Dust Removal Research Articles

Enhancing Surveillance Vision with Multi-Layer Deep Learning Representation

This paper aimed to develop a method for generating sand–dust removal and dehazed images utilizing CycleGAN, facilitating object identification on roads under adverse weather conditions such as heavy dust or haze, which severely impair visibility. Initially, the study addressed the scarcity of paired image sets for training by employing unpaired CycleGAN training. The CycleGAN training module incorporates hierarchical single-scale Retinex (SSR) images with varying sigma sizes, facilitating multiple-scaled trainings. Refining the training data into detailed hierarchical layers for virtual paired training enhances the performance of CycleGAN training. Conventional sand–dust removal or dehazing algorithms, alongside deep learning methods, encounter challenges in simultaneously addressing sand–dust removal and dehazing with a singular algorithm. Such algorithms often necessitate resetting hyperparameters to process images from both scenarios. To overcome this limitation, we proposed a unified approach for removing sand–dust and haze phenomena using a single model, leveraging images processed hierarchically with SSR. The image quality and image sharpness metrics of the proposed method were BRIQUE, PIQE, CEIQ, MCMA, LPC-SI, and S3. In sand–dust environments, the proposed method achieved the highest scores, with an average of 21.52 in BRISQUE, 0.724 in MCMA, and 0.968 in LPC-SI compared to conventional methods. For haze images, the proposed method outperformed conventional methods with an average of 3.458 in CEIQ, 0.967 in LPC-SI, and 0.243 in S3. The images generated via this proposed method demonstrated superior performance in image quality and sharpness evaluation compared to conventional algorithms. The outcomes of this study hold particular relevance for camera images utilized in automobiles, especially in the context of self-driving cars or CCTV surveillance systems.

Examining the effectiveness of municipal street sweeping in removing road-deposited particles and metal(loid)s of respiratory health concern

Road dust is a demonstrated source of urban air pollution. Given this, the implementation of street sweeping strategies that effectively limit road dust accumulation and resuspension should be a public health priority. Research examining the effectiveness of street sweeping for road dust removal in support of good air quality has been limited to date. To address this, the study aimed to assess the use of a regenerative-air street sweeper to efficiently remove road dust particles and metal(loid)s in size fractions relevant for respiratory exposure in Toronto, Canada. As part of this, the mass amounts, particle size distribution and elemental concentrations of bulk road dust before and after sweeping at five arterial sites were characterized.Sweeping reduced the total mass amount of thoracic-sized (<10 µm) road dust particles by 76 % on average. A shift in the size distribution of remaining particles toward finer fractions was observed in post-sweeping samples, together with an enrichment in many metal(loid)s such as Co, Ti and S. Overall, the mass amounts of metal(loid)s of respiratory health concern like Cu and Zn were greatly reduced with sweeping. Traffic volume and road surface quality were predictors of dust loadings and elemental concentrations. Road surface quality was also found to impact street sweeping efficiencies, with larger mass amounts per unit area collected post-sweeping where street surfaces were distressed.This study demonstrates that street sweeping using advanced technology can be highly effective for road dust removal, highlighting its potential to support air quality improvement efforts. The importance of tailoring sweeping service levels and technologies locally as per the quality of road surface and traffic patterns is emphasized. Continued efforts to mitigate non-exhaust emissions that pose a respiratory health risk at their source is essential.

