2.2 Hierarchical merging

Hierarchical merging is a key process in galaxy formation and evolution. Smaller galaxies combine to form larger ones over time, driven by gravitational interactions. This process shapes galaxy morphology, star formation, and chemical composition throughout the universe.

The Lambda-Cold Dark Matter model supports hierarchical merging as a fundamental aspect of cosmic structure formation. Mergers trigger star formation, alter galaxy shapes, and fuel the growth of supermassive black holes, playing a crucial role in galactic development.

Hierarchical merging overview

• Hierarchical merging is a fundamental process in the formation and evolution of galaxies, where smaller structures merge to form larger ones over cosmic time
• This process is driven by gravitational interactions between galaxies and is a key component of the Lambda-Cold Dark Matter (ΛCDM) cosmological model
• Hierarchical merging plays a crucial role in shaping the morphology, star formation history, and chemical composition of galaxies throughout the universe

Definition of hierarchical merging

Top images from around the web for Definition of hierarchical merging

• 

  The Universe in Formation. Hubble Sees 6 Examples of Merging Galaxies - Universe Today View original

  Is this image relevant?

• 

  28.2 Galaxy Mergers and Active Galactic Nuclei | Astronomy View original

  Is this image relevant?

• 

  28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original

  Is this image relevant?

• 

  The Universe in Formation. Hubble Sees 6 Examples of Merging Galaxies - Universe Today View original

  Is this image relevant?

• 

  28.2 Galaxy Mergers and Active Galactic Nuclei | Astronomy View original

  Is this image relevant?

1 of 3

Top images from around the web for Definition of hierarchical merging

• 

  The Universe in Formation. Hubble Sees 6 Examples of Merging Galaxies - Universe Today View original

  Is this image relevant?

• 

  28.2 Galaxy Mergers and Active Galactic Nuclei | Astronomy View original

  Is this image relevant?

• 

  28.5 The Formation and Evolution of Galaxies and Structure in the Universe | Astronomy View original

  Is this image relevant?

• 

  The Universe in Formation. Hubble Sees 6 Examples of Merging Galaxies - Universe Today View original

  Is this image relevant?

• 

  28.2 Galaxy Mergers and Active Galactic Nuclei | Astronomy View original

  Is this image relevant?

1 of 3

• Hierarchical merging refers to the process by which galaxies grow and evolve through successive mergers and accretion events
• In this framework, smaller galaxies merge to form larger ones, which in turn merge with other galaxies to create even more massive systems
• This bottom-up approach to structure formation is a hallmark of the ΛCDM model and is supported by observational evidence and numerical simulations

Role in galaxy formation and evolution

• Hierarchical merging is a primary driver of galaxy formation and evolution, shaping the properties of galaxies across cosmic time
• Mergers can trigger intense episodes of star formation, leading to the rapid growth of stellar mass and the formation of new stellar populations
• Merging events also redistribute angular momentum, alter the morphology of galaxies, and contribute to the growth of central supermassive black holes

Hierarchical merging process

• The hierarchical merging process involves the gravitational interaction and eventual coalescence of two or more galaxies
• This process is governed by the distribution of dark matter halos, which provide the gravitational framework for galaxy mergers
• Merging events occur on various timescales, ranging from hundreds of millions to billions of years, depending on the properties of the galaxies involved

Gravitational interactions between galaxies

• Galaxies experience gravitational attraction towards one another, which can lead to close encounters and eventual mergers
• As galaxies approach each other, tidal forces begin to distort their shapes, creating features such as tidal tails and bridges
• These gravitational interactions can also trigger the inflow of gas towards the central regions of the galaxies, fueling star formation and active galactic nuclei (AGN) activity

Timescales of merging events

• The timescale of a merging event depends on factors such as the relative masses, sizes, and orbital parameters of the galaxies involved
• Major mergers, involving galaxies of comparable mass, typically occur on timescales of several hundred million to a few billion years
• Minor mergers, where a larger galaxy accretes a smaller satellite, can occur more frequently and on shorter timescales

Influence of dark matter halos

• Dark matter halos play a crucial role in the hierarchical merging process, as they provide the gravitational framework for galaxy interactions
• The size and mass of dark matter halos determine the frequency and intensity of merging events
• Galaxies residing in more massive dark matter halos are more likely to experience mergers, as they have a larger gravitational influence on their surroundings

Types of galaxy mergers

• Galaxy mergers can be classified into different types based on the properties of the galaxies involved and the characteristics of the merging process
• The distinction between major and minor mergers, as well as wet and dry mergers, provides insight into the diverse outcomes of hierarchical merging

Major vs minor mergers

• Major mergers involve galaxies of comparable mass, typically with a mass ratio greater than 1:4
• These mergers have a significant impact on the structure and properties of the resulting galaxy, often leading to the formation of elliptical galaxies or disturbed morphologies
• Minor mergers occur when a larger galaxy accretes a smaller satellite galaxy, with a mass ratio less than 1:4
• Minor mergers are more frequent than major mergers and can still influence the properties of the larger galaxy, such as its star formation rate and morphology

