arXiv subjects

H. Liszt

Publications and source records attributed to H. Liszt.

15 recordsLinked to original sources

arXiv2019

Standing in the shadow of dark gas: ALMA observations of absorption from dark CO in the molecular DNM of Chamaeleon

% context We had detected J=1-0 HCO+ absorption in 12 directions lacking detected CO emission in the outskirts of the Chamaeleon complex and on 1 sightline with integrated CO emission 2.4 K-\kms. 8 sightlines had a much larger mean column density of dark neutral medium (DNM)-gas not represented in H I or CO emission-and were found to have much higher mean molecular column density. The 5 other sightlines had little or no DNM and were found to have much smaller but still detectable N(HCO+). % aims To determine N(CO) along previously-observed Chamaeleon sightlines and to determine why CO emission was not detected in directions where molecular gas is present. % methods We took \cotw\ J=1-0 absorption profiles on 5 sightlines having higher DNM and HCO+ column densities and 1 sightline with smaller N(DNM) and N(HCO+). We converted the integrated HCO+ optical depths to N(H2) in the weak-excitation limit using N(HCO+)/N(H2)$=3\times10^{-9}$ and converted the integrated CO optical depths \ICO\ to CO column density N(CO) $= 1.861\times 10^{15}\pcc \ICO^{1.131}$ as found along comparable lines of sight previously studied in J=1-0 and J=2-1 CO absorption&emission. %results CO absorption was detected along the 5 sightlines in the higher-DNM group, with CO column densities $4\times 10^{13} \pcc<$ N(CO) $<10^{15}\pcc$ that are generally below the detectability limit of CO emission surveys. %conclusions In the outskirts of the Chamaeleon complex, the presence of molecular DNM resulted primarily from small CO column densities at the onset of CO formation around the HI/\HH\ transition in diffuse molecular gas. CO relative abundances N(CO)/N(H2) $< 2\times 10^{-6}$ in the outskirts of Chamaeleon are comparable to those seen in UV absorption toward early-type stars, including in Chamaeleon.

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arXiv2018

Molecular ion abundances in the diffuse ISM : CF+, HCO+, HOC+, and C3H+

The transition between atomic and molecular hydrogen is associated with important changes in the structure of interstellar clouds, and marks the beginning of interstellar chemistry. Because of the relatively simple networks controlling their abundances, molecular ions are usually good probes of the underlying physical conditions including for instance the fraction of gas in molecular form or the fractional ionization. In this paper we focus on three possible probes of the molecular hydrogen column density, HCO+, HOC+, and CF+. We presented high sensitivity ALMA absorption data toward a sample of compact HII regions and bright QSOs with prominent foreground absorption, in the ground state transitions of the molecular ions HCO+, HOC+, and CF+ and the neutral species HCN and HNC, and from the excited state transitions of C3H+(4-3) and 13CS(2-1). These data are compared with Herschel absorption spectra of the ground state transition of HF and p-H2O. We show that the HCO+, HOC+, and CF+ column densities are well correlated with each other. HCO+ and HOC+ are tightly correlated with p-H2O, while they exhibit a different correlation pattern with HF depending on whether the absorbing matter is located in the Galactic disk or in the central molecular zone. We report new detections of C3H+ confirming that this ion is ubiquitous in the diffuse matter, with an abundance relative to H2 of ~7E-11. We confirm that the CF+ abundance is lower than predicted by simple chemical models and propose that the rate of the main formation reaction is lower by a factor of about 3 than usually assumed. In the absence of CH or HF data, we recommend to use the ground state transitions of HCO+, CCH, and HOC+ to trace diffuse molecular hydrogen, with mean abundances relative to H2 of 3E-9, 4E-8 and 4E-11.

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arXiv2018

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 HI and CO emission, a phenomenon known as dark neutral medium (DNM) or CO-dark gas for the molecules. %aims To see if molecular gas can be detected in Chamaeleon when gas column densities in the DNM were inferred and CO emission was not detected. % methods We took 3mm absorption profiles of HCO+ and other molecules toward quasars across Chamaeleon, 1 of which had detectable CO emission. We derived N(H2) assuming N(HC+)/N(H2) = 3x10^{-9}. %results With the possible exception of 1 weak continuum target HCO+ absorption was detected in all directions, \cch\ in 8 and HCN in 4 directions. The sightlines divide in 2 groups according to their DNM content with 1 group of 8 directions having N(DNM) \ga 2x10^{20} \pcc and another group of 5 directions having N(DNM) < .5x10^{20}\pcc. The groups have comparable in Chamaeleon 6-7 x 10^{20}\pcc and 0.33 vs 0.18 mag, 3.3 vs .14 x 10^{20}\pcc and <2N(H2)> = 5.6 vs 0.8 x 10^{20} \pcc. Gas at more positive velocities is enriched in molecules and DNM. %conclusion Overall the quantity of H2 inferred from HCO+ fully accounts for the previously-inferred DNM along the studied sightlines. 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 HI emission may occur along 3 of the 4 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.