Dispersion and migration characteristics of multisource respirable dust in development panels during tunnelling processes

Underground miners in Australia are facing continued threats from dust-related diseases. To address these issues, improved knowledge of airflow migration patterns and respirable dust dispersion characteristics within a continuous-miner-driven heading under an exhausting ventilation system is required. Based on site-specific conditions of a development heading in New South Wales, a three-dimensional computational fluid dynamics (CFD) model was constructed and validated with onsite dust monitoring data, where a good agreement was achieved. Three scenarios of coal cutting at the middle, floor and roof positions were considered and simulated, with dust generated at four different sources. The simulation results indicated that the operators on the left-hand-side (LHS) with the extraction duct should be equipped with fit-for-purpose personal protective equipment and stay behind the ventilation duct inlet during coal cutting process, while miners standing at the right-hand-side (RHS) of the continuous miner for roof and rib bolting and machine operating should stay immediately behind the roof and rib bolting rig where dust concentration is relatively low. In addition, an increase in airflow rate through the exhausting ventilation duct or a reduction in the distance from the duct inlet to the heading face assisted in reducing dust levels within the heading, particularly at the LHS of the continuous miners. Finally, compared to the scenario of the current ventilation scheme, an on-board exhausting ventilation system could improve dust removal performance, with dust concentration at the breathing level reducing by approximately 43.6%. This modelling study can advance the understanding of multi-source dust diffusion characteristics in the heading face and provide guidance on dust mitigation, thus improving the health and safety of miners and creating a cleaner underground working environment.

Performance and mechanism of atomizing dust removal from APG12/Active Water: Experimental and molecular simulation study

Coal mining processes produce large amounts of dust particles that seriously pollute the environment and threaten health. Spraying technology is a primary technique for dust removal. To date, experimental methods are mainly applied to study the atomization and dust removal properties of water droplets, but the intermolecular mechanism involved is unclear. First, a test platform was assembled to evaluate the atomization and dust removal performance of APG12/active water. Then, APG12 droplet and APG12 droplet/anthracite molecular models were constructed, and the underlying mechanism was elucidated through molecular simulation. The experimental results showed that the droplet diameter of the APG12/active water spray decreased by 14.15%, and the dust removal efficiencies of total dust and respirable dust increased by 39.55% and 61.51%, respectively. The simulation results indicated that APG12 molecules and H2O molecules exhibited attractive interactions. This interaction weakened the electrostatic attraction between the H2O molecules in the APG12/active water droplets. Moreover, the interaction enhanced the repulsive van der Waals interactions, reduced the number of hydrogen bonds, and increased the diffusion coefficient of H2O molecules. As a result, the droplets were relatively easy to break. In addition, APG12 molecules covered the surfaces of anthracite molecules, thus increasing the surface oxygen content and the electrostatic attraction, van der Waals forces and hydrogen bonding between H2O and coal molecules. This enhancement, in turn, improved the wettability of coal dust. Herein, the molecular mechanism of dust removal from APG12/active water was analysed, providing guidance for the development of new dust suppressants.

Pressure drop model of gas-liquid phase for Venturi scrubber in fog-annular flow

Venturi scrubber is widely used in wet dust removal. Its pressure drop performance determines the dust removal efficiency. The commonly used models, most of them are empirical ones, are hard to accurately predict the friction loss between the gas core and the liquid film near the wall, the proportion of the atomized droplets to the deposited liquid film, and the gas holdup, where the influence of the throat length on throttling and the local resistance formation mechanism were not discussed. Therefore, in this study, we established a pressure drop model for Venturi based on the theoretical derivation of two-phase flow and revision of local resistance pressure drop. It uses the similar theoretical method of straight pipe for Venturi, where the iterative method was employed to calculate the liquid-phase conversion coefficient which is related to the cross-flow ratio and the gas holdup. Meanwhile, the experiments and CFD simulations were carried out to validate the model and judge whether the local resistance should be considered. The results show that if the vena contracta extends out of the throat, the unconstrained expansion flow can cause the boundary layer separation, and the fluctuation of the two-phase flow in throat can induce Kelvin-Helmholtz instability, resulting in apparent energy loss. Accordingly, the critical throat length criterion was developed, and the pressure drop model was modified with the deviation less than 17.14%. It provides a scientific basis for establishing the universal two-phase theoretical pressure drop model of Venturi scrubber.

