arXiv subjects

C. Simmonds

Publications and source records attributed to C. Simmonds.

7 recordsLinked to original sources

arXiv2026

Star-formation variability on the star-forming main sequence during the Epoch of Reionization

Star formation in galaxies is intrinsically stochastic, driven by physical processes operating across a wide range of scales. The scatter in the star-forming main sequence relation provides a window into this variability, but interpreting this scatter in terms of underlying physical mechanisms remains challenging. We present a study of star-formation variability during reionization (redshift z=3-8) using power spectral density (PSD) models to characterize fluctuations in star formation rates (SFRs). We use estimates of the intrinsic scatter in main sequence SFRs at six averaging timescales (10-100 Myr) from a catalogue of ~17000 galaxies presented in Simmonds et al. 2025 to constrain two PSD models, the Simple Harmonic Oscillator (SHO) and the Extended Regulator (ExtReg), with nested sampling and neural network emulators. We find that the regulator component of the ExtReg model is poorly constrained by the present data. However, both the dynamical component of the ExtReg model and the single-component SHO model favour characteristic variability timescales of ~10-30 Myr, comparable to expected galactic dynamical and stellar feedback timescales. At least in the SHO model, and most clearly at z~3-4, the inferred PSD power on ~10 Myr timescales decreases with stellar mass, indicating more bursty, rapidly varying star formation in lower-mass galaxies than in higher-mass systems. We find weak evidence for a transition from a two-component ExtReg-like PSD at lower redshift to a single-component SHO-like PSD at higher redshift in the lowest stellar-mass bin, log M*/M$\odot$ = 8-8.5, although the Bayes factors are small and selection effects at high redshift prevent strong conclusions. Overall, our results suggest that the observed 10-100 Myr scatter of the high-redshift star-forming main sequence is governed primarily by short-timescale variability, consistent with galactic dynamical timescales.

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arXiv2025

A great diversity of spectral shapes in the ionising spectra of z ~ 0.6-1 galaxies revealed by HST/COS and possible detection of nebular LyC emission

We present observations of eleven compact star-forming galaxies in the redshift range z = 0.6145 - 1.0053, with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). We aim to spectroscopically measure for the first time the Lyman continuum (LyC) over a wider rest-frame wavelength range of ~ 600 - 900A compared to ~ 850 - 900A in previous studies of galaxies at z ~ 0.3 - 0.4. The HST data are supplemented by SDSS spectra of all galaxies and by a VLT/Xshooter spectrum of one galaxy, J0232+0025. These data are used to derive the spectral energy distribution in the entire UV and optical range, the stellar mass, and the chemical composition from the nebular emission lines. We detect stellar LyC emission in seven out of eleven galaxies with escape fractions, f_esc(LyC), in the range of ~ 2 - 60%, and establish upper limits for f_esc(LyC) in the remaining galaxies. We discover for the first time nebular LyC emission as a bump just bluewards of the LyC limit at 912A in two galaxies, J0232+0025 and J1021+0436. We find a similar bump among our earlier studies in a less distant galaxy J1243+4646 with z = 0.4317. We conclude that the use of the LyC continuum in the wavelength range close to the LyC limit, which contains both the stellar and nebular continua, requires special consideration to not overestimate the observed f_esc(LyC).

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arXiv2025

Bursting at the seams: the star-forming main sequence and its scatter at z=3-9 using NIRCam photometry from JADES

