Guide to Classification of Galaxies and AGNs

Editor: Nathan Sanders

For a good general reference on galaxy morphology, see arXiv:1102.0550. For quick notes on specific types of galaxies, AGN, and famous examples of the same, search the text below!

Types of galaxies:

Barred spiral galaxy: A spiral galaxy with a bar-shaped distribution of stars in the center, including the Milky Way. The gravitaitonal potential of the bar introduces many interesting dynamic effects (e.g. arXiv:1102.1157) and may prompt star formation (see this Astrobite).

The strongly barred spiral galaxy NGC 613 viewed in the infrared (from 2000AJ....119..536E).

Blue Compact Dwarfs (BCDs): See LCBGs.
Blue Compact Galaxies (BCGs, not to be confused with Brightest Cluster Galaxies): These are intermediate-mass (sim 10^{9-10} M_odot, M_B < -18), low-metallicity, star-forming galaxies that often appear to be interacting with other galaxies or merging. They are starbursting in the sense that they have strong emission lines and they have bright, young clusters with ages <5 Myr.They are considered to be analogs of higher-redshift Lyman Break Galaxies. They are therefore useful tracers of galaxy evolution. See arXiv:1103.5771.

HST image of BCG Mrk 930, demonstrating faetures of interaction. From Figure 1 of arXiv:1103.5771.

Brightest Cluster Galaxy (BCGs, not to be confused with Blue Compact Galaxies): This is simply the galaxy that happens to be the brightest among its galaxy cluster. The BCG is typically a giant elliptical located in the center of the cluster. Because BCGs across all clusters of a given size tend to be of similar brightness, they have been used as standard candles.

The dependence of the brightness of a BCG on the richness of the cluster (number of galaxies above a certain brightness),and its scatter (below). From 2009ApJ...699.1333H

Bright Red Galaxies (BRGs): Synonymous with luminous red galaxies (LRGs).
cD galaxies: Enormous (“supergiant”) early-type galaxies (sometimes r>1 Mpc), typically found at the center of clusters. Their outer brightness profiles (“envelopes”) extend so far that it can be difficult to distinguish them observationally and dynamically from their cluster neighbors. See e.g. 1988ApJ…328..475S.

The central 300 kpc of the cluster Abell 2029 and its cD galaxy. From 1979ApJ...231..659D

