arXiv subjects

R. Gobat

Publications and source records attributed to R. Gobat.

At least 19 recordsLinked to original sources

arXiv2026

Euclid: Disky titans -- surprisingly high star formation efficiency in two brightest group galaxies at $z\sim 0.75$

We present the discovery of two disky titans in the first data release of the Euclid satellite. These sources are massive ($M>10^{11} M_\odot$) star-forming (SFR $\sim 20 M_\odot$/yr) discs located in strong over-densities at intermediate redshift ($z\sim 0.75$). They represent an small fraction of the massive galaxies in over-dense regions (just four candidates in more than 20 deg2 analysed in this study), and their existence is puzzling considering the abundance of passive and bulge-dominated sources commonly found at the centre of groups and clusters at low redshift. Firstly, our analysis shows that these objects are located in massive groups ($M_h\sim 10^{13.8} M_\odot$), where rapid accretion of cold gas should be prevented from the formation of a static hot halo. Despite this, a millimetre follow-up with NOEMA shows significant cold gas reservoirs $M_{h_2} \sim 10^{10.3} M_\odot$) within these sources. Secondly, our morphological analysis shows the presence of a massive and passive bulge in these galaxies, which is expected to stabilise the disc against fragmentation thereby suppressing further star formation. However, these sources lie on the Schmidt-Kennicutt relation or even slightly above. Building on these observations, we propose a scenario where these disky titans are the product of a merger-induced rejuvenation episode, in which the most massive galaxy of a group accretes cold gas from another member and briefly restarts star-formation. Such scenario is supported by a comparison with the TNG300 simulation and easily explains the surviving of star-formation activity in massive galaxies in over-dense environments as temporary stages in a more complex evolution. More in general, our study showcases the ability of Euclid to find rare objects thanks to the unprecedented statistics offered by its surveys and the scientific potential residing in the synergy between Euclid and other facilities observing at longer wavelengths.

astro-ph.GA↗

arXiv2026

A first [CII] view of high-z quiescent galaxies

We present ALMA detections (or stringent upper limits) of the [CII] 158 $\mu m$ emission line and underlying dust continuum from five massive quenched galaxies (QGs) at 2<z<4.7. We find extreme variations in the molecular gas fractions ($\rm{f_g=M_{mol}/M_{\star}}$), spanning 0.1%-25%, if a standard $\rm{\alpha_{[CII]}}$ applies. We attempt a first empirical calibration of $\rm{\alpha_{[CII]}}$ with respect to dust continuum in a $z=2$ lensed QG and with respect to CO(3-2) in a $z=3.1$ QG, finding no evidence of strong deviations from the standard value. Dust continuum measurements, coupled with JWST/MIRI fluxes, suggest higher dust temperatures compared to expectations from $z<2$ QGs, reaching $T_{d}\sim40-50 \,K$ in two galaxies. Coupled with remarkably high total infrared luminosities (LIR) not explained by observed JWST colors not by energy balance based on literature dust extinction measurements, and with [CII] deficits down to $\rm{[CII]/LIR\sim 2\times10^{-4}}$ typical of (Ultra)Luminous Infrared Galaxies, our findings point to additional dust-heating mechanisms other than dust-absorbed stellar radiation. Surprisingly, JWST/NIRCam and ALMA imaging reveal widespread disturbed stellar morphologies and offsets/tails in dust and gas, indicative of ongoing interactions. While larger samples are needed to assess how common these features are in high-z QGs, these findings support a merger-driven origin for the phenomenology observed in these systems, with key similarities with respect to local post-starburst galaxies where low-velocity shocks and turbulence also inject energy into the residual ISM.

astro-ph.GA↗

arXiv2024

The ALPINE-ALMA [CII] Survey: Modelling ALMA and JWST lines to constrain the interstellar medium of $z\sim 5$ galaxies

