arXiv subjects

Mahsa Kohandel

Publications and source records attributed to Mahsa Kohandel.

14 recordsLinked to original sources

arXiv2026

REBELS-25: multi-phase morphology and kinematics at z = 7.31

We present a spatially resolved, multi-wavelength study of the massive, star-forming galaxy REBELS-25 at a redshift of $z=7.31$. We combine new high-resolution ALMA [O III]88$\mu$m observations with resolution-matched [C II], dust continuum, and JWST/NIRSpec IFU spectroscopy, providing a $\lesssim1$ kpc view of the morphology, interstellar medium (ISM) conditions, and multi-tracer gas kinematics of one of the most mature galaxies known in the reionisation era. We find differing morphologies from the rest-frame UV to far-infrared (FIR) emission, with the UV and optical emission appearing clumpy and irregular, whereas the FIR emission is well-described by near-exponential disc profiles, with [C II] being the most extended. Comparing the resolved UV and FIR emission, we find that obscured star formation contributes $\sim55$-$98$% of the total star formation rate across the galaxy, demonstrating that dust obscuration strongly shapes the observed UV and optical morphology. Resolved ionisation-line diagnostics show no significant variation across the source at $\sim1$ kpc resolution, consistent with broadly similar ISM conditions among the identified regions. Kinematic modelling reveals that both the warm ionised gas, traced by [O III]88$\mu$m, and the colder neutral gas, traced by [C II], share the same large-scale rotating structure and are dynamically cold, with ratios of ordered-to-random motion, $V/\sigma$, of $\sim11$ and 4.5, respectively, although we find evidence for non-circular motions that are not well-described by a simple rotating disc. Overall, these results further support a picture in which REBELS-25 hosts a dusty, chemically enriched, and dynamically cold ISM already in place at $z=7.31$.

astro-ph.GA↗

arXiv2026

Intense and extended CIII] emission suggests a strong outflow in JADES-GS-z14-0

JWST has revealed an overabundance of very bright, blue galaxies at z>10, raising fundamental questions about how star formation and feedback operate at Cosmic Dawn. We present new JWST/NIRSpec MSA PRISM/CLEAR spectroscopy of JADES-GS-z14-0 (z=14.18) obtained with the JADES and OASIS programmes. While the rest-frame UV continuum flux level and shape are consistent between the two datasets, the OASIS spectrum shows a 10$\sigma$ detection of the CIII]$\lambda\lambda1907,1909$ emission line, with a luminosity three times higher than that measured in the JADES data. This difference is naturally explained by the offset in shutter placement between OASIS and JADES, implying that the CIII] emission is spatially displaced by $\sim400$ pc from the stellar continuum. The non-detection of CIII] in NIRCam medium-band imaging indicates that the emitting region is extended on scales $\gtrsim165$ pc, with a surface brightness below the detection threshold. Interpreting this diffuse, carbon-enriched gas as the result of ongoing or past outflows, we infer a mass outflow rate of $\dot{M}_{\rm out}\sim160~{\rm M_\odot\,yr^{-1}}$. We compare it with the star-formation rate (SFR) and derive a mass-loading factor of $\eta = \dot{M}_{\rm out}/{\rm SFR} = 4-15$, suggesting highly efficient feedback at very early times. Finally, we show that, if outflows are one of the mechanisms regulating star formation in JADES-GS-z14-0, the instantaneous star-formation efficiency in massive haloes is constrained to $\epsilon_\star\lesssim0.08$. These results support a scenario in which outflows play a crucial role during the earliest phases of galaxy formation. Comparing our results with the current theoretical galaxy formation model, we conclude that a combination of moderate star-formation efficiency and reduced dust attenuation can account for the emergence of luminous galaxies at the highest redshifts.

astro-ph.GA↗

arXiv2026

The ALPINE-CRISTAL-JWST Survey: Chemical Abundance Comparison Between the ISM and CGM of Main-Sequence Galaxies at z=4-6

