Mean Gas Fraction Research Articles

A3COSMOS: Dissecting the gas content of star-forming galaxies across the main sequence at 1.2 ≤ z < 1.6

Aims. We aim to understand the physical mechanisms that drive star formation in a sample of mass-complete (>109.5 M⊙) star-forming galaxies (SFGs) at 1.2 ≤ ɀ < 1.6. Methods. We selected SFGs from the COSMOS2020 catalog and applied a uυ-domain stacking analysis to their archival Atacama Large Millimeter/submillimeter Array (ALMA) data. Our stacking analysis provides precise measurements of the mean molecular gas mass and size of SFGs down to a stellar mass of M★ ~ 109.5 M⊙, even though at these stellar mass galaxies on the main sequence (MS) are no longer detected individually in the archival ALMA data. We also applied an image-domain stacking analysis on their HST i-band and UltraVISTA J - and Ks-band images. This allowed us to trace the distribution of their stellar component. Correcting these rest-frame optical sizes using the Rhalf–stellar–light-to-Rhalf–stellar–mass conversion at rest 5000 Å, we obtain the stellar mass size of MS galaxies and compare them to the sizes of their star-forming component obtained from our ALMA stacking analysis. Results. Across the MS (−0.2 < ∆MS = log(SFR/SFRMS) < 0.2), the mean molecular gas fraction of SFGs increases by a factor of ~1.4, while their mean molecular gas depletion time decreases by a factor of ~1.8. The scatter of the MS could thus be caused by variations in both the star formation efficiency and molecular gas fraction of galaxies. The mean molecular gas fraction of MS galaxies decreases by a factor of ~7 from M★~ 109.7 M⊙ to ~1011.3 M⊙, while their mean molecular gas depletion time remains roughly the same at all stellar masses. This finding could be a hint that the bending of the MS at ɀ ~1.4 is primarily driven by variations in cold gas accretion. The majority of the galaxies lying on the MS have RFIR ≈ Rstellar. Their central regions are subject to large dust attenuation. Starbursts (SBs, ∆MS > 0.7) have a mean molecular gas fraction ~2.1 times larger and mean molecular gas depletion time ~3.3 times shorter than MS galaxies. Additionally, they have more compact star-forming regions (~2.5 kpc for MS galaxies vs. ~1.4 kpc for SBs) and systematically disturbed rest-frame optical morphologies, which is consistent with their association with major-mergers. SBs and MS galaxies follow the same relation between their molecular gas mass and star formation rate surface densities with a slope of ~ 1.1–1.2, that is, the so-called Kennicutt-Schmidt relation.

Revisited Cold Gas Content with Atomic Carbon [C i] in z = 2.5 Protocluster Galaxies

Abstract We revisit the cold gas contents of galaxies in a protocluster at z = 2.49 using the lowest neutral atomic carbon transition [C i] 3 P 1 − 3 P 0 from Atacama Large Millimeter/submillimeter Array observations. We aim to test if the same gas-mass calibration adopted in field galaxies can be applied to protocluster galaxies. Five galaxies out of 16 targeted galaxies are detected in the [C i] line, and these are all previously detected in CO (3–2) and CO (4–3) and three in 1.1 mm dust continuum. We investigate the line luminosity relations between CO and [C i] in the protocluster and compare with other previous studies. We then compare the gas mass based on three gas tracers of [C i], CO(3–2), and dust if at least one of the last two tracers are available. Using the calibration adopted for field main-sequence galaxies, the [C i]-based gas measurements are lower than or comparable to the CO-based gas measurements by −0.35 dex at the lowest with the mean deviation of −0.14 dex. The differences between [C i]- and the dust- based measurements are relatively mild by up to 0.16 dex with the mean difference of 0.02 dex. Taking these all together with calibration uncertainties, with the [C i] line, we reconfirm our previous findings that the mean gas fraction is comparable to field galaxies for a stellar-mass range of . However, at least for these secure five detections, the depletion timescale decreases more rapidly with stellar mass than field galaxies that might be related to earlier quenching in dense environments.

