arXiv subjects

Gabriele Pezzulli

Publications and source records attributed to Gabriele Pezzulli.

At least 37 records · Page 2Linked to original sources

arXiv2024

The density of the Milky Way's corona at $z\approx 1.6$ through ram pressure stripping of the Draco dSph galaxy

Satellite galaxies within the Milky Way's (MW) virial radius $R_{\mathrm{vir}}$ are typically devoid of cold gas due to ram pressure stripping by the MW's corona. The density of this corona is poorly constrained today and essentially unconstrained in the past, but can be estimated using ram pressure stripping. In this paper, we probe the MW corona at $z\approx 1.6$ using the Draco dwarf spheroidal galaxy. We assume that i) Draco's orbit is determined by its interaction with the MW, whose dark matter halo we evolve in time following cosmologically-motivated prescriptions, ii) Draco's star formation was quenched by ram pressure stripping and iii) the MW's corona is approximately smooth, spherical and in hydrostatic equilibrium. We used GAIA proper motions to set the initial conditions and Draco's star formation history to estimate its past gas content. We found indications that Draco was stripped of its gas during the first pericentric passage. Using 3D hydrodynamical simulations at a resolution that enables us to resolve individual supernovae and assuming no tidal stripping, which we estimate to be a minor effect, we find a density of the MW corona $\geq 8\times 10^{-4}$ cm$^{-3}$ at a radius $\approx 0.72R_{\mathrm{vir}}$. This provides evidence that the MW's corona was already in place at $z\approx 1.6$ and with a higher density than today. If isothermal, this corona would have contained all the baryons expected by the cosmological baryon fraction. Extrapolating to today shows good agreement with literature constraints if feedback has removed $\lesssim 30$% of baryons accreted onto the halo.

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arXiv2023

Resolving the physics of Quasar Ly$α$ Nebulae (RePhyNe): I. Constraining Quasar host halo masses through Circumgalactic Medium kinematics

Ly$α$ nebulae ubiquitously found around z>2 quasars can supply unique constraints on the properties of the Circumgalactic Medium, such as its density distribution, provided the quasar halo mass is known. We present a new method to constrain quasar halo masses based on the line-of-sight velocity dispersion maps of Ly$α$ nebulae. By using MUSE-like mock observations obtained from cosmological hydrodynamic simulations under the assumption of maximal quasar fluorescence, we show that the velocity dispersion radial profiles of Ly$α$-emitting gas are strongly determined by gravity and that they are thus self-similar with respect to halo mass when rescaled by the virial radius. Through simple analytical arguments and by exploiting the kinematics of HeII1640Ȧ emission for a set of observed nebulae, we show that Ly$α$ radiative transfer effects plausibly do not change the shape of the velocity dispersion profiles but only their normalisation without breaking their self-similarity. Taking advantage of these results, we define the variable $η^{140-200}_{40-100}$ as the ratio of the median velocity dispersion in two specifically selected annuli and derive an analytical relation between $η^{140-200}_{40-100}$ and the halo mass which can be directly applied to observations. We apply our method to 37 observed quasar Ly$α$ nebulae at 3<z<4.7 and find that their associated quasars are typically hosted by ~$10^{12.16 \pm 0.14}$ M$_{\odot}$ haloes independent of redshift within the explored range. This measurement, which is completely independent of clustering methods, is consistent with the lowest mass estimates based on quasar auto-correlation clustering at z~3 and with quasar-galaxies cross-correlation results.

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arXiv2023

Clouds accreting from the IGM are not able to feed the star formation of low-redshift disc galaxies

Galactic halos accrete material from the intergalactic medium (IGM) and part of this accretion is expected to be in the form of cool ($T\sim10^4$ K) gas. A signature of this process could reside in the detection of numerous clouds in the circumgalactic medium (CGM). However, whether this material is able to accrete onto the galaxies and feed their star formation or, instead, evaporates into the CGM hot phase (corona, $T\sim10^6$ K), is not yet understood. Here, we investigate the evolution of cool CGM clouds accreted from the IGM and falling through the hot corona of low-redshift disc galaxies, using 3D high-resolution hydrodynamical simulations. We include the effects of gravity due to the dark matter halo, isotropic thermal conduction, radiative cooling and an ionizing UV background. We explored different values of parameters such as the halo mass, coronal mass, initial cloud velocity and strength of the thermal conduction. We find that the clouds lose the vast majority of their mass at distances larger than half of the galaxy virial radius and are completely dissolved in the corona before reaching the central galaxy. Resolving the Field length with at least 5-7 cells is crucial to correctly capture the evolution of the infalling cool gas. Our results indicate that cool IGM accretion can not feed star formation in $z\sim0$ star-forming galaxies in halos with masses of $10^{11.9}\ M_{\odot}$ or above. This suggests that present-day massive star-forming galaxies can sustain their star formation only via the spontaneous or induced cooling of their hot corona.

