arXiv subjects

Yohan Dubois

Publications and source records attributed to Yohan Dubois.

At least 127 records · Page 7Linked to original sources

arXiv2020

GalICS 2.1: a new semianalytic model for cold accretion, cooling, feedback and their roles in galaxy formation

Dekel & Birnboim06 proposed that the mass-scale that separates late-type and early-type galaxies is linked to the critical halo mass for the propagation of a stable shock and showed that they could reproduce the observed bimodality scale for plausible values of the metallicity of the accreted gas and the shock radius. Here, we take their analysis one step further and present a new semianalytic model that computes the shock radius from first principles. This advancement allows us to compute the critical mass individually for each halo. Separating cold-mode and hot-mode accretion has little effect on the final galaxy masses if feedback does not preferentially couple to the hot gas. We also present an improved model for stellar feedback where 70% of the wind mass is in a cold galactic fountain with a shorter reaccretion timescale at high masses. The latter is the key mechanism that allows us to reproduce the low-mass end of the mass function of galaxies over the entire redshift range 0<z<2.5. Cooling must be mitigated to avoid overpredicting the number density of galaxies with stellar mass greater than a hundred billion Solar masses but is important to form intermediate-mass galaxies. At virial masses greater than three hundred billion Solar masses, cold accretion is more important at high z, where gas is accreted from smaller solid angles, but this is not true at lower masses because high-z filaments have lower metallicities. Our predictions are consistent with the observed metallicity evolution of the intergalactic medium at 0<z<5.

astro-ph.GA↗

arXiv2020

The impact of AGN feedback on the 1D power spectra from the Ly$α$ forest using the Horizon-AGN suite of simulations

The Lyman-$α$ forest is a powerful probe for cosmology, but it is also strongly impacted by galaxy evolution and baryonic processes such as Active Galactic Nuclei (AGN) feedback, which can redistribute mass and energy on large scales. We constrain the signatures of AGN feedback on the 1D power spectrum of the Lyman-$α$ forest using a series of eight hydro-cosmological simulations performed with the Adaptative Mesh Refinement code RAMSES. This series starts from the Horizon-AGN simulation and varies the sub-grid parameters for AGN feeding, feedback and stochasticity. These simulations cover the whole plausible range of feedback and feeding parameters according to the resulting galaxy properties. AGNs globally suppress the Lyman-$α$ power at all scales. On large scales, the energy injection and ionization dominate over the supply of gas mass from AGN-driven galactic winds, thus suppressing power. On small scales, faster cooling of denser gas mitigates the suppression. This effect increases with decreasing redshift. We provide lower and upper limits of this signature at nine redshifts between $z=4.25$ and $z=2.0$, making it possible to account for it at post-processing stage in future work given that running simulations without AGN feedback can save considerable amounts of computing resources. Ignoring AGN feedback in cosmological inference analyses leads to strong biases with 2\% shift on $σ_8$ and 1\% shift on $n_s$, which represents twice the standards deviation of the current constraints on $n_s$.

astro-ph.GA↗

arXiv2020

Cosmic ray feedback from supernovae in dwarf galaxies

The regulation of the baryonic content in dwarf galaxies is a long-standing problem. Supernovae (SNe) are supposed to play a key role in forming large-scale galactic winds by removing important amounts of gas from galaxies. SNe are efficient accelerators of non-thermal particles, so-called cosmic rays (CRs), which can substantially modify the dynamics of the gas and conditions to form large-scale galactic winds. We investigate how CR injection by SNe impacts the star formation and the formation of large-scale winds in dwarf galaxies, and whether it can produce galaxy star-formation rates (SFR) and wind properties closer to observations. We ran CR magneto-hydrodynamical simulations of dwarf galaxies at high resolution (9 pc) with the adaptive mesh refinement code ramses. Disc galaxies are embedded in isolated halos of mass of $10^{10}$ and $10^{11} \, \rm M_{\odot}$, and CRs are injected by SNe. We included CR isotropic and anisotropic diffusion with various diffusion coefficients, CR radiative losses, and CR streaming. The injection of CR energy into the interstellar medium smooths out the highest gas densities, which reduces the SFR by a factor of 2-3. Mass outflow rates are significantly greater with CR diffusion, by 2 orders of magnitudes for the higher diffusion coefficients. Without diffusion and streaming, CRs are inefficient at generating winds. CR streaming alone allows for the formation of winds but which are too weak to match observations. The formation of galactic winds strongly depends on the diffusion coefficient: for low coefficients, CR energy stays confined in high density regions where CR energy losses are highest, and higher coefficients, which allow for a more efficient leaking of CRs out of dense gas, produce stronger winds. CR diffusion leads to colder and denser winds than without CRs, and brings outflow rates and mass loading factors much closer to observations.

