5.2 HII regions

HII regions are vast clouds of ionized hydrogen gas powered by intense ultraviolet radiation from massive stars. These luminous nebulae serve as valuable tracers of recent star formation and provide insights into the physical conditions and chemical composition of the interstellar medium.

HII regions play a crucial role in galaxy formation and evolution. They act as stellar nurseries, hosting the birth of new stars and shaping galactic structure. The study of HII regions spans various observational techniques and extends to extragalactic environments, offering a window into star formation processes across cosmic time.

Ionized hydrogen clouds

• HII regions are vast clouds of ionized hydrogen gas that play a crucial role in the formation and evolution of galaxies
• These luminous nebulae are powered by the intense ultraviolet radiation emitted by massive, hot stars embedded within them
• HII regions serve as valuable tracers of recent star formation activity and provide insights into the physical conditions and chemical composition of the interstellar medium

Strömgren spheres

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• Strömgren spheres represent the volume of gas that a star can ionize, determined by the balance between ionization and recombination rates
• The size of a Strömgren sphere depends on the luminosity of the central star, the density of the surrounding gas, and the chemical composition of the medium
• Strömgren spheres are typically several parsecs in diameter and have sharp boundaries between the ionized and neutral gas

Density and temperature

• HII regions exhibit a wide range of densities, typically ranging from a few particles per cubic centimeter to several thousand particles per cubic centimeter
• The temperature within HII regions is relatively uniform, with values around 10,000 Kelvin, maintained by the balance between heating by photoionization and cooling through various emission processes
• Density inhomogeneities within HII regions can lead to the formation of compact, high-density substructures known as ultracompact HII regions

Ionization and recombination

• The ionization of hydrogen in HII regions is primarily driven by the absorption of ultraviolet photons with energies greater than 13.6 eV, which corresponds to the ionization potential of hydrogen
• Recombination occurs when free electrons are captured by protons, leading to the formation of neutral hydrogen atoms and the emission of photons
• The balance between ionization and recombination processes determines the overall ionization structure and emission properties of HII regions

Emission and absorption

• HII regions are characterized by their distinct emission and absorption features, which arise from the transitions of electrons between different energy levels in hydrogen and other elements
• These spectral lines provide valuable diagnostic tools for studying the physical conditions, chemical composition, and kinematics of HII regions

Balmer series

• The Balmer series corresponds to the transitions of electrons from higher energy levels (n ≥ 3) to the second energy level (n = 2) in hydrogen
• The most prominent line in the Balmer series is the Hα line, which has a wavelength of 6563 Å and is commonly used to trace HII regions in optical observations
• Other notable lines in the Balmer series include Hβ (4861 Å), Hγ (4340 Å), and Hδ (4102 Å)

Lyman series

• The Lyman series involves transitions of electrons from higher energy levels (n ≥ 2) to the ground state (n = 1) in hydrogen
• The Lyman-α line, with a wavelength of 1216 Å, is the strongest line in the Lyman series but is only observable in the ultraviolet region of the spectrum
• Lyman series lines are important for studying HII regions in the early Universe and in high-redshift galaxies

Paschen series

• The Paschen series corresponds to the transitions of electrons from higher energy levels (n ≥ 4) to the third energy level (n = 3) in hydrogen
• Paschen series lines lie in the near-infrared region of the spectrum, with the most prominent line being Paschen-α at 18751 Å
• Paschen series lines are useful for studying heavily obscured HII regions and penetrating through dusty environments

Brackett series

• The Brackett series involves transitions of electrons from higher energy levels (n ≥ 5) to the fourth energy level (n = 4) in hydrogen
• Brackett series lines are located in the mid-infrared region of the spectrum, with the most notable line being Brackett-α at 40512 Å
• Brackett series lines are valuable for studying HII regions in regions of high extinction and for probing the hot, dense regions near massive stars

Pfund series

• The Pfund series corresponds to the transitions of electrons from higher energy levels (n ≥ 6) to the fifth energy level (n = 5) in hydrogen
• Pfund series lines are situated in the far-infrared region of the spectrum, with the most prominent line being Pfund-α at 74578 Å
• Pfund series lines are important for studying cool, dense regions within HII regions and for probing the interface between ionized and molecular gas

Humphreys series

• The Humphreys series involves transitions of electrons from higher energy levels (n ≥ 7) to the sixth energy level (n = 6) in hydrogen
• Humphreys series lines are located in the far-infrared and submillimeter regions of the spectrum, with the most notable line being Humphreys-α at 123683 Å
• Humphreys series lines are useful for studying the coldest and densest regions within HII regions and for probing the transition from ionized to neutral gas

Stellar nurseries

• HII regions are often referred to as stellar nurseries because they are the birthplaces of new stars
• The intense ultraviolet radiation from massive stars ionizes the surrounding gas, creating the conditions necessary for triggered star formation

Molecular clouds

• HII regions are frequently associated with giant molecular clouds, which are vast reservoirs of cold, dense gas composed primarily of molecular hydrogen
• Molecular clouds provide the raw material for star formation, as gravitational instabilities within these clouds can lead to the collapse of dense cores and the formation of protostars
• The interface between HII regions and molecular clouds is a dynamic and complex environment where various feedback processes shape the structure and evolution of the region

