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1. INTRODUCTION

We start this chapter on Galactic accretion with an introduction, in which we first summarize early measurements of circumgalactic gas and the star-formation history of the Milky Way from a historical perspective. We then shortly discuss the role of gas accretion processes in Milky-Way type galaxies in a cosmological context and highlight their importance for galaxy evolution in general. The introduction section ends with a proper definition of the gas-accretion rate, dMgas / dt, and an assessment of the physical parameters that need to be constrained from observations and simulations to estimate dMgas / dt for the Milky Way.

1.1. Historical remarks

The presence of gas above/below the Milky Way disk has been established already more than 60 years ago, when absorption-line measurements demonstrated the presence of gas clouds at high galactic latitudes that exhibit relatively high radial velocities (Adams 1949; Münch 1952; Münch & Zirin 1961). In 1956, Lyman Spitzer argued that these structures, if located in the halo, must be surrounded by a hot, gaseous medium (he named this medium the Galactic Corona) that provides the necessary pressure-confinement of these clouds, otherwise they should disperse on relatively short timescales (Spitzer 1956).

With the new receiver technologies and the resulting improved sensitivity of radio telescopes in the 1960s, high-velocity H i 21 cm emission at high galactic latitudes was found by Muller et al. (1963), Smith (1963), Dieter (1964), Blaauw & Tolbert (1966), Hulsbosch & Raimond (1966), and Mathewson (1967), supporting the conclusions from the earlier absorption-line measurements. The observed distribution of radial velocities of the 21 cm emission features lead to the definition of two classes of Galactic halo clouds: as “high-velocity clouds” (HVCs) those halo structures were labeled that have radial velocities, |vLSR| > 100 km s−1, while features with somewhat smaller radial velocities (|vLSR| ≈ 30−100 km s−1) were given the name “intermediate-velocity clouds” (IVCs). As will be discussed later, there also might exist a population of halo clouds with very low LSR velocities (“low-velocity clouds” (LVCs), similar to those in the disk.

Several scenarios for the origin of the 21 cm neutral halo clouds were discussed by Jan Oort in 1966, among which the infall of intergalactic gas, the accretion of gas from satellite galaxies, the condensation of neutral gas patches from cooling coronal gas, and the ejection of gaseous material from the Milky Way disk were regarded as plausible scenarios (Oort 1966). Indeed, as we will discuss in this review, these scenarios are still up-to-date.

The need for feeding the Milky Way disk with low-metallicity gas also comes from early studies of the Galaxy's star-formation activity and stellar content. Already in the 1970s it was realized that star-formation in the Milky Way would have come to a halt early on, if the Galaxy was not fed with fresh material from outside. This is because the gas-consumption time scale (even for a moderate star-formation rate of ∼ 1 M yr−1) is short compared to the age of the Milky Way (Larson 1972). Another argument for gas accretion comes from the observed metallicity distribution of low-mass stars in the solar neighborhood, which cannot be reproduced by closed-box models of the chemical evolution of the stellar disk. To match the observations, such models require the continuous accretion of metal-poor gas (van den Bergh 1962; Chiappini et al. 2001). These findings provide additional strong arguments that the Milky Way has accreted (and is still doing so) large amounts of low-metallicity gas to continuously built up its stellar content as observed today.

In conclusion, the observed presence of large amounts of gas above/below the disk, the past and present star-formation rate of the Milky Way, and the metallicity distribution of low-mass stars in the solar neighborhood demonstrate that gas accretion represents an important process that has strong implications for the past, present, and future evolution of our Galaxy.

1.2. Cosmological context

In the overall context of galaxy formation in the Universe, gas accretion and feedback nowadays are regarded as the main processes that regulate the star-formation activity in galaxies. Many of the cosmological aspects of gas accretion in galaxies will be discussed in detail in other chapters of this book. Still, for our discussion on gas accretion in the Milky Way, the main aspects need to be summarized here.

In the conventional sketch of galaxy formation, gas is falling into a dark matter (DM) halo and then is shock-heated to approximately the halo virial temperature (a few 106 K, typically), residing in quasi-hydrostatic equilibrium with the DM potential well (Rees & Ostriker 1977). The gas then cools and sinks into the center of the potential where it is transformed into stars. This model is often referred to as the ‘hot mode' of gas accretion. It has been argued that for smaller DM potential wells the infalling gas may reach the disk directly at much shorter timescales, without being shock-heated to the virial temperature (‘cold mode’ of gas accretion; e.g., White & Rees 1978; Kereš et al. 2005). In the latter case, the star-formation rate of the central galaxy would be directly coupled to its gas-accretion rate (White & Frenk 1991). In these simple pictures, the dominating gas-accretion mode depends on the mass and the redshift of the galaxy (e.g., Birnboim & Dekel 2003; Kereš et al. 2005), where at low redshift the critical halo mass that separates the hot mode from the cold mode is ∼ 1012 M (van de Voort et al. 2011). However, the underlying physics that describes the large-scale flows of multi-phase gas from the outer to the inner regions of a dynamically evolving galaxy is highly complicated (e.g, Mo & Miralda-Escude 1996; Maller & Bullock 2004). To understand these processes, high-resolution hydrodynamical simulations with well-defined initial conditions (e.g, Bauermeister et al. 2010; Fumagalli et al. 2011; van de Voort & Schaye 2012; Vogelsberger et al. 2012; Shen et al. 2013) are required. Therefore, even the most advanced hydrodynamical simulations do not provide tight constraints for the gas-accretion rate of individual galaxies without knowing their exact halo masses, their cosmological environment, and the initial circumgalactic gas distribution (Nuza et al. 2014; hereafter referred to as N14).

