Low Gas Fractions Research Articles

On the origin of star formation quenching in massive galaxies at ≳ in the cosmological simulations IllustrisTNG

ABSTRACT Using the cosmological simulations IllustrisTNG, we perform a comprehensive analysis of quiescent, massive galaxies at $z \gtrsim 3$. The goal is to understand what suppresses their star formation so early in cosmic time, and how other similar mass galaxies remain highly star forming. As a first-order result, the simulations are able to produce massive, quiescent galaxies in this high-redshift regime. We find that active galactic nucleus (AGN) feedback is the primary cause of halting star formation in early, massive galaxies. Not only do the central, supermassive black holes (SMBHs) of the quenched galaxies have earlier seed times, but they also grow faster than in star-forming galaxies. As a result, the quenched galaxies are exposed to AGN feedback for longer, and experience the kinetic, jet mode of the AGN feedback earlier than the star-forming galaxies. The release of kinetic energy reduces inflows of gas while likely maintaining outflows, which keeps a low cold gas fraction and decreases the star formation of the galaxies down to a state of quiescence. In addition to AGN feedback, we also investigate the influence of the large-scale environment. While mergers do not play a significant role in the quenching process, the quenched galaxies tend to reside in more massive haloes and denser regions during their evolution. As this provides a greater initial amount of infalling gas to the galaxies, the large-scale environment can mildly affect the fate of the central SMBH growth and, via AGN feedback, contribute to star formation quenching.

The HYENAS project: a prediction for the X-ray undetected galaxy groups

Abstract Galaxy groups contain the majority of bound mass with a significant portion of baryons due to the combination of halo mass and abundance (Cui 2024). Hence they serve as a crucial missing piece in the puzzle of galaxy formation and the evolution of large-scale structures in the Universe. In observations, mass-complete group catalogues are normally derived from galaxy redshift surveys detected through various three-dimensional group-finding algorithms. Confirming the reality of such groups, particularly in the X-rays, is critical for ensuring robust studies of galaxy evolution in these environments. Recent works have reported numerous optical groups that are X-ray undetected (see, e.g. Popesso et al. 2024), sparking debates regarding the reasons for the unexpectedly low hot gas fraction in galaxy groups. To address this issue, we utilise zoomed-in simulations of galaxy groups from the novel Hyenas project to explore the range of hot gas fractions within galaxy groups and investigate the intrinsic factors behind the observed variability in X-ray emission. We find that the halo formation time can play a critical role – we see that groups in halos that formed earlier exhibit up to an order of magnitude brighter X-ray luminosities compared to those formed later. This suggests that undetected X-ray groups are preferentially late-formed halos and highlights the connection between gas fraction and halo formation time in galaxy groups. Accounting for these biases in galaxy group identification is essential for advancing our understanding of galaxy formation and achieving precision in cosmological studies.

The FLAMINGO project: Galaxy clusters in comparison to X-ray observations

Abstract Galaxy clusters are important probes for both cosmology and galaxy formation physics. We test the cosmological, hydrodynamical FLAMINGO simulations by comparing to observations of the gaseous properties of clusters measured from X-ray observations. FLAMINGO contains unprecedented numbers of massive galaxy groups (>106) and clusters (>105) and includes variations in both cosmology and galaxy formation physics. We predict the evolution of cluster scaling relations as well as radial profiles of the temperature, density, pressure, entropy, and metallicity for different masses and redshifts. We show that the differences between volume-, and X-ray-weighting of particles in the simulations, and between cool-core non cool-core samples, are similar in size as the differences between simulations for which the stellar and AGN feedback has been calibrated to produce significantly different gas fractions. Compared to thermally-driven AGN feedback, kinetic jet feedback calibrated to produce the same gas fraction at R500c yields a hotter core with higher entropies and lower densities, which translates into a smaller fraction of cool-core clusters. Stronger feedback, calibrated to produce lower gas fractions and hence lower gas densities, results in higher temperatures, entropies, and metallicities, but lower pressures. The scaling relations and thermodynamic profiles show almost no evolution with respect to self-similar expectations, except for the metallicity decreasing with redshift. We find that the temperature, density, pressure, and entropy profiles of clusters in the fiducial FLAMINGO simulation are in excellent agreement with observations, while the metallicities in the core are too high.

