arXiv subjects

Jounghun Lee

Publications and source records attributed to Jounghun Lee.

At least 73 records · Page 4Linked to original sources

arXiv2013

How Does the Surface Density and Size of Disk Galaxies Measured in Hydrodynamic Simulations Correlate with the Halo Spin Parameter?

Late-type low surface brightness galaxies (LSBs) are faint disk galaxies with central maximum stellar surface densities below 100 Msun/pc^2. The currently favored scenario for their origin is that LSBs have formed in fast-rotating halos with large angular momenta. We present the first numerical evidence for this scenario using a suite of self-consistent hydrodynamic simulations of a 2.3e11 Msun galactic halo, in which we investigate the correlations between the disk stellar/gas surface densities and the spin parameter of its host halo. A clear anti-correlation between the surface densities and the halo spin parameter, lambda, is found. That is, as the halo spin parameter increases, the disk cutoff radius at which the stellar surface density drops below 0.1 Msun/pc^2 monotonically increases, while the average stellar surface density of the disk within that radius decreases. The ratio of the average stellar surface density for the case of lambda=0.03 to that for the case of lambda=0.14 reaches more than 15. We demonstrate that the result is robust against variations in the baryon fraction, confirming that the angular momentum of the host halo is an important driver for the formation of LSBs.

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arXiv2013

On the Spin Bias of Satellite Galaxies in the Local Group-like Environment

We utilize the Millennium-II simulation databases to study the spin bias of dark subhalos in the Local Group-like systems which have two prominent satellites with comparable masses. Selecting the group-size halos with total mass similar to that of the Local Group (LG) from the friends-of-friends halo catalog and locating their subhalos from the substructure catalog, we determine the most massive (main) and second to the most massive (submain) ones among the subhalos hosted by each selected halo. When the dimensionless spin parameter (lambda) of each subhalo is derived from its specific angular momentum and circular velocity at virial radius, a signal of correlation is detected between the spin parameters of the subhalos and the main-to-submain mass ratios of their host halos at z=0: The higher main-to-submain mass ratio a host halo has, the higher mean spin parameter its subhalos have. It is also found that the correlations exist even for the subhalo progenitors at z=0.5 and z=1. Our interpretation of this result is that the subhalo spin bias is not a transient effect but an intrinsic property of a LG-like system with higher main-to- submain mass ratio, caused by stronger anisotropic stress in the region. A cosmological implication of our result is also discussed.

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arXiv2013

Massive Neutrinos Promote the Size Growth of Early-Type Galaxies

The effect of massive neutrinos on the evolution of the early type galaxies (ETGs) in size ($R_{e}$) and stellar mass ($M_{\star}$) is explored by tracing the merging history of galaxy progenitors with the help of the robust semi-analytic prescriptions. We show that as the presence of massive neutrinos plays a role of enhancing the mean merger rate per halo, the high-$z$ progenitors of a descendant galaxy with fixed mass evolves much more rapidly in size for a $Λ$MDM ($Λ$CDM + massive neutrinos) model than for the $Λ$CDM case. The mass-normalized size evolution of the progenitor galaxies, $R_{e}[M_{\star}/(10^{11}M_{\odot})]^{-0.57}\propto (1+z)^{-β}$, is found to be quite steep with the power-law index of $β\sim 1.5$ when the neutrino mass fraction is $f_ν=0.05$, while it is $β\sim 1$ when $f_ν=0$. It is concluded that if the presence and role of massive neutrinos are properly taken into account, it may explain away the anomalous compactness of the high-$z$ ETGs compared with the local ellipticals with similar stellar masses.

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arXiv2013

Detection of the Vorticity Effect on the Disk Orientations

We present an observational evidence for the vorticity effect on the nonlinear evolution of the galaxy angular momentum. We first calculate the vorticity as the curl of the peculiar velocity field reconstructed from the 2MASS redshift survey. Then, measuring the alignments between the vorticity and the tidal shear fields, we study how the alignment strength and tendency depends on the cosmic web environment. It is found that in the knot and filament regions the vorticity vectors are anti-aligned with the directions of the maximal volume compression while in the void and sheet regions they are anti-aligned with the directions of the minimal compression. Determining the spin axes of the nearby large face-on and edge-on disk galaxies from the Seventh Data Release of the Sloan Digital Sky Survey and measuring their correlations with the vorticity vectors at the galaxy positions, we finally detect a clear signal of the alignments between the galaxy spin and the local vorticity fields. The null hypothesis that there is no alignment between them is rejected at the 99.999% confidence level. Our result supports observationally the recently proposed scenario that although the galaxy angular momentum originates from the initial tidal interaction in the linear regime its subsequent evolution is driven primarily by the cosmic vorticity field.