Gaseous halos around galaxies play an important role in galaxy evolution. The exchange of metals from the interstellar medium (ISM) to the circumgalactic medium (CGM) are caused by the formation, feedback, and/or merging history of galaxies. We study the variation in chemical composition between the ISM ($\lesssim3\,$kpc) and CGM ($\sim5-10\,$kpc) for a sample of $M_{\star}>10^{9.5}\,M_{\odot}$ main-sequence galaxies at $4<z<6$ with both JWST and ALMA observations. Using JWST/NIRSpec integral field spectroscopy, we derive the optical line ratios from the ISM and the CGM for our sample focusing on the typical optical lines used for metallicity studies. Our comparison shows that the ISM and the CGM have similar chemical abundances. This indicates that the CGM of these typical $4<z<6$ galaxies is enriched to the level of their ISM in the early universe. Using statistical tests, we find that some of the line ratios show marginal differences between the ISM and CGM. Combined with \texttt{Cloudy} modeling, our results suggest that a difference in ionization level (higher for the ISM) is the dominant reason for the observed ratio difference of oxygen line ratios. There is also indication of a deficit in the nitrogen abundance with respect to oxygen in the CGM, which suggests a delay in redistribution of secondary nitrogen. Finally, an enhanced $F_{[\rm CII]\rm 158\mu m}/F_{\rm H\alpha}$ ratio is observed in the CGM, suggesting that feedback and/or mergers play a key role in metal mixing.

astro-ph.GA↗

arXiv2025

High-Redshift Galactic Outflows: Orientation Effects, Kinematics, and Metallicity in TNG50 and SERRA

Context: Recently, JWST/NIRSpec observations have provided the first detections of warm ionised outflows in low-mass galaxies at high redshifts (z>3), revealing an occurrence rate of 25-40% depending on the intensity of the emission lines. This fraction is lower than predicted by simulations, which suggest that fast outflowing gas should be a common feature of all star-forming galaxies in the early Universe. Aims: In order to better understand the discrepancies between simulations and observations, we identify and characterize outflows in high-redshift galaxies using the TNG50 cosmological and SERRA zoom-in simulations. Our study examines how outflow detectability depends on the line of sight, explores the properties of the fast gas, and investigates its relationship with key galactic properties. Methods: We analyse approximately 60000 galaxies from TNG50 and 3000 galaxies from SERRA over the redshift ranges z=3-5 and z=4-5, respectively, spanning stellar masses of Mstar=10^7.5-10^11Msun. Outflows in the immediate vicinity of each galaxy are identified using a Gaussian mixture model algorithm that uses the gas velocity, star-formation-rate, and location as input parameters. We subsequently compare the simulated outflows to those observed in the JWST/JADES NIRSpec survey. Results: Outflow masses in both TNG50 and SERRA broadly reproduce the JWST/JADES measurements within roughly 0.5dex, though simulations tend to predict slightly higher values, suggesting that optical emission lines capture only a fraction of the multiphase outflow. However, simulated outflow velocities are typically an order of magnitude lower than those inferred from observations. TNG50 indicates a clear orientation dependence as outflows in face-on galaxies are approximately 15% more likely to be detected than in edge-on systems, with this difference increasing to nearly 40% for more massive, disc-shaped galaxies.

astro-ph.GA↗

arXiv2025

The ALPINE-CRISTAL-JWST Survey: The Fast Metal Enrichment of Massive Galaxies at z~5

We present the stellar mass-metallicity relation (MZR) and mass-metallicity-star formation relation ("fundamental metallicity relation"; FMR) of 18 massive (log(M/Msun) = 9.5 - 11) main-sequence galaxies at z~5 from the ALPINE-CRISTAL-JWST sample. This sample complements recent studies by JWST at up to two orders of magnitude lower stellar masses. The metallicities are derived using strong optical lines, and verified by temperature-based oxygen abundance measurements for five galaxies for which faint auroral lines are detected. We find that the metal abundance evolves, on average, from 40% to 60% solar between z~5 and cosmic noon (z~2) at the massive end of the MZR, suggesting already significant metal enrichment at early times. The FMR at z=5 exhibits a 5x larger scatter (preferentially to lower metallicities) compared the local FMR relation. This scatter can be explained by a bursty star formation and the direct build-up of metals in early galaxies as well as differences in age and outflow efficiencies. Capitalizing on all available samples, we find that the observed MZR and FMR over three orders of stellar mass is generally in good agreement with results from cosmological simulation, although some underestimate the metal enrichment at low stellar masses. This may be due to too efficient metal-rich outflows. We show that the ALPINE-CRISTAL-JWST galaxies likely joined the current FMR at z~10 and will evolve into massive (log(M/Msun)~11.4) galaxies with super-solar metallicities by z=0.

