Density Of H2 Research Articles

Preparation and Reaction Kinetics of Carbon Nanotubes/Aluminum Composite Powders Using Polymer Pyrolysis Method

The method of polymer pyrolysis chemical vapor deposition (PP-CVD) was used to in situ grow carbon nanotubes (CNTs) on the micro and nano sized flake like aluminum powder substrates. The vapor species were in situ produced by pyrolysis of polyethylene glycol (PEG) including the carbon sources, which was the main difference between the PP-CVD and conventional CVD methods, while the catalyst nanoparticles were pro- duced by the reaction between citric acid (CA) and cobalt nitrate (Co(NO3)2) on aluminum powder surfaces. The reaction mechanism of PP-CVD was studied with the analysis of experiments and reaction kinetics mod- eling, revealing the influence of the vapor species produced by pyrolysis of PEG and CA and surface vapor-solid re- actions on catalyst nanoparticles on CNT growth rates. The CNTs growth rates increases with the increase of reaction temperature and initial partial pressure of CO, which is influenced by the content of PEG and CA, and decreases with the increase of catalyst density and initial partial pressure of H2. The variation trends of the simulated CNTs average length with reaction temperature and time are consistent with the experimental results. Thus, this work provides new theoretical basis to the further optimization of fabricating CNTs/aluminum composite powders.

Iridium-decorated Palladium–Platinum core–shell catalysts for oxygen reduction reaction in proton exchange membrane fuel cell

Carbon-supported Pt, Pd, Pd–Pt core–shell (Ptshell–Pdcore/C) and Ir-decorated Pd–Pt core–shell (Ir-decorated Ptshell–Pdcore/C) catalysts were synthesized, and their physical properties, electrochemical behaviors, oxygen reduction reaction (ORR) characteristics and proton exchange membrane fuel cell (PEMFC) performances were investigated herein. From the XRD patterns and TEM images, Ir-decorated Ptshell–Pdcore/C has been confirmed that Pt was deposited on the Pd nanoparticle which had the core–shell structure. Ir-decorated Ptshell–Pdcore/C has more positive OH reduction peak than Pt/C, which is beneficial to weaken the binding energy of Pt–OH during the ORR. Thus, Ir-decorated Ptshell–Pdcore/C has higher ORR activity than Pt/C. The maximum power density of H2–O2 PEMFC using Ir-decorated Ptshell–Pdcore/C is 792.2mWcm−2 at 70°C, which is 24% higher than that using Pt/C. The single-cell accelerated degradation test of PEMFC using Ir-decorated Ptshell–Pdcore/C shows good durability by the potential cycling of 40,000 cycles. This study concludes that Ir-decorated Ptshell–Pdcore/C has the low Pt content, but it can facilitate the low-cost and high-efficient PEMFC.

The density and temperature structure near the exobase of Saturn from Cassini UVIS solar occultations

We analyzed 15 solar occultations observed by the Cassini UVIS instrument to constrain the density and temperature structure near the exobase of Saturn. We retrieved the density of H2 and thus the temperature at altitudes higher than 1900km above the 1bar level by analyzing the ionization continuum of H2 at wavelengths shorter than 804Å. We find that the exospheric temperature ranges from 370K to 540K, with a typical uncertainty of less than 20K. According to our data the temperature increases with latitude from the equator to the poles by 100–150K. At similar latitudes, the temperature varies by 20–50K at different times with no evidence for any systematic diurnal trend so far. Based on our data, the exobase of Saturn is 2700–3000km above the 1bar level and the thermal escape parameter near the exobase ranges from 260 to 340, implying that thermal escape from Saturn is firmly in the Jeans regime. The mixing ratio of H2 is close to unity at all altitudes below the exobase. We find that the pressure levels in the thermosphere deviate significantly from a simple spheroid predicted by potential theory. This is consistent with significant meridional temperature variations in the lower thermosphere. A global analysis of the temperature structure at different depths in the atmosphere is required to constrain both the shape and the deposition and redistribution of energy in the upper atmosphere further.

