arXiv subjects

R. Amorín

Publications and source records attributed to R. Amorín.

At least 19 recordsLinked to original sources

arXiv2026

Wild is the wind from low-luminosity AGN: A jet-driven gas bubble blowing out a massive CO-dark outflow in ESO 420-G13

We present JWST/MIRI mid-infrared integral field spectroscopy combined with ALMA CO(2-1) observations of the post-starburst galaxy ESO 420-G13, hosting a low-luminosity AGN. The unprecedented spatial and spectral resolution of MIRI enables a detailed study of the molecular and ionised gas kinematics, excitation, and energetics in the nuclear kiloparsec, revealing the impact of AGN feedback in a system with modest radiative output. Despite its faint radio and X-ray emission ($L_{2-10keV} \sim 10^{40}$ erg/s), ESO 420-G13 exhibits powerful kinetic feedback in the form of massive molecular and ionised gas outflows, with a total kinetic power of $\sim 1.4 \times 10^{41}$ erg/s. This corresponds to a jet-ISM coupling efficiency of ~3.8%, within the range observed in more powerful AGN. The feedback is driven by a previously undetected compact jet, traced by collimated coronal-line and extended X-ray emission to >870 pc from the nucleus. The interaction is strongest ~370 pc north of the nucleus, where a fast ionised gas stream emerges perpendicular to the jet axis, coinciding with a bend in the jet direction. Enhanced velocity dispersion in warm H$_2$ surrounds this gas stream, consistent with an expanding molecular bubble. Massive molecular outflows are detected at its edges; the blueshifted outflow is devoid of CO emission, likely due to CO destruction in shocks or by cosmic rays from the jet-ISM interaction. About 5% of the central molecular reservoir has already been expelled, and the remaining gas is turbulent and warm, suggesting an ongoing phase of AGN-driven feedback in this post-starburst galaxy. Our results highlight the enormous potential of mid-IR imaging spectroscopy to uncover jet-driven feedback in low-luminosity AGN. Without the spatially resolved MIRI diagnostics, the kinetic power of the AGN in ESO 420-G13 and its role in shaping the host galaxy ISM would have remained hidden.

astro-ph.GA↗

arXiv2026

Charting the expansion of the Universe from z$\sim$0 to z$\sim$14 with HII galaxies

We present an updated Hubble diagram of giant extragalactic H II regions and H II galaxies spanning the redshift interval $z\sim 0$ to $z\sim 14$, extending the use of the $L(\mathrm{H}β)-σ$ relation as an independent cosmological probe into the epoch of cosmic dawn. Our sample comprises 243 objects, including local anchor systems with primary distance measurements, previously published low- and intermediate-redshift H II galaxies, recent JWST/NIRSpec observations, and a new subsample of extremely high-redshift H II galaxies observed with JWST and with ALMA+JWST for the highest-redshift cases. We homogenise the measurements across the full sample, applying consistent corrections for instrumental and thermal broadening, extinction, and, where required, the transformation from [O III]-based to Balmer-line velocity dispersions. We model lensing effects of high redshift using a log-normal magnification formalism and infer cosmological parameters via a nested-sampling analysis. The $L-σ$ relation remains consistent over the full redshift range, showing no evidence for significant evolution even at the highest redshifts currently accessible. For the joint anchor+H II galaxies sample, under a flat $Λ$CDM model, we obtain $h=0.725\pm0.040$ and $Ω_m=0.308^{+0.043}_{-0.053}$. Allowing a constant dark-energy equation of state yields $w_0=-0.96^{+0.53}_{-0.21}$, while a CPL parametrisation gives $w_0=-0.92^{+0.57}_{-0.34}$ and $w_a=-0.48^{+0.60}_{-1.50}$, all consistent, within the uncertainties, with concordance cosmology. These results demonstrate that H II galaxies offer a viable, fully independent tracer of the expansion history across almost the entire age of the Universe, opening a new avenue for testing $Λ$CDM and dark-energy evolution well beyond reionisation.

