arXiv subjects

A. Pallottini

Publications and source records attributed to A. Pallottini.

At least 37 records · Page 2Linked to original sources

arXiv2023

XQR-30: the ultimate XSHOOTER quasar sample at the reionization epoch

The final phase of the reionization process can be probed by rest-frame UV absorption spectra of quasars at z>6, shedding light on the properties of the diffuse intergalactic medium within the first Gyr of the Universe. The ESO Large Programme "XQR-30: the ultimate XSHOOTER legacy survey of quasars at z~5.8-6.6" dedicated ~250 hours of observations at the VLT to create a homogeneous and high-quality sample of spectra of 30 luminous quasars at z~6, covering the rest wavelength range from the Lyman limit to beyond the MgII emission. Twelve quasar spectra of similar quality from the XSHOOTER archive were added to form the enlarged XQR-30 sample, corresponding to a total of ~350 hours of on-source exposure time. The median effective resolving power of the 42 spectra is R~11400 and 9800 in the VIS and NIR arm, respectively. The signal-to-noise ratio per 10 km/s pixel ranges from ~11 to 114 at $λ\simeq 1285$ Årest frame, with a median value of ~29. We describe the observations, data reduction and analysis of the spectra, together with some first results based on the E-XQR-30 sample. New photometry in the H and K bands are provided for the XQR-30 quasars, together with composite spectra whose characteristics reflect the large absolute magnitudes of the sample. The composite and the reduced spectra are released to the community through a public repository, and will enable a range of studies addressing outstanding questions regarding the first Gyr of the Universe.

astro-ph.GA↗

arXiv2023

Dust attenuation law in JWST galaxies at z = 7-8

Attenuation curves in galaxies depend on dust chemical composition, content, and grain size distribution. Such parameters are related to intrinsic galaxy properties such as metallicity, star formation rate, and stellar age. Due to the lack of observational constraints at high redshift, dust empirical curves measured in the local Universe (e.g. Calzetti and SMC curves) have been employed to describe the dust attenuation at early epochs. We exploit the high sensitivity and spectral resolution of the JWST to constrain the dust attenuation curves in high-z galaxies. Our goals are to check whether dust attenuation curves evolve with redshift and quantify the dependence of the inferred galaxy properties on the assumed dust attenuation law. We develop a modified version of the SED fitting code BAGPIPES by including a detailed dust attenuation curve parametrization. Dust parameters are derived, along with galaxy properties, from the fit to the data from FUV to mm bands. Once applied to three star-forming galaxies at z = 7-8, we find that their attenuation curves differ from local templates. One out of three galaxies shows a characteristic MW bump, typically associated to the presence of small carbonaceous dust grains such as PAHs. This is one of the first evidences suggesting the presence of PAHs in early galaxies. Galaxy properties such as stellar mass and SFR inferred from SED fitting are strongly affected by the assumed attenuation curve, though the adopted star formation history also plays a major role. Our results highlight the importance of accounting for the potential diversity of dust attenuation laws when analyzing the properties of galaxies at the EoR, whose dust properties are still poorly understood. The application of our method to a larger sample of galaxies observed with JWST can provide us important insights into the properties of dust and galaxies in the early universe.

astro-ph.GA↗

arXiv2023

ALMA hints at the presence of turbulent disk galaxies at z > 5

High-redshift galaxies are expected to be more turbulent than local galaxies because of their smaller size and higher star formation and thus stronger feedback from star formation, frequent mergers events, and gravitational instabilities. However, this scenario has recently been questioned by the observational evidence of a few galaxies at z~4-5 with a gas velocity dispersion similar to what is observed in the local population. Our goal is to determine whether galaxies in the first Gyrs of the Universe have already formed a dynamically cold rotating disk similar to the local counterparts. We studied the gas kinematic of 22 main-sequence star-forming galaxies at z > 5 and determined their dynamical state by estimating the ratio of the rotational velocity and of the gas velocity dispersion. We mined the ALMA archive and exploited the [CII] and [OIII] observations to perform a kinematic analysis of the cold and warm gas of z>5 main-sequence galaxies. The gas kinematics of the high-z galaxies is consistent within the errors with rotating but turbulent disks. We infer a velocity dispersion that is systematically higher by 4 times than the local galaxy population and the z~5 dust-obscured galaxies reported in the literature. The difference between our results and those reported at similar redshift can be ascribed to the systematic difference in the galaxy properties in the two samples: the disks of massive dusty galaxies are dynamically colder than the disks of dust-poor galaxies. The comparison with the theoretical predictions suggests that the main driver of the velocity dispersion in high-z galaxies is the gravitational energy that is released by the transport of mass within the disk. Finally, we stress that future deeper ALMA high-angular resolution observations are crucial to constrain the kinematic properties of high-z galaxies and to distinguish rotating disks from kpc-scale mergers.

