Galaxy

A galaxy is a system of stars (both "living" and "dead" ones), interstellar gas, dust[notes 1], and dark matter that forms the basic building block of the Universe's structure.

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Some history

Believe it or not, but that galaxies are, well, galaxies is something known with certainly for less than a century. Even if the existence of a "small cloud in the constellation of Andromeda"[notes 2] was known for centuries[notes 3] and once the telescope was invented more of those clouds appeared galore, that were considered as mere nebulae within the Milky Way with the exception of the Magellanic Clouds,[notes 4] that were proved by telescopic observations to be galaxies of their own. Simply put, telescopes of the time lacked enough power to see them resolved into stars and it was not until the 1920s of the 20^th century when variable stars were discovered in Andromeda and later when it was proven without doubt those "nebulae" were actually something very different and much larger and more distant.

Classifying galaxies: putting order in the sky (or at least attempting that)

Of course with galaxies sprinkling the sky it was just matter of time before astronomers decided to classify them. The best known of the schemes used to classify galaxies is the one developed by the American astronomer Edwin Hubble[notes 5], and it's a testament to its usefulness that despite having been expanded and somewhat tweaked it's still in use. Hubble classified galaxies into the following types:

Elliptical galaxies

Elliptical galaxies are, well, galaxies that look more or less elliptical. Its actual shape range is an ellipsoid more or less flattened, from almost spherical to a football (rugby)-shaped system. They're also systems usually predominantly composed of old low-mass stars, giving them a yellow-orange hue on color images, usually with very little — if any — star formation, interstellar dust or (cold) gas — the stuff stars are birth from.[notes 6] They're subclassified depending of how elongated they appear from E0 (spherical) to E7 (very elongated; above it a galaxy would become unstable and would puff up)[notes 7]. These are the unique properties that they have in common.

The diversity of elliptical ellipticals include having the widest range of mass, sizes, and luminosities of all galaxies from tiny systems so faint that are outshined even by luminous stars (read: standard red giants). The faint ellipticals are very hard to detect, and are thought to be composed almost entirely of dark matter, while on the other extreme of the scale there're humongous galaxies that can be tens of times more luminous than ours — which is already a good-sized one — the haloes of which may extend up to millions of light-years away; these galaxies are considered the largest and most luminous ones known. Dynamically, ellipticals also show also a large variety: in the most luminous and massive ellipticals the stars move in random orbits around the center ("boxy ellipticals". Think on a swarm of angry bees), while moderately luminous ones have what seems to be a disk of stars embedded in the center with more ordered orbits ("disky ellipticals")[notes 8] and finally at least some of the less luminous ones have most, if not all, the stars in a disk — these systems are common on galaxy clusters. Density also varies a lot: the already mentioned very low-luminosity systems — just as the haloes of those very luminous galaxies — are so scattered that are very difficult to see, and some recently discovered similar galaxies take this to an extreme, being as large as our galaxy but of such a low surface brightness that very powerful telescopes are needed to see them[1][notes 9] and in the opposite side we've compact ellipticals (most, if not all of them, of low luminosity) where stars are so tightly packed (some recently discovered systems take this also to the extreme, being considered ultracompact) that from our perspective may look star-like[notes 10]. Massive (often supermassive) black holes are usually found in the centers of most ellipticals of moderate luminosity. The most luminous systems are typically hosts to large populations of globular clusters (up to thousands or even more)[2].