astro-ph.CO↗

arXiv2026

Multiphase images of a powerful supernova-driven wind in the early Universe

Galactic winds are considered a likely driver of rapid quenching in early massive galaxies, but until now there has been no direct evidence that such systems drive winds powerful enough to meaningfully suppress their star-formation. We present resolved cold gas and ionized gas observations of a powerful supernova-driven wind in a massive galaxy 1.1 billion years after the Big Bang (at $z$=5.3). The outflow, likely triggered by ongoing merger activity, is removing gas at twice the rate of star-formation and could plausibly eject all the cold gas from the galaxy within 100 Myr. Our results suggest that powerful merger-driven outflows may be a key mechanism to produce abundant massive quiescent galaxies in the early Universe when a large fraction of massive galaxies are interacting. The mass and energetics of this distant outflow are consistent with nearby starburst-driven superwinds, suggesting that the efficiency of stellar feedback has remained relatively constant over the last 12 billion years of cosmic history.

astro-ph.GA↗

arXiv2026

Shape of Direct-Method Mass-Metallicity Relation with JWST: Fast-Track Nitrogen and Helium Enrichment

We investigate the mass-metallicity relation (MZR) from z=1 to z=9 using electron-temperature-based gas-phase metallicities and examine how auroral-line selection, star-formation history, and secondary abundances affect its interpretation in the early Universe. We compile a homogeneous sample of 286 star-forming galaxies observed with JWST/NIRSpec medium resolution spectroscopy, selected through detections of the [O\,III]\,$λ$4363 auroral line from the public DAWN JWST Archive (DJA). We derive electron densities, temperatures, and oxygen abundances using the direct $T_e$ method, along with relative N/O and He/H abundances. Stellar masses are obtained via SED fitting and star-formation rates from reddening-corrected Balmer emission lines. To quantify auroral-line selection biases, we additionally stack galaxy spectra with and without auroral-line detections, extending the MZR into regimes inaccessible to individual measurements. The auroral-line-detected sample spans log(M*/Msun)=6.77-10.5 and 12+log(O/H)=6.9-8.4. A linear fit gives an MZR slope of $γ$=0.38 $\pm$ 0.09. Stacked galaxies without individual $λ$4363 detections define a relation with a similar slope but metallicities higher by ~0.2-0.3 dex at fixed stellar mass. Auroral-line detections also show higher SFRs, larger equivalent widths, and larger offsets from the fundamental metallicity relation, whereas non-detections appear more chemically evolved and closer to it. Several stacked bins also show enhanced N/O and He/H ratios. These results indicate that the low-mass high-redshift MZR traced by JWST is shaped by both recent star-formation history and auroral-line selection effects. Auroral lines preferentially identify high-EW, high-sSFR galaxies in the low-metallicity envelope, whereas non-detections reveal a more enriched sequence closer to the metallicity expected from the FMR.

astro-ph.GA↗

arXiv2026

Not So Isolated: Green Pea Galaxies in Overdense Environments revealed by VLT/MUSE

Context. Green Pea galaxies (GPs) are local starburst galaxies serving as analogues for high-redshift star-forming galaxies, particularly Lyman continuum leakers. It remains debated whether their starbursts are driven by internal secular processes or external triggers. Aims. We aim to constrain the role of environment in this triggering, testing whether external influence comes from close interactions or diffuse processes like gas accretion. Methods. We analyse VLT/MUSE observations of 24 GPs at $z \sim 0.2$ to identify companions via spectral line features. We derive key physical properties (extinction, SFR, stellar mass, age, metallicity) for GPs and companions, and estimate group dynamical masses. Results. We identify 22 emission-line galaxies, 11 being companions ($|Δv| \leq 500$ km s$^{-1}$). We find a high companion fraction ($33^{+11}_{-8}$%) and a $\sim$1 dex number density excess compared to the field, confirming GPs reside in overdense environments. Companions typically lie at projected separations of $\sim$100 kpc with no evidence of ongoing interactions. Physically, GPs form a homogeneous class of young (mass-weighted age $\sim$230 Myr), metal-poor, high-sSFR starbursts with elevated velocity dispersions. In contrast, companions are more evolved ($\sim$1.6 Gyr) and heterogeneous in stellar mass, metallicity, and dust attenuation. Inferred group dynamical masses are $\sim$3 dex higher than total stellar masses, suggesting significant dark matter and neutral gas. Conclusions. GPs do not appear triggered by ongoing major mergers with close (10-30 kpc) companions. Results favor a scenario where GPs are transient starbursts in overdense regions, plausibly sustained by gas accretion. Limited spatial resolution prevents ruling out very close mergers ($\lesssim 10$ kpc). High dynamical-to-stellar mass ratios imply substantial non-stellar mass in these systems.

