arXiv subjects

Nishikanta Khandai

Publications and source records attributed to Nishikanta Khandai.

At least 19 recordsLinked to original sources

arXiv2026

On the Origin of the Ly$α$ Damping Wing in Galaxies at $8\le z \le 10$: Explorations using the NINJA Simulations

Ly$α$ damping wing measurements of galaxies at $7\leq z\leq14$ with the JWST provide a powerful probe of the Epoch of Reionization. We combine the large-scale cosmological hydrodynamical NINJA simulations with an idealized ionized-bubble model at z=8 and 10 to quantify the contributions of the intergalactic medium (IGM), circumgalactic medium (CGM), and interstellar medium (ISM) to the observed Ly$α$ damping wing. We generate mock Ly$α$ absorption spectra, and consider three idealized scenarios: (i) galaxies embedded in a uniformly ionized IGM, (ii) galaxies surrounded by HII regions with radii of 0-400 pkpc, and (iii) the same models including Ly$α$ absorption from partially ionized gas within the virial radius. We explore IGM neutral fractions ($x_{\rm HI}$) of 0.1, 0.8, and 1.0, and compare the distribution of inferred HI column densities (NHI) with observations to constrain the sizes of HII regions and $x_{\rm HI}$ of the IGM and gas in and around galaxies. Galaxies are surrounded by over-dense gas extending to $\sim$60-100 pkpc, with its extent increasing with halo mass and showing little evolution between z=8 and 10. The inferred NHI also increases with halo and stellar mass. While the observed incidence of damped Ly$α$ absorption and the median NHI can be reproduced by different combinations of $x_{\rm HI}$ of the IGM and HII region size, models with a uniformly ionized IGM or HII regions larger than $\sim50$ pkpc fail to reproduce the strongest absorbers (NHI>$10^{22}cm^{-2}$). Including partially ionized gas within the virial radius substantially increases the incidence of these absorbers, although the predicted values remain below the observations, suggesting an additional contribution from unresolved gas in stellar birth clouds. The strongest damped Ly$α$ absorbers therefore provide a unique probe of the ionization state of the ISM, CGM, and IGM.

astro-ph.GA↗

arXiv2026

The Stellar Mass Function of Gas-Rich Galaxies and the Underlying $M_{\rm HI}-M_{\rm star}$ Scaling Relation in the Local Universe

We estimate the Galaxy Stellar Mass Function (GSMF) of HI gas-rich galaxies using the 100\% ALFALFA ($α$) catalog, $\sim 98\%$ of which have optical counterparts in the Sloan Digital Sky Survey (SDSS) and a subset of them have counterparts in GALEX SDSS WISE Legacy Catalogue-2(GSWLC-2). We use the mass estimates from this subset which combines UV, optical and IR bands with individual dust corrections to recalibrate optical stellar mass estimates. We use a non-parametric method to estimate the GSMF of these gas-rich galaxies. The resulting, HI-selected GSMF is consistent with a single Schechter function with best-fit parameters $\left\{ϕ_* (10^{-3}\, h_{70}^{3}\,\mathrm{Mpc}^{-3}\,\mathrm{dex}^{-1}), \log_{10} (M_*/M_{\odot}) + 2\log_{10} h_{70}, α\right\} = \left\{2.30^{+0.12}_{-0.12}, \,10.83^{+0.01}_{-0.01},\, -1.14^{+0.02}_{-0.02}\right\} $. Additionally, the red and blue populations are each well described by a single Schechter function. After correcting for selection effects, we find that the red population accounts for only $\sim18\%$ of gas-rich galaxies by number, yet contributes $\sim54\%$ of the total stellar mass, with the blue population accounting for the rest. Using an optically selected sample and a joint optical-HI sample, we find gas-rich galaxies represent $\sim 33\%$ of the total stellar mass density and $\sim 39\%$ of the total galaxy number counts in the local Universe. We use the GSMF and the HI mass function (HIMF) of the HI-selected sample to obtain the $M_{\rm HI}-M_{\rm star}$ relation, which is free from selection bias.

