arXiv subjects

A. Bik

Publications and source records attributed to A. Bik.

At least 19 recordsLinked to original sources

arXiv2026

Dissecting the Obscured Core of GN20: an Active Galactic Nucleus Outshone by Young Stars

We investigate the relative contributions of star formation and AGN activity to the total energy budget of GN20, one of the most luminous dusty star-forming galaxies known at $z>4$, through spatially resolved spectral energy distribution decomposition. We perform Bayesian SED fitting with CIGALE on two spatially distinct apertures: the nuclear core (r=0.14", $\sim$1kpc physical) and the full galaxy (r=1.4", 9.9 kpc), combining JWST/NIRCam and MIRI broadband imaging, JWST/NIRSpec PRISM IFU pseudo-continuum photometry spanning 42 wavelength bins across rest-frame $0.12$--$1.05\mu$m, and archival HST and millimeter interferometry data from NOEMA and PdBI. The integrated SED is dominated by stellar-heated dust, with only a marginal AGN contribution at galaxy-wide scales ($f_\mathrm{AGN}^\mathrm{int}=0.09\pm0.02$). The nuclear core, however, requires a significant AGN component ($f_\mathrm{AGN}=0.34\pm0.05$) to account for a mid-infrared excess at rest-frame $\sim$2.5--3.6$\mu$m characteristic of AGN-heated torus dust. The AGN accounts for $\sim34\%$ of the nuclear infrared luminosity but only $\sim9\%$ of the total integrated $L_\mathrm{IR}$, explaining its weak signature in integrated diagnostics and its consistency with existing upper limits from Spitzer spectroscopy. The inferred black hole mass places GN20 within the local $M_\mathrm{BH}$--$M_\mathrm{bulge}$ relation at the Eddington limit, and in the overmassive regime at sub-Eddington accretion rates, suggesting early and rapid black hole assembly concurrent with the dominant starburst. GN20 exemplifies a class of systems where nuclear-scale SED decomposition, enabled by the angular resolution and infrared sensitivity of JWST, is the only means to uncover a buried AGN overwhelmed by galaxy-wide star formation.

astro-ph.GA↗

arXiv2026

MIDIS: The identification of deep MIRI-red sources as candidates for extreme Balmer-break and line emitting galaxies at high-z

We investigate the detection and nature of 5.6~\mu m MIRI-red sources in the MIRI Deep Imaging Survey (MIDIS), covering 2.4~arcmin$^2$ in the Hubble Ultra Deep Field. MIDIS is the deepest JWST/MIRI survey to date, probing faint limits and enabling studies of rare high-redshift galaxy populations. We define MIRI-red sources as those detected at 5$\sigma$ significance in MIRI/F560W with red colors: $m_{\rm F444W} - m_{\rm F560W} \ge 0.5$. Using an empirical methodology, we estimate the purity and completeness of MIRI detections and find that a 5-sigma detection at 28.75 mag has a purity of 92\% and completeness of 54\%. We identify seven MIRI-red galaxy candidates, including an F115W dropout consistent with a high-redshift galaxy candidate. We explore possible physical origins for the MIRI-red population, including active galactic nuclei, dust-obscured galaxies, extreme emission-line galaxies, evolved stellar populations, and Little Red Dots (LRDs). Given the proximity of the F444W and F560W filters and the depth of MIDIS, MIRI-red galaxies are consistent with emission-line galaxies with $EW_0(H\alpha) \ge 750$ \AA\ or $EW_0(H\beta + [OIII]) \ge 600$ \AA, or high-redshift Balmer breaks of at least 1.6. We also discuss an extreme MIRI-red galaxy undetected in F444W, a potential MIRI-only source, for which we derive $EW_0(H\alpha) \sim 6000$ \AA\ and $EW_0(H\beta + [OIII]) \sim 4000$ \AA, or high-$z$ LRD analogs with Balmer breaks of 6.3. Finally, we find fewer MIRI-red detections than expected from extrapolations of the H$\alpha$ or H$\beta$+[OIII] line luminosity functions, consistent with previous deep searches, while the absence of $z>10$ LRD candidates agrees with theoretical expectations for the MIDIS volume.

astro-ph.GA↗

arXiv2026

X-ray stellar feedback in low-metallicity starbursts: Insights from the template starburst galaxy ESO 338-IG04 and its halo