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arXiv2017

ALMA observations of molecular absorption in four directions toward the Galactic bulge

Alma Cycle 3 observations showed strong absorption from diffuse molecular gas in the bulge at -200 \kms\ $< {\rm v} < -140$ \kms\ toward J1744-3116 (l,b)= (-2.13d,-1d) We aimed to test if bulge molecular gas could also be seen toward the three other sufficiently strong mm-wave sources seen toward the bulgeat $|b| < 3$°We took absorption profiles of \hcop (1-0) and other species in ALMA Cy 4 toward J1713-3418, J1717-3341, J1733-3722 and J1744-3116. Strong molecular absorption from disk gas at $|\rmv| \la 30$ \kms\ was detected in all directions, and absorption from the 3 kpc arm was newly detected toward J1717 and J1744. However, only the sightline toward J1744 is dominated by molecular gas overall and no other sightlines showed molecular absorption from gas deep inside the bulge. No molecular absorption was detected toward J1717 where H I emission from the bulge was previously known. As observed in \hcop, HCN, \cch\ and CS, the bulge gas toward J1744 at $v < -135$ \kms\ has chemistry and kinematics like that seen near the Sun and in the Milky Way disk generally. We measured isotopologic ratios N(\hcop)/N(H$^{13}$CO\p) $> 51~(3σ)$ for the bulge gas toward J1744 and $58\pm9$ and $64\pm4$ for the disk gas toward J1717 and J1744, respectively, all well above the value of 20-25 typical of the central molecular zone.} %conclusions heading (optional), leave it empty if necessary {The kinematics and chemistry of the bulge gas observed toward J1744 more nearly resemble that of gas in the Milky Way disk than in the central molecular zone.}

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arXiv2017

ALMA hints at the existence of an unseen reservoir of diffuse molecular gas in the Galactic bulge

Aims. We aim to understand the unexpected presence of mm-wave molecular absorption at -200 \kms $< {\rm v} < -140$ \kms\ in a direction that is well away from regions of the Galactic bulge where CO emission at such velocities is prominent. Methods. We compared 89 GHz Cycle 2 ALMA absorption spectra of \hcop, HCN, and HNC toward the extragalactic continuum source B1741-312 at l=-2.14\degr, b=-1.00\degr\ with existing CO, H I, and dust emission and absorption measurements. We placed the atomic and molecular gas in the bulge and disk using circular and non-circular galactic kinematics, deriving N(H I) from a combination of 21cm emission and absorption and we derive N(\HH) from scaling of the \hcop\ absorption. We then inverted the variation of near-IR reddening E(J-K) with distance modulus and scale E(J-K) to a total gas column density N(H) that may be compared to N(H I) and N(\HH). Results. At galactocentric radii \Rgal\ $>$ 1.5 kpc, conventional measures such as the standard CO-\HH\ conversion factor and locally observed N(\hcop)/N(\HH) ratio separately imply that H I and \HH\ contribute about equally to N(H), and the gas-derived N(H) values are in broad agreement with those derived from E(J-K). Within the Galactic bulge at \Rgal $<$ 1.5 kpc, H I contributes less than 10\% of the material inferred from E(J-K), so that the molecular absorption detected here is needed to understand the extinction.

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arXiv2014

[CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane

Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium and its 158 micron fine structure transition [CII] is the most important cooling line of the diffuse interstellar medium (ISM). We combine [CII] absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. We present high resolution [CII] spectra obtained with the Herschel/HIFI instrument towards bright dust continuum sources regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by observations of the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 microns with Herschel/PACS. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution data and that high spectral resolution spectra are needed to interpret the [CII] and [OI] emission and the [CII]/FIR ratio. This phenomenon may explain part of the [CII]/FIR deficit seen in external luminous infrared galaxies. The C+ and C excitation in the diffuse gas is consistent with a median pressure of 5900 Kcm-3 for a mean TK ~100 K. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight: the median value is 2.4 %, in good agreement with the CNM properties. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 x 10^{-26} erg/s/H. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 +/- 0.4 x 10^{-4}.

