= 30%) indicating i) that the ambient gas through which HD34078 is moving is relatively uniform and ii) that the gas flow along the shocked layer is not subject to marked instabilities
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