Compact elliptical (cE) galaxies: Possibly formed when a more massive early-type spiral galaxy is tidally-stripped by a larger neighbor on a timescale of ~1 Gyr. The luminosity of the galaxy may change dramatically during this process, while the color will not, so cEs may appear an order of magnitude dimmer than similarly red galaxies. See e.g. arXiv:1102.1159
Compact Narrow Emission Line Galaxies (CNELG): A subset of Luminous Compact Blue Galaxies with narrow emission line features. See LCBG.
D galaxies:
Dwarf galaxies: Dwarfs generally have little gas, have old stellar populations, and smooth, featureless morphologies that are supported by pressure from random stellar motion. However, this is a diverse class: see other entries for dwarfs on this page, arXiv:1101.2460, and arXiv:1103.1116.
Dwarf irregular (dIrr) galaxies: Dwarf irregulars are interesting laboratories for galaxy evolution, because they do not have dynamical features such as spiral density waves that interfere with the fundamental role of fundamental processes such as turbulence and stellar evolution in driving star formation. However, it has been shown that dIrrs can undergo long-duration (>1 Gyr) starbursts. See e.g. arXiv:1104.0464.
Dwarf ellipticals (dE): Generally the most massive dwarfs, dominated by stars rather than dark matter in their inner regions. They have low gas fractions nad metallicities (Z ~< 0.1 Z_solar). These are common in the universe and dominate cluster populations, but there are only three known in the Local Group (interacting with Andromeda). This suggests that dE formation is strongly environment-dependent (see e.g. arXiv:1101.2460 and arXiv:1103.1116).
Dwarf spheroirdals (dSph): Smaller dwarfs that are dominated by dark matter rather than stars in their central regions. These are ubiquitous in the Local Group (>20 already known), interacting tidally with larger galaxies. They are potentially valuable probes for studying dark matter. It’s unclear if low mass dSph galaxies can be distinguished from globular clusters. See e.g. arXiv:1101.2460, arXiv:1103.0477, and arXiv:1103.1116.
E+A galaxies (“Poststarburst” galaxies, PSGs): Have strong Balmer absorption lines typical of young stars, but lack optical emission features (e.g. O III 5007) typical of star-forming regions and have metallic absoprtion lines (e.g. Ca H and K) indicative of an older population of dwarf stars. The presence of two distinct stellar populations and morphological observations suggest that E+A galaxies are the product of a gas-rich galaxy merger (see e.g. 2000AJ….119.2118G and arXiv:1101.4933)
Early type: Refers to galaxies on the left side of Hubble’s tuning fork diagram – often synonymous with elliptical galaxies (see concepts PDF).
Elliptical: Galaxies typified by a spheroidal distribution of stars, rather than a disk. These are typically “red” and “dead,” with little ongoing star formation.
Faint blue galaxies:
Faint low surface brightness galaxies (fLSBs): Galaxies with a central surface brightness fainter than 24 mag/arcsec^2, but a total r-band magnitude > 21. Because these galaxies are often too faint to be seen, they have been suggested as a contributor to the “missing dwarf problem,” where cosmological models predict more small galaxies than are observed. See arXiv:1101.4136
Giant ellipticals (gE):
Green Pea galaxies (GPs): Very compact (~1-2 kpc half-light radius), low-mass starbursting galaxies in the nearby (z<1) universe that are intermediate in color between early and late type galaxies (green). These may represent high-luminosity blue compact galaxies. GPs are generally metal poor (oxygen abundance ~20% solar), but are relatively enriched in nitrogen.

Green pea galaxies (green dots) are nitrogen-enriched compared to most galaxies (density plot) of the same oxygen abundance. From Amorín et al. 2011 (arXiv:1105.1477).

Green valley galaxies: A small population of galaxies somewhere in between red (late type) and blue (early type) galaxies in the bimodal distribution of galaxy types. It is not clear whether these galaxies are simply outliers among the early and late type distributions, are in transition between the two populations, or are another class of galaxy alltogether. These galaxies tend to have AGN or stellar bars which could be responsible for quenching star formation and causing them to transition from early to late type. See e.g. arXiv:1101.3353 or the 4/5/2011 Astrobite.

Figure 2: A color-magnitude diagram that shows the "red sequence", "Blue Cloud", and less populated "Green Valley".

