arXiv subjects

D. Schaerer

Publications and source records attributed to D. Schaerer.

At least 55 records · Page 3Linked to original sources

arXiv2023

Clues on the presence and segregation of very massive stars in the Sunburst Lyman-continuum cluster at z=2.37

We report the identification of very massive stars (VMS; mass $> 100$\,\msun) that may be segregated in the center of the young massive star cluster at $z$=2.37 hosted in the lensed galaxy called {\tt Sunburst} galaxy. This result is based on two pieces of evidence: (1) VLT/MUSE spectra of several multiple images of the same star cluster show key spectral signatures of VMS, such as the \heii\ broad emission, \nivblue\ emission, and an \niv\ P-Cygni profile. In particular, \heii\ is broad ($\sim1610\pm300$ \kms), with an equivalent width of 3Å,\ and asymmetric profile. These features require an extremely young ($\sim2.5$ Myr) stellar population component in which the masses of the stars exceed 100~\msun. When a Salpeter initial mass function and BPASS models for normal massive stars are assumed, the observed spectral features require $\sim$400 VMS. (2) The same star cluster is detected at a signal-to-noise ratio of~$\sim100$ in the Lyman continuum domain ($λ< 900$Å). The Lyman continuum emission emerges from a region with a radius that is at least twice smaller than what is observed at 1700Å~(independently of magnification) and is located in the center of the cluster. After delensing, the effective radii in absolute scales are R$_{\tt eff}[{\tt LyC}]\sim4.7 \pm 1.5$ pc and R$_{\tt eff}[1700]= 7.8 \pm 1.4$ pc. The Lyman continuum radiation is mainly produced by hot and massive stars, which implies that their spatial distribution (including that of VMS) is preferentially more confined in the central parts of the cluster. Approximately 400 VMS hosted by a cluster of $\sim 10^7$ \msun\ produce $\sim$15\% of the escaping Lyman continuum photons, and the remaining photons are produced by other massive early-type stars.

astro-ph.GA↗

arXiv2023

Search strategies for supermassive stars in young clusters and application to nearby galaxies

Supermassive stars (SMS) with masses $M \gtrsim 10^3-10^4 M_{\odot}$, formed by runaway collisions in young, massive, and dense star clusters have been invoked as a possible solution to the puzzles raised by the presence of multiple stellar populations and peculiar abundance patterns observed in globular clusters. However, such objects have not been observed so far. We developed observational strategies to search for SMS hosted within young massive clusters (thought to be the precursors of globular clusters, GCs), which could be applicable in a relatively general fashion, using both photometric and spectroscopic observations. We used theoretical predictions of spectra of SMS and SMS-hosting clusters, together with predictions from standard simple stellar populations to examine their impact on color-color diagrams and on individual optical spectral lines (primarily Hydrogen emission and absorption lines). As a first step, we apply our search strategies to a sample of $\sim 3000$ young star clusters (YSC) from two nearby galaxies with multi-band observations from the HST and optical integral-field spectroscopy obtained with MUSE on the Very Large Telescope. We focus on models for SMS with large radii (corresponding to $ Teff \lesssim 7000$ K), which predict strong Balmer breaks, and construct proper color-color diagrams to select the corresponding SMS-hosting cluster candidates. We show that their spectrophotometric properties are similar to that of normal clusters with ages of a few hundred Myr, which would, however, show signs of composite stellar populations, in particular the presence of nebular lines (H$α$ and others). Examining the photometry, overall SEDs, and the spectra of $\sim 100$ clusters with strong Balmer breaks, we have found several objects with peculiar SEDs, the presence of emission lines, or other peculiar signatures. [abridged]

astro-ph.GA↗

arXiv2023

The gas and stellar content of a metal-poor galaxy at $z=8.496$ as revealed by JWST and ALMA