How did elliptical galaxies form? Current understanding shows that their origins are very varied: the less luminous systems are expected to be what remains of the numerous low-mass irregular galaxies (see entry below for them) that formed in the earliest moments of the Universe and that coalesced to form large ones as ours. These systems were originally gas-rich, but their weak gravity would not allow them to hold the gas that would be blown apart either by supernovae and/or interactions with nearby, more massive galaxies. No gas means no new stars, so as time passed by and stars died away just low-mass ones would remain. "Disky" low-luminosity elliptical galaxies are expected to have suffered a similar fate, except that their gas was blown away by interactions with the hot, low-density, gas expected to fill the intergalactic space in rich galaxy clusters — plus in some cases interactions with other galaxies of the cluster ("galaxy harassment") would warp them giving an elliptical look[3] — and talking about harassment the compact and ultra-compact galaxies may be all what remain of considerably larger ones that were threshed away by a much larger one.[notes 11]

Going upwards in mass, however, mergers are expected to be responsible of the origin of moderately luminous ellipticals and up. Things begin when two gas-rich spiral galaxies of more or less similar size collide and distort themselves becoming a galactic mess and creating in the process a whole lot of stars and clusters ("wet merger"), including massive ones that will become globular clusters in the future as their stars age, and in the process funneling gas to the center where once the gas is both exhausted by even stronger star formation activity and carried away by supernovae and strong stellar winds (or even a quasar), a dense disk of stars within the elliptical's main body is unveiled[notes 12]. Later they continue when two or more of said "disky" ellipticals collide and merge again — with no or very little available gas there will be very few or no star formation, hence a ("dry merger") — destroying said disks and forming a "boxy" one. In rich galaxy clusters, the pile-up continues and eventually a humongous elliptical galaxy forms, usually in the center that more often than not is surrounded by a very extensive halo of stars.

Fun fact: large elliptical galaxies may be the best ones to develop life. Some research suggests one of those galaxies of the big type could harbor ten thousand times as many Earth-like planets than your typical big spiral galaxy (read: the Milky Way).[4]

Disk galaxies

Disk galaxies are galaxies where stars live in a disk — a big one, not something embedded in a halo of stars as in some ellipticals — that thickens towards the center (a "galactic bulge", where stars are crammed up). However, unlike ellipticals that look bland and simple[notes 13] disk galaxies have a far more varied structure, so we must first describe an expanded system to describe them. Said system was developed by the French astronomer Gérard de Vaucouleurs back in 1959, even if some of the ideas that follow harken back to Hubble's original classification[5]. In all cases this classification uses an "S"[notes 14] followed by what follows is used, being shared by all types of disk galaxies:

Bars. Disk galaxies may have or not a central bar, which is denoted by a "B". Intermediate forms -an oval central region, for example- are classified as "AB" (if there's no bar an "A" is added). After this comes:

Rings. Rings may be present in disk galaxies too, be internal or external. In the de Vaucouleurs' classification sequence inner rings are denoted by (r). Ill defined rings as (rs), and finally no rings as (s). An outer ring is denoted as (R) and is put in front of the "S" above.

Lenticular galaxies

Lenticular galaxies are the first type of disk galaxy. Like ellipticals they tend to have very little, if any, cold gas and thus a similar level star formation even if interstellar dust may be abundant meaning that they're abundant in old stars too. Supermassive black holes may lurk at their centers too.

Like ellipticals, they extend across a large interval of mass, sizes, and luminosities. However, said range is more restricted on both sides, meaning that both very small lenticular galaxies are not known and big ones are rare. In terms of structure they're, well, stellar disks where the most conspicuous structures are that central bulge, more or less luminous compared to its galaxy (it tends to concentrate a significant fraction of the galaxy's luminosity) and especially as explained above a bar of stars (often there are several, one much smaller nested within other).

The origin of lenticular galaxies is still a topic of research, but current understanding suggests that the least luminous systems are spiral galaxies where the interstellar gas was exhausted by star formation or removed[notes 15], while the most luminous ones were born in a past merger[6]