In this work, we devise a model for estimating UV and optical line emission (i.e., CIII] $1909$A, H$\beta$, [OIII] $5007$A, H$\alpha$, [NII] $6583$A) tracing HII regions in the interstellar medium (ISM) of galaxies at $z\sim4-6$ from the ALMA Large Programme ALPINE. The aim is to investigate the impact of binary stars in the stellar population along with an abrupt quenching in the Star Formation History (SFH) on line emission. This is crucial for understanding the ISM's properties in early galaxies and identifying new star formation tracers in high-$z$ galaxies. The model simulates HII+Photodissociation Region (PDR) complexes through radiative transfer in 1D slabs, characterized by gas density ($n$), ionisation parameter ($U$), and metallicity ($Z$). It considers: (a) heating from star formation (SF), simulated with Starburst99 and BPASS to quantify binary stars impact; (b) constant, exponentially declining, and quenched SFH scenarios. For each galaxy, we select theoretical ratios from CLOUDY models between [CII] line emission, tracing PDRs, and nebular lines from HII regions, using these to derive expected optical/UV lines from observed [CII]. We find binary stars strongly impact line emission post-quenching, keeping UV photon flux higher for longer, maintaining free electron temperature and ionised column density in HII regions up to 5 Myr after quenching. We constrain ISM properties of our subsample, finding a low ionisation parameter $\log U{\approx}-3.8\pm 0.2$ and moderate/high densities $\log(n/\rm cm^{-3}){\approx}2.9\pm 0.6$. Finally, we derive UV/optical line luminosities-SFR relations for different burstiness parameters ($k_s$). In the fiducial BPASS model, relations show negligible SFH dependence but depend on $k_s$, while in the SB99 case, dependence is on SFH. We propose their use for characterising the burstiness of high-$z$ galaxies.

astro-ph.GA↗

arXiv2024

The ALPINE-ALMA [CII] Survey: Dust emission effective radius up to 3 kpc in the Early Universe

Measurements of the size of dust continuum emission are an important tool for constraining the spatial extent of star formation and hence the build-up of stellar mass. Compact dust emission has generally been observed at Cosmic Noon (z~2-3). However, at earlier epochs, toward the end of the Reionization (z~4-6), only the sizes of a handful of IR-bright galaxies have been measured. In this work, we derive the dust emission sizes of main-sequence galaxies at z~5 from the ALPINE survey. We measure the dust effective radius r_e,FIR in the uv-plane in Band 7 of ALMA for seven ALPINE galaxies with resolved emission and we compare it with rest-frame UV and [CII]158$μ$m measurements. We study the r_e,FIR-L_IR scaling relation by considering our dust size measurements and all the data in literature at z~4-6. Finally, we compare our size measurements with predictions from simulations. The dust emission in the selected ALPINE galaxies is rather extended (r_e,FIR~1.5-3 kpc), similar to [CII]158 um but a factor of ~2 larger than the rest-frame UV emission. Putting together all the measurements at z~5, spanning 2 decades in luminosity from L_IR ~ 10^11 L_sun to L_IR ~ 10^13 L_sun, the data highlight a steeply increasing trend of the r_e,FIR-L_IR relation at L_IR< 10^12 L_sun, followed by a downturn and a decreasing trend at brighter luminosities. Finally, simulations that extend up to the stellar masses of the ALPINE galaxies considered in the present work predict a sub-set of galaxies (~25% at 10^10 M_sun < M_star < 10^11 M_sun) with sizes as large as those measured.

astro-ph.GA↗

arXiv2023

The ALMA-ALPINE [CII] survey: sub-kpc morphology of 3 main-sequence galaxy systems at z~4.5 revealed by ALMA

Context: From redshift 6 to redshift $\approx$ 4 galaxies grow rapidly from low mass galaxies towards the more mature massive galaxies we see at the cosmic noon. Growth via gas accretion and mergers undoubtedly shape this evolution - however, there currently exists much uncertainty over the contribution of each of these processes to the overall evolution of galaxies. Furthermore, previous characterisations of the morphology of galaxies in the molecular gas phase has been limited by the coarse resolution of previous observations. Aims: The goal of this paper is to derive the morpho-kinematic properties of 3 main-sequence systems at $z\sim4.5$, drawn from the ALPINE survey, using brand new high-resolution ALMA data in band 7. The objects were previously characterised as one merger with three components, and and two dispersion-dominated galaxies. Methods: We use intensity and velocity maps, position-velocity diagrams and radial profiles of [CII], in combination with dust continuum maps, to analyse the morphology and kinematics of the 3 systems.} Results: In general, we find that the high-resolution ALMA data reveal more complex morpho-kinematic properties. We identify in one galaxy interaction-induced clumps, showing the profound effect that mergers have on the molecular gas in galaxies, consistent with what is suggested in recent simulations. A galaxy that was previously classified as dispersion dominated turned out to show two bright [CII] emission regions, that could either be merging galaxies or massive star-forming regions within the galaxy itself. The high resolution data for the other dispersion dominated object also revealed clumps of [CII] that were not previously identified. Within the sample, we might also detect star-formation powered outflows (or outflows from Active Galactic Nuclei) which appear to be fuelling diffuse gas regions and enriching the circumgalactic medium.

