arXiv subjects

Boyuan Liu

Publications and source records attributed to Boyuan Liu.

At least 73 records · Page 4Linked to original sources

arXiv2022

Accelerating early massive galaxy formation with primordial black holes

Recent observations with JWST have identified several bright galaxy candidates at $z\gtrsim 10$, some of which appear unusually massive (up to $\sim 10^{11}\ \rm M_{\odot}$). Such early formation of massive galaxies is difficult to reconcile with standard $Λ\rm CDM$ predictions, demanding very high star formation efficiency (SFE), possibly even in excess of the cosmic baryon mass budget in collapsed structures. With an idealized analysis based on linear perturbation theory and the Press-Schechter formalism, we show that the observed massive galaxy candidates can be explained with lower SFE than required in $Λ\rm CDM$, if structure formation is accelerated/seeded by massive ($\gtrsim 10^{9}\ \rm M_{\odot}$) primordial black holes (PBHs) that make a up a small fraction ($\sim 10^{-6}-10^{-3}$) of dark matter, considering existing empirical constraints on PBH parameters. We also discuss the potential observational signatures of PBH cosmologies in the JWST era. More work needs to be done to fully evaluate the viability of such PBH models to explain observations of the high-$z$ Universe.

astro-ph.CO↗

arXiv2022

Multi-Forgery Detection Challenge 2022: Push the Frontier of Unconstrained and Diverse Forgery Detection

In this paper, we present the Multi-Forgery Detection Challenge held concurrently with the IEEE Computer Society Workshop on Biometrics at CVPR 2022. Our Multi-Forgery Detection Challenge aims to detect automatic image manipulations including but not limited to image editing, image synthesis, image generation, image photoshop, etc. Our challenge has attracted 674 teams from all over the world, with about 2000 valid result submission counts. We invited the Top 10 teams to present their solutions to the challenge, from which three teams are awarded prizes in the grand finale. In this paper, we present the solutions from the Top 3 teams, in order to boost the research work in the field of image forgery detection.

cs.CV↗

arXiv2022

Effects of stellar-mass primordial black holes on first star formation

We use cosmological hydrodynamic zoom-in simulations and semi-analytical models to study the effects of primordial black holes (PBHs) on first star formation. Our models self-consistently combine two competing effects: initial (isocurvature) perturbations induced by PBHs and BH accretion feedback. Focusing on PBHs with masses $\sim 30\ \rm M_{\odot}$, we find that the standard picture of first star formation in molecular-cooling minihaloes is not changed by PBHs, as the simulated star-forming gas clouds in the central parsec are very similar to those in the $\rm ΛCDM$ case when PBHs make up $f_{\rm PBH}\sim 10^{-4}-0.1$ of dark matter. With a dynamical friction timescale of $\sim 2-10\ \rm Myr$ when the central gas density reaches $10^{5}\ \rm cm^{-3}$, it is also unlikely that PBHs can sink into star-forming discs and affect the evolution of protostars, although they may interact with the stars during the main-sequence stage. At larger scales, PBHs tend to shift star formation to more massive haloes, and accelerate structure formation. The latter effect is stronger in regions with higher initial overdensities. For $f_{\rm PBH}\sim 10^{-4}-0.01$ (allowed by observational constraints), the collapsed mass fraction of haloes hosting Population III stars is similar (within a factor of $\sim2$ at $z\lesssim 30$) to that in $\rm ΛCDM$, implying that the impact of stellar-mass PBHs on the cosmic star formation history at $z\gtrsim 10$ is small. We also find that the Lyman-Werner photons from PBH accretion in atomic-cooling haloes may facilitate the formation of direct-collapse BHs.

astro-ph.GA↗

arXiv2022

Some exact results of the generalized Turán numbers for paths

For graphs $H$ and $F$ with chromatic number $χ(F)=k$, we call $H$ strictly $F$-Turán-good (or $(H, F)$ strictly Turán-good) if the Turán graph $T_{k-1}(n)$ is the unique $F$-free graph on $n$ vertices containing the largest number of copies of $H$ when $n$ is large enough. Let $F$ be a graph with chromatic number $χ(F)\geq 3$ and a color-critical edge and let $P_\ell$ be a path with $\ell$ vertices. Gerbner and Palmer (2020, arXiv:2006.03756) showed that $(P_3, F)$ is strictly Turán good if $χ(H)\ge 4$ and they conjectured that (a) this result is true when $χ(F)=3$, and, moreover, (b) $(P_\ell, K_k)$ is Turán-good for every pair of integers $\ell$ and $k$. In the present paper, we show that $(H, F)$ is strictly Turán-good when $H$ is a bipartite graph with matching number $ν(H)=\lfloor \frac{|V(H)|}{2}\rfloor$ and $χ(F)= 3$, as a corollary, this result confirms the conjecture (a); we also prove that $(P_\ell, F)$ is strictly Turán-good for $2\le\ell\leq 6$ and $χ(F)\ge 4$, this also confirms the conjecture (b) for $2\le\ell\leq 6$ and $k\ge 4$.