We present a joint analysis of the galaxy S04590 at $z=8.496$ based on NIRSpec, NIRCam, and NIRISS observations obtained through as part of Early Release Observations programme of the James Webb Space Telescope (JWST) and the far-infrared [CII]-$158μ$m emission line detected by dedicated Atacama Large Millimeter/submillimeter Array (ALMA) observations. We determine the physical properties of S04590 from modelling of the spectral energy distribution (SED) and through the redshifted optical nebular emission lines detected with JWST/NIRSpec. The best-fit SED model reveals a low-mass ($M_\star = 10^{7.2}-10^{8}\,M_{\odot}$) galaxy with a low oxygen abundance of $12+\log{\rm (O/H)} = 7.16^{+0.10}_{-0.12}$ derived from the strong nebular and auroral emission lines. Assuming that [CII] effectively traces the interstellar medium (ISM), we estimate the total gas mass of the galaxy to be $M_{\rm gas} = (8.0\pm 4.0)\times 10^{8}\,M_\odot$ based on the luminosity and spatial extent of [CII]. This yields an exceptionally high gas fraction, $f_{\rm gas} = M_{\rm gas}/(M_{\rm gas} + M_\star) \gtrsim 90\%$, though still consistent within the range expected for its low metallicity. We further derive the metal mass of the galaxy based on the gas mass and gas-phase metallicity, which we find to be consistent with the expected metal production from Type II supernovae. Finally, we make the first constraints on the dust-to-gas (DTG) and dust-to-metals (DTM) ratios of galaxies in the epoch of reionization at $z\gtrsim 6$, showing overall low mass ratios of logDGT $<-3.8$ and logDTM $<-0.5$, though consistent with local scaling relations and in particular the local metal-poor galaxy I Zwicky 18. Our analysis highlights the synergy between ALMA and JWST in characterizing the gas, metal, and stellar content of the first generation of galaxies.

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arXiv2023

Insights into the reionization epoch from cosmic-noon-CIV emitters in the VANDELS survey

Recently, intense emission from nebular C III] and C IV emission lines have been observed in galaxies in the epoch of reionization ($z>6$) and have been proposed as the prime way of measuring their redshift and studying their stellar populations. These galaxies might represent the best examples of cosmic reionizers, as suggested by recent low-z observations of Lyman Continuum emitting galaxies, but it is hard to directly study the production and escape of ionizing photons at such high redshifts. The ESO spectroscopic public survey VANDELS offers the unique opportunity to find rare examples of such galaxies at cosmic noon ($z\sim 3$), thanks to the ultra deep observations available. We have selected a sample of 39 galaxies showing C IV emission, whose origin (after a careful comparison to photoionization models) can be ascribed to star formation and not to AGN. By using a multi-wavelength approach, we determine their physical properties including metallicity and ionization parameter and compare them to the properties of the parent population to understand what are the ingredients that could characterize the analogs of the cosmic reionizers. We find that C IV emitters are galaxies with high photons production efficiency and there are strong indications that they might have also large escape fraction: given the visibility of C IV in the epoch of reionization this could become the best tool to pinpoint the cosmic reioinzers.

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arXiv2022

The ALPINE-ALMA [CII] survey: double stellar population and AGN activity in a galaxy at $z\sim5.5$

GDS J033218.92-275302.7 (here GS-14) is a $z\sim5.5$ galaxy detected in [CII] as part of the ALPINE survey with unusual UV spectral features that have been interpreted as signatures of either a double stellar population or of an active galactic nucleus (AGN). We exploited the multi-wavelength coverage of GS-14 to investigate the properties and the origin of its emission. We performed UV-to-NIR SED-fitting, with single/double stellar population and/or AGN component. We analyzed the VIMOS spectrum, which shows highly-ionized emission lines (Ovi, Nv, and Niv). The line properties have been compared with those observed in galaxies and AGN, and with the predictions from radiation transfer models for star-forming galaxies, AGN, and shocks. The SED-fitting provides a total stellar mass of $M_*=(4 \pm 1) \times 10^{10} M_\odot$, an age of the main stellar population of $\sim670 Myr$ and a recent short (8 Myr) burst of star formation (SF) of $\sim 90 M_\odot yr^{-1}$. The Nv line has a characteristic P-Cygni profile, which suggests a $\sim 3 Myr$ old population of stars with a mass of $\sim 5 \times10^{7} M_\odot$. The Nv profile also shows evidence for an additional component of nebular emission. The comparison of the line ratios with theoretical models allows us to associate the emission with SF or AGN, but the strong radiation field required to ionize the Ovi is more commonly related to AGN activity. We found evidence for an old and already evolved stellar population at $z\sim 5.5$ and show that the galaxy is experiencing a second short burst of SF. In addition, GS-14 carries signatures of obscured AGN activity. The AGN could be responsible for the short depletion time of this galaxy, thus making GS-14 one of the two ALPINE sources with hints of an active nucleus and an interesting target for future follow-ups.