Analysis of parameters for spray-local exhaust ventilation (SLEV) to minimize high-temperature smoke pollutants and reduce energy consumption

Industrial buildings often face limitations in placing local exhaust ventilation (LEV) near pollution sources, which can lead to ineffective control of high-temperature smoke. This study explores the integration of spray with local exhaust ventilation (SLEV) as a solution for smoke control and energy savings. The research identified a design law for spray parameters and developed a formula to calculate SLEV ventilation flow rate that considers the impact of spray. Results indicate that the spray field overlapping coefficient plays a key role in characterizing the position and spacing of multi-nozzles. As the spray field overlapping coefficient increases, the spray dust reduction performance indicator linearly increases, while the capture efficiency follows a pattern that aligns with the Boltzmann curve model. The optimal range for the spray field overlapping coefficient to achieve high efficiency in SLEV is between 0.45 and 0.55. The formula for ventilation flow rate calculation is applicable to common conditions with smoke velocities ranging from 4 to 12 m/s (Ar: 0.0149–0.1339). SLEVs demonstrate a 35.22 % improvement in dust removal efficiency and over 20 % energy savings compared to LEVs, contributing to the development of high-efficiency, energy-saving systems for sustainable industrial production.

Gas-Solid Flow Characteristics of Airflow and Dust Particles in Blasting Excavation of Underground Metal Mine Tunnels.

Effective dust removal has long been a challenge in the blasting mining of underground metal mine tunnels, and uncontrolled dust diffusion seriously endangers workers' respiratory systems and the underground space safety environment. However, the vast majority of existing numerical studies on dust diffusion are focused on coal mine fully mechanized mining, which is different from metal mine blasting excavation in terms of stope structure and dust properties. Furthermore, the mechanism by which the forced and exhaust ventilation modes affect the diffusion characteristics of inhalable particles is unclear. In this work, gas-solid flow characteristics for dust diffusion in a typical metal mine blasting tunnel were numerically investigated based on the Euler-Lagrange method, where the blasting face instantly released 6.37 × 107 particles with 100 different sizes, ranging from 0.8 to 200 μm. The interphase forces between airflow and dust particles are comprehensively modeled, and the particle diffusion effect caused by fluid turbulence is described by a discrete random walk model. Detailed information for airflow turbulence and particle migration was revealed, and dust removal efficiencies for inhalable particulate matter (PM10) by forced, exhaust, and hybrid ventilation were analyzed. Numerical results predict a complex airflow pattern in the working roadway, including the jet-flow region, return airflow core region, airflow disorder region, and secondary flow region. Dust diffusion temporal characteristics can be divided into three stages, namely, the initial stage of dust generation, the efficient ventilation and dust removal stage, and the later stage of dust diffusion. Dust diffusion spatial characteristics indicate that under the Coanda wall attachment effect, the dust concentration exhibits nonuniform distribution in both vertical and horizontal directions of the return air roadway. The dust removal efficiency of hybrid ventilation on inhalable particles above respiratory height is better than that of forced ventilation, especially in the return air roadway. The additional exhaust air duct based on forced ventilation can discharge more inhalable particles from the tunnel.

An empirical study on thermal efficiency and dusty moisture removal efficiency using enthalpy wheel in industrial waste incineration plant

In this work, an empirical study is conducted on the waste heat recovery and moisture removal performance of an enthalpy wheel installed in an industrial waste incineration plant. The enthalpy wheel made of a metal mesh is placed in parallel with the wet scrubber of the incineration plant to exchange heat with high-temperature exhaust at 200 °C. In the enthalpy wheel, exhaust gas and external air exchange heat in the form of a counter flow, and the condensation effect is maximized by installing a water-spray nozzle at the inlet of the enthalpy wheel. Empirical data were collected over 100 days spanning July to October. To evaluate the performance of the enthalpy wheel, thermal efficiency, moisture removal efficiency, and dust removal efficiency were tested under high-temperature humid conditions. In some results, the overall thermal efficiency reached peaked at 61.2 %. Regarding moisture removal performance, the average moisture removal rate was 493 kg/h, affording an efficiency of 49.2 %. In terms of dust removal performance, the average dust removal efficiency was 79.6 %.

