Search

We present ALMA CO(1-0) observations of the nearby LIRG galaxy pair IRAS05054+1718 with a new analysis of X-ray data collected between 2012 and 2021 using NuSTAR, Swift, and XMM-Newton. The western component of the pair, NED01, hosts a Seyfert 1.9 nucleus launching a powerful X-ray UFO. Our X-ray spectral analysis suggests the UFO could be variable or multi-component in velocity and constrains its momentum flux to $\dot p^{X-ray}_{out} \sim (4\pm2)\times 10^{34}$ gcms$^{-2}$. ALMA CO(1-0) observations include also the eastern component of the pair, a LIRG with no clear evidence for an AGN. We study the CO(1-0) kinematics in the two galaxies using the 3D-BAROLO code. In both sources, we can model the bulk of the CO(1-0) emission with rotating disks and, after subtracting the best-fit models, we detect compact residual emission at S/N=15 within $\sim3$kpc from the centre. A molecular outflow in NED01, if present, cannot be brighter than such residuals, implying an upper limit on its outflow rate of $\dot{M}^{mol}_{out} \lesssim 19\pm14~M_{\odot}~yr^{-1}$ and on its momentum rate of $\dot p^{mol}_{out} \lesssim (2.7\pm2.4) \times 10^{34}$gcms$^{-1}$. Combined with the revised energetics of the X-ray wind, we derive an upper limit on the momentum rate ratio of $\dot{p}^{mol}_{out}/\dot{p}^{X-ray}_{out}, Accepted by A&A

The ASTRI Mini-Array is a system of nine imaging atmospheric Cherenkov telescopes to be deployed at the {\itshape Observatorio del Teide} (Tenerife, Spain). In a first phase, the instrument will be operated as an experiment, with an observation schedule focused on primary science cases at multi-TeV energies (origin of cosmic rays, cosmology and fundamental physics, GRBs and multi-messenger astrophysics). Afterwards, a guest-observer observatory phase will follow, in which other significant targets will be pointed at. In this contribution, we focus on this second phase, presenting the observational feasibility of the most relevant extragalactic $\gamma$-ray emitters (high-synchrotron peaked blazars, Seyfert galaxies, self-interacting dark matter dominated dwarf spheroidal galaxies) and astrophysical processes detectable over long-term time scales that best complement and expand the ASTRI Mini-Array core science. In order to derive our results, detailed simulations have been performed by means of the most up-to-date ASTRI Mini-Array instrument response functions. The prospects of observing extragalactic targets with the ASTRI Mini-Array include the characterization of spectral shape and features of the multi-TeV emission from the considered classes of AGN with short-to-long duration (5 h to 200 h) observations, the detection of sources not yet observed at TeV energies and the improvement of the constraints on the cross section and lifetime of dark matter particles with 100 h observations of best-choice dwarf galaxies, and the possibility to simultaneously observe sources falling in the same field of view (up to a few degrees) with dedicated pointings.

Monitoring of vibrational eigenmodes of an elastic body excited by gravitational waves was one of the first concepts proposed for the detection of gravitational waves. At laboratory scale, these experiments became known as resonant-bar detectors first developed by Joseph Weber in the 1960s. Due to the dimensions of these bars, the targeted signal frequencies were in the kHz range. Weber also pointed out that monitoring of vibrations of Earth or Moon could reveal gravitational waves in the mHz band. His Lunar Surface Gravimeter experiment deployed on the Moon by the Apollo 17 crew had a technical failure rendering the data useless. In this article, we revisit the idea and propose a Lunar Gravitational-Wave Antenna (LGWA). We find that LGWA could become an important partner observatory for joint observations with the space-borne, laser-interferometric detector LISA, and at the same time contribute an independent science case due to LGWA's unique features. Technical challenges need to be overcome for the deployment of the experiment, and development of inertial vibration sensor technology lays out a future path for this exciting detector concept., 29 pages, 17 figures

