arXiv subjects

C. Accard

Publications and source records attributed to C. Accard.

6 recordsLinked to original sources

arXiv2026

Multi-tracer exploration of molecular gas in main sequence galaxies at z~4.5

Molecular gas masses in high-z galaxies are inferred from indirect tracers, whose respective reliability remains poorly constrained. In particular, the bright [CII] 158$\mu$m line is now widely used as a molecular gas tracer at z>4, yet direct observational tests against CO remain scarce. We search for CO(4-3), CO(5-4), and [CI](1-0) lines in three of the most [CII]-luminous galaxies at z~4.5 from the ALPINE survey to assess the detectability of these lines in high-z main-sequence (MS) galaxies and to test the reliability of [CII] emission as a molecular gas tracer through the cross-comparison of molecular gas masses inferred from six tracers: CO(4-3), CO(5-4), [CI](1-0), [CII], dust continuum, and [CII]-based dynamical mass, adopting standard calibrations and conversion factors. We detect CO(4-3) and CO(5-4) lines at high significance in the near-solar metallicity galaxy DC873756, obtain a tentative CO(4-3) detection in the merging system DC818760, and detect no CO emission in the half-solar metallicity galaxy VC5110377875. [CI] remains undetected in all three galaxies. In DC873756 the molecular gas masses inferred from the six considered tracers agree within their uncertainties despite different systematics inherent to each tracer. The agreement suggests that, at least for some near-solar metallicity MS galaxies at z~4.5, the CO SLED and Milky Way CO-to-H2 conversion factor adopted for MS galaxies at cosmic noon remain applicable and that mid-J CO transitions trace a substantial fraction of the molecular gas reservoir. The CO non-detection in VC5110377875 is consistent with the reduced CO detectability expected at lower metallicities. In DC818760 we find an inconsistency between the [CII]-based molecular gas mass and masses derived from the other tracers, indicating a [CII] excess possibly reflecting enhanced emission from shocks and/or diffuse ionized gas in merger-driven conditions.

astro-ph.GA↗

arXiv2026

Evaluating star formation rates at z = 5

Inferring the star formation rates (SFR) in high redshift galaxies remains challenging, owing to observational limitations or uncertainties in calibration methods that link luminosities to SFRs. We utilize two state-of-the-art hydrodynamical simulations NewHorizon and NewCluster, post-processed with the radiative transfer code Skirt, to investigate the systematic uncertainties and biases in the inferred SFRs for z=5 galaxies; an epoch where galaxies build-up their stellar mass. We create synthetic observables for widely-used tracers: Halpha nebular line, [CII] 158 micron fine-structure line, total infrared (IR) continuum luminosity, and hybrid (IR + UV). We find that Halpha-inferred SFRs, time-averaged over 10 Myr, are sensitive to the choice of calibration and exhibit substantial scatter driven by dust attenuation, viewing angle, and dust-to-metal ratio. Adopting a steeper attenuation curve reduces this scatter significantly but does not fully eliminate systematic uncertainties. IR continuum-based SFRs trace intrinsic SFRs time-averaged over 100 Myr timescales when a well-sampled continuum emission between restframe 8 and 1000 micron is available and underestimate them with typical approaches when IR data are limited. Nevertheless, IR SFRs display a considerable scatter, largely due to UV photon leakage and strong variations in the star formation history. When UV data are available, hybrid (IR + UV) SFRs provide a more robust estimate, reducing scatter compared to IR-based SFRs while avoiding explicit attenuation corrections. Finally, we derive a [CII]-SFR relation finding a steeper relation than previous studies, however with significant scatter linked to gas density and metallicity. Overall, IR-, hybrid-, and [CII]-based tracers remain more robust than Halpha against variations in optical depth.

astro-ph.GA↗

arXiv2025

The ALPINE-CRISTAL-JWST survey: spatially resolved star formation relations at $z\sim5$

