Galaxies tend to be categorized by their morphology (shape) and emission; however, because every galaxy is unique, the history of galaxy classification has been very messy. The main classifications (based on morphology) used today are: - [[#Spiral Galaxy]] - [[#Elliptical Galaxy]] - [[#Lenticular Galaxy]] - [[#Irregular Galaxy]] / [[#Peculiar Galaxy (Pec or XXp)]] With these classifications, galaxies also tend specify type of they are abnormal (i.e. [[#Dwarf Galaxy|dwarf]], giant, supergiant) There are also plenty of [[#Other Galaxy Types|other]] galaxy classifications that incorporate more attributes of galactic behavior. > [!measure] Typical Galaxy Sizes (diameter) > > The approximate diameter of the visible, [[Anatomy of a Galaxy#Stellar Halo]] of the galaxy... > > - [[#Spiral Galaxy]]: $10-50 \; {\rm kpc}$ > - [[#Elliptical Galaxy]]: $10-200 \; {\rm kpc}$ > - [[#Lenticular Galaxy]]: $5-50 \; {\rm kpc}$ > - [[#Dwarf Galaxy]]: $1-10 \; {\rm kpc}$ > > There are many other "sizes" that can describe a galactic scale: > - $r_{\rm mean}^{200} = \Omega_{m} \rho_{\rm crit}$ > - $r_{\rm crit}^{200} = r_{c}$ > - $r_{\rm splashback}$ > - $r_{\rm halflight}$ > [!measure] Typical Galaxy Mass > Mass: $\sim 10^{9} - 10^{13} \, \pu{M_{\odot}}$ ## Hubble Morphological Sequence *(Also known as the **Hubble tuning-fork diagram**)* ![[hubble_classification.jpg]] ![[galaxy_classes.png|align:center|500]] A morphological classification scheme for galaxies (Edwin Hubble, 1926) that uses the following classifications: - [[#Spiral Galaxy|Spiral]] (S) & [[#Barred Spiral Galaxy|Barred Spiral]] (SB) - The letters "a", "b", "c" indicate how tightly the spiral arms are wound ("a" being most tightly wound) - [[#Elliptical Galaxy|Elliptical]] (E) - The numbers 0-7 indicate how circular/oblate it appears (0 being most circular, 7 being more elongated) - [[#Lenticular Galaxy|Lenticular]] (S0) - Appeared more red and lacked spiral arms, but were still disk-like. (versus E0 that appeared more spherical) - [[#Irregular Galaxy|Irregular]] (Irr) & [[#Peculiar Galaxy|Peculiar]] (Pec) - *Not shown on the Tuning Fork* - A separate classification of galaxies that don't fit into the common types. Hubble thought his tuning form diagram was representative of an evolutionary process for the galaxy formation. We now know this is not true; however, these names and classifications are still used today because they still capture features and trends in galaxy populations. (i.e. ellipticals being pressure supported whereas spirals being rotationally supported) > [!note] > > The difference between **Irregular** and **Peculiar** galaxies is vague. Sometimes, these terms are used interchangeably; however, it seems that there the field is loosely uses the definitions: > - **"Irregular"** - galaxies are those with no distinct shape or structure. > - **"Peculiar"** - galaxies are those that do not fit into any other classification category > > That said, since this classification them only encompasses morphology, many **Peculiar** galaxies have also been identified as... > - a set of interacting galaxies (mergers or collisions) > - galaxies with an [[Black Hole#Active Galactic Nuclei]] > - galaxies with a burst of star formation > > ...but this is not true for all **Peculiar** galaxies. ## Spiral Galaxy ![[galaxy_spiral.jpg|align:center|500]] > [!space] In the Hubble Morphological Sequence... > Spiral galaxies are classified with an "S" and the letters "a", "b", "c" to indicate how tightly the spiral arms are wound ("a" = tight, "c" = loose) **Characteristics:** - Dominate in the field environments (outside [[Galaxy Cluster|galactic clusters]]) - [[#Disk Galaxy]] composed of gas and stars - Young stellar population, with blue color ([[Stellar Populations#Population I (Pop. I)]]) - Rotationally Supported - Exponential Surface Brightness Profile ### Spiral Arm Clustering See [[Question 114]]. ### Barred Spiral Galaxy ![[galaxy_barredspiral.jpg|align:center|500]] > [!space] In the Hubble Morphological Sequence... > Barred Spiral galaxies are classified with an "SB" and the letters "a", "b", "c" to indicate how tightly the spiral arms are wound ("a" = tight, "c" = loose) Follows the same properties of a [[#Spiral Galaxy]]; however, they have a central bar-shaped structure composed of stars, gas, and dust. Bars are thought to be a temporary feature in spiral galaxies that appear from a density wave radiating from the galactic center, reshaping inner stellar orbits. Over time, this effects moves further out causing the self-perpetuating bar to grow until it reaches a "buckling" point. After which, the bar decays and the galaxy transforms into the more regular spiral formation. Since the bar structure channels gas inwards from the spiral arms through [[orbital