astro-ph.GA↗

arXiv2020

Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function

Recent deep surveys have unravelled a population of faint active galactic nuclei (AGN) in the high redshift Universe, leading to various discussions on their nature and their role during the Epoch of Reionization. We use cosmological radiation-hydrodynamics simulations of a bright galaxy at z = 6 ($M_\star \gtrsim 10^{10} M_\odot$) hosting an actively growing super-massive black hole to study the properties of these objects. In particular, we study how the black hole and the galaxy co-evolve and what is the relative contribution of the AGN and of the stellar populations to the luminosity budget of the system. We find that the feedback from the AGN has no strong effect on the properties of the galaxy, and does not increase the total ionizing luminosity of the host. The average escape fraction of our galaxy is around $f_{\rm esc} \sim 5\%$. While our galaxy would be selected as an AGN in deep X-ray surveys, most of the UV luminosity is originating from stellar populations. This confirms that there is a transition in the galaxy population from star forming galaxies to quasar hosts, with bright Lyman-Break Galaxies (LBGs) with $M_{\rm UV}$ around -22 falling in the overlap region. Our results also suggest that faint AGN do not contribute significantly to reionizing the Universe.

astro-ph.GA↗

arXiv2020

Why do extremely massive disc galaxies exist today?

Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios. Since mergers are the principal drivers of disc-to-spheroid morphological transformation, and the most massive galaxies typically have the richest merger histories, it is surprising that discs exist at all at the highest stellar masses (e.g. beyond the knee of the mass function). Using Horizon-AGN, a cosmological hydro-dynamical simulation, we show that extremely massive (M*> 10^11.4 MSun) discs are created via two channels. In the primary channel (accounting for ~70% of these systems and ~8% of massive galaxies) the most recent, significant merger (stellar mass ratio > 1:10) between a massive spheroid and a gas-rich satellite `spins up' the spheroid by creating a new rotational stellar component, leaving a massive disc as the remnant. In the secondary channel (accounting for ~30% of these systems and ~3% of massive galaxies), a system maintains a disc throughout its lifetime, due to an anomalously quiet merger history. Not unexpectedly, the fraction of massive discs is larger at higher redshift, due to the Universe being more gas-rich. The morphological mix of galaxies at the highest stellar masses is, therefore, a strong function of the gas fraction of the Universe. Finally, these massive discs have similar black-hole masses and accretion rates to massive spheroids, providing a natural explanation for why a minority of powerful AGN are surprisingly found in disc galaxies.

astro-ph.GA↗

arXiv2020

Detecting outliers in astronomical images with deep generative networks

With the advent of future big-data surveys, automated tools for unsupervised discovery are becoming ever more necessary. In this work, we explore the ability of deep generative networks for detecting outliers in astronomical imaging datasets. The main advantage of such generative models is that they are able to learn complex representations directly from the pixel space. Therefore, these methods enable us to look for subtle morphological deviations which are typically missed by more traditional moment-based approaches. We use a generative model to learn a representation of expected data defined by the training set and then look for deviations from the learned representation by looking for the best reconstruction of a given object. In this first proof-of-concept work, we apply our method to two different test cases. We first show that from a set of simulated galaxies, we are able to detect $\sim90\%$ of merging galaxies if we train our network only with a sample of isolated ones. We then explore how the presented approach can be used to compare observations and hydrodynamic simulations by identifying observed galaxies not well represented in the models.