The role of molecular gas in galaxy transition in compact groups

Compact groups (CGs) provide an environment in which interactions between galaxies and with the intra-group medium enable and accelerate galaxy transitions from actively star forming to quiescent. Galaxies in transition from active to quiescent can be selected, by their infrared (IR) colors, as canyon or infrared transition zone (IRTZ) galaxies. We used a sample of CG galaxies with IR data from the Wide Field Infrared Survey Explorer (WISE) allowing us to calculate the stellar mass and star formation rate (SFR) for each galaxy. Furthermore, we present new CO(1-0) data for 27 galaxies and collect data from the literature to calculate the molecular gas mass for a total sample of 130 galaxies. This data set allows us to study the difference in the molecular gas fraction (Mmol/Mstar) and star formation efficiency (SFE=SFR/Mmol) between active, quiescent, and transitioning (i.e., canyon and IRTZ) galaxies. We find that transitioning galaxies have a mean molecular gas fraction and a mean SFE that are significantly lower than those of actively star-forming galaxies. The molecular gas fraction is higher than that of quiescent galaxies, whereas the SFE is similar. These results indicate that the transition from actively star-forming to quiescent in CG galaxies goes along with a loss of molecular gas, possibly due to tidal forces exerted from the neighboring galaxies or a decrease in the gas density. In addition, the remaining molecular gas loses its ability to form stars efficiently, possibly owing to turbulence perturbing the gas, as seen in other, well-studied examples such as Stephan's Quintet and HCG~57. Thus, the amount and properties of molecular gas play a crucial role in the environmentally driven transition of galaxies from actively star forming to quiescent.

Intrinsic scatter of caustic masses and hydrostatic bias: An observational study

All estimates of cluster mass have some intrinsic scatter and perhaps some bias with true mass even in the absence of measurement errors for example caused by cluster triaxiality and large scale structure. Knowledge of the bias and scatter values is fundamental for both cluster cosmology and astrophysics. In this paper we show that the intrinsic scatter of a mass proxy can be constrained by measurements of the gas fraction because masses with higher values of intrinsic scatter with true mass produce more scattered gas fractions. Moreover, the relative bias of two mass estimates can be constrained by comparing the mean gas fraction at the same (nominal) cluster mass. Our observational study addresses the scatter between caustic (i.e., dynamically estimated) and true masses, and the relative bias of caustic and hydrostatic masses. For these purposes, we used the X-ray Unbiased Cluster Sample, a cluster sample selected independently from the intracluster medium content with reliable masses: 34 galaxy clusters in the nearby ($0.050<z<0.135$) Universe, mostly with $14<\log M_{500}/M_\odot \lesssim 14.5$, and with caustic masses. We found a 35\% scatter between caustic and true masses. Furthermore, we found that the relative bias between caustic and hydrostatic masses is small, $0.06\pm0.05$ dex, improving upon past measurements. The small scatter found confirms our previous measurements of a highly variable amount of feedback from cluster to cluster, which is the cause of the observed large variety of core-excised X-ray luminosities and gas masses.

Looking for dark matter trails in colliding galaxy clusters

If dark matter interacts, even weakly, via non-gravitational forces, simulations predict that it will be preferentially scattered towards the trailing edge of the halo during collisions between galaxy clusters. This will temporarily create a non-symmetric mass profile, with a trailing over-density along the direction of motion. To test this hypothesis, we fit (and subtract) symmetric halos to the weak gravitational data of 72 merging galaxy clusters observed with the Hubble Space Telescope. We convert the shear directly into excess {\kappa} and project in to a one dimensional profile. We generate numerical simulations and find that the one dimensional profile is well described with simple Gaussian approximations. We detect the weak lensing signal of trailing gas at a 4{\sigma} confidence, finding a mean gas fraction of Mgas/Mdm = 0.13 +/- 0.035. We find no evidence for scattered dark matter particles with a estimated scattering fraction of f = 0.03 +/- 0.05. Finally we find that if we can reduce the statistical error on the positional estimate of a single dark matter halo to <2.5", then we will be able to detect a scattering fraction of 10% at the 3{\sigma} level with current surveys. This poten- tially interesting new method can provide an important independent test for other complimentary studies of the self-interaction cross-section of dark matter.

