arXiv subjects

Xiaohu Yang

Publications and source records attributed to Xiaohu Yang.

At least 289 records · Page 16Linked to original sources

arXiv2007

The Importance of Satellite Quenching for the Build-Up of the Red Sequence of Present Day Galaxies

In the current paradigm, red sequence galaxies are believed to have formed as blue disk galaxies that subsequently had their star formation quenched. Since red-sequence galaxies typically have an early-type morphology, the transition from the blue to the red sequence also involves a morphological transformation. In this paper we study the impact of transformation mechanisms that operate only on satellite galaxies, such as strangulation, ram-pressure stripping and galaxy harassment. Using a large galaxy group catalogue constructed from the SDSS, we compare the colors and concentrations of satellites galaxies to those of central galaxies of the same stellar mass, adopting the hypothesis that the latter are the progenitors of the former. On average, satellites are redder and more concentrated than central galaxies of the same stellar mass. Central-satellite pairs that are matched in both stellar mass and color, however, show no average concentration difference, indicating that the transformation mechanisms affect color more than morphology. The color and concentration differences of matched central-satellite pairs are completely independent of the halo mass of the satellite galaxy, indicating that satellite-specific transformation mechanisms are equally efficient in haloes of all masses. This strongly favors strangulation as the main quenching mechanism for satellite galaxies. Finally, we determine the relative importance of satellite quenching for the build-up of the red sequence. We find that roughly 70 percent of red sequence satellite galaxies with a stellar mass of 10^9 Msun had their star formation quenched as satellites. This drops rapidly to zero with increasing stellar mass, indicating that a significant fraction of red satellites were already quenched before they became a satellite.

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arXiv2007

Ongoing Assembly of Massive Galaxies by Major Merging in Large Groups and Clusters from the SDSS

We investigate the incidence of major mergers creating >10e11 Msun galaxies in present-day groups and clusters more massive than 2.5e13 Msun. We identify 38 pairs of massive galaxies with mutual tidal interaction signatures selected from >5000 galaxies with >5e10 Msun that reside in 845 such groups. We fit the images of each galaxy pair as the line-of-sight projection of symmetric models and identify mergers by the presence of residual asymmetries around each progenitor, such as off-center isophotes, broad tidal tails, and dynamical friction wakes. At the resolution and sensitivity of the SDSS, such mergers are found in 16% of high-mass, galaxy-galaxy pairs with magnitude differences of <1.5 and <30 kpc projected separations. We find that 90% of these mergers have nearly equal-mass progenitors with red-sequence colors and centrally-concentrated morphologies, the hallmarks of dissipationless merger simulations. Mergers at group centers are more common than between 2 satellites, but both are morphologically indistinguishable and we tentatively conclude that the latter are likely located at the dynamical centers of recently accreted subhalos. The frequency of central and satellite merging diminishes with group mass consistent with dynamical friction expectations. Based on reasonable assumptions, the centers of these massive halos are growing in stellar mass by 1-9% per Gyr, on average. Compared to all LRG-LRG mergers, we find a 2-9 times higher rate for their merging when restricted to these dense environments. Our results imply that the massive end of the galaxy population continues to evolve hierarchically at a measurable level, and that the centers of massive groups are the preferred environment for merger-driven galaxy assembly. (abridged)

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arXiv2007

Three Different Types of Galaxy Alignment within Dark Matter Halos

Using a large galaxy group catalogue based on the Sloan Digital Sky Survey Data Release 4 we measure three different types of intrinsic galaxy alignment within groups: halo alignment between the orientation of the brightest group galaxies (BGG) and the distribution of its satellite galaxies, radial alignment between the orientation of a satellite galaxy and the direction towards its BGG, and direct alignment between the orientation of the BGG and that of its satellites. In agreement with previous studies we find that satellite galaxies are preferentially located along the major axis. In addition, on scales r < 0.7 Rvir we find that red satellites are preferentially aligned radially with the direction to the BGG. The orientations of blue satellites, however, are perfectly consistent with being isotropic. Finally, on scales r < 0.1 \Rvir, we find a weak but significant indication for direct alignment between satellites and BGGs. We briefly discuss the implications for weak lensing measurements.

