A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

By Jason Tumlinson, J. Michael Shull, B. Rachford, M. Browning, T. Snow, A. Fullerton, E. Jenkins, B. Savage, P. Crowther, H. Moos and 3 more. 2001

176 citations recorded by the source corpus.

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 Å. Our survey is sensitive to N(H2) ≥ 1014 cm-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions = 0.010 for the SMC and = 0.012 for the LMC, compared with = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lyα in pencil beam sight lines as measures of N(H I). These results imply that the diffuse H2 masses of the LMC and SMC are 8 × 106 and 2 × 106 M☉, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 × 10-18 cm3 s-1, and incident radiation fields ranging from 10-100 times the Galactic mean value. We find that these high-radiation, low formation rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and we find that the distribution of rotational temperatures is similar to Galactic gas, with ⟨T01⟩ = 82 ± 21 K for clouds with N(H2) ≥ 1016.5 cm-2. There is only a weak correlation between detected H2 and far-infrared fluxes as determined by IRAS, perhaps as a result of differences in the survey techniques. We find that the surface density of H2 probed by our pencil beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/f2c351916d0c44ae29d637bde77d6e595bed4642

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Citations in the corpus

Diffuse Atomic and Molecular Clouds (2006, background): “FUSE has allowed the detection and analysis of molecular hydrogen in a wide range of environments not accessible to Copernicus, such as high galactic latitude clouds formed in the halo of the Milky Way (Shull et al. 2000; Richter et al. 2001), the Magellanic Clouds (Tumlinson et al. 2002), and more ”
METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES (2011, background): “…sensitive to the metallicity of the gas, occurring at a very different column density in the Milky Way, at Solar metallicity, than in the Small Magellanic Cloud, at ∼ 20% Solar metallicity (Tumlinson et al. 2002; Bolatto et al. 2007; Leroy et al. 2007; Krumholz et al. 2009b; Bolatto et al. 2011).”
Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database (2008, background): “4), although SMC lines of sight probe larger gas column densities.”
A new look at the large-scale H I structure of the Large Magellanic Cloud (2002, unknown): “Both images use a square root transfer function. mass measured by Tumlinson et al. (2002).”
The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman α systems: physical conditions in the neutral gas (2005, methodology): “Under the LTE assumption, we find that H2 components in DLAs have kinetic temperatures in the range 100–200 K.”
H$_2$ formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations (2017, background): “5 × 10 −3 s −1 , ver the temperature range of 9.2 - 16 K ( Ueta et al., 2016 ).”
Probing the Interstellar Medium near Star-forming Regions with Gamma-Ray Burst Afterglow Spectroscopy: Gas, Metals, and Dust (2007, methodology): “Both samples exhibit a wide range of dust-to-gas ratios relative 14 to the SMC.”
THE STATE OF THE GAS AND THE RELATION BETWEEN GAS AND STAR FORMATION AT LOW METALLICITY: THE SMALL MAGELLANIC CLOUD (2011, background): “That is the procedure we follow, and the H2 results thus obtained are consistent with other observational constraints, such as virial masses on large scales (Leroy et al. 2009) and H2 column densities determined in absorption along diffuse linesof-sight (Tumlinson et al. 2002).”
Some empirical estimates of the H 2 formation rate in photon-dominated regions (2003, background): “Tumlinson et al. (2002) find that to reproduce the reduced molecular fraction and enhanced rotational excitation in the SMC and LMC, a low H2 formation rate (Rf ∼ 3 × 10−18 cm3 s−1) and a high UV field relative to diffuse Galactic medium are required.”
A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds (2002, methodology): “General Criteria for Selecting Target Stars Unlike the case of the diffuse H2 survey, which has been able to use spectra obtained by FUSE for other programs (e.g., the survey of stellar winds in OB stars, the survey of hot gas in the Galactic halo, and the survey of interstellar gas at high latitude”
Stellar and wind properties of LMC WC4 stars A metallicity dependence for Wolf-Rayet mass-loss rates ? (2002, methodology): “Correction for molecular hydrogen in the FUSE spectral region has been made by using H2 column densities determined by Tumlinson et al. (2002): log (H2/cm−2) = 15.45 and 18.94 for HD 37026 and HD 37680, respectively.”
The Very Large Telescope Ultraviolet and Visible Echelle Spectrograph survey for molecular hydrogen in high‐redshift damped Lyman α systems (2003, unknown): “Alternatively, as shown by Tumlinson et al. (2002) in the case of the Magellanic Clouds, most of the observations can be explained by models where the formation rate of H2 onto dust grains is reduced and the ionizing flux is enhanced relative to what is observed in our Galaxy (see also the models…”
New Observations of the Interstellar Medium in the Lyman Break Galaxy MS 1512–cB58 (2001, background): “A recent Far Ultraviolet Spectroscopic Explorer (FUSE) survey of 70 stars in the Small and Large Magellanic Clouds (Tumlinson et al. 2002) has shown that in the interstellar medium of those galaxies the molecular fraction is even lower.”
First detection of CO in a high-redshift damped Lyman-α system (2008, methodology): “…of the H2 rotational levels, and the fine-structure levels of the C i groundstate are used to derive the physical conditions in the gas, such as temperature, gas pressure, and ambient radiation field (Savage et al. 1977; Black & van Dishoeck 1987; Jenkins & Tripp 2000; Tumlinson et al. 2002).”
THE MAGELLANIC MOPRA ASSESSMENT (MAGMA). I. THE MOLECULAR CLOUD POPULATION OF THE LARGE MAGELLANIC CLOUD (2011, background): “However, the presence of a significant mass of H2 residing in CO-dark envelopes surrounding LMC molecular clouds is not strongly excluded by this result, as the Tumlinson et al. (2002) sample contains predominantly sightlines with low extinction.”
THE CO-TO-H2 CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP (2011, background): “Based on UV spectroscopy (e.g., Tumlinson et al. 2002) we consider a pervasive H2 phase unlikely, but our experiment makes no test of this idea one way or the other.”
A Model for Atomic and Molecular Interstellar Gas: The Meudon PDR Code (2006, background): “…came after the launch of theFarUltraviolet Spectroscopic Explorer (FUSE) satellite in 1999, which can detect H2 and HD in absorption in a variety of Galactic and extragalactic environments (Rachford et al. 2002; Tumlinson et al. 2002; Bluhm et al. 2003; Boissé et al. 2005; Lacour et al. 2005).”
THE ATOMIC-TO-MOLECULAR TRANSITION IN GALAXIES. II: H i AND H2 COLUMN DENSITIES (2008, methodology): “Gillmon et al. (2006) report FUSE observations of sightlines at high galactic latitude (b >∼ 20 ◦) using AGN as background sources.”