astro-ph.GA↗

arXiv2026

NISER-IUCAA New Simulations of JWST GAlaxies and Quasars(NINJA): Properties of galaxies at $5 \leq z \leq 10$

We present the NINJA suite of cosmological hydrodynamical simulations developed to investigate galaxy formation and evolution at $z \gtrsim 5$ in the era of JWST. Using our fiducial simulation, we explore a range of spectral synthesis prescriptions and dust attenuation models, demonstrating that suitably chosen parameters can reproduce the observed UV luminosity functions (UVLFs) over $5 \leq z \leq 10$. In all cases, the inferred dust-to-metal ratio evolves with redshift, although its normalization at fixed redshift varies by a factor of $\sim 7$, depending on the adopted dust--metallicity scaling and attenuation curve. These model variations introduce substantial scatter in predictions for the $B$-band luminosity function, the H$α$ luminosity function, the UV slope--UV magnitude relation, the stellar mass--Balmer ratio relation, and the relation between stellar and nebular colour excesses. Simultaneously reproducing these observables across multiple redshifts will therefore be essential for constraining dust models at high redshift with forthcoming observations. Observations of galaxies spanning a broad range of stellar masses with the Atacama Large Millimeter/submillimeter Array (ALMA) will provide particularly strong and independent constraints on dust properties. Our fiducial models underpredict the UV luminosity function at $z \geq 10$ relative to current observations, even when adopting a top-heavy IMF and neglecting dust attenuation. We find that galaxy properties are not fully converged at these redshifts in our simulation, indicating that higher-resolution simulations are required to robustly model galaxies at $z > 10$. We further emphasize that degeneracies between feedback prescriptions used in our simulation and dust properties must be carefully addressed when interpreting high-redshift observations and calibrating galaxy formation models.

astro-ph.GA↗

arXiv2026

Universality of Halo Shape and its Morphological Evolution across Cosmic Time

We investigate the evolution of dark matter halo shapes in cosmological N-body simulations both in scale free Einstein-De Sitter (EdS) and $Λ$CDM cosmologies. We compute the axis ratios ($q=b/a,s=c/a$) of well resolved central halos using the shape tensor. These halos are identified using two different halo finding algorithms, SUBFIND and ROCKSTAR. We find that at fixed mass, halos become more spherical with decreasing redshift. The distribution $P(q,s)$ along with their median values ($q$ and $s$) shows self-similar behaviour as a function of mass scaled by the non-linear mass, $(M/M_{nl})$ across power-law spectral indices for scale free EdS models. However the median $q$ and $s$ show a tighter self-similar evolution as a function of peak height $ν=δ_c/σ(M,z)$. We find that the median $q(ν)$ and $s(ν)$ are consistent with an evolution along a universal curve described by $y=α-δ\tanh \left[ ω\left(\log_{10}(ν) - μ\right)\right]$ across the spectral indices ranging from $n=-1.0$ to $n=-2.2$. Our results hold for both SUBFIND and ROCKSTAR, although there are some differences between them. The universality of the evolution of median $q(ν)$ and $s(ν)$ also holds for the $Λ$CDM runs, although with a different behaviour at small $ν$ compared to the scale free models. The width of the distributions of $P(q)$ and $P(s)$ in both, scale-free and $Λ$CDM, classes of simulations can be reduced further by classifying halos as oblate, triaxial and prolate, each of which also follows a universal behaviour. Although oblate halos are relatively rare at all redshifts, their fraction increases over time at the expense of the other two populations.

astro-ph.CO↗

arXiv2025

Forecasting properties of detectable massive binary black hole mergers in the era of space based gravitational-wave detectors