The X-ray output of low-metallicity starburst galaxies is a key component of stellar feedback, tracing the processes responsible for gas ionization and chemical enrichment. The integrated X-ray luminosity ($L_X$) from high-mass X-ray binaries in star-forming galaxies scales with both the star formation rate (SFR) and host-galaxy metallicity $Z$. Due to the inverse correlation between $L_X/\mathrm{SFR}$ and $Z$, the contribution of X-ray binaries to the ionizing photon budget is enhanced in metal-poor systems and may ionize He II in the surrounding interstellar medium, powering nebular He II $\lambda4686$ emission. The blue compact dwarf galaxy ESO 338-IG04 (ESO 338-4) provides a nearby laboratory for studying stellar feedback in a low-metallicity starburst, combining vigorous recent star formation, low metallicity ($12+\log(\mathrm{O/H})\approx7.9$), and a rich population of massive stellar clusters. We characterize the X-ray emission of ESO 338-4 and its halo using new deep Chandra and XMM-Newton observations. We analyze X-ray spectra, light curves, and images to constrain the nature of its X-ray sources. We identify five ultra-luminous X-ray sources (ULXs) and diffuse hot gas surrounding the galaxy. Two ULXs are spatially associated with stellar clusters. The total X-ray luminosity exceeds $10^{41}\,\mathrm{erg\,s^{-1}}$. The brightest source, ULX1, shows variability on day timescales and lacks a stellar-cluster counterpart. Photoionization modeling shows that X-ray sources significantly impact the ionizing photon budget; models with ULX1 as the ionizing source predict nebular He II $\lambda4686$ luminosities of $\sim10^{39}\,\mathrm{erg\,s^{-1}}$.

astro-ph.HE↗

arXiv2026

Substellar population of the young massive cluster RCW 36 in Vela

The initial mass function (IMF) is a cornerstone of star formation studies, yet its universality remains debated. We investigate the IMF in the young massive cluster RCW 36, located in the Vela Molecular Ridge and comparable to the Orion Nebula Cluster in stellar density. Our goal is to build the most complete census of RCW 36 and derive its first IMF and star-to-brown-dwarf (BD) ratio. We combine new GLAO observations from HAWK-I/VLT with archival data (2MASS, SOFI/NTT) and Gaia DR3 kinematics. Photometric accuracy and source extraction were improved using \textsc{DeNeb}, a deep-learning algorithm that removes complex nebular emission. Membership probabilities were assigned via color-magnitude diagram comparisons with a control field, and stellar masses were estimated using model isochrones. We find a revised distance of $954\pm40\,$pc and determine the IMF down to $\sim0.03\,M_{\odot}$, described by a broken power law ($dN/dM\propto M^{-\alpha}$) with $\alpha=1.62\pm0.03$ for $0.20$-$20\,M_{\odot}$ and $\alpha=0.46\pm0.14$ for $0.03$-$0.20\,M_{\odot}$. The star-BD ratio is $2$-$5$, consistent with other Galactic clusters. Lastly, through a study of the differences in the IMF within and outside $0.2\,$pc and the cumulative mass distributions for low-mass and intermediate to high-mass sources, we also detected signs of possible mass segregation within RCW 36, which should be primordial. RCW 36 shares many characteristics with other young massive clusters, such as a shallower than Salpeter high-mass slope and the possibility of mass segregation. The flatter lower-mass regime of the IMF is similar to most Galactic clusters. The star-BD ratio is also in line with the observed values in other clusters, independent of their inherent properties.

astro-ph.SR↗

arXiv2025

MIRI spectrophotometry of GN-z11: Detection and nature of an optical red continuum component

