arXiv subjects

M. Llerena

Publications and source records attributed to M. Llerena.

32 records · Page 2Linked to original sources

arXiv2024

Physical properties of extreme emission-line galaxies at $z\sim 4-9$ from the JWST CEERS survey

Extreme emission line galaxies (EELGs) are typically characterized by high equivalent widths (EWs) which are driven by elevated specific star formation rates (sSFR) in low-mass galaxies with subsolar metallicities and little dust. Such extreme systems are rare in the local universe, but the number density of EELGs increases with redshift. Such starburst galaxies are currently presumed to be the main drivers of hydrogen reionization over 5.5 0.4-1 which indicates that star formation may be the dominant source of ionization. Based on the photometric fluxes, we find an increase of EW([OIII]+H$β$) with sSFR and $Σ_{SFR}$, and a decrease with age and stellar mass. The sample of EELGs can reach $Σ_{SFR}>$10Msun yr$^{-1}$kpc$^{-2}$ which indicate they are strong candidates of LyC leakers. Another indirect indicator is the high values of O32>5 that can be reached for some galaxies in the sample.

astro-ph.GA↗

arXiv2024

The evolution of the SFR and Sigma-SFR of galaxies in cosmic morning (4 < z < 10)

The galaxy integrated star-formation rate (SFR) surface density ($Σ_{\rm SFR}$) has been proposed as a valuable diagnostic of the mass accumulation in galaxies as being more tightly related to the physics of star-formation (SF) and stellar feedback than other SF indicators. In this paper, we assemble a statistical sample of 230 galaxies observed with JWST in the GLASS and CEERS spectroscopic surveys to estimate Balmer line based dust attenuations and SFRs, and UV rest-frame effective radii. We study the evolution of galaxy SFR and $Σ_{\rm SFR}$ in the first 1.5 Billion years of our Universe, finding that $Σ_{\rm SFR}$ is mildly increasing with redshift with a linear slope of $0.16 \pm 0.06$. We also explore the dependence of SFR and $Σ_{\rm SFR}$ on stellar mass, showing that a SF 'Main-Sequence' and a $Σ_{\rm SFR}$ `Main-Sequence' are in place out to z=10, with a similar slope compared to the same relations at lower redshifts. We find that the specific SFR (sSFR) and $Σ_{\rm SFR}$ are correlated with the [OIII]5007/[OII]3727 ratio and with indirect estimates of the escape fraction of Lyman continuum photons, hence they likely play an important role in the evolution of ionization conditions and in the escape of ionizing radiation. We also search for spectral outflow signatures in a subset of galaxies observed at high resolution, finding an outflow incidence of $2/11$ ($=20\%^{32\%}_{9\%}$) at $z<6$, but no evidence at $z>6$ ($<26\%$). Finally, we find a positive correlation between A$_V$ and $Σ_{\rm SFR}$, and a flat trend as a function of sSFR, indicating that there is no evidence of a drop of A$_V$ in extremely star-forming galaxies between z=4 and 10. This might be at odds with a dust-clearing outflow scenario, which might instead take place at redshifts $z\geq 10$, as suggested by some theoretical models.

astro-ph.GA↗

arXiv2024

Peering into cosmic reionization: the Ly$α$ visibility evolution from galaxies at $z$ = 4.5-8.5 with JWST

The resonant scattering interaction between Ly$α$ photons and neutral hydrogen implies that a partially neutral IGM can significantly impact the detectability of Ly$α$ emission in galaxies. The redshift evolution of the Ly$α$ equivalent width distribution of galaxies thus offers a key probe of the degree of ionization during the Epoch of Reionization (EoR). Previous in-depth investigations at $z$ $\geq$ 7 were limited by ground-based instrument capabilities. We present an extensive study of Ly$α$ emission from galaxies at 4 < $z$ < 8.5, observed from the CEERS and JADES surveys in the JWST NIRSpec/PRISM configuration. The sample consists of 235 galaxies, among which we identify 65 as Ly$α$ emitters. We first measure Ly$α$ escape fractions from Balmer lines, and explore the correlations with the inferred galaxies' physical properties, which are similar to those found at lower redshift. We also investigate the possible connection between the escape of Ly$α$ photons and the inferred escape fractions of LyC photons obtained from indirect indicators. We then analyze the redshift evolution of the Ly$α$ emitter fraction, finding lower average values at $z$ = 5 and 6 compared to ground-based observations. At $z$ = 7 we find a very large difference in Ly$α$ visibility between the EGS and GOODS-South fields, possibly due to the presence of early reionized regions in the EGS. Such large variance is also expected in the Cosmic Dawn II radiation-hydrodynamical simulation. Our findings suggest a scenario in which the ending phase of the EoR is characterized by $\sim$ 1 pMpc ionized bubbles around a high fraction of moderately bright galaxies. Finally, we characterize such two ionized regions found in the EGS at $z$ = 7.18 and $z$ = 7.49 by estimating the radius of the ionized bubble that each of the spectroscopically-confirmed members could have created.