ALLSMOG: an APEX Low-redshift Legacy Survey for MOlecular Gas – I. Molecular gas scaling relations, and the effect of the CO/H2 conversion factor

We present ALLSMOG, the APEX Low-redshift Legacy Survey for MOlecular Gas. ALLSMOG is a survey designed to observe the CO(2-1) emission line with the APEX telescope, in a sample of local galaxies (0.01 < z < 0.03), with stellar masses in the range 8.5 < log(M*/Msun) < 10. This paper is a data release and initial analysis of the first two semesters of observations, consisting of 42 galaxies observed in CO(2-1). By combining these new CO(2-1) emission line data with archival HI data and SDSS optical spectroscopy, we compile a sample of low-mass galaxies with well defined molecular gas masses, atomic gas masses, and gas-phase metallicities. We explore scaling relations of gas fraction and gas consumption timescale, and test the extent to which our findings are dependent on a varying CO/H2 conversion factor. We find an increase in the H2/HI mass ratio with stellar mass which closely matches semi-analytic predictions. We find a mean molecular gas fraction for ALLSMOG galaxies of MH2/M* = (0.09 - 0.13), which decreases with stellar mass. We measure a mean molecular gas consumption timescale for ALLSMOG galaxies of 0.4 - 0.7 Gyr. We also confirm the non-universality of the molecular gas consumption timescale, which varies (with stellar mass) from ~100 Myr to ~2 Gyr. Importantly, we find that the trends in the H2/HI mass ratio, gas fraction, and the non-universal molecular gas consumption timescale are all robust to a range of recent metallicity-dependent CO/H2 conversion factors.

A decrease of the gas exchanges between galaxies and the intergalactic medium, from 12 to 6 billion years ago

Using a representative sample of 65 intermediate mass galaxies at z \sim 0.6, we have inves- tigated the interplay between the main ingredients of chemical evolution: metal abundance, gas mass, stellar mass and SFR. All quantities have been estimated using deep spectroscopy and photometry from UV to IR and assuming an inversion of the Schmitt-Kennicutt law for the gas fraction. Six billion years ago, galaxies had a mean gas fraction of 32% \pm 3, i.e. twice that of their local counterparts. Using higher redshift samples from the literature, we explore the gas-phases and estimate the evolution of the mean gas fraction of distant galaxies over the last 11 Gy. The gas fraction increases linearly at the rate of 4% per Gyr from z \sim 0 to z \sim 2.2. We also demonstrate for a statistically representative sample that < 4% of the z \sim 0.6 galaxies are undergoing outflow events, in sharp contrast with z \sim 2.2 galaxies. The observed co-evolution of metals and gas over the past 6 Gyr favours a scenario in which the population of intermediate mass galaxies evolved as closed-systems, converting their own gas reservoirs into stars.

COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - I. Relations between H2, H i, stellar content and structural properties