We present new MIRI F560W, F770W and F1000W imaging of GN-z11, extending the previous rest-frame coverage from 0.38 to 0.86$\mu$m. We report significant detections (14$\sigma$) in the F560W and F770W images, and a marginal detection (3.2$\sigma$) in F1000W. Here, we analyse its SED combining new MIRI imaging data with archival NIRSpec/Prism and MRS spectroscopy, and NIRCam imaging. The continuum emission shows a flat energy distribution, in f$_\nu$, up to 0.5$\mu$m, compatible with the presence of a mixed stellar population of young (4$\pm$1 Myr) and mature (63$\pm$23 Myr) stars that also account for the [O III], H$\beta$ and H$\alpha$ emission lines. The continuum at rest-frame 0.66$\mu$m shows a 36$\pm$3% flux excess above the predicted flux for a mixed stellar population, pointing to the presence of an additional source contributing at these wavelengths. This excess increases to 91$\pm$28% at rest-frame 0.86$\mu$m, although with a large uncertainty due to the marginal detection in the F1000W filter. We consider that hot dust emission in the dusty torus around a type 2 AGN could be responsible for the observed excess. Alternatively, this excess could be due to hot dust emission or to a photoluminiscence dust process (Extended Red Emission, ERE) under the extreme UV radiation field, as observed in some local metal-poor galaxies and in young compact starbursts. The presence of a type 1 AGN is not supported by the observed SED since high-z QSOs contribute at wavelengths above rest-frame 1$\mu$m, and an additional ad-hoc red source would be required to explain the observed flux excess at 0.66 and 0.86$\mu$m. Additional deep MIRI imaging covering the rest-frame near-IR are needed to confirm the flux detection at 10$\mu$m with higher significance, and to discriminate between the different hot dust emission in the extreme starburst and AGN scenarios with MIRI imaging at longer wavelengths.

astro-ph.GA↗

arXiv2025

MIDIS: Unveiling the Star Formation History in massive galaxies at $1<z<4.5$ with spectro-photometric analysis

We investigate the star formation histories (SFHs) of a sample of massive galaxies ($M_\star \geq 10^{10} \, M_\odot$) in the redshift range $1 < z < 4.5$. We analyze spectro-photometric data combining broadband photometry from HST and JWST with low-resolution grism spectroscopy from JWST/NIRISS, obtained as part of the MIDIS (MIRI Deep Imaging Survey) program. SFHs are derived through spectral energy distribution (SED) fitting using two independent codes, BAGPIPES and Synthesizer, under various SFH assumptions. This approach enables a comprehensive assessment of the biases introduced by different modeling choices. The inclusion of NIRISS spectroscopy, even with its low resolution, significantly improves constraints on key physical parameters, such as the mass-weighted stellar age ($t_M$) and formation redshift ($z_{\mathrm{form}}$), by narrowing their posterior distributions. The massive galaxies in our sample exhibit rapid stellar mass assembly, forming 50\% of their mass between $3 \leq z \leq 9$. The highest inferred formation redshifts are compatible with elevated star formation efficiencies ($\epsilon$) at early epochs. Non-parametric SFHs generally imply an earlier and slower mass assembly compared to parametric forms, highlighting the sensitivity of inferred formation timescales to the chosen SFH model, particularly for galaxies at $z < 2$. Quiescent galaxies are, on average, older ($t_M \sim 1.1$ Gyr) and assembled more rapidly at earlier times than their star-forming counterparts. These findings support the ``downsizing'' scenario, in which more massive and passive systems form earlier and more efficiently.

astro-ph.GA↗

arXiv2025

The z = 9.625 Cosmic Gems Galaxy was a "Compact Blue Monster" Propelled by Massive Star Clusters

The recent discovery of five massive stellar clusters at z=9.625 in the Cosmic Gems has raised the question about the formation mechanism of star clusters in the first half Gyr after the Big-Bang. We infer the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM ~ (n$_0$) $M^\beta$), assuming three different slopes $\beta$ = -1.5, -2.0 and -2.5 and different lower-mass limits between $10^2$ and $10^5$ Msun. The total integrated cluster stellar mass is compared to the stellar mass inferred from the counter-image of the Cosmic Gems, which provides the best, modestly magnified ($\mu$ = 1.84$\pm$0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy is estimated as 3.5$_{-1.8}^{+3.3}$ x$10^7$ Msun, with an effective radius of Reff = 103$_{-15}^{+13}$ parsec and a stellar surface mass density of $\Sigma$mass = 520$_{-225}^{+340}$ Msun pc$^{-2}$. Accounting for normalization uncertainties - including different lensing magnification scenarios for the arc - a modified SCMF, combined with a significantly high star cluster formation efficiency (approaching 100%), appears to be a necessary condition to explain the relatively short formation timescale of both the star clusters and the counter-image, without exceeding the galaxy's stellar mass. By extrapolating the physical properties at the peak of the burst we find that in its recent past (<~ 30 Myr) the Cosmic Gems galaxy has likely experienced a specific star formation rate (sSFR) exceeding 25 Gyr$^{-1}$ and luminosity approaching the ``blue monster'' regime (M$_{UV}$ < -20). Our study provides insights into the extreme clustered nature of star formation in early galaxies and shed light into the formation of bound star clusters that might survive to z = 0 as globular clusters, older than 13 Gyr.