astro-ph.GA↗

arXiv2023

Ionized gas kinematics and chemical abundances of low-mass star-forming galaxies at $z\sim 3$

We selected 35 low-mass SFGs (7.9 1000Å and EW(CIII])>5Å, closely resembling those now seen in EoR galaxies with JWST. We find low gas-phase metallicities 12+log(O/H)~7.5-8.5 and C/O abundances from 23%-128% solar, with no apparent increasing trend with metallicity. From our [OIII]4959,5007 profile modeling, we find that 65% of our sample shows an outflow component, which is shifted relative to the ionized gas systemic velocity, with mean $v_{max}$~280 km/s which correlates with the $Σ_{SFR}$. We find that the mass-loading factor $μ$ of our sample is typically lower than in more massive galaxies from literature but higher than in typical local dwarf galaxies. In the stellar mass range covered, we find that $μ$ increases with $Σ_{SFR}$ thus suggesting that for a given stellar mass, denser starbursts in low-mass galaxies produce stronger outflows. Our results complement the picture drawn by similar studies at lower redshift, suggesting that the removal of ionized gas in low-mass SFGs driven by stellar feedback is regulated by their stellar mass and by the strength and concentration of their star formation, i.e. $Σ_{\rm SFR}$.

astro-ph.GA↗

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.

astro-ph.GA↗

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 $z>6$.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2022

Broad-line Region Clouds orbiting an AGN sample

We present a spectral and temporal analysis of XMM-Newton data from a sample of six galaxies (NGC 3783, Mrk 279, Mrk 766, NGC 3227, NGC 7314, and NGC 3516). Using the hardness-ratio curves, we identify time-intervals in which clouds are eclipsing the central X-ray source in five of the six sources. We detect three occultations in NGC 3227 and one occultation in NGC 3783, NGC 7314, and NGC 3516, together with the well-known occultations in Mrk 766. We estimate the physical properties of the eclipsing clouds. The derived physical size of the X-ray sources ($\sim$(3-28)$\times$10$^{13}$ cm) is less than that of the eclipsing clouds with column densities of $\sim$10$^{22}$-10$^{23}$ cm$^{-2}$, thus a single cloud may block the X-ray source, leading to notorious temporal variability of the X-ray flux. The eclipsing clouds in Mrk 766, NGC 3227, NGC 7314, and NGC 3516 with distances from the X-ray source of $\sim$(0.3-3.6)$\times$10$^4$ $R_g$ are moving at Keplerian velocities $>$1122 km s$^{-1}$, typical parameters of broad-line region clouds, while the eclipsing cloud in NGC 3783 is likely located in the dusty torus. We also find a good anti-correlation with a slope of -187$\pm$62 between the known masses of the supermassive black hole in the center of the galaxies with the equivalent width (EW) of the 6.4 keV Fe line for the five Seyfert 1 galaxies of our sample, while the Seyfert 2 galaxy NGC 7314 shows an average EW value of 100$\pm$11 eV inconsistent with the above anti-correlation.

