arXiv subjects

Paola Severgnini

Publications and source records attributed to Paola Severgnini.

At least 19 recordsLinked to original sources

arXiv2026

Enhanced activity in close dual-AGN systems in the local Universe

We present the study of an X-ray selected sample of active galactic nuclei (AGN) in pairs at projected spatial separations 1 <$ r_p$/kpc < 100 at z < 0.1, using XMM-Newton and Chandra data. The pair sample is derived from an initial pool of approximately 2,000 X-ray-selected AGN, and is composed of both AGN-AGN pairs (so called dual AGN) and AGN-galaxy pairs. From this selection, we find that approximately 10% of AGN reside in pairs, and about 4% are paired with another AGN. We performed a detailed X-ray and SDSS optical spectral analysis for AGN in duals and X-ray analysis for AGN in AGN-galaxy pairs, to characterise their absorption properties and investigate the possible triggering mechanisms. We then investigated how obscuration, luminosity, and Eddington ratio depend on projected separation $r_p$. Amongst all AGN in pairs, we found that ~55% are obscured (with hydrogen column density $N_H$ > $10^{22}$ cm$^{-2}$), amongst which ~6% are Compton-thick ($N_H$ > $10^{24}$ cm$^{-2}$). The fraction of absorbed AGN is significantly higher in late-stage mergers ($r_p$ < 30 kpc) compared to early-stage mergers ($r_p$ > 60 kpc). Amongst the AGN in pairs, we also observed an average excess of AGN pairs with respect to a control sample of inactive galaxies in pairs, and that such excess significantly increases with decreasing $r_p$ only for obscured AGN. Finally, in dual-AGN systems, both the bolometric luminosity and the Eddington ratio of the less massive black hole in the pair increase as the separation decreases. These findings suggest that mergers may have an important role in triggering AGN accretion and activity.

astro-ph.GA↗

arXiv2026

Milky-Way-like stars in a galaxy core 8 billion years ago revealed by gravitational lensing

The assembly of stellar-dominated cores in elliptical galaxies is key to understanding how cosmic structures evolved. Gravitational lensing offers unique insights into the nature of their stars. We report the discovery of the smallest known quadruply lensed quasar (radius ~0.2"), whose lensing galaxy at redshift 1.055 (5.5 billion years after the Big Bang) features a lensing mass of only ~2x10^10 M_sun. A Bayesian analysis, based on the system's exceptional properties and standard scaling relations, allowed us to sample the central galactic initial mass function with unmatched accuracy and in a previously uncharted regime in terms of mass and redshift. We found it consistent with the Milky Way one, while excluding bottom-heavy functions. This suggests that the core either grew slowly or underwent early disruptive events altering its stellar build-up, in contrast with the classical view that bulges form rapidly and remain unchanged by later interactions.

astro-ph.GA↗

arXiv2025

Black hole merger rates for LISA and LGWA from semi-analytical modelling of light seeds

With the upcoming space- and Moon-based gravitational-wave detectors, LISA and LGWA respectively, a new era of GW astronomy will begin with the possibility of detections of the mergers of intermediate-mass black holes (IMBHs) and supermassive black holes (SMBHs). We generate populations of synthetic black hole (BH) binaries with masses ranging from the intermediate ($10^3-10^5 M_\odot$) to the supermassive regime ($>10^5 M_\odot$), formed from the dynamical processes of merging halos and their residing galaxies, assuming that each galaxy is initially seeded with a single black hole at its centre. The aim is to estimate the rate of these BH mergers which could be detected by LISA and LGWA. Using PINOCCHIO cosmological simulation and a semi-analytical model based on GAEA, we construct a population of merging BHs by implementing a "light" seeding scheme and calculating the merging timescales using the Chandrasekhar prescription. We provide upper and lower limits of dynamical friction timescale by varying the mass of the infalling object to create "pessimistic" and "optimistic" merger rates respectively. We find that for our synthetic population of MBHs, both LGWA and LISA are able to detect more than $15$ binary IMBH mergers per year in the optimistic case, while in the pessimistic case less than $\sim5$ detections would be possible considering the entire lifetime of the detectors. For SMBHs, the rates are slightly lower in both cases. Most mergers below $z\approx4$ are detected in the optimistic case, although mergers beyond $z=8$ are also detectable at a lower rate. We find that LGWA is better suited for high-SNR IMBH detections at higher redshift, while LISA is more sensitive to massive SMBHs. Joint observations will probe the full BH mass spectrum and constrain BH formation and seeding models.