Feedback-free starbursts at cosmic dawn. Observable predictions for JWST

We extend the analysis of a physical model within the standard cosmology that robustly predicts a high star-formation efficiency (SFE) in massive galaxies at cosmic dawn due to feedback-free starbursts (FFBs). This model implies an excess of bright galaxies at $z 10$ compared to the standard models based on the low SFE at later epochs, an excess that is indicated by JWST observations. Here we provide observable predictions of galaxy properties based on the analytic FFB scenario. These can be compared with simulations and JWST observations. We use the model to approximate the SFE as a function of redshift and mass, assuming a maximum SFE of $ max 1$ in the FFB regime. From this, we derive the evolution of the galaxy mass and luminosity functions as well as the cosmological evolution of stellar and star-formation densities. We then predict the star-formation history (SFH), galaxy sizes, outflows, gas fractions, metallicities, and dust attenuation, all as functions of mass and redshift in the FFB regime. The major distinguishing feature of the model is the occurrence of FFBs above a mass threshold that declines with redshift. The luminosities and star formation rates in bright galaxies are predicted to be in excess of extrapolations of standard empirical models and standard cosmological simulations, an excess that grows from $z 9$ to higher redshifts. The FFB phase of $ is predicted to show a characteristic SFH that fluctuates on a timescale of $ The stellar systems are compact ($ at $z 10$ and declining with $z$). The galactic gas consists of a steady wind driven by supernovae from earlier generations, with high outflow velocities FWHM low gas fractions ($<\!0.1$), low metallicities ($ and low dust attenuation UV 0.5$ at $z 10$ and declining with $z$). We make tentative comparisons with current JWST observations for initial insights, anticipating more complete and reliable datasets for detailed quantitative comparisons in the future. The FFB predictions are also offered in digital form.

Star Formation Rates in [Ne V] 3426 Å Selected Active Galactic Nuclei: Evidence for a Decrease along the Main Sequence?

Studying the behavior along the galaxy main sequence is key in furthering our understanding of the possible connection between active galactic nucleus (AGN) activity and star formation. We select a sample of 1215 AGN from the catalog of Sloan Digital Sky Survey galaxy properties from the Portsmouth group by detection of the high-ionization [Ne v] 3426 Å emission line. Our sample extends from 1040 to 1042.5 erg s−1 in [Ne v] luminosity in a redshift range z = 0.17 to 0.57. We compare the specific star formation rates (sSFRs; SFR scaled by galaxy mass) obtained from the corrected [O ii] and Hα luminosities, and the spectral energy distribution (SED)–determined values from Portsmouth. We find that the emission-line-based sSFR values are unreliable for the [Ne v] sample due to the AGN contribution, and proceed with the SED sSFRs for our study of the main sequence. We find evidence for a decrease in sSFR along the main sequence in the [Ne v] sample, which is consistent with results from the hard X-ray Burst Alert Telescope AGN sample, which extends to lower redshifts than our [Ne v] sample. Although we do not find evidence that the concurrent AGN activity is suppressing star formation, our results are consistent with a lower gas fraction in the host galaxies of the AGN as compared to that of the star-forming galaxies. If the evacuation of gas, and therefore suppression of star formation, is due to AGN activity, it must have occurred in a previous epoch.

Observed abundance of X-ray low surface brightness clusters in optical, X-ray, and SZ selected samples

The comparison of the properties of galaxy cluster samples selected using observations in different wavebands may shed light on potential biases of the way in which the samples are assembled. For this comparison, we introduce a new observable that does not require previous knowledge of the cluster mass: the X-ray mean surface brightness within the central 300 kpc. We found that clusters with low surface brightness, defined as those with a mean surface brightness below 43.35 erg s−1 Mpc−2, are about one quarter of the whole cluster population in a sample of 32 clusters in the nearby Universe, selected independently of the intracluster medium properties. Almost no example of a low central surface brightness cluster exists instead in two X-ray selected samples, one sample based on XMM-Newton XXL-100 survey data and the other on full-depth eROSITA eFEDS data, although these clusters are known to exist in the range of redshift and mass as probed by these two surveys. Furthermore, the Sunayev–Zeldovich Atacama Cosmology Telescope cluster survey is even more selective than the previous two samples because it does not even include clusters with intermediate surface brightness, which are instead present in X-ray selected samples that explore the same volume of the Universe. Finally, a measure of the mean surface brightness, which is obtained without knowledge of the mass, proves to be effective in narrowing the number of clusters to be followed-up because it recognizes those with a low gas fraction or with a low X-ray luminosity for their mass. Identifying these would otherwise require knowledge of the mass for all clusters.