math.CO↗

arXiv2022

Strong and long-range radiative interaction between resonant transitions

Enhancing the radiative interaction between quantum electronic transitions is of general interest. There are two important properties of radiative interaction: the range and the strength. There has been a trade-off between the range and the strength observed in the literature. Such apparent trade-off arises from the dispersion relation of photonic environments. A general recipe is developed to overcome such trade-off and to simultaneously enhance the range and the strength of radiative interactions.

physics.optics↗

arXiv2021

Detectability of Population III stellar remnants as X-ray binaries from tidal captures in the local Universe

We assess the feasibility of detecting the compact object remnants from Population III (Pop III) stars in nearby dense star clusters, where they become luminous again as X-ray binaries (XRBs) and tidal disruption events (TDEs) via strong tidal encounters. Analytically modelling the formation of Pop III stars, coupled with a top-heavy initial mass function predicted by numerical simulations, we derive the number of (active) Pop III XRBs and TDEs in the present-day Milky Way (MW) nuclear star cluster as $\sim 0.06-0.3$ and $\lesssim 4\times 10^{-6}$, rendering any detection unlikely. The detection probability, however, can be significantly boosted when surveying all massive star clusters from the MW and neighboring galaxy clusters. Specifically, we predict $\sim 1.5-6.5$ and $\sim 40-2800$ active Pop III XRBs in the MW and the Virgo cluster, respectively. Our Pop III XRBs are dominated ($\sim 99\%$) by black holes with a typical mass and luminosity of $\sim 45$ $\rm M_{\odot}$ and $\sim 10^{36}$ $\rm erg\ s^{-1}$. Deep surveys of nearby ($\lesssim 30-300$ $\rm Mpc$) galaxy clusters for such Pop III XRBs are well within reach of next-generation X-ray telescopes, such as ATHENA and LYNX.

astro-ph.HE↗

arXiv2021

Rainbow independent sets in graphs with maximum degree two

Given a graph $G$, let $f_{G}(n,m)$ be the minimal number $k$ such that every $k$ independent $n$-sets in $G$ have a rainbow $m$-set. Let $\mathcal{D}(2)$ be the family of all graphs with maximum degree at most two. Aharoni et al. (2019) conjectured that (i) $f_G(n,n-1)=n-1$ for all graphs $G\in\mathcal{D}(2)$ and (ii) $f_{C_t}(n,n)=n$ for $t\ge 2n+1$. Lv and Lu (2020) showed that the conjecture (ii) holds when $t=2n+1$. In this article, we show that the conjecture (ii) holds for $t\ge\frac{1}{3}n^2+\frac{44}{9}n$. Let $C_t$ be a cycle of length $t$ with vertices being arranged in a clockwise order. An ordered set $I=(a_1,a_2,\ldots,a_n)$ on $C_t$ is called a $2$-jump independent $n$-set of $C_t$ if $a_{i+1}-a_i=2\pmod{t}$ for any $1\le i\le n-1$. We also show that a collection of 2-jump independent $n$-sets $\mathcal{F}$ of $C_t$ with $|\mathcal{F}|=n$ admits a rainbow independent $n$-set, i.e. (ii) holds if we restrict $\mathcal{F}$ on the family of 2-jump independent $n$-sets. Moreover, we prove that if the conjecture (ii) holds, then (i) holds for all graphs $G\in\mathcal{D}(2)$ with $c_e(G)\le 4$, where $c_e(G)$ is the number of components of $G$ isomorphic to cycles of even lengths.