Modification of ultrananocrystalline diamond film microstructure via Fe-coating and annealing for enhancement of electron field emission properties

The interaction between Fe-coatings and ultrananocrystalline diamond (UNCD) films during annealing was investigated in detail using transmission electron microscopy. The thin Fe-coating first formed nanosized Fe-clusters and then catalytically dissociated the diamond, re-precipitating carbon to form nanosized graphite clusters. These clusters formed conducting networks that facilitated electron transport and greatly improved the electron field emission (EFE) properties of the UNCD films. The extent of enhancement varied markedly with annealing temperature and atmosphere. For H2-annealed films, EFE behavior was optimized by annealing at 900 °C. EFE was turned on at (E0)H2 = 1.2 V/μm, attaining EFE current density of (Je)H2 = 772.0 μA/cm2 at an applied field of 8.8 V/mm. These characteristics were superior to those of UNCD films NH3-annealed at 850 °C. The inferior EFE properties for the NH3-annealed samples were attributed to reaction of NH3 with the hydrocarbon phase that encapsulated the nanosized diamond grains, hindering Fe–diamond interaction.

Solubility of molecular hydrogen in silicate melts and consequences for volatile evolution of terrestrial planets

We present experiments from 0.7 to 3GPa that quantify solubility of H2 in silicate melts under controlled hydrogen fugacities (fH2). Two experimental series, one on synthetic basalt+COH and other with a synthetic andesite+OH, were conducted using a double capsule technique to impose a range of fH2, on the samples. Quenched glasses were analyzed by FTIR and SIMS. Both series follow simple solubility laws in which molecular H2 concentrations are proportional to fH2 and with a partial molar volume of molecular H2 of 11cm3/mole. Solubilities in andesitic melt are systematically greater than in basaltic liquid in a relationship consistent with control by the ionic porosity (IP) of the melts. Extrapolation based on IP allows estimation of the solubility of H2 in peridotitic melts applicable to magma oceans. The H2/(H2+H2O) ratio in silicate melts (where H2O includes molecular H2O and OH−) increases as conditions become more reduced, with increasing pressure, and with increasing total H. Under some conditions prevailing in the early Earth and terrestrial planets as well as in the deep Earth today, H2 can be a significant fraction of the dissolved H and at high pressure it may exceed “water” (H2O and OH−). Therefore, magmatic H2 may influence the initial distribution of volatiles and the redox evolution of terrestrial planets, as well as the ongoing formation and fate of hydrous melts in the deep Earth today. Hydrous species in melts in equilibrium with Fe-rich alloy at high pressure, for example during core formation from a magma ocean, could be chiefly H2, rather than H2O. Hence, delivery of H2 to the core by removal of Fe hydride need not be coupled to oxidation of the residual mantle. Although lunar basalts are much reduced, the fraction of H dissolved as molecular H2 is small owing to low total H concentrations. Extrapolation to conditions of potential hydrous partial melting in the deep Earth suggests that the chief magmatic volatile may be H2 rather than H2O. The very small partial specific density of magmatic H2 (0.18g/cm3 at low pressure) may provide significant positive buoyancy to deep partial melts.

Preparation of polytetrafluoroethylene porous membrane based composite alkaline exchange membrane with improved tensile strength and its fuel cell test

A strategy to improve the alkaline exchange membrane tensile strength was introduced based on porous PTFE and quaternized polyvinyl benzyl chloride (qPVBz/OH−). A thin PTFE film was used as the support to prepare the composite AAEM to improve the mechanical strength. Tensile stress tests showed 51.1 MPa which was twice that of the pure anion exchange membrane qPVBz/OH−. SEM observations showed that the pores of the PTFE were successfully filled with qPVBz/OH− polymer and there was no structure defects in the cross-section of the resultant membrane. The synthesized composite membrane had very good thermal stability. The through plane ionic conductivity was in the range of 1.65 × 10−2 to 2.2 × 10−2 S cm−1 in the temperature range of 25 °C–60 °C. The power density of H2 and O2 fuel cell gave 162 mW cm−2.

Molecular clouds under the influence of massive stars in the Galactic Hii region G353.2+0.9

The Galactic HII region G353.2+0.9 is excited by the massive open cluster Pismis-24. By analyzing (sub-)mm molecular-line and -continuum we study the detailed morphology of the gas and dust, as well as their physical parameters and their variation across the PDR. We observed various molecules and transitions to derive the physical properties of the molecular gas through line ratios, and both LTE and non-LTE analyses. The physical properties of the gas were derived with a Bayesian approach for the non-LTE analysis. Based on the continuum data at 870 micron, we derived the dust mass and the column density of H2, and thus the molecular abundances. The total mass of the gas in the region is ca. 2000 Mo, while that of the dust is ca. 21 Mo. A velocity gradient in the region suggests that the expansion of the ionized gas is pushing the molecular gas away from the observer. We unambiguously identify the ionization front, at the location of which we detect an increase in gas density and temperature. We find at least 14 clumps at different positions and LSR velocities. We derive kinetic temperatures in the ranges 11-45 K (CS) and 20-45 K (CN). The H2 number density is typically around 1e5 cm^-3 from CS and few 1e5 cm^-3 from CN, with maxima above 1e6 cm^-3. The abundances of the molecules observed vary across the region, and appear to be higher in regions further away from the ionization front.