HII galaxies: Gas-rich, metal-poor dwarfs undergoing a period of star-formation that dominates its optical spectrum such that it is easily confused for a giant HII region. The galaxy itself is generally low-luminosity (M_V~-17), but there are very bright ~100pc knots in the center. The starburst is thought to last ~1 Gyr. (See e.g. arXiv:1101.4140)
H-alpha emitters (HAWs): Galaxies that have emission at the Balmer line H-alpha, usually found via narrow-band imaging surveys tuned to a given redshift. H-alpha emission is typically associated with star formation. See e.g. arXiv:1104.3608.
Irregular galaxies:
L* and M* galaxies: There is a “knee” in the luminosity function of galaxies that suggests a dividing a line between very bright and less bright galaxies (with L* in between) and very massive and less massive (with M* in between). Galaxies with L~L* are typically M* galaxies and vice versa. The Milky Way is roughly an M* galaxy.
Late type galaxies: Refers to galaxies on the right side of Hubble’s tuning fork diagram – essentially a synonym for spiral galaxies (see concepts PDF).
Lenticular galaxies (S0): These are galaxies that fall between spirals and ellipticals on the Hubble sequence, exhibiting both an old, spherical population and traces of a disk. Originally thought not to contain gas, but some have been found to be gas-rich (see e.g. 1991A&A…243…71V or arXiv:1101.5092).
Low surface brightness galaxies (LSBs): Galaxies with a low surface brightness (e.g. that are not bright for a given area in square arcseconds). The low surface brightness is associated with a low gas surface density. Curiously, these galaxies seem to follow a different star formation efficiency law than the normal Kennicut-Schmidt law. See e.g. arXiv:1103.3711 and also see fLSBs.
Luminous Blue Galaxies (LBGs):
Luminous Compact Blue Galaxies (LCBGs): Galaxies that are bluer (have more star formation) than their small size (half-light radius) would indicate and as bright or brighter than the Milky Way. They are common at z~1, but rare in the local universe, suggesting that this is a vigorous stage of evolution. One possible formation mechanism is the merger of a dwarf elliptical with a gas rich source which could provide fuel for star formation, such as another dwarf galaxy or an HI cloud. See e.g. 2004ApJ…617.1004W and arXiv:1103.0526
Luminous Red Galaxies (LRGs): Very luminous (~3 L^*) early-type galaxies, essentially giant elliptical galaxies. The brightest galaxies in clusters are typically LRGs. LRGs are ofted used to probe hierarchical galaxy formation and the SDSS LRG survey produced a conclusive detection of the baryon acoustic oscillations (BAO). See e.g. 2001AJ….122.2267E.
Lyman Alpha Emitters (LAEs): Strong Lyman alpha emission from star formation, an AGN, or gas infall allows these to be detected at high z (>3). They are traditionally thought of as galaxies in an early burst of star formation, with young stellar populations, very little dust (Av 2.5) and are good tracer sof galaxy evolution. It has been suggested that they can evolve to become Milky-Way like galaxies. See e.g. arXiv:1101.3017 and arXiv:1101.3321.
Lyman break analogs (LBAs): Local-universe (z < 0.2) equivalents of LBGs, very bright in the far-UV due to high star formation rates. They are similar to LBGs in their metallicity, morphology, radio emission, extinction, etc. See arXiv:1102.1740 and see also Blue Compact Galaxies.

HST rest-frame UV/optical images of LBA candidates. From 2009ApJ...706..203O

Lyman break galaxies (LBGs): Starburst galaxies at z>2 which can be detected very efficiently by their unique spectral signature near the Lyman-alpha line at 912 Angstroms (UV, redshifted into the optical). This signature is the product of intense Ly-alpha emission, minimal dust extinction from the ISM, and a discontinuity due to absorption from the atmospheres of massive stars and neutral hydrogen in the ISM. See 2002ARA&A..40..579G.
Polar Ring Galaxies (PRGs): Galaxies that appear to have a disk of gas oriented perpendicular to their stellar disk. In some cases (e.g. IGV 7576, at right), kinematic investigations indicate that the gas disk is actually a ring. Several formation scenarios have been suggested, including a major merger, tidal accretion from a dwarf satellite or nearby spiral, or “cold accretion” of primordial integalactic gas along very long (~1 Mpc) filaments. See e.g. arXiv:1104.2052.

The PRG UGC 7576 in the R band, from Spavone et al. 2011 (arXiv:1104.2052). The gas ring extends from the top left to the bottom right.

