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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 397 records · Page 22
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DOE OSTI2012

FORMATION OF MOLECULAR OXYGEN AND OZONE ON AMORPHOUS SILICATES

Oxygen in the interstellar medium is seen in the gas phase, in ices (incorporated in H{sub 2}O, CO, and CO{sub 2}), and in grains such as (Mg{sub x} Fe{sub 1-x} )SiO{sub 3} or (Mg{sub x} Fe{sub 1-x} ){sub 2}SiO{sub 4}, 0 < x < 1. In this investigation, we study the diffusion of oxygen atoms and the formation of oxygen molecules and ozone on the surface of an amorphous silicate film. We find that ozone is formed at low temperature (<30 K), and molecular oxygen forms when the diffusion of oxygen atoms becomes significant, at around 60 K. This experiment, besides being the first determination of the diffusion energy barrier (1785 {+-} 35 K) for oxygen atoms on a silicate surface, suggests bare silicates as a possible storage place for oxygen atoms in low-A{sub v} environments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

LOW-FREQUENCY RADIO-FIR CORRELATION IN NORMAL GALAXIES AT {approx}1 kpc SCALES

We study the radio-FIR correlation between the nonthermal (synchrotron) radio continuum emission at {lambda}90 cm (333 MHz) and the far-infrared emission due to cool ({approx}20 K) dust at {lambda}70 {mu}m in spatially resolved normal galaxies at scales of {approx}1 kpc. The slope of the radio-FIR correlation significantly differs between the arm and interarm regions. However, this change is not evident at a lower wavelength of {lambda}20 cm (1.4 GHz). We find the slope of the correlation in the arm to be 0.8 {+-} 0.12 and we use this to determine the coupling between equipartition magnetic field (B{sub eq}) and gas density ({rho}{sub gas}) as B{sub eq}{proportional_to}{rho}{sup 0.51{+-}0.12}{sub gas}. This is close to what is predicted by magnetohydrodynamic simulations of turbulent interstellar medium, provided the same region produces both the radio and far-infrared emission. We argue that at 1 kpc scales this condition is satisfied for radio emission at 1.4 GHz and may not be satisfied at 333 MHz. The change of slope observed in the interarm region could be caused by propagation of low energy ({approx}1.5 GeV) and long-lived ({approx}10{sup 8} yr) cosmic-ray electrons at 333 MHz.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

REASSESSMENT OF THE DISSOCIATIVE RECOMBINATION OF N{sub 2}H{sup +} AT CRYRING

The dissociative recombination (DR) of N{sub 2}H{sup +} has been reinvestigated at the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory in Stockholm, Sweden. Thermal rate coefficients for electron temperatures between 10 and 1000 K have been deduced. We show that electron recombination is expected to play an approximately equally important role as CO in the removal of N{sub 2}H{sup +} in dark interstellar clouds. We note that a deeper knowledge on the influence of the ions' rotational temperature in the DR of N{sub 2}H{sup +} would be helpful to set further constraints on the relative importance of the different destruction mechanisms for N{sub 2}H{sup +} in these environments. The branching fractions in the DR of N{sub 2}H{sup +} have been reinvestigated at {approx}0 eV relative kinetic energy, showing a strong dominance of the N{sub 2} + H production channel (93{sup +4}{sub -2}%) with the rest leading to NH + N. These results are in good agreement with flowing afterglow experiments and in disagreement with an earlier measurement at CRYRING.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