astro-ph.GA↗

arXiv2026

Cold molecular gas distribution and kinematics in the low-metallicity dusty starburst of Mrk 996 resolved with ALMA

Detecting cold molecular gas in metal-poor starbursts remains a major challenge. Low carbon and oxygen abundances hinder CO formation, while low dust content reduces shielding against UV photodissociation. Consequently, CO, the main tracer of molecular gas, becomes faint or undetectable. We study the spatial distribution and kinematics of cold molecular gas in Mrk 996, a nearby low-mass Wolf-Rayet galaxy hosting a dense, low-metallicity (about 1/5 solar) and nitrogen-enriched nuclear starburst with complex ionized gas kinematics. Using ALMA observations of CO(1-0) and CO(2-1), we map the morphology and kinematics of the molecular gas and compare them with optical and UV data, tracing the ionized gas and young stellar populations. We detect compact CO clouds within 800 pc of the starburst, spatially offset from the nuclear super star cluster (SSC) and the most highly ionized regions. The CO lines are narrow and supersonic, exhibiting velocity gradients with a mild global blueshift, indicating dynamically perturbed gas without evidence for fast outflows, in contrast with the highly ionized phase. The global CO(2-1)/CO(1-0) ratio is low (R21 ~ 0.3), consistent with subthermal excitation. The millimeter continuum peaks at the SSC, while CO emission is displaced toward obscured regions, suggesting it traces dense shielded clumps. ALMA recovers about half of the single-dish flux, indicating the presence of extended, low-surface-brightness molecular gas. Using a metallicity-dependent CO-to-H2 conversion factor, we infer a molecular gas mass of a few 10^7 solar masses. The molecular gas is only weakly coupled to the stellar feedback that dominates the ionized phase. Our results support a multiphase scenario in which dense molecular clumps survive in shielded regions, while CO is photodissociated in their envelopes, leaving a significant CO-dark H2 component (Abridged).

astro-ph.GA↗

arXiv2026

The k-MENDEL sample of local analogs to reionization galaxies. Spectral identification of EELGs and properties of green peas in DESI

Low-mass galaxies with intense starbursts exhibit spectra dominated by extreme nebular emission and faint stellar continua. These extreme emission-line galaxies (EELGs) are key laboratories to study star formation, feedback, and ionizing photon escape in low-metallicity environments. We exploit the DESI survey to assemble the k-Means of Extreme Nebulae from DEsi outLiers (k-MENDEL), a statistically robust sample of ~16,000 EELGs at 0.01 < z < 0.96 selected via automatic k-means classification. Using SED fitting and Te-based metallicities, we characterize EELGs including "blueberry" and "green pea" galaxies, spanning stellar masses of 10^6-10^10 Msun and SFRs of 0.1-100 Msun/yr. k-MENDEL extends previous SDSS samples toward higher redshifts and lower metallicities (12+log(O/H) ~ 7.0-8.5). EELGs lie systematically above the star-forming main sequence, with sSFRs up to ~100 Gyr^-1. They follow a shallower mass-metallicity relation offset by 0.3-0.5 dex from local relations, closely resembling young galaxies observed with JWST at z > 3-10. The large intrinsic metallicity scatter, even after projecting along the fundamental metallicity relation, indicates strong departures from simple "bathtub" models, suggesting massive inflows of metal-poor gas followed by strong feedback. While ~6% of the sample shows AGN-like signatures, the most extreme star-forming systems reach high ionization (O32 ~ 5-60) comparable to confirmed Lyman-continuum emitters. Our results support the interpretation of EELGs as short-lived, non-equilibrium phases in the evolution of low-mass galaxies and highlight their importance as nearby analogs of galaxies likely driving cosmic reionization (Abridged).

astro-ph.GA↗

arXiv2026

Extreme equivalent width-selected low-mass starbursts at $z=4-9$: insights into their role in cosmic reionization

