Abstract
We propose a new method for finding galaxy protoclusters that is motivated by structure formation theory and also directly applicable to observations. We adopt the conventional definition ...that a protocluster is a galaxy group whose virial mass
M
vir
<
M
cl
at its epoch, where
M
cl
= 10
14
M
⊙
, but would exceed that limit when it evolves to
z
= 0. We use the critical overdensity for complete collapse at
z
= 0 predicted by the spherical top-hat collapse model to find the radius and total mass of the regions that would collapse at
z
= 0. If the mass of a region centered at a massive galaxy exceeds
M
cl
, the galaxy is at the center of a protocluster. We define the outer boundary of a protocluster as the zero-velocity surface at the turnaround radius so that the member galaxies are those sharing the same protocluster environment and showing some conformity in physical properties. We use the cosmological hydrodynamical simulation Horizon Run 5 (
HR5
) to calibrate this prescription and demonstrate its performance. We find that the protocluster identification method suggested in this study is quite successful. Its application to the high-redshift
HR5
galaxies shows a tight correlation between the mass within the protocluster regions identified according to the spherical collapse model and the final mass to be found within the clusters at
z
= 0, meaning that the regions can be regarded as the bona fide protoclusters with high reliability. We also confirm that the redshift-space distortion does not significantly affect the performance of the protocluster identification scheme.
Abstract
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 1010.4 M⊙, 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 (1010.4 < M/M⊙ < 1010.7) 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.
Abstract
Based on the recent advancements in numerical simulations of galaxy formation, we anticipate the achievement of realistic models of galaxies in the near future. Morphology is the most basic ...and fundamental property of galaxies, yet observations and simulations still use different methods to determine galaxy morphology, making it difficult to compare them. We hereby perform a test on the recent
NewHorizon
simulation, which has spatial and mass resolutions that are remarkably high for a large-volume simulation, to resolve the situation. We generate mock images for the simulated galaxies using SKIRT, which calculates complex radiative transfer processes in each galaxy. We measure morphological and kinematic indicators using photometric and spectroscopic methods following observers’ techniques. We also measure the kinematic disk-to-total ratios using the Gaussian mixture model and assume that they represent the true structural composition of galaxies. We found that spectroscopic indicators such as
V
/
σ
and
λ
R
closely trace the kinematic disk-to-total ratios. In contrast, photometric disk-to-total ratios based on the radial profile fitting method often fail to recover the true kinematic structure of galaxies, especially small ones. We provide translating equations between various morphological indicators.
Abstract
We investigate the impact of the surface-brightness (SB) limit on the galaxy stellar mass functions (GSMFs) using galaxy catalogs generated from the
Horizon Run 5
(
HR5
) simulation. We ...compare the stellar-to-halo-mass relation, GSMF, and size–stellar mass relation of the
HR5
galaxies with observational data and other cosmological simulations. The mean SB of simulated galaxies are computed using their effective radii, luminosities, and colors. To examine the cosmic SB dimming effect, we compute
k
-corrections from the spectral energy distributions of individual simulated galaxy at each redshift, apply the
k
-corrections to the galaxies, and conduct mock surveys based on the various SB limits. We find that the GSMFs are significantly affected by the SB limits at the low-mass end. This approach can ease the discrepancy between the GSMFs obtained from simulations and observations at 0.6 ≲
z
≤ 2. We also find that a redshift survey with an SB selection limit of
μ
r
e
=
25 mag arcsec
−2
will miss 20% of galaxies with
M
⋆
g
=
10
9
M
⊙
at
z
= 0.625. The missing fraction of low-surface-brightness galaxies increases to 35%, 55%, and 80% at
z
= 0.9, 1.1, and 1.9, respectively, at the same SB limit.
Abstract
Massive black holes (BHs) inhabit local galaxies, including the Milky Way and some dwarf galaxies. BH formation, occurring at early cosmic times, must account for the properties of BHs in ...today’s galaxies, notably why some galaxies host a BH, and others do not. We investigate the formation, distribution and growth of BH ‘seeds’ by using the adaptive mesh refinement code ramses. We develop an implementation of BH formation in dense, low-metallicity environments, as advocated by models invoking the collapse of the first generation of stars, or of dense nuclear star clusters. The seed masses are computed one-by-one on-the-fly, based on the star formation rate and the stellar initial mass function. This self-consistent method to seed BHs allows us to study the distribution of BHs in a cosmological context and their evolution over cosmic time. We find that all high-mass galaxies tend to host a BH, whereas low-mass counterparts have a lower probability of hosting a BH. After the end of the epoch of BH formation, this probability is modulated by the growth of the galaxy. The simulated BHs connect to low-redshift observational samples, and span a similar range in accretion properties as Lyman-break analogs. The growth of BHs in low-mass galaxies is stunted by strong supernova (SN) feedback. The properties of BHs in dwarf galaxies thus remain a testbed for BH formation. Simulations with strong SN feedback, which is able to quench BH accretion in shallow potential wells, produce galaxies and BHs in better agreement with observational constraints.