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arXiv2022

Strong C IV emission from star-forming galaxies: a case for high Lyman continuum photon escape

Finding reliable indicators of Lyman continuum (LyC) photon leakage from galaxies is essential in order to infer their escape fraction in the epoch of reionisation, where direct measurements of LyC flux are impossible. To this end, here we investigate whether strong C IV $λλ1548,1550$ emission in the rest-frame UV spectra of galaxies traces conditions ripe for ample production and escape of LyC photons. We compile a sample of 19 star-forming galaxies in the redshift range $z=3.1-4.6$ from the VANDELS survey that exhibit strong C IV emission, producing a stacked spectrum where all major rest-UV emission lines are clearly detected. Best-fitting spectral energy distribution models containing both stellar and nebular emission suggest the need for low stellar metallicities ($Z=0.1-0.2\,Z_\odot$), young stellar ages ($\log(\rm{age/yr}) = 6.1-6.5$), a high ionisation parameter ($\log U = -2$) and little to no dust attenuation ($E(B-V)=0.00-0.01$). However, these models are unable to fully reproduce the observed C IV and He II line strengths. We find that the Ly$α$ line in the stacked spectrum is strong and peaks close to the systemic velocity, features that are indicative of significant LyC photon leakage along the line-of-sight. The covering fractions of low-ionisation interstellar absorption lines are also low implying LyC escape fraction in the range $\approx 0.05-0.30$, with signatures of outflowing gas. Finally, C IV/C III] ratios of >0.75 for a subset of individual galaxies with reliable detections of both lines are also consistent with physical conditions that enable significant LyC leakage. Overall, we report that multiple spectroscopic indicators of LyC leakage are present in the stacked spectrum of strong C IV emitting galaxies, potentially making C IV an important tracer of LyC photon escape at $z>6$.

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arXiv2022

An extreme blue nugget, UV-bright starburst at z=3.613 with ninety per cent of Lyman continuum photon escape

We present the discovery and analysis of J1316+2614 at z=3.6130, a UV-bright star-forming galaxy ($M_{\rm UV} \simeq -24.7$) with large escape of Lyman continuum (LyC) radiation. J1316+2614 is a young ($\simeq 10$ Myr) star-forming galaxy with $SFR \simeq 500 M_{\odot}$ yr$^{-1}$ and a starburst mass of log($M_{\star}/M_{\odot}) \simeq 9.7$. It shows a very steep UV continuum, $β_{\rm UV} \simeq -2.59 \pm 0.05$, consistent with residual dust obscuration, $E(B-V)\simeq 0$. LyC emission is detected with high significance ($\simeq 17 σ$) down to $830$Å, for which a very high relative (absolute) LyC escape fraction $f_{\rm esc} \rm (LyC) \simeq 0.92$ ($\simeq 0.87$) is inferred. The contribution of a foreground or AGN contamination to the LyC signal is discussed but is unlikely. J1316$+$2614 is the most powerful ionizing source known among the star-forming galaxy population, both in terms of production ($Q_{\rm H} \approx 10^{56}$ s$^{-1}$) and escape of ionizing photons ($f_{\rm esc} \rm (LyC) \approx 0.9$). Nebular emission in Ly$α$, H$β$, and other rest-frame optical lines are detected, but these are weak ($EW_{0} \rm [Hβ] \simeq 35$Å), with their strengths reduced roughly by $\simeq 90\%$. J1316+2614 is the first case known where the effect of large escape of ionizing photons on the strength of nebular lines and continuum emission is clearly observed. Gas inflows are detected in J1316+2614 from the blue-dominated peak Ly$α$ emission (with a blue-to-red peak line ratio $I_{\rm blue}/I_{\rm red} \simeq 3.7$) and redshifted ISM absorption ($\simeq 100$ km s$^{-1}$). Our results suggest that J1316+2614 is undergoing a gas compaction event, possibly representing a short-lived phase in the evolution of massive and compact galaxies, where strong gas inflows have triggered an extreme star formation episode and nearly $100\%$ LyC photons are escaping.