Gravitational waves (GWs) from massive black hole (MBH) mergers will provide a novel way to probe the high-redshift universe and are key to understanding galactic dynamics and evolution. In this work, we analyze MBH mergers, their GW signals and detectability, as well as their population properties, using the cosmological hydrodynamical simulation - NINJA Simulation Suite. We discuss the effect of resolution and finite volume on the black hole mass function (BHMF), which in turn limits the mergers associated with low mass black holes, $M_{BH} \lesssim 10^{6.5} M_\odot$. We find the upper limit on the total mass of the MBH binaries detectable by LISA to be $\sim 10^{8.4} M_\odot$. We also find that adding time delays pertaining to dissipative processes like dynamical friction and stellar hardening during the final stages of the inspiral for which the simulation lacks sufficient resolution to model, considerably shifts the peak of redshift distribution of detectable binaries from $z\sim0.5$ to $z\sim0.1$. Time delays reduce the number of detectable GW events but on the other hand their signal-to-noise is increased. From the observational point of view, we find a strong correlation between the SFR and $L_{\rm bol}$ at high redshifts for the detectable LISA binaries. This may prove to be a future application in the coincident observation of MBH binaries by GW and electromagnetic observations.

astro-ph.HE↗

arXiv2025

Dispersion in the Hubble-Lemaître constant measurements from gravitational clustering

Measurements of the Hubble-Lemaître constant ($H_0$) require us to estimate the distance and recession velocity of galaxies independently. Gravitational clustering that leads to the formation of galaxies and the large scale structure leaves its imprints in the form of peculiar velocities of galaxies. In general, it is not possible to disentangle the peculiar velocity component from the recession velocities of galaxies, and this introduces an uncertainty in the determination of $H_0$. Using N-body simulations, we quantify the impact of peculiar velocities on the $H_0$ estimation. We consider observers to be located in dark matter halos and compute the distribution of the estimated value of $H_0$ across all such observers. We find that the dispersion of this distribution is large at small scales, and it diminishes as we go to large separations, reaching the level of the quoted statistical error in Planck and SH0ES measurements well beyond $\sim$135 Mpc/h and $\sim$220 Mpc/h, respectively. Measurements at smaller scales are susceptible to errors arising from peculiar motions, and this error can propagate to measurements at larger scales in the distance ladder. Notably, we observe a negative correlation between the local over-density around an observer and the deviation of the local and the global value of $H_0$. We show that deviations more significant than 5% of the global values can be encountered frequently at scales of up to 40 Mpc/h, and this is considerably larger than the statistical errors on local estimates. We also analyse the cumulative effect of such errors on mock measurements of $H_0$ as measured from Milky Way-sized halos. We find that this error is sensitive to the lowest distance at which we use measurements. The distribution of $H_0$ in mock measurements has a large tail, and deviations of a few percent from the global value cannot be ruled out.

astro-ph.CO↗

arXiv2023

Role of ionizing background on the statistics of metal absorbers in hydrodynamical simulations

We study the statistical properties of O VI, C IV, and Ne VIII absorbers at low-$z$ (i.e., $z<0.5$) using Sherwood simulations with "WIND" only and "WIND+AGN" feedback and Massive black simulation that incorporates both "WIND" i.e. outflows driven by stellar feedback and AGN feedbacks. For each simulation, by considering a wide range of metagalactic ionizing UV background (UVB), we show the statistical properties such as distribution functions of column density ($N$), $b$-paramerer and velocity spread ($ΔV_{90}$), the relationship between $N$ and $b$-parameter and the fraction of Lya absorbers showing detectable metal lines as a function of $N$(H I) are influenced by the UVB used. This is because UVB changes the range in density, temperature, and metallicity of gas contributing to a given absorption line. For simulations considered here, we show the difference in some of the predicted distributions between different simulations is similar to the one obtained by varying the UVB for a given simulation. Most of the observed properties of O VI absorbers are roughly matched by Sherwood simulation with "WIND+AGN" feedback when using the UVB with a lower O VI ionization rate. However, this simulation fails to produce observed distributions of C IV and fraction of H I absorbers with detectable metals. Therefore, in order to constrain different feedback processes and/or UVBs, using observed properties of H I and metal ions, it is important to perform simultaneous analysis of various observable parameters.