Lenticular galaxies despite its apparent blandness have quite a variety of features. In addition to bar and ring( s) as commented above, the de Vaucouleurs' system splits them into three types: S0^-, S0⁰, and finally S0⁺, where this denotes how "developed" is the galaxy -a S0^- is hard to distinguish from an elliptical except for a different luminosity gradient from the center to the edges while the S0⁺ has well-defined structures as dust bands and the like. S0⁰ are intermediate between the two (in barred ones, SB0^-, SB0⁰, and finally SB0⁺ this denotes how well formed is the bar: the further one progresses the better it is. In addition to this, there're both galaxies transitional between ellipticals and lenticulars (E⁺, also E/S0) as well as transitional galaxies between lenticulars and spirals (S0/a), and finally the Canadian astronomer Sidney Van den Bergh has developed a system that mimics the one for spirals (see further), depending how well developed is the central bulge (S0a, S0b, and S0c)[6]. So combining all of this with the above section, a barred lenticular galaxy with an both inner and outer rings and somewhat defined bar would be classified as (R)SB(r)0⁰, an unbarred one with an inner ill-defined ring and with visible structure as SA(rs)0⁺, and so on.

Spiral galaxies

Spiral galaxies are disk systems where, as the name implies, more or less-defined spiral arms are present. Unlike lenticulars, significant amounts of gas as well as young stars may be (and usually are) present tipically within the arms, giving them a bluish tinge that contrasts with the orange-yellow color of the bulge, dominated by old stars. They span a narrower range of luminosities than lenticulars and especially ellipticals, but dwarf spiral galaxies are known as well as a few systems of high luminosity and size as NGC 772, UGC 2885, or Andromeda. Our galaxy, the Milky Way, is a spiral galaxy.

Hubble originally divided them into three types depending on how well developed are the central bulge respect to the spiral arms: Sa, Sb, and Sc (plus SBa, SBb, and SBc for barred ones). Sa galaxies have large bulges and tightly wound arms with little star formation, being them defined mainly by interstellar dust, gas, and usually being as red as ellipticals or lenticulars. Sb galaxies have more open arms, smaller bulges, and more star formation and interstellar gas being bluer[notes 16], and finally Sc galaxies have well-defined, open arms with small bulges and are typically forming many stars being rich on interstellar gas and being the bluest of the three. Later developments have added two more classes: the Sd galaxies, with even smaller or non-existent bulges, and finally the Sm galaxies that are borderline irregular galaxies (see further) (in both cases, star formation tends to be more or less abundant as interstellar gas is and are also blue). In addition to this, intermediate forms between those (ab, bc, cd, dm) have been added and remember the presence or not of rings and or bars is accounted for (a galaxy with inner ring, arms between an Sb and an Sc, and a central bar -just like our Milky Way- would be classified as SB(r)bc, a galaxy with a large bulge, no bar, tightly wound spiral arms, and an outer ring as (R)SA(s)a...-. Other classifications are based on the look of the spiral structure: a flocculent spiral galaxy has patchy, discontinuous arms while a Grand design spiral galaxy has long, well-defined arms and there're intermediate forms and finally how are their central regions: some galaxies as Andromeda have what is known as a "classical bulge", that can be broadly described as a (disky) elliptical galaxy in the middle of the spiral galaxy's disk, thought to have been originated in a major merger while others like ours have a pseudobulge, that shares its properties with the galaxy's disk and is thought to have been formed by internal processes relative to galactic bars (see further).

What is the origin of spiral structure (and bars)?. The origin of the former is explained by the Density wave theory, that explains the spiral arms as mere densities on the distribution of stars and interstellar matter (gas and dust). The latter is compressed as they pass across the density waves to the point of starting star formation, so marking the position of the density waves. A competing theory, which not excludes the former and vice-versa, suggests that spiral arm formation is triggered by supernovae and stellar winds of massive stars. Bars are thought to have similar origins, in density waves.

As for the origin of spiral galaxies, it's thought they formed from the merger of many smaller galaxies, but on a more peaceful and ordered manner than ellipticals. Galaxies with large central bulges as Andromeda, however, may have been born in major mergers where there was plenty of gas to reform a disk surrounding the massive bulge[7]

Irregular galaxies

Irregular galaxies are galaxies that, as the name implies, have no marked shape (no nucleus, spiral arms, etc) and often a chaotic aspect. They tend to be small systems, with the largest ones comparable to small or mid-sized spirals (but see below). Two types of them can be defined: the Irr I, that usually are systems resolvable into stars and nebulae -those that have vaguely discernible spiral structure merge with the Sm category described above for spirals, while others that have no structure at all are classified as Im[notes 17], and the Irr II, also often known as I0 that are totally messed up systems with large amounts of dust obscuring them, that are usually the product of a past interaction or merger between two galaxies and thus can be much larger than your standard irregular.

