The immense amount of observational data accumulated in the last decade investigating various properties of galaxies over cosmic time requires a more detailed theoretical understanding of galaxy formation and evolution. The second half of this thesis focuses on galaxy formation in the first billion years of the Universe, known as the reionization era.
INTRODUCTION
Basic physics in galaxy formation
Galaxies form in the central region (~0.1Rvir) of the halo, where the gas can be efficiently cooled by the stars as well. The SN remnant is initially inefficient at cooling, when the hot bubble expands adiabatically and increases the ejection momentum by a factor of several (i.e., the Sedov–Taylor phase).
Common tools for modeling galaxy formation
Left: a slice of cMpc3 from the large-volume cosmological simulation, EAGLE, adapted from Schaye et al. Right: Composite/g/r image of a high-resolution cosmological zoom-in simulation of a MW-like galaxy from the FIRE suite, from Hopkins et al.
Open questions to be addressed in this thesis
- Galactic chemical evolution
- Galaxies in the first billion years of the Universe
Metallicity gradients are also widely found in the stellar component of local galaxies (e.g., Sánchez-Blázquez et al. 2014 ). Understanding the formation of galaxies in the first billion years of the Universe is important for creating a coherent picture of the evolution of galaxies over cosmic time.
THE ORIGIN AND EVOLUTION OF GALAXY MASS–METALLICITY RELATION
Introduction
Meanwhile, galactic winds are ubiquitous (see, for example, Veilleux et al. 2005, for a recent review), carrying metals away from galaxies. In this work, the first of a series, we will study the chemical evolution of galaxies using the FIRE (Feedback in Realistic Environment) simulations (Hopkins et al. 2014).
The Simulations .1 Simulation Details.1Simulation Details
- Halo Identification, Stellar Mass and Metallicity
We follow Wiersma et al. 2009b) and include metal yields from type II SNe, type I SNe and stellar winds. We use the Amiga Halo Finder (AHF; Gill et al. 2004; Knollmann & Knebe 2009) to identify galactic haloes and galaxies in our simulations.
The Mass–Metallicity Relation
- Evolution of the MZR
- Comparison with Observations and Other Models
The red solid and dotted curves represent the median and 2σ dispersion of the SDSS MZR at z ∼ 0.1 (Tremonti et al. 2004). In Figure 2.2 we show the relationship between stellar mass and stellar metallicity atz = 0 and compare our simulations with the SDSS sample from Gallazzi et al. 2005, red solid and dotted lines) and the dwarf galaxies from Kirby et al.
Discussion
- Where are the metals?
- Circumgalactic O vi
- Metal outflows, inflows, and recycling
- Why the MZR evolves with redshift?
Outflow gas escaping the halo at Rvirds is typically less enriched than the gas in the ISM. For consistency, the metallicities in the gas phase, shown in Figure 2-12, are the average metallicities of all the gas in the halo.
Conclusion
In this work, the metallicity in the gas phase is defined as the mass-weighted average metallicity of all gas particles below 104 K, which we call the ISM gas. In the literature, gas-phase metallicity and stellar metallicity are sometimes represented in terms of Zgas and [Fe/H].
WHY DO HIGH-REDSHIFT GALAXIES SHOW DIVERSE GAS-PHASE METALLICITY GRADIENTS?
Introduction
These results support the so-called “inside-out” growth model of galaxy formation (e.g. Bouwens et al. 1997 ). In these systems, rapid gas inflow triggers starbursts in the galactic center (Torrey et al. 2017).
Methodology
- Simulation Details
- Galaxy Identification and Definitions
- Kinematics
- Metallicity Gradients
The SFRs are averaged over 200 Myr to mimic observational measurements based on far-UV luminosity (e.g., Sparre et al. 2017 ). The stellar mass, SFR and R90 for the entire simulated sample are listed in the Appendix. Before presenting the gas-phase metallicity gradients for our simulated sample, we first measure the kinematic properties of these galaxies, as is commonly done in observational studies (e.g., Yuan et al.
Results
- Metallicity gradients: general properties
- The mass-metallicity relation (MZR)
- Metallicity gradient vs stellar mass and sSFR
- Metallicity gradient vs kinematic properties
- Metallicity gradient vs redshift
3.9, we show the relation between gas-phase metallicity gradient (measured over 0.25–1R90) and degree of rotational support, Vc/σ, for the entire simulated sample. The existence of population (3) reflects the observed complex relationship between metallicity gradient and galaxy kinematics (e.g. Jones et al. Our simulations reproduce the observed complexity in the relationship between metallicity gradient and kinematic properties.