astro-ph.GA↗

arXiv2023

The ALMA REBELS Survey: The First Infrared Luminosity Function Measurement at $\mathbf{z \sim 7}

We present the first observational infrared luminosity function (IRLF) measurement in the Epoch of Reionization (EoR) based on a UV-selected galaxy sample with ALMA spectroscopic observations. Our analysis is based on the ALMA large program Reionization Era Bright Emission Line Survey (REBELS), which targets 42 galaxies at $\mathrm{z=6.4-7.7}$ with [CII] 158$\micron$ line scans. 16 sources exhibit a dust detection, 15 of which are also spectroscopically confirmed through the [CII] line. The IR luminosities of the sample range from $\log L_{IR}/L_\odot=11.4$ to 12.2. Using the UVLF as a proxy to derive the effective volume for each of our target sources, we derive IRLF estimates, both for detections and for the full sample including IR luminosity upper limits. The resulting IRLFs are well reproduced by a Schechter function with the characteristic luminosity of $\log L_{*}/L_\odot=11.6^{+0.2}_{-0.1}$. Our observational results are in broad agreement with the average of predicted IRLFs from simulations at $z\sim7$. Conversely, our IRLFs lie significantly below lower redshift estimates, suggesting a rapid evolution from $z\sim4$ to $z\sim7$, into the reionization epoch. The inferred obscured contribution to the cosmic star-formation rate density at $z\sim7$ amounts to $\mathrm{log(SFRD/M_{\odot}/yr/Mpc^{3}) = -2.66^{+0.17}_{-0.14} }$ which is at least $\sim$10\% of UV-based estimates. We conclude that the presence of dust is already abundant in the EoR and discuss the possibility of unveiling larger samples of dusty galaxies with future ALMA and JWST observations.

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arXiv2022

[CII] halos in ALPINE galaxies: smoking-gun of galactic outflows?

ALMA observations have revealed that many high redshift galaxies are surrounded by extended (10-15 kpc) [CII]-emitting halos which are not predicted by even the most advanced zoom-in simulations. Using a semi-analytical model, in a previous work we suggested that such halos are produced by starburst-driven, catastrophically cooling outflows. Here, we further improve the model and compare its predictions with data from 7 star-forming ($10\lesssim \rm SFR/ M_\odot \rm yr^{-1}<100$) galaxies at z=4-6, observed in the ALPINE survey. We find that (a) detected [CII] halos are a natural by-product of starburst-driven outflows; (b) the outflow mass loading factors are in the range $4\lesssimη\lesssim 7$, with higher $η$ values for lower-mass, lower-SFR systems, and scale with stellar mass as $η\propto M_*^{-0.43}$, consistently with the momentum-driven hypothesis. Our model suggests that outflows are widespread phenomena in high-z galaxies. However, in low-mass systems the halo extended [CII] emission is likely too faint to be detected with the current levels of sensitivity.

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arXiv2022

CONCERTO: Simulating the CO, [CII], and [CI] line emission of galaxies in a 117 $\rm deg^2$ field and the impact of field-to-field variance

