arXiv subjects

Karin Cescon

Publications and source records attributed to Karin Cescon.

9 recordsLinked to original sources

arXiv2026

The ALMA REBELS Survey: The [C II] Luminosity Function of UV-Selected Galaxies at $z\sim7$

Luminosity functions (LF) are some of the most basic quantities providing insights into galaxy evolution across cosmic time and enabling a direct comparison between observations and simulations. Here, we present the first [CII]158$μ$m LF at $z\sim7$ from a sizeable UV-selected ALMA sample. Our analysis makes use of 27 galaxies with significant [CII] emission line detections in the epoch of reionization from the REBELS (Reionization Era Bright Emission Line Survey) ALMA Large Program and its pilot programs. We used these line detections and similar from ALPINE to calibrate UV luminosity-$L_{\mathrm{[CII]}}$ and stellar mass-$L_{\mathrm{[CII]}}$ relations at $z\sim7$. Given the challenge in precisely defining a selection volume for the REBELS sample itself, we rely on published UV LF and stellar mass function (SMF) determinations at $z\sim7$ and three complementary approaches to derive the [CII] LF. The UV LF-based method provides a lower limit on the [CII] LF, due to its missing the most dust-obscured systems. Our SMF-based estimate yields a Schechter function with a characteristic luminosity of $L^*_{\mathrm{[CII]}}=10^{9.11^{+0.11}_{-0.27}} L_\odot$ and a steep faint-end slope. From $z\sim7$ to $z\sim5$, our inferred [CII] LF shows an evolution of +0.3 dex in normalisation and +0.1 dex in $L^*_{\mathrm{[CII]}}$. Our results indicate a general underprediction from models of the abundance of [CII] emitters at high-redshift and thus potentially the number densities of massive yet UV-faint systems implied by our SMF-based method. Additionally, they suggest that 63% of the star formation rate density occurs in massive [CII]-bright systems and that $\sim$60-90% of [CII] emitters at $z\sim7$ may be missed by bright UV selections. This highlights the importance of surveys focusing on especially massive, UV-faint sources at $z>6$.

astro-ph.GA↗

arXiv2026

The dawn of the first galaxies

Understanding how the first galaxies assembled their stellar mass at cosmic dawn is a central frontier of modern astrophysics. JWST has now revealed an unexpected population of ultraviolet-bright galaxies at z>10, whose intense emission points towards a rapid stellar mass growth that challenges current models of early galaxy assembly. Yet, the cold gas fueling this growth remains hidden: current ALMA sensitivity restricts routine detection to only the brightest ionized-gas tracers such as [OIII] 88$μ$m, while [CII] 158$μ$m, a key coolant and gas-mass proxy of the neutral interstellar medium (ISM), is undetected beyond $z\sim8$. This leaves fundamental questions unanswered: how quickly primeval galaxies gathered cold gas and turned it into stars; what physical conditions (density, temperature, radiation field) characterized their ISM; how fast metals and dust built up; and how feedback regulated early gas assembly. Only a transformational sub-millimeter facility like ALMA2040 can address these questions, enabling systematic surveys of [CII] in the first galaxies and providing a representative census of gas fractions and depletion times. A simulated galaxy at $z=11.3$ shows that ALMA2040 not only will detect, but also spatially resolve its merging cold-gas structure in [CII] on $\sim$200 pc scales in 5 hours (below 100 pc in 10 hours). ALMA2040 will also spectrally resolve multiple lines simultaneously, akin to integral-field spectroscopy, providing the key diagnostics needed to break density-metallicity-radiation-field degeneracies and map the multi-phase ISM of z>10 galaxies. The combination of dramatically increased line sensitivity and wide instantaneous bandwidth will push the search for the first galaxies out to the earliest cosmic times, with [OIII] detectable up to $z\sim20$. ALMA2040 will thus be transformative for understanding how the first galaxies assembled at cosmic dawn.

astro-ph.IM↗

arXiv2026

What fuels the growth of stars and black holes in galaxies through cosmic time? The physics and chemistry of the interstellar medium in distant galaxies