Analysis of two-phase gas-liquid flow in an Electric Submersible Pump using A CFD approach

This study comprehensively analyzes a mixed-flow Electric Submersible Pump's (ESP) performance under challenging conditions of low flow rates and high gas fractions. The research leverages a computational model built on the Eulerian-Eulerian methodology, adeptly solving the 3D incompressible Navier-Stokes equations for each phase. To ensure an encompassing depiction of the flow dynamics within the ESP, the model considers critical parameters such as the bubble diameter and interphase forces, including drag, lift, and virtual mass force.Key findings reveal that the ESP's performance deteriorates markedly under the studied conditions. One of the primary influencers is the phenomenon of gas locking, where gas accumulation within the impeller hampers the consistent flow of liquid, thereby degrading the pump's efficiency. In addition, the model delineated the occurrence of the surging phenomenon across diverse gas volume fractions and rotational speeds, which further exacerbates the reduction in the ESP's performance. Intriguingly, the bubble diameter was identified as a decisive factor, especially at minuscule flow rates. Large bubble diameters instigate the formation of gas pockets on the suction side of the vane, culminating in a segregated flow pattern and further declining pump efficiency.On the other hand, the drag force is the primary interaction force between phases of gas-liquid flow. Other interfacial forces, such as lift and virtual mass force, change the pump's performance but do not affect the gas flow and distribution patterns for a one-stage configuration. Results show that the Schiller-Neuman drag coefficient offers a better fit than the Bozano-Dente coefficient.Conclusively, the study underscores the indispensable role of nuanced computational models in elucidating the intricate flow dynamics within ESPs, especially under conditions fraught with operational challenges. The insights gleaned enrich our understanding of ESP performance dynamics and provide a roadmap for designing more resilient and efficient pumping systems in the future.

Exploring the angular momentum – atomic gas content connection with eagle and IllustrisTNG

ABSTRACT We use the Evolution and Assembly of GaLaxies and their Environments (eagle) and IllustrisTNG (The Next Generation) cosmological simulations to investigate the properties of the baryonic specific angular momentum (j), baryonic mass (M), and atomic gas fraction (fatm) plane for nearby galaxies. We find EAGLE and TNG to be in excellent agreement with each other. These simulations are also consistent with the results obtained with eXtended GALEX Arecibo SDSS Survey (xGASS) for gas fractions greater than 0.01. This implies that the disagreements previously identified between xGASS and predictions from simple analytical disc stability arguments also holds true for eagle and tng. For lower gas fraction (the regime currently unconstrained by observations), both simulations deviate from the plane but still maintain good agreement with each other. Despite the challenges posed by resolution limits at low gas fractions, our findings suggest a potential disconnect between angular momentum and gas fraction in the gas-poor regime, implying that not all gas-poor galaxies have low specific angular momentum.

The profiles of bars in spiral galaxies

ABSTRACT We present an analysis of major-axis surface-brightness profiles of bars in a volume-limited sample of 182 barred spiral galaxies, using Spitzer 3.6 $\mu$m images. Unlike most previous studies, we use the entire bar profile, and we classify profiles into four categories. These are ‘Peak+Shoulders’ (P+Sh) – updating the classic ‘flat bar’ profile – and three subtypes of the classic ‘exponential’ profile: (true) Exponential, ‘Two-Slope’ (shallow inner slope + steeper outer slope), and ‘Flat-Top’ (constant inner region, steep outer slope). P+Sh profiles are preferentially found in galaxies with high stellar masses, early Hubble types, red colours, and low gas fractions; the most significant factor is stellar mass, and previous correlations with Hubble type can be explained by the tendency of higher-mass galaxies to have earlier Hubble types. The most common type of non-P+Sh profile is Exponential, followed by Flat-Top profiles; all non-P+Sh profiles appear to have similar distributions of stellar mass, Hubble type, colour, and gas fraction. We also morphologically classify the bars of an inclined subsample into those with and without boxy/peanut-shaped (B/P) bulges; as previously reported, the presence of a B/P bulge is very strong function of stellar mass. Essentially all bars with B/P bulges have P+Sh profiles; we associate the profile shoulders with the outer, vertically thin part of the bar. We find a small number of P+Sh profiles in bars without clear B/P bulges, which may indicate that P+Sh formation precedes the formation of B/P bulges.