astro-ph.GA↗

arXiv2025

The ALPINE-CRISTAL-JWST Survey: NIRSpec IFU Data Processing and Spatially-resolved Views of Chemical Enrichment in Normal Galaxies at z=4-6

We present a statistical study of spatially resolved chemical enrichment in 18 main-sequence galaxies at $z=4$--6, observed with \jwst/NIRSpec IFU as part of the ALPINE-CRISTAL-\jwst\ survey. Performing an optimized reduction and calibration procedure, including local background subtraction, light-leakage masking, stripe removal, and astrometry refinement, we achieve robust emission-line mapping on kiloparsec scales. Although line-ratio distributions vary across galaxies in our sample, we generally find mild central enhancements in [O\,\textsc{iii}]/H$\beta$, [O\,\textsc{ii}]/[O\,\textsc{iii}], [S\,\textsc{ii}]$_{6732}$/[S\,\textsc{ii}]$_{6718}$, H$\alpha$/H$\beta$, and $L_{\rm H\alpha}/L_{\rm UV}$, consistent with elevated electron density, dust obscuration, and bursty star formation accompanied by reduced metallicity and ionization parameter. These features point to inside-out growth fueled by recent inflows of pristine gas. Nevertheless, the median metallicity gradient is nearly flat over a few kpc scale, $\Delta \log({\rm O/H}) = 0.02 \pm 0.01$ dex kpc$^{-1}$, implying efficient chemical mixing through inflows, outflows, and mergers. From pixel-by-pixel stellar and emission-line characterizations, we further investigate the resolved Fundamental Metallicity Relation (rFMR). Metallicity is described by a fundamental plane with stellar mass and SFR surface densities, but with a stronger dependence on $\Sigma_{\rm SFR}$ than seen in local galaxies. Our results indicate that the regulatory processes linking star formation, gas flows, and metal enrichment were already vigorous $\sim$1 Gyr after the Big Bang, producing the nearly flat metallicity gradient and a stronger coupling between star formation and metallicity than observed in evolved systems in the local universe.

astro-ph.GA↗

arXiv2025

Lessons Learned from Studying H$\alpha$ Galaxy Kinematics with Mock JWST/NIRSpec IFU Observations at $z > 6$

Galaxies with a disk morphology have been established at $z > 9$ with the James Webb Space Telescope (JWST). However, confirming their disky nature requires studying their gas kinematics, which can be challenging when relying solely on the warm gas observed by JWST. Unlike the cold gas traced by the Atacama Large Millimetre/Submillimetre Array (ALMA), warm gas is sensitive to outflows, complicating the interpretation of the disk dynamics. This elicits the question of how to compare information obtained from varied tracers, as well as how to physically interpret the low angular and spectral resolution observations generally available at high redshift. We address these challenges through comparative kinematic analysis of idealised and realistic NIRSpec/IFU mock observations derived from two galaxies in the SERRA suite of cosmological zoom-in simulations. With these synthetic data, we determine the robustness of dynamical information recovered from typical IFU observations, and test widely-used criteria for identifying disks and gaseous outflows at high redshift. We find that at the typical NIRSpec/IFU spectral and angular resolution ($\sim$ 0.05"/pixel), non-circular motions due to inflows or outflows can mimic the smooth velocity gradient indicative of a disk, and bias measured velocity dispersion upwards by a factor of $2-3\times$. As a result, the level of rotational support may be underestimated in the NIRSpec/IFU observations. However, the recovered dynamical mass appears to be relatively robust despite biases in $v_\text{rot}$ and $\sigma$.