We are conducting COLD GASS, a legacy survey for molecular gas in nearby galaxies. Using the IRAM 30-m telescope, we measure the CO(1−0) line in a sample of ∼350 nearby (⁠ Mpc), massive galaxies (log(M*/M⊙) > 10.0). The sample is selected purely according to stellar mass, and therefore provides an unbiased view of molecular gas in these systems. By combining the IRAM data with Sloan Digital Sky Survey (SDSS) photometry and spectroscopy, GALEX imaging and high-quality Arecibo H i data, we investigate the partition of condensed baryons between stars, atomic gas and molecular gas in 0.1–10L* galaxies. In this paper, we present CO luminosities and molecular hydrogen masses for the first 222 galaxies. The overall CO detection rate is 54 per cent, but our survey also uncovers the existence of sharp thresholds in galaxy structural parameters such as stellar mass surface density and concentration index, below which all galaxies have a measurable cold gas component but above which the detection rate of the CO line drops suddenly. The mean molecular gas fraction of the CO detections is 0.066 ± 0.039, and this fraction does not depend on stellar mass, but is a strong function of (NUV − r) colour. Through stacking, we set a firm upper limit of for red galaxies with NUV − r > 5.0. The average molecular-to-atomic hydrogen ratio in present-day galaxies is 0.3, with significant scatter from one galaxy to the next. The existence of strong detection thresholds in both the H i and CO lines suggests that ‘quenching’ processes have occurred in these systems. Intriguingly, atomic gas strongly dominates in the minority of galaxies with significant cold gas that lie above these thresholds. This suggests that some re-accretion of gas may still be possible following the quenching event.

Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X‐Ray Observations fromChandraandXMM‐Newton

We present radial mass profiles within ~0.3rvir for 16 relaxed galaxy groups—poor clusters (kT range 1-3 keV) selected for optimal mass constraints from the Chandra and XMM-Newton data archives. After accounting for the mass of hot gas, the resulting mass profiles are described well by a two-component model consisting of dark matter, represented by an NFW model, and stars from the central galaxy. The stellar component is required only for eight systems, for which reasonable stellar mass-to-light ratios (M/LK) are obtained, assuming a Kroupa IMF. Modifying the NFW dark matter halo by adiabatic contraction does not improve the fit and yields systematically lower M/LK. In contrast to previous results for massive clusters, we find that the NFW concentration parameter (cvir) for groups decreases with increasing Mvir and is inconsistent with no variation at the 3 σ level. The normalization and slope of the cvir-Mvir relation are consistent with the standard ΛCDM cosmological model with σ8 = 0.9 (considering a 10% bias for early forming systems). The small intrinsic scatter measured about the cvir-Mvir relation implies that the groups represent preferentially relaxed, early forming systems. The mean gas fraction (f = 0.05 ± 0.01) of the groups measured within an overdensity Δ = 2500 is lower than for hot, massive clusters, but the fractional scatter (σf/f = 0.2) for groups is larger, implying a greater impact of feedback processes on groups, as expected.

The Stellar, Gas, and Dynamical Masses of Star‐forming Galaxies atz∼ 2

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z ~ 2. By combining the Hα spectra with photometric measurements from observed 0.3-8 μm, we assess the relationships among kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The Hα line widths give a mean dynamical mass Mdyn = (6.9 ± 0.6) × 1010 M☉ within a typical radius of ~6 kpc, after excluding AGNs. The average dynamical mass is ~2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, ~15% of the sample has Mdyn ≫ M⋆. These objects are best fit by young stellar populations and tend to have high Hα equivalent widths, WHα ≳ 200 Å, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 σ significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation and find a mean gas fraction of ~50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of 3 for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of ~500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of ~40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars.

06/01241 Experimental study of the resulting flow of plume-thermosiphon interaction: application to chimney problems: Zinoubi, J. et al. Applied Thermal Engineering, 2005, 25, (4), 533–544

This paper describes the mean gas fraction distribution in the two-phase flow of a gas–liquid bubble plume set to develop adjacent either to a wall or to another bubble plume. When this happens, the plume exhibits a type of Coanda effect, bending either towards the wall or the other plume. The local mean gas fraction measurements are carried out using the electro-resistivity probe technique in an air–water system. The deflection angle of the plumes is shown to present a dependence on the modified Weber and Froude numbers of the bubbles. A Gaussian distribution for the mean gas fraction profile, observed to exist for axisymmetric single plumes, is shown not to occur in flow geometries where the Coanda effect is allowed to set in. The transition zone between the downstream flow where two Gaussian plumes are observed and the far upstream flow where the plumes are seen to merge onto a single plume is characterized. Photographs of the flow are shown to illustrate the phenomenon. Results are present for two- and three-dimensional mean gas fraction distributions. In addition, a simple theory based on integral methods is advanced for the prediction of the plume deflection angle; the theory considers a variable entrainment co-efficient.