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arXiv2022

First look with JWST spectroscopy: $z \sim 8$ galaxies resemble local analogues

Deep images and near-IR spectra of galaxies in the field of the lensing cluster SMACS J0723.3-7327 were recently taken in the Early Release Observations program of JWST. Among these, two NIRSpec spectra of galaxies at $z=7.7$ and one at $z=8.5$ were obtained, revealing for the first time rest-frame optical emission line spectra of galaxies in the epoch of reionization, including the detection of the important[OIII]4363 auroral line (see JWST PR 2022-035). We present an analysis of the emission line properties of these galaxies, finding that these galaxies have a high excitation (as indicated by high ratios of [OIII]/[OII], [NeIII]/[OII]), strong [OIII]4363/H$γ$, high equivalent widths, and other properties which are typical of low-metallicity star-forming galaxies. Using the direct method we determine oxygen abundances of $12+\log(O/H)=7.9$ in two $z=7.7$ galaxies, and a lower metallicity of $12+\log(O/H)\approx 7.4-7.5$ in the $z=8.5$ galaxy using different strong line methods. More accurate metallicity determinations will require better data. With stellar masses estimated from SED fits, we find that the three galaxies lie close to or below the $z \sim 2$ mass-metallicity relation. Overall, these first galaxy spectra at $z \sim 8$ show a strong resemblance of the emission lines properties of galaxies in the epoch of reionization with those of relatively rare local analogues previously studied from the SDSS. Clearly, the first JWST observations demonstrate already the incredible power of spectroscopy to reveal properties of galaxies in the early Universe.

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arXiv2022

Properties of the interstellar medium in star-forming galaxies at redshifts 2 < z < 5 from the VANDELS survey

Gaseous flows inside and outside galaxies are key to understanding galaxy evolution, as they regulate their star formation activity across cosmic time. We study the ISM kinematics of 330 CIII or HeII emitters, using far-UV ISM absorption lines detected in the VANDELS spectra. These galaxies span a broad range of stellar masses M$_\ast$ from $10^8$ to $10^{11}$ M$_\odot$, and SFRs from 1 to 500 M$_\odot$/yr, in the redshift range between 2 and 5. We find that the bulk ISM velocity v$_{ism}$ is globally in outflow, with v$_{ism}$ of -60 $\pm$ 10 km/s for low ionization gas traced by SiII 1260 Angstrom, CII 1334, SiII 1526, and AlII 1670, and v$_{ism}$ of -160 $\pm$ 30 and -170 $\pm$ 30 km/s for higher ionization gas traced respectively by AlIII 1854-1862 and SiIV 1393-1402. Interestingly, BPASS models are able to better reproduce the stellar continuum around the SiIV doublet than other stellar population templates. For individual galaxies, $34\%$ of the sample has a positive ISM velocity shift, almost double the fraction reported at lower redshifts. Comparing v$_{ism}$ to the host galaxies properties, we find no significant correlations with M$_\ast$ or SFR, and only a marginally significant dependence (at $\sim 2σ$) on morphology-related parameters, with slightly higher velocities in galaxies of smaller size (probed by the equivalent radius), higher concentration, and higher SFR surface density. The outflows are consistent with models of accelerating, momentum-driven winds, with densities decreasing towards the outskirts. Our moderately lower ISM velocities compared to those found in similar studies at lower redshifts suggest that inflows and internal turbulence might play an increased role at $z>2$. We estimate mass outflow rates comparable to the SFRs of the galaxies, and an average escape velocity of 625 km/s, suggesting that most of the ISM will remain bound to the galaxy halo.

