arXiv subjects

Francoise Combes

Publications and source records attributed to Francoise Combes.

At least 73 records · Page 4Linked to original sources

arXiv2022

First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83

We report a CO(3-2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with ALMA. The observed 1kpc^2 region is at about 1.24 times the optical radius (R25) of the disk, where CO(2-1) was previously not detected. The detection and non-detection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A -- with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud and clump masses, estimated from CO(3-2), range from 8.2x10^2 to 2.3x10^4 Msun and from 2.7x10^2 to 7.5x10^3 Msun, respectively. The most massive clouds appear similar to Orion A in star formation activity as well as in mass, as expected if the cloud mass structure is universal. The overall low SF activity in the XUV disk could be due to the relative shortage of gas in the molecular phase. The clouds are distributed like chains up to 600 pc (or longer) in length, suggesting that the trigger of cloud formation is on large scales. The universal cloud mass structure also justifies the use of high-J CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-z universe. This study is the first demonstration that CO(3-2) is an efficient tracer of molecular clouds even in low-metallicity environments.

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arXiv2022

Extended far-ultraviolet emission in distant dwarf galaxies

Blue Compact Dwarfs (BCDs) are low-luminosity (M$_{K} > -21$ mag), metal-poor ($\frac{1}{50}$ $\le Z/Z_{\odot} \le\frac{1}{2}$), centrally concentrated galaxies with bright clumps of star-formation. Cosmological surface brightness dimming and small size limit their detection at high redshifts, making their formation process difficult to observe. Observations of BCDs are needed at intermediate redshifts, where they are still young enough to show their formative stages, particularly in the outer regions where cosmic gas accretion should drive evolution. Here, we report the discovery of excess far-ultraviolet (FUV) emission in the outer regions of 11 BCDs in the GOODS-South field at redshifts between 0.1 and 0.24, corresponding to look back times of 1.3 - 2.8 Gyr in standard cosmology. These observations were made by the Ultra-Violet Imaging Telescope (UVIT) on AstroSat. For ten BCDs, the radial profiles of intrinsic FUV emission, corrected for the instrument point spread function, have larger scale-lengths than their optical counterparts observed with the Hubble Space Telescope. Such shallow FUV profiles suggest extended star-formation in cosmically accreting disks. Clumpy structure in the FUV also suggests the outer FUV disks are gravitationally unstable. Dynamical friction on the clumps drives them inward at an average rate exceeding $10^6~M_{\odot}$Gyr$^{-1}$.

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arXiv2022

Emergence of a new HI 21-cm absorption component at z~1.1726 towards the gamma-ray blazar PKS~2355-106

We report the emergence of a new HI 21-cm absorption at z_abs = 1.172635 in the damped Lyman-alpha absorber (DLA) towards the gamma-ray blazar PKS 2355-106 (z_em~1.639) using science verification observations (June 2020) from the MeerKAT Absorption Line Survey (MALS). Since 2006, this DLA is known to show a narrow HI 21-cm absorption at z_abs = 1.173019 coinciding with a distinct metal absorption line component. We do not detect significant HI 21-cm optical depth variations from this known HI component. A high resolution optical spectrum (August 2010) shows a distinct Mg I absorption at the redshift of the new HI 21-cm absorber. However, this component is not evident in the profiles of singly ionized species. We measure the metallicity ([Zn/H] = -(0.77\pm0.11) and [Si/H]= -(0.96\pm0.11)) and depletion ([Fe/Zn] = -(0.63\pm0.16)) for the full system. Using the apparent column density profiles of Si II, Fe II and Mg I we show that the depletion and the N(Mg I)/N(Si II) column density ratio systematically vary across the velocity range. The region with high depletion tends to have slightly larger N(Mg I)/N(Si II) ratio. The two HI 21-cm absorbers belong to this velocity range. The emergence of z_abs = 1.172635 can be understood if there is a large optical depth gradient over a length scale of ~0.35 pc. However, the gas producing the z_abs = 1.173019 component must be nearly uniform over the same scale. Systematic uncertainties introduced by the absorption line variability has to be accounted for in experiments measuring the variations of fundamental constants and cosmic acceleration even when the radio emission is apparently compact as in PKS 2355-106.