An analytical model for the velocity and gas fraction profiles near gas-evolving electrodes

Understanding multiphase flow close to the electrode surface is crucial to the design of electrolyzers, such as alkaline water electrolyzers for the production of green hydrogen. Vertical electrodes develop a narrow gas plume near their surface. We apply the integral method to the mixture model. Considering both exponentially varying and step-function gas fraction profiles, we derive analytical relations for plume thickness, velocity profile, and gas fraction near the electrode as a function of height and current density. We verify these analytical relations with the numerical solutions obtained using two-dimensional mixture model simulations. We find that for low gas fractions, the plume thickness decreases with an increase in current density for an exponentially varying gas fraction profile. In contrast, the plume thickness increases with increasing current density at high gas fractions for an approximately step-function-shaped gas fraction profile, in agreement with experiments from the literature.

The hidden side of cosmic star formation at z > 3

Our current understanding of the cosmic star formation history at z > 3 is primarily based on UV-selected galaxies (Lyman-break galaxies, i.e., LBGs). Recent studies of H-dropouts (HST-dark galaxies) have revealed that we may be missing a large proportion of star formation that is taking place in massive galaxies at z > 3. In this work, we extend the H-dropout criterion to lower masses to select optically dark or faint galaxies (OFGs) at high redshifts in order to complete the census between LBGs and H-dropouts. Our criterion (H > 26.5 mag & [4.5] < 25 mag) combined with a de-blending technique is designed to select not only extremely dust-obscured massive galaxies but also normal star-forming galaxies (typically E(B − V) > 0.4) with lower stellar masses at high redshifts. In addition, with this criterion, our sample is not contaminated by massive passive or old galaxies. In total, we identified 27 OFGs at zphot > 3 (with a median of zmed = 4.1) in the GOODS-ALMA field, covering a wide distribution of stellar masses with log(M⋆/M⊙) = 9.4 − 11.1 (with a median of log(M⋆med/M⊙) = 10.3). We find that up to 75% of the OFGs with log(M⋆/M⊙) = 9.5 − 10.5 were neglected by previous LBGs and H-dropout selection techniques. After performing an optical-to-millimeter stacking analysis of the OFGs, we find that rather than being limited to a rare population of extreme starbursts, these OFGs represent a normal population of dusty star-forming galaxies at z > 3. The OFGs exhibit shorter gas depletion timescales, slightly lower gas fractions, and lower dust temperatures than the scaling relation of typical star-forming galaxies. Additionally, the total star formation rate (SFRtot = SFRIR + SFRUV) of the stacked OFGs is much higher than the SFRUVcorr (SFRUV corrected for dust extinction), with an average SFRtot/SFRUVcorr = 8 ± 1, which lies above (∼0.3 dex) the 16–84th percentile range of typical star-forming galaxies at 3 ≤ z ≤ 6. All of the above suggests the presence of hidden dust regions in the OFGs that absorb all UV photons, which cannot be reproduced with dust extinction corrections. The effective radius of the average dust size measured by a circular Gaussian model fit in the uv plane is Re(1.13 mm) = 1.01 ± 0.05 kpc. After excluding the five LBGs in the OFG sample, we investigated their contributions to the cosmic star formation rate density (SFRD). We found that the SFRD at z > 3 contributed by massive OFGs (log(M⋆/M⊙) > 10.3) is at least two orders of magnitude higher than the one contributed by equivalently massive LBGs. Finally, we calculated the combined contribution of OFGs and LBGs to the cosmic SFRD at z = 4 − 5 to be 4 × 10−2 M⊙ yr−1 Mpc−3, which is about 0.15 dex (43%) higher than the SFRD derived from UV-selected samples alone at the same redshift. This value could be even larger, as our calculations were performed in a very conservative way.