The Birmingham--CfA cluster scaling project -- I.Gas fraction and the M--TX relation

We have assembled a large sample of virialized systems, comprising 66 galaxy clusters, groups and elliptical galaxies with high-quality X-ray data.To each system we have fitted analytical profiles describing the gas density and temperature variation with radius, corrected for the effects of central gas cooling.We present an analysis of the scaling properties of these systems and focus in this paper on the gas distribution and M—TX relation.In addition to clusters and groups, our sample includes two early-type galaxies, carefully selected to avoid contamination from group or cluster X-ray emission.We compare the properties of these objects with those of more massive systems and find evidence for a systematic difference between galaxy-sized haloes and groups of a similar temperature. We derive a mean logarithmic slope of the M—TX relation within R200 of 1.84 ± 0.06, although there is some evidence of a gradual steepening in the M—TX relation, with decreasing mass.We recover a similar slope using two additional methods of calculating the mean temperature.Repeating the analysis with the assumption of isothermality, we find the slope changes only slightly, to 1.89 ± 0.04, but the normalization is increased by 30 per cent.Correspondingly, the mean gas fraction within R200 changes from (0.13 ± 0.01) h−3/270 to (0.11 ± 0.01) h−3/270, for the isothermal case, with the smaller fractional change reflecting different behaviour between hot and cool systems.There is a strong correlation between the gas fraction within 0.3R200 and temperature.This reflects the strong (5.8σ) trend between the gas density slope parameter, β, and temperature, which has been found in previous work. These findings are interpreted as evidence for self-similarity breaking from galaxy feedback processes, active galactic nuclei heating or possibly gas cooling.We discuss the implications of our results in the context of a hierarchical structure formation scenario.

Bubble plumes and the Coanda effect

This paper describes the mean gas fraction distribution in the two-phase flow of a gas–liquid bubble plume set to develop adjacent either to a wall or to another bubble plume. When this happens, the plume exhibits a type of Coanda effect, bending either towards the wall or the other plume. The local mean gas fraction measurements are carried out using the electro-resistivity probe technique in an air–water system. The deflection angle of the plumes is shown to present a dependence on the modified Weber and Froude numbers of the bubbles. A Gaussian distribution for the mean gas fraction profile, observed to exist for axisymmetric single plumes, is shown not to occur in flow geometries where the Coanda effect is allowed to set in. The transition zone between the downstream flow where two Gaussian plumes are observed and the far upstream flow where the plumes are seen to merge onto a single plume is characterized. Photographs of the flow are shown to illustrate the phenomenon. Results are present for two- and three-dimensional mean gas fraction distributions. In addition, a simple theory based on integral methods is advanced for the prediction of the plume deflection angle; the theory considers a variable entrainment co-efficient.

2D and 3D simulations of an internal airlift loop reactor on the basis of a two-fluid model

Two- and three-dimensional (2D and 3D) simulations of an airlift reactor under steady state conditions at low gas flow rates are presented. The simulations are based on a two-fluid model with a k– ε model for the turbulence and as little as possible ad hoc closure terms. The results are compared with an one-dimensional mechanical energy balance and are found to be in good agreement. The 2D results show sensitivity to the gas inlet geometry: whether or not the gas is partially sparged into the liquid directly next to a wall affects the liquid velocity distribution and thereby the gas disengagement at the top of the airlift. The three-dimensional calculations make a more realistic geometry possible. The friction in the system is found to be about a factor of two larger in the 3D case at the same gas inlet conditions. For a given gas flow rate, the mean gas fraction in the riser is the same for the 2D and 3D simulations, the liquid circulation rate is about 30% higher in the 2D case than in the 3D one. A comparison is made with experimental data obtained in an airlift of the same dimensions. The simulated overall gas fraction is in agreement with the experimental findings. The simulated superficial velocity in the riser is compared to LDA data. For the lowest superficial velocities the LDA data coincide with the results from the 2D simulations, for higher gas flow rates the LDA results switch over towards the 3D results.

