We investigate the observability of cold accretion streams at redshift 3 via Lyα emission and the feasibility of cold accretion as the main driver of Lyα blobs (LABs). We run cosmological zoom ...simulations focusing on three haloes spanning almost two orders of magnitude in mass, roughly from 1011 to
. We use a version of the ramses code that includes radiative transfer of ultraviolet (UV) photons, and we employ a refinement strategy that allows us to resolve accretion streams in their natural environment to an unprecedented level. For the first time in a simulation, we self-consistently model self-shielding in the cold streams from the cosmological UV background, which enables us to predict their temperatures, ionization states and Lyα luminosities with improved accuracy. We find the efficiency of gravitational heating in cold streams in a
halo to be around 10-20 per cent throughout most of the halo but reaching much higher values close to the centre. As a result, most of the Lyα luminosity comes from gas which is concentrated at the central 20 per cent of the halo radius, leading to Lyα emission which is not extended. In more massive haloes, of
, cold accretion is complex and disrupted, and gravitational heating does not happen as a steady process. Ignoring the factors of Lyα scattering, local UV enhancement and supernovae feedback, the cold 'messy' accretion alone in these massive haloes can produce LABs that largely agree with observations in terms of morphology, extent and luminosity. Our simulations slightly and systematically overpredict LAB abundances, perhaps hinting that the interplay of these ignored factors may have a negative net effect on extent and luminosity. We predict that a factor of a few increase in sensitivity from current observational limits should unambiguously reveal continuum-free accretion streams around massive galaxies at z= 3.
We present a new implementation of radiation hydrodynamics (RHD) in the adaptive mesh refinement (AMR) code ramses. The multigroup radiative transfer (RT) is performed on the AMR grid with a ...first-order Godunov method using the M1 closure for the Eddington tensor, and is coupled to the hydrodynamics via non-equilibrium thermochemistry of hydrogen and helium. This moment-based approach has the great advantage that the computational cost is independent of the number of radiative sources - it can even deal with continuous regions of emission such as bound-free emission from gas. As it is built directly into ramses, the RT takes natural advantage of the refinement and parallelization strategies already in place. Since we use an explicit advection solver for the radiative transport, the time-step is restricted by the speed of light - a severe limitation that can be alleviated using the so-called reduced speed of light approximation. We propose a rigorous framework to assess the validity of this approximation in various conditions encountered in cosmology and galaxy formation. We finally perform with our newly developed code a complete suite of RHD tests, comparing our results to other RHD codes. The tests demonstrate that our code performs very well and is ideally suited for exploring the effect of radiation on current scenarios of structure and galaxy formation.
We use semi-analytic techniques to study the formation and evolution of brightest cluster galaxies (BCGs). We show the extreme hierarchical nature of these objects and discuss the limitations of ...simple ways to capture their evolution. In a model where cooling flows are suppressed at late times by active galactic nucleus (AGN) activity, the stars of BCGs are formed very early (50 per cent at z∼ 5, 80 per cent at z∼ 3) and in many small galaxies. The high star formation rates in these high-z progenitors are fuelled by rapid cooling, not by merger-triggered starbursts. We find that model BCGs assemble surprisingly late: half their final mass is typically locked up in a single galaxy after z∼ 0.5. Because most of the galaxies accreted on to BCGs have little gas content and red colours, late mergers do not change the apparent age of BCGs. It is this accumulation of a large number of old stellar populations – driven mainly by the merging history of the dark matter halo itself – that yields the observed homogeneity of BCG properties. In the second part of the paper, we discuss the evolution of BCGs to high redshifts, from both observational and theoretical viewpoints. We show that our model BCGs are in qualitative agreement with high-z observations. We discuss the hierarchical link between high-z BCGs and their local counterparts. We show that high-z BCGs belong to the same population as the massive end of local BCG progenitors, although they are not in general the same galaxies. Similarly, high-z BCGs end up as massive galaxies in the local Universe, although only a fraction of them are actually BCGs of massive clusters.
A large-scale hydrodynamical cosmological simulation, Horizon-AGN, is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more ...than 150 000 galaxies per time step in the redshift range 1.2 < z < 1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3 × 1010 M⊙. Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.
Context.
Resonant lines are powerful probes of the interstellar and circumgalactic medium of galaxies. Their transfer in gas being a complex process, the interpretation of their observational ...signatures, either in absorption or in emission, is often not straightforward. Numerical radiative transfer simulations are needed to accurately describe the travel of resonant line photons in real and in frequency space, and to produce realistic mock observations.
Aims.
This paper introduces RASCAS, a new public 3D radiative transfer code developed to perform the propagation of any resonant line in numerical simulations of astrophysical objects. RASCAS was designed to be easily customisable and to process simulations of arbitrarily large sizes on large supercomputers.
Methods.
RASCAS performs radiative transfer on an adaptive mesh with an octree structure using the Monte Carlo technique. RASCAS features full MPI parallelisation, domain decomposition, adaptive load-balancing, and a standard peeling algorithm to construct mock observations. The radiative transport of resonant line photons through different mixes of species (e.g. H
I
, Si
II
, Mg
II
, Fe
II
), including their interaction with dust, is implemented in a modular fashion to allow new transitions to be easily added to the code.
Results.
RASCAS is very accurate and efficient. It shows perfect scaling up to a minimum of a thousand cores. It has been fully tested against radiative transfer problems with analytic solutions and against various test cases proposed in the literature. Although it was designed to describe accurately the many scatterings of line photons, RASCAS may also be used to propagate photons at any wavelength (e.g. stellar continuum or fluorescent lines), or to cast millions of rays to integrate the optical depths of ionising photons, making it highly versatile.