A third type, the dIrr (dwarf irregular) is also often added. As the name implies, they're small irregular systems rich in gas (or very rich for their small size), and are thought to evolve into dwarf spheroidals (see above) once they run out of gas to make new stars. Transitional systems between both as LGS 3 are known to exist. They're thought to be the basic building blocks from which larger galaxies would form, as these tiny (in cosmological terms) systems collided and merged among themselves in the early Universe.

Peculiar galaxies

Peculiar galaxies can be described as the freaks of the Universe, in some cases being so bizarre that cannot be classified in one of the categories mentioned above. Other times are still recognizable, but have their share of peculiarities such as a more or less distorted shape, tails connecting them to other galaxy/galaxies, and the like. They're designed by adding the suffix "pec" to their classification, except in some cases so messed up that they're simply known as "Pec".

In most cases those weird shapes are caused by interactions with other galaxies, up to and including mergers (galaxy sex is a very hazardous activity for the galaxies involved, often more than two). Other times the peculiarities are jets expelled by active galactic nuclei as quasars, such as the nearby Messier 87 in the Virgo Cluster.

Halton Arp's Atlas of Peculiar Galaxies, despite being almost sixty years-old is still the galactic freak show for astronomers.

Active galactic nuclei

Some galaxies have something funny going on in their centers, with a very small part of it shing a lot more than it should be (often a lot means outshining the rest of the galaxy several times or more): these are galaxies with active nuclei ("AGN" for short).

The consensus is that said activity is caused by a supermassive black hole located in the galactic center, or rather the accretion disk that surrounds it. Other times there's also a starburst (star formation galore) complimenting it, especially in those faraway systems during the early days of the Universe when things were far more lively than now. As described in the next section, galactic interactions and mergers are thought to be what causes a galaxy to go AGN, funneling gas to its center where it has no other place to go, except the black hole's maw (plus some matter being expelled in the form of luminous jets -see Messier 87, already mentioned-) and/or having said gas compressed to form stars in a starburst.

The brightest AGNs are quasars, where the galactic nucleus is so bright that not only it can be up to dozens of times more luminous than a typical large galaxy, but also the rest of the galaxy is swamped by its light and one needs the Hubble telescope and/or special techniques to uncover it[notes 18].

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Galaxy interactions and mergers

Needless to say, with galaxies living (relatively) so close to each other, interactions between them as well as mergers are quite common. As with so many things, the small ones in proportion get the worst part.

Mergers are perhaps the most violent events in the life of a galaxy and their outcomes are very different depending on the relative size of the involved systems: a small galaxy may be devoured by a much larger one with the latter being totally fine except for the remnants of the victim -for a time- ("galactic cannibalism"), while the closer in sizes and mass colliding galaxies are, the more interesting things become.

Galaxy mergers have been simulated in computers (see here, for example) showing how the orbits of the stars in the previous galaxies are destroyed being replaced by new, randomized ones (in other words, in a collision of two disk galaxies, the disks are destroyed)[notes 19]. Gas clouds, if present, suffer the worst being compressed making stars galore and sent to the centers of their galaxies (and the merger remnant when the latter merge), where they'll make even more stars still being more compressed and/or will feed an active galactic nucleus.

As time passes by, that gas is blown away either by supernovae and stellar winds or (often and) that active galactic nucleus leaving behind a galaxy that looks like an elliptical galaxy, albeit surrounded by shells and long tails of stars and young stars before finally settling as a normal-looking elliptical[notes 20]. The Toomre sequence illustrates this quite well.

