Yougang Wang

DISTRIBUTION OF SATELLITE GALAXIES IN HIGH-REDSHIFT GROUPS

We use galaxy groups at redshifts between 0.4 and 1.0 selected from the Great Observatories Origins Deep Survey to study the color-morphological properties of satellite galaxies and investigate possible alignment between the distribution of the satellites and the orientation of their central galaxy. We confirm the bimodal color and morphological-type distribution for satellite galaxies at this redshift range: the red and blue classes correspond to the early and late morphological types, respectively, and the early-type satellites are on average brighter than the late-type ones. Furthermore, there is a morphological conformity between the central and satellite galaxies: the fraction of early-type satellites in groups with an early-type central is higher than those with a late-type central galaxy. This effect is stronger at smaller separations from the central galaxy. We find a marginally significant signal of alignment between the major axis of the early-type central galaxy and its satellite system, while for the late-type centrals no significant alignment signal is found. We discuss the alignment signal in the context of shape evolution of groups.

ALIGNMENTS OF GROUP GALAXIES WITH NEIGHBORING GROUPS

Using a sample of galaxy groups found in the Sloan Digital Sky Survey Data Release 4, we measure the following four types of alignment signals: (1) the alignment between the distributions of the satellites of each group relative to the direction of the nearest neighbor group (NNG); (2) the alignment between the major axis direction of the central galaxy of the host group (HG) and the direction of the NNG; (3) the alignment between the major axes of the central galaxies of the HG and the NNG; and (4) the alignment between the major axes of the satellites of the HG and the direction of the NNG. We find strong signal of alignment between the satellite distribution and the orientation of central galaxy relative to the direction of the NNG, even when the NNG is located beyond 3r vir of the host group. The major axis of the central galaxy of the HG is aligned with the direction of the NNG. The alignment signals are more prominent for groups that are more massive and with early-type central galaxies. We also find that there is a preference for the two major axes of the central galaxies of the HG and NNG to be parallel for the system with both early central galaxies, however, not for the systems with both late-type central galaxies. For the orientation of satellite galaxies, we do not find any significant alignment signals relative to the direction of the NNG. From these four types of alignment measurements, we conclude that the large-scale environment traced by the nearby group affects primarily the shape of the host dark matter halo, and hence also affects the distribution of satellite galaxies and the orientation of central galaxies. In addition, the NNG directly affects the distribution of the satellite galaxies by inducing asymmetric alignment signals, and the NNG at very small separation may also contribute a second-order impact on the orientation of the central galaxy in the HG.

BRIGHTEST SATELLITE GALAXY ALIGNMENT OF SLOAN DIGITAL SKY SURVEY GALAXY GROUPS

We study the alignment signal between the distribution of brightest satellite galaxies (BSGs) and the major axis of their host groups using SDSS group catalog constructed by Yang et al. (2007). After correcting for the effect of group ellipticity, a statistically significant (∼ 5σ) major-axis alignment is detected and the alignment angle is found to be 43.0 ± 0.4. More massive and richer groups show stronger BSG alignment. The BSG alignment around blue BCGs is slightly stronger than that around red BCGs. And red BSGs have much stronger major-axis alignment than blue BSGs. Unlike BSGs, other satellites do not show very significant alignment with group major axis. We further explore the BSG alignment in semi-analytic model (SAM) constructed by Guo et al. (2011). We found general good agreement with observations: BSGs in SAM show strong major-axis alignment which depends on group mass and richness in the same way as observations; and none of other satellites exhibit prominent alignment. However, discrepancy also exists in that the SAM shows opposite BSG color dependence, which is most probably induced by the missing of large scale environment ingredient in SAM. The combination of two popular scenarios can explain the detected BSG alignment. The first one: satellites merged into the group preferentially along the surrounding filaments, which is strongly aligned with the major axis of the group. The second one: BSGs enter their host group more recently than other satellites, then will preserve more information about the assembling history and so the major-axis alignment. In SAM, we found positive evidence for the second scenario by the fact that BSGs merged into groups statistically more recently than other satellites. We also found that most of BSGs (80%) were BCGs before they merged into groups and earlier merged BSGs tend to be closer to their BCGs than other BSGs. On the other hand, although is opposite in SAM, the BSG color dependence in observation might indicate the first scenario as well. Subject headings: BSG – alignment – group of galaxies – SAM – SDSS

TWO-DIMENSIONAL TOPOLOGY OF COSMOLOGICAL REIONIZATION

We study the two-dimensional topology of the 21-cm differential brightness temperature for two hydrodynamic radiative transfer simulations and two semi-numerical models. In each model, we calculate the two dimensional genus curve for the early, middle and late epochs of reionization. It is found that the genus curve depends strongly on the ionized fraction of hydrogen in each model. The genus curves are significantly different for different reionization scenarios even when the ionized faction is the same. We find that the two-dimensional topology analysis method is a useful tool to constrain the reionization models. Our method can be applied to the future observations such as those of the Square Kilometer Array.

Peculiar in-plane velocities in the outer disc of the Milky Way

We present the peculiar in-plane velocities derived from the LAMOST red clump stars. From the variations of the in-plane velocity with the Galactocentric radius for the young and old red clump stars, we are able to identify two types of peculiar velocities: 1) both the two red clump populations show that the radial velocity is negative within

Probing satellite galaxies in the Local Group by using FAST

R=9.0

The Local Spiral Arm in the LAMOST-Gaia Common Stars?

\,kpc and becomes positive beyond (denoted as the \emph{long-wave} mode); and 2) the young red clump stars show larger mean radial velocity than the old population by about 3\,km

Study on the spectrum of the injected relativistic protons

\rm s^{-1}

The jet-disk symbiosis model for gamma ray bursts: cosmic ray and neutrino background contribution

between

Black hole to bulge mass correlation in Active Galactic Nuclei: a test for the simple unified formation scheme

R=9