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arXiv2013

An Extended Zel'dovich Model for the Halo Mass Function

A new way to construct a fitting formula for the halo mass function is presented. Our formula is expressed as a solution to the modified Jedamzik matrix equation that automatically satisfies the normalization constraint. The characteristic parameters expressed in terms of the linear shear eigenvalues are empirically determined by fitting the analytic formula to the numerical results from the high-resolution N-body simulation and found to be independent of scale, redshift and background cosmology. Our fitting formula with the best-fit parameters is shown to work excellently in the wide mass-range at various redshifts: The ratio of the analytic formula to the N-body results departs from unity by up to 10% and 5% over 10^{11}<= M/(M_sun/h)<= 5x10^{15} at z=0,\ 0.5 and 1 for the FoF-halo and SO-halo cases, respectively.

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arXiv2012

Modified Gravity Spins Up Galactic Halos

We investigate the effect of modified gravity on the specific angular momentum of galactic halos by analyzing the halo catalogs at z=0 from high-resolution N-body simulations for a f(R) gravity model that meets the solar-system constraint. It is shown that the galactic halos in the f(R) gravity model tend to acquire significantly higher specific angular momentum than those in the standard LCDM model. The largest difference in the specific angular momentum distribution between these two models occurs for the case of the isolated galactic halos with mass less than 10^{11}M_sun/h, which are likely least shielded by the chameleon screening mechanism. As the specific angular momentum of galactic halos is rather insensitive to the other cosmological parameters, it can in principle be an independent discriminator of modified gravity. We speculate a possibility of using the relative abundance of the low surface brightness galaxies (LSBGs) as a test of GR given that the formation of the LSBGs befalls in the fast spinning dark halos.

astro-ph.CO↗

arXiv2012

The Relative Abundance of Isolated Clusters as a Probe of Dark Energy

Those galaxy clusters which do not belong to the superclusters are referred to as the isolated clusters. Their relative abundance at a given epoch may be a powerful constraint of the dark energy equation of state since it depends strongly on how fast the structures grow on the largest scale in the Universe. We note that the mass function of the isolated clusters can be separately evaluated through the modification of the recently developed Corasaniti-Achitouv (CA) theory according to which the stochastic collapse barrier is quantified by two coefficients: the drifting average coefficient (β) and the diffusion coefficient (D_B). Regarding $β$ in the CA formalism as an adjustable parameter and assuming that the formation of isolated clusters corresponds to the case of D_B=0, we determine the mass function of the isolated clusters by fitting the numerical results from the MICE simulations to the modified CA formula. It is found that the best-fit value of beta changes with redshift and that the CA mass function with D_B=0 agrees very well with the numerical results at various redshifts. Defining the relative abundance of the isolated clusters, xi_I, as the ratio of the cumulative mass function of the isolated clusters to that of the non-isolated clusters at a given epoch, we finally show how sensitively xi_I changes with the dark energy equation of state. It is also discussed how xi_I can help to break the degeneracy between the dark energy equation of state and the other key cosmological parameters.

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arXiv2012

The Spin Alignments in Galaxy Pairs as a Test of Bouncing Coupled Dark Energy

We investigate the effect of coupled dark energy (cDE) on the spin alignments in isolated pairs of galactic halos, using the publicly available data from the hydrodynamical cDE simulations (H-CoDECs) which were run for various cDE models such as EXP001, EXP002, EXP003 (with exponential potential and constant coupling), EXP008e3 (with exponential potential and exponential coupling) and SUGRA003 (with supergravity potential and negative constant coupling) as well as for a standard LCDM cosmology (with the WMAP7 parameters). Measuring the cosines of the angles between the spin axes in isolated pairs of galactic halos for each model and determining its probability density distribution, we show that for the SUGRA003 model with bouncing cDE the null hypothesis of no spin alignment in pairs of galactic halos is rejected at 99.999% confidence level. In contrast, the LCDM cosmology yields no significant signal of spin alignment and the other four cDE models also exhibit only weak signals of spin-alignments. The strength of the spin alignment signal is found to be almost independent of the total halo mass and separation distance in galaxy pairs. Showing also that no signal is detected from the Sloan Digital Sky Survey DR 7, we conclude that the spin alignments in galaxy pairs is in principle a unique test of bouncing cDE models.

