A BOUND VIOLATION ON THE GALAXY GROUP SCALE: THE TURN-AROUND RADIUS OF NGC 5353/4

Lee, Jounghun,Kim, Suk,Rey, Soo-Chang IOP Publishing 2015 The Astrophysical journal Vol.815 No.1

<P>The first observational evidence for the violation of the maximum turn-around radius on the galaxy group scale is presented. The NGC 5353/4 group is chosen as an ideal target for our investigation of the bound-violation because of its proximity, low-density environment, optimal mass scale, and the existence of a nearby thin straight filament. Using the observational data on the line-of-sight velocities and three-dimensional distances of the filament galaxies located in the bound zone of the NGC 5353/4 group, we construct their radial velocity profile as a function of separation distance from the group center and then compare it to the analytic formula obtained empirically by Falco et al. to find the best-fit value of an adjustable parameter with the help of the maximum likelihood method. The turn-around radius of NGC 5353/4 is determined to be the separation distance where the adjusted analytic formula for the radial velocity profile yields zero. The estimated turn-around radius of NGC 5353/4 turned out to substantially exceed the upper limit predicted by the spherical model based on the ACDM cosmology. Even when the restrictive condition of spherical symmetry is released, the estimated value is found to be only marginally consistent with the ACDM expectation.</P>

Intrinsic Inclination of Galaxies embedded in Cosmic Sheets and Its Cosmological Implications: An Analytic Calculation

Jounghun Lee 한국천문학회 2004 天文學會報 Vol.29 No.2

The three-dimensional structural shape of the gravitational potential in the Local Group

Lee, Bomee,Lee, Jounghun Blackwell Publishing Ltd 2008 Monthly notices of the Royal Astronomical Society Vol.389 No.2

<P>ABSTRACT</P><P>The Local Group is a small galaxy cluster with a membership of 62 nearby galaxies including the Milky Way and M31. Although the Local Group has yet to be virialized, it interacts with the surrounding matter as one gravitationally bound system. To understand the formation and evolution of the Local Group as well as its member galaxies, it is important to reconstruct the gravitational potential field from the surrounding matter distribution in the local cosmic web. By measuring the anisotropy in the spatial distribution of the Local Group galaxies, which is assumed to be induced by the local gravitational tidal field, we resolve the three-dimensional structure of the gravitational potential in the vicinity of the Milky Way smoothed on the Local Group mass scale. Our results show that (i) the minor principal axis of the Local Group tidal field is in the equatorial direction of α<SUB>p</SUB>= 15<SUP>h</SUP>00<SUP>m</SUP> and δ<SUB>p</SUB>= 20°; (ii) it has a prolate shape with axial ratio of 0.5 ± 0.13; (iii) the global tides in the Local Group are quite strong, which may provide a partial explanation for the low abundance of dwarf galaxies in the Local Group.</P>

Quantifying the cosmic web – I. The large-scale halo ellipticity–ellipticity and ellipticity–direction correlations

Lee, Jounghun,Springel, Volker,Pen, Ue-Li,Lemson, Gerard Blackwell Publishing Ltd 2008 Monthly notices of the Royal Astronomical Society Vol.389 No.3

<P>ABSTRACT</P><P>The formation of dark matter haloes tends to occur anisotropically along the filaments of the cosmic web, which induces ellipticity–ellipticity (EE) correlations between the shapes of haloes, as well as ellipticity–direction (ED) cross-correlations between halo shapes and the directions to neighbouring haloes. We analyse the halo catalogue and the semi-analytic galaxy catalogue of the recent Millennium Run Simulation to measure the EE and ED correlations numerically at four different redshifts (<I>z</I>= 0, 0.5, 1 and 2). For the EE correlations, we find that (i) the major-axis correlation is strongest while the intermediate-axis correlation is weakest; (ii) the signal is significant at distances out to 10 <I>h</I><SUP>−1</SUP> Mpc; (iii) the signal decreases as <I>z</I> decreases and (iv) its behaviour depends strongly on the halo mass scale, with larger masses showing stronger correlations at large distances. For the ED correlations, we find that (i) the correlations are much stronger than the EE correlations, and are significant even out to distances of 50 <I>h</I><SUP>−1</SUP> Mpc; (ii) the signal also decreases as <I>z</I> decreases and (iii) it increases with halo mass at all distances. We also provide empirical fitting functions for the EE and ED correlations. The EE correlations are found to scale linearly with the linear density correlation function, ξ(<I>r</I>), while the ED cross-correlation is found to scale as ξ<SUP>1/2</SUP>(<I>r</I>) at large distances beyond 10 <I>h</I><SUP>−1</SUP> Mpc. The best-fitting values of the fitting parameters for the EE and the ED correlations are all determined through χ<SUP>2</SUP>-statistics. Our results may be useful for quantifying the filamentary distribution of dark matter haloes over a wide range of scales.</P>

ALIGNMENTS OF THE GALAXIES IN AND AROUND THE VIRGO CLUSTER WITH THE LOCAL VELOCITY SHEAR

Lee, Jounghun,Rey, Soo Chang,Kim, Suk IOP Publishing 2014 The Astrophysical journal Vol.791 No.1