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arXiv2007

The cross-correlation between galaxies of different luminosities and Colors

We study the cross-correlation between galaxies of different luminosities and colors, using a sample selected from the SDSS Dr 4. Galaxies are divided into 6 samples according to luminosity, and each of these samples is divided into red and blue subsamples. Projected auto-correlation and cross-correlation is estimated for these subsample. At projected separations r_p > 1\mpch, all correlation functions are roughly parallel, although the correlation amplitude depends systematically on luminosity and color. On r_p < 1\mpch, the auto- and cross-correlation functions of red galaxies are significantly enhanced relative to the corresponding power laws obtained on larger scales. Such enhancement is absent for blue galaxies and in the cross-correlation between red and blue galaxies. We esimate the relative bias factor on scales r > 1\mpch for each subsample using its auto-correlation function and cross-correlation functions. The relative bias factors obtained from different methods are similar. For blue galaxies the luminosity-dependence of the relative bias is strong over the luminosity range probed (-23.0<M_r < -18.0),but for red galaxies the dependence is weaker and becomes insignificant for luminosities below L^*. To examine whether a significant stochastic/nonlinear component exists in the bias relation, we study the ratio R_ij= W_{ii}W_{jj}/W_{ij}^2, where W_{ij} is the projected correlation between subsample i and j. We find that the values of R_ij are all consistent with 1 for all-all, red-red and blue-blue samples, however significantly larger than 1 for red-blue samples. For faint red - faint blue samples the values of R_{ij} are as high as ~ 2 on small scales r_p < 1 \mpch and decrease with increasing r_p.

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arXiv2007

The Alignment between Satellites and Central Galaxies: Theory vs. Observations

Recent studies have shown that the distribution of satellite galaxies is preferentially aligned with the major axis of their central galaxy. The strength of this alignment has been found to depend strongly on the colours of the satellite and central galaxies, and only weakly on the mass of the halo in which the galaxies reside. In this paper we study whether these alignment signals, and their dependence on galaxy and halo properties, can be reproduced in a hierarchical structure formation model of a $Λ$CDM concordance cosmology. To that extent we use a large $N$-body simulation which we populate with galaxies following a semi-analytical model for galaxy formation. We find that if the orientation of the central galaxy is perfectly aligned with that of its dark matter halo, then the predicted central-satellite alignment signal is much stronger than observed. If, however, the minor axis of a central galaxy is perfectly aligned with the angular momentum vector of its dark matter halo, we can accurately reproduce the observed alignment strength as function of halo mass and galaxy color. Although this suggests that the orientation of central galaxies is governed by the angular momentum of their dark matter haloes, we emphasize that any other scenario in which the minor axes of central galaxy and halo are misaligned by $\sim 40^{\circ}$ (on average) will match the data equally well. Finally, we show that dependence of the alignment strength on the color of the central galaxy is most likely an artefact due to interlopers in the group catalogue. The dependence on the color of the satellite galaxies, on the other hand, is real and owes to the fact that red satellites are associated with subhaloes that were more massive at their time of accretion.