astro-ph.GA↗

arXiv2025

Spatially resolved H$\alpha$ emission in B14-65666: compact starbursts, ionizing efficiency and gas kinematics in an advanced merger at the Epoch of Reionization

We present MIRI/JWST medium resolution spectroscopy (MRS) and imaging (MIRIM) of B14-65666, a Lyman-break and interacting galaxy at redshift $z$=7.15. We detect the H$\alpha$ line emission in this system, revealing a spatially-resolved structure of the H$\alpha$ emitting gas, which consists of two distinct galaxies, E and W, at a projected distance of 0.4". Galaxy E is very compact in the rest-frame UV, while W galaxy is more extended, showing a clumpy structure reminiscent of a tidal tail. The total H$\alpha$ luminosity implies that the system is forming stars at a Star Formation Rate (SFR) of 76$\pm$8 M$_{\odot}$ yr$^{-1}$ and 30$\pm$4 M$_{\odot}$ yr$^{-1}$ for E and W, respectively. The ionizing photon production efficiency is within the range measured in galaxies at similar redshifts. The high values derived for the H$\alpha$ equivalent widths (EW) and the distinct locations of the E and W galaxies in the $\log(\zeta_\mathrm{ion}$) $-$ EW (H$\alpha$) plane, indicate that the system is dominated by a young (less than 10 Myr) stellar population. The overall spectral energy distribution suggests that in addition to a young stellar population, the two galaxies may have mature stellar population and very different dust attenuation. The derived SFR and stellar masses identify the two galaxies as going through a starburst phase. The kinematics of the ionized gas traced by the H$\alpha$ line show a velocity difference of 175 $\pm$ 28 km s$^{-1}$ between the two components of B14-65666. The in-depth study of systems like B14-65666 reveal how galaxy mergers in the early Universe drive intense star formation, shape the interstellar medium, and influence the buildup of stellar mass, just 700 $-$ 800 Myr after the Big Bang.

astro-ph.GA↗

arXiv2025

MIDIS: Quantifying the AGN component of X-ray-detected galaxies

We combine the deepest X-ray survey from the Chandra Deep Field-South (CDF-S) `7-Ms' survey with the deepest mid-infrared (5.6$ \mu m$) image from the JWST/MIRI Deep Imaging Survey (MIDIS) in the Hubble Ultra-Deep Field (HUDF) to study the infrared counterparts and point-source emission of 31 X-ray sources with a median, intrinsic, rest-frame X-ray luminosity of $\log_{10}(L_{\rm Xc}^{\rm 0.5-7keV})$=42.04$\pm$0.22 erg $\rm s^{-1}$. The sample includes 24 AGN with a redshift range, as set by the X-ray detectability, of $z \simeq 0.5-3$. Through a multi-wavelength morphological decomposition, employing three separate classifications (visual, parametric and non-parametric) we separate (where present) the luminosity of the point-like AGN component from the remainder of the host-galaxy emission. The unprecedented mid-infrared sensitivity and imaging resolution of MIRI allows, in many cases, the direct characterisation of point-like (i.e. unresolved) components in the galaxies' emission. We establish a broad agreement between the three morphological classifications. At least 70% of the X-ray sources, including some classified as galaxies, show unresolved emission in the MIRI images, with the unresolved-to-total flux fraction at rest-frame 2$\mu m$ ranging from $\sim$0.2 to $\sim$0.9. At high X-ray luminosities ($\log_{10}(L_{\rm Xc}$)>43 erg $\rm s^{-1}$) we derive a consistent rest-frame near-infrared 2$ \mu m$ point-source luminosity to that derived for local AGN, whilst at lower X-ray luminosity we identify an excess in the 2$ \mu m$ emission compared to pre-JWST studies. We speculate this offset may be driven by a combination of Compton-thick AGN components and nuclear starburst, merger driven activity. Our observations highlight the complex nature of X-ray sources in the distant Universe and demonstrate the power of JWST/MIRI in quantifying their nuclear infrared emission. (Abridged)

