67 research outputs found
Explore the Nature of Dark Matter in the Context of Galaxy Formation
The nature of dark matter (DM) is a fundamental question in modern cosmology. Despite its significant role in various physical processes throughout the Universe, the particle nature of DM remains elusive. With the non-detection of classical candidates (e.g. WIMPs), the theoretical space for DM is becoming increasingly open. This thesis revolves around studying the nature of DM in the context of structure formation and we will focus on a category of DM with self-interactions (SIDM), which can be constrained only through astrophysical probes if DM has no coupling with the standard model particles. Utilizing advanced cosmological hydrodynamical simulations, we examine the effects of DM elastic and dissipative self-interactions on galaxy structure and their interplay with baryonic physics processes. Our numerical studies encompass a range of systems, such as Local dwarf galaxies, massive galaxy clusters in the Local Universe, and rare massive quasar-host galaxies at high redshift (z ≳ 6). In Local dwarf galaxies, we analyze the unique signatures of dissipative self-interacting DM (dSIDM) with typical self-interaction cross-section σ/m ~ 0.1-10 cm² g⁻¹ and dissipation factor ~ 0.5. We find a universal cuspy central density profile and systematic changes in halo morphology in dSIDM. By comparing our results with observations, we derive constraints for effective parameters of dSIDM and identify the parameter space where it remains viable and exhibits interesting observational implications. For a similar type of dSIDM with fairly low σ/m ≾ 0.05 cm² g⁻¹, we also explore the possibility that the direct collapse of dSIDM halos at high redshift can seed supermassive black holes and serve as progenitors for massive bright quasars observed at high redshift. This scenario predicts a large population of quiescent supermassive black holes (SMBHs) at high redshift, which could be tested by future LISA observations. Lastly, in Local massive galaxy clusters, we compare the X-ray morphology of hot gas in observed clusters with simulations of elastic SIDM. Although SIDM models with large interaction cross-sections (σ/m ≳ 0.5 cm² g⁻¹) are favored, uncertainties from cooling and feedback physics in galaxy clusters must be taken into account. This thesis summarizes the findings and constraints on DM properties, with a particular emphasis on its potential self-interactions, as derived from a combination of research projects
Seen and unseen: bursty star formation and its implications for observations of high-redshift galaxies with JWST
Both observations and simulations have shown strong evidence for highly
time-variable star formation in low-mass and/or high-redshift galaxies, which
has important observational implications because high-redshift galaxy samples
are rest-UV selected and therefore particularly sensitive to the recent star
formation. Using a suite of cosmological "zoom-in" simulations at from
the Feedback in Realistic Environments (FIRE) project, we examine the
implications of bursty star formation histories for observations of
high-redshift galaxies with JWST. We characterize how the galaxy observability
depends on the star formation history. We also investigate selection effects
due to bursty star formation on the physical properties measured, such as the
gas fraction, specific star formation rate, and metallicity. We find the
observability to be highly time-dependent for galaxies near the survey's
limiting flux due to the SFR variability: as the star formation rate
fluctuates, the same galaxy oscillates in and out of the observable sample. The
observable fraction at and to for a JWST/NIRCam survey reaching a limiting
magnitude of -, representative of
surveys such as JADES and CEERS. JWST-detectable galaxies near the survey limit
tend to have properties characteristic of galaxies in the bursty phase: on
average, they show approximately 2.5 times higher cold, dense gas fractions and
20 times higher specific star formation rates at a given stellar mass than
galaxies below the rest-UV detection threshold. Our study represents a first
step in quantifying selection effects and the associated biases due to bursty
star formation in studying high-redshift galaxy properties.Comment: 8 pages, 4 figures, resubmitted after incorporating referee's
comments; analysis expanded to include more galaxies and some quantitative
results correcte
Dissipative Dark Matter on FIRE: I. Structural and kinematic properties of dwarf galaxies
We present the first set of cosmological baryonic zoom-in simulations of
galaxies including dissipative self-interacting dark matter (dSIDM). These
simulations utilize the Feedback In Realistic Environments (FIRE-2) galaxy
formation physics, but allow the dark matter to have dissipative
self-interactions analogous to Standard Model forces, parameterized by the
self-interaction cross-section per unit mass, , and the
dimensionless degree of dissipation, . We survey this
parameter space, including constant and velocity-dependent cross-sections, and
focus on structural and kinematic properties of dwarf galaxies with . Central density profiles of simulated
dwarfs become cuspy when (and as fiducial). The power-law slopes
asymptote to in low-mass dwarfs independent of
cross-section, which arises from a dark matter "cooling flow". Through
comparisons with dark matter only simulations, we find the profile in this
regime is insensitive to the inclusion of baryons. However, when
, baryonic effects can
produce cored density profiles comparable to non-dissipative cold dark matter
(CDM) runs but at smaller radii. Simulated galaxies with develop significant coherent rotation of dark matter,
accompanied by halo deformation, but this is unlike the well-defined thin "dark
disks" often attributed to baryon-like dSIDM. The density profiles in this high
cross-section model exhibit lower normalizations given the onset of halo
deformation. For our surveyed dSIDM parameters, halo masses and galaxy stellar
masses do not show appreciable difference from CDM, but dark matter kinematics
and halo concentrations/shapes can differ.Comment: Accepted by MNRAS. 23 pages, 19 figure
X-ray morphology of cluster-mass haloes in self-interacting dark matter
We perform cosmological zoom-in simulations of relaxed cluster-mass
haloes with the inclusion of adiabatic gas in the cold dark matter (CDM) and
self-interacting dark matter (SIDM) models. These clusters are selected as
dynamically relaxed clusters from a parent simulation with . Both the dark matter and
the intracluster gas distributions in SIDM appear more spherical than their CDM
counterparts. Mock X-ray images are generated based on the simulations and are
compared to the real X-ray images of relaxed clusters selected from the
Chandra and ROSAT archives. We perform ellipse fitting for the isophotes of
mock and real X-ray images and obtain the ellipticities at cluster-centric
radii of . The X-ray isophotes in
SIDM models with increasing cross-sections are rounder than their CDM
counterparts, which manifests as a systematic shift in the distribution
function of ellipticities. Unexpectedly, the X-ray morphology of the observed
non-cool-core clusters agrees better with SIDM models with cross-section
than CDM and SIDM with
. Our statistical analysis indicates that
the latter two models are disfavored at the confidence level (as
conservative estimates). This conclusion is not altered by shifting the radial
range of measurements or applying temperature selection criterion. However, the
primary uncertainty originates from the lack of baryonic physics in the
adiabatic model, such as cooling, star formation and feedback effects, which
still have the potential to reconcile CDM simulations with observations.Comment: Accepted by MNRA
Bursty Star Formation Naturally Explains the Abundance of Bright Galaxies at Cosmic Dawn
Recent discoveries of a significant population of bright galaxies at cosmic
dawn have enabled critical tests of cosmological
galaxy formation models. In particular, the bright end of the galaxy UV
luminosity function (UVLF) appears higher than predicted by many models. Using
approximately 25,000 galaxy snapshots at in a suite of
FIRE-2 cosmological "zoom-in'' simulations from the Feedback in Realistic
Environments (FIRE) project, we show that the observed abundance of UV-bright
galaxies at cosmic dawn is reproduced in these simulations with a multi-channel
implementation of standard stellar feedback processes, without any fine-tuning.