Li and Na Co-decorated carbon nitride nanotubes as promising new hydrogen storage media

The capacity of Li and Na co-decorated carbon nitride nanotube (CNNT) for hydrogen storage is studied using first-principles density functional theory. The results show that with two H2 molecules attached to per Li and four H2 molecules per Na the Li and Na co-decorated CNNT gains a gravimetric density of H2 as high as 9.09 wt% via electrostatic interaction without the clustering of the deposited metal atoms (at T=0 K). The average adsorption energy of hydrogen molecule is in the range of 0.09–0.22 eV/H2, which is suitable for practical hydrogen storage at ambient temperatures.

Relating dust, gas, and the rate of star formation in M 31

We derive distributions of dust temperature and dust opacity across M31 at 45" resolution using the Spitzer data. With the opacity map and a standard dust model we de-redden the Ha emission yielding the first de-reddened Ha map of M31. We compare the emissions from dust, Ha, HI and H2 by means of radial distributions, pixel-to-pixel correlations and wavelet cross-correlations. The dust temperature steeply decreases from 30K near the center to 15K at large radii. The mean dust optical depth at the Ha wavelength along the line of sight is about 0.7. The radial decrease of the dust-to-gas ratio is similar to that of the oxygen abundance. On scales<2kpc, cold dust emission is best correlated with that of neutral gas and warm dust emission with that of ionized gas. Ha emission is slightly better correlated with emission at 70um than at 24um. In the area 6kpc<R< 17kpc, the total SFR is ~0.3Msun/yr. The Kennicutt-Schmidt law between SFR and total gas has a power-law index of 1.30+-0.05 in the radial range of R=7-11kpc increasing by about 0.3 for R=11-13kpc. The lack of H2 in the central region could be related to the lack of HI and the low opacity/high temperature of the dust. Since neither SFR nor SFE is well correlated with the surface density of H2 or total gas, other factors than gas density must play an important role in the formation of massive stars in M31. The molecular depletion time scale of 1.1 Gyr indicates that M31 is about three times less efficient in forming young massive stars than M33.

Methanol maps of low-mass protostellar systems

Observations of Serpens have been performed at the JCMT using Harp-B. Maps over a 4.5'x5.4' region were made in a frequency window around 338 GHz, covering the 7-6 transitions of methanol. Emission is extended over each source, following the column density of H2 but showing up also particularly strongly around outflows. The rotational temperature is low, 15-20 K, and does not vary with position within each source. The abundance is typically 10^-9 - 10^-8 with respect to H2 in the outer envelope, whereas "jumps" by factors of up to 10^2 -10^3 inside the region where the dust temperature exceeds 100 K are not excluded. A factor of up to ~ 10^3 enhancement is seen in outflow gas. In one object, SMM4, the ice abundance has been measured to be ~ 3x10^-5 with respect to H2 in the outer envelope, i.e., a factor of 10^3 larger than the gas-phase abundance. Comparison with C18O J=3-2 emission shows that strong CO depletion leads to a high gas-phase abundance of CH3OH not just for the Serpens sources, but for a larger sample of protostars. The observations illustrate the large-scale, low-level desorption of CH3OH from dust grains, extending out to and beyond 7500 AU from each source, a scenario which is consistent with non-thermal (photo-)desorption from the ice. The observations also illustrate the usefulness of CH3OH as a tracer of energetic input in the form of outflows, where methanol is sputtered from the grain surfaces. Finally, the observations provide further evidence of CH3OH formation through CO hydrogenation proceeding on grain surfaces in low-mass envelopes.

Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration

AbstractInterest in sustainable non‐hydrocarbon‐based fuels for transportation has grown as the realization that the supply of fossil fuels is limited and the deleterious environmental effects of burning them has come into public focus. The use of hydrogen (H2) has been proposed as an alternative, but its use in pure form is undesirable due to the high pressures or low temperatures required to store useful quantities. Approaches to ameliorate this issue, which stem from the low volumetric energy density of H2, include the pursuit of sorbents capable of containing H2 in greater density than liquid H2 at reasonable temperatures and pressures, metal hydrides such as NaBH4, and chemical hydrides such as ammonia borane (AB, NH3BH3). AB contains 19.6 wt.‐% H2, which is well suited to practical applications, but issues with extracting the optimal quantities of H2 at reasonable temperatures and at useful rates as well as recycling of the spent fuel back into AB with good efficiency and reasonable cost remain to be solved. These problems are inextricably intertwined, as the needs of the regeneration process dictate which catalysts are used (i.e. what kind of spent fuel is generated). This Microreview focuses on recent developments in transition metal complexes for the catalytic dehydrogenation of AB. Neither solvolysis of AB nor thermolysis of AB in the absence of a catalyst are covered in this review.

Odin observations of water in molecular outflows and shocks

Aims. We investigate the ortho-water abundance in outflows and shocks in order to improve our knowledge of shock chemistry and of the physics behind molecular outflows. Methods. We have used the Odin space observatory to observe the H2O(110-101) line. We obtain strip maps and single pointings of 13 outflows and two supernova remnants where we report detections for eight sources. We have used RADEX to compute the beam averaged abundances of o-H2O relative to H2. In the case of non-detection, we derive upper limits on the abundance. Results. Observations of CO emission from the literature show that the volume density of H2 can vary to a large extent, a parameter that puts severe uncertainties on the derived abundances. Our analysis shows a wide range of abundances reflecting the degree to which shock chemistry is affecting the formation and destruction of water. We also compare our results with recent results from the SWAS team. Conclusions. Elevated abundances of ortho-water are found in several sources. The abundance reaches values as high as what would be expected from a theoretical C-type shock where all oxygen, not in the form of CO, is converted to water. However, the high abundances we derive could also be due to the low densities (derived from CO observations) that we assume. The water emission may in reality stem from high density regions much smaller than the Odin beam. We do not find any relationship between the abundance and the mass loss rate. On the other hand, there is a relation between the derived water abundance and the observed maximum outflow velocity.

On the origin of giant planets and their hosts

AbstractThe correlation between stellar metallicity and giant planets has been tentatively explained by the possible increase of planet formation probability in stellar disks with enhanced amount of metals. There are two caveats to this explanation. First, giant stars with planets do not show a metallicity distribution skewed towards metal-rich objects, as found for dwarfs. Second, the correlation with metallicity is not valid at intermediate metallicities, for which it can be shown that giant planets are preferentially found orbiting thick disk stars.None of these two peculiarities is explained by the proposed scenarios of giant planet formation. We contend that they are galactic in nature, and probably not linked to the formation process of giant planets. It is suggested that the same dynamical effect, namely the migration of stars in the galactic disk, is at the origin of both features, with the important consequence that most metal-rich stars hosting giant planets originate from the inner disk. A planet-metallicity correlation similar to the observed one is easily obtained if stars from the inner disk have a higher percentage of giant planets than stars born at the solar radius, with no specific dependence on metallicity. We propose that the density of H2 in the inner galactic disk (the molecular ring) could play a role in setting the high percentage of giant planets that originate from this region.

ON THE CORRELATION BETWEEN METALLICITY AND THE PRESENCE OF GIANT PLANETS

The correlation between stellar metallicity and the presence of giant planets is well established. It has been tentatively explained by the possible increase of planet-formation probability in stellar disks with enhanced amount of metals. However, there are two caveats to this explanation. First, giant stars with planets do not show a metallicity distribution skewed toward metal-rich objects, as found for dwarfs. Second, the correlation with metallicity is not valid at intermediate metallicities, for which it can be shown that giant planets are preferentially found orbiting thick disk stars. Neither of these two peculiarities is explained by the proposed scenarios of giant planet formation. We contend that they are galactic in nature, and probably not linked to the formation process of giant planets. It is suggested that the same dynamical effect, namely the migration of stars in the galactic disk, is at the origin of both features, with the important consequence that most metal-rich stars hosting giant planets originate from the inner disk, a property that has been largely neglected until now. We illustrate that a planet–metallicity correlation similar to the observed one is easily obtained if stars from the inner disk have a higher percentage of giant planets than stars born at the solar radius, with no specific dependence on metallicity. We propose that the density of H2 in the inner galactic disk (the molecular ring) could play a role in setting the high percentage of giant planets that originate from this region.