astro-ph.GA↗

arXiv2020

Horizon-AGN virtual observatory -- 2: Template-free estimates of galaxy properties from colours

Using the Horizon-AGN hydrodynamical simulation and self-organising maps (SOMs), we show how to compress the complex data structure of a cosmological simulation into a 2-d grid which is much easier to analyse. We first verify the tight correlation between the observed 0.3$\!-\!5μ$m broad-band colours of Horizon-AGN galaxies and their high-resolution spectra. The correlation is found to extend to physical properties such as redshift, stellar mass, and star formation rate (SFR). This direct mapping from colour to physical parameter space is shown to work also after including photometric uncertainties that mimic the COSMOS survey. We then label the SOM grid with a simulated calibration sample and estimate redshift and SFR for COSMOS-like galaxies up to $z\sim3$. In comparison to state-of-the-art techniques based on synthetic templates, our method is comparable in performance but less biased at estimating redshifts, and significantly better at predicting SFRs. In particular our "data-driven" approach, in contrast to model libraries, intrinsically allows for the complexity of galaxy formation and can handle sample biases. We advocate that obtaining the calibration for this method should be one of the goals of next-generation galaxy surveys.

astro-ph.GA↗

arXiv2020

Properties of simulated galaxies and supermassive black holes in cosmic voids

Cosmic voids, the under-dense regions of the cosmic web, are widely used to constrain cosmology. Voids contain few, isolated galaxies, presumably expected to be less evolved and preserving memory of the pristine Universe. We use the cosmological hydrodynamical simulation Horizon-AGN coupled to the void finder {\sc \texttt{VIDE}} to investigate properties of galaxies in voids at z=0. We find that, closer to void centers, low-mass galaxies are more common than their massive counterparts. At fixed dark matter halo mass, they have smaller stellar masses than in denser regions. The star formation rate of void galaxies diminishes when approaching void centers, but their sSFR slightly increases, suggesting that void galaxies form stars more efficiently with respect to their stellar mass. We find that this can not only be attributed to the prevalence of low-mass galaxies. The inner region of voids also predominantly host low-mass BHs. However, the BH mass to galaxy mass ratios resemble those of the whole simulation at z=0. Our results suggest that even if the growth channels in cosmic voids are different than in denser environments, voids grow their galaxies and BHs in a similar way. While a large fraction of the BHs have low Eddington ratios, we find that 20\% could be observed as AGN with log10 L=41.5-42.5 erg/s in hard X-ray (2-10 keV). These results pave the way to future work with larger next-generation hydro simulations, aiming to confirm our findings and prepare the application on data from upcoming large surveys such as PFS, Euclid and WFIRST.

astro-ph.GA↗

arXiv2020

Real galaxy mergers from galaxy pair catalogs

Mergers of galaxies are extremely violent events shaping their evolution. Such events are thought to trigger starbursts and, possibly, black hole accretion. Nonetheless, it is still not clear how to know the fate of a galaxy pair from the data available at a given time, limiting our ability to constrain the exact role of mergers. In this paper we use the lightcone of the Horizon-AGN simulation, for which we know the fate of each pair, to test three selection processes aiming at identifying true merging pairs. We find that the simplest one (selecting objects within two thresholds on projected distance $d$ and redshift difference $Δz$) gives similar results than the most complex one (based on a neural network analyzing $d$, $Δz$, redshift of the primary, masses/star formation rates/aspect ratio of both galaxies). Our best thresholds are $d_\mathrm{th}\sim100\mathrm{\, kpc}$ and $Δz_\mathrm{th} \sim 10^{-3}$, in agreement with recent results.

astro-ph.GA↗

arXiv2020

The impact of AGN feedback on galaxy intrinsic alignments in the Horizon simulations