astro-ph.GA↗

arXiv2024

Insight into the Starburst Nature of Galaxy GN-z11 with JWST MIRI Spectroscopy

This paper presents a deep MIRI/JWST medium resolution spectroscopy (MRS) covering the rest-frame optical spectrum of the GN-z11 galaxy. The [OIII]5008 and H$\alpha$ emission lines are detected and spectroscopically resolved. The line profiles are well-modeled by a narrow Gaussian component with intrinsic FWHMs of 189$\pm$25 and 231$\pm$52 kms$^{-1}$, respectively. We do not find any evidence of a dominant broad H$\alpha$ emission line component tracing a Broad Line Region in a type 1 active galactic nuclei (AGN). However, a broad ($\sim$430-470 kms$^{-1}$) and weak ($<$ 20-30%) H$\alpha$ line component, tracing a minor AGN contribution in the optical, cannot be ruled out completely with the sensitivity of the present data. The physical and excitation properties of the ionized gas are consistent with a low-metallicity starburst forming stars at a rate of SFR(H$\alpha$)$=$24 $\pm$3$M_{\odot}$yr$^{-1}$. The electron temperature of the ionized gas is $T_{\mathrm{e}}$(O$^{++}$)$=$14000$\pm$2100K, while the direct-$T_{\mathrm{e}}$ gas-phase metallicity is 12+$\log$(O/H)$=$7.91$\pm$0.07 (Z=0.17$\pm$0.03Z$_{\odot}$). The optical line ratios locate GN-z11 in the starburst or AGN region but more consistent with those of local low-metallicity starbursts and high-$z$ luminous galaxies detected at redshifts similar to GN-z11. We conclude that the MRS optical spectrum of GN-z11 is consistent with that of a massive, compact, and low-metallicity starburst galaxy. Due to its high SFR and stellar mass surface densities, close to that of the densest stellar clusters, we speculate that GN-z11 could be undergoing a feedback-free, highly efficient starburst phase. Additional JWST data are needed to validate this scenario, and other recently proposed alternatives, to explain the existence of bright compact galaxies in the early Universe.

astro-ph.GA↗

arXiv2024

ANDES, the high resolution spectrograph for the ELT: science goals, project overview and future developments

The first generation of ELT instruments includes an optical-infrared high-resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs ([U]BV, RIZ, YJH) providing a spectral resolution of $\sim$100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 $\mu$m with the goal of extending it to 0.35-2.4 $\mu$m with the addition of a U arm to the BV spectrograph and a separate K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Modularity and fibre-feeding allow ANDES to be placed partly on the ELT Nasmyth platform and partly in the Coud\'e room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases, there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of almost 300 scientists and engineers which include the majority of the scientific and technical expertise in the field that can be found in ESO member states.

astro-ph.IM↗

arXiv2024

JWST/MIRI unveils the stellar component of the GN20 dusty galaxy overdensity at $z$=4.05

Despite the importance of the dusty star-forming galaxies (DSFGs) at $z$>2 for understanding the galaxy evolution in the early Universe, their stellar distributions traced by the near-IR emission were spatially unresolved until the arrival of the JWST. In this work we present, for the first time, a spatially-resolved morphological analysis of the rest-frame near-IR (~1.1-3.5$\mu$m) emission in DSFGs traced with the JWST/MIRI. In particular, we study the mature stellar component for the three DSFGs and a Lyman-break galaxy (LBG) present in an overdensity at $z$=4.05. Moreover, we use MIRI images along with UV to (sub)-mm ancillary photometric data to model their SEDs and extract their main physical properties. The sub-arcsec resolution MIRI images have revealed that the stellar component present a wide range of morphologies, from disc-like to compact and clump-dominated structures. These near-IR structures contrast with their UV emission, which is usually diffuse and off-centered. The SED fitting analysis shows that GN20 dominates the total SFR with a value ~2500 $M_\odot$yr$^{-1}$ while GN20.2b has the highest stellar mass in the sample ($M_*$~2$\times$10$^{11}$ $M_\odot$). The two DSFGs classified as LTGs (GN20 and GN20.2a) show high specific SFR (sSFR>30 Gyr$^{-1}$) placing them above the star-forming main sequence (SFMS) at z~4 by >0.5 dex while the ETG (i.e.,GN20.2b) is compatible with the high-mass end of the main sequence. When comparing with other DSFGs in overdensities at $z$~2-7 we observe that our objects present similar SFRs, depletion times and projected separations. Nevertheless, the effective radii computed for GN20 and GN20.2a are up to two times larger than those of isolated galaxies observed in CEERS and ALMA-HUDF at similar redshifts. We interpret this difference in size as an effect of rapid growth induced by the dense environment.