astro-ph.GA↗

arXiv2023

Uncovering the MIR emission of quiescent galaxies with $JWST$

We present a study of the mid-IR (MIR) emission of quiescent galaxies (QGs) beyond the local universe. Using deep $JWST$ imaging in the SMACS-0723 cluster field we identify a mass limited ($M_{*} >10^{9}$M$_{\odot}$) sample of intermediate redshift QGs ($0.2 5 μ$m, can be attributed to different dust continuum levels and/or the presence of Polycyclic Aromatic Hydrocarbons (PAHs) features. The latter would indicate enhanced 11.3- and 12.7 $μ$m PAHs strengths with respect to those at 6.2- and 7.7$ μ$m, consistent with the observed spectra of local ellipticals and indicative of soft radiation fields. Finally, we augment the average UV-to-MIR spectrum of the population with cold dust and gas emission in the far-IR/mm and construct a panchromatic UV-to-radio SED that can serve as a template for the future exploration of the interstellar medium of $z>0$ QGs with ALMA and $JWST$.

astro-ph.GA↗

arXiv2023

The [CII] 158 $μ$m emission line as a gas mass tracer in high redshift quiescent galaxies

Many efforts have been done in recent years to probe the gas fraction evolution of massive quiescent galaxies (QGs); however, a clear picture has not yet been established. Recent spectroscopic confirmations at z>3 offer the chance to measure the residual gas reservoirs of massive galaxies a few hundreds of Myr after their death and to study how fast quenching proceeds in a highly star-forming Universe. Even so, stringent constraints at z$>$2 remain hardly accessible with ALMA when adopting molecular gas tracers commonly used for the quenched population. In this letter, we propose overcoming this impasse by using the carbon [CII] 158 $μ$m emission line to systematically probe the gaseous budget of unlensed QGs at z>2.8, when these galaxies could still host non-negligible star formation on an absolute scale and when the line becomes best observable with ALMA (Bands 8 and 7). So far predominantly used for star-forming galaxies, this emission line is the best choice to probe the gas budget of spectroscopically confirmed QGs at $z>3$, reaching 2-4 and 13-30 times deeper than dust continuum (ALMA band 7) and CO(2-1)/(1-0) (VLA K-K$α$ bands), respectively, at fixed integration time. Exploiting archival ALMA observations, we place conservative 3$σ$ upper limits on the molecular gas fraction (f$_{\rm{mol}}=M_{\rm{H_2}}/M_{\star}$) of ADF22-QG1 (f$_{\rm{mol}}$<21%), ZF-COS-20115 (f$_{\rm{mol}}$<3.2%), two of the best-studied high-z QGs in the literature, and GS-9209 (f$_{\rm{mol}}$<72%), the most distant massive QG discovered to date. The deep upper limit found for ZF-COS-20115 is 3 times lower than previously anticipated for high-z QGs suggesting, at best, the existence of a large scatter in the f$_{\rm{mol}}$ distribution of the first QGs. Lastly, we discuss the current limitations of the method and propose ways to mitigate some of them by exploiting ALMA bands 9 and 10.

astro-ph.GA↗

arXiv2023

The Gas Mass Reservoir of Quiescent Galaxies at Cosmic Noon

We present a 1.1mm stacking analysis of moderately massive (log($M_{*}$/$M_{\odot}$) = 10.7 $\pm$ 0.2) quiescent galaxies (QGs) at $\langle z\rangle \sim1.5$, searching for cold dust continuum emission, an excellent tracer of dust and gas mass. Using both the recent GOODS-ALMA survey as well as the full suite of ALMA Band-6 ancillary data in the GOODS-S field, we report the tentative detection of dust continuum equivalent of dust mass log($M_{dust}$/$M_{\odot}$) = 7.47 $\pm$ 0.13 and gas mass log($M_{gas}$/$M_{\odot}$) = 9.42 $\pm$ 0.14. The emerging gas fraction is $f_{gas}$ = 5.3 $\pm$ 1.8%, consistent with the results of previous stacking analyses based on lower resolution sub(mm) observations. Our results support the scenario where high-z QGs have an order of magnitude larger $f_{gas}$ compared to their local counterparts and have experienced quenching with a non negligible gas reservoir in their interstellar medium - i.e. with gas retention. Subsequent analysis yields an anti-correlation between the $f_{gas}$ and the stellar mass of QGs, especially in the high mass end where galaxies reside in the most massive haloes. The $f_{gas}$ - $M_{*}$ anti-correlation promotes the selection bias as a possible solution to the tension between the stacking results pointing towards gas retention in high-z QGs of moderate $M_{*}$ and the studies of individual targets that favour a fully depleted ISM in massive (log($M_{*}$/$M_{\odot}$) high-z QGs.

