1.1 Hubble sequence

The Hubble sequence is a fundamental tool for classifying galaxies based on their visual appearance. Developed by Edwin Hubble in 1926, it organizes galaxies into categories like elliptical, spiral, and irregular, providing insights into their structure and composition.

This classification system reveals important trends in galaxy properties along the sequence. From elliptical to spiral galaxies, we observe changes in star formation rates, gas content, and stellar populations, reflecting the diverse evolutionary paths of these cosmic structures.

Hubble's galaxy classification system

• Developed by Edwin Hubble in 1926, this system categorizes galaxies based on their morphological appearance
• Provides a framework for understanding the diversity of galaxy shapes and structures observed in the universe

Tuning fork diagram

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• Visual representation of Hubble's classification scheme resembling a tuning fork
• Places elliptical galaxies on the "handle" and spiral galaxies on the two "prongs" (normal and barred)
• Arranges galaxies along a sequence from elliptical to spiral, with lenticular galaxies as a transitional class

Elliptical galaxies

• Designated as "E" galaxies followed by a number indicating their ellipticity (E0 to E7)
• Smooth, featureless appearance lacking well-defined structures like spiral arms
• Range from nearly spherical (E0) to highly elongated (E7) shapes
• Contain mostly old, red stars with minimal ongoing star formation

Spiral galaxies

• Divided into two main classes: normal spirals (S) and barred spirals (SB)
• Characterized by a central bulge surrounded by a flat, rotating disk with spiral arms
• Subclassified as Sa, Sb, or Sc based on the tightness and prominence of spiral arms
  • Sa galaxies have tightly wound, smooth arms and large central bulges
  • Sc galaxies have loosely wound, patchy arms and small central bulges

Barred spiral galaxies

• Contain an elongated bar-like structure extending from the central bulge
• Bar composed of older stars and acts as a mechanism for funneling gas into the galaxy center
• Subclassified as SBa, SBb, or SBc based on the bar strength and spiral arm appearance

Irregular galaxies

• Lack any distinct regular structure or symmetry (neither elliptical nor spiral)
• Often appear chaotic and fragmented, with no clear central bulge or organized spiral arms
• Typically smaller in size compared to other galaxy types
• Examples include the Large and Small Magellanic Clouds, satellite galaxies of the Milky Way

Lenticular galaxies

• Designated as "S0" and considered a transitional class between elliptical and spiral galaxies
• Contain a central bulge and a disk component but lack well-defined spiral arms
• Minimal ongoing star formation and gas content compared to spiral galaxies
• May represent an evolutionary stage as spirals lose their gas and spiral structure

Properties along the Hubble sequence

Bulge vs disk dominance

• Relative prominence of the central bulge and surrounding disk varies along the Hubble sequence
• Elliptical galaxies are bulge-dominated with no significant disk component
• Spiral galaxies exhibit a range of bulge-to-disk ratios, with Sa galaxies being more bulge-dominated and Sc galaxies more disk-dominated

Gas and dust content

• Amount of interstellar gas and dust decreases from late-type spirals (Sc) to early-type spirals (Sa) and is minimal in elliptical galaxies
• Higher gas and dust content in spiral galaxies supports ongoing star formation
• Elliptical galaxies have little to no cold gas and dust, indicating a lack of star formation

Star formation rates

• Star formation activity follows the gas and dust content along the Hubble sequence
• Highest star formation rates observed in late-type spirals (Sc) and irregular galaxies
• Early-type spirals (Sa) have lower star formation rates, while elliptical galaxies show minimal to no ongoing star formation

Stellar populations

• Age and composition of stellar populations vary along the Hubble sequence
• Elliptical galaxies dominated by old, low-mass stars with minimal young, blue stars
• Spiral galaxies contain a mix of old and young stellar populations, with ongoing star formation in the disk
• Late-type spirals (Sc) have a higher proportion of young, blue stars compared to early-type spirals (Sa)

Color differences

• Galaxies exhibit a color gradient along the Hubble sequence due to differences in stellar populations
• Elliptical galaxies appear redder due to the predominance of older, low-mass stars
• Spiral galaxies show a range of colors, with late-type spirals (Sc) appearing bluer due to the presence of young, massive stars
• Color differences reflect the varying star formation histories and stellar content of galaxies