Mapping Warm Molecular Hydrogen with theSpitzerInfrared Array Camera (IRAC)

Photometric maps, obtained with the Spitzer Infrared Array Camera (IRAC), can provide a valuable probe of warm molecular hydrogen within the interstellar medium. IRAC maps of the supernova remnant IC 443, extracted from the Spitzer archive, are strikingly similar to spectral line maps of the H2 pure rotational transitions that we obtained with the Infrared Spectrograph (IRS) instrument on Spitzer. IRS spectroscopy indicates that IRAC bands 3 and 4 are indeed dominated by the H2 v = 0-0 S(5) and S(7) transitions, respectively. Modeling of the H2 excitation suggests that bands 1 and 2 are dominated by H2 v = 1–0 O(5) and v = 0-0 S(9). Large maps of the H2 emission in IC 443, obtained with IRAC, show band ratios that are inconsistent with the presence of gas at a single temperature. The relative strengths of IRAC bands 2, 3, and 4 are consistent with pure H2 emission from shocked material with a power-law distribution of gas temperatures. CO vibrational emissions do not contribute significantly to the observed band 2 intensity. Assuming that the column density of H2 at temperatures T to T + dT is proportional to T−b for temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The power-law index, b, shows variations over the range ~3-6 within the set of different sight lines probed by the maps, with the majority of sight lines showing b in the range 4-5. The observed power-law index is consistent with the predictions of simple models for paraboloidal bow shocks.

Where Is the Molecular Hydrogen in Damped Lyα Absorbers?

We show in this paper why molecular millimeter absorption line searches in DLAs have been unsuccessful. We use CO emission-line maps of local galaxies to derive the H2 column density distribution function f(N) at z = 0. We show that it forms a natural extension to f(N): the H2 distribution exceeds f(N) at NH ≈ 1022cm-2 and exhibits a power-law dropoff with slope ~-2.5. Approximately 97% of the H2 mass density ρ is in systems above N = 1021 cm-2. We derive a value ρ = 1.1 × 107 h70 M☉ Mpc-3, which is ≈25% the mass density of atomic hydrogen. Yet the redshift number density of H2 above this N limit is only ≈ 3 × 10-4, a factor 150 lower than that for H I in DLAs at z = 0. Furthermore, we show that the median impact parameter between a N > 1021 cm-2 absorber and the center of the galaxy hosting the H2 gas is only 2.5 kpc. On the basis of arguments related to the Schmidt law, we argue that H2 gas above this column density limit is associated with a large fraction of the integral star formation rate density. Even allowing for an increased molecular mass density at higher redshifts, the derived cross sections indicate that it is very unlikely to identify the bulk of the molecular gas in present quasar absorption lines samples. We discuss the prospects for identifying this molecular mass in future surveys.

Large‐Scale CO Observations of a Far‐Infrared Loop in Pegasus: Detection of a Large Number of Very Small Molecular Clouds Possibly Formed via Shocks

We have performed large-scale CO observations with the millimeter/submillimeter telescope NANTEN toward a FIR looplike structure whose angular extent is ~20° × 20° around (l,b) ~ (109°, - 45°) in Pegasus. Its diameter corresponds to ~25 pc at a distance of 100 pc, adopted from that of the star HD 886 (B2 IV) near the center of the loop. We covered the looplike structure in the 12CO (J = 1-0) emission at 4'-8' grid spacing and in the 13CO (J = 1-0) emission at 2' grid spacing for the 12CO-emitting regions. The 12CO distribution is found to consist of 78 small clumpy clouds of mass 0.04-11 M☉, of which ~83% have very small masses <1.0 M☉. 13CO observations revealed significant 13CO emission in 18 of the 78 12CO clouds. 13CO emission was detected in the region where the column density of H2 derived from 12CO is >5 × 1020 cm-2, corresponding to AV ~ 1 mag, which takes into account that of H I. We find no indication of star formation in these clouds in the IRAS and 2MASS point source catalogs. The very low mass clouds, M ≤ 1 M☉, identified are unusual in that they have a very weak 12CO peak temperature of 0.5-2.7 K and that they aggregate in a region of a few parsecs with no main massive clouds; in contrast, similar low-mass clouds <1 M☉ in other regions previously observed including those at high Galactic latitude are all associated with more massive main clouds of ~100 M☉. Comparison with a theoretical work on molecular cloud formation suggests that the very low mass clouds may have been formed in the shocked layer through thermal instability. HD 886 may be the source of the mechanical luminosity via stellar winds to create shocks, forming the looplike structure where the very low mass clouds are embedded.