astro-ph.GA↗

arXiv2022

The 700 ks Chandra Spiderweb Field II: Evidence for inverse-Compton and thermal diffuse emission in the Spiderweb galaxy

We present the X-ray imaging and spectral analysis of the diffuse emission around the Spiderweb galaxy at z=2.16 and of its nuclear emission, based on a deep (700 ks) Chandra observation. We characterize the nuclear emission and computed the contamination in the surrounding regions due to the wings of the instrument PSF. Then, we quantified the extended emission within 12". We find that the Spiderweb galaxy hosts a mildly absorbed quasar, with modest yet significant variability on a timescale of ~1 year. We find that the emission in the jet regions is well described by a power law with Gamma~2-2.5, and it is consistent with IC upscattering of the CMB photons by the relativistic electrons. We also find a roughly symmetric, diffuse emission within a radius of ~100 kpc. This emission is consistent with thermal bremsstrahlung from a hot ICM with a temperature of kT=2.0_{-0.4}^{+0.7} keV, and a metallicity of Z<1.6Z_sun. The average electron density within 100 kpc is n_e=(1.51+-0.24+-0.14)E-2 cm^{-3}, corresponding to an upper limit for the total ICM mass of <=(1.76+-0.30+-0.17)E+12 M_sun (where error bars are 1 sigma statistical and systematic, respectively). If we apply hydrostatic equilibrium to the ICM, we measure a total gravitational mass M(<100 kpc)=(1.5^{+0.5}_{-0.3})E+13 M_sun and, extrapolating at larger radii, we estimate a total mass M_{500}=(3.2^{+1.1}_{-0.6})E+13 M_sun within a radius of r_{500}=(220+-30) kpc. We conclude that the Spiderweb protocluster shows significant diffuse emission within a radius of 12 arcsec, whose major contribution is provided by IC scattering associated with the radio jets. Outside the jet regions, we also identified thermal emission within a radius of ~100 kpc, revealing the presence of hot, diffuse baryons that may represent the embryonic virialized halo of the forming cluster.

astro-ph.GA↗

arXiv2022

The 700 ks Chandra Spiderweb Field I: evidence for widespread nuclear activity in the Protocluster

(Abridged) We present an analysis of the 700 ks Chandra ACIS-S observation of the field around the Spiderweb Galaxy at z=2.156, focusing on the nuclear activity in the associated large-scale environment. We identify unresolved X-ray sources down to flux limits of 1.3X10^{-16} and 3.9X10^{-16} erg/s/cm^2 in the soft and hard band, respectively. We search for counterparts in the optical, NIR and submm bands to identify X-ray sources belonging to the protocluster. We detect 107 X-ray unresolved sources within 5 arcmin (corresponding to 2.5 Mpc) of J1140-2629, among which 13 have optical counterparts with spectroscopic redshift 2.11 ~1.84+-0.04. The best-fit intrinsic absorption for 5 protocluster X-ray members is N_H>10^{23} cm^{-2}, while other 6 have upper limits of the order of fewX10^{22} cm^{-2}. Two sources can only be fitted with very flat Γ<=1, and are therefore considered Compton-thick candidates. Their 0.5-10 keV rest frame luminosities are larger than 2X10^{43} erg/s, significantly greater than X-ray luminosities expected from star formation activity. The X-ray luminosity function of AGN in the volume associated to the Spiderweb protocluster in the range 10^{43} 10.5, corresponding to an enhancement of 6.0^{+9.0}_{-3.0} with respect to the COSMOS field at comparable redshifts and stellar mass range. We conclude that the galaxy population in the Spiderweb Protocluster is characterized by enhanced X-ray nuclear activity triggered by environmental effects on Mpc scales.