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arXiv2014

HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds

%methods {We used the EMIR receiver and FTS spectrometer at the IRAM 30m to construct absorption spectra toward bright extra-galactic background sources at 195 kHz spectral resolution ($\approx$ 0.6 \kms). We used the IRAM Plateau de Bure interferometer to synthesize absorption spectra of \hthcop\ and HCO toward the galactic HII region W49.} %results {HCO, \cc3h\ and CF\p\ were detected toward the blazars \bll\ and 3C111 having \EBV\ = 0.32 and 1.65 mag. HCO was observed in absorption from ``spiral-arm'' clouds in the galactic plane occulting W49. The complement of detectable molecular species in the 85 - 110 GHz absorption spectrum of diffuse/translucent gas is now fully determined at rms noise level $δ_τ\approx 0.002$ at \EBV\ = 0.32 mag (\AV\ = 1 mag) and $δ_τ$/\EBV\ $\approx\ 0.003$ mag$^{-1}$ overall.} %conclusions {As with OH, \hcop\ and \cch, the relative abundance of \cc3h\ varies little between diffuse and dense molecular gas, with N(\cc3h)/N({\it o-c}-\c3h2) $\approx$ 0.1. We find N(CF\p)/N(H$^{13}$CO\p) $\approx 5$, N(CF\p)/N(\cch) $\approx$ 0.005-0.01 and because N(CF\p) increases with \EBV\ and with the column densities of other molecules we infer that fluorine remains in the gas phase as HF well beyond \AV\ = 1 mag. We find N(HCO)/N(H$^{13}$CO\p) = 16 toward \bll, 3C111 and the 40 km/s spiral arm cloud toward W49, implying X(HCO) $\approx 10^{-9}$, about 10 times higher than in dark clouds. The behaviour of HCO is consistent with previous suggestions that it forms from C\p\ and \HH, even when \AV\ is well above 1 mag. The survey can be used to place useful upper limits on some species, for instance N(\hhco)/N(\HH CS) $>$ 32 toward 3C111, compared to 7 toward TMC-1, confirming the possibility of a gas phase formation route to \hhco.}

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arXiv2004

Mm-wave HCO+, HCN and CO absorption toward NGC1052

We used the Plateau de Bure Interferometer to observe $λ$3mm J=1-0 absorption lines of \hcop, HCN and CO toward the core of the nearby elliptical, megamaser-host galaxy NGC1052. The lines are relatively weak, with peak optical depths 0.03 for \hcop and HCN and 0.1 for CO. Nonetheless the inferred column density of molecular gas 2N(\HH) $\simeq 5\times10^{21}~\pcc$ is consistent with the degree of reddening inferred toward the nucleus from observations of the Balmer series of hydrogen. Mm-wave absorption line profiles are somewhat broader than those of H I and OH, perhaps because lower free-free opacity at mm-wavelengths exposes higher-velocity material nearer the nucleus. Overall, the OH/\hcop ratio in NGC1052 is as expected from the strong relationship established in local diffuse clouds but the optical depth ratio varies strongly over the line profiles. Similar variations are also seen toward Cen A, which has very different line ratios among H I, OH and \hcop for very nearly the same amount of OH absorption.

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arXiv2004

The abundance of HOC+ in diffuse clouds

We used the Plateau de Bure Interferometer to search for $λ$3mm absorption lines of HOC+ from local diffuse and translucent clouds occulting compact extragalactic mm-wave continuum sources. We detected HOC+ in three directions with column densities only 70-120 times below those of the HCO+ isomer, a factor 5-50 higher than typically found in dense dark gas but comparable to recent observations of dense photon-dominated regions. The observed amounts of HOC+, N(HOC+)/N(\HH) $ = 3-6\times 10^{-11} $, can be made in quiescent diffuse gas at thermal gas-kinetic rates if the {\HH O}/OH ratio is of order unity, in mild violation of extant observational limits. %

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arXiv2002

Gas-phase recombination, grain neutralization and cosmic-ray ionization in diffuse gas

Atomic ions are mostly neutralized by small grains (or PAH molecules) in current theories of heating and cooling in cool diffuse clouds; in the main they do not recombine with free electrons. This alters the ionization balance by depressing n(H+) and n(He+) while carbon generally remains nearly fully once-ionized: charge exchange with atomic oxygen and formation of H2 and OH also depress n(H+) in partly molecular gas. Seemingly restrictive empirical limits on the cosmic ray ionization rate of hydrogen ($ζ_H$) are relaxed and faster rates are favored in a wide range of circumstances, when grain neutralization is recognized. Maintenance of the proton density at levels needed to reproduce observations of HD requires $ζ_H$ at least 2x10^{-16} s^{-1}, but such models naturally explain the presence of both HD and H3^+ in relatively tenuous H I clouds. In dense gas, a higher ionization rate can account for high observed fractions of atomic hydrogen, and recognition of the effects of grain neutralization can resolve a major paradox in the formation of sulfur-bearing compounds.

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arXiv2002

Noise and the strong signal limit in radio astronomical measurement

The random error of radioastronomical measurements is usually computed in the weak-signal limit, which assumes that the system temperature is sensibly the same on and off source, or with and without a spectral line. This assumption is often very poor. We give examples of common situations in which it is important to distinguish the system noise in signal-bearing and signal-free regions.