astro-ph.CO↗

arXiv2025

Variability in the supermassive black hole binary candidate SDSS J2320+0024: No evidence of periodic modulation

Supermassive black hole binaries (SBHBs) are a natural outcome of galaxy mergers, and they are expected to be among the loudest gravitational-wave sources at low frequencies. The source SDSS J2320+0024 was recently proposed as a promising SBHB candidate due to a possible periodicity in its light curve and variability in the MgII emission line. In this work, we reanalysed the optical (g, r, and i bands) light curves of J2320+0024 within the framework of Bayesian model selection. When periodicity was searched for together with red noise, analysis of the g-band light curve reveals a peak in the posterior of the period at ~290 days. The posterior profile is too broad to yield a preference for periodic models over models that include only red noise. Furthermore, the same peak is not present in the analysis of the r-band and i-band light curve. A periodic model without red noise identified a different (~1100 days) periodicity, but this model is statistically significantly disfavoured relative to the other models tested. In summary, we find no significant evidence in favour of a true periodic signal over red-noise variability. Our analysis questions the robustness of the previously proposed periodicity and emphasises the importance of rigorous statistical treatment. While our findings challenge the binary interpretation for J2320+0024, they do not rule it out. A statistically robust joint analysis of the photometric light curves and evolving broad-line profiles would shed further light on the true nature of this object.

astro-ph.GA↗

arXiv2025

ESPRESSO reveals a single but perturbed broad-line region in the supermassive black hole binary candidate PG 1302-102

We present a new, Bayesian analysis of the highest-resolution optical spectrum of the supermassive black hole (SMBH) binary candidate PG 1302-102, obtained with ESPRESSO@VLT (R \simeq 138, 000). Our methodology, based on robust Bayesian model selection, reveals the presence of multiple narrow emission lines at the expected redshift of the source and confirms (for H\{beta}) and detects (for Hγ) the presence of redshifted broad components. Additionally, we have discovered a very broad and, if it is associated with the H\{beta}, very redshifted component at λ \simeq 5000Å. We evaluate two scenarios for explaining the observed broad emission line (BEL) in PG 1302-102. In the case in which the redshifted BEL asymmetry arises from the orbital motion of a putative binary, our measurements coupled with simple estimates of the broad-line region (BLR) sizes suggest that the individual black hole BLRs are either settled in a single BLR or in the process of merging and, therefore truncated and highly disturbed. Alternatively, in the scenario of a single SMBH, we explain the distorted emission of the BELs with a nonsymmetric distribution of the BLR clouds; namely, a thin disk with a spiral perturbation. This BLR configuration is statistically preferred over any empirical multi-Gaussian fit and simultaneously explains the asymmetric emission of the Hβ and Hγ close to the bulk of the line and any additional excess (or the lack of it, in the case of the Hγ) at much longer wavelengths. The physical origins of the perturbation are unclear, and a connection with the possible presence of a black hole binary cannot be ruled out. Given the growing evidence from theoretical and observational works demonstrating the common presence of disturbed BLRs in active galactic nuclei, we argue that an origin related to self-gravitating instabilities may be more plausible.

astro-ph.GA↗

arXiv2024

The Lunar Gravitational-wave Antenna: Mission Studies and Science Case

The Lunar Gravitational-wave Antenna (LGWA) is a proposed array of next-generation inertial sensors to monitor the response of the Moon to gravitational waves (GWs). Given the size of the Moon and the expected noise produced by the lunar seismic background, the LGWA would be able to observe GWs from about 1 mHz to 1 Hz. This would make the LGWA the missing link between space-borne detectors like LISA with peak sensitivities around a few millihertz and proposed future terrestrial detectors like Einstein Telescope or Cosmic Explorer. In this article, we provide a first comprehensive analysis of the LGWA science case including its multi-messenger aspects and lunar science with LGWA data. We also describe the scientific analyses of the Moon required to plan the LGWA mission.

gr-qc↗

arXiv2024

Atacama Large Aperture Submillimeter Telescope \mbox{(AtLAST)} Science: Probing the Transient and Time-variable Sky

