arXiv subjects

Zohreh Ghaffari

Publications and source records attributed to Zohreh Ghaffari.

6 recordsLinked to original sources

arXiv2026

Do Submillimeter Galaxies Trace Megaparsec Large-scale Structures? -- An Overdensity Analysis of 449 Submillimeter Galaxies in COSMOS

We present an environmental analysis and overdensity catalog for a large sample of submillimeter galaxies (SMGs) in the COSMOS field in order to understand whether SMGs are signposts of megaparsec-scale overdensity structures. We apply the Poisson Probability Method (PPM) protocluster finder to characterize the significance ($σ$) and overdensity value ($δ$) of any overdense structures surrounding specific 'beacon' galaxies. These beacons include 449 SMGs from A3COSMOS (including 168 with spectroscopic redshifts, 52 of which have CO or [CII] 158$μ$m line detections), as well as a control sample of over 5000 main-sequence star-forming galaxies (SFGs) selected from the 40-band photometric-redshift catalog in COSMOS2025. We successfully reproduce 15 known protoclusters and clusters in COSMOS (including Hyperion large-scale structure, CL J1001+0220 at z~2.5, and AzTEC-3 protocluster at z~5.3), and report 26 new PPM-based high-overdensity ($δ$ $\geq$ 5), high-significance ($σ$ $\geq$ 5), robust candidates. We measure an overdensity fraction of 11.5-71.2% (1.3-61.0%) in the CO/[CII] 52 SMG (full 449 SMG) sample with different criterion categories, confirming that SMGs are signposts of overdensities compared to the control SFG sample. We also notice that the fraction is lower than 100%, indicating that not all SMGs are associated with overdensities. We thus inspect the JWST morphology and argue that the triggering of starburst in SMGs must have non-environmental mechanisms, for instance, merger and violent disk instability. Finally, we further conduct a spatial distribution analysis, finding that the location of SMGs in overdensities has a redshift evolution, with SMGs being more concentrated in the inner regions of overdensities at higher redshift.

astro-ph.GA↗

arXiv2026

X-ray Characterization of Galaxy Groups and Hickson Groups in COSMOS-Web to z~3.8 I. X-ray Luminosity Function Evolution

Galaxy groups are the dominant halo environment in the Universe and a sensitive laboratory for feedback and gas-cycling physics, yet a uniform, intermediate-redshift X-ray characterization of the group population has been limited by the availability of deep, wide-area imaging. We present a uniform X-ray characterization of the COSMOS-Web galaxy group population: 1,678 groups (CW-All) identified with the AMICO algorithm applied to the JWST-selected COSMOS-Web photometric catalog ($0.08 \lesssim z \lesssim 3.7$), and a parallel sample of 993 Hickson compact groups (CW-HCG; $0.08 \lesssim z \lesssim 3.82$) over $0.45~\mathrm{deg}^2$. We apply a uniform aperture-photometry X-ray pipeline to the combined Chandra and XMM-Newton COSMOS mosaic, measuring fluxes, luminosities, temperatures, and halo masses and radii for every group, together with detections, upper limits, and quality flags. We detect X-ray emission from 499 CW-All groups (29.7\%) and 332 CW-HCG groups (33.4\%) at SNR~$\geq 2.0$, and present the X-ray luminosity function in four redshift bins. The characteristic luminosity $L^\star$ rises monotonically by $\sim$1.9--2.1~dex from the lowest to highest redshift bin in both catalogs, a trend that persists under two independent cross-check binning schemes and is not explained by mass or richness selection of the detected population. A targeted search for diffuse X-ray emission from the AmicOne protocluster candidate at $2.50 \lesssim z < 3.09$ finds no statistically significant signal at the current depth. We publicly release the X-ray measurements and derived properties for both catalogs.