astro-ph.GA↗

arXiv2025

Amaryllis: a digital twin of the earliest galaxies in the Universe

Synergies between JWST and ALMA are unveiling a population of bright, super-early ($z>10$) galaxies, including systems like GS-z14-0 ($z=14.2$) and GHZ2 ($z=12.3$) with extreme FIR line ratios ([OIII] 88$\,$um / [CII] 158$\,$um $>3$) that challenge galaxy formation models. To address this, we identify a synthetic analog of these sources, "Amaryllis", within the SERRA zoom-in simulations, and track its evolution from $z=16$ to $z=7$. During this period, Amaryllis grows from $\log(M_\star/M_{\odot}) \sim 7.4$ to $10.3$, linking super-early progenitors to the massive galaxy population at the end of reionization. At $z \sim 11.3$, Amaryllis closely matches the observed properties of GS-z14-0, including $M_\star$, SFR, and the luminosity of FIR ([OIII] 88$\,$um) and UV (e.g. CIII]$\,1908$) lines. We find that high [OIII]/[CII] ratios appear during short, merger-driven starburst episodes, when low metallicity ($Z \sim 0.1\,Z_{\odot}$) and high ionization conditions ($U_{\mathrm{ion}} \sim 0.3$) push the ISM far from equilibrium. These extreme FIR line ratios are thus transient and linked to major mergers that ignite strong ionized gas outflows. Strikingly, despite this dynamical activity, Amaryllis develops a rotation-supported gaseous disk ($V/\sigma \sim 4$-6) by $z \sim 11$, while stars remain dispersion-dominated. This coexistence of ordered gas rotation and merger-driven disturbances occurs within a massive yet typical $\Lambda$CDM halo, enabling disk formation even at cosmic dawn.

astro-ph.GA↗

arXiv2024

An optically-dark merging system at z~6 detected by JWST

Near- to mid-Infrared observations (from Spitzer and JWST) have revealed a hidden population of galaxies at redshift z=3-6, called optically-dark objects, which are believed to be massive and dusty star-formers. While optically-dark sources are widely recognized as a significant component of the stellar mass function, the history of their stellar mass assembly remains unexplored. However, they are thought to be the progenitors of the more massive early-type galaxies found in present-day groups and clusters. It is thus important to examine the possible connection between dark sources and merging events, in order to understand the environment in which they live. Here, we report our search for close companions in a sample of 19 optically-dark objects identified in the SMACS0723 JWST deep field. They were selected in the NIRCam F444W band and undetected below 2mu. We restrict our analysis to the reddest (i.e. F277W-F444W> 1.3) and brightest (F444W< 26 mag) objects. We have identified an optically-dark source showing a very close companion (<0.5"). The spatially resolved SED fitting procedure indicates that all components lying within 1.5" from the dark source are indeed at z~5.7. Tidal features (leading to a whale shaped morphology) corroborate the hypothesis that the dark source is the most massive (log(M/Msun)>10.3) and dusty (Av~3 at the core) system of an ongoing merger with a mass ratio of ~10. Similar merging systems are identified in the SERRA simulations, allowing us to reconstruct their stellar mass assembly history and predict their molecular gas properties The discovery of mergers within dark galaxies at the end of the Epoch of Reionization underscores the importance of conducting a statistical search for additional candidates in deep NIRCam fields. Such research will aid in understanding the role of merging processes during the obscured phase of stellar mass accumulation.

astro-ph.GA↗

arXiv2023

Dynamically cold disks in the early Universe: myth or reality?