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arXiv2023

Characterizing the Circumgalactic Medium of Quasars at z $\sim$ 2.2 through H$α$ and Ly$α$ Emission

The discovery of giant quasar Ly$α$ nebulae at $z>2$ has opened up the possibility to directly study in emission the Circumgalactic and Intergalactic Medium (CGM/IGM). However, the resonant nature of the Ly$α$ line and its different emission mechanisms hamper the ability to constrain both the kinematics and physical properties of the CGM/IGM. Here, we present results of a pilot project aiming at the detection of CGM H$α$ emission, a line which does not suffer from these limitations. To this end, we first used KCWI to detect Ly$α$ emission around three bright quasars with $2.25 10^{12}$M$_{\odot}$).

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arXiv2023

Fountain-driven gas accretion feeding star formation over the disc of NGC 2403

We use a dynamical model of galactic fountain to study the neutral extraplanar gas (EPG) in the nearby spiral galaxy NGC 2403. We have modelled the EPG as a combination of material ejected from the disc by stellar feedback (i.e. galactic fountain) and gas accreting from the inner circumgalactic medium (CGM). This accretion is expected to occur because of cooling/condensation of the hot CGM (corona) triggered by the fountain. Our dynamical model reproduces the distribution and kinematics of the EPG H$\mathrm{\scriptsize{I}}$ emission in NGC 2403 remarkably well and suggests a total EPG mass of $4.7^{+1.2}_{-0.9}\times10^8\mathrm{M}_\odot$, with a typical scale height of around 1 kpc and a vertical gradient of the rotation velocity of $-10.0\pm2.7\,\mathrm{km\,s^{-1}\,kpc^{-1}}$. The best-fitting model requires a characteristic outflow velocity of $50\pm10\,\mathrm{km\,s^{-1}}$. The outflowing gas starts out mostly ionised and only becomes neutral later in the trajectory. The accretion rate from the condensation of the inner hot CGM inferred by the model is 0.8$\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$, approximately equal to the star formation rate in this galaxy (0.6$\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$). We show that the accretion profile, which peaks at a radius of about 4.5$\,$kpc, predicts a disc growth rate compatible with the observed value. Our results indicate that fountain-driven corona condensation is a likely mechanism to sustain star formation as well as the disc inside-out growth in local disc galaxies.

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arXiv2022

(Re)Solving Reionization with Lyα: How Bright Lyα Emitters account for the $z\approx2-8$ Cosmic Ionizing Background

The cosmic ionizing emissivity from star-forming galaxies has long been anchored to UV luminosity functions. Here we introduce an emissivity framework based on Ly$α$ emitters (LAEs), which naturally hones in on the subset of galaxies responsible for the ionizing background due to the intimate connection between the production and escape of Ly$α$ and LyC photons. Using constraints on the escape fractions of bright LAEs ($L_{\rm{Lyα}}>0.2 L^{*}$) at $z\approx2$ obtained from resolved Ly$α$ profiles, and arguing for their redshift-invariance, we show that: (i) quasars and LAEs together reproduce the relatively flat emissivity at $z\approx2-6$, which is non-trivial given the strong evolution in both the star-formation density and quasar number density at these epochs and (ii) LAEs produce late and rapid reionization between $z\approx6-9$ under plausible assumptions. Within this framework, the $>10\times$ rise in the UV population-averaged $f_{\rm{esc}}$ between $z\approx3-7$ naturally arises due to the same phenomena that drive the growing Ly$α$ emitter fraction with redshift. Generally, a LAE dominated emissivity yields a peak in the distribution of the ionizing budget with UV luminosity as reported in latest simulations. Using our adopted parameters ($f_{\rm{esc}}=50\%$, $ξ_{\rm{ion}}=10^{25.9}$ Hz erg$^{-1}$ for half the bright LAEs), a highly ionizing minority of galaxies with $M_{\rm UV}<-17$ accounts for the entire ionizing budget from star-forming galaxies. Rapid flashes of LyC from such rare galaxies produce a "disco" ionizing background. We conclude proposing tests to further develop our suggested Ly$α$-anchored formalism.