Orbital Electrowetting-on-Dielectric for Droplet Manipulation on Superhydrophobic Surfaces.

Electrowetting-on-dielectric (EWOD), recognized as the most successful electrical droplet actuation method, is essential in diverse applications, ranging from thermal management to microfluidics and water harvesting. Despite significant advances, it remains challenging to achieve repeatability, high speed, and simple circuitry in EWOD-based droplet manipulation on superhydrophobic surfaces. Moreover, its efficient operation typically requires electrode arrays and sophisticated circuit control. Here, a newly observed droplet manipulation phenomenon on superhydrophobic surfaces with orbital EWOD (OEW) is reported. Due to the asymmetric electrowetting force generated on the orbit, flexible and versatile droplet manipulation is facilitated with OEW. It is demonstrated that OEW droplet manipulation on superhydrophobic surfaces exhibits higher speed (up to 5 times faster), enhanced functionality (antigravity), and manipulation of diverse liquids (acid, base, salt, organic, e.g., methyl blue, artificial blood) without contamination, and good durability after 1000 tests. It is envisioned that this robust droplet manipulation strategy using OEW will provide a valuable platform for various processes involving droplets, spanning from microfluidic devices to controllable chemical reactions. The previously unreported droplet manipulation phenomenon and control strategy shown here can potentially upgrade EWOD-based microfluidics, antifogging, anti-icing, dust removal, and beyond.

Effect of Different Outlet Structure of Axial Flow Cyclone Separator in Drying Tower

The axial flow cyclone separator used in drying tower is characterized by the diameter of the dust outlet at the tail is larger than that of the cylinder, and the dust removal efficiency is increased by a series dust outlet. The numerical analysis of its separation characteristics is carried out by using the RSM model of CFD. The results show that the separation efficiency can be increased by appropriately increasing the diameter of the dust outlet, and the separation efficiency will not change when the diameter of the dust outlet increases to a certain value. When the insertion depth of the air outlet is reduced from 150mm to 100mm, the average separation efficiency of particles with a diameter of 15-30um is reduced by 4.5%. When the insertion depth of the air outlet is greater than the width of the dust outlet, the insertion depth has no effect on the dust removal efficiency. The tandem dust outlet and parallel dust outlet have the same separation effect on small particles, and the gas flow rate is equivalent to twice the gas flow rate of a single dust outlet. The average particle separation efficiency of 1 - 30um is increased by 16.5% compared with that of a single dust outlet. The flow velocity of the tandem dust outlet is higher than that of the parallel dust outlet.

WaveletFormerNet: A Transformer-based wavelet network for real-world non-homogeneous and dense fog removal

Although deep convolutional neural networks have achieved remarkable success in removing synthetic fog, it is essential to be able to process images taken in complex foggy conditions, such as dense or non-homogeneous fog, in the real world. However, the haze distribution in the real world is complex, and downsampling can lead to color distortion or loss of detail in the output results as the resolution of a feature map or image resolution decreases. Moreover, the over-stacking of convolutional blocks might increase the model complexity. In addition to the challenges of obtaining sufficient training data, overfitting can also arise in deep learning techniques for foggy image processing, which can limit the generalization abilities of the model, posing challenges for its practical applications in real-world scenarios. Considering these issues, this paper proposes a Transformer-based wavelet network (WaveletFormerNet) for real-world foggy image recovery. We embed the discrete wavelet transform into the Vision Transformer by proposing the WaveletFormer and IWaveletFormer blocks, aiming to alleviate texture detail loss and color distortion in the image due to downsampling. We introduce parallel convolution in the Transformer block, which allows for the capture of multi-frequency information in a lightweight mechanism. Such a structure reduces computational expenses and improves the effectiveness of the network. Additionally, we have implemented a feature aggregation module (FAM) to maintain image resolution and enhance the feature extraction capacity of our model, further contributing to its impressive performance in real-world foggy image recovery tasks. Through extensive experiments on real-world fog datasets, we have demonstrated that our WaveletFormerNet achieves superior performance compared to state-of-the-art methods, as shown through quantitative and qualitative evaluations of minor model complexity. Additionally, our satisfactory results on real-world dust removal and application tests showcase the superior generalization ability and improved performance of WaveletFormerNet in computer vision-related applications compared to existing state-of-the-art methods, further confirming our proposed approach's effectiveness and robustness. Our code is available at https://github.com/shengli666666/WaveletFormerNet.