astro-ph.GA↗

arXiv2023

Halo mass function in scale invariant models

Sheth-Tormen mass function has been widely used to quantify the abundance of dark matter halos. It is a significant improvement over the Press-Schechter mass function as it uses ellipsoidal collapse in place of spherical collapse. Both of these mass functions can be written in a form that is universal, i.e., independent of cosmology and power spectrum when scaled in suitable variables. However, cosmological simulations have shown that this universality is approximate. In this paper, we investigate the power spectrum dependence of halo mass function through a suite of dark-matter-only N-body simulations of seven power-law models in an Einstein-de Sitter cosmology. This choice of cosmology and a power-law power spectrum ensures the self-similar evolution of dark matter distribution, allowing us to isolate the power spectrum dependence of mass function. We find that the mass function shows a clear non-universality. We present fits for the parameters of the Sheth-Tormen mass function for a range of power-law power-spectrum indices. We find a mild evolution in the overall shape of the mass function with the epoch. Finally, we extend our result to LCDM cosmology. We show that the Sheth-Tormen mass function with parameter values derived from a matched power-law EdS cosmology provides a better fit to the LCDM mass function than the standard Sheth-Tormen mass function. Our results indicate that an improved analytical theory is required to provide better fits to the mass function.

astro-ph.CO↗

arXiv2021

The Dark Matter Halos of HI Selected Galaxies

We present the neutral hydrogen mass ($M_{\text{HI}}$) function (HIMF) and velocity width ($w_{50}$) function (HIWF) based on a sample of 7857 galaxies from the 40% data release of the ALFALFA survey ($α.40$). The low mass (velocity width) end of the HIMF (HIWF) is dominated by the blue population of galaxies whereas the red population dominates the HIMF (HIWF) at the high mass (velocity width) end. We use a deconvolution method to estimate the HI rotational velocity ($V_{\text{rot}}$) functions (HIVF) from the HIWF for the total, red, and blue samples. The HIWF and HIVF for the red and blue samples are well separated at the knee of the function compared to their HIMFs. We then use recent stacking results from the ALFALFA survey to constrain the halo mass ($M_{\text{h}}$) function of HI-selected galaxies. This allows us to obtain various scaling relations between $M_{\text{HI}} - w_{50} - V_{\text{rot}} - M_{\text{h}}$, which we present. The $M_{\text{HI}} - M_{\text{h}}$ relation has a steep slope ~2.10 at small masses and flattens to ~0.34 at masses larger than a transition halo mass, $\log_{10}(M_{\text{ht}} h^2_{70}/M_{\odot})=10.62$. Our scaling relation is robust and consistent with a volume-limited sample of $α.40$. The $M_{\text{HI}} - M_{\text{h}}$ relation is qualitatively similar to the $M_{\text{star}} - M_{\text{h}}$ relation but the transition halo mass is smaller by ~1.4 dex compared to that of the $M_{\text{star}} - M_{\text{h}}$ relation. Our results suggest that baryonic processes like heating and feedback in larger mass halos suppress HI gas on a shorter time scale compared to star-formation.

astro-ph.GA↗

arXiv2021

Redshift space three-point correlation function of IGM at $z<0.48$

The Ly$α$ forest decomposed into Voigt profile components allow us to study clustering properties of the intergalactic medium and its dependence on various physical quantities. Here, we report the first detections of probability excess of low-z (i.e z<0.48) Ly$α$ absorber triplets over a scale of $r_\parallel\leq8$pMpc with a maximum amplitude of $8.76^{+1.96}_{-1.65}$ at a longitudinal separation of 1-2pMpc. We measure non-zero three-point correlation ($ζ=4.76^{+1.98}_{-1.67}$) only at this scale with reduced three-point correlation value of Q=$0.95^{+0.39}_{-0.38}$. The measured $ζ$ shows an increasing trend with increasing HI column density ($N_{HI}$) while Q does not show any $N_{HI}$ dependence. About 88% of the triplets contributing to $ζ$ (at z$\le0.2$) have nearby galaxies (whose distribution is known to be complete for 0.1L$_*$ at z<0.1 and for L$_*$ at z~0.25 and within 20' to the quasar sightlines) within a velocity separation of 500$kms^{-1}$ and a median impact parameter of 405pkpc. The measured impact parameters are consistent with appreciable number of triplets at z$\le0.2$ not originating from individual galaxies but tracing the underlying galaxy distribution. Frequency of occurrence of high-b absorbers in triplets (~85%) is a factor~3 higher than that found among the full sample (~32%). Using four different cosmological simulations, we quantify the effect of peculiar velocities, feedback effects and show that most of the observed trends are broadly reproduced. However, $ζ$ at small scales ($r_\parallel<1$pMpc) and b-dependence of $ζ$ in simulations are found inconsistent with the observations. This could either be related to the fact that none of these simulations reproduce the observed b-distribution and $N_{HI}$ distribution for $N_{HI}>10^{14}$cm$^{-2}$ self-consistently or to the widespread of signal-to-noise ratio in the observed data.