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arXiv2006

Towards a Concordant Model of Halo Occupation Statistics

We use the conditional luminosity function (CLF) and data from the 2dFGRS to constrain the average relation between light and mass in a LCDM cosmology with Omega_m=0.23 and sigma_8=0.74 (hereafter WMAP3 cosmology). Reproducing the observed luminosity dependence of the galaxy two-point correlation function results in average mass-to-light ratios that are about 35 percent lower than in a LCDM cosmology with Omega_m=0.3 and sigma_8=0.9 (hereafter WMAP1 cosmology). This removes an important problem with previous halo occupation models which had a tendency to predict cluster mass-to-light ratios that were too high. For the WMAP3 cosmology our model yields average mass-to-light ratios, central galaxy luminosities, halo occupation numbers, satellite fractions, and luminosity-gap statistics, that are all in excellent agreement with those obtained from a 2dFGRS group catalogue and from other independent studies. We also use our CLF model to compute the probability distribution P(M|L_cen), that a central galaxy of luminosity L_cen resides in a halo of mass M. We find this distribution to be much broader than what is typically assumed in HOD models, which has important implications for the interpretation of galaxy-galaxy lensing data. Finally, reproducing the luminosity dependence of the pairwise velocity dispersions in the 2dFGRS requires relatively low mass-to-light ratios for clusters and a satellite fraction that decreases strongly with increasing luminosity. This is only marginally consistent with our CLF constraints. We argue that a cosmology with parameters between those of the WMAP1 and WMAP3 cosmologies is likely to yield results with a higher level of consistency.

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arXiv2006

Weak Lensing by Galaxies in Groups and Clusters: I.--Theoretical Expectations

Galaxy-galaxy lensing is rapidly becoming one of the most promising means to accurately measure the average relation between galaxy properties and halo mass. In order to obtain a signal of sufficient signal-to-noise, one needs to stack many lens galaxies according to their property of interest, such as luminosity or stellar mass. Since such a stack consists of both central and satellite galaxies, which contribute very different lensing signals, the resulting shear measurements can be difficult to interpret. In the past, galaxy-galaxy lensing studies have either completely ignored this problem, have applied rough isolation criteria in an attempt to preferentially select `central' galaxies, or have tried to model the contribution of satellites explicitely. However, if one is able to {\it a priori} split the galaxy population in central and satellite galaxies, one can measure their lensing signals separately. This not only allows a much cleaner measurement of the relation between halo mass and their galaxy populations, but also allows a direct measurement of the sub-halo masses around satellite galaxies. In this paper, we use a realistic mock galaxy redshift survey to show that galaxy groups, properly selected from large galaxy surveys, can be used to accurately split the galaxy population in centrals and satellites. Stacking the resulting centrals according to their group mass, estimated from the total group luminosity, allows a remarkably accurate recovery of the masses and density profiles of their host haloes. In addition, stacking the corresponding satellite galaxies according to their projected distance from the group center yields a lensing signal that can be used to accurate measure the masses of both sub-haloes and host haloes. (Abridged)

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arXiv2006

The Dependence of Galaxy Type on Host Halo Mass

We examine the relation between galaxy properties and environment in the SDSS DR2, quantifying environment in terms of the mass of the host halo, which is obtained with a new iterative group finder. We find that galaxy type fractions scale strongly and smoothly with halo mass, but, at fixed mass, not with luminosity. We compare these findings with the semi-analytical galaxy formation model of Croton et al. (2006). The discrepancies we find can be explained with an oversimplified implementation of strangulation, the neglect of tidal stripping, and shortcomings in the treatments of dust extinction and/or AGN feedback.

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arXiv2006

Properties of Galaxy Groups in the SDSS: II.- AGN Feedback and Star Formation Truncation

Successfully reproducing the galaxy luminosity function and the bimodality in the galaxy distribution requires a mechanism that can truncate star formation in massive haloes. Current models of galaxy formation consider two such truncation mechanisms: strangulation, which acts on satellite galaxies, and AGN feedback, which predominantly affects central galaxies. The efficiencies of these processes set the blue fraction of galaxies as function of galaxy luminosity and halo mass. In this paper we use a galaxy group catalogue extracted from the Sloan Digital Sky Survey (SDSS) to determine these fractions. To demonstrate the potential power of this data as a benchmark for galaxy formation models, we compare the results to the semi-analytical model for galaxy formation of Croton et al. (2006). Although this model accurately fits the global statistics of the galaxy population, as well as the shape of the conditional luminosity function, there are significant discrepancies when the blue fraction of galaxies as a function of mass and luminosity is compared between the observations and the model. In particular, the model predicts (i) too many faint satellite galaxies in massive haloes, (ii) a blue fraction of satellites that is much too low, and (iii) a blue fraction of centrals that is too high and with an inverted luminosity dependence. In the same order, we argue that these discrepancies owe to (i) the neglect of tidal stripping in the semi-analytical model, (ii) the oversimplified treatment of strangulation, and (iii) improper modeling of dust extinction and/or AGN feedback. The data presented here will prove useful to test and calibrate future models of galaxy formation and in particular to discriminate between various models for AGN feedback and other star formation truncation mechanisms.