astro-ph.CO↗

arXiv2012

Voids in Redshift Space

We study the ellipticity probability distribution function (PDF) of voids in redshift space with galaxies as tracers of the shapes of voids. We find that the redshift space distortion on the shape of voids statistically increases the ellipticities of voids, and leaves a prominent feature on the ellipticity PDF as a substantial reduction in the probability of having voids with small ellipticity. The location of this characteristic cutoff of the ellipticity PDF is an explicit function of the logarithmic growth rate, and it can be used as a probe of cosmology once the radial density profile of voids is better understood. However, the biggest limiting factor for the use of ellipticity PDF as a probe of cosmology lies in the Poisson noise from a small number of galaxies to define the shape of a given void. This Poisson noise creates a significant contamination of the resulting ellipticity PDF so that the shape of the original PDF is almost washed-out. Nevertheless, there is a way to overcome the Poisson noise via the Alcock Paczynski test on the shape of stacked voids. In redshift space, since the void is elongated toward the line of sight, the stacked void has non-zero ellipticity, which can be a tell-tale of the logarithmic growth rate. Although some useful information of void ellipticity will be lost by stacking, in this way, we can see the effect of redshift space distortion as a source of anisotropy in the stacked void ellipticity. We think that the stacking analysis of the voids in redshift space is potentially a powerful tool to probe the cosmology.

astro-ph.CO↗

arXiv2012

Modeling the Alignment Profile of Satellite Galaxies in Clusters

Analyzing the halo and galaxy catalogs from the Millennium simulations at redshifts $z=0,\ 0.5,\ 1$, we determine the alignment profiles of cluster galaxies by measuring the average alignments between the major axes of the pseudo inertia tensors from all satellites within cluster's virial radius and from only those satellites within some smaller radius as a function of the top-hat scale difference. The alignment profiles quantify how well the satellite galaxies retain the memory of the external tidal fields after merging into their host clusters and how fast they lose the initial alignment tendency as the cluster's relaxation proceeds. It is found that the alignment profile drops faster at higher redshifts and on smaller mass scales. This result is consistent with the picture that the faster merging of the satellites and earlier onset of the nonlinear effect inside clusters tend to break the preferential alignments of the satellites with the external tidal fields. Modeling the alignment profile of cluster galaxies as a power-law of the density correlation coefficient that is independent of the power spectrum normalization ($σ_{8}$) and demonstrating that the density correlation coefficient varies sensitively with the density parameter ($Ω_{m}$) and neutrino mass fraction ($f_ν$), we suggest that the alignment profile of cluster galaxies might be useful for breaking the $Ω_{m}$-$σ_{8}$ and $f_ν$-$σ_{8}$ degeneracies.

astro-ph.CO↗

arXiv2011

Can Coupled Dark Energy Speed Up the Bullet Cluster?

It has been recently shown that the observed morphological properties of the Bullet Cluster can be accurately reproduced in hydrodynamical simulations only when the infall pairwise velocity V_{c} of the system exceeds 3000km/s (or at least possibly 2500 km/s) at the pair separation of 2R_{vir}, where R_{vir} is the virial radius of the main cluster, and that the probability of finding such a bullet-like system is extremely low in the standard ΛCDM cosmology. We suggest here the fifth-force mediated by a coupled Dark Energy (cDE) as a possible velocity-enhancing mechanism and investigate its effect on the infall velocities of the bullet-like systems from the CoDECS (COupled Dark Energy Cosmological Simulations) public database. Five different cDE models are considered: three with constant coupling and exponential potential, one with exponential coupling and exponential potential, and one with constant coupling and supergravity potential. For each model, after identifying the bullet-like systems, we determine the probability density distribution of their infall velocities at the pair separations of (2-3)R_{vir}. Approximating each probability density distribution as a Gaussian, we calculate the cumulative probability of finding a bullet-like system with V_{c}>=3000 km/s or V_{c}>=2500 km/s. Our results show that in all of the five cDE models the cumulative probabilities increase compared to the ΛCDM case and that in the model with exponential coupling P(V_{c}>=2500 km/s) exceeds 10^{-4}. The physical interpretations and cosmological implications of our results are provided.

astro-ph.CO↗

arXiv2011

Merging Rates of the First Objects and the Formation of First Mini-Filaments in Models with Massive Neutrinos