We report the discovery of diffuse extended Ly α emission from redshift 3.1 to 4.5, tracing cosmic web filaments on scales of 2.5−4 cMpc. These structures have been observed in overdensities of Ly α ...emitters in the MUSE Extremely Deep Field, a 140 h deep MUSE observation located in the Hubble Ultra-Deep Field. Among the 22 overdense regions identified, five are likely to harbor very extended Ly α emission at high significance with an average surface brightness of 5 × 10 −20 erg s −1 cm −2 arcsec −2 . Remarkably, 70% of the total Ly α luminosity from these filaments comes from beyond the circumgalactic medium of any identified Ly α emitter. Fluorescent Ly α emission powered by the cosmic UV background can only account for less than 34% of this emission at z ≈ 3 and for not more than 10% at higher redshift. We find that the bulk of this diffuse emission can be reproduced by the unresolved Ly α emission of a large population of ultra low-luminosity Ly α emitters (< 10 40 erg s −1 ), provided that the faint end of the Ly α luminosity function is steep ( α ⪅ −1.8), it extends down to luminosities lower than 10 38 − 10 37 erg s −1 , and the clustering of these Ly α emitters is significant (filling factor < 1/6). If these Ly α emitters are powered by star formation, then this implies their luminosity function needs to extend down to star formation rates < 10 −4 M ⊙ yr −1 . These observations provide the first detection of the cosmic web in Ly α emission in typical filamentary environments and the first observational clue indicating the existence of a large population of ultra low-luminosity Ly α emitters at high redshift.
We discuss the dynamics of parton cascades that develop in dense QCD matter, and contrast their properties with those of similar cascades of gluon radiation in vacuum. We argue that such cascades ...belong to two distinct classes that are characterized respectively by an increasing or a constant (or decreasing) branching rate along the cascade. In the former class, of which the BDMPS, medium-induced, cascade constitutes a typical example, it takes a finite time to transport a finite amount of energy to very soft quanta, while this time is essentially infinite in the latter case, to which the DGLAP cascade belongs. The medium induced cascade is accompanied by a constant flow of energy towards arbitrary soft modes, leading eventually to the accumulation of the initial energy of the leading particle at zero energy. It also exhibits scaling properties akin to wave turbulence. These properties do not show up in the cascade that develops in vacuum. There, the energy accumulates in the spectrum at smaller and smaller energy as the cascade develops, but the energy never flows all the way down to zero energy. Our analysis suggests that the way the energy is shared among the offsprings of a splitting gluon has little impact on the qualitative properties of the cascades, provided the kernel that governs the splittings is not too singular.
Abstract
Absorption line spectroscopy offers one of the best opportunities to constrain the properties of galactic outflows and the environment of the circumgalactic medium. Extracting physical ...information from line profiles is difficult; however, for the physics governing the underlying radiation transfer is complicated and depends on many different parameters. Idealized analytical models are necessary to constrain the large parameter spaces efficiently, but are typically plagued by model degeneracy and systematic errors. Comparison tests with idealized numerical radiation transfer codes offer an excellent opportunity to confront both of these issues. In this paper, we present a detailed comparison between SALT, an analytical radiation transfer model for predicting UV spectra of galactic outflows, with the numerical radiation transfer software, RASCAS. Our analysis has led to upgrades to both models including an improved derivation of SALT and a customizable adaptive mesh refinement routine for RASCAS. We explore how well SALT, when paired with a Monte Carlo fitting procedure, can recover flow parameters from nonturbulent and turbulent flows. Overall we find that turbulence leads to biases in the recovery of kinematic parameters and the optical depth, but find that derived quantities (e.g., mass outflow rates, column density, etc.) are still well recovered. From the analysis, we estimate average uncertainties in our ability to measure metal flow rates spanning 0.65 (0.95) dex in
M
⊙
yr
−1
and uncertainties spanning 0.54 (0.94) dex in cm
−2
for column densities at a resolution of 20 (100) km s
−1
and signal-to-noise ratio = 10.
We report the detection of extended Lyα emission around individual star-forming galaxies at redshifts z = 3−6 in an ultradeep exposure of the Hubble Deep Field South obtained with MUSE on the ...ESO-VLT. The data reach a limiting surface brightness (1σ) of ~1 × 10-19 erg s-1 cm-2 arcsec-2 in azimuthally averaged radial profiles, an order of magnitude improvement over previous narrowband imaging. Our sample consists of 26 spectroscopically confirmed Lyα-emitting, but mostly continuum-faint (mAB ≳ 27) galaxies. In most objects the Lyα emission is considerably more extended than the UV continuum light. While five of the faintest galaxies in the sample show no significantly detected Lyα haloes, the derived upper limits suggest that this is due to insufficient S/N. Lyα haloes therefore appear to be ubiquitous even for low-mass (~ 108−109 M⊙) star-forming galaxies at z > 3. We decompose the Lyα emission of each object into a compact component tracing the UV continuum and an extended halo component, and infer sizes and luminosities of the haloes. The extended Lyα emission approximately follows an exponential surface brightness distribution with a scale length of a few kpc. While these haloes are thus quite modest in terms of their absolute sizes, they are larger by a factor of 5−15 than the corresponding rest-frame UV continuum sources as seen by HST. They are also much more extended, by a factor ~5, than Lyα haloes around low-redshift star-forming galaxies. Between ~40% and ≳90% of the observed Lyα flux comes from the extended halo component, with no obvious correlation of this fraction with either the absolute or the relative size of the Lyα halo. Our observations provide direct insights into the spatial distribution of at least partly neutral gas residing in the circumgalactic medium of low to intermediate mass galaxies at z > 3.
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