We present a comprehensive study of the star-forming main sequence (SFMS) and its scatter at redshifts $3 \leq z \leq 9$, using NIRCam photometry from the JADES survey in the GOODS-S and GOODS-N fields. Our analysis is based on a sample of galaxies that is stellar mass complete down to $\log \left(M_{\star}/M_{\odot}\right) \approx 8.1$. The redshift evolution of the SFMS at an averaging timescale of 10 Myr follows a relation, quantified by the specific star-formation rates (sSFR$_{10}$), of $\mathrm{sSFR}\propto(1+z)^{\mu}$ with $\mu = 2.30^{+0.03}_{-0.01}$, in good agreement with theoretical predictions and the specific mass accretion rate of dark matter halos. We find that the SFMS normalisation varies in a complex way with the SFR averaging timescale, reflecting the combined effects of bursty star formation and rising star formation histories (SFHs). We quantify the scatter of the SFMS, revealing that it decreases with longer SFR averaging timescales, from $\sigma_{\rm{int}} \approx 0.4-0.5~\mathrm{dex}$ at 10 Myr to $\sigma_{\rm{int}} \approx 0.2~\mathrm{dex}$ at 100 Myr, indicating that shorter-term fluctuations dominate the scatter, although long-term variations in star formation activity are also present. Our findings suggest that bursty SFHs are more pronounced at lower stellar masses. Furthermore, we explore the implications of our results for the observed over-abundance of UV-bright galaxies at $z > 10$, concluding that additional mechanisms, such as top-heavy initial mass functions, increased star-formation efficiencies, or increased burstiness in star formation are needed to explain these observations. Finally, we emphasize the importance of accurate stellar mass completeness limits when fitting the SFMS, especially for galaxies with bursty SFHs.

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arXiv2024

Ionising properties of galaxies in JADES for a stellar mass complete sample: resolving the cosmic ionising photon budget crisis at the Epoch of Reionisation

We use NIRCam imaging from the JWST Advanced Deep Extragalactic Survey (JADES) to study the ionising properties of a sample of 15721 galaxies at $3 \leq z_{\rm{phot}} \leq 9$, 90\% complete in stellar mass down to log(M$_{\star}$/[M$_{\odot}$])$\approx 7.5$. Out of the full sample, 1620 of the galaxies have spectroscopic redshift measurements from the literature. We use the spectral energy distribution fitting code \texttt{Prospector} to fit all available photometry and infer galaxy properties. We find a significantly milder evolution of the ionising photon production efficiency (\xion\/) with redshift and UV magnitude than previously reported. Interestingly, we observe two distinct populations in \xion\/, distinguished by their burstiness (given by SFR$_{10}$/SFR$_{100}$). Both populations show the same evolution with $z$ and M$_{\rm{UV}}$, but have a different \xion\/ normalisation. We convolve the more representative $\log(\xi_{\rm{ion}} (z,\text{M}_{\rm{UV}}))$ relations (accounting for $\sim96$\% of the sample), with luminosity functions from literature, to place constraints on the cosmic ionising photon budget. By combining our results, we find that one of our models can match the observational constraints from the \lya\/ forest at $z\lesssim6$. We conclude that galaxies with M$_{\rm{UV}}$ between $-16$ and $-20$, adopting a reasonable escape fraction, can produce enough ionising photons to ionise the Universe, without exceeding the required ionising photon budget.

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arXiv2024

The impact of nebular Lyman-Continuum on ionising photons budget and escape fractions from galaxies

Several Lyman Continuum (LyC) emitters have been detected so far, but their observed ionising spectra sometimes differ from attenuated stellar spectra predicted by stellar population synthesis modelling. This discrepancy may be due to a significant contribution of LyC nebular emission. We aim to quantify the importance this emission in LyC leakers: its contribution to the ionising photons budget, and to measurements of LyC escape. To estimate the nebular contribution to the LyC spectra of galaxies, we run photoionisation models with Cloudy for a range of BPASS templates, varying the column density of the surrounding gas, from density-bounded (log(NH$_{\rm{stop}}$/cm$^{-2}$)=16) to ionisation-bounded (log(NH$_{\rm{stop}}$/cm$^{-2}$)=19) regimes. In the limits of very optically thin (f$_{\rm{esc}}$ = 1), or thick configurations (f$_{\rm{esc}}$ = 0), there is no nebular contribution to the emergent LyC spectra. This contribution matters only at intermediate LyC opacities ($0 <$ f$_{\rm{esc}}$ $< 1$), where it alters the shape of the LyC spectrum chromatically, so that escape fractions estimates are highly sensitive to the wavelength range over which they are calculated. We propose a formula to estimate integrated escape fractions using f$_{\lambda 700}$/f$_{\lambda 1100}$ flux ratios, since this wavelength range is not affected by nebular emission. Regarding simulations, the boost of hydrogen ionising photons escaping galaxies is inversely proportional to the stellar escape fractions, but since typical simulated escape fractions are low, LyC photons escape is important. Nebular LyC is a non-negligible additional source of ionising photons from galaxies, which contribution has been overlooked so far in observations and in cosmic reionisation simulations.

