arXiv subjects

Saeed Tavasoli

Publications and source records attributed to Saeed Tavasoli.

18 recordsLinked to original sources

arXiv2026

Cosmic Environment as the Primary Driver of Dwarf Satellite Statistics

Context: Satellite dwarf galaxies provide key constraints on galaxy formation and evolution, since their abundance and spatial distribution reflect both the host properties and the large-scale environment. Aims: This study quantifies the dependence of satellite populations on the host stellar mass, morphology, and star formation activity across different environments, and traces their evolution with cosmic time within the $\Lambda$CDM framework. Methods: The Millennium-II simulation combined with the G11 semi-analytic model is used to construct consistent samples of host galaxies brighter than $M_{r}<-16$ and their satellites ($M_{\ast}\geq 3\times10^{5}\,M_{\odot}$, $M_{r}<-9$) within the virial radius. Satellite abundance and radial profiles are analysed in cluster, group, and void environments, and their evolution is traced from $z=2$ to $z=0$ across three host stellar mass bins. Results: Satellite abundance is correlated strongly with host stellar and bulge mass, whereas host morphology has little independent effect once stellar mass is accounted for. Dense environments suppress satellite populations relative to voids. Correlations between satellite abundance, specific star formation rate, and disk scale length become evident only in groups and clusters. At $z=0$, radial profiles show strong central concentrations in voids, flattened distributions in clusters, and intermediate trends in groups. Their redshift evolution reveals progressive flattening for low- and intermediate-mass hosts in dense environments, stability for massive hosts, and increasing central concentration in voids. The cosmic evolution of satellite abundance further highlights distinct pathways: gradual accumulation in voids, mass-dependent trends in groups, and strong late-time suppression in clusters.

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arXiv2026

Evolution of Cosmic Voids: Structure, Galaxies, and Dynamics

We investigate the structural, photometric, and dynamical evolution of cosmic voids and their galaxy populations from $z=2.09$ to the present, focusing on void size as a key evolutionary parameter. Using void catalogs from four Millennium Simulation snapshots and SDSS data at $z<0.04$, we perform a unified analysis of void demographics, galaxy properties, and internal kinematics. Our analysis reveals clear evidence that cosmic voids exhibit a significant evolutionary trend of becoming progressively emptier toward low redshift, accompanied by a marked decline in the brightness and clustering of their galaxy populations. The void galaxy luminosity function evolves significantly: $M^{*}$ fades and $\alpha$ flattens with time, with large voids hosting brighter, more rapidly evolving galaxies than small voids. Stacked density profiles exhibit a universal shape when scaled by void radius, deepening and building more pronounced walls toward $z=0$. Galaxy spatial distributions reveal persistent size-dependent segregation, with galaxies in large voids lying farther from the center and more strongly clustered. Dynamical analysis of simulations shows coherent outward flows in all voids, with amplitudes decreasing toward $z=0$, providing a physical basis for observed redshift-space distortions. Comparison with SDSS broadly confirms these evolutionary trends but uncovers a non-zero central galaxy population in observed voids -- absent in $\Lambda$CDM predictions -- that may challenge current galaxy formation models in extreme underdensities. Future comparisons with additional simulations and deeper high-redshift surveys will provide stronger tests of $\Lambda$CDM in the most underdense regions.

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arXiv2025

The Filament Rift: $\Lambda$CDM's Structural Challenge Against Observation

This study presents the first extended comparison of cosmic filaments identified in SDSS DR10 observations ($z < 0.05$) and the IllustrisTNG300-1 $\Lambda$CDM simulation ($z = 0$), utilizing the novel GrAviPaSt filament-finder method. The analyses are performed on both macro- and micro-filaments, each characterized by their length, thickness, and contrast in mass density. In addition to total sample comparisons, two subcategories of micro-filaments, GG (linking galaxy groups) and CC (linking galaxy clusters), are introduced to further analyze discrepancies between the $\Lambda$CDM model and observation. While $\Lambda$CDM produces extended macro-filaments, such structures are largely absent in SDSS, and where present, they exhibit higher densities than their simulated counterparts. Micro-filaments also show notable density discrepancies: at fixed length and thickness, observational filaments are significantly denser than those in the simulation. Employing radial density profiles reveal that micro-filaments in the $\Lambda$CDM simulation exhibit higher contrasts in mass density relative to the background compared to their observational counterparts. Notably, CC type micro-filaments displayed enhanced density contrasts over GG types in the simulation, while observational data showed the opposite trend. Furthermore, SDSS galaxies in both GG and CC micro-filaments exhibit lower specific star formation rates (sSFR) and older stellar populations, while TNG300-1 micro-filaments host more actively star-forming galaxies within the intermediate stellar mass range. These results reveal persistent discrepancies between observational data and the $\Lambda$CDM reconstruction of cosmic filaments, pointing to possible tensions in our current understanding of large-scale structures and their environmental effects on galaxy evolution.