The intrinsic correlations of galaxy shapes and orientations across the large-scale structure of the Universe are a known contaminant to weak gravitational lensing. They are known to be dependent on galaxy properties, such as their mass and morphologies. The complex interplay between alignments and the physical processes that drive galaxy evolution remains vastly unexplored. We assess the sensitivity of intrinsic alignments (shapes and angular momenta) to Active Galactic Nuclei -AGN- feedback by comparing galaxy alignment in twin runs of the cosmological hydrodynamical Horizon simulation, which do and do not include AGN feedback respectively. We measure intrinsic alignments in three dimensions and in projection at z=0 and z=1. We find that the projected alignment signal of all galaxies with resolved shapes with respect to the density field in the simulation is robust to AGN feedback, thus giving similar predictions for contamination to weak lensing. The relative alignment of galaxy shapes around galaxy positions is however significantly impacted, especially when considering high-mass ellipsoids. Using a sample of galaxy "twins" across simulations, we determine that AGN changes both the galaxy selection and their actual alignments. Finally, we measure the alignments of angular momenta of galaxies with their nearest filament. Overall, these are more significant in the presence of AGN as a result of the higher abundance of massive pressure-supported galaxies.

astro-ph.CO↗

arXiv2019

On the number density of "direct collapse" black hole seeds

Supermassive black holes (BHs) of millions solar masses and above reside in the center of most local galaxies, but they also power active galactic nuclei and quasars, detected up to z=7. This observational evidence puts strong constraints on the BH growth and the mass of the first BH seeds. The scenario of "direct collapse" is very appealing as it leads to the formation of large mass BH seeds in the range 10^4-10^6 Msun, which eases explaining how quasars at z=6-7 are powered by BHs with masses >10^9 Msun. Direct collapse, however, appears to be rare, as the conditions required by the scenario are that gas is metal-free, the presence of a strong photo-dissociating Lyman-Werner flux, and large inflows of gas at the center of the halo, sustained for 10-100 Myr. We performed several cosmological hydrodynamical simulations that cover a large range of box sizes and resolutions, thus allowing us to understand the impact of several physical processes on the distribution of direct collapse BHs. We identify halos where direct collapse can happen, and derive the number density of BHs. We also investigate the discrepancies between hydrodynamical simulations, direct or post-processed, and semi-analytical studies. We find that for direct collapse to account for BHs in normal galaxies, the critical Lyman-Werner flux required for direct collapse must be much less than predicted by 3D simulations that include detailed chemical models. However, when supernova feedback is relatively weak, enough direct collapse BHs to explain z=6-7 quasars can be obtained for more realistic values of the critical Lyman Werner flux.

astro-ph.GA↗

arXiv2019

The diverse galaxy counts in the environment of high-redshift massive black holes in Horizon-AGN

High-redshift quasars are believed to reside in highly biased regions of the Universe, where black hole (BH) growth is sustained by an enhanced number of mergers and by being at the intersection of filaments bringing fresh gas. This assumption should be supported by an enhancement of the number counts of galaxies in the field of view of quasars. While the current observations of quasar environments do not lead to a consensus on a possible excess of galaxies, the future missions JWST, WFIRST, and Euclid will provide new insights on quasar environments, and will substantially increase the number of study-cases. We are in a crucial period, where we need to both understand the current observations and predict how upcoming missions will improve our understanding of BH environments. Using the large-scale simulation Horizon-AGN, we find that statistically the most massive BHs reside in environments with the largest number counts of galaxies. However, we find a large variance in galaxy number counts, and some massive BHs do not show enhanced counts in their neighborhood. Interestingly, some massive BHs have a very close galaxy companion but no further enhancement of the galaxy number counts at larger scales, in agreement with recent observations. We find that AGN feedback in the surrounding galaxies is able to decrease their luminosity and stellar mass, and therefore to make them un-observable when using restrictive galaxy selection criteria. Radiation from the quasars can spread over large distances, which could affect the formation history of surrounding galaxies, but a careful analysis of these processes requires radiative transfer simulations.

astro-ph.GA↗

arXiv2019

Cosmological evolution of orientations of cluster-sized dark matter haloes and their central galaxies in the Horizon-AGN simulation