Research of wet string grid dust removal vehicle and creation of dust control area on tunnel working face

The spread of blast dust throughout the tunnel becomes a common problem in drill and blast tunneling,the key to breaking through the problem is the creation of a dust control area on the working face.In view of this key problem, a wet string grid dust removal crawler vehicle was developed, the power of the vehicle came from the diesel generator, and further, the air cooler of the diesel generator was used to generate airflow, and the suction process formed by the on-board axial flow fan was coupled to create a dust control area of the working face after blasting.The results show that when the frequency of the axial flow fan is adjusted to 30 Hz, the airflow speed of the wet chord grid section reaches 3.34 m/s, and the dust removal efficiency is the highest, with a value of 94.3%.Compared with the non-use of the dust removal vehicle, when the air outlet of the air cooler is front, horizontal front, horizontal rear, the dust concentration is reduced by 74.37, 92.39 and 50.53%.Finally, the optimized wet grid dust removal crawler was installed in the Dading tunnel, and the actual dust reduction efficiency was about 78.49%. The results obtained provide an important technical way to improve the working environment of the drilling and blasting construction tunnel.

Efficiency Improvement and Market Value Analysis of Guide Vane Electrostatic Precipitators

Urbanization and industrialization accelerate, the issue of atmospheric environmental pollution has become increasingly severe, particularly the impact of indoor air quality on human health and well-being. This study introduces a novel guide vane electrostatic precipitator that significantly enhances dust removal efficiency through optimized airflow channels and electrostatic field design. Experimental data demonstrate that this device can effectively reduce PM2.5 concentrations, improving dust removal efficiency by approximately 10% compared to traditional electrostatic dust removal technologies. Applications in various scenarios, including hotels, dust workshops, and steel foundries, indicate that the device not only improves air quality, enhancing employee productivity and well-being, but also features low energy consumption and intelligent operation. Furthermore, it holds considerable potential market value, contributing to the promotion of green and low-carbon development objectives. This research provides a comprehensive analysis and evaluation of the structure, working principle, mechanism of efficiency improvement, practical application effects, and market value of the guide vane electrostatic precipitator, looking forward to its application prospects in the field of air purification.

Research and application of bag filter system for railway ballast bed coal suction vehicles: An optimization and application study.

The current bag filter system used by railway ballast bed coal suction vehicles for cleaning coal dust from railway tunnels has low operational efficiency and generates significant volumes of dust. This paper describes a simulation test unit designed to enhance the dust removal performance in railway tunnels. The flow field inside the simulation test unit is investigated under different operating conditions through numerical simulations, and the variations in air volume and working resistance, total dust collection efficiency, and optimal operating parameters of a pulse cleaning system are identified through a series of experiments. The numerical results show that the pulse cleaning system does not significantly affect the uniformity of the flow field distribution at the bottom of the filter cartridge during the process of operation. The experimental research indicates that the simulation test unit satisfies the design requirements, achieving an average total dust removal efficiency of 99.93%. A field application shows that the total dust mass concentration at the operator position can be reduced from 335.8 mg∙m-3 to 4.2 mg∙m-3, effectively improving the operating environment within the tunnel.