The LX--T relation and intracluster gas fractions of X-ray clusters

We re-examine the X-ray luminosity–temperature relation using a nearly homogeneous data set of 24 clusters selected for statistically accurate temperature measurements and absence of strong cooling flows. The data exhibit a remarkably tight power-law relation between bolometric luminosity and temperature with a slope 2.88 ± 0.15. With reasonable assumptions regarding cluster structure, we infer an upper limit on fractional variations in the intracluster gas fraction 〈(δ fgas fgas)2〉1/2≤ 15 per cent. A strictly homogeneous Ginga subset of 18 clusters places a more stringent limit of 9 per cent. Imaging data from the literature are employed to determine absolute values of fgas within spheres encompassing density contrasts δc=500 and 200 with respect to the critical density. Comparing binding mass estimates based on the virial theorem (VT) and the hydrostatic β-model (BM), we find a temperature-dependent discrepancy in fgas between the two methods caused by systematic variation of the outer slope parameter β with temperature. Mean values (for H0=50 km s−1 Mpc−1) range from f¯gas=0.10 for cool (T 4 keV) clusters using the BM at δc=200. There is evidence that cool clusters have a lower mean gas fraction than hot clusters, but it is not possible to assess the statistical significance of this effect in the present data set. The T dependence of the intracluster medium (ICM) density structure, coupled with the increase of the gas fraction with T in the VT approach, explains the steepening of the LX–T relation. The small variation about the mean gas fraction within this majority subpopulation of clusters presents an important constraint for theories of galaxy formation and supports arguments against an Einstein–de Sitter universe based on the population mean gas fraction and conventional, primordial nucleosynthesis. The apparent trend of lower gas fractions and more extended atmospheres in low-temperature systems is consistent with expectations of models incorporating the effects of galactic winds on the ICM.

A new ventilation inhomogeneity index from multiple breath indicator gas washout tests in mechanically ventilated patients

a) To determine the validity of a new method to analyze indicator gas washout tests on mechanically ventilated patients. This method takes into account the difference between the end-expiratory gas fraction and the mean gas fraction in the lung and provides the end-expiratory lung volume and a new index of ventilation inhomogeneity called volumes regression index. b) To determine the validity of this index as a predictor of chronic obstructive pulmonary disease. c) To compare this index with the moment ratio index and Becklake index. Prospective study of diagnostic test. Criterium standards: Closed-circuit indicator gas dilution technique and Tiffeneau index. Surgical intensive care unit of a university hospital. A total of 38 mechanically ventilated postoperative patients, divided into two groups: the obstructive group (n = 21) and the nonobstructive group (n = 17), based on their preoperative lung function. None. a) The mean coefficient of variation of all lung volume measurements in a group of nine healthy volunteers was 5%, and the difference between this technique and the closed-circuit helium dilution measurements was -2 +/- 5%. In patients, the mean coefficient of variation of the lung volume measurements was 3.5%. The volumes regression index was measured as 0.02 +/- 0.04 in a dummy lung, 0.37 +/- 0.08 in the healthy volunteers, 0.64 +/- 0.23 in the nonobstructive patients, and 1.1 +/- 0.3 in the obstructive patients. The volumes regression index provided a better correlation (r2 = .46) with preoperatively determined Tiffeneau index than the Becklake index (r2 = .11) or the moment ratio index (r2 = .18). The proposed technique provides a means for accurate measurement of the end-expiratory lung volume and the amount of ventilation inhomogeneity in mechanically ventilated intensive care unit patients.