Gravitational collapse

• Star formation within HII regions is triggered by the gravitational collapse of dense cores within molecular clouds
• As a dense core collapses under its own gravity, it fragments into smaller cores, which may eventually form individual stars or multiple star systems
• The gravitational collapse process is regulated by the interplay between gravity, thermal pressure, turbulence, and magnetic fields

Protostellar disks

• During the gravitational collapse of a dense core, conservation of angular momentum leads to the formation of a protostellar disk around the central protostar
• Protostellar disks are flattened structures composed of gas and dust that surround young stars and serve as the sites of planet formation
• Accretion of material from the protostellar disk onto the central protostar drives the early evolution and growth of the young star

Stellar winds and outflows

• Massive stars within HII regions generate powerful stellar winds and outflows that can significantly impact their surroundings
• Stellar winds are streams of high-velocity particles that are ejected from the surfaces of massive stars, driven by the intense radiation pressure
• Bipolar outflows are highly collimated jets of gas that emanate from the poles of young stellar objects, likely driven by magnetic fields and disk accretion processes
• Stellar winds and outflows can clear away surrounding gas, create cavities and bubbles, and trigger further star formation in the neighboring regions

Massive star formation

• HII regions are particularly associated with the formation of massive stars, which have masses greater than 8 solar masses
• Massive stars play a crucial role in shaping the evolution of galaxies through their intense radiation, stellar winds, and supernova explosions

OB associations

• Massive stars often form in clusters or associations known as OB associations, named after the predominance of O- and B-type stars
• OB associations are loose groupings of young, massive stars that share a common origin and are gravitationally unbound
• The presence of OB associations within HII regions indicates recent and ongoing massive star formation activity

Feedback effects

• Massive stars within HII regions exert significant feedback effects on their surroundings through various mechanisms
• Radiative feedback occurs when the intense ultraviolet radiation from massive stars heats and ionizes the surrounding gas, creating expanding ionization fronts and driving the evolution of the HII region
• Mechanical feedback arises from the powerful stellar winds and outflows generated by massive stars, which can sweep up and compress the surrounding gas, triggering further star formation
• Chemical feedback involves the enrichment of the interstellar medium with heavy elements synthesized by massive stars and released through stellar winds and supernova explosions

Triggering mechanisms

• HII regions can trigger the formation of new stars through several mechanisms that compress and destabilize the surrounding gas
• The collect and collapse mechanism occurs when the expanding ionization front of an HII region sweeps up and compresses the surrounding neutral gas, creating dense shells that can fragment and form new stars
• Radiation-driven implosion takes place when the ionizing radiation from massive stars penetrates and compresses pre-existing dense clumps, inducing their gravitational collapse and star formation
• Shock-induced star formation can occur when the supersonic expansion of an HII region generates shocks that compress and destabilize the surrounding gas, leading to the formation of new stars

Expansion and dissipation

• HII regions undergo a continuous process of expansion and dissipation over time, driven by the energy input from massive stars
• The expansion of an HII region is initially driven by the pressure difference between the hot, ionized gas and the surrounding neutral medium
• As the HII region expands, it can break out of its parent molecular cloud and create champagne flows, where the ionized gas escapes into the surrounding low-density medium
• The dissipation of an HII region occurs when the massive stars that power it evolve and die, leading to a gradual recombination and cooling of the ionized gas

Galactic structure

• HII regions are important tracers of the structure and evolution of galaxies, particularly in spiral galaxies
• The distribution and properties of HII regions provide insights into the star formation history, gas dynamics, and chemical evolution of galaxies

Spiral arms

• In spiral galaxies, HII regions are predominantly found along the spiral arms, which are regions of enhanced gas density and star formation activity
• The concentration of HII regions in spiral arms is a result of the compression and shock of gas as it flows into the arms, triggering the formation of massive stars
• The presence and morphology of HII regions along spiral arms can be used to trace the structure and dynamics of the arms

Star formation rates

• HII regions are excellent indicators of the current star formation rate in galaxies, as they are associated with the most massive and short-lived stars
• The total Hα luminosity of HII regions in a galaxy can be used as a proxy for the star formation rate, as it directly traces the ionizing photon output from massive stars
• Variations in the star formation rate across different regions of a galaxy can provide insights into the factors that regulate and trigger star formation on galactic scales

Luminosity functions

• The luminosity function of HII regions describes the distribution of HII regions as a function of their luminosity
• The shape of the HII region luminosity function can provide information about the initial mass function of stars, the star formation history, and the evolutionary stage of the galaxy
• Differences in the luminosity functions of HII regions between galaxies can reflect variations in the star formation processes and the physical conditions of the interstellar medium

Metallicity gradients

• HII regions can be used to study the chemical composition and metallicity gradients within galaxies
• The gas-phase metallicity of HII regions can be determined by measuring the relative abundances of heavy elements through emission line ratios
• Radial metallicity gradients, where the metallicity decreases with increasing distance from the galactic center, are commonly observed in spiral galaxies
• Metallicity gradients provide insights into the chemical evolution of galaxies, the interplay between gas inflow and outflow, and the effects of stellar feedback on the enrichment of the interstellar medium