We investigate the properties of extreme emission line galaxies (EELGs) at $z=4-9$ and their role in reionization. Compact, low-mass galaxies with intense optical emission lines are linked to elevated specific star formation rates (sSFRs) and recent bursts of star formation. Feedback in these systems may enable the leakage of ionizing radiation into the intergalactic medium. Using JWST/NIRSpec spectroscopy from the CAPERS, CEERS, and RUBIES surveys, we compile 160 NIRCam-selected EELGs in the EGS field. These galaxies show extreme rest-frame equivalent widths (EWs), with a median EW([O III]+H$β$)=1616Å and EW(H$α$)=763Å. They are low-mass (median log(M$_{\star}$/M$_{\odot}$)=8.26) with high sSFRs (median 43 Gyr$^{-1}$), above the $z\sim6$ main sequence. UV slopes are diverse, with a mean $β=-2.0$, and only 7% have extremely blue continua ($β<-2.6$). Emission-line diagnostics suggest stellar populations as the primary ionizing source, although an AGN fraction of 14% is cannot be entirely ruled out. These galaxies are efficient ionizing photon producers, with mean log($ξ_{\rm ion}$ [Hz erg$^{-1}$])=25.37, exceeding typical values at similar redshifts. Escape fractions, however, are heterogeneous: 16% of EELGs at $z<7$ show escape fractions $>$5% for both Ly$α$ and LyC photons, while 82% lack detectable Ly$α$ emission. The median inferred LyC escape fraction is modest (5%) but enhanced in compact super-Eddington systems with sSFR >25 Gyr$^{-1}$. These results indicate that EELGs contribute approximately 16-40% of the total ionizing emissivity required to sustain hydrogen reionization. EELGs are extremely compact, with a median effective radius of 0.49 kpc, and exhibit a recent star-formation burst. Our analysis indicates that sSFR and star-formation rate surface density are the primary drivers of their extreme emission line strengths.

astro-ph.GA↗

arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Northern Local Star-forming Dwarf Galaxies. Analogues of the First Galaxies and Probes of the Cosmic Metallicity Scale

Star-forming dwarf galaxies in the local Universe, especially extremely metal-poor ones, can be considered analogous to early galaxies of the Epoch of Reionization (z >= 6). Currently available telescopes cannot adequately detect and measure heavy element recombination lines and certain faint collisionally excited lines, which are essential for exploring the effects and biases that potential inhomogeneities in electron temperature and density of the ionized gas may have on determining the chemical composition of these galaxies. On the other hand, the origin of very high-ionization lines (e.g. He II, [Ne V], C IV]) measured in the spectra of an important fraction of these objects remains unknown and a challenge to current stellar models, suggesting the presence of Population III-like stars and/or the existence of non-conventional ionizing sources. Obtaining very deep spectra for a selected sample of local star-forming dwarf galaxies would provide unprecedented constraints on their nature, ionization and true chemical abundances, and could change the metallicity scale we assume to understand the chemical evolution of galaxies over cosmic time.

astro-ph.IM↗

arXiv2025

J-PAS: First Identification, Physical Properties and Ionization Efficiency of Extreme Emission Line Galaxies

Extreme emission line galaxies (EELGs) are key tracers of intense star formation and potential analogues of the sources that reionized the early Universe. Their low-redshift counterparts offer a unique opportunity to study the physical conditions that enable high ionizing-photon escape fractions. We present a robust method to photometrically identify EELGs in the J-PAS survey, which provides 56 optical bands over 8500 deg^2. Using data from a fully observed 30 deg^2 region, we combine narrow-band equivalent widths with machine-learning techniques to select galaxies with emission lines above 300 Å. The method achieves 95% purity and 96% completeness for $i_\mathrm{SDSS}<22.5$ mag. We identify 917 EELGs up to $z=0.8$; spectroscopic cross-matching with DESI/DR1 confirms the reliability of our redshifts and emission-line measurements. The selected galaxies show strong correlations between $ξ_\mathrm{ion}$ and EW([OIII]), consistent with previous low- and high-z studies. Most sources exceed the ionizing efficiency threshold required for reionization, reinforcing their role as local analogues of early-Universe galaxies.

astro-ph.GA↗

arXiv2025

J-PAS: A value-added catalogue of optical line intensities for nebular emission galaxies (JOLINES)

