arXiv subjects

I. Shivaei

Publications and source records attributed to I. Shivaei.

24 records · Page 2Linked to original sources

arXiv2023

Identification and properties of intense star-forming galaxies at redshifts z>10

Surveys with James Webb Space Telescope (JWST) have discovered candidate galaxies in the first 400 Myr of cosmic time. Preliminary indications have suggested these candidate galaxies may be more massive and abundant than previously thought. However, without confirmed distances, their inferred properties remain uncertain. Here we identify four galaxies located in the JWST Advanced Deep Extragalactic Survey (JADES) Near-Infrared Camera (NIRCam) imaging with photometric redshifts z~10-13. These galaxies include the first redshift z>12 systems discovered with distances spectroscopically confirmed by JWST in a companion paper. Using stellar population modelling, we find the galaxies typically contain a hundred million solar masses in stars, in stellar populations that are less than one hundred million years old. The moderate star formation rates and compact sizes suggest elevated star formation rate surface densities, a key indicator of their formation pathways. Taken together, these measurements show that the first galaxies contributing to cosmic reionisation formed rapidly and with intense internal radiation fields.

astro-ph.GA↗

arXiv2023

Nuclear high-ionisation outflow in the Compton-thick AGN NGC6552 as seen by the JWST mid-infrared instrument

During the commissioning of the James Webb Space Telescope (JWST), the mid-infrared instrument (MIRI) observed NGC6552 with the MIRI Imager and the medium-resolution spectrograph (MRS). NGC6552 is an active galactic nucleus (AGN) at redshift 0.0266 classified as a Seyfert 2 nucleus in the optical, and Compton-thick AGN in X-rays. This work exemplifies and demonstrates the MRS capabilities to study the mid-infrared (mid-IR) spectra and characterize the physical conditions and kinematics of the ionized and molecular gas in the nuclear regions of nearby galaxies. We obtained the nuclear, circumnuclear, and central mid-IR spectra of NGC6552. They provide the first clear observational evidence for a nuclear outflow in NGC6552. The outflow contributes to 67$\pm$7% of the total line flux independent of the ionization potential (27 to 187 eV) and critical densities (10$^4$ to 4$\times$10$^{6}$ cm$^{-3}$), showing an average blue-shifted peak velocity of -127$\pm$45 kms$^{-1}$ and an outflow maximal velocity of 698$\pm$80 kms$^{-1}$. Since the mid-IR photons penetrate dusty regions as efficiently as X-ray keV photons, we interpret these results as the evidence for a highly ionized, non-stratified, AGN-powered, and fast outflowing gas in a low density environment (few 10$^{3}$ cm$^{-3}$) located very close (<0.2kpc) to the Compton-thick AGN. Nine pure rotational molecular Hydrogen lines are detected and spectrally resolved, and exhibit symmetric Gaussian profiles, consistent with the galactic rotation, and with no evidence of outflowing H$_{2}$ material. We detect a warm H$_{2}$ mass of $1.9\pm1.1\times10^7 M_{\odot}$ in the central region (1.8 kpc in diameter) of the galaxy, with almost 30% of that mass in the circum-nuclear region. Line ratios confirm that NGC6552 has a Seyfert nucleus with a black hole mass estimated in the range of 0.6 to 6 million solar masses.

astro-ph.GA↗

arXiv2022

The average dust attenuation curve at z~1.3 based on HST grism surveys

We present the first characterisation of the average dust attenuation curve at $z\sim1.3$ by combining rest-frame ultraviolet through near-IR photometry with Balmer decrement ($\mathrm{H}α$/$\mathrm{H}β$) constraints for $\sim$900 galaxies with $8\lesssim\log (M_\star /M_\odot)<10.2$ at $0.75<z<1.5$ in the HST WFC3 IR Spectroscopic Parallel (WISP) and 3D-HST grism surveys. Using galaxies in SDSS, we establish that the ($\mathrm{H}α$+[NII])/[OIII] line ratio and stellar mass are good proxies for the Balmer decrement in low-spectral resolution grism data when only upper-limits on $\mathrm{H}β$ are available and/or $\mathrm{H}α$ is blended with [NII]. The slope of the $z\sim1.3$ attenuation curve ($A(0.15μm)/A(V)=3.15$) and its normalization ($R_V=3.26$) lie in-between the values found for $z=0$ and $z\sim2$ dust attenuation curves derived with similar methods. These provide supporting evidence that the average dust attenuation curve of star forming galaxies evolves continuously with redshift. The $z\sim1.3$ curve has a mild 2175Å feature (bump amplitude, $E_b=0.83$; $\sim$25% that of the MW extinction curve), which is comparable to several other studies at $0<z\lesssim3$, and suggests that the average strength of this feature may not evolve significantly with redshift. The methods we develop to constrain dust attenuation from HST grism data can be applied to future grism surveys with JWST, Euclid, and RST. These new facilities will detect millions of emission line galaxies and offer the opportunity to significantly improve our understanding of how and why dust attenuation curves evolve.