math.CO↗

arXiv2021

Stellar winds and metal enrichment from fast-rotating Population III stars

Stellar winds from fast-rotating Population III (Pop III) stars have long been suspected to make important contributions to early metal enrichment, as features in the nucleosynthesis of such `spinstars' are consistent with the chemical abundance patterns of some metal-poor stars in the local Universe. Particularly, stellar winds rich in light elements can provide another pathway towards explaining the carbon enhancement in carbon-enhanced metal-poor (CEMP) stars. In this work, we focus on the feedback of Pop III stellar winds combined with supernovae (SNe), and derive the resulting chemical signatures in the enriched medium. We explore a large parameter space of Pop III star formation, feedback, yields from winds and SNe with a semi-analytical model. The predicted pattern of carbon and iron abundances of second-generation stars agrees well with observations of CEMP-no stars ($[\rm Ba/Fe]<0$) at $[\rm Fe/H]\lesssim -3$ and $A(\mathrm{C})\lesssim 7$, under the optimistic assumption of significant mass loss by winds from massive ($\gtrsim 25\ \rm M_{\odot}$) stars that collapse into BHs without SNe. In this scenario, carbon-rich but iron-free second-generation stars can form in systems dominated by enrichment from winds, gaining trace amounts of iron by accretion from the interstellar medium, to become the most iron-poor and carbon-enhanced stars seen in observations ($[\rm Fe/H]\lesssim -4$, $[\rm C/Fe]\gtrsim 2$). We conclude that the observed CEMP-no stars can be explained by both our winds + ISM accretion channel as well as the well-studied faint SN scenario. Wind feedback from Pop~III spinstars deserves more detailed modelling in early cosmic structure formation.

astro-ph.GA↗

arXiv2021

Gravitational waves from the remnants of the first stars in nuclear star clusters

We study Population III (Pop III) binary remnant mergers in nuclear star clusters (NSCs) with a semi-analytical approach for early structure formation. Within this framework, we keep track of the dynamics of Pop III binary (compact object) remnants during cosmic structure formation, and construct the population of Pop III binary remnants that fall into NSCs by dynamical friction of field stars. The subsequent evolution within NSCs is then derived from three-body encounters and gravitational-wave (GW) emission. We find that 7.5% of Pop III binary remnants will fall into the centres ($< 3\ \rm pc$) of galaxies. About $5-50\%$ of these binaries will merge at $z>0$ in NSCs, including those with very large initial separations (up to 1~pc). The merger rate density (MRD) peaks at $z\sim 5-7$ with $\sim 0.4-10\ \rm yr^{-1}\ \rm Gpc^{-3}$, leading to a promising detection rate $\sim 170-2700\ \rm yr^{-1}$ for 3rd-generation GW detectors that can reach $z\sim 10$. Low-mass ($\lesssim 10^{6}\ \rm M_{\odot}$) NSCs formed at high redshifts ($z\gtrsim 4.5$) host most ($\gtrsim 90\%$) of our mergers, which mainly consist of black holes (BHs) with masses $\sim 40-85\ \rm M_{\odot}$, similar to the most massive BHs found in LIGO events. Particularly, our model can produce events like GW190521 involving BHs in the standard mass gap for pulsational pair-instability supernovae with a MRD $\sim 0.01-0.09\ \rm yr^{-1}\ Gpc^{-3}$ at $z\sim 1$, consistent with that inferred by LIGO.

astro-ph.GA↗

arXiv2021

DFGC 2021: A DeepFake Game Competition

This paper presents a summary of the DFGC 2021 competition. DeepFake technology is developing fast, and realistic face-swaps are increasingly deceiving and hard to detect. At the same time, DeepFake detection methods are also improving. There is a two-party game between DeepFake creators and detectors. This competition provides a common platform for benchmarking the adversarial game between current state-of-the-art DeepFake creation and detection methods. In this paper, we present the organization, results and top solutions of this competition and also share our insights obtained during this event. We also release the DFGC-21 testing dataset collected from our participants to further benefit the research community.

cs.CV↗

arXiv2021

Coherent optical communications using coherence-cloned Kerr soliton microcombs

Dissipative Kerr soliton microcomb has been recognized as a promising on-chip multi-wavelength laser source for fiber optical communications, as its comb lines possess frequency and phase stability far beyond independent lasers. In the scenarios of coherent optical transmission and interconnect, a highly beneficial but rarely explored target is to re-generate a Kerr soliton microcomb at the receiver side as local oscillators that conserve the frequency and phase property of the incoming data carriers, so that to enable coherent detection with minimized optical and electrical compensations. Here, by using the techniques of pump laser conveying and two-point locking, we implement re-generation of a Kerr soliton microcomb that faithfully clones the frequency and phase coherence of another microcomb sent from 50 km away. Moreover, leveraging the coherence-cloned soliton microcombs as carriers and local oscillators, we demonstrate terabit coherent data interconnect, wherein traditional digital processes for frequency offset estimation is totally dispensed with, and carrier phase estimation is substantially simplified via slowed-down phase estimation rate per channel and joint phase estimation among multiple channels. Our work reveals that, in addition to providing a multitude of laser tones, regulating the frequency and phase of Kerr soliton microcombs among transmitters and receivers can significantly improve coherent communication in terms of performance, power consumption, and simplicity.