Sparging-based fission gas separation technology for molten salt reactor

Thorium molten salt reactor is one of the GEN-IV Molten Salt Reactors currently developed in China. The online removal of fission gases like Xenon and Krypton is a crucial technique to achieve high burnup of the liquid nuclear fuel. One of the promising candidates for practical application is the bubble sparging method, where tiny bubbles are first introduced into the liquid fuel, then the fission gases migrate to the carrier bubbles by interfacial mass transfer, and finally, a gas–liquid separator is used to remove the carrier bubbles along with the absorbed fission gases from the liquid. A thorough understanding of the mechanism and accurate modeling for the three sub-processes mentioned-above, namely bubble generation, mass transfer, and gas–liquid separation, are necessary for quantitative control of the fission gas removal performance.As for the bubble injection, the microbubble generating approach based on turbulent fragmentation mechanism is proposed, the predicting model of bubble diameter and the design criterion of bubble generator were established, constructing a generation and control technology of microbubbles for molten salt medium. For the mass transfer process, a novel numerical method simulating the interfacial migration of dissolved gases in bubbly flow is developed. Besides, a compact gas–liquid reactor with enhanced mass transfer efficiency applied to nuclear installations is proposed. In terms of the separation of bubbles, the separation length and stability of the swirling flow gas–liquid separator are investigated, and a solution of gas–liquid separation with ultra-high separation efficiency under extremely low gas fraction conditions is proposed. The feasibility and efficiency of the fission gas separation technology integrating all three procedures have been evaluated experimentally, providing a solid foundation for the future online post-processing of both off-gas and liquid fuel.

There and back again: Understanding the critical properties of backsplash galaxies

ABSTRACT Backsplash galaxies are galaxies that once resided inside a cluster, and have migrated back outside as they move towards the apocentre of their orbit. The kinematic properties of these galaxies are well understood, thanks to the significant study of backsplashers in dark matter-only simulations, but their intrinsic properties are not well-constrained due to modelling uncertainties in subgrid physics, ram pressure stripping, dynamical friction, and tidal forces. In this paper, we use the IllustrisTNG300-1 simulation, with a baryonic resolution of Mb ≈ 1.1 × 107 M⊙, to study backsplash galaxies around 1302 isolated galaxy clusters with mass 1013.0 < M200,mean/M⊙ < 1015.5. We employ a decision tree classifier to extract features of galaxies that make them likely to be backsplash galaxies, compared to nearby field galaxies, and find that backsplash galaxies have low gas fractions, high mass-to-light ratios, large stellar sizes, and low black hole occupation fractions. We investigate in detail the origins of these large sizes, and hypothesize their origins are linked to the tidal environments in the cluster. We show that the black hole recentring scheme employed in many cosmological simulations leads to the loss of black holes from galaxies accreted into clusters, and suggest improvements to these models. Generally, we find that backsplash galaxies are a useful population to test and understand numerical galaxy formation models due to their challenging environments and evolutionary pathways that interact with poorly constrained physics.

An experimental investigation of the stochastic nature of slug flow

Slug flow is widely encountered in oil and gas systems. It has an unsteady hydrodynamic behavior that motivated the development of mechanistic models to better understand and quantify the parameters of the flow. The present work investigated several slug flow conditions through an experimental campaign that was performed in a 2-in ID horizontal pipe facility. The experimental data were statistically analyzed and they indicated the suitability of a lognormal distribution to model the flow characteristics, such as slug length, film length, and frequency. Based on the mean results, slug and film lengths tend to increase as more gas is present in the flow. The mean frequency indicated that higher frequencies are obtained with lower gas fractions. All the slugs from each test were individually studied using different approaches to analyze the data. The results suggest the existence of a relationship between the slug and film lengths when the slug unit is identified starting from a film region followed by a slug region. The values of the coefficient of determination for the relation between slug length and film length indicate a possible influence of other flow parameters of stochastic nature, such as slug liquid holdup, that could affect the behavior of the slugs.