We study the effect of massive neutrinos on the formation and evolution of the first filaments containing the first star-forming halos of mass M~10^{6}M_sun at z~20. With the help of the extended Press-Schechter formalism, we evaluate analytically the rates of merging of the first star-forming halos into zero-dimensional larger halos and one-dimensional first filaments. It is shown that as the neutrino mass fraction f_ν increases, the halo-to-filament merging rate increases while the halo-to-halo merging rate decreases sharply. For f_ν<=0.04, the halo-to-filament merging rate is negligibly low at all filament mass scales, while for f_ν>=0.07 the halo-to-filament merging rate exceeds 0.1 at the characteristic filament mass scale of ~10^{9}M_sun. The distribution of the redshifts at which the first filaments ultimately collapse along their longest axes is derived and found to have a sharp maximum at z~8. We also investigate the formation and evolution of the second generation filaments which contain the first galaxies of mass 10^{9}M_sun at z=8 as the parent of the first generation filaments. A similar trend is found: For f_ν>= 0.07 the rate of clustering of the first galaxies into the second-generation filaments exceeds 0.3 at the characteristic mass scale of ~10^{11}M_sun. The longest-axis collapse of these second-generation filaments are found to occur at z~3. The implications of our results on the formation of massive high-z galaxies and the early metal enrichment of the intergalactic media by supernova-driven outflows, and possibility of constraining the neutrino mass from the mass distribution of the high-z central blackholes are discussed.

astro-ph.CO↗

arXiv2011

The Effect of Coupled Dark Energy on the Alignment between Dark Matter and Galaxy Distributions in Clusters

We investigate the effects of a coupled Dark Energy (cDE) scalar field on the alignment between satellites and matter distributions in galaxy clusters. Using high-resolution N-body simulations for LCDM and cDE cosmological models, we compute the probability density distribution for the alignment angle between the satellite galaxies and underlying matter distributions, finding a difference between the two scenarios. With respect to LCDM, in cDE cosmologies the satellite galaxies are less preferentially located along the major axis of the matter distribution, possibly reducing the tension with obersevational data. A physical explanation is that the coupling between dark matter and dark energy acts as an additional tidal force on the satellite galaxies diminishing the alignments between their distribution and the matter one. Through a Wald test based on the generalized chi-square statistics, the null hypothesis that the two probability distributions come from the same parent population is rejected at the 99 % confidence level. It is concluded that the galaxy-matter alignment in clusters may provide a unique probe of dark sector interactions as well as the nature of dark energy.

astro-ph.CO↗

arXiv2011

On the Intrinsic Alignments of the Late-Type Spiral Galaxies from the Sloan Digital Sky Survey Data Release 7

A robust detection of the tidally induced intrinsic alignments of the late-type spiral galaxies with high statistical significance is reported. From the spectroscopic galaxy sample of SDSS DR7 compiled by Huertas-Company et al. which lists each galaxy's probabilities of being in five Hubble types, P(E),P(Ell), P(S0),P(Sab), P(Scd), we select the nearby large late-type spiral galaxies which have redshifts of 0<=z<=0.02, probabilities of P(Scd)>=0.5 and angular sizes of D>=7.92 arcsec. The spin axes of the selected nearby large late-type spiral galaxies are determined up to the two-fold ambiguity with the help of the circular thin-disk approximation and their spatial correlations are measured as a function of the separation distance r. A clear signal of the intrinsic correlation as high as 3.4 sigma and 2.4 sigma is found at the separation distance of r~1Mpc/h and r~2Mpc/h, respectively. The comparison of this observational results with the analytic model based on the tidal torque theory reveals that the spin correlation function for the late-type spiral galaxies follow the quadratic scaling of the linear density correlation and that the intrinsic correlations of the galaxy spin axes are stronger than that of the underlying dark halos. We investigate a local density dependence of the galaxy spin correlations and found that the correlations are stronger for the galaxies located in dense regions having more than 10 neighbors within 2Mpc/h. We also attempt to measure a luminosity dependence of the galaxy spin correlations, but find that it is impossible with our magnitude-split samples to disentangle a luminosity from a redshift dependence. We provide the physical explanations for these observational results and also discuss the effects of possible residual systematics on the results.

astro-ph.CO↗

arXiv2010

The Misalignments between Matter and Galaxy Distributions in Triaxial Clusters: A Signature of a Possible Fifth Force?