The interplay between the baryonic matter in galaxies (gas, dust, stars) has a fundamental role in our understanding of galaxy formation and evolution through cosmic time. The (sub)mm regime provides a unique window to measure the physical conditions and chemistry in the cold and dusty gas reservoirs that fuel the formation of new stars and the growth of massive black holes in distant galaxies. Large sub(mm) interferometers have made a revolutionary impact mapping the cold gas and dust in galaxies beyond the local universe. However, significant progress is hampered by limited spectral-line sensitivity. After more than a decade of surveys, molecular gas (traced by 12CO) has been measured only in a few tens of typical, massive main-sequence star-forming galaxies at z>1, while access to fainter gas-tracing species has been limited to only the very brightest galaxies. Thereby it restricts access to the detailed conditions in the cold gas clouds in typical galaxies in the distant universe. A revolutionary, flagship sub-mm interferometer with a massive increase in instantaneous spectral line sensitivity and broad spectral grasp (ALMA2040) fills this gap by enabling rich, multi-transition, multi-species and multi-isotopologue studies of the atomic and molecular interstellar medium for thousands of typical galaxies across galaxy populations and redshift. This will unveil how the ISM evolves across the galaxy population for statistical samples in different redshift bins, detailed studies of the physical conditions and chemistry in the multi-phase interstellar medium of typical galaxies, and how the cold ISM regulates and responds to star- and black-hole formation, providing a tremendous leap forward in the study of galaxy assembly over cosmic time.

astro-ph.IM↗

arXiv2026

REBELS-25: multi-phase morphology and kinematics at z = 7.31

We present a spatially resolved, multi-wavelength study of the massive, star-forming galaxy REBELS-25 at a redshift of $z=7.31$. We combine new high-resolution ALMA [O III]88$μ$m observations with resolution-matched [C II], dust continuum, and JWST/NIRSpec IFU spectroscopy, providing a $\lesssim1$ kpc view of the morphology, interstellar medium (ISM) conditions, and multi-tracer gas kinematics of one of the most mature galaxies known in the reionisation era. We find differing morphologies from the rest-frame UV to far-infrared (FIR) emission, with the UV and optical emission appearing clumpy and irregular, whereas the FIR emission is well-described by near-exponential disc profiles, with [C II] being the most extended. Comparing the resolved UV and FIR emission, we find that obscured star formation contributes $\sim55$-$98$% of the total star formation rate across the galaxy, demonstrating that dust obscuration strongly shapes the observed UV and optical morphology. Resolved ionisation-line diagnostics show no significant variation across the source at $\sim1$ kpc resolution, consistent with broadly similar ISM conditions among the identified regions. Kinematic modelling reveals that both the warm ionised gas, traced by [O III]88$μ$m, and the colder neutral gas, traced by [C II], share the same large-scale rotating structure and are dynamically cold, with ratios of ordered-to-random motion, $V/σ$, of $\sim11$ and 4.5, respectively, although we find evidence for non-circular motions that are not well-described by a simple rotating disc. Overall, these results further support a picture in which REBELS-25 hosts a dusty, chemically enriched, and dynamically cold ISM already in place at $z=7.31$.

astro-ph.GA↗

arXiv2026

Breathing Fire: Hot Dust in the Big Three Dragons at z = 7.15

We present new Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 ($λ_\mathrm{obs} = 0.45\,\mathrm{mm}$) and 4 ($λ_\mathrm{obs} = 2.2\,\mathrm{mm}$) observations towards the Big Three Dragons, a pair of merging Lyman-break galaxies (LBGs) at $z=7.15$. The system was previously detected in ALMA Bands 6, 7 and 8 ($λ_\mathrm{obs} = 0.73 - 1.32\,\mathrm{mm}$), which, combined with our new observations, allows us to robustly constrain its dust temperature and obscured SFR. The unresolved Band 4 observations yield a $3.5σ$ detection, and the $0.4''$ Band 9 observations detect the Eastern and Western LBGs at $3.7$ and $3.5σ$, respectively. Through modified blackbody fitting, we infer a global dust temperature of $T_d = 78_{-23}^{+35}\,\mathrm{K}$ for the system, which implies a high IR luminosity of $\log(L_\mathrm{IR}/L_\odot) = 12.32_{-0.41}^{+0.43}$. This makes the Big Three Dragons one of the most IR-luminous systems known at $z>7$, with a total $\mathrm{SFR}_\mathrm{UV+IR} = 267_{-153}^{+418}\,M_\odot\,\mathrm{yr}^{-1}$ that is almost completely obscured ($f_\mathrm{obs} = 0.94_{-0.09}^{+0.04}$). Using resolved ALMA observations in Bands 6, 8 and 9, we confirm both LBGs have hot dust temperatures ($T_d \approx 67 - 84\,\mathrm{K}$) and correspondingly high obscured fractions ($f_\mathrm{obs} \approx 0.88 -0.95$). We find the Western LBG to fall $\sim1\,\mathrm{dex}$ above the canonical IRX-$β_\mathrm{UV}$ relation, suggesting patchy dust obscuration. The compact Eastern component, on the other hand, is consistent with a Calzetti- or SMC-like dust screen within the uncertainties. Together with the similarly hot dust temperature recently reported for the $z=8.31$ galaxy MACS0416-Y1, our results suggest a non-negligible fraction of star formation at the bright end of the UV luminosity function is highly dust-obscured, even at $z\gtrsim7$.