We report on the direct observation of an extended X-ray jet in the $z$=6.1 radio-loud Active Galactic Nucleus PSO J030947.49+271757.31 from a deep Chandra X-ray observation (128 ksec). This detection represents the most distant kpc off-nuclear emission resolved in the X-rays to date. The angular distance of the emission is $\sim$4" (corresponding to $\sim$20 kpc at $z$=6.1), along the same direction of the jet observed at parsec scales in previous VLBA high-resolution radio observations. Moreover, the 0.5-7.0 keV isophotes coincide with the extended radio emission as imaged by the VLA Sky Survey at 3 GHz. The rest-frame 2-10 keV luminosity of the extended component is L$_{2-10keV}$=5.9$\times$10$^{44}$ erg s$^{-1}$, about 8% of the core: this makes it one of the most luminous jets resolved in the X-rays so far. Through Spectral Energy Distribution modelling we find that this emission can be explained by the Inverse Compton interaction with the photons of the Cosmic Microwave Background assuming that the jet's physical parameters are similar to those in the local Universe. At the same time, we find that the radiation produced by a putative population of high-energetic electrons through the synchrotron process observed at low redshift is quenched at high redshift, hence becoming negligible., Comment: Accepted for publication on A&A

Investigations of H2O maser galaxies at X-ray energies reveal that most harbor highly absorbed AGN. Possible correlations between the intrinsic X-ray luminosity and the properties of water maser emission have been suggested. With the aim of looking into these correlations on a more solid statistical basis, we have search for maser emission in a well-defined sample of Compton-thick AGN. Here we report the results of the survey, which yielded a surprisingly high maser detection rate, with a particular focus on the newly discovered luminous water maser in the lenticular (field) S0 galaxy IRAS 15480-0344. Recently, VLBI observations have been obtained to image the line and continuum emission in the nucleus of this galaxy. The radio continuum emission at VLBI scales is resolved into two compact components that are interpreted as jet knots. Based on the single-dish profile, the variability of the maser emission, and the position of the maser spots with respect to these continuum sources, we favor of a jet/outflow origin for the maser emission, consistent with similar cases found in other radio-quiet AGN. This scenario is consistent with the hypothesis of the presence of strong nuclear winds recently invoked to explain the main characteristics of field S0 galaxies.

Ricorre sabato prossimo il 56esimo anniversario del lancio della sonda spaziale statunitense Mariner 4, avvenuto il 28 novembre 1964 da Cape Canaveral. Sue le prime immagini ravvicinate – 22 fotografie, una delle quali incompleta – del suolo marziano, ricorda oggi su Media Inaf Roberto Della Ceca, responsabile dell’Unità per la gestione dei progetti spaziali dell’Inaf

Il 12 aprile 1961, con un volo orbitale di un’ora e 48 minuti a bordo della capsula spaziale Vostok 1, Yuri Gagarin diventa il primo cosmonauta della storia. Una storia che già all’epoca vedeva l’Italia in prima linea nell’impresa spaziale, come ci raccontano Roberto Della Ceca e Frida Paolella in questo articolo

Lunedì prossimo, 6 aprile 2020, ricorre il cinquecentenario della morte di Raffaello Sanzio. Per l’occasione, Frida Paolella e Roberto Della Ceca ripercorrono la fortissima connessione tra arte e scienza – e in particolare l’astronomia – in alcuni capolavori del grande artista, al quale è stato dedicato anche un modulo della Stazione spaziale internazionale

Elusive supermassive black hole binaries (SMBHBs) are thought to be the penultimate stage of galaxy mergers, preceding a final coalescence phase. SMBHBs are sources of continuous gravitational waves, possibly detectable by pulsar timing arrays; the identification of candidates could help in performing targeted gravitational wave searches. Due to their origin in the innermost parts of active galactic nuclei (AGN), X-rays are a promising tool to unveil the presence of SMBHBs, by means of either double Fe K$\alpha$ emission lines or periodicity in their light curve. Here we report on a new method to select SMBHBs by means of the presence of a periodic signal in their Swift-BAT 105-months light curves. Our technique is based on the Fisher's exact g-test and takes into account the possible presence of colored noise. Among the 553 AGN selected for our investigation, only the Seyfert 1.5 Mrk 915 emerged as possible candidate for a SMBHB; from the subsequent analysis of its light curve we find a period $P_0=35\pm2$ months, and the null hypothesis is rejected at the $3.7\sigma$ confidence level. We also present a detailed analysis of the BAT light curve of the only previously X-ray-selected binary candidate source in the literature, the Seyfert 2 galaxy MCG+11-11-032. We find $P_0=26.3\pm0.6$ months, consistent with the one inferred from previously reported double Fe K$\alpha$ emission lines., Comment: 11 pages, 10 figures; accepted for publication in The Astrophysical Journal