The study of transient and variable events, including novae, active galactic nuclei, and black hole binaries, has historically been a fruitful path for elucidating the evolutionary mechanisms of our universe. The study of such events in the millimeter and submillimeter is, however, still in its infancy. Submillimeter observations probe a variety of materials, such as optically thick dust, which are hard to study in other wavelengths. Submillimeter observations are sensitive to a number of emission mechanisms, from the aforementioned cold dust, to hot free-free emission, and synchrotron emission from energetic particles. Study of these phenomena has been hampered by a lack of prompt, high sensitivity submillimeter follow-up, as well as by a lack of high-sky-coverage submillimeter surveys. In this paper, we describe how the proposed Atacama Large Aperture Submillimeter Telescope (AtLAST) could fill in these gaps in our understanding of the transient universe. We discuss a number of science cases that would benefit from AtLAST observations, and detail how AtLAST is uniquely suited to contributing to them. In particular, AtLAST's large field of view will enable serendipitous detections of transient events, while its anticipated ability to get on source quickly and observe simultaneously in multiple bands make it also ideally suited for transient follow-up. We make theoretical predictions for the instrumental and observatory properties required to significantly contribute to these science cases, and compare them to the projected AtLAST capabilities. Finally, we consider the unique ways in which transient science cases constrain the observational strategies of AtLAST, and make prescriptions for how AtLAST should observe in order to maximize its transient science output without impinging on other science cases.

astro-ph.CO↗

arXiv2024

SDSS J222428.53+261423.2: unique emission lines properties unveil a sub-kiloparsec dual AGN candidate

In this paper, we presented a detailed analysis of the Sloan Digital Sky Survey optical spectrum of a new sub-kpc scale dual AGN candidate SDSS J222428.53+261423.2 (=SDSS J2224). The target is one of the few AGNs with all the optical narrow emission lines characterized by double-peaked profiles and with peak separations in velocity units of about 930 km/s. If the double-peaked narrow emission lines (DPNELs) are due to a dual AGN in \obj, the estimated physical separation between the two cores is about 500 pc. Meanwhile, three alternative explanations are also discussed in this paper, however, we can not find solid evidence to completely rule them out. Our results support the presence of a sub-kpc dual AGN with DPNELs in all lines, indicating a key episode of galaxy merging evolution at sub-kpc scale.

astro-ph.GA↗

arXiv2023

The NuSTAR view of the changing look AGN ESO 323-G77

The presence of an obscuring torus at pc-scale distances from the central black hole is the main ingredient for the Unified Model of Active Galactic Nuclei (AGN), as obscured sources are thought to be seen through this structure. However, the Unified Model fails to describe a class of sources that undergo dramatic spectral changes, transitioning from obscured to unobscured and vice-versa through time. The variability in such sources, so-called Changing Look AGN (CLAGN), is thought to be produced by a clumpy medium at much smaller distances than the conventional obscuring torus. ESO 323-G77 is a CLAGN that was observed in various states through the years with Chandra, Suzaku, Swift-XRT and XMM-Newton, from unobscured ($N_{\rm H}<3\times10^{22}$ cm$^{-2}$) to Compton-thin ($N_{\rm H}\sim1-6\times10^{23}$ cm$^{-2}$) and even Compton-thick ($N_{\rm H}>1\times10^{24}$ cm$^{-2}$), with timescales as short as one month. We present the analysis of the first NuSTAR monitoring of ESO 323-G77, consisting of 5 observations taken at different timescales (1, 2, 4 and 8 weeks from the first one) in 2016-2017, in which the AGN was caught in a persistent Compton-thin obscured state ($N_{\rm H}\sim2-4\times10^{23}$ cm$^{-2}$). We find that a Compton-thick reflector is present ($N_{\rm H,refl}=5\times10^{24}$ cm$^{-2}$), most likely associated with the presence of the putative torus. Two ionized absorbers are unequivocally present, located within maximum radii of $r_{\rm max,1}=1.5$ pc and $r_{\rm max,2}=0.01$ pc. In one of the observations, the inner ionized absorber is blueshifted, indicating the presence of a possible faster ($v_{\rm out}=0.2c$) ionized absorber, marginally detected at $3σ$. Finally, we are able to constrain the coronal temperature and the optical depth of ESO 323-G77, obtaining $kT_e=38$ keV or $kT_e=36$ keV, and $τ=1.4$ or $τ=2.8$, depending on the coronal geometry assumed.