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arXiv2006

The Alignment between the Distribution of Satellites and the Orientation of their Central Galaxy

We use galaxy groups selected from the Sloan Digital Sky Survey to examine the alignment between the orientation of the central galaxy (defined as the brightest group member) and the distribution of satellite galaxies. By construction, we therefore only address the alignment on scales smaller than the halo virial radius. We find a highly significant alignment of satellites with the major axis of their central galaxy. This is in qualitative agreement with the recent study of Brainerd (2005), but inconsistent with several previous studies who detected a preferential minor axis alignment. The alignment strength in our sample is strongest between red central galaxies and red satellites. On the contrary, the satellite distribution in systems with a blue central galaxy is consistent with isotropic. We also find that the alignment strength is stronger in more massive haloes and at smaller projected radii from the central galaxy. In addition, there is a weak indication that fainter (relative to the central galaxy) satellites are more strongly aligned. We present a detailed comparison with previous studies, and discuss the implications of our findings for galaxy formation.

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arXiv2006

Observational Evidence for an Age Dependence of Halo Bias

We study the dependence of the cross-correlation between galaxies and galaxy groups on group properties. Confirming previous results, we find that the correlation strength is stronger for more massive groups, in good agreement with the expected mass dependence of halo bias. We also find, however, that for groups of the same mass, the correlation strength depends on the star formation rate (SFR) of the central galaxy: at fixed mass, the bias of galaxy groups decreases as the SFR of the central galaxy increases. We discuss these findings in light of the recent findings by Gao et al (2005) that halo bias depends on halo formation time, in that halos that assemble earlier are more strongly biased. We also discuss the implication for galaxy formation, and address a possible link to galaxy conformity, the observed correlation between the properties of satellite galaxies and those of their central galaxy.

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arXiv2005

Properties of Galaxy Groups in the SDSS: I.-- The Dependence of Colour, Star Formation, and Morphology on Halo Mass

Using a large galaxy group catalogue constructed from the SDSS, we investigate the correlation between various galaxy properties and halo mass. We split the population of galaxies in early types, late types, and intermediate types, based on their colour and specific star formation rate. At fixed luminosity, the early type fraction increases with increasing halo mass. Most importantly, this mass dependence is smooth and persists over the entire mass range probed, without any break or feature at any mass scale. We argue that the previous claim of a characteristic feature on galaxy group scales is an artefact of the environment estimators used. At fixed halo mass, the luminosity dependence of the type fractions is surprisingly weak: galaxy type depends more strongly on halo mass than on luminosity. We also find that the early type fraction decreases with increasing halo-centric radius. Contrary to previous studies, we find that this radial dependence is also present in low mass haloes. The properties of satellite galaxies are strongly correlated with those of their central galaxy. In particular, the early type fraction of satellites is significantly higher in a halo with an early type central galaxy than in a halo of the same mass but with a late type central galaxy. This phenomenon, which we call `galactic conformity', is present in haloes of all masses and for satellites of all luminosities. Finally, the fraction of intermediate type galaxies is always ~20 percent, independent of luminosity, independent of halo mass, independent of halo-centric radius, and independent of whether the galaxy is a central galaxy or a satellite galaxy. We discuss the implications of all these findings for galaxy formation and evolution.