astro-ph.GA↗

arXiv2022

Evidence for Cold-stream to Hot-accretion Transition as Traced by Lyα Emission from Groups and Clusters at 2 < z < 3.3

We present Keck Cosmic Web Imager (KCWI) observations of giant Lya halos surrounding 9 galaxy groups and clusters at 2<z<3.3, including five new detections and one upper limit. We find observational evidence for the cold-stream to hot-accretion transition predicted by theory by measuring a decrease in the ratio between the spatially extended Lya luminosity and the expected baryonic accretion rate (BAR), with increasing elongation above the transition mass Mstream). This implies a modulation of the share of BAR that remains cold diminishing quasi-linearly (logarithmic slope of 0.97+-0.19, 5 sigma significance) with the halo to Mstream mass ratio. The integrated star-formation rates (SFRs) and AGN bolometric luminosities display a potentially consistent decrease, albeit significant only at 2.6 sigma and 1.3 sigma, respectively. The higher scatter in these tracers suggests the Lya emission might be mostly a direct product of cold accretion in these structures rather than indirect, mediated by outflows and photo-ionization from SFR and AGNs; this is also supported by energetics considerations. Below Mstream (cold-stream regime) we measure LLya/BAR=10^{40.51+-0.16}~erg/s/Msun*yr, consistent with predictions, and SFR/BAR=10^{-0.54+-0.23}: on average 30_{-10}^{+20}% of the cold streams go into stars. Above Mstream (hot-accretion regime), LLya is set by Mstream (within 0.2~dex scatter in our sample), independent of the halo mass but rising tenfold from z=2 to 3.

astro-ph.CO↗

arXiv2021

CLASH-VLT: Abell~S1063. Cluster assembly history and spectroscopic catalogue

Using the CLASH-VLT survey, we assembled an unprecedented sample of 1234 spectroscopically confirmed members in Abell~S1063, finding a dynamically complex structure at z_cl=0.3457 with a velocity dispersion σ_v=1380 -32 +26 km s^-1. We investigate cluster environmental and dynamical effects by analysing the projected phase-space diagram and the orbits as a function of galaxy spectral properties. We classify cluster galaxies according to the presence and strength of the [OII] emission line, the strength of the H$δ$ absorption line, and colours. We investigate the relationship between the spectral classes of galaxies and their position in the projected phase-space diagram. We analyse separately red and blue galaxy orbits. By correlating the observed positions and velocities with the projected phase-space constructed from simulations, we constrain the accretion redshift of galaxies with different spectral types. Passive galaxies are mainly located in the virialised region, while emission-line galaxies are outside r_200, and are accreted later into the cluster. Emission-lines and post-starbursts show an asymmetric distribution in projected phase-space within r_200, with the first being prominent at Delta_v/sigma <~-1.5$, and the second at Delta_v/ sigma >~ 1.5, suggesting that backsplash galaxies lie at large positive velocities. We find that low-mass passive galaxies are accreted in the cluster before the high-mass ones. This suggests that we observe as passives only the low-mass galaxies accreted early in the cluster as blue galaxies, that had the time to quench their star formation. We also find that red galaxies move on more radial orbits than blue galaxies. This can be explained if infalling galaxies can remain blue moving on tangential orbits.

astro-ph.GA↗

arXiv2021

HST grism spectroscopy of z~3 massive quiescent galaxies: Approaching the metamorphosis

Tracing the emergence of the massive quiescent galaxy (QG) population requires the build-up of reliable quenched samples. We present Hubble Space Telescope WFC3/G141 grism spectra of 10 quiescent galaxy candidates selected at $2.5<z<3.5$ in the COSMOS field. Spectroscopic confirmation for the whole sample is obtained within 1-3 orbits based on the presence of strong spectral breaks and Balmer absorption lines. Combining their spectra with optical to near-IR photometry, star-forming solutions are formally rejected for the entire sample. Broad spectral indices are consistent with the presence of young A-type stars, which implies that the last major episode of star formation has taken place no earlier than $\sim$300-800 Myr prior to observation. Marginalising over three different slopes of the dust attenuation curve, we obtain short mass-weighted ages and an average peak star formation rate of SFR$\sim10^3$ M$_{\odot}$ yr$^{-1}$ at $z_{formation}\sim3.5$. Despite mid- and far-IR data are too shallow to determine the obscured SFR on a galaxy-by-galaxy basis, the mean stack emission from 3GHz data constrains the level of residual obscured SFR to be globally below 50 M$_{\odot}$ yr$^{-1}$, hence three times below the scatter of the coeval main sequence. Alternatively, the very same radio detection suggests a widespread radio-mode feedback by active galactic nuclei (AGN) four times stronger than in z$\sim$1.8 massive QGs. This is accompanied by a 30% fraction of X-ray luminous AGN with a black hole accretion rate per unit SFR enhanced by a factor of $\sim30$ with respect to similarly massive QGs at lower redshift. The average compact, high Sérsic index morphologies of our galaxies, coupled with their young mass-weighted ages, suggest that the mechanisms responsible for the development of a spheroidal component might be concomitant with (or preceding) those causing their quenching.