astro-ph.GA↗

arXiv2022

The X-ray view of optically selected dual AGN

We present a study of optically selected dual AGN with projected separations of 3--97~kpc. Using multi-wavelength (MWL) information (optical, X-rays, mid-IR), we characterized the intrinsic nuclear properties of this sample and compared them with those of isolated systems. Among the 124 X-ray detected AGN candidates, 52 appear in pairs and 72 as single X-ray sources. Through MWL analysis, we confirmed the presence of the AGN in a fraction >80\% of the detected targets in pairs (42 over 52). X-ray spectral analysis confirms the trend of increasing AGN luminosity with decreasing separation, suggesting that mergers may have contributed in triggering more luminous AGN. Through X/mid-IR ratio $vs$ X-ray colors, we estimated a fraction of Compton-thin AGN (with 10$^{22}$ cm$^{-2}$ $<$ N$_{\rm H} <$10$^{24}$ cm$^{-2}$) of about 80\%, while about 16\% are Compton thick (CT, with N$_{\rm H}>$10$^{24}$ cm$^{-2}$) sources. These fractions of obscured sources are larger than those found in samples of isolated AGN, confirming that pairs of AGN show higher obscuration. This trend is further confirmed by comparing the de-reddened [O\ III] emission with the observed X-ray luminosity. However, the derived fraction of Compton-thick sources in this sample at early stage of merging is lower than reported for late-merging dual-AGN samples. Comparing N$_{\rm H}$ from X-rays with that derived from E(B-V) from Narrow Line Regions, we find that the absorbing material is likely associated with the torus or the Broad Line Regions. We also explored the X-ray detection efficiency of dual-AGN candidates, finding that, when observed properly (at on-axis positions and with long exposures), X-ray data represent a powerful way to confirm and investigate dual-AGN systems.

astro-ph.GA↗

arXiv2022

Unveiling the population of dual- and lensed- AGNs at sub-arcsec separations

All cosmological models of structure formation predict the existence of a widespread population of dual supermassive black holes in-spiralling inside their common host galaxy, eventually merging and giving rise to intense gravitational waves. These systems can be identified as dual AGNs at kilo parsec separations, but only very few have been confirmed at z>0.5. The appearance of multiple AGNs at small angular separations can also be due to gravitational lensing of single AGNs, which are themselves very important systems for many astrophysical topics. Here we present a novel technique, dubbed the Gaia Multipeak (GMP) method, to obtain large and reliable samples of dual/lensed AGN candidates with sub-arcsec separations by looking for AGNs showing multiple peaks in the light profiles observed by the Gaia satellite. All the GMP-selected sources with high resolution images (26 from the HST archive and 5 from dedicated adaptive-optics assisted imaging at the Large Binocular Telescope) show multiple components with sub-arcsec separation pointing toward a very high reliability of the method. By sampling separations down to ~2 kpc at z>1, this method allows us to probe the physical processes that drive the inspiralling of a pair of SMBHs inside a single galaxy.

astro-ph.GA↗

arXiv2022

The central engine of the highest redshift blazar

We report on a LUCI/Large Binocular Telescope near-infrared (NIR) spectrum of PSO J030947.49+271757.31 (hereafter PSO J0309+27), the highest redshift blazar known to date (z$\sim$6.1). From the C$\rm IV$$λ$1549 broad emission line we found that PSO J0309+27 is powered by a 1.45$^{+1.89}_{-0.85}$$\times$10$^9$M$_{\odot}$ supermassive black hole (SMBH) with a bolometric luminosity of $\sim$8$\times$10$^{46}$ erg s$^{-1}$ and an Eddington ratio equal to 0.44$^{+0.78}_{-0.35}$. We also obtained new photometric observations with the Telescopio Nazionale Galileo in J and K bands to better constrain the NIR Spectral Energy Distribution of the source. Thanks to these observations, we were able to model the accretion disk and to derive an independent estimate of the black hole mass of PSO J0309+27, confirming the value inferred from the virial technique. The existence of such a massive SMBH just $\sim$900 million years after the Big Bang challenges models of the earliest SMBH growth, especially if jetted Active Galactic Nuclei are associated to a highly spinning black hole as currently thought. Indeed, in a Eddington-limited accretion scenario and assuming a radiative efficiency of 0.3, typical of a fast rotating SMBH, a seed black hole of more than 10$^6$ M$_{\odot}$ at z = 30 is required to reproduce the mass of PSO J0309+27 at redshift 6. This requirement suggests either earlier periods of rapid black hole growth with super-Eddington accretion and/or that only part of the released gravitational energy goes to heat the accretion disk and feed the black hole.

astro-ph.HE↗

arXiv2021

Minute-timescale variability in the X-ray emission of the highest redshift blazar