The standard structure formation model based on a LCDM cosmology predicts that the galaxy clusters have triaxial shapes and that the cluster galaxies have a strong tendency to be located preferentially along the major axes of host cluster's dark matter distributions due to the gravitational tidal effect. The predicted correlations between dark matter and galaxy distributions in triaxial clusters are insensitive to the initial cosmological parameters and to the galaxy bias, and thus can provide a unique test-bed for the nonlinear structure formation of the LCDM cosmology. Recently, Oguri et al. determined robustly the dark matter distributions in the galaxy clusters using the two dimensional weak lensing shear fitting and showed that the orientations of the cluster galaxy distributions are only very weakly correlated with those of the underlying dark matter distributions determined robustly, which is in contrast to with the LCDM-based prediction. We reanalyze and compare quantitatively the observational result with the LCDM-based prediction from the Millennium Run simulation with the help of the bootstrap resampling and generalized chi^{2}-statistics. The hypothesis that the observational result is consistent with the LCDM-based prediction is ruled out at the 99% confidence level. A local fifth force induced by a non-minimal coupling between dark energy and dark matter might be responsible for the observed misalignments between dark matter and galaxy distributions in triaxial clusters.

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arXiv2010

Detection of Large-Scale Cosmic Magnetic Fields

Observational evidence for the existence of cosmic magnetic fields in intergalactic space coherent over Mpc scales is presented. Using an unprecedentedly large sample of Faraday rotation measures of radio sources from the NRAO VLA Sky Survey data and the photometric redshift galaxy catalog from the sixth Data Release of the Sloan Digital Sky Survey (SDSS), we measure the cross-correlations between the rotation measures and the galaxy density field distributed along the source sightlines. It is shown that the rotation measures for sightlines passing through high density regions at separation r >= 1 Mpc/h from the locations of background radio sources are significantly enhanced. We discuss possible generators of this enhancement and interpret it to be intergalactic magnetic fields coherent over 1 Mpc/h with mean field strength B~30 nG.

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arXiv2010

Bullet Cluster: A Challenge to LCDM Cosmology

To quantify how rare the bullet-cluster-like high-velocity merging systems are in the standard LCDM cosmology, we use a large-volume 27 (Gpc/h)^3 MICE simulation to calculate the distribution of infall velocities of subclusters around massive main clusters. The infall-velocity distribution is given at (1-3)R_{200} of the main cluster (where R_{200} is similar to the virial radius), and thus it gives the distribution of realistic initial velocities of subclusters just before collision. These velocities can be compared with the initial velocities used by the non-cosmological hydrodynamical simulations of 1E0657-56 in the literature. The latest parameter search carried out recently by Mastropietro and Burkert showed that the initial velocity of 3000 km/s at about 2R_{200} is required to explain the observed shock velocity, X-ray brightness ratio of the main and subcluster, and displacement of the X-ray peaks from the mass peaks. We show that such a high infall velocity at 2R_{200} is incompatible with the prediction of a LCDM model: the probability of finding 3000 km/s in (2-3)R_{200} is between 3.3X10^{-11} and 3.6X10^{-9}. It is concluded that the existence of 1E0657-56 is incompatible with the prediction of a LCDM model, unless a lower infall velocity solution for 1E0657-56 with < 1800 km/s at 2R_{200} is found.

astro-ph.CO↗

arXiv2010

A Scaling Relation of the Evolving Tidal Fields in a LCDM Cosmology

We report the finding of a scaling relation among the cosmic-web anisotropy parameter $A$, the linear density rms fluctuation sigma(r) and the linear growth factor D(z). Using the tidal field derived from the Millennium Simulation on 512^{3} grids at z=0, 2, 5 and 127, we calculate the largest eigenvalues $λ$ of the local tidal tensor at each grid resolution and measure its distance-averaged two-point correlation function, xi_{lambda}, as a function of the cosines of polar angles cos(theta) in the local principal axis frame. We show that xi_{lambda} is quite anisotropic, increasing toward the directions of minimal matter compression, and that the anisotropy of xi_{lambda} increases as the redshift, z, decreases and as the upper distance cutoff r_{c} decreases. Fitting the numerical results to an analytic fitting model ξ_λ(\cosθ)\propto (1+A\cos^{n}θ)^{-1}, it is found that the best fit value of A, dubbed the cosmic-web anisotropy parameter, varies systematically with sigma(r_{c}) and D(z), allowing us to determine the simple empiral scaling relation A(r_{c},z)=0.8, D^{0.76}(z), sigma (r_{c})$.

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