Next to the feeding of the halos of Milky-Way type galaxies through intergalactic gas, galactic-fountain type processes (from supernova (SN) feedback; Fraternali & Binney 2008) and mergers with satellite galaxies (Di Teodoro & Fraternali 2014) need to be considered. The vast amounts of neutral and ionized gas carried by the Magellanic Stream underline the importance of merger processes for the Milky Way's gas-accretion rate (D'Onghia & Fox 2016; see Sect. 2). The preconditions under which such gas clouds are generated and falling toward the disk are different from those for clouds being accreted from the intergalactic medium (IGM; e.g., Peek 2009) and thus they need to be explored separately by both observations and simulations. Finally, it is important to keep in mind that the Milky Way is not an isolated galaxy, but is embedded in the Local Group, being close to another galaxy of similar mass, M31. The original distribution of gas that is entering the virial radius of the Milky Way from outside thus depends on the spatial distribution of satellite galaxies and the distribution of intragroup gas in the cosmological filament that builds the Local Group. In Fig. 1 we sketch the local galaxy distribution in the Local Group and the gas distribution around the Milky Way and M31 from a hypothetical external vantage point.

Figure 1

Figure 1. Sketch of the expected distribution of galaxies and multi-phase gas in the Local Group from an external vantage point. The Milky Way (left big galaxy) and M31 (right big galaxy) are surrounded by their populations of satellite galaxies and by large amounts of multi-phase gas. Both galaxies are interconnected by a gaseous bridge, which spatially falls together with the Local Group barycenter (see, e.g., N14). Figure produced by the author for this review article.

Turning back to the Milky Way, we know that since z = 1 the Milky Way has produced ∼ 8 × 109 M of stars, while the current star-formation rate of the Milky Way is ∼ 0.7−2.3 M yr−1 (Levine, Blitz & Heiles 2006; Robitaille & Whitney 2010; Chomiuk & Povich 2011; see also Peek 2009). To relate this stellar mass and star-formation rate to the gas-accretion rate it is important to remember that as much as 50 percent of the initial material from which a generation of stars is formed will be returned back to the ISM and will be recycled in later stellar generations (Rana 1991). This means that for one accreted mass unit of gas, two mass units of evolved stars will have emerged after several star-formation cycles. For the Milky Way, this implies that is has accreted a gas mass of ≥ 4 × 109 M during the last 8 Gyr.

1.3. Parameterization of gas accretion

Before we start to discuss in detail the various observational and theoretical aspects of gas-accretion processes in the Milky Way, we need a proper definition of the most important parameters involved, in particular the accretion rate. From a cosmological perspective (e.g., in studies using cosmological simulations), the growth and evolution of galaxies through cosmic times is governed by the gain of gas mass within their gravitational sphere of influence (i.e., within their virial radius, Rvir) through mergers and gas infall. In this context, one can simply define the gas accretion rate of a galaxy as the net mass inflow of gas through an imaginary sphere with radius Rvir. From a galaxy-evolution perspective, in contrast, the only relevant accretion rate is that of the disk, where the infalling gas is being transformed into stars, while the total amount of gas cycling within the galaxy's extended halo is relatively unimportant. One of the most burning questions in gas accretion research therefore is, how much of the gas entering the virial radius of a Milky-Way type galaxy actually makes it to the disk and what are the typical timescales for this process?

In the following, we address these conceptional issues in two steps. First, we define the overall current-day gas-accretion rate of the Milky Way simply by relating the total mass of infalling gas, Mgas, with its infall velocity, vinfall, and its galactocentric distance, d, so that

Equation 1

(1)

Because of the cloud's passage through the halo and the interaction with the ambient hot coronal gas, only a fraction of this initial gas mass will end up in the disk to power star-formation therein. The future disk gas accretion rate thus can be defined as

Equation 2

(2)

where η ≤ 1 represents the fueling parameter that modulates the disk's gas accretion and star-formation rate at the time of impact. The accretion time of each infalling gas cloud from its initial position seen today to the disk is tacc = d / ⟨ vinfall⟩. Here, ⟨ vinfall⟩ represents the average infall velocity along the cloud's passage towards the disk. In conclusion, it is the today's 3D distribution and space motion of gas around the Milky Way (parameterized by the current-day halo gas-accretion rate) that governs the future star-formation activity in the Milky Way disk.

From equations (1) and (2) it becomes immediately clear, which parameters need to be constrained by observations to get an insight into the gas accretion rate of the Milky Way.

In the following sections, we will discuss our understanding of these individual parameters in detail, summarizing past and recent observational and theoretical studies.

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