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arXiv2022

Molecular gas dynamics around nuclei of galaxies

Recent molecular line observations with ALMA in several nearby Seyferts have revealed the existence of molecular tori, and the nature of gas flows at 10-20~pc scale. At 100~pc scale, or kpc-scale, previous NOEMA work on gravitational torques had shown that only about one third of Seyfert galaxies experienced molecular inflow and central fueling, while in most cases the gas was stalled in rings. At higher resolution, i.e. 10-20~pc scale, it is possible now to see in some cases AGN fueling due to nuclear trailing spirals, influenced by the black hole potential. This brings smoking gun evidence for nuclear fueling. In our sample galaxies, the angular resolution of up to 60~mas allows us to reach the black hole (BH) sphere of influence and the BH mass can be derived more directly than with the M-sigma relation.

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arXiv2022

Central star formation in double-peak gas rich radio galaxies

In a recent work, a large sample of double-peak (DP) emission line galaxies have been identified from the SDSS. While the two peaks could represent two kinematic components, they may be linked to the large bulges which their host galaxies tend to have. Star-forming DP galaxies display a central star formation enhancement and have been discussed to be compatible with a sequence of recent minor mergers. In order to probe merger induced star formation mechanisms, we conducted observations of the molecular gas content of 35 star-forming DP galaxies in the upper part of the main sequence (MS) of star formation with the IRAM 30m telescope. Including similar galaxies 0.3 dex above MS and with existing molecular gas observations from the literature, we finally obtain a sample of 52 such galaxies. We succeed in fitting the same kinematic parameters to the optical ionised and molecular gas emission lines for 10 (19%) galaxies. We find a central star formation enhancement resulting most likely from a galaxy merger or galaxy interaction, which is indicated by an excess of gas extinction found in the centre. This star formation is traced by radio continuum emissions at 150MHz, 1.4 and 3GHz, which are all three linearly correlated in log with the CO luminosity with the same slope. The 52 DP galaxies are found to have a significantly larger amount of molecular gas and larger depletion times, hence a lower star formation efficiency, than the expected values at their distance of the MS. This is consistent with a scenario of minor mergers increasing the mass of bulges and driving gas to the centre. We also exclude the inwards directed gas migration and central star formation enhancement to be the origin of a bar morphology. Hence, these 52 DP galaxies could be the results of recent minor mergers that funnelled molecular gas towards their centre, triggering star formation, but with a moderate efficiency.

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arXiv2022

ALMA Lensing Cluster Survey: ALMA-Herschel Joint Study of Lensed Dusty Star-Forming Galaxies across $z\simeq0.5-6$

We present an ALMA-Herschel joint analysis of sources detected by the ALMA Lensing Cluster Survey (ALCS) at 1.15 mm. Herschel/PACS and SPIRE data at 100-500 $μ$m are deblended for 180 ALMA sources in 33 lensing cluster fields that are either detected securely (141 sources; in our main sample) or tentatively at S/N$\geq$4 with cross-matched HST/Spitzer counterparts, down to a delensed 1.15-mm flux density of $\sim0.02$ mJy. We performed far-infrared spectral energy distribution modeling and derived the physical properties of dusty star formation for 125 sources (109 independently) that are detected at $>2σ$ in at least one Herschel band. 27 secure ALCS sources are not detected in any Herschel bands, including 17 optical/near-IR-dark sources that likely reside at $z=4.2\pm1.2$. The 16-50-84 percentiles of the redshift distribution are 1.15-2.08-3.59 for ALCS sources in the main sample, suggesting an increasing fraction of $z\simeq1-2$ galaxies among fainter millimeter sources ($f_{1150}\sim 0.1$ mJy). With a median lensing magnification factor of $μ= 2.6_{-0.8}^{+2.6}$, ALCS sources in the main sample exhibit a median intrinsic star-formation rate of $94_{-54}^{+84}\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$, lower than that of conventional submillimeter galaxies at similar redshifts by a factor of $\sim$3. Our study suggests weak or no redshift evolution of dust temperature with $L_\mathrm{IR}<10^{12}\,\mathrm{L}_\odot$ galaxies within our sample at $z \simeq 0 - 2$. At $L_\mathrm{IR}>10^{12}\,\mathrm{L}_\odot$, the dust temperatures show no evolution across $z \simeq 1 -4$ while being lower than those in the local Universe. For the highest-redshift source in our sample ($z=6.07$), we can rule out an extreme dust temperature ($>$80 K) that was reported for MACS0416 Y1 at $z=8.31$.