It is known observationally that the major axes of galaxy clusters and their brightest cluster galaxies are roughly aligned with each other. To understand the origin of the alignment, we identify 40 cluster-sized dark matter (DM) haloes with masses higher than $5\times10^{13}~M_{\odot}$ and their central galaxies (CGs) at $z\approx 0$ in the Horizon-AGN cosmological hydrodynamical simulation. We trace the progenitors at 50 different epochs between $0 1$. The alignment becomes tighter with cosmic time; the major axes of the CGs and their host DM haloes at present are aligned on average within $\sim 30^{\circ}$ in the three dimensional space and $\sim 20^{\circ}$ in the projected plane. The orientations of the major axes of DM haloes on average follow one of the eigen-vectors of the surrounding tidal field that corresponds to the {\it slowest collapsing} (or even stretching) mode, and the alignment with the tidal field also becomes tighter. This implies that the orientations of CGs and DM haloes at the present epoch are largely imprinted in the primordial density field of the Universe, whereas strong dynamical interactions such as mergers are important to explain their mutual alignment at each epoch.

astro-ph.CO↗

arXiv2019

When galaxies align: intrinsic alignments of the progenitors of elliptical galaxies in the Horizon-AGN simulation

Elliptical galaxies today appear aligned with the large-scale structure of the Universe, but it is still an open question when they acquire this alignment. Observational data is currently insufficient to provide constraints on the time evolution of intrinsic alignments, and hence existing models range from assuming that galaxies gain some primordial alignment at formation, to suggesting that they react instantaneously to tidal interactions with the large-scale structure. Using the cosmological hydrodynamical simulation Horizon-AGN, we measure the relative alignments between the major axes of galaxies and eigenvectors of the tidal field as a function of redshift. We focus on constraining the time evolution of the alignment of the main progenitors of massive $z=0$ elliptical galaxies, the main weak lensing contaminant at low redshift. We show that this population, which at $z=0$ has a stellar mass above $10^{10.4}$ M$_\odot$, transitions from having no alignment with the tidal field at $z=3$, to a significant alignment by $z=1$. From $z=0.5$ they preserve their alignment at an approximately constant level until $z=0$. We find a mass-dependence of the alignment signal of elliptical progenitors, whereby ellipticals that are less massive today ($10^{10.4}<M/{\rm M}_\odot<10^{10.7}$) do not become aligned till later redshifts ($z<2$), compared to more massive counterparts. We also present an extended study of progenitor alignments in the parameter space of stellar mass and galaxy dynamics, the impact of shape definition and tidal field smoothing.

astro-ph.CO↗

arXiv2019

Comparing Galaxy Clustering in Horizon-AGN Simulated Lightcone Mocks and VIDEO Observations

Hydrodynamical cosmological simulations have recently made great advances in reproducing galaxy mass assembly over cosmic time - as often quantified from the comparison of their predicted stellar mass functions to observed stellar mass functions from data. In this paper we compare the clustering of galaxies from the hydrodynamical cosmological simulated lightcone Horizon-AGN, to clustering measurements from the VIDEO survey observations. Using mocks built from a VIDEO-like photometry, we first explore the bias introduced into clustering measurements by using stellar masses and redshifts derived from SED-fitting, rather than the intrinsic values. The propagation of redshift and mass statistical and systematic uncertainties in the clustering measurements causes us to underestimate the clustering amplitude. We find then that clustering and halo occupation distribution (HOD) modelling results are qualitatively similar in Horizon-AGN and VIDEO. However at low stellar masses Horizon-AGN underestimates the observed clustering by up to a factor of ~3, reflecting the known excess stellar mass to halo mass ratio for Horizon-AGN low mass haloes, already discussed in previous works. This reinforces the need for stronger regulation of star formation in low mass haloes in the simulation. Finally, the comparison of the stellar mass to halo mass ratio in the simulated catalogue, inferred from angular clustering, to that directly measured from the simulation, validates HOD modelling of clustering as a probe of the galaxy-halo connection.