Pseudobulge galaxies: The term pseudobulge refers to a feature that looks like the central, spheroidal, “classical” bulge feature common to spiral galaxies, but is different in some significant way. One definition is a central brightening of the disc that is not extended vertically. Galaxies with pseudobulges have become of great interest because they seem to fall outside of the traditional velocity dispersion – supermassive black hole mass relationship ($M_{BH} – sigma$). The presence of a pseudobulge seems to be associated with a strong bar feature and pseudobulges and classical bulges can both be present in the same galaxy. See e.g. arXiv:1103.0525.
Quiescent galaxies: These are simply galaxies that are not observed to have ongoing star formation.
Satellite galaxies: Smaller galaxies gravitationally bound to a larger primary galaxy. Usually this refers to satellites of an isolated galaxy as opposed to members of a large cluster. The “missing satellites problem” refers to the disparity between the large number of satellites predicted by ΛCDM cosmological simulations and the relatively small number of satellites observed (see e.g. arXiv:1101.2674)
Spiral galaxies: The most common type of galaxy (as defined by a magnitude-limited survey of the local universe). Named for the apparently spiral structure of their thin-disk stars, caused by stellar density waves. They may also have nuclear spheroidal population (bulge) and an extended spheroidal population (halo). Spiral galaxies typically have ongoing star formation, particularly in their spiral arms. The Milky Way is a spiral galaxy.
Starbursting dwarf galaxies (SBDGs): Dwarf galaxies with gas fractions and star formation rates on the order of giant spiral galaxies (implying the gas will be consumed in less than a hubble time), but low metallicity. It may be that galactic winds carry away heavy elements formed in the galaxy out of its shallow potential well. See e.g. arXiv:1103.1116.
Starburst galaxies: A generic term for galaxies with regions undergoing a high rate of star formation that is roughly as luminous as the rest of the galaxy combined. Starburst phases are often short-lived (the starburst is highly variable). See also SMGs and ULIRGs. (See e.g. arXiv:1101.4140)
Sub-Millimeter Galaxies (SMGs): These are among the most vigorously star-forming galaxies in the universe (1000 Msol/yr), are very bright (L~10^13 Lsol), and typically found at high redshift (z~2.5). They are perhaps the result of mergers of gas-rich galaxies and are the progenitors of the massive ellipticals in the local universe. This phase of evolution apparently lasts only ~100 Myr. See e.g. 2008ApJ…680..246T.
Tadpole galaxies:
Tidal Dwarf Galaxies (TDGs): Dwarf galaxies that are created from material stripped from larger galaxies rather than simply clouds collapsing in small halos, proposed by Zwicky in 1956. Unlike other dwarf galaxies, TDGs could have no dark matter halo and may be metal rich because they are formed from the recycled material of more massive galaxies. They range from $sim 10^7-10^9 M_odot$, with larger TDGs typically formed by accumulation at the tip of tidal tails. Tidal interaction can cause starbursts in TDGs, and smaller TDGs may be easily disrupted by their parent galaxies, living <10 Gyr. See eg. arXiv:1103.2546.

Tidal dwarf galaxies (boxed in red) identified in the tidal tail of TDG 45115, from arXiv:1103.2546

Ultra-compact dwarfs (UCDs): These are similar to dSph/dE/cE galaxies, but more compact. Some UCDs are perhaps more like massive globular clusters than small galaxies and have been called giant globular clusters (GGCs). It is proposed that other UCDs are the remnant nuclei of galaxies which have been tidally stripped by more massive neighbors (see e.g. arXiv:1101.2460 and arXiv:1102.0001).

UCDs (boxed in green) are intermediate in brightness between brighter dwarf elliptical galaxies and dimmer globular clusters. This is a color-magnitude diagram of extended sources in the Hydra 1 cluster from Figure 2 of arXiv:1103.5463.