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arXiv2023

Lyman Continuum Leaker Candidates at $z\sim3-4$ in the HDUV Based on a Spectroscopic Sample of MUSE LAEs

In recent years, a number of Lyman continuum (LyC) leaker candidates at intermediate redshifts have been found, providing insight into how the Universe was reionised at early cosmic times. Here we identify new LyC leaker candidates at $z\approx 3-4.5$ and compare them to objects from the literature to get an overview of the different observed escape fractions and their relation to the properties of the Lyman $\alpha$ (Ly$\alpha$) emission line. The aim of this work is to test indicators for LyC leakage and to improve our understanding of the kind of galaxies from which LyC radiation can escape. We use data from the Hubble Deep Ultraviolet (HDUV) legacy survey to search for LyC emission based on a sample of $\approx 2000$ Ly$\alpha$ emitters (LAEs) detected previously in two surveys with the Multi-Unit Spectroscopic Explorer (MUSE), MUSE-Deep and MUSE-Wide. Based on their known redshifts and positions, we look for potential LyC leakage in the WFC3/UVIS F336W band of the HDUV. The escape fractions are measured and compared based on spectral energy distribution (SED) fitting performed using the CIGALE software. We add twelve objects to the sample of known LyC leaker candidates, one of which was previously known, and compare their Ly$\alpha$ properties to their escape fractions. We find escape fractions between $\sim 20\%$ and $\sim 90\%$, assuming a high transmission in the intergalactic medium (IGM). We show a method to use the number of LyC leaker candidates we find to infer the underlying average escape fraction of galaxies, which is $\approx 12\%$. Based on their Ly$\alpha$ properties, we conclude that LyC leakers are not very different from other high-z LAEs and suggest that most LAEs could be leaking LyC even if this can not always be detected due to the direction of emission and the transmission properties of the IGM.

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arXiv2023

Low-mass bursty galaxies in JADES efficiently produce ionising photons and could represent the main drivers of reionisation

We study galaxies in JADES Deep to study the evolution of the ionising photon production efficiency, $\xi_{\rm{ion}}$, observed to increase with redshift. We estimate $\xi_{\rm{ion}}$ for a sample of 677 galaxies at $z \sim 4 - 9$ using NIRCam photometry. Specifically, combinations of the medium and wide bands F335M-F356W and F410M-F444W to constrain emission lines that trace $\xi_{\rm{ion}}$: H$\alpha$ and [OIII]. Additionally, we use the spectral energy distribution fitting code \texttt{Prospector} to fit all available photometry and infer galaxy properties. The flux measurements obtained via photometry are consistent with FRESCO and NIRSpec-derived fluxes. Moreover, the emission-line-inferred measurements are in tight agreement with the \texttt{Prospector} estimates. We also confirm the observed $\xi_{\rm{ion}}$ trend with redshift and M$_{\rm{UV}}$, and find: $\log \xi_{\rm{ion}} (z,\text{M}_{\rm{UV}}) = (0.05 \pm 0.02)z + (0.11 \pm 0.02) \text{M}_{\rm{UV}} + (27.33 \pm 0.37)$. We use \texttt{Prospector} to investigate correlations of $\xi_{\rm{ion}}$ with other galaxy properties. We see a clear correlation between $\xi_{\rm{ion}}$ and burstiness in the star formation history of galaxies, given by the ratio of recent to older star formation, where burstiness is more prevalent at lower stellar masses. We also convolve our $\xi_{\rm{ion}}$ relations with luminosity functions from the literature, and constant escape fractions of 10 and 20\%, to place constraints on the cosmic ionising photon budget. By combining our results, we find that if our sample is representative of the faint low-mass galaxy population, galaxies with bursty star formation are efficient enough in producing ionising photons and could be responsible for the reionisation of the Universe.

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