astro-ph.GA↗

arXiv2019

The impact of the connectivity of the cosmic web on the physical properties of galaxies at its nodes

We investigate the impact of the number of filaments connected to the nodes of the cosmic web on the physical properties of their galaxies using the Sloan Digital Sky Survey. We compare these measurements to the cosmological hydrodynamical simulations Horizon-(no)AGN and Simba. We find that more massive galaxies are more connected, in qualitative agreement with theoretical predictions and measurements in dark matter only simulation. The star formation activity and morphology of observed galaxies both display some dependence on the connectivity of the cosmic web at fixed stellar mass: less star forming and less rotation supported galaxies also tend to have higher connectivity. These results qualitatively hold both for observed and virtual galaxies, and can be understood given that the cosmic web is the main source of fuel for galaxy growth. The simulations show the same trends at fixed halo mass, suggesting that the geometry of filamentary infall impacts galaxy properties beyond the depth of the local potential well. Based on simulations, it is also found that AGN feedback is key in reversing the relationship between stellar mass and connectivity at fixed halo mass. Technically, connectivity is a practical observational proxy for past and present accretion (minor mergers or diffuse infall).

astro-ph.GA↗

arXiv2019

Shock-accelerated cosmic rays and streaming instability in the adaptive mesh refinement code Ramses

Cosmic rays (CRs) are thought to play a dynamically important role in several key aspects of galaxy evolution, including the structure of the interstellar medium, the formation of galactic winds, and the non-thermal pressure support of halos. We introduce a numerical model solving for the CR streaming instability and acceleration of CRs at shocks with a fluid approach in the adaptive mesh refinement code ramses. CR streaming is solved with a diffusion approach and its anisotropic nature is naturally captured. We introduce a shock finder for the ramses code that automatically detects shock discontinuities in the flow. Shocks are the loci for CR injection, and their efficiency of CR acceleration is made dependent on the upstream magnetic obliquity according to the diffuse shock acceleration mechanism. We show that the shock finder accurately captures shock locations and estimates the shock Mach number for several problems. The obliquity-dependent injection of CRs in the Sedov solution leads to situations where the supernova bubble exhibits large polar caps (homogeneous background magnetic field), or a patchy structure of the CR distribution (inhomogeneous background magnetic field). Finally, we combine both accelerated CRs with streaming in a simple turbulent interstellar medium box, and show that the presence of CRs significantly modifies the structure of the gas.

astro-ph.GA↗

arXiv2019

New Horizon: On the origin of the stellar disk and spheroid of field galaxies at $z=0.7$

The origin of the disk and spheroid of galaxies has been a key open question in understanding their morphology. Using the high-resolution cosmological simulation, New Horizon, we explore kinematically decomposed disk and spheroidal components of 144 field galaxies with masses greater than $\rm 10^9\,M_{\odot}$ at $z=0.7$. The origins of stellar particles are classified according to their birthplace (in situ or ex situ) and their orbits at birth. Before disk settling, stars form mainly through chaotic mergers between proto-galaxies and become part of the spheroidal component. When disk settling starts, we find that more massive galaxies begin to form disk stars from earlier epochs; massive galaxies commence to develop their disks at $z\sim1-2$, while low-mass galaxies do after $z\sim1$. The formation of disks is affected by accretion as well, as mergers can trigger gas turbulence or induce misaligned gas infall that prevents galaxies from forming co-rotating disk stars. The importance of accreted stars is greater in more massive galaxies, especially in developing massive spheroids. A significant fraction of the spheroids comes from the disk stars that are perturbed, which becomes more important at lower redshifts. Some ($\sim12.5\%$) of our massive galaxies develop counter-rotating disks from the gas infall misaligned with the existing disk plane, which can last for more than a Gyr until they become the dominant component, and flip the angular momentum of the galaxy in the opposite direction. The final disk-to-total ratio of a galaxy needs to be understood in relation to its stellar mass and accretion history. We quantify the significance of the stars with different origins and provide them as guiding values.

astro-ph.GA↗