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arXiv2022

Genesis of morpho-kinematic lopsidedness in minor merger of galaxies

An $m=1$ lopsided asymmetry is common in disc galaxies. Here, we investigate the excitation of an $m=1$ lopsidedness in host galaxies during minor mergers while choosing a set of 1:10 merger models (with varying orbital configurations, morphology of the host galaxy) from the GalMer galaxy merger library. We show that a minor merger triggers a prominent $m=1$ lopsidedness in stars of the host galaxy. The strength of the $m=1$ lopsidedness undergoes a transient amplification phase after each pericenter passage of the satellite, in concordance with past findings of exciting an $m=1$ lopsidedness by tidal encounters. However, once the merger happens, and the post-merger remnant readjusts itself, the lopsidedness disappears in short time-scale ($\sim$ 500-850 Myr). Furthermore, a delayed merger can drive a prolonged ($\sim$2 Gyr) lopsidedness in the host galaxy. We demonstrate that the $m=1$ lopsidedness rotates with a well-defined pattern speed which is much slower than the $m=2$ bar pattern speed, and is retrograde with respect to the bar. This gives rise to a dynamical scenario where the Inner Lindblad resonance of the $m=1$ lopsidedness falls in between the corotation and the Outer Lindblad resonance of the $m=2$ bar mode. A kinematic lopsidedness also arises in the host galaxy; the resulting temporal variation closely follows that of the density lopsidedness. The minor merger also triggers a transient off-centred stellar disc-dark matter halo configuration due to the tidal encounter with the satellite.

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arXiv2021

Hidden in Plain Sight: UVIT and MUSE Discovery of a Large, Diffuse Star-Forming Galaxy

We report the discovery of a nearby large, diffuse galaxy that shows star formation, using Ultra Violet Imaging Telescope (UVIT) far-UV observations, archival optical data from Multi-Unit Spectroscopic Explorer (MUSE) and Dark Energy Camera Legacy Survey (DECaLS), and InfraRed Survey Facility (IRSF) near-infrared observations. The galaxy was not detected earlier due to its superposition with the background galaxy, NGC 6902A. They were together mistakenly classified as an interacting system. NGC 6902A is at a redshift of 0.05554, but MUSE observations indicate that the interacting tail is a separate star-forming, foreground galaxy at a redshift of 0.00980. We refer to the new galaxy as UVIT J202258.73-441623.8 (UVIT J2022). The near-infrared observations show that UVIT J2022 has a stellar mass of 8.7$\times$10$^{8}$M$_{\odot}$. Its inner disk (R$<$4 kpc) shows UV and H$α$ emission from ongoing massive star formation. The rest of the disk is extremely low luminosity, has a low stellar surface density, and extends out to a radius of R$\sim$9 kpc. The velocity and metallicity distribution maps and the star formation history indicate that UVIT J2022 has undergone three bursts of star formation. The latest episode is ongoing, which is supported by the presence of widespread H$α$ and UV emission in its inner disk. The galaxy also shows patchy spiral arms in far-UV, and there is a metallicity enhancement along a bar-like feature. UVIT J2022 is thus a unique example of triggered star formation in a diffuse galaxy, resulting in the growth of its inner stellar disk. Our study raises the intriguing possibility that (i) there could be similar diffuse galaxies that have been mistakenly interpreted as interacting galaxies due to their superposition, and (ii) UV or H$α$ could be a way to detect such diffuse galaxies in our local universe.

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arXiv2021

SAGAN -- III: New insights into giant radio quasars

Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs), propelling megaparsec-scale jets. In order to understand GRQs and their properties, we have compiled all known GRQs ("the GRQ catalogue"), and a subset of small (size <700 kpc) radio quasars (SRQs) from the literature. In this process, we have found 10 new FR-II GRQs, in the redshift range of 0.66 < z < 1.72, which we include in the GRQ catalogue. Using the above samples, we have carried out a systematic comparative study of GRQs and SRQs, using optical and radio data. Our results show that the GRQs and SRQs statistically have similar spectral index and black hole mass distributions. However, SRQs have higher radio core power, core dominance factor, total radio power, jet kinetic power and Eddington ratio compared to GRQs. On the other hand, when compared to giant radio galaxies (GRGs), GRQs have higher black hole mass and Eddington ratio. The high core dominance factor of SRQs is an indicator of them lying closer to the line of sight than GRQs. We also find a correlation of the accretion disc luminosity with the radio core and jet power of GRQs, which provides evidence for disc-jet coupling. Lastly, we find the distributions of Eddington ratios of GRGs and GRQs to be bi-modal, similar to that found in small radio galaxies (SRGs) and SRQs, which indicate that size is not strongly dependent on the accretion state. Using all of these, we provide a basic model for the growth of SRQs to GRQs.

