In this chapter we calculate the merger rates of dark matter halos and we investigate the role of smooth accretion versus mergers to their growth. We extract the merger rates and accretion histories from the Millennium and Millennium-II Simulations, combined with two additional, smaller, cosmological simulations. We use the ”splitting” merger tree construction algorithm described in §3.3.1 and in G09, and verify its reliability by reproducing our results by following individual particle histories alone, independent of merger tree construction algorithms.
We find that the contributions of all resolved mergers (up to mass ratios ≈105) to the
total growth rate of halos do not exceed 60%, regardless of halo mass and redshift. Most of
the merger contribution comes from small mass ratio (”major”) mergers (e.g. 1< x < 10
contribute ≈ 30% of the total growth), while ”very minor” mergers add very little mass (e.g. 103 < x <105contribute just a few percent to the total halo growth). We find that the
power-law index of the merger rate is such that if the merger rate is extrapolated beyond the maximum resolved mass ratio ≈ 105, the total contribution of all mergers saturates
at ≈ 60%. This suggests that a significant mass fraction of halos may be accreted in a genuinely smooth way.
Our results have important implications for galaxy formation models and the modes of baryonic accretion. If ≈ 40% of the dark matter that is accreted onto a halo was never previously bound in any merging smaller halos, then at least ≈40% of the baryons must also be accreted smoothly - as gas that was never previously heated by feedback processes or converted to stars. For the baryons, this ≈40% is a strong lower limit since halos with Tvir <104K likely cannot retain their gas. The common assumption that halos
with Tvir <104K cannot retain their gas also makes our results insensitive to the limited
resolution of the simulations we use, because we resolve all halos above this limit (atz .3). The implication is that a very large fraction of the baryonic matter falling into a halo must be pristine ”cold” IGM gas, with T & 104K set by IGM photoheating. This gas is not
expected to have formed stars or to have become significantly enriched with metals, no matter what the star-formation efficiency and history of the baryons in any of the merging halos.
Chapter 4
The Millennium Simulation
Compared to
z
≈
2
Galaxies
Note: This chapter has been published in Genel et al. (2008).
4.1
Abstract
Recent observations of UV-/optically selected, massive star forming galaxies at z ≈ 2 indicate that the baryonic mass assembly and star formation history is dominated by continuous rapid accretion of gas and internal secular evolution, rather than by major mergers. We use the Millennium Simulation to build new halo merger trees, and extract halo merger fractions and mass accretion rates. We find that even for halos not undergoing major mergers the mass accretion rates are plausibly sufficient to account for the high star formation rates observed in z ≈ 2 disks. On the other hand, the fraction of major mergers in the Millennium Simulation is sufficient to account for the number counts of submillimeter galaxies (SMGs), in support of observational evidence that these are major mergers. When following the fate of these two populations in the Millennium Simulation to z = 0, we find that subsequent mergers are not frequent enough to convert all z ≈ 2 turbulent disks into elliptical galaxies at z = 0. Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at z ≈2 into observed massive cluster ellipticals at z = 0. We argue therefore, that secular and internal evolution must play an important role in the evolution of a significant fraction ofz ≈2 UV-/optically and submillimeter selected galaxy populations.
4.2
Introduction
In the cold dark matter model of hierarchical structure formation (Blumenthal et al., 1984; Davis et al., 1985; Springel et al., 2006) mergers are believed to play an important role in galaxy formation and evolution (Steinmetz & Navarro, 2002). Mergers induce starbursts (Hernquist & Mihos, 1995) and transform galactic morphology (Naab & Burkert, 2003).
Major mergers may drive the buildup of the red sequence (Toomre, 1977; Hopkins et al., 2008a). Dark matter models and many observations show that mergers are more frequent at high redshift (Fakhouri & Ma, 2008; Conselice, 2003).
However, there is growing evidence that a smoother growth mode may be important for the baryonic mass assembly and star formation history at high redshift. For example, the tight correlation between star formation rate (SFR) and stellar mass in UV-/optically se- lected star forming galaxies is indicative of buildup by continuous gas inflow (Daddi et al., 2007; Noeske et al., 2007). As part of the SINS survey (F¨orster Schreiber et al., 2006, 2009), integral field spectroscopy of more than 50 UV-/optically selected z ≈2 star form- ing galaxies show a preponderance of thick gas-rich rotating disks and only a minority of major mergers (F¨orster Schreiber et al., 2006; Genzel et al., 2006, 2008; Shapiro et al., 2008). In contrast, SMGs are probably short-lived maximum-starburst galaxies undergoing dissipative major mergers (Tacconi et al., 2006; Bouch´e et al., 2007; Tacconi et al., 2008). Table 4.2 summarises key properties of these z ≈2 galaxy samples.
How do these observations fit into the concordance cosmological model? Modern simu- lations of dark matter structure formation are robust and fixed by the cosmological param- eters. However, complicated baryonic physics makes it difficult to model the evolution of galaxies and reproduce, for example, the high SFRs of these z ≈2 galaxies (Daddi et al., 2007).
Galaxies at z ≈ 2 differ significantly from local galaxies. The central mass densities of SMGs and of massive quiescent galaxies at the same redshift (van Dokkum et al., 2008 and references therein) are an order of magnitude greater than those of local spheroids and disks (Tacconi et al., 2008). Also, the z ≈ 2 rotating disks are thick and turbulent, unlike local disk galaxies. These differences raise the question: what are the local Universe descendants of these z ≈2 galaxies?
In this chapter we use the cosmological dark matter Millennium Simulation (Springel et al., 2005b;§4.3) to investigate the possible role of major mergers in galaxy formation at z ≈2 (§4.4), and to consider the evolution of thez ≈2 galaxies to z = 0 (§4.5).
Galaxy SFR Halo Comoving number Major
sample [ M⊙yr
−1
] mass density merger
[ M⊙] [h30.7Mpc −3] fraction SINS ≈30−3001 1011.84v3 200× 1−2.2×10 −434 ≈0.35 (1+z 3.2 ) −1.5h−1 0.72 SMGs6 ≈750±3007 - 1−2×10−53 ≈13
Table 4.1: Properties of galaxy samples at z ≈ 2. (1) F¨orster Schreiber et al., 2009 ; (2) F¨orster Schreiber et al., 2006 ; (3) Tacconi et al., 2008 and references therein ; (4) BX/BM & sBzK galaxies with K ≤20 ; (5) Shapiro et al., 2008 ; (6)S(850µm)≥5mJy ; (7) Genzel, priv. comm.