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 environmental dependence of the stellar and gas-phase mass-metallicity relation at 2 < z < 4

In the local universe, galaxies in clusters show different properties compared to more isolated systems. Understanding how this difference originates and whether it is already in place at high redshift is still a matter of debate. Thanks to uniquely deep optical spectra from the VANDELS survey, we investigate environmental effects on the stellar mass-metallicity relation (MZR) for a sample of ~1000 star-forming galaxies at 2<z<4. We complement our dataset with MOSFIRE follow-up of 21 galaxies to study the environmental dependence of the gas-phase MZR. Robust stellar and gas metallicities are derived, respectively, from well-calibrated photospheric absorptions features at 1501 and 1719 Åin the stacked spectra, and from optical emission lines ([OII]3726-3729, [OIII]5007, and Hbeta) in individual systems. We characterize the environment through multiple criteria by using the local galaxy density maps previously derived in VANDELS. We find that environmental effects are weak at these redshifts, and more important around the densest overdensity structures, where galaxies have a lower stellar Z (by 0.2 dex) and a lower gas-phase Z (by 0.1 dex) compared to the field, with a significance of 1 and 2 sigma, respectively. Crucially, this offset cannot be explained by a selection effect due to a higher SFR, a fainter UV continuum, or different dust attenuations and stellar ages. Despite the still low S/N of our results, we propose a combination of increased mergers and high-speed encounters, more efficient AGN feedback in dense cores, and cold gas inflows as viable mechanisms diluting the metal content of overdense galaxies or expelling their metals to the IGM. Finally, some tensions remain with semi-analytic models and hydrodynamical simulations, which predict no significant offset as a function of host halo mass, suggesting that an explicit implementation of environmental processes is needed.

astro-ph.GA↗

arXiv2022

No strong dependence of Lyman continuum leakage on physical properties of star-forming galaxies at $\mathbf{3.1 \lesssim z \lesssim 3.5}$

We present Lyman continuum (LyC) radiation escape fraction $f_{\rm{esc}}$ measurements for 183 spectroscopically confirmed star-forming galaxies in the redshift range $3.11 < z < 3.53$ in the \textit{Chandra} Deep Field South. We use ground-based imaging to measure $f_{\rm{esc}}$, and use ground- and space-based photometry to derive galaxy physical properties using spectral energy distribution (SED) fitting. We additionally derive [O III]+H$β$ equivalent widths (that fall in the observed K band) by including nebular emission in the SED fitting. After removing foreground contaminants, we report the discovery of 11 new candidate LyC leakers, with absolute LyC escape fractions, $f_{\rm{esc}}$ in the range $0.14-0.85$. From non-detections, we place $1σ$ upper limits of $f_{\rm{esc}}<0.12$, where the Lyman-break selected galaxies have $f_{\rm{esc}} < 0.11$ and `blindly' discovered galaxies with no prior photometric selection have $f_{\rm{esc}}<0.13$. We find a slightly higher $1σ$ limit of $f_{\rm{esc}}<0.20$ for extreme emission line galaxies with rest-frame [O III]+H$β$ equivalent widths $>300$A. For candidate LyC leakers, we find a weak negative correlation between $f_{\rm{esc}}$ and galaxy stellar masses, no correlation between $f_{\rm{esc}}$ specific star-formation rates (sSFRs) and a positive correlation between $f_{\rm{esc}}$ and EW$_0$([O III]+H$β$). The weak/no correlations between stellar mass and sSFRs may be explained by misaligned viewing angles and/or non-coincident timescales of starburst activity and periods of high $f_{\rm{esc}}$. Alternatively, escaping radiation may predominantly occur in highly localised star-forming regions, or $f_{\rm{esc}}$ measurements may be impacted by stochasticity of the intervening neutral medium, obscuring any global trends with galaxy properties. These hypotheses have important consequences for models of reionisation.

astro-ph.GA↗

arXiv2021

The VANDELS survey: global properties of CIII]$λ$1908Å emitting star-forming galaxies at z$\sim$3