AN IN-DEPTH STUDY OF THE ABUNDANCE PATTERN IN THE HOT INTERSTELLAR MEDIUM IN NGC 4649

We present our X-ray imaging spectroscopic analysis of data from deep Suzaku and XMM-Newton Observatory exposures of the Virgo Cluster elliptical galaxy NGC 4649 (M60), focusing on the abundance pattern in the hot interstellar medium (ISM). All measured elements show a radial decline in abundance, with the possible exception of O. We construct steady-state solutions to the chemical evolution equations that include infall in addition to stellar mass return and Type Ia supernova (SNIa) enrichment, and consider recently published SNIa yields. By adjusting a single model parameter to obtain a match to the global abundance pattern in NGC 4649, we infer that introduction of subsolar metallicity external gas has reduced the overall ISM metallicity and diluted the effectiveness of SNIa to skew the pattern toward low {alpha}/Fe ratios, and estimate the combination of SNIa rate and level of dilution. Evidently, newly introduced gas is heated as it is integrated into, and interacts with, the hot gas that is already present. These results indicate a complex flow and enrichment history for NGC 4649, reflecting the continual evolution of elliptical galaxies beyond the formation epoch. The heating and circulation of accreted gas may help reconcile this dynamic history with the mostly passive evolution of elliptical stellar populations. In an Appendix, we examine the effects of the recent updated atomic database AtomDB in spectral fitting of thermal plasmas with hot ISM temperatures in the elliptical galaxy range.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DRIVING OUTFLOWS WITH RELATIVISTIC JETS AND THE DEPENDENCE OF ACTIVE GALACTIC NUCLEUS FEEDBACK EFFICIENCY ON INTERSTELLAR MEDIUM INHOMOGENEITY

We examine the detailed physics of the feedback mechanism by relativistic active galactic nucleus (AGN) jets interacting with a two-phase fractal interstellar medium (ISM) in the kpc-scale core of galaxies using 29 three-dimensional grid-based hydrodynamical simulations. The feedback efficiency, as measured by the amount of cloud dispersal generated by the jet-ISM interactions, is sensitive to the maximum size of clouds in the fractal cloud distribution but not to their volume filling factor. Feedback ceases to be efficient for Eddington ratios P{sub jet}/L{sub edd} {approx}< 10{sup -4}, although systems with large cloud complexes {approx}> 50 pc require jets of Eddington ratio in excess of 10{sup -2} to disperse the clouds appreciably. Based on measurements of the bubble expansion rates in our simulations, we argue that sub-grid AGN prescriptions resulting in negative feedback in cosmological simulations without a multi-phase treatment of the ISM are good approximations if the volume filling factor of warm-phase material is less than 0.1 and the cloud complexes are smaller than {approx}25 pc. We find that the acceleration of the dense embedded clouds is provided by the ram pressure of the high-velocity flow through the porous channels of the warm phase, flow that has fully entrained the shocked hot-phase gas it has swept up, and is additionally mass loaded by ablated cloud material. This mechanism transfers 10% to 40% of the jet energy to the cold and warm gas, accelerating it within a few 10 to 100 Myr to velocities that match those observed in a range of high- and low-redshift radio galaxies hosting powerful radio jets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MOLECULAR AND ATOMIC LINE SURVEYS OF GALAXIES. II. UNBIASED ESTIMATES OF THEIR STAR FORMATION MODE

We make use of our 'minimal' cold interstellar medium emission line model that predicts the molecular and atomic line emission per unit dense, star-forming gas mass to examine the utility of key line ratios in surveys of the so-called star formation 'mode' as traced by {xi}{sub SF} = M{sub dense}(H{sub 2})/M{sub total}(H{sub 2}). We argue that {xi}{sub SF} and its proxies provide very sensitive, extinction-free discriminators of rapid starburst/merger-driven versus secular quiescent/disk-like stellar mass assembly, with the most promising diagnostic to be applied in the near-future being CO J(4 {yields} 3)/ [C I]({sup 3} P{sub 1} {yields} {sup 3} P{sub 0}). These lines are accessible across nearly the full range 0 < z < 2 (thus covering the bulk of galaxy evolution) with the Atacama Large Millimeter Array. In addition to their diagnostic power, another advantage of this combination is the similar observed frequencies ({Delta}{nu}{sub 0} Almost-Equal-To 30 GHz) of the lines, resulting in nearly spatially matched beams for a fixed aperture, thus mitigating the effects of resolution/morphology bias in the interpretation of galaxy-averaged line ratios. Finally, we discuss the capability of deep blind redshift surveys with the high-frequency component of the Square Kilometer Array (SKA) in discovering H{sub 2}-rich galaxies with very low {xi}{sub SF} values. These could be the progenitors of starburst galaxies seen prior to the onset of star formation; such galaxies could be a class of extreme outliers from local (gas surface density)-(star formation rate) scaling laws, which would exclude them from current star formation or stellar-mass-selected samples. Our conservative model suggests that SKA could detect such systems residing at z {approx} 3 at a rate of 20-200 hr{sup -1}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CHEMICAL ANALYSIS OF A DIFFUSE CLOUD ALONG A LINE OF SIGHT TOWARD W51: MOLECULAR FRACTION AND COSMIC-RAY IONIZATION RATE