In the submm regime, spectral line scans and line intensity mapping (LIM) are new promising probes for the cold gas content and star formation rate of galaxies across cosmic time. However, both of these two measurements suffer from field-to-field variance. We study the effect of field-to-field variance on the predicted CO and [CII] power spectra from future LIM experiments such as CONCERTO, as well as on the line luminosity functions (LFs) and the cosmic molecular gas mass density that are currently derived from spectral line scans. We combined a 117 $\rm deg^2$ dark matter lightcone from the Uchuu cosmological simulation with the simulated infrared dusty extragalactic sky (SIDES) approach. We find that in order to constrain the CO LF with an uncertainty below 20%, we need survey sizes of at least 0.1 $\rm deg^2$. Furthermore, accounting for the field-to-field variance using only the Poisson variance can underestimate the total variance by up to 80%. The lower the luminosity is and the larger the survey size is, the higher the level of underestimate. At $z$<3, the impact of field-to-field variance on the cosmic molecular gas density can be as high as 40% for the 4.6 arcmin$^2$ field, but drops below 10% for areas larger than 0.2 deg$^2$. However, at $z>3$ the variance decreases more slowly with survey size and for example drops below 10% for 1 deg$^2$ fields. Finally, we find that the CO and [CII] LIM power spectra can vary by up to 50% in $\rm 1 deg^2$ fields. This limits the accuracy of the constraints provided by the first 1 deg$^2$ surveys. The level of the shot noise power is always dominated by the sources that are just below the detection thresholds. We provide an analytical formula to estimate the field-to-field variance of current or future LIM experiments. The code and the full SIDES-Uchuu products (catalogs, cubes, and maps) are publicly available.

astro-ph.CO↗

arXiv2022

Interpreting ALMA non-detections of JWST super-early galaxies

Recent attempts to detect [OIII] 88$μ$m emission from super-early ($z>10$) galaxy candidates observed by JWST have been unsuccessful. By using zoom-in simulations, we show that these galaxies are faint, and mostly fall below the local metal-poor $\rm [OIII]-SFR$ relation as a result of their low ionization parameter, $U_{\rm ion}\lesssim 10^{-3}$. Such low $U_{\rm ion}$ values are found in galaxies that are in an early assembly stage, and whose stars are still embedded in high-density natal clouds. However, the most luminous galaxy in our sample ($\rm{log}[L_{\rm{[OIII]}}/L_\odot] = 8.4$, $U_{\rm ion} \approx 0.1$) could be detected by ALMA in only $2.8$ hrs.

astro-ph.GA↗

arXiv2022

A new look at the infrared properties of z $\sim$ 5 galaxies

Recent ALMA large surveys unveiled the presence of significant dust continuum emission in star-forming galaxies at $z>4$. Unfortunately, such large programs -- i.e. ALPINE ($z\sim 5$) and REBELS ($z \sim 7$) -- only provide us with a single Far-Infrared (FIR) continuum data point for their individual targets. Therefore, high-$z$ galaxies FIR spectral energy densities (SEDs) remain mostly unconstrained, hinging on an assumption for their dust temperature ($T_{\rm d}$) in the SED fitting procedure. This introduces uncertainties in the inferred dust masses ($M_{\rm d }$), infrared luminosities ($L_{\rm IR}$), and obscured Star Formation Rate (SFR) fraction at $z > 4$. In this work we use a method that allows us to constrain $T_{\rm d}$ with a single band measurement by combining the $158\ \mathrm{μm}$ continuum information with the overlying [CII] emission line. We analyse the $21$ [CII] and FIR continuum detected $z\sim 5$ galaxies in ALPINE, finding a range of $T_{\rm d}=25-60\ \mathrm{K}$ and $M_{\rm d} = 0.6-25.1\ \times 10^{7}\ \mathrm{M_{\odot}}$. Given the measured stellar masses of ALPINE galaxies, the inferred dust yields are around $M_{\rm d}/M_{\star} = (0.2-8) \times 10^{-3}$, consistent with theoretical dust-production constraints. We find that $8$ out of $21$ ALPINE galaxies have $L_{\rm IR} \geq 10^{12}\ \mathrm{L_{\odot}}$, comparable to UltraLuminous IR Galaxies (ULIRGs). Relying on ultraviolet-to-optical SED fitting, the SFR was underestimated by up to $2$ orders of magnitude in $4$ of these $8$ ULIRGs-like galaxies. We conclude that these $4$ peculiar sources should be characterised by a two-phase interstellar medium structure with "spatially-segregated" FIR and ultraviolet emitting regions.

