37 research outputs found
Modeling Lyman-α Forest Cross-Correlations with LyMAS
No full textWe use the Ly- Mass Association Scheme (LyMAS; Peirani et al. 2014) to predict cross-correlations at between dark matter halos and transmitted flux in the Ly- forest, and compare to cross-correlations measured for quasars and damped Ly- systems (DLAs) from the Baryon Oscillation Spectroscopic Survey (BOSS) by Font-Ribera et al. (2012, 2013). We calibrate LyMAS using Horizon-AGN hydrodynamical cosmological simulations of a comoving volume. We apply this calibration to a simulation realized with dark matter particles. In the 100 Mpc box, LyMAS reproduces the halo-flux correlations computed from the full hydrodynamic gas distribution very well. In the 1 Gpc box, the amplitude of the large scale cross-correlation tracks the halo bias as expected. We provide empirical fitting functions that describe our numerical results. In the transverse separation bins used for the BOSS analyses, LyMAS cross-correlation predictions follow linear theory accurately down to small scales. Fitting the BOSS measurements requires inclusion of random velocity errors; we find best-fit RMS velocity errors of 399 km s and 252 km s for quasars and DLAs, respectively. We infer bias-weighted mean halo masses of and for the host halos of quasars and DLAs, with dex systematic uncertainty associated with redshift evolution, IGM parameters, and selection of data fitting range
How do galaxies build up their spin in the cosmic web?
No full textUsing the Horizon-AGN simulation we find a mass dependent spin orientation trend for galaxies: the spin of low-mass, rotation-dominated, blue, star-forming galaxies are preferentially aligned with their closest filament, whereas high-mass, velocity dispersion- supported, red quiescent galaxies tend to possess a spin perpendicular to these filaments. We explore the physical mechanisms driving galactic spin swings and quantify how much mergers and smooth accretion re-orient them relative to their host filaments
The role of minor mergers in the recent star formation history of early-type galaxies
No full textInternational audienceWe demonstrate that the large scatter in the ultraviolet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation (RSF) in these objects can be reproduced by minor mergers in the standard Λ cold dark matter (ΛCDM) cosmology. Numerical simulations of mergers with mass ratios =20 per cent. Early-types that satisfy (NUV - r) <~ 3.8 are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ~1.5 Gyr, while those that satisfy 3.8 < (NUV - r) < 5.5 are consistent with either recent mergers with mass ratios <=1:6 or mergers with higher mass ratios that occurred more than ~1.5 Gyr in the past. We demonstrate that the early-type colour-magnitude relations and colour distributions, in both the UV and optical spectral ranges, are consistent with the expected frequency of minor merging activity in the standard ΛCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level RSF observed in early-type galaxies in the nearby Universe
Statistics of the projected angles between the black-hole spin and the host-galaxy rotation axes from NewHorizon
No full textInternational audienceUnderstanding the alignment between AGN jets and their host galaxies is crucial for interpreting AGN unification models, jet feedback processes, and the co-evolution of galaxies and their central black holes (BH). In this study, we use the high-resolution cosmological zoom-in simulation NewHorizon, which self-consistently evolves BH mass and spin, to statistically examine the relationship between AGN jet orientation and host galaxy structure. Building upon our previous work, we extend the analysis of projected (2-d) alignment angles to facilitate more direct comparisons with recent observational studies. In our methodology, galaxy orientations are estimated using optical position angles derived from synthetic DESI-LS and Euclid images, while BH spin vectors serve as proxies for AGN jet directions. From a carefully selected sample of 100 BH-galaxy systems at low redshift, we generate a catalog of 5,000 mock optical images using a Monte Carlo approach that samples random viewing angles and redshifts. Our results reveal a statistically significant tendency for AGN jets to align with the orientation of their host galaxies, consistent with recent observations combining Very Long Baseline Interferometry (VLBI) and optical imaging of nearby AGNs. Furthermore, we find a slightly stronger alignment when using kinematic position angles derived from synthetic MaNGA-like stellar velocity fields. These findings underscore the importance of combining morphological, kinematic, and polarimetric information to disentangle the complex interplay between black hole spin evolution, accretion mode, and the galactic environment in shaping the direction of relativistic jets
The rise and fall of stellar discs across the peak of cosmic star formation history: mergers versus smooth accretion
No full textBuilding galaxy merger trees from a state-of-the-art cosmological hydrodynamics simulation, Horizon-AGN, we perform a statistical study of how mergers and smooth accretion drive galaxy morphologic properties above . More specifically, we investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that smooth accretion tends to flatten small galaxies over cosmic time, leading to the formation of disks. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar disks, confirming the origin of elliptical galaxies. We also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size-mass evolution r \prop M^{1.2} instead of r \prop M^{-0.5} - M^{0.5} depending on the merger mass ratio. The gas content drive the size-mass evolution due to merger with a faster size growth for gas-poor galaxies r \prop M^2 than for gas-rich galaxies r \prop M
Projected Axis Ratios of Galaxy Clusters in the Horizon-AGN Simulation: Impact of Baryon Physics and Comparison with Observations