Theoretical models struggle to reproduce dynamically cold disks with significant rotation-to-dispersion support($V_{\rm{rot}}/\sigma$) observed in star-forming galaxies in the early Universe, at redshift $z>4$. We aim to explore the possible emergence of dynamically cold disks in cosmological simulations and to understand if different kinematic tracers can help reconcile the tension between theory and observations. We use 3218 galaxies from the SERRA suite of zoom-in simulations, with $8<\log(M_*/M_{\odot})<10.3$ and SFR$<128\,M_{\odot}{yr}^{-1}$, within $4 10^9\,M_{\odot}$ galaxies, for which $\sigma_{H\alpha}>2\sigma_{CII}$ for a significant fraction of the sample. Regardless of the tracer, our predictions suggest the existence of massive ($M_*>10^{10}M_{\odot}$) galaxies with $V_{rot}/\sigma>10$ at $z>4$, maintaining cold disks for >10 orbital periods (200Myr). Furthermore, we do not find any significant redshift dependence for $V_{rot}/\sigma$ ratio in our sample. Our simulations predict the existence of dynamically cold disks in the early Universe. However, different tracers are sensitive to different kinematic properties. While [CII] effectively traces the thin, gaseous disk of galaxies, H$\alpha$ includes the contribution from ionized gas beyond the disk, characterized by prevalent vertical or radial motions that may be associated with outflows. The presence of H$\alpha$ halos could be a signature of such galactic outflows. This emphasizes the importance of combining ALMA and JWST/NIRspec studies of high-z galaxies.

astro-ph.GA↗

arXiv2023

Spatially resolved Kennicutt-Schmidt relation at z~7 and its connection with the interstellar medium properties

We exploit moderately resolved [OIII], [CII] and dust continuum ALMA observations to derive the gas density ($n$), the gas-phase metallicity ($Z$) and the deviation from the Kennicutt-Schmidt (KS) relation ($\kappa_s$) on ~sub-kpc scales in the interstellar medium (ISM) of five bright Lyman Break Galaxies at the Epoch of Reionization ($z\approx 7$). To do so, we use GLAM, a state-of-art, physically motivated Bayesian model that links the [CII] and [OIII] surface brightness ($\Sigma_{\rm [CII]}$, $\Sigma_{\rm [OIII]}$) and the SFR surface density ($\Sigma_{\rm SFR}$) to $n$, $\kappa_s$, and $Z$. All five sources are characterized by a central starbursting region, where the $\Sigma_{\rm gas}$ vs $\Sigma_{\rm SFR}$ align ~10x above the KS relation ($\kappa_s\approx10$). This translates into gas depletion times in the range $t_{\rm dep}\approx 80-250$ Myr. The inner starbursting centers are characterized by higher gas density ($\log (n/{\rm cm^{-3}}) \approx 2.5-3.0$) and higher metallicity ($\log (Z/Z_{\odot}) \approx -0.5$) than the galaxy outskirts. We derive marginally negative radial metallicity gradients ($\nabla \log Z \approx -0.03 \pm 0.07$dex/kpc), and a dust temperature ($T_d\approx$32-38 K) that anticorrelates with the gas depletion time.

astro-ph.GA↗

arXiv2022

Blue Monsters. Why are JWST super-early, massive galaxies so blue?

The recent JWST tentative discovery of a population of super-early (redshift $z> 10$), relatively massive (stellar mass $M_* = 10^{8-9} M_{\odot}$) and evolved (metallicity $Z \approx 0.1 Z_{\odot}$) galaxies, which nevertheless show blue ($\beta \simeq -2.6$) spectra, and very small dust attenuation ($A_{\rm V} \leq 0.02$), challenges our interpretation of these systems. To solve the puzzle we propose two solutions in which dust is either (a) ejected by radiation pressure, or (b) segregated with respect to UV-emitting regions. We clarify the conditions for which the two scenarios apply, and show that they can be discriminated by ALMA observations, such as the recent non-detection of the $88\mu m$ dust continuum in GHZ2 ($z\simeq 12$) favouring dust ejection.

astro-ph.GA↗

arXiv2017

Unitary Inequivalent Representations and Quantum Physics

In this paper we discuss the unitary inequivalentness in quantum physics. Then based on some of the current outstanding problems in theoretical physics, we will show the important role of this concept to better understand the physical theories.

quant-ph↗

arXiv2013

On the Alternative Theories of Cosmology

In this article first we present a brief review of some alternative theories of cosmology. Then by referring to some of the main works done in these theories we show that how they can describe the phenomena which are explained nowadays in the framework of standard model of cosmology(SM).

physics.gen-ph↗