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arXiv2022

CONCERTO: High-fidelity simulation of millimeter line emissions of galaxies and [CII] intensity mapping

The intensity mapping of the [CII] 158um line redshifted to the sub-mm window is a promising probe of the z>4 star formation and its spatial distribution into the large-scale structure. To prepare the first-generation experiments (e.g., CONCERTO), we need realistic simulations of the sub-mm extragalactic sky in spectroscopy. We present a new version of the SIDES simulation including the main sub-mm lines around 1mm (CO, [CII], [CI]). This approach successfully reproduces the observed line luminosity functions. We then use our simulation to generate CONCERTO-like cubes (125-305GHz) and forecast the power spectra of the fluctuations caused by the various astrophysical components at those frequencies. Depending on our assumptions on the relation between star formation rate and [CII] luminosity, and the star formation history, our predictions of the z~6 [CII] power spectrum vary by two orders of magnitude. This highlights how uncertain the predictions are and how important future measurements will be to improve our understanding of this early epoch. SIDES can reproduce the CO shot noise recently measured at ~100 GHz by the mmIME experiment. Finally, we compare the contribution of the different astrophysical components at various redshift to the power spectra. The continuum is by far the brightest, by a factor of 3 to 100 depending on the frequency. At 300GHz, the CO foreground power spectrum is higher than the [CII] one for our base scenario. At lower frequency, the contrast between [CII] and extragalactic foregrounds is even worse. Masking the known galaxies from deep surveys should allow to reduce the foregrounds to 20% of the [CII] power spectrum up to z~6.5. However, this masking method will not be sufficient at higher redshifts. The code and the products of our simulation are released publicly and can be used for both intensity mapping experiments and sub-mm continuum and line surveys.

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arXiv2022

On the stunning abundance of super-early, massive galaxies revealed by JWST

The earliest JWST observations have revealed an unexpected abundance of super-early ($z>10$), massive ($M_*\approx 10^9\, M_\odot$) galaxies at the bright-end ($M_{\rm UV}\approx -21$) of the ultraviolet luminosity function (UV LF). We present a minimal physical model that explains the observed galaxy abundance at $z=10-14$. The model primarily combines (a) the halo mass function, with (b) an obscured star formation fraction prescription that is consistent with findings of the ALMA REBELS dusty galaxy survey. It has been successfully tested on well-known UV LFs up to $z=7$. The weak evolution from $z=7$ to $z\approx 14$ of the LF bright-end arises from a conspiracy between a decreasing dust attenuation, making galaxies brighter, that almost exactly compensates for the increasing shortage of their host halos. The model also predicts that galaxies at $z > 11$ should contain negligible amounts of dust. We speculate that dust could have been efficiently ejected during the very first phases of galaxy build-up.

astro-ph.GA↗

arXiv2022

The black hole and host galaxy growth in an isolated $z\sim 6$ QSO observed with ALMA

The outstanding mass growth of supermassive black holes (SMBHs) at the Reionisation Epoch and how it is related to the concurrent growth of their host galaxies, poses challenges to theoretical models aimed at explaining how these systems formed in short timescales (<1 Gyr). To trace the average evolutionary paths of quasi-stellar objects (QSOs) and their host galaxies in the BH mass-host mass ($M_{\rm dyn}$) plane, we compare the star formation rate (SFR), derived from the accurate estimate of the dust temperature and the dust mass ($T_{\rm dust}, M_{\rm dust}$), with the BH accretion rate. To this aim, we analysed a deep, $900$ pc resolution ALMA observation of the sub-mm continuum, [CII] and H$_2$O of the $z\sim 6$ QSO J2310+1855, enabling a detailed study of dust properties and cold gas kinematics. We performed an accurate SED analysis obtaining a dust temperature of $T_{\rm dust} = 71$ K and a dust mass of $M_{\rm dust}= 4.4 \times 10^8\ \rm M_{\odot}$. The implied AGN-corrected SFR is $1240 \ \rm M_{\odot}yr^{-1}$, a factor of 2 smaller than previously reported for this QSO. We derived the best estimate of the dynamical mass $M_{\rm dyn} = 5.2\times 10^{10}\ \rm M_{\odot}$ within $r = 1.7$ kpc, based on a dynamical model of the system. We found that ${\rm SFR}/M_{\rm dyn}>\dot M_{\rm BH}/M_{\rm BH}$, suggesting that AGN feedback might be efficiently acting to slow down the SMBH accretion, while the stellar mass assembly is still vigorously taking place in the host galaxy. In addition, we were also able to detect high-velocity emission on the red and blue sides of the [CII] emission line, that traces a gaseous outflow, and for the first time, we mapped a spatially-resolved water vapour disk through the H$_2$O v=0 $3_{(2,2)}-3_{(1,3)}$ emission line detected at $ν_{\rm obs} = 274.074$ GHz, whose kinematic properties and size are broadly consistent with those of the [CII] disk.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey: The Cosmic HI Gas Mass Density in Galaxies at $z\approx 7$