Absorption lines from the molecules OH{sup +}, H{sub 2}O{sup +}, and H{sup +} {sub 3} have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH{sup +} and H{sub 2}O{sup +} are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H{sup +} {sub 3} is used to infer the cosmic-ray ionization rate of H{sub 2} in the molecular interior of the cloud, which we find to be {zeta}{sub 2} = (4.8 {+-} 3.4) Multiplication-Sign 10{sup -16} s{sup -1}. Combining the results from all three species we find an efficiency factor-defined as the ratio of the formation rate of OH{sup +} to the cosmic-ray ionization rate of H-of {epsilon} = 0.07 {+-} 0.04, much lower than predicted by chemical models. This is an important step in the future use of OH{sup +} and H{sub 2}O{sup +} on their own as tracers of the cosmic-ray ionization rate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE LOW-TEMPERATURE NUCLEAR SPIN EQUILIBRIUM OF H{sup +} {sub 3} IN COLLISIONS WITH H{sub 2}

Recent observations of H{sub 2} and H{sup +} {sub 3} in diffuse interstellar sightlines revealed a difference in the nuclear spin excitation temperatures of the two species. This discrepancy comes as a surprise, as H{sup +} {sub 3} and H{sub 2} should undergo frequent thermalizing collisions in molecular clouds. Non-thermal behavior of the fundamental H{sup +} {sub 3}/H{sub 2} collision system at low temperatures was considered as a possible cause for the observed irregular populations. Here, we present measurements of the steady-state ortho/para ratio of H{sup +} {sub 3} in collisions with H{sub 2} molecules in a temperature-variable radiofrequency ion trap between 45 and 100 K. The experimental results are close to the expected thermal outcome and they agree very well with a previous micro-canonical model. We briefly discuss the implications of the experimental results for the chemistry of the diffuse interstellar medium.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

SEARCHING FOR INTERSTELLAR MOLECULE BUTATRIENYLIDENE IN REACTION C{sub 2} + C{sub 2}H{sub 4}

We investigated the reaction C{sub 2}(X {sup 1}{Sigma}{sup +} {sub g}/a {sup 3}{Pi} {sub u}) + C{sub 2}H{sub 4} at collision energy 5.0 kcal mol{sup -1} in a crossed molecular-beam apparatus using selective photoionization. Time-of-flight and photoionization spectra of products C{sub 4}H{sub 3} and C{sub 4}H{sub 2} were measured. From the best simulation of product time-of-flight spectra, a low-energy-biased translational-energy distribution and an isotropic angular distribution are derived for product channels C{sub 4}H{sub 3} + H and C{sub 4}H{sub 2} + H{sub 2} that have average translational-energy releases of 11 and 20 kcal mol{sup -1}, respectively. Product C{sub 4}H{sub 3} is identified as H{sub 2}CCCCH because its ionization threshold 8.0 {+-} 0.2 eV and maximal translational-energy release 42 kcal mol{sup -1} coincide with that of product channel H{sub 2}CCCCH + H. H{sub 2}CCCC (butatrienylidene) and HCCCCH (diacetylene) might have contributions to product C{sub 4}H{sub 2}; both isomers have ionization energies near the measured ionization threshold 10.0 {+-} 0.2 eV and the maximal translational-energy release 62 kcal mol{sup -1} is within the energetic limits of both isomeric product channels. Nonetheless, channel H{sub 2}CCCC + H{sub 2} is suggested to be more dominant than channel HCCCCH + H{sub 2} because the maximal translational-energy release is in good agreement with the available energy of the former channel and the former channel is 3.8 times the branching ratio of the later channel predicted by Rice-Ramsperger-Kassel-Marcus calculations. C{sub 4}H{sub 2} is identified for the first time in the barrierless reaction C{sub 2} + C{sub 2}H{sub 4} which has never been considered in any astronomical chemical networks. This work sheds new light on the formation of butatrienylidene/diacetylene in cold interstellar media where C{sub 2} and C{sub 2}H{sub 4} are abundant.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