Ultra-Luminous Infrared Galaxies (ULIRGs): Galaxies with far-IR L(8-1000 microns) > 10^12 L_solar due to dust heated by young stars and, in about half of known ULIRGs, an AGN. These are perhaps the local-universe (z~2) analogy to SMGs, being galaxies with very high star formation rates. They are typically galaxies undergoing major mergers (a collission between two galaxies of similar mass) and are thought to represent a transitionary phase between gas-rich spirals and ellipticals. See e.g. arXiv:1101.5262 and arXiv:1102.1974.
Void galaxies: Galaxies that are found in voids, the nominally-empty tens-of-Mpc regions of space in the large scale structure of the universe. These essentially isolated galaxies comprise only about 7% of all galaxies. They are generally blue (star-forming, or “young”) and gas-rich. They are valuable tracers of the dependence of galaxy evolution, and especially gas accretion, on environment, See e.g. arXiv:1101.4187, arXiv:1103.4156, and arXiv:1103.5798.
Wolf-Rayet (WR) galaxies: These are simply galaxies which have recently undergone a starburst such that they exhibit spectral signatures of Wolf-Rayet stars (massive stars with strong mass loss). The existence of WR stars limits the timeframe since the starburst to roughly 2<t<5 Myr. See e.g. 2010A&A…517A..85L

Types of active galaxies and active galactic nuclei (AGN):

Active Galactic Nuclei (AGN): Active nuclei are common to galaxies in the early universe. AGN have diverse observed properties, but most of these can described by a single unified model. The unified model consists of a supermassive black hole with a superheated accretion disk that radiates in the optical through soft X-ray. The disk is surrounded by broad and narrow line regions (BLR, NLR) consisting of high and lower velocity gas, respectively. A dusty torus can obscure all of these components if viewed edge on. A relativistic jet can extend from the supermassive black hole to hundreds of kpc. Depending on the viewing angle of this system, the AGN can appear to have many different properties – hence the classification scheme given below. (see e.g. arXiv:astro-ph/0312545). AGN may correspond to periods of growth in supermassive black holes due to galaxy mergers (see e.g. 2006ApJS..163….1H).

Blazars: The most extreme class of AGN, they comprise the majority of extragalactic gamma ray sources. They are AGN whose relativistic jets happen to point toward Earth, producing both synchrotron and inverse Compton gamma rays. They are extremely variable over short timescales and their emission is highly polarized. They are often divided into two major subtypes based on the strength of their emission lines, FSRQs and BL Lac objects. See e.g. arXiv:1101.2764.
BL Lacertae (BL Lac) objects: A subclass of blazars with very weak optical emission lines (equivalent width < 5 Angstroms). XBL and RBL refer to BL Lac objects identified by either their X-ray or radio emission, respectively. XBLs and RBLs have different spectral properties which may be explained by viewing angle if radio-emission is more highly beamed than X-ray. See also HBLs and e.g. arXiv:1101.2764.
Broad-Line Seyfert 1 galaxies (BLS1s): Seyfert 1 galaxies with broad emission lines, which appear to follow the normal supermassive black hole to bulge size relationship. Contrast with NLS1s and see and e.g. arXiv:1102.0537
Composite galaxies: These galaxies fall on the border between Seyfert galaxies and LINERs in their emission line ratios.
Compton thick AGN: These are simply AGN with column densities of N_H>10^24 cm^{-2} in the X-ray. Apparently roughly a quarter of AGN in the local universe are Compton thick. See e.g. arXiv:1101.3478.
Double Radio source AGN (DRAGNs): AGN that produce narrow jets extending for hundreds of kpc out of either side of the galactic nucleus. The jets terminate in hugely dispersed radio lobes. Both the lobes and the jets are powerful radio emitters due to synchrotron radiation (see e.g. Alan Bridle’s page).
Dual AGNs: AGN with double-peaked emission lines are suspected of being dual AGN: hosts to two accreting supermassive black holes. The dual AGN phase is expected to be associated with galaxy mergers. However, double-peaked emission lines can have other explanations, including asymmetric outflows or extinction of a single AGN. See e.g. arXiv:1102.1733, 2011ApJ…727…71F, and arXiv:1103.2597.
Fanaroff and Riley types (FR Is and FR IIs): A classification scheme for blazar jets. FR II jets are highly collimated and terminate at bright hotspots far from the galaxy center, while FR I jets terminate nearer to the galaxy center. FR Is are associated with BL Lac objects and FR IIs with FSRQs. See 1974MNRAS.167P..31F and e.g. arXiv:1101.5342.
Forbidden High Ionization Line (FHIL) regions: Some AGN have emission originating from atoms much more highly ionized than is typical of AGN narrow line regions (NLRs). It is uncertain whether these regions are simply extremely photoionized NLRs (perhaps due to extreme SMBH accretion events or supernovae) or if the emission is due to an entirely different process, such as high-temperature collisional excitation. The FHIL regions are apparently high density regions very near to the nucleus, perhaps forming the inner wall of the torus. Examples of FHILs include [Fe VII], [Fe X], and [Ne V]. See arXiv:1103.0660