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arXiv2022

Inferring the HII region escape fraction of ionizing photons from infrared emission lines in metal-poor star-forming dwarf galaxies

(abridged) Quantifying the ISM porosity to ionizing photons in nearby galaxies may improve our understanding of the mechanisms leading to Lyman Continuum photons leakage from galaxies. Primitive galaxies with low metal and dust content have been shown to host a more patchy and porous ISM than their high-metallicity counterparts. To what extent this peculiar structure contributes to the leakage of ionizing photons remains to be quantitatively studied. To address these questions we build a refined grid of models including density-bounded regions and a possible contribution of an X-ray source. Using MULTIGRIS, a new Bayesian code based on Monte Carlo sampling, we combine the models as sectors under various assumptions to extract the probability density distributions of the parameters and infer the corresponding escape fractions from H II regions (fesc,HII). We apply this new code to a sample of 39 well-know local starbursting dwarf galaxies from the Dwarf Galaxy Survey. We confirm previous results hinting at an increased porosity to ionizing photons of the ISM in low-metallicity galaxies and provide, for the first time, quantitative predictions for fesc,HII. The predicted fesc,HII for low-metallicity objects span a large range of values, up to 60%, while the values derived for more metal-rich galaxies are globally lower. We also examine the influence of other parameters on the escape fractions, and find that the specific star-formation rate correlates best with fesc,HII . Finally, we provide observational line ratios which could be used as tracers of the photons escaping from density-bounded regions. Although this multi-sector modelling remains too simple to fully capture the ISM complexity, it can be used to preselect galaxy samples with potential leakage of ionizing photons based on current and up-coming spectral data in unresolved surveys of local and high-redshift galaxies.

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arXiv2022

The Far-Ultraviolet Continuum Slope as a Lyman Continuum Escape Estimator at High-redshift

Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high-redshifts. While observations have established that reionization occurred, observational constraints on the emissivity of ionizing photons at high-redshift remains elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and archival observations, a combined sample of 89 star-forming galaxies at z~0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6$σ$ significant) inverse correlation between the continuum slope at 1550Å (defined as F$_λ\proptoλ^β$) and both the LyC escape fraction (f$_{esc}$) and f$_{esc}$ times the ionizing photon production efficiency ($ξ_{ ion}$). On average, galaxies with redder continuum slopes have smaller f$_{esc}$ than galaxies with bluer slopes due to higher dust attenuation. More than 5% (20%) of the LyC emission escapes galaxies with $β$<-2.1 (-2.6). We find strong correlations between $β$ and the gas-phase ionization ([OIII]/[OII] flux ratio; at 7.5$σ$ significance), galaxy stellar mass (at 5.9$σ$), the gas-phase metallicity (at 4.6$σ$), and the observed FUV absolute magnitude (M$_{UV}$ at 3.4$σ$). Using previous observations of $β$ at high-redshift, we estimate the evolution of f$_{esc}$ with both $z$ and M$_{UV}$. The LzLCS suggest that fainter and lower mass galaxies dominate the ionizing photon budget at higher redshift, possibly due to their rapidly evolving metal and dust content. Finally, we use our correlation between $β$ and f$_{ esc}\timesξ_{ion}$ to predict the ionizing emissivity of galaxies during the epoch of reionization. Our estimated emissivities match IGM observations, and suggest that star-forming galaxies emit sufficient LyC photons into the IGM to exceed recombinations near redshifts of 7-8.

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arXiv2022

The ionizing and heating power of ultraluminous X-ray sources under the geometrical beaming model

While there is now a consensus that X-ray binaries (XRBs) are the dominant X-ray sources in the early Universe and play a significant role during the epoch of heating of the intergalactic medium (IGM), recent studies report contradicting results regarding their contribution in the nebular emission of local Universe galaxies. Ultraluminous X-ray sources (ULXs), which dominate the X-ray budget of normal galaxies, may be important interstellar-medium (ISM) ionizing sources. However, their output in the extreme UV (EUV) and soft--X-ray part of the spectrum remains observationally unconstrained. In this paper, we predict the ionizing and heating power from ULX populations under the geometrical beaming scenario, and three models describing the emission from super-critical accretion disks. We find that our theoretical spectra for ULX populations cannot (can) explain the HeII (NeV) emission observed in some galaxies, with their contribution being less (more) important than the underlying stellar population. Stochastic fluctuations in the number of ULXs may allow for equal contributions in the HeII emission, in a fraction of galaxies. We provide average spectra of ULX populations as an input to local, and early-Universe studies. We find that the soft--X-ray emission arising from super-critical accretion is significant for the heating of the IGM, and consistent with recent constraints from the 21-cm cosmic signal. Based on the dependence on the adopted compact-object (CO) mass and accretion model, we encourage efforts in modeling ULX spectra via simulations, and their combination with detailed binary population synthesis models.