STAR FORMATION IN ATOMIC GAS

Observations of nearby galaxies have firmly established, over a broad range of galactic environments and metallicities, that star formation occurs exclusively in the molecular phase of the interstellar medium (ISM). Theoretical models show that this association results from the correlation between chemical phase, shielding, and temperature. Interstellar gas converts from atomic to molecular only in regions that are well shielded from interstellar ultraviolet (UV) photons, and since UV photons are also the dominant source of interstellar heating, only in these shielded regions does the gas become cold enough to be subject to Jeans instability. However, while the equilibrium temperature and chemical state of interstellar gas are well correlated, the timescale required to reach chemical equilibrium is much longer than that required to reach thermal equilibrium, and both timescales are metallicity-dependent. Here I show that the difference in timescales implies that, at metallicities below a few percent of the solar value, well shielded gas will reach low temperatures and proceed to star formation before the bulk of it is able to convert from atomic to molecular. As a result, at extremely low metallicities, star formation will occur in a cold atomic phase of the ISM rather than a molecular phase. I calculate the observable consequences of this result for star formation in low-metallicity galaxies, and I discuss how some current numerical models for H{sub 2}-regulated star formation may need to be modified.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A UNIFIED MICROSCOPIC-MACROSCOPIC MONTE CARLO SIMULATION OF GAS-GRAIN CHEMISTRY IN COLD DENSE INTERSTELLAR CLOUDS

For the first time, we report a unified microscopic-macroscopic Monte Carlo simulation of gas-grain chemistry in cold interstellar clouds in which both the gas-phase and the grain-surface chemistry are simulated by a stochastic technique. The surface chemistry is simulated with a microscopic Monte Carlo method in which the chemistry occurs on an initially flat surface. The surface chemical network consists of 29 reactions initiated by the accreting species H, O, C, and CO. Four different models are run with diverse but homogeneous physical conditions including temperature, gas density, and diffusion-barrier-to-desorption energy ratio. As time increases, icy interstellar mantles begin to grow. Our approach allows us to determine the morphology of the ice, layer by layer, as a function of time, and to ascertain the environment or environments for individual molecules. Our calculated abundances can be compared with observations of ices and gas-phase species, as well as the results of other models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ENERGY DEPOSITION PROFILES AND ENTROPY IN GALAXY CLUSTERS

We report the results of our study of fractional entropy enhancement in the intracluster medium (ICM) of the clusters from the representative XMM-Newton cluster structure survey. We compare the observed entropy profile of these clusters with that expected for the ICM without any feedback, as well as with the introduction of preheating and cooling. We make the first estimate of the total, as well as radial, non-gravitational energy deposition up to r {sub 500} for this large, nearly flux-limited, sample of clusters. We find that the total energy deposition corresponding to the entropy enhancement is proportional to the cluster temperature (and hence cluster mass). The energy deposition per particle scaled by T {sub sp}, {Delta}E/T {sub sp} has a similar profile in all clusters, and is more pronounced in the central regions. Our results support models of entropy enhancement through active galactic nucleus feedback.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DISCOVERY OF X-RAY EMISSION FROM AND DISTANCE TO THE SUPERNOVA REMNANT G84.2-0.8