astro-ph.GA↗

arXiv2023

Clumpy star formation and an obscured nuclear starburst in the luminous dusty z=4 galaxy GN20 seen by MIRI/JWST

Dusty star-forming galaxies emit most of their light at far-IR to mm wavelengths as their star formation is highly obscured. Far-IR and mm observations have revealed their dust, neutral and molecular gas properties. The sensitivity of JWST at rest-frame optical and near-infrared wavelengths now allows the study of the stellar and ionized gas content. We investigate the spatially resolved distribution and kinematics of the ionized gas in GN20, a dusty star forming galaxy at $z$=4.0548. We present deep MIRI/MRS integral field spectroscopy of the near-infrared rest-frame emission of GN20. We detect spatially resolved \paa, out to a radius of 6 kpc, distributed in a clumpy morphology. The star formation rate derived from \paa\ (144 $\pm$ 9 \msunperyear) is only 7.7 $\pm 0.5 $\% of the infrared star formation rate (1860 $\pm$ 90 \msunperyear). We attribute this to very high extinction (A$_V$ = 17.2 $\pm$ 0.4 mag, or A$_{V,mixed}$ = 44 $\pm$ 3 mag), especially in the nucleus of GN20, where only faint \paa\ is detected, suggesting a deeply buried starburst. We identify four, spatially unresolved, clumps in the \paa\ emission. Based on the double peaked \paa\ profile we find that each clump consist of at least two sub-clumps. We find mass upper limits consistent with them being formed in a gravitationally unstable gaseous disk. The UV bright region of GN20 does not have any detected \paa\ emission, suggesting an age of more than 10 Myrs for this region of the galaxy. From the rotation profile of \paa\ we conclude that the gas kinematics are rotationally dominated and the $v_{rot}/\sigma_{m} = 3.8 \pm 1.4$ is similar to low-redshift LIRGs. We speculate that the clumps seen in GN20 could contribute to building up the inner disk and bulge of GN20.

astro-ph.GA↗

arXiv2023

MIDIS: Unveiling the Role of Strong Ha-emitters during the Epoch of Reionization with JWST

By using the ultra-deep \textit{JWST}/MIRI image at 5.6 $\mu m$ in the Hubble eXtreme Deep Field, we constrain the role of strong H$\alpha$-emitters (HAEs) during Cosmic Reionization at $z\simeq7-8$. Our sample of HAEs is comprised of young ($<35\;\rm Myr$) galaxies, except for one single galaxy ($\approx 300\;\rm Myr$), with low stellar masses ($\lesssim 10^{9}\;\rm M_{\odot}$). These HAEs show a wide range of UV-$\beta$ slopes, with a median value of $\beta = -2.15\pm0.21$ which broadly correlates with stellar mass. We estimate the ionizing photon production efficiency ($\xi_{ion,0}$) of these sources (assuming $f_{esc,LyC} = 0\%$), which yields a median value $\rm log_{10}(\xi_{ion,0}/(Hz\;erg^{-1})) = 25.50^{+0.10}_{-0.12}$. We show that $\xi_{ion,0}$ positively correlates with EW$_{0}$(H$\alpha$) and specific star formation rate (sSFR). Instead $\xi_{ion,0}$ weakly anti-correlates with stellar mass and $\beta$. Based on the $\beta$ values, we predict $f_{esc, LyC}=4\%^{+3}_{-2}$, which results in $\rm log_{10}(\xi_{ion}/(Hz\;erg^{-1})) = 25.55^{+0.11}_{-0.13}$. Considering this and related findings from the literature, we find a mild evolution of $\xi_{ion}$with redshift. Additionally, our results suggest that these HAEs require only modest escape fractions ($f_{esc, rel}$) of 6$-$15\% to reionize their surrounding intergalactic medium. By only considering the contribution of these HAEs, we estimated their total ionizing emissivity ($\dot{N}_{ion}$) as $\dot{N}_{ion} = 10^{50.53 \pm 0.45}; \text{s}^{-1}\text{Mpc}^{-3}$. When comparing their $\dot{N}_{ion}$ with "non-H$\alpha$ emitter" galaxies across the same redshift range, we find that that strong, young, and low-mass emitters may have played an important role during Cosmic Reionization.

