HCG 5

Key takeaways

HCG 5 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect HCG 5 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HCG 5 from memory before moving on to harder problems.

Reference excerpt

Hickson Compact Group 5 is a compact group of galaxies located in the constellation Pisces. It was identified using a systematic search by Palomar Observatory under a research of compact groups lead by astronomer Paul Hickson. In this group, only three members are interacting, HCG 3A, 3B and 3C, while HCG 3D is much closer to the Milky Way and has a much lower velocity than the rest of the group, indicating it is likely not interacting with the other galaxies in the group and it is not part of the merger.

Properties HCG 5 has a mean redshift of approximately 0.04, having a distance of 570 million light years, and it is located at a galactic latitude (gal z) of 0.04. A study suggests that HCG 5A is the brightest member in the group, having absolute magnitude (-M_B + 5 log h) 20.7, followed by HCG 5B, HCG 5C and HCG 5D, with an absolute magnitude of 19.9, 19.2 and 18.7 respectively. Another study suggests that HCG 5A is the luminous galaxy in the group, as HCG 5A has a blue minus red (B-R) and corrected total blue magnitude (B_TC) of 14.55 and 1.58, signifying that it is a luminous galaxy and has a large amount of interstellar dust and is undergoing vigorous amount of star birth and star formation. In contrast, HCG 5B is the reddest galaxy in this group, having a 2.03 (B-R), displaying that it is a galaxy populated with old, ageing stars and is currently not undergoing high amounts of star formation. HCG 5B also has a Radial Velocity of 12489 km/s (another study suggests 11944 km/s), and a velocity dispersion of 205.6 km/s, indicating it is a relatively massive galaxy, containing a high amount of dark matter. HCG 5 has a radial velocity dispersion (km/s) of 147.9 (another paper suggests 174) and a crossing time (H0tc) of 0.0132, suggesting that this group is undergoing rapid evolution and each member of the group is travelling at moderate speeds. This group also has a median galaxy separation of 25.7 kpc and a mass-to-light ratio (M☉/L☉) of 19.95, indicating it is a very dense group and has a moderate amount of dark matter.

Morphology

HCG 5A and 5B are currently undergoing a galaxy merger, as HCG 5A has a distorted spiral arm that is caused by gravitational interaction between galaxies. Another study shows that the total out of 4 galaxies, 2 are spiral galaxies, having the spiral fraction of HCG 5 at 0.5 or 50%. A total of 9 foreground stars and 44 foreground galaxies have been detected in the background of galaxy group HCG 5, with 0.2 or 20% of the field galaxies are spiral galaxies. An analysis of the environment around Hickson Compact Group 5 has been conducted, revealing that HCG 5 is relatively isolated and has a low surface density of background objects in the outer field, having a background surface density (σ_O arcmin⁻²) of 0.01762. HCG 5A, while nearly face-on, has two visible spiral arms, indicating that it is a spiral galaxy, even without a galactic bulge visible. HCG 5A also having a major axis diameter (arcminutes") of 24.40 and a minor axis diameter of 24.30" revealing that it is a proportionately circular galaxy, due to having an ellipticity (ε) of 0.004. On the other hand, HCG 5D is edge-on, and is a lenticular morphology galaxy. It has a major axis diameter and minor axis diameter of 12.30" and 5.0", having an ellipticity (ε) of 0.593, showing that it is a galaxy that has an elongated structure. HCG 5B and 5C are two nearly face-on galaxies, the same as HCG 5A.

Dynamics The radial velocity dispersion of a Hickson Compact Group is determined by the elongation of the group, suggesting that every member is rotating around the gravitational center of the group, proposing that Hickson Compact Groups are dynamically stable. A 21 cm radio survey has been conducted for HCG 5. The survey data has shown that HCG 5 has an observable neutral hydrogen (HI) content (log[M(HI)_obs]) of 9.77, and a predicted HI amount (log[M(HI)_pred]) of 10.05, indicating HCG 5 has a deficient amount of HI gas. This is due to HCG 5 having undergone significant dynamical evolution, as gravitational interaction or gas stripping from other companion galaxies, caused a deficient amount of Neutral Hydrogen (HI) and a moderate amount of spiral galaxies, due to HI gas stripping, transforming spirals into ellipticals. In another study, using Infrared Astronomical Satellite (IRAS), no far infrared (FIR) is detected, this is common due to deficiency of HI gas. HCG 5A has a integrated carbon monoxide (CO) intensity (I_CO(1-0) [K km/s]) of 5.73 km/s, indicating a significant amount of molecular gas and has a Spread Velocity of Molecular Gas (ΔV_CO(1-0) [km/s]) of 275 km/s, indicating that the gas is rotating around the galaxy at moderate speed.

Members Hickson Compact Group 5 has a total of 4 galaxy members. Their details are stated in the table below.

Gallery

References

Illustrations

Worked examples

Example 1 — a first encounter with HCG 5

Start with the simplest possible case. Write down what HCG 5 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 HCG 5 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 HCG 5 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 HCG 5

In research: HCG 5 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 HCG 5 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: HCG 5 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Galaxy clusters, Hickson Compact Groups, so understanding it makes those chapters shorter.
In everyday life: Look for HCG 5 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.

How to study HCG 5 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HCG 5 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 HCG 5 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HCG 5 in simple terms?

Hickson Compact Group 5 is a compact group of galaxies located in the constellation Pisces. It was identified using a systematic search by Palomar Observatory under a research of compact groups lead by astronomer Paul Hickson.

Why does HCG 5 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 HCG 5?

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 HCG 5.