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arXiv2021

Inflow of low-metallicity cool gas in the halo of the Andromeda galaxy

As the closest $L^{\ast}$ galaxy to our own Milky Way, the Andromeda galaxy (M31) is an ideal laboratory for studies of galaxy evolution. The AMIGA project has recently provided observations of the cool ($T\sim10^4$ K) phase of the circumgalactic medium (CGM) of M31, using HST/COS absorption spectra along $\sim40$ background QSO sightlines, located up to and beyond the galaxy virial radius. Based on these data, and by the means of semi-analytic models and Bayesian inference, we provide here a physical description of the origin and dynamics of the cool CGM of M31. We investigate two competing scenarios, in which (i) the cool gas is mostly produced by supernova(SN)-driven galactic outflows or (ii) it mostly originates from infall of gas from the intergalactic medium. In both cases, we take into account the effect of gravity and hydrodynamical interactions with a hot corona, which has a cosmologically motivated angular momentum. We compare the outputs of our models to the observed covering factor, silicon column density and velocity distribution of the AMIGA absorbers. We find that, to explain the observations, the outflow scenario requires an unphysically large (> 100\%) efficiency for SN feedback. Our infall models, on the other hand, can consistently account for the AMIGA observations and the predicted accretion rate, angular momentum and metallicity are consistent with a cosmological infall from the intergalactic medium.

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arXiv2021

The Synchrony of Production & Escape: Half the Bright Ly$α$ Emitters at $z\approx2$ have Lyman Continuum Escape Fractions $\approx50\%$

The ionizing photon escape fraction (LyC $f_{\rm{esc}}$) of star-forming galaxies is the single greatest unknown in the reionization budget. Stochastic sightline effects prohibit the direct separation of LyC leakers from non-leakers at significant redshift. Here we circumvent this uncertainty by inferring $f_{\rm{esc}}$ with resolved (R>4000) LyA profiles from the X-SHOOTER LyA survey at z=2 (XLS-z2). We select leakers ($f_{\rm{esc}}>20$%) and non-leakers ($f_{\rm{esc}}<5$%) from a representative sample of $>0.2 L^{*}$ LyA emitters (LAEs). With median stacked spectra of these subsets covering 1000-8000 Å (rest-frame) we investigate the conditions for LyC $f_{\rm{esc}}$. We find the following differences between leakers vs. non-leakers: (i) strong nebular CIV and HeII emission vs. non-detections, (ii) O32~8.5 vs. ~3, (iii) Ha/Hb indicating no dust vs. E(B-V)~0.3, (iv) MgII emission close to the systemic velocity vs. redshifted, optically thick MgII, (v) LyA $f_{\rm{esc}}$ of ~50% vs. ~10%. The extreme EWs in leakers (O3+Hb~1100 Å) constrain the characteristic timescale of LyC escape to ~3-10 Myr bursts when short-lived stars with the hardest ionizing spectra shine. The defining traits of leakers -- extremely ionizing stellar populations, low column densities, a dust-free, high ionization state ISM -- occur simultaneously in the $f_{\rm{esc}}>20\%$ stack, suggesting they are causally connected, and motivating why indicators like O32 may suffice to constrain $f_{\rm{esc}}$ at z>6 with JWST. The leakers comprise half our sample, have a median LyC $f_{\rm{esc}}$~50%, and an ionising production efficiency $\log({ξ_{\rm{ion}}/\rm{Hz\ erg^{-1}}})$~25.9. These results show LAEs -- the type of galaxies rare at z=2, but that become the norm at higher redshift -- are highly efficient ionizers, with extreme $ξ_{\rm{ion}}$ and prolific $f_{\rm{esc}}$ occurring in sync. (ABRIDGED)

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arXiv2021

Implications of a spatially resolved main sequence for the size evolution of star forming galaxies

Two currently debated problems in galaxy evolution, the fundamentally local or global nature of the main sequence of star formation and the evolution of the mass-size relation of star forming galaxies (SFGs), are shown to be intimately related to each other. As a preliminary step, a growth function $g$ is defined, which quantifies the differential change in half-mass radius per unit increase in stellar mass ($g = d \log R_{1/2}/d \log M_\star$) due to star formation. A general derivation shows that $g = K Δ(sSFR)/sSFR$, meaning that $g$ is proportional to the relative difference in specific star formation rate between the outer and inner half of a galaxy, with $K$ a dimensionless structural factor for which handy expressions are provided. As an application, it is shown that galaxies obeying a fundamentally local main sequence also obey, to a good approximation, $g \simeq γn$, where $γ$ is the slope of the normalized local main sequence ($sSFR \propto Σ_\star^{-γ}$) and $n$ the Sersic index. An exact expression is also provided. Quantitatively, a fundamentally local main sequence is consistent with SFGs growing along a stationary mass-size relation, but inconsistent with the continuation at $z=0$ of evolutionary laws derived at higher $z$. This demonstrates that either the main sequence is not fundamentally local, or the mass-size relation of SFGs has converged to an equilibrium state some finite time in the past, or both.