astro-ph.GA↗

arXiv2026

The Infrared Glow of Galactic Outskirts: A New Window with NASIM

The outskirts of galaxies retain fossil records of their assembly history and are dominated by old stellar populations shaped by tidal interactions, accretion events, and environmental processes. Observing these low-surface-brightness (LSB) regions in the near-infrared (NIR), particularly in the $K_s$ band, is crucial for tracing evolved stars and minimising the effects of dust. Yet, ground-based NIR studies of faint extended structures remain challenging because of the bright and variable sky background and complex detector systematics. To overcome these limitations, we developed NASIM, the Near-infrared Automated low Surface brightness reduction In Maneage pipeline, a fully automated and reproducible framework designed to recover faint structures in VISTA/VIRCAM data. Using GNU Astronomy Utilities (Gnuastro), NASIM corrects large-scale flat-field patterns and instrumental signatures across the VIRCAM detectors while preserving LSB emission that can be strongly suppressed in conventional NIR reductions. We apply NASIM to $K_s$-band imaging of the Euclid Deep Field South through the KEDFS programme, a key region also targeted by Rubin/LSST and Euclid. The final reduction reaches a surface-brightness limit of $μ_{K_s} \simeq 27.7$ mag arcsec$^{-2}$, measured at $3σ$ over 100 arcsec$^2$, approximately 67 times deeper than 2MASS and 11 times deeper than VHS in surface-brightness sensitivity. Direct comparisons with publicly available VISTA products show that NASIM preserves diffuse emission without substantially compromising compact-source depth. These results open a deep NIR view of faint galaxy outskirts, LSB galaxies, and intracluster light, providing a reproducible ground-based foundation for future studies of the faint Universe.

astro-ph.GA↗

arXiv2025

NASIM: Revealing the low surface brightness Universe from legacy VISTA data

Near-infrared imaging is a powerful technique in observational astronomy, but the bright background, primarily from the Earthś atmosphere, makes the detection of faint features particularly challenging. To recover low surface brightness (LSB) structures in such data, we present NASIM (Near-infrared Automated low Surface brightness reduction In Maneage), a fully automated and reproducible data reduction pipeline optimised for VISTA/VIRCAM observations. NASIM builds on GNU Astronomy Utilities (Gnuastro) to effectively remove large-scale instrumental artefacts while preserving faint, diffuse emission. As a key science application, we focus on deep Ks-band observations of the Euclid Deep Field South (KEDFS), one of the deepest VISTA/VIRCAM datasets and a high-priority field for synergy with current and future facilities, including Euclid, JWST, LSST, Roman, Spitzer, and ALMA. With VIRCAM no longer operational, KEDFS now stands as a unique legacy dataset. We release selected tiles from the KEDFS survey and highlight science cases, including galaxy outskirts, LSB galaxies, and intracluster light, that demonstrate NASIMś ability to recover diffuse structures. A direct comparison with conventional VISTA data reduction pipelines demonstrates the advantages of NASIM in preserving diffuse emission without compromising compact source detection. All quantitative results presented in this paper are fully reproducible with Maneage (commit 4d32667).

astro-ph.IM↗

arXiv2023

Gnuastro: Estimating the Zero Point Magnitude in Astronomical Imaging

Calibration of pixel values is a fundamental step for accurate measurements in astronomical imaging. In astronomical jargon this is known as estimating zero point magnitude. Here, we introduce a newly added script in GNU Astronomy Utilities (Gnuastro) version 0.20 for the zero point magnitude estimation, named: astscript-zeropoint. The script offers numerous features, such as the flexibility to use either image(s) or a catalog as the reference dataset. Additionally, steps are parallelized to enhance efficiency for big data. Thanks to Gnuastro's minimal dependencies, the script is both flexible and portable. The figures of this research note are reproducible with Maneage, on the Git commit c89275e.

astro-ph.IM↗

arXiv2021

Clustering of red and blue galaxies around high-redshift 3C radio sources as seen by the Hubble Space Telescope

To properly understand the evolution of high-redshift galaxy clusters, both passive and star-forming galaxies have to be considered. Here we study the clustering environment of 21 radio galaxies and quasars at 1 1.6, red overdensities at 1.2<z<1.6, and red overdensities with an increased deficit of central blue galaxies at z<1.2. Only a few 3C sources show a blue overdensity tracing active star-formation in the cluster centers; this rarity could indicate that the powerful quasar activity may quench star-formation in the vicinity of most radio sources. The derived number of central luminous red galaxies and the radial density profiles are comparable to those found in local clusters, indicating that some 3C clusters are already mass-rich and compact.

astro-ph.GA↗