astro-ph.GA↗

arXiv2023

Spatially-resolved H$\alpha$ and ionizing photon production efficiency in the lensed galaxy MACS1149-JD1 at a redshift of 9.11

We present MIRI/JWST medium-resolution spectroscopy (MRS) and imaging (MIRIM) of the lensed galaxy MACS1149-JD1 at a redshift of $z$=9.1092$\pm$0.0002 (Universe age about 530 Myr). We detect, for the first time, spatially resolved H$\alpha$ emission in a galaxy at a redshift above nine. The structure of the H$\alpha$ emitting gas consists of two clumps, S and N. The total H$\alpha$ luminosity implies an instantaneous star-formation of 5.3$\pm$0.4 $M_{\odot}$ yr$^{-1}$ for solar metallicities. The ionizing photon production efficiency, $\log(\zeta_\mathrm{ion})$, shows a spatially resolved structure with values of 25.55$\pm$0.03, 25.47$\pm$0.03, and 25.91$\pm$0.09 Hz erg$^{-1}$ for the integrated galaxy, and clumps S and N, respectively. The H$\alpha$ rest-frame equivalent width, EW$_{0}$(H$\alpha$), is 726$^{+660}_{-182}$ \'Angstrom for the integrated galaxy, but presents extreme values of 531$^{+300}_{-96}$ \'Angstrom and $\geq$1951 \'Angstrom for clumps S and N, respectively. The spatially resolved ionizing photon production efficiency is within the range of values measured in galaxies at redshift above six, and well above the canonical value (25.2$\pm$0.1 Hz erg$^{-1}$). The extreme difference of EW$_{0}$(H$\alpha$) for Clumps S and N indicates the presence of a recent (<5 Myrs) burst in clump N and a star formation over a larger period of time (e.g., $\sim$50 Myr) in clump S. Finally, clump S and N show very different H$\alpha$ kinematics with velocity dispersions of 56$\pm$4 km s$^{-1}$ and 113$\pm$33 km s$^{-1}$, likely indicating the presence of outflows or increased turbulence in the clump N. The dynamical mass, $M_\mathrm{dyn}$= (2.4$\pm$0.5)$\times$10$^{9}$ $M_{\odot}$, is within the range measured with the spatially resolved [OIII]88$\mu$m line.

astro-ph.GA↗

arXiv2023

Massive pre-main-sequence stars in M17: $1^{\rm st}$ and $2^{\rm nd}$ overtone CO bandhead emission and the thermal infrared

Recently much progress has been made in probing the embedded stages of massive star formation, pointing to formation scenarios akin to a scaled up version of low-mass star formation. However, the latest stages of massive star formation have rarely been observed. Using 1st and 2nd overtone CO bandhead emission and near- to mid-infrared photometry we aim to characterize the remnant formation disks around 5 unique pre-main-sequence (PMS) stars with masses $6-12~\rm M_{\odot}$, that have constrained stellar parameters thanks to their detectable photospheres. We seek to understand this emission and the disks it originates from in the context of the evolutionary stage of the studied sources. We use an analytic LTE disk model to fit the CO bandhead and the dust emission, found to originate in different disk regions. For the first time we modeled the 2nd overtone emission. Furthermore, we fit continuum normalized bandheads and show the importance of this in constraining the emission region. We also include $^{13}\rm CO$ in our models as an additional probe of the young nature of the studied objects. We find that the CO emission originates in a narrow region close to the star (<1 AU) and under very similar disk conditions (temperatures and densities) for the different objects. This is consistent with previous modeling of this emission in a diverse range of young stellar objects. We discuss these results in the context of the positions of these PMS stars in the Hertzsprung-Russel diagram and the CO emission's association with early age and high accretion rates in (massive) young stellar objects. We conclude that, considering their mass range and for the fact that their photospheres are detected, the M17 PMS stars are observed in a relatively early formation stage. They are therefore excellent candidates for longer wavelength studies to further constrain the end stages of massive star formation.

