Binary Aggregations in Hierarchical Galaxy Formation: The Evolution of the Galaxy Luminosity Function

Abstract

We develop a semi-analytic model of hierarchical galaxy formation with an improved treatment of the evolution of galaxies inside dark matter haloes. We take into account not only dynamical friction processes building up the central dominant galaxy, but also binary aggregations of satellite galaxies inside a common halo. These deplete small to intermediate mass objects, affecting the slope of the luminosity function at its faint end, with significant observable consequences. We model the effect of two-body aggregations using the kinetic Smoluchowski equation. This flattens the mass function by an amount which depends on the histories of the host haloes as they grow by hierarchical clustering. The description of gas cooling, star formation and evolution, and Supernova feedback follows the standard prescriptions widely used in semi-analytic modelling. We find that binary aggregations are effective in depleting the number of small/intermediate mass galaxies over the redshift range 1 < z < 3, thus flattening the slope of the luminosity function at the faint end. At z ≈ 0 the flattening occurs for −20 < M B < −18, but an upturn is obtained at the very faint end for M B > −16. We compare our predicted luminosity functions with those obtained from deep multicolor surveys in the HDF-N, HDF-S, NTT-DF in the rest-frame B and UV bands for the redshift ranges 0 < z < 1 and 2.5 < z < 3.5, respectively. The comparison shows that the discrepancy of the predictions of other semi-analytic models with the observations is considerably reduced at z > 1 and even more at z ≈ 3 by the effect of binary aggregations. The predictions from our dynamical model are discussed and compared with the effects of complementary processes (additional starburst recipes, alternative sources of feedback, different mass distribution of the dark matter haloes) which may conspire in affecting the shape of the luminosity function.

A European Southern Observatory Very Large Telescope Survey of Near‐Infrared (Z≤ 25) Selected Galaxies at Redshifts 4.5 <z< 6: Constraining the Cosmic Star Formation Rate near the Reionization Epoch

“…At the brightest magnitudes, the CDM model underpredicts the number of bright sources, while it overpredicts the number of fainter sources dominating the deeper sample. This is analogous to what has already been found at z ' 3 and confirms a general trend of this version of CDM models to underpredict the amount of SFR in high-redshift massive objects (Somerville & Primack 2001;Poli et al 2001;Menci et al 2002;Cimatti et al 2002).…”

“…To discuss this point, we present in both panels of Figure 3 the UV LD predicted by the CDM semianalytic model described in Menci et al (2002). This model is based on the Cole et al (2000) recipes, with an additional improved treatment of aggregation of satellite galaxies in common dark matter halos.…”

“…The main goal behind the study of high-redshift galaxies is the construction of a coherent picture of the physical processes that led to galaxy evolution. Within the framework of gravitational instability driven by primordial fluctuation, simple but physically motivated prescriptions have been used over the last years to describe the main processes involved (e.g., White & Frenk 1991;Cole et al 1994Cole et al , 2000Somerville & Primack 1999;Menci et al 2002). These models are not as '' tunable '' as commonly believed since they cannot be modified freely to match the high-redshift observables without worsening the local fits (e.g., the local luminosity function or local Tully-Fisher relation).…”

TheB‐Band Luminosity Function of Red and Blue Galaxies up toz= 3.5

“…Starting from primordial density perturbations in a ÃCDM universe, the model describes the collapse and merging histories of the DM haloes that originate from such perturbations. In previous papers (see, e.g., Menci et al 2002) we have described our model in detail; it includes the dynamical friction and the binary aggregations, determining the fate of clumps included into larger DM haloes along the hierarchy of mergers. The baryonic processes (gas cooling, star formation, and supernova feedback) are linked to the dynamical history of the galaxies through the simple recipes adopted by current SAMs (Kauffmann et al 1993;Cole et al 1994;Somerville & Primack 1999;Poli et al 1999;Wu et al 2000;Cole et al 2000).…”

Integrated spectroscopy of bulge globular clusters and fields

“…Current renditions of hierarchical galaxy formation in CDM dominated universes have so far failed to predict these empirical findings, favoring instead a late formation with major activity even below z ∼ 1. The culprit is probably in the ways in which star formation and feedback processes have been parameterized and implemented in the so-called semi-analytic models of galaxy formation and evolution (e.g., Kauffmann & Charlot 1998;Cole et al 2001;Somerville et al 2001;Menci et al 2002).…”