We analyze X-ray and radio observations of the supernova remnant G84.2-0.8. 1420 MHz atomic hydrogen (H I) line and radio continuum data yield H I absorption spectra and a new H I absorption distance of 5.8-6.2 kpc. Archival X-ray observations from ROSAT and Chandra which cover the area including G84.2-0.8 are analyzed to show extended X-ray emission from G84.2-0.8. Fits to X-ray spectra from Chandra, with the new H I distance of 5.8-6.2 kpc, are used to determine the Sedov parameters of the supernova remnant. G84.2-0.8 is large (16-18 pc radius), middle aged ({approx}9000 yr), expanding in low-density interstellar medium (0.02 cm{sup -3}), and consistent with a low explosion energy (0.8-6.5 Multiplication-Sign 10{sup 50} erg).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE INTERSTELLAR MAGNETIC FIELD CLOSE TO THE SUN. II

The magnetic field in the local interstellar medium (ISM) provides a key indicator of the galactic environment of the Sun and influences the shape of the heliosphere. We have studied the interstellar magnetic field (ISMF) in the solar vicinity using polarized starlight for stars within 40 pc of the Sun and 90 Degree-Sign of the heliosphere nose. In Frisch et al. (Paper I), we developed a method for determining the local ISMF direction by finding the best match to a group of interstellar polarization position angles obtained toward nearby stars, based on the assumption that the polarization is parallel to the ISMF. In this paper, we extend the analysis by utilizing weighted fits to the position angles and by including new observations acquired for this study. We find that the local ISMF is pointed toward the galactic coordinates l, b =47 Degree-Sign {+-} 20 Degree-Sign , 25 Degree-Sign {+-} 20 Degree-Sign . This direction is close to the direction of the ISMF that shapes the heliosphere, l, b =33 Degree-Sign {+-} 4 Degree-Sign , 55 Degree-Sign {+-} 4 Degree-Sign , as traced by the center of the 'Ribbon' of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX) mission. Both the magnetic field direction and the kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. A nearby ordered component of the local ISMF has been identified in the region l Almost-Equal-To 0 Degree-Sign {yields} 80 Degree-Sign and b Almost-Equal-To 0 Degree-Sign {yields} 30 Degree-Sign , where PlanetPol data show a distance-dependent increase of polarization strength. The ordered component extends to within 8 pc of the Sun and implies a weak curvature in the nearby ISMF of {approx}0.{sup 0}25 pc{sup -1}. This conclusion is conditioned on the small sample of stars available for defining this rotation. Variations from the ordered component suggest a turbulent component of {approx}23 Degree-Sign . The ordered component and standard relations between polarization, color excess, and H{sup o} column density predict a reasonable increase of N(H) with distance in the local ISM. The similarity of the ISMF directions traced by the polarizations, the IBEX Ribbon, and pulsars inside the Local Bubble in the third galactic quadrant suggest that the ISMF is relatively uniform over spatial scales of 8-200 pc and is more similar to interarm than spiral-arm magnetic fields. The ISMF direction from the polarization data is also consistent with small-scale spatial asymmetries detected in GeV-TeV cosmic rays with a galactic origin. The peculiar geometrical relation found earlier between the cosmic microwave background dipole moment, the heliosphere nose, and the ISMF direction is supported by this study. The interstellar radiation field at {approx}975 A does not appear to play a role in grain alignment for the low-density ISM studied here.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE LOCAL INTERSTELLAR SPECTRUM BEYOND THE HELIOPAUSE: WHAT CAN BE LEARNED FROM VOYAGER IN THE INNER HELIOSHEATH?

The local interstellar spectrum (LIS) is one of the most important but unknown parameters used in all modeling efforts to describe the modulation of Galactic cosmic rays on their way from the galaxy through a possible bow shock, heliosheath, and heliosphere toward the Earth. Because it has not been measured thus far, several LIS models derived from numerical simulations or data on Earth were developed. A new method to determine the LIS was introduced when the Voyager spacecraft crossed the termination shock and entered the heliosheath. Webber and Higbie derived a new LIS, which is lower than all previous LIS models over the entire energy range, on the basis of these measurements. Numerical simulations by Scherer et al. showed that particles already in the outer heliosheath (OHS) are modulated, suggesting that the LIS by Webber and Higbie is a heliopause spectrum (HPS) rather than the ''true'' LIS. By using the same simplified simulation model, we estimate the diffusion coefficient in the OHS to be consistent with several 10{sup 26} to 10{sup 27} cm{sup 2} s{sup -1} for all LIS models under consideration by mapping them to this HPS and conclude that the Voyager measurements will not be able to determine the LIS in the near future. We then discuss the circumstances under which the terrestrial archive can be used to at least exclude LIS models, unless one awaits a dedicated mission like e.g., the Interstellar Probe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

