astronomy

Type-cD galaxy

Type-cD galaxy is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Type-cD galaxy rather than just read about it. In short: The type-cD galaxy (also cD-type galaxy, cD galaxy) is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy. Characterized by a large halo of stars, they can be found near the centres of some rich galaxy clusters.

Key takeaways

Type-cD galaxy belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Type-cD galaxy to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Type-cD galaxy from memory before moving on to harder problems.

Reference excerpt

The type-cD galaxy (also cD-type galaxy, cD galaxy) is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy. Characterized by a large halo of stars, they can be found near the centres of some rich galaxy clusters. They are also known as supergiant ellipticals or central dominant galaxies.

Characteristics The cD-type is a classification in the Yerkes galaxy classification scheme, one of two Yerkes classifications still in common use, along with D-type. The "c" in "cD" refers to the fact that the galaxies are very large, hence the adjective supergiant, while the "D" refers to the fact that the galaxies appear diffuse. A back-formation of "cD" is frequently used to indicate "central Dominant galaxy". cDs are also frequently considered the largest galaxies. cD galaxies are similar to lenticular galaxies (S0) or elliptical galaxies (E#), but many times larger, some having envelopes that exceed one million light years in radius. They appear elliptical-like, with large low surface brightness envelopes which may belong as much to the galaxy cluster as the cD galaxy. It is currently thought that cDs are the result of galaxy mergers. Some cDs have multiple galactic nuclei. cD galaxies are one of the types frequently found to be the brightest cluster galaxy (BCG) of a cluster. Many fossil group galaxies are similar to cD BCG galaxies, leading some to theorize that the cD results from the creation of a fossil group, and then the new cluster accumulating around the fossil group. However, cDs themselves are not found as field galaxies, unlike fossil groups. cDs form around 20% of BCGs.

Importance Massive galaxies such as supergiant elliptical galaxies are important to understanding the evolution of the universe, because they, along with other large-early type galaxies, account for half of the Universe's stellar mass, contribute significantly to its chemical enrichment and provide clues to the star formation history of the universe.

Growth cD galaxies are believed to grow via mergers of galaxies that spiral in to the center of a galaxy cluster, a theory first proposed by Herbert J. Rood in 1965. This "cannibalistic" mode of growth leads to the large diameter and luminosity of the cDs. The second-brightest galaxy in the cluster is usually under-luminous, a consequence of its having been "eaten". Remains of "eaten" galaxies sometimes appear as a diffuse halo of gas and dust, or tidal streams, or undigested off-center nuclei in the cD galaxy. The envelope or halo may also consist of the "intra-cluster light", originating from stars stripped away from their original galaxy, and it can be up to 3 million light years in diameter. It is estimated that the cD galaxy alone contributes 1–7%, depending on the cluster mass, of the total baryon mass within 12.5 virial radii.

Dynamical friction Dynamical friction is believed to play an important role in the formation of cD galaxies at the centres of galaxy clusters. This process begins when the motion of a large galaxy in a cluster attracts smaller galaxies and dark matter into a wake behind it. This over-density follows behind the larger galaxy and exerts a constant gravitational force on it, causing it to slow down. As it loses kinetic energy, the large galaxy gradually spirals toward the centre of the cluster. Once there, the stars, gas, dust and dark matter of the large galaxy and its trailing galaxies will join with those of other galaxies who preceded them in the same fate. A giant or supergiant diffuse or elliptical galaxy will result from this accumulation. The centers of merged or merging galaxies can remain recognizable for long times, appearing as multiple "nuclei" of the cD galaxy.

cD clusters Type-cD galaxies are also used to define clusters. A galaxy cluster with a cD at its centre is termed a "cD cluster" or "cD galaxy cluster" or "cD cluster of galaxies".

Examples 3C 401 Abell 1201 BCG Abell 1413 BCG Abell 2261 BCG Abell 2390 BCG ESO 383-76, the large central galaxy of Abell 3571 ESO 444-46, the central galaxy of Shapley Supercluster Holmberg 15A (home to one of the largest black holes currently known) IC 1101, the large central galaxy of the massive cluster Abell 2029. Messier 87, the central galaxy in the Virgo Cluster NeVe 1, the host galaxy of the Ophiuchus Supercluster eruption event, the most energetic outburst known. NGC 1399 in the Fornax Cluster NGC 4889, is also known as the Caldwell 35, a supergiant galaxy, a class-4 elliptical galaxy, it is the brightest within Caldwell Objects in the constellation Coma Berenices NGC 6086 NGC 6166 Perseus A QSO 0957, the first identified gravitationally lensed object

See also Elliptical galaxy (E#) Giant elliptical galaxy (gE) Lenticular galaxy (S0, SA0, SAB0, SB0, E9) Seyfert galaxy Type-D galaxy List of largest galaxies - a broader list containing type-cD galaxies, arranged by size.

References

Further reading PDF—"A Preliminary Classification of the Forms of Galaxies According to Their Stellar Population", W. W. Morgan, Yerkes Obeservatory, 1958, doi:10.1086/127415, Bibcode:1959PASP...71..394M (PASP 70) "Intracluster light and the extended stellar envelopes of cD galaxies: an analytical description", Marc S. Seigar, Alister W. Graham, Helmut Jerjen, July 2007 doi:10.1111/j.1365-2966.2007.11899.x, Bibcode:2007MNRAS.378.1575S, arXiv:astro-ph/0612229v2 (MNRAS 07/2007)

Illustrations

Worked examples

Example 1 — a first encounter with Type-cD galaxy

Start with the simplest possible case. Write down what Type-cD galaxy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Type-cD galaxy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Type-cD galaxy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Type-cD galaxy

In research: Type-cD galaxy appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Type-cD galaxy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Type-cD galaxy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Morphological types of galaxy, so understanding it makes those chapters shorter.
In everyday life: Look for Type-cD galaxy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Type-cD galaxy in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Type-cD galaxy means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Type-cD galaxy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Type-cD galaxy in simple terms?

Why does Type-cD galaxy matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Type-cD galaxy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Type-cD galaxy.