<P>Observational evidence is presented for the alignment between the cosmic sheet and the principal axis of the velocity shear field at the position of the Virgo cluster. The galaxies in and around the Virgo cluster from the Extended Virgo Cluster Catalog that was recently constructed by Kim et al. are used to determine the direction of the local sheet. The peculiar velocity field reconstructed from the Sloan Digital Sky Survey Data Release 7 is analyzed to estimate the local velocity shear tensor at the Virgo center. Showing first that the minor principal axis of the local velocity shear tensor is almost parallel to the direction of the line of sight, we detect a clear signal of alignment between the positions of the Virgo satellites and the intermediate principal axis of the local velocity shear projected onto the plane of the sky. Furthermore, the dwarf satellites are found to appear more strongly aligned than their normal counterparts, which is interpreted as an indication of the following. (1) The normal satellites and the dwarf satellites fall in the Virgo cluster preferentially along the local filament and the local sheet, respectively. (2) The local filament is aligned with the minor principal axis of the local velocity shear while the local sheet is parallel to the plane spanned by the minor and intermediate principal axes. Our result is consistent with the recent numerical claim that the velocity shear is a good tracer of the cosmic web.</P>

Testing Modified Gravity with the Universal Effect of Large Scale Velocity Shear on the Satellite Infall Directions

Jounghun Lee,Yun-Young Choi 한국천문학회 2014 天文學會報 Vol.39 No.2

AN OBSERVATIONAL SIGNAL OF THE VOID SHAPE CORRELATION AND ITS LINK TO THE COSMIC WEB

Lee, Jounghun,Hoyle, Fiona IOP Publishing 2015 The Astrophysical journal Vol.803 No.1

<P>The shapes of cosmic voids are prone to distortions caused by external tidal forces since their low densities imply a lower internal resistance. This susceptibility of the void shapes to tidal distortions makes them useful as indicators of large-scale tidal and density fields, despite the practical difficulty in defining them. Using the void catalog constructed by Pan et al. from the Seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7), we detect a clear 4 sigma signal of spatial correlations of the void shapes on a scale of 20 h(-1) Mpc and show that the signal is robust against the projection of the void shapes onto the plane of sky. By constructing a simple analytic model for the void shape correlation, within the framework of tidal torque theory, we demonstrate that the void shape correlation function scales linearly with the two-point correlation function of the linear density field. We also find direct observational evidence for the cross-correlation of the void shapes with the large-scale velocity shear field that was linearly reconstructed by Lee et al. from SDSS DR7. We discuss the possibility of using the void shape correlation function to break the degeneracy between the density parameter and the power spectrum amplitude and to independently constrain the neutrino mass as well.</P>

FORMALISM FOR THE SUBHALO MASS FUNCTION IN THE TIDAL-LIMIT APPROXIMATION

LEE JOUNGHUN The Korean Astronomical Society 2005 Journal of The Korean Astronomical Society Vol.38 No.2

We present a theoretical formalism by which the global and the local mass functions of dark matter substructures (dark subhalos) can be analytically estimated. The global subhalo mass function is defined to give the total number density of dark subhalos in the universe as a function of mass, while the local subhalo mass function counts only those sub halos included in one individual host halo. We develop our formalism by modifying the Press-Schechter theory to incorporate the followings: (i) the internal structure of dark halos; (ii) the correlations between the halos and the subhalos; (iii) the subhalo mass-loss effect driven by the tidal forces. We find that the resulting (cumulative) subhalo mass function is close to a power law with the slope of ${\~}$ -1, that the subhalos contribute approximately $10\%$ of the total mass, and that the tidal stripping effect changes the subhalo mass function self-similarly, all consistent with recent numerical detections.

MODIFIED GRAVITY SPINS UP GALACTIC HALOS

Lee, Jounghun,Zhao, Gong-Bo,Li, Baojiu,Koyama, Kazuya IOP Publishing 2013 The Astrophysical journal Vol.763 No.1

<P>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 Lambda CDM model. The largest difference in the specific angular momentum distribution between these two models occurs for the case of isolated galactic halos with mass less than 10(11) h(-1) M-circle dot, which are likely least shielded by the chameleon screening mechanism. As the specific angular momentum of galactic halos is rather insensitive to other cosmological parameters, it can in principle be an independent discriminator of modified gravity. We speculate a possibility of using the relative abundance of low surface brightness galaxies (LSBGs) as a test of general relativity given that the formation of the LSBGs occurs in fast spinning dark halos.</P>

THE ANISOTROPIC TWO-POINT CORRELATION FUNCTIONS OF THE NONLINEAR TRACELESS TIDAL FIELD IN THE PRINCIPAL-AXIS FRAME

Lee, Jounghun,Hahn, Oliver,Porciani, Cristiano IOP Publishing 2009 The Astrophysical journal Vol.705 No.2

<P>Galaxies on the largest scales of the universe are observed to be embedded in the filamentary cosmic web, which is shaped by the nonlinear tidal field. As an efficient tool to quantitatively describe the statistics of this cosmic web, we present the anisotropic two-point correlation functions of the nonlinear traceless tidal field in the principal-axis frame, which are measured using numerical data from an N-body simulation. We show that both the nonlinear density and traceless tidal fields are more strongly correlated along the directions perpendicular to the eigenvectors associated with the largest eigenvalues of the local tidal field. The correlation length scale of the traceless tidal field is found to be similar to 20 h(-1) Mpc, which is much larger than that of the density field similar to 5 h(-1) Mpc. We also provide analytic fitting formulae for the anisotropic correlation functions of the traceless tidal field, which turn out to be in excellent agreement with the numerical results. We expect that our numerical results and analytical formula are useful to disentangle cosmological information from the filamentary network of the large-scale structures.</P>