Blue spectrum of the AGN Q1131+16 displaying many FHIL features, Fig. 2 of Rose et al. 2011 (arXiv:1103.0660).

Flat-spectrum radio quasars (FSRQs): A subclass of blazars with recognizable broad emission lines, similar to other quasars and in contrast to BL Lac objects. FSRQ seems to be a catch-all classification for such AGN, which can also be referred to by a variety of other names depending on how they were discovered, including “optically violently variable quasars,” “highly polarized quasars” (HPQs), and “core-dominant radio quasars”. See also FR II jets and e.g. 1996ApJ…463..444S.
FRI radio galaxies: The radio emission is concentrated at the core – the jets fade after a short distances (see e.g. arXiv:1101.3223).
FRII radio galaxies: The jets are much more radio-bright and extended than FRI sources. FRII galaxies tend to be more luminous than FRI in general, but are much less common (see e.g. arXiv:1101.3223).
Hidden broad-line regions (HBLRs): Type II AGN are about evenly divided in number as HBLR or non-HBLR based on polarized light observations. The difference might be because HBLRs are dominated by their AGN and non-HBLRs are dominated by starbursts. See e.g. arXiv:1101.4132.
High-frequency peaked BL Lac objects (HBLs): A subclass of BL Lac objects whose high energy peak is located around 100 GeV, also typically bright in X-Rays and radio. Most >100 GeV sources are HBLs. (see e.g. arXiv:1101.2764)
High-redshift quasars (HZQs): Extremely distant quasars with z>~6, which corresponds to a lookback time at roughly the end of cosmic reionization. About 50 are known as of January 2011. See e.g. arXiv:1101.4965
Infrared-Faint Radio Sources (IFRSs): A rare (~7 per square degree) class of sources that are fairly bright in the radio (gtrsim 0.1 mJy at 20cm), but very faint in the infrared. It is possible that IFRSs are relatively nearby (1<z<2) radio-loud AGN or high-redshift SMGs with high-extinction, lobes of undetected radio galaxies, or high-latitude pulsars. However, perhaps the most favored explanation is that they are radio-loud AGN at high redshifts (zgtrsim4). See e.g. arXiv:1105.0960

CS0194, an IFRS. The contours are from 20cm radio emission, clearly showing the source. The infrared image demonstrates the non-detection of the source. From Figure 2 of Norris et al. 2011 (arXiv:1105.0960).

Low and High-excitation radio AGN (LERAGN and HERAGN): LERAGN do not have strong emission lines, unlike typical high-luminosity AGN (e.g. HERAGN). This may be because the supermassive black holes of LERAGN do not accrete in a radiatively effictient manner. LERAGN tend to appear on the red sequence, while HERAGN appear in the green valley. Consistent with that, LERAGN tend to have lower gas masses, but larger stellar masses, older stars, and larger supermassibe black holes. (see above).See e.g. arXiv:1101.4935.
Low-Ionization Narrow Emission Line Regions (LINERs): LINER galaxies have strong narrow emission lines similar to AGN, but are characterized by softer ionization. The discriminant used in classification is often that LINERs have less doubly-ionized O emission than AGN. Usually synonymous with Low Luminosity AGN (LLAGN). See e.g. 2006MNRAS.372..961K.