Wet vs dry mergers

• Wet mergers involve galaxies with significant amounts of cold gas, which can fuel intense episodes of star formation during the merging process
• These mergers often result in the formation of star-forming galaxies with blue colors and disturbed morphologies
• Dry mergers, on the other hand, involve galaxies with little to no cold gas reservoirs
• Dry mergers typically result in the formation of red, quiescent galaxies with elliptical morphologies and minimal star formation activity

Merger classifications and properties

• Galaxy mergers can be further classified based on the properties of the galaxies involved, such as their morphologies (spiral, elliptical, or irregular)
• The relative orientations of the galaxies' rotational axes and their orbital parameters also influence the outcome of the merger
• Prograde mergers, where the rotational axes of the galaxies are aligned, tend to result in more pronounced tidal features and star formation activity compared to retrograde mergers

Impact on galaxy morphology

• Hierarchical merging has a profound impact on the morphology of galaxies, reshaping their appearance and structural properties
• Mergers can transform the morphology of galaxies, creating elliptical galaxies, disturbed disks, and tidal features

Formation of elliptical galaxies

• Major mergers between spiral galaxies can result in the formation of elliptical galaxies
• During the merging process, the ordered rotational motion of the stars in the spiral galaxies is disrupted, leading to a more random and pressure-supported system
• The resulting elliptical galaxy typically has a smooth, featureless appearance and lacks the prominent spiral arms and disk structure of its progenitors

Creation of tidal features and streams

• Gravitational interactions during mergers can create striking tidal features, such as tidal tails and bridges
• Tidal tails are elongated streams of stars and gas that extend from the merging galaxies, often spanning tens to hundreds of kiloparsecs
• Tidal streams are narrower, more coherent structures that can trace the orbital path of the merging galaxies
• These tidal features serve as important observational signatures of ongoing or past merging activity

Reshaping of galactic disks and bulges

• Mergers can significantly alter the structure of galactic disks and bulges
• Minor mergers can thicken and heat the disk, leading to the formation of a more pronounced bulge component
• Major mergers can completely disrupt the disk structure, resulting in a more spheroidal morphology
• The growth of central bulges through mergers can also influence the overall size and mass distribution of galaxies

Merger-induced star formation

• Galaxy mergers can trigger intense episodes of star formation, known as starbursts, which can significantly increase the star formation rate (SFR) of the merging system
• The enhanced star formation activity is driven by the compression and cooling of gas during the merging process, leading to the formation of new stellar populations

Starburst activity in merging galaxies

• Merging galaxies often exhibit enhanced star formation activity compared to isolated galaxies
• The gravitational interactions and tidal forces during mergers can compress and shock the interstellar medium (ISM), triggering the collapse of gas clouds and the formation of new stars
• Starburst galaxies, such as ultra-luminous infrared galaxies (ULIRGs), are often associated with ongoing or recent merging activity

Influence on star formation rates

• Merger-induced starbursts can significantly increase the SFR of galaxies, sometimes by orders of magnitude compared to their pre-merger levels
• The peak SFR during a merger depends on factors such as the gas content of the galaxies, the mass ratio of the merger, and the orbital parameters
• The enhanced SFR can contribute to the rapid growth of stellar mass and the chemical enrichment of the ISM

Creation of super star clusters

• Merging galaxies can host the formation of super star clusters (SSCs), which are compact, massive star clusters with exceptional luminosities
• SSCs are thought to form in the high-pressure, high-density environments created by galaxy mergers and interactions
• These clusters can contain thousands to millions of young, massive stars and can significantly contribute to the overall star formation activity of the merging system

Active galactic nuclei (AGN) triggering

• Galaxy mergers can trigger the activation and growth of active galactic nuclei (AGN), which are powered by accretion onto central supermassive black holes (SMBHs)
• The gravitational interactions and gas inflows during mergers can fuel the SMBHs, leading to enhanced AGN activity

Fueling of central supermassive black holes

• Mergers can drive gas and dust towards the central regions of galaxies, providing a source of fuel for the growth of SMBHs
• Gravitational torques and tidal forces can remove angular momentum from the gas, allowing it to be accreted onto the SMBH
• The increased gas supply can lead to a significant growth in the mass of the SMBH and the formation of a luminous AGN

AGN feedback effects on host galaxies

• AGN activity can have significant feedback effects on the host galaxy, influencing its star formation activity and gas content
• Radiative feedback from the AGN can heat and ionize the surrounding gas, suppressing star formation in the host galaxy
• Mechanical feedback, in the form of jets and outflows, can expel gas from the galaxy and regulate the growth of the SMBH