The interconnection between galaxy mergers, AGN activity, and rapid quenching of star formation in simulated post-merger galaxies

ABSTRACT We investigate the role of galaxy mergers on supermassive black hole (SMBH) accretion and star formation quenching in three state-of-the-art cosmological simulations with contrasting physics models: EAGLE, Illustris, and IllustrisTNG (TNG). We find that recently coalesced ‘post-mergers’ in all three simulations have elevated SMBH accretion rates by factors of ∼2–5. However, rapid (within 500 Myr of coalescence) quenching of star formation is rare, with incidence rates of $0.4{{\ \rm per\ cent}}$ in Illustris, $4.5{{\ \rm per\ cent}}$ in EAGLE, and $10{{\ \rm per\ cent}}$ in TNG. The rarity of quenching in post-mergers results from substantial gas reservoirs that remain intact after the merger. The post-mergers that do successfully quench tend to be those that had both low pre-merger gas fractions as well as those that experience the largest gas losses. Although rare, the recently quenched fraction of post-mergers is still elevated compared to a control sample of non-mergers by factors of two in TNG and 11 in EAGLE. Conversely, quenching is rarer in Illustris post-mergers than in their control. Recent observational results by Ellison et al. have found rapid quenching to be at least 30 times more common in post-mergers, a significantly higher excess than found in any of the simulations. Our results therefore indicate that whilst merger-induced SMBH accretion is a widespread prediction of the simulation, its link to quenching depends sensitively on the physics models, and that none of the subgrid models of the simulations studied here can fully capture the connection between mergers and rapid quenching seen in observations.

Formation of a barred galaxy in a major merger: The role of AGN feedback

Among the many processes involved in galaxy evolution, those of bar formation, quenching, and feedback from an active galactic nucleus (AGN) seem to be connected, however, the nature of these relations remains unclear. In this work, we aim to elucidate them by studying the formation of a barred galaxy in a major merger of two disks in the IllustrisTNG simulations. This merger involves a coalescence of two supermassive black holes and a sudden switch to the kinetic mode of AGN feedback implemented in the simulations, which leads to the removal of the gas from the inner part of the galaxy, followed by quenching of star formation and the formation of the bar. This causal relation between AGN feedback and bar formation explains a number of correlations observed in the data, such as the higher frequency of bars among red spirals and the presence of central gas holes in barred galaxies. In such a picture, the bars do not feed the black holes, so their presence does not increase the AGN strength, and they do not cause quenching. However, bars do form in regions characterized by a low gas fraction resulting from AGN feedback. This scenario is probably applicable to many barred galaxies, not only those formed in major mergers.

Simulation view of galaxy clusters with low X-ray surface brightness

Context. X-ray selected samples are known to miss galaxy clusters that are gas poor and have a low surface brightness. This is different for the optically selected samples such as the X-ray Unbiased Selected Sample (XUCS). Aims. We characterise the origin of galaxy clusters that are gas poor and have a low surface-brightness by studying covariances between various cluster properties at fixed mass using hydrodynamic cosmological simulations. Methods. We extracted ≈1800 galaxy clusters from a high-resolution Magneticum hydrodynamic cosmological simulation and computed covariances at fixed mass of the following properties: core-excised X-ray luminosity, gas fraction, hot gas temperature, formation redshift, matter density profile concentration, galaxy richness, fossilness parameter, and stellar mass of the bright central galaxy. We also compared the correlation between concentration and gas fractions in non-radiative simulations, and we followed the trajectories of particles inside galaxy clusters to assess the role of AGN depletion on the gas fraction. Results. In simulations and in observational data, differences in surface brightness are related to differences in gas fraction. Simulations show that the gas fraction strongly correlates with assembly time, in the sense that older clusters are gas poor. Clusters that formed earlier have lower gas fractions because the feedback of the active galactic nucleus ejected a significant amount of gas from the halo. When the X-ray luminosity is corrected for the gas fraction, it shows little or no covariance with other quantities. Conclusions. Older galaxy clusters tend to be gas poor and possess a low X-ray surface brightness because the feedback mechanism removes a significant fraction of gas from these objects. Moreover, we found that most of the LX covariance with the other quantities is explained by differences in the gas fraction.