Star formation governs galaxy evolution, shaping stellar mass assembly and gas consumption across cosmic time. The Kennicutt-Schmidt (KS) relation, linking star formation rate (SFR) and gas surface densities, is fundamental to understand star formation regulation, yet remains poorly constrained at $z > 2$ due to observational limitations and uncertainties in locally calibrated gas tracers. The [CII] $158 {\rm \mu m}$ line has recently emerged as a key probe of the cold ISM and star formation in the early Universe. We investigate whether the resolved [CII]-SFR and KS relations established at low redshift remain valid at $4 < z < 6$ by analysing 13 main-sequence galaxies from the ALPINE and CRISTAL surveys, using multi-wavelength data (HST, JWST, ALMA) at $\sim2$ kpc resolution. We perform pixel-by-pixel spectral energy distribution (SED) modelling with CIGALE on resolution-homogenised images. We develop a statistical framework to fit the [CII]-SFR relation that accounts for pixel covariance and compare our results to classical fitting methods. We test two [CII]-to-gas conversion prescriptions to assess their impact on inferred gas surface densities and depletion times. We find a resolved [CII]-SFR relation with a slope of $0.87 \pm 0.15$ and intrinsic scatter of $0.19 \pm 0.03$ dex, which is shallower and tighter than previous studies at $z\sim5$. The resolved KS relation is highly sensitive to the [CII]-to-gas conversion factor: using a fixed global $\alpha_{\rm [CII]}$ yields depletion times of $0.5$-$1$ Gyr, while a surface brightness-dependent $W_{\rm [CII]}$, places some galaxies with high gas density in the starburst regime ($<0.1$ Gyr). Future inputs from both simulations and observations are required to better understand how the [CII]-to-gas conversion factor depends on local ISM properties. We need to break this fundamental limit to properly study the KS relation at $z\gtrsim4$.

astro-ph.GA↗

arXiv2025

Spatially resolved [CII]-gas conversion factor in early galaxies

Determining how efficiently gas collapses into stars at high-redshift is key to understanding galaxy evolution in the Epoch of Reionization (EoR). Globally, this process is quantified by the gas depletion time ($t_{dep}$); on resolved scales, by the slope and normalization of the Kennicutt-Schmidt (KS) relation. This work explores the global ($\alpha_{[CII]}$) and spatially resolved ($W_{[CII]}$) [CII]-to-gas conversion factors at high-$z$ and their role in inferring reliable gas masses, surface densities, and $t_{dep}$ in the EoR. We select galaxies at 4<z<9 from the SERRA cosmological zoom-in simulation, that features on-the-fly radiative transfer and resolves interstellar medium properties down to $\approx$30 pc. The [CII] emission modelling from photodissociation regions allow us to derive global $\alpha_{ [CII]}$, and maps of $W_{[CII]}$. We study their dependence on gas metallicity (Z), density (n), Mach number (M), and burstiness parameter ($k_s$), and provide best fit relations. The $\alpha_{[CII]}$ decreases with increasing $Z$ and galaxy compactness, while the resolved $W_{[CII]}$ shows two regimes: at $Z< 0.2 Z_\odot$, it anticorrelates with n and Z, but not with $k_s$; above this threshold, it also depends on $k_s$, with more bursty regions showing lower conversion factors. This implies $W_{[CII]}\propto \Sigma_{[CII]}^{-0.5}$, as dense, metal-rich, and bursty regions exhibit higher [CII] surface brightness. Applying a constant $\alpha_{[CII]}$ overestimates $\Sigma_{gas}$ in bright $\Sigma_{[CII]}$ patches, thus flattening the KS slope and overestimating $t_{dep}$ by a factor of $\approx$4.

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

The ALMA-ALPINE [CII] survey: Kennicutt-Schmidt relation in four massive main-sequence galaxies at z~4.5

The Kennicutt-Schmidt (KS) relation between the gas and the star formation rate (SFR) surface density ($\Sigma_{\rm gas}$-$\Sigma_{\rm SFR}$) is essential to understand star formation processes in galaxies. So far, it has been measured up to z~2.5 in main-sequence galaxies. In this letter, we aim to put constraints at z~4.5 using a sample of four massive main-sequence galaxies observed by ALMA at high resolution. We obtained ~0.3"-resolution [CII] and continuum maps of our objects, which we then converted into gas and obscured SFR surface density maps. In addition, we produced unobscured SFR surface density maps by convolving Hubble ancillary data in the rest-frame UV. We then derived the average $\Sigma_{\rm SFR}$ in various $\Sigma_{\rm gas}$ bins, and estimated the uncertainties using a Monte Carlo sampling. Our galaxy sample follows the KS relation measured in main-sequence galaxies at lower redshift and is slightly lower than predictions from simulations. Our data points probe the high end both in terms of $\Sigma_{\rm gas}$ and $\Sigma_{\rm gas}$, and gas depletion timescales (285-843 Myr) remain similar to z~2 objects. However, three of our objects are clearly morphologically disturbed, and we could have expected shorter gas depletion timescales (~100 Myr) similar to merger-driven starbursts at lower redshifts. This suggests that the mechanisms triggering starbursts at high redshift may be different than in the low- and intermediate-z Universe.

astro-ph.GA↗