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arXiv2005

Preheating by Previrialization and its Impact on Galaxy Formation

We use recent observations of the HI-mass function to constrain galaxy formation. The data conflicts with the standard model where most of the gas in a low-mass dark matter halo is assumed to settle into a disk of cold gas that is depleted by star formation and supernova-driven outflows until the disk becomes gravitationally stable. A consistent model can be found if low-mass haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev cm^2. Such a model simultaneously matches the faint-end slope of the galaxy luminosity function. We propose a preheating model where the medium around low-mass haloes is preheated by gravitational pancaking. Since gravitational tidal fields suppress the formation of low-mass haloes while promoting that of pancakes, the formation of massive pancakes precedes that of the low-mass haloes within them. We demonstrate that the progenitors of present-day dark matter haloes with M<10^{12}h^{-1}\msun were embedded in pancakes of masses $~5x10^{12}h^{-1}\msun$ at z~2. The formation of such pancakes heats the gas to a temperature of 5x10^5K and compresses it to an overdensity of ~10. Such gas has a cooling time that exceeds the age of the Universe at z~2, and has a specific entropy of ~15Kev cm^2, almost exactly the amount required to explain the stellar and HI mass functions. (Abridged)

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arXiv2005

The Cross-Correlation between Galaxies and Groups: Probing the Galaxy Distribution in and around Dark Matter Haloes

We determine the cross-correlation function between galaxies and galaxy groups, using both the Two-Degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS). We study the cross-correlation as a function of group mass, and as a function of the luminosity, stellar mass, colour, spectral type and specific star formation rate of the galaxies. All these cross-correlation functions show a clear transition from the `1-halo' to the `2-halo' regimes on a scale comparable to the virial radius of the groups in consideration. On scales larger than the virial radius, all cross-correlation functions are roughly parallel, consistent with the linear bias model. In particular, the large scale correlation amplitudes are higher for more massive groups, and for brighter and redder galaxies. In the `1-halo' regime, the cross-correlation function depends strongly on the definition of the group center. We consider both a luminosity-weighted center (LWC) and a center defined by the location of the brightest group galaxy (BGC). With the first definition, the bright early-type galaxies in massive groups are found to be more centrally concentrated than the fainter, late-type galaxies. Using the BGC, and excluding the brightest galaxy from the cross correlation analysis, we only find significant segregation in massive groups ($M \gta 10^{13}h^{-1}\msun$) for galaxies of different spectral types (or colours or specific star formation rates). In haloes with masses $\la 10^{13}h^{-1}\msun$, there is a significant deficit of bright satellite galaxies. Comparing the results from the 2dFGRS with those obtained from realistic mock samples, we find that the distribution of galaxies in groups is much less concentrated than dark matter haloes predicted by the current $Λ$CDM model. (Abridged)

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arXiv2005

The Phase-Space Parameters of Brightest Halo Galaxies

The brightest galaxy in a dark matter halo is expected to reside at rest at the center of the halo. In this paper we test this `Central Galaxy Paradigm' using group catalogues extracted from the 2dFGRS and the SDSS. For each group we compute a parameter, R, defined as the difference between the velocity of the brightest group galaxy and the average velocity of the other group members (hereafter satellites), normalized by the unbiased estimator of the velocity dispersion of the satellite galaxies. Using detailed mock galaxy redshift surveys, we show that the central galaxy paradigm is inconsistent with the data at high confidence, and that instead the brightest halo galaxies have a specific kinetic energy that is about 25 percent of that of the satellites. This indicates that either central galaxies reside at the minimum of the dark matter potential, but that the halo itself is not yet fully relaxed, or, that the halo is relaxed, but that the central galaxy oscillates in its potential well. The former is consistent with the fact that we find a weak hint that the velocity bias of brightest halo galaxies is larger in more massive haloes, while the latter may be indicative of cored, rather than cusped, dark matter haloes. We discuss several implications of these findings, including mass estimates based on satellite kinematics, strong gravitational lensing, halo occupation models, and the frequency and longevity of lopsidedness in disk galaxies.