Si celebra domani la ricorrenza del primo viaggio spaziale di una donna, la cosmonauta sovietica Valentina Vladimirovna “Valya” Tereshkova, avvenuto il 16 giugno 1963 a bordo della capsula Vostok 6. Da allora circa una sessantina di donne sono andate nello spazio. Un numero ben lontano dalla parità con i colleghi maschi. Ne scrive oggi su Media Inaf Roberto Della Ceca, responsabile dell’Unità per la gestione dei progetti spaziali dell’Inaf

Il prossimo 10 ottobre ricorre il 53esimo anniversario dell’entrata in vigore dell'Outer Space Treaty, il primo trattato internazionale nel campo spaziale e il primo testo internazionale in cui vengano riconosciuti come punti cardine la pace e il benessere di tutto il genere umano. Ma la situazione, da allora, è molto cambiata. Ce ne parla Roberto Della Ceca, responsabile dell’Unità per la gestione dei progetti spaziali dell’Inaf

Fondata il 30 maggio 1975, domani l’Agenzia spaziale europea festeggia il suo 45esimo compleanno. Celebriamo la ricorrenza con questo articolo di taglio storico – dagli anni dei primi esperimenti su satellite ai giorni nostri – firmato da Roberto Della Ceca, responsabile dell’Unità per la gestione dei progetti spaziali dell’Inaf

Ricorre domani, venerdì 17 luglio, il 45esimo anniversario dell’aggancio in orbita fra la navicella spaziale Apollo e la capsula Sojuz, coronamento della prima collaborazione tra gli Stati Uniti e l’Unione Sovietica. Ripercorriamo con Roberto Della Ceca, responsabile dell’Unità per la gestione dei progetti spaziali dell’Inaf, le tappe di quella straordinaria missione spaziale e politica

Cinquantasei anni fa, con il lancio del satellite San Marco 1 dalla base americana di Wallops Island, il nostro Paese entrava di diritto nell’era spaziale. Come arrivò a tagliare quel traguardo? Lo racconta oggi su Media Inaf Roberto Della Ceca, responsabile dell’Unità per la gestione dei progetti spaziali dell’Inaf

Concepito intorno alla metà degli anni ‘60 in ambiente Esro dal gruppo di lavoro su Cosmic Rays and Trapped Radiation, il cui coordinatore e responsabile era Beppo Occhialini, il satellite per lo studio del cielo gamma Cos-B fu il primo a essere lanciato dall’Agenzia spaziale europea. Ne ripercorriamo la storia con Roberto Della Ceca, responsabile dell’Unità per la gestione dei progetti spaziali dell’Inaf

Il 26 aprile 1967, alle ore 10:04 Utc, sotto gli occhi stupiti di chi dalla spiaggia osservava i preparativi, si alzò in volo dalla base italiana di lancio al largo di Malindi – in pieno oceano, unica al mondo – il satellite San Marco 2. Roberto Della Ceca e Frida Paolella ripercorrono le fasi principali di quell’evento, interamente – tranne il vettore Scout messo a disposizione dagli Stati Uniti – opera italiana

We present ALMA and ACA [CI]$^{3}P_{1}-^{3}P_{0}$ ([CI](1-0)) observations of NGC6240, which we combine with ALMA CO(2-1) and IRAM PdBI CO(1-0) data to study the physical properties of the massive molecular (H$_2$) outflow. We discover that the receding and approaching sides of the H$_2$ outflow, aligned east-west, exceed 10 kpc in their total extent. High resolution ($0.24"$) [CI](1-0) line images surprisingly reveal that the outflow emission peaks between the two AGNs, rather than on either of the two, and that it dominates the velocity field in this nuclear region. We combine the [CI](1-0) and CO(1-0) data to constrain the CO-to-H$_2$ conversion factor ($\alpha_{\rm CO}$) in the outflow, which is on average $2.1\pm1.2~\rm M_{\odot} (K~km~s^{-1}~pc^2)^{-1}$. We estimate that $60\pm20$ % of the total H$_2$ gas reservoir of NGC6240 is entrained in the outflow, for a resulting mass-loss rate of $\dot{M}_{\rm out}=2500\pm1200~M_{\odot}~yr^{-1}$ $\equiv 50\pm30$ SFR. This energetics rules out a solely star formation-driven wind, but the puzzling morphology challenges a classic radiative-mode AGN feedback scenario. For the quiescent gas we compute $\langle\alpha_{\rm CO}\rangle = 3.2\pm1.8~\rm M_{\odot} (K~km~s^{-1}~pc^2)^{-1}$, which is at least twice the value commonly employed for (U)LIRGs. We observe a tentative trend of increasing $r_{21}\equiv L^{\prime}_{\rm CO(2-1)}/L^{\prime}_{\rm CO(1-0)}$ ratios with velocity dispersion and measure $r_{21}>1$ in the outflow, whereas $r_{21}\simeq1$ in the quiescent gas. We propose that molecular outflows are the location of the warmer, strongly unbound phase that partially reduces the opacity of the CO lines in (U)LIRGs, hence driving down their global $\alpha_{\rm CO}$ and increasing their $r_{21}$ values., Comment: Accepted for publication in ApJ