Limitations of the Hubble sequence

Observed vs intrinsic properties

• Hubble classification based on observed morphology, which can be affected by viewing angle and distance
• Galaxies with similar intrinsic properties may appear different depending on their orientation relative to the observer
• Edge-on spiral galaxies can be misclassified as lenticular or elliptical due to the obscuration of spiral arms

Impact of galaxy orientation

• Inclination of a galaxy's disk relative to the line of sight can impact its observed morphology
• Face-on spiral galaxies reveal clear spiral arm structures, while edge-on spirals appear as elongated, featureless disks
• Orientation effects can lead to misclassification or ambiguity in Hubble types

Difficulty classifying distant galaxies

• At large distances, galaxies appear smaller and fainter, making morphological classification challenging
• Lack of resolution and signal-to-noise ratio can hinder the detection of fine structures like spiral arms or bars
• Distant galaxies observed at earlier cosmic epochs may have different morphologies compared to nearby galaxies

Existence of peculiar galaxies

• Some galaxies exhibit unusual or distorted morphologies that do not fit well into the Hubble sequence
• Peculiar galaxies often result from galaxy mergers, interactions, or other disruptive events
• Examples include ring galaxies, tadpole galaxies, and galaxies with tidal tails or bridges
• Presence of peculiar galaxies highlights the limitations of a purely morphological classification scheme

Revisions to the Hubble sequence

De Vaucouleurs classification system

• Extends the Hubble sequence by adding more detailed subclasses and criteria
• Introduces intermediate classes (Sab, Sbc) and finer distinctions within elliptical and spiral categories
• Considers additional features such as inner and outer ring structures, lenses, and asymmetries

Yerkes classification system

• Developed by W. W. Morgan and based on the central concentration of light in galaxies
• Classifies galaxies into seven categories (a, af, f, fg, g, gk, k) based on the degree of central concentration
• Provides a quantitative measure of galaxy structure independent of the Hubble sequence

Importance of bar structures

• Presence of bars recognized as a significant morphological feature in galaxy classification
• Bars can influence gas dynamics, star formation, and the overall evolution of galaxies
• Inclusion of barred spiral categories (SBa, SBb, SBc) in the Hubble sequence acknowledges the importance of bars

S0 galaxies as a transitional class

• Lenticular (S0) galaxies initially placed between elliptical and spiral galaxies in the Hubble sequence
• Subsequent research suggests S0 galaxies may represent an evolutionary stage rather than a strict morphological class
• S0 galaxies could result from the transformation of spiral galaxies through gas stripping or merger processes

Physical interpretation of the Hubble sequence

Evolutionary sequence vs morphological sequence

• Hubble sequence originally interpreted as an evolutionary sequence, with galaxies evolving from elliptical to spiral
• Modern understanding recognizes the Hubble sequence primarily as a morphological classification scheme
• Evolutionary connections between different galaxy types are more complex and multifaceted than a simple linear sequence

Relation to galaxy formation and evolution

• Morphology of galaxies reflects their formation and evolutionary histories
• Elliptical galaxies likely formed through mergers or rapid collapse of gas clouds, leading to a burst of star formation and subsequent quiescence
• Spiral galaxies formed through more gradual accretion of gas and angular momentum, allowing for prolonged star formation in the disk

Environmental effects on morphology

• Galaxy morphology can be influenced by the local environment, such as galaxy clusters or groups
• Tidal interactions, ram-pressure stripping, and harassment can alter the morphology and gas content of galaxies
• Elliptical and S0 galaxies more common in dense cluster environments, while spirals dominate in less dense regions

Mergers and interactions

• Galaxy mergers and interactions play a crucial role in shaping galaxy morphology and evolution
• Major mergers between galaxies of comparable mass can disrupt spiral structures and lead to the formation of elliptical galaxies
• Minor mergers and tidal interactions can trigger bar formation, enhance star formation, and cause morphological distortions
• Hubble sequence does not fully capture the diversity and complexity of galaxy morphologies resulting from mergers and interactions