resonance]], it can help fuel star birth in the vicinity of its galaxy center. This process is thought to explain why many barred spiral galaxies have [[Black Hole#Active Galactic Nuclei|AGNs]] (i.e. the Southern Pinwheel Galaxy). ## Elliptical Galaxy ![[galaxy_elliptical.jpg|align:center|500]] > [!space] In the Hubble Morphological Sequence... > Elliptical galaxies are classified with an "E" and the numbers 0-7 to indicate the oblateness of spherical shape (0 = spherical, 7 = oblong) **Characteristics:** - Dominate in [[Galaxy Cluster|cluster]] environment (less found in the field) - Spheroidal structure - Old stellar population, with red color ([[Stellar Populations#Population II (Pop. II)]]) - Pressure Supported - de Vaucouleurs Surface Brightness Profile ## Lenticular Galaxy ![[galaxy_lenticular.jpg|align:center|500]] > [!space] In the Hubble Morphological Sequence... > Lenticular galaxies are classified with an "S0" and were originally through to be the transition point between [[#Elliptical Galaxy|elliptical galaxies]] and [[#Spiral Galaxy|spiral galaxies]], because they were round disk-like shapes without spiral arms. **Characteristics:** - Large [[#Disk Galaxy]] without spiral arms - Used/lost a significant amount of [[Interstellar Medium|ISM]] - Low star formation rate --> composed of old stellar population ([[Stellar Populations#Population II (Pop. II)]]) - Retains significant dust in their disks - Named so because they looked like "lenses" (or "lentils"?) - In morphology, similar to [[#Spiral Galaxy|spiral galaxies]] - In spectra & scaling, similar to [[#Elliptical Galaxy|elliptical galaxies]] ## Irregular Galaxy ![[galaxy_irregular.jpg|align:center|500]] Galaxies that are observed to be without a distinct shape or structure. ## Peculiar Galaxy ![[galaxy_peculiar.jpg|align:center|500]] Galaxies that do not fit into any other classification category. There is quite a diversity to the types of galaxies that are categorized as *peculiar*. Many of them have been identified to be galaxies with... - strange shapes - strange halos - rings - plumes and tails - unusual dark lanes - unusual placement of HII regions - excessively bright or peculiar nuclei - apparent explosions - particularly distorted interactions with other galaxies - possible interactions with intergalactic clouds of dust or gas ## Other Galaxy Classifications ### Disk Galaxy Following the [[Anatomy of a Galaxy#Galactic Disk]] description, a **disk galaxy** is a broad overall classification for may type of galaxies; whether they showcase [[Anatomy of a Galaxy#Spiral Arms|spiral arm]] structures or not. The presence of a disk in a galaxy is determined more by its formation history and environment than by its age. From the [[Question 107|Morphology-Density relationship]], they are less likely to be seen denser regions (like [[Galaxy Cluster|galaxy clusters]]) due to: 1. **Environmental Effects:** ("galaxy strangulation" & "ram pressure stripping") The hot, dense gas found in galaxy clusters can exert strong tidal forces and gravitational interactions on galaxies, stripping away their gas and dust and disrupting their orbits. This can disrupt the formation and maintenance of disk structures and lead to the formation of elliptical galaxies. 2. **Mergers and Interactions:** ("galaxy harassment") Galaxy clusters are very crowded environments, and the high relative speeds of galaxies can lead to frequent mergers and interactions. These violent interactions can disrupt disk structures and lead to the formation of elliptical galaxies. The more time spent in the cluster, the more potential mergers can occur. ### Dwarf Galaxy Dwarf galaxies are very similar to [[Stellar Clusters#Globular Cluster|globular clusters]] in mass, but are significantly less compact, and thus, less luminous. This is possible because they are thought to have a higher dark matter content, relative to [[Stellar Clusters#Globular Cluster|globular clusters]], that allows them to remain gravitationally bound. $R_{\rm glob} \sim \mathcal{O}(10 \; {\rm pc}) \hspace{2cm} R_{\rm dwarf} \sim \mathcal{O}(100 \; {\rm pc})$ Often, the challenge with dwarf galaxies is identifying the membership stars. Observationally, this is done by comparing their radial velocity with the systemic velocity of the dwarf galaxy itself. Similarly, it could also be done through the star's proper motion compared to background stars. Relative to [[#Ultra-Faint Dwarf (UFD) Galaxy|UFDs]], the membership stars can be seen as overdensities on the sky, making member star identification straightforward in comparison. Regarding the stellar composition, these systems showcase all characteristic signs of galactic evolution, including multiple bursts of star formation and chemical evolution (i.e. from metal-poor to metal-rich