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arXiv2002

Comparative Chemistry of Diffuse Clouds IV: CH

We observed the 3335 MHz (9cm) F=1-1 line of CH toward a sample of diffuse clouds occulting compact extragalactic mm-wave continuum sources, using the old NRAO 43m telescope. Because radiofrequency observations of CH really must be calibrated with reference to a known CH abundance, we begin by deriving the relationships between CH, EB-V, H2 and other hydrides found by optical spectroscopy. No simple relationship exists between N(CH) and EB-V, since N(CH) is strongly bimodal with respect to reddening for EB-V < 0.3 mag and the typical range in the N(CH)/EB-V ratio is an order of magnitude or more at any given EB-V > 0.3 mag. However, N(CH)/N(H2) = 4.3 +/- 1.9 x 10^-8 in the mean and N(CH) varies as N(H2)^(1.00+/-0.06) for 10^{19} < N(H2) < 10^{21} cm^-2. If CH is a good predictor of H2, 40-45% of the hydrogen in the local diffuse/translucent ISM is in the molecular form at the accepted mean density, higher than previous estimates found in samples of lower-than-average mean density. Optical observations of the population ratios in the upper and lower halves of the CH lambda-doublet suggest that the brightness of the 3335 MHz CH line should be double-valued at a given CH column density in diffuse gas: double-valuedness is noticeable in our data when comparing CH with CO or HCO+. The CH brightness at 3335 MHz is mildly bimodal with respect to CO emission in our diffuse cloud data but much more strongly bimodal when comparing diffuse or translucent gas and dark gas. The CH Lambda-doublet is generally inverted in diffuse gas but we did not succeed in measuring the excitation temperature except toward 3C123 where we confirm one older value Tex ~ -10 K.

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arXiv2002

Two-phase equilibrium and molecular hydrogen formation in damped Lyman-alpha systems

Molecular hydrogen is quite underabundant in damped Lyman-alpha systems at high redshift, when compared to the interstellar medium near the Sun. This has been interpreted as implying that the gas in damped Lyman-alpha systems is warm. like the nearby neutral intercloud medium, rather than cool, as in the clouds which give rise to most H I absorption in the Milky Way. Other lines of evidence suggest that the gas in damped Lyman-alpha systems -- in whole or part -- is actually cool; spectroscopy of neutral and ionized carbon, discussed here, shows that the damped Lyman-alpha systems observed at lower redshift z $<$ 2.3 are largely cool, while those seen at z $>$ 2.8 are warm (though not devoid of H2). To interpret the observations of carbon and hydrogen we constructed detailed numerical models of H2 formation under the conditions of two-phase thermal equilibrium, like those which account for conditions near the Sun, but with varying metallicity, dust-gas ratio, $etc$. We find that the low metallicity of damped Lyman-alpha systems is enough to suppress H2 formation by many orders of magnitude even in cool diffuse clouds, as long as the ambient optical/uv radiation field is not too small. For very low metallicity and under the most diffuse conditions, H2 formation will be dominated by slow gas-phase processes not involving grains, and a minimum molecular fraction in the range $10^{-8}-10^{-7}$ is expected.

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arXiv2001

Comparative Chemistry of Diffuse Clouds II: CN, HCN, HNC, CH3CN & N2H+

Using the Plateau de Bure interferometer, we observed the 3mm absorption lines of CN, HCN and HNC from some of the diffuse clouds which lie toward our well-studied sample of compact extragalactic mm-wave continuum sources. The column densities of these species all vary by a factor of about fifty and are prominent in only a limited subset of the clouds seen in the most ubiquitous species such as OH, HCO+, C2H and C3H2. The CN, HCN, and HNC column densities vary in fixed proportion to each other, with = 0.21+/-0.05 and = 6.8+/-1. We searched unsuccessfully for CH3CN and N2H+, which are underabundant compared to dark clouds, by factors of at least 10 and 100, respectively.

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arXiv2001

The Spin Temperature of Warm Interstellar H I

Collisional excitation of the 21cm HI hyperfine transition is not strong enough to thermalize it in warm neutral (``intercloud'') interstellar gas, which we show by simultaneously solving the equations of ionization and collisional equilibrium under typical conditions. Coupling of the 21cm excitation temperature and local gas motions may be established by the Ly-alpha radiation field, but only if strong Galactic Ly-alpha radiation permeates the gas in question. The Ly-alpha radiation tends to impart to the gas its own characteristic temperature, which is determined by the range of gas motions that occur on the spatial scale of the Ly-alpha scattering. In general, the calculation of H I spin temperatures is a more difficult and interesting problem than might have been expected, as is any interpretation of H I spin temperature measurements.

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