Notably, we find no need to invoke previously suggested modifications such as a
non-standard cosmology, a top-heavy stellar initial mass function, or a
strongly enhanced star formation efficiency. We contrast the UVLFs predicted by
bursty star formation in these original simulations to those derived from star
formation histories (SFHs) smoothed over prescribed timescales (e.g., 100 Myr).
The comparison demonstrates that the strongly time-variable SFHs predicted by
the FIRE simulations play a key role in correctly reproducing the observed,
bright-end UVLFs at cosmic dawn: the bursty SFHs induce order-or-magnitude
changes in the abundance of UV-bright () galaxies
at . The predicted bright-end UVLFs are consistent with both the
spectroscopically confirmed population and the photometrically selected
candidates. We also find good agreement between the predicted and
observationally inferred integrated UV luminosity densities, which evolve more
weakly with redshift in FIRE than suggested by some other models.Comment: 12 pages, 4 figures + 1 table, submitted to ApJ
Multi-scale distribution of coal fractures based on CT digital core deep learning
In order to realize high-precision and high-efficiency identification of multi-scale distribution characteristics of coal fractures, carry out the study of multi-scale distribution characteristics identification methods based on CT digital core deep learning. Industrial CT scanning system is used to collect a large number of coal original CT digital core information array, the CT digital core information array is converted into a two-dimensional gray-scale image and then it is divided into square images of different scales and the image brightness is enhanced to different levels as training samples, Finally, the construction and optimization of model parameters of AlexNet, ResNet-18, GoogLeNet and Inception-V3 models for the identification of CT-containing fractures are realized by Matlab platform. Study the recognition accuracy and verification accuracy of different model training under different number of training samples; Study the accuracy, calculation efficiency and training time of different models for images with different scales and brightness under the same training sample, obtain the optimal model for calculating the fractal dimension of two-dimensional CT images with fractures, then, the fractal distribution characteristics of each fracture image are calculated according to the statistical method of box-counting dimension, compared with the traditional binarization method and human eye recognition method, The applicability of the multi-scale distribution characteristics identification method of coal fractures based on CT digital core deep learning is verified. The result shows: ① ResNet-18 model is the optimal model for calculating the fractal dimension of two-dimensional CT images with cracks when the image sample is brightness 4 and the scale is 3.5 mm to 21 mm, the model has high accuracy and short training time in calculating the fractal dimension of two-dimensional CT fracture images. ② Compared with the traditional binarization method, the multi-scale recognition method of coal fracture based on CT digital core deep learning has the advantages of fast speed, high accuracy and is not easily affected by impurities in coal
Early-type galaxy density profiles from IllustrisTNG – II. Evolutionary trend of the total density profile
We study the evolutionary trend of the total density profile of early-type galaxies (ETGs) in IllustrisTNG. To this end, we trace ETGs from z = 0 to 4 and measure the power-law slope γ′ of the total density profile for their main progenitors. We find that their slopes γ′ steepen on average during z ∼ 4–2, then becoming shallower until z = 1, after which they remain almost constant, aside from a residual trend of becoming shallower towards z = 0. We also compare to a statistical sample of ETGs at different redshifts, selected based on their luminosity profiles and stellar masses. Due to different selection effects, the average slopes of the statistical samples follow a modified evolutionary trend. They monotonically decrease since z = 3, and after z ≈ 1, they remain nearly invariant with a mild increase towards z = 0. These evolutionary trends are mass dependent for both samples, with low-mass galaxies having in general steeper slopes than their more massive counterparts. Galaxies that transitioned to ETGs more recently have steeper mean slopes as they tend to be smaller and more compact at any given redshift. By analysing the impact of mergers and AGN feedback on the progenitors’ evolution, we conjecture a multiphase path leading to isothermality in ETGs: dissipation associated with rapid wet mergers tends to steepen γ′ from z = 4 to 2, whereas subsequent AGN feedback (especially in the kinetic mode) makes γ′ shallower again from z = 2 to 1. Afterwards, passive evolution from z = 1 to 0, mainly through gas-poor mergers, mildly decreases γ′ and maintains the overall mass distribution close to isothermal
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