astro-ph.HE↗

arXiv2022

Lyman alpha and Lyman continuum emission of MgII-selected star-forming galaxies

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of seven compact low-mass star-forming galaxies at redshifts, z,in the range 0.3161-0.4276, with various O3Mg2=[OIII]5007/MgII 2796+2803 and Mg2=MgII 2796/MgII 2803 emission-line ratios. We aim to study the dependence of leaking Lyman continuum (LyC) emission on the characteristics of MgII emission together with the dependences on other indirect indicators of escaping ionizing radiation. LyC emission with escape fractions fesc(LyC)=3.1-4.6 per cent is detected in four galaxies, whereas only 1sigma upper limits of fesc(LyC) in the remaining three galaxies were derived. A strong narrow Ly-alpha emission line with two peaks separated by Vsep~298-592 km/s was observed in four galaxies with detected LyC emission and very weak Ly-alpha emission is observed in galaxies with LyC non-detections. Our new data confirm the tight anti-correlation between fesc(LyC) and Vsep found for previous low-redshift galaxy samples. Vsep remains the best indirect indicator of LyC leakage among all considered indicators. It is found that escaping LyC emission is detected predominantly in galaxies with Mg2>1.3. A tendency of an increase of fesc(LyC) with increasing of both the O3Mg2 and Mg2 is possibly present. However, there is substantial scatter in these relations not allowing their use for reliable prediction of fesc(LyC).

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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.

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arXiv2022

ALPINE: A Large Survey to Understand Teenage Galaxies

A multiwavelength study of galaxies is important to understand their formation and evolution. Only in the recent past, thanks to the Atacama Large (Sub) Millimeter Array (ALMA), were we able to study the far-infrared (IR) properties of galaxies at high redshifts. In this article, we summarize recent research highlights and their significance to our understanding of early galaxy evolution from the ALPINE survey, a large program with ALMA to observe the dust continuum and 158um C+ emission of normal star-forming galaxies at z = 4-6. Combined with ancillary data at UV through near-IR wavelengths, ALPINE provides the currently largest multiwavelength sample of post-reionization galaxies and has advanced our understanding of (i) the demographics of C+ emission; (ii) the relation of star formation and C+ emission; (iii) the gas content; (iv) outflows and enrichment of the intergalactic medium; and (v) the kinematics, emergence of disks, and merger rates in galaxies at z > 4. ALPINE builds the basis for more detailed measurements with the next generation of telescopes, and places itself as an important post-reionization baseline sample to allow a continuous study of galaxies over 13 billion years of cosmic time.

astro-ph.GA↗

arXiv2022

No correlation of the Lyman continuum escape fraction with spectral hardness

The properties that govern the production and escape of hydrogen ionizing photons (Lyman continuum, LyC; with energies >13.6 eV) in star-forming galaxies are still poorly understood, but they are key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relationship between the hardness of ionizing radiation and the LyC leakage in a large sample of low-$z$ star-forming galaxies from the recent Hubble Space Telescope Low-$z$ Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep XShooter observations, we investigate the hardness of the ionizing spectra ($Q_{\rm He^+}/Q_{\rm H}$) between 54.4 eV (He$^{+}$) and 13.6 eV (H) from the optical recombination lines HeII 4686A and H$β$ 4861A for galaxies with LyC escape fractions spanning a wide range, $f_{\rm esc} \rm (LyC) \simeq 0 - 90\%$. We find that the observed intensity of HeII/H$β$ is primarily driven by variations in the metallicity, but is not correlated with LyC leakage. Both very strong ($ \simeq 0.5$) and nonleakers ($ < f_{\rm esc} \rm (LyC) > \simeq 0$) present similar observed intensities of HeII/H$β$ at comparable metallicity, between $\simeq 0.01$ and $\simeq 0.02$ for $12 + \log({\rm O/H}) > 8.0$ and $<8.0$, respectively. Our results demonstrate that $Q_{\rm He^+}/Q_{\rm H}$ does not correlate with $f_{\rm esc} \rm (LyC)$, which implies that strong LyC emitters do not show harder ionizing spectra than nonleakers at similar metallicity.