astro-ph.SR↗

arXiv2023

Uncovering the stellar structure of the dusty star-forming galaxy GN20 at z=4.055 with MIRI/JWST

Luminous infrared galaxies at high redshifts ($z$>4) include extreme starbursts that build their stellar mass over short periods of time (>100 Myr). These galaxies are considered to be the progenitors of massive quiescent galaxies at intermediate redshifts ($z\sim$2) but their stellar structure and buildup is unknown. Here, we present the first spatially resolved near-infrared imaging of GN20, one of the most luminous dusty star-forming galaxies known to date, observed at an epoch when the Universe was only 1.5 Gyr old. The 5.6$\mu$m image taken with the JWST Mid-Infrared Instrument (MIRI/JWST) shows that GN20 is a very luminous galaxy (M$_\mathrm{1.1\mu m,AB}$=$-$25.01), with a stellar structure composed of a conspicuous central source and an extended envelope. The central source is an unresolved nucleus that carries 9% of the total flux. The nucleus is co-aligned with the peak of the cold dust emission, and offset by 3.9 kpc from the ultraviolet stellar emission. The diffuse stellar envelope is similar in size to the clumpy CO molecular gas distribution. The centroid of the stellar envelope is offset by 1 kpc from the unresolved nucleus, suggesting GN20 is involved in an interaction or merger event supported by its location as the brightest galaxy in a proto-cluster. The stellar size of GN20 is larger by a factor of about 3-5 than known spheroids, disks, and irregulars at $z\sim$4, while its size and low S\'ersic index are similar to those measured in dusty, infrared luminous galaxies at $z\sim$2 of the same mass. GN20 has all the ingredients necessary for evolving into a massive spheroidal quiescent galaxy at intermediate $z$: it is a large, luminous galaxy at $z$=4.05 involved in a short and massive starburst centred in the stellar nucleus and extended over the entire galaxy, out to radii of 4 kpc, and likely induced by the interaction or merger with a member of the proto-cluster.

astro-ph.GA↗

arXiv2023

MIDIS: Strong (Hb + [OIII]) and Ha emitters at redshift $z \simeq 7-8$ unveiled with JWST/NIRCam and MIRI imaging in the Hubble eXtreme Deep Field (XDF)

We make use of \textit{JWST} medium and broad-band NIRCam imaging, along with ultra-deep MIRI $5.6 \rm \mu m$ imaging, in the Hubble eXtreme Deep Field (XDF) to identify prominent line emitters at $z\simeq 7-8$. Out of a total of 58 galaxies at $z\simeq 7-8$, we find 18 robust candidates ($\simeq$31\%) for (H$\beta$ + [OIII]) emitters, based on their enhanced fluxes in the F430M and F444W filters, with EW$_{0}$(H$\beta$ +[OIII]) $\simeq 87 - 2100$ {\AA}. Among these emitters, 16 lie in the MIRI coverage area and 12 exhibit a clear flux excess at $5.6 \, \rm \mu m$, indicating the simultaneous presence of a prominent H$\alpha$ emission line with EW$_{0}$(H$\alpha$) $\simeq 200-3000$ {\AA}. This is the first time that H$\alpha$ emission can be detected in individual galaxies at $z>7$. The H$\alpha$ line, when present, allows us to separate the contributions of H$\beta$ and [OIII] to the (H$\beta$ +[OIII]) complex, and derive H$\alpha$-based star formation rates (SFRs). We find that in most cases [OIII]/H$\beta > 1$. Instead, two galaxies have [OIII]/H$\beta < 1$, indicating that the NIRCam flux excess is mainly driven by H$\beta$. This could potentially imply extremely low metallicities. Most prominent line emitters are very young starbursts or galaxies on their way to/from the starburst cloud. They make for a cosmic SFR density $\rm log_{10}(\rho_{SFR_{H\alpha}}) \simeq -2.35$, which is about a quarter of the total value ($\rm log_{10}(\rho_{SFR_{tot}}) \simeq -1.76$) at $z\simeq 7-8$. Therefore, the strong H$\alpha$ emitters likely had a significant role in reionization.

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