Abundances in the Neutral Interstellar Medium of I Zw 18 fromFar Ultraviolet Spectroscopic ExplorerObservations

We report on new FUSE far-UV spectroscopy of the most metal-poor blue compact dwarf galaxy, I Zw 18. The new data represent an improvement over previous FUSE spectra by a factor of 1.7 in the signal-to-noise ratio. Together with a larger spectral coverage (λλ = 917-1188 A), this allows us to characterize absorption lines in the interstellar medium with unprecedented accuracy. The kinematics averaged over the large sampled region shows no clear evidence of gas inflows or outflows. The H I absorption is interstellar with a column density of 2.2 × 1021 cm-2. A conservative 3 σ upper limit of 5.25 × 1014 cm-2 is derived for the column density of diffuse H2. From a simultaneous fitting of metal absorption lines in the interstellar medium, we infer the following abundances: [Fe/H] = -1.76 ± 0.12, [O/H] = -2.06 ± 0.28, [Si/H] = -2.09 ± 0.12, [Ar/H] = -2.27 ± 0.13, and [N/H] = -2.88 ± 0.11. This is in general several times lower than in the H II regions. The only exception is iron, whose abundance is the same. The abundance pattern of the interstellar medium suggests ancient star formation activity with an age of at least 1 Gyr that enriched the H I phase. Around 470 SNe Ia are required to produce the iron content. A more recent episode that started 10 to several hundred megayears ago is responsible for the additional enrichment of α-elements and nitrogen in the H II regions.

CO Molecular Gas in Infrared‐luminous Galaxies

We present the first statistical survey of the properties of the 12CO(1-0) and 12CO(3-2) line emission from the nuclei of a nearly complete subsample of 60 infrared (IR) luminous galaxies selected from SCUBA Local Universe Galaxy Survey (SLUGS). This subsample is flux limited at S60 ?m ? 5.24 Jy with far-IR (FIR) luminosities mostly at LFIR > 1010 L?. We compare the emission line strengths of 12CO(1-0) and (3-2) transitions at a common resolution of ~15''. The measured 12CO(3-2) to (1-0) line intensity ratios r31 vary from 0.22 to 1.72, with a mean value of 0.66 for the sources observed, indicating a large spread of the degree of excitation of CO in the sample. These CO data, together with a wide range of data at different wavelengths obtained from the literature, allow us to study the relationship between the CO excitation conditions and the physical properties of gas/dust and star formation in the central regions of galaxies. Our analysis shows that there is a nonlinear relation between CO and FIR luminosities, such that their ratio LCO/LFIR decreases linearly with increasing LFIR. This behavior was found to be consistent with the Schmidt law relating star formation rate to molecular gas content, with an index N = 1.4 ? 0.3. We also find a possible dependence of the degree of CO gas excitation on the efficiency of star-forming activity. Using the large velocity gradient (LVG) approximation to model the observed data, we investigate the CO-to-H2 conversion factor X for the SLUGS sample. The results show that the mean value of X for the SLUGS sample is lower by a factor of 10 compared to the conventional value derived for the Galaxy, if we assume the abundance of CO relative to H2, ZCO = 10-4. For a subset of 12 galaxies with H I maps, we derive a mean total face-on surface density of H2 + H of about 42 M? pc-2 within about 2 kpc of the nucleus. This value is intermediate between that in galaxies like our own and those with strong star formation.

The Cosmic Density of Massive Black Holes from Galaxy Velocity Dispersions

Supermassive black holes are thought to be relics of quasars, and their numbers and masses are therefore related to the quasar luminosity function and its evolution with redshift. We have used the relationship between black hole mass and bulge velocity dispersion (the •-σ relation) to make an improved estimate of the mass density and mass spectrum of supermassive black holes. Uncertainties in the •-σ relation have little effect on the mass density. We find a mass density of (4.8 ± 1.6)h2 × 105 M⊙ Mpc-3. Some of the variance in published density estimates comes from the use of different values of the Hubble constant.