We present the value-added catalogue JOLINES (J-PAS optical line intensities for nebular emission galaxies), which provides emission-line fluxes in galaxies at from the spectrophotometric catalogues of miniJPAS, J-NEP and the J-PAS early data release (EDR). This catalogue will be updated with future data releases, offering a growing resource for the study of emission-line galaxies. To obtain reliable emission-line fluxes from narrow-band photometry, we employed spectral energy distribution (SED) fitting using CIGALE, a robust tool that reconstructs the continuum emission and ensures accurate flux measurements. This method effectively mitigates uncertainties associated with direct continuum subtraction techniques, and systematics such as absorption components in the emission lines. We validate our approach using simulated observations of galaxy spectra with added noise, testing the method's performance across different equivalent width (EW) regimes and emission-line strengths. Additionally, we compare the recovered emission-line fluxes with spectroscopic measurements from the Sloan Digital Sky Survey (SDSS) and the Dark Energy Spectroscopic Instrument (DESI). Our results show a tight correlation between photometric and spectroscopic fluxes, particularly for bright emission lines, with a typical dispersion of $\sim$0.3 dex. Reliable fluxes are obtained for emission lines with EW $\gtrsim20\, \rm{\mathring{A}}$, in agreement with previous empirical studies. The current catalogue comprises approximately 13,900 sources with reliable flux measurements in the H$α$+[NII] complex and 7,200 in [OIII]$λ5007$, ensuring statistically robust samples for the brightest optical emission lines. This resource will be expanded in future J-PAS releases, facilitating large-scale studies of star formation, AGN activity, and galaxy evolution.

astro-ph.GA↗

arXiv2025

J-PAS: Discovery of RaJav, a bright spatially extended Ly$α$ Nebula at z=2.25

We report the discovery of a massive and potentially largest Ly$α$ Nebula, RaJav, at z=2.25, associated with a quasar pair: the bright SDSS~J162029.07+433451.1 (hereafter J1620+4334) and the faint newly discovered quasar JPAS-9600-10844, at 2.265 $\pm$ 0.021 using the early data release (17 deg$^{2}$) of the J-PAS. The quasar JPAS-9600-10844 embedded in the nebula is located at ~ 60.2 kpc (7.3'') from J1620+4334, and shows a compact structure with broad emission lines (> 3000 km/s), typical of active galactic nuclei. At a 2$σ$ surface brightness (SB) contour of $\sim 1.86 \times 10^{-16}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$, the nebula extends > 100 kpcs and has a total Ly$α$ luminosity of $\sim 5.8 \pm 0.7 \times 10^{44}$ erg s$^{-1}$ signify the presence of a giant Enormous Ly$α$ Nebula (ELAN). The nebula traces an over density of quasars at redshift of 2.2-2.3 consistent with the progenitor of a massive galaxy cluster. The extended CIV emission indicates that the circum-galactic medium (CGM) is metal-enriched and not primordial. The current J-PAS observations suggest photoionization and shocks due to outflows as possible ionization mechanisms. The faint extended FUV and NUV continuum emission likely points to ongoing star formation around the two quasars, suggesting a complex interaction in their environments. These findings provide new insights into the environment of quasars and their role in shaping the dynamics and evolution of the CGM at cosmic noon. Further spectroscopic observations will be required to fully characterize the object's nature and its kinematic properties. This study demonstrates the unique capability of J-PAS to detect massive and rare Ly$α$ nebulae, providing new insights into their properties, environments, and connections to large-scale structures in the cosmic web such as filaments and overdensities in a large cosmological volume.

astro-ph.GA↗

arXiv2025

The ionizing photon production efficiency of star-forming galaxies at $z\sim 4-10$