astro-ph.GA↗

arXiv2020

Simulated X-ray Emission in Galaxy Clusters with Feedback from Active Galactic Nuclei

To investigate the effect of feedback from active galactic nuclei (AGN) on their surrounding medium, we study the diffuse X-ray emission from galaxy groups and clusters by coupling the Astrophysical Plasma Emission Code (APEC) with the cosmological hydrodynamic simulation involving AGN feedback. We construct a statistical sample of synthetic Chandra X-ray photon maps to observationally characterize the effect of AGN on the ambient medium. We show that AGN are effective in displacing the hot X-ray emitting gas from the centers of groups and clusters, and that these signatures remain evident in observations of the X-ray surface brightness profiles.

astro-ph.GA↗

arXiv2020

The Distribution of Neutral Hydrogen in the Color-Magnitude Plane of Galaxies

We present the conditional HI (neutral hydrogen) Mass Function (HIMF) conditioned on observed optical properties, $M_{\text{r}}$ ($r$-band absolute magnitude) and $C_{\text{ur}}$ ($u-r$ color), for a sample of 7709 galaxies from ALFALFA (40% data release - $α.40$) which overlaps with a common volume in SDSS DR7. Based on the conditional HIMF we find that the luminous red, luminous blue and faint blue populations dominate the total HIMF at the high-mass end, knee and the low-mass end respectively. We use the conditional HIMF to derive the underlying distribution function of $Ω_{\text{HI}}$ (HI density parameter), $p(Ω_{\text{HI}})$, in the color-magnitude plane of galaxies. The distribution, $p(Ω_{\text{HI}})$, peaks in the blue cloud at $M_{\text{r}}^{\text{max}}=$ $-19.25, C_{\text{ur}}^{\text{max}}=1.44$ but is skewed. It has a long tail towards faint blue galaxies and luminous red galaxies. We argue that $p(Ω_{\text{HI}})$ can be used to reveal the underlying relation between cold gas, stellar mass and the star formation rate (SFR) in an unbiased way; that is the derived relation does not suffer from survey or sample selection.

astro-ph.GA↗

arXiv2020

The Population of Galaxies that Contribute to The HI Mass Function

We look at the contribution of different galaxy populations to the atomic hydrogen (HI) mass function (HIMF) and the HI density parameter, $Ω_{\text{HI}}$, in the local Universe. Our analysis is based on a sample of 7857 HI-selected galaxies selected from a volume common to the SDSS and ALFALFA surveys (40$\%$ catalog -- $α.40$). We define different populations of galaxies in the color(u-r)-magnitude($M_{\text{r}}$) plane and compute the HIMF for each of them. Additionally we compute the HIMF for dark galaxies; these are undetected in SDSS and represent $\sim 2\%$ of the total sample. We find that the luminous red population dominates the total HIMF for $\log_{10}(M_{\text{HI}}h^2_{70}/M_{\odot}) \geq 10.4$. The full red population -- luminous and faint -- represents about $\sim 17\%$ of the $Ω_{\text{HI}}$ budget, while that of the dark population is $\sim 3\%$. The HIMF about the knee, $\log_{10}(M_{\text{HI}}h^2_{70}/M_{\odot}) \in [8,10.4]$, is dominated by the faint and luminous blue populations, the latter dominating at larger masses in this interval. Their total contribution to $Ω_{\text{HI}}$ is $\sim 55-70\%$, the variation depending on the definition of population. The dominant populations at the low mass end, $\log_{10}(M_{\text{HI}}h^2_{70}/M_{\odot}) \leq 8.0$ are the faint blue and faint bluer populations, the latter's dominance being sensitive to its definition. The full blue (blue--bluer luminous and faint) population represents $\sim 80\%$ of $Ω_{\text{HI}}$. A bimodal HIMF suggested by our results is however not seen since the amplitude of the HIMF of the luminous red population is small compared to that of the luminous blue population.