We present here some initial results from the ongoing XMM-Newton bright serendipitous survey. The survey is aimed at selecting and spectroscopically identifying a large and statistically representative sample of bright (fx ≳ 7× 10−14 c.g.s) serendipitous X-ray sources in the 0.5–4.5 keV energy band (BSS) and a complementary (smaller) sample in the 4.5–7.5 keV energy band (HBSS).

One of the key ingredients of the Unified Model of Active Galactic Nuclei (AGN) is the presence of a torus-like optically thick medium composed by dust and gas around the putative supermassive black hole. However, the structure, size and composition of this circumnuclear medium are still matter of debate. To this end, the search for column density variations through X-ray monitoring on different timescales (months, weeks and few days) is fundamental to constrain size, kinematics and location of the X-ray absorber(s). Here we describe our project of mining the Swift-XRT archive to assemble a sample of AGN with extreme column density variability and determining the physical properties of the X-ray absorber(s). We also present the results obtained from a daily-weekly Swift-XRT follow-up monitoring recently performed on one of the most interesting new candidates for variability discovered so far, Mrk 915., Comment: 6 pages, 3 figures. To appear in Proceedings of Science for the "Swift: 10 years of Discovery" meeting, held in Rome (2-5 December 2014)

We investigate the possible link between mergers and the enhanced activity of supermassive black holes (SMBHs) at the centre of galaxies, by comparing the merger fraction of a local sample (0.003 =< z < 0.03) of active galaxies - 59 active galactic nuclei (AGN) host galaxies selected from the all-sky Swift BAT (Burst Alert Telescope) survey - with an appropriate control sample (247 sources extracted from the Hyperleda catalogue) that has the same redshift distribution as the BAT sample. We detect the interacting systems in the two samples on the basis of non-parametric structural indexes of concentration (C), asymmetry (A), clumpiness (S), Gini coefficient (G) and second order momentum of light (M20). In particular, we propose a new morphological criterion, based on a combination of all these indexes, that improves the identification of interacting systems. We also present a new software - PyCASSo (Python CAS Software) - for the automatic computation of the structural indexes. After correcting for the completeness and reliability of the method, we find that the fraction of interacting galaxies among the active population, 20(+7/-5) per cent, exceeds the merger fraction of the control sample, 4(+1.7/-1.2) per cent. Choosing a mass-matched control sample leads to equivalent results, although with slightly lower statistical significance. Our findings support the scenario in which mergers trigger the nuclear activity of supermassive black holes., 17 pages, 6 figures, 4 tables, accepted for publication in MNRAS