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arXiv2021

CO kinematics unveil outflows plausibly driven by a young jet in the Gigahertz Peaked Radio Core of NGC6328

We report the detection of outflowing molecular gas in the center of the nearby (z=0.014) massive radio galaxy NGC6328. The radio core of the galaxy, PKS B1718-649, is identified as a Gigahertz Peaked Spectrum source with a compact (2 pc) double radio lobe morphology. We used ALMA CO(2-1) and CO(3-2) observations at 100 pc resolution to study the gas kinematics up to ~5 kpc from the galaxy center. While the bulk of the molecular gas is settled in a highly warped disk, in the inner 300 pc of the disk and along with the orientation of the radio jet, we identified high-excitation and high-velocity gas that cannot be attributed to any regular kinematic component based on our detailed 3D modeling of the ALMA data. The high-velocity dispersion in the gas also suggests that it is not part of an inflowing, shredding structure. These results suggest the presence of a molecular outflow of 3 to 8 solar masses per year. The outflow possibly originated from the interaction of the jet with the dense interstellar medium, even though the radio emission is detected closer to the center than the outflow. In this sense, this source resembles NGC1377, 4C31.04 and ESO420-G13, in which the outflows are linked to faint or past jet activity.

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arXiv2021

1321+045: a Compact Steep Spectrum radio source in a cool-core galaxy cluster

Cluster-central gigahertz peak and compact steep spectrum (CSS) sources offer an opportunity to study the earliest phases of AGN feedback, but few have yet been examined in detail. We present results from radio and X-ray observations of 1321+045, a CSS source in a 4.4 keV cluster at z=0.263. The cluster has a strongly cooling core, and disturbances from a minor cluster merger may have triggered a period of jet activity which formed the 16 kpc radio lobes 2.0 [+0.3,-0.2] Myr ago. However, new VLBA imaging shows a ~20 pc jet on a different projected axis, which is probably only a few hundred years old. We consider possible histories for the system, with either one or two periods of jet activity. While this single system is informative, a broader study of the youngest cluster-central radio sources is desirable.

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arXiv2021

Extensive Lensing Survey of Optical and Near-Infrared Dark Objects (El Sonido): HST H-Faint Galaxies behind 101 Lensing Clusters

We present a Spitzer/IRAC survey of H-faint ($H_{160} \gtrsim 26.4$, $<5σ$) sources in 101 lensing cluster fields. Across a CANDELS/Wide-like survey area of $\sim$648 arcmin$^2$ (effectively $\sim$221 arcmin$^2$ in the source plane), we have securely discovered 53 sources in the IRAC Channel-2 band (CH2, 4.5 $\mathrm{μm}$; median CH2$=22.46\pm0.11$ AB mag) that lack robust HST/WFC3-IR F160W counterparts. The most remarkable source in our sample, namely ES-009 in the field of Abell 2813, is the brightest H-faint galaxy at 4.5 $\mathrm{μm}$ known so far ($\mathrm{CH2}=20.48\pm0.03$ AB mag). We show that the H-faint sources in our sample are massive (median $M_\mathrm{star} = 10^{10.3\pm 0.3}$ $M_\odot$), star-forming (median star formation rate $=100_{-40}^{+60}$ $M_\odot$yr$^{-1}$) and dust-obscured ($A_V=2.6\pm0.3$) galaxies around a median photometric redshift of $z=3.9\pm0.4$. The stellar continua of 14 H-faint galaxies can be resolved in the CH2 band, suggesting a median circularized effective radius ($R_\mathrm{e,circ}$; lensing corrected) of $1.9\pm0.2$ kpc and $<1.5$ kpc for the resolved and whole samples, respectively. This is consistent with the sizes of massive unobscured galaxies at $z\sim4$, indicating that H-faint galaxies represent the dusty tail of the distribution of a wider galaxy population. Comparing with the ALMA dust continuum sizes of similar galaxies reported previously, we conclude that the heavy dust obscuration in H-faint galaxies is related to the compactness of both stellar and dust continua ($R_\mathrm{e,circ}\sim 1$ kpc). These H-faint galaxies make up $16_{-7}^{+13}$% of the galaxies in the stellar mass range of $10^{10}-10^{11.2}$ $M_\odot$ at $z=3\sim5$, contributing to $8_{-4}^{+8}$% of the cosmic star formation rate density in this epoch and likely tracing the early phase of massive galaxy formation.