Observational techniques

• The study of HII regions relies on a variety of observational techniques that probe different aspects of their structure, composition, and evolution
• These techniques span a wide range of wavelengths, from radio to X-rays, and provide complementary information about the physical conditions and processes within HII regions

Hα imaging

• Hα imaging is a widely used technique for identifying and studying HII regions in the optical wavelength range
• Narrow-band filters centered on the Hα emission line are employed to selectively capture the light from ionized hydrogen gas
• Hα images reveal the distribution, morphology, and luminosity of HII regions within galaxies, enabling the study of star formation rates and the structure of galaxies

Radio observations

• Radio observations are crucial for studying HII regions, as they can penetrate through dusty regions and probe the ionized gas at low frequencies
• Radio continuum emission from HII regions arises from free-free emission, which is produced by the interaction of free electrons with ions in the ionized gas
• Radio recombination lines, which result from the capture of electrons by protons, provide information about the density, temperature, and velocity structure of HII regions
• Radio observations also allow the detection of ultracompact HII regions, which are deeply embedded in dense molecular clouds and are difficult to observe at optical wavelengths

Infrared observations

• Infrared observations are valuable for studying HII regions, particularly in regions of high dust extinction
• Dust grains within HII regions absorb ultraviolet and optical radiation from massive stars and re-emit it in the infrared
• Near-infrared observations can penetrate through dusty environments and reveal the embedded stellar populations and the structure of HII regions
• Mid- and far-infrared observations provide information about the dust content, temperature, and composition within HII regions, as well as the properties of the associated molecular clouds

Spectroscopy

• Spectroscopic observations are essential for understanding the physical conditions, chemical composition, and kinematics of HII regions
• Optical and near-infrared spectroscopy can measure the relative strengths of emission lines from different elements, providing diagnostics of the temperature, density, and metallicity of the ionized gas
• High-resolution spectroscopy allows the determination of the velocity structure and dynamics of HII regions, including the presence of outflows, inflows, and turbulence
• Spectroscopic observations also enable the study of the stellar populations within HII regions, including the identification of massive stars and the determination of their spectral types and evolutionary stages

Interferometry

• Interferometric observations, particularly in the radio and submillimeter wavelengths, provide high angular resolution images of HII regions
• Radio interferometers, such as the Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), can resolve the fine structure and kinematics of HII regions on scales of a few astronomical units
• Interferometric observations can reveal the presence of ultracompact HII regions, the structure of ionization fronts, and the interaction between HII regions and their surrounding molecular clouds
• Interferometry also enables the study of the earliest stages of star formation within HII regions, including the detection of protostellar disks and outflows associated with individual massive stars

Extragalactic HII regions

• HII regions are not limited to the Milky Way galaxy but are also observed in a wide range of external galaxies
• The study of extragalactic HII regions provides insights into the star formation processes, chemical evolution, and physical conditions in different galactic environments

Giant HII regions

• Giant HII regions are extremely luminous and massive HII regions that are found in some galaxies, particularly in starburst galaxies and interacting systems
• These regions can have sizes of several hundred parsecs and luminosities equivalent to hundreds or thousands of typical HII regions in the Milky Way
• Giant HII regions are powered by the collective effect of numerous massive stars and are thought to represent the most extreme sites of star formation in the Universe
• Examples of well-known giant HII regions include 30 Doradus in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy

Starburst galaxies

• Starburst galaxies are galaxies undergoing an exceptionally high rate of star formation, often triggered by galaxy mergers or interactions
• These galaxies exhibit a high density of HII regions and a correspondingly high Hα luminosity, indicating the presence of numerous massive stars
• The study of HII regions in starburst galaxies provides insights into the extreme star formation conditions and the feedback processes that regulate the evolution of these systems
• Examples of starburst galaxies include M82, NGC 253, and Arp 220

Lyman-break galaxies

• Lyman-break galaxies are high-redshift galaxies that are identified by the presence of a strong break in their ultraviolet spectra, caused by the absorption of Lyman-continuum photons by neutral hydrogen
• These galaxies are thought to be actively forming stars at a high rate and contain a significant population of massive stars and HII regions
• The study of HII regions in Lyman-break galaxies provides insights into the star formation processes and the chemical enrichment of galaxies in the early Universe
• Lyman-break galaxies are typically found at redshifts greater than 2 and are important probes of the cosmic star formation history

High-redshift galaxies

• High-redshift galaxies are galaxies that are observed at large distances and early cosmic times, corresponding to redshifts greater than 1
• The study of HII regions in high-redshift galaxies provides a window into the star formation processes and the chemical evolution of galaxies in the early Universe
• Observations of high-redshift galaxies often rely on the detection of strong emission lines from HII regions, such as Lyman-α and Hα, which are redshifted into the optical and near-infrared wavelength ranges
• The properties of HII regions in high-redshift galaxies can provide constraints on the initial mass function, the star formation rates, and the chemical enrichment history of galaxies at