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arXiv2025

From Voids to Clusters: Mergers and Evolutionary Pathways of Star-Forming and Quenched Low-Mass Galaxies

The evolution of low-mass galaxies is shaped by both internal processes and environmental factors, yet the role of environment and mergers in regulating their growth and star formation rates remains poorly understood, especially in the low-density regime. This study aims to compare the evolutionary pathways and merger histories of star-forming and quenched galaxies in dense (cluster) and under-dense (void) environments, focusing on galaxies with stellar masses in the range $10^{8.5} \leq M_\star/M_\odot \leq 10^{10.5}$. It presents the first statistical analysis within this stellar mass range, explicitly distinguishing between mini, minor, and major mergers across varying environments. Using the high-resolution TNG300-1 simulation from the IllustrisTNG project, we classify galaxies as star-forming and quenched based on sSFR and UVJ criteria. We track their physical properties over the last $\sim 10.5$ Gyr ($z < 2$), follow their complete merger histories, distinguishing between major, minor, and mini mergers, and assess the statistical impact of these mergers on star formation and gas content.

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arXiv2025

GrAviPaSt's Lens to the Past: Unveiling the Evolution of Filamentary Structures

This paper examines the evolution of cosmic filaments across redshifts 1, 0.5, and 0 using the IllustrisTNG100-1 magneto-hydrodynamical simulation. To achieve this, we introduce GrAviPaSt, a simple, efficient and parameter-free filament identification method that leverages gravitational potential, an A*-like path-finding algorithm, and spanning trees. Applying this method to galaxy distributions at different redshifts allows us to analyze various filament properties, including their length, thickness, mass density contrast, and radial profile. Additionally, we investigate dynamic characteristics such as the mean distance of filament galaxies from the skeleton, their weighted mean velocity, and velocity trends normalized by their positions within the filaments. Our findings reveal the evolution of cosmic filaments from redshift 1 to 0, highlighting key differences across classifications. In particular, we examine the mass density contrast radial profile of filaments connecting two galaxy groups and those linking two galaxy clusters, identifying distinct differences in profile shape between these categories. Furthermore, in the context of weighted mean velocity, we analyze cosmic filaments exhibiting either negative or positive weighted mean velocity, demonstrating their differing evolutionary trends in terms of the mean distance of filament galaxies from the skeleton.

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arXiv2025

Star-Forming vs. Quenched Galaxies in Voids: Insights into the Role of Mergers

Cosmic voids, the largest under-dense structures in the Universe, are crucial for exploring galaxy evolution. These vast, sparsely populated regions are home to void galaxies -- predominantly gas-rich, star-forming, and blue -- that evolve more slowly than those in denser environments. Additionally, the correlation between galaxy mergers and specific properties of galaxies, such as the star formation rate (SFR), is not fully understood, particularly in these under-dense environments. Quenched void galaxies exhibit high SFRs at high redshifts, significantly decreasing at lower redshifts (z < 0.5). These galaxies have higher dark matter halos than star-forming galaxies across all redshifts, leading to rapid gas consumption. They formed earlier and experienced more major mergers in earlier epochs but fewer recent mergers, resulting in a lack of fresh gas for sustained star formation. Also, star-forming and high-mass quenched void galaxies show higher SFRs in mergers compared to non-merger galaxies. This study highlights that formation time, merger rates, and dark matter halos play a crucial role in the star formation history of void galaxies. Rapid and earlier gas consumption due to earlier formation time and the absence of recent mergers could lead to quenched void galaxies at lower redshifts, providing valuable insights into galaxy evolution in low-density environments.