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arXiv2004

Galaxy Occupation Statistics of Dark Matter Haloes: Observational Results

We study the occupation statistics of galaxies in dark matter haloes using galaxy groups identified from the 2-degree Field Galaxy Redshift Survey with the halo-based group finder of Yang et al. The occupation distribution is considered separately for early and late type galaxies, as well as in terms of central and satellite galaxies. The mean luminosity of the central galaxies scales with halo mass approximately as $L_c\propto M^{2/3}$ for haloes with masses $M<10^{13}h^{-1}\msun$, and as $L_c\propto M^{1/4}$ for more massive haloes. The characteristic mass of $10^{13} h^{-1} \Msun$ is consistent with the mass scale where galaxy formation models suggest a transition from efficient to inefficient cooling. Another characteristic halo mass scale, $M\sim 10^{11} h^{-1}\msun$, which cannot be probed directly by our groups, is inferred from the conditional luminosity function (CLF) that matches the observed galaxy luminosity function and clustering. For a halo of given mass, the distribution of $L_c$ is rather narrow. The satellite galaxies are found to follow a Poissonian number distribution. The central galaxies in low-mass haloes are mostly late type galaxies, while those in massive haloes are almost all early types. We also measure the CLF of galaxies in haloes of given mass. Over the mass range that can be reliably probed with the present data ($13.3 \lta {\rm log}[M/(h^{-1}\Msun)] \lta 14.7$), the CLF is reasonably well fit by a Schechter function. Contrary to recent claims based on semi-analytical models of galaxy formation, the presence of central galaxies does not show up as a strong peak at the bright end of the CLF. (Abridged)

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arXiv2004

The Galaxy Dark Matter Connection

What galaxy lives in what halo? The answer to this simple question holds important information regarding galaxy formation and evolution. We describe a new statistical technique to link galaxies to their dark matter haloes, or light to mass, using the clustering properties of galaxies as function of their luminosity. The galaxy-dark matter connection thus established, and parameterized through the conditional luminosity function, indicates the presence of two characteristic scales in galaxy formation: one at ~10^{11} Msun, where galaxy formation is most efficient, and another at ~10^{13} Msun, where a transition occurs from systems dominated by one brightest, central galaxy to systems with several dominant galaxies of comparable luminosity. We show that the relation between light and mass established from galaxy clustering alone is in excellent agreement with the dynamical masses inferred from satellite kinematics. We also present a new (halo-based) galaxy-group finder, and show that the multiplicity function of galaxy groups identified in the 2dFGRS suggests a relatively high mass-to-light ratio on the scales of galaxy clusters, or, alternatively, a relatively low value of sigma_8. These findings are also supported by our studies of pairwise peculiar velocities and satellite abundances.

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arXiv2004

The Two-Point Correlation of Galaxy Groups: Probing the Clustering of Dark Matter Haloes

We analyze the 2-point correlation function (2PCF) of galaxy groups identified from the 2-degree Field Galaxy Redshift Survey with the halo based group finder recently developed by Yang et al (2004b). With this group catalogue we are able to estimate the 2PCFs for systems ranging from isolated galaxies to rich clusters of galaxies. The real-space correlation length obtained for these systems ranges from $\sim 4\mpch$ to $\sim 15\mpch$, respectively. The observed correlation amplitude (and the corresponding bias factor) as a function of group abundance is well reproduced by associating galaxy groups with dark matter haloes in the standard $Λ$CDM model. Redshift distortions are clearly detected in the redshift-space correlation function, the degree of which is consistent with the assumption of gravitational clustering and halo bias in the cosmic density field. In agreement with previous studies we find a strong increase of the correlation length with the mean inter-group separation. Although well determined observationally, we show that current theoretical predictions are not yet accurate enough to allow for stringent constraints on cosmological parameters. Finally, we use our results to explore the power-law nature of the 2PCF of galaxies. We split the 2PCF in 1-group and 2-group terms, equivalent to the 1-halo and 2-halo terms in halo occupation models, and show that the power-law form of the 2PCF is broken, when only including galaxies in the more massive systems.

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