No full textInternational audienceWe characterize the non-sphericity of galaxy clusters by the projected axis ratio of spatial distribution of star, dark matter, and X-ray surface brightness (XSB). We select 40 simulated groups and clusters of galaxies with mass larger than 5 × 10^13 M_⊙ from the Horizon simulation that fully incorporates the relevant baryon physics, in particular, the active galactic nucleus feedback. We find that the baryonic physics around the central region of galaxy clusters significantly affects the non-sphericity of dark matter distribution even beyond the central region, approximately up to half of the virial radius. Therefore it is very difficult to predict the probability density function (PDF) of the projected axis ratio of XSB from dark-matter-only N-body simulations as attempted in previous studies. Indeed, we find that the PDF derived from our simulated clusters exhibits much better agreement with that from the observed X-ray clusters. This indicates that our present methodology to estimate the non-sphericity directly from the Horizon simulation is useful and promising. Further improvements in both numerical modeling and observational data will establish the non-sphericity of clusters as a cosmological test complementary to more conventional statistics based on spherically averaged quantities
LyAl-Net: A high-efficiency Lyman- forest simulation with a neural network
No full textInternational audienceThe inference of cosmological quantities requires accurate and large hydrodynamical cosmological simulations. Unfortunately, their computational time can take millions of CPU hours for a modest coverage in cosmological scales (). The possibility to generate large quantities of mock Lyman- observations opens up the possibility of much better control on covariance matrices estimate for cosmological parameters inference, and on the impact of systematics due to baryonic effects. We present a machine learning approach to emulate the hydrodynamical simulation of intergalactic medium physics for the Lyman- forest called LyAl-Net. The main goal of this work is to provide highly efficient and cheap simulations retaining interpretation abilities about the gas field level, and as a tool for other cosmological exploration. We use a neural network based on the U-net architecture, a variant of convolutional neural networks, to predict the neutral hydrogen physical properties, density, and temperature. We train the LyAl-Net model with the Horizon-noAGN simulation, though using only 9% of the volume. We also explore the resilience of the model through tests of a transfer learning framework using cosmological simulations containing different baryonic feedback. We test our results by analysing one and two-point statistics of emulated fields in different scenarios, as well as their stochastic properties. The ensemble average of the emulated Lyman- forest absorption as a function of redshift lies within 2.5% of one derived from the full hydrodynamical simulation. The computation of individual fields from the dark matter density agrees well with regular physical regimes of cosmological fields. The results tested on IllustrisTNG100 showed a drastic improvement in the Lyman- forest flux without arbitrary rescaling
Formation of Warped Disks by Galactic Flyby Encounters. I. Stellar Disks
No full textInternational audienceWarped disks are almost ubiquitous among spiral galaxies. Here we revisit and test the "flyby scenario" of warp formation, in which impulsive encounters between galaxies are responsible for warped disks. Based on N-body simulations, we investigate the morphological and kinematical evolution of the stellar component of disks when galaxies undergo flyby interactions with adjacent dark matter halos. We find that the so-called "S"-shaped warps can be excited by flybys and sustained for even up to a few billion years, and that this scenario provides a cohesive explanation for several key observations. We show that disk warp properties are governed primarily by the following three parameters: (1) the impact parameter, i.e., the minimum distance between two halos; (2) the mass ratio between two halos; and (3) the incident angle of the flyby perturber. The warp angle is tied up with all three parameters, yet the warp lifetime is particularly sensitive to the incident angle of the perturber. Interestingly, the modeled S-shaped warps are often non-symmetric depending on the incident angle. We speculate that the puzzling U- and L-shaped warps are geometrically superimposed S-types produced by successive flybys with different incident angles, including multiple interactions with a satellite on a highly elongated orbit
The rise and fall of stellar across the peak of cosmic star formation history: effects of mergers versus diffuse stellar mass acquisition
No full textInternational audienceBuilding galaxy merger trees from a state-of-the-art cosmological hydrodynamical simulation, Horizon-AGN, we perform a statistical study of how mergers and diffuse stellar mass acquisition processes drive galaxy morphologic properties above z > 1. By diffuse mass acquisition here, we mean both accretion of stars by unresolved mergers (relative stellar mass growth smaller than 4.5 per cent) as well as in situ star formation when no resolved mergers are detected along the main progenitor branch of a galaxy. We investigate how stellar densities, galaxy sizes and galaxy morphologies (defined via shape parameters derived from the inertia tensor of the stellar density) depend on mergers of different mass ratios. We investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that diffuse stellar accretion and in situ formation tend to flatten small galaxies over cosmic time, leading to the formation of discs. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar discs, confirming the origin of elliptical galaxies. We confirm that mergers grow galaxy sizes more efficiently than diffuse processes ( and on average, respectively) and we also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size–mass evolution instead of for discs depending on the merger mass ratio. The gas content drives the size–mass evolution due to merger with a faster size growth for gas-poor galaxies than for gas-rich galaxies r_0.5 ∝ M_s