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arXiv2021

Void Galaxy Distribution: A Challenge for $Λ$CDM

We extract void catalogs from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16) survey and also from the Millennium simulation. We focus our comparison on distribution of galaxies brighter than $M_r < -18$ inside voids and study the mean separation of void galaxies, distance from the void center, and the radial density profile. We find that mean separation of void galaxies depends on void size, as bigger voids have lower mean separation in both samples. However, void galaxies in the observation sample seem to have generally larger mean-distance than simulated ones at any given void size. In addition, observed void galaxies tend to reside closer to the void center than those in the simulation. This discrepancy is also shown in the density profile of voids. Regardless of the void size, the central densities of real void profiles are higher than the ones in the predicted simulated catalog.

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arXiv2019

Comparing IMF-sensitive indices of intermediate-mass quiescent galaxies in various environments

Using samples drawn from the Sloan Digital Sky Survey, we study for the first time the relation between large-scale environments (Clusters, Groups and Voids) and the stellar Initial Mass Function (IMF). We perform an observational approach based on the comparison of IMF-sensitive indices of quiescent galaxies with similar mass in varying environments. These galaxies are selected within a narrow redshift interval ($ 0.020 < z < 0.055 $) and spanning a range in velocity dispersion from 100 to 200 kms$^{-1}$. The results of this paper are based upon analysis of composite spectra created by stacking the spectra of galaxies, binned by their velocity dispersion and redshift. The trends of spectral indices as measured from the stacked spectra, with respect to velocity dispersion, are compared in different environments. We find a lack of dependence of the IMF on the environment for intermediate-mass galaxy regime. We verify this finding by providing a more quantitative measurement of the IMF variations among galactic environments using MILES stellar population models with a precision of $ΔΓ_{b}\sim0.2$.

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arXiv2019

Role of environment on nuclear activity

Motivated by the apparently conflicting results reported in the literature on the effect of environment on nuclear activity, we have carried out a new analysis by comparing the fraction of galaxies hosting active galactic nuclei (AGNs) in the most overdense regions (rich galaxy clusters) and the most underdense ones (voids) in the local universe. Exploiting the classical BPT diagnostics, we have extracted volume limited samples of star forming and AGN galaxies. We find that, at variance with star-forming galaxies, AGN galaxies have similar distributions of specific star formation rates and of galactic ages (as indicated by the Dn4000 parameter) both in clusters and in voids. In both environments galaxies hosting AGNs are generally old, with low star formation activity. The AGN fraction increases faster with stellar mass in clusters than in voids, especially above 10^10.2 M(sun). Our results indicate that, in the local universe, the nuclear activity correlates with stellar mass and galaxy morphology and is weakly, if at all, affected by the local galaxy density.

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arXiv2018

Stellar Mass Profiles of Quiescent Galaxies in Different Environments at $z\sim0$

We present the stellar mass profiles of 147 isolated quiescent galaxies in very low-density environments (i.e., void regions) in the local Universe ($0.01<z<0.06$) from the Sloan Digital Sky Survey. These galaxies have stellar masses between $ 9.8\lesssim \log(M_{\ast}/M_{\odot})\lesssim11.2$ and they represent $\sim15\%$ of the whole galaxy population in the void regions down to $M_{r} = -19$. We do not find any isolated quiescent galaxies with $\log(M_{\ast}/M_{\odot})\gtrsim11.2$. We compare the stellar mass profiles of these isolated quiescent galaxies with the profiles of stellar mass-matched samples of the quiescent galaxies in group and cluster environments. We find that, at fixed mass, quiescent galaxies in voids have similar central ($1$ kpc) mass densities ($Σ_1$) and central velocity dispersions ($σ_1$) compared to their counterparts in groups and clusters. We show that quiescent galaxies in voids have at most $10-25\%$ smaller half-mass (and half-light) sizes compared to quiescent galaxies in groups and clusters. We conclude that for the intermediate stellar mass range of $10^{10}-10^{11}M_{\odot}$ in the local Universe, environmental mechanisms have no significant additional effect on the mass profiles of the quiescent galaxies.