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arXiv2021

Science with SKA

Highlights are presented about the science to be done with SKA. as well as state of the art science already done today with its precursors (MeerKAT, ASKAP) and pathfinders (LOFAR, NenuFAR), with accent on the expected breakthroughs.

astro-ph.CO↗

arXiv2021

A Triple AGN in the NGC 7733-7734 Merging Group

Context: Galaxy interactions and mergers can lead to supermassive black hole (SMBH) binaries which become active galactic nuclei (AGN) pairs when the SMBHs start accreting mass. If there is a third galaxy involved in the interaction, then a triple AGN system can form. Aims: Our goal is to investigate the nature of the nuclear emission from the galaxies in the interacting pair NGC\,7733--NGC\,7734 using archival VLT/MUSE Integral field spectrograph data and study its relation to the stellar mass distribution traced by near-infrared (NIR) observations from the South African Astronomical Observatory (SAAO). Methods:We conducted near-infrared observations using the SAAO and identified the morphological properties of bulges in each galaxy. We used MUSE data to obtain a set of ionized emission lines from each galaxy and studied the ionization mechanism. We also examined the relation of the galaxy pair with any nearby companions with Far-UV observations using the UVIT. Conclusions: The emission line analysis from the central regions of NGC\,7733 and NGC\,7734 show Seyfert and LINER type AGN activity. The galaxy pair NGC\,7733--34 also shows evidence of a third component, which has Seyfert-like emission. Hence, the galaxy pair NGC\,7733--34 forms a triple AGN system. We also detected an Extended Narrow-line region (ENLR) associated with the nucleus of NGC\,7733.

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arXiv2021

Fate of stellar bars in minor merger of galaxies

Minor merger of galaxies are common during the evolutionary phase of galaxies. Here, we investigate the dynamical impact of a minor merger (mass ratio 1:10) event on the final fate of a stellar bar in the merger remnant. To achieve that, we choose a set of minor merger models from the publicly available GalMer library of galaxy merger simulations. The models differ in terms of their orbital energy, orientation of the orbital spin vector, and morphology of the satellite galaxy (discy/spheroidal). We demonstrate that the central stellar bar, initially present in the host galaxy, undergoes a transient bar amplification phase after each pericentre passage of the satellite; in concordance with past studies of bar excitation due to tidal encounter. However, once the merger happens, the central stellar bar weakens substantially in the post-merger remnants. The accumulation of satellite's stars in the central region of merger remnant plays a key role in the bar weakening process; causing a net increase in the central mass concentration as well as in the specific angular momentum content. We find that the efficiency of mass accumulation from the satellite in the central parts of merger remnants depends on the orbital parameters as well as on the satellite's morphology. Consequently, different minor merger models display different degrees of bar weakening event. This demonstrates that minor merger of galaxies is a plausible avenue for bar weakening in disc galaxies.

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arXiv2021

ALMA 1.3 mm Survey of Lensed Submillimeter Galaxies (SMGs) Selected by Herschel: Discovery of Spatially Extended SMGs and Implications

We present an ALMA 1.3 mm (Band 6) continuum survey of lensed submillimeter galaxies (SMGs) at $z=1.0\sim3.2$ with an angular resolution of $\sim0.2$". These galaxies were uncovered by the Herschel Lensing Survey (HLS), and feature exceptionally bright far-infrared continuum emission ($S_\mathrm{peak} \gtrsim 90$ mJy) owing to their lensing magnification. We detect 29 sources in 20 fields of massive galaxy clusters with ALMA. Using both the Spitzer/IRAC (3.6/4.5 $\mathrm{μm}$) and ALMA data, we have successfully modeled the surface brightness profiles of 26 sources in the rest-frame near- and far-infrared. Similar to previous studies, we find the median dust-to-stellar continuum size ratio to be small ($R_\mathrm{e,dust}/R_\mathrm{e,star} = 0.38\pm0.14$) for the observed SMGs, indicating that star formation is centrally concentrated. This is, however, not the case for two spatially extended main-sequence SMGs with a low surface brightness at 1.3 mm ($\lesssim 0.1$ mJy arcsec$^{-2}$), in which the star formation is distributed over the entire galaxy ($R_\mathrm{e,dust}/R_\mathrm{e,star}>1$). As a whole, our SMG sample shows a tight anti-correlation between ($R_\mathrm{e,dust}/R_\mathrm{e,star}$) and far-infrared surface brightness ($Σ_\mathrm{IR}$) over a factor of $\simeq$ 1000 in $Σ_\mathrm{IR}$. This indicates that SMGs with less vigorous star formation (i.e., lower $Σ_\mathrm{IR}$) lack central starburst and are likely to retain a broader spatial distribution of star formation over the whole galaxies (i.e., larger $R_\mathrm{e,dust}/R_\mathrm{e,star}$). The same trend can be reproduced with cosmological simulations as a result of central starburst and potentially subsequent "inside-out" quenching, which likely accounts for the emergence of compact quiescent galaxies at $z\sim2$.