The neutral atomic gas content of individual galaxies at large cosmological distances has until recently been difficult to measure due to the weakness of the hyperfine HI 21-cm transition. Here we estimate the HI gas mass of a sample of main-sequence star-forming galaxies at $z\sim 6.5 - 7.8$ surveyed for [CII]$-158μ$m emission as part of the Reionization Era Bright Emission Line Survey (REBELS), using a recent calibration of the [CII]-to-HI conversion factor. We find that the HI gas mass excess in galaxies increases as a function of redshift, with an average of $M_{\rm HI} / M_\star \approx 10$, corresponding to HI gas mass fractions of $f_{\rm HI} = M_{\rm HI} / (M_\star + M_{\rm HI}) = 90\%$, at $z\approx 7$. Based on the [CII]-$158μ$m luminosity function (LF) derived from the same sample of galaxies, we further place constraints on the cosmic HI gas mass density in galaxies ($ρ_{\rm HI}$) at this redshift, which we measure to be $ρ_{\rm HI} = 7.1^{+6.4}_{-3.0} \times 10^{6}\,M_{\odot}\,{\rm Mpc^{-3}}$. This estimate is substantially lower by a factor of $\approx 10$ than that inferred from an extrapolation of damped Lyman-$α$ absorber (DLA) measurements, and largely depend on the exact [CII] LF adopted. However, we find this decrease in $ρ_{\rm HI}$ to be consistent with recent simulations and argue that this apparent discrepancy is likely a consequence of the DLA sightlines predominantly probing the substantial fraction of HI gas in high-$z$ galactic halos, whereas [CII] traces the HI in the ISM associated with star formation. We make predictions for this build-up of neutral gas in galaxies as a function of redshift, showing that at $z\gtrsim 5$ only $\approx 10\%$ of the cosmic HI gas content is confined in galaxies and associated with the star-forming ISM.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey: Average [CII] $158\,\rm{μm}$ sizes of Star-Forming Galaxies from $z\sim 7$ to $z\sim 4$

We present the average [CII] $158\,\rm{μm}$ emission line sizes of UV-bright star-forming galaxies at $z\sim7$. Our results are derived from a stacking analysis of [CII] $158\,\rm{μm}$ emission lines and dust continua observed by ALMA, taking advantage of the large program Reionization Era Bright Emission Line Survey (REBELS). We find that the average [CII] emission at $z\sim7$ has an effective radius $r_e$ of $2.2\pm0.2\,\rm{kpc}$. It is $\gtrsim2\times$ larger than the dust continuum and the rest-frame UV emission, in agreement with recently reported measurements for $z\lesssim6$ galaxies. Additionally, we compared the average [CII] size with $4<z<6$ galaxies observed by the ALMA Large Program to INvestigate [CII] at Early times (ALPINE). By analysing [CII] sizes of $4<z<6$ galaxies in two redshift bins, we find an average [CII] size of $r_{\rm e}=2.2\pm0.2\,\rm{kpc}$ and $r_{\rm e}=2.5\pm0.2\,\rm{kpc}$ for $z\sim5.5$ and $z\sim4.5$ galaxies, respectively. These measurements show that star-forming galaxies, on average, show no evolution in the size of the [CII] $158\,{\rm μm}$ emitting regions at redshift between $z\sim7$ and $z\sim4$. This finding suggest that the star-forming galaxies could be morphologically dominated by gas over a wide redshift range.