Galactic interactions are milder compared with galaxy mergers, ranging from distant approaches like two strangers in the night where the two galaxies will go away almost unscathed to far more serious encounters that may royally mess up things on the affected galaxy(es) -high star formation, distorted shapes... name it-[notes 21] (they usually are a prelude for a final merger, as those glancing blows slow galaxies in their orbits-), up to collisions without merging that are even more hazardous for the affected system(s). On rich galaxy clusters, as mentioned above, multiple encounters at relative high speeds between galaxies may transform late-type systems into early-type ones[notes 22].

Groups and clusters: the places where galaxies live

A galaxy group is an aggregation of no more than 50 galaxies bound by gravity against the Universe's expansion. They've a size of no more than 1-2 megaparsecs (ie: 3-6 million light-years), a low velocity dispersion[notes 23], and in cosmological terms are low-mass (no more than 10¹³ solar masses). Our galaxy together with Andromeda and a bunch of usually much smaller ones lives in one of those, the Local Group. In some galaxy groups, galaxies live in closeness one to each other, meaning that the interactions among them are very common. A compilation of these groups ("compact groups") was published in 1982 by the astronomer Paul Hickson, the Hickson Compact Group. Eventually all those galaxies will forever embrace into one much larger galaxy ("fossil group"), with at best a couple of much smaller remaining galaxies[notes 24].

Galaxy clusters are much larger complexes, where up to thousands of galaxies exist together. They're the largest gravitationally bound structures in the Universe[notes 25], and their estimated masses range from 10¹⁴ to 10¹⁵ solar masses. They're much larger than groups, between 2 and 10 megaparsecs and those high mass gives them higher velocity dispersions.

Dark matter aside, that forms around 90% of the cluster's total mass, the most conspicuous ingredient of a galaxy cluster is very hot plasma detectable thanks to its X-ray emission that fills its intergalactic space and contains most of the cluster's (visible with especialized instruments) mass (up to 95%).

The largest and most luminous galaxies known, the cD-type ellipticals (see above), live in galaxy clusters and tend to be situated in their centers. The mechanism for their growth is, as commented above, mergers between smaller galaxies produced by fall into the cluster's center due to interaction with others and/or the cluster's gravity potential (that pesky dark matter). Another feature of most clusters is how their central regions are usually dominated by early-type galaxies (remember: ellipticals and lenticulars), while late-type ones (spirals and irregulars) live at the outskirts. This is thought to be caused by how harmful is for a late-type galaxy to live in the central regions of a galaxy cluster, where in one hand as commented above for the origin of dwarf ellipticals interactions with other galaxies of the cluster will mess up them[notes 26] and in the other interactions with that mentioned intergalactic hot gas will strip them of cold gas and thus raw matter to form new stars, transforming them into early-type galaxies, either dwarf ellipticals or lenticulars[notes 27].

Galaxy clusters are classified depending of how are galaxies that form them in the Bautz–Morgan classification, that ranges from "Type I", with one luminous and large elliptical galaxy dominating the cluster, to "Type III", where the most luminous galaxies are similar in brightness (the "Type II" contains two or more ellipticals galaxies less luminous than those of "Type I", but more than those at "Type III" clusters).

The Abell catalog is the hitch-hiker's guide astronomer's catalog to galaxy clusters.

The nearest galaxy cluster to the Local Group is the Virgo cluster, in the constellation of the same name. It contains around 2,000 galaxies, most of them dwarfs, and its most famous member (but not the most luminous one) is the already mentioned giant elliptical galaxy Messier 87, famous for the jet that is emitted from its center. In the opposite side of the sky and a bit further away we can find the Fornax cluster, considerably smaller and less massive but much more compact.