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arXiv2018

Peculiar velocity measurement in a clumpy universe

In this work we address the issue of peculiar velocity measurement in a perturbed Friedmann universe using the deviations from measured luminosity distances of standard candles from background FRW universe. We want to show and quantify the statement that in intermediate redshifts ($0.5< z < 2$), deviations from the background FRW model are not uniquely governed by peculiar velocities. Luminosity distances are modified by gravitational lensing. We also want to indicate the importance of relativistic calculations for peculiar velocity measurement at all redshifts. For this task we discuss the relativistic correction on luminosity distance and redshift measurement and show the contribution of each of the corrections as lensing term, peculiar velocity of the source and Sachs-Wolfe effect. Then we use the SNe Ia sample of Union 2, to investigate the relativistic effects we consider. We show that, using the conventional peculiar velocity method, that ignores the lensing effect, will result in an overestimate of the measured peculiar velocities at intermediate redshifts. Here we quantify this effect. We show that at low redshifts the lensing effect is negligible compare to the effect of peculiar velocity. From the observational point of view, we show that the uncertainties on luminosity of the present SNe Ia data prevent us from precise measuring the peculiar velocities even at low redshifts ($z<0.2$).

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arXiv2015

The Galaxy Population In Voids: Are All Voids The Same?

The influence of under-dense environments on the formation and evolution of galaxies is studied by analysing the photometric properties of ~200 galaxies residing in voids, taken from our SDSS DR10 void catalogue up to z ~0.055. We split void galaxies into two subsamples based on the luminosity density contrast of their host voids: 'sparse void' δ_s = δ < -0.95 and 'populous void' δ_p = δ > -0.87. We find that galaxies in sparse voids are less massive than galaxies in populous voids. The luminosity distribution of galaxies in populous voids follows the same distribution observed across the SDSS survey in the same redshift range. Galaxies in the sparse voids are also bluer suggesting that they may be going through a relatively slow and continuous star formation. Additionally, we find that the luminosity function of galaxies in populous voids is represented with the Schechter function whereas the same does not hold for sparse voids. Our analysis suggests that the properties of a host void plays a significant role in the formation and evolution of the void galaxies and determining the large scale evolution of voids is an important step to understand what processes regulate the evolution of galaxies.

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arXiv2014

Probing the Local Bubble with Diffuse Interstellar Bands. II. The DIB properties in the Northern hemisphere

We present a new high signal to noise ratio spectroscopic survey of the Northern hemisphere to probe the Local Bubble and its surroundings using the $λ5780$ Å and $λ5797$ Å Diffuse Interstellar Bands (DIBs). We observed 432 sightlines to a distance of 200 pc over a duration of 3 years. In this study, we establish the $λ5780$ and $λ5797$ correlations with Na I, Ca II and E(B-V), for both inside and outside the Local Bubble. The correlations show that among all neutral and ionized atoms, the correlation between Ca II and $\lambda5780$ is stronger than its correlation with $\lambda5797$, suggesting that $\lambda5780$ is more associated with regions where Ca$^{+}$ is more abundant. We study the $\lambda5780$ correlation with $\lambda5797$, which shows a tight correlation within and outside the Local Bubble. In addition we investigate the DIB properties in UV irradiated and UV shielded regions. We find that, within and beyond the Local Bubble, $\lambda5797$ is located in denser parts of clouds, protected from UV irradiation, while $\lambda5780$ is located in the low density regions of clouds.

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arXiv2014

Unraveling the nature of Gravity through our clumpy Universe

We propose a new probe to test the nature of gravity at various redshifts through large-scale cosmological observations. We use our void catalog, extracted from the Sloan Digital Sky Survey (SDSS, DR10), to trace the distribution of matter along the lines of sight to SNe Ia that are selected from the Union 2 catalog. We study the relation between SNe Ia luminosities and convergence and also the peculiar velocities of the sources. We show that the effects, on SNe Ia luminosities, of convergence and of peculiar velocities predicted by the theory of general relativity and theories of modified gravities are different and hence provide a new probe of gravity at various redshifts. We show that the present sparse large-scale data does not allow us to determine any statistically- significant deviation from the theory of general relativity but future more comprehensive surveys should provide us with means for such an exploration.