astro-ph.GA↗

arXiv2026

A first systematic study of [OIII] 88$μ$m at $z>8$: two luminous oxygen lines and a powerful ionized outflow in the first 600 million years

We present deep ALMA Band 7 observations of the [OIII] $88μ$m line and underlying dust continuum emission in four UV-bright, gravitationally lensed (magnification $μ= 1.4-3.8$), JWST-selected galaxies at $z = 8.5 - 10.3$, with observed magnitudes $-22.5 \lesssim M_\mathrm{UV} \lesssim -20.5$. [OIII] $88μ$m is confidently detected in UNCOVER-10646 at $z=8.5080 \pm 0.0011$ ($15σ$) and DHZ1 at $z=9.3113 \pm 0.0006$ ($6σ$), with both being intrinsically luminous systems [$L_\text{[OIII]} = (1.1 - 1.6) \times 10^9\,L_\odot$] that follow the local [OIII]-SFR relation. [OIII] $88μ$m remains undetected in the two $z>10$ targets, including in the $z=10.07$ X-ray AGN UHZ1, where we obtain a deep limit of $L_\text{[OIII]} < 6 \times 10^7\,L_\odot$. Dust emission is not detected in any individual source nor in a stack ($<3σ$). The high S/N [OIII] $88μ$m detection in UNCOVER-10646 uniquely reveals an additional broad component ($\mathrm{FWHM} = 1366_{-329}^{+473}\,\mathrm{km/s}$; $Δ\mathrm{BIC}\approx20$) indicative of an ionized outflow. We infer a high outflow rate of $\dot{M}_\mathrm{out} = 128_{-46}^{+80}\,M_\odot\,\mathrm{yr}^{-1}$, corresponding to a mass loading factor $η= \dot{M}_\mathrm{out}/\mathrm{SFR} = 2.9_{-1.0}^{+1.8}$ that matches or exceeds theoretical predictions and JWST-based studies of ionized outflows at high redshift. While high-resolution ALMA follow-up is required to confirm and spatially resolve the outflow, this first systematic study at $z>8$ highlights the unique diagnostic power of [OIII] $88μ$m in characterizing galaxies in the early Universe.

astro-ph.GA↗

arXiv2026

Direct detection of cool molecular gas in a star-forming galaxy at $z=7.31$

We investigate the molecular gas content and interstellar medium (ISM) conditions of REBELS-25, a massive, star-forming galaxy at $z=7.31$. Deep VLA Q-band and ALMA Band 3 observations reveal CO(3-2) and CO(7-6) emission (both at $\sim3.5σ$), and provide an upper limit on [C I](2-1). From the CMB-corrected CO(3-2) flux-representing the highest-redshift detection of a low-$J$ CO transition to date-we derive a molecular gas mass of $M_{\rm mol}=(1.0\pm0.4)\times10^{11}\,(α_{\rm CO}/(3\,$M$_{\odot}$(K$\,$\kms$\,$pc$^2)^{-1}))\,$M$_{\odot}$, directly confirming the presence of a very massive gas reservoir only $\simeq700\,$Myr after the Big Bang. This implies an extreme gas fraction of $f_{\rm gas}\simeq0.95$, a gas-to-dust ratio of $δ_{\rm GDR}\simeq6\times10^2$, and a depletion timescale of $τ_{\rm dep}\simeq1.2\,$Gyr, broadly consistent with extrapolated scaling relations for main-sequence galaxies at lower redshift. Using the radiative transfer code TUNER, we self-consistently model CO and dust continuum emission in the context of the significant CMB background, constraining ISM properties and recovering $M_{\rm mol}= (1.8^{+1.0}_{-0.9})\times10^{11}\,$M$_{\odot}$, independent of assumptions about $r_{31}$ and $α_{\rm CO}$. We further discuss the use of alternative molecular gas tracers at early epochs. Combining CO and [C II] measurements, we infer an empirical [C II]-to-H$_2$ conversion factor of $α_{\rm [C II]}=(60\pm25)\,$M$_{\odot}$/L$_{\odot}$, suggesting [C II] remains a viable molecular gas tracer in the Epoch of Reionization. These results demonstrate the detectability of low-$J$ CO emission even at $z>7$, paving the way for next-generation facilities, and provide critical insights into the rapid mass assembly of galaxies during the first billion years of cosmic history.