To better understand the impact of supernova (SN) explosions on the evolution of galaxies, we perform a suite of high-resolution (12 pc), zoom-in cosmological simulations of a Milky Way-like galaxy ...at z = 3 with adaptive mesh refinement. We find that SN explosions can efficiently regulate star formation, leading to the stellar mass and metallicity consistent with the observed mass–metallicity relation and stellar mass–halo mass relation at z ∼ 3. This is achieved by making three important changes to the classical feedback scheme: (i) the different phases of SN blast waves are modelled directly by injecting radial momentum expected at each stage, (ii) the realistic time delay of SNe is required to disperse very dense gas before a runaway collapse sets in, and (iii) a non-uniform density distribution of the interstellar medium (ISM) is taken into account below the computational grid scale for the cell in which an SN explodes. The simulated galaxy with the SN feedback model shows strong outflows, which carry approximately 10 times larger mass than star formation rate, as well as smoothly rising circular velocity. Although the metallicity of the outflow depends sensitively on the feedback model used, we find that the accretion rate and metallicity of the cold flow around the virial radius is impervious to SN feedback. Our results suggest that understanding the structure of the turbulent ISM may be crucial to assess the role of SN and other feedback processes in galaxy formation theory.
Abstract
We compare five sub-grid models for supernova (SN) feedback in adaptive mesh refinement (AMR) simulations of isolated dwarf and L-star disc galaxies with 20–40 pc resolution. The models are ...thermal dump, stochastic thermal, ‘mechanical’ (injecting energy or momentum depending on the resolution), kinetic and delayed cooling feedback. We focus on the ability of each model to suppress star formation and generate outflows. Our highest resolution runs marginally resolve the adiabatic phase of the feedback events, which correspond to 40 SN explosions, and the first three models yield nearly identical results, possibly indicating that kinetic and delayed cooling feedback converge to wrong results. At lower resolution all models differ, with thermal dump feedback becoming inefficient. Thermal dump, stochastic and mechanical feedback generate multiphase outflows with mass loading factors β ≪ 1, which is much lower than observed. For the case of stochastic feedback, we compare to published SPH simulations, and find much lower outflow rates. Kinetic feedback yields fast, hot outflows with β ∼ 1, but only if the wind is in effect hydrodynamically decoupled from the disc using a large bubble radius. Delayed cooling generates cold, dense and slow winds with β > 1, but large amounts of gas occupy regions of temperature–density space with short cooling times. We conclude that either our resolution is too low to warrant physically motivated models for SN feedback, that feedback mechanisms other than SNe are important or that other aspects of galaxy evolution, such as star formation, require better treatment.
ABSTRACT
Recent deep surveys have unravelled a population of faint active galactic nuclei (AGNs) 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 (${M_\star } \gtrsim 10^{10}\, {\rm M}_{\odot }$) hosting an actively growing supermassive black hole to study the properties of these objects. In particular, we study how the black hole and the galaxy coevolve and what is the relative contribution of the AGNs 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 $f_{\rm esc} \sim 5{{\ \rm per\ cent}}$. While our galaxy would be selected as an AGN in deep X-ray surveys, most of the ultraviolet (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 with MUV around −22 falling in the overlap region. Our results also suggest that faint AGNs do not contribute significantly to reionizing the Universe.
Super-Eddington accretion is one scenario that may explain the rapid assembly of ∼10
9
M
⊙
supermassive black holes (BHs) within the first billion year of the Universe. This critical regime is ...associated with radiatively inefficient accretion and accompanied by powerful outflows in the form of winds and jets. By means of hydrodynamical simulations of BH evolution in an isolated galaxy and its host halo with 12 pc resolution, we investigate how super-Eddington feedback affects the mass growth of the BH. It is shown that super-Eddington feedback efficiently prevents BH growth within a few Myr. The super-Eddington accretion events remain relatively mild with typical rates of about 2−3 times the Eddington limit, because of the efficient regulation by jets in that regime. We find that these jets are powerful enough to eject gas from the centre of the host galaxy all the way up to galactic scales at a few kpc, but do not significantly impact gas inflows at those large scales. By varying the jet feedback efficiency, we find that weaker super-Eddington jets allow for more significant BH growth through more frequent episodes of super-Eddington accretion. We conclude that effective super-Eddington growth is possible, as we find that simulations with weak jet feedback efficiencies provide a slightly larger BH mass evolution over long periods of time (∼80 Myr) than that for a BH accreting at the Eddington limit.
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