Adhesion Behavior of Underground Coal Dust with Fused Silica: Effects of Relative Humidity and Particle Size

Coal dust particles adhering to a camera lens reduce its light transmittance, which deteriorates the performance of the camera and may lead to serious problems with mining equipment that requires visual ability. Aiming at improving coal dust removal and cleaning technologies, the adhesion behavior of coal dust with fused silica is studied here. Experiments were conducted from microscopic and statistical points of view. The adhesion force between a single coal dust particle and fused silica is tested using atomic force microscopy (AFM), and the number and size distribution of large amounts of coal dust particles on fused silica are tested using a home-made adhesion experimental platform and image processing method. The results show that the adhesion force increases at high relative humidity (RH); it is dominated by van der Waals forces at low RH and capillary forces at high RH. The fused silica glass surface is predominantly covered by small-sized coal dust particles, and the total number of particles as well as the proportion of large-sized particles increases with RH. The theoretical values of van der Waals and capillary forces are significantly larger than the experimental values, owing to the irregular shape and roughness of the surface of the coal dust.

Thermally Driven Winds on Mars: A Review and a Slope Effect Numerical Study

AbstractRegional and local thermally driven winds are planetary boundary layer phenomena frequently observed on Mars over sloping and flat regions, when diurnal surface temperature variations are significant and large‐scale winds are sufficiently weak. In particular, slope flows are prevalent in many areas on Mars, where they can reach high speeds and have a substantial impact on the near‐surface wind patterns, pressure diurnal cycle, aeolian activity, etc. This work first reviews literature on Martian winds, listing wind speeds obtained either numerically or from in situ measurements, with special emphasis on slope flows. Second, a methodology is presented to perform numerical simulations of slope flows on Mars using the open‐source computational fluid dynamics code OpenFOAM. Slope winds are then studied in a simplified Martian mountain‐valley configuration, using realistic values for various parameters, such as the slope angle and temperature variation. The incompressible Navier‐Stokes equations with Boussinesq approximation are used, along with the k‐ɛ Reynolds‐averaged Navier‐Stokes turbulence model. Radiant heat transfer is included and proven to be paramount for a correct description of Martian slope winds. Results of velocity and temperature profiles are obtained for various slope angles. These simulations aim to provide further insight on the behavior of slope winds on Mars, to simplify the process of assessing numerous potential landing sites for upcoming missions, since these highly regular winds could be useful, for instance, for dust removal applications.

Synergistic Humidification and Chemical Agglomeration to Improve Capturing the Fine Particulate Matter by Electrostatic Precipitator

The wet electrostatic precipitator (WESP) overcomes the shortcomings of traditional electrostatic precipitators, such as dust re-entrainment and back corona. It can effectively remove high-specific-resistivity dust, with a good removal effect on PM2.5. It is proposed to adopt chemical agglomeration and humidification agglomeration technology in the wet electrostatic precipitators to achieve ultra-low dust emissions from coal-fired power plants. The results show that the addition of chemical agglomerates, surfactants, and water vapor all affect the dust diameter of coal-fired power plants. After adding sesbania gum (SG), the D50 of dust particles increases from 28.29 μm to 48.22 μm. And the D50 of dust particles is 36.46 μm when spraying 3.6 kg/h water vapor only. With the cooperation of chemical agglomeration agents and water vapor, the dust agglomeration effect and removal efficiency can be further improved. When 10 mg/L SG is synergistically combined with 2.9 kg/h water vapor, the D50 is 64.75 μm, and the dust removal efficiency reaches 97.88%. On this basis, by adding 5 mg/L of Hexadecyltrimethylammonium bromide (CTAB), the D50 is 83.06 μm, and the dust removal efficiency increases to 98.62%. The synergistic effect of chemical agglomeration and humidification agglomeration promotes the aggregation of dust from coal-fired power plants. It can improve the removal efficiency of WESP for fine particulate matter but has little impact on the operation of existing equipment. The synergistic effects of multiple agglomeration technologies are also the direction for future research on the removal efficiency of fine particulate matter.