astro-ph.GA↗

arXiv2022

A survey of high-$z$ galaxies: SERRA simulations

We introduce SERRA, a suite of zoom-in high-resolution ($\sim 10\,\rm pc$) cosmological simulations including non-equilibrium chemistry and on-the-fly radiative transfer. The outputs are post-processed to derive galaxy UV+FIR continuum and emission line properties. Results are compared with available multi-wavelength data to constrain the physical properties (e.g., star formation rates, stellar/gas/dust mass, metallicity) of high-redshift $6 \lesssim z \lesssim 15$ galaxies. This flagship paper focuses on the $z=7.7$ sub-sample, including 202 galaxies with stellar mass $10^7 M_\odot \lesssim M_\star \lesssim 5\times 10^{10}M_\odot$, and specific star formation ranging from ${\rm sSFR} \sim 100\,{\rm Gyr}^{-1}$ in young, low-mass galaxies to $\sim 10\,{\rm Gyr}^{-1}$ for older, massive ones. At this redshift, SERRA galaxies are typically bursty, i.e. they are located above the Schmidt-Kennicutt relation by a factor $κ_s = 3.03^{+4.9}_{-1.8}$, consistent with recent findings for [OIII] and [CII] emitters at high-$z$. They also show relatively large ${\rm IRX} = L_{\rm FIR}/L_{\rm UV}$ values as a result of their compact/clumpy morphology effectively blocking the stellar UV luminosity. Note that this conclusion might be affected by insufficient spatial resolution at the molecular cloud level. We confirm that early galaxies lie on the standard $\rm [CII]-SFR$ relation; their observed $L_{\rm [OIII]}/L_{\rm [CII]} \simeq 1-10$ ratios can be reproduced by a part of the SERRA galaxies without the need of a top-heavy IMF and/or anomalous C/O abundances. [OI] line intensities are similar to local ones, making ALMA high-$z$ detections challenging but feasible ($\sim 6\,\rm hr$ for a SFR of $50\,M_\odot\,{\rm yr}^{-1}$).

astro-ph.GA↗

arXiv2022

Dynamical characterization of galaxies up to $z \sim 7$

The characterization of the dynamical state of galaxies up to z~7 is crucial for constraining the mechanisms driving the mass assembly in the early Universe. However, it is unclear whether the data quality of current and future observations is sufficient to perform a solid dynamical analysis. This paper defines the angular resolution and S/N required for a robust characterization of the dynamical state of galaxies up to the EoR. The final aim is to help design spatially-resolved surveys targeting emission lines of primeval galaxies. We investigate the [CII]-158um emission from z~6-7 LBGs from the SERRA cosmological simulation, covering a range of dynamical states: from disks to major mergers. We create ALMA mock observations with various data quality and apply the kinematic classification methods used in the literature. These tests allow us to quantify the performances of such methods as a function of angular resolution and S/N. We find that barely-resolved observations do not allow the correct dynamical characterization of a galaxy, resulting in the misclassification of all disks in our sample. However, even when using spatially-resolved observations with data quality typical of high-z galaxies, the standard kinematic classification methods, based on the analysis of the moment maps, fail to distinguish a merger from a disk. The high angular resolution and S/N needed to apply these standard methods successfully can be achieved with current data only for a handful of bright galaxies. We propose a new classification method, called PVsplit, that quantifies the asymmetries and morphological features in position-velocity diagrams using three empirical parameters. We test PVsplit on our mock data concluding that it can predict whether a galaxy is a disk or a merger provided that S/N $\gtrsim10$, and the major axis is covered by $\gtrsim3$ independent resolution elements.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey: the dust content of $z \sim 7$ Lyman Break Galaxies

We include a fully coupled treatment of metal and dust enrichment into the Delphi semi-analytic model of galaxy formation to explain the dust content of 13 Lyman Break Galaxies (LBGs) detected by the Atacama Large millimetre Array (ALMA) REBELS Large Program at $z\simeq 7$. We find that the galaxy dust mass, $M_d$, is regulated by the combination of SNII dust production, astration, shock destruction, and ejection in outflows; grain growth (with a standard timescale $τ_0= 30$ Myr) plays a negligible role. The model predicts a dust-to-stellar mass ratio of $\sim 0.07-0.1\%$ and a UV-to-total star formation rate relation such that $log (ψ_{\rm UV}) = -0.05 ~[log (ψ)]^{2} + 0.86 ~log(ψ) -0.05$ (implying that 55-80\% of the star formation is obscured) for REBELS galaxies with stellar mass $M_* = 10^{9-10} M_\odot$. This relation reconciles the intrinsic UV luminosity of LBGs with their observed luminosity function at $z=7$. However, 2 out of the 13 systems show dust-to-stellar mass ratios ($\sim 0.94-1.1\%$) that are up to $18\times$ larger than expected from the fiducial relation. Due to the physical coupling between dust and metal enrichment, even decreasing $τ_0$ to very low values (0.3 Myr) only increases the dust-to-stellar mass ratio by a factor $ \sim 2$. Given that grain growth is not a viable explanation for such high observed ratios of the dust-to-stellar mass, we propose alternative solutions.