We report on two Chandra observations of the quasar PSO J0309+27, the most distant blazar observed so far (z=6.1), performed eight months apart, in March and November 2020. Previous Swift-XRT observation showed that this object is one of the brightest X-ray sources beyond redshift 6.0 ever observed so far. This new data-set confirmed the high flux level and unveiled a spectral change occurred on a very short timescale (250s rest-frame), caused by a significant softening of the emission spectrum. This kind of spectral variability, on a such short interval, has never been reported in the X-ray emission of a flat spectrum radio quasar. A possible explanation is given by the emission produced by the inverse Compton scatter of the quasar UV photons by the cold electrons present in a fast shell moving along the jet. Although this bulk comptonization emission should be an unavoidable consequence of the standard leptonic jet model, this would be the first time that it is observed.

astro-ph.HE↗

arXiv2020

Lunar Gravitational-Wave Antenna

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.

gr-qc↗

arXiv2020

Unveiling sub-parsec supermassive black hole binary candidates in active galactic nuclei

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$α$ 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 $3.7σ$ 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$α$ emission lines.

astro-ph.HE↗

arXiv2019

Testing the blast-wave AGN feedback scenario in MCG-03-58-007

We report the first Atacama large millimeter/submillimeter array observations of MCG-03-58-007, a local ($z=0.03236\pm0.00002$, this work) AGN ($L_{AGN}\sim10^{45}~\rm erg~s^{-1}$), hosting a powerful X-ray ultra-fast ($v=0.1c$) outflow (UFO). The CO(1-0) line emission is observed across $\sim18\,$kpc scales with a resolution of $\sim 1\,\rm kpc$. About 78\% of the CO(1-0) luminosity traces a galaxy-size rotating disk. However, after subtracting the emission due to such rotating disk, we detect with a S/N=20 a residual emission in the central $\sim 4\,$kpc. Such residuals may trace a low velocity ($v_{LOS}=170\,\rm km\,s^{-1}$) outflow. We compare the momentum rate and kinetic power of such putative molecular outflow with that of the X-ray UFO and find $\dot{P}_{out}/\dot{P}_{UFO}=0.3\pm0.2$ and $\dot{E}_{mol}/\dot{E}_{UFO}\sim4\cdot10^{-3}$. This result is at odds with the energy-conserving scenario suggested by the large momentum boosts measured in some other molecular outflows. An alternative interpretation of the residual CO emission would be a compact rotating structure, distinct from the main disk, which would be a factor of $\sim10-100$ more extended and massive than typical circumnuclear disks revealed in Seyferts. However, in both scenarios, our results rule out the hypothesis of a momentum-boosted molecular outflow in this AGN, despite the presence of a powerful X-ray UFO. [Abridged]

astro-ph.GA↗

arXiv2019

The space density of z>4 blazars

High redshift blazars are an important class of Active Galactic Nuclei (AGN) that can provide an independent estimate of the supermassive black-hole mass function in high redshift radio-loud AGN without the bias due to absorption along the line-of-sight. Using the Cosmic Lens All Sky Survey (CLASS) we built a complete radio flux-limited sample of high redshift (z>4) blazars suitable for statistical studies. By combining dedicated optical observations and the SDSS spectroscopic database, we obtained a sample of 26 blazar candidates with a spectroscopic redshift above 4. On the basis of their radio spectrum we distinguish between blazars and QSO with a Gigahertz Peaked Spectrum (GPS) like spectrum. Out of the 18 confirmed blazars 14 constitute a completely identified, flux-limited sample down to a magnitude of 21 (AB). Using this complete sample we derive a space density of blazars with 4<z<5.5 of rho=0.13 (+0.05,-0.03) Gpc^-3. This is the first actual estimate of the blazar space density in this range of redshift. This value is in good agreement with the extrapolation of the luminosity function and cosmological evolution based on a sample of flat-spectrum radio quasars selected at lower redshifts and it is consistent with a cosmological evolution peaking at z$\sim$2 similar to radio-quiet QSO. We do not confirm, instead, the presence of a peak at z~4 in the space density evolution, recently suggested using an X-ray selected sample of blazars. It is possible that this extreme peak of the evolution is present only among the most luminous blazars.

astro-ph.GA↗

arXiv2018

ALMA [CI]$^{3}P_{1}-^{3}P_{0}$ observations of NGC6240: a puzzling molecular outflow, and the role of outflows in the global $α_{\rm CO}$ factor of (U)LIRGs

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 ($α_{\rm CO}$) 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 $\langleα_{\rm CO}\rangle = 3.2\pm1.8~\rm M_{\odot} (K~km~s^{-1}~pc^2)^{-1}$, 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 $α_{\rm CO}$ and increasing their $r_{21}$ values.

astro-ph.GA↗