Quasar activity in merging systems

• Merging galaxies can host luminous quasars, which are highly energetic AGN that can outshine their host galaxies
• Quasars are often associated with gas-rich mergers, where the abundant fuel supply can power the rapid growth of the SMBH
• The peak of quasar activity is thought to occur during the final stages of a major merger, as the SMBHs of the merging galaxies coalesce

Observational evidence of mergers

• Observational studies provide compelling evidence for the occurrence of galaxy mergers and their impact on galaxy evolution
• Various observational signatures, such as disturbed morphologies and tidal features, can be used to identify merging systems

Tidal tails and bridges

• Tidal tails and bridges are distinctive features that indicate ongoing or recent merging activity
• These structures are created by the gravitational interactions between merging galaxies and can extend far beyond the main bodies of the galaxies
• Examples of galaxies with prominent tidal tails include the Antennae Galaxies (NGC 4038/4039) and the Mice Galaxies (NGC 4676)

Disturbed galaxy morphologies

• Merging galaxies often exhibit disturbed and asymmetric morphologies, deviating from the regular spiral or elliptical shapes of isolated galaxies
• These morphological disturbances can include warped disks, shells, and rings, which are indicative of gravitational interactions
• The peculiar galaxy Centaurus A (NGC 5128) is an example of a galaxy with a disturbed morphology, likely the result of a recent merger

Multi-wavelength signatures of merging activity

• Merging galaxies can be studied across multiple wavelengths, from radio to X-rays, to probe different aspects of the merging process
• Radio observations can reveal the presence of synchrotron emission from relativistic particles, which can be associated with merger-induced star formation or AGN activity
• Infrared observations can trace the dust-obscured star formation activity, which is often enhanced in merging systems
• X-ray observations can detect the presence of hot gas and AGN activity, providing insights into the energetics of the merging process

Simulations of hierarchical merging

• Numerical simulations play a crucial role in understanding the hierarchical merging process and its impact on galaxy evolution
• These simulations can model the complex gravitational interactions and gas dynamics involved in galaxy mergers, providing insights into their outcomes and observable properties

Numerical modeling techniques

• Various numerical techniques are employed to simulate galaxy mergers, including N-body simulations and hydrodynamical simulations
• N-body simulations focus on the gravitational interactions between particles representing stars and dark matter, while hydrodynamical simulations additionally model the gas dynamics and star formation processes
• Adaptive mesh refinement (AMR) and smoothed particle hydrodynamics (SPH) are commonly used methods for hydrodynamical simulations of galaxy mergers

Reproducing observed merger properties

• Simulations of galaxy mergers aim to reproduce the observed properties of merging systems, such as their morphologies, star formation rates, and AGN activity
• By comparing simulations with observations, researchers can constrain the initial conditions and physical processes that govern the merging process
• Simulations have successfully reproduced features such as tidal tails, starburst activity, and the formation of elliptical galaxies through mergers

Insights into merger dynamics and outcomes

• Simulations provide valuable insights into the dynamics of galaxy mergers and the factors that influence their outcomes
• By exploring a range of initial conditions and merger parameters, simulations can investigate the impact of mass ratios, gas fractions, and orbital configurations on the resulting galaxy properties
• Simulations can also predict the observable signatures of mergers at different stages, aiding in the interpretation of observational data

Role in cosmological context

• Hierarchical merging is a fundamental process within the broader cosmological context, playing a crucial role in the formation and evolution of large-scale structure
• The ΛCDM model, which is the current standard cosmological model, provides the framework for understanding the role of mergers in galaxy evolution

Hierarchical merging in the Lambda-CDM model

• In the ΛCDM model, structure formation proceeds in a hierarchical manner, with smaller structures merging to form larger ones over cosmic time
• Dark matter halos, which are the gravitational scaffolding for galaxies, grow through mergers and accretion, driving the hierarchical assembly of galaxies
• The merger rate of galaxies is predicted to evolve with redshift, with more frequent mergers occurring at higher redshifts when the universe was denser

Building up large-scale structure

• Hierarchical merging plays a key role in the formation and evolution of large-scale structure in the universe
• Mergers between galaxies and galaxy clusters contribute to the growth of the cosmic web, which consists of filaments, walls, and voids
• The properties of galaxies and their distribution within the cosmic web are influenced by their merger histories and the environment in which they reside

Implications for galaxy evolution over cosmic time

• The hierarchical nature of structure formation has significant implications for the evolution of galaxies over billions of years
• Mergers drive the growth of galaxies in terms of their stellar mass, size, and morphology, shaping the diverse population of galaxies observed today
• The merger rate and its evolution with redshift have important consequences for the star formation history and the emergence of different galaxy types (ellipticals, spirals, and irregulars) over cosmic time
• Understanding the role of hierarchical merging in galaxy evolution is crucial for interpreting observations of galaxies at different epochs and for constructing a comprehensive picture of galaxy formation and evolution