Potential of ozone addition on dethrottling of a gasoline/ethanol blend-fueled direct injection spark ignition engine in part load

Several efforts are required to improve engine efficiency and decrease global carbon dioxide (CO2) emissions and local pollutant products. In countries like Brazil, with a four-decade long experience with ethanol, the use of fossil fuels, although widespread, is being put under intense scrutiny. Governmental programs like ROTA 2030 stimulate engine research and development focused on environment-friendly fuels such as bioethanol-gasoline blends. Brazilian gasoline has about a quarter of ethanol, reducing the carbon footprint while maintaining suitable energy density. Regardless of the fuel, the load control method in part load operation of spark ignition engine causes a considerable penalty in fuel conversion efficiency. For these reasons, ozone addition was tested as an ignition enhancer that would allow higher de-throttling to reduce part-load pumping losses. The residual gas was used to dilute the mixture due to the possibility of keeping the three-way catalyst working properly with the stoichiometric mixture. An experimental investigation on efficiency, emissions and combustion related parameters was carried out in a downsized 1.0 l turbocharged direct-injected engine. Experimental tests were performed at 3 bar of indicated mean effective pressure (IMEP), 1500 rpm and stoichiometric air-fuel ratio. Spark timing (ST) was adjusted to achieve maximum indicated efficiency and the fuel used was Brazilian gasoline. Different ozone concentrations were used as a mixture with the intake air to overcome unstable engine operation by enhancing ignition. The results showed that it is possible to increase the gas exchange efficiency with ozone addition by promoting de-throttling operation with residual gases. Furthermore, the ozone addition exhibits the potential to promote autoignition of the end gas with spark assistance, even with a low compression ratio and residual gas fraction higher than 30%. However, the combustion efficiency is impaired by the higher residual gas fraction in some operation points that leave room for improvements.

ALMA/ACA CO Survey of the IC 1459 and NGC 4636 Groups: Environmental Effects on the Molecular Gas of Group Galaxies

We present new results of a 12CO(J = 1–0) imaging survey using the Atacama Compact Array (ACA) for 31 H i detected galaxies in the IC 1459 and NGC 4636 groups. This is the first CO imaging survey for loose galaxy groups. We obtained well-resolved CO data (∼0.7–1.5 kpc) for a total of 16 galaxies in two environments. By comparing our ACA CO data with the H i and UV data, we probe the impacts of the group environment on the cold gas components (CO and H i gas) and star formation activity. We find that CO and/or H i morphologies are disturbed in our group members, some of which show highly asymmetric CO distributions (e.g., IC 5264, NGC 7421, and NGC 7418). In comparison with isolated galaxies in the xCOLD GASS sample, our group galaxies tend to have low star formation rates and low H2 gas fractions. Our findings suggest that the group environment can change the distribution of cold gas components, including the molecular gas and star formation properties of galaxies. This is supporting evidence that preprocessing in the group-like environment can play an important role in galaxy evolution.

Probing Cold Gas in a Massive, Compact Star-forming Galaxy at z = 6

Observations of low-order 12C16O transitions represent the most direct way to study galaxies’ cold molecular gas, the fuel of star formation. Here we present the first detection of CO(J = 2 → 1) in a galaxy lying on the main-sequence of star-forming galaxies at z > 6. Our target, G09-83808 at z = 6.03, has a short depletion timescale of τ dep ≈ 50 Myr and a relatively low gas fraction of M gas/M ⋆ ≈ 0.30 that contrasts with those measured for lower-redshift main-sequence galaxies. We conclude that this galaxy is undergoing a starburst episode with a high star formation efficiency that might be the result of gas compression within its compact rotating disk. Its starburst-like nature is further supported by its high star formation rate surface density, thus favoring the use of the Kennicutt–Schmidt relation as a more precise diagnostic diagram. Without further significant gas accretion, this galaxy would become a compact, massive quiescent galaxy at z ∼ 5.5. In addition, we find that the calibration for estimating interstellar medium masses from dust continuum emission satisfactorily reproduces the gas mass derived from the CO(2 → 1) transition (within a factor of ∼2). This is in line with previous studies claiming a small redshift evolution in the gas-to-dust ratio of massive, metal-rich galaxies. In the absence of gravitational amplification, this detection would have required of order 1000 hr of observing time. The detection of cold molecular gas in unlensed star-forming galaxies at high redshifts is thus prohibitive with current facilities and requires a tenfold improvement in sensitivity, such as that envisaged for the Next-Generation Very Large Array .