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arXiv2021

A tight angular-momentum plane for disc galaxies

The relations between the specific angular momenta ($j$) and masses ($M$) of galaxies are often used as a benchmark in analytic models and hydrodynamical simulations as they are considered to be amongst the most fundamental scaling relations. Using accurate measurements of the stellar ($j_\ast$), gas ($j_{\rm gas}$), and baryonic ($j_{\rm bar}$) specific angular momenta for a large sample of disc galaxies, we report the discovery of tight correlations between $j$, $M$, and the cold gas fraction of the interstellar medium ($f_{\rm gas}$). At fixed $f_{\rm gas}$, galaxies follow parallel power laws in 2D $(j,M)$ spaces, with gas-rich galaxies having a larger $j_\ast$ and $j_{\rm bar}$ (but a lower $j_{\rm gas}$) than gas-poor ones. The slopes of the relations have a value around 0.7. These new relations are amongst the tightest known scaling laws for galaxies. In particular, the baryonic relation ($j_{\rm bar}-M_{\rm bar}-f_{\rm gas}$), arguably the most fundamental of the three, is followed not only by typical discs but also by galaxies with extreme properties, such as size and gas content, and by galaxies previously claimed to be outliers of the standard 2D $j-M$ relations. The stellar relation ($j_{\ast}-M_{\ast}-f_{\rm gas}$) may be connected to the known $j_\ast-M_\ast-$bulge fraction relation; however, we argue that the $j_{\rm bar}-M_{\rm bar}-f_{\rm gas}$ relation can originate from the radial variation in the star formation efficiency in galaxies, although it is not explained by current disc instability models.

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arXiv2021

The X-SHOOTER Lyman-$α$ survey at z=2 (XLS-z2) I: What makes a galaxy a Lyman-$α$ emitter?

We present the first results from the X-SHOOTER Lyman-$α$ survey at $z=2$ (XLS-$z2$). XLS-$z2$ is a deep spectroscopic survey of 35 Lyman-$α$ emitters (LAEs) utilising $\approx90$ hours of exposure time with VLT/X-SHOOTER and covers rest-frame Ly$α$ to H$α$ emission with R$\approx4000$. We present the sample selection, the observations and the data reduction. Systemic redshifts are measured from rest-frame optical lines for 33/35 sources. In the stacked spectrum, our LAEs are characterised by an interstellar medium with little dust, a low metallicity and a high ionisation state. The ionising sources are young hot stars that power strong emission-lines in the optical and high ionisation lines in the UV. The LAEs exhibit clumpy UV morphologies and have outflowing kinematics with blue-shifted SiII absorption, a broad [OIII] component and a red-skewed Ly$α$ line. Typically 30 % of the Ly$α$ photons escape, of which one quarter on the blue side of the systemic velocity. A fraction of Ly$α$ photons escapes directly at the systemic suggesting clear channels enabling a $\approx10$ % escape of ionising photons, consistent with an inference based on MgII. A combination of a low effective HI column density, a low dust content and young star-burst determine whether a star forming galaxy is observed as a LAE. The first is possibly related to outflows and/or a fortunate viewing angle, while we find that the latter two in LAEs are typical for their stellar mass of 10$^9$ M$_{\odot}$.

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arXiv2021

Constraining the cosmic UV background at z>3 with MUSE Lyman-α emission observations

The intensity of the Cosmic UV background (UVB), coming from all sources of ionising photons such as star-forming galaxies and quasars, determines the thermal evolution and ionization state of the intergalactic medium (IGM) and is, therefore, a critical ingredient for models of cosmic structure formation. Most of the previous estimates are based on the comparison between observed and simulated Lyman-$α$ forest. We present the results of an independent method to constrain the product of the UVB photoionisation rate and the covering fraction of Lyman limit systems (LLSs) by searching for the fluorescent Lyman-$α$ emission produced by self-shielded clouds. Because the expected surface brightness is well below current sensitivity limits for direct imaging, we developed a new method based on three-dimensional stacking of the IGM around Lyman-$α$ emitting galaxies (LAEs) between 2.9 3 that are consistent with previous measurements, with a preference for relatively low UVB intensities at z=3, and which suggest a non-monotonic decrease of $Γ$HI with increasing redshift between 3<z<5. This could suggest a possible tension between some UVB models and current observations which however require deeper and wider observations in Lyman-$α$ emission and absorption to be confirmed. Assuming instead a value of UVB from current models, our results constrain the covering fraction of LLSs at 3<z<4.5 to be less than 25% within 150kpc from LAEs.