We study the mean properties of a large representative sample of 217 galaxies showing CIII] emission at $2<z<4$, selected from a parent sample of $\sim$750 main-sequence star-forming galaxies in the VANDELS survey. These CIII] emitters have a broad range of UV luminosities, thus allowing a detailed stacking analysis to characterize their stellar mass, star formation rate (SFR) and stellar metallicity, as a function of the UV emission line ratios, EWs, and the carbon-to-oxygen (C/O) abundance ratio. Reliable CIII] detections represent $\sim$30% of the parent sample. Extreme CIII] emitters (EW(CIII])$\gtrsim$8Å) are exceedingly rare ($\sim$3%) in VANDELS. The UV line ratios of the sample suggest no ionization source other than massive stars. Stacks with larger EW(CIII]) show larger EW(Ly$α$) and lower metallicity, but not all CIII] emitters are Ly$α$ emitters. The stellar metallicities of CIII] emitters are not significantly different from that of the parent sample, increasing from $\sim$10% to $\sim$40% solar for stellar masses $\log$(M$_{\star}$/M$_{\odot})\sim$9-10.5. The stellar mass-metallicity relation of the CIII] emitters is consistent with previous works showing strong evolution from $z=0$ to $z\sim3$. The C/O abundances of the sample range 35%-150% solar, with a noticeable increase with FUV luminosity and a smooth decrease with the CIII] EW. We discuss the CIII] emitters in the C/O-Fe/H and the C/O-O/H planes and find they follow stellar and nebular abundance trends consistent with those of Milky Way halo and thick disc stars and local HII galaxies, respectively. A qualitative agreement is also found with chemical evolution models, which suggests that CIII] emitters at $z\sim$3 are experiencing an active phase of chemical enrichment.

astro-ph.GA↗

arXiv2020

On the effects of UV photons/X-rays on the chemistry of the Sgr B2 cloud

The lines of HOC$^+$, HCO and CO$^+$ are considered good tracers of photon-dominated regions (PDRs) and X-ray dominated regions. We study these tracers towards regions of the Sgr B2 cloud selected to be affected by different heating mechanisms. We find the lowest values of the column density ratios of HCO$^+$ versus HOC$^+$, HCO and CO$^+$ in dense HII gas, where UV photons dominate the heating and chemistry of gas. HOC$^+$, HCO and CO$^+$ abundances and the above ratios are compared with those of chemical modeling, finding that high temperature chemistry, a cosmic-ray ionization rate of 10$^{-16}$ s$^{-1}$ and timescales $>$10$^{5.0}$ years explain well the HOC$^+$ abundances in quiescent Sgr B2 regions, while shocks are also needed to explain the highest HCO abundances derived for these regions. CO$^+$ is mainly formed in PDRs since the highest CO$^+$ abundances of $\sim$(6-10)$\times$10$^{-10}$ are found in HII regions with electron densities $>$540 cm$^{-3}$ and that CO$^+$ emission is undetected in quiescent gas. Between the ratios, the HCO$^+$/HCO ratio is sensitive to the electron density as it shows different values in dense and diffuse HII regions. We compare SiO J=2-1 emission maps of Sgr B2 with X-ray maps from 2004 and 2012. One known spot shown on the 2012 X-ray map is likely associated with molecular gas at velocities of 15-25 km s$^{-1}$. We also derive the X-ray ionization rate of $\sim$10$^{-19}$ s$^{-1}$ for Sgr B2 regions pervaded by X-rays in 2004, which is quite low to affect the chemistry of the molecular gas.

astro-ph.GA↗

arXiv2018

Study of diffuse HII regions potentially forming part of the gas streams around Sgr A*

We present a study of diffuse extended ionised gas toward three clouds located in the Galactic Centre (GC). One line of sight (LOS) is toward the 20 km s$^{-1}$ cloud (LOS$-$0.11) in the Sgr A region, another LOS is toward the 50 km s$^{-1}$ cloud (LOS$-$0.02), also in Sgr A, while the third is toward the Sgr B2 cloud (LOS+0.693). The emission from the ionised gas is detected from H$nα$ and H$mβ$ radio recombination lines (RRLs). He$nα$ and He$mβ$ RRL emission is detected with the same $n$ and $m$ as those from the hydrogen RRLs only toward LOS+0.693. RRLs probe gas with positive and negative velocities toward the two Sgr A sources. The H$mβ$ to H$nα$ ratios reveal that the ionised gas is emitted under local thermodynamic equilibrium conditions in these regions. We find a He to H mass fraction of 0.29$\pm$0.01 consistent with the typical GC value, supporting the idea that massive stars have increased the He abundance compared to its primordial value. Physical properties are derived for the studied sources. We propose that the negative velocity component of both Sgr A sources is part of gas streams considered previously to model the GC cloud kinematics. Associated massive stars with what are presumably the closest HII regions to LOS$-$0.11 (positive velocity gas), LOS$-$0.02 and LOS+0.693 could be the main sources of UV photons ionising the gas. The negative velocity components of both Sgr A sources might be ionised by the same massive stars, but only if they are in the same gas stream.

astro-ph.GA↗