FIRE M03 Y11 S12
J15 L16 W16 MUGS
The effects of feedback: a case study
Middle: Gas morphology at the four epochs marked by the vertical gray dashed lines in the top panel (a–d). Rapid gas inflow triggers a starburst in the disc and a negative metallicity gradient rapidly builds (b, see the argument in Section 3.3.4). A negative metallicity gradient builds up rapidly as soon as the disc settles and remains almost unchanged after this time.
Discussion and Conclusions
This has important implications for the interpretation of metallicity gradients observed in high-redshift galaxies. A large fraction of high-redshift massive galaxies would be expected to have significant negative stellar metallicity gradients, even if they exhibit a wide range of gas-phase kinematic properties and metallicity gradients. Negative stellar metallicity gradients have been observed in local galaxies (e.g. Sánchez-Blázquez et al. 2014), although it is difficult to measure the metallicity of stars at higher redshifts.
THE STRUCTURE AND DYNAMICAL EVOLUTION OF THE STELLAR DISK OF A SIMULATED MILKY WAY-MASS
GALAXY
Introduction
The bimodality, if real, implies that the MW may have a hiatus in its star formation history at high redshift (e.g. Chiappini et al. Likewise, many authors have also found a two-component disc structure and similar MAP properties in their simulations ( Brook et al. 2012b; In this paper, we study a simulation from the Feedback in Realistic Environments (FIRE; Hopkins et al. 2014)1 project, which produces a disc galaxy of stellar mass equivalent to the MW at z = 0, to study the structure and the abundance pattern of stars in the galactic disk.
Simulation and Methods
A detailed description of the simulations, numerical recipes, and physics involved is presented in Hopkins et al. The 'zoom-in' cosmological initial conditions for m12i are adopted by the AGORA project (Kim et al. 2014). We follow Wiersma et al. 2009b ) and calculate the metal production from Type-II SNe, Type-Ia SNe, and stellar winds using the metal yields in Woosley & Weaver (1995) , Iwamoto et al.
Results
- General Picture
- Disk Structure
One common argument for the presence of a thick disc in the MW and nearby disc galaxies is that the stellar density profile along the |Z| direction cannot be well described by a single component profile. Note that the stellar densities, thin and thick disc scaleheights and the mass fraction of the thick disc in our simulation are in good agreement with observations and other simulations in the literature. Stars younger than 4 Gyr contribute more than 90% of the mass in the 'thin disc', while the 'thick disc' consists almost entirely of stars older than 4 Gyr.
Age and Metallicity Gradients
The negative radial metallicity gradient for stars younger than 6 Gyr (those formed in the disc) near the midplane is inherited from the parent star-forming gas disc (Ma et al. 2017b). A negative radial metallicity gradient is expected from coevolution between the gas disc and stellar disc (e.g. Ho et al. 2015 ). 13.5 < R < 14.5 kpc Figure 4.6: Top: Radial metallicity profile in a layer with | Z and 2.0–2.5 kpc. Bottom: Vertical metallicity profile at radii R = 6.10, 14 kpc. We show metallicity profiles for all stars (black solid lines) as well as in bins of different stars. Flattening and inversion of the radial metallic gradient on the thigh | Z| it follows a negative age gradient at these elevations. The steepening of the vertical metallicity gradient at smaller radii is the result of a stronger age gradient. The stellar gradient is a natural consequence These results are in line with predictions in Minchevetal. (2014, Fig. 10).
Dynamical Evolution of the Stellar Disk
Gravitational perturbation of satellites can, for example, induce bars and spiral arms (e.g. Purcell et al. 2011 ), which further result in kinematic heating and radial migration (Lynden-Bell & Kalnajs 1972 ). In our simulation, the increase in disc thickness and velocity dispersion is roughly a linear function of time, indicating that spiral arms may be the dominant heating mechanism, as suggested by an analysis of a large sample of disc galaxy simulations (Grand et al. 2016). During their formation time, stars have inherited the flare from their parent gas disk, which is probably a natural consequence of hydrostatic equilibrium in a galactic potential (e.g. Olling 1995; O'Brien et al.
Discussion
The burning of the stellar disc is present 'at birth' and is preserved during kinematic heating.
- The thin and thick disks
- Stellar Migration in the Disk
- Abundance Patterns and Mono-abundance Populations
- Conclusions
First, about two-thirds of the stars in the thick disc are older than 6 Gyr (formation redshift z & 0.7). In the right panel, we show the distribution of these stars in the [Mg/Fe]–[M/H] plane. The gradient of [Mg/Fe] is also a consequence of the age gradient in the disk.
THE DIFFICULTY OF GETTING HIGH ESCAPE FRACTIONS OF IONIZING PHOTONS FROM HIGH-REDSHIFT GALAXIES
Introduction
For example, Finkelstein et al. 2013) suggested fesc > 13% and fesc > 20%, respectively, assuming that all the ionization photons are contributed by galaxies brighter than MUV = -13. For example, Finkelstein et al. 2012) deduced that reionization requires a much higher escape fraction fesc > 34% if one only accounts for the contribution of galaxies brighter than MUV = -18. Razoumov & Sommer-Larsen (2010) also found fesc decreases from z = 4–10 at fixed halo mass, while Yajima et al. 2011) found no dependence of fesc on redshift.