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arXiv2020

Most of the cool CGM of star-forming galaxies is not produced by supernova feedback

The characterization of the large amount of gas residing in the galaxy halos, the so called circumgalactic medium (CGM), is crucial to understand galaxy evolution across cosmic time. We focus here on the the cool ($T\sim10^4$ K) phase of this medium around star-forming galaxies in the local universe, whose properties and dynamics are poorly understood. We developed semi-analytical parametric models to describe the cool CGM as an outflow of gas clouds from the central galaxy, as a result of supernova explosions in the disc (galactic wind). The cloud motion is driven by the galaxy gravitational pull and by the interactions with the hot ($T\sim10^6$ K) coronal gas. Through a bayesian analysis, we compare the predictions of our models with the data of the COS-Halos and COS-GASS surveys, which provide accurate kinematic information of the cool CGM around more than 40 low-redshift star-forming galaxies, probing distances up to the galaxy virial radii. Our findings clearly show that a supernova-driven outflow model is not suitable to describe the dynamics of the cool circumgalactic gas. Indeed, to reproduce the data, we need extreme scenarios, with initial outflow velocities and mass loading factors that would lead to unphysically high energy coupling from the supernovae to the gas and with supernova efficiencies largely exceeding unity. This strongly suggests that, since the outflows cannot reproduce most of the cool gas absorbers, the latter are likely the result of cosmological inflow in the outer galaxy halos, in analogy to what we have previously found for early-type galaxies.

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arXiv2020

Revealing the Impact of Quasar Luminosity on Giant Ly$α$ Nebulae

We present the results from a MUSE survey of twelve $z\simeq3.15$ quasars, which were selected to be much fainter (20<i<23) than in previous studies of Giant Ly$α$ Nebulae around the brightest quasars (16.6<i<18.7). We detect HI Ly$α$ nebulae around 100% of our target quasars, with emission extending to scales of at least 60 physical kpc, and up to 190 pkpc. We explore correlations between properties of the nebulae and their host quasars, with the goal of connecting variations in the properties of the illuminating QSO to the response in nebular emission. We show that the surface brightness profiles of the nebulae are similar to those of nebulae around bright quasars, but with a lower normalization. Our targeted quasars are on average 3.7 magnitudes (~30 times) fainter in UV continuum than our bright reference sample, and yet the nebulae around them are only 4.3 times fainter in mean Ly$α$ surface brightness, measured between 20 and 50 pkpc. We find significant correlations between the surface brightness of the nebula and the luminosity of the quasar in both UV continuum and Ly$α$. The latter can be interpreted as evidence for a substantial contribution from unresolved inner parts of the nebulae to the narrow components seen in the Ly$α$ lines of some of our faint quasars, possibly from the inner CGM or from the host galaxy's ISM.

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arXiv2020

The impact of the halo spin-concentration relation on disc scaling laws

Galaxy scaling laws, such as the Tully-Fisher, mass-size and Fall relations, can provide extremely useful clues on our understanding of galaxy formation in a cosmological context. Some of these relations are extremely tight and well described by one single parameter (mass), despite the theoretical existence of secondary parameters such as spin and concentration, which are believed to impact these relations. In fact, the residuals of these scaling laws appear to be almost uncorrelated with each other, posing significant constraints on models where secondary parameters play an important role. Here, we show that a possible solution is that such secondary parameters are correlated amongst themselves, in a way that removes correlations in observable space. In particular, we focus on how the existence of an anti-correlation between the dark matter halo spin and its concentration -- which is still debated in simulations -- can weaken the correlation of the residuals of the Tully-Fisher and mass-size relations. Interestingly, using simple analytic galaxy formation models, we find that this happens only for a relatively small portion of the parameter space that we explored, which suggests that this idea could be used to derive constraints to galaxy formation models that are still unexplored.