Investigating the ionizing emission of star-forming galaxies is critical to understanding their contribution to reionization and their impact on the surrounding environment. The number of ionizing photons available to reionize the intergalactic medium (IGM) depends not only on the abundance of galaxies but also on their efficiency in producing ionizing photons ($ξ_{ion}$). We aim to estimate the $ξ_{ion}$ using Balmer lines in a sample of 761 galaxies at $4\leq z \leq 10$ selected from different JWST surveys. We used the available HST and JWST photometry to perform a SED fitting in the sample to determine their physical properties. We used the BAGPIPES code and assumed a delayed exponential model for the star formation history. We used the NIRSpec spectra from prism or grating configurations to estimate Balmer luminosities and then constrained $ξ_{ion}$ values after dust correction. We find a mean value of 10$^{25.22}$Hz erg$^{-1}$ for $ξ_{ion}$ in the sample with an observed scatter of 0.42dex. We find an increase in the median values of $ξ_{ion}$ which confirms the redshift evolution of $ξ_{ion}$ found in other works. Regarding the relation with physical properties, we find a decrease of $ξ_{ion}$ with increasing stellar mass, indicating that low-mass galaxies are efficient producers of ionizing photons. We also find an increase of $ξ_{ion}$ with increasing specific star formation rate (sSFR) and increasing UV absolute magnitude, which indicates that faint galaxies and with high sSFR are also efficient producers. We also investigated the relation of $ξ_{ion}$ with the EW([OIII]$λ$5007) and find that galaxies with the higher EW([OIII]) are the more efficient producers of ionizing photons. Similarly, we find that galaxies with higher O32 ratios and lower gas-phase metallicities (based on the R23 calibration) show higher $ξ_{ion}$ values.

astro-ph.GA↗

arXiv2025

Exploring the hardness of the ionizing radiation with the infrared softness diagram. II. Bimodal distributions in both the ionizing continuum slope and the excitation in active galactic nuclei

After exploring the infrared softness diagram to characterize the hardness of the incident ionizing radiation in star-forming regions, we exploit the availability of high-excitation lines in the same spectral regime to explore its use for studying the narrow-line regions in AGN. We adapted the IR softness diagram to consider very high-excitation lines, such as [Ne V] 14.3,24.3 $μ$m or [O IV] 25.9$μ$m. The measured emission-line ratios were included as inputs for the code HCm-Teff-IR, in order to provide a quantification for both the ionization and the $α_{\rm OX}$ parameters. The latter is sensitive to the spectral shape of the incident continuum in AGN. We applied this to a large AGN sample including different spectral types with available Spitzer/IRS, Herschel/PACS and/or SOFIA/FIFI-LS mid-IR spectroscopic observations. The combination of the ([Ne II] 12.8 $μ$m+[Ne III] 15.6$μ$m)/[Ne V] 14.3$μ$m and [O III] 52,88 $μ$m/[O IV] 25.9$μ$m emission line ratios is a robust proxy for the shape of the ionizing continuum in AGN. An alternative based on the [S III] 18.7 $μ$m+[S IV] 10.5$μ$m lines can be used instead. The inclusion of very high-excitation lines in HCm-Teff-IR, to derive both $α_{\rm OX}$ and $U$ for the studied sample, points to a bimodal distribution of galaxies. One of the peaks is characterized by relatively harder values of $α_{\rm OX}$ around -1.4 in combination with low values for log $U$ around -2.4, while the other peak shows a softer $α_{\rm OX}$ around -1.7 and high values of log $U$ around-1.5. This result is consistent with the existence of two very marked AGN populations., one with a softer ionizing continuum, possibly dominated by a radiatively efficient accretion disk in bright Seyfert nuclei. In contrast, we observe a harder radiation field in low-luminosity AGN, where the accretion disk is expected to recede.

astro-ph.GA↗

arXiv2024

Physical properties of circumnuclear ionising clusters. III. Kinematics of gas and stars in NGC 7742

In this third paper of a series, we study the kinematics of the ionised gas and stars, calculating the dynamical masses of the circumnuclear star-forming regions in the ring of of the face-on spiral NGC 7742. We have used high spectral resolution data from the MEGARA instrument attached to the Gran Telescopio Canarias (GTC) to measure the kinematical components of the nebular emission lines of selected HII regions and the stellar velocity dispersions from the CaT absorption lines that allow the derivation of the associated cluster virialized masses. The emission line profiles show two different kinematical components: a narrow one with velocity dispersion $\sim$ 10 km/s and a broad one with velocity dispersion similar to those found for the stellar absorption lines. The derived star cluster dynamical masses range from 2.5 $\times$ 10$^6$ to 10.0 $\times$ 10$^7$ M$_\odot$. The comparison of gas and stellar velocity dispersions suggests a scenario where the clusters have formed simultaneously in a first star formation episode with a fraction of the stellar evolution feedback remaining trapped in the cluster, subject to the same gravitational potential as the cluster stars. Between 0.15 and 7.07 % of the total dynamical mass of the cluster would have cooled down and formed a new, younger, population of stars, responsible for the ionisation of the gas currently observed.