The Simulations
This SF prescription adaptively selects the largest overdensities and automatically predicts clustered SF (Hopkins et al. 2013b). Previous studies have shown that it has little effect on the properties of global galaxies (e.g. Hopkins et al. 2014 and references therein). We emphasize that the instantaneous photoionization is treated in an approximate way in our simulations – we move radially outward from the star and ionize every nearest neutral gas particle until the photon budget is exhausted.
Galaxy Properties .1 Halo Identification.1Halo Identification
- Multi-phase ISM Structure
We compare our results with the simulations of Hopkins et al. 2014) atz = 6 and the relation obtained by observation from Behroozi et al. We compare the relation with the simulations of Hopkins et al. 2014) atz =6 (small black dots) and the relationship found by observation in Behrooza et al. These simulations are consistent with those in Hopkins et al. 2014), although the latter have a much lower resolution.
Escape Fraction of Ionizing Photons
- Radiative Transfer Calculation
- Instantaneous and Time-averaged Escape Fraction
Qint was relatively low at the time and the time-averaged escape fraction is just that. However, in our z5m10e run, in which we form stars in diffuse gas, the time-averaged escape fraction is usually over 20%. We find that there is no significant dependence of the escape fraction on cosmic time or the mass of stars.
Discussion
- UV Background
- Star Formation Criteria
This is consistent with the argument that the escape of ionizing photons is limited by the small-scale structures of the ISM surrounding young stellar populations. This confirms that the diffuse low-density gas in the galactic halo (influenced by the UV background) does not significantly affect the escape of ionizing photons10. However, as shown in Figure 5.6, the escape fraction from z5m10e is significantly higher than in the other simulations.
Runaway Stars
Stellar Population Models
Conclusions
According to standard stellar population models, most of the intrinsic ionizing photons are produced by newly formed stellar particles younger than 3 Myr. However, if the gas distribution is highly asymmetric around an isolated star particle (see e.g. the middle column in Figure 5.2), the gas ionization conditions will not be accurately captured. Overall, both results agree fairly well on large-scale pattern of the neutral gas distribution, although radiative transfer calculations reveal more small-scale structures in star-forming regions.
BINARY STARS CAN PROVIDE THE “MISSING PHOTONS”
NEEDED FOR REIONIZATION
- Introduction
- Method
- Results
- Discussion and Conclusions
Planck Collaboration et al. 2014 ), assuming that most of the ionizing photons come from star-forming galaxies brighter than MUV= −13. Moreover, binary evolution not only produces more ionizing photons, but can also significantly increase the escape fractions (Ma et al. 2015). Most of the escaping photons come from stellar populations ~3–10 million years old, but they contribute only a very small fraction of the intrinsic ionizing photons in single-star models (Ma et al. 2015).
SIMULATING GALAXIES IN THE REIONIZATION ERA WITH FIRE-2: GALAXY SCALING RELATIONS, STELLAR MASS
FUNCTIONS, AND LUMINOSITY FUNCTIONS
Introduction
2015; Finkelstein et al. 2015a), but the dim-end behavior of the UV luminosity function remains highly uncertain. These faint galaxies contribute a non-trivial fraction of the ionizing photons required for reionization (e.g., Finkelstein et al. 2012; therefore, the stellar mass functions reported by different authors have significant discrepancies (e.g. eg, figure 9 in Song et al. 2016).
The Simulations
- Initial conditions
- Baryonic physics
- Halo selection and definitions
- Halo abundances
3We also include other well-resolved haloes in the zoomed-in regions in our analysis (see Section 7.2.3). These haloes, by design, reside in the vicinity of a more massive halo (the target halo in the zoomed-in region). We use HMFcalc(Murray et al. 2013)9 to calculate the halo mass functions, applying the same cosmological parameters and virial overdensities as those assumed in the simulations.
Galaxies in the reionization era .1 Morphology.1Morphology
- The stellar mass–halo mass relation
We find little evolution in the mass-halomass relationship of stars at these redshifts, consistent with recent empirical limitations (e.g. Rodríguez-Puebla et al. 2017; however, see Behroozi & Silk 2015). Nevertheless, a halo mass-dependent scattering exists in the relationship between stellar mass and halo mass at low redshift in both observations and FIRE-2 simulations (e.g., Garrison-Kimmel et al. 2014; Fitts et al. 2017) . We show our best fit 1σ star mass halo mass relationship in each panel in Figure 7.4 (the red dotted lines).