stars). Stars can be found with [[Metallicity|metallicities]] of $\ce{[Fe/H]} \sim -4.0$ up to solar levels ($\ce{[Fe/H]} \sim 0$). > [!measure] Typical Parameters > - Mass Range: $\sim 10^{6} - 10^{9} \; {\rm M_{\odot}}$  > - Size Range: $\sim \mathcal{O}(100 \; {\rm pc})$  ![[globularCluster_dwarfGalaxy_distribution.png|align:center]] #### Dwarf Spheroidal A **dwarf spheroidal galaxies (dSph)** are small, low-luminosity galaxies with very little dust and an older stellar population. While similar to *dwarf elliptical galaxies* in appearance and properties (such as little to no gas or dust or recent star formation), they are approximately spheroidal in shape and generally have lower luminosity. They are found in the Local Group as companions to the Milky Way and the Andromeda Galaxy (M31) -- the two most observable areas for faint/low-luminosity systems. #### Ultra-Faint Dwarf (UFD) Galaxy A **ultra-faint dwarf galaxy (UFD)** are the smallest and faintest galaxies with very small stellar populations of only a few of thousand of stars ($M \sim 10^{4} - 10^{6} \; \pu{M_{\odot}}$). They are the oldest, most dark matter-dominated, most metal-poor, and least chemically evolved stellar systems known. The stellar composition of these UFDs has [[Metallicity|metallicities]] ranging from $-4.0 \lesssim \ce{[Fe/H]} \lesssim -1.0$, with more metal-rich stars being completely absent from the system, due to star formation quenching early on during the reionization process. Thus, after the first couple generations of star formation took place, chemical evolution stopped such that they metal-poor system was preserved over time. The observed UFDs are relatively nearby (within $150 - 250 \, \pu{kpc}$), though their low-luminosity makes them hard to find. Some of the more nearby systems are beginning to undergo tidal disruption, though some remain intact as well. ### Starburst Galaxy (SBG) Starburst Galaxies are the galaxies where massive stars are forming at a very high rate (about $10^3$ times greater than in a normal galaxy like our Milky way). - The optical and infrared [[Luminosity|luminosity]] observed from these sources are dominated by radiation from these young massive stars, indicating that there is a high concentration of gas and radiation in localized regions. - These massive stars have relatively shorter lifetimes and at the end of their lifetime, they explode as [[Stellar Explosions#Supernova|supernovae]]. Thus starburst regions are an ideal environment for the acceleration of comic rays. - These supernovae enrich the central star-forming regions with relativistic cosmic rays (protons, electrons and positrons). - Cosmic ray energy densities in SBGs are orders of magnitude higher compared to normal galaxies (see [[Question 82]]). **Example:** M82 ### Radio Galaxy (See [[Active Galactic Nuclei|AGN]]) ### Jellyfish Galaxy ### Brightest Cluster Galaxy *(Also known as **BCG** or **cD galaxies**)* The **brightest cluster galaxy** are the largest/brightest galaxies found in the center of [[Galaxy Cluster|galaxy clusters]]. They are also known as **cD galaxies**, where the "c" refers to very large galaxies and "D" refers to being diffuse. Sometimes more commonly, cD also refers to the backronym "central Dominant". > [!measure] Typical Parameters > - Mass: $M \sim 10^{13} \; {\rm M_{\odot}}$ > - Diameter: $D \sim 300-1000 \; {\rm kpc}$ **Propterties** - A type of [[#Elliptical Galaxy]] - Contains tens of thousands of [[Stellar Clusters#Globular Cluster|globular clusters]] - High mass-light ratio $\implies$ large quantity of [[Dark Matter]] - Large amounts of [[Electromagnetic Spectrum|x-ray]] emission - Some have multiple galactic nuclei from mergers **Formation Theories:** - Form through multiple merger events in a dense cluster environment (Ostriker + Tremaine, 1975). - Dynamical friction of nearby galaxies cause them to lose energy and fall into the central cD galaxy - Theory supported by the large presence of [[Stellar Clusters#Globular Cluster|globular clusters]] in cD galaxies, captured during the mergers **Observation Strategies**: - Can observe molecular gas emission of the BCG, but it must contain $M \sim 10^{8} \; {\rm M_{\odot}}$ of stellar mass to observe this confidently (can do with [[Instruments#ALMA]]) - Can use molecular absorption of [[Active Galactic Nuclei|AGN]] backlight in radio to probe the BCG mass scales of ~10s to 100s of $M_{\odot}$ - Very strong absorption lines from $\ce{CO}$, $\ce{HCO^{+}}$, $\ce{HNC}$, $\ce{H_{2}}$, $\ce{CO}$, and $\ce{SiO}$. - Width of absorption feature increases with [[Velocity Dispersion|velocity dispersion]] $\rightarrow$ proximity to an [[#AGN]]'s [[Black Hole#Supermassive Black Hole|SMBH]]. - Does require the presence AGN and good alignment with the BCG disk