We investigate the relative occurrence of radio--loud and radio-quiet quasars in the first billion years of the Universe, powered by black holes heavier than one billion solar masses. We consider the sample of high-redshfit blazars detected in the hard X-ray band in the 3-years all sky survey performed by the Burst Alert Telescope (BAT) onboard the Swift satellite. All the black holes powering these blazars exceed a billion solar mass, with accretion luminosities close to the Eddington limit. For each blazar pointing at us, there must be hundreds of similar sources (having black holes of similar masses) pointing elsewhere. This puts constraints on the density of billion solar masses black holes at high redshift (z>4), and on the relative importance of (jetted) radio-loud vs radio-quiet sources. We compare the expected number of high redshift radio--loud sources with the high luminosity radio-loud quasars detected in the Sloan Digital Sky Survey (SDSS), finding agreement up to z~3, but a serious deficit at z>3 of SDSS radio-loud quasars with respect to the expectations. We suggest that the most likely explanations for this disagreement are: i) the ratio of blazar to misaligned radio-sources decreases by an order of magnitude above z=3, possibly as a result of a decrease of the average bulk Lorentz factor; ii) the SDSS misses a large fraction of radio-loud sources at high redshifts, iii) the SDSS misses both radio-loud and radio-quiet quasars at high redshift, possibly because of obscuration or because of collimation of the optical-UV continuum in systems accreting near Eddington. These explanations imply very different number density of heavy black holes at high redshifts, that we discuss in the framework of the current ideas about the relations of dark matter haloes at high redshifts and the black hole they host.

The Energetic X-ray Imaging Survey Telescope (EXIST) will continuously survey the full sky in scanning mode for 2- years followed by a 3-years pointing phase. The mission includes three instruments: a High Energy coded mask Telescope; a 1.1m aperture optical-IR Telescope; and a Soft X-ray Imager (SXI), sensitive in the 0.1-10 keV band. SXI is proposed as a contribution of ASI-Italy, fully developed by Italian institutes. Here we will present the optical and mechanical design of the SXI mirror module, that includes also a pre-collimator and a magnetic diverter to ensure a low background on the detector. In particular we will describe the mirror module characteristics in term of effective area, imaging capability, thermal requirement and mechanical properties. The current optical design foresees 26 shells providing an effective area comparable to one XMM-Newton mirror module up to 3 keV. The realization of these shells is based on the well-proven Nickel replication-process technology.

We aim to test a method of efficiently selecting X-ray obscured AGN in the 2XMM-Newton EPIC Serendipitous Source Catalogue. By means of a strong correlation established using the XMM-Newton Hard Bright Sample between the intrinsic absorption and the hardness ratio to the 0.5-2.0 keV and 2.0-4.5 keV bands, an efficient way of selecting absorbed sources has been worked out. A hardness ratio selection based on the 2XMM-Newton pre-release Source Catalogue led us to the definition of candidates likely to be obscured in X-rays. X-ray and optical spectral analysis were performed for three objects. Strong absorption (NH > 10^{22} cm^{-2}) was detected from the X-ray analysis, confirming the efficiency of the method used to select obscured sources. The presence of absorption is also revealed in the optical band, although at a significantly lower level than inferred from the X-ray band., 6 pages, 9 PostScript figures, 2 tables, LaTeX - Accepted for publication in Astronomy & Astrophysics, elisabetta.memola@brera.inaf.it. The original version of the manuscript is available at http://www.brera.inaf.it/utenti/memola/publications.html

ROSAT deep survey X‐ray sources have been identified with active galaxies (Active Galactic Nuclei and Narrow Emission‐Line galaxies) and also with early‐type, ‘‘passive galaxies’’. In this paper we discuss preliminary results of the galaxy content, both early‐type and emission‐line galaxies, of 5 ROSAT fields.

A detailed study of the cosmological properties of X-ray selected AGN is presented. The data are analyzed within the framework of a pure luminosity evolution (PLE) model and the two most population evolutionary forms. Evidence is found for luminosity-dependent luminosity evolution if the evolution function has the exponential form. The simpler PLE model is more acceptable if the data are fitted with a power-law evolution function. Similar results are obtained in the optical domain from an analysis of a sample of optically selected QSOs with z less than 2.2 and B less than 20.

X-ray luminosity functions (XLFs) and their implications for cosmological evolution are determined for 420 X-ray-selected active galactic nuclei (AGN). The local XLFs flatten considerably when L(x) is less than or equal to 5 x 10 to the 42nd ergs/s, and higher-luminosity XLF are demonstrated to fit a power-law approximation. Cosmological evolution in terms of two pure-luminosity evolutionary models is directly supported by model-independent AGN XLF, showing weak evolution when z is less than 0.4 and more pronounced evolution at higher z. The parameter C is determined for the two models, and the values support the 'slower' evolution of these AGN relative to QSOs selected optically. The deevolved XLF best-fit slope is found to be 3.05 for high luminosities and 1.35 for low luminosities, with no evidence of slope changes for other luminosities. 38 refs.