astro-ph.GA↗

arXiv2022

The ALMA REBELS Survey. Epoch of Reionization giants: properties of dusty galaxies at $z \approx 7$

We analyse FIR dust continuum measurements for 14 galaxies ($z\approx 7$) in the ALMA REBELS LP to derive their physical properties. Our model uses three input data: (a) the UV spectral slope, $β$, (b) the observed UV continuum flux at $1500$A, $F_{\rm UV}$, (c) the observed continuum flux at $\approx 158μ$m, $F_{158}$, and considers Milky Way (MW) and SMC extinction curves, along with different dust geometries. We find that REBELS galaxies have (28-90.5)% of their star formation obscured; the total (UV+IR) star formation rates are in the range $31.5 < {\rm SFR}/ (M_\odot {\rm yr}^{-1}) < 129.5$. The sample-averaged dust mass and temperature are $(1.3\pm 1.1)\times 10^7 M_\odot$ and $52 \pm 11$ K, respectively. In some galaxies dust is abundant (REBELS-14, $M'_d \approx 3.4 \times 10^7 M_\odot$), or hot (REBELS-18, $T'_d \approx 67$ K). The dust distribution is compact ($<0.3$ kpc for 70% of the galaxies). The dust yield per supernova is $0.1 \le y_d/M_\odot \le 3.3$, with 70% of the galaxies requiring $y_d < 0.25 M_\odot$. Three galaxies (REBELS-12, 14, 39) require $y_d > 1 M_\odot$. With the SFR predicted by the model and a MW extinction curve, REBELS galaxies detected in [CII] nicely follow the local $L_{\rm CII}-$SFR relation, and are approximately located on the Kennicutt-Schmidt relation. The sample-averaged gas depletion time is of $0.11\, y_P^{-2}$ Gyr, where $y_P$ is the ratio of the gas-to-stellar distribution radius. For some systems a solution simultaneously matching the observed ($β, F_{\rm UV}, F_{158}$) values cannot be found. This occurs when the index $I_m = (F_{158}/F_{\rm UV})/(β-β_{\rm int})$, where $β_{\rm int}$ is the intrinsic UV slope, exceeds $I_m^*\approx 1120$ for a MW curve. For these objects we argue that the FIR and UV emitting regions are not co-spatial, questioning the use of the IRX-$β$ relation.

astro-ph.GA↗

arXiv2022

The REBELS ALMA Survey: cosmic dust temperature evolution out to z $\sim$ 7

ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature $T_d$ remains mostly unconstrained due to the few available FIR continuum data at redshift $z>5$. This introduces large uncertainties in several properties of high-$z$ galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous [CII] 158$μ$m line and underlying dust continuum measurements, we derive $T_ d$ in the continuum and [CII] detected $z\approx 7$ galaxies in the ALMA Large Project REBELS sample. We find $39\ \mathrm{K} < T_d < 58\ \mathrm{K}$, and dust masses in the narrow range $M_d = (0.9-3.6)\times 10^7 M_{\odot}$. These results allow us to extend for the first time the reported $T_d(z)$ relation into the Epoch of Reionization. We produce a new physical model that explains the increasing $T_ d(z)$ trend with the decrease of gas depletion time, $t_{dep}=M_g/\mathrm{SFR}$, induced by the higher cosmological accretion rate at early times; this hypothesis yields $T_d \propto (1+z)^{0.4}$. The model also explains the observed $T_d$ scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, $T_d$ only depends on the gas column density (metallicity), $T_d \propto N_H^{1/6}$ ($T_d \propto Z^{-1/6}$). REBELS galaxies are on average relatively transparent, with effective gas column densities around $N_H \simeq (0.03-1)\times 10^{21} \mathrm{cm}^{-2}$. We predict that other high-$z$ galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated $T_d \gg 60$ K, are significantly obscured, low-metallicity systems. In fact $T_d$ is higher in metal-poor systems due to their smaller dust content, which for fixed $L_{ IR}$ results in warmer temperatures.

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