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Register a new userAssembly bias evidence in close galaxy pairs
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The growth channel of massive galaxies involving mergers can be studied via close pairs as putative merger progenitors, where the stellar populations of the satellite galaxies will be eventually incorporated into the massive primaries. We extend our recent analysis of the GAMA-based sample of close pairs presented in Ferreras et al. to the general spectroscopic dataset of SDSS galaxies (DR14), for which the high S/N of the data enables a detailed analysis of the differences between satellite galaxies with respect to the mass of the primary galaxy. A sample of approximately two thousand satellites of massive galaxies is carefully selected within a relatively narrow redshift range (0.07<z<0.14). Two main parameters are considered as major drivers of the star formation history of these galaxies, namely: the stellar velocity dispersion of the satellite ($sigma$), as a proxy of local drivers, and the ratio between the stellar mass of the satellite and the primary, $mu=M_{rm SAT}/M_{rm PRI}$, meant to serve as an indicator of environment. Consistently with the independent, GAMA-based work, we find that satellites around the most massive primaries appear older, at fixed velocity dispersion, with respect to satellites of lower mass primaries. This trend is more marked in lower mass satellites ($sigma$~100 km/s), with SSP-equivalent age differences up to ~0.5 Gyr, and can be interpreted as a one-halo assembly bias, so that satellites corresponding to smaller values of the mass ratio $mu$ represent older structures, akin to fossil groups.
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We analyze the spectra of 300,000 luminous red galaxies (LRGs) with stellar masses $M_* gtrsim 10^{11} M_{odot}$ from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). By studying their star-formation histories, we find two main evolutionary paths converging into the same quiescent galaxy population at $zsim0.55$. Fast-growing LRGs assemble $80%$ of their stellar mass very early on ($zsim5$), whereas slow-growing LRGs reach the same evolutionary state at $zsim1.5$. Further investigation reveals that their clustering properties on scales of $sim$1-30 Mpc are, at a high level of significance, also different. Fast-growing LRGs are found to be more strongly clustered and reside in overall denser large-scale structure environments than slow-growing systems, for a given stellar-mass threshold. Our results imply a dependence of clustering on stellar-mass assembly history (naturally connected to the mass-formation history of the corresponding halos) for a homogeneous population of similar mass and color, which constitutes a strong observational evidence of galaxy assembly bias.
We use a highly complete subset of the GAMA-II redshift sample to fully describe the stellar mass dependence of close-pairs and mergers between 10^8 Msun and 10^12 Msun. Using the analytic form of this fit we investigate the total stellar mass accreting onto more massive galaxies across all mass ratios. Depending on how conservatively we select our robust merging systems, the fraction of mass merging onto more massive companions is 2.0%-5.6%. Using the GAMA-II data we see no significant evidence for a change in the close-pair fraction between redshift $z = 0.05-0.2$. However, we find a systematically higher fraction of galaxies in similar mass close-pairs compared to published results over a similar redshift baseline. Using a compendium of data and the function $gamma_M =A(1+z)m$ to predict the major close-pair fraction, we find fitting parameters of $A = 0.021 pm 0.001$ and $m = 1.53 pm 0.08$, which represents a higher low-redshift normalisation and shallower power-law slope than recent literature values. We find that the relative importance of in-situ star-formation versus galaxy merging is inversely correlated, with star-formation dominating the addition of stellar material below Mstar and merger accretion events dominating beyond Mstar. We find mergers have a measurable impact on the whole extent of the GSMF, manifest as a deepening of the dip in the GSMF over the next Gyr and an increase in Mstar by as much as 0.01-0.05 dex.
Understanding the galaxy-halo connection is fundamental for contemporary models of galaxy clustering. The extent to which the haloes assembly history and environment impact galaxy clustering (a.k.a. galaxy assembly bias; GAB), remains a complex and challenging problem. Using a semi-analytic galaxy formation model, we study the individual contributions of different secondary halo properties to the GAB signal. These are obtained by comparing the clustering of stellar-mass selected samples to that of shuffled samples where the galaxies are randomly reassigned to haloes of fixed mass and a specified secondary halo property. We explore a large range of internal halo properties and environmental measures. We find that commonly used properties like halo age or concentration amount to only 20-30 per cent of the signal, while the smoothed matter density or the tidal anisotropy can account for the full level of GAB (though care should be given to the specific definition). For the successful measures, we examine the occupancy variations and the associated changes in the halo occupation function parameters. These are used to create mock catalogues that reproduce the full level of GAB. Finally, we propose a practical modification of the standard halo occupation distribution model, which can be tuned to any level of assembly bias. Fitting the parameters to our semi-analytic model, we demonstrate that the corresponding mock catalogue recovers the target level of GAB as well as the occupancy variations. Our results enable producing realistic mock catalogues and directly inform theoretical modelling of assembly bias and attempts to detect it in the Universe.
Context. Studies of galaxy pairs can provide valuable information to jointly understand the formation and evolution of galaxies and galaxy groups. Consequently, taking into account the new high precision photo-z surveys, it is important to have reliable and tested methods that allow us to properly identify these systems and estimate their total masses and other properties. Aims. In view of the forthcoming Physics of the Accelerating Universe Survey (PAUS) we propose and evaluate the performance of an identification algorithm of projected close isolated galaxy pairs. We expect that the photometric selected systems can adequately reproduce the observational properties and the inferred lensing mass - luminosity relation of a pair of truly bound galaxies that are hosted by the same dark matter halo. Methods. We develop an identification algorithm that considers the projected distance between the galaxies, the projected velocity difference and an isolation criteria in order to restrict the sample to isolated systems. We apply our identification algorithm using a mock galaxy catalog that mimics the features of PAUS. To evaluate the feasibility of our pair finder, we compare the identified photometric samples with a test sample that considers that both members are included in the same halo. Also, taking advantage of the lensing properties provided by the mock catalog, we apply a weak lensing analysis to determine the mass of the selected systems. Results. Photometric selected samples tend to show high purity values, but tend to misidentify truly bounded pairs as the photometric redshift errors increase. Nevertheless, overall properties such as the luminosity and mass distributions are successfully reproduced. We also accurately reproduce the lensing mass - luminosity relation as expected for galaxy pairs located in the same halo.
We use the {sc Illustris TNG300} magneto-hydrodynamic simulation, the {sc SAGE} semi-analytical model, and the subhalo abundance matching technique (SHAM) to examine the diversity in predictions for galaxy assembly bias (i.e. the difference in the large scale clustering of galaxies at a fixed halo mass due to correlations with the assembly history and other properties of host haloes). We consider samples of galaxies selected according to their stellar mass or star formation rate at various redshifts. We find that all models predict an assembly bias signal of different magnitude, redshift evolution, and dependence with selection criteria and number density. To model these non-trivial dependences, we propose an extension to the standard SHAM technique so it can include arbitrary amounts of assembly bias. We do this by preferentially selecting subhaloes with the same internal property but different {it individual} large-scale bias. We find that with this model, we can successfully reproduce the galaxy assembly bias signal in either {sc SAGE} or the {sc TNG}, for all redshifts and galaxy number densities. We anticipate that this model can be used to constrain the level of assembly bias in observations and aid in the creation of more realistic mock galaxy catalogues.
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