A BPT plot illustrating the different ionization regimes for AGN, LINERs, and HII regions ("stellar" ionization). From Figure 9 of arXiv:1103.4134.

Low Luminosity AGN (LLAGN): Synonymous with LINER.
Microquasar: These are actually not galaxies or AGN, but possibly a high-mass X-ray binary consisting of a black hole in orbit around a more massive stellar companion (see e.g. arXiv:1101.3486).
Narrow-Line Seyfert 1 galaxies (NLS1s): A subclass of Seyfert 1 galaxies with Hbeta full-width at half maximum less than 2000 km/s. A subset of these galaxies seem to be accreting near the Eddington rate and lie below the M_BH/bulge relation of normal galaxies (i.e. they’re supermassive black holes are smaller than one would expect based on their stellar mass). This may suggest that the nuclear black holes in these galaxies grow by secular processes rather than by mergers (producing “psudobulges ” rather than bulges). See also BLS1s and e.g. arXiv:1102.0537
Quasars (QSO): A historical term for distant, very bright AGN. Derives from their discovery as “quasi-stellar radio sources.” See e.g. Gene Smith’s UCSD page.
Radio galaxies: A general term for bright radio sources that usually refers to AGN viewed edge on. The dusty taurus may block blackbody emission from the accretion disk, but is a strong synchrotron emitter (see e.g. a Barbara Ryden’s page). See also DRAGNs. Radio galaxies at high redshift are sometimes called HzRGs, and radio galaxies that are not bright at visible wavelengths may be called Optically Fait Radio Galaxies (OFRGs) (arXiv:1104.3608).

Illustrating the difference in the spectral energy distributions of radio-loud and radio-quiet quasars. From 1994ApJS...95....1E.

Seyfert galaxies: Spiral galaxies with strong nuclear emission. Seyferts are usually less luminius and at lower redshift than quasars. The ‘type’ I or II classificaition used for AGN is also applied to Seyfert galaxies (eg. Type I AGN are also called Seyfert 1 galaxies or a Sy1s, ditto Sy2s). See e.g. Gene Smith’s UCSD page.
Types I and II AGN: Type I refers to AGN whose nucleus is visible (the spectra has both narrow and broad emission lines), while in type II AGN, the broad line region (BLR) is obscured and the lines are very narrow. This may be due either to the viewing angle or some intrinsic difference in structure (see arXiv:1101.3335).

Types of galaxy clusters:

Cool core (CC) and non-cool core (NCC): Thermal bremsstrahlung and line emission from the intracluster medium can cause significant energy loss (a cooling flow) from the brightest region of a cluster (it’s core) on timescales shorter than the age of the universe. Something like 80% of clusters have such cooling flows and are classified as cool core (CC) clusters, distinguished by optical line emission (e.g. Halpha). Mechanisms proposed to explain this difference include AGN heating and disruption by merger in NCC clusters. NCC clusters have flat abundance profiles, perhaps due to frequent galaxy mergers. (See e.g. arXiv:1101.3317 and )
Galaxy groups: This simply refers to a lower-mass system of galaxies than a cluster, but there are differences besides the mass. Gravitational interaction dominates the energetics of clusters, while various forms of feedback (radio jets, starburst wdriven winds) and mergers can dominate groups. Clusters are dominated by dark matter, have ~10% of their mass in gas (the intracluster medium, ICM), and relatively little mass in stars; groups have ~10% of their mass in stars and a negligable ICM. Moreover, clusters have typically formed since z~1, while groups are thought to have formed as far back as z~10. See e.g. arXiv:1102.1972 and arXiv:1104.4888.
Radio relics: Diffuse synchrotron radiation from cosmic ray electrons in the magnetic field of the intracluster medium shock-heated by a galaxy cluster merger.
Proto-clusters: Over-densities of massive galaxies in the very early (z~5) universe that are expected to merge heirarchically to form galaxy clusters in the late universe. See arXiv:1101.3586