With its large collection area, broad-band energy coverage from optical/NIR (0.3 to 2.2 micron) to soft/hard X-ray (0.1-600 keV), all-sky monitoring capability, and on-board follow-up, the proposed Energetic X-ray Imaging Survey Telescope mission (EXIST, see L. Natalucci contribution at this conference) has been designed to properly tackle the study of the AGN phenomenon and the role that SMBH play in the Universe. In particular EXIST will carry out an unprecedented survey above 10 keV (a factor ~20 increase in hard X-ray sensitivity compared to current and prior X-ray missions) of SMBH activity, not just in space but also in time and over a significant expanded energy range/ this strategy will overcome previous selection biases, will break the 'multi-wavelength' identification bottleneck and will dramatically increase the number of AGN detected above 10 keV that are amenable to detailed follow-up studies (~50000 AGN are expected). We discuss here on few selected AGN science topics enabled by the unique combination of EXIST's instruments. In particular EXIST will enable major progress in understanding: i) when and where SMBH are active in the Universe (by revealing and measuring heavily obscured accretion in the local - z

We present and discuss the properties of a complete and well defined sample of X-ray selected type 1 AGNs including 26 Narrow-Line Seyfert 1 and 129 Broad-Line Seyfert 1 galaxies derived from the XMM-Newton Bright Serendipitous Survey (XBS). We study the differences between the two classes of AGNs for what concerns the X-ray properties (the photon-index) and the main physical parameters, like the Black-Hole mass and the Eddington ratio. We then consider the two classes together and look for statistical correlations between observed and physical parameters. We find a significant dependence of the photon-index with the Eddington ratio and a weaker (possibly secondary) correlation between the photon index and the Black-Hole Mass., 8 pages, 3 figures, 1 table. Proceedings of the Workshop "Narrow-Line Seyfert 1 Galaxies and Their Place in the Universe", Milano, April 4-6, 2011, (Proceedings of Science)

With 2D hydrodynamical simulations, we study the evolution of the hot gas flows in early-type galaxies, focussing on the effects of galaxy rotation on the thermal and dynamical status of the ISM. The galaxy is modelled as a two-component axisymmetric system (stars and dark matter), with a variable amount of azimuthal velocity dispersion and rotational support; the presence of a counter rotating stellar disk is also considered. It is found that the ISM of the rotationally supported (isotropic) model is more prone to thermal instabilities than the fully velocity dispersion counterpart, while its ISM temperature and X-ray luminosity are lower. The model with counter rotation shows an intermediate behaviour., 2 pages, 2 figures. Proceedings of the International Conference "X-ray Astronomy: towards the next 50 years!", Milan, 1-5 Oct 2012

Elliptical galaxies have hot coronae with X-ray luminosities and mean gas temperatures that span over wide ranges. This variation can be partially due to the energy budget of the hot gas, that depends on the host galaxy structure and internal kinematics. With the aid of realistic axisymmetric galaxy models, we performed a diagnostic study focussed on the effects of galaxy flattening and rotational support on the hot gas temperature., Comment: 2 pages, 1 figure. Proceedings of the International Conference "X-ray Astronomy: towards the next 50 years!", Milan, 1-5 Oct 2012

With a deep Chandra ACIS pointing of the compact radio elliptical NGC4278, representative of the less explored class of low/medium mass ellipticals, we studied the origin of the very sub-Eddington nuclear emission, the radiative and mechanical accretion energy outputs, and the unusual sharp increase of the gas temperature at the center. Extended hot gas was also detected, misaligned with the stars and aligned with the ionized gas and dust, an effect possibly due to accreting cold gas triggering the hot gas cooling.

To explore the connection between AGN activity and environment in the nearby Universe, we have carried out a spectroscopic study to determine the frequency and nature of the nuclear activity in two well-defined samples of compact groups (CGs) of galaxies: Hickson Compact Groups and Updated Zwicky Catalogue of Compact Groups. We found that a large fraction of the member-galaxies, 63% of the HCG galaxies and 67% of the UZC-CG galaxies, show emission lines. From them, low luminosity AGN is the most frequent activity type found in Compact Groups. Only 8 BLAGNs have been detected, showing a clear deficiency of broad line AGNs that must be related to the compact group environment, where galaxies suffer slow encounters and morphological transformations that result in an important decrease of the total content of gas.