Non-Thermal Plasma Technology for Further Purification of Flue Gas in the Resource Utilization Process of Waste Mercury Catalyst: A Case Study in Xinjiang, China

This study aims to advance the recycling of mercury-containing waste and promote sustainable development within the polyvinyl chloride (PVC) industry. Our innovative system integrates pre-treatment technology (spraying potassium permanganate and demisting and dust removal) with efficient non-thermal plasma oxidation, resulting in excellent treatment efficiency, low cost, and simple operation. With a processing capacity of 3000 m3/h, the concentration of mercury emissions in flue gas can achieve the target of &lt;0.01 mg/m3, boasting a removal efficiency exceeding 98%, which satisfies the standard “Emission standard of air pollutants for industrial kiln and furnace” (GB 9078-1996). Our results can provide technical support for the comprehensive purification of mercury-containing flue gas during the resource recovery process from mercury-containing waste. The application of our system can contribute to reducing mercury emissions in the PVC industry, lowering occupational exposure risks for workers, and promoting China’s better compliance with “the Minamata Convention on Mercury”.

Improving the cleaning uniformity of rectangular flat pleated filter with pulse-jet airflow by the new type of intensive injection nozzle

The rectangular flat pleated filter is widely used in the industrial field due to its compactness and high efficiency in dust removal. However, due to the large number of folds and the extremely small gaps between folds, achieving complete cleaning is a key technical challenge. A new type of injection nozzle has been proposed, the effects of different injection nozzles on the static pressure acting on the inner wall of the rectangular flat pleated filter were investigated through multi-condition cleaning experiments and industrial powder coating verification experiments. The performance of the uniform pressure distribution can be enhanced by 17.67% compared to the traditional circular nozzle, especially 88 × Ф2.4 mm injection pipe, which has the optimum cleaning performance at 0.2 MPa and 20 mm. The relationship between the comprehensive pressure and number of the injection nozzles is put forward: Pζ=0.2n2−21.59n+1322.02, which is also applicable to traditional circular nozzle. The filtration velocity has more apparent impact on the cleaning efficiency of the filter, while the influence of gas-solid concentration is not significant. According to the requirements of different industrial scenes, the filtration velocity and the gas-solid concentration can be appropriately reduced within a certain range to make the filter more efficient.

The Fate of the Interstellar Medium in Early-type Galaxies. III. The Mechanism of Interstellar Medium Removal and the Quenching of Star Formation

Understanding how galaxies quench their star formation is crucial for studies of galaxy evolution. Quenching is related to a decrease of cold gas. In the first paper we showed that the dust removal timescale in early-type galaxies (ETGs) is about 2.5 Gyr. Here we present carbon monoxide and 21 cm hydrogen line observations of these galaxies and measure the timescale of removal of the cold interstellar medium (ISM). We find that all the cold ISM components (dust and molecular and atomic gas) decline at similar rates. This allows us to rule out a wide range of potential ISM-removal mechanisms (including starburst-driven outflows, astration, or a decline in the number of asymptotic giant branch stars), and artificial effects like the stellar mass–age correlation, environmental influence, mergers, and selection bias, leaving ionization by evolved low-mass stars and ionization/outflows by Type Ia supernovae or active galactic nuclei as viable mechanisms. We also provide evidence for an internal origin of the detected ISMs. Moreover, we find that the quenching of star formation in these galaxies cannot be explained by a reduction in the gas amount alone, because the star formation rates (SFRs) decrease faster (on a timescale of about 1.8 Gyr) than the amount of cold gas. Furthermore, the star formation efficiency (SFE) of the ETGs ( SFE≡SFR/MH2 ) is lower than that of star-forming galaxies, whereas their gas mass fractions ( fH2≡MH2/M* ) are normal. This may be explained by the stabilization of gas against fragmentation, for example due to morphological quenching, turbulence, or magnetic fields.