eess.SP↗

arXiv2020

Dynamical evolution of Population III stellar systems and the resulting binary statistics

We use N-body simulations to study the dynamical evolution of Population III (Pop III) stellar systems and the resulting binary statistics. We design a physically-motivated framework for the initial conditions of Pop III star clusters, based on small-scale hydrodynamic simulations and the scale-free nature of disk evolution during Pop III star formation. Our novel approach enables us to explore the dependence of binary statistics on initial conditions and arrive at more robust predictions for the signals of Pop III X-ray binaries (XRBs) and binary black hole (BBH) mergers, compared to simple extrapolations of Pop III protostar systems. We find that binary properties are highly sensitive to the initial cluster size and distribution of binary separation, while the effect of initial mass function is relatively minor. Our simulations predict less close binaries, and thus, significantly lower efficiencies (by a factor of $\sim 10-10^{4}$) for the formation and accretion of Pop III XRBs, than found in previous studies, implying that the contribution of Pop III XRBs to the cosmic X-ray background is negligible and their feedback effects are unimportant. We estimate the efficiency of Pop III BBH mergers as $\sim 10^{-5}-10^{-4}\ \rm M_{\odot}^{-1}$, for which 3-body hardening by surrounding stars in dense star clusters or close binary interactions is required to facilitate in-spirals of BBHs. All simulation data, including catalogs of Pop III binaries and multiple systems, are publicly available.

astro-ph.GA↗

arXiv2020

The Population III origin of GW190521

We explore the possibility that the recently detected black hole binary (BHB) merger event GW190521 originates from the first generation of massive, metal-free, so-called Population III (Pop III), stars. Based on improved binary statistics derived from N-body simulations of Pop III star clusters, we calculate the merger rate densities of Pop III BHBs similar to GW190521, in two evolution channels: classical binary stellar evolution and dynamical hardening in high-redshift nuclear star clusters. Both channels can explain the observed rate density. But the latter is favoured by better agreement with observation and less restrictions on uncertain parameters. Our analysis also indicates that given the distinct features of the two channels (with merger rates peaked at $z\lesssim2$ and $z\sim 10$, respectively), future observation of BHB mergers similar to GW190521 with third-generation gravitational wave detectors will greatly improve our knowledge of the evolution of Pop III BHBs, especially for their dynamics during cosmic structure formation.

astro-ph.GA↗

arXiv2020

The Turán number for the edge blow-up of trees

The edge blow-up of a graph $F$ is the graph obtained from replacing each edge in $F$ by a clique of the same size where the new vertices of the cliques are all different. In this article, we concern about the Turán problem for the edge blow-up of trees. Erdős et al. (1995) and Chen et al. (2003) solved the problem for stars. The problem for paths was resolved by Glebov (2011). Liu (2013) extended the above results to cycles and a special family of trees with the minimum degree at most two in the smaller color class (paths and proper subdivisions of stars were included in the family). In this article, we extend Liu's result to all the trees with the minimum degree at least two in the smaller color class. Combining with Liu's result, except one particular case, the Turán problem for the edge blow-up of trees is completely resolved. Moreover, we determine the maximum number of edges in the family of $\{K_{1,k}, kK_2, 2K_{1,k-1}\}$-free graphs and the extremal graphs, which is an extension of a result given by Abbott et al. (1972).

math.CO↗

arXiv2020

When did Population III star formation end?