Notes

1. And in "dust" we include junk such as planets, asteroids, comets… that while they outnumber in number stars one cannot say the same in what refers to their mass
2. The famous Andromeda Galaxy, known for centuries as the "Andromeda Nebula",
3. Same for the Clouds of Magellan.
4. Named so by the Portuguese explorer Ferdinand Magellan, who described them in the 16^th century. However they were known since prehistoric times by people who lived in the Southern Hemisphere
5. Yes, his name is where the name of that famous space telescope comes from
6. Hot gas, detectable thanks to its emission of X-Rays may be much more abundant
7. However, some flattened ellipticals can look spherical seen from certain angles
8. In addition to that, other differences include in the former a "core" (central missing light (read: relative lack of stars in the very center)), something that does not happen in the latter. See review here
9. That those galaxies have been found on rich galaxy clusters, may hold clues to their origins
10. M32, a satellite galaxy of the Andromeda Galaxy, is the nearest and best studied of those compact systems
11. Gravitational interactions would have funneled gas to the center of the doomed galaxy, feeding a powerful starburst that would have produced those tightly packed stars
12. See, for example, the merger remnant NGC 7252
13. As described by Hubble: "small patches of obscuring material are occasionally silhouetted against the luminous background, but otherwise these nebulae [elliptical galaxies] present no structural details"
14. From Spiral, as in Hubble's original sequence there were just spiral galaxies
15. For example stripped away by the hot, tenuous, gas that fills the intergalactic medium of rich galaxy clusters. This idea is reinforced by the presence of so-called "Anemic galaxies" with ill-defined arms in those places
16. Remember that young, massive stars are blue/white while old stars (red giants) and small ones orange/red
17. m is for "Magellanic", as the Clouds of Magellan are prototypical examples
18. 3C 273 is the brightest quasar, bright enough to be spotted by amateur telescopes, and (usually) is the farthest object amateur astronomers can see -is quite a feeling to know that anonymous faint star actually shines with the light of more than two hundred Milky Ways from a distance of more than two billion light-years-
19. Not the stars due to their small sizes and even planetary orbits (if the Sun was a marble, Neptune's orbit would be comparable in diameter to the wingspan of a Boeing 747-400 and the closest star would be at more than 300 kilometers away)
20. In some cases, depending of the remaining amount of gas, a disk galaxy either lenticular or spiral can be created instead
21. Including to cause the affected galaxies, if they're diskies, to usually develop grand design patterns and bars]
22. A fine example of this, that may actually be a sort of galactic "ménage à trois", are the nearby Eyes Galaxies
23. The relative speed of galaxies forming it
24. As our galaxy is doomed to interact and merge with Andromeda plus the Triangulum Galaxy, the third most important member of the Local Group, within few billion years, the Local Group would become a "compact group", followed by being a "fossil group" once everything had fused in
25. Most Superclusters are thought not to be gravitationally bound, at the very least totally
26. Mergers in galaxy clusters are not very common due to the high relative speed of the galaxies that form them
27. With, for spiral galaxies, transitional forms known as the already mentioned Anemic spiral galaxies and/or passive spiral galaxies

References

1. Galaxies at the Extremes: Ultra-diffuse Galaxies in the Virgo Cluster by J. Christopher Mihos et al. (2015) The Astrophysical Journal Letters, Volume 809, Number 2 .
2. A list can be seen here
3. Kinematic Properties as Probes of the Evolution of Dwarf Galaxies in the Virgo Cluster by E. Toloba et al. (2009) The Astrophysical Journal Letters, Volume 707, Number 1 .
4. The Quest For Cradles Of Life: Using The Fundamental Metallicity Relation To Hunt For The Most Habitable Type Of Galaxy by Pratika Dayal et al. (2015) The Astrophysical Journal Letters, Volume 810, Number 1 .
5. Galaxy Classification
6. Luminosities of Barred and Unbarred S0 Galaxies by Sidney van den Bergh. (2012) The Astrophysical Journal, Volume 754, Number 1.
7. The Recent Stellar Archeology of M31—The Nearest Red Disk Galaxy by T. J. Davidge et al. (2012) The Astrophysical Journal, Volume 751, Number 1.