astro-ph.GA↗

arXiv2019

Cosmological Simulation of Galaxy Groups and Clusters-I: Global Effect of Feedback from Active Galactic Nuclei

In this study we quantify the properties of the gas and dark matter around active galactic nuclei (AGN) in simulated galaxy groups and clusters and analyze the effect of AGN feedback on the surrounding intra-cluster (group) medium. Our results suggest downsizing of AGN luminosity with host halo mass, supporting the results obtained from clustering studies of AGN. By examining the temperature and density distribution of the gas in the vicinity of AGN we show that due to feedback from the central engine, the gas gets displaced from the centre of the group/cluster resulting in a reduction of the density but an enhancement of temperature. We show that these effects are pronounced at both high and low redshifts and propose new observables to study the effect of feedback in higher redshift galaxies. We also show that the average stellar mass is decreased in halos in the presence of AGN feedback confirming claims from previous studies. Our work for the first time uses a fully cosmological-hydrodynamic simulation to evaluate the global effects of AGN feedback on their host dark matter halos as well as galaxies at scales of galaxy groups and clusters.

astro-ph.GA↗

arXiv2016

Large-scale 3D mapping of the intergalactic medium using the Lyman Alpha Forest

Maps of the large-scale structure of the Universe at redshifts 2-4 can be made with the Lyman-alpha forest which are complementary to low redshift galaxy surveys. We apply the Wiener interpolation method of Caucci et al. to construct three-dimensional maps from sets of Lyman-alpha forest spectra taken from cosmological hydrodynamic simulations. We mimic some current and future quasar redshift surveys (BOSS, eBOSS and MS-DESI) by choosing similar sightline densities. We use these appropriate subsets of the Lyman-alpha absorption sightlines to reconstruct the full three dimensional Lyman-alpha flux field and perform comparisons between the true and the reconstructed fields. We study global statistical properties of the intergalactic medium (IGM) maps with auto-correlation and cross-correlation analysis, slice plots, local peaks and point by point scatter. We find that both the density field and the statistical proper- ties of the IGM are recovered well enough that the resulting IGM maps can be meaningfully considered to represent large-scale maps of the Universe in agreement with Caucci et al., on larger scales and for sparser sightlines than had been tested previously. Quantitatively, for sightline parameters comparable to current and near future surveys the correlation coefficient between true and reconstructed fields is r > 0.9 on scales > 30 h^-1 Mpc. The properties of the maps are relatively insensitive to the precise form of the covariance matrix used. The final BOSS quasar Lyman-alpha forest sample will allow maps to be made with a resolution of ~ 30 h^-1 Mpc over a volume of ~ 15 h^-3 Gpc^3 between redshifts 1.9 and 2.3.

astro-ph.CO↗

arXiv2015

Luminosity function of [OII] emission-line galaxies in the MassiveBlack-II simulation

We examine the luminosity function (LF) of [OII] emission-line galaxies in the high-resolution cosmological simulation MassiveBlack-II (MBII). From the spectral energy distribution of each galaxy, we select a sub-sample of star-forming galaxies at $0.06 \le z \le 3.0$ using the [OII] emission line luminosity L([OII]). We confirm that the specific star formation rate matches that in the GAMA survey. We show that the [OII] LF at z=1.0 from the MBII shows a good agreement with the LFs from several surveys below L([OII])=$10^{43.0}$ erg/s while the low redshifts ($z \le 0.3$) show an excess in the prediction of bright [OII] galaxies, but still displaying a good match with observations below L([OII])=$10^{41.6}$ erg/s. Based on the validity in reproducing the properties of [OII] galaxies at low redshift ($z \le 1$), we forecast the evolution of the [OII] LF at high redshift ($z \le 3$), which can be tested by upcoming surveys such as the HETDEX and DESI. The slopes of the LFs at bright and faint ends range from -3 to -2 showing minima at z=2. The slope of the bright end evolves approximately as 1/(z+1) at z=2 while the faint end evolves as ~3/(z+1) at $0.6 \le z \le 2$. In addition, a similar analysis is applied for the evolution of [OIII] LFs, which is to be explored in the forthcoming survey WFIRST-AFTA. Finally, we show that the auto-correlation function of [OII] and [OIII] emitting galaxies shows a rapid evolution from z=2 to 1.