astro-ph.GA↗

arXiv2024

Evolution of the UV slope of galaxies at cosmic morning (z > 4): the properties of extremely blue galaxies

We present an analysis of the UV continuum slope, beta, using a sample of 733 galaxies selected from a mixture of JWST ERS/GTO/GO observational programs and with z > 4. We consider spectroscopic data obtained with the low resolution PRISM/CLEAR NIRSpec configuration. Studying the correlation of beta with M_UV we find a decreasing trend of beta = (-0.056 +- 0.017) M_UV - (3.01 +- 0.34), consistent with brighter galaxies having redder beta as found in previous works. However, analysing the trend in separate redshift bins, we find that at high redshift the relation becomes much flatter, consistent with a flat slope. Furthermore, we find that beta decreases with redshift with an evolution as beta = (-0.075 +- 0.010) z - (1.496 +- 0.056), consistent with most previous results that show a steepening of the spectra going at higher z. We then select a sample of galaxies with extremely blue slopes (beta < -2.6): such slopes are steeper than what is predicted by stellar evolution models, even for dust free, young, metal poor populations, when the contribution of nebular emission is included. We select 51 extremely blue galaxies (XBGs) and we investigate the possible physical origin of their steep slopes, comparing them to a sub-sample of redder galaxies (matched in redshift and M_UV). We find that XBGs have younger stellar populations, stronger ionization fields, lower dust attenuation, and lower but not pristine metallicity (~ 10% solar) compared to red galaxies. However, these properties alone cannot explain the extreme beta values. By using indirect inference of Lyman continuum escape, using the most recent models, we estimate escape fractions f_esc > 10% in at least 25% of XBGs, while all the red sources have smaller f_esc. A reduced nebular continuum contribution as due to either a high escape fraction or to a bursty star-formation history is likely the origin of the extremely blue slopes.

astro-ph.GA↗

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 NIRVANDELS Survey: the stellar and gas-phase mass-metallicity relations of star-forming galaxies at z = 3.5

We present determinations of the gas-phase and stellar metallicities of a sample of 65 star-forming galaxies at $z \simeq 3.5$ using rest-frame far-ultraviolet (FUV) spectroscopy from the VANDELS survey in combination with follow-up rest-frame optical spectroscopy from VLT/KMOS and Keck/MOSFIRE. We infer gas-phase oxygen abundances ($Z_{\mathrm{g}}$; tracing O/H) via strong optical nebular lines and stellar iron abundances ($Z_{\star}$; tracing Fe/H) from full spectral fitting to the FUV continuum. Our sample spans the stellar mass range $8.5 < \mathrm{log}(M_{\star}/\mathrm{M}_{\odot}) < 10.5$ and shows clear evidence for both a stellar and gas-phase mass-metallicity relation (MZR). We find that our O and Fe abundance estimates both exhibit a similar mass-dependence, such that $\mathrm{Fe/H}\propto M_{\star}^{0.30\pm0.11}$ and $\mathrm{O/H}\propto M_{\star}^{0.32\pm0.09}$. At fixed $M_{\star}$ we find that, relative to their solar values, O abundances are systematically larger than Fe abundances (i.e., $α$-enhancement).We estimate an average enhancement of $\mathrm{(O/Fe)} = 2.65 \pm 0.16 \times \mathrm{(O/Fe)_\odot}$ which appears to be independent of $M_{\star}$. We employ analytic chemical evolution models to place a constraint on the strength of galactic-level outflows via the mass-outflow factor ($η$). We show that outflow efficiencies that scale as $η\propto M_{\star}^{-0.32}$ can simultaneously explain the functional form of of the stellar and gas-phase MZR, as well as the degree of $α$-enhancement at fixed Fe/H. Our results add further evidence to support a picture in which $α$-enhanced abundance ratios are ubiquitous in high-redshift star-forming galaxies, as expected for young systems whose interstellar medium is primarily enriched by core-collapse supernovae.

astro-ph.GA↗