astro-ph.GA↗

arXiv2025

REBELS-IFU: on the origin of the elevated [OIII]/[CII] ratios in the early Universe

We present new ALMA [OIII]$_{88}$ observations of eight previously [CII]$_{158}$-detected galaxies at $6.8 \lesssim z \lesssim 7.7$. Six of our targets -- the primary sample -- are massive, UV-luminous galaxies drawn from the REBELS survey, while the remaining two are UV-fainter galaxies that were previously serendipitously detected through their luminous [CII] lines in the REBELS fields. We detect [OIII]$_{88}$ emission in all eight galaxies at $6.2 - 17.7σ$ significance, and find them to be consistent with the local dwarf galaxy relation between $L_\mathrm{[OIII]}$ and star formation rate. Our sample spans [OIII]/[CII] $\approx 1.9 - 9.6$, which is typical for the high-redshift galaxy population. Five of the primary targets benefit from JWST/NIRSpec observations, enabling a direct comparison of the [OIII]/[CII] ratio against rest-optical ISM diagnostics. We supplement our high-redshift sample with eleven $z\approx6-14$ galaxies in the literature for which similar ALMA and JWST observations are available, and furthermore compare to the [OIII]/[CII] ratios measured for local dwarf galaxies. We find that, at fixed metallicity and ionization parameter, $z>6$ galaxies show elevated [OIII]/[CII] ratios compared to local dwarfs. Instead, we find that a large [OIII]$_{4959,5007}$+H$β$ equivalent width -- a proxy for burstiness -- is the main driver of the high [OIII]/[CII] ratios seen in the early Universe, which is primarily due to [CII] being suppressed in bursty galaxies. Given the apparent validity of the [OIII]$_{88}$-SFR relation across most of cosmic time, as well as the abundance of young, bursty galaxies at high redshift, [OIII]$_{88}$ is set to remain a powerful ISM tracer at the cosmic dawn.

astro-ph.GA↗

arXiv2025

REBELS-IFU: Evidence for metal-rich massive galaxies at z~6-8

Metallicity is a crucial tracer of galaxy evolution, providing insights into gas accretion, star formation, and feedback. At high redshift, these processes reveal how early galaxies assembled and enriched their interstellar medium. In this work, we present rest-frame optical spectroscopy of 12 massive ($\log(M_*/\mathrm{M_{\odot}})>9$) galaxies at $z\sim 6$-$8$ from the REBELS ALMA large program, observed with JWST NIRSpec/IFU in the prism mode. These observations span emission lines from [OII]$λ$3727,9 to [SII]$λ$6716,31, providing key information on nebular dust attenuation, ionisation states, and chemical abundances. We find lower O32 ratios (average $\sim3.7$) and [OIII]$λ$5007 equivalent widths (average ${EW_{[OIII]}}\sim390$Å) than are generally found in existing large spectroscopic surveys at $z>6$, indicating less extreme ionising conditions. Strong-line diagnostics suggest that these systems are some of the most metal-rich galaxies observed at $z>6$ (average $Z_{\mathrm{gas}}\sim 0.4 Z_{\odot}$), including sources with near-solar oxygen abundances, in line with their high stellar masses (average $\log{M_*/\mathrm{M_{\odot}}}\sim9.5$). Supplementing with literature sources at lower masses, we investigate the mass-metallicity and fundamental metallicity relations (MZR and FMR, respectively) over a 4 dex stellar mass range at $6 3$ studies.

astro-ph.GA↗