astro-ph.GA↗

arXiv2021

Three Lyman-alpha emitting filaments converging to a massive galaxy group at z=2.91: discussing the case for cold gas infall

We have discovered a 300kpc-wide giant Lya nebula centered on the massive galaxy group RO-1001 at z=2.91 in the COSMOS field. Keck Cosmic Web Imager observations reveal three cold gas filaments converging into the center of the potential well of its ~4x10^13Msun dark matter halo, hosting 1200Msun/yr of star formation as probed by ALMA and NOEMA observations. The nebula morphological and kinematics properties and the prevalence of blueshifted components in the Lya spectra are consistent with a scenario of gas accretion. The upper limits on AGN activity and overall energetics favor gravity as the primary Lya powering source and infall as the main source of gas flows to the system. Although interpretational difficulties remain, with outflows and likely also photoionization with ensuing recombination still playing a role, this finding provides arguably an ideal environment to quantitatively test models of cold gas accretion and galaxy feeding inside an actively star-forming massive halo at high redshift.

astro-ph.GA↗

arXiv2021

Multiwavelength dissection of a massive heavily dust-obscured galaxy and its blue companion at z $\sim $2

In this work we study a system of two galaxies, Astarte and Adonis, at z $\sim $2 when the Universe was undergoing its peak of star formation activity. Astarte is a dusty star-forming galaxy at the massive-end of the main sequence (MS) and Adonis is a less-massive, bright in ultraviolet (UV), companion galaxy with an optical spectroscopic redshift. We analyse the physical properties of this system, and probe the gas mass of Astarte with its ALMA CO emission, to investigate whether this ultra-massive galaxy is quenching or not. We use CIGALE - a spectral energy distribution modeling code - to derive the key physical properties of Astarte and Adonis, mainly their star formation rates (SFRs), stellar masses, and dust luminosities. We inspect the variation of the physical parameters depending on the assumed dust attenuation law. We also estimate the molecular gas mass of Astarte from its CO emission, using different $α_{CO}$ and transition ratios ($r_{31}$) and discuss the implication of the various assumptions on the gas mass derivation. We find that Astarte exhibits a MS-like star formation activity, while Adonis is undergoing a strong starburst (SB) phase. The molecular gas mass of Astarte is far below the gas fraction of typical star-forming galaxies at z=2. This low gas content and high SFR, result in a depletion time of $0.22\pm0.07$ Gyrs, slightly shorter than what is expected for a MS galaxy at this redshift. The CO luminosity versus the total IR luminosity suggests a MS-like activity assuming a galactic conversion factor and a low transition ratio. The SFR of Astarte is of the same order using different attenuation laws, unlike its stellar mass that increases using shallow attenuation laws. We discuss these properties and suggest that Astarte might be experiencing a recent decrease of star formation activity and is quenching through the MS following a SB epoch.