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arXiv2020

Massive molecular gas reservoir around the central AGN in the CARLA J1103+3449 cluster at z=1.44

Passive early-type galaxies dominate cluster cores at z $\lesssim$1.5. At higher redshift, cluster core galaxies are observed to have still on-going star-formation, fuelled by cold molecular gas. We measure the molecular gas reservoir of the central region around the radio-loud AGN in the cluster CARLA J1103+3449 at z=1.44 with NOEMA. The AGN synchrotron emission dominates the continuum emission at 94.48 GHz, and we measure its flux at the AGN position and at the position of two radio jets. Combining our measurements with published results over the range 4.71 GHz-94.5 GHz, we obtain a flat spectral index $α= 0.14 \pm 0.03$ for the AGN core emission, and a steeper index $α= 1.43 \pm 0.04$ and $α= 1.15 \pm 0.04$ at positions close to the western and eastern lobe, respectively. The total spectral index is $α= 0.92 \pm 0.02$ over the range 73.8 MHz-94.5 GHz. We detect two CO(2-1) emission lines, both blue-shifted with respect to the AGN. Their emission corresponds to two regions, ~17 kpc south-east and ~14 kpc south-west of the AGN, not associated with galaxies. In these two regions, we find a total massive molecular gas reservoir of $M_{gas}$ = 3.9 $\pm$ 0.4 $10^{10} M_{\odot}$, which dominates (~ 60%) the central total molecular gas reservoir. These results can be explained by massive cool gas flows in the center of the cluster. The AGN early-type host is not yet quenched; its star formation rate is consistent with being on the main sequence of star-forming galaxies in the field (SFR~30-140 $M_{\odot}$/yr), and the cluster core molecular gas reservoir is expected to feed the AGN and the host star-formation before quiescence. The other cluster confirmed members show star formation rates at ~2 $σ$ below the field main sequence at similar redshifts and do not have molecular gas masses larger than galaxies of similar stellar mass in the field.

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arXiv2020

SAGAN-II : Molecular gas content of giant radio galaxies

Radio galaxies with jets of relativistic particles are usually hosted by massive elliptical galaxies with active nuclei powered by accretion of interstellar matter onto a supermassive black hole. In some rare cases (<5%), their jets drive the overall structure to sizes larger than 700 kpc, and they are called giant radio galaxies (GRGs). A very small fraction of the population of such radio galaxies contains molecular and atomic gas in the form of rings or discs that can fuel star formation. The origin of this gas is not well known; it has sometimes been associated with a minor merger with a gas-rich disc galaxy (e.g. Centaurus A) or cooling of material from a hot X-ray atmosphere (e.g. cooling flows). The giant radio jets might be the extreme evolution of these objects, and they can teach us about the radio galaxy evolution. We selected 12 targets from a catalogue of 820 GRGs that are likely to be in a gas-accretion and star formation phase. The targets were selected from the mid-infrared to contain heated dust. We report here the results of IRAM-30m observations, the molecular gas content, and the star formation efficiency, and we discuss the origin of the gas and disc morphology. Three out of our 12 targets are detected, and for the others, we report significant upper limits. We combine our three detections and upper limits with four additional detected GRGs from the literature to discuss the results. Most of the GRG targets belong to the main sequence, and a large fraction are in the passive domain. Their star formation efficiency is comparable to normal galaxies, except for two galaxies that are deficient in molecular gas with a short (~200Myr) depletion time, and a quiescent gas-rich giant spiral galaxy. In general, the depletion time is much longer than the lifetime of the giant radio jet.

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