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arXiv2014

Ultimate age-dating method for galaxy groups; clues from the Millennium Simulations

There have been a number of studies dedicated to identification of fossil galaxy groups, arguably groups with a relatively old formation epoch. Most of such studies identify fossil groups, primarily based on a large luminosity gap, which is the magnitude gap between the two most luminous galaxies in the group. Studies of these types of groups in the millennium cosmological simulations show that, although they have accumulated a significant fraction of their mass, relatively earlier than groups with a small luminosity gap, this parameter alone is not highly efficient in fully discriminating between the "old" and "young" galaxy groups, a label assigned based on halo mass accumulation history. We study galaxies drawn from the semi-analytic models of Guo et al. (2011), based on the Millennium Simulation. We establish a set of four observationally measurable parameters which can be used in combination, to identify a subset of galaxy groups which are old, with a very high probability. We thus argue that a sample of fossil groups selected based on luminosity gap will result in a contaminated sample of old galaxy groups. By adding constraints on the luminosity of the brightest galaxy, and its offset from the group luminosity centroid, we can considerably improve the age-dating.

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arXiv2013

The challenge of large and empty voids in the SDSS DR7 redshift survey

We present catalogues of voids for the SDSS DR7 redshift survey and for Millennium I simulation mock data. We aim to compare the observations with simulations based on a $Λ$CDM model and a semi-analytic galaxy formation model. We use the void statistics as a test for these models. We assembled a mock catalogue that closely resembles the SDSS DR7 catalogue and carried out a parallel statistical analysis of the observed and simulated catalogue. We find that in the observation and the simulation, voids tend to be equally spherical. The total volume occupied by the voids and their total number are slightly larger in the simulation than in the observation. We find that large voids are less abundant in the simulation and the total luminosity of the galaxies contained in a void with a given radius is higher on average than observed by SDSS DR7 survey. We expect these discrepancies to be even more significant in reality than found here since the present value of $σ_8$ given by WMAP7 is lower than the value of 0.9 used in the Millennium I simulation. The reason why the simulation fails to produce enough large and dark voids might be the failure of certain semi-analytic galaxy formation models to reduce the small-scale power of $Λ$CDM and to produce sufficient power on large scales.

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arXiv2010

A statistical study of the luminosity gap in galaxy groups

The luminosity gap between the two brightest members of galaxy groups and clusters is thought to offer a strong test for the models of galaxy formation and evolution. This study focuses on the statistics of the luminosity gap in galaxy groups, in particular fossil groups, e.g. large luminosity gap, in an analogy with the same in a cosmological simulation. We use spectroscopic legacy data of seventh data release (DR7) of SDSS, to extract a volume limited sample of galaxy groups utilizing modified friends-of-friends (mFoF) algorithm. Attention is paid to galaxy groups with the brightest group galaxy (BGG) more luminous than \Mr = -22. An initial sample of 620 groups in which 109 optical fossil groups, where the luminosity gap exceeds 2 magnitude, were identified. We compare the statistics of the luminosity gap in galaxy groups at low mass range from the SDSS with the same in the Millennium simulations where galaxies are modeled semi-analytically. We show that the BGGs residing in galaxy groups with large luminosity gap, i.e. fossil groups, are on average brighter and live in lower mass halos with respect to their counter parts in non-fossil systems. Although low mass galaxy groups are thought to have recently formed, we show that in galaxy groups with 15 galaxies brighter than $M_r\ge -19.5$, evolutionary process are most likely to be responsible for the large luminosity gap. We also examine a new probe of finding fossil group. In addition we extend the recently introduced observational probe based on the luminosity gap, the butterfly diagram, to galaxy groups and study the probe as a function of halo mass. This probe can, in conjunction with the luminosity function, help to fine tune the semi-analytic models of galaxies employed in the cosmological simulations.

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arXiv2009

The Spatial Distribution Function of Galaxies at High Redshift

This is the first exploration of the galaxy distribution function at redshifts greater than about 0.1. Redshifts are based on the North and South GOODS Catalogs. In each catalog we examine clustering in the two redshift bands 0.47 < z < 0.8 and 0.9 < z < 1.5. The mean redshifts of the samples in these bands are about 0.6 and 1.1. Our main result is that at these redshifts the galaxy spatial distribution function f_V(N) has the form predicted by gravitational quasi-equilibrium dynamics for cosmological many-body systems. This constrains related processes such as galaxy merging and the role of dark matter in the range of these redshifts.

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