astro-ph.GA↗

arXiv2021

The Interstellar Medium of Quiescent Galaxies and its Evolution With Time

We characterise the basic far-IR (FIR) properties and the gas mass fraction of massive ( ~ 11.0) quiescent galaxies (QGs) and explore how these evolve from z = 2.0 to the present day. We use robust, multi-wavelength (mid- to far-IR and sub-millimetre to radio) stacking ensembles of homogeneously selected and mass complete samples of log(M*/Msun) > 10.8 QGs. We find that the dust to stellar mass ratio (Md/M*) rises steeply as a function of redshift up to z~1.0 and then remains flat at least out to z = 2.0. Using Md as a proxy of gas mass (Mgas), we find a similar trend for the evolution of the gas mass fraction (fgas) with z > 1.0 QGs having fgas ~ 7.0% (for solar metallicity). This fgas is 3 - 10 times lower than that of normal star forming galaxies (SFGs) at their corresponding redshift but ~3 and ~10 times larger compared to that of z = 0.5 and local QGs. Furthermore, the inferred gas depletion time scales are comparable to that of local SFGs and systematically longer than that of main sequence galaxies at their corresponding redshifts. Our analysis also reveals that the average dust temperature (Td) of massive QGs remains roughly constant (< Td > = 21.0 \pm 2.0K) at least out to z ~ 2.0 and is substantially colder (~ 10K) compared to that of z > 0 SFGs. This motivated us to construct and release a redshift-invariant template IR SED, that we use to make predictions for ALMA observations and to explore systematic effects in the Mgas estimates of massive, high-z QGs. Finally, we discuss how a simple model that considers progenitor-bias can effectively reproduce the observed evolution of Md/M* and fgas. Our results indicate universal initial interstellar medium conditions for quenched galaxies and a large degree of uniformity in their internal processes across cosmic time.

astro-ph.GA↗

arXiv2020

The Typical Massive Quiescent Galaxy at z$\sim$3 is a Post-starburst

We have obtained spectroscopic confirmation with Hubble Space Telescope WFC3/G141 of a first sizeable sample of nine quiescent galaxies at 2.4<z<3.3. Their average near-UV/optical rest-frame spectrum is characterized by low attenuation (Av$\sim$0.6 mag) and a strong Balmer break, larger than the 4000 A break, corresponding to a fairly young age of $\sim$300 Myr. This formally classifies a substantial fraction of classically selected quiescent galaxies at z $\sim$ 3 as post-starbursts, marking their convergence to the quenching epoch. The rapid spectral evolution with respect to z $\sim$ 1.5 quiescent galaxies is not matched by an increase of residual star-formation, as judged from the weak detection of [O II]3727A emission, pointing to a flattening of the steep increase in gas fractions previously seen from z $\sim$ 0 to 1.8. However, radio 3GHz stacked emission implies either much stronger dust-obscured star formation or substantial further evolution in radio-mode AGN activity with respect to z $\sim$ 1.5.

astro-ph.GA↗

arXiv2020

A hyper luminous starburst at z=4.72 magnified by a lensing galaxy pair at z=1.48

[Abridged] We discovered in the Herschel Reference Survey an extremely bright IR source with $S_{500}$~120mJy (Red Virgo 4 - RV4). Based on IRAM/EMIR and IRAM/NOEMA detections of the CO(5-4), CO(4-3), and [CI] lines, RV4 is located at z=4.724, yielding a total observed L$_{IR}$ of 1.1+/-0.6x0$^{14}$L$_{\odot}$. At the position of the Herschel emission, three blobs are detected with the VLA at 10cm. The CO(5-4) line detection of each blob confirms that they are at the same redshift with the same line width, indicating that they are multiple images of the same source. In Spitzer and deep optical observations, two sources, High-z Lens 1 (HL1) West and HL1 East, are detected at the center of the three VLA/NOEMA blobs. These two sources are placed at z=1.48 with XSHOOTER spectra, suggesting that they could be merging and gravitationally lensing the emission of RV4. HL1 is the second most distant lens known to date in strong lensing systems. The Einstein radius of the lensing system is 2.2"+/-0.2 (20kpc). The high redshift of HL1 and the large Einstein radius are highly unusual for a strong lensing system. We present the ISM properties of the background source RV4. Different estimates of the gas depletion time yield low values suggesting that RV4 is a SB galaxy. Among all high-z SMGs, this source exhibits one of the lowest L$_{[CI]}$ to L$_{IR}$ ratios, 3.2+/-0.9x10$^{-6}$, suggesting an extremely short gas tdepl of only 14+/-5Myr. It also shows a relatively high L$_{[CI]}$ to L$_{CO(4-3)}$ ratio (0.7+/-0.2) and low L$_{CO(5-4)}$ to L$_{IR}$ ratio (only ~50% of the value expected for normal galaxies) hinting a low density of gas. Finally, we discuss that the short tdepl of RV4 can be explained by either a very high SFE, which is difficult to reconcile with major mergers simulations of high-z galaxies, or a rapid decrease of SF, which would bias the estimate of tdepl toward low value.

astro-ph.GA↗