Particularly famous individual galaxies and clusters:

Antennae galaxies (NGC 4038 and 4039): The nearest (~15 Mpc) and youngest of interacting galaxies. It is undergoing a burst of star formation that is producing new, compact star clusters that may be the progenitors of globular clusters. See also 2010AJ….140…75W and NED.
Abell 1367: A nearby (~90 Mpc) galaxy cluster. See e.g. NED.
Andromeda (M31): The closest Milky Way-like spiral galaxy, at a distance of only 770 kpc, and a dominant member of our local group. See NED
Arp220: One of the finest local (z=0.018, D=73 Mpc) examples of a local ULIRG. It is a system of gas-rich merging galaxies, complete with tidal tails and galactic outflows. An old (>1 Gyr) stellar population dominates its mass, while a young (<10 Myr) populations essentially dominates its luminosity. It is so gas-rich that extinction A_v~50-1000! This extinction limits the study of its nucleus (potentially an AGN) and starbursting clusters. See e.g. arXiv:1102.1974.
BL Lacertae (BL Lac): Host to the prototypical Bl Lac AGN at z=0.07. See NED.
The Coma Cluster: A massive galaxy cluster at ~100 Mpc. It contains many late-type galaxies with ongoing star formation, despite clear signs of ram-pressure stripping that typically robs cluster galaxies of gas. See also arXiv:0601108 and NED.
The Fornax Cluster: One of the most massive nearby (~20 Mpc) galaxy clusters, named for the constellation it is nearest to. See also NED
Large and Small Magellanic Clouds (LMC and SMC): Two large satellite galaxies of the Milky Way, so named because they are so near as to actually look like clouds in the Southern hemisphere. Apparently it is unusual for a galaxy to have two satellites as large as these (see e.g. arXiv:1011.2255).
The Triangulum Galaxy (M33, NGC 598): A large spiral galaxy in the Local Group, although smaller than the Milky Way and Andromeda. It is at a distance of about ~0.8-1 Mpc. Famous for its beautiful face-on spiral and for being, barely, a naked-eye object. See NED

M33 in a drawing from 1850 (left) and in the UV (from Landsman et al. 1992), from Fig. 18 of Javadi et al. 2011 (arXiv:1103.0755).

The Whirlpool galaxy (AKA the Question Mark Galaxy, M51, NGC 5194): A face-on interacting spiral galaxy at about 8 Mpc. Its interacting companion is NGC 5195. See NED.

Hubble ACS image of M51. Image Credit: NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler

M82 (NGC 3034): The nearest (3-5 Mpc) massive starbursting galaxy, producing a new supernova every ~3 years. It is interacting with the neighboring M81, with a close pass about 10^8 years ago. See e.g. arXiv:1101.4942

X-ray/optical/IR image of M82 from Chandra/Hubble/Spitzer, composite by NASA/CXC/SAO.

M83: Another gorgeous target for amateurs, at ~4.5 Mpc this is the nearest face-on grand design spiral (Hubble type SAB(s)c). It has a prominant central bar and a bright starburst. See e.g. arXiv:1102.2444
M87 (NGC 4486): A giant elliptical galaxy with a LINER at about 17 Mpc, the brightest galaxy in the Virgo cluster. See NED.
Mrk231 (Markarian 231)(: A ULIRG/QSO with many AGN outflows that seem to indicate a major merger. See e.g. arXiv:1102.4349 and 2007 SAO press release and NED.
The Milky Way: Our own galaxy.
The Sombrero galaxy (M104, NGC 4594: A large spiral at ~10 Mpc in Virgo; a favorite of amateurs for its attractive inclination. Famous among astronomers for its large globular cluster system, supermassive black hole, and prominent dust lanes. See NED
The Virgo Cluster: The largest nearby(~14 Mpc) cluster of galaxies, it is at the center of the Local Supercluster. Its BCG is M87. See also NED.

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