TWO-STAGE FRAGMENTATION FOR CLUSTER FORMATION: ANALYTICAL MODEL AND OBSERVATIONAL CONSIDERATIONS

Linear analysis of the formation of protostellar cores in planar magnetic interstellar clouds shows that molecular clouds exhibit a preferred length scale for collapse that depends on the mass-to-flux ratio and neutral-ion collision time within the cloud. We extend this linear analysis to the context of clustered star formation. By combining the results of the linear analysis with a realistic ionization profile for the cloud, we find that a molecular cloud may evolve through two fragmentation events in the evolution toward the formation of stars. Our model suggests that the initial fragmentation into clumps occurs for a transcritical cloud on parsec scales while the second fragmentation can occur for transcritical and supercritical cores on subparsec scales. Comparison of our results with several star-forming regions (Perseus, Taurus, Pipe Nebula) shows support for a two-stage fragmentation model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE 217.5 nm BAND, INFRARED ABSORPTION, AND INFRARED EMISSION FEATURES IN HYDROGENATED AMORPHOUS CARBON NANOPARTICLES

We report on the preparation of hydrogenated amorphous carbon nanoparticles whose spectral characteristics include an absorption band at 217.5 nm with the profile and characteristics of the interstellar 217.5 nm feature. Vibrational spectra of these particles also contain the features commonly observed in absorption and emission from dust in the diffuse interstellar medium. These materials are produced under ''slow'' deposition conditions by minimizing the flux of incident carbon atoms and by reducing surface mobility. The initial chemistry leads to the formation of carbon chains, together with a limited range of small aromatic ring molecules, and eventually results in carbon nanoparticles having an sp {sup 2}/sp {sup 3} ratio Almost-Equal-To 0.4. Spectroscopic analysis of particle composition indicates that naphthalene and naphthalene derivatives are important constituents of this material. We suggest that carbon nanoparticles with similar composition are responsible for the appearance of the interstellar 217.5 nm band and outline how these particles can form in situ under diffuse cloud conditions by deposition of carbon on the surface of silicate grains. Spectral data from carbon nanoparticles formed under these conditions accurately reproduce IR emission spectra from a number of Galactic sources. We provide the first detailed fits to observational spectra of Type A and B emission sources based entirely on measured spectra of a carbonaceous material that can be produced in the laboratory.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

CAN DUST EMISSION BE USED TO ESTIMATE THE MASS OF THE INTERSTELLAR MEDIUM IN GALAXIES-A PILOT PROJECT WITH THE HERSCHEL REFERENCE SURVEY

The standard method for estimating the mass of the interstellar medium (ISM) in a galaxy is to use the 21 cm line to trace the atomic gas and the CO 1-0 line to trace the molecular gas. In this paper, we investigate the alternative technique of using the continuum dust emission to estimate the mass of gas in all phases of the ISM. Using Herschel observations of 10 galaxies from the Herschel Reference Survey and the Herschel Virgo Cluster Survey, we show that the emission detected by Herschel is mostly from dust that has a temperature and emissivity index similar to that of dust in the local ISM in our galaxy, with the temperature generally increasing toward the center of each galaxy. We calibrate the dust method using the CO and 21 cm observations to provide an independent estimate of the mass of hydrogen in each galaxy, solving the problem of the uncertain ''X-factor'' for the CO observations by minimizing the dispersion in the ratio of the masses estimated using the two methods. With the calibration for the dust method and the estimate of the X-factor produced in this way, the dispersion in the ratio of the two gas masses is 25%. The calibration we obtain for the dust method is similar to those obtained from Herschel observations of M31 and from Planck observations of the Milky Way. We discuss the practical problems in using this method.

79 ASTRONOMY AND ASTROPHYSICS↗