Galaxies���intergalactic medium interaction calculation ��� I. Galaxy formation as a function of large-scale environment

Abstract

We present the first results of hydrodynamical simulations that follow the formation of galaxies to the present day in nearly spherical regions of radius ∼20 h−1 Mpc drawn from the Millennium Simulation (Springel et al.). The regions have mean overdensities that deviate by (−2, −1, 0, +1, +2)σ from the cosmic mean, where σ is the rms mass fluctuation on a scale of ∼20 h−1 Mpc at z= 1.5. The simulations have mass resolution of up to ∼106 h−1 M⊙, cover the entire range of large‐scale cosmological environments, including rare objects such as massive clusters and sparse voids, and allow extrapolation of statistics to the (500 h−1 Mpc)3 Millennium Simulation volume as a whole. They include gas cooling, photoheating from an imposed ionizing background, supernova feedback and galactic winds, but no AGN. In this paper, we focus on the star formation properties of the model. We find that the specific star formation rate density at z≲ 10 varies systematically from region to region by up to an order of magnitude, but the global value, averaged over all volumes, closely reproduces observational data. Massive, compact galaxies, similar to those observed in the GOODS fields (Wiklind et al.), form in the overdense regions as early as z= 6, but do not appear in the underdense regions until z∼ 3. These environmental variations are not caused by a dependence of the star formation properties on environment, but rather by a strong variation of the halo mass function from one environment to another, with more massive haloes forming preferentially in the denser regions. At all epochs, stars form most efficiently in haloes of circular velocity vc∼ 250 km s−1. However, the star formation history exhibits a form of ‘downsizing’ (even in the absence of AGN feedback): the stars comprising massive galaxies at z= 0 have mostly formed by z= 1−2, whilst those comprising smaller galaxies typically form at later times. However, additional feedback is required to limit star formation in massive galaxies at late times.

Star formation sustained by gas accretion

“…The decrease at late times is due to the quenching of SF by lower cooling rates in hotter and lower density gas, in addition to gas exhaustion and self-regulated feedback from stars and black holes. Similar results are provided by, e.g., White and Frenk (1991), Hernquist and Springel (2003), Somerville et al (2008), Crain et al (2009), andChoi andNagamine (2009). Schaye et al (2010) stress that without AGN feedback it is difficult to match the steep decline in the cosmic SFR below z = 2.…”

Magnetic fields in Local Group dwarf irregulars

“…We also note that sometimes even relatively massive galaxies with a total mass of up to ≈10 11 M are called dwarf galaxies (e.g. Crain et al 2009) in the sense that they have a sub-galactic mass value compared to the typical galactic systems in the present Universe, such as the Milky Way. However, primordial systems of this mass, as well as LBGs, would not have evolved to become the typical LG dIrrs that we investigate in this paper.…”

“…Continuous star formation agrees with the detection of significant amounts of gas in these systems (Mateo 1998). We adopted a higher ambient gas temperature (T IGM = 10 5 K), as the IGM in the LG is not pristine and likely to have a hot component due to galactic feedback, structure formation heating, and the large potential wells of massive spirals (Davé & Oppenheimer 2007;Crain et al 2009). There are also some predictions that the density of the local IGM is about 10 −4 cm −3 (Pildis & McGaugh 1996;Rasmussen et al 2003;Sembach 2006), which we use to estimate the ambient pressure P IGM .…”

The Lockman Hole project: gas and galaxy properties from a stacking experiment

“…Cold gas is known to play an important role in the formation and evolution of galaxies (Kereš et al 2005;Crain et al 2009;van de Voort et al 2011). Therefore, our understanding of the structure, properties and evolution of galaxies can not be complete without knowing about the various phases of the gas, including atomic hydrogen (H I), in and around galaxies, and their dependence on other galaxy characteristics.…”