We construct a theoretical framework to study Population III (Pop III) star formation in the post-reionization epoch ($z\lesssim 6$) by combining cosmological simulation data with semi-analytical models. We find that due to radiative feedback (i.e. Lyman-Werner and ionizing radiation) massive haloes ($M_{\rm halo}\gtrsim 10^{9}\ \rm M_{\odot}$) are the major ($\gtrsim 90$%) hosts for potential Pop III star formation at $z\lesssim 6$, where dense pockets of metal-poor gas may survive to form Pop III stars, under inefficient mixing of metals released by supernovae. Metal mixing is the key process that determines not only when Pop III star formation ends, but also the total mass, $M_{\rm PopIII}$, of \textit{active} Pop III stars per host halo, which is a crucial parameter for direct detection and identification of Pop III hosts. Both aspects are still uncertain due to our limited knowledge of metal mixing during structure formation. Current predictions range from early termination at the end of reionization ($z\sim 5$) to continuous Pop III star formation extended to $z=0$ at a non-negligible rate $\sim 10^{-7}\ \rm M_{\odot}\ yr^{-1}\ Mpc^{-3}$, with $M_{\rm PopIII}\sim 10^{3}-10^{6}\ \rm M_{\odot}$. This leads to a broad range of redshift limits for direct detection of Pop III hosts, $z_{\rm PopIII}\sim 0.5-12.5$, with detection rates $\lesssim 0.1-20\ \rm arcmin^{-2}$, for current and future space telescopes (e.g. HST, WFIRST and JWST). Our model also predicts that the majority ($\gtrsim 90$%) of the cosmic volume is occupied by metal-free gas. Measuring the volume filling fractions of this metal-free phase can constrain metal mixing parameters and Pop III star formation.

astro-ph.GA↗

arXiv2020

Gravitational waves from Population III binary black holes formed by dynamical capture

We use cosmological hydrodynamic simulations to study the gravitational wave (GW) signals from high-redshift binary black holes (BBHs) formed by dynamical capture (ex-situ formation channel). We in particular focus on BHs originating from the first generation of massive, metal-poor, so-called Population~III (Pop~III) stars. An alternative (in-situ) formation pathway arises in Pop~III binary stars, whose GW signature has been intensively studied. In our optimistic model, we predict a local GW event rate density for ex-situ BBHs (formed at z>4) of ~0.04 /(yr Gpc^3). This is comparable to or even higher than the conservative predictions of the rate density for in-situ BBHs ~0.01-0.1 /(yr Gpc^3), indicating that the ex-situ formation channel may be as important as the in-situ one for producing GW events. We also evaluate the detectability of our simulated GW events for selected planned GW instruments, such as the Einstein Telescope (ET). For instance, we find the all-sky detection rate with signal-to-noise ratios above 10 to be <100 per year for the xylophone configuration of ET. However, our results are highly sensitive to the sub-grid models for BBH identification and evolution, such that the GW event efficiency (rate) is reduced by a factor of 4 (20) in the pessimistic case. The ex-situ channel of Pop III BBHs deserves further investigation with better modeling of the environments around Pop III-seeded BHs.

astro-ph.CO↗

arXiv2020

Lessons on early structure formation from a mature galaxy cluster observed at cosmic noon

We demonstrate a new approach of indirectly constraining both early star and structure formation via mature galaxy clusters at cosmic noon (z~2), using the cluster XLSSC 122 as an example. With the standard Press Schechter formalism, we infer a rapid evolution of the star formation efficiency (the ratio of stellar to halo mass) from 10^-4 to 0.01 during z~20-13, based on the age distribution of stars in post-starburst galaxies of XLSSC 122, measured by HST photometry assuming no dust extinction. Here, we consider all low-mass haloes, including minihaloes, that host the first stars and galaxies (5*10^5 Msun < M_halo < 10^10 Msun). We also place new constraints on fuzzy dark matter models of m_a < 5*10^-21 eV/c^2 for the ultra-light boson mass, from the abundance of galaxies with star formation at z > 13 in XLSSC 122. Our exploratory results are consistent with existing constraints. More comprehensive results will be obtained if our approach is extended to a large sample of clusters or field post-starburst galaxies at cosmic noon, with improved modelling of halo and stellar populations.

astro-ph.CO↗

arXiv2020

Maximizing the number of independent sets of fixed size in $K_n$-covered graphs

A graph $G$ is $H$-covered by some given graph $H$ if each vertex in $G$ is contained in a copy of $H$. In this note, we give the maximum number of independent sets of size $t\ge 3$ in $K_n$-covered graphs of size $N\ge n+t-1$ and determine its extremal graph. The result answers a question proposed by Chakraborit and Loh. The proof uses an edge-switching operation of hypergraphs which remains the number of independent sets nondecreasing.

math.CO↗