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arXiv2020

The volumetric star formation law for nearby galaxies -- Extension to dwarf galaxies and low-density regions

In the last decades, much effort has been put into finding the star formation law which could unequivocally link the gas and the star formation rate (SFR) densities measured on sub-kiloparsec scale in star-forming galaxies. The conventional approach of using the observed surface densities to infer star formation laws has however revealed a major and well-known issue, as such relations are valid for the high-density regions of galaxies but break down in low-density and HI-dominated environments. Recently, an empirical correlation between the total gas (HI+H$_2$) and the star formation rate (SFR) volume densities was obtained for a sample of nearby disc galaxies and for the Milky Way. This volumetric star formation (VSF) law is a single power-law with no break and a smaller intrinsic scatter with respect to the star formation laws based on the surface density. In this work, we explore the VSF law in the regime of dwarf galaxies in order to test its validity in HI-dominated, low-density, and low-metallicity environments. In addition, we assess this relation in the outskirts of spiral galaxies, which are low-density and HI-dominated regions similar to dwarf galaxies. Remarkably, we find the VSF law, namely $ρ_\mathrm{SFR} \propto ρ_\mathrm{gas}^α$ with $α\approx 2$, is valid for both these regimes. This result indicates that the VSF law, which holds unbroken for a wide range of gas ($\approx 3$ dex) and SFR ($\approx 6$ dex) volume densities, is the empirical relation with the smallest intrinsic scatter and is likely more fundamental than surface-based star formation laws.

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arXiv2020

The nature of CR7 revealed with MUSE: a young starburst powering extended Lyman-$α$ emission at z=6.6

CR7 is among the most luminous Lyman-$α$ emitters (LAEs) known at $z = 6.6$ and consists of at least three UV components that are surrounded by Lyman-$α$ (Ly$α$) emission. Previous studies have suggested that it may host an extreme ionising source. Here, we present deep integral field spectroscopy of CR7 with VLT/MUSE. We measure extended emission with a similar halo scale length as typical LAEs at $z\approx5$. CR7's Ly$α$ halo is clearly elongated along the direction connecting the multiple components, likely tracing the underlying gas distribution. The Ly$α$ emission originates almost exclusively from the brightest UV component, but we also identify a faint kinematically distinct Ly$α$ emitting region nearby a fainter component. Combined with new near-infrared data, the MUSE data show that the rest-frame Ly$α$ equivalent width (EW) is $\approx100$ Å. This is a factor four higher than the EW measured in low-redshift analogues with carefully matched Ly$α$ profiles (and thus arguably HI column density), but this EW can plausibly be explained by star formation. Alternative scenarios requiring AGN powering are also disfavoured by the narrower and steeper Ly$α$ spectrum and much smaller IR to UV ratio compared to obscured AGN in other Ly$α$ blobs. CR7's Ly$α$ emission, while extremely luminous, resembles the emission in more common LAEs at lower redshifts very well and is likely powered by a young metal poor starburst.

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arXiv2020

Evidence for supernova feedback sustaining gas turbulence in nearby star-forming galaxies

It is well known that gas in galaxy discs is highly turbulent, but there is much debate on which mechanism can energetically maintain this turbulence. Among the possible candidates, supernova (SN) explosions are likely the primary drivers but doubts remain on whether they can be sufficient in regions of moderate star formation activity, in particular in the outer parts of discs. In this paper, we measure the SN efficiency $η$, namely the fraction of the total SN energy needed to sustain turbulence in galaxies, and verify that SNe can indeed be the sole driving mechanism. The key novelty of our approach is that we take into account the increased turbulence dissipation timescale associated to the flaring in outer regions of gaseous discs. We analyse the distribution and kinematics of HI and CO in 10 nearby star-forming galaxies to obtain the radial profiles of the kinetic energy per unit area, for both the atomic gas and the molecular gas. We use a theoretical model to reproduce the observed energy with the sum of turbulent energy from SNe, as inferred from the observed star formation rate (SFR) surface density, and the gas thermal energy. We find that the observed kinetic energy is remarkably well reproduced by our model across the whole extent of the galactic discs, assuming $η$ constant with the galactocentric radius. Taking into account the uncertainties on the SFR surface density and on the atomic gas phase, we obtain that the median SN efficiencies for our sample of galaxies are $\langle η_\mathrm{atom} \rangle=0.015_{-0.008}^{+0.018}$ for the atomic gas and $\langle η_\mathrm{mol} \rangle = 0.003_{-0.002}^{+0.006}$ for the molecular gas. We conclude that SNe alone can sustain gas turbulence in nearby galaxies with only few percent of their energy and that there is essentially no need for any further source of energy.

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