astro-ph.GA↗

arXiv2019

The Strength of the 2175Å Feature in the Attenuation Curves of Galaxies at 0.1<z<3

We update the spectral modeling code MAGPHYS to include a 2175Å absorption feature in its UV-to-near-IR dust attenuation prescription. This allows us to determine the strength of this feature and the shape of the dust attenuation curve in ~5000 star-forming galaxies at 0.1<z<3 in the COSMOS field. We find that a 2175Å absorption feature of ~1/3 the strength of that in the Milky Way is required for models to minimize residuals. We characterize the total effective dust attenuation curves as a function of several galaxy properties and find that the UV slopes of the attenuation curve for COSMOS galaxies show a strong dependence with star formation rate (SFR) and total dust attenuation ($A_V$), such that galaxies with higher SFR and $A_V$ have shallower curves and vice versa. These results are consistent with expectations from radiative transfer that attenuation curves become shallower as the effective dust optical depth increases. We do not find significant trends in the strength of the 2175Å absorption feature as a function of galaxy properties, but this may result from the high uncertainties associated with this measurement. The updated code is publicly available online.

astro-ph.GA↗

arXiv2019

Searches After Gravitational-waves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo's Third Observing Run

We present Searches After Gravitational-waves Using ARizona Observatories (SAGUARO), a comprehensive effort dedicated to the discovery and characterization of optical counterparts to gravitational wave (GW) events. SAGUARO utilizes ground-based facilities ranging from 1.5m to 10m in diameter, located primarily in the Northern Hemisphere. We provide an overview of SAGUARO's telescopic resources, pipeline for transient detection, and database for candidate visualization. We describe SAGUARO's discovery component, which utilizes the $5$~deg$^2$ field-of-view optical imager on the Mt. Lemmon 1.5m telescope, reaching limits of $\approx 21.3$~AB mag while rapidly tiling large areas. We also describe the follow-up component of SAGUARO, used for rapid vetting and monitoring of optical candidates. With the onset of Advanced LIGO/Virgo's third observing run, we present results from the first three SAGUARO searches following the GW events S190408an, S190425z and S190426c, which serve as a valuable proof-of-concept of SAGUARO. We triggered and searched 15, 60 and 60 deg$^{2}$ respectively, 17.6, 1.4 and 41.8 hrs after the initial GW alerts. We covered 7.8, 3.0 and 5.1\% of the total probability within the GW event localizations, reaching 3$σ$ limits of 19.8, 21.3 and 20.8 AB mag, respectively. Although no viable counterparts associated with these events were found, we recovered 6 known transients and ruled out 5 potential candidates. We also present Large Binocular Telescope spectroscopy of PS19eq/SN2019ebq, a promising kilonova candidate that was later determined to be a supernova. With the ability to tile large areas and conduct detailed follow-up, SAGUARO represents a significant addition to GW counterpart searches.

astro-ph.HE↗

arXiv2016

Dust attenuation of the nebular regions and optical emission lines of $z\sim2$ star-forming galaxies

We use a sample of 149 spectroscopically confirmed UV-selected galaxies at $z\sim 2$ to investigate the relative dust attenuation of the stellar continuum and the nebular emission lines. For each galaxy in the sample, at least one rest-frame optical emission line (H$α$/[NII]$\lambda6583$ or [OIII]$\lambda5007$) measurement has been taken from the litterature, and 41 galaxies have additional Spitzer/MIPS 24$μ$m observations that are used to infer infrared luminosities. We use a spectral energy distribution (SED) fitting code that predicts nebular line strengths when fitting the stellar populations of galaxies in our sample, and we perform comparisons between the predictions of our models and the observed/derived physical quantities. We find that on average our code is able to reproduce all the physical quantities (e.g., UV $β$ slopes, infrared luminosities, emission line fluxes), but we need to apply a higher dust correction to the nebular emission compared to the stellar emission for the largest SFR ($\log\mathrm{(SFR/M}_\odot\mathrm{yr}^{-1})>1.87$, Salpeter IMF). We find a correlation between SFR and the difference in nebular and stellar color excesses, which could resolve the discrepant results regarding nebular dust correction at $z\sim2$ from previous results.

astro-ph.GA↗