astro-ph.GA↗

arXiv2015

Galaxy shapes and alignments in the MassiveBlack-II hydrodynamic and dark matter-only simulations

We compare the shapes and intrinsic alignments of galaxies in the MassiveBlack-II cosmological hydrodynamic simulation (MBII) to those in a dark matter-only (DMO) simulation performed with the same volume (100$h^{-1}$Mpc)$^{3}$, cosmological parameters, and initial conditions. Understanding the impact of baryonic physics on galaxy shapes and alignments and their relation to the dark matter distribution should prove useful to map the intrinsic alignments of galaxies from hydrodynamic to dark matter-only simulations. We find that dark matter subhalos are typically rounder in MBII, and the shapes of stellar matter in low mass galaxies are more misaligned with the shapes of the dark matter of the corresponding subhalos in the DMO simulation. At $z=0.06$, the fractional difference in the mean misalignment angle between MBII and DMO simulations varies from $\sim 28 \% - 12 \%$ in the mass range $10^{10.8} - 6.0 \times 10^{14} h^{-1}M_{\odot}$. We study the dark matter halo shapes and alignments as a function of radius, and find that while galaxies in MBII are more aligned with the inner parts of their dark matter subhalos, there is no radial trend in their alignments with the corresponding subhalo in the DMO simulation. This result highlights the importance of baryonic physics in determining the alignment of the galaxy with respect to the inner parts of the halo. Finally, we compare the ellipticity-direction (ED) correlation for galaxies to that for dark matter halos, finding that it is suppressed on all scales by stellar-dark matter misalignment. In the projected shape-density correlation ($w_{δ+}$), which includes ellipticity weighting, this effect is partially canceled by the higher mean ellipticities of the stellar component, but differences of order $30-40\%$ remain on scales $> 1$ Mpc over a range of subhalo masses, with scale-dependent effects below $1$ Mpc.

astro-ph.CO↗

arXiv2015

Intrinsic alignments of galaxies in the MassiveBlack-II simulation: analysis of two-point statistics

The intrinsic alignment of galaxies with the large-scale density field is an important astrophysical contaminant in upcoming weak lensing surveys. We present detailed measurements of the galaxy intrinsic alignments and associated ellipticity-direction (ED) and projected shape ($w_{g+}$) correlation functions for galaxies in the cosmological hydrodynamic MassiveBlack-II (MB-II) simulation. We carefully assess the effects on galaxy shapes, misalignment of the stellar component with the dark matter shape and two-point statistics of iterative weighted (by mass and luminosity) definitions of the (reduced and unreduced) inertia tensor. We find that iterative procedures must be adopted for a reliable measurement of the reduced tensor but that luminosity versus mass weighting has only negligible effects. Both ED and $w_{g+}$ correlations increase in amplitude with subhalo mass (in the range of $10^{10} - 6.0\times 10^{14}h^{-1}M_{\odot}$), with a weak redshift dependence (from $z=1$ to $z=0.06$) at fixed mass. At $z \sim 0.3$, we predict a $w_{g+}$ that is in reasonable agreement with SDSS LRG measurements and that decreases in amplitude by a factor of $\sim 5$--18 for galaxies in the LSST survey. We also compared the intrinsic alignments of centrals and satellites, with clear detection of satellite radial alignments within their host halos. Finally, we show that $w_{g+}$ (using subhalos as tracers of density) and $w_{δ+}$ (using dark matter density) predictions from the simulations agree with that of non-linear alignment models (NLA) at scales where the 2-halo term dominates in the correlations (and tabulate associated NLA fitting parameters). The 1-halo term induces a scale dependent bias at small scales which is not modeled in the NLA model.

astro-ph.CO↗