astro-ph.GA↗

arXiv2022

The VANDELS survey: a measurement of the average Lyman-continuum escape fraction of star-forming galaxies at z=3.5

We present a study designed to measure the average LyC escape fraction ($\langle f_{\rm esc}\rangle$) of star-forming galaxies at z=3.5. We assemble a sample of 148 galaxies from the VANDELS survey at $3.35\leq z_{\rm spec}\leq3.95$, selected to minimize line-of-sight contamination of their photometry. For this sample, we use ultra-deep, ground-based, $U-$band imaging and HST $V-$band imaging to robustly measure the distribution of $\mathcal{R_{\rm obs}}$ $=(L_{\rm LyC}/L_{\rm UV})_{\rm obs}$. We then model the distribution as a function of $\langle f_{\rm esc}\rangle$, carefully accounting for attenuation by dust, and the IGM (and CGM). A maximum likelihood fit to the $\mathcal{R_{\rm obs}}$ distribution returns a best-fitting value of $\langle f_{\rm esc}\rangle =0.07\pm0.02$, a result confirmed using an alternative Bayesian inference technique (both exclude $\langle f_{\rm esc}\rangle=0.0$ at $> 3σ$). By splitting our sample in two, we find evidence that $\langle f_{\rm esc}\rangle$ is positively correlated with Ly$α$ equivalent width, with high and low sub-samples returning best fits of $\langle f_{\rm esc}\rangle=0.12^{+0.06}_{-0.04}$ and $\langle f_{\rm esc} \rangle=0.02^{+0.02}_{-0.01}$, respectively. In contrast, we find evidence that $\langle f_{\rm esc}\rangle$ is anti-correlated with intrinsic UV luminosity and UV dust attenuation; with low UV luminosity and dust attenuation sub-samples returning best fits in the range $0.10 \leq \langle f_{\rm esc}\rangle \leq 0.22$. We do not find evidence for a clear correlation between $f_{\rm esc}$ and galaxy stellar mass, suggesting it is not a primary indicator of leakage. Although larger samples are needed to further explore these trends, they suggest that it is entirely plausible that the low dust and metallicity galaxies found at z > 6 will display the $\langle f_{\rm esc}\rangle\geq0.1$ required to drive reionization.

astro-ph.GA↗

arXiv2022

The VIMOS Ultra Deep Survey: The reversal of the star-formation rate $-$ density relation at $2 < z < 5$

Utilizing spectroscopic observations taken for the VIMOS Ultra-Deep Survey (VUDS), new observations from Keck/DEIMOS, and publicly available observations of large samples of star-forming galaxies, we report here on the relationship between the star formation rate (SFR) and the local environment ($δ_{gal}$) of galaxies in the early universe ($2<z<5$). Unlike what is observed at lower redshifts ($z<2$), we observe a definite, nearly monotonic increase in the average SFR with increasing galaxy overdensity over more than an order of magnitude in $δ_{gal}$. The robustness of this trend is quantified by accounting for both uncertainties in our measurements and galaxy populations that are either underrepresented or not present in our sample and find that the trend remains significant under all circumstances. This trend appears to be primarily driven by the fractional increase of galaxies in high density environments that are more massive in their stellar content and are forming stars at a higher rate than their less massive counterparts. We find that, even after stellar mass effects are accounted for, there remains a weak but significant SFR-$δ_{gal}$ trend in our sample implying that additional environmentally-related processes are helping to drive this trend. We also find clear evidence that the average SFR of galaxies in the densest environments increases with increasing redshift. These results lend themselves to a picture in which massive gas-rich galaxies coalesce into proto-cluster environments at $z\sim3$, interact with other galaxies or with a forming large-scale medium, subsequently using or losing most of their gas in the process, and begin to seed the nascent red sequence that is present in clusters at slightly lower redshifts.

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